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 (<b>2a</b>), Pd (<b>3a</b>); 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 (<b>1a</b>), Pd (<b>1b</b>)), 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 (<b>2a</b>) and 674 nm (<b>3a</b>) 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 <b>2a</b> were replaced by H^–, I^–, and CO to afford [Pt₆(μ-H)­(H)₂(μ-dpmp)₄]⁺ (<b>4</b>), [Pt₆(μ-H)­I₂(μ-dpmp)₄]­(PF₆) (<b>5</b>), and [Pt₆(μ-H)­(μ-dpmp)₄(CO)₂]­(PF₆)₃ (<b>6</b>). 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 <b>2a</b>, <b>4</b>, <b>5</b>, and <b>6</b>. The nature of the axial ligands influences the redox activities of the hexanuclear complexes; <b>2a</b>, <b>3a</b>, and <b>5</b> 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 (<b>7a</b>), Pd (<b>8a</b>)). The present results are important in developing bottom-up synthetic methodology to create nanostructured metal strings by utilizing fine-tunable metallic building blocks