MAGNETO STRUCTURAL CORRELATIONS IN MACROCYCLIC DINICKEL(II) COMPLEXES- TUNING OF SPIN-EXCHANGE BY VARYING STEREOCHEMISTRY AND AUXILIARY LIGANDS

Variable-temperature (5-300 K) susceptibility measurements carried out for the binuclear nickel(II) complexes of a tetraamino diphenol macro cyclic ligand (H2L) of compositions [Ni2L(H2O)4](ClO4)2.4NH2CONH2 (1), [Ni2L(NCS)2(H2O)2].2Me2NCHO (2), [Ni2L(MeOH)2(ClO4)2].2NHEt3ClO4 (3), [Ni2L(im)2](ClO4)2 (4), [Ni2L(py)2](ClO4)2 (5), and [Ni2L(mu-O2CCH2NH 3)(H2O)2](ClO4)2.2H2O (6) are reported. All the complexes behave antif erromagnetically, and the exchange parameter, J, varies in the followi ng way: 1 (-17 cm-1), 2 (-21 cm-1), 3 (-29 cm-1), 4 (-50 cm-1), 5 (-67 cm-1), and 6 (-1 cm-1). The decrease in the value of J from 1 to 3 is attributed to the increase in tetragonal distortion around the metal centers. The change in stereochemistry of nickel(II) to the square pyr amidal configuration leads to the significant decrease in the value of J for 4 and 5. The net effective exchange coupling constant of 6, in which two orthogonal superexchange pathways are involved, is very smal l. The X-ray structure of 2 has been determined. The complex crystalli zes in the monoclinic space group P2(1)/c with a = 10.091(1) angstrom, b = 7.957(1) angstrom, c = 24.569(3) angstrom, beta = 92.77(1)-degree s, and Z = 2. The structure was solved by direct methods and refined t o R = 0.039 and R(w) = 0.042. Complex 2 undergoes two reversible one-e lectron oxidation steps (E1/2 = 0.90, 1.05 V vs SCE, Pt/MeCN) with the formation of Ni(II)-Ni(III) and Ni(III)-Ni(III) species.