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

A series of compounds [Cp*Ti-2(III)OR'] where R' is (Pr3Si)-Pr-i (2), Ph3Si (3), ((BuO)-Bu-t)(3)Si (4), (c-C5H9)(7)Si8O12 (5), and Bu-t (6) were prepared by protolysis of the titanium-methylene bond in singly tucked-in permethyltitanocene [Cp*Ti(III)(eta(5):eta(1)-C5Me4CH2)] 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 pi-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*Ti-2(III)OH] (7), and [Cp*Ti-2(III)OMe] (8) reproduced the dependence of experimental lambda(1a(1) -> b(2)) on electron donation/attraction properties of R' and revealed that the decrease of the oxygen T-donation is accompanied with an increase in negative natural charge on the OR' group. The observed increase of lambda(exp)(1a(1) -> b(2)) in the order of substituents R', Me < Bu-t < H < Ph < (SiPr3)-Pr-i < SiPh3 similar to Si((OBu)-Bu-t)(3) < (c-C5H9)(7)Si8O12 (SIPOSS cluster), thus indicates the decrease of Ti-O pi-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 pi-donation.