Abstract
Two novel complexes, [CuCl~2~(Mebdmpza)] (1) and [Cu~2~(μ‐O~4~C~2~)Cl~2~(Mebdmpza)~2~] (2) [Mebdmpza = methyl bis(3,5‐dimethylpyrazol‐1‐yl)acetate], were synthesized by reaction of CuCl~2~ with the precursor ligand bis(3,5‐dimethylpyrazol‐1‐yl)acetic acid (Hbdmpza) in methanol. The X‐ray crystal structures of 1 and 2 revealed that the initial Hbdmpza ligand had reacted with methanol during the crystallization process to yield the esterified ligand Mebdmpza. A reverse Mebdmpza to bdmpza reaction is observed by dissolving [CuCl~2~(Mebdmpza)] (1) in water to yield [Cu(bdmpza)~2~]·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 1 shows significant changes upon variation of the temperature, from a broad pseudo‐isotropic symmetry signal at 298 K (g~av.~ = 2.16) to a signal characterized by an axially distorted Cu^II^ below 200 K (g~∥~ = 2.34; no A~∥~ resolved; g~⟂~ = 2.11). The green, dinuclear, oxalato‐bridged compound 2 is strongly antiferromagnetically coupled with 2__J__ = –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 Cu^II^ is observed at any temperature.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)