Synthesis of Neutral and Cationic Monocyclopentadienyl Tantalum Alkoxo Complexes and Polymerization of Methyl Methacrylate

Abstract

The alkoxo compounds [TaCp*Me 3 (OR)] (R ) SiiPr 3 , 1a; 2,6-Me 2 C 6 H 3 , 1b; 2,6-iPr 2 C 6 H 3 , 1c) were obtained by reaction of [TaCp*Me 4 ] with the corresponding alcohol ROH, at room temperature for R ) SiiPr 3 and upon heating at 100 °C over 2 and 3 days for R ) 2,6-Me 2 C 6 H 3 and 2,6-iPr 2 C 6 H 3 , respectively. The alkoxo cations [TaCp*Me 2 (OR)] + (R ) SiiPr 3 , 2a; 2,6-Me 2 C 6 H 3 , 2b; 2,6-iPr 2 C 6 H 3 , 2c) were formed when one of the methyl groups of the neutral trimethyl complexes 1 was dissociated by reaction with 1 equiv of B(C 6 F 5 ) 3 to generate the borate anion [MeB(C 6 F 5 ) 3 ] -(B) and with 1 and 2 equiv of Al(C 6 F 5 ) 3 to give the mononuclear [MeAl(C 6 F 5 ) 3 ] -(Al) and dinuclear [Me{Al(C 6 F 5 ) 3 } 2 ] -(Al2) aluminate anions, respectively. A similar reaction of the neutral compound [TaCp*Me 4 ] with 1 and 2 equiv of Al(C 6 F 5 ) 3 gave salts of the [MeAl(C 6 F 5 ) 3 ] -(Al) and [Me{Al(C 6 F 5 ) 3 } 2 ] -(Al2) anions containing the [TaCp*Me 3 ] + (3) cation. Compounds 1 and [TaCp*Me 4 ] did not show catalytic activity for MMA polymerization when B(C 6 F 5 ) 3 was used as cocatalyst but were active in the presence of Al(C 6 F 5 ) 3 , yielding high molecular weight syndiotactic PMMA.

Synthesis of neutral and cationic monocyclopentadienyl alkyl niobium and tantalum complexes

“…This result can be interpreted in terms of the steric hindrance imposed by the presence of two aryloxo groups that avoid the proximity of cation and anion necessary for the initiation step, as was also observed for [TaCp * -Me 3 (OAr)] (Ar ¼ 2,6-iPr 2 C 6 H 3 ) [42].…”

“…The 19 F NMR spectrum of 5B confirmed this behaviour, with Dd (m,p-F) < 3 [53]. All of these ionic complexes 4 and 5 decomposed within several hours in BrC 6 D 5 solutions and much faster in CD 2 Cl 2 , in contrast to the stability observed for related tantalum derivatives [16,42].…”

“…H 3 ) aryloxo ligand of compound 2c might be the reason for the higher stability of the active species, favoring the polymerization process [42]. In contrast to the related tantalum compound [TaCp * Me 3 X] (X ¼ Me, OR), increasing temperature did not improve polymerization results with any of these compounds.…”

“…Recently, we showed that protonolysis of a TaeMe bond in complex [TaCp * Me 4 ] with several alcohol and silanol compounds was a suitable method for the synthesis of new monocyclopentadienyl complexes [TaCp * Me 3 (OR)] (R ¼ SiiPr 3 , 2,6-Me 2 C 6 H 3 , 2,6-iPr 2 C 6 H 3 ) [42]. We also reported their reactions with the Lewis acids E(C 6 F 5 ) 3 (E ¼ B, Al) and studies on polymerization of MMA with the systems [TaCp * Me 3 X]/E(C 6 F 5 ) 3 , observing that the polymerization was dependent on the X group (X ¼ Me, OR) and E (E ¼ B, Al).…”

Synthesis and Reactivity of Oxametallacyclic Niobium Compounds by Using α,β‐Unsaturated Carbonyl Ligands

“…[12][13][14][15][16] Although the nucleophilicity of the oxygen atom bonded to the niobium atom could compete with that of the carbon atom, we have previously observed that alkoxido complexes of the type [Cp*TaMe 3 (OR)] reacted with these Lewis acids E(C 6 F 5 ) 3 at the Ta-C bond. [32] Furthermore, Mashima and co-workers also reported that reaction of the MMA complex [TaCp*(DAD)(MMA)] [DAD = N(R) CH=CHN(R)] with AlMe 3 produced the activation of the Ta-C bond of the coordinated MMA ligand. [11] Addition of 1 equiv.…”