Binuclear Palladium Complexes Supported by Bridged Pincer Ligands

A series of binucleating ligands each containing two tridentate pincer sites based on a central, monoanionic aryl (C) donor flanked by neutral phosphinito (P) and imino (N) donors, [(PCN)-(CH2)n-(PCN)] (1), or a central, monoanionic amido (N) flanked by neutral phosphine (P) and imino donors (N), [(PNN)-(CH2)n-(PNN)] (4), are presented. The metalating sites were linked through the condensation of two equivalents of m-hydroxybenzaldehyde or the purposively built asymmetric diarylamine [(H)­N­(2-C­(O)­H-4-Me-C6H3)­(2-P­(iPr)2-4-Me-C6H3)] (3) with primary α,ω-diamines (1,2-diaminoethane, n = 2; 1,4-diaminobutane, n = 4), which simultaneously constructed the imino arms and bridged the pincer cores. Ligands 1 and 4 were then used in the synthesis of neutral, square-planar palladium­(II) complexes 5 and 6, [(PCN-Cn)­Pd2X2, (PNN-Cn)­Pd2X2; n = 2, 4; X = Cl, OAc, OTf]. The difference in the trans influence of the central donor was demonstrated through X-ray crystal structures of 5b-Cl, (PCN-C4)­Pd2Cl2, and 6b-Cl, (PNN-C4)­Pd2Cl2. The (PNN-Cn)­Pd2X2 complexes (6) proved to be redox-active, presumably via oxidation of the ligand, and cyclic voltammetry illustrated the extent to which electronic communication between the two pincer sites is mediated by the length of the bridge between them. (PCN-Cn)­Pd2OTf2 (5-OTf) and (PNN-Cn)­Pd2OTf2 (6-OTf) complexes reacted with hydride donors such as Et3SiH or β-hydride-containing alkoxides such as NaOiPr to generate bridging-hydride monocations [(PCN)­Pd-H-Pd­(PCN)-Cn]­[OTf] (5-H, n = 2, 4) and [(PNN)­Pd-H-Pd­(PNN)-C2]­[OTf] (6a-H), where the supported Pd–Pd separation was also found to be affected by the bridge length. In the case of the (CH2)4-bridged (PNN-C4)­PdOTf (6b-OTf), a bridging-hydride monocation was not observed. Instead, the formation of [(PN­(H)­N)­Pd-Pd­(PNN)-C4]­[OTf] (6b-H), protonated at the amido N of the ligand with a direct Pd­(I)–Pd­(I) bond, was recorded.