Reactions of oxygen containing ligands in the coordination sphere of permethylhafnocene and permethyltantalocene

Abstract

A series of terl-butylperoxide complexes of hafnium, Cp* 2 Hf(R)(OOCMe 3 ) (Cp* = ((η 5 -C 5 Me 5 ); R = Cl, H, CH 3 , CH 2 CH 3 , CH 2 CH 2 CH 3 , CH 2 CH 2 CH 2 CH 3 , CH 2 CHMe 2 , CH=CHCMe 3 , C 6 H 5 , meta-C 6 H 3 (CH 2 )2) and Cp*(η 5 -C 5 (CH 3 ) 4 CH 2 CH 2 CH 2 )Hf(OOCMe 3 ), has been synthesized. One example has been structurally characterized, Cp* 2 Hf(OOCMe 3 )CH 2 CH 3 crystallizes in space group P2 1 /c, with a = 19.890(7)A, b = 8.746(4)A, c = 17.532(6)A, β = 124.987(24)°, V = 2498(2)A 3 , Z = 4 and R F = 0.054 (2222 reflections, I > 0). Despite the coordinative unsaturation of the hafnium center, the terl-butylperoxide ligand is coordinated in a mono-dentate ligand. The mode of decomposition of these species is highly dependent on the substituent R. For R = H, CH 2 CH 3 , CH 2 CH 2 CH 3 , CH 2 CH 2 CH 2 CH 3 , CH 2 CHMe 2 a clean first order conversion to Cp* 2 Hf(OCMe 3 )(OR) is observed (for R CH 2 CH 3 , ΔHǂ = 19.6 kcal•mol -1 , ΔSǂ = -13 e.u.). These results are discussed in terms of a two step mechanism involving η 2 -coordination of the terl-butylperoxide ligand. Homolytic O-O bond cleavage is observed upon heating of Cp* 2 Hf(OOCMe 3 ) R (R = C 6 H 6 , meta-C 6 H 3 (CH 3 ) 2 ). In the presence of excess 9,10-dihydroanthracene thermolysis of Cp* 2 Hf(OOCMe 3 )C 6 H 6 cleanly affords Cp* 2 Hf(C 6 H 6 )OH and HOCMe 3 (ΔHǂ = 22.6 kcal•mol -1 , ΔSǂ = -9 e.u.). The O-O bond strength in these complexes is thus estimated to be 22 kcal•mol -1 . Cp* 2 Ta(CH 2 )H, Cp* 2 Ta(CHC 6 H 5 )H, Cp* 2 Ta(C 6 H 4 )H, Cp* 2 Ta(CH 2 =CH 2 )H and Cp* 2 Ta(CH 2 =CHMe)H react, presumably through Cp* 2 Ta-R intermediates, with H 2 O to give Cp* 2 Ta(O)H and alkane. Cp* 2 Ta(O)H was structurally characterized: space group P2 1 /n, a= 13.073(3)A, b = 19.337(4)A, c = 16.002(3)A, β = 108.66(2)°, V = 3832(1)A 3 , Z = 8 and R F = 0.0672 (6730 reflections). Reaction of terlbutylhydroperoxide with these same starting materials ultimately yields Cp* 2 Ta(O)R and HOCMe 3 . Cp* 2 Ta(CH 2 =CHR)OH species are proposed as intermediates in the olefin hydride reactions. Cp* 2 Ta(O 2 )R species can be generated from the reaction of the same starting materials and O 2 . Lewis acids have been shown to promote oxygen insertion in these complexes.