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 (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)