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.
The magnetization of stabilized cobalt colloids in tetrahydrofuran has been studied by a superconducting quantum interference device (SQUID) and magnetic balance measurements in dependence of applied magnetic field and temperature. The colloids are generated by a newly developed electrochemical method which allows one to generate clusters containing about 1000 atoms with a narrow size distribution. The final size distribution of the clusters is examined by high resolution transmission electron microscopy and small angle x-ray scattering. The magnetization curves have been determined with special emphasis on changes at the freezing point of the solution. The curves of the liquid phase can be reasonably described by the Langevin function and the magnetic moments of isolated cobalt clusters that have been recently measured by Stern–Gerlach experiments. Deviations that appear at the freezing point can be understood in terms of magnetic anisotropy effects. It is shown that the cluster sizes and the susceptibilities of the dispersions are related. Therefore the growth of the clusters during the electrolysis can be directly observed by measuring the susceptibility in dependence of the charge transport in the cell.
Lithiation of trovacene, (η7-C7H7)V(η5-C5H5), Li/I exchange, and Pd(dppf)Cl2-catalyzed
homocoupling affords [5-5]bitrovacene 5
2•. Intermetallic communication manifests itself in
typical biradical features of the EPR spectrum and in the resolution of redox splittings of
147 mV for the two subsequent oxidations and 224 mV for the reductions, respectively. 5
2•
adopts a trans conformation in the single crystal (X-ray diffraction) as well as in rigid solution
(EPR spectroscopy). A magnetic susceptibility study of 5
2• reveals weak antiferromagnetic
coupling (J = −2.78 cm-1) of the two V(d5) centers.
The crystal structures of the thioether complexes [MCl4(SEt2)2] with M = Mo and W have been solved by X-ray methods. Both compounds crystallize isotypically in the triclinic space group P1̄ with two formula units per cell unit. The metal atoms are octahedrally coordinated by four chlorine atoms and by the two sulfur atoms of the thioether molecules in transposition (symmetry Ci) with bond lengths (average): Mo-Cl 233.1, Mo-S 253.4, W-Cl 233.1, and W-S 251.7 pm. Both complexes were also characterized by 1H and 13C NMR spectroscopy as well as by measurement of the magnetic susceptibilities in the temperature range from 1.8 to 350 K.
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.
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