Synthesis, Crystal Structures, and Hirshfeld Surface Analysis of Nickel and Cobalt Complexes From an in Situ Hydrolyzed Amino-Pyrazole Ester Ligand

The synthesis and solid-state characterization of the dissymmetric amino-pyrazole ester ligand methyl-6-(5-amino-1-(pyridine-2-yl)-1H-pyrazol-3-yl)picolinate, L1-CH3, and three Ni(II) and Co(II) complexes are reported. Under protic solvent conditions in the presence of transition-metal salts, L1-CH3 undergoes apparent in situ ester hydrolysis to generate the monoanionic carboxylate ligand L1, which supports the formation of [(L1)2Ni]·MeOH·H2O (1), [(L1)3Ni2]Cl (2), and [(L1)5Co3](NO3) (3). Single-crystal X-ray diffraction reveals that 1 is a mononuclear Ni(II) complex with a distorted octahedral N4O2 coordination environment, while 2 is a dinuclear Ni(II) complex in which three L1 ligands bridge the two metal centres and generate two distinct N/O donor environments. Compound 3 is a trinuclear Co(II) complex containing an approximately linear Co3 arrangement, with terminal distorted octahedral Co(II) centres and a central distorted square-pyramidal Co(II) centre. Hirshfeld surface analysis shows that metal complexation reduces the contribution from C···C contacts relative to L1-CH3, consistent with loss of ligand planarity and diminished π-stacking. These results demonstrate that ester/carboxylate functionalization can alter both ligand reactivity and coordination outcomes in aminopyrazole systems, enabling access to mono-, di-, and trinuclear architectures and providing a basis for future mixed-donor and heterometallic ligand design.