NMR-STUDIES OF THE STRUCTURES OF P-ARYL-SUBSTITUTED CHIRAL LIGANDS INRHODIUM(I) AND PLATINUM(II) COMPLEXES

The solution-phase stereochemistry of the chelate rings and aryl group arrangement of some p-aryl-substituted DIOP, BDPP, and Chiraphos deri vatives is studied in various complexes by H-1, C-13, and P-31 NMR. Th e complexes are of the general composition [Rh(diene)L2] BF4, PtL2CI2, where L2 = DIOP-(pNMe2)4, la, BDPP-(pNMe2)4,2a, Chirophos-(pNMe2)4,3a ; diene = NBD or COD and [Rh(diene)L2] (BF4)5 where L2 = DIOP-(pNMe3+) 4, lb, DIOP-(pNHMe2+)4, 1c, BDPP-(pNMe3+)4, 2b, BDPP-(pNHMe2+)4, 2c, C hiraphos-(pNMe3+)4, 3b; diene = NBD or COD. The characterization of co mplexes such as [Rh(2b)(CH3CN)2](BF4)5, Ni(3a)(SCN)2, and Pt(2a)Cl(SnC l3) is also included. With the exception of the latter, each complex s hows C2 symmetry; also, with the exception of Pt(1a)Cl2, each complex shows two, different aryl group environments. The similarity in the ar yl resonances of these complexes, the C-13 resonances and proton-proto n couplings in the chelate rings, and variable-temperature measurement s are consistent with the presence of rigid, or highly preferred, chel ate ring conformations. Accordingly, the aryl groups of these complexe s are fixed in a chiral quasi-axial quasi-equatorial arrangement in so lution. The complex, Pt(1a)Cl2 was found to be conformationally labile by NMR. The higher conformational stability of Rh-diene complexes of p-aryl-substituted DIOP derivatives compared to the ones with nonsubst ituted DIOP ligand and to Pt(la)CI2 is attributed to the stabilizing e ffect of the bulky diene ligands and the p-aryl substituents on the ch elate conformation.