Hydride-Bridged Pt₂M₂Pt₂ Hexanuclear Metal Strings (M = Pt, Pd) Derived from Reductive Coupling of Pt₂M Building Blocks Supported by Triphosphine Ligands

Abstract

Linear Pt2M2Pt2 hexanuclear clusters [Pt4M2(μ-H)(μ-dpmp)4(XylNC)2](PF6)3 (M = Pt (2a), Pd (3a); dpmp = bis(diphenylphosphinomethyl)phenylphosphine) were synthesized by site-selective reductive coupling of trinuclear building blocks, [Pt2M(μ-dpmp)2(XylNC)2](PF6)2 (M = Pt (1a), Pd (1b)), and were revealed as the first example of low-oxidation-state metal strings bridged by a hydride with M–H–M linear structure. The characteristic intense absorption bands around 583 nm (2a) and 674 nm (3a) were assigned to the HOMO–LUMO transition on the basis of a net three-center/two-electron (3c/2e) bonding interaction within the central M2(μ-H) part. The terminal ligands of 2a were replaced by H–, I–, and CO to afford [Pt6(μ-H)(H)2(μ-dpmp)4]+ (4), [Pt6(μ-H)I2(μ-dpmp)4](PF6) (5), and [Pt6(μ-H)(μ-dpmp)4(CO)2](PF6)3 (6). The electronic structures of these hexaplatinum cores, {Pt6(μ-H)(μ-dpmp)4}3+, are varied depending on the σ-donating ability of axial ligands; the characteristic HOMO–LUMO transition bands interestingly red-shifted in the order of CO < XylNC < I– < H–, which was in agreement with calculated HOMO–LUMO gaps derived from DFT optimizations of 2a, 4, 5, and 6. The nature of the axial ligands influences the redox activities of the hexanuclear complexes; 2a, 3a, and 5 were proven to be redox-active by the cyclic voltammograms and underwent two-electron oxidation by potentiostatic electrolysis to afford [Pt4M2(μ-dpmp)4(XylNC)2](PF6)4 (M = Pt (7a), Pd (8a)). The present results are important in developing bottom-up synthetic methodology to create nanostructured metal strings by utilizing fine-tunable metallic building blocks.