Synthesis and characterization of the cycloheptatrienyl tantalum "mixed-sandwich" compounds (C₅R₅)Ta(C ₇H₇)

The molecule (cycloheptatrienyl)(cyclopentadienyl)tantalum, (C ₅H₅)Ta(C₇H₇) (1), and its methylcyclopentadienyl and pentamethylcyclopentadienyl analogues (C ₅H₄Me)Ta(C₇H7) (2), and (C₅Me ₅)Ta(C₇H₇) (3) have been synthesized by magnesium reduction of the corresponding (C₅R₅)TaCl ₄ species in the presence of cycloheptatriene. The crystal structures of 2 and 3 show that the two rings are planar and essentially parallel to each other. Interestingly, the Ta-C distances to the C₇H₇ ring are significantly shorter (by about 0.1 Å) than those to the cyclopentadienyl ring; the difference reflects stronger bonding to the C ₇H₇ ring. A comparison with the structures of other (C₅R₅)M(C₇H₇) shows that the M-C distances to the seven-membered ring are especially sensitive to the d-orbital energies of the metal center and its ability to engage in δ bonding with the ring. For 1-3, the EPR spectra at room temperature consist of octets due to the tantalum nuclear spin. Both A_iso and g_iso increase as the number of methyl groups on the cyclopentadienyl ring increases. EPR spectra of 1-3 as frozen glasses correspond to axial symmetry, and the hyperfine couplings and g factors are deduced from simulations.