Reactions of α-diimine-aluminum complexes with sodium alkynides: versatile structures of aluminum σ-alkynide complexes

Reaction of AlCl₃ with the monoanionic α-diimine ligand [NaL] yielded the complex [L^•-Al^IIICl₂^-] (1, L = [(2,6-iPr₂C₆H₃)NC(Me)]₂), and subsequent reduction of 1 by sodium metal afforded the mononuclear [L^2-Al^IIICl^-(THF)] (2) and binuclear [L^2-(THF)Al^II-Al^II(THF)L^2-] (3). Compounds 2 and 3 exhibit interesting reactivities to sodium alkynides at room temperature. Treatment of dialumane 3 with 1 equiv. of 4-methylphenylacetylene in the presence of sodium metal yielded the asymmetric Al-Al-bonded compound [Na(Et₂O)][LAl-Al(C≡C(C₆H₄-Me))L] (4) containing an alkynyl group attached to one of the Al atoms. The reaction of 2 with 4-methylphenylacetylene and Na (or sodium 4-methylphenylacetylide) resulted in the mononuclear product [L(THF)Al(C≡C-(C₆H₄-Me))] (5) containing a single terminal acetylide ligand. Precursor 2 reacted with 2 equiv. of phenylacetylene (or 4-methylphenylacetylene, trimethylsilylacetylene) and Na to give the tweezer "ate" complexes, [Na(THF)(DME)][LAl(C≡CR)₂] (R = C₆H₅, 6a; C₆H₄-Me, 6b; Si(Me)₃, 6c), [Na(THF)]₂[LAl(C≡CPh)₂]₂(μ-C₇H₈) (7), [Na(C₇H₈)][(μ-Na)][LAl(C≡CSi(Me)₃)₂]₂ (8), as well as the polymeric [LAl(C≡CPh)₂Na]_n (9). In the products, two alkynyl groups coordinate terminally to one Al center and a sodium ion is embedded between these two alkynyls. Interestingly, both cycloaddition and terminal acetylide coordination of three equiv. of alkyne occurred in the reaction of 2 with 1-hexyne, resulting in the unique dialuminum complex [Na(Et₂O)]₂[{L(C(C₄H₉)=CH)}Al(C≡C(C₄H₉))₂]₂ (10). Complexes 1-10 have been characterized by NMR (¹H, ¹³C) and IR spectroscopy, elemental analysis, and X-ray diffraction, and their electronic structures were studied by DFT calculations.