NMR study of L-shaped (quinoxaline)platinum(II) complexes - Crystal structure of [Pt(DMeDPQ)(bipy)](PF6)(2)

A H-1 and C-13 NMR study of nine Pt-II complexes of DMeDPQ [6,7-dimethyl-2,3-bis(2-pyridyl)quinoxaline] and BDPQ [2,3-bis(2-pyridyl)benzo[g]quinoxaline], and the crystal structure of one of them, are reported. The results are consistent with C-s symmetry of "L-shaped square-planar complexes". The rigid seven-membered chelated quinoxaline ligand holds the fused aromatic rings nearly perpendicular to the Pt-II coordination plane, generating the peculiar L-shaped structure. Ancillary ligands in the residual coordination sites are: a) bidentate flexible-planar 2,2'-bipyridine (bipy; complexes 1 and 2); b) bidentate rigid-planar dipyrido[3,2-h:2'3'-j]phenazine (dppz) or benzo[b]dipyrido[3,2-h:2',3'-j]phenazine (bdppz; complexes 3-6); or c) 3-substituted monodentate pyridines (3-Rpy; complexes 7-9). The L-shaped geometry has been exploited to gain insight into the steric and dynamic features that regulate the noncovalent interactions of these square-planar complexes with DNA. We have shown previously, for [Pt(bipy)(n-Rpy)(2)](2+), that bipy twisting can be frozen out on the NMR timescale below 260 K. Preservation of the C, symmetry at low temperature indicates a lack of bipy fluxionality within these L-shaped structures. The static butterfly-like symmetric orientation of the quinoxaline pyridyl rings accounts for the hampered twisting of Pt(bipy), which is otherwise assisted by the synchronous "windscreen wiper" conrotatory rocking of the ancillary pyridine rings. The L-geometry can also be used to monitor the ancillary n-Rpy rotation by NMR spectroscopy. The quasi-vertical quinoxaline pyridyl rings alignment leave room in the coordination plane for the crossing of the opposite pyridine rings, thereby reducing their rotational barriers about the Pt-N bond.