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 formatio
n of (NBzEt(3))(2)[V5O9Cl(O(2)CPh)(4)] 4a; a similar reaction employin
g (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 f
ollowing unit cell dimensions at -148 degrees C: a = 13.863(13) Angstr
om, b = 34.009(43) Angstrom, c = 12.773(11) Angstrom, and Z = 4. The r
eaction between (NEt(4))(2)[VOBr4] (2a) and NaO(2)CPh under similar co
nditions gives (NEt(4))(2)[V5O9Br(O(2)CPh)(4)] (6a), and the use of (P
Ph(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 fol
lowing 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 symme
try. 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 conditio
ns gives (NEt(4))(2)[V4O8(NO3)(tca)(4)] (7). Complex 7 . H2O crystalli
zes in space group C2/c with the following unit cell dimensions at -17
0 degrees C: a = 23.606(4) Angstrom, b = 15.211(3) Angstrom, c = 23.99
9(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 u
nit, 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 elec
tronically delocalized system. Variable-temperature magnetic susceptib
ility 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 slig
ht decrease to similar to 4.2 mu(B) at 2.00 K. The data were fit to th
e theoretical expression for a V-4(IV) square with two exchange parame
ters J = J(cis) and J' = J(trans) ((H) over cap = -2JS(i)S(j)); fittin
g 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 fie
ld 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 te
mperature-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 ar
e consistent with retention of their pentanuclear structure or dissolu
tion. The EPR spectrum of 7 in a toluene/MeCN (1:2) solution at simila
r 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.