Synthesis, crystal structure and spin exchange coupling in polynuclear carboxylates with {Li 2 (VO) 2 } metal core

Crystals of binuclear complex [(VO) 2 ( µ -OH)( µ -Piv) 2 (bpy) 2 ](OTf) ( 1 ) have been obtained by the reaction of [VO(H 2 O) 5 ](OTf) 2 (OTf = CF 3 SO 3 − , Piv = (CH 3 ) 3 CCO 2 − ) with 2,2′-bipyridine (bpy) and lithium trimethylacetate (pivalate), Li(Piv), in a ratio of 1:1:2 in MeCN. Change of the reagent ratio to 1:1:3 resulted in the bimetallic compound [Li 2 (VO) 2 ( µ -Piv) 6 (bpy) 2 ] ( 2 ), which was isolated in the form of 2 ·MeCN and 2 ·CH 2 Cl 2 solvates. Replacement of Li(Piv) with lithium trifluoroacetate Li(Tfac) (Tfac = CF 3 CO 2 − ) results in the bimetallic compound [Li 2 (VO) 2 ( µ -Tfac) 6 (bpy) 2 ] ( 3 ). A similar reaction with lithium trichloroacetate, Li(Tcac) (Tcac = CCl 3 CO 2 − ), in ethanol gives the mononuclear complex [VO(Tcac)(bpy) 2 ](OTf) ( 4 ). The structures of 1 – 4 were determined by single crystal X-ray diffraction. The magnetic properties (2–300 K) and EPR spectra of compounds 1 – 3 have been studied. Using the ESEEM (electron spin echo envelope modulation) method it has been shown that the mononuclear cation [VO(Tcac)(bpy) 2 ] + of complex 4 has partial spin density transfer from the V IV atom to donor ligands (in particular, the bpy nitrogen atoms). According to EPR spectroscopy data and quantum-chemical calculations, a similar spin density distribution is observed for 2 and 3 , though spin exchange in these compounds is hindered by lithium ions, which contain no spin density and block electron density delocalization in the bridging system between the “magnetic” vanadium ions. Electrochemical properties of compounds 2 and 3 confirm the vanadium(IV) oxidation state.