THE [CO4O4](4+) CUBANE AS A QUADRUPLY-BRIDGING UNIT - THE MIXED-VALENCE CLUSTER [CO8O4(O(2)CPH)(12)(SOLV)(4)] (SOLV=DMF, MECN, H2O)

被引:65
作者
DIMITROU, K
SUN, JS
FOLTING, K
CHRISTOU, G
机构
[1] INDIANA UNIV,DEPT CHEM,BLOOMINGTON,IN 47405
[2] INDIANA UNIV,CTR MOLEC STRUCT,BLOOMINGTON,IN 47405
关键词
D O I
10.1021/ic00120a021
中图分类号
O61 [无机化学];
学科分类号
070301 ; 081704 ;
摘要
Synthetic procedures are described that allow access to an octanuclear, mixed-valence 4Co(II),4Co(III) molecular aggregate. Oxidation of Co(O(2)CPh)(2) in DMF with aqueous H2O2 leads to a dark emerald-green solution from which can be isolated [Co8O4(O(2)CPh)(12)(DMF)(3)(H2O)] (1a) in similar to 60% yield. The same reaction in MeCN leads to [Co8O4(O(2)CPh)(12)(MeCN)(3)(H2O)] (1b) Complex 1b . 2MeCN crystallizes in triclinic space group P $($) over bar$$ 1 with the following unit cell dimensions at -153 degrees C: a = 15.056(4) Angstrom, b = 23.548(6) Angstrom, c = 14.697(4) Angstrom, alpha = 90.74(1)degrees, beta = 114.75(1)degrees, gamma = 75.12(1)degrees, V = 4546.5 Angstrom(3), Z = 2. A total of 10 207 unique data with F > 3 sigma(F) were used to solve the structure, which refined to R and R(w) values of 4.01 and 4.24%, respectively. The structure consists of a central [(C4O4)-O-III](4+) cubane unit, with each mu(3)-O2- ion becoming mu(4) by attachment to a Co-II center to give a [Co8O4](12+) core; the [Co4O4](4+) cubane is thus bridging the four Co-II ions. Twelve PhCO(2)(-) groups bridge the Co-II.. Co-III pairs, and terminal MeCN/H2O groups complete the peripheral ligation at the five-coordinate, high-spin Co-II centers. The Co-III centers are low-spin. Excluding differences in terminal ligation (MeCN or H2O), the molecule has idealized Td symmetry. Variable-temperature, solid-state magnetic susceptibility studies on in have been carried out in the range 4.00-320 K. The effective magnetic moment decreases from 8.35 mu(B)/Co-8 (4.18 mu(B) per Co-II) at 320 K to 5.29 mu(B)/Co-8 (2.65 mu B/COII) at 4.00 K. The data were fit to the theoretical expression for an axial symmetry Co-II ion undergoing zero-field splitting. The fitting parameters were \D\ = 23.5(2.0) cm(-1), Theta = -1.76(8) K, and g = 2.05(2), with TIP set at 700 x 10(-6) cm(3) mol(-1). Low-temperature (<20 K) deviation between experimental and calculated mu(eff) values is taken as evidence of weak, antiferromagnetic intra- and intermolecular interactions being present, the latter mediated by hydrogen-bonding contacts between two Cos units. H-1 NMR spectra recorded in (CD3)(2)CO, CD2Cl2, or CD3OD show only one set of paramagnetically-shifted benzoate resonances suggesting effective T-d solution symmetry and retention of the [Co8O4] core On dissolution; this conclusion is supported by electronic spectral studies. Cyclic and differential pulse voltammetric studies show a quasi-reversible oxidation at 1.54 V vs SCE for 1c; however, for 1a, the oxidation appears irreversible.
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页码:4160 / 4166
页数:7
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