Allyl Ligand Reactivity in Tantalum(V) Compounds: Experimental and Computational Evidence for Allyl Transfer to the Formamidinate Ligand in fac-Ta(NMe₂)₃(η¹-allyl)[ⁱPrNC(H)NⁱPr] via a Metallo-Claisen Rearrangement

Abstract

The first alkyl-amido tantalum(V) compounds having the composition Ta(NMe 2 ) 4 R (where R = Et, i Pr, t Bu, CH 2 SiMe 3 ) were reported by Chisholm et al. in 1982. 1 These monoalkyl derivatives were prepared from TaCl(NMe 2 ) 4 and the appropriate organolithium reagent. The molecular structure of the t Bu derivative revealed a square-pyramidal tantalum compound, and the alkyl group was situated at the apical site. More recently, Xue and co-workers have prepared the related silyl-substituted Ta-(NMe 2 ) 4 R derivatives (where R = SiMe 3 , Si t BuPh2), and the molecular structure of Ta(NMe 2 ) 4 (Si t BuPh2) was found to exhibit a trigonal-bipyramidal geometry with the silyl substituent residing at an equatorial site. 2 To our knowledge, Ta(NMe 2 ) 4 -( t Bu) and Ta(NMe 2 ) 4 (Si t BuPh2), whose structures are shown below, represent the only structurally characterized derivatives for this genre of compounds.Industrial demands for new, single-source tantalum compounds remain high. In particular, new tantalum compounds

Reaction mechanisms: pericyclic reactions

“…Calculations (meta-GGA) TPSS and B3LYP methods revealed that allyl group transfer from tantalum in the fac-Ta(NMe 2 ) 3 (Z 1 -allyl)[ i PrNC(H)N i Pr] Ta(NMe 2 ) 3 -[ i PrNCH(allyl)N i Pr] rearrangement occurs via a [3,3]-sigmatropic rearrangement (Scheme 19). 34 The allyl and the Ta-N-C(formamidinate) fragments participate via a transition structure that adopts a chair-like geometry. The TPSS calculated activation barrier for the metallo-Claisen rearrangement is 5.1 kcal mol À1 lower and in better agreement with the results of NMR experiment than the barrier determined by the B3LYP calculations.…”

Vanadium, niobium and tantalum

“…The former complex reacts with i PrNC(H)NH i Pr to give the five coordinate [Ta(NMe 2 ) 3 { i PrNCH-(allyl)N i Pr}] via a potential metallo-Claisen rearrangement. 52 The reactions between the tantalum dimethyl and diphenyl complexes, [(ONO cat )TaR 2 ] (R = Me, Ph; ONO cat = N,N-bis(3,5-t Bu 2 -2-phenoxide)amide) and aryl azides and 1,2-diphenylhydrazine, were reported. 53 55 The mechanism for the reaction of H 2 with ''end-on-bridged'' dinitrogen bimetallic complex of [Cp*Ta(MeC(N i Pr) 2 ] was examined by DFT studies.…”