Sterically Bulky Tris(triazolyl)borate Ligands as Water-Soluble Analogues of Tris(pyrazolyl)borate

Abstract

The recently synthesized 3-tert-butyl-5-methyl-1,2,4-triazole reacted with KBH4 to give the new potassium tris(3-tert-butyl-5-methyl-1,2,4-triazolyl)borate K(Ttz(tBu,Me)) ligand. Ttz(tBu,Me) formed a four-coordinate (Ttz(tBu,Me))CoCl complex and five-coordinate (Ttz(tBu,Me))CoNO3 and (Ttz(tBu,Me))ZnOAc complexes. When these complexes were compared to their Tp(tBu,Me) analogues, it was found that Ttz(tBu,Me) resulted in negligible steric differences. K(Ttz(tBu,Me)) is more water-soluble than K(Tp(tBu,Me)), so bulky tris(triazolyl)borate ligands should lead to functional models for enzyme active sites in an aqueous environment and the creation of water-soluble analogues of Tp catalysts.

Tris(triazolyl)borate ligands of intermediate steric bulk for the synthesis of biomimetic structures with hydrogen bonding and solubility in hydrophilic solvents

“…3-phenyl-5-methyl-1,2,4-triazole (Htz Ph,Me ) Htz Ph,Me was synthesized by modified literature procedures [11]; in particular we found it was necessary to dissolve the final product in hot toluene, filter, and recrystallize to produce an analytically pure sample. Characterization of Htz Ph,Me : 1 Htz Ph,Me (21.010 g, 131.9 mmol) and potassium borohydride (1.272 g, 23.6 mmol) were combined in a flask under N 2 , and gradually heated to 210°C. The reaction mixture generated a gas, presumably H 2 , which was measured by water displacement.…”

“…Excess triazole was removed by performing five sublimations on small batches of the mixture under vacuum with the heat gradually being raised from 90 to 140°C and the mixture was ground in a mortar and pestle between sublimations. Once the mixture was nearly pure by 1 …”

“…After filtration, rotary evaporation of CH 2 Cl 2 gave (Ttz Ph,Me )ZnCl which was dried under vacuum and isolated in 72% yield (0.402 g, 0.687 mmol). Characterization of Ttz Ph,Me ZnCl: 1 …”

“…Additionally, through the synthesis of novel tris(triazolyl)borate (Ttz) ligands with sterically bulky groups at the third position of the triazole ring, we can control the coordination geometry and obtain late first row transition metal complexes with coordination numbers four and five. In the course of our studies on tris(3-t-butyl-5-methyl-1,2,4-triazolyl)borate (Ttz tBu,Me ) [1,2], we became interested in synthesizing sterically less demanding tris(triazolyl)-borates for diversifying the types of biomimetic geometries we can obtain, including larger exogenous ligands and perhaps creating bimetallic complexes. Our most recent addition to the class of tris(triazolyl)borate ligands has been based on 3-phenyl-5-methyl-1,2,4-triazole (Htz Ph,Me ), and the resulting ligand tris(3-phenyl-5-methyl-1,2,4-triazolyl)borate (Ttz Ph,Me ) represents the first Ttz ligand of intermediate steric bulk, since only Ttz H,H and Ttz Me,Me were known prior to our work [3][4][5][6].…”

Theoretical investigation of the scope of sequential ligand tuning using a bifunctional scorpionate tris(1,2,4-triazolyl)borate-based architecture

“…[3] Analogous scorpionate ligands can also be easily prepared through the use of other heterocycles, for example hydrotris(1,2,4-triazolyl)borate (Ttz) which has similar coodination properties to Tp but yields greater solubility of its complexes in protic and aqueous media and weaker donor capacity. [4][5][6][7][8] Since the first isolation of a free imidazole based N-heterocyclic carbene by Arduengo et al, [9] this ligand class has also become extremely common. [10] Regarded as analogues and replacements of more traditional phosphine ligands, N-heterocyclic carbenes have found widespread utility as supporting ligands in catalysis yielding more stable and catalytically active complexes due to their higher electron donor strengths and stronger metal-ligand bonds.…”