Tetranuclear and pentanuclear vanadium(IV/V) carboxylate complexes: [V4O8(NO3)(O(2)CR)(4)](2-) and [V(5)O(9)X(O(2)CR)(4)](2-) (X=Cl-,Br-) salts

The syntheses and properties of tetra- and pentanuclear vanadium(IV, V) carboxylate complexes are reported. Reaction of (NBzEt(3))(2)[VOCl4] (1a) with NaO(2)CPh and atmospheric H2O/O-2 in MeCN leads to formation of (NBzEt(3))(2)[V5O9Cl(O(2)CPh)(4)] 4a; a similar reaction employing (NEt(4))(2)[VOCl4] (1b) gives (NEt(4))(2)[V5O9Cl(O-2-CPh)(4)] (4b). Complex 4a . MeCN crystallizes in space group P2(1)2(1)2(1) with the following unit cell dimensions at -148 degrees C: a = 13.863(13) Angstrom, b = 34.009(43) Angstrom, c = 12.773(11) Angstrom, and Z = 4. The reaction between (NEt(4))(2)[VOBr4] (2a) and NaO(2)CPh under similar conditions gives (NEt(4))(2)[V5O9Br(O(2)CPh)(4)] (6a), and the use of (PPh(4))(2)[VOBr4] (2b) likewise gives (PPh(4))(2)[V5O9Br(O(2)CPh)(4)] (6b). Complex 6b crystallizes in space group P2(1)2(1)2(1) with the following unit cell dimensions at -139 degrees C: a = 18.638(3) Angstrom, b = 23.557(4) Angstrom, c = 12.731(2) Angstrom, and Z = 4. The anions of 4a and 6b consist of a V-5 square pyramid with each vertical face bridged by a mu(3)-O2- ion, the basal face bridged by a mu(4)-X(-) (X = Cl, Br) ion, and a terminal, multiply-bonded O2- ion on each metal. The RCO(2)(-) groups bridge each basal edge to give C-4v virtual symmetry. The apical and basal metals are V-V and V-IV, respectively (i.e., the anions are trapped-valence). The reaction of 1b with AgNO3 and Na(tca) (tca = thiophene-2-carboxylate) in MeCN under anaerobic conditions gives (NEt(4))(2)[V4O8(NO3)(tca)(4)] (7). Complex 7 . H2O crystallizes in space group C2/c with the following unit cell dimensions at -170 degrees C: a = 23.606(4) Angstrom, b = 15.211(3) Angstrom, c = 23.999(5) Angstrom, and Z = 4. The anion of 7 is similar to those of 4a and 6b except that the apical [VO] unit is absent, leaving a V-4 square unit, and the mu(4)-X(-) ion is replaced with a mu(4) eta(1)-NO3- ion. The four metal centers are now at the V-IV, 3V(V) oxidation level, but the structure indicates four equivalent V centers, suggesting an electronically delocalized system. Variable-temperature magnetic susceptibility data were collected on powdered samples of 4b, 6a, and 7 in the 2.00-300 K range in a 10 kG applied field. 4b and 6a both show a slow increase in effective magnetic moment (mu(eff)) from similar to 3.6-3.7 mu(B) at 320 K to similar to 4.5-4.6 mu(B) at 11.0 K and then a slight decrease to similar to 4.2 mu(B) at 2.00 K. The data were fit to the theoretical expression for a V-4(IV) square with two exchange parameters J = J(cis) and J' = J(trans) ((H) over cap = -2JS(i)S(j)); fitting of the data gave, in the format 4b/6a, J = +39.7/+46.4 cm(-1), J' = -11.1/-18.2 cm(-1) and g = 1.83/1.90, with the compelxes possessing S-T = 2 ground states. The latter were confirmed by magnetization vs field studies in the 2.00-30.0 K and 0.500-50.0 kG ranges: fitting of the data gave S-T = 2 and D = 0. 00 cm(-1) for both complexes, where D is the axial zero-field splitting parameter. Complex 7 shows a nearly temperature-independent mu(eff) (1.6-2.0 mu(B)) consistent with a single d electron per V-4 unit. The H-1 NMR spectra of 4b and 6a in CD3CN are consistent with retention of their pentanuclear structure or dissolution. The EPR spectrum of 7 in a toluene/MeCN (1:2) solution at similar to 25 degrees C yields an isotropic signal with a 29-line hyperfine pattern assignable to hyperfine interactions with four equivalent I = 7/2 V-51 nuclei.