5522 Inorg. Chem. zyxwvuts 1994, zyxwvu 33, 5522-5521 Synthetic Models for Low-Molecular-Weight Chromium-Binding Substance: Synthesis and Characterization of Oxo-Bridged Tetranuclear Chromium(II1) Assemblies Truitt Ellis,' Miriam Glass,l Anthony Harton,' Kristen Felting: John C. Huffman: and John B. Vincent*J Department of Chemistry, University of Alabama, Tuscaloosa, Alabama 35487-0336, and Molecular Structure Center, Indiana University, Bloomington, Indiana 47405 Received April zyxwvutsrq 21, 1994@ The structural and spectroscopic properties of a series of tetranuclear chromium-oxo complexes are reported. One example, [Cr402(02CMe)7(phen)$l-l SCHzC126HzO (1) crystallizes in the orthorhombic space group C2221 with a zyxwvutsr = 18.652(4) A, zyxwvutsrq b = 25.200(7) A, c = 12.559(3) A, V= 5903.23 A3, and Z = 4. The structure was refined with 2043 reflections having F > 3.0o(F), giving final R factors of 0.0609 and 0.0623 for R and R,, respectively. The [Cr402l8+ core consists of a "butterfly" arrangement and is structurally akin to other recently reported [M40#+ (M = Fe, Mn) complexes. These complexes were synthesized by heating a PhCN solution of a bidentate ligand and a trinuclear complex of the composition [Cr30(02CR)&#+. A combination of 'H and 2H NMR studies allows for assignment of all proton (deuteron) resonances. A n-spin delocalization mechanism is the dominant contributor to the 'H and *H chemical shifts. Fast atom bombardment mass spectrometry has been shown to be a valuable tool in the characterization of the chromium-oxo assemblies. The results of electronic and room temperature magnetic studies and the biological relevance of these materials is discussed. Introduction Interest in multinuclear chromium-carboxylate assemblies stems from several areas including serving as models of the metal center of the protein Low-Molecular-Weight Chromium- Binding Substance (LMWCr) and as sources of fundamental information about magnetic exchange in multinuclear as- semblies. Cr has been determined to be required for normal mammalian carbohydrate and lipid metab~lism.~ Cr deficiency in humans results in symptoms comparable with those associated with adult-onset diabetes and cardiovascular diseases4 Yet as much as 90% of the American population have a daily intake less than the recommended safe and adequate quantities of Cr.5 Despite its importance, essentially nothing is known about the composition and structure of the biologically active form of Cr. The best and perhaps only current candidate is LMWCr. LMWCr is an anionic, carboxylate-rich polypeptide which possesses a molecular weight of ca. 1500 and binds four chromic ions.6 Charge balance and other considerations suggest that LMWCr possesses an anion-bridged, multinuclear Cr-carboxyl- ate assembly.' Trinuclear oxo-centered carboxylate assemblies of the general composition [M3O(02CR)&3lflf (especially where M = Cr(II1)) have been of intense interest for three decades as they have served as models to test theories of magnetic coupling betweeen metal ions in multinuclear systems.* Larger, more complex systems are required to further test and extend these theories. This is especially important as assemblies become appreciably larger such that the paramagnet/superparamagnet/ ferromagnet boundaries can be studied? Discrete, multinuclear oxo-bridged Mn and Fe carboxylate assemblies have been found to approach properties of superparamagneti~m;~~~~ a series of well-studied high nuclearity assemblies with different electron counts could lead to a better understanding of this phenomena. With these goals in mind, this laboratory has recently turned its attention to the synthesis and characterization of such tetranuclear assemblies. Experimental Section Syntheses. All manipulations were performed under aerobic condi- tions, and all chemicals were used as received. Elemental analyses were performed by Galbraith Laboratories, Knoxville, TN. [CrsO- (OAc)6(H20)31C1," [Cr3O(OAc)6(pY)31C1," [C~~O(O~CCD~)(~Y)~IC~,'~ [Cr~0(0*CPh)6(py)3]Clo4,12 and [ C ~ ~ O ( O A C ) ~ ( ~ ~ ~ ) ~ I P F & ~ were pre- pared as described in the literature or were available from previous work. Caution! Appropriate care should be taken whenever perchlorate salts are manipulated or heated. [Cr402(0Ac)~(phen)2]C1-1.5CH2C126Hz0 (1). A solution of 1.75 g (2.84 m o l ) of [Cr30(OAc)6(H20)3]C1and 1.05 g (5.83 mmol) of 1,lO-phenanthroline in 60 mL of PhCN was heated to reflux for 4 h. After the resulting slurry was allowed to cool, a gray solid was isolated by filtration. This material was dissolved in CH2C12 and layered with hexanes to give black crystals in 42% yield (based on Cr). Anal. Calcd (found) for C39,5H52N4O22C4Cr4: C, 36.93 (37.18); H, 4.08 (4.35); N, 4.36 (3.95); C1, 11.04 (11.72); Cr, 16.19 (16.65). Selected IR data: 3400 (br), 1610 (s), 1560 (m), 1230 (m), 1150 (m), 1110 (w), 1030 (m), 880 (m), 850 (m), 775 (w), 725 (vs), 675 (s), 620 (s), 570 (m) cm-'. @ Abstract published in Advance ACS Abstracts, October 15, 1994. (1) University of Alabama. (2) Indiana University. (3) (a) Anderson, R. A. Clin. Physiol. Biochem. 1986, 4, 31. (b) Mertz, W. J. J. Nutr. 1993, 123, 626. (4) Anderson, R. A. In Trace Elements in Human and Animal Nutrition; (5) Anderson, R. A.; Kozlovsky, A. zyxwvutsr S. Am. J. Clin. Nutr. 1985,41, 1177. (6) Yamamoto, A.; Wada, 0.; Ono, T. Eur. J. Biochem. 1987, 165, 627. (7) Vincent, J. B. Encyclopedia of Inorganic Chemistry; John Wiley & (8) Cannon, R. D.; White, R. P. Prog. Inorg. Chem. 1988, 36, 195. Academic Press: Orlando, FL, 1987; Vol. 1; pp 225-244. Sons: Chichester, England, 1994; Vol. 2; pp 661-665. 0020-166919411333-5522$04.50/0 (9) Taft, K. L.; Papaefthymiou, G. C.; Lippard, S. J. Science 1993, 259, 1302. (10) (a) Sessoli, R.; Tsai, H.-L.; Schake, A. R.; Wang, S.; Vincent, J. B.; Folting, K.; Gatteschi, D.; Christou, G.; Henderickson, D. N. J. Am. Chem. SOC. 1993, 115, 1804. (b) Boyd, P. D. W.; Li, Q.; Vincent, J. B.; Folting, K.; Chang, H.-R.; Streib, W.; Huffman, J. C.; Christou, G.; Hendrickson, D. N. J. Am. Chem. SOC. 1988, 110, 8537. (11) Johnson, M. K.; Powell, D. B.; Cannon, R. D. Spectrochim.Acta 1981, 37A, 995. (12) Glass, M. M.; Belmore, K.; Vincent, J. B. Polyhedron 1993,12, 133. (13) Bino, A,; Chayat, R.; Pedersen, E.; Schneider,A. Inorg. Chem. 1991, 30, 856. 0 1994 American Chemical Society