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

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) (9Ni), exhibited no discernible communication between the redox sites. The homobimetallic complexes (PNP)­M–CCCC–M­(PNP) (6M) displayed a lower degree of communication in comparison to the −CC– linker analogues (PNP)­M–CC–M­(PNP) (3M). 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 6M can be assigned class I for M = Pd and class II for M = Ni, Pt. Complex 3Pd also falls into class II, while 3Ni and 3Pt 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.