Synthesis and characterization of the cycloheptatrienyl tantalum "mixed-sandwich" compounds (c5r5)ta(c7h7).

The molecule (cycloheptatrienyl)(cyclopentadienyl)tantalum, (C(5)H(5))Ta(C(7)H(7)) (1), and its methylcyclopentadienyl and pentamethylcyclopentadienyl analogues (C(5)H(4)Me)Ta(C(7)H(7)) (2), and (C(5)Me(5))Ta(C(7)H(7)) (3) have been synthesized by magnesium reduction of the corresponding (C(5)R(5))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 C(7)H(7) ring are significantly shorter (by about 0.1 A) than those to the cyclopentadienyl ring; the difference reflects stronger bonding to the C(7)H(7) ring. A comparison with the structures of other (C(5)R(5))M(C(7)H(7)) 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 delta 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.