SYNTHESIS OF CYCLOMETALATED ARYLTANTALUM(V) COMPLEXES - AMINE C-H ACTIVATION VIA SIGMA-BOND METATHESIS

Cyclometalated aryltantalum(V) complexes containing a five-membered Ta-C-C-C-N chelate ring can be prepared by transmetalation reactions. The 1:2 reaction of [Zn(C6H4CH2NMe2-2)2] with TaCl5 affords [TaCl4(C6H4CH2NMe2-2)] (1a) in high yield. [TaCl4{C6H4CH(Me)NMe2-2}] (1b) (S enantiomer) and [TaCl4(C6H3CH2NMe2-2-Me-6)] (lc) can be prepared similarly. The reaction of la and 1c with a half-equiv of [Zn(CH2Ph)2] affords the monobenzyl substituted analogues [TaCl3(C6H4CH2NMe2-2)(CH2Ph)] (2a) and [TaCl3(C6H3CH2NMe2-2-Me-6((CH2Ph)] (2c). The 2:1 reaction of [TaCl3(CH2Ph)2] With [Zn(C6H4CH2NMe2-2)2] affords the dibenzyl species [TaCl2(C6H4CH2NMe2-2)(CH2Ph)2] (3a) in good yield. [TaCl2{C6H4CH(Me)NMe2-2}(CH2Ph)2] (3b) and [TaCl2(CH2C6H4Me2-2)(CH2Ph)2] (3d) can be prepared similarly. An analogous transmetalation of [TaCl3(CH2Ph)2] with/2 [Li(l-C10H6Me2-8)]2 affords [TaCl2(1-C10H6NMe-8)(CH2Ph)2] (3e). [TaCl2(C6H4CH2NMe2-2)(CH2Ph)2] (3a) belongs to the space group P1BAR with a = 9.055 (2) angstrom, b = 9.614 (1) angstrom, c = 13.910 (3) angstrom, alpha = 72.93 (1)-degrees, beta = 80.91 (2)-degrees, gamma = 72.74 (1)-degrees, and Z = 2. The overall structure is distorted octahedral with the Ta-C-C-N chelate ring and both benzyl ligands in the equatorial plane. In solution all complexes 3 are fluxional: at room temperature the benzyl ligands are rendered equivalent by a sequence of pseudorations that does not involve dissociation of the Ta-N coordinative bond. For 3b and 3e in solution rupture of the Ta--N bond (at ca. 60-degrees-C) leads to a (stereoselective) NMe C-H activation, proceeding via a sigma-bond metathesis step, that gives [TaCl{C6H4(CH(Me)N(Me)CH2)-2}(CH2Ph)(THF)] (4b) and [TaCl2{1-C10H6(N(Me)CH2)-8}(CH2Ph)(THF)2] (4e), respectively, in moderate yield. 4b belongs to the space group P1BAR with a = 9.218 (1) angstrom, b = 9.974 (1) angstrom, c = 25.930 (1) angstrom, alpha = 85.75 (1)-degrees, beta = 85.70 (2)-degrees, gamma = 64.97 (1)-degrees, and Z = 4. The overall structure is distorted pentagonal bipyramidal. The chirality of the arylamine leads to a single, structurally characterized diastereoisomer in which intramolecular coordination of the modified amine group produces a triangular Ta-CH2-N unit.