Tetranuclear osmium complexes of tetracyanoquinodimethane [TCNQ, 2,2'-(cyclohexa-2,5-diene-1,4-diylidene)bis(propane-1,3-dinitrile)] and 1,2,4,5-tetracyanobenzene (TCNB). Synthesis, spectroelectrochemistry and magnetism

The new complexes [(mu(4),eta(4)-TCNX) {Os(PR3)(2)(CO)(H)Cl)(4)], R = isopropyl, TCNX = tetracyanoquinodimethane [TCNQ, 2,2'-(cyclohexa-2,5-diene-1,4-diylidene)bis(propane-1,3-dinitrile)] or 1,2,4,5-tetracyanobenzene (TCNB), were studied by spectroelectrochemistry in the UV/VIS/NIR and IR regions and by EPR spectroscopy. Both compounds are reduced in two reversible steps and oxidized in a two-electron process (reversible for TCNQ, quasi-reversible for TCNB). In all oxidation states studied (-, 0,2+) the TCNQ complexes exhibited intense long-wavelength absorptions in the near IR region (lambda(max) >1000 nm). The EPR spectra of the monoanionic forms exhibit hardly any g anisotropy and thus very little metal participation, suggesting an oxidation state formulation [(TCNX.-)(Os-II)(4). The dication of the TCNQ system is formulated as [(TCNQ)(Os-2.5)(4)] with an intervalence; transfer transition at lambda(max) = 1245 nm (epsilon = 50 000 M-1 cm(-1)). In the solid state, the neutral complexes show temperature dependent paramagnetism that could be fitted with a model implying two coupled S = 1/2 entities.