Synthesis and reactivity studies of a [Cp*Rh] complex supported by a methylene-bridged hybrid phosphine-imine ligand

[Cp*Rh] complexes (Cp* = eta(5)-pentamethylcyclopentadienyl) supported by bidentate chelating ligands are useful in studies of redox chemistry and catalysis, but little information is available for derivatives bearing hybrid [P,N] chelates. Here, the preparation, structural characterization, and chemical and electrochemical properties of a [Cp*Rh] complex bearing the kappa(2)-[P,N]-2-[(diphenylphosphino)methyl] pyridine ligand (PN) are reported. Cyclic voltammetry data reveal that [Cp*Rh(PN)Cl]PF6 (1) undergoes a chemically reversible, net two-electron reduction at -1.28 V vs. ferrocenium/ferrocene, resulting in generation of a rhodium(I) complex (3) that is stable on the timescale of the voltammetry. However, H-1 and P-31{H-1} NMR studies reveal that chemical reduction of 1 generates a mixture of products over a 1 h timescale; this mixture forms as a result of deprotonation of the methylene group of 1 by 3 followed by further reactivity. The analogous complex [Cp*Rh(PQN)Cl]PF6 (2; PQN = kappa(2)-[P,N]-8-(diphenylphosphino) quinoline) does not undergo self-deprotonation or further reactivity upon two-electron reduction, confirming the reactivity of the acidic backbone methylene C-H bonds in the PN complexes. Comparison of the electrochemical properties 1 and 2 also shows that the extended conjugated system of PQN contributes to an additional ligand-centered redox event for 2 that is absent for 1. (C) 2020 Elsevier B.V. All rights reserved.