Synthesis, structure, electrochemical and catalytic behaviour of new cyclopalladated complexes
Creators: name: Kar, Gopa
Publisher: name: RMIT University
Year: 2024

description: The halide-bridged cyclopalladated complexes [Pd2(m-X)2(k2-2-C6F4PPh2)2] (X = Cl, Br) were prepared in high yields from the reaction of equimolar amounts of 2-Me3SnC6F4PPh2 and [PdX2(cod)] (cod = 1,5-cyclooctadiene). These complexes are not thermodynamically stable and slowly dimerise in solution over several days to give tetranuclear [Pd4(m-X)4(m-2-C6F4PPh2)4] in which the four 2-C6F4PPh2 groups bridge two pairs of palladium atoms. Attempts to prepare protio analogues of the dinuclear complexes by treating [PdX2(cod)] (X = Cl, Br) with 2-Me3SnC6H4PPh2 gave [Pd4(m-X)4(m-2-C6H4PPh2)4] as the only isolable product.<br><br>Anation of [Pd2(m-X)2(k2-2-C6F4PPh2)2] with the appropriate alkali metal or silver salt gave the corresponding iodo-, acetato-, benzoato- and thiocyanato-complexes which are also unstable with respect to ring-opening reactions. For example, heating a solution of [Pd2(m-SCN)2(k2-2-C6F4PPh2)2] gave [Pd4(m-SCN)4(k2-2-C6F4PPh2)2(m-2-C6F4PPh2)2] in which only two of the four chelate rings are opened, and solutions of [Pd2(m-OAc)2(k2-2-C6F4PPh2)2] rearranged to give [Pd2(m-OAc)2(m-2-C6F4PPh2)2]. The halide bridges in [Pd2(m-X)2(k2-2-C6F4PPh2)2] (X = Cl, Br) are cleaved by acetone and THF with retention of the chelate rings to give species tentatively identified as [PdX(solvent)(k2-2-C6F4PPh2)]; evaporation of the solvent in vacuo regenerated the original dimers. In contrast, irreversible ring opening occurs in acetonitrile, the products being [Pd2X2(NCMe)2(m-2-C6F4PPh2)2].<br><br>Reaction of [Pd2(m-Br)2(k2-2-C6F4PPh2)2] with [Na(acac)] gives initially monomeric [Pd(acac)(k2-2-C6F4PPh2)], which dimerises by chelate ring-opening in dichloromethane/methanol to [Pd2(acac)2(µ-2-C6F4PPh2)2]. The tertiary phosphines L react with [Pd2(m-Br)2(k2-2-C6F4PPh2)2] to give monomeric complexes [PdBr(k2-2-C6F4PPh2)L] (L = PPh3, PPh2Fc); both these and the derived iodo-complexes exist as cis/trans isomers in solution (predominantly trans in non-polar solvents, predominantly cis in acetonitrile).<br><br>The reduction of many of these 2-C6F4PPh2 compounds has been studied by cyclic voltammetry and rotating disk electrochemical techniques in dichloromethane and acetonitrile. The results indicate that, in the first step, one electron is added to each metal atom to generate a transient palladium(I) species. Corresponding 2-C6H4PPh2 complexes are reduced at higher potentials (by ca. 0.4 V) and the course of the reduction could not be defined. However, attempts to detect the known dipalladium(I) complex [Pd2(µ-2-C6F4PPh2)2(PPh3)2] by controlled potential reduction of [PdBr(k2-2-C6F4PPh2)(PPh3)] failed, possibly due to over-reduction. Ill defined products with complex 31P NMR spectra were obtained, these possibly being palladium(0) clusters. Cyclic voltammetry of [PdX(k2-2-C6F4PPh2)(PPh2Fc)] (X = Br, I) led initially to the mono-cation which rapidly released [PPh2Fc]+ by cleavage of the Pd-P bond. The voltammetric behaviour of [PPh2Fc]+ is considerably more complex than that of ferrocenium and probably involves oxidation at the phosphorus centre but the species generated have not been completely characterised.<br><br>The catalytic properties of selected 2-C6F4PPh2 complexes have been investigated for Sonogashira, Heck and Suzuki reactions with moderate yields obtained for aryl iodide substrates.