BIS(CYCLOOCTATETRAENE)VANADIUM - X-RAY STRUCTURE AND STUDY OF MOLECULAR MOTIONS BY EPR AND ENDOR SPECTROSCOPY IN FROZEN SOLUTION

The paramagnetic V(cot)2 (cot = eta(8)C8H8) was obtained in high yields from VCl3(THF)3 and a mixture of cyclooctatetraene with its potassium salt. The X-ray structure shows that the metal is bound to one ring in an eta(8) and to the other ring in a eta(4) fashion, with two different molecules in the unit cell. The complex crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a = 825.9 (1) pm, b = 1145.6 (2) pm, c = 2560.4 (4) pm, V = 2422.5 angstrom 3, and Z = 8. Refinement of 5392 reflections yielded R = 0.0388 and R(w) = 0.0364. In frozen solution, EPR parameters show that the unpaired electron resides in a nonbonding orbital having mainly a d(z)2 character. Proton hyperfine interactions were determined by ENDOR for the different types of (eta(8)-C8H8) and (eta(4)-C8H8) ring protons, in agreement with the bonding scheme revealed by the crystal structure. Variable-temperature ENDOR studies from 9 to 150 K gave information about molecular motion such as ring fluxionality and molecular tumbling in the solid state. It is also shown that upon slow cooling partial alignment of the complex along the magnetic field axis occurs.