Ligand-tetrahydrofuran coupling in chelated aluminum phosphinates

When the reagents LAIMe (L = N,N-(alkylene or arylene)bis(3,5-di-tert-butyl)salicylideneimine (alkylene = ethylene (Salen(tBu)) (1); propylene (Salpen(tBu)) (2), and butylene (Salben(tBu)) (3); arylene = phenylene (Salophen(tBu) (4), 3,4-dimethylphenylene (Salomphen(tBu) (5)) are combined with Ph(H)P(O)OH in tetrahydrofuran (thf) the unique aluminophosphinate compounds, [L(tBu)Al{O₂P(H)Ph}]_n with L, n = Salen, ∞ (6), Salpen, 2 (7), Salben, 2 (8), Salophen, ∞ (9) and Salomphen, ∞ (10) are produced. The yields for the latter two reactions are low and it was subsequently found that the unique thf-coupled compounds appear in the thf filtrates of the original reaction mixture. These compounds are, [L-thf(tBu)Al{O₂P(H)Ph}]₂, L = Salophen (13) and Salomphen (14). The thf connects through an α-carbon to only one of the two possible imine carbons of the ligand. While trying to determine how this coupling proceeds, the six-coordinate, solution-state species LAIMe(thf) (L = Salophen (11) and Salomphen (12) were discovered and implicated as intermediates. All of the compounds are characterized by melting point, NMR, IR, and X-ray analyses for 5-8, 13, and 14. A possible mechanism for the thf coupling event is presented.