Ethene Complexes of Bulky Titanocenes, Their Thermolysis, and Their Reactivity toward 2-Butyne

Ethene complexes of titanocenes [Ti­(II)­(η2-C2H4)­(Cp′)2] for Cp′ = η5-C5Me5 (1), η5-C5Me4t-Bu (2), η5-C5Me4SiMe3 (3), and η5-C5HMe4 (4) were prepared by reduction of corresponding titanocene dichlorides with magnesium in THF in the presence of ethene. Thermolysis of 1–3 in toluene solution at a maximum of 100 °C resulted in elimination of ethane, affording cleanly doubly tucked-in titanocene compounds 5–7, respectively. Experiments with 2 and 3 in NMR tubes proved that symmetrical isomers 6a and 7a were formed first, and these thermally isomerized to thermodynamically more stable asymmetrical isomers 6b and 7b. The energy difference between 7a and 7b calculated by DFT methods was 15.3 kJ/mol. Thermolysis of 4 in m-xylene required a temperature of 135 °C, affording a mixture of 8b > 8a and “dimeric dehydro-titanocene” 9 as a concurrent product of hydrogen abstraction. In contrast to thermolysis in solvents, heating of 1 and 2 in high vacuum to 135 °C resulted in sublimation of known titanocenes [Ti­(C5Me5)2] (10) and [Ti­(η5-C5Me4t-Bu)2] (13) (Chirik et al. J. Am. Chem. Soc. 2004, 126, 14688–14689), respectively. The former isomerized in hexane solution to the tucked-in hydride [TiH­{C5Me4(CH2)}­(C5Me5)] (10A) as described by Bercaw (J. Am. Chem. Soc. 2004, 126, 14688–14689). A mixture of 10/10A decayed within days to give major paramagnetic products [TiH­(C5Me5)2] (11) and singly tucked-in titanocene [Ti­{C5Me4(CH2)}­(C5Me5)] (12) and minor diamagnetic 5 and its so far unknown, less stable isomer [Ti­{C5Me4(CH2)}2] (10B), identified by NMR spectra and corroborated by DFT calculations. Solid 3 eliminated ethene at only 80 °C, leaving titanocene 14, whereas compound 4 sublimed at 135 °C mostly without decomposition. Cocrystals of 10 with [TiCl­(C5Me5)2] (1:2) (10C) afforded an X-ray single-crystal structure with linear geometry for 10. The ethene complexes 1–4 differed in their reactivity toward but-2-yne: compounds 1 and 4 yielded the respective [Ti­(IV)­(η1: η1-CH2CH2CMeCMe)­(Cp′)2] 2,3-dimethyltitanacyclopent-2-ene complexes 15 and 16, whereas 2 and 3 replaced ethene with but-2-yne, affording the [Ti­(II)­(η2-MeCCMe)­(Cp′)2] complexes 17 and 18, respectively. Crystal structures of 2, 4, 10C, 15, 17, and 18 have been determined by X-ray crystallography.