Substituent effects in the zinc(II) coordination chemistry of isomeric pyridylpyrazole ligands

Abstract

Pyrazoles with unsymmetric substitution are useful ligands in coordination chemistry, but are under-developed due to synthetic challenges in accessing the pure isomers. We have prepared four new structurally related N-(2-pyridyl)-3,5-dialkylpyrazole ligands, L1–L4, and probed their coordination chemistry in the crystalline phase and in solution to elucidate a relationship between steric influence of the alkyl substituents, the stability of the subsequent metal complexes, and their crystal packing influences. We find that L1 and L2, bearing linear or branched alkyl substituents, show similar stabilities and crystal packing motifs featuring π···π and C-H···Cl interactions in the crystalline complexes 1 and 2, respectively. The cyclohexyl-fused species L3 and L4 vary both in the solution stability of complexes 3 and 4, respectively, and in their crystal packing. Complex ZnCl₂(L3)] (3) is a mononuclear complex similar to 1 and 2, albeit with π···π interactions disallowed by the bulk of the cyclohexyl ring. Reaction of isomeric L4 with ZnCl₂ gives two polymorphic complexes, 4α and 4β, of the form Zn₂Cl₂(μ₂-Cl)₂(L4)₂], varying only in their long-range packing modes. These results show the importance of understanding the steric influences in substituted pyridylpyrazoles, which determine both stability in solution and speciation in the crystalline phase.