Discovery of New Reactivity at Complexes of Rhenium Supported by PCP, PNN, and PNP Pincer Ligands and the Study of Pincers Containing Ancillary Antimony Donors

The chemistry of pincer ligands is such that a wide variety of transition metals and main group elements are capable of being supported, stabilized, and isolated in often, highly reactive forms. The pincer ligands which have found the most utility are those based on the 1,3- bis(dialkylphosphinomethyl)benzene and 2,2’-bis(dialkylphosphine)diphenylamine ligand backbones. Herein, we have taken steps and report on the development of complexes of rhenium supported by PCP and PNP-related pincer ligands. Described is the formation and characterization of the first complexes of rhenium supported by the parent PCP pincer ligand and the subsequent formation of multiple polyhydrides. The substitution of dihydrogen allowed access to mixed ligand species supported by the PCP pincer. The general salt metathesis reactivity as well as the generation of supposed five-coordinate (PCP)Re+ cations and their insertion reactivity was explored. Attention was then turned to the unusual and counter-intuitive reactivity of a novel rhenium-oxo-acetate supported by the PCP pincer. Against expected pKa values, the rhenium-acetate undergoes completely irreversible hydrolysis to form a highly fluxional rhenium dioxo with an acetic acid adduct, featuring a significant hydrogen-bonding interaction. Aside from the substitution chemistry at (PCP)Re, the parent ligand was also found to, for the first time, be capable of undergoing a base-assisted dearomatization forming a “pseudo-carbene” which performs a metal-ligand cooperative activation of small molecules such as carbon dioxide. The nature of the rhenium-carbon bonding was studied spectroscopically, crystallographically, and computationally to determine significant Re-C π-bonding interactions. PNP and proton-responsive PNN-type pincers were introduced into the coordination sphere of high-valent rhenium as well. They exhibited pronounced rotameric isomerism which was able to be studied on the NMR timescale. The reduction of high-valent (PNP)Re fragments also allowed for the isolation of Re(I)/Mn(I) dicarbonyls which exhibit metal-ligand cooperativity in the activation of acidic substrates such as formic acid. Finally, pincer ligands containing antimony donors in varying oxidation states were examined and their direct comparisons to phosphorous were made as well as their possible application in the binding of small molecules and formation of novel aromatics