DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1998, Pages 263–270 263 New mono- and di-nuclear complexes of PdII, PtII and NiII of PNNP ligands with a 2,29-biaryl bridging unit Alette G. J. Ligtenbarg,a Esther K. van den Beuken,a Auke Meetsma,a Nora Veldman,b Wilberth J. J. Smeets,b Anthony L. Spek b and Ben L. Feringa *,a a Department of Organic and Molecular Inorganic Chemistry, Groningen Center for Catalysis and Synthesis, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands b Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, University of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands The Schiff bases 2,29-bis{N-[(2-diphenylphosphino)phenyl]formimidoyl}biphenyl (L1) and 2,29-bis{[(2-diphenylphosphino) benzylidene]amino}biphenyl (L2) were synthesized from the appropriate amine and aldehyde. Their co-ordination behaviour towards PdII, PtII and NiII has been studied. Dinuclear complexes were formed when treating L1 with 2 equivalents of [PdCl(Me)(cod)] (cod = cycloocta-1,5-diene) and Pd(O2CMe)2.In the case of the reaction of L1 with Pd(O2CMe)2 an unexpected double cyclopalladated complex was formed. However, treatment of L1 with 2 equivalents of [PtCl2(MeCN)2] resulted in hydrolysis of the ligand and a mononuclear complex of (2-diphenylphosphino)benzenamine was obtained. The structure of [Pd2L1Cl2Me2] 1 has been established by an X-ray diffraction study. The dihedral angle between the biphenyl rings is 49.2(8)8 and the intramolecular Pd ? ? ? Pd distance is 7.165(3) Å.The biphenyl bridging moiety shows a remarkable bending of 169.2(8)8. Treatment of 1 with carbon monoxide resulted in a double CO insertion, yielding the bis(acetyl)palladium compound [Pd2L1Cl2- {C(O)Me}2] 2. The structure of [Pd2L1(O2CMe)2]?4CH2Cl2 3 was also determined by X-ray diffraction. The dihedral angle between the biphenyl rings is 75.9(14)8 while the intramolecular Pd ? ? ? Pd distance is 8.655(2) Å.The reaction of L2 with 2 equivalents of anhydrous [PdCl2(MeCN)2] or NiCl2 afforded mononuclear complexes. The crystal and molecular structure of [PdL2]Cl2?3.5CH2Cl2 was determined. The compound has an approximately square-planar P2N2 geometry. The chemistry of dinuclear transition-metal complexes has attracted considerable interest in recent years.1 In particular dinuclear models of metalloproteins 2 have received great attention 3 and a variety of new homo-4 and hetero-dinuclear 5 complexes has been synthesized.In the design of these types of compounds the choice of the ligand system is of prime importance. 6 When the two metal centres are incorporated in close proximity the complexes have the potential of catalysing reactions either more efficiently, or with different chemo-, regio- or stereo-selectivity than those of corresponding mononuclear ones, due to co-operation of the two metals in the transition state of the catalytic reaction.7 An excellent example of a dinuclear catalyst which exhibits higher regioselectivity and reactivity than those of the related mononuclear system by bimetallic co-operation is the dirhodium system reported by Stanley and co-workers 8 for the hydroformylation of a-olefins.In our group oxygen activation on dinuclear copper complexes contain a meta-arene-bridged ligand system was investigated.9 Recently dinuclear rhodium phosphite complexes based on a tetranaphthol skeleton were studied as catalysts in hydroformylation reactions.10 In the systems studied so far ligands are often either highly flexible or rigid; in the second case the two metal centres are in a fixed orientation towards each other.It was envisaged that when the distance between the two metal ions is flexible this might result in optimum co-operation of the metal centres in the transition state of a catalytic reaction. A building block for this approach is a biphenyl unit, which can act as a bridging moiety between two ligating groups.Réglier et al.11 have reported a dinuclear complex of a bis(pyridyl)- substituted biphenyl ligand which acts as a functional mimic of tyrosinase, a copper-containing monooxygenase. However, in most cases where a biaryl backbone is used, mononuclear complexes are obtained e.g. with NiII, CuII, CoII, ZnII and PdII.12,13 In the approach described here a biphenyl ligand system is developed consisting of two fairly rigid arms connected to the backbone which has flexibility at point a.Owing to the bulky 2,29 substituents, however, hindered rotation is expected around the biaryl bond (atropisomerism), which might result in stereoisomers of these ligands or complexes. As donor sets phosphorus and imino-nitrogen atoms were chosen, and the co-ordination behaviour of these ligands with metals of Group VIII, e.g. PdII, PtII and NiII, was studied. The co-ordination geometry of the metals will largely be determined by the angle between the two phenyl rings in the biphenyl moiety (a) and by rotation around the single bonds connecting the side-arms to this unit (b).Results Synthesis of 2,29-bis{N-[(2-diphenylphosphino)phenyl]- formimidoyl}biphenyl (L1) The tetradentate diimine ligand 2,29-bis{N-[(2-diphenylphosphino) phenyl]formimidoyl}biphenyl (L1) was synthesized by the Schiff-base formation of 2 equivalents of biphenyl-2,29- dicarbaldehyde with (2-diphenylphosphino)benzenamine14 (Scheme 1).The synthesis of biphenyl-2,29-dicarbaldehyde by phenanthrene ozonolysis is a known literature procedure 15 but the yields are often low. Therefore we developed a more convenient route towards this compound starting from diphenic acid ([1,19-biphenyl]-2,29-dicarboxylic acid).16 In a first step the dimethyl ester I was formed in 80% yield. Subsequently reduction with lithium aluminium hydride gave the corresponding N N P P b b a264 J.Chem. Soc., Dalton Trans., 1998, Pages 263–270 diol II in 90% yield. Next, a mild Swern oxidation was performed, affording the dialdehyde III in high yield (90%). The synthesis of L1 was carried out under a nitrogen atmosphere in dry benzene. The reaction was followed by 31P NMR spectroscopy, which revealed that imine formation was rather slow and was complete after 24 h. Besides a Dean–Stark trap, addition of molecular sieves was necessary in order to remove all the water produced during the reaction and to drive it to completion. Even small amounts of water in the reaction mixture caused oxidation of the product.The compound L1 was purified by crystallisation from a dichloromethane–pentane solvent mixture. It showed in the 31P NMR spectrum an absorption at d 214.9 and is air-stable in the solid state. Furthermore, the ligand was characterised by 1H NMR spectroscopy, elemental analysis and mass spectrometry (see Experimental section). Synthesis of 2,29-bis{[2-(diphenylphosphino)benzylidene]amino}- biphenyl (L2) A related tetradentate Schiff-base ligand (L2) with phosphorus and imino-nitrogen donor sets was also synthesized.In L2 the imino nitrogen atom is attached to the biphenyl moiety, instead of the imino carbon atom in L1. We expected this ligand to be more stable towards oxidation and more easily synthesized. Furthermore, upon co-ordination of the phosphorus and nitrogen atoms to a metal ion, a six-membered chelate ring will be formed instead of a five-membered ring in the case of L1.Ligand L2 was synthesized by a Schiff-base reaction of 2,29- diaminobiphenyl 17 and 2-(diphenylphosphino)benzaldehyde (Scheme 2).18 After 4 h reflux in benzene under Dean–Stark conditions with a catalytic amount of toluene-p-sulfonic acid L2 was obtained in 92% yield. It showed an absorption at d Scheme 1 (i) MeOH, H2SO4, reflux, 16 h, yield 80%; (ii) LiAlH4, tetrahydrofuran (thf), reflux, 17 h, 90%; (iii) ClCOCOCl, dimethyl sulfoxide (dmso), 260 8C, 45 min; (iv) NEt3, 90%; (v) (2-diphenylphosphino) benzenamine, toluene-p-sulfonic acid, benzene, molecular sieves (4 Å), Dean–Stark, reflux, 24 h, 65% CO2H CO2H CO2Me CO2Me ( iii), ( iv) ( i) ( ii) CHO CHO N N PPh2 PPh2 OH OH ( v) I II III L1 Scheme 2 (i) NaN3, H2SO4, 30–40 8C, 95 min, yield 65%; (ii) 2- (diphenylphosphino)benzaldehyde, toluene-p-sulfonic acid, benzene, Dean–Stark, reflux, 4 h, 92% CO2H CO2H NH2 NH2 N N PPh2 PPh2 ( i) (ii) L2 215.2 in the 31P NMR spectrum.The product was also characterised as for L1. Compound L2 was air-stable in the solid state as well as in solution. The Schiff-base reaction yielding it was much faster than the imine formation affording L1 as only 4 h for complete conversion for L2 and 24 h for L1 were required. In addition, a larger overall yield for L2 compared to L1 (60 and 45%, respectively) was found. Palladium and platinum complexes based on L1 Ligand L1 was allowed to react with 2 equivalents of [PdCl(Me)(cod)]19 (cod = cycloocta-1,5-diene) in tetrahydrofuran for 16 h at room temperature yielding a precipitate, which was crystallised from dichloromethane–pentane as pale yellow crystals in 65% yield.The cod group of the starting material was replaced by L1, giving rise to dinuclear complex 1 (Scheme 3). The 31P NMR spectrum shows a singlet at d 35.4 (CDCl3), which indicates that the two phosphorus atoms are identical.Compared with the free biphenyl (d 214.9, CDCl3) a strong downfield shift is observed. The 1H NMR spectrum of 1 revealed that the methyl groups on the metal ions are equivalent giving a sharp doublet at d 0.86 due to phosphorus proton coupling [J(P]H) = 2.93 Hz]. The magnitude of this coupling indicates a cis relationship 20 between two groups having a large trans influence,21 i.e. the phosphorus atom and the methyl group. The crystal structure of [Pd2L1Cl2Me2] 1 was established by an X-ray diffraction study (Fig. 1). Selected bond lengths, angles and torsion angles are listed in Table 1. The notion that the ligand has imposed C2 symmetry on the palladium complex was confirmed by the X-ray diffraction analysis showing a two-fold axis dividing the C(25)]C(25a) Scheme 3 (i) thf, room temperature (r.t.), 16 h, yield 65%; (ii) CDCl3, r.t., 15 min; (iii) CH2Cl2, r.t.; (iv) CH2Cl2, r.t., 1 h N N Pd Ph2P Cl Me Pd PPh2 Cl Me N Pd Ph2 P MeCO2 N Pd P Ph2 O2CMe P Ph2 H2 N Pt Ph2 P NH2 1 L1 + 2[PdCl(Me)(cod)] L1 + 2Pd(O2CMe)2 3 2+ 2 Cl– 1 + 2CO N N Pd Ph2P Cl MeC Pd PPh2 Cl CMe L1 + 2[PtCl2(MeCN)2] 4 2 O O ( iv) ( iii) ( ii) ( i)J.Chem. Soc., Dalton Trans., 1998, Pages 263–270 265 bond. The metals adopt a distorted square-planar coordination geometry. The P]Pd]N angle of 79.70(11)8 is signifi- cantly smaller than the ideal 908 angle, whereas the Cl]Pd]N angle of 98.57(11)8 is significantly larger than 908. The distances between palladium and the donor atoms are within the normal range;20b,23,24a Pd]N and Pd]P distances are 2.237(4) and 2.1963(16) Å, respectively whereas Pd]Cl and Pd]CH3 are 2.3625(15) and 2.020(6) Å, respectively.The Pd ? ? ? Pd distance is 7.165(3) Å and the metal ions are directed to the outside (anti-folding, see Scheme 5, structure A). The two phenyl rings of the biphenyl are twisted with respect to each other resulting in a dihedral angle of 49.2(8)8 for C(20)]C(25)]C(25a)]C(20a).A remarkable feature of the biphenyl moiety is the significant non-linearity as shown by the orientation of the atoms C(22), C(25), C(25a) and C(22a). The angle C(22)]C(25)]C(22a) is 169.2(8)8 instead of the ideal 1808. The carbon atoms of the phenyl ring of C(20) to C(25) deviate 0.022 Å from the mean plane. Carbon atom C(22a) is 0.366(6) Å and C(25a) 0.123(5) Å out of that plane. Complexes containing a metal–carbon s bond can undergo insertion reactions with e.g. carbon monoxide 24 and isocyanides. 25 Treatment of a solution of 1 with carbon monoxide in CDCl3 at room temperature for 15 min resulted smoothly and nearly quantitatively in double CO insertion leading to the formation of the dinuclear acetylpalladium compound [Pd2L1Cl2{C(O)Me}2] 2 which was characterised by IR and NMR spectroscopy. The CO stretching vibration at 1697 cm21 indicates inserted carbon monoxide.24a In the 1H NMR spectrum the methyl absorption is shifted to d 2.25 compared with d 0.86 in 1 and the coupling pattern has disappeared. The 13C NMR spectrum shows an additional carbonyl absorption at d 210.8 and an acetyl methyl signal at d 10.93 whereas the methyl resonance in 1 was located at d 21.42.The 31P resonance has shifted upfield to d 14.29. Reaction of 1 with tert-butyl Fig. 1 An ORTEP22 plot of complex 1 (50% probability level) Table 1 Selected bond distances (Å) and angles (8) and torsion angles (8) of complex 1 with estimated standard deviations (e.s.d.s) in parentheses Pd]N Pd]Cl Pd]P Cl]Pd]N P]Pd]N P]Pd]C(26) Cl]Pd]C(26) 2.237(4) 2.3625(15) 2.1963(16) 98.57(11) 79.70(11) 92.0(2) 89.8(2) Pd]C(26) C(25)]C(25a) Pd(1) ? ? ? Pd(1a) Cl]Pd]P N]Pd]C(26) C(20)]C(25)]C(25a) C(24)]C(25)]C(25a) 2.020(6) 1.486(6) 7.165(3) 178.21(5) 169.6(2) 126.0(4) 116.5(4) C(20)]C(25)]C(25a)]C(20a) 49.2(8) isocyanate resulted in the formation of some palladium(0) and a complex reaction mixture, according to NMR spectroscopy, which was not further identified.Reaction of 2 equivalents of Pd(O2CMe)2 with L1 was carried out in dichloromethane at room temperature under a nitrogen atmosphere for 1.5 h. Subsequently, the solvent was removed in vacuo and the resulting dark yellow-brown oil was crystallised from dichloromethane–pentane. Dark red crystals of the cyclopalladated complex 3 were obtained after a few weeks and its structure confirmed by X-ray crystallography (Fig. 2). The 31P NMR spectrum shows a singlet at d 44.9 (CDCl3), which indicates that the two phosphorus atoms are identical.The complex has C2 symmetry with a two-fold axis dividing the C(25)]C(25a) bond. The two phenyl rings of the biphenyl moiety are twisted with respect to each other with a dihedral angle of 75.9(14)8 for C(20)]C(25)]C(25a)]C(20a). Atoms C(22), C(25) and C(25a), C(22a) show a nearly linear arrangement. For example the angle between C(22), C(25) and C(25a) is 179.1(9)8. The distances between palladium and the coordinated donor atoms in complex 3 are in the normal range, Pd]N and Pd]P are 2.000(7) and 2.319(2) Å, respectively and Pd]O and Pd]C are respectively 2.051(7) and 1.978(10) Å.26 The intramolecular metal–metal distance is 8.655(2) Å.The metal adopts a distorted square-planar co-ordination geometry: the angles P(1)]Pd(1)]N(1) [84.5(2)8] and N(1)]Pd(1)] C(21) [82.0(3)8] are significantly less than the ideal 908. The angles P(1)]Pd(1)]O(1) and O(1)]Pd(1)]C(21) are respectively 100.0(2) and 93.4(3)8.Selected bond lengths and angles are listed in Table 2. From 31P and 1H NMR studies it can be concluded that cyclometallation of L1 with Pd(O2CMe)2 in dichloromethane is a very slow process. Initially, upon addition of Pd(O2CMe)2, a mixture of products is formed. These products eventually rearrange to give the more stable cyclopalladated compound together with acetic acid. The formation of [Pd2(O2CMe)2L1] is irreversible. The binding of L1 to PtII was briefly examined by treating a solution of L1 in dichloromethane with 2 equivalents of bis- (acetonitrile)platinum(II) chloride. After 1 h at room temperature 31P NMR spectroscopy showed the presence of various compounds.After 1 night this composition had not changed. Attempts to isolate either a mono- or a di-nuclear complex of L1 failed. The only complex that was isolated according to 1H, 13C NMR spectroscopy and elemental analysis was the platinum complex 4 of (2-diphenylphosphino)benzenamine IV.Obviously, the complexes formed initially are highly water sensitive. During work-up by filtering the solution in the presence of air the ligand was hydrolysed, followed by complexation of Pt to form 4. Palladium and nickel complexes based on L2 To study the co-ordination behaviour of L2, 2 equivalents of [PdCl2(MeCN)2] were added to a solution of L2 in dichloromethane at room temperature. Remarkably, in this case no Table 2 Selected bond distances (Å) and angles (8) and torsion angles (8) of complex 3 with e.s.d.s in parentheses Pd(1)]N(1) Pd(1)]O(1) Pd(1)]P(1) P(1)]Pd(1)]O(1) P(1)]Pd(1)]N(1) O(1)]Pd(1)]C(21) N(1)]Pd(1)]C(21) 2.000(7) 2.051(7) 2.319(2) 100.0(2) 84.5(2) 93.4(2) 82.0(3) Pd(1)]C(21) C(25)]C(25a) Pd(1) ? ? ? Pd(1a) O(1)]Pd(1)]N(1) P(1)]Pd(1)]C(21) C(20)]C(25)]C(25a) C(24)]C(25)]C(25a) 1.978(10) 1.508(14) 8.655(2) 172.7(3) 166.6(3) 122.5(8) 119.8(9) C(20)]C(25)]C(25a)]C(20a) 75.9(14)266 J.Chem.Soc., Dalton Trans., 1998, Pages 263–270 Fig. 2 An ORTEP plot of complex 3 (50% probability level) formation of a dinuclear species was observed. According to 31P NMR spectroscopy, mononuclear complex 5 (Scheme 4) was formed quantitatively, giving rise to a 31P NMR absorption at d 31.1 (free biphenyl d 215.2). Crystallisation from dichloromethane–pentane afforded orange crystals suitable for X-ray analysis. The crystal structure of 5 is shown in Fig. 3. Selected bond lengths, angles and torsion angles are listed in Table 3.Complex 5 exists in two forms in the unit cell which slightly differ from one another, together with seven solvent molecules Scheme 4 (i) CH2Cl2, r.t., 1.5 h, yield 66%; (ii) absolute EtOH, reflux, 3 h; (iii) CH2Cl2, 4 NaBF4, r.t., 2 h, 53% N N P Ph2 P Ph2 Pd L2 + [PdCl2(MeCN)2] 5 N N P Ph2 P Ph2 L2 + NiCl2 Ni 6 2+ 2+ 2BF4 – 2Cl– ( i) ( ii), ( iii) Table 3 Selected bond distances (Å) and angles (8) and torsion angles (8) of complex 5 * with e.s.d.s in parentheses Pd(1)]N(1) Pd(1)]N(2) Pd(1)]P(1) P(1)]Pd(1)]N(2) P(1)]Pd(1)]P(2) N(1)]Pd(1)]N(2) P(2)]Pd(1)]N(1) 2.086(6) 2.164(6) 2.243(2) 85.08(18) 96.56(8) 90.0(2) 88.29(17) Pd(1)]P(2) Pd(1) ? ? ? Cl(1) C(251)]C(261) P(1)]Pd(1)]N(1) P(2)]Pd(1)]N(2) C(201)]C(251)]C(261) C(241)]C(251)]C(261) 2.262(2) 2.8025(19) 1.470(11) 175.08(17) 161.98(16) 122.6(7) 119.5(7) C(241)]C(251)]C(261)]C(271) 51.4(11) * Values for only one crystallographically independent molecule are given.The dimensions of the second are very similar. (dichloromethane). In both structures the chlorides are nonco- ordinating. The palladium ion adopts a slightly distorted square-planar geometry. The twist in the biphenyl moiety is 51.4(11)8 for C(241)]C(251)]C(261)]C(271) and 51.5(11)8 for the corresponding carbon atoms in the second structure. The distances between palladium and the donor atoms are within the normal range.23 The distances between Pd(1) and P(1) and P(2) are 2.243(2) and 2.262(2) Å, respectively. The Pd]N distances are 2.086(6) Å for Pd(1)]N(1) and 2.164(6) Å for Pd(1)]N(2).Also in this compound the atoms C(221)]C(251)] C(261)]C(291) show a non-linear arrangement, but the bending in the biaryl unit is significantly less than in complex 1 (see above). The angle between C(251), C(261) and C(291) is 175.5(6)8 and for the corresponding atoms in the second structure 175.2(6)8. For a related mononuclear rhodium(I) complex of an asymmetric 6,69-dimethyl-2,29-bis{[2-diphenylphosphino) benzylidene]amino}biphenyl ligand 27 no bend in the biphenyl moiety was reported, whereas the dihedral angle between the rings was larger (698).Upon reaction of L2 with anhydrous nickel(II) chloride mononuclear complex 6 was formed which was characterised by 31P and 1H NMR spectroscopy, elemental analysis and mass spectrometry (see Experimental section). The complex shows an absorption in the 31P NMR spectrum at d 31.3.Fig. 3 An ORTEP plot of one of the two independent molecules in the crystals of complex 5 (50% probability level)J. Chem. Soc., Dalton Trans., 1998, Pages 263–270 267 Discussion From these results it can be concluded that L1 is an excellent dinucleating ligand for palladium. When [PdCl(Me)(cod)] is added to L1 a geometry is adopted in which the ligating groups at each phenyl ring point to the ‘inside’ and the two metal centres are directed to the ‘outside’ as illustrated in Scheme 5, structure A.Probably steric interactions between the two sidearms disfavour the formation of a syn-folded geometry as indicated in structure B, although it must be emphasised that in solution rotation around the single bonds connecting the sidearms to the backbone is still possible. In geometry B the two metals are in closer proximity, yielding a complex in which they might be able to co-operate in a catalytic reaction. However, in the double cyclometallated complex 3 (Fig. 2), despite the biaryl rotation, the formation of a syn-folded geometry is not possible because the palladium ions are directly attached to the biphenyl backbone via C(21)]Pd(1) [C(21a)]Pd(1a)] aryl–metal bonds. In the case of L2 only mononuclear complexes were formed with the general structure as shown in Scheme 5, C. In spite of the possibility of adopting a conformation able to accommodate two metal ions simultaneously, by rotation around the single bond in the biphenyl backbone, only mononuclear complexes are formed.Apparently, these are very stable. Once a metal ion is complexed between the four donor atoms in the ligand, the binding of a second metal ion is disfavoured. Conclusion Two new ligand systems, L1 and L2, were developed containing a PNNP donor-atom sequence and a biphenyl backbone. We have shown that L1 can readily accommodate two palladium ions. Furthermore, a mild double cyclometallation reaction was observed with Pd(O2CMe)2 although the process is rather slow.In contrast, L2 only affords mononuclear complexes of palladium and nickel. Notably, the crystal structures of [Pd2- L1Cl2Me2] 1 and [Pd2L2]Cl2 5 obtained by X-ray analyses showed that the biphenyl moiety is twisted to approximately the same degree (about 508) for both complexes. However, the twist in the biphenyl moiety of [Pd2L1(O2CMe)2] 3 is much larger (75.98). Another remarkable feature of these compounds is the significant non-linearity in the biphenyl entity leading to significant biphenyl bending.Finally with complex 1 a successful double-insertion reaction was performed with carbon monoxide yielding [Pd2L1Cl2{C(O)Me}2] 2. Experimental Melting points (uncorrected) were determined on a Mettler FP- 2 apparatus equipped with a FP-21 microscope and are uncorrected. Proton NMR spectra were recorded on a Varian Gemini-200 (at 200 MHz), on a VXR-300 (at 300 MHz) or on a Unity 500 spectrometer (at 500 MHz).Chemical shifts are reported in d units (ppm) relative to the solvent and converted into the SiMe4 scale (d 0) using d (CHCl3) 7.26. Carbon-13 NMR spectra were recorded on a Varian Gemini-200 (at 50.3 MHz), on a VXR-300 (at 75.4 MHz) or on a Unity 500 spectrometer (at 125 MHz), 31P NMR spectra on a Varian Gemini- Scheme 5 Representation of the geometries of mono- and di-nuclear complexes based on PNNP ligands N N M N M P P P N N P P M M N P M A B C 200 spectrometer (at 80.95 MHz) with triphenyl phosphate as an external reference (d 218).High-resolution mass spectra were obtained on an AEI MS-902 spectrometer by electron impact (EI), electron spray (ES) mass spectra on a NERMAG mass spectrometer. Infrared spectra were recorded on a Perkin- Elmer 841 spectrometer. Elemental analyses were performed in the Microanalytical Department of our laboratory. All manipulations were carried out under an atmosphere of dry dinitrogen except for the synthesis of compound I.Dichloromethane, pentane and hexane were distilled from phosphorus pentaoxide, methanol from magnesium, benzene from sodium wire and tetrahydrofuran from sodium– benzophenone. Dimethyl sulfoxide was dried by stirring over barium oxide during 18 h before use. (2-Diphenylphosphino)- benzenamine,14 2,29-diaminobiphenyl,17 2-(diphenylphosphino) benzaldehyde 18 and [PdCl(Me)(cod)]19 were synthesized using literature procedures. Diphenic acid, 2-bromobenzaldehyde and chlorodiphenylphosphine obtained from Aldrich were used without further purification.Preparations Biphenyl-2,29-dicarboxylic acid dimethyl ester I. To biphenyl- 2,29-dicarboxylic acid (25.0 g, 103 mmol) in methanol (250 cm3) was added sulfuric acid (4 cm3) and the solution was heated under reflux for 16 h. The resulting dark brown reaction mixture was poured into a water–ice mixture (300 cm3) and subsequently ethyl acetate (300 cm3) was added. Evaporation of most of the methanol yielded a two-phase system.The dark green aqueous layer was extracted with ethyl acetate (2 × 75 cm3) and the combined organic layers were subsequently washed with saturated sodium hydrogencarbonate solution (2 × 75 cm3), water (1 × 75 cm3) and brine (1 × 50 cm3). Drying (MgSO4) and evaporation of the solvent yielded a pale brown solid which was crystallised from ethyl acetate (10 cm3). Yield: 22.6 g as white crystals (80%). M.p. 76.7–77.7 8C; 1H NMR (CDCl3) d 8.05–8.00 (m, 2 H), 7.60–7.40 (m, 4 H), 7.24–7.20 (m, 2 H) and 3.63 (s, 6 H); 13C NMR (CDCl3) d 167.4 (C), 143.3 (C), 131.4 (CH), 130.2 (CH), 129.8 (CH), 129.3 (C), 127.2 (CH) and 51.8 (CH3). 2,29-Di(hydroxymethyl)biphenyl II.Dimethyl ester I (7.0 g, 25.9 mmol) in thf (100 cm3) was added slowly to a suspension of LiAlH4 (2.0 g, 0.21 mol) in thf (150 cm3) at 0 8C. The reaction mixture was refluxed for 17 h. After cooling to 0 8C, aqueous 1.4 M KOH (20 cm3) was added carefully. After refluxing for 1 h, filtering, drying (MgSO4) and evaporation of the solvent, II was obtained in 94% yield as a white solid which was recrystallised from ethyl acetate (10 cm3).Yield: 5.0 g as white needles (90%). M.p. 112.4–113.5 8C; 1H NMR (CDCl3) d 7.36 (m, 6 H), 7.13 (dd, J = 0.9, J = 3.6 Hz, 2 H), 4.27 (s, 4 H) and 3.66 (s, OH, 2 H); 13C NMR (CDCl3) d 140.0 (C), 138.6 (C), 129.61 (CH), 129.55 (CH), 128.0 (CH), 127.6 (CH) and 62.7 (CH2). Biphenyl-2,29-dicarbaldehyde III. A solution of oxalyl chloride (2.75 cm3, 0.032 mol, 2.7 equivalents) in dichloromethane (50 cm3) was cooled to 260 8C and a mixture of dmso (3 cm3, 0.042 mol, 3.5 equivalents) and dichloromethane (10 cm3) was added carefully.After stirring for 5 min, II (2.6 g, 0.012 mol) in dmso (1 cm3) and dichloromethane (50 cm3) was added in less than 5 min. The resulting white solution was stirred for 45 min at 260 8C. Next a large excess of triethylamine (10 cm3, 0.072 mol, 6.0 equivalents) was added carefully.The mixture was allowed to warm to room temperature and subsequently washed with 5% HCl solution (30 cm3), aqueous NaHCO3 (saturated, 30 cm3) and brine (30 cm3). Drying (MgSO4) and evaporation of the solvent gave dialdehyde III as a pale yellow oil that solidified upon standing. Yield: 2.3 g as a yellow solid (90%). M.p. 63.2–64.3 8C (lit.,15 62.5–63.5 8C); 1H NMR268 J. Chem. Soc., Dalton Trans., 1998, Pages 263–270 (CDCl3) d 9.80 (s, CHO, 2 H), 8.02 (d, J = 4, 2 H), 7.70–7.57 (m, 4 H) and 7.33 (d, J = 3 Hz, 2 H); 13C NMR (CDCl3) d 191.0 (CH), 141.2 (C), 134.5 (C), 133.4 (CH), 131.7 (CH), 128.8 (CH) and 128.5 (CH). 2,29-Bis{N-[(2-diphenylphosphino)phenyl]formimidoyl}- biphenyl L1. To a mixture of biphenyl-2,29dicarbaldehyde III (0.31 g, 1.49 mmol), (2-diphenylphosphino)benzenamine IV (0.83 g, 2.98 mmol, 2.0 equivalents) and a catalytic amount of toluene-p-sulfonic acid in dry benzene (40 cm3) were added molecular sieves (4 Å).The reaction mixture was refluxed under Dean–Stark conditions for 16 h. After 90% completion of the reaction according to 31P NMR spectroscopy (16 h), the mixture was filtered over Celite and the yellow solution refluxed for 6 h. Evaporation of the solvent yielded a pale yellow foam. Pure L1 was obtained upon crystallisation from dichloromethane– pentane (1 : 2). Yield: 0.76 g as a pale yellow powder (65%). M.p. 183.3–185.3 8C; 1H NMR (CDCl3, 200 MHz) d 8.10–8.05 (m, 2 H), 7.87 (s, 2 H), 7.41–7.32 (m, 4 H), 7.31–7.20 (m, 22 H), 7.08 (t, J = 7.5, 2 H), 7.00–6.95 (m, 2 H), 6.79 (d, J = 1.2, 1 H), 6.77 (d, J = 1.5, 1 H), 6.74 (d, J = 4.4, 1 H), 6.71 (d, J = 1.0, 2 H) and 6.69 (d, J = 1.2 Hz, 1 H); 13C NMR (CDCl3, 50.3 MHz) d 157.9 (CH, 4JPC = 1.5), 154.0 (C, 1JPC = 17.9), 140.3 (C), 136.8 (C, 2JPC = 10.7), 134.4 (C), 134.0 (CH, 2JPC = 20.6), 132.6 (C, 1JPC = 11.4), 132.3 (CH), 130.7, 130.2 (CH), 129.7 (CH), 128.4 (CH), 128.2 (CH, 3JPC = 7.3 Hz), 128.0 (CH), 127.2 (CH), 125.8 (CH) and 117.0 (CH); 31P NMR (CDCl3, 80.95 MHz) d 214.9. High-resolution mass spectrum: m/z 728.251 (calc.for C50H38N2P2: 728.251) (Found: C, 82.04; H, 5.47; N, 3.75; P, 8.29. Calc. for C25H19NP: C, 82.40; H, 5.26; N, 3.84; P, 8.50%). 2,29-Bis{[2-(diphenylphosphino)benzylidene]amino}biphenyl L2. A mixture of 2-(diphenylphosphino)benzaldehyde (1.01 g, 3.44 mmol), 2,29-diaminobiphenyl (0.322 g, 1.72 mmol) and a catalytic amount of toluene-p-sulfonic acid in benzene (40 cm3) was refluxed for 4 h under Dean–Stark conditions. The solvent was evaporated and the product crystallised from absolute ethanol (15 cm3).Yield: 1.15 g as a pale yellow powder (92%). M.p. 170.8–171.2 8C; 1H NMR (CDCl3, 200 MHz) d 8.98 (d, J = 5.6 Hz, 2 H), 7.86–7.79 (m, 2 H), 7.38–7.15 (m, 30 H), 6.85– 6.78 (m, 2 H) and 6.39 (m, 2 H); 13C NMR (CDCl3, 50.3 MHz) d 158.4 (CH, 3JPC = 25.2), 150.6 (C), 139.4 (C, 1JPC = 17.2), 138.0 (C, 1JPC = 19.5), 136.2 (C, 2JPC = 10.7), 134.0 (CH, 2JPC = 20.2), 133.7 (C), 132.7 (CH), 131.0 (CH), 130.4 (CH), 129.0 (CH), 128.7 (CH), 128.5 (CH, 3JPC = 7.3), 128.1 (CH), 127.4 (CH, 2JPC = 4.2 Hz), 124.9 (CH) and 118.2 (CH); 31P NMR (CDCl3, 80.95) d 215.2.High-resolution mass spectrum: m/z 728.251 (calc. for C50H38N2P2: 728.251) (Found: C, 82.08; H, 5.43; N, 3.82; P, 8.35. Calc. for C25H19NP: C, 82.40; H, 5.26; N, 3.84; P, 8.50%). [Pd2L1Cl2Me2] 1. To L1 (100 mg, 0.14 mmol) in thf (1.5 cm3) was added [PdCl(Me)(cod) (72.7 mg, 0.28 mmol, 2.0 equivalents).After stirring for 16 h at r.t., the yellow solution was concentrated under reduced pressure until a white precipitate was formed. Filtration and crystallisation from dichloromethane –pentane yielded pale yellow crystals, 95 mg (65%). 31P NMR (CDCl3): d 35.4. 1H NMR (CDCl3, 200 MHz): d 9.16 (d, J = 5.1, 2 H), 8.86 (s, 2 H), 7.65–7.34 (m, 30 H), 7.21–7.15 (m, 2 H), 7.05–7.00 (m, 2 H) and 0.86 (d, J = 2.9 Hz, 6 H). 13C NMR (CDCl3, 75.4 MHz): d 167.43 (CH), 155.89 (d, C, 2JPC = 15.9), 140.90 (C), 134.32 (s, CH), 133.56 (d, CH, 2JPC = 13.4), 133.40 (d, CH, 2JPC = 11.0), 133.51 (C), 132.39 (CH), 131.97 (CH), 131.32 (CH), 131.06 (CH), 131.03 (CH), 130.45 (CH), 130.14 (CH), 129.53 (d, C, 1JPC = 40.3 Hz), 129.01 (d, CH, 3JPC = 11.0), 128.85 (d, CH, 3JPC = 11.0), 128.81 (d, C, 1JPC = 37.9), 128.35 (d, CH, 2JPC = 6.1), 128.07 (C), 120.51 (d, CH, 2JPC = 7.3 Hz) and 21.42 (CH3). ES mass spectrum: m/z 1007 (M 2 Cl), 1039 (M 2 Cl 1 MeOH) and 486 (M 2 2Cl) (Found: C, 59.29; H, 4.26; N, 2.64.Calc. for C26H22ClNPPd: C, 59.90; H, 4.25; N, 2.69%). [Pd2L1Cl2{C(O)Me}2] 2. The complex [Pd2L1Cl2Me2] (25 mg, 0.02 mmol) in CDCl3 was stirred for 15 min under 1 bar (105 Pa) CO at r.t. According to NMR data the conversion was nearly quantitative. IR (CDCl3); nCO 1687 cm21. 31P NMR (CDCl3): d 14.3. 1H NMR (CDCl3, 200 MHz): d 9.35 (d, J = 7.8 Hz, 2 H), 8.57 (s, 2 H), 7.76–7.42 (m, 30 H), 7.36–7.26 (m, 2 H), 7.22–7.09 (m, 2 H) and 2.25 (s, 6 H). 13C NMR (CDCl3, 75.4 MHz): d 223.39 (d, C, 2JPC = 7.3), 167.10 (CH), 155.03 (d, C, 2JPC = 15.9), 140.83 (C), 134.37 (CH), 133.57 (d, CH, 2JPC = 13.4), 133.17 (C), 133.11 (d, C, 2JPC = 13.4), 132.60 (CH), 132.15 (CH), 131.31 (CH), 131.00 (CH), 130.39 (CH), 129.98 (CH), 129.66 (CH), 129.15 (d, CH, 3JPC = 11.0), 129.01 (C), 128.91 (d, CH, 3JPC = 9.8), 128.41 (CH), 128.36 (CH), 120.50 (d, CH, 2JPC = 7.3) and 37.43 (d, CH3, 3JPC = 21.8 Hz); one Cq was not resolved.[Pd2L1(O2CMe)2] 3. To L1 (100 mg, 0.14 mmol) in CH2Cl2 (4 cm3) was added Pd(O2CMe)2 (62 mg, 0.28 mmol, 2.0 equivalents) and the resulting clear dark red solution was stirred for 1.5 h at r.t. The product was precipitated as a yellow powder by adding the solution dropwise to pentane while stirring vigorously. The pentane was decanted and the powder crystallised from dichloromethane–pentane to yield orange-red crystals suitable for X-ray analysis. 31P NMR (CDCl3): d 44.9. 1H NMR (CDCl3, 200 MHz): d 8.14–7.98 (m, 4 H), 7.90–7.76 (m, 6 H), 7.60–7.21 (m, 24 H), 7.20–7.00 (m, 4 H) and 1.92 (s, 6 H). [Pt(H2NC6H4PPh2-2)2]Cl2 4. To L1 (100 mg, 0.14 mmol) in CH2Cl2 (1 cm3) was added [PtCl2(MeCN)2] (49 mg, 0.14 mmol, 2.0 equivalents). The resulting yellow solution was stirred for 1 h at r.t. The solution was quickly filtered in the presence of air to remove undissolved material. Evaporation of the solvent and crystallisation from dichloromethane–pentane yielded pale yellow crystals. 31P NMR (CDCl3): d 25.8 (s), 25.8 (d, JPPt = 3334 Hz). 1H NMR (CDCl3, 200 MHz): d 7.79 (dd, J = 3.4 Hz, 2 H), 7.46 (m, 2 H), 7.40–7.30 (m, 14 H) and 7.26–7.11 (m, 14 H). 13C NMR (CDCl3, 50.3 MHz): d 147.30 (C), 134.12 (CH), 133.13 (t, CH, 2JPC = 11.8), 132.16 (CH), 132.06 (CH), 103.19 (d, C, 1JPC = 64.9), 128.95 (t, CH, 3JPC = 11.8), 128.34 (t, CH, 3JPC = 7.6), 127.01 (t, CH, 3JPC = 14.1) and 125.84 (d, C, 1JPC = 67.1 Hz) (Found: C, 51.26; H, 4.02; N, 3.34.Calc. for C36H32Cl2N2P2Pt?1.5CH2Cl2: C, 51.35; H, 3.85; N, 3.31%). [PdL2]Cl2 5. To L2 (100 mg, 0.14 mmol) in CH2Cl2 (2 cm3) was added [PdCl2(MeCN)2] (35.6 mg, 0.14 mmol, 1.0 equivalent) and the clear red solution was stirred for 1.5 h at r.t. Crystallisation from dichloromethane–pentane afforded large red crystals suitable for X-ray analysis. Filtration and drying of the crystals in vacuo gave 82 mg of analytically pure [PdL2]Cl2 as an orange-red powder (66%). 31P NMR (CDCl3): d 31.3. 1H NMR (CDCl3, 500 MHz): d 8.17 (d, J = 11.5, 2 H), 8.02 (br, 4 H), 7.76 (m, 2 H), 7.67 (t, J = 7.6, 4 H), 7.60–7.55 (m, 8 H), 7.45 (t, J = 7.6, 2 H), 7.35 (m, 2 H), 7.30 (m, 6 H), 6.94 (br, 2 H), 6.88 (m, 4 H) and 6.58 (d, J = 7.7 Hz, 2 H). 13C NMR (CDCl3, 125 MHz): d 168.96 (CH), 148.45 (C), 138.77 (CH), 135.60 (CH), 134.80 (CH), 133.63 (t, C, 2JPC = 26.1), 133.48 (CH), 132.89 (C), 131.87 (CH), 131.68 (CH), 131.34 (C), 131.07 (CH), 129.56 (CH), 129.45 (CH), 129.37 (t, C, 2JPC = 11.8), 129.12 (t, CH), 126.3 (d, CH, 1JPC = 52.5), 124.52 (d, C, 1JPC = 53.6), 123.66 (CH) and 123.30 (d, CH, 1JPC = 65.5); one CH was not resolved due to overlap.ES mass spectrum: m/z 869 (M 2 Cl) and 417 (M 2 2Cl). [NiL2][BF4]2 6. To a suspension of L2 (100 mg, 0.14 mmol) in absolute EtOH (2 cm3) was added dry NiCl2 (15 mg, 0.14 mmol, 1.0 equivalent) and the resulting clear dark brown solution was warmed to reflux for 3 h. After evaporation of theJ.Chem. Soc., Dalton Trans., 1998, Pages 263–270 269 EtOH, CH2Cl2 (2 cm3) was added. To the resulting pale brown suspension was added NaBF4 (60 mg, 0.55 mmol, 4 equivalents). After stirring for an additional 2 h at r.t., the reaction mixture was filtered over Celite to remove the excess of NaBF4 and evaporation of the solvent gave a dark brown solid that was crystallised from dichloromethane–pentane to afford complex 6 as dark brown crystals: 63 mg (53%). 31P NMR (CDCl3): d 31.3. 1H NMR (CDCl3, 200 MHz): d 8.2–8.0 (br, 2 H), 7.9–7.3 (m, 28 H), 7.3–7.0 (br, 6 H) and 7.0–6.8 (br, 2 H). ES mass spectrum: m/z 873 (M 2 BF4) and 394 (M 2 2BF4) (Found: C, 62.50; H, 4.28; N, 3.12. Calc. for C50H38B2F8N2NiP2: C, 62.48; H, 3.99; N, 2.91%). Crystallography X-Ray data for compound 1 were collected at 130 K on an Enraf-Nonius CAD-4F2 diffractometer with Mo-Ka radiation (l = 0.710 73 Å). The orange crystal was quickly removed from the wall of the vessel and mounted on top of a glass fibre.The structure was solved by Patterson methods and extension of the model was accomplished by direct methods applied to difference structure factors using the program DIRDIF.28 The positional and anisotropic thermal displacement parameters for the non-hydrogen atoms were refined on F 2 with full-matrix least-squares procedures minimising the Q = Sh[w(|Fo)2 2 k(Fc)2|2], where w = 1/[s2(Fo)2 1 (aP)2 1 bP], P = [max(Fo,02) 1 2Fc 2]/3. Reflections were stated observed if satisfying the F 2 > 0 criterion of observability. A subsequent Fourier-difference synthesis resulted in the location of all hydrogen atoms, the coordinates and isotropic thermal displacement parameters of which were refined.Final refinement on F 2 carried out by fullmatrix least-squares techniques converged at wR(F 2) = 0.1298 for 4269 reflections with F 2 > 0 and R(F ) = 0.0471 for 3921 reflections with F > 4.0s(F ) and 359 parameters. A final Fourier-difference map showed residual densities between 21.14 and 11.22(13) e Å23 and one peak of 3.13 e Å23 located near (1.86 Å) the Pd atom.No other significant peaks having chemical meaning were observed in the final Fourier-difference syntheses. Geometrical calculations were done with PLATON.29 X-Ray data for compounds 3 and 5 were collected at 150 K on an Enraf-Nonius CAD4T diffractometer with rotating anode and graphite monochromated Mo-Ka radiation. Orange crystals were cut to size and covered by inert oil to avoid deterioration due to loss of solvent.A total of 12 388 reflections for 3 and 19 488 for 5 were scanned to qmax of 26.0 and Table 4 Summary of crystal data for compounds 1, 3 and 5 Formula M Crystal system Space group a/Å b/Å c/Å a/8 b/8 g/8 U/Å3 Dc/g cm23 Z F(000) Crystal size/mm m(Mo-Ka)/cm21 Ra wR2 b S 1 (C26H22- ClNPPd)2 1042.6 Monoclinic C2/c 17.789(5) 14.170(4) 19.423(7) 113.02(2) 4506(3) 1.537 4 2104 0.19 × 0.44 × 0.50 10.26 0.0471 0.1298 1.083 3?4CH2Cl2 (C27H21NO2- PPd)2 1397.45 Trigonal P3121 13.009(2) 13.009(2) 29.706(6) 4353.7(13) 1.599 3 2106 0.40 × 0.50 × 0.50 10.9 0.0653 0.156 0.988 5?3.5CH2Cl2 C50H38Cl2N2- P2Pd 906.1 Triclinic P1� 15.1653(18) 19.369(2) 20.3003(17) 100.755(10) 99.828(8) 111.470(10) 5262.1(10) 1.519 4 2436 0.45 × 0.70 × 0.70 9.1 0.0673 0.174 1.008 a R = S(|Fo| 2 |Fc|)/S|Fo|.b wR2 = [Sw(Fo 2 2 Fc 2)2/Sw(Fo 2)2]� �� . 25.48 respectively. Intensity data for 3 were corrected for absorption PLATON/DIFABS.29 The structures of 3 and 5 were solved by Patterson methods (DIRDIF) 28 and direct methods (SHELXS 8630), respectively.Least-squares refinement of F 2 was done with SHELXL 96.31 Hydrogen atoms were taken into account at calculated positions riding on their carrier atoms. Convergence was reached at wR2 = 0.1560 [5513 reflections, R1 = 0.0653 for 3793 reflections with I > 2s(I)] and 0.1735 [18 889 reflections, R1 = 0.0673 for 11 771 reflections with I > 2s(I)].The final difference map did not show any significant residual features. Geometrical calculations and the ORTEP illustrations were done with PLATON.29 The final results were checked for missed symmetry with the PLATON/ MISSYM option and solvent-accessible voids with the PLATON/SOLV option. CCDC reference number 186/754. See http://www.rsc.org/suppdata/dt/1998/263/ for crystallographic files in .cif format. Acknowledgements This work was supported in part (A.L. 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