Site-Isolated Main-Group Tris (2-pyridyl)borate Complexes by Pyridine Substitution and Their Ring-Opening Polymerization Catalysis.

Tris (2-pyridyl)borates are an emerging class of scorpionate ligands, distinguished as exceptionally robust and electron-donating. However, the rapid formation of inert homoleptic complexes with divalent metals has so far limited their catalytic utility. We report site-isolating tris (2-pyridyl)borate ligands, bearing isopropyl, tert -butyl, and mesityl substituents at the pyridine 6-position to suppress the formation of inert homoleptic complexes. These ligands form the first 1:1 complexes between tris (2-pyridyl)borates and Mg 2+ , Zn 2+ , or Ca 2+ , with isopropyl-substituted Tpy iPr H showing the most generality. Single-crystal X-ray diffraction analysis of the resulting complexes and comparison to density functional theory (DFT) models showed geometric distortions driven by steric repulsion between the pyridine 6-substituents and the hexamethyldisilazide (HMDS - , - N(SiMe 3 ) 2 ) anion. We show that this steric profile is a feature of the six-membered pyridine ring and contrasts with more established tris (pyrazolyl)borate and tris (imidazoline)borate scorpionate complexes. Tpy iPr Mg(HMDS) ( 1 ) and its zinc analogue are moderately active for the controlled polymerization of l-lactide, ε-caprolactone, and trimethylene carbonate. Furthermore, 1 gives controlled polymerization under more demanding melt-phase polymerization conditions at 100 °C, and block copolymerization of ε-caprolactone and trimethylene carbonate. These results will enable useful catalysis and coordination chemistry studies with tris (2-pyridyl)borates, and characterizes their structural complementarity to more familiar scorpionate ligands.