HETERONUCLEAR TRINUCLEAR COMPLEX-IONS OF THE TYPE [M(II)(CR(A)4(OH)2)2]4+

The syntheses and characterization of a series of trinuclear complex ions of the types [M(II)Cr(A)4(OH)2}2]4+, where (A)4 represents four ligating nitrogen atoms, are described. The zinc(II)/chromium(III) complex [Zn{(OH)2Cr-(cyclam)}2] (S2O6)2.2.5H2O (5h), where cyclam is 1,4,8,11-tetraazacyclotetradecane, crystallizes in space group P2(1)/c of the monoclinic system with four trinuclear species in a cell of dimensions a = 15.719 (12) angstrom, b = 16.805 (15) angstrom, c = 16.800 (10) angstrom, and beta = 116.86 (5)-degrees. The structure has been refined to a final R factor of 0.065 based on 3472 observed independent reflections. The central zinc(II) atom is bonded to the oxygen atoms of four bridging hydroxo groups and exhibits axial coordination to a dithionate anion leading to approximate square pyramidal geometry. The axial Zn-O bond of 2.217 angstrom indicates strong anion coordination. The hydroxo groups bridge, in pairs, to two chromium atoms, so the complex can be viewed as consisting of two cis-[Cr(cyclam)(OH)2]+ units which act as bidentate ligands on the central zinc atom. Each chromium atom is in a roughly octahedral environment, bonded to two oxygen atoms from the bridging hydroxo groups and four nitrogen atoms from the ligands. The manganese(II)/chromium(III) complex [Mn{(OH)Cr(pico)2(OH)12(OH2)4](S2O6)2-0.5H2O (2g), where pico is (2-pyridylmethyl)amine, crystallizes in space group C2/c of the monoclinic system with four trinuclear species in a cell of dimensions a = 16.969 (3) angstrom, b = 14.587 (1) angstrom, c = 17.014 (1) angstrom, and beta = 99.13 (1)-degrees. The structure has been refined to a final R factor of 0.037 based on 1849 observed independent reflections. The cobalt analogue, [Co{(OH)Cr(pico)2(OH)}2(OH2)4](S2O6)2-0.5H2O (3g) is isomorphous with the manganese complex, with cell constants a = 16.968 (7) angstrom, b = 14.490 (3) angstrom, c = 16.996 (3) angstrom, and beta = 98.76 (2)-degrees, and has been refined to a final R factor of 0.069 based on 1183 observed independent reflections. The central metal atoms its on a crystallographic inversion center which relates one half of the molecule to the other. The central M nor Co atom exhibits approximate octahedral geometry, being bonded to four terminal water molecules and two hydroxo bridges. Hence, in the present cases, each of the chromium complexes is linked to the central metal by only a single hydroxo bridge. The bridging Mn-O-Cr and Co-O-Cr angles are 134.3 (2) and 135.2 (5)-degrees, respectively. The magnetic susceptibility data for the complexes demonstrate that the ''open-form'' mu-hydroxo complexes 2g and 3g and their analogues exhibit antiferromagnetic interactions while the 'closed-form'' bis(mu-hydroxo) cyclam complexes with paramagnetic central metal ions either are weakly ferromagnetic or exhibit no magnetic interaction. The zinc cyclam complex 5h exhibits a weak antiferromagnetic interaction between the two chromium(III) centers.