Synthesis of Zwitterionic Tris(pyrazolyl) Ligands and Their Mn(II) Bis-Chelates Exhibiting Charge–Transfer-Induced Spin Crossover

Abstract

Hydro-tris(pyrazolyl)borate-based scorpionates are cornerstone ligands in coordination chemistry, offering precise control over the steric and electronic environment of metal centers. In contrast, their emerging zwitterionic analogs remain largely unexplored. With intermediate ligand donor strength, these ligands provide a distinctive handle for fine-tuning the electronic structures of metal complexes. Expanding this nascent class opens new avenues for probing fundamental processes in coordination chemistry. In the present study, the synthesis and characterization of new zwitterionic scorpionates with the general formula [base]⁺-[B–Pz₃]⁻ are presented. In addition, a synthetic procedure for preparing their bis-chelates with divalent manganese ions is provided; this topic remains largely unexplored. Systematic modification of the zwitterionic ligand series (1a–1f) allows precise fine-tuning of their sigma-donor strength, establishing a direct correlation with the Mn(II/III) oxidation potentials  determined by cyclic voltammetry. These complexes and their chemically oxidized counterparts are characterized using single-crystal, magnetic, and electrochemical techniques to study the charge–transfer-induced spin crossover inherent to this class of compounds.