Aluminium(iii) dialkyl 2,6-bisimino-4: R -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'(4-R-PrBIPH)] (R = Bn, Allyl; R′ = Me, Et, Bu) 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 H- C-NMR, elemental analysis and in the case 2′a, also by X-ray diffraction studies. Upon heating in toluene solution at 110 °C, 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 [MeAl(4-R-PrHBIP)] (R = Bn (3a); allyl (3′a)). 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-PrHBIPH) and (4-allyl-PrHBIPH), 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 [AlEt(4-Bn-PrBIPH)] (2b) was heated at 110 °C, followed by the addition of methanol in excess, it yields a mixture of the dimer (4-Bn-PrHBIPH) and the aromatized base 4-Bn-PrBIP, in ca. 12 ratio, indicating that the dimerization of 2b competes with its spontaneous dehydrogenation, yielding a paramagnetic complex containing a AlEt unit and a non-innocent (4-Bn-PrBIP) radical-anion ligand. Similar NMR monitoring experiments on the thermal behavior of [AlEt(4-allyl-PrBIPH)] (2′b) and [AlBu(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-PrBIP and non-alkylated PrBIP ligand, suggesting that dehydrogenation and deallylation reactions take place competitively.Peer Reviewe

Kinetic and mechanism of alkene polymerization

Triphenylmethyl salts of the very weakly-coordinating borate anions [CN{B(C6F5)3}2]- ( 1 ), [H2N{(C6F5)3}2]- and [M{CNB(C6F5)3}4]2- (M = Ni, Pd) have been prepared in simple one-pot reactions. Mixtures of (SBI)ZrMe2/ 1 /AlBu 3 i (SBI = rac-Me2Si(Ind)2) are 30–40 times more active in ethylene polymerizations at 60–100°C than (SBI)ZrCl2/MAO. The quantification of anion effects on propene polymerization activity at 20°C gives the order [CN{B(C6F5)3}2]- > [H2N{(C6F5)3}2]- ˜ B(C6F5) 4 - » [MeB(C6F5)3]-. The highest productivities were of the order of ca. 3.0 × 108 g PP (mol Zr)-1 h-1 [C3H6]-1, about 1.3–1.5 times higher than with B(C6F5) 4 - as the counter anion. The titanium system CGCTiMe2/ 1 /AlBu 3 i gave activities that were very similar to the zirconocene catalyst. The concentration of active species [C*] as determined by quenched-flow kinetic techniques indicates typical values of around 10%, independent of the counter anion, for both the borate and MAO systems. Pulsed field-gradient spin echo and nuclear Overhauser effect NMR experiments on systems designed to be more realistic models for active species with longer polymeryl chains, (SBI)M(CH2SiMe3)(µ-Me)B(C6F5)3 and [(SBI)MCH2SiMe 3 + ...B(C6F5) 4 - ] (M = Zr, Hf), demonstrated the influence of bulky alkyl chains on the ion pair solution structures: while the MeB(C6F5)3 compound exists as a simple inner-sphere ion-pair, the B(C6F5) 4 - compound is an outer-sphere ion pair (OSIP), a consequence of the relegation of the anion into the second coordination sphere by the ?-agostic interaction with the alkyl ligand. The OSIP aggregates to ion hextuples (10 mM) or quadruples (2 mM). Implications for the polymerization mechanism are discussed; the process follows an associative interchange (I a) pathway