SYNTHESES AND STRUCTURES OF THE POTASSIUM-AMMONIUM DIOXOCITRATOVANADATE(V) AND SODIUM OXOCITRATOVANADATE(IV) DIMERS

The two dimers potassium-ammonium dioxocitratovanadate (V), K-2(NH4)(4 )[VO2(cit)](2) . 6H(2)O (1) and sodium oxocitratovanadate (IV), Na-4[V O(cit)](2) . 6H(2)O (2) have been prepared by the reactions of citric acid and metavanadate in neutral solution. Complex 1 crystallizes in t he triclinic space group P ($) over bar 1 with unit cell parameters: a = 8.894(1), b = 9.783(1), c = 9.930(2) Angstrom, alpha = 70.00(1), be ta = 88.09(1), gamma = 68.42(1)degrees, V = 750.8 Angstrom(3), Z = 1, R = 0.035 for 2622 observed reflections. The dimeric anion contains a centrosymmetric planar four-member V2O2 ring with the bridging hydroxy l oxygens. The citrate ion is coordinated via oxygen atoms of the hydr oxyl and alpha-carboxylato groups, and the two acetato branches are no t coordinated to vanadium. The principal V-O dimensions are: V-O(hydro xy), 1.961(2), 2.005(2) Angstrom; V-O(alpha-carboxy), 1.981(3) Angstro m. Complex 2 crystallizes in the monoclinic space group P2(1)/n with u nit cell parameters: a = 10.120(2), b = 10.822(4), c = 11.934(4) Angst rom, beta = 111.57(2)degrees, V = 1215.4 Angstrom(3), Z = 2, R = 0.035 for 2255 observed reflections. The dimer contains a similar planar V2 O2 ring with bridging hydroxyl oxygens. The tetradentate citrate ligan ds coordinate via hydroxyl and alpha-carboxylato oxygens to one vanadi um, and via two acetato branches to the two vanadiums in the dimer. Th e principal V-O dimensions are: V-O(hydroxy), 1.971(2), 2.206(2) A; V- O(alpha-carboxy), 2.038(2) Angstrom: V-O(beta-carboxy), 2.017(2), 2.03 2(2) Angstrom. The coordination number of the vanadium ions in complex 1 and complex 2 is therefore five and six, respectively.