Hydrogenation of titanocene and zirconocene bis(trimethylsilyl)acetylene complexes

Abstract

Reactions following the addition of dihydrogen under maximum atmospheric pressure to bis(trimethylsilyl)acetylene (BTMSA) complexes of titanocenes, [(η⁵-C₅H_5−nMe_n)₂Ti(η²-BTMSA)] (n = 0, 1, 3, and 4) (1A–1D), and zirconocenes, [(η⁵-C₅H_5−nMe_n)₂Zr(η²-BTMSA)] (n = 2–5) (4A–4D), proceeded in diverse ways and, depending on the metal, afforded different products. The former complexes lost, in all cases, their BTMSA ligand via its hydrogenation to bis-1,2-(trimethylsilyl)ethane when reacted at 80 °C for a prolonged reaction time. For n = 0, 1, and 3, the titanocene species formed in situ dimerised via the formation of fulvalene ligands and two bridging hydride ligands, giving known green dimeric titanocenes (2A–2C). For n = 4, a titanocene hydride [(η⁵-C₅HMe₄)₂TiH] (2D) was formed, similarly to the known [(η⁵-C₅Me₅)₂TiH] (2E) for n = 5; however, in contrast to this example, 2D in the absence of dihydrogen spontaneously dehydrogenated to the known Ti(III)–Ti(III) dehydro-dimer [{Ti(η⁵-C₅HMe₄)(μ-η¹:η⁵-C₅Me₄)}₂] (3B). This complex has now been fully characterised via spectroscopic methods, and was shown through EPR spectroscopy to attain an intramolecular electronic triplet state. The zirconocene-BTMSA complexes 4A–4D reacted uniformly with one hydrogen molecule to give Zr(IV) zirconocene hydride alkenyls, [(η⁵-C₅H_5−nMe_n)₂ZrH{C(SiMe₃)CH(SiMe₃)}] (n = 2–5) (5A–5D). These were identified through their ¹H and ¹³C NMR spectra, which show features typical of an agostically bonded proton, CH(SiMe₃). Compounds 5A–5D formed equilibria with the BTMSA complexes 4A–4D depending on hydrogen pressure and temperature.