New sandwich complexes containing 6,6-dimethylcyclohexadienyl (dmCh): (dmCh)2M (M = Ti, V, Cr, Fe) and (dmCh)2M-CO (M = Ti, V)

Article abstract of DOI:10.1021/om00152a020

Deprotonation of 5,5-dimethylcyclohexadiene ((dmCh)H) with KO-t-Bu/n-BuLi afforded potassium 6,6-dimethylcyclohexadienyl (K(dmCh)). Treatment of MCl_n(THF)_m with n equiv of K(dmCh) afforded the new sandwich complexes (dmCh)₂M (M = Ti (1), V (2), Cr (3), Fe (5)) in moderate yield (35-68%). The Ti and Fe complexes are diamagnetic, while 2 and 3 possess one and two unpaired electrons. When exposed to CO, 1 and 2 formed the monocarbonyl adducts (dmCh)₂M-CO (M = Ti (6), V (7)); EPR spectra of thermally unstable (dmCh)₂V-PMe₃ (8) and (dmCh)₂V-PF₃ (10) and NMR evidence for (dmCh)₂Ti-PF₃ (9) were also obtained. While Ch₂Fe (11, Ch = η⁵-C₆H₇) could be prepared from FeCl₂ and LiCh, generated via LDA and 1,4-C₆H₈ (-78°C), this procedure was not general. The isomerization of 11 to (η⁶-C₆H₆)-Fe(η⁴-1,3-C ₆H₈) (12), concomitant with Fe⁰, was also noted. The expected C_2v symmetry of 6 was confirmed via an X-ray crystallographic investigation. Crystal data: monoclinic, P2₁./n, a = 7.372 (1) A?, b = 9.900 (1) A?, c = 22.101 (3) A?, β = 105.100 (8)°, Z = 4, T = 25°C. Standard refinement procedures yielded an R of 0.074 and R_w = 0.076 from 1356 reflections where |F_o| ≥ 3σ(F_o). The structures, bonding, and stability of 1-10 are discussed in relation to Cp and pentadienyl analogues.