Aluminum Tris(2-pyridyl)borates: Structure, reactivity and catalysis

Abstract

Tris(2-pyridyl)borates are emerging scorpionate ligands distinguished by their robustness and modularity, as well as their strong electron-donicity. Metal tris(2-pyridyl)borates show a strong tendency toward six-coordinate, pseudooctahedral metal geometries with static κ3-ligation. Most exceptions involve sterically hindered tris(2-pyridyl)borates. We report four-coordinate alkylaluminum complexes with the sterically unhindered ligand phenyl tris(2-pyridyl)borate (Tpy–), using single-crystal XRD, variable-temperature NMR, and computational modelling to characterize their structure and dynamics. Thus proton exchange between TpyH and trialkylaluminum gave four-coordinate complexes TpyAlR2 (–R = methyl, ethyl, and isobutyl), the first κ2-complexes of sterically unhindered tris(2-pyridyl)borates. Moreover, pyridine coordination in TpyAlR2 complexes is fluxional, showing NMR symmetry down to −80 °C. Computational modelling suggests that pyridine exchange occurs through a higher energy five-coordinate κ3-conformation. By contrast, the analogous gallium complex TpyGaMe2 is also κ2, but NMR does not show pyridine interconversion even up to 70 °C. Alternatively, proton exchange involving TpyH, trialkylaluminums, and acids gives κ3, four-coordinate cations with the composition [TpyAlR]+ (–R = methyl, ethyl, and isobutyl), in addition to an unusual protonated tris(2-pyridyl)borate complex [TpyHAliBu2]+. Furthermore, reaction of TpyH with two equivalents of AlMe3 gave the first bimetallic tris(2-pyridyl)borate complex TpyAl2Me5, with distinct κ2- and κ1-bound aluminum sites. Finally, we characterize controlled polymerization of ε-caprolactone by these complexes at high temperatures.