Electro- and magnetocommunication in [5,5]ditrovacenyls, [(eta7-C7H7)V(eta5-C5H4-X-eta5-C5H4)V(eta7-C7H7)], mediated by the spacers X = (Z)-CH=CH-, (E)-CH=CH-, > C=CH2, -CH2CH2-, and -CH2-.

Five new paramagnetic dinuclear complexes containing [5]trovacenyl groups, (eta7-C7H7)V(eta5-C5H4-), have been prepared and characterized, including by single-crystal X-ray diffraction. As intervening spacers, ethenediyl units in the geminal and vicinal (Z)- and (E)-bridging modes as well as methanediyl and ethanediyl units have been included with the aim of studying their propensity to transmit electric and magnetic information. It is found that redox splitting of consecutive electron-transfer steps is resolved for reduction (0-->1- -->2-) only, unsaturation of the C2 bridge not being requisite, since the -CH2CH2- spacer also gives rise to a small redox splitting. Magnetic communication is quantified in terms of the exchange coupling constant J, accessible from the EPR hyperfine pattern in solution and from magnetic susceptometry in the solid state. The results obtained from these methods generally differ; this fact is not surprising in view of conformational differences in the respective states of aggregation. It is concluded that orientation-dependent mechanisms of spin-spin interactions (pi-orbital overlap, hyperconjugation) contribute extensively although, as implied by sizeable J values for -CH2- and -C2H4- linked di[5]trovacenyl groups, coupling mediated by the sigma-orbital chain must also be considered.

Di([5]trovacenyl)ethyne, di([5]trovacenyl)butadiyne, and di-1,4-([5]trovacenylethynyl)benzene: electrocommunication and magnetocommunication mediated by --C triple bond C--, --C triple bond C--C triple bond C-- and --C triple bond C--C6H4--C triple bond C-- spacers.

The synthesis of dinuclear derivatives of trovacene (eta7-C7H7)V(eta5-C5H5) (1.) is reported, in which ethynyl (6..), butadiynyl (7..), and 1,4-di(ethynyl)phenyl (8..) groups serve as spacers between paramagnetic (S = 1/2) [5]trovacenyl units. The mononuclear precursors [5]trovacenylcarbaldehyde (2.) and [5]trovacenylacetylene (4.) are also described. Structural characterization by X-ray diffraction has been performed for 4., 6.., 7.., and 8.. Electronic communication as gleaned from cyclic voltammetry only manifests itself in the reduction processes where redox splitting deltaE1/2(0/ 1-, 1-/2-) is resolved for 6.. (deltaE1/2 = 150 mV) and indicated for 7.. (deltaE1/2 < or approximately 80 mV). Magnetocommunication leads to exchange coupling of the two electron spins which reside in vanadium centered orbitals. The values JEPR(6..) = (-)0.92, JEPR(7..) = (-)0.56, and JEPR(8..) = (-)0.005 cm(-1) are derived from the 51V hyperfine patterns. Accordingly, attenuation of exchange interaction by oligoalkyne spacers is weak, corresponding to a factor of 0.6 only per added --C triple bond C-- unit. In the determination of very weak long distance exchange interactions, EPR excels because of the range 5 x 10(-4) < or approximately J < or approximately 1.5 cm(-1) accessible in the case of 51V as a reporting magnetic nucleus and because competing intermolecular exchange is quenched in dilute fluid solution. This is demonstrated by the value Jchi(7..) = -3.84 cm(-1) obtained from a magnetic susceptibility study, which exceeds JEPR(7..) by a factor of 7. The small magnitude of spin exchange interaction between trovacene units reflects the fact that the spin bearing V3dz2 orbital is virtually orthogonal to the pi-perimeter ligand orbitals and weakly overlapping only with the a1g(sigma) ring orbitals, creating two bottlenecks for spin-exchange in the spacer-containing ditrovacenes.

Increasing the ketone selectivity of the cobalt-catalyzed radical chain oxidation of cyclohexane.

A variety of heterogeneous catalysts for the radical chain oxidation of cyclohexane has been prepared by immobilization of the well-defined cobalt acetate oligomers [py(3)Co(3)(mu(3)-O)(OH)(O(2)CCH(3))(5)](PF(6)) (1) and [py(4)Co(2)(OH)(2)(O(2)CCH(3))(3)](PF(6)) (2) on carboxy-modified mesoporous silica supports A-D by carboxylate exchange. The catalytic oxidation of cyclohexane with tert-butyl hydroperoxide (TBHP) in the presence of these homogeneous and immobilized cobalt acetate complexes afforded the corresponding alcohol and ketone in high yield. The immobilization of 1 and 2 results in a significant increase of catalytic activity. TBHP acts as a radical initiator and as source of molecular oxygen, which is also involved in the overall oxidation process. The rate of cyclohexane conversion is limited by the diffusion of molecular oxygen, and steady-state concentrations of cyclohexanone (K, ketone) and cyclohexanol (A, alcohol) are established; these determine the maximum K:A ratio.

Heterogeneous Dinuclear Rhodium(II) Hydroformylation Catalysts-Performance Evaluation and Silsesquioxane-Based Chemical Modeling.

Supported, air stable, and reusable hydroformylation catalysts have been prepared by immobilizing dinuclear rhodium(II) complexes bearing ortho-metalated arylphosphane ligands on amorphous silica and mesoporous MCM-41 supports by phosphane tethers. The oligosilsesquioxane model complex of the catalytic site 1 has been prepared analogously and characterized by single-crystal X-ray diffraction analysis.