Synthesis of Biomimetic Zinc Complexes for CO2 Activation and the Influence of Steric Changes in the Ttz Ligands [Ttz = Tris(triazolyl)borate]

Carbon dioxide fixation with zinc complexes of the anionic tris(triazolyl)borate (Ttz) ligand offers biomimetic structures in a hydrophilic environment. However, the synthetic result depends greatly on the steric bulk of the Ttz ligand. Synthetic routes adapted from the synthesis of (Tp)ZnOH have been applied to bulky Ttz ligands, but side products often result because Ttz frequently forms complexes with +1 metals or cations used in the synthesis. Nonetheless, (TtztBu,Me)ZnOH (1) has been successfully synthesized and crystallographically characterized, but only via the hydrolysis of the zinc (hexamethylsilyl)azide complex (6). With less bulky ligands, namely TtzPh,Me, routes expected to form LZnOH lead instead to bis(ligand) products, namely [(TtzPh,Me)2Zn] (12). The use of ligands of intermediate steric bulk has led to the isolation of (TtziPr2)ZnOH (2a) which fixes atmospheric CO2 to produce [(TtziPr2)Zn]2(μ-η1-η2-CO3) (2b). This activation of CO2 from air is irreversible. We report the crystal structures of 2b and related molecules used for synthesis and structural comparison: (TtziPr2)ZnCl (7), [(TtziPr2)Cu(μ-OH)]2 (9), and [(TtziPr2)Cu(μ-Cl)]2 (10).