摘要:
Laying traps for elusive prey: recent advances in the non-covalent binding of anions Jerry L. Atwood,*a K. Travis Holmanu and Jonathan W. Steed*b a Department of Chemistry, University of Missouri-Columbia, Columbia, MO 65211, USA Department of Chemistry, King’s College London, Strand, London, UK WC2R 2LS The structure and function of a new class of host molecules for the supramolecular complexation of anionic guest species are analysed within the context of other recent advances in the field. In particular, organometallic hosts based upon the calixarenes, and the related macrocycle cyclotriveratrylene (CTV), are examined. X-Ray crystallographic results clearly demonstrate the inclusion of anionic guest species such as BF4-, I-, CF3S03-, Re04- etc.within the ostensibly electron-rich bowl-shaped cavities of both types of host as a result of cooperative effects arising from the presence of two or more metal centres arranged around a common binding pocket. Solution radiochemical studies show that hosts based upon CTV in particular are selective for largetetrahedral anions such as M04- (M = Tc, Re). It is anticipated that the ability to discriminate between anions on a size and shape selective basis by means of manipulation of host cavity dimensions will pave the way towards new sensor devices and methods of environmental waste remediation. Introduction With the advent of macrocyclic ligands such as the crown ethers, cryptands and an enormous range of other multidentate hosts, supramolecular cation coordination chemistry has pro- gressed rapidly.1-4 In comparison, progress in the non-covalent complexation of anions has been more slow, probably as a consequence of the large ionic radii of anions, high free energy of solvation and the wide variety of topologies encountered, resulting in great difficulty in designing multidentate receptors with appropriately situated Lewis-acidic or other acceptor sites.In recent years, however, increasing attention has focused upon supramolecular anion complexation,5-23 perhaps not in little part because of the important environmental consequences of the presence of excess nutrients such as nitrate and phos- phate.24-27 Also relevant are anionic products of nuclear fuel reprocessing such as 99T~04-,27,28as well as the extreme importance of anionic substrates in biochemi~try.~~ One of the problems which most plagues chemists involved in the design and synthesis of anion hosts, however, is that of introducing anion selectivity.Factors such as anion size and topology, charge density, hydrogen-bond donor/acceptor prop- erties and Lewis-basic character must all be considered and such properties are often less easily defined than in analogous examples in cation complexation.5 Recent X-ray crystallo- graphic studies have demonstrated that, in nature, the selective binding of anions such as phosphate and sulfate is achieved by complex hydrogen-bond donor/acceptor arrays.3s32 It still remains to be seen if such arrays may be duplicated with the same success in a synthetic system.Several comprehensive reviews of recent progress in various forms of multidentate anion complexation have been published and the interested reader is referred to these works as a primary source for further reading.33-35 A book dealing solely with the topic of anion complexation is also planned.5 The purpose of this account is to survey some new methods of anion complexation, most notably those involving the calixarenes and other polycyclic, aromatic hosts, as a means towards introduc- tion of anion selectivity and to introduce our own work on organometallic macrocyclic hosts based upon the calixarenes and other bowl-shaped m0lecules.3~~ Historical Perspective One of the earliest reports of anion complexation dates back to 1967 (the same year as the first reports of the alkali-metal complexation properties of the crown ethers42) and involves the chelation of the bidentate Lewis acids X2B(CH2)2BX2 1 (X = F, Cl) to methoxide ions.43 In the following year Park and Simmons described the size selective binding of halide ions by in,in-1,ll-diazabicyclo[9.9.9]nonacosaneand related bicyclic, bidentate ammonium ion based receptors.44 After these early results, however, it was not until 1976 that work by Graf and Lehn upon related tetraprotonated, tricyclic, spheroidal cryp- tands such as 2 demonstrated a high affinity for C1- and Br-.45 This work was followed by the synthesis of a cylindrical bis(tren) macrocycle selective for linear anions such as azide46 and recently, by even larger cyclophane receptors capable of encompassing large carboxylate anions such as tere~hthalate.~~ Since then work upon macrocyclic polyammonium based receptors related to cryptands and spherands has progressed rapidly5,7,*,33-35,48-50and includes the binding of biologically important anions such as ATP and anionic transition-metal species such as MC142- (M = Pd, Pt).Recent work has also demonstrated that macrotricyclic zwitterions involving macro- tricyclic borane-amine adducts may act as electrically neutral anion hosts, thus eliminating the need for competition of the guest with the counter ions of the host.6 Uncharged, bidentate Lewis acids related to 1 have also received considerable attention.In particular receptors such as those based upon the 1,8-diborylnaphthalene unit 3 18733-35951 and the o-phenylenedimercurials 420952 have been shown to readily chelate hydride and halide anions. A recent report deals with carborane supported cyclic o-dimercurials which represent anion complexation analogues of the crown ethers.9 Reinhoudt and coworkers have also recently reported a range of neutral uranyl salene complexes which form novel hosts for a range of anions, notably H2PO4, incorporating both Lewis-acidic and hydrogen-bond acceptor sites. H BR2 BR:, CI H &I ?gCI H 2 3 4 Chem. Commun., 1996 1401 The Calixarenes The calixarenes are a class of phenolic macrocycles more generally termed [1,Jmetacyclophanes consisting of four or more phenolic units bridged by methylenic spacer groups, Fig.1.53354 With an electron-rich molecular cavity and 'lower rim' consisting of a cyclic array of oxygen donors, the calixarenes are, in general, much more suited to cation complexation that the binding of anions, and indeed numerous such examples have been reported.55-58 Only a single report involving the binding of an anion by a native calixarene has appeared, involving the inclusion of a methyl sulfate anion in the solid state within the cavity of [p-sulfonatocalix[4]arene]5-. This inclusion of an 'anion by an anion' is, however, a weak interaction based solely upon hydrophobic considerations and does not represent a significant attraction.59 The calixarenes do, however, represent a synthetically malleable framework upon which charged, Lewis-acidic or hydrogen-bond donor or acceptor functionalities may be placed in order to design anion binding hosts of very specific dimensions and selectivities.Recently Puddephatt and coworkers have developed a series of rigid, resorcinol-based calixarene ligands (L) such as 5, incorporating trivalent phosphorous substituents. Reaction of these macrocycles with complexes of the coinage metals such as [{M(CCPh)),] (M = Cu, Ag) in the presence of pyridinium chloride gives the metal complexes [C5H5NH] [M4(L)- (pC1)4(pn-Cl)] (n = 3, M = Cu 6a; n = 4, M = Ag 6b). In the case of 6a the guest chloride anion bridges across three of the four copper centres, but is too small to adopt the full p4-binding mode possible in the rigid cavity.The guest anion in 6a may be selectively exchanged for bromide or iodide by addition of the appropriate alkali-metal halides and, in the case of the iodide salt, the guest iodide anion then adopts the full p4-I symmet- rically bridging mode. In the case of the silver complex 6b the presence of the larger metal ion enables even chloride to bridge symmetrically between the four metal centres. l7m As part of a research programme aimed at the design of new spectroelectrochemical sensory reagents for anions, Beer and coworkers have recently reported a range of calixarene-based R a R R = But, Y = H,7a b R = Y = H, 7b R = H, Y = Et, 7~ Fig.1 Generalized structures of the calix[n]arenes a (n = 4-8) and calix [4]arenes b 5 1402 Chem. Commun., 1996 hosts containing electrochemically active cobaltocenium, ferro- cene or tris(bipyridy1)ruthenium moieties in conjunction with amide NH groups,13-15 as well as a range of related non-calixarene species.61-63 Solution electrochemical and lH NMR measurements indicate a significant selectivity of these hosts for H2PO4-even over tenfold excesses of HS04-and C1-. Also, competition experiments between calixarene and non-calix- arene hosts indicate that the former possess a higher affinity for dihydrogen phosphate.14 These same workers have also devel- oped fluorescent sensors for chloride and phosphate based upon porphyrin and other N-donor macrocycle m0ieties.6~9~~ Related work, especially involving biologically important polyanions, has been carried out by Czarnik.66 Within our own group we have recently embarked upon a programme of research aimed at the synthesis of new anion hosts, incorporating redox-active transition-metal centres by direct attachment of cationic metal ions to the calixarene aromatic rings.It is anticipated that such materials will have application in environmental waste remediation and in the sensing of environmental pollutants. We reasoned that the reaction of calixarenes with cationic metal centres may well result in cationic hosts displaying not only anion complexation affinity, but also important properties such as air- and moisture- stability, water solubility and synthetic accessibility under relatively mild conditions.There is ample literature precedent for the stability and facile synthesis of arene compounds of the second- and third-row late transition metals, notably Ru, Os, Rh and Ir,67"9 and indeed in many cases [e.g. hydrido triphenylphosphine complexes of Ru and pentamethylcyclopentadienyl and norbornadiene com-pounds of Rh and Ir] stable n complexes are formed with phenol, suggesting that analogous chemistry may also exist for the calixarenes.73-75 Also, Shinkai and coworkers have reported a range of neutral n-chromium tricarbonyl complexes of especially rigidified calix[4]arene n-propyl ethers.7&7* While useful in connection with calixarene modification by nucleo- philic addition reactions at the metallated rings, these materials have not been studied with a view to anion complexation and it seems unlikely that these electron-rich chromium(0) sDecies will display aiy significant anion coordination behaviour.We began very simply with the reaction of p-tert-butyl- calix[4]arene 7a with the chloride-bridged dimer complexes [{ M(q5-C5Me5)Cl(y-Cl)}2] (M = Rh 8a, Ir 8b),76 pretreated with silver salts according to the reaction shown in Scheme 1. This strategy was immediately successful in as much as the bimetallic hosts [{ M(q5-C5Me5)} &-tert-butylcalix-[4]arene)][BF4I4(M = Rh 9a, Ir 9b) were produced in excellent yield.37 Unfortunately, as the X-ray crystal structure of the deprotonated analogue of 9b revealed, none of the tetra-fluoroborate anions are included within the calixarene bowl (Fig.2), but rather they occupy lattice voids to either side of the metal centres, while the narrow molecular cavity is filled by a molecule of diethyl ether in a fashion more reminiscent of classical calixarene inclusion chemistry.53.54 This lack of anion inclusion is unsurprising given the relatively electron-rich nature of the calixarene cavity, formed from four aromatic rings, and it was deemed necessary to metallate all four rings, as well as to reduce the steric bulk at the upper rim of the cavity, in order to maximize the possibility of anion inclusion. Me2CO +L'+ AgX --[(ML)"L'Imc[{MLCI(J.L-CI)}~J -AgCI CF3C02H,reflux [ML(Me2C0),l2+ -acetone M = Ru, Rh, Ir L = C6H6, pMeC6H4CHMep, C6Me6, C5Me5 L' = calix[4]arene,ptert-butylcalix[4]arene,pterf-butylcalix[5]arene, CTV efc.X = BF4, PF6, CF3S03, '/2SO4 n = 1-4, m=2-6 Scheme 1General synthesis of metallated host cornpo~nds67"~ Under mild conditions (refluxing acetone in the presence of CF3C02H) analogous results were obtained for the debutylated calixarene 7b (which possesses a less sterically congested cavity), giving the bimetallic host [{ Ir(q5-C5Mes) )2(calix- [4]arene)]4+ for which an X-ray crystal structure demonstrating the inclusion of a -CF3 substituent of the neutral silver complex [{ Ag(p-O2CCF3)}2] (present as a result of the use of excess silver salt in the early stages of the reaction, Scheme 1) was obtained.77 While surprising, this result was unproductive from the point of view of anion complexation.However, under more forcing conditions (refluxing CF3C02H) we were able to cleanly isolate the tetrametallic complex [{ Ir(q5-CSMes)}4-(calix[4]arene -2H)][BF4I6 The fact that this material has only a 6+ (as opposed to 8+) charge is a result of the high acidity of two of the phenolic oxygen atoms of the metallated calixarene. High acidity of at least one phenolic proton was also noted for complexes of type 9 and even related phenol specie^.^^-^^ More importantly, however, the X-ray crystal structure of 10 revealed the inclusion of one of the BF4- anions deeply within the calixarene bowl with F( lA)-C contacts as low as 2.91, 8, demonstrating that the calixarene cavity is ideally suited to the inclusion of the small, tetrahedral tetrafluoroborate anion.Furthermore, in contrast to the shallow methylsulfate inclusion observed for [p-sulfonato-calix[4]arene]5- in which the methyl substituent lies between 4.15 and 5.03 8, from the four calixarene oxygen atoms,59 the intracavity fluorine atom F( 1A) is situated only 2.58 8, from the plane containing the four phenolic oxygen atoms with F( lA).-O distances in the range 3.12-3.25 A,demonstrating conclusively that the interaction in this case is of electrostatic origin, between an electropdeficient host and an electron-rich guest. Indeed, there is ample precedent for the build-up of partial positive charge upon the carbon atoms of coordinated unsaturated hydrocarbons.78.79 Reaction of 7b with the ruthenium complex [{ Ru(q6-p-MeC6H4CHMe2)C1(p-C1) 12] 11 which is isoelectronic with complexes 8, as in Scheme 1 also resulted in the clean isolation of tetrametallic complexes, namely [{ Ru(q6-MeC6H4CH-Me2) )4(calix[4]arene -2H)][BF4]6 12a.36 As with complex 10, the X-ray crystal structure of 12a demonstrates that one of the BF4- anions is found deeply embedded within the calixarene cavity (Fig. 3) with F( lA)--Ophenolic distances of 3.23-3.37 8,.The fluorine atom F(1A) lies 2.70 8, above the plane of the phenolic oxygen atoms, whilst the shortest F( 1A)...C distance is to C(1A) and C(lD), 2.85 A, compared with distances >3.10 8, for the remaining, non-included anions, clearly suggesting a cooperative effect arising from the proximity of the four metallated rings. Also noteworthy are the low crystallographic thermal parameters of the included anion.The small, near- spherical shape of BF4- often results in a very high degree of thermal motion even in the solid state. In contrast, the included anion in both 10 and 12a exhibits thermal motion similar to the much less mobile atoms of the host, emphasizing the tight fit of the anion into the intracavity void. Substitution of Ag[CF3S03] and Ag2[S04] in place of Ag[BF4] resulted in the ready synthesis of the analogous triflate and hydrogen sulfate species [{ Ru( q6-MeC6H4CH- Me2) )4(calix[4]arene -2H)]X6 (X = CF3S03 12b, X = HS04 12c).Complex 12b in particular has proved versatile since it may be cleanly prepared in exceptionally high yield (ca. 85%) and the triflate counter ion is readily exchanged for other anions such as C1-, Br-, I-, H2PO4-, SCN-, HS04-, Re04- etc. as their tetraalkylammonium salts in polar solvents such as nitromethane or dimethylformamide, or with alkali-metal salts in water, although a number of these salts are highly insoluble precluding the synthesis of X-ray diffraction quality crystals. In solution, preliminary radiochemical studies also indicate some selectivity of 12b for the radioisotope 99T~04- over the triflate anion, possibly as a consequence of the size of the cavity in host 12 which may well be more suited to the inclusion of tetrahedral or spherical rather than cylindrical anions, as evidenced by the structure of 12a.36 In particular, the iodide salt [{ Ru(q6-MeC6H4CHMe2) j4(calix[4]arene -2H)]I6 has been crystallographically characterized and, as expected, contains an iodide anion (again exhibiting low thermal motion) within the 16+ RU \ 12 Fig.2 X-Ray crystal structure of the bimetallic host [{Ir(qW5Me5)J2- Fig. 3 X-Ray crystal structure of the tetrametallic host [(Ru(q6-(p-tert-butylcalix[4]arene -H)][BF4I3.Et20.MeNO2 showing the inclusion p-MeC6H4CHMe2) )4(calix[4]arene -2H)]6+ showing the included BF4- of a neutral solvent molecule anion Chem. Commun., 1996 1403 calixarene cavity.77 The closest iodide+arbon approach is to C(4B) at the upper rim of the calixarene, I( 1)..C(4B) 3.73 A,whilst the height of the iodide anion above the plane of the phenolic oxygen atoms of 4.00 8, is consistent with the large ionic radius of I-and suggests that iodide is slightly too large a guest to fit comfortably within the calix[4]arene cavity.It should be noted, however, that the iodide penetrates the cavity more deeply than the central (i.e.boron) atom of the BF4- anion in 12a (the boron is situated at 4.1 1 8, above the plane of the phenolic oxygen atoms) and the penetration is still markedly deeper than those based solely upon hydrophobic inter-act ions .s9 Interestingly, the synthesis of tetrametallic complexes of type 12 is not general, and is strongly dependent upon the choice of silver salt added.For example, attempts to make analogues of 12 containing phosphate and carboxylate anions have resulted only in the isolation of bimetallic species such as [{Ru(q6-arene) },(calix[4]arene)][H(CF3CO2)2]413 (arene = p-cymene, hexamethylbenzene) related to complexes 9. Bimetallic com- pounds were also obtained from the reaction of 10 and Ag[BF4] with calix[4]arene ethyl ether 7c. Overall these results suggest that the formation of tetrametallic complexes capable of anion complexation is sensitive both to subtle ion-pairing effects and to the stereochemistry about the calixarene as a whole. Given the apparently excellent fit of tetrahedral anions within the calix[4]arene cavity we attempted the reaction of the iridium complex 8b with p-rer-r-butylcalix[5]arene, which possesses a larger cavity than its tetrameric analogue 7a.As with 7a we were unable to metallate all of the aromatic rings of the macrocycle; however, the isolated product [( Ir(qs-CSMeS)} &-rer:f-butylcalix[5]arene -H)][BF4]S did exhibit anion inclusion by virtue of two of the three metallated rings of the macrocycle (rings C and D, Fig. 4) adjacent to one another. In this case the included tetrafluoroborate anion is situated asymmetrically in the large calix[5]arene cavity, close to the rings C and D, with relatively long F..C contacts in the region of 2.95 A.41This result serves to illustrate that it is not necessary to metallate all the rings of the calixarene hosts in order to observe anion binding, but inclusion within the calixarene cavity is not observed unless at least two transition-metal containing moieties are adjacent to one another and hence able to exert a cooperative effect, favouring intracavity inclusion.A number of attempts have been made to metallate various other calixarenes and their derivatives with some degree of success. Notably, the bimetallic species [ { Ir(qs-CsMeS)] 2(q6:q6-C32H3004N4Me8)]6+ derived from the dipro- tonated form of the Mannich base dimethylaminomethylcalix-[4]arenesS has been synthesized, offering increased promise for anion complexation as a consequence of the -NHMe2+ potential anion binding sites.%" A bimetallic calix[4]resorcarene deriva- tive, [{ Ru(q6-MeC6H4CHMe2)] 2(calix[4]resorcarene-2H)]-[BF4I2, has also been synthesized and its X-ray crystal structure determined, Fig. 5.While this bimetallic complex does not bind anions as a consequence of its narrow molecular cavity, it does offer scope for further variation of the host size and shape in order to fine-tune anion selectivity.80 Cyclotriveratrylene Another bowl-shaped macrocycle which contains electron-rich aromatic rings liable to act as good ligands for transition metals, is the trimeric veratrole derivative cyclotriveratrylene (CTV) 14.81-85 Solid-state inclusion complexes of CTV have been known for many years but have always been of the channel variety with neutral guest molecules occupying lattice voids between stacks of hosts. The wide, shallow CTV cavity in these materials is occupied by the base of another CTV unit.x2.x3W& reasoned that attachment of transition-metal centres to the outer surface of CTV in the same way as the calixarenes should result in potential anion complexation hosts, and furthermore, should prevent the stacking of one CTV unit into another in the solid state, thus modifying the inclusion chemistry of CTV.In practice, reaction of the chloro complexes 8b and 11 with Ag[BF4] and CTV in a similar way to that outlined in Scheme 1 resulted in the clean isolation of a wide range of mono-, di- and tri-metallated CTV compounds [ { Ru( qh-arene),, ] -(CTV)]2'1+ (arene = p-cymene, n = I 15, 2 16, 3 17; arene = benzene, n = 1 or 3; arene = hexamethylbenzene, II = 1) and [{Ir(qs- CSMes)],,(CTV)]2"+ (n = 1 18, 2 19, 3 20) with the number of metal centres (and hence charge of the cation) essentially dependent solely upon the relative stoichiometries of starting materials added at the beginning of the Furthermore, these materials proved to be more readily handled than their calixarene analogues, being generally more soluble in polar organic solvents and not subject to deprotonation. Fig.4 Tetrafluoroborate inclusion within a calix[S]arene derivative 14 1404 Chem. Commun., 1996 A range of the monometallic complexes of types 15 and 18 have been synthesized with various counter anions. The results of X-ray crystal structure determinations indicate that as expected, only neutral solvent molecules are included within the CTV cavity.The one exception is the [H(CF3CO2)2]- salt [Ru( q.6-MeC6H4CHMe2)( q6-CTV)] [H(CF3C02)2] 2 15 b which exhibits the inclusion of the CF3 substituent of one of the [H(CF3C02)2]-anions within the CTV cavity. Anion-cation contacts are long, however (upwards of 3.3 A), clearly indicative of a hydrophobic type interaction rather than true anion binding.39 The situation is very different, however, for the bi- and tri- metallic CTV complexes of type 16,17, 19 and 20 which act as true anion hosts in the same way as their calixarene analogues. In the case of the bimetallic complex [{ Ru(q6-MeC6H4CH-Me2)J2(q6:q6-CTV)]4+ 16 we have characterized both the triflate and mixed perrhenate/triflate salts by X-ray crystallo- graphy, Fig.6, as well as the tetrafluoroborate salt of the analogous iridium complex [{ Ir(qs-CSMe5) }2(q6 :q6-CTV)][BF4I4 19a. In the case of [{ Ru(q6-MeC6H4CH-Me2) }2(q6 :qe6-CTV)][CF3S03]416a the triflate guest is firmly embedded within the CTV cavity with the negatively charged sulfonate head group seated between the two metallated rings whilst the hydrophobic-CF3 substituent lies at the centre of the cavity. The closest -S03-.CTV contact, 0(2)-C(2B), of 2.95( 1) A is relatively long in comparison to those observed for 12a, reflecting the shallowness of the CTV bowl, which is not -0 0-\/ 14+0 I-16 Fig. 6 X-Ray crystal structure of the bimetallic host 16 with included anionic guests (a) trifluoromethanesulfonate, (b) perrhenate ideally suited to the triflate anion, and the presence of only two metal centres.40 Metathesis of 16a with [NBun4][Re04] results in the isolation of the perrhenate salt [{Ru(q6-MeC6H4CHMe2)} 2(q6:q6-CTV)][Re04]4 16b as a poorly crystalline powder.In the presence of a limited quantity of [NBun4] [Re04], however, single crystals of a mixed anion complex [{ Ru(q6-MeC6H4CH-Me2)}2(q6: q6-CTV)][Re04]3[CF3S03] 16c may be Once again the structure clearly demonstrates the binding of the Re04- ion within the CTV cavity, slightly displaced from the pseudo-threefold axis of the CTV ligand towards the two metallated rings. This result is especially important because of the close relationship between [Re04] -and the isoelectronic pertechnetate ion, [Tc04]-.The radioisotope 99Tc (tl/2 2.13 X 105 y) is formed in high fission yield (6.13% from 235U) and, during nuclear fuel reprocessing, is converted to [99Tc04]- by aqueous nitric acid. Releases from the nuclear fuel cycle as well as atomic weapons testing and use of 99Mo pertechnetate generators in medical applications all contribute to the presence of 99Tc in the environment, where it is concentrated in the food chain by absorption into lichens which are in turn consumed by grazing animals such as reindeer.27.28 One of the potential applications of selective anion complexation hosts is the removal of such dangerous contaminants and it is clear that the design of hosts with anion complementary cavity dimensions may well lead to such selectivity.A number of radiochemical studies have been carried out upon host 16 which have demonstrated a marked selectivity of the wide CTV bowl for large, tetrahedral anions such as M04-. In particular, competition studies with 3-10-fold excesses of C1-, NO3-, S042- and C104- over 188Re04- in two-phase extraction experiments (saline-nitromethane) indicate that the extractability of the perrhenate anion is inhibited only by C104-, which is structurally very similar. The fact that S042- has little effect in spite of its tetrahedral shape and higher negative charge may be attributable to its high free energy of hydration, encouraging it to remain within the aqueous layer. Most surprisingly of all, direct competition studies between 188Re04-and 99mTcO4- suggest a small but statistically significant difference in the extractability of the two anions by host 16 (71 vs.84%).40 It is possible that the rigid CTV bowl is somewhat better suited to the inclusion of Tc04- than the rhenium analogue, in spite of the relatively small size difference between the two.The situation may well be significantly affected by the different hydrated radii of the anions. The anion binding properties of host 16 (as the triflate salt) were also investigated by means of cyclic voltammetry. In the absence of any externally added anion, the cyclic voltammo- gram (CV) of 16 displayed two irreversible reduction waves, -670 and -825 mV. A slight, concentration dependent, cathodic shift was noted for [NBu4] [Re041 with the reduction wave at -825 mV especially, shifting to -809 mV.Strikingly, however, addition of the HS04-anion in molar ratios increasing from 1 :10 to 2: 1 caused a marked change in the appearace of the CV, such that both reduction waves merged into a single, irreversible peak at -753 mV. A significant effect was also noted for the addition of [NBu4][H2P04] with a net cathodic shift of 43 mV for the second reduction wave, gradually increasing over the same range of molar ratios. These results are typical of the redox potential shifts noted by Beer and coworkers for systems based upon the Co3+-Co2+ couple in cobaltocenium derivatives. 13.15 The fact that the most striking effects on the CV are obtained from large, tetrahedral anions such as H2PO4- and HS04- strongly supports the contention that these anions are bound within the macrocyclic cavity, which is of complementary dimensions to these anions (little effect was observed upon addition of C1-, I- or BPh4-).This contention is also supported by the radiochemical data reported above. While a smaller effect is noted for Re04-, it is important to note the difference in solvent between the cyclic voltammetric and radiochemical Chem. Commun., 1996 1405 experiments. It is highly probable that the affinity of the host for the target anions is significantly influenced by solvation effects. Thus, in saline-nitromethane little sulfate selectivity is noted, whereas HS04- exerts a strong influence in MeCN solution, as may be expected from the high affinity of the hydrogen sulfate anion for aqueous media.It is likely that the opposite effect is in evidence for Re04-. It should be noted that the effect of the HS04- anion on the CV of 16 may also result in part from its high acidity, though this is not the case with H2P04-. The inclusion of the tetrahedral [Re04]- in 16c is also related to the structure observed for the bimetallic iridium complex 19a, with the smaller [BF4]-anion situated even more asymmetrically in the wide CTV cavity. The size mismatch between the small anion and large CTV cavity is further exemplified by the timetallic species [{ Ru(q6-MeC6H4CH-Me2)}3(q6:q6 :v~-CTV)][BF~]~17a.38 In the solid state the complex belongs to the cubic space group Pu3, with the hexacation situated upon a threefold rotation axis, implying that the CTV host adopts its full C3,,symmetry (as opposed to C, observed for the free ligand) as a result of reduced repulsion between the aromatic rings and/or steric effects arising from the presence of the bulky cymene ligands.However, there is room for only one tetrafluoroborate anion within the central cavity and rather than lie in the centre of the CTV bowl, on the threefold rotation axis, the entire guest is disordered over three sites, each similar to the single intracavity anion binding site in 19a. A similar off-axis binding of BF4- within the large CTV bowl is observed for the iridium analogue [{Ir(q5-C5Me5)}3(q6:q6:q6-CTV)][BF4]620, although fortunately the complex is crystallographically ordered, Fig.7.39 Conclusions This survey has demonstrated both the generality and the ready synthesis of anion complexation hosts based on the previously unprecedented inclusion of anionic guests within an ostensibly electron-rich macrocyclic cavity. From these results four general statements may be made. (i) Anion binding occurs in hosts containing two or more cationic transition-metal centres bound to adjacent rings of polyaromatic macrocycles. (ii) Anion inclusion is favoured by an open cavity that is not severely sterically hindered. (iii) Size compatibility between anion and host also favours cooperativity between metal centres and hence tight binding. Fig. 7 X-Ray crystal structure of the trimetallic host 20 showing the off- centre position of the included tetrafluoroborate anion 1406 Chem.Commun., 1996 (iv) Host-guest interactions are strongly influenced by additional factors, notably anion solvation energy within the medium of study and the magnitude of the electrostatic charge on the host and anion Given these rules it should now be possible to design anion complexation hosts specific to particular target anions (e.g. H2P04-, Tc04-, ATP etc.) and apply them to real anion sensing and binding applications in the field. Acknowledgments We are indebted to our coworkers whose names appear in the references, with whom it has been a great pleasure to be associated. We are also extremely grateful to the continued support of the US National Science Foundation, the University of Missouri-Columbia and NATO for providing the funding without which this work would not be possible.Jerry Atwood was born in Missouri, where he received a B.S. In 1964. Following the Ph.D. from the University of Illinois in 1968, he took a position as Assistant Professor at the University of Alabama. He rose to the position of University Research Professor in 1987 and moved to the University of Missouri-Columbia as Professor and Chair in 1994. His research interests currently are based on supramolecular chemistry, and they comprise both fundamental studies and applications to industrial and environmental chemistry. He is founder and co-editor of the journal Supramoleculur Chemistry and was recently appointed as an Associate Editor for Chemical Communications.Jonathan Steed was born in Wimbledon, England. He received his B.Sc. in 1990 and Ph.D. three years later from University College London, under Dr Derek A. Tocher. In 1993 he joined the group of Professor Jerry L. Atwood at the University of Alabama and, more recently, at the University of Missouri-Columbia, as a NATO Postdoctoral fellow. 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ISSN:1359-7345
DOI:10.1039/CC9960001401
出版商:RSC
年代:1996
数据来源: RSC