Optimised syntheses and purifications of 3-aryl/heterocyclic dihydrobis- and hydrotris-(pyrazolyl)borate ligands as their alkali 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 Tp^R (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 Bp^R 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.