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†

Abstract

The new complexes [(µ₄,η⁴-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 (λ_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 λ_max = 1245 nm (ε = 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 = ½ entities.