Aluminium (III) dialkyl 2,6-bisimino-4R-dihydropyridinates(-1): Selective Synthesis, Structure and Controlled Dimerization.
A family of stable and otherwise selectively unachievable 2,6-bisimino-4-R-1,4-dihydropyridinate aluminium (III) dialkyl complexes [AlR’2(4-R´-iPrBIPH)] (R= Bn or Allyl; R’ Me, Et or iBu) have been synthesized, taking advantage of a method for the preparation of the corresponding 4-R-1,4-dihydropiridine precursors developed in our group. All the dihydropyrdinate(-1) dialkyl aluminium complexes have been fully characterized by 1H- 13C-NMR, elemental analysis and in the case 2´a, also by X-ray diffraction studies. Upon heating in toluene solution at 110 ºC, the dimethyl derivatives 2a and 2´a dimerize selectively through a double cycloaddition. This reaction leads to the formation of two new C-C bonds that involve the both meta positions of the two 4-R-1,4-dihydropyridinate fragments, resulting the binuclear aluminium species [Me2Al(4-R-iPrHBIP)]2 (R = Bn (3a); Allyl (2a)). Experimental kinetics showed that the dimerization of 2´a obeys second order rate with negative activation entropy, which is consistent with a bimolecular rate-determining step. Controlled methanolysis of both 3a and 3´a release the metal-free dimeric bases, (4-Bn-iPrHBIPH)2 and (4-Allyl-iPrHBIPH)2, providing a convenient route to these potentially useful ditopic ligands. When the R’ groups are bulkier than Me (2b, 2´b and 2´c), the dimerization is hindered or fully disabled, favoring the formation of paramagnetic NMR-silent species, which have been identified on the basis of a controlled methanolysis of the final organometallic products. Thus, when a toluene solution of [AlEt2(4-Bn´-iPrBIPH)] (2b) was heated at 110ºC, followed by the addition of methanol in excess, it yields a mixture of the dimer (4-Bn-iPrHBIPH)2 and the aromatized base 4-Bn-iPrBIP , in ca. 1:2 ratio, indicating that the dimerization of 2b competes with its spontaneous dehydrogenation, yielding a paramagnetic complex containing a AlEt2 unit and a non-innocent (4-Bn-iPrBIP)·- radical-anion ligand. Similar NMR monitoring experiments on the thermal behavior of [AlEt2(4-Allyl-iPrBIPH)] (2´b) and [AlBui2(4-Allyl-iPrBIPH)] (2´c) showed that these complexes do not dimerize, but afford exclusively NMR silent products. When such thermally treated samples were subjected to methanolysis, they resulted in mixtures of the alkylated 4-allyl-iPrBIP and non-alkylated iPrBIP ligand, suggesting that dehydrogenation and deallylation reactions take place competitively.
- This article is part of the themed collection: Inorganic chemistry of the p-block elements