Misdirected π-donor ligands: (η-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 (η⁵-C₅H₅)₂Zr(Cl) (SR) (R = −CH₃, −CH₂CH₃, −CH(CH₃)₂, −C(CH₃)₃) (1a–d) have been measured by dynamic ¹H NMR methods: 32, 33, 35 and 26 kJ mol⁻¹, respectively. The ground-state orientation about the Zr-S bonds of 1 that maximizes Spπ → Mdπ bonding (Cl-Zr-S-R ≈ 90°) also maximizes CpR steric interaction, whereas the rotational transition-state orientation (Cl-Zr-S-R ≈ 0°) is one that minimizes Spπ → Mdπ bonding and maximizes ClR steric interaction. Deviation from a ground-state orientation that is ideal for Spπ → Mdπ bonding might be expected as the size of the R group and CpR 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 Mdπ-Spπ bonding, but a favorable orientation with respect to CpR and ClR steric interactions. However, the solid-state structures of (η⁵-C₅H₅)₂Zr(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° and S-Zr-S′ bond angles of 97, 99, 100 and 106°, 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 dπ acceptor orbital and the thiolate S pπ 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.