CHEMISTRY OF MONONUCLEAR AND BINUCLEAR OXIDIC V(V), V(V)V(V), AND V(V)V(IV) AZOPHENOLATES

The aerial reaction of 2,2'-dihydroxyazobenzene (H2L) with bis(acetylacetonato)oxovanadium(IV) in methanol affords binuclear VV2O3L2. In the presence of KOH, KVVO2L.1.5H2O is isolated. Dilute acids convert VO2L- to V2O3L2 via V(V)O(OH)L. The electronic and IR spectra of the complexes are described. In crystalline KVO2L.1.5H2O, the vanadium(V) atom has square pyramidal geometry. The base is formed by coordinated ONO atoms of L2- and an oxo oxygen atom. The metal atom is displaced by approximately 0.5 angstrom from the base toward the apical oxo oxygen atom. The O-V-O angle is > 107-degrees, which is a characteristic feature of VO2+ complexes with pentacoordinated vanadium. The VO2L- units are interlinked into an infinite network by K+ ions (bonded to water, oxo, and phenolic oxygen atoms). In crystalline V2O3L2, two VOL fragments are bridged by an oxygen atom lying on a 2-fold axis. The geometry of the pentacoordinated metal atom is similar to that in VO2L-, the bridging oxygen atom defining the shared comer of the bases of the two metal pyramids. The two terminal oxygen atoms are trans-directed lying on opposite sides of the VOV plane. The V-O(phenolic) length in V2O3L2 is shorter than that in KVO2L.1.5H2O by approximately 0.1 angstrom due to stronger metal-phenol pi-bonding in the former complex. Crystal data for KVO2L.1.5H2O: monoclinic; space group C2/c; a = 33.834(9) angstrom, b = 7.245(2) angstrom, c = 23.708(4) angstrom; beta = 103.56(2)degrees; V = 5649(2) angstrom3; Z = 16; R = 3.64%; R(w) = 4.07%. Crystal data for V2O3L2: orthorhombic; space group Pcnb; a = 7.392(2) angstrom, b = 12.868(2) angstrom, c = 24.371(7) angstrom; V = 2318(1) angstrom3; Z = 4; R = 3.84%, R(w) = 4.38%. In dichloromethane solution this complex displays a one-electron couple, V2O3L2-V2O3L2-, having an E1/2 value of 0.30 V vs SCE. The electrogenerated mixed-valence (V(V)(IV)) complex V2O3L2- shows intervalence bands at 760 and 980 run. At room temperature its solution (1.77 mu(B)) EPR spectrum consists of 15 lines (A = 49.9 G; g = 1.98) corresponding to the delocalization of the unpaired electron over the two equivalent metal atoms. In frozen glass (77 K) the electron is localized at one center (A(parallel-to) = 174.3 G, A(perpendicular-to) = 64.2 G; g(parallel-to) = 1.96; g(perpendicular-to) = 1.98). From variable-temperature EPR spectra the activation energy and rate constant (298 K) for the delocalization process are estimated to be 3.1 kcal mol-1 and 3.2 X 10(10) s-1, respectively.