Diffusional and Internal Rotation in Single and Linked Trinuclear Bis(carbyne) Cluster Complexes Determined by (13)C NMR Relaxation Time Measurements.

Carbon-13 NMR relaxation times have been measured as a function of temperature for a family of bis(carbyne) cluster complexes: (CpCo)(3)(&mgr;(3)-CPh)(2) (1a), (CpCo)(3)(&mgr;(3)-CH)(&mgr;(3)-CPh) (1b), and X(1,4-C(6)H(4))Y with X = Y = (CpCo)(3)(&mgr;(3)-CPh)(&mgr;(3)-C-) (2); X = (CpCo)(3)(&mgr;(3)-CPh)(&mgr;(3)-C-), Y = -C&tbd1;CPh (3); or X = Y = -C&tbd1;CPh (4). These molecules can be regarded as symmetric top molecules in which the phenyl rings are adjacent to zero, one, or two Cp(3)Co(3) metal cluster units. Molecular rotational diffusion coefficients about the different axes of the molecules (D( perpendicular)and D(s)) obtained from the relaxation times indicate that the motions of the molecules are highly anisotropic. Rapid internal rotation of the phenyl rings described by the rotation rates R contributes significantly to that anisotropy. Phenyl rings adjacent to an alkyne group rotate at a rate comparable to that of free benzene whereas phenyl rings bound to a (CpCo)(3) group through a &mgr;(3)-carbyne ligand show significantly reduced rates of rotation. The observed difference in rotation rates is believed to be due primarily to steric interactions between phenyl rings and their substituents.