Nickel, Ruthenium, and Rhodium NCN-Pincer Complexes Featuring a Six-Membered N-Heterocyclic Carbene Central Moiety and Pyridyl Pendant Arms

Abstract

NCN pincer ligand precursor 1·HX (X = Cl, PF6), having a six-membered N-heterocyclic carbene central moiety and pyridyl pendant arms, was prepared via N–C cross-coupling of 2-bromo-6-tert-butylpyridine with propane-1,3-diamine, followed by ring closure with triethylorthoformate. The free ligand 1 was not accessible via deprotonation, yet its copper(I) complex 2 was prepared by reacting 1·HCl with CuI and potassium hexamethyldisilazide. It was used further as a transmetalation agent to prepare nickel(II) and ruthenium(II) dihalide NCN-pincer complexes 3a, 4a, and 4b, featuring a distorted square-pyramidal geometry at the metal, with the carbene ligand in the axial position. The rhodium(III) NCN-pincer complex 6b was obtained from 2 and [Rh(cod)Cl]2 via transmetalation followed by oxidation with Cu(I) in THF. The rhodium(III) NCC-pincer complex 9b was prepared under similar conditions in acetonitrile, where a spontaneous rollover cyclometalation occurred. Complex 9b has a distorted octahedral geometry, with the acetonitrile ligand trans to the carbene. Very short Ru–CNHC and Rh–CNHC bonds were measured in complexes 4a, 4b, and 9b. Halogen exchange was observed by 1H and 13C NMR spectroscopy in both ruthenium and rhodium systems during the transmetalation stage, and the purification was accomplished by pushing the respective equilibrium with excess halide.