Di([5]trovacenyl)ethyne, Di([5]trovacenyl)butadiyne, and Di-1,4-([5]trovacenylethynyl)benzene:  Electrocommunication and Magnetocommunication Mediated by C⋮C, C⋮CC⋮C and C⋮CC6H4C⋮C Spacers

The synthesis of dinuclear derivatives of trovacene (η7-C7H7)V(η5-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 δE 1/2(0/1−, 1−/2−) is resolved for 6 • • (δE 1/2 = 150 mV) and indicated for 7 • • (δE1/2 ≲ 80 mV). Magnetocommunication leads to exchange coupling of the two electron spins which reside in vanadium centered orbitals. The values J EPR(6 • • ) = (−)0.92, J EPR(7 • • ) = (−)0.56, and J EPR(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⋮C unit. In the determination of very weak long distance exchange interactions, EPR excels because of the range 5 × 10-4 ≲ J ≲ 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 J χ(7 • • ) = −3.84 cm-1 obtained from a magnetic susceptibility study, which exceeds J EPR(7 • • ) by a factor of 7. The small magnitude of spin exchange interaction between trovacene units reflects the fact that the spin bearing V3d z 2 orbital is virtually orthogonal to the π-perimeter ligand orbitals and weakly overlapping only with the a1g(σ) ring orbitals, creating two bottlenecks for spin-exchange in the spacer-containing ditrovacenes.