SYNTHESIS, CHARACTERIZATION, AND CRYSTAL-STRUCTURE OF A VANADIUM(V)-VANADIUM(IV)-VANADIUM(V) MIXED-VALENCE TRINUCLEAR COMPLEX WITH A TRIDENTATE QUINATO LIGAND, (NH4)2([V-V(O)2](2)[V-IV(O)](MU-(-)-QUINATO(3-))(2)).H2O

The synthesis, characterization, and crystal structure of the first mixed-valence V(V)-V(IV)-V(V) trinuclear complex is described. Crystals of (NH4)(2){[V-V(O)(2)](2)[V-IV(O)](mu-(-)-quinato(3-))(2)}.H2O are monoclinic, space group P2(1), a = 9.837(3) Angstrom, b = 8.180(3) Angstrom, c = 14.872(3) Angstrom, beta = 99.65(3)degrees, Z = 2, and R = 0.022 (678 F). Each trianion of quinic acid (1(R),3(R),4(R),5(R)-tetrahydroxycyclohexanecarboxylic acid) coordinates to all three vanadium ions. The ligand acts as a 2-hydroxylate chelate toward one V(V) ion and as a monodentate alcoholate toward the other V(V) ion. The alcoholate donors form a 1,3-diolato chelate and act as bridging groups between the central V(IV) ion and the outer V(V) ions. The geometry about all of the five-coordinate vanadium ions tends more toward an idealized square pyramid than toward a trigonal bipyramid, with the central V(IV) ion [V(O)(OR')(2)(OR'')(2)](2-), having the most square-pyramidal character. The greater degree of trigonal character found in the coordination sphere of the V(V) centers [V(O)(2)(O(2)CR)(OR')(OR'')](2-), compared to the V(IV) center, approximates the geometry of the active site of V(V) inhibitor complexes of enzymes, including ATPases and ribonucleases. The V=O bond lengths range between 1.611 and 1.624 Angstrom. The bridging V(IV)-O(alcoholate) bond lengths range between 1.930 and 1.959 Angstrom and are significantly shorter than the bridging V(V)-O(alcoholate) bond lengths, ranging between 1.984 and 2.041 Angstrom, probably due to the steric constraints of the ligand. This is indicated by the much shorter terminal V(V)-O(carboxylate) bonds at 1.957 and 1.982 Angstrom. The EPR spectrum (g(iso) = 1.9725, A(iso) = 99.3 x 10(-4) cm(-1)) is typical of a localized V(IV) system showing that the complex is a trapped-valence system. This is supported by a bond valence sum analysis (BVSA). The electronic spectrum shows a weak broad transition between 900 and 1000 nm, which is assigned to the intervalence transition of the complex.