Central N-heterocyclic carbene moieties in protic pincer-type bis(pyrazole) ligands: Perturbation on steric and electronic properties of ruthenium center

Abstract

Chelation-assisted C–H bond cleavage of an N,N’-bis(pyrazol-3-ylmethyl)imidazolium salt with RuCl2(PPh3)3 and subsequent anion exchange afforded the cationic pincer-type ruthenium complex [RuCl(PPh3)2(5NCN-H2)]PF6 (3) bearing a five-membered N-heterocyclic carbene (NHC) furnished with two protic pyrazole arms (5NCN-H2). On the other hand, the uncharged dichlorido complex [RuCl2(PPh3)(6NCN-H2)] (4) with a six-membered carbene (6NCN-H2) was obtained by a similar treatment of an N,N′-bis(pyrazol-3-ylmethyl)perimidinium cation. X-ray analysis of 3 and 4 suggested that the different outcome is ascribed to the larger twist of the 6NCN-H2 pincer ligand. Cyclic voltammetry showed that these NHC-centered pincer complexes are oxidized more easily than the pyridine-centered analogue in line with the stronger electron-donating property of the central NHC unit in the pincer ligands. In contrast, the CO stretching frequency of the carbonyl complex [Ru(CO)(PPh3)2(5NCN)] (5) derived from the five-membered NHC complex 3 is unexpectedly higher than that of the pyridine analogue. Computational studies revealed that effective π-back donation to the carbonyl ligand in the NHC-centered pincer complex 5 is retarded by the twist of the pincer plane with six-membered chelation, which allows the d electrons of the metal to spread over the pyrazole π-orbitals.