SYNTHETIC MODELS FOR LOW-MOLECULAR-WEIGHT CHROMIUM-BINDING SUBSTANCE - SYNTHESIS AND CHARACTERIZATION OF OXO-BRIDGED TETRANUCLEAR CHROMIUM(III) ASSEMBLIES

The structural and spectroscopic properties of a series of tetranuclea r chromium-oxo complexes are reported. One example, Cr4O2(O(2)CMe)(7)( phen)(2)]Cl.1.5CH(2)Cl(2).6H(2)O (1) crystallizes in the orthorhombic space group C222(1) with a = 18.652(4) Angstrom, b = 25.200(7) Angstro m, c = 12.559(3) Angstrom, V = 5903.23 Angstrom(3), and Z = 4. The str ucture was refined with 2043 reflections having F > 3.0 sigma(F), givi ng final R factors of 0.0609 and 0.0623 for R and R(w), respectively. The [Cr4O2](8+) core consists of a ''butterfly'' arrangement and is st ructurally akin to other recently reported [M(4)O(2)](8+) (M = Fe, Mn) complexes. These complexes were synthesized by heating a PhCN solutio n of a bidentate ligand and a trinuclear complex of the composition [C r3O(O(2)CR)(6)L(3)](n+). A combination of H-1 and H-2 NMR studies allo ws for assignment of all proton (deuteron) resonances. A pi-spin deloc alization mechanism is the dominant contributor to the H-1 and H-2 che mical shifts. Fast atom bombardment mass spectrometry has been shown t o be a valuable tool in the characterization of the chromium-oxo assem blies. The results of electronic and room temperature magnetic studies and the biological relevance of these materials is discussed.