(6,6‘-Dimethoxybiphenyl-2,2‘-diyl)bis(diphenylphosphine) (MeO-BIPHEP) as a Six-Electron Donor in [Ru(η⁵-C₈H₁₁)(MeO-BIPHEP)]⁺ Cations. Coordination of a Biaryl Double Bond, As Shown by ¹³C NMR and X-ray Crystallography

The reaction of the MeO-BIPHEP complex Ru(OAc) 2 ( 1a ) ( 1a = (6,6‘-dimethoxybiphenyl-2,2‘-diyl)bis(bis(3,5-di- tert -butylphenyl)phosphine), with HBF 4 and 1,5-COD affords [Ru(η 5 -C 8 H 11 )( 1a )]BF 4 ( 4 ), in which 1a functions as a 6e donor to Ru(II) via an unexpected coordination of one of the biaryl double bonds. The isopropyl analog [Ru(η 5 -C 8 H 11 )( 1b )]CF 3 CO 2 ( 6; 1b = (6,6‘-dimethoxybiphenyl-2,2‘-diyl)bis(diisopropylphosphine)) was prepared by starting from [Ru(CF 3 CO 2 ) 2 (1,5-COD)] 2 and reveals the same η 4 -bonding mode. Both complexes were characterized by detailed multidimensional NMR studies, and the X-ray structure for 6 is reported. Although the 31 P chemical shifts for this new η 4 -bonding mode are informative, the 13 C 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 13 C measurements, and a long-range 13 C, 1 H-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 double bond dissociates; however, complex 6 does not show an analogous exchange process at ambient temperature.