Bidentate and tridentate coordination modes of bis(3-methylindolyl)-2-pyridylmethane in complexes of aluminum and gallium: Structural characterization of bridging N-indolide in a dialuminum complex

Add time:09/10/2019    Source:sciencedirect.com

Reactions of a slight excess of trialkylaluminum or tri-tert-butylgallium with bis(3-methylindolyl)-2-pyridylmethane (1) produced monomeric, tripodal complexes (2-C5H4N)HC(3-CH3C8H4N)2MR (M = Al, R = Me (2a), Et (2b), iBu (2c), tBu (2d); M = Ga, R = tBu (3)). Under more mild reaction conditions and with a stoichiometric amount of tri-tert-butylaluminum or tri-tert-butylgallium, complexes (2-C5H4N)HC(3-CH3C8H4N)(3-CH3C8H4NH)MtBu2 (M = Al (4), Ga (5)) were isolated. Whereas di-deprotonated 1 adopts a tridentate coordination mode in 2a-2d and 3, mono-deprotonated 1 adopts a bidentate coordination mode in 4 and 5. Reaction of 1 with two equivalents of trimethylaluminum or triethylaluminum yields dialuminum complexes (2-C5H4N)HC(3-CH3C8H4N)2Al2R4 (R = Me (6a), Et (6b). In addition to NMR (1H, 13C) spectroscopy, compounds 1, 2b·0.25C6H6, 4, and 6a·C7H8 were further characterized by X-ray crystallography. The solid-state structure for 6a·C7H8 confirms the presence of a bridging μ2-η1:η1-N-indolide moiety. Variable-temperature NMR spectra of toluene solutions of 6a and 6b are consistent with retention of a bridging μ2-η1:η1-N-indolide group at low temperature, which becomes fluxional in solution at higher temperatures. A mechanism for the fluxional process is proposed.

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