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

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Title: Aluminum Tris(2-pyridyl)borates: Structure, reactivity and catalysis.
Authors: Qian, Jin1 (AUTHOR), Simran, Simran1 (AUTHOR), Comito, Robert J.1 (AUTHOR) rjcomito@central.uh.edu
Source: Polyhedron. May2024, Vol. 254, pN.PAG-N.PAG. 1p.
Subjects: Borates, Living polymerization, Coordinate covalent bond, Aluminum, Ring-opening polymerization, Catalysis, Coordination polymers
Abstract: We characterize alkylaluminum complexes of phenyl tris (2-pyridyl)borate (Tpy–) with coordination modes previously not reported among tris (2-pyridyl)borate ligands. These include fluxional hapticity (TpyAlMe 2) and bimetallic coordination (TpyAl 2 Me 5). We compare the reactivity of these complexes in the polymerization of ε-caprolactone and discuss their broader implications for catalysis and coordination chemistry. [Display omitted] • Aluminum. • Scorpionate ligand. • Ring-opening polymerization. • tris (2-pyridyl)borate. 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 TpyAlR 2 (–R = methyl, ethyl, and isobutyl), the first κ2-complexes of sterically unhindered tris (2-pyridyl)borates. Moreover, pyridine coordination in TpyAlR 2 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 TpyGaMe 2 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 [TpyHAliBu 2 ]+. Furthermore, reaction of TpyH with two equivalents of AlMe 3 gave the first bimetallic tris (2-pyridyl)borate complex TpyAl 2 Me 5 , with distinct κ2- and κ1-bound aluminum sites. Finally, we characterize controlled polymerization of ε-caprolactone by these complexes at high temperatures. [ABSTRACT FROM AUTHOR]
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Abstract:We characterize alkylaluminum complexes of phenyl tris (2-pyridyl)borate (Tpy–) with coordination modes previously not reported among tris (2-pyridyl)borate ligands. These include fluxional hapticity (TpyAlMe 2) and bimetallic coordination (TpyAl 2 Me 5). We compare the reactivity of these complexes in the polymerization of ε-caprolactone and discuss their broader implications for catalysis and coordination chemistry. [Display omitted] • Aluminum. • Scorpionate ligand. • Ring-opening polymerization. • tris (2-pyridyl)borate. 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 TpyAlR 2 (–R = methyl, ethyl, and isobutyl), the first κ2-complexes of sterically unhindered tris (2-pyridyl)borates. Moreover, pyridine coordination in TpyAlR 2 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 TpyGaMe 2 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 [TpyHAliBu 2 ]+. Furthermore, reaction of TpyH with two equivalents of AlMe 3 gave the first bimetallic tris (2-pyridyl)borate complex TpyAl 2 Me 5 , with distinct κ2- and κ1-bound aluminum sites. Finally, we characterize controlled polymerization of ε-caprolactone by these complexes at high temperatures. [ABSTRACT FROM AUTHOR]
ISSN:02775387
DOI:10.1016/j.poly.2024.116931