Optimised syntheses and purifications of 3-aryl/heterocyclic dihydrobis- and hydrotris-(pyrazolyl)borate ligands as their alkali salts

TL;DR

This paper presents improved methods for making and purifying specific borate ligands used in chemistry, avoiding toxic materials and achieving high purity and yield.

Contribution

The paper introduces optimized and safer synthesis and purification techniques for poly(pyrazolyl)borate ligands.

Findings

Fraction crystallisation from MeCN allows purification of TpR ligands with high purity.

An optimized procedure enables safe isolation of alkali salts of BpR ligands in high yields.

The methods avoid the use of highly toxic thallium(i) salts.

Abstract

Though poly(pyrazolyl)borate ligands, namely dihydrobis(pyrazolyl)borate (Bp) and hydrotris(pyrazolyl)borate (Tp), have been used in coordination chemistry for decades, their synthesis and purification are of great importance, when targeting high purity and yield of metal complexes. As borohydride substitutions is temperature and pyrazolyl dependent, determining the reaction conditions for each ligand is non-trivial and does not always result in a temperature where only the desired products form. Herein we report the purification of two known TpR (R = phenyl, 2′-thienyl), as their tris-substituted temperatures coincide with their tetrakis-temperatures via fraction crystallisation from MeCN, and an optimised safe procedure for 3-aryl/heterocyclic BpR ligands (R = 2′-pyridyl, 2′-furyl, 2′-thineyl). These novel techniques allow for the safe isolation of alkali salts of poly(pyrazolyl)borate ligands in high yields and purities without the use of highly toxic thallium(i) salts.

The optimised synthesis and purification of poly(pyrazolyl)borate ligands is detailed for their use in coordination chemistry, to which we implore others to adopt.

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