Synthesis of asymmetric alkyne-bridged dipalladium complexes from the paramagnetic cation [Pd2(µ-PhC2Ph)(η-C5Ph5)2]+; crystal structures of [Pd2(bipy)(µ-PhC2Ph)(η-C5Ph5)][Pd2(NCMe)(bipy)(µ-PhC2Ph)(η-C5Ph5)][PF6]2·OEt2 and [Pd2(bipy){P(OPh)3}(µ-PhC2Ph)(η-C5Ph5)][PF6]

Abstract

The paramagnetic complex [Pd₂(µ-PhC₂Ph)(η-C₅Ph₅)₂]⁺1⁺ dissolves in acetone or acetonitrile to gives diamagnetic [Pd₂L₃(µ-PhC₂Ph)(η-C₅Ph₅)][PF₆](L = acetone, 2 or MeCN, 5). Compound 2 reacts with bipyridyl-like ligands to give [Pd₂(OCMe₂)(L–L)(µ-PhC₂Ph)(η-C₅Ph₅)][PF₆]3[L–L = 2,2′-bipyridyl(bipy), 4,4′-dimethyl-2,2′-bipyridyl(dmbipy) or 1,10-phenanthroline(phen)] and 5 reacts with bipy to give [Pd₂(bipy)(µ-PhC₂Ph)(η-C₅Ph₅)][Pd₂(NCMe)(bipy)(µ-PhC₂Ph)(η-C₅Ph₅)][PF₆]₂6. Treatment of 3 or 6 with P-donor ligands gives [Pd₂L(L–L)(µ-PhC₂Ph)(η-C₅Ph₅)][PF₆]4[L = PPh₃, L–L = bipy or dmbipy; L = P(OPh)₃, L–L = bipy]; the direct reaction between 5 and PPh₃ gave [Pd₂(NCMe)(PPh₃)(µ-PhC₂Ph)(η-C₅Ph₅)][PF₆]7. The X-ray structure of complex 6(as its diethyl ether solvate) showed it to contain two different cations A and B. In A one palladium atom carries a bipyridyl ligand and the second an η-C₅Ph₅ ring, the Pd–Pd vector being transversely bridged by the alkyne; cation B differs from A in that an additional acetonitrile ligand is co-ordinated to the first palladium atom. The X-ray structure of 4[L = P(OPh)₃, L–L = bipy] shows its cation to be broadly similar to cation B of 6 with the P(OPh)₃ ligand replacing the acetonitrile {albeit with the Pd(bipy)L [L = MeCN or P(OPh)₃] units orientated differently}. The geometries and bonding of these µ-alkyne dipalladium cations are analysed in terms of isolobal analogies with [Co₂(CO)₆(µ-alkyne)](for 4 and B) and [PtL₂(cyclopropenium)] species (for A).