Misdirected π-donor ligands:: (η5-C5H5)2Zr(Cl)(SR) with sterically more demanding R groups have lower rotational barriers

Rotational barriers about the M-S bonds of 16-electron bent metallocene monothiolates (eta(5)-C5H5)(2)Zr(Cl)(SR) (R =-CH3, -CH2CH3, -CH(CH3)(2), -C(CH3)(3)) (1a-d) have been measured by dynamic H-1 NMR methods: 32, 33, 35 and 26 kJ mol(-1), respectively. The ground-state orientation about the Zr-S bonds of 1 that maximizes Sp pi --> Md pi bonding (Cl-Zr-S-R approximate to 90 degrees) also maximizes Cp <-> R steric interaction, whereas the rotational transition-state orientation (Cl-Zr-S-R approximate to 0 degrees) is one that minimizes Sp pi --> Md pi bonding and maximizes Cl <-> R steric interaction. Deviation from a ground-state orientation that is ideal for Sp pi --> Md pi bonding might be expected as the size of the R group and Cp <-> R steric interaction increases. Thus, the aberrant trend for the R = -C(CH3)(3) derivative could be attributed to a ground-state steric effect where the sterically demanding -C(CH3)(3) group forces an unfavorable (misdirected) orientation for Md pi-Sp pi bonding, but a favorable orientation with respect to Cp <-> R and Cl <-> R steric interactions. However, the solid-state structures of (eta(5)-C5H5)(2)Zr(SR)(2) (R=-CH3, -CH2CH3, -CH(CH3)(2), -C(CH3)(3)) (2a-d) exhibit regular variation of their metric parameters as evidenced by their Zr-S-C bond angles of 108, 109, 113, and 124 degrees and S-Zr-S' bond angles of 97, 99, 100 and 106 degrees, respectively. Neither the S'-Zr-S-R torsion angles nor the dihedral angles that describe the relationship between the S/Zr/S' and Cp( centroid) /Zr/Cp'(centroid) planes (both indicators of the relative orientation of the Zr d pi acceptor orbital and the thiolate S p pi donor orbital) reflect the steric demand of the R group. Thus, the size of the R group imposes a measured effect on the geometry of 2 and the tert-butyl group is not extraordinary. Although the enthalpic and entropic effects could not be deconvoluted for rotation about the Zr-S bond of 1 in the present study, literature precedents suggest that both enthalpic and entropic effects may play a role in determining the irregular trend that is observed. (C) 1998 Elsevier Science S.A.