(6,6'-Dimethoxybiphenyl-2,2'-diyl)bis(diphenylphosphine) (MeO-BIPHEP) as a six-electron donor in [Ru(eta(5)-C8H11)(MeO-BIPHEP)](+) cations. Coordination of a biaryl double bond, as shown by C-13 NMR and X-ray crystallography

The reaction of the MeO-BIPHEP complex Ru(OAc)(2)(1a) (1a = (6,6'-dimethorrybiphenyl-2,2'-diyl)bis(bis(3,5-di-tert-butylphenyl)phosphine), with HBF4 and 1,5-COD affords [RU(eta(5)-C8H11)(1a)]BF4 (4), in which la functions as a 6e donor to Ru(II) via an unexpected coordination of one of the biaryl double bonds. The isopropyl analog [RU(eta(5)-C8H11)(1b)]CF3-CO2 (6; 1b = (6,6'-dimethoxybiphenyl-2,2'-diyl)bis(diisopropylphosphine)) was prepared by starting from [Ru(CF3CO2)(2)(1,5-COD)](2) and reveals the same eta(4)-bonding mode. Both complexes were characterized by detailed multidimensional NMR studies, and the X-ray structure for 6 is reported. Although the P-31 chemical shifts for this new eta(4)-bonding mode are informative, the C-13 resonance positions for the coordinated biaryl carbons are a more reliable criterion for recognizing this type of interaction. These chemical shift data are difficult to obtain using routine C-13 measurements, and a long-range C-13,H-1-correlation is recommended as the method of choice. Complex 4 exhibits dynamic behavior in solution, as shown by 2-D NOESY. This exchange process can be rationalized by assuming that the doubt bond dissociates; however, complex 6 does not show an analogous exchange process at ambient temperature.