Outer sphere anion participation can modify the mechanism for conformer interconversion in Pd pincer complexes

Abstract

Interconversion of the two chiral conformations of the square planar Pd(II) CCC pincer carbene complex, 1 (η3-C,C′,C″) (2,6-bis{[N-methyl-N′-methylene]imidazol-2-ylidene}phenyl)bromopalladium(II), and the CNC cation, 2, (η3-C,C′,N)(2,6-bis{[N-methyl-N′-methylene]imidazol-2-ylidene}pyridine)bromopalladium(II)(1+), is characterized by VT NMR spectroscopy. Combined DFT/experimental work indicates two alternative mechanisms. In the case of 1, having no counterion, and several derivatives of 2 with weakly nucleophilic counterions, the fluxional process goes in two steps via an unsymmetrical cationic 4-coordinate intermediate. In this case one carbene ring moves through the square plane before the other. In some cases for 2 with more nucleophilic counterions, such as [{CNC}PdI]I, a second lower-barrier process takes over that depends on the nature of the counterion. We propose that the outer sphere anion reversibly displaces the central N (pyridine) unit of the pincer in a rate limiting step to form a neutral dihalo intermediate that undergoes rapid conformer interconversion. This accounts for the counterion dependence and constitutes an unusual type of fluxionality that couples anion substitution at the metal with the conformational change of the ligand. A pyridine, even when present as the central element of a CNC pincer ligand, can therefore be labile even under mild conditions and reaction mechanisms involving decoordination of such group are therefore possible.