Copper Complexes with the Ligand Methyl Bis(3,5-dimethylpyrazol-1-yl)acetate (Mebdmpza), Generated by In Situ Methanolic Esterification of Bis(3,5-dimethylpyrazol-1-yl)acetic Acid

Two novel complexes, [CuCl(2)(Mebdmpza)] (1) and [CU(2)(mu-O(4)C(2))Cl(2)(Mebdmpza)2] (2) [Mebdmpza = methyl bis(3,5-dimethylpyra zol-1-yl)acetate], were synthesized by reaction of CuCl, with the precursor ligand bis(3,5-dimethylpyrazol-1-yl)acetic acid (Hbdmpza) in methanol. The X-ray crystal structures of I and 2 revealed that the initial Hbdmpza ligand had reacted with methanol during the crystallization process to yield the esterified ligand Medmpza. A reverse Mebdmpza to bdmpza reaction is observed by dissolving [CuCl(2)(Mebdmpza)] (1) in water to yield [Cu(bdmpza)(2)]center dot 2H(2)O. After drying, the latter gives water-free [Cu(bdmpza)(2)]. The copper ion in the orange mononuclear complex 1 is tetrahedrally coordinated by two nitrogen atoms (from one ligand) and two chloride ions. The light-green dinuclear complex 2 exhibits a square-pyramidal geometry, defined by two ligand nitrogen atoms, two oxalato oxygen atoms, and one apical chloride anion. The EPR spectrum of I shows significant changes upon variation of the temperature, from a broad pseudo-isotropic syrnmetry signal at 298 K (g(av) = 2.16) to a signal characterized by an axially distorted Cu(II) below 200 K g(parallel to) = 2.34; no A(parallel to) resolved; g(perpendicular to) = 2.11). The green, dinuclear, oxalato-bridged compound 2 is strongly antiferromagnetically coupled with 2J = -342 cm(-1). Its EPR spectrum shows only a broad line (g(av) = 2.12), which is ascribed to mononuclear impurities, as it. does not disappear even at very low temperatures. No signal for a triplet Of dinuclear Cull is observed at any temperature. (C) Wiley-VCH Verlag GmbH & Co. KGdA, 69451 Weinheim, Germany, 2008)