Redox-Active Bisphosphines Based on a W(II)-Propargyl Complex Backbone

Abstract

Three bisphosphine ligands based on propargyl-type alkyne complexes [Tp*W(CO)(OR){η²-C₂(CH₂PPh₂)(PPh₂)}] {Tp* = hydridotris(3,4,5-trimethylpyrazolyl)borate} with R = Me, Et, ⁱPr have been synthesized and characterized. The pK_b values of the two phosphine groups were determined by conversion into the selenide derivative and determination of the ³¹P/⁷⁷Se coupling constants. The methoxy-substituted compound was utilized as a metalla-ligand for preparation of a dinuclear bisphosphine κ²-PdCl₂ complex. The electrochemical properties were investigated using cyclic voltammetry (CV) and IR spectroelectrochemistry (IR-SEC). The CV experiments of both the ligand and the Pd complex showed an electrochemically reversible W(II/III) redox couple, indicating that the W(III) species is stable on the voltammetry timescale. However, in both cases, IR-SEC measurements disclosed a follow-up process upon oxidation, showing that the W(III) compound undergoes a fast chemical transformation. Pure component decomposition of the IR-SEC data was performed for kinetic analysis of the reaction profile. IR and mass spectrometric monitoring after stoichiometric oxidation as well as DFT calculations point to the formation of a [W=O] moiety by cleavage of the methoxy group leading to a subsequent disproportionation equilibrium.