Binuclear chromium(III) complexes bridged by hydroxide and acetate groups

Three acetato-bridged binuclear Cr(III) complexes, [(tren)Cr(mu-OH)(mu-O2CCH3)Cr(tren)](ClO4)(4) . 4H(2)O (1), [(trenH)Cr(mu-OH)(mu-O2CCH3)(2)Cr(trenH)](ClO4)(5) . 6H(2)O (2) and [(H2O)(4)Cr(mu-OH)(mu-O2CCH3)Cr(OH2)4]((CH3)(3)C6H2SO3)(4) . 8H(2)O(3), have been prepared and characterised using a range of physico-chemical techniques. The crystal structure of [(H2O)(4)Cr(mu-OH)(mu-O2CCH3) Cr(OH2)(4)]((CH3)(3)C6H2SO3)(4) . 8H(2)O has been determined from single crystal X-ray diffraction studies. The compound crystallises in the space group P2(1)/c with a=18.350(4), b=8.756(3), c=36.373(6) Angstrom, beta=98.54(2)degrees and Z=4. Least squares refinement gave R and R-w values of 0.122 and 0.126, respectively for 3965 observed reflections. The complex consists of two Cr(III) centres in a distorted octahedral environment held together by one acetate and one hydroxide bridge. The Cr Cr distance of 3.55(1) Angstrom is longer than in the Cr(III) hydrolytic dimer due to the widening of the Cr-OH-Cr angle caused by the bridging acetate group. The Cr-OH2 bond lengths are typical of Cr(III) aqua complexes while the Cr-OH and Cr-O(acetate) distances are also in the normal range for such complexes. The temperature dependence of the magnetic susceptibility for complexes 1-3 indicates that the two Cr( III) centres are weakly antiferromagnetically coupled with coupling constants J of -19.9, -15.7 and -7.7 cm(-1) for 1, 2 and 3, respectively.