Alkylidene-centered rearrangement of a tantalum alkylidene alkoxide species with the N,C,N-bis-ortho-chelated 1,2,6-trisubstituted aryldiamine ligand [C6H3(CH2NMe2)2-2,6] - to a product with a C,N-mono-ortho-chelated 1,2,4-Trisubstituted aryldiamine ligand

The reaction of 2 equiv of LiO-t-Bu with [TaCl₂(=CH-t-Bu){C₆H₃(CH₂NMe ₂)₂-2,6}] (1), in which there is an N,C,N-chelated 1,2,6-trisubstituted aryldiamine ligand, affords in a one-pot procedure at 80 °C the new rearranged product [Ta(=CH-t-Bu){C₆H₃(CH₂NMe₂) ₂-2,4}-(O-t-Bu)₂] (2), in which there is a C,N-chelated 1,2,4-trisubstituted aryldiamine ligand. Complex 2 is a yellow solid that has been isolated in 69% yield. The reaction mechanism for the formation of 2 involves a crucial isomerization of the intermediate [TaCl(=CH-t-Bu){C₆H₃(CH₂NMe₂) ₂-2,6}(O-t-Bu)] (3) to the rearranged intermediate [TaCl(=CH-t-Bu)-{C₆H₃(CH₂NMe₂) ₂-2,4}(O-t-Bu)] (4). The intermediate complexes 3 and 4 have been independently prepared and characterized. The known complex 3 can be obtained by reaction of 1 with LiO-t-Bu at room temperature. Complex 4 is obtained exclusively, as revealed by ¹H NMR spectroscopy, by heating a benzene solution of 3 to 80 °C and has been isolated as a purple solid in 77% yield. In solution 4 exists as two rotational isomers for the Ta=C-(H)-t-Bu moiety; ΔG^‡ = 71 kJ mol^-1. The X-ray molecular structure of 3 shows it to be a pentacoordinate Ta(V) species in which the aryl C_ipso atom, the alkylidene functionality, and the alkoxide group define the meridional plane of a trigonal bipyramid, with one of the NMe₂ nitrogen donors of the C,N-bidentate-bonded aryldiamine and the chloride occupying the axial positions. The lone pair of the N-donor atom of the second ortho amine substituent is oriented toward Ta (Ta⋯N = 2.629.4 A°), providing incipient η³(N,C,N) facial bonding of the aryldiamine ligand. The structure of 3 also shows a pseudo-parallel orientation of the alkylidene C_α-H_α bond and the C_ipso-Ta bond that points to potential C-H activation via a four-membered metallacyclic ring that contains Ta, C_ipso, C_α, and H_α. The involvement of the alkylidene functionality in the sequence of highly regiospecific C-H bond-making and -breaking processes necessary to produce complex 4 from 3 was confirmed by deuterium-labeling experiments. The mechanism probably involves an a-H abstraction from the alkylidene moiety in 3 (assisted by the weakly coordinated dimethylamino group) that leads to an intermediate which has a geometry similar to that of a known aryltantalum(V) zinc alkylidene adduct.