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Synthesis, structure and magnetic properties of an asymmetric dinuclear oxocitratovanadate(IV) complex
ELSEVIER
InorganicaChimicaActa251 (1996) 75-79
Synthesis, structure and magnetic properties of an asymmetric dinuclear oxocitratovanadate(IV) complex Suzana Burojevic ~. Itzhak Shweky ~, Avi Bino a.,. David A. Summers b. Robert C. Thompson b., Department of Inorganic and Analytical Chemistry. The Hebrew Universityof Jerusalem. 91904Jerusalem. Israel hDepartmentof Chemistry, Universityof British Columbia. 2036Main Mall, Vancouver.BC. V6T 17,1. Canada
Received21 February 1996;revised 13 May 1996
Abstract Evaporation of an aqueous solution containing VO2+, citric acid and neocuproine (2,9-dimethyl- l,t0-phcnanthroline = nee) produces blue crystals of (Hneo)3[ (VO)2(cit) (Hcit)] .4H,O (1). An X-ray structural analysis showed that 1 consists of a discrel¢, asymmetric, dinudear oxocitratovanadate(1V) complex and three neocuproinium cations. The complex contains one six-coordinate V(IV) atom with a distorted octahedral geometry and one five-coordinateV(IV) atom with a distorted square pyramidal geometry. The two metal atoms are bridged by two aikoxide oxygen atoms of one cit'*- group and one Hcit~- group which has one protonated, non-coordinating carboxylic group. Tim V---V distance was found to be 2.949(1) A. Compound 1 is orthorhombic, space group Pea21, with a=22.223(3), b-10.499(I), c = 22.818(2) A, V= 5324( 1) A3and Z = 4. Magnetic susceptibility measurements of a powdered sample ~f I over the range 2-300 K showed a very strong antiferromagnetic coupling between the two d ~systems with - J value of 212 cm-~. Keywords: Crystalstructures; Vanadylcomplexes;Citratecomplexes;Asymmetriccomplexes;Dinuclearcomplexes
1. Introduction Complexes of vanadium citrate in solutions and solids have been studied extensively in many laboratories by EPR, UVVis, IR, saV NMR spectroscopy and potentiometry [ 1 ]. Part of the interest in this system comes from the recent structural report of the FeMo cofactor in nitrogenase, in which homocitrate is chelated to a molybdenum atom [ 2]. It was reported that the FeV cofactor in vanadium nitrogenase possesses a similar structure and that citrate or homocitrate play an important role in the mechanism of these cofactors [3]. Another area of interest is the use of vanadium citrates as precursors in the preparation of mixed oxides which serve as selective oxidation catalysts [4]. A few years ago, Djordjevic et al. reported the structure ofa citrato-peroxo vanadium(V) complex, namely, [ (VO) 2(02 ) 2(H2cit )2 ] 2- [ 5 ]. More recently, Zhou et al. reported the structures of V(V) and V(IV) citrate complexes, [(V02)2(cit)2] 6- and [(V0)2(¢it)2] 4-, in which the citrate ligands are fully deprotonated [6]. The vanadyl complex, as found in Na4[ (VO)2(cit)2]" 6H20, was prepared in aqueous solution of sodium metavanadate and excess citric acid and obtained by a slow reduction ofV(V). * Correspondingauthors. 0020-16931961515.00© 1996ElsevierScienceS.A. All rights reserved Pll S0020-1693 ( 96 ) 05254- !
Crystals of this compound were obtained from solutions at pH = 7. All these complexes are dimeric and possess acentrosymmetric structure. We now report the synthesis, structure and magnetic properties of a new asymmetric vanadyl citrate complex, namely, [ (VO)2(cit)(Hcit)] 3- , which contains one six-coordinate V(IV) atom and one five-coordina~ V(IV) atom.
2. Experimental 2.1. Prepare~'on o f (Hneo)z [(VO)2(cit)(Hcit)]'4H20(1)
Vanadium(IV) sulfate oxide hydrate (0A g, 2.5 retool), citric acid (0.52 g, 2.5 retool) and neocuproin¢ hydrate (2,9dimethyl-l,10-phenanthroline=noo) (0.51 g, 2.5 retool) were dissolved in 15 ml of H20. The bile solution was filtered and placed in an open beaker for slow evaporation. After a few days, large blue crystals of I were obtained. Tim crystals were collected, washed with cold water, acetone and ether. Yield 0.13 g, 9%. Anal. Calc. for Cs4Hs~1602oV2: C, 53.56; H, 4.66; N, 6.94. Found: C, 53.37; H, 4.72; N, 7.25%.
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S. Burojevic et al. / inorganica Chimica Acta 251 (1996) 75-79
3. Results and discussion
Table I
Crystallographic data for (Hneoh [ (VO)2(cil) (Hcit) 1.4H20 (1) Formula Forraula weight Space group a (/~) b (A) c (A) V (A -~) Z p,~: (g cm -~) /~ (era- ~) Range of 20 (*) No. unique data Data with Fo2> 3o'(F,,2) No. variables R
C.~HstNtO2oV2 1210.94 22.223(3) 10.499( l ) 22.818(2) 5324( ! ) 4 1.511 36.89 4--120 4443 3654 419 0.046
Rw
0.064
Pca2j
2.2. X-ray crystallography
A crystal of 1 of dimensions 0.2 X0.3 x0.3 mm was attached to the end of a glass fiber and mounted on a CAD4 diffractometer. Cu Ka (h=1.5418 A,) radiation with a graphite crystal monoehromator in the incident beam was used. The unit cell dimensions were obtained by a leastsquares fit of 24 reflections in the range of 23 < 0 < 29 °. Data were measured at 22 °(2 by using an to-20 motion. Lorentz and polarization corrections were applied. Intensity data were corrected for absorption using the empirical absorption program DIFABS, incorporated to TEXSAN. The structure was solved by using the results of SHELXS86 direct method analysis. The structure was refined ~ to convergence using anisotropic then~tal parameters for the vanadium atoms and for all oxygen atoms of the dinuclear complex. All the hydrogen atoms of the citrate ligands and the neocuproinium ions were introduced in calculated positions using the riding model except H(45--48) (the Hneo + N-H hydrogens) and H(48) (the/3-COOH hydrogen), which were located from the differenc~ maps and their positions were fixed. The discrepancy indices R = E I i Fol - IEel I/E IFol and Rw = [Ew( IFol - IFJ )ZlEwlFol 2] tlz are listed in Table 1. 2.3. Magnetic susceptibilities
Magnetic susceptibilities were me?.sured on a powdered sample, prepared from a crystalline batch, over the 2-300 K range and at a field of 10000 Oe using a Quantum Design (MPMS) SQUID magnetometer. The sarhpi¢ holder, made from PVC, was specially designed to possess a constant crosssectional area. Magnetic susceptibilities were corrected from sample holder background signal over the entire temperature range studied and a diamagnetic correction of 333× 10 -6 cm 3 tool - ~was applied to susceptibilities calculated on a per mole of vanadium basis.
The atomic positional parameters are listed in Table 2. Table 3 presents the important bond lengths and angles and Fig. 1 shows the numbering scheme in 1. With Z--4 in the unit cell of space group Pca21 there is no crystallographic symmetry imposed upon the dinuclear complex. The two vanadium(IV) atoms are bridged by two alkoxide oxygen atoms of a fully deprotonated citrate ligand, O(13), and of one Hcit3- ligand, 0(23). As can be seen from Fig. 1, one vanadium atom, V( 1), is six coordinate with a distorted octahedral geometry. The second vanadium atom, V(2), i~ five coordinate with a distorted square pyramidal geometry, in which O(13), O(14), 0(26) and 0(23) form the basal plane and O(2), the vanadyl oxygen atom, caps the pyramid. The two V=O distances, V(1)--O(I) and V(2)--O(2) of 1.590(5) and 1.595(4) A, respectively, are slightly shorter than that reported for [(VO)2(cit)2] 4-, 1.610(2) Pt [6]. All V-O(citrate) bond distances in 1 range from 1.948(4) to 2.020(4) A except for the V( 1)-O(16) bond length of 2.303(5) A, which is significantly longer, probably due to the trans effect of the V=O system. A similar phenomenon exists in [ (VO)2(cit)2] 4- in which one of the V--O(alkoxo bridge) bonds is trans to the V=O system. As a result, the V202 system is not symmetrical and the two different V-O distances are 1.971 (2) and 2.206(2)/~ [6]. In 1, on the other hand, the V-O(alkoxo bridge) bond lengths range form 1.957(4) to 2.016( 1) A and this may explain the relatively 024
C24
025 027 C22 022
C26 ~,026
021
02
Oil
012
~
~
.mi,ACl5
~ CL2
Cla All crystallography computing was done on a VAX-9000 computer at the Hebrew Univerisly, using the TEXSAN structure determination package.
Fig. !. The structure of [ (VO) 2(cit) (Hcit) ] 3~ as found in 1.
77
S. Burojevic et at./lnorganica Chimica Acta 251 (1996) 75-79
Table 2 Positionalparametersfor ( Hneo)3[ ( VO): (tit) (Hcit) ]. 4H:O ( 1) Atom
x
V( I ) V(2) O( 1) 0(2) O(11) O(12) O(13) O(14) O(15) O(16)
0.58273(4) 0.55920(4) 0.5918(2) 0.5517(2) 0.5296(2) 0.4459(2) 0.5113(2) 0.4999(2) 0.4109(2) : 0.5579(2)
O(17) O(21 ) 0(22) 0(23)
0.4727(2) 0.6513(2) 0.7285(2) 0.6283(2)
02121(4) 0.3017(4) 0.1886(4) 0.5237(4)
0(24) O(25) O(26) 0(27) C( 11) C(12) C(13) C(14) C(t5) C(16) C(21 ) C(22) C(23) C(24) C(25) C( 26) N( I1 )
0.7631(3) 0.7092(2) 0.6186(2)
0.7488(5) 0.7556(4) 0.5647(4) 0.5274(5) 0.2980(6) 0.3642(6) 0.4108(6) 0.4702(6) 0.5023(6) 0.3009(6) 0.2753(6) 0.3449(6) 0.4905(6) 0.5536(6) 0.6962(6) 0.5322( 6 ) 0.0581(5) 0.2396(5) -0.0241(6) -0.1538(6) -0.1970(6) - O.1088(6) - O.1467(7 ) -0.0578(7) 0.0740(6) 0.1675(7) 0.2927(7) 0.3258(6) 0.1153(6) 0.0202(6) 0.0243(7)
0.7147(2)
C(IOI) C(102) C(103) C(104) C( 105) C(!06) C(107) C(108) C(109) C(110) C( I I I ) C(112) C(113) C(114) N(21 )
0.4725(3) 0.4344(3) 0.4675(3) 0.4207(3 ) 0,4442(3) 0.5014(3) 0.6916(3) 0.6914(3) 0.6816(3) 0.7361(3) 0.7352(3) 0.6725( 3) 0.6331(2) 0.6667(2) 0.6156(3) 0.6185(3) 0.6388(3) 0.6569(3) 0.6786(3) 0.6981(3) 0.6952(3) 0,7138(3) 0.7086(3) 0.6848(3) 0.6726(3) 0.6536(3) 0.5938(3) 0,6783(4) 0,5021(2)
N(22 )
0.4708(3)
C(201 ) C(202)
0.5168(3) 0.5382(4)
C(203)
0.5459(4)
C(204) C(205) C(206) C(207) C(208) C(209) C(210) C(211 ) C(212) C(213)
0.5310(3) 0.5389(4) 0.5250(4) 0.5016(3) 0.4865(4) 0,4656(4) 0.4564(3) 0.4929(3) 0.5089(3) 0.5086(4)
N(12)
Table2 (continued) Atom
x
y
z
C(214) N(31 ) N(32) C(301 ) C(302) C(303)
0.4310(5) 0.3314(3) 0.3336(2) 0.3266(3) 0.3424(4) 0.3640(4)
-0.061(I) 0.0645(5) -0.1912(5) 0.1889(7) 0.2415(8) 0.1665(8)
0.1452(5) 0.3219(2) 0.3002(3) 0.3297(3) 0.3840(4) 0.4269(4)
C(304) C(305 ) C(306) C(307) C(308) C(309) C(310) C(311 ) C(312) C(313) C(314) .O(IW) O(2W) O(3W) O(4W)
0.3700(4) 0,3933(4) 0.3982(4) 0.3783(3) 0.3816(4) 0.3618(4) 0.3372(3) 0.3547(3) 0.3519(3) 0.3040(3) 0.3124(4) 0.2502(2) 0.2794(2) 0.1819(2) 0.3560(3)
0.0338(7) - 0.0525(9) -0.1756(8) -0.2299(7) -0.3610(8) -0.3990(8) -0.3151(7) -0.1507(7) -0.0158(7) 0.2694(7) -0.3583(8) 0.0063(4) 0,0130(5) 0.3285(5) 0.1265(6)
0.4187(4) 0.4616(4) 0.4514(4) 0.3968(3) 0.382I(4) 53303(4) 0.2897(3) 0.3532(3) 0.3639(3) 0.2802(4) 0.2310(4) 0.0545(3) 0.2092(2) 0.4666(3) 0.0348(3)
y 0.42419(9) 0.5849( I ) 0,5136(5) 0.7344(4) 0.2934(4) 0.2475(5) 0.5051(4) 0.5370(4) 0.4945(6) 0.3097(4)
-
0A626(8) 0.I181(5) -
0.0430(6) 0.2035(7) 0.1593(8)
0.0334(8) -0.0574(7) - O.1907(8) - 0.2702(7 ) - 0.2263(7) - 0.3037(8) - 0.2536(9) -0.1191(8) - 0.0950(6) - 0.0107(6) 0.3410(8)
0.161 I 0.05764(6) 0.2167(2) 0.0676(2) 0.2003(2) 0.2475(2) 0,1214(2) - 0.0012(2) -0.0391(2) 0.0785(2)
0.0528(2) 0.1667(2) 0.1356(2) 0.I026(2) 0.1468(3) 0,0637(2) - 0.0059(2)
-0.0275(2) 0.2064(3) 0.1600(3) 0.1050(3) 0.0634(3) 0.0034(3) 0.0759(3) 0.1278(3) 0.0697(3) 0.0722(3) 0.1013(3) 0.1071(3) 0.0077( 3 ) 0.2301(2) 0.3104(2) 0.1891(3) 0.2005(32) 0.2548(3) 0.2982(3) 0.3538(3) 0.3931(3) 0.3804(3) 0.4193(3) 0.4052(3) 0.3493(3) 0.3266(3) 0.2842(3) 0.1323(4)
0.3327(4) 0.3321(2) 0.2430(3) 0.3732(3) 0.4265(4)
0.4356(.'~) 0.3919(3) 0.3987(4) 0.3568(4) 0.3022(3) 0.2531(4) 0.2040(4) 0.1994(4) 0.2930(3 ) 0.3396(3) 0.3591(4) (continued)
short V--.V distance of 2.949(1) in 1 versus 3.316(1) in [ (VO)2(cit)2] 4-. The two citrate ligands in I differ in their coordination mode, one c i P - group uses all its depcotonated carboxylate groups for metal binding, whereas the H c i d ligand has orie protonated and non-coordinating/g-carboxylic group. The two C - O bond distances within this group are
C(25)ffiO(24), 1.196(8) A and C(25)-O(25)(H), 1.306(8) A. Similar coordination modes of citrateligands were recentlyobserved indinuclearFe (HI) citratecomplexes
[7]. The overall 3 - charge of the dinuclear complex in 1 is balanced by three neocupronium (Hneo + ) cations. An extensive network of hydrogen bonding in the latticeincludes
carboxylate oxygen atoms, the/3-carboxylicgroup, the four w.aer molecules of crystallizationand the threeHnco" cations. The O-.-O distancesrange from 2.670(7) to 2.928(7) A and the N O distaacesfrom 2.670(7) to 2.967(6) ]L Compound I crystallizesfrom slightlyacidicsolutionsbut can also be obtained in lower yields at pH=7, whereas Na4[ (VO)2 (cit)2]"6H~O was obtained only from a neutral solution [6]. Itisreasonableto assume thatboth complexes, [(VO)2(cit)2] 4- and [(VO)2(Hcit)2] 3-, existin equilibrium in aqueous solutionsince the two differentcompounds selectivelycrystallizedepending upon which cationisadded to obtainsolidmaterial. Magnetic susceptibilities(per mole of vanadium) areplotted versus temperature in F!-. 2. The pIot shows the approach to a broad maximum in susceptibility around 300 K, consistent with the presence c~f significant antiferromagnctic coupling. The increase t|~ susceptibility with decreasing temperature at the lowest temperatures arises fiom a paramagnetic impurity and is commonly seen in studies ea strongly coupled antiferromagnetic systems [8]. The mag-
78
$. Burojevic et al. / hwrganica Chimica Acre 251 (1996) 75-79
Table 3 (continued)
Table 3 Selected bona distances (A) and angles (°) for I Atom
Atom
Distance
V(I) V(I) V(I) V(I) V(l) V(1) 7(I) 7(2) V(2) 7(2) V(2) V(2) 0(11) 0(12) O(13) O(14) O(15) O(16) 0(t7) O(21) 0(22) 0(23) 0(24)
0(25) 0(26) 0(27) C(It) C(12) C(13) C(13) C(14) C(21) C(22) C(23) C(23) C(24)
V(2) O(1) O(!1) O(13) O(t6) O(21) 0(23) 0(2) U(13) O(14) 0(23) 0(26) C(II) C(II) C(13) C(15) C(15) C(16) C(16) C(21) C(21) C(23) C(25) C(25) C(26) C(26) C(12) C(13) C(14) C(16) C(15) C(22) C(23) C(24) C(26) C(25)
2.949(i) 1.590(5) 2.020(4) 2.016(4) 2.303(5) 1.998(4) 1.974(4) 1.595(4) 1.989(4) 1.948(4) 1.957(4) 1.971(4) 1.278(7) t,227(8) 1.437(7) 1.293(9) 1.223(8) 1.260(7) 1.246(7) 1.290(8) i.239(7) 1.416(7) !.196(8) 1.306(8) 1.282(7) 1.235(8) 1.523(9) 1.534(9) 1.542(9) 1.530(8) 1.50(I) 1,51(I) 1.545(9) t.532(9) 1.550(9) 1,50(I)
Atom
Atom
Atom
Angle
O(1) O(!) O(l) O(1) O(1) O(ll) O(li) O(I I ) O(11 ) O(13) 0(13) 0(13) O(16) 0(16) 0(21) 0(2) 0(2) 0(2) 0(2) 0(13) 0(13) O(13)
V(l) V(l) V(I) V(I) V(l) V(I) V( ! ) V( 1) V(l ) 7(1) V(I) V(i) V(l) V(I) V(I) V(2) V(2) V(2) 7(2) V(2) 7(2) V(2)
O(11) O(13) O(16) O(21) 0(23) O(13) O(16) O(21 ) 0(23) O(16) 0(21) 0(23) O(21) 0(23) 0(23) 0(13) O(14) 0(23) 0(26) 0(14) 0(23) 0(26)
97.0(2) 102.1(2) 172.3(2) 103A(2) 99.3(2) 91.4(2) 82.4(2) 88.9(2) 163,5(2) 70.3(2) 154.3(2) 83,0(2) 84.2(2) 812(2) 89.5(2) 104.7(2) 106.4(2) 109.3(2) 106,3(2) 91.9(2) 84.1(2) 148.3(2)
,
(cmu#med)
Atom
Atom
Atom
Angle
O(14) O(t4) 0(23) V(I) V(I) V(I) 7(2) V(2) V(I) V(I) V(I) V(I) V(2) V(2) O(!1) O(11) O(12) C(ll) O(13) O(13) O(13) C(12) C(12) C(14) C(13) O(14) O(14) O(15) O(16) O(16) O(17) O(21) O(21) 0(22) C(21) 0(23) 0(23) 0(23) C(22) 9(22) C(24) C(23) 0(24) 0(24) 0(25) 0(26) 0(26) 0(27)
V(2) 7(2) V(2) O(ll) O(13) O(13) O(13) O(14) O(16) O(21) 0(23) 0(23) 0(23) 0(26) C(ll) C(II) C(II) C(12) C(13) C(13) C(13) C(13) C(13) C(13) C(14) C(15) C(15) C(15) C(16) C(16) C(16) C(21) C(21) C(21) C(22) C(23) C(23) C(23) C(23) C(23) C(23) C(24) C(25) C(25) C(25) C(26) CL26) C(26)
0(23) 0(26) 0(26) C(ll) 7(2) C(13) C(13) C(15) C(16) C(21) V(2) C(23) C(23) C(26) O(12) C(12) C(12) C(13) C(12) C(14) C(16) C(14) C(16) C(16) C(15) O(15) C(14) C(14) O(17) C(13) C(13) 0(22) C(22) C(22) C(23) C(22) C(24) C(26) C(24) C(26) C(26) C(25) 0(25) C(24) C(24) O(27) C(23) C(23)
143.9(2) 85,4(2) 79.9(2) 127.1(4) 94.8(2) 111.1(3) 117,5(3) 131.6(4) 108.4(4) 128.6(4) 97.2(2) 129.1(3) 118.7(4) 118.5(4) 122.9(6) !19.6(6) !17.5(6) 116.6(5) 109.4(5) 109.8(5) 107.5(5) t08.0(5) 110.5(5) 111.7(5) 114.6(5) 122,2(6) 118,0(6) 119.8(6) 125.7(6) 114.5(5) 119.8(5) 121.2(6) 1i9.8(5) 118.9(6) 116.5(5) 112.3(5) 110.0(5) 106.7(5) 109.4(5) 105,3(5) 113.1(5) 117.4(5) 124.0(6) 121.4(6) 114.4(6) t24.3(6) 115.2(5) 120.4(5)
E.s.d.s in the least significant figure are given in parentheses.
netic susceptibilities were fitted to the theoretical. ~xpression given by Bleany and Bowers for antiferromagnetically coupled, S = !/2, dimers [9] with allowance for a paramagnetic component, P. The equation used is:
Ng2132 [ "- exp(2J/kT) l X=--kf-L[I-P][I~T)]+P/Z
-
t
where J is the exchange coupling constant. In modelling the data g was set at 2.00 and J and P were allowed to vary. This gave best fit values of - J and P of 212 cm -t and 0.012,
S. Burojevic et at./lnorganica Chimica Acta 251 (1996) 75-79 0.0025
' -o"
0.0020
u
0.0015
~
~
~
t
~
~ ""
L
E
79
pied dimers and has a shorter V-V distance and stronger coupling than any of the compounds listed previously.
4. Supplementary material Tables of structure factors, thermal parameters, non-essential bond c~istances and angles and positional parameters for hydrogen atoms (34 pages) are available from author A.B.
K o.~1o io o 0.0005
Acknowledgements R.C.T. thanks the Natural Sciences and Engineering Research Council of Canada for financial support. i
i
._J..
o
50
100
L 150
I .... 200
I 250
I 300
Temperature (K) Fig. 2. Magnetic susceptibility (per mole of vanadium ) vs. temperazurcplot. Line is best fit of theory as described in the text.
respectively, with the fitting function of F=0.00012. The function F: ~__1-! n r, i ~
i .12-11/2
r / v/x°"'~-x°"q / Ln~L
X~,b,, d J
is minimized in the fitting procedure and provides a measure of agreement between the experimental data and the model. The magnitude of the coupling constant is large in comparison, for example, with the value - J = 30 cm- t reported for the structurally related oxo-vanadium(IV) dimer of 3hydroxy-3-methylglutanate [ 10]. The magnetic orbitals on each of the metals in these oxo-vanadium complexes are the d~. orbitals (convention, ligands on x and y axes) and this configuration favors direct metal-metal exchange over ligand-mediated superexchange [ 11 ]. Consistent with this, the V-V distance in the compound studied here is significantly shorter (2.949 A) than that in the 3-hydroxy-3-methylglutanate compound (3.107 ,~). Castro et al. list several vanadium dimers a~d place them in four different categories based on their magnedc properties [ 10]. The compound studied here belongs to tN-~,category of antiferromagneticallycou-
References [I] (a) B.M. Nikolova and G. St. Nikolov, J. Inorg. Nucl. Ch~nL, 29 (1967) 1013; (b) R.H. Danhill and TD. Smith, J. Chem. Soc. A, (1968) 2189; (c) Yu.K. Tselinskii, L.V. Shevcheko and I.L Kussel'man, Ukr. Khim. 7,h., 46 (1980) 656; (d) P.M- Ehd¢, I. Anderson and L. Penersson,Acta Chem. Scana[. 143 (1989) !.~; (e) S.G. Vul'fson, A.N. Glebov, O. Yu. Tarasov and Yu.I. Sar nikov, Dok/. Akad. Nauk~ SSSR, 314 (1990) 386; (f) S.P. Arya and P.K. Sharma, J. Indian Chem. Soc., 69 (1992) 793; (g) T.A. Dyachkova, R.S. Satin, A.N. Glebov and G.K. Badnikov, Zh. Neorg. Khim.. 38 (1993) 482; (h) T. Kiss, Bugtyo, D. Sanna, G. Micera, P. Decock and D. Dewae~, inorg. Chim. Acta, 239 (1995) 145. [21 J. Kim and D.C. Rees, Biochemistry. 33 (1994) 389. [3] B.K. Burgess, Chem. Rev.. 90 (1990) 1377. [4] (a) Ph. Courty, H. Ajot, Ch. Matciily and B. F~'.mon, Pe:.,'der Technol., 7 (1973) 21; (b) X.T. Gao, D. Ruitz, XX'. Guo and B. Delmon, Catal. Lett., 23 (1994) 321. [5] C. Djordjevic, M. Lee and E. Sinn, lnorg. Chem.. 28 (1989) 719. 16] ZH. ghou, H E Wan, S.g. Hu and K.R. Tsai, Inorg. Chir~ Acta, 237 (1995) 193. [7] t. Shweky, A. Bino, D.P. Goldberg and SJ. Lippatd, inorg. Chew, 33 (1994) 5161. [8] T. Ofieno, SJ. Rettig, R.C. Thompson and J. Troner, Inorg. Chen~, 32 (1993) 4384. [9] B. Bleany and KD. Bowers, Prec. R. Soc. Lo,',don. Set. A. 226 (1952) 95. [ 10] S.L. Castro, ME. Cass, F.J. Hollander and $..I. Bartley, inorg. Chent, 34 (1995) 466. i I I I E.F. Hasty, T.H. Colbum and D.N. Headrickson, lnorg. Chem., IO (1973) 2414,
InorganicaChimicaActa251 (1996) 75-79
Synthesis, structure and magnetic properties of an asymmetric dinuclear oxocitratovanadate(IV) complex Suzana Burojevic ~. Itzhak Shweky ~, Avi Bino a.,. David A. Summers b. Robert C. Thompson b., Department of Inorganic and Analytical Chemistry. The Hebrew Universityof Jerusalem. 91904Jerusalem. Israel hDepartmentof Chemistry, Universityof British Columbia. 2036Main Mall, Vancouver.BC. V6T 17,1. Canada
Received21 February 1996;revised 13 May 1996
Abstract Evaporation of an aqueous solution containing VO2+, citric acid and neocuproine (2,9-dimethyl- l,t0-phcnanthroline = nee) produces blue crystals of (Hneo)3[ (VO)2(cit) (Hcit)] .4H,O (1). An X-ray structural analysis showed that 1 consists of a discrel¢, asymmetric, dinudear oxocitratovanadate(1V) complex and three neocuproinium cations. The complex contains one six-coordinate V(IV) atom with a distorted octahedral geometry and one five-coordinateV(IV) atom with a distorted square pyramidal geometry. The two metal atoms are bridged by two aikoxide oxygen atoms of one cit'*- group and one Hcit~- group which has one protonated, non-coordinating carboxylic group. Tim V---V distance was found to be 2.949(1) A. Compound 1 is orthorhombic, space group Pea21, with a=22.223(3), b-10.499(I), c = 22.818(2) A, V= 5324( 1) A3and Z = 4. Magnetic susceptibility measurements of a powdered sample ~f I over the range 2-300 K showed a very strong antiferromagnetic coupling between the two d ~systems with - J value of 212 cm-~. Keywords: Crystalstructures; Vanadylcomplexes;Citratecomplexes;Asymmetriccomplexes;Dinuclearcomplexes
1. Introduction Complexes of vanadium citrate in solutions and solids have been studied extensively in many laboratories by EPR, UVVis, IR, saV NMR spectroscopy and potentiometry [ 1 ]. Part of the interest in this system comes from the recent structural report of the FeMo cofactor in nitrogenase, in which homocitrate is chelated to a molybdenum atom [ 2]. It was reported that the FeV cofactor in vanadium nitrogenase possesses a similar structure and that citrate or homocitrate play an important role in the mechanism of these cofactors [3]. Another area of interest is the use of vanadium citrates as precursors in the preparation of mixed oxides which serve as selective oxidation catalysts [4]. A few years ago, Djordjevic et al. reported the structure ofa citrato-peroxo vanadium(V) complex, namely, [ (VO) 2(02 ) 2(H2cit )2 ] 2- [ 5 ]. More recently, Zhou et al. reported the structures of V(V) and V(IV) citrate complexes, [(V02)2(cit)2] 6- and [(V0)2(¢it)2] 4-, in which the citrate ligands are fully deprotonated [6]. The vanadyl complex, as found in Na4[ (VO)2(cit)2]" 6H20, was prepared in aqueous solution of sodium metavanadate and excess citric acid and obtained by a slow reduction ofV(V). * Correspondingauthors. 0020-16931961515.00© 1996ElsevierScienceS.A. All rights reserved Pll S0020-1693 ( 96 ) 05254- !
Crystals of this compound were obtained from solutions at pH = 7. All these complexes are dimeric and possess acentrosymmetric structure. We now report the synthesis, structure and magnetic properties of a new asymmetric vanadyl citrate complex, namely, [ (VO)2(cit)(Hcit)] 3- , which contains one six-coordinate V(IV) atom and one five-coordina~ V(IV) atom.
2. Experimental 2.1. Prepare~'on o f (Hneo)z [(VO)2(cit)(Hcit)]'4H20(1)
Vanadium(IV) sulfate oxide hydrate (0A g, 2.5 retool), citric acid (0.52 g, 2.5 retool) and neocuproin¢ hydrate (2,9dimethyl-l,10-phenanthroline=noo) (0.51 g, 2.5 retool) were dissolved in 15 ml of H20. The bile solution was filtered and placed in an open beaker for slow evaporation. After a few days, large blue crystals of I were obtained. Tim crystals were collected, washed with cold water, acetone and ether. Yield 0.13 g, 9%. Anal. Calc. for Cs4Hs~1602oV2: C, 53.56; H, 4.66; N, 6.94. Found: C, 53.37; H, 4.72; N, 7.25%.
76
S. Burojevic et al. / inorganica Chimica Acta 251 (1996) 75-79
3. Results and discussion
Table I
Crystallographic data for (Hneoh [ (VO)2(cil) (Hcit) 1.4H20 (1) Formula Forraula weight Space group a (/~) b (A) c (A) V (A -~) Z p,~: (g cm -~) /~ (era- ~) Range of 20 (*) No. unique data Data with Fo2> 3o'(F,,2) No. variables R
C.~HstNtO2oV2 1210.94 22.223(3) 10.499( l ) 22.818(2) 5324( ! ) 4 1.511 36.89 4--120 4443 3654 419 0.046
Rw
0.064
Pca2j
2.2. X-ray crystallography
A crystal of 1 of dimensions 0.2 X0.3 x0.3 mm was attached to the end of a glass fiber and mounted on a CAD4 diffractometer. Cu Ka (h=1.5418 A,) radiation with a graphite crystal monoehromator in the incident beam was used. The unit cell dimensions were obtained by a leastsquares fit of 24 reflections in the range of 23 < 0 < 29 °. Data were measured at 22 °(2 by using an to-20 motion. Lorentz and polarization corrections were applied. Intensity data were corrected for absorption using the empirical absorption program DIFABS, incorporated to TEXSAN. The structure was solved by using the results of SHELXS86 direct method analysis. The structure was refined ~ to convergence using anisotropic then~tal parameters for the vanadium atoms and for all oxygen atoms of the dinuclear complex. All the hydrogen atoms of the citrate ligands and the neocuproinium ions were introduced in calculated positions using the riding model except H(45--48) (the Hneo + N-H hydrogens) and H(48) (the/3-COOH hydrogen), which were located from the differenc~ maps and their positions were fixed. The discrepancy indices R = E I i Fol - IEel I/E IFol and Rw = [Ew( IFol - IFJ )ZlEwlFol 2] tlz are listed in Table 1. 2.3. Magnetic susceptibilities
Magnetic susceptibilities were me?.sured on a powdered sample, prepared from a crystalline batch, over the 2-300 K range and at a field of 10000 Oe using a Quantum Design (MPMS) SQUID magnetometer. The sarhpi¢ holder, made from PVC, was specially designed to possess a constant crosssectional area. Magnetic susceptibilities were corrected from sample holder background signal over the entire temperature range studied and a diamagnetic correction of 333× 10 -6 cm 3 tool - ~was applied to susceptibilities calculated on a per mole of vanadium basis.
The atomic positional parameters are listed in Table 2. Table 3 presents the important bond lengths and angles and Fig. 1 shows the numbering scheme in 1. With Z--4 in the unit cell of space group Pca21 there is no crystallographic symmetry imposed upon the dinuclear complex. The two vanadium(IV) atoms are bridged by two alkoxide oxygen atoms of a fully deprotonated citrate ligand, O(13), and of one Hcit3- ligand, 0(23). As can be seen from Fig. 1, one vanadium atom, V( 1), is six coordinate with a distorted octahedral geometry. The second vanadium atom, V(2), i~ five coordinate with a distorted square pyramidal geometry, in which O(13), O(14), 0(26) and 0(23) form the basal plane and O(2), the vanadyl oxygen atom, caps the pyramid. The two V=O distances, V(1)--O(I) and V(2)--O(2) of 1.590(5) and 1.595(4) A, respectively, are slightly shorter than that reported for [(VO)2(cit)2] 4-, 1.610(2) Pt [6]. All V-O(citrate) bond distances in 1 range from 1.948(4) to 2.020(4) A except for the V( 1)-O(16) bond length of 2.303(5) A, which is significantly longer, probably due to the trans effect of the V=O system. A similar phenomenon exists in [ (VO)2(cit)2] 4- in which one of the V--O(alkoxo bridge) bonds is trans to the V=O system. As a result, the V202 system is not symmetrical and the two different V-O distances are 1.971 (2) and 2.206(2)/~ [6]. In 1, on the other hand, the V-O(alkoxo bridge) bond lengths range form 1.957(4) to 2.016( 1) A and this may explain the relatively 024
C24
025 027 C22 022
C26 ~,026
021
02
Oil
012
~
~
.mi,ACl5
~ CL2
Cla All crystallography computing was done on a VAX-9000 computer at the Hebrew Univerisly, using the TEXSAN structure determination package.
Fig. !. The structure of [ (VO) 2(cit) (Hcit) ] 3~ as found in 1.
77
S. Burojevic et at./lnorganica Chimica Acta 251 (1996) 75-79
Table 2 Positionalparametersfor ( Hneo)3[ ( VO): (tit) (Hcit) ]. 4H:O ( 1) Atom
x
V( I ) V(2) O( 1) 0(2) O(11) O(12) O(13) O(14) O(15) O(16)
0.58273(4) 0.55920(4) 0.5918(2) 0.5517(2) 0.5296(2) 0.4459(2) 0.5113(2) 0.4999(2) 0.4109(2) : 0.5579(2)
O(17) O(21 ) 0(22) 0(23)
0.4727(2) 0.6513(2) 0.7285(2) 0.6283(2)
02121(4) 0.3017(4) 0.1886(4) 0.5237(4)
0(24) O(25) O(26) 0(27) C( 11) C(12) C(13) C(14) C(t5) C(16) C(21 ) C(22) C(23) C(24) C(25) C( 26) N( I1 )
0.7631(3) 0.7092(2) 0.6186(2)
0.7488(5) 0.7556(4) 0.5647(4) 0.5274(5) 0.2980(6) 0.3642(6) 0.4108(6) 0.4702(6) 0.5023(6) 0.3009(6) 0.2753(6) 0.3449(6) 0.4905(6) 0.5536(6) 0.6962(6) 0.5322( 6 ) 0.0581(5) 0.2396(5) -0.0241(6) -0.1538(6) -0.1970(6) - O.1088(6) - O.1467(7 ) -0.0578(7) 0.0740(6) 0.1675(7) 0.2927(7) 0.3258(6) 0.1153(6) 0.0202(6) 0.0243(7)
0.7147(2)
C(IOI) C(102) C(103) C(104) C( 105) C(!06) C(107) C(108) C(109) C(110) C( I I I ) C(112) C(113) C(114) N(21 )
0.4725(3) 0.4344(3) 0.4675(3) 0.4207(3 ) 0,4442(3) 0.5014(3) 0.6916(3) 0.6914(3) 0.6816(3) 0.7361(3) 0.7352(3) 0.6725( 3) 0.6331(2) 0.6667(2) 0.6156(3) 0.6185(3) 0.6388(3) 0.6569(3) 0.6786(3) 0.6981(3) 0.6952(3) 0,7138(3) 0.7086(3) 0.6848(3) 0.6726(3) 0.6536(3) 0.5938(3) 0,6783(4) 0,5021(2)
N(22 )
0.4708(3)
C(201 ) C(202)
0.5168(3) 0.5382(4)
C(203)
0.5459(4)
C(204) C(205) C(206) C(207) C(208) C(209) C(210) C(211 ) C(212) C(213)
0.5310(3) 0.5389(4) 0.5250(4) 0.5016(3) 0.4865(4) 0,4656(4) 0.4564(3) 0.4929(3) 0.5089(3) 0.5086(4)
N(12)
Table2 (continued) Atom
x
y
z
C(214) N(31 ) N(32) C(301 ) C(302) C(303)
0.4310(5) 0.3314(3) 0.3336(2) 0.3266(3) 0.3424(4) 0.3640(4)
-0.061(I) 0.0645(5) -0.1912(5) 0.1889(7) 0.2415(8) 0.1665(8)
0.1452(5) 0.3219(2) 0.3002(3) 0.3297(3) 0.3840(4) 0.4269(4)
C(304) C(305 ) C(306) C(307) C(308) C(309) C(310) C(311 ) C(312) C(313) C(314) .O(IW) O(2W) O(3W) O(4W)
0.3700(4) 0,3933(4) 0.3982(4) 0.3783(3) 0.3816(4) 0.3618(4) 0.3372(3) 0.3547(3) 0.3519(3) 0.3040(3) 0.3124(4) 0.2502(2) 0.2794(2) 0.1819(2) 0.3560(3)
0.0338(7) - 0.0525(9) -0.1756(8) -0.2299(7) -0.3610(8) -0.3990(8) -0.3151(7) -0.1507(7) -0.0158(7) 0.2694(7) -0.3583(8) 0.0063(4) 0,0130(5) 0.3285(5) 0.1265(6)
0.4187(4) 0.4616(4) 0.4514(4) 0.3968(3) 0.382I(4) 53303(4) 0.2897(3) 0.3532(3) 0.3639(3) 0.2802(4) 0.2310(4) 0.0545(3) 0.2092(2) 0.4666(3) 0.0348(3)
y 0.42419(9) 0.5849( I ) 0,5136(5) 0.7344(4) 0.2934(4) 0.2475(5) 0.5051(4) 0.5370(4) 0.4945(6) 0.3097(4)
-
0A626(8) 0.I181(5) -
0.0430(6) 0.2035(7) 0.1593(8)
0.0334(8) -0.0574(7) - O.1907(8) - 0.2702(7 ) - 0.2263(7) - 0.3037(8) - 0.2536(9) -0.1191(8) - 0.0950(6) - 0.0107(6) 0.3410(8)
0.161 I 0.05764(6) 0.2167(2) 0.0676(2) 0.2003(2) 0.2475(2) 0,1214(2) - 0.0012(2) -0.0391(2) 0.0785(2)
0.0528(2) 0.1667(2) 0.1356(2) 0.I026(2) 0.1468(3) 0,0637(2) - 0.0059(2)
-0.0275(2) 0.2064(3) 0.1600(3) 0.1050(3) 0.0634(3) 0.0034(3) 0.0759(3) 0.1278(3) 0.0697(3) 0.0722(3) 0.1013(3) 0.1071(3) 0.0077( 3 ) 0.2301(2) 0.3104(2) 0.1891(3) 0.2005(32) 0.2548(3) 0.2982(3) 0.3538(3) 0.3931(3) 0.3804(3) 0.4193(3) 0.4052(3) 0.3493(3) 0.3266(3) 0.2842(3) 0.1323(4)
0.3327(4) 0.3321(2) 0.2430(3) 0.3732(3) 0.4265(4)
0.4356(.'~) 0.3919(3) 0.3987(4) 0.3568(4) 0.3022(3) 0.2531(4) 0.2040(4) 0.1994(4) 0.2930(3 ) 0.3396(3) 0.3591(4) (continued)
short V--.V distance of 2.949(1) in 1 versus 3.316(1) in [ (VO)2(cit)2] 4-. The two citrate ligands in I differ in their coordination mode, one c i P - group uses all its depcotonated carboxylate groups for metal binding, whereas the H c i d ligand has orie protonated and non-coordinating/g-carboxylic group. The two C - O bond distances within this group are
C(25)ffiO(24), 1.196(8) A and C(25)-O(25)(H), 1.306(8) A. Similar coordination modes of citrateligands were recentlyobserved indinuclearFe (HI) citratecomplexes
[7]. The overall 3 - charge of the dinuclear complex in 1 is balanced by three neocupronium (Hneo + ) cations. An extensive network of hydrogen bonding in the latticeincludes
carboxylate oxygen atoms, the/3-carboxylicgroup, the four w.aer molecules of crystallizationand the threeHnco" cations. The O-.-O distancesrange from 2.670(7) to 2.928(7) A and the N O distaacesfrom 2.670(7) to 2.967(6) ]L Compound I crystallizesfrom slightlyacidicsolutionsbut can also be obtained in lower yields at pH=7, whereas Na4[ (VO)2 (cit)2]"6H~O was obtained only from a neutral solution [6]. Itisreasonableto assume thatboth complexes, [(VO)2(cit)2] 4- and [(VO)2(Hcit)2] 3-, existin equilibrium in aqueous solutionsince the two differentcompounds selectivelycrystallizedepending upon which cationisadded to obtainsolidmaterial. Magnetic susceptibilities(per mole of vanadium) areplotted versus temperature in F!-. 2. The pIot shows the approach to a broad maximum in susceptibility around 300 K, consistent with the presence c~f significant antiferromagnctic coupling. The increase t|~ susceptibility with decreasing temperature at the lowest temperatures arises fiom a paramagnetic impurity and is commonly seen in studies ea strongly coupled antiferromagnetic systems [8]. The mag-
78
$. Burojevic et al. / hwrganica Chimica Acre 251 (1996) 75-79
Table 3 (continued)
Table 3 Selected bona distances (A) and angles (°) for I Atom
Atom
Distance
V(I) V(I) V(I) V(I) V(l) V(1) 7(I) 7(2) V(2) 7(2) V(2) V(2) 0(11) 0(12) O(13) O(14) O(15) O(16) 0(t7) O(21) 0(22) 0(23) 0(24)
0(25) 0(26) 0(27) C(It) C(12) C(13) C(13) C(14) C(21) C(22) C(23) C(23) C(24)
V(2) O(1) O(!1) O(13) O(t6) O(21) 0(23) 0(2) U(13) O(14) 0(23) 0(26) C(II) C(II) C(13) C(15) C(15) C(16) C(16) C(21) C(21) C(23) C(25) C(25) C(26) C(26) C(12) C(13) C(14) C(16) C(15) C(22) C(23) C(24) C(26) C(25)
2.949(i) 1.590(5) 2.020(4) 2.016(4) 2.303(5) 1.998(4) 1.974(4) 1.595(4) 1.989(4) 1.948(4) 1.957(4) 1.971(4) 1.278(7) t,227(8) 1.437(7) 1.293(9) 1.223(8) 1.260(7) 1.246(7) 1.290(8) i.239(7) 1.416(7) !.196(8) 1.306(8) 1.282(7) 1.235(8) 1.523(9) 1.534(9) 1.542(9) 1.530(8) 1.50(I) 1,51(I) 1.545(9) t.532(9) 1.550(9) 1,50(I)
Atom
Atom
Atom
Angle
O(1) O(!) O(l) O(1) O(1) O(ll) O(li) O(I I ) O(11 ) O(13) 0(13) 0(13) O(16) 0(16) 0(21) 0(2) 0(2) 0(2) 0(2) 0(13) 0(13) O(13)
V(l) V(l) V(I) V(I) V(l) V(I) V( ! ) V( 1) V(l ) 7(1) V(I) V(i) V(l) V(I) V(I) V(2) V(2) V(2) 7(2) V(2) 7(2) V(2)
O(11) O(13) O(16) O(21) 0(23) O(13) O(16) O(21 ) 0(23) O(16) 0(21) 0(23) O(21) 0(23) 0(23) 0(13) O(14) 0(23) 0(26) 0(14) 0(23) 0(26)
97.0(2) 102.1(2) 172.3(2) 103A(2) 99.3(2) 91.4(2) 82.4(2) 88.9(2) 163,5(2) 70.3(2) 154.3(2) 83,0(2) 84.2(2) 812(2) 89.5(2) 104.7(2) 106.4(2) 109.3(2) 106,3(2) 91.9(2) 84.1(2) 148.3(2)
,
(cmu#med)
Atom
Atom
Atom
Angle
O(14) O(t4) 0(23) V(I) V(I) V(I) 7(2) V(2) V(I) V(I) V(I) V(I) V(2) V(2) O(!1) O(11) O(12) C(ll) O(13) O(13) O(13) C(12) C(12) C(14) C(13) O(14) O(14) O(15) O(16) O(16) O(17) O(21) O(21) 0(22) C(21) 0(23) 0(23) 0(23) C(22) 9(22) C(24) C(23) 0(24) 0(24) 0(25) 0(26) 0(26) 0(27)
V(2) 7(2) V(2) O(ll) O(13) O(13) O(13) O(14) O(16) O(21) 0(23) 0(23) 0(23) 0(26) C(ll) C(II) C(II) C(12) C(13) C(13) C(13) C(13) C(13) C(13) C(14) C(15) C(15) C(15) C(16) C(16) C(16) C(21) C(21) C(21) C(22) C(23) C(23) C(23) C(23) C(23) C(23) C(24) C(25) C(25) C(25) C(26) CL26) C(26)
0(23) 0(26) 0(26) C(ll) 7(2) C(13) C(13) C(15) C(16) C(21) V(2) C(23) C(23) C(26) O(12) C(12) C(12) C(13) C(12) C(14) C(16) C(14) C(16) C(16) C(15) O(15) C(14) C(14) O(17) C(13) C(13) 0(22) C(22) C(22) C(23) C(22) C(24) C(26) C(24) C(26) C(26) C(25) 0(25) C(24) C(24) O(27) C(23) C(23)
143.9(2) 85,4(2) 79.9(2) 127.1(4) 94.8(2) 111.1(3) 117,5(3) 131.6(4) 108.4(4) 128.6(4) 97.2(2) 129.1(3) 118.7(4) 118.5(4) 122.9(6) !19.6(6) !17.5(6) 116.6(5) 109.4(5) 109.8(5) 107.5(5) t08.0(5) 110.5(5) 111.7(5) 114.6(5) 122,2(6) 118,0(6) 119.8(6) 125.7(6) 114.5(5) 119.8(5) 121.2(6) 1i9.8(5) 118.9(6) 116.5(5) 112.3(5) 110.0(5) 106.7(5) 109.4(5) 105,3(5) 113.1(5) 117.4(5) 124.0(6) 121.4(6) 114.4(6) t24.3(6) 115.2(5) 120.4(5)
E.s.d.s in the least significant figure are given in parentheses.
netic susceptibilities were fitted to the theoretical. ~xpression given by Bleany and Bowers for antiferromagnetically coupled, S = !/2, dimers [9] with allowance for a paramagnetic component, P. The equation used is:
Ng2132 [ "- exp(2J/kT) l X=--kf-L[I-P][I~T)]+P/Z
-
t
where J is the exchange coupling constant. In modelling the data g was set at 2.00 and J and P were allowed to vary. This gave best fit values of - J and P of 212 cm -t and 0.012,
S. Burojevic et at./lnorganica Chimica Acta 251 (1996) 75-79 0.0025
' -o"
0.0020
u
0.0015
~
~
~
t
~
~ ""
L
E
79
pied dimers and has a shorter V-V distance and stronger coupling than any of the compounds listed previously.
4. Supplementary material Tables of structure factors, thermal parameters, non-essential bond c~istances and angles and positional parameters for hydrogen atoms (34 pages) are available from author A.B.
K o.~1o io o 0.0005
Acknowledgements R.C.T. thanks the Natural Sciences and Engineering Research Council of Canada for financial support. i
i
._J..
o
50
100
L 150
I .... 200
I 250
I 300
Temperature (K) Fig. 2. Magnetic susceptibility (per mole of vanadium ) vs. temperazurcplot. Line is best fit of theory as described in the text.
respectively, with the fitting function of F=0.00012. The function F: ~__1-! n r, i ~
i .12-11/2
r / v/x°"'~-x°"q / Ln~L
X~,b,, d J
is minimized in the fitting procedure and provides a measure of agreement between the experimental data and the model. The magnitude of the coupling constant is large in comparison, for example, with the value - J = 30 cm- t reported for the structurally related oxo-vanadium(IV) dimer of 3hydroxy-3-methylglutanate [ 10]. The magnetic orbitals on each of the metals in these oxo-vanadium complexes are the d~. orbitals (convention, ligands on x and y axes) and this configuration favors direct metal-metal exchange over ligand-mediated superexchange [ 11 ]. Consistent with this, the V-V distance in the compound studied here is significantly shorter (2.949 A) than that in the 3-hydroxy-3-methylglutanate compound (3.107 ,~). Castro et al. list several vanadium dimers a~d place them in four different categories based on their magnedc properties [ 10]. The compound studied here belongs to tN-~,category of antiferromagneticallycou-
References [I] (a) B.M. Nikolova and G. St. Nikolov, J. Inorg. Nucl. Ch~nL, 29 (1967) 1013; (b) R.H. Danhill and TD. Smith, J. Chem. Soc. A, (1968) 2189; (c) Yu.K. Tselinskii, L.V. Shevcheko and I.L Kussel'man, Ukr. Khim. 7,h., 46 (1980) 656; (d) P.M- Ehd¢, I. Anderson and L. Penersson,Acta Chem. Scana[. 143 (1989) !.~; (e) S.G. Vul'fson, A.N. Glebov, O. Yu. Tarasov and Yu.I. Sar nikov, Dok/. Akad. Nauk~ SSSR, 314 (1990) 386; (f) S.P. Arya and P.K. Sharma, J. Indian Chem. Soc., 69 (1992) 793; (g) T.A. Dyachkova, R.S. Satin, A.N. Glebov and G.K. Badnikov, Zh. Neorg. Khim.. 38 (1993) 482; (h) T. Kiss, Bugtyo, D. Sanna, G. Micera, P. Decock and D. Dewae~, inorg. Chim. Acta, 239 (1995) 145. [21 J. Kim and D.C. Rees, Biochemistry. 33 (1994) 389. [3] B.K. Burgess, Chem. Rev.. 90 (1990) 1377. [4] (a) Ph. Courty, H. Ajot, Ch. Matciily and B. F~'.mon, Pe:.,'der Technol., 7 (1973) 21; (b) X.T. Gao, D. Ruitz, XX'. Guo and B. Delmon, Catal. Lett., 23 (1994) 321. [5] C. Djordjevic, M. Lee and E. Sinn, lnorg. Chem.. 28 (1989) 719. 16] ZH. ghou, H E Wan, S.g. Hu and K.R. Tsai, Inorg. Chir~ Acta, 237 (1995) 193. [7] t. Shweky, A. Bino, D.P. Goldberg and SJ. Lippatd, inorg. Chew, 33 (1994) 5161. [8] T. Ofieno, SJ. Rettig, R.C. Thompson and J. Troner, Inorg. Chen~, 32 (1993) 4384. [9] B. Bleany and KD. Bowers, Prec. R. Soc. Lo,',don. Set. A. 226 (1952) 95. [ 10] S.L. Castro, ME. Cass, F.J. Hollander and $..I. Bartley, inorg. Chent, 34 (1995) 466. i I I I E.F. Hasty, T.H. Colbum and D.N. Headrickson, lnorg. Chem., IO (1973) 2414,