Evaluation of the Oxygen π-Donation in Permethyltitanocene Silanolates and Alcoholates

A series of compounds [Cp* 2 Ti(III)OR′] where R′ is i Pr 3 Si ( 2 ), Ph 3 Si ( 3 ), ( t BuO) 3 Si ( 4 ), ( c -C 5 H 9 ) 7 Si 8 O 12 ( 5 ), and t Bu ( 6 ) were prepared by protolysis of the titanium−methylene bond in singly tucked-in permethyltitanocene [Cp*Ti(III)(η 5:η 1 -C 5 Me 4 CH 2 )] with the respective silanols or tert -butanol. Their electronic transitions from the ground-state molecules to their first excited states (dominantly a 1a 1 → b 2 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 1a 1 → b 2 transition for optimized structures of 2, 4, 6, [Cp* 2 Ti(III)OH] ( 7 ), and [Cp* 2 Ti(III)OMe] ( 8 ) reproduced the dependence of experimental λ(1a 1 → b 2 ) 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 exp (1a 1 → b 2 ) in the order of substituents R′, Me < t Bu < H < Ph < Si i Pr 3 < SiPh 3 ∼ Si(O t Bu) 3 < ( c -C 5 H 9 ) 7 Si 8 O 12 (SIPOSS cluster), thus indicates the decrease of Ti−O π-interaction with an increased polarity of the Ti−O bond. The DFT calculations of 7 with the naturally bent and collinear Ti−O−H conformation showed only a small effect of bending on the oxygen π-donation.