Kinetics and mechanism of the stereochemical isomerization of an arene-ruthenium complex of the atropisomeric ligand 1,1′-biphenyl-2,2′-diamine

(eta(6)-Benzene)(delta/lambda-1,1'-biphenyl-2,2'-diamine)chlorometal(II) hexafluorophosphate (1; metal ruthenium, osmium) have been synthesized. The rigid nature of the seven-membered chelate ring formed by the 1,1'-biphenyl-2,2'-diamine (dabp) ligand renders the complexes chiral. The resulting C-1 molecular symmetry of 1 (M=Ru) that we have observed in the solid state by single-crystal X-ray crystallography is preserved in solution on the NMR time scale. The four N-H protons of 1 (M=Ru,Os) are chemically inequivalent in the H-1 NMR spectrum at 20 degrees C. Spin-perturbation NMR experiments in acetone solutions reveal pairwise exchange of the resonances that correspond to the N-H protons on the spin-relaxation time scale. The three mechanisms that would account for such an exchange (atropisomerization of the dabp ligand, inversion of stereochemistry at the metal center, and simultaneous inversion of the stereochemistry at the metal and the ligand) are distinguishable, provided a proper assignment of the four N-H protons can be made in the NMR spectra. Having made that assignment, we conclude from 2D EXSY NMR spectroscopy that the mechanism of exchange is inversion of stereochemistry at the dabp ligand center. This observation contrasts with previous reports that conformational isomers of dabp can be resolved.