Redox Communication between Two Diarylamido/Bis(phosphine) (PNP)M Moieties Bridged by Ynediyl Linkers (M = Ni, Pd, Pt)

Abstract

A series of binuclear homo- and heterobimetallic complexes of the general type (PNP)M-[linker]-M(PNP) have been prepared (M = Ni, Pd, Pt; PNP = a diarylamido/bis(phosphine) pincer ligand; -[linker]- = -C≡C-, -C≡CC≡C-, -C≡CC₆H₄C≡C-). Each (PNP)M site can be oxidized by one electron, and this work reports the investigation of the mixed-valence behavior in terms of the communication between the two redox sites and the degree of the delocalization in the monooxidized cation. The compounds were evaluated using cyclic voltammetry, UV-vis-NIR and EPR spectroscopy, X-ray crystallography, and DFT calculations. The complex with the longest examined linker, (PNP)Ni-C≡CC₆H₄C≡C-Ni(PNP) (<b>9Ni</b>), exhibited no discernible communication between the redox sites. The homobimetallic complexes (PNP)M-C≡CC≡C-M(PNP) (<b>6M</b>) displayed a lower degree of communication in comparison to the -C≡C- linker analogues (PNP)M-C≡C-M(PNP) (<b>3M</b>). Within each of these two subsets, the relative degree of communication and delocalization was determined to be Pd < Ni ≤ Pt. On the Robin-Day scale, compounds <b>6M</b> can be assigned class I for M = Pd and class II for M = Ni, Pt. Complex <b>3Pd</b> also falls into class II, while <b>3Ni</b> and <b>3Pt</b> may be viewed as borderline class II/III cases. It is likely that the communication in the Ni systems has the advantage of the smaller size of Ni, resulting in a greater physical proximity of the redox sites, while the 5d metal Pt possesses the greatest ability for orbital interaction with the -C≡C- linker.