Interconversion between the Enantiomers of Chiral Five-Coordinate Me3Pt(IV) Complexes

The dinuclear Me2Pt(II) complexes of 3,4-bis(quinolin-8-yl)thiophene (1a), 3,4-bis(6 trifluoromethoxyquinolin-8-yl)thiophene (1b), and 3,4-bis(2-methylquinolin-8-yl)thiophene (1c) react with MeOTf (OTf = trifluoromethanesulfonate) to afford the corresponding chiral mononuclear five-coordinate Me3Pt(IV) complexes [PtMe3(1a)]OTf (3a), [PtMe3(1b)]OTf (3b), and [PtMe3(1c)]OTf (3c), respectively. [PtMe3(1c)]BArF4 (3d) (where BArF4 = [B{C6H3-3,5-(CF3)2}4]) has also been synthesized for structural study. While 3a appears to be symmetric in solution and asymmetric in solid state, 3c and 3d are asymmetric in both solution and solid state. The chirality originates from interligand repulsion, rather than any unsymmetrical ligand. Variable-temperature NMR and computational studies suggest a ligand-twisting isomerization pathway for the interconversion of the enantiomers, rather than the rotational exchange of three CH3 ligands on the metal center.