MONONUCLEAR AND BINUCLEAR CHROMIUM(III) PICOLINATE COMPLEXES

The chromium(III) picolinate complexes [Cr(pic)3] (1) and [Cr(PiC)2OH]2 (2) (where pic = 2-carboxypyridine) have been isolated from the reaction of chromium(III) chloride and picolinic acid in aqueous solution. The monomeric complex 1.H2O (CrC18H14N3O7) crystallizes in the monoclinic space group C2/c with a = 30.219 (3) angstrom, b = 8.529 (1) angstrom, c = 13.942 (2) angstrom, beta = 95.141 (1)-degrees, V = 3579 (2) angstrom3, Z = 4, R = 0.032, and R(W) = 0.043 for 1815 reflections (I > 3sigma(I)). The binuclear complex 2.5H2O (Cr2C24H28N4O15) crystallizes in the monoclinic space group P2(1)/n with a = 12.569 (2) angstrom, b = 18.839 (3) angstrom, c = 13.000 (4) angstrom, beta = 105.823 (3)-degrees, V = 2962 (2) angstrom3, Z = 8, R = 0.046, R(W) = 0.067 for 3250 reflections (I > 3sigma(I)). The mononuclear complex 1 is the meridional isomer, not the facial isomer as previously assumed. For 2 the bridging geometry is as follows: Cr-Cr = 2.999 (1) angstrom; Cr-O(bridge) range = 1.934(3)-1.980 (3) angstrom; Cr-O-Cr = 99.0 (1), 101.6(1)-degrees; O-Cr-O = 79.4(1), 79.9(1)-degrees. Magnetic susceptibility data for 2 were measured from 280 to 6 K and can be fit using an isotropic spin-exchange Hamiltonian, H = -2JS1.S2, with a weak antifeffomagnetic interaction (J = -8.02 (4) cm-1), g = 1.844, and a 2.23 (4) mol % paramagnetic impurity. The extent of magnetic coupling in 2 is compared to predictions made by the Glerup-Hodgson-Pedersen (GHP) equation. Compounds 1 and 2 are discussed in the general context of biologically active chromium.