Configurational Isomerization of Dinuclear Iridium and Rhodium Complexes with a Series of NPPN Ligands, 2-PyCH2(Ph)P(CH2)nP(Ph)CH2-2-Py (Py = Pyridyl, n = 2–4)

Abstract

New heterodonor NPPN tetradentate ligands, 2-PyCH2(Ph)P(CH2)nP(Ph)CH2-2-Py (meso- and rac-Ln; n = 2–4, Py = pyridyl), were prepared and reacted with [Cp*MCl2]2 (M = Ir, Rh; Cp* is pentamethylcyclopentadienyl) in the presence of NH4BF4 to afford a series of dinuclear complexes [(Cp*MCl)2(meso-Ln)](BF4)2 (M = Ir, n = 2 (2a), 3 (3a), 4 (4a); M = Rh, n = 2 (2c), 3 (3c), 4 (4c)) and [(Cp*MCl)2(rac-Ln)](BF4)2 (M = Ir, n = 2 (2b), 3 (3b), 4 (4b); M = Rh, n = 2 (2d), 3 (3d), 4 (4d)), which were characterized by IR, 1H and 31P{1H} NMR, and ESI mass spectroscopic techniques and X-ray crystallography. The configurations around the two metal centers were controlled by the configuration of the coordinated P atoms so as to avoid repulsive interaction between the phenyl group on P and the chloride ligand, resulting in the formation of stereospecific isomers; a meso configuration of the metal centers is induced from meso-Ln (abbreviated as meso-P2/meso-M2), and in contrast, a rac configuration is induced from rac-Ln (rac-P2/rac-M2). Furthermore, inversion of metal centers for the Ir2 complexes occurred in DMSO at higher temperatures (60–100 °C), generating equilibrium mixtures of minor diastereomers (meso-P2/rac-M2 or rac-P2/meso-M2) in low ratios together with the major isomers (meso-P2/meso-M2 or rac-P2/rac-M2). The equilibrium constants, K = [minor isomer]/[major isomer], varied appreciably depending on the lengths of the methylene chains as well as configurations of the NPPN ligands; the overall propensity for the K values was observed to be L2 < L3 < L4 and meso-Ln < rac-Ln, while rac-L3, rac-L4, and meso-L4 showed almost identical equilibrium constants, presumably resulting from no steric influence between the two metal centers.