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

Abstract

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]...