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 δE1/2(0/1−, 1−/2−) is resolved for 6•• (δE1/2 = 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 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⋮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 Jχ(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 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.