A DFT study of Zr-S rotational barriers of (η5-C5H5)2Zr(Cl)(SR); the origin of an inverse steric effect

Abstract

We have previously reported the experimental rotational barriers about the M–S bonds of 16e– bent metallocene monothiolates (η5-C5H5)2Zr(Cl)(SR) (R = CH3, CH2CH3, CH(CH3)2, C(CH3)3] (1a–d): 32, 33, 35, and 26 kJ mol–1, respectively. The ground-state orientation about the Zr–S bonds of 1 that maximizes S(pπ) [Formula: see text] M(dπ) bonding (Cl-Zr-S-R [Formula: see text] 90°) also maximizes Cp « R (Cp = η5-C5H5) steric interaction, whereas the rotational transition-state orientation (Cl-Zr-S-R [Formula: see text] 0°) is one that minimizes S(pπ) [Formula: see text] M(dπ) bonding and maximizes Cl « R steric interaction. Deviation from a ground-state orientation that is ideal for S(pπ ) [Formula: see text] M(dπ ) bonding might be expected as the size of the R-group and Cp « R steric interaction increases. The present study employs hybrid density function computational methods to reproduce the experimental rotational barriers. The computational rotational barriers decrease in the order 1a (R = CH3) > 1b (R = CH2CH3) > 1c (R = CH(CH3)2) at low temperature. However, opposing enthalpic factors reverse this trend at higher temperatures. The aberrant trend (unexpectedly low rotational barrier) that is observed for the 1d (R = C(CH3)3) derivative is due to dominant ground-state steric (enthalpic) effects. We conclude the thiolate ligand of 1d is misdirected in the ground-state with respect to the other thiolate derivatives.Key words: misdirected ligands, metallocene, thiolate, structure, bonding, dynamics.