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

Abstract

Linear Pt 2 M 2 Pt 2 hexanuclear clusters [Pt 4 M 2 (μ-H)(μ-dpmp) 4 (XylNC) 2 ](PF 6 ) 3 (M = Pt ( 2a ), Pd ( 3a ); dpmp = bis(diphenylphosphinomethyl)phenylphosphine) were synthesized by site-selective reductive coupling of trinuclear building blocks, [Pt 2 M(μ-dpmp) 2 (XylNC) 2 ](PF 6 ) 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 M 2 (μ-H) part. The terminal ligands of 2a were replaced by H –, I –, and CO to afford [Pt 6 (μ-H)(H) 2 (μ-dpmp) 4 ] + ( 4 ), [Pt 6 (μ-H)I 2 (μ-dpmp) 4 ](PF 6 ) ( 5 ), and [Pt 6 (μ-H)(μ-dpmp) 4 (CO) 2 ](PF 6 ) 3 ( 6 ). The electronic structures of these hexaplatinum cores, {Pt 6 (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 [Pt 4 M 2 (μ-dpmp) 4 (XylNC) 2 ](PF 6 ) 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.