First crystal structures of metal complexes with a 4‐nitropyrazole‐3‐carboxylic acid ligand and the third crystal form of the ligand

Pyrazole (pz)‐derived ligands can, besides exhibiting a strong coordination ability toward different metal ions, exhibit a great diversity in their coordination geometry and nuclearity, which can be achieved by varying the type and position of the pz substituents. The present study reports the synthesis and crystal structure of two binuclear complexes, namely bis(μ‐4‐nitro‐1H‐imidazol‐1‐ide‐5‐carboxylato)‐κ3N1,O:N2;κ3N2:N1,O‐bis[aqua(dimethylformamide‐κO)copper(II)], [Cu2(C4HN3O4)2(C3H7NO)2(H2O)2], (II), and bis(μ‐4‐nitro‐1H‐imidazol‐1‐ide‐5‐carboxylato)‐κ2N1,O:N2;κ2N2:N1,O‐bis[triaquacobalt(II)] dihydrate, [Co2(C4HN3O4)2(H2O)6]·2H2O, (III). These compounds represent rare examples of metal complexes comprising 3,4‐substituted pz derivatives as a bridging ligand and also the first crystal structures of transition‐metal complexes with ligands derived from 4‐nitropyrazole‐3‐carboxylic acid. Recently, the crystal structures of the same ligand in the neutral and mixed neutral/anionic forms have been reported. We present here the third form of this ligand, where it is present in a fully deprotonated anionic form within a salt, i.e. ammonium 4‐nitropyrazole‐3‐carboxylate, NH4+·C4H2N3O4−, (I). Single‐crystal X‐ray diffraction revealed that in the present complexes, the CuII and CoII centres adopt distorted square‐pyramidal and octahedral geometries, respectively. In both cases, the N,N′,O‐coordinated pz ligand shows simultaneously chelating and bridging coordination modes, leading to the formation of a nearly planar six‐membered M2N4 metallocycle. In all three crystal structures, the supramolecular arrangement is controlled by strong hydrogen bonds which primarily engage the carboxylate O atoms as acceptors, while the nitro group adopts the role of an acceptor only in structures with an increased number of donors, as is the case with CoII complex (III). The electrostatic potential, as a descriptor of reactivity, was also calculated in order to examine the changes in ligand electrostatic preferences upon coordination to metal ions. The title structures represent rare examples of metal complexes comprising a 3,4‐disubstituted pyrazole derivative as a bridging ligand and the first crystal structures of metal complexes with a 4‐nitropyrazole‐3‐carboxylic acid‐based ligand.