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DALTON PERSPECTIVE J. Chem. Soc., Dalton Trans., 1999, 815–822 815 The chemistry of volatile waste from silicon wafer processing Peter L. Timms School of Chemistry, University of Bristol, Bristol, UK BS8 1TS Received 28th August 1998, Accepted 30th November 1998 The semiconductor industry uses extremely pure gases and vapours to produce solid-state devices based on silicon wafers of great chemical and physical sophistication under very clean conditions. However, the waste volatiles from the processes pose a serious environmental threat.This article attempts to show how an increasing armoury of chemical methods is being applied to abate any pollution and to highlight areas where ideal solutions to problems have yet to be found. 1 Introduction The production of semiconductors for use as computer chips has grown phenomenally in the last twenty five years and the semiconductor industry is now very large and of major economic importance. The industry is rightly regarded as “high tech” with its use of ultra-pure materials handled and processed under very clean conditions using sophisticated equipment.It is still dominated by semiconductors based on silicon and a new “Fab”, a unit for the large scale production of silicon chips through processing of 200 mm diameter silicon wafers, will cost upwards of $1 bn. Even higher costs are predicted when the industry moves on from its current recession to make chips on 300 mm diameter wafers.Chemistry plays a part in the industry in a variety of ways: in the production of ultra-pure, single crystal, silicon ingots which are cut into wafers; in the choice and design of chemicals which can dope, etch or coat wafers to help achieve an intricate architecture with millions of individually accessible sub-micron domains from which the final chips are cut; in developing methods to make ultra-pure forms of the chemicals used to process silicon under vacuum or in liquid media at atmospheric pressure.Many of these chemical aspects of the semiconductor industry have been reviewed.1,2 Peter Timms graduated from Oxford and spent two years in industry gaining experience with high temperature chemistry. This shaped much of his subsequent work in synthetic main group and transition element chemistry at Oxford, Rice, Berkeley and at Bristol where he is a Reader in Inorganic Chemistry, and led him to become involved with pollution control in the semiconductor industry through a successful use of hot solids to destroy waste gases.He is a consultant to BOC Edwards. Peter L. Timms Less obvious is the fact that the ultra-clean semiconductor industry has a significant waste disposal problem arising from its use of a wide range of gases and vapours in low pressure processing of wafers. It is the chemical challenge of how to deal with this gaseous waste which is the topic of this perspective. 2 The range of process gases and their reaction products Table 1 shows a list of gases and vapours used in processing of semiconductors.This is a list which changes with time under the pressure of new process developments on the one hand and environmental considerations on the other. For example, the demands of semiconductor device miniaturisation are dictating a change from aluminium to copper as the main metal for electrical and heat conduction and this is spurring the use of volatile metal–organic precursors of copper.A few years ago, tetrachloromethane and other carbon–chlorine compounds would have been in the list for plasma etching of aluminium. These are no longer used both on account of the Montreal agreement on the release of ozone depleting substances and because of a perceived risk of dioxin production from a malfunction of equipment employing thermal methods to destroy these gases. Table 2 shows some additional gases and vapours which arise from the use of the compounds of Table 1 in semiconductor device processing using plasma etching or chemical vapour deposition.For example, plasma etching of silicon using per- fluoroalkanes mixed with oxygen at low pressure generates SiF4, COF2 and traces of perfluoroalkenes which are far more dangerous than the starting materials. Each vacuum chamber where processing of wafers occurs will be using some of the gases or vapours in Table 1 in short Table 1 Some of the gases and vapours used in semiconductor processing (number in parentheses show approximate amounts in metric tons per annum used by the semiconductor industry) Hydrides SiH4 (300) Si2H6 (2) GeH4 B2H6 (2) NH3 PH3 (13) AsH3 (7) Fluorides CF4 (800), C2F6 (400) C3F8 CHF3 (160) (CF3CO)2O NF3 (190) SF6 (50) ClF3 (50) WF6 (40) Other inorganics SiH2Cl2 BCl3 (200) HCl (1000) HBr (40) Cl2 (100) O2, NO, N2O (300) H2, H2O2 Organoderivatives Si(OEt)4 (150) B(OMe)3 P(OMe)3 (Me2AlH)2 But 3Al Me3Ga [Cu(Hfac)- (TMVS)] a a Hfac = Hexafluoroacetonate, TMVS = trimethylvinylsilane.Table 2 Some volatile products formed during semiconductor processing SiF4 SiCl4 (SiOCl2)n (SiH2NH)n HF COF2 C2F4 C4F8 AlCl3 WOF4 NH4Cl NH4F (NH4)2SiF6816 J. Chem. Soc., Dalton Trans., 1999, 815–822 bursts, interspersed with permanent gas purging and cleaning operations. The percentage of each process gas which is used eVectively varies widely with the nature of the process and the design of equipment but currently figures of 25 to 50% consumed are typical.Approximate values for the amounts in metric tons of some of the compounds which are used annually in the industry are included in parentheses in Table 1. It follows from above that the waste gases will contain 25–50% of these amounts so that, overall, there will be many tons of waste to deal with. To put these quantities into context, the waste CF4 from the semiconductor industry is <10% of the amount produced as a byproduct of extracting aluminium by electrolysis in fluoride containing melts with carbon anodes.On the other hand, the semiconductor industry is probably the major producer of waste containing Group 14 and 15 hydrides, or fluorides such as NF3, ClF3 and WF6, or some metal–organic compounds. In the last category, as has been mentioned there is a great potential for growth of copper precursors which could generate tons per annum of waste in a few years times. 3 Methods of dealing with gaseous waste 3.1 Destruction or recycling? It might seem that the hazardous gaseous waste from semiconductor processing could be dealt with in either of two ways.It could all be destroyed and converted into products which are much less hazardous or it could be collected, fractionated and the useful components recycled. The latter would seem to be the obvious environmentally friendly approach. Unfortunately, recycling is not an easy option for the reasons given below and destruction is currently the dominant technology.The semiconductor industry is based around the use of very pure substances mainly because the intrinsic properties of silicon are aVected beneficially or adversely by such low concentrations of additives. So all gases used in processing are extremely pure, at least 99.995% and upwards to 99.99999%. This purity is achieved by gas manufacturers by very careful selection of source materials to avoid compromising impurity elements as far as possible since these elements are often very hard to remove completely from the process gas.Gases recovered after use in semiconductor processing are likely to contain a broader spectrum of impurities than is compatible with their easy and economic reconversion to very high purity starting materials. So it is often more expensive to recycle gases than to use new gases and destroy what is unused in wafer processing. Of course, future legislation may force the industry into recycling but at present only a few opportunities for economic recycling can be seen.Examples include some of the perfluoroalkanes, compounds so stable that they could be put through vigorous purification procedures to remove possible impurities (see section 4) and some metal–organic compounds which are so expensive to synthesize that recovery and recycling may be economically attractive. 3.2 Where destruction takes place Fig. 1 shows a schematic of a vacuum chamber for semiconductor processing and its associated pumping equipment.The waste gases can be intercepted for destruction/recycling at any point between the main exit valve on the vacuum process chamber and the exhaust ducts of the Fab. At the present time the great majority of waste destruction is done at essentially atmospheric pressure, i.e. after the pumps and before the waste gases enter the ducts. This region is chosen because it is simple to work at atmospheric pressure and because any chemistry carried out on waste there is well removed from the process chamber which is so sensitive to contamination.Fig. 1 indicates a local waste destruction system, specific to one process chamber. Local destruction is usually preferred over using a centralised destruction system which would abate the waste output from the many diVerent process chambers in a Fab. There are several reasons for this. First, local systems reduce the risk of accidentally mixing chemically incompatible waste streams.Secondly, the local system can be designed to cope with a particular group of waste components. Thirdly, it is usually easier to destroy waste products before they have become too diluted by other gas streams. Finally, if a local waste abatement system fails, it has less eVect on the running of the Fab than if there is a problem with a centralised system. When waste gases are destroyed at atmospheric pressure it means that all the waste has had to pass through the vacuum pumps, commonly a combination of a turbomolecular or Roots pump to raise the pressure to ca. 1022 mbar, backed by a foreline pump to take gases from ca. 1022 mbar to atmospheric pressure. The industry began using conventional oil filled rotary pumps to back the higher vacuum pumps but this was largely abandoned for two reasons. Using hydrocarbon pump oils there were often chemical reactions between the oil and the waste gases and vapours. This resulted in corrosion, pump seizure and even the risk of fire.When hydrocarbon pump oils were replaced by chemically inert perfluorinated oils (e.g. Fomblin) there was still a risk of retention of compounds in the oil which could react in dangerous ways with other compounds subsequently pumped through the oil. There was a rapid move towards multi-stage “dry-pumps” employing a series of closetolerance mechanical rotors with no lubricant. The final stages of these pumps are normally purged with nitrogen to reduce the likelihood of reactions between waste products depositing solid residues (which could jam the pump) as the pressure in the gas is increased, but this has the disadvantage of giving a waste stream highly diluted by nitrogen. 3.3 Required levels of destruction While the semiconductor industry would like to have a “zero emissions” policy this is very hard to achieve in practice. A more realistic aim is to require that the eZuent gas stream from each device used to destroy waste contains no species at a concentration above its “threshold limiting value” (t.l.v.).The range of t.l.v. values for the compounds of Tables 1 and 2 is large. Some values are <1 ppm, particularly among the hydrides; values in the range 3–10 ppm are typical among the acid gases but values >1000 ppm are found for a few compounds like sulfur hexafluoride and the perfluoroalkanes. Scrubbing gases so that their t.l.v. values are not exceeded is an attainable target with many commercial destruction devices working completely to specification.However, various factors, including operating errors, can contribute to situations when gases at much above their t.l.v. are released from the scrubbing device. Mostly these gases enter the exhaust ducts of the whole Fab where they experience such considerable dilution that the emission from the Fab may be below the t.l.v. of each compound. Analysis of the gaseous eZuent composition is easily achieved using mass spectrometry but less costly devices are needed for continuous monitoring of the output from individual scrubbing devices.Techniques include infrared detectors, Fig. 1 Schematic showing where waste volatiles from semiconductor processing are commonly treated to remove environmentally harmful products.J. Chem. Soc., Dalton Trans., 1999, 815–822 817 paper-tape detectors using colorimetry to detect reactions between reagents on the tape and eZuent gases, and electrochemical detectors.Spot sampling of the concentrations of individual gases using Draeger (or equivalent) tubes is also used as an additional check.3 3.4 Types of methods of destruction 4 Any inorganic chemist looking at the compounds listed in Tables 1 and 2 might reasonably ask, “What is the problem?” For, apart from some of the perfluorinated compounds, disposal of the compounds does not seem to be too diYcult. The first challenge is that the destruction system must be able to cope successively with a range of waste from the diVerent process stages such as chemical vapour deposition, etching and cleaning.The second challenge is the jump from laboratory conditions, where experiments tend to be short term and under the control of chemists, to the situation in a Fab which works virtually continuously under the control of process engineers. So any destruction system has to be capable of running with the minimum of attention. Three main types of methods for destroying waste gases are currently in use based on (i) water based absorbents, (ii) solid absorbants or (iii) combustion.Very limited use is made of plasmas for scrubbing as described in section 5.3. In ideal situations, methods (i) and (ii) will lead to total removal of environmentally harmful components of a gas stream whereas method (iii) always requires some follow-up treatment. For example, combustion of halides will leave hydrogen halides in the gas stream which have to be subsequently taken out by liquid or solid absorbents. 3.4.1 Water based absorbents.Treating waste gas streams with water or aqueous solutions of bases is widely used for removing acid gases in many industries. Looking at the molecules listed in Tables 1 and 2, it is clear that this method is not universally applicable for waste from semiconductor processing as several of the gases shown will either not react at all or not quickly enough to be useful.Nevertheless, water-based scrubbers of various designs to maximise the contact of gas with water or with solutions of bases (mainly KOH or aqueous ammonia but seldom NaOH because sodium is a feared impurity in silicon based semiconductors) have important uses where only a limited range of gases have to be scrubbed. A problem in their use arises from hydrolysis of dichlorosilane, silane, tetraethyl orthosilicate or other silicon containing compounds which generates silica, as this comes out of solution as a suspension capable of restricting gas or liquid flow and of blocking many types of filters.Direct coupling of a vacuum system to a water-based scrubber calls for careful design of fail-safe systems. The industry has known costly accidents when the contents of a water based scrubber have been sucked back through the pumps into the process chamber! Water treatments are also used to follow-up combustion processes which convert the least reactive compounds into simple molecules which can be absorbed by water or base.All silicon compounds are converted into SiO2 in the flames and this solid creates the problems described above when it is collected in a wet scrubber and its concentration builds up in recirculating wash water. Many Fabs have a large water scrubber on their exhaust ducts before exit to atmosphere. This may be in addition to local exhaust scrubbers and is intended to catch not only waste breaking through local systems but also vapours of many kinds which enter the ducts from diverse operations throughout the Fab.A general problem with water based scrubbing is the eventual disposal of large volumes of used wash liquor. Sometimes this can be put into municipal sewers at controlled rates but sometimes costly processing of the wash liquor has to be carried out on site before release. The semiconductor industry is quite familiar with waste wash liquors, because aqueous chemistry is also used for a variety of etching and cleaning operations on wafers which produce an eZuent containing HF, NH4HF2, HNO3, H2O2 and other chemicals.In this eZuent and in the eZuent from wet scrubbing of vapours the fluoride ion concentration is currently of greatest concern to regulatory authorities in Europe and the USA and the industry is seeking to find better methods for control of aqueous fluoride emissions. 3.4.2 Solid absorbents. Solid absorbents are more versatile than water based liquid absorbents as they can be used either at room temperature or at elevated temperatures.Absorbents for room temperature use will be considered first. Activated charcoal or molecular sieves which physically absorb vapours only delay the problem of destroying waste and there can be dangerous reactions with these solids (see section 4.2). However, they do find uses in systems designed to avert catastrophic release of dangerous gases in the event of leakage from cylinders housed in gas cabinets and in final stages of chemical absorber systems.Calcium hydroxide is a commonly used solid base either by itself or intimately mixed with a few percent of sodium hydroxide (or potassium hydroxide) as porous, granular soda lime. Soda lime is a versatile material, the simple acid-absorbing properties of which can be extended by many diVerent additives, e.g. with addition of oxidants such as sodium or potassium permanganate it can also destroy germane, phosphine and arsine.It is fairly cheap and disposal of part-used material gives only limited environmental problems. The disadvantages associated with its use are that it cannot attack all perfluorinated compounds, particularly NF3, SF6 or perfluorocarbons, and that typically <30% of the theoretical capacity of a soda lime absorber can be used before its reactivity is reduced enough to allow vapours to breakthrough at unacceptably high concentrations. A quite diVerent range of solids used to purify gases, especially hydrides and organometallics, was described in a lengthy patent by Tom et al.5 Polyvinylphenoxylithium on alumina or other high surface inorganic support was recommended for destroying Group 13 alkyls and silane and chlorosilanes.Dibutylmagnesium on alumina, calcium fluoride or polytetrafluoroethylene was recommended for destroying Group 15 hydrides and alkyls (except NH3), and Group 16 and 17 hydrides. Supported KMnO4 was recommended as a back-up oxidant for hydride scrubbing.An acid ion-exchange resin was proposed for ammonia scrubbing. The patent specified conditions under which the heat of reaction with waste gases could be removed from the solids. This technology was licensed and some is still in use (NovaPure Dry Scrubber). The much higher cost of the reagents compared with simple bases like soda lime is oVset by the fact that they react very quickly with waste gases so that short contact times and almost complete usage of the reagents is possible before substantial breakthrough occurs. 3.4.3 Hot solids. Using heated solids gives increased possibilities for destroying waste gases for a number of reasons. First and most obvious is that the rate of reaction increases. Secondly, solid-state diVusion eVects may help counter coating of the surface of a solid by reaction products. Thirdly, thermal decomposition of gases, particularly some hydrides, may occur on hot solids irrespective of any particular reaction between the solid and the gas. Hot soda lime is not an ideal absorber of waste gases as its eYciency at room temperature depends critically on water absorbed in the structure which is lost on heating.More eVective is solid CaO which is a very powerful base. The reactivity of CaO varies greatly with the way in which it is formed. Perhaps the highest reactivity CaO has been prepared by Klabunde and818 J. Chem. Soc., Dalton Trans., 1999, 815–822 co-workers 6 by vacuum dehydration of Ca(OH)2 prepared under carefully controlled conditions.The surface area of such powdered material can be up to 120 m2 g21 and it has been proposed for a number of applications.7 More practicable for large scale use is granular CaO of surface area of 2–5 m2 g21, prepared commercially by heating selected limestone in a rotary lime kiln to 900–1100 8C. At 500 8C a column of such granular lime will react with high eYciency with many gases.The reactivity of granular lime is exploited in the Gas Reactor Column, marketed by BOC Edwards.8,9 As shown in Fig. 2(a), the column is a vertically mounted stainless steel tube (the replaceable “cartridge”, commonly 150 mm diameter, 1.5 m long) electrically heated from the outside, which contains in the bottom third granular metallurgical grade silicon and granular CaO in the upper two thirds. It is designed to be used with gas streams which contain waste gases mixed with a large excess of nitrogen at flow rates which give a residence time of ca. 6 s for molecules passing through the packed zone. The silicon acts both as a heat transfer medium and a reactant. It reacts eYciently with free halogens or ClF3 forming silicon tetrahalides and, rather less completely, with NF3, SF6 and with hydrogen halides. Reaction of ClF3 with silicon is highly exothermic but, provided the amount of gas is controlled, this is a recommended, safe method for destroying the compound.10 The calcium oxide stage then reacts with silicon tetrahalides forming calcium halides and calcium silicate and it is a powerful reagent for destroying nearly all the compounds of Tables 1 and 2 except the perfluoroalkanes. Silane and diborane undergo both thermal decomposition to the elements and some reaction with the CaO as a base.The column has the ability to retain small amounts of phosphorus or arsenic formed by thermal decomposition of PH3 or AsH3, but if large amounts of these gases are to be absorbed, air can be blown into the column at the bottom of the lime stage as shown in Fig. 2(b), allowing complete oxidation of P or As to calcium phosphate or arsenate Fig. 2 (a) The normal form of the Gas Reactor Column (GRC). (b) A special form of GRC with an inlet to add air or water vapour to improve destruction of some compounds. (c) A bank of GRCs enclosed in cabinets in a Fab (Photograph courtesy of Texas Instruments, Dallas, USA).on the lime. In favourable cases, up to 60% of the theoretical capacity of the lime part of the cartridge can be used before an acid gas such as silicon tetrachloride, introduced at a concentration of 1% (10 000 ppm) in nitrogen, breaks through at unacceptable concentrations. This is a higher percent usage of the CaO than has been reported for commercial lime by other users.11 The reason is probably the long period over which the lime is being treated when the gas stream contains only 1% of an acid gas.This allows solid-phase diVusion to occur to a greater extent than if the column is exposed to a higher concentration of an acid gas for correspondingly less time. A bank of Gas Reactor Columns in use in the Texas Instruments Fab in Dallas, USA, is shown in Fig. 2(c). Over the next few years, the existing Gas Reactor Column will need to be resized to cope with the higher gas flows associated with processing of larger diameter silicon wafers.A good feature of the Gas Reactor Column is that spent cartridges are often acceptable as landfill as the waste gases they have trapped have been converted mainly into calcium salts which are either water insoluble or, if soluble like calcium chloride, are not too harmful. If a cartridge is contaminated with arsenic (as calcium arsenate) it is classified as a special waste the disposal of which is strictly controlled. Dealing with waste arsenic in solid or aqueous eZuents is an unsolved problem for the industry.The situation would become worse if the industry wanted to make more use of gallium arsenide as a semiconductor. The use of heated layered silicates for perfluoroalkane destruction is discussed in section 4.2.4. 3.4.4 Destruction in flames and related methods. Incineration of waste is a well established process in many industries and it has been adapted to the needs of the semiconductor industry. Combustion devices which are used range from very simple burners to more sophisticated inwardly fired burners.Successful use of combustion has to overcome a number of problems. First, the flame into which the waste stream is to be injected must provide suYcient enthalpy and suYcient mixing of gases that the waste gases all become hot and are all exposed to reactive flame components for suYcient time to bring about complete chemical decomposition and oxidation. This is dif- ficult because the large excess of nitrogen mixed with the waste gases from semiconductor processing tends to cool a flame.Secondly, combustion can only convert waste into the most stable, oxidised forms of the elements it contains. So the oV- gases from combustion may contain HF, HCl, SO2 and other acidic gases which must be absorbed in water in an adjacent scrubber. Thirdly, combustion of silicon, boron, phosphorus or metal compounds will create solid oxide residues which must not be allowed to block the burner.Finally, the combustion temperature must be kept as low as possible to minimise NOx formation and the flame made suYciently oxidising to yield CO2 not CO, so the eZuent gases after water scrubbing are as environmentally acceptable as possible. A range of commercial burners have been developed which meet the above requirements to a greater or lesser extent. Many involve passing the waste gases (usually heavily diluted by nitrogen as mentioned above) through the centre of a hydrogen or methane plus air or oxygen flame.Such flames tend to have a wide range of temperatures within them and the residence time of the waste gases in the hottest parts is short. Most waste gases undergo complete decomposition/oxidation/hydrolysis to products which can be washed out in a water scrubber. However, to achieve complete destruction of CF4 requires the highest flame temperatures (see section 4.1.3) and, under these conditions, the amount of NOx also formed can be quite high.The flame temperature can be kept lower if the residence time in the flame can be increased. This has been eVectively achieved by passing the waste gases through the hot zone of an inward fired burner as in the Thermal Processing Unit 12 made by BOCJ. Chem. Soc., Dalton Trans., 1999, 815–822 819 Edwards. As shown in Fig. 3, a mixture of methane (or other hydrocarbon fuel) is forced through a porous ceramic matrix lining of the cylindrical combustion chamber and ignited to create a structureless flame just above the surface of the matrix which fills the cylinder with hot gas.Waste gases are blown axially through this hot gas and combustion and decomposition products are fed into a high eYciency water scrubber. The temperature can be controlled in the range 800–1100 8C by varying the fuel/air/oxygen ratios. An alternative approach to passing waste gases through a flame is to pass them through a heated tube in the presence of air or oxygen.Thermal oxidation may then occur with development of a flame only with high concentrations of oxidisable wastes gases, particularly silane. Pioneers of this simple but useful approach have been Delatech Inc. in the USA. An inconel tube is used (in one model, 15 cm diameter and 90 cm long) heated electrically to 900–1000 8C. An inlet manifold feeds air, nitrogen and the waste gas stream down the hot tube, with the flows being designed to try to prevent blocking of the inlet manifold or the tube by solid oxidation products, e.g.silica. A wet scrubber immediately following absorbs solid and gaseous products. The system is fairly successful with airoxidisable gases such as SiH4, PH3, Si(OEt)4 or BCl3, and, with the addition of hydrogen, it will destroy NF3 and C2F6 but not CF4. The main problem with the method is blocking of the lower part of the tube although this can be overcome in part with a mechanical scraper device. 4 The problem of perfluorocarbons 4.1 Thermodynamics and kinetics The perfluorocarbons have a well established role in plasmabased dry etching and process chamber cleaning operations and are very eVective.They are safe because they are of low toxicity but are environmentally damaging on account of their eVectiveness as greenhouse gases and their long atmospheric lifetimes.13 So the only problem with their use is the diYculty of destroying any that remains unused and is pumped out of the process chamber with other waste gases.14,15 The inertness of simple perfluorocarbons arises from a combination of co-ordinative saturation and strong C–F bonds.This is most extreme with tetrafluoromethane which is the least reactive of known molecular compounds with an extraordinary resistance to attack by dissociative or associative mechanisms. Unlike many other highly fluorinated compounds, CF4 has little aYnity for thermal electrons 16 or F217 but its proton aYnity is comparable with that of CH4 or CO2.18 It is also a thermodynamic and kinetic sink compound which is formed as a by-product from treating perfluorinated or highly fluorinated carbon compounds, e.g.CHF3 or (CF3CO)2O, with oxygen in plasmas. So, whenever highly fluorinated carbon compounds are used the problem of dealing with CF4 still has to be faced. The reactivity of perfluoroalkanes containing C–C bonds is definitely higher than that of CF4. Among the compounds in current use in the semiconductor industry, reactivity decreases down the series C3F8 > C2F6 > CF4, in proportion to the num- Fig. 3 Schematic of an inward fired burner which can convert all waste volatiles into simple combustion products. ber of C–C bonds. The reactivity of C3F8 and C2F6 is still very low as none of the methods for attacking perfluorinated organic compounds reviewed by Saunders involving electron transfer reactions from organometallic compounds 19 or from hot sodium oxalate 20 is applicable to their rapid destruction.However, as described below, methods have been developed which allow destruction of these compounds as components of waste gas streams containing a large excess of nitrogen. 4.2 Methods for destroying perfluoroalkanes 4.2.1 Reaction with alkali metals at elevated temperatures. Perfluorocarbons are sensitive to attack by Group 1 metals as is evident from ease of activating the surface of Teflon by attack with sodium in liquid ammonia. A Japanese patent describes destruction of CF4 by sodium in liquid ammonia at 233 8C but this is not a method that could readily be used in conjunction with an eZuent stream from semiconductor production.21 The rate of reaction of CF4 with gaseous Na, K, Rb and Cs atoms has been measured at temperatures around 500 8C.The results show some inconsistencies but all suggest that the rate constant for the endothermic, primary defluorination, eqn. (1), CF4 1 M(g) æÆ CF3 ? 1 MF(g) (1) is ca. 103 times less than for the corresponding exothermic reaction involving CF3Cl or 105 times less than for SF6.22 Nevertheless, the complete defluorination of CF4 by sodium vapour is highly exothermic and Dufaux and Zachariah 22 have destroyed CF4 to >96% completion in a flame created by mixing it with a two-fold excess of sodium vapour pre-heated to temperatures >700 8C. The destruction of perfluorocarbons present in low concentrations in nitrogen streams, simulating the conditions of the eZuent from semiconductor processing, has been explored at Bristol.24 When a gas stream containing 1% CF4 in N2 was passed over dispersed sodium metal supported on alumina reaction was first detectable at 420 8C and, with a contact time of ca. 6 s, destruction of the CF4 was >99% complete at 595 8C. Comparable figures for C2F6 were 370 and 510 8C respectively. It was concluded that the reaction was occurring mainly on the surface of molten sodium not with sodium vapour, which is consistent with the reported low reaction rate of sodium atoms with CF4.However, the use of metallic sodium or potassium as liquid or vapour to destroy waste gases from semiconductor processing raises such serious safety issues that it is extremely unlikely to be acceptable to the industry. An alternative, safer approach is to pass the waste gas stream containing perfluorocarbons over heated mixtures which can generate sodium or potassium. Before electrolysis became the dominant route for making metallic sodium or potassium, numerous thermal processes were proposed by which the metals were liberated by high temperature reduction of their compounds.25 At Bristol, a range of such mixtures have been studied including Na2SiO3/Si/CaO, NaF/Si/CaO, NaF/Al and corresponding mixtures using potassium salts.24,26 They have been used to remove perfluorocarbons from nitrogen streams containing 1% CF4 or C2F6 at temperatures of 500–800 8C, which are substantially lower than the temperatures proposed to liberate alkali metals on a laboratory or manufacturing scale.25 The NaF/Si/CaO mixture heated to 700 8C proved particularly eVective as the NaF was engaged partly in a catalytic role as in eqns.(2) and (3). Adding reaction (2) and (3) gives (4) with elimination of the “catalyst” NaF. In 4NaF 1 Si 1 3CaO æÆ 4Na 1 CaSiO3 1 2CaF2 (2) CF4 1 4Na æÆ 4NaF 1 C (3) CF4 1 Si 1 3CaO æÆ CaSiO3 1 2CaF2 1 C (4)820 J.Chem. Soc., Dalton Trans., 1999, 815–822 practice, it was found that in order to maintain the level of CF4 destruction at >98% and to make eVective use of >50% of the available Si and CaO, a NaF :Si :CaO mole ratio of 1:1:2 was best. The calculated equilibrium vapour pressure of sodium over the mixture at 700 8C is ca. 5 mbar, but the observed transport of sodium in a pure nitrogen stream was very slight. This implies that the rate of production of free sodium from the solid-phase reactions was quite low and that CF4 was involved in reactions on the solid surfaces not just with liberated sodium vapour. 4.2.2 Catalytic destruction of perfluorocarbons over hot iron. Other electropositive metals in Groups 2–4 cannot be used to destroy perfluoroalkanes diluted with nitrogen as all the metals form nitrides when hot which do not then seem to react readily with perfluoroalkanes. The tendency to nitride formation diminishes across the transition series and at Bristol heated iron has been useful for destroying perfluoroalkanes.24 By adding hydrogen and an oxidising gas such as O2, H2O or CO2, iron can be used as a catalyst for conversion of perfluorocarbons into carbon oxides and HF.Temperatures about 950 8C are required to get >99% reaction with C2F6 and about 1050 8C for CF4. The rate determining step is the fluorination of the metal by the perfluorocarbon; subsequent reduction of the fluoride by hydrogen to give HF and the metal and oxidative removal of carbon from the metal surface are processes which occur easily above 700 8C. 4.2.3 Flame destruction. Destruction of perfluorocarbons other than CF4 in air/hydrocarbon or air/hydrogen flames at temperatures of >1000 8C occurs fairly readily through thermal breaking of the C–C bonds and attack on the molecules by flame generated radicals. The comparable reactions with CF4 are significantly slower so that higher temperatures and/or longer residence time in the flame are needed to achieve near total destruction and this increases the likelihood of generating significance amounts of NOx.The greatest success in flame destruction of CF4 has been obtained with an inward fired burner (Fig. 3 and section 3.4.4) but even with this design extra fuel and oxygen have to be added completely to destroy CF4. In all cases, HF is a reaction product which has to be scrubbed out of the oV-gases from the combustion. 4.2.4 Destruction on active sites in heated oxides.Both CF4 and C2F6 will react with hot silica but the reactions are not very fast below 1000 8C so this is not a practicable method of destroying these gases.27,28 A quite diVerent approach to destroying CF4 or C2F6 is to use high surface area, layered silicates to retain and destroy the perfluoroalkanes.29,30 Full details of the chemistry of these products which are available commercially from CS Clean Systems have not been published but it is claimed that they are successful and, remarkably, that they destroy CF4 at a lower temperature than that for C2F6.Activated layered clays and silicates are reported irreversibly to absorb CF4 at above 300 8C while temperatures of above 500 8C are required for C2F6. The fluorine is said to be retained as metal fluorides from Group 1 and 2 and transition metal ions originally present or intercalated into the structure, with carbon dioxide and some water as the only volatile products.The capacity of the system for CF4 or C2F6 is not given but the paper implies that it is fairly low.30 A similar eVect has been observed in experiments at Bristol in which it was shown that chromatographic grade alumina will destroy CF4 at 450–500 8C and C2F6 at 500–550 8C. The capacity of the alumina for destroying either gas is very small, ca. 2% of the theoretical assuming that the alumina could all be converted into AlF3. Pretreatment of the alumina with acids or the use of acid-treated Al2O3/TiO2 mixtures caused a slight lowering of the temperature of reaction but caused little change in the absorptive capacity of the solid.31 This observed greater ease of destruction of CF4 than C2F6 is contrary to the order of their reactivity under all other situations.The implication is that there are specific sites on the oxide surfaces which hold CF4 more strongly than C2F6 so that it can undergo an overall reaction to yield a metal fluoride (i.e.AlF3 on alumina or aluminium, iron or other metal fluorides from metal cations in the activated clays or layered silicates), carbon dioxide (or an alkali carbonate) and some water (from hydroxyl groups on the oxide). It is still unclear if this reaction is triggered by proton transfer at a superacid site, by Lewis-acid interactions between lone pairs on the fluorine atoms and a metal cation, or by some other mechanism. 4.3 Other perfluorinated compounds The other two perfluorinated compounds which are used and which can be diYcult to scrub because of their low reactivity are NF3 and SF6.They are both used for plasma activated etching and NF3 is widely used for the cleaning stage of plasma enhanced chemical vapour deposition processes. While both gases show considerable kinetic stability and are entirely untouched by room temperature wet scrubbing processes, NF3 is the less thermodynamically stable. Violent explosions can arise if NF3 is absorbed at low temperatures in charcoal or in molecular sieves and then the solid is warmed, through formation of N2 1 CF4 or of N2 1 SiF4 1 AlF3.The gases can be destroyed safely on proprietary solid absorbents based on activated layered silicates or aluminosilicates (CS Clean Systems) at temperatures of >150 8C,30 or in the BOC Edwards Gas Reactor Column through contact with silicon and CaO at 450–550 8C.8 The reactivity of NF3 towards the hot silicon can be improved by precoating the silicon with copper which acts catalytically through the cycle shown in eqns.(5) and (6). Cu 1 2NF3 æÆ CuF2 1 2NF2 (5) 2CuF2 1 Si æÆ 2Cu 1 SiF4 (6) Further reaction of NF2 with the silicon or copper occurs much more readily so complete destruction of NF3 can be achieved. The relative ease of flame destruction of perfluorocompounds decreases in the order NF3 @ C2F6 > SF6@CF4. So, destruction of NF3 occurs easily in flames; SF6 is more diYcult and is only achieved with eYcient burner designs.The combustion products from SF6 are HF, SO2 and SO3 which are easily removed by post-flame scrubbing but with NF3 the flame conditions have to be carefully controlled to prevent too much NO formation as this is not trapped in water based scrubbers. 4.4 Recovery of perfluorinated compounds The inertness of the perfluoroalkanes and SF6 and to a lesser extent NF3 makes them attractive targets for recycling. Details of a recycling system have been published.32 This involves complete removal of reactive waste products from the processing using conventional wet and dry scrubbing, followed by low temperature condensation and fractionation of the perfluorinated compounds which have passed through the scrubbing stages. The fractionation is aided by a cryogenic “wash liquid” such as C3F8 (mp 2148, bp 237 8C).Pure compounds can be recovered in high yield which is a remarkable achievement. However, the eZuent gases have to be extensively pre-purified to remove all compounds apart from the perfluorinated compounds so the cost-of-ownership of the system is likely to be higher than for competitive systems which destroy everything including the perfluorinated compounds. 5 Disposing of silanes 5.1 Processes using silanes Silane, SiH4, is consumed in large quantities by the semi-J. Chem. Soc., Dalton Trans., 1999, 815–822 821 conductor industry for deposition by thermal or plasma methods of “polysilicon” (polycrystalline or amorphous silicon), of boron or phosphorus doped silicon, of silica, of tungsten silicide and in numerous other applications.Bulk deliveries of up to three tons of ultra-high purity compressed gaseous silane are available to users in the USA. Dichlorosilane, SiH2Cl2, is also used as a source of polysilicon and, in conjunction with ammonia and nitrogen, for depositing silicon nitride usually by Low Pressure Chemical Vapour Deposition. The problems that the residual silanes cause when pumped out of the process chambers are considered below. 5.2 Silane; a pyrophoric gas that is hard to oxidise Silane is well known to be pyrophoric so its destruction by controlled oxidation or combustion does not appear diYcult. However, the reaction of silane with oxygen is a very complex process. The classic study of Emeleus and Stewart 33 on the upper and lower explosion limits of silane and oxygen showed that silane and oxygen in a 1 : 2.3 mol ratio at a total pressure of ca. 0.6 bar do not react appreciably even when kept at 70 8C for many days. Yet at a slightly higher partial pressure of silane or at a slightly higher temperature an explosion occurred immediately. Much more detailed studies have been done since, extensively reviewed by Koda.34 Many of the species contributing to a radical chain oxidation as well the kinetics of their reactions have been documented. Nevertheless, the initiation of the reaction of oxygen with silane seems to require the intermediacy of reactive surfaces or of traces of gaseous radicals otherwise it does not occur.This makes silane/air mixtures treacherous, with spontaneous ignition/explosion likely but not certain. It also means that when silane at concentrations of <2% in nitrogen is brought into contact with air there is no ignition as radical quenching predominates. There is available “burn-box” technology in which eZuent SiH4/N2 mixtures are brought into contact with a large volume of air with the intention of allowing either controlled, spontaneous combustion or air oxidation without combustion.The idea works to some extent. High concentrations of silane will ignite and burn safely within the box but lower concentrations of silane pass through and are released without ignition into the atmosphere where oxidation/hydrolysis will occur very slowly. Many other of the methods described in section 3 can be used successfully, the main complication being the solid oxidation or decomposition products generated. Thus, injection of SiH4 into a flame generally results in complete decomposition but premature decomposition depositing silicon in the inlet tubes and excessive deposition of silica on burner components can occur.Silane can be destroyed rapidly by bases such as supported polyvinylphenoxylithium 5 or less rapidly and completely with sodium or potassium hydroxide solutions 35 but these create the problems described in section 3.4.1.Soda lime will also readily destroy SiH4 but its capacity is rather limited. Silane is completely destroyed in the Gas Reactor Column although blocking may eventually occur as silane is largely thermolysed at 450–550 8C to powdered silicon which can build up on the hot silicon or CaO stages. 5.3 Explosive by-products Deposition of polysilicon by thermal or plasma enhanced chemical vapour deposition using SiH2Cl2 or SiHCl3 in the presence of hydrogen, followed by cleaning processes using HCl gas, leads to the formation of a complex mixture of solid and gaseous by-products. The solids carried out of the process chamber collect in the pumps or in the immediate exhaust from the pumps.Initially, the solids contain Si–Si, Si–H and Si–Cl bonds but they may be exposed subsequently to limited amounts of oxygen and water. Mostly, the solids are simply a nuisance but on rare occasions they explode violently. The chemistry of these explosions is still a matter of conjecture but a clue may come from old literature on hydrolysis of polychlorosilanes; for example, hydrolysis of Si3Cl8 is said to give an explosive solid, fancifully named as “silico mesooxalic acid”.36 This is likely to be a partial condensation product of Si3(OH)8, which may owe its explosive power to an exothermic internal oxidation which is a possible alternative to the expected elimination of water in species containing both Si–Si and Si–OH bonds.The process is shown schematically below. Further energy could be derived from oxidation of Si–H bonds by water liberated from condensation of Si–OH groups. When the right amount of hydrolysis of the primary solid has occurred the final solid could have the potential to explode, liberating hydrogen which may in turn be ignited in contact with air. 5.4 Problems with the SiH2Cl2/NH3 reaction The reaction of SiH2Cl2 with NH3 under chemical vapour deposition conditions gives excellent layers of insulating silicon nitride.However, the waste products are messy because ammonium chloride and solid complexes of SiH2Cl2 and NH3 are formed and may condense in and block vacuum pumps or the pipework on the atmospheric pressure side of the pumps. The problem has been addressed in two ways. The DryScrub Systems device, which is one of the few examples of low pressure scrubbing, is positioned between the process chamber and the dry-pump. It exposes the eZuent vapours to a powerful plasma between concentric aluminium electrodes. Silicon nitride is deposited on the electrodes, completing the reaction which began in the process chamber.The residual gaseous pollutant is then HCl. The system needs to be stripped down periodically to clean the silicon nitride deposit oV the electrodes. Corrosion of the aluminium electrodes by HCl and other chlorine-containing species is a longer term problem. The second solution is simply a water cooled trap on the outlet to the dry-pump which finally condenses and collects the solids.If stoichiometric mixtures of SiH2Cl2 and NH3 are used in the process all the final products are solids and no further scrubbing than the cold trap is required. 6 Concluding observations Methods of controlling the waste volatiles from the semiconductor industry may not yet appear to compare in sophistication to the chemistry of wafer processing. In part, this is a consequence over the years of a lower research eVort by the industry and its suppliers on pollution control than on wafer production, for the profits of the industry are generated from sales of processed wafers.Despite its elegance, much of the highly developed process chemistry is quite ineYcient in its use of valuable chemicals. When the industry moves into the next scale of production processing 300 mm instead of 200 mm + 2H2 ( higher activation energy but quite exothermic) + 2H2O (low activation energy, only slightly exothermic) possible reactions within products created by hydrolysis of a solid containing -SiCl2-SiCl2- units Si OH Si OH OH Si Si OH Si O Si O Si Si Si O O Si Si O Si O822 J.Chem. Soc., Dalton Trans., 1999, 815–822 silicon wafers it is committed to increase the eYciency of the process chemistry as well as further reducing emissions of all environmentally harmful gases and to promote recycling where practicable. There are undoubtedly many opportunities for research on both process chemistry and emission control.However, because of the way the semiconductor industry has developed, there are perhaps now easier opportunities for developing new methods of emission control. 7 Acknowledgements Thanks are due to Dr A. J. Seeley and Mr J. R. Smith of BOC Edwards, Nailsea, for their help with preparing this paper and to the Company for its support of research in the University of Bristol. 8 References 1 The Chemistry of the Semiconductor Industry, eds.S. J. Moss and A. Ledwith, Blackie, London, 1987. 2 S. C. O’Brien, Chem. Soc. Rev., 1996, 393. 3 Encyclopaedia of Analytical Science, ed. A. Townsend, Academic Press, London, 1995, p. 63. 4 M. Hayes and K. Woods, Solid State Technol., 1996, 39, 141. 5 G. M. Tom, J. V. McManus and B. A. Luxon, U.S. Pat., 5 037 624, 1991. 6 O. Koper, Y.-X. Li and K. J. Klabunde, Chem. Mater., 1993, 5, 500. 7 Y.-X. Li, H. Li and K. J. Klabunde, Environ. Sci. Technol., 1994, 28, 1248. 8 J. R. Smith and P. L. Timms, U.S. Pat., 5 213 767, 1993. 9 D. Baker, P. J. Mawle and J. R. Smith, Solid State Technol., 1995, 38, 79. 10 R. G. Czerepinski and J. L. Margrave, Inorg. Chem., 1963, 2, 875. 11 S. Decker and K. J. Klabunde, J. Am. Chem. Soc., 1996, 118, 12465. 12 J. Van Gompel and T. Walling, Semiconductor Int., 1997, 20, 95. 13 P. Maroulis, J. Langan, A. Johnson, R. Ridgeway and H. Withers, Semiconductor Int., 1994, 17, 107. 14 M. T. Mocella, Mater. Res. Soc. Symp. Proc., 1997, 447, 29. 15 PFC Emissions Reduction from Semiconductor Processing Tools, Sixth Status Report on Technology and Industry Activities, E. I. du Pont de Nemours and Co., January 1998 (www.dupont.com/zyron/ techinfo/status6.html). 16 G. L. Gutsev and L. Adamowicz, J. Chem. Phys., 1995, 102, 9309. 17 K. Hiraoka, M. Nasu, S. Fujinaki, E. W. Ignacio and S. Yamabe, Chem. Phys. Lett., 1995, 245, 14. 18 N. G. Adams, D. Smith, M. Tichy, G. Javahevy, N. D. Twiddy and E. E. Fergusson, J. Chem. Phys., 1989, 91, 4037. 19 G. C. Saunders, Angew. Chem., Int. Ed. Engl., 1996, 35, 2615. 20 J. Burdenuic and R. H. Crabtree, Science, 1996, 271, 340. 21 Jap. Pat., JP 59 10 329, 1984. 22 R. S. Clay and D. Husain, J. Chem. Res. (S), 1990, 384. 23 D. P. Dufaux and M. Zachariah, Environ. Sci. Technol., 1997, 31, 2223. 24 R. M. T. Lott, Ph.D. Thesis, University of Bristol, 1997. 25 J. P. Quin, in Mellors Comprehensive Treatise on Inorganic and Theoretical Chemistry, Longmans, London, 1956, vol. II, Suppl. 1, p. 308. 26 R. M. T. Lott and P. L. Timms, Eur. Pat., EP 0 663 233 B1, 1998. 27 H. Vogt, A. Fischer, G. Grosser and L. Riesel, Z. Anorg. Allg. Chem., 1987, 551, 223. 28 L. White and O. K. Rice, J. Am. Chem. Soc., 1947, 69, 267. 29 Ger. Pat., DE 4 404 329 A1, 1995. 30 C. Scholz and K. Markert, Semicond. Fabtech, ICG Publishing, 4th edn., June 1996, p. 131. 31 A. Dakhil, R. M. T. Lott and P. L. Timms, University of Bristol, unpublished work. 32 M. T. McClear and P. A. Taylor, Eur. Semiconductor, July 1996, 42. 33 H. J. Emeleus and K. Stewart, J. Chem. Soc., 1935, 1182. 34 S. Koda, Prog. Energy Combust. Sci., 1992, 18, 513. 35 E. Sada, H. Kumazawa and S. Hattori, Chem. Eng. Commun., 1987, 57, 95. 36 L. Gatterman and E. Ellery, Chem. Ber., 1899, 32, 1114. Paper 8/06743K

ISSN:1477-9226    

DOI:10.1039/a806743k

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 823–824 823 Spectroscopic and structural characterisation of fac-[Mn(CO)3- {MeC(CH2TeMe)3}]CF3SO3: the first transition metal complex of a tritelluroether William Levason, Simon D. Orchard and Gillian Reid Department of Chemistry, University of Southampton, Highfield, Southampton, UK SO17 1BJ Received 22nd January 1999, Accepted 8th February 1999 The preparation and crystal structure of the first complex of a multidentate telluroether, fac-[Mn(CO)3{MeC(CH2- TeMe)3}]CF3SO3 are described, and multinuclear NMR spectroscopic studies suggest that the telluroether is a significantly better Û-donor to Mn(I) compared to its selenoether analogue. Since a range of ditelluroethers was reported by one of us 1,2 ca. 10 years ago the coordination chemistry of these ligands with a variety of transition metals has been studied in some detail, although much less so compared to analogous thio- and seleno-ether ligands.3 Very few tri-, tetra- or higher polytelluroethers have been reported in the literature, reflecting dif- ficulties in developing suitable synthetic routes to these sensitive compounds.Examples are limited to the tripodal MeC(CH2- TeMe)3, spirocyclic C(CH2TePh)4 and one recently reported macrocyclic tritelluroether [12]aneTe3 (1,5,9-tritelluracyclododecane) which was structurally characterised as its hexachloro derivative,4 and no metal complexes of any of these have been reported.However, some years ago work by Schumann and HoVmann and co-workers 5 using Me2E led to the conclusion that for low valent centres metal–E bonding follows the series E = S < Se!Te. This is also supported by our own recent work which shows that telluroether ligands are significantly better s-donors to low-valent metal centres compared to the lighter Group 16 congeners.6 In light of this we have begun a study of the coordination chemistry of MeC(CH2TeMe)3 and related multidentate telluroethers and we report here the preparation, full spectroscopic and structural characterisation of the first isolated tritelluroether complex, fac-[Mn(CO)3- {MeC(CH2TeMe)3}]CF3SO3.The title compound is readily prepared as a yellow solid by treatment of fac-[Mn(CO)3(Me2CO)3]CF3SO3 with MeC(CH2- TeMe)3 at room temperature in Me2CO solution, followed by evaporation to dryness and recrystallisation from CH2Cl2–light petroleum (bp 40–60 8C). The reaction was monitored by solution IR spectroscopy which showed the disappearance of the bands due to the tris(acetone) Mn(I) precursor and the appearance of strong bands at 2023 and 1947 cm21 associated with the product, indicative of a fac-tricarbonyl unit (a1 1 e).Electrospray mass spectrometry (MeCN) shows peaks with the correct isotopic distribution for [Mn(CO)3{MeC(CH2TeMe)3}]1 as well as peaks associated with loss of CO ligands, and, together with microanalysis and 1H and 13C-{1H} NMR spectroscopy,† this supports the formulation [Mn(CO)3{MeC(CH2TeMe)3}]CF3- SO3 for the product.fac-[Mn(CO)3{MeC(CH2SeMe)3}]CF3SO3 was prepared and characterised similarly for comparison [IR spectrum: n(CO) 2039, 1962 cm21]. Yellow rod-like single crystals of the telluroether complex were obtained by vapour diVusion of light petroleum (bp 40–60 8C) into a solution of the complex in CH2Cl2. The crystal structure ‡ shows that the cation and anion are both disordered across a crystallographic mirror plane.In the [Mn(CO)3{MeC(CH2TeMe)3}]1 cation (Fig. 1) the central Mn centre is coordinated to three mutually fac carbonyl ligands and all three Te donors from one tritelluroether ligand, Mn–Te(1) 2.601(1), Mn–Te(2) 2.6063(8), Mn–C(1) 1.795(6), Mn–C(2) 1.790(8) Å. However, the disorder leads to two alternative sites for each of the Te-bound Me groups, and hence it is not possible to establish which diastereoisomer (invertomer) occurs in the solid state. Similar Mn–Te and Mn–C bond distances have been observed for fac- [MnCl(CO)3{o-C6H4(TeMe)2}],6 d(Mn–Te) 2.598(1), 2.613(1), d(Mn–C) 1.795(6), 1.790(8) Å and the Te–Mn–Te angles in the title compound are very close to the 908 expected for a regular octahedron.The 125Te-{1H} NMR spectrum (CDCl3) of [Mn(CO)3{MeC- (CH2TeMe)3}]CF3SO3 shows a single resonance at d 112, indicative of three equivalent Te donors and hence factridentate coordination in solution [free MeC(CH2TeMe)3 d(125Te) 21].Since pyramidal inversion at a coordinated Te donor atom is expected to be slow on the NMR timescale, this also implies that the ligand is in the syn configuration, with all three terminal Me groups pointing in the same direction giving a propeller-like arrangement. The 55Mn (100% I = 5/2) NMR spectrum also shows a single strong resonance at d 21509, w1/2 ca. 1200 Hz [a very weak resonance at d 21465 is attributed to a minor (<3%) quantity of the anti isomer]. This is ca. 800 ppm Fig. 1 View of the structure of [Mn(CO)3{MeC(CH2TeMe)3}]1 with the numbering scheme adopted. Ellipsoids are shown at the 40% probability level and H-atoms are omitted for clarity. The figure shows the syn arrangement established spectroscopically in solution, although we cannot be certain which isomer occurs in the solid state due to the disorder. Selected bond lengths (Å) and angles (8): Mn(1)–Te(1) 2.601(1), Mn(1)–Te(2) 2.6063(8), Mn–C(1) 1.795(6), Mn(1)–C(2) 1.790(8), C(1)–O(1) 1.153(6), C(2)–O(2) 1.157(9); Te(1)–Mn(1)–Te(2) 90.08(3), Te(2)–Mn(1)–Te(2*) 89.31(4).824 J.Chem. Soc., Dalton Trans., 1999, 823–824 to low frequency of the 55Mn NMR shifts for the most closely related neutral ditelluroether complexes fac-[MnX(CO)3{Me- Te(CH2)3TeMe}]; X = Cl, d 2644, 2594, 2581 (invertomers); X = Br, d 2753, 2690,6 and also very considerably to low frequency compared to the tripodal selenoether analogue [Mn(CO)3{MeC(CH2SeMe)3}]CF3SO3 [d(55Mn) 2721 (syn) and 2672 (anti, <5% by 55Mn NMR); d(77Se) 48 (syn)].The d(125Te) : d(77Se) ratio for the tripodal species is therefore ca. 2.3:1 compared to the more usual 1.7–1.8 : 17 and this also supports the conclusion that there is a considerable increase in electron density at the Mn centre in the cationic tritelluroether complex compared to the other species. For the bidentate selenoether species [MnX(CO)3{MeSe(CH2)3SeMe}] shows8 d(55Mn) 2175, 2190, 2219 (X = Cl), 2257, 2317 (X = Br), i.e.the tripodal selenoether complex is only ca. 500 ppm to low frequency of these. The much larger shifts in both the 125Te and 55Mn NMR of [Mn(CO)3{MeC(CH2TeMe)3}]1 may be attributed to the enhanced s-donation from TeÆMn as a consequence of the positive charge on the Mn centre. Acknowledgements We thank the EPSRC and the University of Southampton for support. Notes and references † [Mn(CO)3{MeC(CH2SeMe)3}]CF3SO3: fac-[Mn(CO)3(Me2CO)3]CF3- SO3 (0.22 mmol) was stirred with MeC(CH2SeMe)3 (0.071 g, 0.22 mmol) in acetone (15 cm3) under N2 for 16 h.The solvent was removed in vacuo and CH2Cl2 (2 cm3) was added to dissolve the residue. Ice cold light petroleum (bp 40–60 8C) was then added to precipitate a yellow powder which was filtered and dried in vacuo (yield: 47%) (Calc. for C12H18F3MnO6SSe3: C, 22.5; H, 2.8. Found: C, 23.2; H, 3.0%). Electrospray mass spectrum (MeCN): m/z 491, [Mn(CO)3{MeC(CH2- SeMe)3}]1; 437, [Mn(CO){MeC(CH2SeMe)3}]1; 407, [Mn{MeC(CH2- SeMe)3}]1. 1H NMR spectrum: d 2.70 (s, 6H, CH2), 2.38 (s, 9H, SeMe), 1.27 (s, 3H, CCH3). 13C-{1H} NMR spectrum: d 215.4–217.7 (CO), 40.8 (C), 38.9 (CH2), 34.7 (SeCH3), 25.5 (CCH3). fac-[Mn(CO)3{MeC(CH2TeMe)3}]CF3SO3: this compound was prepared in the same way (yield: 78%) (Calc. for C12H18F3MnO6STe3: C, 18.3; H, 2.3. Found: C, 18.9; H, 2.6%). Electrospray mass spectrum (MeCN): m/z 639, [Mn(CO)3{MeC(CH2TeMe)3}]1; 583, [Mn(CO)- {MeC(CH2TeMe)3}]1; 555, [Mn{MeC(CH2TeMe)3}]1. 1H NMR spectrum: d 3.00 (br, 6H, CH2), 2.06 (s, 9H, TeCH3), 1.28 (s, 3H, CCH3). 13C-{1H} NMR spectrum: d 216.5–222.1 (CO), 39.5 (C), 31.8 (CH2), 29.0 (CCH3), 28.3 (TeCH3). ‡ Crystal data for C12H18F3MnO6STe3, M = 785.06, monoclinic, space group P21/m, a = 8.989(3), b = 10.033(2), c = 12.086(2) Å, b = 104.85(1)8, V = 1053.6(4) A3, Z = 2, Dc = 2.474 g cm23, m(Mo-Ka) = 48.47 cm21. A pale yellow rod (0.28 × 0.10 × 0.04 mm) was grown by diVusion of light petroleum into a solution of the compound in CH2Cl2.Data collection used a Rigaku AFC7S four-circle diVractometer, T = 150 K, Mo-Ka X-radiation (l = 0.71073 Å), 1977 unique reflections (Rint = 0.026) of which 1603 with F > 4s(F) were used in all calculations. The structure was solved using heavy atom methods9 and developed by iterative cycles of least-squares refinement10 and diVerence Fourier synthesis. The cation and anion are both disordered across a crystallographic mirror plane, although we were able to model this very satisfactorily. In the cation Mn(1), Te(1), O(2), C(2) and C(4) lie on the plane, although there are two equally populated alternative positions for each of the terminal Me substituents.The disorder in the triflate anion also leads to two equally populated arrangements, such that S(1), F(1) and O(3) lie on the mirror plane and are common to both, with one 50% occupied triflate defined by S(1), O(3), O(4), O(5), F(1), F(2), F(3) and C(10), while the other is defined by S(1), O(3), O(4), O(5*), C(10*), F(1), F(2*) and F(3*).All non-hydrogen atoms were refined anisotropically and H atoms were included in fixed, calculated positions. Final R = 0.025, Rw = 0.034, S = 1.08 for 169 parameters. CCDC reference number 186/1345. See http://www.rsc.org/suppdata/ dt/1999/823/ for crystallographic files in .cif format. 1 E. G. Hope, T. Kemmitt and W. Levason, Organometallics, 1987, 6, 206; 1988, 7, 78. 2 T. Kemmitt and W. Levason, Organometallics, 1989, 8, 1303. 3 E. G. Hope and W. Levason, Coord. Chem. Rev., 1993, 122, 109. 4 Y. Takaguchi, E. Horn and N. Furukawa, Organometallics, 1996, 15, 5112. 5 H. Schumann, A. M. Arif, A. L. Rheingold, C. Janiak, R. HoV- mann and N. Kuhn, Inorg. Chem., 1991, 30, 1618, and refs. therein. 6 W. Levason, S. D. Orchard and G. Reid, Organometallics, 1999, in the press. 7 N. P. Luthra and J. D. Odom, in The Chemistry of Organic Selenium and Tellurium Compounds, ed. S. Patai and Z. Rappoport, Wiley, New York, 1986, vol. 1, ch. 6. 8 J. Connolly, M. K. Davies and G. Reid, J. Chem. Soc., Dalton Trans., 1998, 3833. 9 P. T. Beurskens, G. Admiraal, G. Beurskens, W. P. Bosman, S. Garcia-Granda, R. O. Gould, J. M. M. Smits and C. Smykalla, PATTY, The DIRDIF Program System, Technical Report of the Crystallography Laboratory, University of Nijmegen, 1992. 10 TeXsan, Crystal Structure Analysis Package, Molecular Structure Corporation, The Woodlands, TX, 1995. Communication 9/00615J

ISSN:1477-9226    

DOI:10.1039/a900615j

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 825–826 825 New ring-closure reaction involving co-ordinated amide groups Igor O. Fritsky Department of Chemistry, Shevchenko University, 252033 Kiev, Ukraine. E-mail: kokozay@chem.kiev.ua Received 12th January 1999, Accepted 1st February 1999 The condensation of [NiL1(HL1)]2 [H2L1 5 2-(hydroxyimino) propanamide] with formaldehyde and methylamine in methanol solution yields a stable anionic complex based on a tetradentate open-chain ligand and represents the first example of a ring-closure reaction featuring the coordinated amide groups.Template synthesis employing ring-closure reactions between transition metal polyamine complexes, formaldehyde and suitable C–H or N–H acids as locking fragments is widely used for macrocyclic 1 and cage 2 compounds. Among the recent advances in this field is the use of new types of C–H and N–H padlocks, such as diethylmalonate and barbituric acid 3 or primary amides and sulfonamides,4 respectively.The latter appeared more eVective as locking fragments than primary amines due to the higher acidity of their N–H protons which causes them to be significantly more reactive towards formaldehyde. A simple inversion of nitrogen-bearing functions in the template and the padlock in this reaction (i.e., use of coordinated secondary amide, formaldehyde and amine) can lead to a very attractive route for amide-containing macrocyclic or acyclic open-chain ligands.Co-ordination compounds with the above-mentioned ligands are under continuous investigation, in particular, due to the remarkable ability of amide macrocyclic donors to stabilise high oxidation states of transition metals 5 and because of the extensive use of mononuclear amidato complexes as “building blocks” for polynuclear assemblies.6 However, the tetraamide macrocyclic ligands reported to date were obtained by multi-step organic synthesis with relatively small yields.5 Polydentate amide-containing ligands used in molecular magnetism often undergo hydrolytic destruction in solution on co-ordination, so N-substituted amides should be employed instead.6 Both of these inconveniences can be avoided by the use of simple ring-closure reactions involving amidato groups.The only example of the metal-directed synthesis of tetraamide macrocyclic or open-chain acyclic complexes is the condensation of non-co-ordinated NH2 moieties of the hydrazide groups with aliphatic aldehydes.7 Thus, no ring-closure reactions based on co-ordinated amide groups have been reported to date.Interaction of primary amides with formaldehyde and amines is known as the Einhorn reaction of amidomethylation. 8 Taking into account the higher acidity of the amide N–H protons as compared to the amine N–H protons, we suggested that cis-disposed co-ordinated amidato groups in thermodynamically stable square-planar metal complexes can react readily with formaldehyde and primary amines with ring closure.Here we describe an implementation of this idea and the structure and spectral properties of the obtained products. We chose the square-planar nickel(II) complex K[NiL1- (HL1)]?2H2O [H2L1 = 2-(hydroxyimino)propanamide] (Scheme 1) to begin with since it is stable enough in solution and because of the important stabilisation of its co-ordination sphere by the short intramolecular H-bond between the oximato oxygen atoms which leads to retention of the cis-disposition of the ligands in reactions involving co-ordinated amide groups.7c,9 Secondly, such a template has only one pair of amide groups for ring closing thus making it possible to avoid the problem of the isolation of macrocyclic and open-chain products which normally arises when two pairs of cis-disposed reacting groups are present.Relatively fast and eYcient reaction was carried out in methanol solution under mild conditions and in high yield.† As a result of cyclisation with involvement of the co-ordinated amidato groups, two formaldehyde and one methylamine molecules had condensed with [NiL1(HL1)]2 giving the product K[Ni(HL2)]?2H2O 1, in which the six-membered chelate ring which forms is fused with two five-membered chelates (Scheme 1).Thus, the resulting ligand [{[{[2-(hydroxyimino)propanoyl]- amino}methyl](methyl)amino}methyl]-2-(hydroxyimino)- propanamide (H4L2) exhibits an open-chain structure.The structures of the resulting complexes 1 and Tl[Ni(HL2)] 2† were established by elemental analysis and NMR spectroscopy and then confirmed by single crystal X-ray analysis of 2 (Fig. 1). The 13C NMR spectrum of 1 in comparison with that of the initial complex showed the presence of two new signals at d 37.13 and 62.48 characteristic of N-methyl and Scheme 1 N N O O CH3 N N N N N O O O O CH3 H3C CH3 N N O O H3C CH2O CH3NH2 _ H Ni Ni H _ H H [NiL1(HL1)]– [Ni(HL2)]– Fig. 1 Structure of the complex anion [NiL2]2 in 2. Selected bond lengths (Å) and angles (8) for anion A (B in parentheses): Ni–N(1) 1.867(5) (1.869(5)), Ni–N(2) 1.866(5) (1.859(5)), Ni–N(3) 1.872(5) (1.863(5)), Ni–N(4) 1.850(5) (1.860(5)), N(1)–C(2) 1.302(8) (1.271(8)), N(2)–C(3) 1.308(8) (1.318(8)), N(2)–C(4) 1.462(8) (1.459(8)), N(3)–C(8) 1.287(8) (1.279(8)), N(4)–C(6) 1.462(8) (1.451(8)), N(4)–C(7) 1.326(8) (1.313(8)), N(5)–C(4) 1.450(8) (1.462(8)), N(5)–C(5) 1.461(8) (1.470(8)), N(5)–C(6) 1.453(8) (1.445(8)); N(1)–Ni–N(2) 82.9(2) (82.8(2)), N(1)– Ni–N(3) 97.6(2) (97.9(2)), N(1)–Ni–N(4) 178.6(2) (178.2(2)), N(2)–Ni– N(3) 179.4(2) (178.1(2)), N(2)–Ni–N(4) 95.9(2) (96.1(2)), N(3)–Ni– N(4) 83.6(2) (83.2(2)).826 J. Chem.Soc., Dalton Trans., 1999, 825–826 methylene groups, respectively. Disappearance of the signals corresponding to labile NH protons in the 1H NMR spectra of 1 and 2, as well as the sharp bands corresponding to the n(N–H) stretching mode vibrations of the secondary amide groups in the IR spectrum of 1, suggests the substitution of these protons with the methylene bridges.At the same time, new resonances for the N-methyl and methylene groups appeared in the 1H NMR spectra of 1 and 2. The centrosymmetric unit cell of 2‡ contains two crystallographically independent complex anions, [Ni(HL2)]2 (A and B) which diVer insignificantly in their geometrical parameters, and two thallium cations. The central atom in the complex anion is square-planar with four nitrogen donors from the deprotonated amide and oxime groups of the tetradentate open-chain ligand.The latter is triply deprotonated and forms three condensed chelate rings. The central and the donor atoms define the same plane (the deviations do not exceed 0.01 Å), the five-membered chelate rings are in fact planar. The sixmembered ring NiN(2)C(4)N(5)C(6)N(4) formed as a result of template condensation indicates clear envelope conformation with the N(5) atoms lying 0.648(7) and 0.678(7) Å above the mean plane defined by the five other atoms, for anions A and B, respectively.The corresponding dihedral angles along the C(4)–C(6) vector are 54.9(4) (for A) and 57.7(4)8 (for B). The square planar co-ordination of the ligand is additionally stabilised by the short intramolecular H bond between oxime oxygen atoms thus forming a closed pseudo-macrocyclic structure. The O ? ? ? O separations [2.478(7) and 2.452(7)Å for anions A and B, respectively] are typical for nickel(II) oximato complexes.10 The ring-closure reaction reported here points the way to the elaboration of convenient synthetic methods for amidecontaining macrocycles using metal complexes with bidentate amide ligands (e.g., oxamide, biuret, amino acid amides) or tetradentate open-chain amide ligands as starting compounds.Another possibility is the use of suitable C–H acids (e.g., nitroalkanes) as padlocks in this reaction.Such research is in progress and its results are to be reported in a full paper. Acknowledgements This work was partially supported by a grant from the International Soros Science Education Program (grant No. APU073113). We also thank Dr E. B. Rusanov for collecting X-ray data. Notes and references † K[Ni(HL2)]?2H2O (1). The compound K[NiL1(HL1)]?2H2O {0.335 g, 1 mmol, prepared analogously to Li[NiL1(HL1)]?5H2O as described in ref. 10(b) using KOH instead of LiOH} was dissolved in 10 ml of methanol, then paraformaldehyde (0.075 g, 2.5 mmol, depolymerised in 10 ml of methanol), methylamine (33% solution in ethanol, 0.14 ml, 1.1 mmol) and KOH (0.056 g, 1 mmol, dissolved in 5 ml of methanol) were added.The mixture was heated under reflux with continuous stirring for 30 min, then the volume was reduced to 5 ml on a rotary evaporator. In 12 h the product was obtained as a clear yellow crystalline precipitate, which was washed with methanol and dried over CaCl2.Yield 0.285 g (73%) [Calc. for KNiC9H14N5O4?2H2O (390.06): C, 27.71; H, 4.65; N, 19.95; Ni, 15.05. Found: C, 27.56; H, 4.81; N, 19.81; Ni, 16.29%]. 1H NMR (DMSO-d6): d 1.775 (s; 6H, CH3), 2.773 (s; 3H, NCH3), 4.081 (s; 4H, CH2), 18.613 (s, 1H, NOH). 13C NMR (DMSOd6): d 10.65 (CH3); 37.13 (NCH3); 62.48 (CH2); 149.10 (C]] N), 169.29 (C]] O). IR (KBr pellet, n/cm21): 1126 [n(N–O)]; 1632 [n(C]] O), Amide I]; 2890, 2925, 2985 [n(C–H)]; 3455br [n(O–H)]. Tl[Ni(HL2)] 2.Amber-yellow prismatic crystals of 2 [yield 0.089 g (86%)] were grown by evaporation at room temperature of a solution prepared by metathesis of 1 (0.078 g, 0.2 mmol) with Tl2CO3 (0.047 g, 0.1 mmol) in water [Calc. for TlNiC9H14N5O4 (519.31): C, 20.82; H, 2.72; N, 13.49; Ni, 11.30. Found: C, 20.66; H, 4.79; N, 13.61; Ni, 11.12%]. 1H NMR (DMSO-d6): d 1.717 (s; 6H, CH3), 2.450 (s; 3H, NCH3), 3.681 (s; 4H, CH2), 11.154 (s, 1H, NOH). ‡ Crystal data for 2: C9H14N5NiO4Tl, M = 519.33, triclinic, space group P1� , a = 8.586(1), b = 10.256(1), c = 16.422(3) Å, a = 78.40(1), b = 83.36(1), g = 70.11(1)8, U = 1330.3(3) Å3, Z = 4, Dc = 2.593 g cm23, m(Mo-Ka) = 13.534 mm21, F(000) = 976, T = 293 K, 4193 measured reflections, 3911 independent reflections, structure solution by direct methods using SHELXL-93.11 R1 = 0.0274, wR2 = 0.0658 for 3350 reflections with I > 2s(I) and R1 = 0.0367, wR2 = 0.0723 for all unique reflections.CCDC reference number 186/1340.See http://www.rsc.org/ suppdata/dt/1999/825/ for crystallographic files in .cif format. 1 M. P. Suh and S.-G. Kang, Inorg. Chem., 1988, 27, 2544; P. V. Bernardt and G. A. Lawrance, Coord. Chem. Rev., 1990, 104, 297; S. V. Rosokha, Y. D. Lampeka and I. M. Maloshtan, J. Chem. Soc., Dalton Trans., 1993, 631. 2 A. Sargeson, Pure Appl. Chem., 1984, 56, 1603; 1986, 58, 1511; Coord. Chem. Rev., 1996, 151, 89. 3 L. Fabbrizzi, M. Licchelli, A. Poggi, O. Vassalli, L. Ungaretti and N.Sardone, Inorg. Chim. Acta, 1996, 246, 379; Y. D. Lampeka, A. I. Prikhod’ko, A. Y. Nazarenko and E. B. Rusanov, J. Chem. Soc., Dalton Trans., 1996, 2017. 4 F. Abba, G. De Santis, L. Fabbrizzi, M. Licchelli, A. Lanfredi, P. Pallavicini, A. Poggi and F. Ugozzoli, Inorg. Chem., 1994, 33, 1366. 5 F. C. Anson, T. J. Collins, T. G. Richmond, B. D. Santarsiero, J. E. Toth and B. G. R. T. Treco, J. Am. Chem. Soc., 1987, 109, 2974; T. J. Collins, K. L. Kostka, E. S. UVelman and T. L.Weinberger, Inorg. Chem., 1991, 30, 4204; T. J. Collins, R. D. Powell, C. Slebodnick and E. S. UVelman, J. Am. Chem. Soc., 1991, 113, 8419. 6 F. Lloret, M. Julve, J. A. Real, J. Faus, R. Ruiz, M. Mollar, I. Castro and C. Bois, Inorg. Chem., 1992, 31, 2956; J. L. Sans, B. Cervera, R. Ruiz, C. Bios, J. Faus, F. Lloret and M. Julve, J. Chem. Soc., Dalton Trans., 1996, 1359; B. Cervera, J. L. Sanz, M. J. Ibánez, G. Vila, F. Lloret, M. Julve, R. Ruiz, X. Ottenwaelder, A. Aukauloo, S. Poussereau, Y. Journaux and M. C. Munoz, J. Chem. Soc., Dalton Trans., 1998, 781. 7 (a) G. R. Clark, B. W. Skelton and T. N. Waters, J. Chem. Soc., Chem. Commun., 1972, 1163; (b) G. R. Clark, B. W. Skelton and T. N. Waters, J. Chem. Soc., Dalton Trans., 1976, 1528; K. J. Oliver and T. N. Waters, J. Chem. Soc., Chem. Commun., 1982, 1111; (c) I. O. Fritsky, H. Kozlowski, P. J. Sadler, O. P. Yefetova, J. Swiatek-Kozlowska, V. A. Kalibabchuk and T. Glowiak, J. Chem. Soc., Dalton Trans., 1998, 3269. 8 B. C. Challis and J. A. Challis, in Comprehensive Organic Chemistry, ed. D. Barton and W. D. Ollis, Pergamon Press, Oxford, 1979, vol. 2, ch. 9.9.3.4. 9 I. O. Fritsky, H. Kozlowski, E. V. Prisyazhnaya, Z. Rza.czyn� ska, A. Karaczyn, T. Yu. Sliva and T. Glowiak, J. Chem. Soc., Dalton Trans., 1998, 3629. 10 (a) A. M. Duda, A. Karaczyn, H. Kozlowski, I. O. Fritsky, T. Glowiak, E. V. Prisyazhnaya, T. Yu. Sliva and J. Swiatek- Kozlowska, J. Chem. Soc., Dalton Trans., 1997, 3853; (b) T. Yu. Sliva, T. Kowalik-Jankowska, V. M. Amirkhanov, T. Glowiak, C. O. Onindo, I. O. Fritsky and H. Kozlowski, J. Inorg. Biochem., 1997, 65, 287. 11 G. M. Sheldrick, SHELXL-93, University of Göttingen, 1993. Communication 9/00

ISSN:1477-9226    

DOI:10.1039/a900349e

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 827–829 827 Chromium ethylene polymerisation catalysts bearing reduced SchiV-base N,O-chelate ligands Vernon C. Gibson,* Claire Newton, Carl Redshaw, Gregory A. Solan, Andrew J. P. White and David J. Williams Department of Chemistry, Imperial College, South Kensington, London, UK SW7 2AY. E-mail: V.Gibson@ic.ac.uk Received 5th January 1999, Accepted 8th February 1999 Treatment of CrCl3(THF)3 with the lithium and sodium salts of the reduced Schiff-base ligand 3,5-(tBu)2-2-(OH)C6- H2CH2NH(2,6-Me2C6H3) affords bis-chelate chromium(II) and mono-chelate chromium(III) complexes respectively; both give active ethylene polymerisation catalysts upon treatment with alkylaluminium activators.There is currently much academic and industrial interest in the development of highly eYcient molecular a-olefin polymerisation catalysts.1 In these systems, it is the steric and electronic properties of ancillary ligands that allow control over the molecular weight and microstructure of the resultant polymers. 2,3 Industrially, chromium supported on silica plays an important role in the global production of polyethylene.4 Examples of molecular systems are, however, scarce 5–10 and of those that have been reported, low valent half-sandwich chromium compounds predominate.7,8 We have been investigating non-cyclopentadienyl chromium systems as potential well-defined catalysts for ethylene polymerisation.In recent reports, we 5 and others 6 have described new chromium catalysts bearing monoanionic N,N-chelate ligands. We now report new procatalysts based upon the bulky monoanionic N,O-chelating ligand I derived from reduction of the corresponding SchiV-base precursor; ethylene polymerisation tests reveal the highest activities to date for a noncyclopentadienyl chromium system. Treatment of [CrCl3(THF)3] with two equivalents of the lithium salt of 3,5-(tBu)2-2-(OH)C6H2CH2NH(2,6-Me2C6H3) HI in THF at 278 8C leads to reduction and formation of the bis-chelated red chromium(II) complex {Cr[3,5-(tBu)2-2-(O)C6- H2CH2NH(2,6-Me2C6H3)]2} 1 in 45% yield (Scheme 1).† Use of only one equivalent of the lithium salt again aVords 1, albeit in reduced yield (23%).The single crystal X-ray structure ‡ of 1 shows the ligands to have cis-coordinated oxygen and nitrogen atoms respectively (Fig. 1) and to have retained their amino hydrogen atoms, hence indicating a formal 12 oxidation state at the chromium centre.The molecule has crystallographic C2 symmetry about an axis bisecting the O–Cr–O9 angle. The geometry at chromium is distorted square planar, there being a 148 twist about the C2 axis between the O–Cr–O9 and N–Cr–N9 planes. The bite of the N,O-chelating ligand I appears to be near optimal, the angle subtended at chromium being 90.3(1)8. The six-membered chelate ring has a folded boat-like conformation with O and C(1) as “prow” and “stern” respectively; the out of plane fold angle about the O ? ? ? C(1) vector is ca. 228. The two 2,6- O– tBu tBu N Me Me H I dimethylphenyl rings are sheared, there being no p ? ? ?p stacking interaction between them. By contrast, reaction of one equivalent of the sodium salt Fig. 1 The molecular structure of 1. Selected bond lengths (Å) and angles (8): Cr–O 1.913(3), Cr–N 2.100(3), N–C(1) 1.463(5), N–C(16) 1.444(5), O–C(3) 1.336(4); O–Cr–O9 90.9(2), O–Cr–N 90.3(1), O–Cr– N9 169.9(1), N–Cr–N9 90.3(2).Scheme 1 Preparation of chromium complexes 1 and 2 featuring ligand I. Reagents and conditions: (i) 2LiI, 278 8C, THF, 12 h; (ii) NaI, 278 8C, THF, 12 h, followed by recrystallisation from heptane– CH3CN. O tBu tBu N Cr Cl NCMe O tBu tBu N Cr O tBu tBu N H H H Cl NCMe Me Me Me Me Me Me (ii) (i) 1 CrCl3(THF)3 2828 J. Chem. Soc., Dalton Trans., 1999, 827–829 of 3,5-(tBu)2-2-(OH)C6H2CH2NH(2,6-Me2C6H3) in THF at 278 8C with CrCl3(THF)3 aVords, upon recrystallisation from acetonitrile–heptane, green blocks of the mono-chelated octahedral chromium(III) complex {Cr[3,5-(tBu)2-2-(O)C6H2- CH2NH(2-6-Me2C6H3)](h1-NCCH3)2Cl2} 2 † (51%) (Scheme 1).The crystal structure ‡ of 2 shows the geometry at chromium to be distorted octahedral with trans-chlorides and cis-acetonitriles (Fig. 2) and angles ranging from 82.7(1)–98.0(1)8 and 171.1(2)–179.3(1)8. Interestingly, the bite of I is here reduced to 82.7(1)8, cf. 90.3(1)8 in 1. The geometry of the six-membered chelate ring is also diVerent, adopting here a half-chair conformation with the N(1)–Cr–C(7) plane being folded by ca. 588 out of the Cr–O(1)–C(1)–C(6)–C(7) plane. The Cr–O(1) and Cr–N(1) distances are comparable to those observed in 1 and in other related systems.11 The infrared spectra of complexes 1 and 2 both exhibit absorption bands between 3312 and 3222 cm21 consistent with n(N–H) stretching modes while complex 2 shows, in addition, strong bands at 2319 and 2291 cm21 due to the symmetric and asymmetric nitrile stretches.12 Both complexes are paramagnetic with the Cr(II) complex 1 displaying a magnetic moment of 2.6 mB (consistent with an S = 1 ground state) and the Cr(III) complex, 2, 3.9 mB (Evans balance).The results of the ethylene polymerisation runs are collected in Table 1. Compounds 1 and 2 are both active as procatalysts in ethylene polymerisation and aVord polymers with high molecular weight and virtually no branching by NMR.§ The highest activity is observed using a combination of 2 and Fig. 2 The molecular structure of 2. Selected bond lengths (Å) and angles (8): Cr–O(1) 1.865(3), Cr–N(1) 2.123(3), Cr–N(2) 2.102(4), Cr–N(3) 2.100(4), Cr–Cl(1) 2.311(2), Cr–Cl(2) 2.330(2), O(1)–C(1) 1.341(5), N(1)–C(7) 1.486(6), N(1)–C(8) 1.468(5); O(1)–Cr–N(3) 88.9(1), O(1)–Cr–N(2) 179.3(1), N(3)–Cr–N(2) 90.4(2), O(1)–Cr–N(1) 82.7(1), N(3)–Cr–N(1) 171.1(2), N(2)–Cr–N(1) 98.0(1), O(1)–Cr–Cl(1) 92.7(1), N(3)–Cr–Cl(1) 86.8(1), N(2)–Cr–Cl(1) 87.4(1), N(1)–Cr–Cl(1) 96.5(1), O(1)–Cr–Cl(2) 91.6(1), N(3)–Cr–Cl(2) 89.0(1), N(2)–Cr–Cl(2) 88.3(1), N(1)–Cr–Cl(2) 88.3(1), Cl(1)–Cr–Cl(2) 174.0(1).Table 1 Results of ethylene polymerisation runs using procatalysts 1 and 2a Run 1234 Procatalyst/ mmol 1 (0.017) 1 (0.017) 2 (0.025) 2 (0.025) Activator b/ mmol (equiv.) MAO (12/700) Et2AlCl (0.6/35) MAO (10/400) Et2AlCl (0.5/20) Yield PEc/g 0.26 1.02 0.11 3.26 Activity/ g mmol21 h21 bar21 15 60 4 130 § a General conditions: 1 bar ethylene, Schlenk test carried out in toluene (40 cm3) at 25 8C, over 60 min, reaction quenched with dilute HCl and the solid washed with methanol (50 cm3) and dried in a vacuum oven at 40 8C.b MAO = Methylaluminoxane. c Solid polyethylene. Et2AlCl (130 g mmol21 h21 bar21, run 4). Under related conditions, the chromium(II) species 1 results in an activity less than half that of 2 (60 g mmol21 h21 bar21, run 2). As we have reported elsewhere 5 dialkylaluminium chlorides appear to be more compatible co-catalysts (runs 2, 4) than MAO (runs 1, 3) for chromium systems of this type.In conclusion, two chromium complexes incorporating the bulky reduced-SchiV-base ligand I have been prepared and their role in ethylene polymerisation has been examined. The higher activity observed for 2 relative to 1 may be attributed to a more accessible chromium centre, possibly aided by the lability of the ancillary acetonitrile ligands.Further studies are in progress to obtain a greater understanding of the factors influencing the activity and selectivity of these and related molecular chromium polymerisation catalysts. Acknowledgements BP Chemicals Ltd is thanked for financial support. Drs G. Audley and J. Boyle are thanked for GPC and NMR measurements, respectively. Notes and references † Synthesis of 1: to a THF (20 cm3) solution of HI (1.62 g, 4.76 mmol) was added nBuLi (3.2 cm3, 5.0 mmol) at 278 8C.The solution was allowed to warm to room temperature and stirred for 1 h. On cooling to 278 8C, solid CrCl3(THF)3 (0.90 g, 2.38 mmol) was added. The reaction mixture was then allowed to warm to room temperature and stirred for 12 h. Following removal of the volatile components, the residue was extracted into pentane (50 cm3) and taken to dryness. Recrystallisation from heptane aVorded, on prolonged standing (1–2 d) at ambient temperature, dark red prisms of 1 in 45% yield (0.62 g) (Found: C, 75.8; H, 8.2; N, 3.5.Calc. for C46H64N2O2Cr 1: C, 75.8; H, 8.8; N, 3.8%). Synthesis of 2: HI (2.00 g, 5.89 mmol) and NaH (0.31 g, 12.96 mmol) were refluxed in THF (45 cm3) for 12 h. On cooling, the suspension was filtered into a solution of CrCl3(THF)3 (2.21 g, 5.89 mmol) in THF (25 cm3) at 278 8C. The solution was stirred at room temperature for 12 h. Following removal of the volatile components, the solid residue was extracted into toluene (75 cm3) and taken to dryness.Recrystallisation from acetonitrile–heptane (1 : 3) aVorded 2 as green blocks on prolonged standing (3–4 d). Yield 51% (1.65 g) (Found: C, 59.4; H, 7.7; N, 7.0. Calc. for C27H38N3OCl2Cr 2: C, 59.7; H, 7.0; N, 7.7%). ‡ Crystal data for 1: C46H64N2O2Cr, M = 729.0, monoclinic, I2/a (no. 15), a = 13.645(5), b = 18.382(4), c = 17.243(8) Å, b = 102.73(2)8, V = 4219(3) Å3, Z = 4 (the molecule has C2 symmetry), Dc = 1.148 g cm–3, m(Mo-Ka) = 3.08 cm–1, F(000) = 1576, T = 203 K; red blocks, 0.50 × 0.43 × 0.37 mm, Siemens P4/PC diVractometer, w-scans, 3669 independent reflections. The structure was solved by direct methods and the non-hydrogen atoms were refined anisotropically using full matrix least-squares based on F2 to give R1 = 0.060, wR2 = 0.125 for 2158 independent observed reflections [|Fo| > 4s(|Fo|), 2q < 508] and 235 parameters.Crystal data for 2: C27H38N3OCl2Cr, M = 543.5, monoclinic, P21/n (no. 14), a = 9.566(1), b = 13.048(2), c = 23.432(4) Å, b = 95.83(1)8, V = 2909.7(8) Å3, Z = 4, Dc = 1.241 g cm–3, m(Cu-Ka) = 50.9 cm–1, F(000) = 1148, T = 293 K; green plates, 0.27 × 0.27 × 0.10 mm, Siemens P4/PC diVractometer, w-scans, 4438 independent reflections.The structure was solved by direct methods and the major occupancy nonhydrogen atoms were refined anisotropically using full matrix leastsquares based on F2 to give R1 = 0.057, wR2 = 0.127 for 3139 independent observed absorption corrected reflections [|Fo| > 4s(|Fo|), 2q £ 1288] and 324 parameters.CCDC reference number 186/1346. See http:// www.rsc.org/suppdata/dt/1999/827 for crystallographic files in .cif format. § As a representative example, GPC analysis of the polyethylene obtained from run 4 aVorded Mw 827000, Mn 84000, Mw/Mn 9.8; 13C NMR (C2D2Cl4–1,2,4-trichlorobenzene at 130 8C) gave 0.6 Me per 1000 C atoms. 1 W. Kaminsky, J. Chem. Soc., Dalton Trans., 1998, 1413; K. Soga and T. Shiono, Prog. Polym. Sci., 1997, 22, 1503; R.G. Harvan, Chem. Ind., 1997, 212; R. F. Jordan, Adv. Organomet. Chem., 1991, 32, 325; G. P. J. Britovsek, V. C. Gibson and D. F. Wass, Angew. Chem., Int. Ed., 1999, 38, 428. 2 G. J. P. Britovsek, V. C. Gibson, B. S. Kimberley, P. J. Maddox, S. J. McTavish, G. A. Solan, A. J. P. White and D. J. Williams, Chem. Commun., 1998, 849; B. L. Small, M. Brookhart and A. M. A. Bennett, J. Am. Chem. Soc., 1998, 120, 4049.J. Chem. Soc., Dalton Trans., 1999, 827–829 829 3 L.K. Johnson, C. M. Killian and M. Brookhart, J. Am. Chem. Soc., 1995, 117, 6414; L. K. Johnson, C. M. Killian, S. D. Arthur, J. Feldman, E. F. McCord, S. J. McLain, K. A. Kreutzer, M. A. Bennett, E. B. Coughlin, S. D. Ittel, A. Parthasarathy, D. J. Tempel and M. S. Brookhart (DuPont), Pat. WO 96/23010, 1996; Chem Abstr., 1996, 125, 222773t. 4 F. J. Karol, G. L. Karapinka, C. Wu, A. W. Dow, R. N. Johnson and W. I. Carrick, J. Polym. Sci., Part A, 1972, 10, 2621; J. P. Hogan, J. Polym. Sci, Part A, 1972, 8, 2637. 5 V. C. Gibson, P. J. Maddox, C. Newton, C. Redshaw, G. A. Solan, A. J. P. White and D. J. Williams, Chem. Commun., 1998, 1651. 6 W.-K. Kim, M. J. Fevola, L. M. Liable-Sands, A. L. Rheingold and K. H. Theopold, Organometallics, 1998, 17, 4541. 7 For recent reviews see, K. H. Theopold, Eur. J. Inorg. Chem., 1998, 1, 15; K. H. Theopold, CHEMTECH, 1997, 27, 26. 8 R. Emrich, O. Heinemann, P. W. Jolly, C. Krüger and G. P. J. Verhovnik, Organometallics, 1997, 16, 1511. 9 M. P. Coles, C. I. Dalby, V. C. Gibson, W. Clegg and M. R. J. Elsegood, J. Chem. Soc., Chem. Commun., 1995, 1709. 10 F. J. Feher and R. L. Blanski, J. Chem. Soc., Chem. Commun., 1990, 1614. 11 G. Wilkinson, C. Redshaw, B. Hussain-Bates and M. B. Hursthouse, J. Chem. Soc., Dalton Trans., 1992, 1803. 12 S. J. Anderson, F. J. Wells, G. Wilkinson, B. Hussain and M. B. Hursthouse, Polyhedron, 1988, 7, 2615. Communication 9/00118B

ISSN:1477-9226    

DOI:10.1039/a900118b

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 831–834 831 Reaction of tris[(diphenylphosphino)dimethylsilyl]methane with molybdenum hexacarbonyl and deprotonation to give a salt with a planar carbanion. Crystal structures of (Ph2PMe2Si)3CH and [Li(tmen)2][C(SiMe2PPh2)3], tmen 5 N,N,N9,N9-tetramethylethane- 1,2-diamine Anthony G. Avent, Dominique Bonafoux, Colin Eaborn,* Sushil K. Gupta, Peter B. Hitchcock and J. David Smith * School of Chemistry, Physics and Environmental Science, University of Sussex, Brighton, UK BN1 9QJ.E-mail: C.Eaborn@sussex.ac.uk; J.D.Smith@sussex.ac.uk Received 17th November 1998, Accepted 21st January 1999 The compound (Ph2PMe2Si)3CH I reacted (i) with [Mo(CO)6] to give cis-[Mo(CO)4{(Ph2PMe2Si)3CH}] 1 in which two phosphine groups are co-ordinated to molybdenum and one is free, and (ii) with LiBu in the presence of N,N,N9,N9-tetramethylethane-1,2-diamine (tmen) to give [Li(tmen)2][C(SiMe2PPh2)3] 2, which contains discrete planar carbanions and no Li–P co-ordination. The crystal structures of compounds I and 2 have been determined and 1 has been characterised spectroscopically.We have been able to isolate a wide range of novel types of organometallic compounds by attaching the bulky ligands C(SiMe3)3 or C(SiMe2Ph)3 to metal centres.1 Recently the emphasis in our research has moved towards use of ligands of the type C(SiMe3)2(SiMe2X) and C(SiMe2X)3 that have similar bulk around the metal centre to which they are attached but also contain groups X bearing lone pairs capable of coordinating intra- or inter-molecularly to the metal.Those used previously include C(SiMe3)2(SiMe2X) with X = OMe2 or SMe 3 and C(SiMe2X)3 with X = OMe4 or NMe2.5,6 In particular, the lithium derivative LiC(SiMe2NMe2)3, made by metallation of the ligand precursor (Me2NMe2Si)3CH, adopts a linear polymeric structure in which the planar lithium ion, co-ordinated by three NMe2 groups, is well separated from the planar carbanionic centre.Moreover, the highly unusual Grignard reagent (Me2NMe2Si)3CMgI has a planar carbanionic centre without specific interaction with the magnesium atom. We now describe the synthesis of the analogous phosphorus-containing ligand precursor I, the molybdenum complex 1, and the lithium derivative 2 obtained from the reaction between I and LiBu/tmen (tmen = N,N,N9,N9-tetramethylethane-1,2-diamine). Results and discussion The precursor I was obtained in good yield from the reaction between (BrMe2Si)3CH7 and KPPh2 in tetrahydrofuran (thf) and characterised by multinuclear NMR spectroscopy.The 1H NMR spectrum showed the expected signals assigned to SiMe2, CH and Ph protons. The 13C and 29Si spectra showed complex multiplets which were simulated by the PANIC program. The signal from the ipso-carbon of the phenyl group was analysed as the A part of an AXY2 system, with the 12C/13C isotope shift at phosphorus |dX 2 dY| = 0.028.The signal from the ortho carbon was also analysed as the A part of an AXY2 spin system but with |dX 2 dY| = 0.009; that from the meta-carbon was analysed as the A part of an AXX92 spin system, as the isotope shift is attenuated over three bonds. The presence of a 13C nucleus in a methyl site makes the two remote 31P nuclei magnetically inequivalent so that the CH3 signal appears as the A part of an AXYY9 system with |dX 2 dY| = 0.006. The 29Si signal was analysed as the A part of an AXY2 system with |dX 2 dY| (arising from the 29Si/28Si isotope shift at P) = 0.011.The 29Si satellites from the quartet assigned to the tertiary carbon showed that the coupling constant 1JSiC is 33.2 Hz [cf. 38.7 Hz for (Me3Si)3CH8 and 42 Hz for (Me2NSiMe2)3CH5]. The phosphorus chemical shift, d 251, is close to that, d 256.8, for diphenyl(trimethylsilyl)phosphine.9 Crystals of compound I suitable for an X-ray study were obtained from heptane–thf. The reaction between compound I and molybdenum hexacarbonyl gave a white solid which was judged to be the complex cis-[Mo(CO)4{(Ph2PMe2Si)3CH}] 1 from (i) the mass spectrum, which showed the successive loss of four carbonyl groups from the molecular ion, (ii) the presence of four bands in the carbonyl stretching region of the infrared spectrum as required for C2v symmetry, and (iii) the presence in the 29Si and 31P NMR spectra of two signals with intensities in the ratio 2 : 1.These are attributed respectively to complexed SiA,BMe2PPh2 fragments (with dP 221, about 30 ppm to higher frequency than the signal for I), and uncomplexed SiCMe2PPh2 fragments (with dP close to that for I).The bidentate co-ordination of the ligand in 1 gives a six-membered MoPSiCSiP metallacycle and leaves one free SiMe2PPh2 fragment. The presence of three signals in each region of the 1H and 13C spectra, corresponding to the methyl and phenyl substituents in uncomplexed SiCMe2PPh2 groups and SiA,BMe2PPh2 groups on either side of the ring, shows that the chelate structure is preserved in solution and is not fluxional on the NMR timescale.In several other complexes MoL(CO)4 the ligand L containing three phosphorus centres, e.g. (Ph2P)3CH10 or (Ph2PCH2)3CCH3,11,12 is bidentate. Compound I was readily metallated by LiBu in the presence of tmen. The product 2 was obtained as pale yellow832 J. Chem. Soc., Dalton Trans., 1999, 831–834 Table 1 Bond lengths (Å) and angles (8) in (Ph2PSiMe2)3CH I and [(Ph2PSiMe2)3C]2 Si–C Si–Me P–Si P–Ph (Ph2PSiMe2)3CH I 1.898(5) a 1.871(6) a 2.275(2) a 1.837(5) a [(Ph2PSiMe2)3C]2 1.809(7) a 1.879(8) a 2.323(3) a 1.837(8) a Si–C–Si Ph–P–Ph Si–P–Ph Me–Si–Me C–Si–Me C–Si–P Me–Si–P Si1 114.7(3) 101.9(2) 101.1(2) 106.5(3) 107.1(3) 116.1(2) 111.5(3) 107.9(2) 103.4(2) 110.1(2) Si2 112.1(3) 101.9(2) 107.9(2) 98.7(2) 106.3(3) 116.1(2) 110.8(3) 108.0(2) 111.5(2) 103.4(2) Si3 114.5(3) 103.0(2) 97.7(2) 106.8(2) 104.6(3) 118.1(2) 110.6(3) 107.3(2) 109.4(2) 106.3(2) Si1 120.7(4) 102.4(3) 106.0(3) 101.4(3) 104.0(4) 114.9(4) 116.6(3) 112.7(2) 105.3(3) 101.8(3) Si2 120.2(4) 105.2(4) 106.3(3) 102.2(3) 105.9(4) 114.6(3) 117.2(3) 114.1(3) 103.3(2) 99.8(3) Si3 118.6(4) 102.2(4) 104.2(3) 106.8(3) 103.4(4) 116.6(3) 114.7(3) 110.8(3) 102.3(3) 107.8(3) a Mean value.Numbers in parentheses indicate the precision of individual measurements, none of which diVered significantly from the mean.crystals from warm toluene and shown by an X-ray study to be ionic, with a lattice consisting of discrete [Li(tmen)2]1 cations and [C(SiMe2PPh2)3]2 anions. The CSi3 core of the carbanion is planar, like those in [Li(thf)4][C(SiMe2- C6H4Me-o)3] 13 and [Li(12-crown-4)2][C(SiMe3)(SiMeBut 2)- (SiMe2F)].14 The ionic structure of 2 contrasts with the polymeric structure of the amino derivative LiC(SiMe2NMe2)3,5 and the molecular structure of the recently described compound [LiP{C(SiMe3)2}(C6H4CH2NMe2)2] 3,15 which has a planar carbanionic centre and lithium co-ordinated both by the lone pair on phosphorus and by those on the two nitrogen atoms.The 31P NMR spectrum of a thf solution of compound 2 consisted of a singlet at d 239.4. The absence of Li–P coupling and the small diVerence between the chemical shifts of 2 and of the precursor I indicate that in solution, as in the solid, the phosphorus atoms in 2 are not co-ordinated to lithium. The structures of the anion of compound 2 and the corresponding protonated species I are shown in Figs. 1 and 2. (The cation in 2 is similar to that described in several other compounds and is not discussed further here.) Bond lengths and angles are given in Table 1. For each species the individual Si–C, Si–Me, P–Si and P–C bond lengths diVer insignificantly from the corresponding mean values, but there is considerably more variation in the bond angles, which means that neither the Fig. 1 Molecular structure of (Ph2PMe2Si)3CH I.anion in 2 nor the corresponding protonated species I has any crystallographic symmetry. The anion has one SiMe2PPh2 group on one side of the CSi3 plane and two on the other. {There is a similar configuration of SiMe2C6H4Me-o groups in the anion [C(SiMe2C6H4Me-o)3]2.13} In contrast, the molecule I is propeller-shaped, with approximate C3 symmetry and all the SiMe2PPh2 groups lying on the side opposite the C–H bond. The Si–C1 distances are much shorter in the anion (mean 1.809 Å) than in the protonated species I (mean 1.898 Å) showing that the anionic charge is delocalised into C1–Si bonds, probably by negative hyperconjugation.16 Data for a number of related species are given in Table 2.The Si–C1 distances in the silyl-stabilised carbanions are all short compared with those in the corresponding silyl-substituted methanes. The bond lengths show little systematic variation with the nature of the substituent X.The Si–P bonds are longer in the anion than in I, and in other organosilylphosphines or organosilylphosphine complexes. 20 The P–C bond lengths are in the normal range, 2.20– 2.29 Å, and all the Si–Me bond lengths are, within experimental error, the same as those in SiMe4 [1.875(2) Å].21 The Si–C–Si angles are larger in the anion than in the protonated species, as expected if the ionic charge is delocalised over the CSi3 system [cf. C(SiMe3)3 17 and C(SiMe2Ph)3 1b derivatives].The other mean bond angles in I are similar (within 48) to those in the anion of 2 but there is more scatter in the protonated than Fig. 2 Structure of the anion of [Li(tmen)2][C(SiMe2PPh2)3] 2.J. Chem. Soc., Dalton Trans., 1999, 831–834 833 in the unprotonated species. The geometry at phosphorus is pyramidal with C–P–C and C–P–Si angles less than the tetrahedral value, as found in other silylphosphine complexes.20 The chemistry of the compounds described here shows several novel features.(a) Although the organolithium compound 2 reacted in the normal way with a stoichiometric amount of MeOH-d4 to give the expected (Ph2PMe2Si)3CD in high yield, reactions with other electrophiles e.g. MeI or I2 resulted in cleavage of Si–P bonds. Similar cleavages have been observed for Ph2PSiMe3 20 but it is remarkable that attack at the Si–P bond in 2 appears to occur more readily rather than that at the carbanionic centre. (b) There is a considerable diVerence in reactivity between Si–N or Si–S bonds on the one hand and Si–P bonds on the other.Whereas the compounds LiC(SiMe2NMe2)3 4,5 and LiC- (SiMe3)2(SiMe2SMe)3 could each be used as a reagent for the transfer of the C-centred ligands to other metals, reactions of compound 2 with HgBr2 and PtCl2 did not proceed cleanly. Tetraphenyldiphosphine P2Ph2 was always obtained as the principal product but the additional presence of PPh2H in some cases may indicate that PPh2Li and PPh2Br are intermediates in the ligand degradation.It has not been possible to isolate the silicon-containing products in a pure state. (c) The isolation of complex 1 suggests that in the absence of electrophiles the compound I has some potential as a mono-, di- or tri-dentate ligand towards transition metals. This area of chemistry is however likely to be restricted by ligand degradation and by slow attack of I on the thf commonly used as a solvent. (d) The stability of compound I in the absence of electrophiles is also shown by the fact that it reacts in the normal way with metal methyl derivatives, e.g.with LiBu to give the compound 2. Experimental Air and moisture were excluded as far as possible from all reactions by the use of standard Schlenk techniques and Ar as blanket gas. Solvents were dried by normal procedures and distilled immediately before use. The NMR spectra were recorded at 300.13 (1H), 125.8 (13C), 99.4 (29Si), 121.4 (31P) and 32.53 MHz (95Mo) and chemical shifts are relative to SiMe4 for H, C and Si, H3PO4 for P, and X 6.515 for Mo.The 29Si spectra were obtained by inverse gated decoupling and signals from ternary carbon were detected by the DEPT procedure. Coupling constants derived from PANIC simulation are accurate to ±0.2 Hz. The EI mass spectra were recorded at 70 eV: m/z values are given for 1H, 12C, 28Si and 98Mo. Syntheses (Ph2PMe2Si)3CH I. A solution of KPPh2 (130 cm3, 0.5 M in thf) was added dropwise to (BrMe2Si)3CH (9.08 g, 21.3 mmol) in thf (100 cm3) at room temperature and the mixture stirred for 4 h.The solvent was removed under vacuum, the residue extracted with benzene (3 × 20 cm3), the extract filtered, the solvent removed, and the residue recrystallised from thf– heptane (10 : 1) to give colourless needles of compound I (14.2 Table 2 Si–C1 Distances (Å) in compounds HC(SiMe2X)3 and anions [C(SiMeX)3]2 X Me Ph PPh2 NMe2 OMe Br HC(SiMe2X)3 1.887(6) 1.895(1) 1.898(5) 1.884(5) Ref. 17 19(a) This work 19(b) [C(SiMeX)3]2 1.818(10)–1.822(10) 1.800(3)–1.812(3) 1.809(7) 1.793(6) 1.805(4) Ref. 17 13 a This work 54 a For C(SiMe2C6H4Me-o)3. g, 89%), mp 192 8C (Found: C, 68.1; H, 6.4; P, 13.8. C43H49P3Si3 requires C, 69.5; H, 6.4; P, 12.5%). No carbon- or phosphoruscontaining impurity was detected by NMR spectroscopy. dH (C6D6) 0.51 (18 H, s, SiMe2), 1.20 (1 H, q, 3JPH 3.5 Hz, CH), 6.98 (18 H, m, m- and p-H) and 7.52 (12 H, m, o-H). dC 1.9 (DEPT q, 1JSiC 33.2, 2JCP 11.1, CH), 2.0 (m, 2JCP 8.1, 4JCP 9.4, 0.4, 4JPP 9.4, SiMe2), 127.7 (p-C), 128.8 (m, 3JCP 6.5, 4JPP 9.4, m-C), 134.6 (m, 2JCP 18.4, 4JPP 9.4, o-C) and 136.5 (m, ipso-C, 1JCP 18.6, 5JCP 20.5, 4JPP 9.4 Hz).dSi 2.8 (1JSiP 27.2, 3JSiP 23.9 Hz). dP 250.6. m/z 557 (5, M 2 PPh2), 370 (75, P2Ph4), 185 (80, PPh2) and 183 (100%, PPh2 2 H2). Compound I reacted slowly (during 1 week) with thf at room temperature with formation of some P2Ph4, the presence of which was deduced from the 31P NMR spectrum.cis-[Mo(CO)4{(Ph2PMe2Si)3CH}] 1. A suspension of [Mo- (CO)6] (0.712 g, 2.69 mmol) and I (2.00 g, 2.69 mmol) in toluene (150 cm3) was slowly heated to reflux, then maintained at reflux for 3 h to give a red solution. The solution was allowed to cool to room temperature and the solvent removed to leave a yellow solid which was judged to be complex 1 (2.34 g, 90%), mp 101 8C (Found: C, 52.9; H, 5.4; Mo, 9.9; P, 9.4. C47H49- MoO4P3Si3 requires C, 59.3; H, 5.2; Mo, 10.1; P, 9.7%).The low value for the carbon analysis is puzzling since the NMR data were fully consistent with the proposed structure. n& max/cm21 2018s, 1950 (sh), 1925s and 1880s. dH (C6D6) 0.15 (6 H, d, 3JPH 3.5, SiMe), 0.31 (6 H, d, 3JPH 6.6, SiMe), 0.36 (6 H, d, 3JPH 2.3, SiMe), 0.93 (1 H, d, 3JPH 5.3 Hz, CH), 6.8–7.2 (18 H, m, m- and p-H), 7.26 (4 H, m, o-H), 7.50 (4 H, m, o-H) and 7.76 (4 H, m, o-H). dC 0.64 (d, 2JCP 14.4, SiMe2), 2.16 (dd, 2JCP 9.6, 4JCP 1.2, SiMe2), 3.25 (d, 2JCP 9.3, SiMe2), 3.45 (q, 2JCP 7.2, CH), 128.06 (A of AXX9, 3JCP 9.6, 2JPP ca. 2, m-CA,B), 128.42 (s, p-C), 128.46 (A of AXX9, 3JCP 9.4, 2JPP ca. 2, m-CA,B), 128.57 (s, p-C), 129.05 (s, p-C), 129.14 (d, 3JCP 7.0, m-CC), 133.61 (A of AXX9, 2JCP 10.3, 2JPP ca. 2, o-CA,B), 134.69 (d, 2JCP 18.5, o-CC), 134.84 (d, 1JCP 17.5, ipso-C), 135.24 (d, 1JCP 24.0, ipso-C), 135.36 (A of AXX9, 2JCP 12.3, 2JPP ca. 2, o-CA,B), 137.56 (d, 1JCP 25.6, ipso- C), 206.4 (t, 2JCP 7.6, cis-CO), 215.9 (dd, 2JCP-trans 22.2, 2JCP-cis 9.4, trans-CO) and 216.3 (t, 2JCP 7.6 Hz, cis-CO).dSi 22.2 (2 Si, d, 1JSiP 18.7, SiA,B) and 0.1 (1 Si, dt, 1JSiP 30.9, 3JSiP 7.2 Hz, SiC). dP 248.7 (1 P, t, 4JPP 2.0, PC) and 217.7 (2 P, d, 4JPP 2.0 Hz, PC). dMo 21290 (Dn2� 1 500 Hz). m/z 924 (8, M 2 CO), 896 (30 M 2 2CO), 868 (100, M 2 3CO), 840 (55, M 2 4CO) and 682 (50, M 2 3CO 2 PHPh2) and 654 (80%, M 2 4CO 2 PHPh2). [Li(tmen)2][C(SiMe2PPh2)3] 2. A solution of LiBu (3.23 mmol) in hexane (1.3 cm3) was added to a mixture of I (2.0 g, 2.7 mmol) and tmen (4.5 cm3, 30 mmol) in toluene (30 cm3) at room temperature.After about 30 min an orange solid separated. This was filtered oV, washed first with light petroleum (bp 40–60 8C, 2 × 10 cm3) then with benzene (3 × 10 cm3), and recrystallised from warm toluene to give pale yellow air- and moisture-sensitive plates of compound 2 (1.73 g, 65%), mp 187 8C (decomp.) (Found: C, 66.1; H, 8.2; N, 5.7.C55H80- LiN4P3Si3 requires C, 67.3; H, 8.2; N, 5.7%). dH (thf-d8) 0.16 (18 H, s, SiMe2), 25 (24 H, s, NMe2), 2.31 (8 H, s, CH2), 6.91–7.37 (20 H, m, Ph) and 7.66–7.73 (10 H, m, Ph). dC 4.75 (m, SiMe2), 4.8 (DEPT, q, 1JSiC 59.7, 2JCP 20.3 Hz, CSi3), 46.1 and 58.8 (free tmen, displaced by solvent), 125.3 (s), 127.8 (m) and 134.8 (m). dSi 20.9. dP 239.4. Reactions of complex 2 With CD3OD. The compound CD3OD (0.4 cm3) in benzene (5 cm3) was added to a suspension of 2 (0.94 g, 0.95 mmol) in benzene (10 cm3) to give a clear solution immediately.The solvent was removed to leave the deuteriated species (Ph2PMe2- Si)3CD as a white solid (0.54 g, 76%). m/z 558.188 (M 2 PPh2); C31H38DP2Si3 requires m/z 558.190. The 13C and 31P NMR spectra were identical with those for I; the proportion of I834 J. Chem. Soc., Dalton Trans., 1999, 831.834 estimated from the signal at d 1.2 in the 1H NMR spectrum was less than 10%. With MeI. A solution of MeI (0.36 mmol) in thf (0.72 cm3) was added to compound 2 (0.32 mmol) in thf (10 cm3) at 270 8C.The mixture was allowed to warm to room temperature and stirred for 2 h. After removal of the solvent the residue was extracted with light petroleum to give, according to the 31P NMR spectrum and integration of the 1H spectrum, a mixture of PPh2Me (51%), and PPh2H/P2Ph4 (49%), identified by comparison of the 31P chemical shifts with those of commercially available samples and with values in the literature.23 With an eight-fold excess of MeI, 2 gave a white solid which was shown by its 1H, 13C and 31P NMR spectra18 to be [PMe2Ph2]I, isolated in 87% yield.Positive FAB MS: m/z 557 (2, [PPh2Me2]2I) and 215 (90, PPh2Me2). With I2. A solution of I2 (0.7 mmol) in thf (0.8 cm3) was added to compound 2 (0.7 mmol) in thf (10 cm3) at 278 8C, and the mixture allowed to warm to room temperature. A white solid was filtered oV and the 31P NMR spectrum of the filtrate showed the presence of P2Ph4 as the only detectable phosphorus-containing thf-soluble product.The solid was not investigated further. With metal halides. The reaction of compound 2 with PtCl2 gave P2Ph4 and that with HgBr2 gave Hg, P2Ph4 (75% of thfsoluble products) and PPh2H (25%). Unidentified white solids were obtained in both reactions and their NMR spectra showed that they contained neither phosphorus nor aromatic protons. Crystallography Details are given in Table 3. All non-hydrogen atoms were refined anisotropically. The H atoms were included in riding mode with Uiso(H) = 1.2Ueq(C) except for Me groups which were fixed at idealised geometry but with the torsion angles defining the H atom position refined and Uiso(H) = 1.5Ueq(C).The high value of R for complex 2 is a consequence of the weak diVraction from a thin plate. Table 3 Crystallographic data and details of structure refinement for compounds I and 2 Empirical formula Formula weight T/K l/A Crystal system Space group a/A b/A c/A b/8 U/A3 Z m/mm21 q Range/8 Reflections collected Unique reflections Reflections with I > 2s(I) R1, wR2 [I > 2s(I)] (all data) Data/restraints/parameters I C43H49P3Si3 743.0 173(2) 0.71073 Monoclinic P21/n (no. 14) 18.314(4) 10.398(3) 21.859(7) 100.61(2) 4091(2) 4 0.26 2.22 5170 4993 (Rint = 0.0357) 3229 0.053, 0.107 0.105, 0.136 4992/0/446 2 C55H80LiN4P3Si3 981.4 173(2) 0.71073 Monoclinic P21/n (no. 14) 15.023(4) 16.592(4) 23.174(11) 90.78(3) 5776(3) 4 0.20 2.23 8371 8027 (Rint = 0.0698) 4106 0.090, 0.145 0.189, 0.179 8026/0/595 CCDC reference number 186/1330.See http://www.rsc.org/suppdata/dt/1999/831/ for crystallographic files in .cif format. Acknowledgements The authors thank the EPSRC for financial support, the EU for the award of a Marie-Curie Fellowship to D. B. and the Royal Society for a Commonwealth Visiting Fellowship for S. K. G. References 1 (a) C. Eaborn, K. Izod and J. D. Smith, J. Organomet.Chem., 1995, 500, 89; (b) C. Eaborn and J. D. Smith, Coord. Chem. Rev., 1996, 154, 125; (c) C. Eaborn, W. Clegg, P. B. Hitchcock, M. Hopman, K. Izod, P. N. O¡�Shaughnessy and J. D. Smith, Organometallics, 1997, 16, 4728; (d ) C. Eaborn, P. B. Hitchcock, K. Izod, Z.-R. Lu and J. D. Smith, Organometallics, 1996, 15, 4783. 2 C. Eaborn, P. B. Hitchcock, A. Kowa©©ewska, Z.-R. Lu, J. D. Smith and W. A. Stan¢¥ czyk, J. Organomet. Chem., 1996, 521, 113. 3 D. A. Antonov, C. Eaborn, J.D. Smith, P. B. Hitchcock, E. Molla, V. I. Rozenberg, W. A. Stan¢¥czyk and A. Kowa©©ewska, J. Organomet. Chem., 1996, 521, 109. 4 F. I. Aigbirhio, N. H. Buttrus, C. Eaborn, S. H. Gupta, P. B. Hitchcock, J. D. Smith and A. C. Sullivan, J. Chem. Soc., Dalton Trans., 1992, 1015. 5 C. Eaborn, A. Farook, P. B. Hitchcock and J. D. Smith, Chem. Commun., 1996, 741. 6 C. Eaborn, A. Farook, P. B. Hitchcock and J. D. Smith, Organometallics, 1997, 16, 503. 7 C. Eaborn, P. B. Hitchcock and P.D. Lickiss, J. Organomet. Chem., 1983, 252, 281. 8 B. Wrackmeyer and W. BiVar, Z. Naturforsch., Teil B, 1979, 34, 1270. 9 H. Schumann and H.-J. Kroth, Z. Naturforsch., Teil B, 1977, 32, 513. 10 J. T. Mague and S. E. Dessens, J. Organomet. Chem., 1984, 262, 347. 11 J. Chatt, G. J. Leigh and N. Thankarajan, J. Organomet. Chem., 1971, 29, 105. 12 E. J. Fernandez, M. C. Gimeno, P. G. Jones, A. Laguna, M. Laguna and E. Olmos, J. Chem. Soc., Dalton Trans., 1996, 3603. 13 A. I. Almansour, C. Eaborn, S. A. Hawkes, P. B. Hitchcock and J. D. Smith, Organometallics, 1997, 16, 6035. 14 N. Wiberg, G. Wagner, G. Reber, J. Riede and G. Muller, Organometallics, 1987, 6, 35. 15 W. Clegg, S. Doherty, K. Izod and P. O¡�Shaughnessy, Chem. Commun., 1998, 1129. 16 P. v. R. Schleyer, T. Clark, A. J. Kos, G. W. Spitznagel, C. Rohde, D. Arad, K. N. Houk and N. G. Rondan, J. Am. Chem. Soc., 1984, 106, 6467; E. A. Brinkman, S. Berger and J. I. Brauman, J. Am. Chem. Soc., 1994, 116, 8304. 17 P. T. Brain, M. Mehta, D. W. H. Rankin, H. E. Robertson, C. Eaborn, J. D. Smith and A. D. Webb, J. Chem. Soc., Dalton Trans., 1995, 349. 18 C. Eaborn, N. Retta and J. D. Smith, J. Chem. Soc., Dalton Trans., 1983, 905; H.-J. Cristau and Y. Ribeill, Synthesis, 1988, 911. 19 (a) C. Eaborn, P. B. Hitchcock and P. D. Lickiss, J. Organomet. Chem., 1984, 269, 235; (b) A. Farook, D.Phil. Thesis, University of Sussex, 1998. 20 E. Lukevics, O. Pudova and R. Sturkovich, Molecular Structure of Organosilicon Compounds, Ellis Horwood, Chichester, 1989, p. 93; W. S. Sheldrick and A. Borkenstein, Acta Crystallogr., Sect. B, 1977, 33, 2916; J. C. Calabrese, R. T. Oakley and R. West, Can. J. Chem., 1979, 57, 1909. 21 B. Beagley, J. J. Monaghan and T. G. Hewitt, J. Mol. Struct., 1971, 8, 401. 22 E. W. Abel, R. A. N. McLean and I. H. Sabherwal, J. Chem. Soc. A, 1968, 2371; P. J. Manning, L. K. Peterson, F. Wada and R. S. Dhami, Inorg. Chim. Acta, 1986, 114, 15. 23 G. Mavel, Annu. Rep. N.M.R. Spectrosc., 1973, 5B, 1. Paper 8/08

ISSN:1477-9226    

DOI:10.1039/a808971j

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

Atomistic computer modeling of [Ru(bpy)3]21 and [Ru(phen)3]21 intercalated into low charged smectites † Josef Breu,*a Nilesh Raj b and C. Richard A. Catlowb a Institut für Anorganische Chemie der Universität Regensburg, D 93040 Regensburg, Germany. E-mail: josef.breu@chemie.uni-regensburg.de b Davy Faraday Research Laboratory, 21 Albemarle Street, London, UK W1X 4BS Received 24th November 1998, Accepted 26th January 1999 Lattice energy minimization techniques have been used to study the 2D molecular organization of [Ru(bpy)3]21 and [Ru(phen)3]21 confined in the interlamellar space of low charged smectites with respect to the stereochemistry of the pillars and the charge distribution within the host.The simulation results underline the complexity of the interplay of long range and short range host–guest and guest–guest interactions in controlling the structure of the interlamellar space.With racemic pillars, favourable p stackings lead to clustering of pillars even with homogeneously charged smectites. Long range and/or short range ordering of the isomorphous substitution within the silicate layer strongly influences the interlayer structure, since host–guest interactions are dominated by electrostatics. The heterogeneity of natural clays rationalises apparently contradicting experimental results on the chiral discrimination by achiral clays.Introduction There is considerable interest in the synthesis of chiral microporous materials for use as stationary phases in separations or as enantioselective heterogenous catalysts, as such materials may combine good size-, shape-, and stereo-selectivity with various types of catalytic activity.2–7 In particular, photocatalytic synthesis of chiral compounds has so far only been subjected to limited investigation, because of the lack of photocatalytically eYcient metal complexes with high asymmetric induction ability which are not prone to photoracemization or photodecomposition.8–10 But photochemical reactions in heterogeneous media may diVer from analogous reactions in a homogeneous solution due to the restricted geometry of the reaction environment.11 Construction of highly organized photocatalytic systems is a prerequisite to obtain high eYciency and selectivity.In principle there are two diVerent routes to arrive at stable chiral inorganic microporous solids.One is the use of chiral templates in the synthesis of framework structures like aluminosilicates or aluminophosphates (ALPOs).2–7 Such a template synthesis will yield solids with 3D framework structures and it remains to be seen how much of the chiral information is preserved in the framework after the template is removed to generate the microporous solid.A very promising alternative route is the intercalation of chiral pillars between stable preformed layered solids where microporosity can be controlled via the pillar density. Here the 2-dimensionally ordered framework of intercalation hosts is used as a template to control the arrangement and orientation of catalysts and educts in the interlamellar region.Clay materials of the so called 2 : 1 class, e.g. vermiculites or smectites (for nomenclature of clay minerals see Martin et al.12), distinguish themselves as host materials by the pronounced corrugation of their surfaces, by the rigidity of the layers towards † Chiral recognition among trisdiimine-metal complexes, Part 4.Part 3; ref. 1. Dedicated to Prof. Dr. K.-J. Range on the occasion of his 60th birthday. Supplementary data available: Tables of potential parameters used. For direct electronic access see http://www.rsc.org/suppdata/dt/1999/835, otherwise available from BLDSC (No. SUP 57490, 7 pp.) or the RSC Library. See Instructions for Authors, 1999, Issue 1 (http://www.rsc.org/ dalton).transverse distortion, and by the tuneability of the charge density of the silicate layers which in turn determines the densities of the cationic pillars.13,14 With chiral tris(1,10-phenanthroline)- or tris(2,29-bipyridyl)- metal complexes ([M(phen)3]n1, [M(bpy)3]n1, M = Fe21, Ru21, Ni21) as guest molecules, remarkable chiral recognition phenomena have been observed.Adsorption capacities, UV/VIS absorption and emission spectra diVer depending on whether the complexes are added as a racemic mixture or as a pure enantiomer.15–20 However, since clay minerals are achiral, there is no simple explanation for the underlying chiral recognition mechanism. Despite this lack of a deeper understanding, clay intercalation compounds (CICs) of optically active [Ru- (bpy)3]21 have been used to photooxidize alkyl phenyl sulfides to sulfoxides with an enantiomeric excess of 15–20%.21 It is expected that the selectivity will increase with our understanding of the complex interplay of host–guest and guest–guest interactions that control the structure of the interlamellar region. Unfortunately, it is notoriously diYcult to ascertain, by direct experiment, the atomic detail of the interlamellar region.Intercalation compounds of natural smectites display turbostratic disorder. In only a very small number of cases of intercalation compounds of the higher charged vermiculites has it been possible to obtain a three-dimensional structure by single crystal X-ray diVraction.22–25 Even in these cases the interlamellar regions were only partially resolved.For intercalation compounds of smectites even 2D long range order of the interlamellar region is hardly ever observed 26–28 and interpretation of the basal spacing at best indicates the orientation of pillars relative to the silicate sheets.19,29 Other experimental techniques like absorption, emission,30–39 Mössbauer,40,41 and vibrational spectroscopy,42 NMR,43 electric dichroism,29 ESR,44 and XANES/EXAFS45 probe the local environment and their structural interpretation is rarely conclusive because of the complexity of the system.Further progress in this promising field suVers badly from this limited ability to characterize the system experimentally and the consequent lack of a deeper understanding of host– guest and guest–guest interactions controlling the structure of intercalation compounds. This problem is probably the main reason why there is an increasing range of applications of molecular-scale simulations of such compounds using J. Chem.Soc., Dalton Trans., 1999, 835–845 DALTON 835 FULL PAPERFig. 1 Schematic representation of possible arrangements of pillars in the interlamellar region of homogeneously charged smectites.diVerent computational techniques (Monte Carlo, Molecular Dynamics, Lattice Energy Minimizations) and levels of approximation.1,46–56 Following our earlier work1 we have performed atomistic simulations for [Ru(bpy)3]21 and [Ru(phen)3]21 intercalated in trioctahedral smectites with a low layer charge originating from either the tetrahedral layers (saponite, [Mg3]oct[Si3.78Al0.22]tet- O10(OH)20.222) or the octahedral layers (hectorite, [Mg2.78- Li0.22]oct[Si4]tetO10(OH)20.222). With this charge density the complex cation pillars do not use up the available interlamellar space completely.Our main objective was to examine the in-plane structure of the interlamellar region, especially with respect to the given pillar system, stereochemistry of the guest complexes and the charge distribution within the host framework. Using large simulation boxes that contain 8 pillars, it was possible to study diVerent pillar distributions and to probe the relevance of diVerent host–guest and guest–guest interactions to the structure of these intercalation compounds.331] 3]21 Methodology General considerations Given a uniform charge density of the corrugated clay substrate we can conceive three possible limiting cases for the arrangement of interlayer cations (Fig. 1): (a) Short-range host–guest interactions are negligible; the guest–guest interactions are dominated by unscreened electrostatic repulsion of the cations, which will consequently arrange in a hexagonal 2-dimensional lattice (Fig. 1a). The lattice constant is a function of the loading and hence of the charge density of the clay which is related to the cation exchange capacity (CEC) because of the charge-neutrality condition.Host lattice and interlamellar structure will only be commensurate for specific loading levels. Experimental support for this model is claimed from adsorptive and diVusive properties of clay systems pillared with [Cr(en) x[Co(en)231 2 (en)]12x (en = ethylenediamine).28 The formation of this typically incommensurate hexagonal lattice of pillars is also supported by an interpretation of the in-plane XRD peaks of smectites exchanged with Ir(diamsar)31, Hg(diamsarH2)41 and Hg(diamsar)21 (diamsar = 1,8-diamino-3,6,10,13,16,19-hexaazabicyclo[6.6.6]- eicosane).27 (b) Again, short range host–guest interactions are negligible.However, attractive lateral interactions between guests are strong enough to induce close-packed islands of interlayer cations despite the electrostatic repulsion.Regardless of the mismatch between non-uniform positive charge density in the interlamellar region and uniform negative charge density in the silicate sheets (Fig. 1b) molecules cluster in the interlamellar region with voids in between. Such oasis/desert scenarios might seem unlikely, but layers of complex cations are a common building block in crystal structures of [M(L–L) compounds (L–L = phen, bpy or 4,49-bipyrimidine).57–59 Also it is well known that [Ru(phen)3]21 and protonated phen form associates in aqueous solution.60,61 Furthermore, emission spectra show that (pyrenylbutyl)trimethylammonium tends to cluster on the clay surface and is not adsorbed randomly.62 (c) There is a considerable short range host–guest interaction.The molecular recognition between host and guest ensures that the interlamellar structure is always commensurable with the host lattice. Charge neutrality is preserved by creating defects 836 J. Chem. Soc., Dalton Trans., 1999, 835–845 Fig. 2 Match between the arrangement of peripheral hydrogen atoms (dark) of the pillars and the molecular imprinting pattern on the corrugated silicate surface.Fig. 3 Labelling scheme used in Table 1. in the packing of the interlayer cations. The molecular/chiral recognition between guests is altered by the host–guest interaction. Thus the structure of the interlamellar region is determined by both interactions (Fig. 1c). We have previously performed atomistic simulations of [Ru(bpy)3]21 intercalated in saponite at a loading level high enough to yield a close packed monolayer.1 The simulation results showed that indeed, for this system, neither host–guest nor guest–guest interactions may be neglected.The delicate molecular imprinting on the silicate surfaces is reinforced by the electrostatic interaction between negative host layers and positive interlayer to the extent that the clay substrate controls the orientations and relative positions of the complex cations in the interlamellar space based on a match between the corrugation of the silicate layer and the shape of the van der Waals surface of the interlayer species (Fig. 2). The lattice energy minima are observed with perfect host–guest fit where all peripheral H atoms that terminate the pillars along their C3-axis (H4 in Fig. 3) protrude into hexagonal hollows on the host surface.Given this restriction, the molecular recognition based on lateral guest–guest interactions leads to completely diVerent 2D packing patterns for racemic and enantiomerically pure monolayers of complex cations.However, at this high loading level the CICs are not microporous. For the design of microporous materials the concentration of the complex cation pillars in the interlamellar region and hence the charge density of the clay has to be reduced. It is therefore crucial to know how changing the clay charge will aVect the interlamellar structure. Even under the restriction of a perfect host–guest fit, the key question remains open as to whether short range attractive forces will induce clustering of complex pillars.As outlined above, the interplay of the diVerent host–guest and guest–guest interactions may induce diVerent pillar distributions in the interlamellar space, which in turn determine the size and shape of pores in this microporous material.Therefore the relative energies of clusters of diVerent size and shape need to be compared to an evenly spaced (hexagonal) arrangement of pillars. Since the lateral interactions and consequently the relative strengths of the contributingFig. 4 Local lattice energy minima for diVerent configurations of enantiomeric (E) and racemic (R) pillars confined between homogeneously charged smectite layers.3]21 energy terms vary with respect to the particular pillar system, this problem has to be investigated for diVerent stereochemistries (enantiomeric and racemic) and for the particular ligand system. The results may well be diVerent for [Ru(bpy) and [Ru(phen)3]21. It is the aim of this paper to answer these questions using the static lattice atomistic simulation technique which we apply to very large periodic systems.Size of the simulation box and charge density Regarding the charge density we are somewhat restricted, as in order to be able to use periodic boundary conditions we can only choose those specific loading levels that yield commensurate hexagonal patterns (as in case (a) above).This is the case for a 2a2b supercell of phlogopite with two complex cations and a charge density of 0.5 per formula unit as used in the previous simulation.1 For the current simulation we are using a lower charge density of 0.22 per formula unit, which corresponds to a 3a3b supercell again with two complex cations. These two charge densities roughly span the range of CECs found for natural smectites.However, with two complexes in the cell, only a dimer can be modeled, therefore we use a yet bigger cell (6a6b) with eight complexes in the simulation box. For the starting structures an approximate host–guest fit is provided. Each complex ‘occupies’ three hexagonal hollows on the clay surface. Since only 8 triples out of 72 hollows on each side of the gallery are ‘occupied’, there are thousands of diVerent starting structures.Given the size of the models we had to make sensible choices. Ten representative starting structures with increasing degrees of clustering of pillars were chosen (Fig. 4), which were expected to be energetically favourable and likely to be close to a local minimum based on previous modeling experience 1 and a survey of published packing motifs 57 for this kind of molecular pillar.Computational methods Probing the energy hypersurface for the ionic, metal-organic/ inorganic composite materials under investigation requires a sound treatment of both long range electrostatic interactions and the short range interactions between guest molecules and between guests and the host lattice.This was achieved in the present study with a classical description of the intracrystalline forces based on the Born model: a covalent ‘molecular mechanics’ potential was used to model the intramolecular forces, while atom–atom pair potentials described the intermolecular interactions. We continue using the method of static lattice energy minimization with periodic boundary conditions, which takes a starting structure and a set of interatomic potentials, and calculates the structure corresponding to the nearest energy minimum.46,63–66 This bears the inherent limitation that con- figurational space has to be sampled by starting from diVerent points; but minimisation methods allow us to model routinely the polarizability of ions in an electric field using a dipolar shell model which has proved crucial to the success in modeling oxide materials.67 Employing the program GULP,68 the lattice energy was calculated by standard summation procedures, using the Ewald method69 for evaluation of the electrostatic term, and real space summation for the short range components of the interaction.The lattice energy was minimized at constant pressure using the Newton–Raphson method starting with a unit Hessian and subsequently updating it with the Broyden–Fletcher–Goldfarb–Shanno algorithm.70 To ensure that a minimum has indeed been reached, the Rational Function Optimisation 71 was used in the final cycles, which removes imaginary modes from the Hessian, thus forcing it to be positive definite.Temperature is not explicitly considered; the simulations are basically athermal, but some thermal eVects may have been subsumed into the interatomic potentials during their fitting to experimental observables.67 We should stress that relaxation of the structure was not limited to the pillars, but rather the host framework was fully flexible during the minimization.In particular, the interlayer spacings and the relative shifts of the silicate layers were allowed to alter; hence the shape and size of the unit cell is a variable during minimization. 837 J. Chem. Soc., Dalton Trans., 1999, 835–845Table 1 MEP derived partial charges [Ru(phen) [Ru(bpy) 3]21 3]21 0.2828 20.1014 0.1366 20.1956 0.0079 20.0664 20.0819 0.2864 20.1197 0.0817 0.1536 20.1414 20.0654 20.0567 20.1975 — 0.1686 0.1339 0.1430 0.1415 — 0.1625 0.1495 0.1360 0.1830 2.8 — 2.8 Ru N C1 C2 C3 C4 C5 C6 H2 H3 H4 H5 H6 rrms (%) For the clay, ionic model potentials with formal charges on Si/Al, Mg/Li and O and a shell model treatment of the polarisability 72 of the oxygen atom were used.Such force fields have been used successfully to model static and dynamic properties of micas as described in detail elsewhere.63 The molecular electrostatic potential (MEP) of the complexes was represented by point charges positioned at the nuclei of the atoms. Following a widely used procedure, point charges were fitted to the quantum chemically calculated MEP73 employing the program POL.74 The net charge was constrained using a Lagrange multiplier and least squares fits were performed for points given on a rectangular grid (0.2 Å spacing) in a 0.7 Å thick layer outside the van der Waals surface.The charge of the well-buried central atom was fixed to its Hirshfeld partition value 75 and values for symmetry equivalent atoms were averaged.The partial charges listed in Table 1 reproduce the MEP well as indicated by the low rrms values for the fits and should be suited to represent the electrostatic interactions between pillars and between the pillars and host lattice. The MEP was calculated employing the density functional code DMOL.76,77 Molecular geometries for the single point calculations were taken from published XRD structures with H atoms recalculated at idealized positions (C–H = 1.09 Å) and no symmetry restrictions were applied. We used “DNP” basis sets with inner cores frozen, a “FINE” integration grid, and the Vosko, Wilk, Nusair 78 parameterization of the exchange correlation energy in the homogeneous electron gas.The local approximation was used in SCF iterations and gradient corrections were added in a pertubative approach using the functionals proposed by Perdew and Wang79 and Becke 80 for the correlation and exchange, respectively. During lattice energy minimization, the molecular geometry of the complexes was restrained at that found in the crystal structure of b-[Ru(bpy)3](PF6)2 81,82 and [Ru(phen)3](PF6)2,59 respectively, by using strong harmonic potentials between the atoms to fix the bond length and three-body potentials to fix the bond angles.This is well justified in view of the small variations observed for these intramolecular parameters in diVerent crystal environments (e.g.b-[Ru(bpy)3](PF6)2,81,82 a-[Ru(bpy)3]- (PF6)2,83 racemic [Ru(bpy)3](ClO4)2,84,85 and enantiomeric [Ru- (bpy)3](ClO4)2 85). The only molecular parameter that shows significant variation within this series is the torsional angle around the Cl]Cl9 bond and realistic energy terms from the cvV-forcefield 86 were used to model torsional motions. We note that force field parameters that are suYciently reliable and accurate are unavailable for the Ru]N interactions.Since, however, the interactions for the intercalation system are dominated by electrostatic and steric factors, this approach is more adequate for the purpose of this study than introducing uncertainties connected with the intramolecular energetics. Non-bonding interactions between the complex cations and 838 J.Chem. Soc., Dalton Trans., 1999, 835–845 both the framework and adjacent complexes are represented using Buckingham potentials fitted by Oie et al.87 to a large range of organic crystal structures. These parameters have been supplemented by Lennard-Jones potentials for the Si]C and Si]Hcomplex cation interactions.88 The cut-oV distance for the nonbonding terms was 16 Å.Ru]Ru, Ru–framework, as well as octahedral cation–complex cation interactions were neglected. Silicate layer charge models The permanent negative charge of trioctahedral smectites is generated by isomorphous substitution of higher valent cations by lower valent cations in the octahedral (hectorite) or tetrahedral (saponite) layer.The diVerent cations that occupy a given type of structural site may do so in either a regular or a disordered manner. A truly random distribution is required to meet the definition of a solid solution, which is best represented by a hybrid atom that is statistically part atom A, part atom B. This disordered state seems to be prevalent.89,90 But there may be also a tendency instead for complete or partial ordering. Long range ordering is indicated in diVraction experiments by lower symmetry or by observation of a superlattice.91 Ordering may also just occur in small domains.For this short range ordering a second possible ordering scheme, besides symmetry reduction, may be observed, i.e. the segregation of isomorphous cations into clusters.92,93 It has long been known that charge densities in smectites vary from silicate layer to silicate layer in a crystal,94 but this fundamentally diVerent type of ordering leads to a heterogenous distribution of negative charges within a single silicate layer.The charge density is segregated into low and high density areas. Such short range domains may diVer in size, in the nature of the predominant cation and/or the cation ordering.95 With a theoretical approach, these diVerent charge distributions at constant CEC can easily be simulated.Computer simulations therefore can provide valuable information on the impact of charge location, charge ordering, and charge distribution on the structure of the interlamellar region.For the main body of the present simulations, a homogeneous saponitic charge distribution using a hybrid species (Al0.056Si0.944) of charge 13.9444 was used. For selected configurations, additionally a homogeneous hectoritic charge distribution using a (Li0.074Mg0.926) hybrid species of charge 11.9259 and various ordered and clustered substitution patterns of Al for Si within the tetrahedral layers were studied.Potential parameters for Al in the latter simulations were taken from Gale and Henson.96 Results and discussion Minimized lattice energies and lattice parameters for diVerent pillar arrangement patterns, pillar systems and host lattice charge models are given in Tables 2–5. All results correspond to the same charge density of the smectite and therefore the same pillar concentration in the interlamellar space.Generally, there is little correlation between density, volume, basal spacing (c^) and the total lattice energy of the diVerent components (electrostatic, short range, etc.) which contribute to it. The dimensions of a and b deviate little from 6× the experimental values found for the related trioctahedral mica phlogopite 97 (5.316(1), 9.221(1) Å) and g stays close to 90.08.Even for the clustered arrangements of pillars (e.g. Fig. 8, right) with large voids in between the islands, the rigidity of the silicate layers is correctly represented. Both observations reassure confidence in the forcefield used. All calculated basal spacings are slightly smaller than the experimental values 19,29 (c^ exp.= 17.9 and 17.8 Å for [Ru(bpy)3]21 and [Ru(phen)3]21 intercalates, respectively). This trend was already observed in previous simulations 1,63 and is expected for a treatment in which thermal motions are neglected. Since the839 J. Chem. Soc., Dalton Trans., 1999, 835–845 Table 2 [Ru(bpy)3]-saponite: calculated energies and structural parameters intercala Enantiomeric te Pattern Total lattice energy/eV DEa/kJ mol21 r/g cm23 Da/Å Db/Å c^/Å a/Å b/Å c/Å a/8 b/8 g/8 V/Å3 a Energy relative to the hexagonal patterns E 1 and R 1, respectively. Table 3 [Ru(phen)3]-saponite: calculated energies and structural parameters intercala Enantiomeric te Pattern Total lattice energy/eV DEa/kJ mol21 r/g cm23 Da/Å Db/Å c^/Å a/Å b/Å c/Å a/8 b/8 g/8 V/Å3 a Energy relative to the hexagonal patterns E 1 and R 1, respectively.E 2 E 1 245508.792 245509.659 83.65 1.701 3.546 0 1.700 3.545 6.208 17.428 32.091 6.179 17.430 32.091 55.638 18.838 70.76 55.637 18.830 70.84 79.15 90.00 31118.1 79.15 90.00 31119.5 E 2 E 1 245509.873 245510.242 22.48 1.739 3.550 0 1.739 3.546 6.196 17.349 32.093 6.178 17.347 32.092 55.642 18.762 70.71 55.639 18.752 70.77 79.09 90.00 30980.8 79.10 90.00 30973.6 E 4 E 3 245499.023 1026.23 245506.010 352.08 1.707 3.572 6.946 1.700 3.559 6.241 17.370 32.118 55.582 17.432 32.085 55.652 19.045 68.61 79.19 18.855 70.67 79.12 89.99 31008.2 90.00 31127.3 E 4 E 3 245501.380 809.72 245508.041 173.00 1.740 3.551 6.446 1.738 3.558 6.247 17.339 32.116 55.602 17.354 32.088 55.655 18.837 69.98 79.13 18.784 70.58 79.08 89.99 30962.8 90.00 30991.8 Racemic intercalate R 2 R 1 245509.433 245509.517 8.10 1.702 3.552 0 1.700 3.542 6.234 17.421 32.091 6.181 17.431 32.091 55.630 18.841 70.68 55.639 18.831 70.84 79.13 90.00 31100.2 79.16 90.00 31122.9 Racemic intercalate R 2 R 1 245511.032 2121.38 245509.893 0 1.739 3.543 1.740 3.554 6.251 6.176 17.349 32.092 17.346 32.093 55.635 55.643 18.753 70.77 18.777 70.55 79.09 79.11 90.00 30979.5 90.00 30970.6 R 4 R 3 245508.039 142.61 245508.060 140.58 1.704 3.537 6.217 1.703 3.565 6.282 17.395 32.091 55.626 17.409 32.089 55.634 18.808 70.70 79.16 18.848 70.53 79.10 89.99 31052.4 90.00 31079.3 R 4 R 3 245510.631 294.56 245510.643 290.41 1.742 3.536 6.179 1.740 3.564 6.290 17.325 32.096 55.628 17.341 32.091 55.638 18.731 70.74 79.12 18.788 70.44 79.07 90.00 30962.8 89.99 30933.0 R 6 R 5 245508.249 122.35 245503.187 610.76 1.702 3.558 6.208 1.702 3.567 6.270 17.413 32.088 55.635 17.419 32.097 55.623 18.826 70.74 79.11 18.853 70.57 79.09 90.00 31086.3 89.99 31098.7 R 6 R 5 245510.286 254.03 245505.841 360.09 1.740 3.557 6.202 1.739 3.557 6.277 17.341 32.091 55.640 17.345 32.099 55.631 18.757 70.69 79.07 18.786 70.48 79.08 90.00 30961.9 89.99 30972.5 Table 4 [Ru(bpy)3]-hectorite: calculated energies and structural parameters Enantiomeric intercalate Racemic intercalate Pattern Total lattice energy/eV DEa/kJ mol21 r/g cm23 Da/Å Db/Å c^/Å a/Å b/Å c/Å a/8 b/8 g/8 V/Å3 a Energy relative to the hexagonal patterns E 1 and R 1, respectively.Table 5 [Ru(bpy)3]-saponite, diVerent charge models: calculated energies and structural parameters Pattern Total lattice energy/eV DEa/kJ mol21 r/g cm23 Da/Å Db/Å c^/Å a/Å b/Å c/Å a/8 b/8 g/8 V/Å3 a Energy relative to the hexagonal patterns E 1.Fig. 5 DiVerent stacking vectors providing the desired host–guest fit for a 2a2b supercell. bonding in the clay layers is much stronger than that perpendicular to it, c^ exp. will increase more with temperature than will a and b.Influence of layer stacking faults Our simulation assumes a perfect crystal while CICs exhibit a wide variety of ordered, partially ordered and fully randomized c axis stacking arrangements with the latter being predominant. Fig. 5 schematically depicts this stacking fault problem for the racemic monolayer previously identified as the global minimum at high loading level 1 (2 cations per 2a2b supercell).Upper (white/dark gray) and lower (shaded) tetrahedral layers belonging to the same silicate layer are shown. The relative positions of the six peripheral hydrogen atoms (H4 in Fig. 3) of each pillar are an extension of the coordination sphere; they are arranged in two trigonal planes (large shaded and black tri- 840 J.Chem. Soc., Dalton Trans., 1999, 835–845 E 1 245955.636 0 1.708 3.540 6.157 17.475 31.982 55.436 18.864 70.95 79.18 90.00 30983.2 Charge model 1 E 4 E 1 245703.842 769.38 1.730 3.554 9.464 245711.816 0 1.708 3.543 6.167 17.195 32.023 55.509 19.947 61.67 79.74 17.420 32.027 55.518 18.816 70.87 79.15 89.99 30566.0 90.01 30973.2 R 2 R 1 E 4 245955.422 6.37 1.709 3.544 6.213 245955.488 0 1.708 3.536 6.158 245945.190 1007.90 1.714 3.560 6.928 17.467 31.983 55.428 18.875 70.781 79.178 17.479 31.982 55.438 18.866 70.95 79.20 17.415 32.012 55.380 19.077 68.704 79.245 89.999 30965.3 90.00 30990.1 89.990 30872.9 Charge model 3 Charge model 2 E 4 E 1 E 4 E 1 245692.701 21141.92 1.711 3.507 6.114 245680.866 0 1.703 3.545 5.858 245708.774 2270.36 1.709 3.526 6.050 245705.972 0 1.725 3.580 20.014 17.371 32.038 55.541 18.747 70.96 79.22 17.460 32.046 55.507 18.755 71.80 79.10 17.396 32.036 55.520 18.752 71.18 79.16 17.250 32.032 55.491 17.617 90.04 78.27 89.98 30909.8 89.99 31057.2 90.01 30940.6 90.00 30660.5 angles), which in turn form a trigonal distorted octahedron.Complex cations (only 2 represented) are located above all dark gray tetrahedra and the three lower peripheral H atoms (shaded triangles) of each pillar protrude into three hollows on the upper tetrahedral layer.Note that the black triangles representing the upper three peripheral H atoms of the pillars are not located above the hexagonal hollows of the lower tetrahedral layer. Therefore an orthogonal stacking of consecutive silicate layers would not provide the same host–guest fit for the upper three peripheral H atoms. Rather, the next silicate layer bordering the interlamellar space along c* has to be shifted to provide the same host–guest fit for the upper three peripheral H atoms. However, many possible stacking vectors match this requirement; for this small 2a2b simulation box, there are 8 alternative stacking vectors (Fig. 5) which manifest themselves in diVerent triclinic simulation cells (Table 6). Because of the pseudohexagonal symmetry of the host lattice and its smaller repeat distance, the relative arrangements of consecutive layers and of host lattice and pillars do not change for the diVerent stackings.Dissimilar shifts rather imply discrete mutual alignments of the complex cation monolayers in successive interlayers. Despite the contrasting shift vectors the lattice energies are in fact the same (Table 6).The interaction of guest species in diVerent interlayers is too weak to aVect the lattice energy and the minima for the 2D arrangement of interlamellar species will not be perturbed by the long range ordering along the stacking direction and any arbitrarily chosen stacking vector. On the other hand, this result implies that for this intercalation system, one would not expect 3D ordering as observed for some vermiculite intercalates with much shorter c-stackingTable 6 Influence of diVerent stackings on the lattice energy b/Å a/Å Db/Å Da/Å c^ 18.597 18.596 18.597 18.596 18.596 18.595 10.727 10.727 10.727 10.727 10.728 10.728 17.343 17.335 17.342 17.335 17.339 17.339 1.450 23.197 1.456 23.191 27.850 6.112 0.866 21.816 6.236 3.547 0.866 21.813 1: 2: 3: 4: 5: 6: 18.597 18.596 10.727 10.727 17.338 17.338 27.852 6.113 6.233 3.550 7: 8: a a Stacking vector used in previous 1 and this work.Fig. 6 Shifted p-stacking arrangements between aromatic ligands of neighbouring cations as observed in [Ru(phen)3](PF6)2 59 (bottom) and in the lattice energy minimum for pattern R4 (top).distances.22–25 The favourable host–guest interaction bridging the interlamellar space might induce fixed phase relationships for the host layers, but guest ordering will only be twodimensional. Distribution of pillars in the interlamellar space With [Ru(bpy)3]21 intercalated into a homogeneously charged saponite (Table 2) the energy minimum for both enantiomeric and racemic interlayers is observed with the hexagonal arrangement of pillars (E 1 and R 1).Due to the large electrostatic repulsion in E 4, the cationic pillars move a little further apart along b with Ru]Ru distances increasing from 10.7 to 11.0–11.3 Å in this direction during minimization.As a consequence, 6 out of 48 peripheral H atoms are forced out of the hexagonal hollows on the clay substrate in the local minimum of E 4. However, despite large energy diVerences for some configurations, all other local minima are observed with perfect host–guest fit emphasizing the importance of short range host–guest interactions with this intercalation system.Surprisingly, the picture changes when switching to the supposedly very similar pillar [Ru(phen)3]21. Here, for enantiomeric interlayers, E 1 is still energetically favoured over any clusters investigated (Table 3). However, with racemic intercalates, shifted p stacks can be realized between aromatic c/Å ELatt./eV 24961.469 24961.482 24961.469 24961.483 24961.474 24961.475 17.425 17.721 18.487 17.980 19.053 18.474 b/8 87.2 95.9 70.3 78.6 87.4 95.6 71.9 79.1 a/8 85.2 100.4 85.5 100.2 114.3 70.7 113.1 70.9 24961.476 24961.480 20.028 18.724 g/8 90.0 90.0 90.0 90.0 90.0 90.0 90.0 90.0 ligands of neighbouring cations. This intermolecular packing pattern is also observed for [Ru(phen)3](PF6)2 59 (Fig. 6). As a consequence of chirality this penetration of two complexes with parallel C3-axes, as required by the host–guest interaction, is only feasible with a racemic pair. Note the structure directing eVect of the peculiar anion clay, which forces the C3-axis to be exactly parallel, while in [Ru(phen)3](PF6)2 they are slightly tilted.Owing to the inherent polarity of aromatic systems, the electron rich core being surrounded by an electron poor torus of H atoms, this is a favourable ‘p–p-interaction’.98 Lateral interactions of this kind are popular motifs in crystal packings of aromatic compounds 99,100 and their general importance in molecular recognition has been acknowledged.101,102 Apparently with the cation–cation distances and orientations induced by the clay surface corrugation, this interaction is stronger for the larger p system (phen as compared to bpy) and even large clusters like one dimensional chains of complex cations running along a (R 4) are lower in energy than the hexagonal pattern (R 1).These results suggest that racemic [Ru(phen)3]21 will cluster in the interlamellar space of saponites even if the host layer charge density is homogeneous.Admittedly, the energy diVerences are relatively small and raise the question of significance. While it is diYcult to quantify precisely the uncertainties associated with the interatomic potentials, the errors associated with computational and numerical aspects (e.g.cutoVs in summations) are low (<1 kJ mol21). We consider that these will not influence the main conclusions drawn from our results. Interestingly, enantiomeric and racemic hexagonal patterns diVer already in lattice energy. The central atoms in these con- figurations are still 16 Å apart, but the molecules already sense their MEP and the cooperative long-range electrostatic interaction for this structure is in favour of the enantiomers (by 14.67 and 35.51 kJ mol21 for [Ru(bpy)3]21 and [Ru(phen)3]21, respectively).Moving the origin of the permanent silicate charge from the tetrahedral to the octahedral layer and hence further away from the interlayer cations alters the relative weight of host–guest and guest–guest interactions (Table 4), which makes clustering more feasible.The diVerence between E 4 and E 1 declines from 1026.26 to 1007.90 kJ mol21 when going from saponite to hectorite. Moreover, for the hectorite intercalate, pairs of [Ru(bpy)3]21 (R 2) have energies that are so close to that of the hexagonal arrangement of pillars (R 1) that a change in model ordering cannot be ruled out, especially when we recall that entropy terms are not included in our assessment of the relative stability of diVerent structures.This result emphasizes the complexity of the interplay of long-range and short-range, host–guest and guest–guest interactions in controlling the structure of the interlamellar space. Presumably minor changes in the pillar or host system may completely alter the structure.Influence of the charge distribution within the clay layers The picture becomes even more complicated and subtly differentiated when taking into account alternative charge distri- J. Chem. Soc., Dalton Trans., 1999, 835–845 841Fig. 7 Local lattice energy minima for pattern E 1 (left) and E 4 (right) with charge model 1 viewed perpendicular to the silicate layers and along a.Al-containing tetrahedra are shown in blue. The protruding hydrogen atoms are red and depicted with realistic van der Waals radii. Fig. 8 Local lattice energy minima for pattern E 1 (left) and E 4 (right) with charge model 2 viewed perpendicular to the silicate layers and along a. Al-containing tetrahedra are shown in blue.The protruding hydrogen atoms are red and depicted with realistic van der Waals radii. butions in the host material. We have investigated three diVerent charge models in combination with two starting configurations, E 1 and E 4 (Table 5, Figs. 7–9). For charge model 1, Al was explicitly assigned in a fully ordered hexagonal superlattice (magenta tetrahedra). The substitution pattern follows the pillar locations in configuration E 1.Charge models 2 and 3 represent cases for short range ordering with segregation of isomorphous substitution into higher charged islands. With charge model 2, isomorphous substitution follows the pillar distribution in configuration E 4, while charge model 3 reflects the maximum Löwensteinian clustering of isomorphous substitution.Starting structures with a prevalent mismatch between the isomorphous substitution pattern in the host and the pillar arrangement in the interlayer (charge model 1/E 4, charge model 2/E 1, and charge model 3/E 1) are far from any local minimum. This expresses itself in substantial changes in shifting vectors and in large displacements of pillars from their initial positions during minimization. This observation gives us some confidence that we have covered configurational space more thoroughly than suggested by the limited selection of starting structures.Clearly, the host–guest interactions are 842 J. Chem. Soc., Dalton Trans., 1999, 835–845 dominated by the electrostatics and the molecular imprinting manifested in the short-range interactions is overruled. Consequently the host–guest fit for the local minima identified for these configurations is not strictly obeyed.The C3-axis of some pillars is no longer exactly perpendicular to the silicate surface. Surprisingly this tilting does not show in the basal spacings. On the other hand, configurations where pillar arrangement and substitution pattern correspond (charge model 1/E 1, charge model 2/E 4, and charge model 3/E 4) have local minima close to the starting structures.Despite the increased charge clustering, the pillar arrangement does not change any further for the latter as compared to charge model 2/E 4, because short range guest–guest and/or host–guest interactions will not allow any denser packing.The host–guest fit for both is perfect, while the Ru]Ru distances marginally decrease from 10.65–10.75 to 10.47–10.68 Å when going from charge model 2 to charge model 3. Looking at the relative energies for these ordered systems, several important conclusions can be drawn. As might have been anticipated, interlayer cation distribution will follow any long-range and/or short-range ordering of substitution in the host lattice, because the electrostatic energy is the lead-Fig. 9 Local lattice energy minima for pattern E 1 (left) and E 4 (right) with charge model 3 viewed perpendicular to the silicate layers and along a. Al-containing tetrahedra are shown in blue. The protruding hydrogen atoms are red and depicted with realistic van der Waals radii.ing term in host–guest interactions. Constructing supercells in the manner done with charge model 1 creates highly ordered structures which do not truly represent the solid solution hosts most frequently encountered in nature. Unless there is good reason to assume such an ordering, this approach will introduce severe artefacts and involves the inherent risk to arrive at wrong conclusions regarding the structure of CICs.On the other hand, the results show convincingly that a heterogenous distribution of negative charges within the silicate layer as proposed by Muller et al.93 for montmorillonite will inevitably induce clustering of pillars in the interlamellar space. Note that we have deliberately selected the cluster configuration that gave the poorest lattice energy compared to the hexagonal arrangement of pillars between homogeneously charged smectite layers. Still there is a strong energetic preference for pillar clustering with clay charge models 2 and 3.This result puts the focus back on the importance of charge characterization and charge control of the host system in intercalation chemistry.It will be essential to synthesize monophasic, homogeneously charged host materials in order to be able to construct highly organized intercalation compounds. Even then, lateral interactions between certain pillars might cause a non-uniform pillar distribution. When working with natural smectites, characterization of exchange properties solely by bulk CECs is insuYcient and will make a conclusive interpretation of experimental results and their reproduction diYcult.This point can best be illustrated by looking at the many papers published on the luminescence behaviour of [Ru(bpy)3]21 and [Ru(phen)3]21 intercalated in smectites. For example, while Joshi et al. found higher emission intensities for D,L-[Ru(bpy)2]21* than for the enantiomers, but higher emission intensities for enantiomeric [Ru(phen)3]21* as compared to the racemates,18 Shimizu et al.37 report that emission intensities of racemates are higher than for enantiomers for both pillar systems.It has been found that self-quenching or concentration quenching readily occurs with increasing loading, indicating that clustering, rather than random distribution of [Ru(bpy)3]21 does occur.Moreover, this clustering is more pronounced for the racemic mixture than for the enantiomers.30,31,36 Kamat et al.36 have recognized that the spectroscopic diVerences are based on distinct host–guest and guest– guest interactions. However, their interpretation is inconclusive in view of the lack of an in depth characterization of the charge distribution of the host material used.Therefore it is unclear whether the driving force for clustering was a heterogeneous charge density of the host or ‘p–p-interactions’ between racemic pillars or a combination of both. Our simulation results, for the first time, rank the diVerent host–guest and guest–guest interactions and oVer a firmer base for the interpretation of the experimental observations.In the light of the delicate balancing of diVerent interactions demonstrated for the idealized limiting cases apparently contradicting experimental facts may be rationalised. Conclusions The simulations described in this paper highlight the complexity of the interplay of diVerent host–guest and guest–guest interactions controlling the structure of the interlamellar region.Calculations of the type reported here allow a detailed and accurate treatment of the competing terms and may be used to probe the energetics and structures of such composite materials. On the other hand, this will allow a more conclusive interpretation of experimental data, but on the other hand it will also foster synthetic approaches that lead to materials with improved properties.Already, supposedly minor changes in host (hectorite vs. saponite) or pillar system ([Ru(bpy)3]21 vs. [Ru(phen)3]21) may result in diVerent pillar arrangement and clustering. Moreover, the corrugation of the host layers may not be neglected.It is essential to allow simultaneous relaxation of both the relative shift of silicate layers and the basal spacing during minimization to develop a realistic picture of the energetics of intercalation chemistry. In his recent review Schoonheydt states that “all the data taken together show that an adequate explanation for chiral discrimination by chiral clays is not yet available.”39 We consider that simulations of the type reported here can yield considerable information on the mechanism of the observed chiral recognition phenomena.The results suggest that the clay mineral controls the orientations and relative positions of the complex cations in the interlamellar space based on the match between host and guest shape and the charge density distribution if ordering in the isomorphous substitution pattern is present.From there on, everything is determined by lateral interactions between the chiral pillars. In particular, favourable shifted p stacks can only be realized for racemic pairs. It should be stressed that this interaction leads to diVerent interlamellar structures at the same loading level.For the intercalation systems where clustering is preferred, these diVerences will occur even at very low loading levels. Even though we did not con- J. Chem. Soc., Dalton Trans., 1999, 835–845 843sider structural quenchers like Fe31, our simulation results oVer a satisfactory explanation of published experimental observations, especially in the light of the heterogeneity of natural clays. Acknowledgements We are indebted to the Leibniz-Rechezentrum and the S.E.R.C. for generous allocation of computer time.We are grateful to Dr. J. D. Gale for many useful discussions and code modifi- cations and to G. Fuchs for helping with code implementation. Finally, J. B. would like to thank Prof. K.-J. 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ISSN:1477-9226    

DOI:10.1039/a809173k

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 847–852 847 Reactions of a ditungsten-capped tetrayne with cobalt carbonyls: molecular structures of {W(CO)3Cp}2{Ï-C8[Co2(Ï-dppm)m- (CO)6 2 2m]n} (m 5 0, 1; n 5 1, 2) Michael I. Bruce,a Brian D. Kelly,a Brian W. Skelton b and Allan H. White b a Department of Chemistry, University of Adelaide, Adelaide, South Australia 5005. E-mail: mbruce@chemistry.adelaide.edu.au b Department of Chemistry, University of Western Australia, Nedlands, Western Australia 6907 Received 15th December 1998, Accepted 12th January 1999 The carbon-rich complex {W(CO)3Cp}2(m-C8) reacts with Co2(m-dppm)m(CO)8 2 2m (m = 0, 1) to give several complexes formed by addition of Co2(m-dppm)m(CO)6 2 2m moieties to one or two C]] ] C triple bonds.X-Ray structure determinations on {W(CO)3Cp}2{m-C8[Co2(m-dppm)m(CO)6 2 2m]n} [m = 0, n = 1, 2; m = 1, n = 1, 2 (two isomers)] confirm the presence of the C8 chain linking the two W(CO)3Cp groups.Introduction There is much current interest in molecules containing carbon chains capped by transition metal–ligand groups.1 These materials have potential utility as quasi-one dimensional conductors, 2 poly-yne systems allowing electronic interactions over relatively long distances through p-delocalisation.3–5 Unsaturated carbon chains with up to 20 carbon atoms have been used to link redox-active metal centres.6–8 While p-bonding of MLn groups to complexes of this type is thought to reduce communication along the chain,9,10 recent studies of diyne complexes of cobalt have suggested that both through-bond and through-space interactions may occur.11 Three groups have described compounds containing C8 chains bridging two W(CO)3Cp,12 Re(NO)(PPh3)Cp*,13 or Fe(dppe)Cp* groups.14 However, complexes of this type have proved diYcult to characterise crystallographically, as crystals of suitable size and quality have not been obtained.The use of Co2(CO)6 and Co2(m-dppm)(CO)4 as protecting groups for C]] ] C triple bonds is well-established as these groups can be easily displaced to regenerate the parent alkyne.15 Consequently, the preparation of similar derivatives of metal complexes containing carbon chains would provide independent evidence for the existence of these chains, although distortions in the Cn geometry as a result of complexation do not give any useful structural information about the uncomplexed polyalkynes.In the limit, it is possible to envisage a novel form of carbon consisting of Cn chains which might be stabilised by formation of the dicobalt derivatives.Indeed, just such a material is considered to form the black insoluble polymer obtained from {Co2(CO)6}2(m,m-Me3SiC2C2SiMe3) on standing in methanol.16 In this paper we describe several complexes which we have prepared from the recently described C8 complex {W(CO)3Cp}2- (m-C8) 1 12 and the dicobalt carbonyl complexes. Results and discussion Reactions between 1 and Co2(CO)8 were carried out at room temperature in thf.The black reaction products were purified by preparative TLC, initial separation into two black fractions occurring. Subsequent crystallisation aVorded X-ray quality crystals of mono- and di-adducts, which were shown by the X-ray structural studies to have Co2(CO)6 groups attached to the C(3)–C(4) (2) and C(3)–C(4), C(39)–C(49) triple bonds (3), respectively. Numbering the C]] ] C triple bonds along the chain from one tungsten allows these complexes to be formulated as {W(CO)3Cp}2{m-C8[Co2(CO)6]-2} 2 and {W(CO)3Cp}2{m-C8- [Co2(CO)6]2-2,3} 3, respectively.The IR n(CO) spectra contained only terminal n(CO) bands between 2096 and 1951 cm21. The 1H NMR spectra contained singlet resonances for the Cp protons at d 5.64 and 5.67 (2) and at d 5.59 (3). Similar reactions between 1 and Co2(m-dppm)(CO)6 aVorded a mono-adduct {W(CO)3Cp}2{m-C8[Co2(m-dppm)(CO)4]} 4 and two isomeric products, again characterised by single crystal X-ray studies as the bis-adducts, containing Co2(m-dppm)(CO)4 groups attached to the C(1)–C(2) and C(19)–C(29) or to the C(3)–C(4) and C(39)–C(49) triple bonds, respectively, namely {W(CO)3Cp}2{m-C8[Co2(m-dppm)(CO)4]2-1,4} 5 and {W(CO)3- Cp}2{m-C8[Co2(m-dppm)(CO)4]2-2,3} 6.Complexes 5 and 6 have essentially indistinguishable IR spectra (Table 1) with terminal n(CO) bands between 2038 and 1933 cm21, at lower energies than those in 2 and 3; those for 4 are found some 3–10 cm21 higher.The 1H NMR spectra contained singlet resonances for the Cp protons at d 5.56 and 5.81, respectively, while the CH2 protons of the dppm ligands occurred at d 3.19 and 3.91 (5) and at d 3.30 and 3.82 (6). Molecular structures of 2–6 Representations of the five molecular structures are given in848 J. Chem. Soc., Dalton Trans., 1999, 847–852 Fig. 1–5, while significant structural parameters are summarised in Table 2.Molecules of 3, 5 and 6 are centrosymmetric. Comparison of the W(CO)3Cp groups with that found in Fig. 1 Projection of {W(CO)3Cp}2{m-C8[Co2(CO)6]} 2. For this and subsequent figures, 20% thermal ellipsoids are shown for the nonhydrogen atoms, hydrogen atoms having arbitrary radii of 0.1 Å. Table 1 IR n(CO) spectra Complex 17238456 n(CO)/cm21 2043s, 1959vs 2101m, 2082s, 2062vs, 2037s, 2028s (sh), 1984w (br) 2090w, 2057s, 2038s, 2031m (sh), 1967vs, 1955s 2096w, 2079s, 2057vs, 2038s, 2025s, 1965s, 1951s 2029m, 2002vs, 1968s 2047w, 2038m, 2010m, 1995s, 1967s, 1955s, 1943m 2037m, 2030m, 2004s, 1992vs, 1960m, 1942m, 1933m 2038m, 2034m, 2011vs, 1996m, 1972s (br), 1960s (br), 1940m (sh) W(C]] ] CC]] ] CSiMe3)(CO)3Cp12 show no significant diVerences to result from the coordination of the dicobalt fragments to the C]] ] C triple bonds.Thus the W–C(Cp) distances all fall in the range 2.27–2.37(1) Å, with the W–CO distances being between 1.929(8) and 2.02(1) Å.Fig. 2 Projection of {W(CO)3Cp}2{m-C8[Co2(CO)6]2} 3. Fig. 3 Projection of {W(CO)3Cp}2{m-C8[Co2(m-dppm)(CO)4]} 4. Fig. 4 Projection of {W(CO)3Cp}2{m-C8[Co2(m-dppm)(CO)4]2} (1,4- isomer) 5.J. Chem. Soc., Dalton Trans., 1999, 847–852 849 The C2Co2 tetrahedra are similar to those found in many other related complexes, with Co–Co separations of 2.4595(8)– 2.480(1) Å, Co–C distances of between 1.951(8) and 2.023(5) Å, and C-C bonds now between 1.33(1) and 1.358(6) Å.The uncoordinated C]] ] C triple bond lengths are between 1.20(1) and 1.214(5) Å. These data may be compared with the other similarly characterised pair of derivatives, {Co2(CO)6}2(m,m-PhC2- C2Ph) 7 17 and {Co2(m-dppm)(CO)4}2(m,m-PhC2C2Ph) 8,18 in which the Co–Co and Co–C distances are between 2.438 and 2.469(4) Å, and between 1.94 and 1.98(1) Å, respectively. The separation of the centre-points of the two Co–Co bonds in 3 and 6 are both 4.373 Å, very similar to the values of 4.43 and 4.36 Å found for 7 and 8.As expected, significant distortions of the C8 chains from linearity occur in these complexes. The bend-back angles of the coordinated C]] ] C triple bonds range from 33.8 to 38.2(5)8. Comparison of 2 and 4, in which the Co2 units are attached to the C(3)–C(4) and C(1)–C(2) bonds, respectively, shows that the total bending is greater in the latter (79.3 vs. 70.38), suggesting that steric pressure from the bulky W(CO)3Cp group may play a role here.As a result of the symmetry of the bis-adducts, the C8 chains describe transoid or S-shaped conformations, with the W–C(1) vectors being approximately orthogonal to the central C(4)–C(49) vectors (range 82.6–92.68). Although the dppm ligands are much larger than the CO groups which they replace, comparison of the bending of the C8 chains shows essentially no diVerence, with angles at C(3) and C(4) in 3 and 6 summing to 276.88 and 276.18, respectively. In 3 and 6, the W–C(1)–C(2)–C(3) sequences are approximately linear, with angles at C(1) and C(2) being 176.9(4) and 175.7(5)8 (for 3) and 174.5(4) and 176.1(4)8 (for 6).The central sequence in 5 has angles at C(3) and C(4) of 170.9(4) and 179.3(4)8, respectively. Closer structural comparisons can be made between 5 and the centrosymmetric molecule Me3SiC2{Co2(m-dppm)- (CO)4}(C]] ] C)2C2{Co2(m-dppm)(CO)4}SiMe3 9,19 the only other tetrayne–dicobalt complex to have been structurally characterised. The C–C bond lengths are 1.343(11), 1.386(9), 1.210(10) and 1.372(14) Å for the bonds between atoms C(1)–C(2)–C(3)– C(4)–C(49), all closely similar to those found in 5.In 9, angles at atoms C(1–4) are 147.0(7), 144.4(9), 171.1(11) and 178.8(3)8, respectively, resulting in the two Si–C(1,19) vectors forming angles of 68.68 with the central C(4)–C(49) bond. Again, this suggests that the bulk of the substituent at C(1) has an eVect on Fig. 5 Projection of {W(CO)3Cp}2{m-C8[Co2(m-dppm)(CO)4]2} (2,3- isomer) 6.the bend-back angle, in additon to any electronic influence of the dicobalt fragment. In the case of 6, in spite the bulk of the Co2(m-dppm)(CO)4 moiety, the angles subtended by the CO and diynyl groups at the W atoms are not significantly diVerent from those in 5. As expected, the W–C(1) distance in the latter has lengthened to 2.213(4) Å compared with 2.123(6) Å in 6, consistent with rehybridisation of this carbon towards sp2. The structural results enable a rationalisation of the 1H NMR data to be obtained.Coordination of a Co2 group to the inner C]] ] C triple bonds results in the Cp signal being at lower field than the resonance observed for the complex in which the outer C]] ] C triple bond is coordinated. For 1, the Cp resonance is at d 5.67; in the case of the mono-adduct 2, two Cp signals separated by 0.03 ppm are found. The presence of the dppm ligands results in a shift of ca. 0.3 ppm to low field.Small amounts of other products were also present in the reaction mixtures, as evidenced by several other bands developing on the TLC plate. However, we have not been able to characterise these compounds. Their IR n(CO) spectra contained bands at significantly lower energies, suggesting that monoadducts were present, while their 1H NMR spectra contained multiple Cp resonances, perhaps indicating that there were up to three other, unsymmetrical isomers of the mono-adducts.Finally, the Co2(CO)6 groups may be removed from the complexed diyndiyl complexes by treatment with ammonium cerium(IV) nitrate in acetone, a method which has been used for more conventional alkyne–Co2(CO)6 complexes.15 Thus, treatment of 2 with [NH4]2[Ce(NO3)6] in acetone resulted in lightening of the colour to orange; conventional work-up aVorded 1 in 50% yield. Conclusions We have shown that it is possible to prepare and characterise derivatives of the C8 complex 1 containing dicobalt carbonyl groups attached to one or two C]] ] C triple bonds; in contrast to the parent complex, crystalline samples of four of these complexes were readily obtained, for which single crystal X-ray studies showed that apparent preferential coordination to the inner C]] ] C triple bonds occurred with Co2(CO)8.However, with Co2(m-dppm)(CO)6, two isomeric complexes containing two dicobalt units attached to the two outer (5) or the two inner C]] ] C triple bonds (6) were obtained. It is of interest that we have been able to isolate derivatives containing Co2(CO)6 moieties attached only to the ‘inner’ C]] ] C triple bonds, whereas with the Co2(m-dppm)(CO)4-substituted complexes the ‘outer’ C]] ] C triple bonds can also coordinate.The dicobalt carbonyl moiety may be removed by oxidation with cerium(IV). Experimental General reaction conditions Reactions were carried out under an atmosphere of nitrogen, but no special precautions were taken to exclude oxygen during work-up.Instrumentation IR: Perkin-Elmer 1700X FT IR. NMR: Bruker CXP300 or ACP300 (1H NMR at 300.13 MHz, 13C NMR at 75.47 MHz). ES MS: Finnegan LCQ: solutions were directly infused into the instrument. Chemical aids to ionisation were used as required.20 Reagents Complex 1 12 and Co2(m-dppm)(CO)6 21 were prepared by the literature methods; Co2(CO)8 (Strem) was used as received. Reaction of {W(CO)3Cp}2(Ï-C8) with Co2(CO)8 A mixture of {W(CO)3Cp}2(m-C8) (100 mg, 0.13 mmol) and850 J.Chem. Soc., Dalton Trans., 1999, 847–852 Table 2 Selected bond lengths (Å) and angles (8) W–CO (av.) W–C(Cp) (av.) W–C(1) Co(2)–Co(3) Co–CO (av.) Co(2)–P(1) Co(3)–P(2) Co(2)–C(1/3) Co(2)–C(2/4) Co(3)–C(1/3) Co(3)–C(2/4) P(1)–C(0) P(2)–C(0) C(1)–C(2) C(2)–C(3) C(3)–C(4) C(4)–C(49) C(11)–W–C(12) C(11)–W–C(13) C(12)–W–C(13) C(1)–W–C(11) C(1)–W–C(12) C(1)–W–C(13) C(21)–Co(2)–C(22) C(21)–Co(2)–C(23) C(22)–Co(2)–C(23) C(31)–Co(3)–C(32) C(31)–Co(3)–C(33) C(32)–Co(3)–C(33) Co(2)–C(4)–C(49) Co(3)–C(3)–C(2) W–C(1)–C(2) C(1)–C(2)–C(3) C(2)–C(3)–C(4) C(3)–C(4)–C(49) Co(2)–P(1)–C(0) P(1)–C(0)–P(2) C(0)–P(2)–Co(3) P(1)–Co(2)–C(21) P(1)–Co(2)–C(22) P(2)–Co(3)–C(31) P(2)–Co(3)–C(32) 2 a 1.99–2.02(1) 1.998 2.27–2.37(1) 2.33 2.143(8) [2.118(8)] 2.461(1) 1.802–1.838(9) 1.821 1.951(8) 1.987(7) 1.978(7) 1.963(7) 1.19(1), 1.23(1) 1.43(1), 1.36(1) 1.33(1), 1.21(1) 1.40(1) 111.8(4) [107.5(4)] 77.4(4) [79.6(4)] 79.4(4) [76.6(4)] 72.5(4) [75.6(3)] 76.5(3) [73.6(3)] 130.5(3) [132.8(4)] 105.3(4) 103.8(4) 98.3(4) 108.6(4) 98.6(4) 98.3(4) 131.5(46) 131.3(6) 173.4(7) 171.6(8) 146.2(8) 143.5(7) 3 1.992–2.006(6) 1.998 2.302–2.351(7) 2.329 2.127(4) 2.4794(9) 1.794–1.825(6) 1.812 1.995(4) 1.964(4) 1.978(5) 1.970(5) 1.213(5) 1.398(5) 1.353(5) 1.425(6) 113.0(3) 78.6(2) 77.8(2) 75.2(2) 73.4(2) 128.8(2) 102.3(2) 103.6(2) 97.6(3) 105.2(2) 102.2(2) 99.0(2) 134.2(3) 137.4(3) 176.9(4) 175.7(5) 142.7(5) 133.6(5) 4 1.96–2.02(1) 1.977 2.293(8)–2.370(9) 2.327 2.216(7), 2.108(9) 2.476(1) 1.765(9)–1.79(1) 1.774 2.243(3) 2.232(2) 2.019(7) 1.954(7) 2.023(5) 1.961(6) 1.827(8) 1.834(9) 1.36(1), 1.22(1) 1.40(1), 1.36(1) 1.20(1), 1.20(1) 1.37(1) 76.0(4), 109.7(4) 77.1(3), 77.6(4) 106.9(4), 79.9(4) 132.1(4), 75.4(4) 73.2(3), 74.3(4) 77.9(3), 133.2(3) 101.8(4) 99.7(4) 141.9(4) 138.8(5) 177.6(8) 178.1(8) 107.7(3) 108.7(5) 108.8(2) 97.6(3) 107.8(3) 96.0(3) 112.9(2) 5 1.955–1.997(5) 1.977 2.307–2.368(6) 2.337 2.213(4) 2.4595(8) 1.752–1.787(4) 1.773 2.221(1) 2.244(1) 1.996(4) 1.969(4) 2.022(3) 1.983(3) 1.830(4) 1.842(4) 1.353(5) 1.398(5) 1.214(5) 1.372(5) 78.2(2) 76.7(2) 105.7(2) 132.8(2) 76.0(2) 73.3(2) 99.6(2) 97.7(2) 143.7(3) 143.5(4) 170.9(4) 179.3(4) 108.5(1) 110.5(2) 110.2(1) 104.4(2) 103.7(2) 112.1(2) 96.5(2) 6 1.929–2.002(8) 1.972 2.28–2.35(1) 2.31 2.123(6) 2.480(1) 1.750–1.783(7) 1.767 2.219(1) 2.232(1) 1.975(5) 1.955(5) 1.964(6) 1.973(5) 1.821(5) 1.834(5) 1.219(8) 1.395(8) 1.358(6) 1.424(6) 78.9(3) 78.0(3) 110.7(3) 131.7(3) 77.0(2) 72.5(3) 103.9(3) 101.6(3) 134.1(3) 132.7(4) 174.5(4) 176.1(4) 142.2(5) 141.8(6) 108.6(2) 109.3(2) 108.9(2) 95.9(2) 112.2(2) 97.2(2) 109.8(2) a This molecule has no crystallographic centre of symmetry; second entries correspond to the counterpart atoms in the primed/second ‘half’ of the molecule.Co2(CO)8 (54 mg, 0.16 mmol) in thf (10 mL) was left to stir for 1 h at r.t., then concentrated under reduced pressure. The resulting black residue was extracted with CH2Cl2 and purified by TLC (silica gel; hexane–CH2Cl2 3 : 2).The top two black bands were removed. Band 1 (Rf 0.7) contained {W(CO)3Cp}2{m-C]] ]CC2[Co2- (CO)6]C2[Co2(CO)6]C]] ] C} 3 (10 mg, 6%). Crystals suitable for X-ray study were obtained from CH2Cl2–pentane (Found: C, 31.38; H, 0.59. C36H10Co4O18W2?CH2Cl2 calcd.: C, 31.32; H, 0.85%; M, 1334). IR (cyclohexane) n(CO) 2096w, 2079s, 2057vs, 2038s, 2025s, 1965s, 1951s cm21. 1H NMR (CDCl3): d 5.59 (s, Cp).Band 2 (Rf 0.6) aVorded {W(CO)3Cp}2{m-C]] ] CC2[Co2(CO)6]- (C]] ] C)2} 2 (37 mg, 37%). Crystals suitable for X-ray study were obtained from CH2Cl2–pentane (Found: C, 34.38; H, 0.96. C30H10Co2O12W2 calcd.: C, 34.40; H, 1.16%; M, 1048). IR (cyclohexane) n(CO) 2090w, 2057s, 2038s, 1967vs, 1955s cm21. 1H NMR (CDCl3): d 5.64, 5.67 (2s, Cp). ES MS (with NaOMe in MeOH): m/z 1071, [M 1 Na]1. Reaction of {W(CO)3Cp}2(Ï-C8) with Co2(Ï-dppm)(CO)6 A stirred mixture of {W(CO)3Cp}2(m-C8) (56 mg, 0.07 mmol) and Co2(m-dppm)(CO)6 (100 mg, 0.15 mmol) in benzene (20 mL) was refluxed for 1 h.The mixture was allowed to cool, concentrated under reduced pressure and the resulting black residue extracted with CH2Cl2 and purified by TLC (silica gel; hexane–CH2Cl2 1 : 1). The top three black bands were removed. Band 1 (Rf 0.8) aVorded {W(CO)3Cp}2{m-C]] ] CC2[Co2- (m-dppm)(CO)4]C2[Co2(m-dppm)(CO)4]C]] ] C} 6 (45 mg, 31%). Crystals suitable for X-ray study were obtained from CH2Cl2– pentane (Found: C, 46.13; H, 2.74.C82H54Co4O14P4W2?2CH2- Cl2 calcd.: C, 46.70; H, 2.71%; M, 1991). IR (cyclohexane) n(CO) 2026m, 1997s, 1974s, 1958m, 1941m, 1925s, 1912s cm21. 1H NMR (CDCl3): d 3.30, 3.82 (2m, 4H, CH2P2), 5.81 (s, 10H, Cp), 7.09–7.39 (m, 40H, Ph). Band 2 (Rf 0.7) contained {W(CO)3Cp}2{m-C2[Co2(m-dppm)- (CO)4](C]] ] C)2C2[Co2(m-dppm)(CO)4]} 5 (25 mg, 17%). Crystals suitable for X-ray study were obtained from CH2Cl2–benzene– pentane (Found: C, 48.48; H, 2.42.C82H54Co4O14P4W2 calcd.: C, 47.77; H, 2.64%; M, 1991. IR (cyclohexane) n(CO) 2031m, 1991s, 1956s, 1947m, 1937m, 1922s, 1913s cm21. 1H NMR (CDCl3): d 3.19, 3.91 (2m, 4H, CH2P2), 5.56 (s, 10H, Cp), 6.87– 7.58 (m, 40H, Ph). Band 3 (Rf 0.5) contained {W(CO)3Cp}2{m-C2[Co2(m-dppm)- (CO)4](C]] ] C)3}?C6H6 4 (10 mg, 7%). Crystals of the hemi-J. Chem. Soc., Dalton Trans., 1999, 847–852 851 Table 3 Crystal and refinement data Compound Formula M Crystal system Space group a/Å b/Å c/Å a/8 b/8 g/8 V/Å3 Z DC/g cm23 F(000) Crystal size/mm T (min, max) m cm21 NN r (Rint) No RR w 2 C30H10Co2O12W2 1048.0 Triclinic P1� 6.9570(8) 12.280(1) 18.768(2) 99.267(2) 96.708(2) 94.144(2) 1564.9 2 2.224 976 0.37 × 0.10 × 0.10 0.53, 0.91 84 18066 7621 (0.048) 6299 0.048 0.056 3 C36H10Co4O18W2? CH2Cl2 1418.8 Triclinic P1� 9.753(2) 11.498(1) 12.046(2) 108.760(2) 103.562(2) 111.034(2) 1096.3 1 2.149 668 0.55 × 0.35 × 0.12 0.47, 0.77 69.0 12368 5322 (0.025) 4442 0.026 0.032 4 C53H32Co2O10P2W2? 0.5C6H6 1415.4 Triclinic P1� 13.656(2) 14.520(2) 15.450(2) 100.616(3) 103.489(3) 113.166(3) 2605 2 1.804 1366 0.20 × 0.10 × 0.04 0.65, 0.88 52 30621 12876 (0.029) 7498 0.040 0.039 5 C82H54Co4O14P4W2? 2CH2Cl2 2160.5 Monoclinic C2/c 19.256(2) 14.656(2) 30.588(4) 105.234(2) 8321 4 1.724 4232 0.2 (cuboid) 0.76, 0.89 38.0 44221 10489 (0.023) 7746 0.034 0.041 6 C82H54Co4O14P4W2? 2C6H6 2146.9 Monoclinic P21/c 18.304(1) 14.0866(8) 18.729(1) 115.739(1) 4350 2 1.639 2116 0.20 × 0.18 × 0.14 0.582, 0.773 35.2 44160 10855 (0.045) 6205 0.040 0.041 benzene solvate suitable for the X-ray study were obtained from CH2Cl2–benzene–hexane (Found: C, 47.23; H, 2.70; C53H32Co2- O10P2W2?0.5C6H6 calcd.: C, 47.52; H, 2.49%).IR (cyclohexane) n(CO) 2047w, 2038m, 2010m, 1995s, 1967s, 1955s, 1943m cm21. 1H NMR (CDCl3); d 3.30, 3.65 (2 × m, 2H, CH2P), 5.66, 5.78 (2 × s, 10H, Cp), 6.97–7.36 (m, 20H, Ph). Decomplexation of 2 A mixture of 2 (20 mg, 0.019 mmol) and [NH4]2[Ce(NO3)6] (35 mg, 0.064 mmol) in acetone (10 ml) was stirred at r.t.for 2 h. The initial black solution became orange over this time. Evaporation, extraction of the residue with CH2Cl2 (3 × 50 ml) and washing the extracts with water (2 × 100 ml) and evaporation of the dried (MgSO4) organic phase gave 1 (7 mg, 50%), identi- fied by 1H NMR [d(CDCl3) 5.63 (Cp); lit.,12 d 5.67]. Crystallography Full spheres of data were measured at ca. 300 K to 2qmax = 588 using a Bruker AXS CCD instrument (monochromatic Mo-Ka radiation, l 0.71073 Å); N data were measured and reduced to Nr independent reflections, No with |F| > 4s(F) being considered ‘observed’ and used in the full matrix least squares refinement after ‘absorption correction’ (proprietary software SADABS).22 Anisotropic thermal parameters were refined for the nonhydrogen atoms; (x, y, z, Uiso)H were included constrained at estimated values. Conventional residuals R, R9 on |F| are quoted, statistical weights derivative of s2(I) = s2(Idiff) 1 0.0004s4(Idiff) being used.Computation used the XTAL 3.4 program system23 implemented by S. R. Hall; neutral atom complex scattering factors were employed. Pertinent results are given in the figures and Table 3. Special features 3. DiVerence map residues were modelled in terms of a molecule of dichloromethane, population constrained at unity after trial refinement, but disordered about an inversion centre. 4 DiVerence map residues were modelled in terms of benzene of solvation, disposed about a crystallographic centre of symmetry, site occupancy set at unity after trial refinement. 5 DiVerence map residues were modelled in terms of a molecule of dichloromethane disordered over two sets of sites, total occupancy constrained at unity after trial refinement, occupancies of the individual components being x, 1 2 x, with x = 0.57(1). 6 DiVerence map residues were modelled in terms of benzene of solvation, site occupancy set at unity after trial refinement.CCDC reference number 186/1311. See http://www.rsc.org/suppdata/dt/1999/847/ for crystallographic files in .cif format. Acknowledgements We thank the Australian Research Council for financial support. References 1 U. H. F. Bunz, Angew. Chem., 1996, 108, 1047; Angew. Chem., Int. Ed. Engl., 1996, 35, 969. 2 G. Frapper and M. Kertesz, Inorg. Chem., 1993, 32, 732. 3 W. Beck, B. Niemer and M. Wieser, Angew.Chem., 1993, 105, 969; Angew. Chem., Int. Ed. Engl., 1993, 32, 923. 4 H. Lang, Angew. Chem., 1994, 106, 569; Angew. Chem., Int. Ed. Engl., 1994, 33, 547. 5 J. S. Schumm, D. L. Pearson and J. M. Tour, Angew. Chem., 1994, 106, 1445; Angew. Chem., Int. Ed. Engl., 1994, 33, 1360. 6 N. Le Narvor, L. Toupet and C. Lapinte, J. Am. Chem. Soc., 1995, 117, 7129. 7 M. Brady, W. Weng, Y. Zhigou, J. W. Seyler, A. J. Amoroso, A. M. Arif, M. Böhme, G. Frenking and J. A. Gladysz, J. Am. Chem. Soc., 1997, 119, 775. 8 T. Bartik, B. Bartik, M. Brady, R. Dembinski and J. A. Gladysz, Angew. Chem., 1996, 108, 467; Angew. Chem., Int. Ed. Engl., 1996, 35, 414. 9 D. Osella, L. Milone, C. Nervi and M. Ravera, J. Organomet. Chem., 1995, 488, 1. 10 N. DuVy, J. McAdam, C. Nervi, D. Osella, M. Ravera, B. H. Robinson and J. Simpson, Inorg. Chim. Acta, 1996, 247, 99. 11 D. Osella, L. Milone, C. Nervi and M. Ravera, Eur. J. Inorg. Chem., 1998, 1473. 12 M. I. Bruce, M. Ke, P. J. Low, B. W. Skelton and A. H. White, Organometallics, 1998, 17, 3539. 13 M. Brady, W. Weng and J. A. Gladysz, J. Chem. Soc., Chem. Commun., 1994, 2655. 14 F. Coat and C. Lapinte, Organometallics, 1996, 15, 477. 15 D. Seyferth, M. O. Nestle and A. T. Wehman, J. Am. Chem. Soc., 1975, 97, 7417. 16 P. Magnus and D. P. Becker, J. Chem. Soc., Chem. Commun., 1985, 640. 17 B. F. G. Johnson, J. Lewis, P. R. Raithby and D. A. Wilkinson, J. Organomet. Chem., 1991, 408, C9. 18 C. J. McAdam, N. W. DuVy, B. H. Robinson and J. Simpson, Organometallics, 1996, 15, 3935.852 J. Chem. Soc., Dalton Trans., 1999, 847–852 19 J. Lewis, B. Lin, M. S. Khan, M. R. A. Al-Mandhary and P. R. Raithby, J. Organomet. Chem., 1994, 484, 161. 20 W. Henderson, J. S. McIndoe, B. K. Nicholson and P. J. Dyson, J. Chem. Soc., Dalton Trans., 1998, 519. 21 L. S. Chia and W. R. Cullen, Inorg. Chem., 1975, 14, 482. 22 G. M. Sheldrick, SADABS, University of Göttingen, 1996. 23 S. R. Hall, G. S. D. King and J. M. Stewart (Editor), The XTAL 3.4 Users’ Manual, University of Western Australia, Lamb, Perth, 1994. Paper 8/

ISSN:1477-9226    

DOI:10.1039/a809749f

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 853–859 853 Reaction of 1-aryl-2-methylenecyclopropanes with rhodium(I) complexes leading to ring opening isomerization and � co-ordination of the C] C double bond Kohtaro Osakada,* Hisami Takimoto and Takakazu Yamamoto* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Received 16th November 1998, Accepted 27th January 1999 1-Aryl-2-methylenecyclopropanes reacted with [RhCl(PPh3)3] at 50 8C to give [RhCl(h4-CH2]] CArCH]] CH2)(PPh3)2] (Ar = C6H5 1a, C6H4F-p 1b, C6H4Me-p 1c or C6H4OMe-p 1d) via ring opening isomerization of the substrate and its subsequent co-ordination to Rh.The diene-co-ordinated rhodium complexes have been characterized by X-ray crystallography and NMR spectroscopy. Similar reaction at 0 8C aVorded the rhodium(I) complexes with p-coordinated 1-aryl-2-methylenecyclopropane, [RhCl(h2-CH2]] CCH2CHAr)(PPh3)2] (Ar = C6H5 2a, C6H4F-p 2b, C6H4Me-p 2c or C6H4OMe-p 2d).Exchange of the ligand of 2a with added 1-aryl-2-methylenecyclopropanes occurs reversibly at 30–45 8C with the thermodynamic parameters of the reactions 2a 1 CH2]] CCH2CHC6H4X-p 2b (or 2c) 1 CH2]] CCH2CHC6H5 being DH8 = 210.3 kJ mol21 and DS8 = 232 J K21 mol21 for X = F and DH8 = 2.2 kJ mol21 and DS8 = 22.6 J K21 mol21 for X = Me, respectively, at 298 K. The structure of a PEt3 co-ordinated analog, [RhCl(h2-CH2]] CCH2CHC6H4Me-p)(PEt3)2] 3c, has been determined by X-ray crystallography.The reaction of 1-methylene-2-phenylcyclopropane with [RhCl(PPh3)3] at 25 8C gave a mixture of 1a and 2a. Heating of a benzene solution of 2a at 50 8C turned it into 1a in low yield (<7%), while the reactions of 1-methylene-2-phenylcyclopropane with 2a at the same temperature gave 1a (10%) and 2-phenylbuta-1,3-diene (14%). The amounts of the products formed via ring opening isomerization in these reactions are much smaller than those in the reaction of 1-methylene- 2-phenylcyclopropane with [RhCl(PPh3)3] at 50 8C.Methylenecyclopropane with a high strain energy (DHf larger than that of cyclopropane by ca. 35 kcal mol21) 1 has been regarded as a useful synthetic equivalent of butadiene and trimethylenemethane 2,3 since upon reaction with transition metal complexes it is easily turned into the ring-opened isomers that are free from the ring strain. The reactions of methylenecyclopropane and its derivatives with organotransition metal complexes give various products such as 1,3-diene (Sc),4 trimethylenemethane-co-ordinated metal complexes (Fe, Mo),5 and organometallic compounds formed via insertion of a C]] C double bond into M–C or M–Cl bonds (Ti, Pd).6,7 Rhodium(I) complexes, which often cause C–C bond activation of small-membered ring molecules,8 react also with methylenecyclopropanes to form 1,3-dienes 9,10 or organic products through formation of a trimethylenemethane-co-ordinated rhodium complex and its further reaction with olefins.11 Complexes containing h2-co-ordinated methylenecyclopropane were also prepared.12 There have been few reports on elucidation of the detailed mechanism of the reactions.Ring opening isomerization of vinylcyclopropane and cyclopropene promoted by d9 metal (Co, Ir) complexes was proposed to involve preco- ordination of the C]] C double bond of the substrate and ensuing C–C bond cleavage of the three-membered ring of the molecule.13,14 Cobalt and rhodium complex promoted reactions of methylenecyclopropane and its derivatives giving 1,3-dienes have also been believed to proceed via initial p co-ordination followed by C–C bond activation of the resulting intermediate (A) as shown in Scheme 1.10,15 Although such an anchoring eVect of the C]] C double bond of methylenecyclopropanes may facilitate the ring-opening isomerization, no experimental results have been presented to support the above reaction pathway. In this paper we report the reaction of 1-aryl-2-methylenecyclopropanes with [RhCl(PPh3)3], leading to ring opening isomerization or h2-co-ordination of a C]] C double bond of the substrate depending on the conditions.The reaction pathway of the ring opening isomerization is discussed based on several reactions of rhodium complexes with and without p-coordinated 1-methylene-2-phenylcyclopropane. Part of this work was reported in a preliminary form.16 Scheme 1 H H R R M R M R H H H H H H H H R H M M + (A) or854 J.Chem. Soc., Dalton Trans., 1999, 853–859 Results and discussion Preparation and characterization of the rhodium complexes 1-Methylene-2-phenylcyclopropane reacts with [RhCl(PPh3)3] (5 : 1 molar ratio) at 50 8C to give [RhCl(h4-CH2]] CPhCH]] CH2)(PPh3)2] 1a in 95% yield after 16 h. The reactions of 1-aryl- 2-methylenecyclopropanes with [RhCl(PPh3)3] give analogous complexes 1b–1d as summarized in eqn. 1. Fig. 1 depicts the molecular structure of 1c determined by X-ray crystallography. The molecule has a distorted piano-stool co-ordination around the Rh that is bonded to a h4-2-(p-methylphenyl)buta-1,3-diene ligand and to Cl and PPh3 ligands. The diene ligand adopts an s-cis conformation similarly to other rhodium(I) complexes with 1,3-diene ligands.18 Table 1 summarizes selected bond dis- Fig. 1 An ORTEP17 drawing of [RhCl{CH2]] C(C6H4Me-p)CH]] CH2]- (PPh3)2]?0.5C6H14 1c?0.5C6H14 with 50% thermal ellipsoidal plotting.Atoms of the solvated hexane and hydrogen atoms were omitted for simplicity. H H Ar Ar Rh Cl PPh3 Ph3P RhCl(PPh3)3 + 50 °C 1a: Ar = C6H5; 1b: Ar = C6H4F- p; 1c: Ar = C6H4Me- p; 1d: Ar = C6H4OMe- p (1) (isolated) + PPh3 Table 1 Selected bond distances (Å) and angles (8) for complexes 1a and 1c Rh–Cl Rh–P1 Rh–P2 Rh–Cl Rh–C2 Rh–C3 Rh–C4 C1–C2 C2–C3 C2–C5 C3–C4 C1–Rh–P1 C1–Rh–P2 P1–Rh–P2 C1–C2–C3 C1–C2–C5 C3–C2–C5 C2–C3–C4 1a a 2.459(3) 2.354(3) 2.368(3) 2.159(10) 2.21(1) 2.144(9) 2.082(10) 1.42(1) 1.44(1) 1.51(1) 1.40(1) 96.90(10) 88.1(1) 107.6(1) 114.(1) 127.0(7) 121.0(1) 118.1(10) 1c b 2.470(1) 2.339(2) 2.370(1) 2.152(3) 2.212(3) 2.155(3) 2.111(3) 1.423(5) 1.411(5) 1.492(5) 1.414(4) 97.12(3) 87.82(4) 108.27(5) 115.0(3) 122.7(4) 122.0(3) 117.8(3) a Taken from ref. 16. b This work. tances and angles of 1c and of the preliminarily reported 1a.16 The C2–C3 bond distances of the diene ligand [1.44(1) Å for 1a and 1.411(5) Å for 1c] are shorter than that of a single bond of an unco-ordinated 1,3-diene molecule.The above results as well as elongation of the C]] C double bonds (C1]] C2 and C3]] C4) are consistent with partial contribution of a metallacyclopentene structure to co-ordination of the diene ligand as proposed for many transition metal diene complexes.19 Complexes 1a–1d were characterized also by NMR (1H, 13C and 31P) spectroscopy. The 1H NMR spectra show two signals at d 3.44–3.62 and 5.45–5.59 due to two hydrogens of the diene ligand.Signals of the other three hydrogens of the ligand appear at significantly higher magnetic field (d 20.56 to 20.49, 0.56–0.58 and 0.77–0.82). The 13C-{1H} NMR spectrum of 1a contains signals due to the diene carbons at d 38.7, 48.3, 87.8 and 113.0. The two former signals of ]] CH2 carbons are split by PC or RhC coupling. These 1H and 13C NMR signals were assigned by 1H–1H and 1H–13C COSY technique although the assignment of a part of the ]] CH2 hydrogens is ambiguous.The molecular structures of 1a and 1c suggest close contact of three ]] CH2 hydrogens (one attached to C1 and two to C4 in Fig. 1) to phenyl planes of the PPh3 ligands. Thus, the high magnetic field positions of the three 1H NMR signals can be attributed to a magnetic anisotropy eVect of p electrons of the phenyl groups. The reactions of 1-aryl-2-methylenecyclopropanes with [RhCl(PPh3)3] at 0 8C aVorded complexes with p-co-ordinated 1-aryl-2-methylenecyclopropane, [RhCl(h2-CH2]] CCH2CHAr)- (PPh3)2] 2a–2d, which were separated from the solution during the reaction, eqn.(2). The complexes were characterized by NMR spectroscopy. e 13C-{1H} NMR signal of the ]] CH2 carbon of 2a appears at high magnetic field (d 34.2) with splitting due to RhC coupling (J 13 Hz). The signal due to the other vinylic carbon shows coupling with Rh also (d 61.5, J 22 Hz), whereas the remaining two carbon signals of the cyclopropane ring are free from such coupling.These results indicate h2 coordination of the double bond to Rh. Upfield shift of the 1H NMR signals of two vinylic hydrogens (d 2.18–2.34) from those of unco-ordinated 1-methylene-2-phenylcyclopropane (d 5.50) also indicates co-ordination of the olefin group. The single 31P- {1H} NMR signal with J(RhP) of 133 Hz suggests a structure with PPh3 ligands at mutually trans positions, similar to other RhCl(olefin)(PR3)2-type complexes.20 Complexes 2b–2d show quite similar NMR data to those of 2a and are considered to have the same four-co-ordinated structure.Introduction of ambient pressure of CO to a solution of 2a led to formation of quantitative amounts of 1-methylene-2-phenylcyclopropane and [RhCl(CO)(PPh3)2] which were identified by GLC and by IR and NMR spectroscopy, respectively, eqn. (3). Recovery of the organic product in a high yield indicates that the complex contains 1-methylene-2-phenylcyclopropane as the ligand.H H Ar Rh Cl PPh3 Ph3P H H Ar RhCl(PPh3)3 (2) (isolated) + 0 °C 2a: Ar = C6H5; 2b: Ar = C6H4F- p; 2c: Ar = C6H4Me- p; 2d: Ar = C6H4OMe- p + PPh3 H H Ph Rh Ph3P PPh3 Cl Ph H H 2a + CO (3) + RhCl(CO)(PPh3)2J. Chem. Soc., Dalton Trans., 1999, 853–859 855 Although X-ray crystallography of complexes 2a–2d was not feasible due to insuYcient quality of the crystals, the PEt3- co-ordinated analogue, [RhCl(h2-CH2]] CCH2CHC6H4Me-p)- (PEt3)2] 3c was prepared from the reaction of 1-methylene-2- (p-methylphenyl)cyclopropane with [RhCl(PEt3)3] and characterized by X-ray crystallography.The reaction shown in eqn. (4) does not give any other products via ring opening isomerization of the substrate. Fig. 2 depicts the molecular structure of 3c. It reseals square-planar co-ordination around the Rh that is bonded to the h2-olefinic group of the ligand. The p-methylphenyl group of the ligand is situated at the opposite side of the Rh.The C]] C double bond is orientated perpendicular to the co-ordination plane and elongated from a typical C]] C double bond [C1–C2 1.405(9) Å]. Deviation of the C]] C double bond from the cyclopropane plane (ca. 1358) caused by back donation from Rh to the ligand seems to release a part of the strain energy of unco-ordinated 1-methylene-2-(p-methylphenyl) cyclopropane. The NMR data of complex 3c shown below indicate its similar structure to those of PPh3 co-ordinated complexes 2a– 2d.The 13C-{1H} NMR signals of two vinylic carbons at d 26.6 and 56.1 are accompanied by large J(RhC) (66 and 22 Hz, respectively). The vinylic hydrogen signals are observed at high magnetic field similarly to those of 2a–2d, while the position of one of the signals was determined from the 1H–13C COSY spectrum due to their severe overlapping with the signals of phosphine ligands. The 31P-{1H} NMR spectrum of 3c contains an AB pattern with J(PP) = 415 Hz as depicted in Fig. 3. Two P nuclei are magnetically inequivalent arising from h2 coordination of unsymmetrically substituted methylenecyclopropane. Fig. 2 An ORTEP drawing of [RhCl(CH2]] CCH2CHC6H4Me-p)- (PEt3)2] 3c with 50% thermal ellipsoidal plotting. Hydrogen atoms except for those in vinyl and cyclopropyl groups were omitted for simplicity. Selected bond distances (Å) and angles (8): Rh–Cl 2.373(2), Rh–P1 2.323(2), Rh–P2 2.319(2), Rh–C1 2.105(7), Rh–C2 2.047(7), C1–C2 1.405(9), C2–C3 1.497(9), C2–C4 1.465(10) and C3–C4 1.528(10); Cl– Rh–P1 86.45(8), Cl–Rh–P2 86.31(8), P1– Rh–P2 171.18(7), C1–Rh–C2 39.5(3), Rh–C1–C2 68.0(4), Rh–C2–C1 72.4(4), C1–C2–C3 136.8(7), C1–C2–C4 136.5(7), C2–C3–C4 57.9(5), C2–C4–C3 60.0(5), C3–C2–C4 62.1(5), H1–C1–H2 107, C2–C1–H1 119 and C2–C1–H2 120.H H C6H4Me- p Rh Cl PEt3 Et3P H H C6H4Me- p RhCl(PEt3)3 (4) 3c + + PEt3 Associative exchange of �-co-ordinated ligand of complex 2a Addition of 1-aryl-2-methylenecyclopropanes to a benzene-d6 solution of complex 2a caused partial conversion of the complex into 2b–2d accompanied by liberation of 1-methylene-2- phenylcyclopropane.The 1H NMR spectra of solutions containing 2a, 1-methylene-2-phenylcyclopropane and a 1-aryl-2- methylenecyclopropane show reversible and rapid exchange between the olefin co-ordinated to Rh and that in solution as shown in eqn. (5). The equilibrium constants were obtained by comparison of the 1H NMR peak area ratios of the mixtures in the temperature range 30–45 8C.The temperature dependence of the equilibrium constants shown in Fig. 4 gives the thermodynamic parameters of the reactions, DH8 = 210.3 kJ mol21 and DS8 = 232 J K21 mol21 for Ar = C6H4F-p and DH8 = 2.2 kJ mol21 and DS8 = 22.6 J K21 mol21 for Ar = C6H4Me-p, respectively, at 298 K. Although the reaction of 1-(p-methoxyphenyl)- 2-methylenecyclopropane with 2a proceeds smoothly to result in exchange of the ligand, an accompanying ring opening isomerization of the substrate to give 1d prevented determin- Fig. 3 The 31P-{1H} NMR spectrum of complex 3c (160 MHz in C6D6) at room temperature. Chemical shifts are referenced to external H3PO4. Fig. 4 Van’t HoV plots of the reactions (a) 2a 1 CH2]] CCH2CHC6H4F- p æÆ 2b 1 CH2]] CCH2CHC6H5 and (b) 2a 1 CH2]] CCH2CHC6H4Me- p æÆ 2c 1 CH2]] CCH2CHC6H5. Rh Ph3P PPh3 Cl Ph H H H H Ar Rh Ph3P PPh3 Cl Ar H H H H Ph + + (5) 2a 2b: Ar = C6H4F- p; 2c: Ar = C6H4Me- p; 2d: Ar = C6H4OMe- p856 J.Chem. Soc., Dalton Trans., 1999, 853–859 ation of the precise equilibrium constants. Comparison of DH8 values of the two above reaction systems indicates that the h2 co-ordination of 1-aryl-2-methylenecyclopropanes to Rh is stabilized by the presence of the electron withdrawing substituent on the aryl group. Mechanism of the ring opening isomerization As shown above, the reactions of 1-aryl-2-methylenecyclopropanes with [RhCl(PPh3)3] at 50 and at 0 8C gave the 2- arylbuta-1,3-diene- and the 1-aryl-2-methylenecyclopropaneco- ordinated rhodium complexes, respectively. 1-Methylene-2- phenylcyclopropane reacts with [RhCl(PPh3)3] at 25 8C to give a mixture of 1a and 2a in a 47 : 53 molar ratio. Similar reaction of 1-(p-fluorophenyl)-2-methylenecyclopropane and of 1-(pmethoxyphenyl)- 2-methylenecyclopropane with [RhCl(PPh3)3] aVorded a mixture of 1b and 2b (54 : 46) and 1d and 2d (55 : 45), respectively. Several additional experiments were conducted to elucidate detailed pathways of formation of these products.The change in the amounts of organic and inorganic products during the reaction of 1-methylene-2-phenylcyclopropane with [RhCl(PPh3)3] was monitored at 50 8C by 1H NMR spectroscopy to obtain mechanistic insights into the ring opening isomerization pathway. Fig. 5 shows plots of the increase in 1a and 2-phenylbuta-1,3-diene which reached 55 and 165% per Rh, respectively, after the reaction for 4.5 h. The 1H and 31P-{1H} NMR signals of [Rh2(m-Cl)2(PPh3)4] and polymer of 2-phenylbuta-1,3-diene 21 were also observed during the reaction although the amounts were not included in Fig. 5. The reactions of 2a at 50 8C were examined to compare its reactivity with [RhCl(PPh3)3] toward 1,3-diene formation. Heating of a benzene solution of 2a for 7 h at 50 8C led to the formation of 1a in a low NMR yield (<7%) and a negligible amount of 2-phenylbuta-1,3-diene. The reaction of 2a with 1-methylene- 2-phenylcyclopropane (1: 5 molar ratio) at 50 8C gave 1a (10%) and 2-phenylbuta-1,3-diene (14% per Rh) after 6 h.The amounts of the products formed via ring-opening isomerization of the substrate are much smaller than those in the reaction of 1-methylene-2-phenylcyclopropane with [RhCl(PPh3)3] under similar conditions (55 and 165%). Scheme 2 summarizes several possible routes for the ng opening isomerization of 1-aryl-2-methylenecyclopropanes promoted by rhodium(I) complexes. Oxidative addition of a C–C bond of the substrate to [RhCl(PPh3)3] (i) will give 1 directly.Reactions (ii) and (iii) involving initial co-ordination of the C]] C double bond to Rh and the reaction of the 1-aryl-2- methylenecyclopropane with the formed 2 also account for formation of the product. The reaction (iii) should be accom- Fig. 5 Plots of the products of the reaction of 1-methylene-2-phenylcyclopropane with [RhCl(PPh3)3] (5 : 1) at 50 8C. Relative amounts of (a) 1-methylene-2-phenylcyclopropane, (b) 2-phenylbuta-1,3-diene, (c) 1a, and (d) 2a to the initial amount of [RhCl(PPh3)3] are shown.panied by more rapid associative exchange of the ligand (iii9) because complex 2a undergoes exchange of the p-co-ordinated ligand even at room temperature. Direct conversion of 2 into 1 via intramolecular C–C bond activation of the co-ordinated 1- aryl-2-methylenecyclopropane (iv) will provide the ring opening isomerization products. The reactions (ii)–(iv) are slower than (i) since heating of 2a and of a mixture of 2a and 1-methylene- 2-phenylcyclopropane forms the 1,3-diene product in low yields.As suggested in Fig. 5, initially formed 2 is further converted into 1 or 2-phenylbuta-1,3-diene under the conditions. Another pathway involving regeneration of [RhCl(PPh3)3] from the reaction of 2 and PPh3 and its reaction with the substrate is also to be considered. However, addition of PPh3 to the reaction mixture of 2a and 1-methylene-2-phenylcyclopropane Scheme 2 H H Ar Ar Rh Ph3P PPh3 Cl H H Ar* Rh Ph3P PPh3 Cl Ar H H H H Ar H H Ar* *Ar Rh Ph3P PPh3 Cl Rh Ph3P PPh3 Cl Ar H H H H Ar Rh Ph3P PPh3 Cl Ar* H H H H Ar Rh Ph3P PPh3 Cl Ar H H Ar Rh Ph3P PPh3 Cl Rh Ph3P PPh3 Cl Ar H H Ar* Rh Ph3P PPh3 Cl Rh Ph3P PPh3 Cl Ar H H H H Ar* (i) 2 1 [RhCl(PPh3)3] + [RhCl(PPh3)3] + [RhCl(PPh3)3] (v) (iii') + + (iii) + + PPh3 + [RhCl(PPh3)3] + (ii) (iv) 1 2 2 1 2 1 (vi) + PPh3 + PPh3 + PPh3J.Chem. Soc., Dalton Trans., 1999, 853–859 857 at 50 8C caused inhibition of formation of 1a and of 2-phenylbuta-1,3-diene.The mechanism in Scheme 3 accounts for all the above results. Dissociation of a PPh3 ligand from [RhCl(PPh3)3] and of 1-aryl-2-methylenecyclopropane ligand from 2 gives [RhCl(PPh3)2(solv)] (solv = solvent) that is responsible for oxidative addition of a C–C bond of the substrate giving 1 or 2-phenylbuta-1,3-diene. Addition of PPh3 to the reaction mixture will turn a labile [RhCl(PPh3)2(solv)] species present into [RhCl(PPh3)3] that shows much less reactivity toward the activation of the C–C bond.The present study has revealed the reaction of 1-aryl-2- methylenecyclopropanes with [RhCl(PPh3)3] to give the ring opening isomerization product or a complex having the substrate as the h2-bonded ligand depending on the conditions. Ring opening isomerization occurs at higher temperature than simple p co-ordination of the substrate, but heating of the p-co-ordinated rhodium complex does not give the ring opened products.Experimental General considerations, measurements and materials Manipulations of the rhodium complexes were carried out under nitrogen or argon using standard Schlenk techniques. The NMR spectra (1H, 13C, and 31P) were recorded on a JEOL EX-400 spectrometer at 25 8C unless otherwise stated; 31P-{1H} NMR peaks were referenced to external 85% H3PO4. Elemental analyses were carried out by a Yanaco MT-5 CHN autocorder.The complex [RhCl(PPh3)3] and 1-aryl-2-methylenecyclopropanes were prepared according to the literature,22 [RhCl(PEt3)3] from the reaction of PEt3 with [{RhCl(C8H14)}2] (C8H14 = cyclooctene).23 1-(p-Fluorophenyl)-2-methylenecyclopropane: dH (C6D6) 0.90 (1 H, dddd, cyclo-C3H3, J 2, 2, 5 and 10), 1.41 (1 H, dddd, cyclo-C3H3, J 2, 2, 9 and 10), 2.26 (1 H, dddd, cyclo- C3H3, J 2, 2, 5 and 9 Hz), 5.52 (m, 2 H, vinyl) and 6.72–6.83 (4 H, m, C6H4). 1-Methylene-2-(p-methylphenyl)cyclopropane: dH (C6D6) 1.05 (1 H, dddd, cyclo-C3H3, J 2, 2, 5, and 9), 1.47 (1 H, dddd, cyclo-C3H3, J 2, 2, 9 and 10), 2.11 (3 H, s, Me), 2.42 (dddd, 1 H, cyclo-C3H3, J 2, 2, 5 and 10 Hz), 5.52–5.55 (2 H, m, vinyl) and 7.02–7.04 (4 H, m, C6H4). 1-(p-Methoxyphenyl)-2- methylenecyclopropane: dH (C6D6) 1.05 (1 H, dddd, cyclo-C3H3, J 2, 2, 5, and 9), 1.46 (1 H, dddd, cyclo-C3H3, J 2, 2, 9 and 10), 2.41 (1 H, dddd, cyclo-C3H3, J 2, 2, 5 and 10 Hz), 3.31 (3 H, s, OMe), 5.52–5.55 (2 H, m, vinyl) and 6.73–7.04 (4 H, m, C6H4). Preparations Complexes 1a–1d.To a toluene (4 cm3) solution of [RhCl(PPh3)3] (193 mg, 0.21 mmol) was added 1-methylene-2- phenylcyclopropane (135 mg, 1.04 mmol) at 50 8C. The solution changed from red to orange during the reaction. After 16 h the Scheme 3 H H Ar Ar* Rh Ph3P PPh3 Cl Rh Ph3P PPh3 Cl Ar H H H H Ar* [RhCl(PPh3)3] 1 2 [RhCl(PPh3)2(solv)] [RhCl(PPh3)2(solv)] + + PPh3 [RhCl(PPh3)2(solv)] + (solv = solvent) solvent was removed by evaporation.Addition of hexane to the orange product led to separation of a yellow solid, which was collected by filtration and dried in vacuo to give complex 1a (157 mg, 95%) (Found: C, 69.75; H, 5.95; Cl, 4.22. C46H40ClP2Rh requires C, 69.66; H, 5.08, Cl, 4.47%). dH (C6D6) 20.56 (1 H, br, Hd or He), 0.58 (1 H, br, Hb), 0.77 (1 H, br, He or Hd), 3.56 (1 H, br, Ha), 5.45 (1 H, br, Hc), 6.81–6.96 (21 H, m), 7.43 (6 H, t, J 8 Hz), 7.78 (6 H, br) and 8.08 (2 H, br). dC (C6D6) 38.7 (d, C4, J 11), 48.3 (dd, C1, J 53 and 11 Hz), 87.8 (s, C3), 113.0 (s, C2), 128.2, 128.7, 128.8, 129.1, 129.4, 131.8, 132.6, 132.7, 134.9, 135.7, 136.0, 137.5, 137.8 and 138.0.dP (C6D6) 23.1 [d, J(PRh) 121] and 35.1 [d, J(PRh) 180 Hz]. Coupling due to a small J(PP) (<8 Hz) was observed depending on the measurement conditions (Chart 1). The reactions of 1-methylene-2-(p-methylphenyl)cyclopropane and of 1-(p-methoxyphenyl)-2-methylenecyclopropane with [RhCl(PPh3)3] gave similar butadiene co-ordinated complexes 1c and 1d which were recrystallized from CH2Cl2– hexane.Complex 1b was isolated from the reaction of 1-(p- fluorophenyl)-2-methylenecyclopropane with [RhCl(PPh3)3] at 25 8C followed by repeated recrystallization of the product. Complex 1b (yield 24%) (Found: C, 68.29; H, 5.43; Cl, 4.23; F, 2.16. C46H39ClFP2Rh requires C, 68.12; H, 4.85; Cl, 4.37; F, 2.34%): dH (C6D6) 20.56 (1 H, br, Hd or He), ca. 0.5 (1 H, br, Hb), ca. 0.8 (1 H, br, He or Hd), 3.44 (1 H, br, Ha), 5.34 (1 H, br, Hc), 6.83–7.12 (20 H, m), 7.41 (6 H, br) and 7.72–7.87 (8 H, br); dP (C6D6) 22.9 [d, J(PRh) 121] and 34.7 [d, J(PRh) 180 Hz].Complex 1c (yield 72%) (Found: C, 67.74; H, 5.39; Cl, 7.71. C47H42ClP2Rh?0.5 CH2Cl2 requires C, 67.15; H, 4.99; Cl, 8.35%): dH 20.49 (1 H, br, Hd or He), 0.56 (1 H, br, Hb), 0.82 (1 H, br, He or Hd), 3.55 (3 H, s, Me), 3.59 (1 H, br, Ha), 5.48 (1 H, br, Hc), 6.84 (10 H, br), 6.86 (10 H, br), 7.45 (6 H, br), 7.78 (6 H, br) and 8.02 (2 H, br); dP (C6D6) 23.6 [d, J(PRh) 117] and 34.7 [d, J(PRh) 184 Hz].Complex 1d (yield 77%) (Found: C, 63.12; H, 5.02. C47H42ClOP2Rh?CH2Cl2 requires C, 63.49; H, 4.88%): dH 20.53 (1 H, br, Hd or He), 0.56 (1 H, br, Hb), 0.80 (1 H, br, He or Hd), 3.62 (1 H, br, Ha), 5.49 (1 H, br, Hc), 6.86– 6.97 (20 H, m), 7.45 (6 H, t, J 7 Hz), 7.79 (6 H, br) and 8.02 (2 H, br); dP (C6D6) 24.9 [d, J(PRh) 121] and 36.4 [d, J(PRh) 180 Hz]. Complexes 2a–2d. To a toluene (7 cm3) solution of [RhCl(PPh3)3] (860 mg, 0.93 mmol) was added 1-methylene-2- phenylcyclopropane (605 mg, 4.6 mmol) at 0 8C.Stirring the solution for 4 h at that temperature caused precipitation of a yellow solid. After 24 h the solid product was collected by filtration, washed with Et2O and then with hexane, and dried in vacuo to give complex 2a as a yellow microcrystalline solid (500 mg, 68%) (Found: C, 69.55 ; H, 5.15. C46H40ClP2Rh requires C, 69.66; H, 5.08%). dH (C6D6) 0.37 [1 H, dd, Hd, J(HcHd) 6, J(HdHe) 5], 1.89 [1 H, dd, Hc, J(HcHd) 6, J(HcHe) 9], 2.27 (1 H, br, Ha or Hb), 2.34 (1 H, br, Ha or Hb), 2.71 [1 H, dd, He, J(HcHe 9, J(HdHe) 5], 6.57–6.69 (2 H, m), 6.81–7.08 (21 H, m), 7.87 (6 H, t, J 8) and 8.01 (6 H, t, J 7 Hz).dC (C6D6) 27.2 [d, C3, J(CRh) 6], 29.6 [d, C4, J(CRh) 4], 34.2 [d, C1, J(CRh) 13], 61.5 [d, C2, J(CRh) 22], 124.8 (C6H5 of 1-methylene-2-phenylcyclopropane), 125.8 (d, J 33), 127.8, 128.5, 129.3 (C6H5 of 1- methylene-2-phenylcyclopropane), 129.8 (d, J 25), 133.0 (t, J 21 and 9), 133.3 (t, J 21 and 9), 135.6 (dd, J 9 and 4), 135.9 (dd, J 9 and 4 Hz), 137.8 (C6H5 of 1-methylene-2-phenylcyclopropane) and 144.7 (C6H5 of 1-methylene-2-phenylcyclopropane); dP C3 C4 C2 C1 Ar Hc He Hd Rh Hb Ha Cl PPh3 Ph3P Chart 1 Numbering scheme of complexes 1a–1d.858 J.Chem. Soc., Dalton Trans., 1999, 853–859 (C6D6) 33.1 and 36.7 [AB pattern, J(PRh) = 133, J(PP) = 426 Hz] (Chart 2). Complex 2b (yield 80%) (Found: C, 68.40 ; H, 5.07; Cl, 4.19; F, 2.12.C46H39ClFP2Rh requires C, 68.12; H, 4.85; Cl, 4.37; F, 2.34%): dH (C6D6) 0.22 [1 H, dd, Hd, J(HcHd) = 5, J(HdHe) = 6], 1.83 [1 H, dd, Hc, J(HcHd) = 6, J(HcHe) = 9], 2.27 (1 H, br, Ha or Hb), 2.31 (1 H, br, Ha or Hb), 2.59 [1 H, dd, He, J(HcHe) = 9, J(HdHe) = 5], 6.34 (2 H, dd, J 8 and 6), 6.46 (2 H, t, J = 8), 6.95 (9 H, d, J 6), 7.08 (9 H, d, J 6), 7.84 (6 H, t, J 7) and 8.01 (6 H, t, J 7 Hz); dP(C6D6) 33.4 and 36.7 [AB pattern, J(PRh) = 133, J(PP) = 426 Hz].Complex 2c (yield 66%) (Found: C, 70.56; H, 5.46; Cl, 4.23. C47H42ClP2Rh requires C, 69.94; H, 5.24; Cl, 4.39%): dH (C6D6) 0.39 [1 H, dd, Hd, J(HcHd) = 5, J(HdHe) = 6], 1.90 [1 H, dd, Hc, J(HcHd) = 5, J(HcHe) = 7], 2.02 (3 H, s, Me) 2.18 (1 H, br, Ha or Hb), 2.19 (1 H, br, Ha or Hb), 2.70 [1 H, dd, He, J(HcHe) = 9, J(HdHe) = 5], 6.53 (2 H, d, J 8), 6.67 (2 H, d, J 8), 6.97 (9 H, d, J 8), 7.07 (9 H, d, J 6), 7.88 (6 H, t, J 8) and 8.01 (6 H, t, J 8 Hz); dP(C6D6) 34.7 and 35.3 [AB pattern, J(PRh) = 133, J(PP) = 426 Hz].Complex 2d (yield 79%) (Found: C, 68.68 ; H, 5.20; Cl, 4.31. C47H42ClOP2Rh requires C, 68.58; H, 5.14; Cl, 4.31%): dH (C6D6) 0.34 [1 H, dd, Hd, J(HcHd) = 5, J(HdHe) = 5], 1.87 [1 H, dd, Hc, J(HcHd) = 5, J(HcHe) = 9], 2.29 (1 H, br, Ha or Hb), 2.34 (1 H, br, Ha or Hb), 2.70 [1 H, dd, He, J(HcHe) = 9, J(HdHe) = 5], 3.26 (3 H, s, OMe), 6.44 (2 H, d, J 8), 6.50 (2 H, d, J 8), 6.98 (9 H, d, J 6), 7.08 (9 H, d, J 6), 7.88 (6H, t, J 7) and 8.02 (6 H, t, J 7 Hz); dP(C6D6) 33.0 and 36.7 [AB pattern, J(PRh) = 133, J(PP) = 426 Hz].Complex 3c. To a toluene (4 cm3) solution of [RhCl(PEt3)3] (369 mg, 0.75 mmol) was added 1-methylene-2-(p-methylphenyl) cyclopropane (257 mg, 1.78 mmol) at 0 8C. The reaction mixture was warmed to room temperature gradually. After 22 h the solvent was removed under vacuum. The residue was recrystallized from Et2O to aVord complex 3c as orange crystals (263 mg, 68%) (Found: C, 52.82; H, 8.18; Cl, 6.92.C23H42- ClP2Rh requires C, 53.24; H, 8.16; Cl, 6.83%). dH(C6D6) 0.88 (1 H, br, Hc or Hd), 0.90–1.05 (18 H, m, PCH2CH3), 1.41–1.69 (13 H, m, PCH2 and Hc (or Hd)), 2.15 (3 H, s, Me), 2.37–2.41 (3 H, br, Ha, Hb and He) and 7.03 (4 H, s, C6H4). dC(C6D6) 8.47 (s, CH3), 8.52 (s, CH3), 13.4 (dd, CH2, J 16 and 5), 13.8 (dd, CH2, J 16 and 7), 21.0 (s, p-Me), 26.3 (s, C4), 26.6 [d, C1 J(CRh) = 66], 30.8 (s, C3), 56.1 [d, C2 J(CRh) = 22 Hz], 125.7, 129.3, 134.4 and 141.7.dP(C6D6) 21.2 and 24.3 [AB pattern, J(RhP) 121, J (PP) 415 Hz] (Chart 3). Reaction of CO with complex 2a A Schlenk flask was charged with a THF (2 cm3) solution of complex 2a (43 mg). After one freeze–pump–thaw cycle the solution was contacted with CO (1 atm = 101.325 Pa) at room temperature. The solution changed from orange to pale yellow. Evaporation of the solvent to ca. 1 cm3 caused separation of an C1 C2 C4 C3 Ar Ha Hb Rh He Hc Hd Chart 2 Numbering scheme of complexes 2a–2d (Cl and PPh3 are omitted.C1 C2 C4 C3 Ar Ha Hb Rh He Hc Hd Chart 3 Numbering scheme of complex 3c (Cl and PEt3 are omitted). oV-white solid which was collected by filtration and dried in vacuo. The IR spectrum of the solid product was identical with that of authentic [RhCl(CO)(PPh3)2]. The 1H NMR analyses of the filtrate showed formation of 1-methylene-2-phenylcyclopropane in a quantitative amount. Reaction of 1-aryl-2-methylenecyclopropanes with [RhCl(PPh3)3] at 25 8C To a toluene (4 cm3) solution of [RhCl(PPh3)3] (141 mg, 0.15 mmol) was added 1-methylene-2-phenylcyclopropane (99 mg, 0.76 mmol) at 25 8C.The solution changed from red to orange. After 20 h the solvent was removed under vacuum. Addition of hexane to the product led to separation of a yellow solid (132 mg) whose 1H NMR spectrum indicated the presence of complexes 1a and 2a in a 47 : 53 ratio. Similar reaction of 1-(p- fluorophenyl)-2-methylenecyclopropane and of 1-(p-methoxyphenyl)- 2-methylenecyclopropane with [RhCl(PPh3)3] aVorded a mixture of 1b and 2b (54 :46) and 1d and 2d (55 : 45), respectively.Equilibrium constant measurement To a benzene-d6 (0.426 g) solution of complex 2a (19.4 mg, 0.024 mol) were added 1-methylene-2-phenylcyclopropane (29.8 mg, 0.23 mmol) and 1-(p-fluorophenyl)-2-methylenecyclopropane (38.3 mg, 0.26 mmol) in an NMR sample tube. Then the NMR spectra were recorded at 30, 35, 40 and 45 8C.The molar ratio of 2a and 2b in the equilibrium mixtures was determined by comparison of the peak area of cyclopropane ring hydrogens (d 2.59 and 2.71). Equilibrium constants between 2a and 2c were obtained from the peak intensity of the methyl hydrogens of 2c and the other signals. The equilibrium constants at each temperature in the reaction of 1-(p- fluorophenyl)-2- methylenecyclopropane and of 1-methylene-2- (p-methylphenyl)cyclopropane with 2a shown in eqn. (5) were 1.206 (30), 1.118 (35), 1.048 (40) and 0.988 (45) and 0.303 (30), 0.310 (35), 0.312 (40) and 0.317 (45 8C), respectively. NMR study of the reaction of 1-methylene-2-phenylcyclopropane with [RhCl(PPh3)3] To a benzene-d6 (ca. 0.5 cm3) solution of [RhCl(PPh3)3] (31.8 mg, 0.034 mmol) was added 1-methylene-2-phenylcyclopropane (22.6 mg, 0.17 mmol) at 0 8C. The NMR spectra were recorded every 10 min at 50 8C. Table 2 Crystallographic data for complexes 1c and 3c Formula M Dimensions/mm Crystal system Space group a/Å b/Å c/Å a/8 b/8 g/8 V/Å3 Z Dc/g cm23 F(000) m(Mo-Ka)/mm21 Reflections measured Unique reflections Used reflections [I > 3.0 s(I)] Variables R(R9) 1c a C50H49ClP2Rh 850.24 1.0 × 0.3 × 0.2 Triclinic P1� (no. 2) 13.582(9) 16.643(5) 10.100(2) 94.94(2) 104.42(3) 73.21(4) 2116(1) 2 1.334 882 0.573 9286 8864 (Rint = 0.026) 6094 487 0.036 (0.029) 3c C23H42ClP2Rh 518.89 0.8 × 0.3 × 0.2 Monoclinic P21/c (no. 14) 12.856(5) 11.455(4) 18.717(6) 101.51(3) 2700(1) 4 1.276 1088 0.854 6764 6485 (Rint = 0.181) 2985 244 0.045 (0.049) a Hexane solvated form.J.Chem. Soc., Dalton Trans., 1999, 853–859 859 Crystallography Recrystallization of complex 1c from a THF–hexane mixture gave single crystals in a hexane solvated form, 1c?0.5 C6H14. Orange single crystals of 3c were obtained by recrystallization from Et2O. The crystals were sealed in a glass capillary tube under argon and applied to data collection at 25 8C. Full-matrix least squares refinement was carried out with all the nonhydrogen atoms anisotropic.Vinyl hydrogens of 3c were located in the final electron density map, while other hydrogens were situated at calculated positions. The hydrogens were included in the structure calculation without further refinement of the positional parameters. Crystal data and details of refinement are summarized in Table 2. CCDC reference number 186/1335. See http://www.rsc.org/suppdata/dt/1999/853/ for crystallographic files in .cif format.Acknowledgements This work was financially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan. References 1 K. B. Wiberg, Angew. Chem., Int. Ed. Engl., 1986, 25, 312; S. M. Bachrach, J. Phys. Chem., 1993, 97, 4496. 2 W. A. Donaldson, Adv. Met.-Org. Chem., 1991, 2, 269; R. Noyori, T. Odagi and H. Takaya, J. Am. Chem. Soc., 1970, 92, 5780; R. Noyori, Y. Kumagai, I. Umeda and H. Takaya, J. Am.Chem. Soc., 1972, 94, 4018; R. Noyori, T. Ishigami, N. Hayashi and H. Takaya, J. Am. Chem. Soc., 1973, 95, 1674; P. Binger and U. Schuchardt, Chem. Ber., 1980, 113, 3334; S. Yamago and E. Nakamura, J. Chem. Soc., Che Commun., 1988, 1112; W. B. Motherwell and M. Shipman, Tetrahedron Lett., 1991, 32, 1103; M. Lautens, Y. Ren and P. H. M. Delanghe, J. Am. Chem. Soc., 1994, 116, 8821; M. Lautens, C. Meyer and A. Lorenz, J. Am. Chem. Soc., 1996, 118, 10676; N. Tsukada, A. Shibuya, I.Nakamura and Y. Yamamoto, J. Am. Chem. Soc., 1997, 119, 8123; R. J. BoVey, M. Santagostino, W. G. Whittingham and J. D. Kilburn, Chem. Commun., 1998, 1875. 3 L. Jia, X. Yang, A. M. Seyam, I. D. L. Albert, P. Fu, S. Yang and T. J. Marks, J. Am. Chem. Soc., 1996, 118, 7900 and refs. therein. 4 G. Parkin, E. Bunel, B. J. Burger, M. S. Trimmer, A. van Asselt and J. E. Bercaw, J. Mol. Catal., 1987, 41, 21. 5 W. E. Billups, L.-P. Lin and B.A. Baker, J. Organomet. Chem., 1973, 61, C55; T.H. Whitesides and R. W. Slaven, J. Organomet. Chem., 1974, 67, 99; T. H. Whitesides, R. W. Slaven and J. C. Calabrese, Inorg. Chem., 1974, 13, 1895; A. R. Pinhas, A. G. Samuelson, R. Risemberg, E. V. Arnoid, J. Clardy and B. K. Carpenter, J. Am. Chem. Soc., 1981, 103, 1668; S. R. Allen, S. G. Barnes, M. Green, G. Moran, L. Trollope, N. W. Murrall, A. J. Walch and D. M. Sharaiha, J. Chem. Soc., Dalton Trans., 1984, 1157. 6 M. Green and R. P. Hughes, J. Chem. Soc., Chem.Commun., 1974, 686. 7 K. Mashima and H. Takaya, Organometallics, 1985, 4, 1464; K. Mashima, N. Sakai and H. Takaya, Bull. Chem. Soc. Jpn., 1991, 64, 2475. 8 D. M. Roundhill, D. N. Lawson and G. Wilkinson, J. Chem. Soc. A, 1968, 845; H. C. Volger, H. Hogeveen and M. M. P. Gaasbeek, J. Am. Chem. Soc., 1969, 91, 218 and 2137; T. J. Katz and S. Cerefice, J. Am. Chem. Soc., 1969, 91, 2405; L. Cassar and J. Halpern, Chem. Commun., 1970, 1082; F. J. McQuillin and K. C. Powell, J.Chem. Soc., Dalton Trans., 1972, 2129; H. Ogoshi, J.-I. Setsune and Z.-I. Yoshida, J. Chem. Soc., Chem. Commun., 1975, 572; N. W. Alcock, J. M. Brown, J. A. Conneely and D. H. Williamson, J. Chem. Soc., Perkin Trans. 2, 1979, 962; R. A. Periana and R. G. Bergman, J. Am. Chem. Soc., 1984, 106, 7272. 9 J. M. Brown and A. G. Kent, J. Chem. Soc., Perkin Trans. 2, 1987, 1597; G. P. Chiusoli, M. Costa and L. Meli, J. Organomet. Chem., 1988, 358, 495. 10 C.-H. Jun and Y.-G. Lim, Bull.Korean Chem. Soc., 1989, 10, 468. 11 G. P. Chiusoli, M. Costa, P. Schianchi and G. Salerno, J. Organomet. Chem., 1986, 315, C45. 12 M. Green, J. A. K. Howard, R. P. Hughes, S. C. Kellett and P. Woodward, J. Chem. Soc., Dalton Trans., 1975, 2007. 13 J. Foerstner, A. Kakoschke, D. Stellfeldt, H. Butenschon and R. Wartchow, Organometallics, 1998, 17, 893. 14 M. Murakami, K. Itami, M. Ubukata, I. Tsuji and Y. Ito, J. Org. Chem., 1998, 63, 4. 15 P. Binger, T. R. Martin, R. Benn, A. Rufinska and G. Schroth, Z. Naturforsch., Teil B, 1984, 39, 993. 16 K. Osakada, H. Takimoto and T. Yamamoto, Organometallics, 1998, 17, 4532. 17 C. K. Johnson, ORTEP, Report ORNL-5138, Oak Ridge National Laboratory, Oak Ridge, TN, 1976. 18 L. Porri, A. Lionetti, G. Allegra and A. Immirzi, Chem. Commun., 1965, 336; L. Porri and A. Lionetti, J. Organomet. Chem., 1966, 6, 422; S. M. Nelson, M. Sloan and M. G. B. Drew, J. Chem. Soc., Dalton Trans., 1973, 2195; R. R. Schrock and J. A. Osborn, J. Am. Chem. Soc., 1971, 93, 2397; B. F. G. Johnson, J. Lewis and D. J. Yarrow, J. Chem. Soc., Dalton Trans., 1972, 2084; E. W. Abel, T. Blackmore and R. J. Whitley, J. Chem. Soc., Dalton Trans., 1976, 2484; P. Caddy, M. Green, J. A. K. Howard, J. M. Squire and N. J. White, J. Chem. Soc., Dalton Trans., 1981, 400; J. Moreto, K. Maruya, P. M. Bailey and P. M. Maitlis, J. Chem. Soc., Dalton Trans., 1982, 1341; P. Powell, M. Stephens, A. Muller and M. G. B. Drew, J. Organomet. Chem., 1986, 310, 255; F. Claret and P. Vogel, Organometallics, 1990, 9, 2785; M. Murakami, K. Itami and Y. Ito, J. Am. Chem. Soc., 1996, 118, 11672; 1997, 119, 2950. 19 H. Yasuda, K. Tatsumi and A. Nakamura, Acc. Chem. Res., 1985, 18, 120 and refs. therein. 20 J. A. Osborn, F. H. Jardine, J. F. Young and G. Wilkinson, J. Chem. Soc. A, 1966, 1711; J. T. Mague and G. Wilkinson, J. Chem. Soc. A, 1966, 1736; C. A. Tolman, P. Z. Meakin, D. L. Lindner and J. P. Jesson, J. Am. Chem. Soc., 1974, 96, 2762; H. L. M. van Gaal and F. L. A. van den Bekerom, J. Organomet. Chem., 1977, 134, 237; Y. Ohtani, A. Yamagishi and M. Fujimoto, Bull. Chem. Soc. Jpn., 1979, 52, 2149. 21 T. Suzuki, Y. Tsuji, Y. Takegami and H. J. Harwood, Macromolecules, 1979, 12, 234. 22 N. Ahmad, J. J. Levison, S. D. Robinson and M. F. Uttley, Inorg. Synth., 1974, 15, 58; J. A. Osborn and G. Wilkinson, Inorg. Synth., 1990, 28, 77; S. Arora and P. Binger, Synthesis, 1974, 801. 23 S. Montelatici, A. van der Ent, J. A. Osborn and G. Wilkinson, J. Chem. Soc. A, 1968, 1054. Paper 8/08906J

ISSN:1477-9226    

DOI:10.1039/a808906j

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 861–865 861 The reaction of trimethylsilyldiazomethane with complexes of the type [PtX(CH3)(diphosphine)] (X 5 Cl, Br, I). Some observations on ‚-hydrogen migrations in PtCHRCH3 species and organoplatinum(II)-catalysts for alkene formation from trimethylsilyldiazomethane Paola Bergamini,*a Emiliana Costa,b Christian Ganter,b A. Guy Orpen b and Paul G. Pringle *b a Dipartimento di Chimica dell’Università di Ferrara e Centro di Studio su Fotoreattività e Catalisi del CNR, via L.Borsari 46, 44100 Ferrara, Italy b School of Chemistry, University of Bristol, Cantocks Close, Bristol, UK BS8 1TS Received 5th October 1998, Accepted 16th December 1998 Treatment of [PtX(CH3)(diphos)] 1 {X = Cl, Br, I; diphos = (4R,5R)-4,5-bis(diphenylphosphinomethyl)-2,2-dimethyl- 1,3-dioxolane (diop), (2S,4S)-2,4-bis(diphenylphosphino)pentane (skewphos), (2S,3S)-2,3-bis(diphenylphosphino)- butane (chiraphos)} with N2CHSiMe3 gives two series of products: “a-products“, [PtX(CH2SiMe3)(diphos)] 2 and “b-products“ [PtX(CH2CH2SiMe3)(diphos)] 3.Which product is formed and their stability depends on the ancillary ligands X and diphos. Treatment of [PtCl(CH3)(diop)] 1a with an excess of N2CHSiMe3 gives the a-product [PtCl(CH2SiMe3)(diop)] 2a in high yield. The structure of 2a was confirmed by X-ray crystallography. Under similar conditions [PtCl(CH3)(skewphos)] 1d reacts with an excess of N2CHSiMe3 to give the b-product [PtCl(CH2CH2SiMe3)(skewphos)] 3d as shown unambiguously by a combination of 1H-COSY and 31P NMR spectroscopy.It is established that the reaction sequence is 1 æÆ 3 æÆ 2 and the conversion of 3 æÆ 2 is via a b-hydrogen migration and elimination of CH2]] CHSiMe3. The stability of 3 with respect to b-hydrogen elimination is in the order Cl > Br > I and chiraphos > skewphos > diop; a mechanism is proposed based on five-coordinate platinum(II) intermediates to rationalize these trends.The reactions of [PtX(CH3)(diphos)] with N2CHSiMe3 and N2CHCOOEt are contrasted and it is concluded that in PtCHRMe species, a SiMe3 group facilitates b-hydrogen migration while a CO2Et group retards b-hydrogen migration. The complexes 2 are catalysts for the conversion of N2CHSiMe3 to Me3SiCH=CHSiMe3. Introduction The formation of a C–C bond at a metal centre and b-hydrogen elimination are two of the most important steps in many transition metal complex catalysed processes.1 We and others have previously shown that carbenes generated from diazocarbonyls insert into Pt–CH3 bonds [eqn.(1)] to give branched alkylplatinum( II) species which are remarkably kinetically stable with respect to b-hydrogen transfer.2,3 Moreover carbene insertions into the Pt–CH3 bonds in optically active complexes of the type [PtX(CH3)(diphos)] occurred with modest diastereoselectivity and no epimerisation at the a-carbon was observed.2 We wanted to investigate whether the bulky, electron rich carbene CHSiMe3 would insert into Pt–CH3 with greater diastereoselectivity.However we report here that when trimethylsilyldiazomethane is the reagent, Pt–CH3 insertion is followed by rapid b-hydrogen migration. The significance of these Pt L CH3 C H L X R Pt CH3 X L L C CH3 Pt X L H R L L L X R + N2CHR (1) = cod, R, R-diop, S, S-skewphos or S, S-chiraphos = Cl, Br or I = CO2Et, COPh observations to the mechanism of b-hydrogen migration in PtCHRCH3 species is discussed.Results and discussion Treatment of [PtCl(CH3)(R,R-diop)] 1a [R,R-diop = (4R,5R)- 4,5-bis(diphenylphosphinomethyl)-2,2-dimethyl-1,3-dioxolane] with more than two equivalents of N2CHSiMe3 in CH2Cl2 gave exclusively [PtCl(CH2SiMe3)(R,R-diop)] 2a (see Scheme 1) as shown by a combination of elemental analysis, IR, 31P, 1H NMR spectroscopy (see Experimental section and Table 1 for the data) and X-ray crystallography (see below).Moreover treatment of [PtCl(CH2SiMe3)(cod)] 4 with R,R-diop also gave the complex 2a [eqn. (2)] as shown by 31P NMR spectroscopy. The crystal structure of 2a was determined by conventional single crystal methods. Fig. 1(a) and (b) show the molecular structures of the two independent molecules of complex 2a present in the crystal and Table 2 gives important structural parameters for these molecules. The structure determination Pt P CH2SiMe3 P Cl Pt CH2SiMe3 Cl 2a P P = R,R-diop P P (2) P P = S,S-skewphos P P = S,S-chiraphos 2d 2g862 J.Chem. Soc., Dalton Trans., 1999, 861–865 unambiguously confirms the absolute configuration shown. As expected the platinum is square planar with small deviations from ideal coordination geometry and the Pt–P distances Scheme 1 Pt P CH2SiMe3 P X Pt CH3 X P P Pt P CH2CH2SiMe3 P X 2a 1a-i P P = R, R-diop 2 N2CHSiMe3 2b P P = R, R-diop 2c P P = R, R-diop 2e P P = S, S-skewphos 2d P P = S, S-skewphos a-products b-products X = Cl X = Br X = I X = Br X = Cl 2f X = I P P = S, S-skewphos ii i 3a P P = R, R-diop 3b P P = R, R-diop 3c P P = R, R-diop 3e P P = S, S-skewphos 3d P P = S, S-skewphos X = Cl X = Br X = I X = Br X = Cl 3f X = I P P = S,S-skewphos 3h P P = S, S-chiraphos 3g P P = S, S-chiraphos X = Br X = Cl 3i X = I P P = S, S-chiraphos 2i X = I P P = S, S-chiraphos N2CHSiMe3 N2CHSiMe3 Table 1 31P NMR data a Compound 2a 2b 2c 2d 2e 2f 2g 2i 3a 3b 3c 3d 3e 3f 3g 3h 3i d(PA) 5.5 5.7 3.2 16.2 16.0 13.2 45.0 41.3 7.7 8.0 4.5 18.4 18.5 14.7 44.4 45.1 42.7 1J(PtPA) 4329 4323 4157 4224 4222 4064 4152 4040 4684 4676 4473 4545 4545 4361 4520 4516 4355 d(PB) 8.5 5.6 0.6 15.6 13.7 9.5 42.5 42.2 8.3 6.3 1.3 15.2 14.2 10.5 44.6 44.4 42.2 1J(PtPB) 1745 1763 1799 1730 1746 1765 1780 1769 1492 1528 1567 1466 1493 1518 1504 1509 1511 2J(PAPB) 14 14 15 23 22 24 13 22 12 12 12 22 22 12 10 12 12 a Spectra (81 MHz) measured in MeCN at 21 8C; chemical shifts (d) in ppm (±0.1) to high frequency of 85% H3PO4.Coupling constants (J) in Hz (±3). PA is trans to the halogen and PB is trans to the carbon. reflect the higher trans influence of the CH2SiMe3 group compared with chloride. As shown in the torsion angle data given in Table 2, there is considerable variation in the conformation of the R,R-diop ligand (and of the CH2SiMe3 ligand) in the two independent molecules. Treatment of [PtCl(CH3)(S,S-skewphos)] 1d [S,S-skewphos = (2S,4S)-2,4-bis(diphenylphosphino)pentane] with an excess of N2CHSiMe3 gave (after 15 min) a new species quantitatively which has been isolated and assigned the structure 3d (see Scheme 1) on the basis of elemental analysis, 31P NMR, 13C DEPT, 1H and particularly 1H-COSY NMR, (see Table 1 and Experimental section for the data).Fig. 1 (a) Molecular structure of first independent molecule of 2a showing labelling scheme, all hydrogen atoms have been omitted for clarity. (b) Molecular structure of second independent molecule of 2a showing labelling scheme, all hydrogen atoms have been omitted for clarity.Table 2 Selected bond distances (Å), angles (8) and torsion angles (8) from the crystal structure of 2a Pt–P(1) (trans to C) Pt–P(2) (trans to Cl) C–Pt–Pcis C–Pt–Ptrans P–Pt–P Cl–Pt–C–Si (P)C–C–C–C(P) Ptrans to C–C–C–C Pcis to C–C–C–C 2.36(2) 2.215(11) 93.5(12) 168.6(12) 96.2(5) 271.3 282.0 224.6 66.6 2.321(13) 2.242(13) 92.8(8) 169.5(8) 97.7(5) 71.1 289.6 107.8 61.6 Pt–C Pt–Cl C–Pt–Cl Ptrans–Pt–Cl Pcis–Pt–Cl Pt–Ptrans to C–C–C Pt–Pcis to C–C–C P–Pt–Ptrans to C–C P–Pt–Pcis to C–C 2.18(4) 2.371(13) 83.6(12) 175.9(5) 87.0(4) 70.0 39.7 28.6 258.7 2.09(3) 2.366(10) 83.6(8) 176.3(5) 86.0(5) 257.0 277.4 26.4 60.2J.Chem. Soc., Dalton Trans., 1999, 861–865 863 Thus there is apparently a sharp contrast in the products 2a, with a SiMe3 group on the a-carbon (an “a-product“) and 3d, with a SiMe3 group on the b-carbon (a “b-product“) formed by addition of an excess of Me3SiCHN2 to [PtCl(CH3)(R,Rdiop)] and [PtCl(CH3)(S,S-skewphos)] respectively.However further study by 31P NMR spectroscopy revealed that if 1 equivalent of Me3SiCHN2 is added to [PtCl(CH3)(R,R-diop)] 1a, a complex having 31P parameters similar to 3d is observed (Table 1) in the mixture of species present and is assigned the structure 3a; this b-product 3a appears stable in CDCl3 for at least 24 h. After 2 h treatment of [PtCl(CH3)- (S,S-skewphos)] 1d with an excess of Me3SiCHN2, a small amount of a second species with the same 31P NMR parameters as 2d [generated via the route shown in eqn.(2)] was observed (Table 1). Therefore it is apparent that 1a and 1d with an excess of Me3SiCHN2 undergo the following sequence of transformations: Pt–CH3 æÆ Pt–CH2CH2SiMe3 æÆ PtCH2SiMe3 (see Scheme 1). The same pattern is seen for the reactions of the complexes [PtX(CH3)(R,R-diop)] (X = Br 1b or I 1c) and [PtX(CH3)- (S,S-skewphos)] (X = Br 1e or I 1f) with Me3SiCHN2.Thus 31P NMR spectra show that upon treatment of 1a–f with 1 equivalent of Me3SiCHN2 the b-products, assigned structures 3a–f on the basis of the similarity of their 1J(PtP) values (Table 1), are formed which are stable in the absence of an excess of Me3SiCHN2. When an excess of Me3SiCHN2 is used, the aproducts, assigned structures 2a–f again on the basis of the similarity of their 1J(PtP) values, are formed (Table 1). In these reactions, with the exception of 2a and 3d (see above), the products were not isolated. It was noticed that the chloro complexes undergo the second step in Scheme 1 (3 æÆ 2) more slowly than their bromo and iodo analogues.The six-membered chelate (skewphos) complexes also undergo the second step more slowly than the seven-membered chelate (diop) analogues which prompted us to investigate whether there was a trend in reactivity with chelate ring size. Thus the reactions of the five-membered chelate complexes 1g–i [chiraphos, (2S,3S)-2,3-bis(diphenylphosphino) ethane] with Me3SiCHN2 were followed by 31P NMR spectroscopy and it was revealed that the reactions were slower than 1a–f and, with 1g and 1h, the only products Scheme 2 Pt P CH2SiMe3 P X Pt H X P P Pt P CHSiMe3 P X Pt X P P H CH H2C SiMe3 Pt P CH2CH2SiMe P X Pt X CH3 P P SiMe3 CH3 1 2 N2CHSiMe3 N2CHSiMe3 2 3 i ii iii v N2CHSiMe3 iv C A B – observed (even after 20 h in the presence of an excess of Me3- SiCHN2) were the b-products 3g and 3h, i.e.the second step in Scheme 2 with the five-membered chelates was not observed; 2g has been generated via the route shown in eqn. (2). With the iodo complex 1i, a species assigned to the a-product 2i (identified from its 31P parameters only, see Table 1) was detected when an excess of N2CHSiMe3 was used, consistent with the greater tendency of the iodo complexes to form the a products. A mechanism to explain the formation of 2a–f and 3a–i from the corresponding 1a–i and N2CHSiMe3 is proposed in Scheme 2.Insertion of the CHSiMe3 into the Pt–CH3 bond of 1a–i (step i) to give the transient species A (the mechanism for insertions of this type we have previously discussed in detail 2) is followed by b-hydrogen migration (step ii) to give a five-coordinate hydridoplatinum(II) species B.5 A b-hydrogen migration to the substituted alkenyl carbon (step iii) would give the observed b-products 3a–i. Alternatively, dissociation of vinyltrimethylsilane from B (step iv) would give the four-coordinate hydrido species C which would then rapidly insert more CHSiMe3 (step v) to give the observed a-products 2a–f.Further insight into the reactions discussed above was gained from observations made with the deuterium-labelled complex d3-1a. Monitoring the reaction of d3-1a with 2 equivalents of N2CHSiMe3 in CHCl3 by 2H NMR spectroscopy revealed that vinyltrimethylsilane was the only detected organic product and that the deuterium was distributed over all three alkenyl sites equally (3 equally intense singlets at d 6.18, 5.93 and 5.67 in agreement with the 1H shifts measured for a genuine sample of CH2]] CHSiMe3); a deuterium resonance at d 0.51 was also detected and assigned to the a-methylene of the trimethylsilylmethyl ligand [see eqn.(3)]. Scheme 3 shows how this SiMe3 Pt P C(H/D)2SiMe3 P Cl Pt CD3 Cl P P H/D H/D H/D + d3-2a (3) N2CHSiMe3 Scheme 3 Pt P D P Cl Pt P CHSiMe3 P Cl Pt Cl P P D CH D2C SiMe3 Pt P CD2CHDSiMe3 P Cl Pt CD3 Cl P P CD3 Pt Cl P P H CD D2C SiMe3 Pt P CDSiMe3 P Cl CD2H Pt Cl P P D CD DHC SiMe3 C C SiMe3 H D D C C SiMe3 D D D C C SiMe3 D H D C C SiMe3 D D H Pt P CHDSiMe3 P Cl d3 -2a + N2CHSiMe3864 J.Chem. Soc., Dalton Trans., 1999, 861–865 scrambling of the deuterium can be rationalised by invoking a series of b-hydrogen migrations. These results are consistent with the b-hydrogen migrations from B (steps ii and iii in Scheme 2) being reversible and occurring rapidly relative to the alkene elimination from B (step iv in Scheme 2).The deuteriation studies show that the complexes [PtX- (CHMeSiMe3)(diphos)] and [PtX(CH2CH2SiMe3)(diphos)] can interconvert rapidly via b-hydrogen migrations. It is of interest to consider how the substituents R may influence the thermodynamics and kinetics of the systems shown in eqn. (4). We have never observed species containing the branched alkyl Pt–CHMeSiMe3 nor species containing the linear alkyl Pt–CH2CH2CO2Et and therefore we are not able to determine whether the instability is kinetic or thermodynamic.Young et al. 6 have recently shown that the Pt–C bond in [PtCl(CH2SiMe3)(PMe3)2] is anomalously weak and moreover have reported that 1J(PtP) for the trans phosphorus is inversely correlated with the Pt–alkyl bond strength. Our 1J(PtP) data (Table 1) are consistent with the Pt–CH2SiMe3 bonds being weaker than the Pt–CH2CH2SiMe3 bonds.Therefore one of the thermodynamic driving forces for the transformation of Pt–CHMeSiMe3 into Pt–CH2CH2SiMe3 [eqn. (4)] is the greater Pt–C bond strength. Furthermore it has been shown7 that for steric and electronic reasons, primary alkyl metal complexes MCH2R generally contain stronger M-C bonds than secondary alkyl metal complexes MCHR2 and thus when R is the bulky SiMe3 group it is not surprising that the linear Pt–CH2- CH2SiMe3 species is the only one observed. The stability of the branched species PtCHMeCO2Et may be due in part to the stabilisation of the d2 charge on the a-carbon by the electronegative CO2Et.The greater kinetic lability of the Pt–CHMeSiMe3 species than the Pt–CHMeCO2Me species can be rationalised by consideration of the associative mechanism proposed in Scheme 4 which is consistent with previously reported 5,8,9 mechanisms for b-hydrogen migration in alkylplatinum(II) complexes; in coordinating solvents such as methanol, dissociation of halide or phosphine to give three-coordinate platinum(II) species as intermediates in b-hydrogen elimination have also been proposed 9 but since our reactions were carried out in CDCl3, we have not considered this possibility further.Species D and E are transition states en route to the five-coordinate intermediate B. From previous work,5 we would predict that intermediate B would be more stable when R = CO2Et since this yields the Pt P R CH P Cl CH3 Pt P CH2 P Cl CH2 R (4) P P = R, R-diop R = SiMe3 or CO2Et Scheme 4 Pt P R CH P Cl CH3 Pt P CH2 P Cl CH2 R Pt P R CH P Cl CH2 H Pt P R CH P Cl CH2 H Pt P CH2 P Cl CH H R E d+ d- D B better p-acceptor alkene ligand.However we suggest that the lower activation energy observed when R = SiMe3 than when R = CO2Et is a consequence of two factors: (i) the lower stability of the PtCHRMe species when R = SiMe3 (see above) and (ii) the stabilization of the developing d1 on the carbon b to the SiMe3 group in D by hyperconjugation.10 From the mechanism presented in Scheme 4, it might also be expected that ancillary ligands which can stabilise the five-coordinate species B would promote the conversion of 3 æÆ 2.It is known11 that the stability of trigonal bipyramidal platinum(II) complexes PtX2L3 increases in the order Cl < Br < I and thus if B is trigonal bipyramidal, this would explain why our bromo and iodo complexes favour the formation of 2 more than the chloro analogues.Moreover the strain caused by the 1208 angle in the diphosphine chelates in B would decrease in the order chiraphos (five-membered) > skewphos (six-membered) > diop (seven-membered) consistent with the preference of the diop complexes to form 2. The conversion of N2CHSiMe3 to Me3SiCH]] CHSiMe3 is catalysed by complexes of type 2. Hence addition of N2CHSiMe3 to a solution of 2a (0.1 equivalent) in CDCl3 led to evolution of N2 and formation of trans alkene (as shown by 1H NMR).The catalysis turnover was slow: ca. 2 h21 with 2a and ca. 1 h21 with complexes 2d and 2g [generated according to eqn. (2)]. The rate of the catalysis with 2a was similar in CD3CN and furthermore, when the cationic species [Pt(NCMe)- (CH2SiMe3)(diop)]O3SCF3 was generated in situ and used as the catalyst, only trace amounts of Me3SiCH]] CHSiMe3 were detected among the many products that formed very rapidly. Hence we favour the mechanism shown in Scheme 5 involving uncharged intermediates.In conclusion we have shown that the rate of b-hydrogen migration in [PtX(CHRCH3)(diphos)] is a function of the stereoelectronic eVects of R. When R = SiMe3, b-hydrogen migration is facilitated while for R = CO2Et, b-hydrogen migration is eVectively arrested at ambient temperatures. Experimental All reactions were carried out in air at ca. 20 8C. The phosphines and Me3SiCHN2 solution were used as purchased from Aldrich. The complexes [PtX(CH3)L2] (L2 = R,R-diop, S,S-skewphos, R,R-chiraphos) 2 and [PtCl(CH2SiMe3)(cod)] 4 were made as previously described. 31P-{1H} (81 MHz), 13C- {1H} (50 MHz), and 1H (200 MHz) spectra were measured using a Bruker AM200 spectrometer at 22 8C. Preparation of [PtCl(CH2SiMe3)(R,R-diop)] 2a from [PtCl(CH3)(R,R-diop)] The complex [PtCl(CH3)(R,R-diop)] (230 mg, 0.30 mmol) was dissolved in CH2Cl2 or CHCl3 (3 cm3) and vigorously stirred while Me3SiCHN2 (0.45 cm3 of a 2 M solution in n-hexane, Scheme 5 Pt P CH2SiMe3 P Cl Cl Pt CHSiMe3 P P CH2SiMe3 Cl Pt H P P CHSiMe3 CHSiMe3 Pt P Cl P CHSiMe3 CH2SiMe3 N2CHSiMe3 2 N2CHSiMe3J.Chem. Soc., Dalton Trans., 1999, 861–865 865 0.90 mmol) was added. The mixture was stirred for 2 h and then the solvent removed under reduced pressure to give a white solid which was triturated with diethyl ether (2 cm3) and then filtered oV to give 208 mg (80%) of product. Elemental analysis (calculated for C35H43ClO2P2PtSi): C, 51.00 (51.55); H, 5.45 (5.25%); 1H NMR (200 MHz, CDCl3): d 20.20 [s, 9H, Si(CH3)3]; 0.4–0.7 (complex multiplets, 2H, diastereotopic PtCH2Si); 1.12 (s, 3H, CH3), 1.14 (s, 3H, CH3), other diop resonances are complex multiplets centred at the following shifts: d 2.28 (1H), 2.74 (1H), 2.91 (1H), 3.23 (1H), 3.81 (2H) 3.90 (1H) and 7.6–8.0 (20H); 13C NMR (50 MHz, CDCl3, assignments from DEPT): d 4.5 [Si(CH3)3]; 17.3 [d, J(PC) 88.8, J(PtC) 632, PtCH2Si]; 27.3 (s, CH3 of diop); 30.7 [d, J(PC) 23.5, CH2P]; 33.4 [d, J(PC) 37.3 Hz, CH2P]; 128–135 (complex multiplets); IR (CsI disc) n(PtCl) at 280 cm21.Crystals for the X-ray crystallography were obtained by slow diVusion of Et2O into a CH2Cl2 solution of 2a. Preparation of [PtCl(CH2SiMe3)(R,R-diop)] 2a from [PtCl(CH2SiMe3)(cod)] To a stirred solution of the complex [PtCl(CH2SiMe3)(cod)] (40 mg, 0.094 mmol) in CH2Cl2 (1 cm3) was added a solution of diop (47 mg, 0.094 mmol) in CH2Cl2 (2 cm3) dropwise over 5 min via a syringe.After 15 min, the solvent was removed under reduced pressure and then the residue triturated with Et2O (3 cm3) to give the product 2a (60 mg, 78%) as a fluVy white solid. The 31P and 1H NMR spectra were identical to the product 2a obtained above. The complexes [PtCl(CH2- SiMe3)(skewphos)] 2d and [PtCl(CH2SiMe3)(chiraphos)] 2g were made similarly from the appropriate diphosphine in ca. 80% yields. Preparation of [PtCl(CH2CH2SiMe3)(S,S-skewphos)] 3d The complex [PtCl(CH3)(R,R-skewphos)] (60 mg, 0.088 mmol) was dissolved in CH2Cl2 (1 cm3) which had been previously stirred over NaHCO3 for 10 h to remove traces of acid.Me3- SiCHN2 (0.175 cm3 of a 2 M solution in n-hexane, 0.35 mmol) was added. The mixture was stirred for 1 h and then the solvent removed under reduced pressure to give a white solid which was triturated with n-pentane (2 cm3) and then filtered oV to give 52 mg (80%) of product. Elemental analysis (calculated for C34H43ClP2PtSi): C, 52.85 (52.90); H, 5.60 (5.40%); 1H NMR (200 MHz, CDCl3): d 20.40 [s, 9H, Si(CH3)3]; 0.15 (1H), 0.55 (1H), 1.0 (2H) (complex multiplets, assigned to two of the four PtCH2CH2Si protons based on the observation of cross-peaks in the COSY spectrum) other skewphos resonances are centred at the following shifts: d 0.98 and 1.05 (2 overlapping dd, 7.1, 13.0 Hz, 6H, CH3), 2.88 (1H), 2.72 (1H), 1.86 (2H), and 7.2–8.0 (20H); 13C NMR (50 MHz, CDCl3, assignments from DEPT): d 21.3 [Si(CH3)3]; 24.0 [d, J(PC) 98.0 Hz, J(PtC) not resolved, PtCH2CH2Si]; 23.3 (s, PtCH2CH2Si); other skewphos signals at 36.8 (s), 17.8 (s), 19.4 (s), 26–28 (complex multiplets), 126–137 (complex multiplets).NMR characterisation of the other complexes The other complexes were characterised in solution by 31P NMR spectroscopy only and assigned their structures as aproducts (2) or b-products (3) by comparison of their 1J(PtP) values with those for 2a, 2d, 2g and 3d (see Table 1). In a typical experiment 0.025 mmol of complex was dissolved in CDCl3 (0.4 cm3) and then 12.6 cm3 of Me3SiCHN2 in hexane (2 M, 0.025 mmol) added.When the nitrogen evolution had subsided the 31P NMR spectrum was recorded. More Me3SiCHN2 (typically 0.1 mmol) was then added and the spectrum recorded at appropriate intervals over 24 h. Catalysis of alkene formation from N2CHSiMe3 A solution of 2a in CDCl3 (100 mL, 0.20 M, 0.02 mmol) was added to a solution of Me3SiCHN2 in hexane (100 mL, 2 M, 0.20 mmol) diluted with CDCl3 (0.4 cm3).Nitrogen evolution was observed and the formation of trans-Me3SiCH]] CHSiMe3 monitored by 1H NMR spectroscopy (dH 6.60) after 10, 50, 90, 120 min and 24 h. The 31P NMR spectrum showed that 2a was the only detected P-containing species present after 24 h. Similar experiments with 2d and 2g in CDCl3 and 2a in CD3CN were also carried out. Structure determination of [PtCl(CH2SiMe3)(R,R-diop)] 2a Crystal data for [PtCl(CH2SiMe3)R,R-diop)] 2a, C35H43- ClO2P2PtSi, M = 852.76, monoclinic, space group P21 (no. 4), a = 15.026(4), b = 14.802(6), c = 16.712(7) Å, b = 104.43(3)8, V = 4026(4) Å3, Z = 4, Dx = 1.658 Mg m23, T = 293 K, m(Mo- Ka) = 4.123 mm21, 6262 reflections collected, 6178 unique (Rint 0.048) all used in refinement, q £ 22.58. R1 = 0.064, wR2 0.113. Phenyl rings were constrained to idealised geometry and all silicon and carbon atoms assigned isotropic displacement parameters. The absolute structure was confirmed by refinement of the Flack parameter [x = 0.05(3)].12 CCDC reference number 186/1302.Acknowledgements We thank EPSRC for support, NATO for a travel grant, Johnson-Matthey for a loan of platinum compounds, C. N. R. for the use of Bruker 200 MHz NMR instrument, P. G. P. thanks Ciba-Geigy for a Senior Research Fellowship, and C. G. thanks the DAAD for a post-doctoral fellowship. References 1 J. P. Collman, L. S. Hegedus, J. R. Norton and R. G. Finke, Principles of Organotransition Metal Chemistry, University Science Books, Mill Valley, CA, 1987. 2 P. Bergamini, E. Costa, A. G. Orpen, P. G. Pringle and M. B. Smith, Organometallics, 1995, 14, 3178. 3 T. Kégl, L. Kollár and L. Radics, Inorg. Chim. Acta, 1997, 267, 249. 4 B. Wozniak, J. D. Ruddick and G. Wilkinson, J. Chem. Soc. A , 1971, 3116. 5 G. Alibrandi, L. M. Scolaro, D. Minniti and R. Romeo, Inorg. Chem., 1990, 29, 3467 and refs. therein. 6 R. Kapadia, J. B. Pedley and G. B. Young, Inorg. Chim. Acta, 1997, 265, 235. 7 H. E. Bryndza, L. K. Fong, R. A. Paciello, W. Tam and J. E. Bercaw, J. Am. Chem. Soc., 1987, 109, 1444 and refs. therein. 8 (a) G M. Whitesides, J. F. Gaash and E. R. Stedronsky, J. Am. Chem. Soc., 1972, 94, 5258; (b) P. J. Davidson, M. F. Lappert and R. Pearce, Chem. Rev., 1996, 219; (c) H. E. Bryndza, J. C. Calabrese, M. Marsi, D. C. Roe, W. Tam and J. E. Bercaw, J. Am. Chem. Soc., 1986, 108, 4805; (d) G. Alibrandi, D. Minniti, R. Romeo and P. Vitarelli, Inorg. Chim. Acta, 1984, 81, L23; (e) G. Alibrandi, D. Minniti, R. Romeo, G. Cum and R. Gallo, J. Organomet. Chem., 1985, 291, 133. 9 (a) T. J. McCarthy, R. G. Nuzzo and G. M. Whitesides, J. Am. Chem. Soc., 1981, 103, 1676; (b) G. Alibrandi, M. Cusumano, D. Minniti, L. M. Scolaro and R. Romeo, Inorg. Chem., 1989, 28, 342; (c) R. Romeo, G. Alibrandi and L. M. Scolaro, Inorg. Chem., 1993, 32, 4688. 10 W. Colvin, Chem. Soc. Rev., 1978, 7, 15. 11 (a) R. Favez, R. Roulet, A. A. Pinkerton and D. Schwarzenbach, Inorg. Chem., 1980, 19, 1356; (b) F. P. Fanizzi, F. P. Intini, L. Maresca, G. Natile, M. Lanfranchi and A. Tiripicchio, J. Chem. Soc., Dalton Trans., 1991, 1007 and refs. therein. 12 G. Bernardinelli and H. D. Flack, Acta Crystallogr., Sect. A, 1985, 41, 500. Paper 8/07719C

ISSN:1477-9226    

DOI:10.1039/a807719c

出版商:RSC    

年代:1999

数据来源: RSC

摘要:

DALTON PERSPECTIVE J. Chem. Soc., Dalton Trans., 1998, Pages 865–872 865 Crystallography of molecular excited states. Transition-metal nitrosyl complexes and the study of transient species Philip Coppens, Dmitry V. Fomitchev, Michael D. Carducci and Kirby Culp Department of Chemistry, Natural Sciences and Mathematics Complex, State University of New York at BuValo, BuValo, New York 14260-3000, USA The study of photoinduced processes in crystals is a frontier area of crystallographic research, which requires development of novel experimental and computational methods.In a series of studies, long-lived metastable states, generated upon photoirradiation of transition-metal nitrosyl complexes, have been identified as Á2 nitrosyl and isonitrosyl linkage isomers, and their detailed geometry has been determined. Calculations using density functional theory indicate that the species correspond to minima on the ground-state potential energy surface. The time-structure of synchrotron sources opens the possibility of time-resolved studies of transient species in crystals at the atomic level.Possible strategies for such experiments are described. 1 Introduction Photochemical reactions in crystals have been an active area of research since the pioneering studies of Schmidt and coworkers, 1 who showed that the nature of the products of a solidstate reaction is topochemically-controlled by the geometry of the crystal structure. The work opened a new field of solid-state chemistry, typified by its close co-ordination of crystallographic and chemical methods.Since that time, dramatic new technical developments in optical and X-ray technology have occurred. We now have at our disposal much more advanced instrumentation, including highly intense pulsed synchrotron sources, pulsed lasers, fiber optic light guides, cryostats, helium gas-flow systems and area detectors. At this point in time only our imagination limits the application of the new technologies.In combination, they oVer the possibility of stroboscopic experiments on transient species with lifetimes of ms and less.2 As dynamic processes are central to chemistry, such studies will constitute a highly relevant extension of structure determination techniques, which until now have been essentially restricted to the elucidation of ground-state structures. How- Philip Coppens received his Doctorate in Chemistry in 1960 from the University of Amsterdam with Professor Carolyn MacGillavry, based on work done at the Weizmann Institute of Science under the guidance of Professor Gerhard Schmidt. He has worked at Brookhaven National Laboratory, and at present carries the rank of Distinguished Professor of Chemistry at the State University of New York at BuValo.His research interests include X-ray charge density analysis, chemical applications of synchrotron radiation crystallography and the study of photoinduced states in molecular crystals.He is a past President of the International Union of Crystallography. Dmitry Fomitchev was born in 1968 in Moscow, Russia. He received his MS in Inorganic Chemistry in 1992 from the Moscow State University under the supervision of Professors Leonid M. Kovba and Eugene V. Antipov. Since 1994, he has been a graduate student at the State University of New York at BuValo in the laboratory of Professor Philip Coppens where he is involved in the study of the light-induced metastable states of transition-metal nitrosyls.Inorganic chemistry is the major field of his research interests. Michael Carducci, born in 1965 in California, received his Doctorate in Inorganic Chemistry during 1994 from the University of Arizona under the supervision of John Enemark and his Bachelors of Science from the University of California at Irvine working for Robert Doedens. Since 1995, he has been a postdoctoral associate at the State University of New York at BuValo in the laboratory of Philip Coppens where he has studied photoinduced metastable species and time-resolved crystallography.His research interests extend throughout the fields of molecular and electronic structure determination. Kirby Culp is finishing his undergraduate studies at the State University of New York at BuValo. He has previously studied at SUNY Plattsburgh and the University of Maryland extension in Munich, Germany. Philip Coppens Dmitry Fomitchev Michael Carducci Kirby Culp866 J.Chem. Soc., Dalton Trans., 1998, Pages 865–872 ever, at present the use of X-ray diVraction in the analysis of metastable and transient species is still in its infancy. Because the percentage of the molecules in a crystal that is excited depends on the beam intensity, the quantum eYciency, and the rate of the deactivation process, in general not all molecules in a crystal will be converted to the excited state. It is therefore necessary that approaches for analysis and interpretation of diVraction patterns of partially excited crystals be developed.Some such methods are illustrated below, and used in the diVraction analysis of novel configurations of transitionmetal nitrosyl compounds. While the metastable states of the transition-metal nitrosyl complexes have been described in the literature as electronically excited states 3,4 with unusually long lifetimes at reduced temperatures, the crystallographic experiments lead to a diVerent conclusion.A strategy for stroboscopic experiments on transient species, using the time-structure of synchrotron sources,5 is discussed in the second part of this Perspective. 2 The Photochemistry of Nitrosyl Complexes The photosensitivity of transition-metal nitrosyl compounds of iron was discovered in 1977 by Hauser et al.,6 who studied optical dispersion changes in iron-containing photo-irradiated solids, using Mössbauer spectroscopy at low temperature.The prototype of the iron complexes is sodium nitroprusside (SNP), Na2[Fe(CN)5NO], which in the solid state occurs as a dihydrate. The eVect is not limited to solids, but also occurs in glassy matrices.7 It was subsequently found that similar long-lived states can be generated by irradiation of nitrosyl complexes of Ru or Os,8,9 with diVerent ligands, diVerent cations, and varying numbers of solvent molecules. In the same year as the discovery by Hauser et al., Crichton and Rest 10 reported changes in IR frequencies that occur on irradiation of cyclopentadienylnitrosylnickel [Ni(NO)(h5-Cp)], in Ar, N2, CO, and methane matrices at 20 K.According to the classification of Enemark and Feltham,11 in which the d electrons are counted together with those electrons on the ligand which occupy s* or p* levels, the ground state of the photosensitive nitrosyl complexes of Fe, Ru and Os is described as {M(NO)}6, while [Ni(NO)(h5-Cp)] and other complexes, such as [Mn(CO)4(NO)] and [Mn(CO)(NO)3], subsequently described by Crichton and Rest, have the {M(NO)}8 or {M(NO)}10 configurations.DiVerential scanning calorimetry (Fig. 1) and lowtemperature spectroscopic studies of SNP show that in addition to the first photoinduced state, MS1, a second metastable state, labeled MS2, exists. Infrared and Raman measurements show C]N, N]O, Fe]N stretching modes and the Fe]N]O bending mode to be downshifted on formation of the metastable states, the shifts for the modes associated with the M]N]O group Fig. 1 DiVerential scanning calorimetry curve for a laser-irradiated crystal of sodium nitroprusside being an order of magnitude larger than the small (ª10 cm21) downshifts for the other vibrational frequencies.12 The metastable states decay to the ground state at temperatures around 195 K (MS1) and 151 K (MS2) (Fig. 1). Thus, the MS2 state can be eliminated by warming above 151 K, while an MS1-free excited crystal can be obtained by converting the MS1 state to MS2 by irradiation with 1064 nm photons. The spectroscopic and thermal analyses indicate that the metastable states are not populated directly, but that population occurs through a transient higher energy state, and that at least some of the MS1 species are formed through the MS2 state as intermediate.13,14 Various interconversion processes involving the ground and metastable, and corresponding excited states are illustrated in Fig. 2. It has usually been assumed that the initial transition is an electronic excitation from the HOMO, 2b2 (dxy) orbital to the LUMO, 7e (p* NO) orbital (using the approximate C4v point group symmetry),16 leading to an excited complex which subsequently relaxes into two diVerent metastable states,17 though d–d transfer to a metal dz2 orbital has also been proposed.18 In any case, the states are not triplet states, as Mössbauer 19 and ESR18 evidence indicate that both MS1 and MS2 are diamagnetic.It is also noteworthy that decay of the metastable states is radiationless.20 In 1990 Güdel 21 pointed out that the longevity of the metastable states is inconsistent with any oneelectron transfer model, and that either a large structural change, or a multi-electron promotion is required to explain the stability of the species. The {M(NO)}10 complexes appear to be stable only at much lower temperatures, but the postulated mechanism similarly involves photoelectron transfer to the NO1 ligand.Far fewer studies have been reported on these complexes. 3 Photocrystallographic Experiments In order to perform excited-state diVraction experiments, instrumentation and methods for such studies must be developed. The crystal must usually be kept at reduced temperatures, often below that of liquid N2, and be simultaneously accessible to X-rays and the laser beam. In our equipment this is accomplished with a DISPLEX cryorefrigerator, mounted in the c-circle of a Huber 512 diVractometer (Fig. 3). The optics Fig. 2 Proposed relationship and interconversion pathways between the diVerent states of SNP. Straight vertical arrows are electronic transitions. Slanted arrows combine an electronic transition with nuclear motion. Curved arrows are radiationless thermal decay (ref. 15)J. Chem. Soc., Dalton Trans., 1998, Pages 865–872 867 are attached to the edge of the c-circle with a specially designed optical plate. When the oscillation method is used, a simple mirror system is adequate because the crystal rotation during data collection is limited to one axis.However, for conventional data collection with a point detector both the c and w diVractometer angles must be adjusted for each reflection, and flexible fiber optics and a lens system are needed.22 The vacuum chamber (XTRANS, Anholt Technologies Inc.), which surrounds the crystal, is made of an X-ray transparent carbonaceous material,23 with UV-transparent quartz optical windows mounted in its roof for access of the laser beam.For long-lived species, such as the nitrosyl complexes described above, the irradiation period can preceed the entire X-ray exposure. For short-lived transient species, the X-ray and laser pulses must be synchronized to allow the probing of the sample during or just after excitation, when the population of the photoinduced species is at a maximum.24 In both cases the experiment requires a comparison of the photoexcited and the ground-state crystal, both in reciprocal and real space.The reciprocal space comparison of reflection intensities is particularly important when conversion percentages are small, because a diVerence between two intensities measured in rapid succession can be measured to a better accuracy than the intensity itself. In the diVerential experiment, systematic eVects with a longer time-scale, such as beam intensity or position variation, will not aVect the results.For a crystal in which only part of the molecules are excited, the structure factor expression, assuming random distribution of the excited molecules, and the presence of only two species, is given in equation (1) where the subscripts gs and es represent F = (1 2 P)F9gs 1 PFes 1 Frest (1) the ground and metastable states respectively, P is the conversion percentage, and the subscript ‘rest’ represents inert moieties such as water of crystallization or counter ions not involved in the excitation.The term F9gs may not be identical to Fgs, the structure factor of the ground-state crystal, as the ground-state molecules may move or rotate slightly due to the changed molecular environment. However, in studies we have done so far such eVects appear minor, even when conversion percentages are close to 50%. We note that unit cell changes upon irradiation will also lead to slight diVerences between F9gs and Fgs, and must therefore be taken into account. Simplifi- Fig. 3 Schematic drawing of the experimental arrangement used in the photocrystallography experiments cations may be introduced in the model, such as the assumption that the thermal parameters of the excited state are equal to those of the ground state. Especially when only a small fraction of the molecules are excited, such constraints may be essential to achieve convergence in the refinement. Expression (1) allows the parametrization of the excited state molecule in terms of structural and charge density parameters, and therefore makes it possible to plot the charge density of the excited-state molecule.25 Such detailed charge density experiments are only in the planning stage, but potentially allow identification of the electronic structure of an excited complex.When conversion percentages are small, a better procedure is to analyze the response ratio, defined first in studies of the eVect of an external electric field on the X-ray diVraction intensities.26 The response ratio is given by expression (2) where Ion is based hhkl = Ion(hkl) 2 Ioff(hkl) Ioff(hkl) = F2 on(hkl) 2 F2 off(hkl) F2 off(hkl) = F2 on(hkl) F2 off(hkl) 2 1 (2) on the structure factor as defined in (1), and Ioff is related to the conventional ground-state structure factor expression.The response ratio thus gives the relative change of a reflection intensity as a result of the external perturbation. The parameters of the excited complex can be refined directly from the observed response ratios.The derivative expressions that are needed in the least-squares analysis of h are given in ref. 24. 4 Studies on Nitrosyl Complexes 4.1 Crystallographic studies and theoretical calculations A first test of changes induced by photoirradiation is a diVerence map, in which the ground-state density is subtracted from the experimental density of the excited crystal. For centrosymmetric crystals, the calculated signs of the structure factors can be used with impunity, for acentric systems a phase error will be introduced, which may have to be considered.Sections of the Fourier-diVerence maps for centrosymmetric sodium nitroprusside, after the initial refinement of the rigid-body groundstate anion, for both MS1 and MS2 are shown in Fig. 4.15 As the section in this figure contains two of the equatorial ligands, it is inclined by 458 to the crystallographic mirror plane that bisects the anion in the space group Pnnm.In both cases the iron atom is located on a pronounced slope in the dr diVerence maps, flanked by a peak and a deep trough. This indicates that the atom has moved on irradiation in the direction of the peak position. It is interesting that in MS1 the shift is away from the NO ligand, while in MS2 the Fe is displaced towards this ligand, as indicated by the opposite slope of dr at the Fe position in the two maps. On the other hand, the equatorial ligands are displaced towards and away from the nitrosyl group respectively, as indicated by peaks adjacent to the equatorial CN atoms.Some of the density remaining in the bonds is overlap density, not accounted for in the spherical atom formalism of this part of the analysis. The remaining features of Fig. 4 are indicative of the nitrosyl groups. For MS2 they appear near the ground-state N4 nitrogen position on both sides of the N]O bond. Corresponding minima at the ground-state N4 and O1 positions indicate a reorientation of the nitrosyl group atoms upon excitation.In agreement with the diVerence map features, subsequent least-squares refinement of the MS2 data leads to the sidewaysbound h2 geometry of NO shown in Fig. 5. It indicates a hitherto unknown species, with the NO group in an eclipsed, rather than the sterically less demanding staggered conformation. Since the NO group is located in a mirror plane in the ground-state crystal, two mirror-related eclipsed positions are868 J.Chem. Soc., Dalton Trans., 1998, Pages 865–872 occupied after excitation. Clearly MS2 is a linkage isomer rather than an electronically excited state. Can MS1 similarly be a linkage isomer? Geometric changes resulting from refinement of the MS1 data are the lengthening of the Fe](NO) distance by 0.053(6) Å, accompanied by an increase in the C(2)]Fe]C(3) angle by 1.1(3)8 to 170.0(3)8, while the Fe]N]O angle and N]O bond length are not significantly changed from the ground-state values. A more pronounced change, however, occurs for the temperature parameters of the atoms of the NO ligand.Invariably, in this and other analogous studies we have performed (Table 1), the proximal atom has abnormally small values of the mean-square displacement parameters, while the distal atom shows excessively large values. It is well known, and has recently again been emphasized,30 that assignment of an incorrect element-type to an atomic position leads to anomalous temperature parameters.In the present case, the anomaly is related to the observed decrease of density on the proximal, and increase of electron density on the distal Fig. 4 DiVerence in electron density between the photoirradiated sodium nitroprusside crystal and ground-state molecules. Contours at 0.1 e Å23, zero contour omitted, negative contours broken. Crosses indicate the ground-state species atomic positions; (a) for the first metastable state MS1, (b) for the second state MS2.The insert shows a section perpendicular to the ground-state NO bond, chosen such as to contain the residual peaks (ref. 15) atom, as described above. Since the interchange of the O and N atoms of the nitrosyl group removes the anomaly for SNP and for all other cases we have studied, the indication is that MS1 is an isonitrosyl complex, in analogy with well known isocyanides. The disappearance of the anomaly is illustrated for the [Ru(OH)(NO)(NO2)4]22 anion in Fig. 6.28 When the isonitrosyl configuration is introduced, the metastable state temperature parameters become normal, with a terminal/proximal ratio comparable to that of the ground state atoms, as shown in Table 1 for SNP, K2[Ru(OH)(NO)(NO2)4], and [Ru(Cl)(NO)(py)4]- [PF6]2.29 In all four cases (there are two independent molecules in the cell of the last complex), the ratio of the thermal parameters with the nitrosyl assignment of MS1 is four or more, while it is quite normal for the isonitrosyl configuration. Evidence for the ‘inverted’ isonitrosyl configuration also comes from a neutron diVraction study.As nitrogen is a stronger neutron scatterer than oxygen, in this case the incorrect assumption leads to an anomalously small thermal parameter for the terminal atom. As expected, examination of peak heights in Fig. 5 An ORTEP27 diagram of the MS2 anion of sodium nitroprusside. Both mirror-related NO conformations are displayed, 50% probability ellipsoids are shown (ref. 15) Table 1 Isotropic ‘thermal’ mean-square displacements (Å2), 50 K, for ground-state and MS1 metastable state complexes, the latter both in the nitrosyl and isonitrosyl formulation Na2[Fe(CN)5(NO)] (ref. 15) Proximal Distal Ground state 0.0056(1) 0.0116(1) MS1 Fe(NO) 0.0040(4) 0.0163(5) MS1 Fe(ON) 0.088(5) 0.0102(4) K2[Ru(OH)(NO)(NO2)4] (ref. 28) Proximal Distal Ground state 0.0072(5) 0.0141(5) MS1 Ru(NO) 0.0037(13) 0.022(2) MS1 Ru(ON) 0.008(1) 0.015(2) [Ru(Cl)(NO)(py)4][PF6]2 (ref. 29) Proximal Distal Ground state 0.0081(4) 0.0128(5) 0.0135(4) 0.0203(5) MS1 Ru(NO) 0.0042(5) 0.0061(5) 0.0232(7) 0.0293(8) MS1 Ru(ON) 0.0108(6) 0.0175(7) 0.0117(6) 0.0151(6)J. Chem. Soc., Dalton Trans., 1998, Pages 865–872 869 Fourier maps based on the X-ray data invariably gives corroborating information. At this stage we concluded that both purportedly electronically excited states of the {M(NO)}6 (Fe or Ru) nitrosyl complexes are linkage isomers, with MS2 an intermediate on the pathway from the ground state to MS1.We therefore decided to examine the {M(NO)}10 complexes studied by Crichton and Rest. As [Ni(NO)(h5-Cp)] is a liquid at room temperature, we synthesized and determined the structure of (pentamethylcyclopentadienyl) nitrosylnickel, [Ni(NO)(h5-Cp*)].31 In agreement with what was reported in EXAFS studies of [Ni(NO)(h5-Cp)],32 we find that photoexcitation to a long lifetime state requires very low temperatures, below about 60 K.Irradiation of a diVractometer-mounted single crystal of [Ni(NO)(h5-Cp*)] with laser light (458 nm) at 25 K, produces the change in intensities and cell dimensions typical for photoexcitation in the solid state. There are two molecules in the asymmetric unit of the ground-state monoclinic [Ni(NO)(h5- Cp*)] crystals, labelled here A and B, both with the NO axis oriented along the b axis of the unit cell. The observed shortening upon irradiation of the axis along which NO is oriented (from 14.355 to 14.078 Å) 31 is typical for formation of an MS2- type species, and also observed for SNP, though in that case the NO axis is not perfectly aligned with the crystallographic axes.When an MS1 species is formed, on the other hand, the corresponding cell dimension will lengthen, reflecting the increased distance from the metal to the proximal atom of the NO ligand. The formation of an MS2 species is confirmed by the photodiVerence map, shown in Fig. 7, which very clearly indicates the depletion of density along the axial direction, and the build-up in oV-axis regions. Subsequent analysis shows that the NO group of the second molecule has two diVerent orientations, both approximately parallel to the Cp* ring. The conversion percentage achieved in this experiment, as obtained in the least-squares analysis, is much higher than that for SNP, 47% vs. 9.5%. The diVerence is likely due to the experimental conditions, specifically the necessity, in the case of SNP, to eliminate MS1 by subsequent irradiation with light of 1064 nm.The high conversion percentage allows a reasonably accurate determination of the geometry of the excited [Ni(NO)(h5-Cp*)] species (Fig. 8). Bond lengths of the nitrosyl ligand and those involving the Ni atom are listed in Table 2. The average metastable state Ni]O distance is 2.087 Å, the Ni]N distance is lengthened by about 0.09 Å, and there is an increase of about 0.02 Å in the distance from the Ni atom to the Cp* plane.As in SNP, no significant change occurs in the NO distance. The latter is surprising and unexplained, given the large decrease of 446 cm21 in the IR stretching frequency of NO. For comparison, the EXAFS studies led to the conclusion that the Ni]N bond was elongated by 0.12(3) Å, and the Ni]N]O angle Fig. 6 The ORTEP drawings of the Ru]NO (left) and Ru]ON (right) models for the MS1 excited state of the [Ru(OH)(NO)(NO2)4]22 anion, showing the diVerences in the nitrosyl thermal parameters.Thermal parameters for the other atoms are the same as those for the groundstate molecules, 50% probability ellipsoids are shown. The hydroxyl hydrogen atom has been omitted (ref. 28) bent from 1808 to 1608 or even 1338. While the lengthening of the Ni]N bond is confirmed by the crystallographic structure determination, the bending is more pronounced than inferred from the EXAFS study.Additional support for the existence of the linkage isomers comes from theoretical calculations, which confirm that the Fig. 7 DiVerence in electron density between the photoirradiated [Ni(NO)(h5-Cp*)] crystal and ground-state molecules. Contours at 0.4 e Å23. Negative contours dotted (ref. 31) Fig. 8 The geometry of the photogenerated [Ni(h2-NO)(h5-Cp*)] species (based on ref. 31). Some of the interatomic distances and angles are indicated. Top line: experimental values, lower line from theoretical optimization with the B3LYP functional and the LANL2DZ basis set870 J.Chem. Soc., Dalton Trans., 1998, Pages 865–872 observed structures correspond to local minima on the groundstate potential energy surface. For SNP this is shown by an extended-Hückel based Walsh diagram of the end-to-end interconversion of the nitrosyl and isonitrosyl configurations, and more accurately, by a DFT calculation of Delley et al.33 The DFT calculation correctly predicts the lower energy of the MS2 configuration compared with MS1, which is observed experimentally.For [Ni(NO)(h5-Cp*)] we have performed a complete DFT geometry optimization of three diVerent species: the ground state, the observed [Ni(h2-NO)(h5-Cp*)] metastable state and a hypothetical [Ni(ON)(h5-Cp*)] isonitrosyl configuration. The theoretical geometry of [Ni(h2-NO)(h5-Cp*)] agrees rather well with the observed values (Table 3, Fig. 8), except for the NO bond, which is calculated to be considerably lengthened compared with the ground state, by an amount equal to 0.064 Å.For SNP, the lengthening of the NO bond is predicted to be much smaller according to the DFT calculation: 0.031 Å for MS2 and a small shortening of 0.006 Å for MS1. For MS2 this diVerence correlates with the downshift of the NO stretching frequency, which is 284 cm21 for SNP, compared with 446 cm21 for [Ni(h2-NO)(h5-Cp)]. It is noteworthy that, notwithstanding the shortening of the NO bond in MS1 of SNP, the Table 2 Selected bond lengths (Å) for ground and metastable state structures of the [Ni(NO)(h5-Cp*)] at 25 K (ref. 31) Ground state Metastable state Bond Ni]N Ni]O Ni]plane a Ni]C(1) Ni]C(2) Ni]C(3) Ni]C(4) Ni]C(5) N]O Molecule A 1.620(3) 1.719(1) 2.107(3) 2.106(3) 2.109(3) 2.106(3) 2.105(3) 1.177(3) Molecule B 1.614(3) 1.718(1) 2.110(3) 2.110(3) 2.108(3) 2.104(4) 2.110(3) 1.181(4) Molecule A 1.697(18) 2.096(18) 1.732(6) 2.092(12) 2.132(12) 2.142(11) 2.142(12) 2.106(12) 1.134(18) Molecule B 1.724(10)/ 1.716(10) 2.077(10)/ 2.079(10) 1.750(6) 2.138(12) 2.132(11) 2.137(12) 2.120(12) 2.165(12) 1.126(30)/ 1.128(32) a The mean plane through the C(1), C(2), C(3), C(4), C(5) atoms of the Cp* ring.Table 3 Calculated (B3LYP functional–LANL2DZ basis set) and experimental values for some of the bond lengths (Å) and angles (8) for the metastable structure of [Ni(h2-NO)(h5-Cp*)]. The experimental values for molecule A are listed (ref. 31) Ni]C(1) Ni]C(2) Ni]C(5) Ni]C(3) Ni]C(4) Ni]N Ni]O N]O Ni]N]O Theoretical 2.117 2.200 2.236 1.740 2.121 1.278 87.9 Experimental 2.092(12) 2.132(12) 2.106(12) 2.142(11) 2.142(12) 1.697(18) 2.096(18) 1.134(18) 93(1) Table 4 Relative energies (eV a) of the ground MS1 and MS2 states from theoretical calculations SNP Theory Experimental (ref. 20) [Ni(NO)(h5-Cp*)] Theory (ref. 31) Ground state 000 MS2 1.368(1.465) b 1.0 0.993 MS1 1.677(1.639) b 1.1 1.847 a eV ª 1.602 × 10219 J.b Numbers in brackets refer to solid state calculations. calculation predicts a small decrease of the NO stretching frequency. The energy diVerences that are predicted are of the same order of magnitude as those deduced from the thermodynamic and spectroscopic measurements (Table 4), in both cases MS2 is found to be the more stable linkage isomer, but the destabilization of MS1 relative to MS2 is much more pronounced for [Ni(NO)(h5-Cp*)]. The fact that MS1 has not been observed for the Ni complex, either crystallographically or spectroscopically, may be related to a correspondingly smaller barrier to deactivation for this species. 4.2 Solid-state photosensitivity of other nitrosyl complexes Nitrosyl complexes such as SNP have been proposed as molecular storage devices in which information can be optically written, read and erased.34 The dependence of the thermal depopulation temperature on chemical substitution is therefore of particular interest, as species stable at room temperature would be most attractive for applications.Variation of the cation in the nitroprusside salt series leads to only small variations in the transition temperature, which varies from 186 K for the tetramethylammonium salt to 218 K for K2[Fe(CN)5- NO]?2.5H2O.35 However, we have found that chemical substitution in the complex cation of the Ru series leads to considerable variation in the deactivation temperature. The first metastable state (MS1) of [Ru(Cl)(NO)(py)4][PF6]2, for example, only decays at 256 K (217.2 8C), while even higher decay temperatures have been observed for trans-[Ru(H2O)(en)2NO]Cl3 and [Ru(NH3)5(NO)]Cl3, though conversion percentages appear small in these cases. A summary of our results and those reported by Morioka and co-workers 36 is given in Table 5.The limited amount of data perhaps indicates that cationic complexes in general have higher decay temperatures. There are unexplained variations, such as the diVerence in behaviour upon substitution of two bipyridyl ligands for the four pyridines of the [Ru(Cl)(NO)(py)4]21 cation (third and fourth entries in Table 5).It is possible that the crystalline environment plays a role in determining the stability of the metastable states. Photoinduced deactivation is also likely to diVer from complex to complex. It is clear that additional experimental and theoretical work is needed before the factors influencing the relative stability of the metastable nitrosyl complexes can be understood. Table 5 Transition temperatures and conversion percentages reached for several transition-metal nitrosyl complexes Tc a/8C Conversion (%) (may not be Compound Na2[Fe(CN)5(NO)] K2[Ru(OH)(NO)(NO2)4] [Ru(Cl)(NO)(py)4][PF6]2 [Ru(Cl)(NO)(bipy)2][PF6]2 [Ru(OH)(N3)(NO)(py)3][PF6] [Ru(NO)(N3)3(py)2] [Ru(NH3)5(NO)]Cl3 Ni(NO)(h5-Cp*)] trans-[Ru(Cl)(en)2(NO)]Cl2 cis-[Ru(Cl)(en)2(NO)]Cl2 trans-[Ru(Br)(en)2(NO)]Br2 cis-[Ru(Br)(en)2(NO)]Br2 trans-[Ru(H2O)(en)2(NO)]Cl3 [Ru(Cl)3(en)(NO)] (mixture of fac and mer isomers) MS1 278 265 217.2 no 238.4 no 0 no 227 267 244 262 26 268 MS2 2122 2100 2102 (weak) no no no no 2240 to 2200 no no no no no no maximalb) 48/ª30 ª16/1 30 (MS1) – 12 – ª2 47 ª5 Ref. 37 28 29 29 29 29 29 31 36 36 36 36 36 36 a no Indicates that no metastable state has been observed. b First value MS1, second value MS2. Only one value is given if only one metastable state is observed.J.Chem. Soc., Dalton Trans., 1998, Pages 865–872 871 5 Perspectives for the Study of Transient Species 5.1 Time-resolved experiments using pulsed synchrotron sources Many species of interest in chemistry have lifetimes of ms, ns or shorter. There are also a great many fast processes, such as electron transfer within and between molecules, which are of crucial importance in chemistry and biology. To study such processes, the use of non-steady state methods becomes imperative.Rather than establish a steady-state concentration by relatively long laser irradiation, an instantaneous nonequilibrium concentration is established and probed before significant decay occurs. Such experiments have now been performed to study processes like the photodissociation and recombination of myoglobin and carbon monoxide.38,39 In these experiments the polychromatic Laue technique is used with a single synchrotron bunch of 60 ps length. By varying the delay between the exciting pump pulse and the probe pulse, the reversible dynamic process can be followed on a very fine timescale, and a ‘motion picture’ of the reaction can be obtained.For experiments that aim at detailed atomic and even electronic resolution, monochromatic methods promise better accuracy. To perform the experiments in a reasonable time span, parallel, rather than sequential reflection measurement techniques must be used. Electronic detectors, such as CCD’s now have read-out times of a few seconds, and allow a rapid succession of light-on and light-oV measurements at each angular oscillation range of the crystal.For such experiments to be successful, the number of photons in the exciting laser pulse must be of the same order of magnitude as the number of molecules in the crystal. This is well within reach of current technology. A 5 kHz Nd-YAG laser with a 50 mJ pulse energy gives, for example, about 10 14 photons per pulse at 355 nm. For a typical complex, the number of 10 14 molecules corresponds to a 40 × 40 × 40 m crystal, which is what is used in many synchrotron diVraction experiments.Working with small sample crystals has the further advantage that illumination is more uniform, and heat dissipation to the surroundings is enhanced. A reasonably uniform illumination requires that only a fraction of the incident beam is absorbed, so that not all photons will lead to excitation. To counter this eVect, the number of active molecules can be reduced further by embedding the molecules as hosts in a spectroscopically inert matrix.Thus, the active molecule–photon ratio and the transparency requirement all indicate that very small samples must be used, which means that experiments are most likely to be successful at the brightest synchrotron sources. At the Advanced Photon Source at Argonne National Laboratory, where our experiments are to be carried out, the orbit flight time is 3.68 ms, or a frequency of about 0.27 MHz. This means that only a small fraction of the X-ray pulses can be used in the experiment, a not so serious limitation given the brightness of the sources available, but nevertheless a factor that will limit the possibilities for use of even smaller samples.An X-ray free electron laser with its much lower repeat rate may eventually be the optimal source for stroboscopic lasertriggered experiments! For the synchrotron source, the need to eliminate a large fraction of the X-ray pulses adds an additional level of complexity, as it dictates the use of a very fast shutter or pulse selector.Several designs for such devices are now being developed. A detector with very fast read-out capability, now being tested,40 may oVer an alternative to the fast shutter, and produce a concomitant simplification of the experimental setup. A schematic description of the experiment is given in Fig. 9. 5.2 What systems can be studied? Though the relevance of time-resolved studies is expected to be very extensive, and involve a variety of chemical processes, we will, for illustration, give a few examples of possible applications.(a) Transition-metal complexes. Many transition-metal complexes have excited triplet states with long lifetimes. Prime examples are complexes of rhodium studied more than twenty years ago by Crosby and co-workers.41 The light-induced triplet state of [RhBr2(C5D5N)4]Br, for instance, has a lifetime of 1.5 ms in the solid state.With such long lifetimes, even conventional sources oVer some possibilities, as we have recently demonstrated in a series of test experiments in preparation for the synchrotron studies.24 In these experiments a steady-state concentration of excited molecules was maintained during a period of about 20 ms. In the synchrotron experiments, on the other hand, no equilibrium is to be established, and a much shorter lifetime species can be studied.Examples of transition-metal complexes with photoinduced excited triplet states having ms lifetimes are the binuclear metal d8–d8 complexes of rhodium and platinum. They show unusual reactivity, and are, in solution, intermediates in metal-catalyzed photoreactions of small molecules and organic substrates.42 Since for these d8–d8 complexes the excitation corresponds to a ds* æÆ ps transition, very large contractions of the metal– metal distance occur if the bridging ligands are suYciently flexible.An example is Rh2(dimen)4 21 (dimen = 1,8-diisocyanomenthane) for which metal–metal bond shortening from 4.8 Å to 3.2 Å upon excitation to a 21 ms lifetime state has been deduced from spectroscopic information. On the other hand, single metal–metal bond Rh0 2 and Rh0RhII complexes with the bridging ligand bis(difluorophosphine)methylamine, (PF2)CH3- N(PF2), undergo excitation into a ds* orbital, and are expected to show bond lengthening upon excitation.43 As illustrated above for the transition-metal nitrosyl complexes, the diVraction experiment is capable of yielding the full three-dimensional structure of the photoinduced state. At present such information is completely lacking for shorter lived species. (b) Photoinduced electron transfer and charge-separated states.An intriguing field for time-resolved studies is photoinduced electron transfer. For such studies, the time resolution Fig. 9 Schematic of the time-resolved synchrotron experiment (numerical information based on the Advanced Photon Source parameters). The electron bunches travel around the storage ring with a flight time of 3.68 m, and produce flashes of X-rays incident on the sample crystal.Either a single bunch, or a ‘superbunch’ consisting of multiple bunches may be used872 J. Chem. Soc., Dalton Trans., 1998, Pages 865–872 may need to be increased to nanoseconds or better. Factors limiting time resolution in the techniques described above are jitter in the laser pulses and the width of the probing X-ray pulse, which is of the order of 100 ps.However, other experimental arrangements have been proposed.44 Possible candidates are triad molecules, containing, for instance, a carotenoid donor, porphyrin or metal–porphyrin bridge and quinone acceptor(s).45 We note that such molecules have been studied extensively because of their relevance for energy conversion and the storage of information,46,47 and that they have been proposed as components of molecular computing devices.48 Information on geometry changes along the reaction path of the photochemical charge separation and recombination would shed new light on the mechanism of electron transfer and of photoinduced processes in chemical and biologically important systems. 6 Concluding remarks We may expect that within the next decade time-resolved experiments will become widespread, thus opening a novel field of diVraction studies of broad relevance to chemistry.Such experiments may yield presently not accessible information at the atomic level and beyond, on processes that are at the center of current interest. 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ISSN:1477-9226    

DOI:10.1039/a708604k

出版商:RSC    

年代:1998

数据来源: RSC