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 - X-RAY MOLECULAR-STRUCTURE OF [TACL(=CH-T-BU)(C6H3(CH2NME2)(2)-2,4)(O-T-BU)]

The reaction of 2 equiv of LiO-t-Bu with [TaCl2(=CH-t-Bu){C6H3(CH2NMe2 )(2)-2,6}] (1), in which there is an N,C,N-chelated 1,2,6-trisubstitut ed aryldiamine ligand, affords in a one-pot procedure at 80 degrees 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 aryldi amine ligand. Complex 2 is a yellow solid that has been isolated in 69 % yield. The reaction mechanism for the formation of 2 involves a cruc ial 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) {C6H 3(CH2NMe2)(2)-2,4}(O-t-Bu)] (4). The intermediate complexes 3 and 4 ha ve been independently prepared and characterized. The known complex 3 can be obtained by reaction of 1 with LiO-t-Bu at room temperature, Co mplex 4 is obtained exclusively, as revealed by H-1 NMR spectroscopy, by heating a benzene solution of 3 to 80 degrees C and has been isolat ed as a purple solid in 77% yield. in solution 4 exists as two rotatio nal isomers for the Ta=C-(H)-t-Bu moiety; Delta G(double dagger) = 71 kJ mol(-1). The X-ray molecular structure of 3 shows it to be a pentac oordinate 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-b identate-bonded aryldiamine and the chloride occupying the axial posit ions. The lone pair of the N-donor atom of the second ortho amine subs tituent is oriented toward Ta (Ta ... N = 2.629.4 Angstrom), providing incipient eta(3)(N,C,N) facial bonding of the aryldiamine ligand. The structure of 3 also shows a pseudo-parallel orientation of the alkyli dene C-alpha-H-alpha bond and the C-ipso-Ta bond that points to potent ial C-H activation via a four-membered metallacyclic ring that contain s Ta, C-ipso, C-alpha, and H-alpha. 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 conf irmed by deuterium-labeling experiments. The mechanism probably involv es 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.