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

Abstract

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 f 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 f 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 f 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 f 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.

Synthesis and structure of dinuclear dimethylene- or 1,4-phenylene-linked bis(decamethyltitanoceneoxide) (TiIII) complexes

“…The TieO valence vibration can be attributed only a weak absorption band at 522 cm À1 , the only absorption band observed in the 800e450 cm À1 range. The UVenear IR spectrum of 3 displayed in addition to an absorption band at 450 nm and a shoulder at 640 nm a well-discernible absorption band at 1500 nm of very low intensity which can be assigned to the 1a 1 / b 2 transition well-known from electronic absorption spectra of Cp* 2 TiOR compounds [10,12]. This transition which reflects the extent of oxygen p-donation to the TieO bond is quite close to the analogous transition for Cp* 2 TiOPh found at 1528 nm [12].…”

“…As concerns the TieO valence vibration which was assigned to the band at 528 cm À1 for [(Cp* 2 Ti(Ot-Bu))] [10] and 571 cm À1 for [Cp* 2 TiOH] [11f] there is no absorption band at least of weak intensity in the range 500e1000 cm À1 , hence the medium-to-strong intensity band at 496 cm À1 should be attributed to this vibration. Compound 2 was only very slightly soluble in tetrahydrofuran (THF) or 2-methyltetrahydrofuran (MTHF), however, using 1.0 cm cuvette its UVevis spectrum could be identified showing similar features as the spectrum of 3 and similar permethyltitanocene alcoholates or silanolates [10]. The low concentration of 2 did not allow to observe the expected 1a 1 / b 2 transition of very low intensity in nearinfrared region [12], however.…”

“…The bulky alcoholate or silanolate substituents, however, resulted in TieOeC or TieOeSi angles close to 174 (AE2 ) [10] whereas a smaller steric hindrance between the links and the C 5 Me 5 ligands in 2 and 3 allowed for the TieOeC angle close to 167 (see Table 1). The sp 3 dimethylene carbon atoms in 2 form longer bonds to the oxygen atoms than sp 2 carbon atoms of 3 (see Table 1) that is in agreement with the lower frequency of infrared n (CeO) vibration (1127 cm À1 versus 1256 cm À1 ).…”

“…The mononuclear one with the rhombic g-tensor (g 1 ¼ 2.000, g 2 ¼ 1.982, g 3 ¼ 1.956, g av ¼ 1.979) which is typical of Cp* 2 TiOR compounds [10,12], and dinuclear one showing features of triplet state spectra. The evidence for its presence is given by outer resonances (denoted z,z) whose separation (DH zz ) determines a two-fold value of zero-field splitting (D) [16].…”

“…We have recently prepared a number of Ti(III) siloxy and alkoxy permethyltitanocenes [(h 5 -C 5 Me 5 ) 2 Ti(OR)] (R ¼ Si(OtBu) 3 , SiPh 3 , Si i Pr 3 , t Bu) [10] using a straightforward protonolysis of the TieC bond in the singly tucked-in permethyltitanocene [Ti(III)(h 5 -C 5 Me 5 )(h 5 :h 1 -C 5 Me 4 CH 2 )] (1) [11] with silanols or alcohols. Here, we extend our previous investigations by reacting 1 with two different diols, ethylene glycol and hydroquinone, respectively, with the aim to study crystal structures of the obtained dinuclear titanocene(TiIII) complexes and electron spinespin coupling therein as observed by EPR spectroscopy.…”