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

Linear Pt2M2Pt2 hexanuclear clusters [Pt4M2(mu-H)(mu-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(mu-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 M-2(mu-H) part. The terminal ligands of 2a were replaced by H-, I-, and CO to afford [Pt-6(mu-H)(H)(2)(mu-dpmp)(4)](+) (4), [Pt-6(mu-H)I-2(mu-dpmp)(4)](PF6) (5), and [Pt-6(mu-H)(mu-dpmp)(4)(CO)(2)](PF6)(3) (6). The electronic structures of these hexaplatinum cores, {Pt-6(mu-H)(mu-dpmp)(4)}(3+), are varied depending on the sigma-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(mu-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.