The Chemistry of Carbene and Carbonyl Complexes of Platinum

An extensive series of electroneutral alkoxy(organo)carbene complexes of platinum(II) has been prepared, the compounds having "been characterised by elemental analysis, 1H and 31P NMR and ir. spectroscopies and, in the case of cis-[PtCl2{C(OEt)CHpPh}-(PMe2Ph)] , by X-ray crystal structure determination. The complex cis-[PtCl2-{C(OMe)CH2Ph}(PMe2Ph)] reacted with CD3OD via rapid H/D exchange of its benzyl-CH2-hydrogens, no alkoxy group exchange being observed. Reaction with dimethylamine proceeded by proton abstraction to give an alkoxy-vinylplatinum complex. These reactions have been compared with those of cationic alkoxy(organo)carbene complexes. Reaction with chloride ions involved elimination of alkyl chloride, and produced the chloride-bridged complexes [Pt2(u-Cl)2-(COCH2R)2(PR'3)2]. These complexes, as well as terminal carbonyl species, were also produced by thermolysis of the electroneutral carbene complexes. Treatment of the neutral carbene complexes with tertiary phosphine produced acyIbis(tertiary phosphine)platinum complexes, but if the reaction was performed in the presence of AgSO3CF3 the cationic carbene complexes, trans- [PtCl{C(OR")CH2R} (RP'3)2]+ SO3CF3-, could be isolated. These derivatives were also prepared by alkylation of trans-[PtCl(COCH2R)(PR') species. Addition of AgSO3CF3 to the complexes cis-[PtX2{C(OR")CH2R}(PR'3)] produced dimeric, cationic carbene complexes, which reacted further by alkylation of the SO3CF3- anion. The aroylplatinum complex, [Pt2(mu-Cl)2(COPh)2(PMePh2)2] , was found to exist in solution in equilibrium with one isomer of [PtClPh(CO)(PMePh2)] (I). The other two isomers (II and III) were produced by passing CO through a solution of [Pt2(mu-Cl)2Ph2(PMePh2)2] hut both were resistant to attempts to promote carbonyl insertion. The three isomers were identified by 1H, 13C and 31P NMR spectroscopy, and I was shown to have Ph and PMePh2 in mutually trans-positions. Reaction of cis-[PtCl2(CO)(PMePh2)] with HgPh2 at low temperature produced I only which, on warming, equilibrated with [Pt2(mu-Cl)2(COPh)2(PMePh2)] (IV). The I/lV equilibrium position for the complexes [Pt2(mu-X)2-(COR)2L2] was shown to be dependent on the natures of R, X and L. The halide-bridged dimer was favoured by strong bridging ligands X, while the position of equilibrium was dependent on the electronic nature and, for bulky ligands, the size of the neutral ligands L. Thermodynamic parameters were obtained for the complexes [Pt2(u-Cl)2-(COR)2(PMePh2)2] (R = Et, Me or Ph), though the dependence of the equilibrium position on the nature of R is not understood. These results are suggested to be consistent with an alkyl or aryl migration mechanism for the CO insertion reaction. The novel trans-[PtX2(CO)L] complexes have been prepared and characterised by 13C and 31P NMR spectroscopies. They isomerised spontaneously to their cis-isomers, and the isomerisation could be catalysed by the addition of CO, X or L. The isomerisation process was shown to involve free CO. The interconversion of [PtClPh(CO)-(PMePh2)] (II and III) also involved CO elimination, but was inhibited by excess CO. Possible isomerisation pathways have been proposed, and the mechanisms of isomerisation reactions of platinum complexes in general have been discussed. The halide-bridged dimers, [Pt2(u-X)2R2(PR'3)2] (R=Ph, acyl or aroyl), exist in solution as cis- and trans-isomers and a mechanism for their interconversion, involving reversible cleavage of one platinum-halide bridge bond, has been presented. The 13C and 31P NMR data for all the carbonyl, acyl and aroyl, and halide-bridged complexes have been tabulated, and the effects of the cis- and trans-ligands on chemical shifts and coupling constants have been discussed. Assignments of cis- and trans-geometries for the halide-bridged complexes have been made on the basis of solvent-dependenoe. The signs of the 1J(Pt-C) and 2J(P-C) coupling constants were determined for a typical member of each series of carbonyl complexes. The results of crystal structure determinations for the complexes cis-[PtCl2(PEt3)L (L= CO, P(OPh)3 and P(NMe2)3) were discussed, and both cis- and trans-influences of L were found to be significant. An X-ray structural investigation of trans-[Pt2(mu-Cl)2(COEt)2(PMe2Ph)2] permitted some comparison of the relative importance of trans-influence in mono- and binuclear complexes to be made