COOPERATIVE EFFECTS IN PI-LIGAND-BRIDGED DINUCLEAR COMPLEXES .11. 3 DIFFERENT POSITIONAL ISOMERS OF PARAMAGNETIC (MU-[ETA-7-7-BI(CYCLOOCTATRIENYL)]BIS[ETA-5-CYCLOPENTADIENYL)VANADIUM] - STRONG INFLUENCE ON MAGNETIC-PROPERTIES CAUSED BY THE DIFFERENT KINDS OF LINKAGES

Attempts to prepare the mononuclear sandwich complex CpVCot or its anion [CpVCot]- (Cp = cyclopentadienyl; Cot = cyclooctatetraene) reveal one red (2) and two green (3, 5) paramagnetic dinuclear complexes of the general composition [CpV(C8H8)]2. X-ray structures of the red isomer 2 (space group P2(1)/c, with a = 8.2797 (6) angstrom, b = 7.9257 (5) angstrom, c = 14.3273 (9) angstrom, beta = 93.416 (7)-degrees, V = 989.33 angstrom 3, Z = 2, and R = 0.035) and the green isomer 5 (space group C2/c, with a = 21.7603 (12) angstrom, b = 7.7366 (6) angstrom, c = 12.3510 (9) angstrom, beta = 92.434 (6)-degrees, V = 2077.42 angstrom 3, Z = 4, and R = 0.038) establish different linkages between the C8 rings of two mononuclear CpV(C8H8) sandwich units: via two metal attached carbon atoms for the red isomer 2 and via an sp3-sp3 carbon-carbon single bond endo-positioned with respect to the metal centers for the green isomer 5. The linkage in the third but thermally unstable isomer 3, which isomerizes to the red one, is explained by an sp3-sp3 carbon-carbon single bond in the exo position relative to the metal centers. Magnetic susceptibility measurements and ESR spectra manifest the importance of the linkage between the two mononuclear subunits: (i) the exchange interaction in the red compound 2 (J = -62 cm-1) is enhanced by 1 order of magnitude relative to the green endo isomer 5 (J = -5 cm-1); (ii) fluid-solution ESR spectra show highly resolved V-51-hyperfine structures for the green isomers 3 and 5 whereas a corresponding solution ESR spectrum for 2 fails; (iii) from the solid-solution ESR spectrum of 2 a zero-field-splitting parameter D can be estimated, which is much larger than that calculated from the distance of the paramagnetic centers taking into account the point dipole approximation.