Evaluation of the oxygen π-donation in permethyltitanocene silanolates and alcoholates

A series of compounds [Cp*Ti(III)OR] where R is PrSi (2), PhSi (3), (BuO) Si (4), (c-CH)Si o (5), and Bu (6) were prepared by protolysis of the titanium-methylene bond in singly tucked-in permethyltitanocene [Cp*Ti(III)(η:η-C MeCH)] with the respective silanols or terf-butanol. Their electronic transitions from the ground-state molecules to their first excited states (dominantly a lai - b transition) occur in the range 1300-1800 nm, originating from π-donation from oxygen lone pair electrons to the Ti-O bond (as found by Andersen et al. J. Am. Chem. Soc. 1996, 118, 1719). The X-ray crystal structures of 2-4 and 6 revealed that steric effects of substituents R change the geometry of the titanocene moiety only negligibly. DFT calculations of the lai - b transition for optimized structures of 2, 4, 6, [Cp*Ti(III)OH] (7), and [Cp*Ti(III)OMe] (8) reproduced the dependence of experimental λ(lai → b) on electron donation/attraction properties of R and revealed that the decrease of the oxygen π-donation is accompanied with an increase in negative natural charge on the OR group. The observed increase of λ(lai → b) in the order of substituents R, Me