Cationic aluminum alkyl complexes incorporating aminotroponiminate ligands

Abstract

The synthesis, structures, and reactivity of cationic aluminum complexes containing the N,N'-diisopropylaminotroponiminate ligand (iPr2-ATI-) are described. The reaction of (iPr2-ATI)A1R2 (1a-e,g,h; R = H (a), Me (b), Et (c), Pr (d), iBu (e), Cy (g), CH2Ph (h)) with [Ph3C][B(C6F5)4] yields (iPr2-ATI)A1R+ species whose fate depends on the properties of the R ligand. 1a and 1b react with 0.5 equiv of [Ph3C]-[B(C6F5)4] to produce dinuclear monocationic complexes [{(iPr2-ATI)AIR}2(μ-R)][ (C6F5)4] (2a,b). The cation of 2b contains two (iPr2-ATI)AlMe+ units linked by an almost linear Al-Me-Al bridge; 2a is presumed to have an analogous structure. 2b does not react further with [Ph3C][B(C6F5)4]. However, 1a reacts with 1 equiv of [Ph3C][B(C6F5)4] to afford (iPr2-ATI)Al(C6F5) (μ-H)2B(C6F5)2 (3) and other products, presumably via C6F5- transfer and ligand redistribution of a [(iPr2-ATI)AlH] [(C6F5)4] intermediate. 1c-e react with 1 equiv of [Ph3C]-[B(C6F5)4] to yield stable base-free [(iPr2-ATI)AIR][B (C6F5)4] complexes (4c-e). 4c crystallizes from chlorobenzene as 4c(ClPh)·0.5PhCl, which has been characterized by X-ray crystallography. In the solid state the PhCl ligand of 4c(ClPh) is coordinated by a dative PhCl-Al bond and an ATI/Ph π-stacking interaction. 1g,h react with [Ph3C][B(C6F5)4] to yield (iPr2-ATI)Al(R)(C6F5)(5g,h) via C6F5- transfer of [(iPr2-ATI)A1R]-[ (BC6F5)4] intermediates. 1c,h react with B(C6F5)3 to yield [(iPr2-ATI)Al(R)](C6F5) (5c,h) via C6F5- transfer of [iPr2-ATI)A1R][RB (C6F5)3] intermediates. The reaction of 4c-e with MeCN or acetone yields [(iPr2-ATI)-Al(R)(L)][B (C6F5)4] adducts (L = MeCN (8c-e), acetone (9 c-e)), which undergo associative intermolecular L exchange. 9c-e undergo slow β-H transfer to afford the dinuclear dicationic alkoxide complex [{(iPr2 -ATI)Al(μ-OiPr)}2][B (C6F5)4]2 (10) and the corresponding olefin. 4c-e catalyze the head-to-tail dimerization of tert-butyl acetylene by an insertion/σ-bond metathesis mechanism involving [(iPr2-ATI)Al(C≡CtBu)]- [B(C6F5)4] (13) and [(iPr2-ATI)Al(CH =C(tBu)C≡Ct(Bu)] [B(C6F5)4] (14) intermediates. 13 crystallizes as the dinuclear dicationic complex [{(iPr2-ATI)Al (μ-C≡tBu)}2] [B(C6F5)4]2·5PhCl from chlorobenzene. 4e catalyzes the polymerization of propylene oxide and 2a catalyzes the polymerization of methyl methacrylate. 4c,e react with ethylene-d4 by β-H transfer to yield [(iPr2- ATI)AlCD2CD2H][B (C6F5)4 initially. Polyethylene is also produced in these reactions by an unidentified active species.