Site-Isolated Main-Group Tris(2-pyridyl)borate Complexes by Pyridine Substitution and Their Ring-Opening Polymerization CatalysisClick to copy article linkArticle link copied!
- Jin QianJin QianDepartment of Chemistry, University of Houston, Houston, Texas 77204-5003, United StatesMore by Jin Qian
- Robert J. Comito*Robert J. Comito*Email: [email protected]Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United StatesMore by Robert J. Comito
Abstract
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 Mg2+, Zn2+, or Ca2+, with isopropyl-substituted TpyiPrH 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(SiMe3)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. TpyiPrMg(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.
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