Allyl Ligand Reactivity in Tantalum(V) Compounds: Experimental and Computational Evidence for Allyl Transfer to the Formamidinate Ligand in fac-Ta(NMe2)3(1-allyl)[iPrNC(H)NiPr] via a Metallo-Claisen Rearrangement
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abstract
Treatment of TaCl(NMe2)4 (1) with allylMgCl furnishes the allyl-substituted compound Ta(NMe2) 4(1-allyl) (2) in moderate yield. The X-ray structure of 2 reveals a trigonal-bipyramidal geometry at the tantalum center with an equatorially situated 1-allyl moiety. VT 1H NMR measurements confirm that the molecule is fluxional in solution over the temperature range 298-193 K, and DFT calculations indicate that the time-averaged environment exhibited by the allyl moiety in fluid solution derives from a rapid 1-to-3 equilibration, with Ta(NMe2)4(3-allyl) serving as the transition state for this process. 1 reacts rapidly with the formamidine iPrNC- (H)NHiPr to yield fac-TaCl(NMe2) 3[iPrNC(H)NiPr] (5) and Me2NH, and the tantalum product has been characterized by NMR spectroscopy and X-ray diffraction analysis. The five-coordinate compound Ta(NMe2) 3[iPrNCH(allyl)NiPr] (7), whose origin is traced to the putative octahedral species fac-Ta(NMe2) 3(1-allyl)[iPrNC(H)NiPr] (6), has been obtained from the reaction of 2 with iPrNC(H)NH iPr; 7 may also be prepared from the reaction of 5 with allylMgCl. The rearrangement of the allyl moiety in fac- Ta(NMe2) 3(1-allyl)[iPrNC(H)NiPr] to the formamidinate carbon atom in 7 has been investigated by DFT calculations. Here the DFT calculations have provided crucial insight into the reaction mechanism and the composition of those transient species that do not lend themselves to direct spectroscopic observation. The computed barrier for this metallo-Claisen rearrangement is sensitive to the nature of the density functional employed, and the barrier computed using the meta-GGA TPSS functional provides the best agreement with the experimental conditions. The related alkenyl derivatives Ta(NMe2)4(1-3-butenyl) (3) and Ta(NMe 2)3(1-3- butenyl)[iPrNC(H) NiPr] (8) have been synthesized, and their reactivity is contrasted with the corresponding allyl-substituted analogues. 2011 American Chemical Society.
