Reactivity of tantalum hydride aryloxide complexes toward organic isocyanide reagents

The tantalum monohydride compound [Ta(OC6H3Pr2i-2,6)(2)Cl-2(H)(PMe(2)Ph)(2)] (1) reacts with organic isocyanides RNC to produce the phosphine adduct eta(2)-iminoformyl complexes [Ta(OC6H3Pr2i-2,6)(2)Cl-2{HC(PMe(2)Ph)NR}] (R=2,6-diisopropylphenyl (2a), 2,6-dimethylphenyl (2b), tert-butyl (2c)). Compounds 2b,c were only detected spectroscopically as intermediates, while 2a was isolated and reacted with PMe(3) to produce the complex [Ta(OC6H3Pr2i,2,6)(2)Cl-2{HC(PMe(3))NC6H3Pr2i-2,6}] (3), which was structurally characterized. The structure of 3 is best described as trigonal bipyramidal about tantalum with a chloride atom and the phosphine adduct of an eta(2)-iminoformyl group occupying axial sites. Important structural parameters for 3 are Ta-N=1.956(4), Ta-C=2.210(5), and N-C=1.421(7) Angstrom. Compounds 2a and 3 do not react with a further 1 equiv of 2,6-diisopropylphenyl isocyanide. In contrast 2b reacts with 2,6-dimethylphenyl isocyanide to produce the ylide derivative [Ta(OC6H3Pr2i-2,6)(3)Cl-2{RN=CHC(=PMe(2)Ph)NR}] (4) (R=C(6)H(3)Me(2)-2,6), while 2c reacts with an additional 2 equiv of Bu(t)NC to produce the compound [Ta(OC6H3Pr2i-2,6)(3)Cl-2{Bu(t)N=CHC(=C=NBu(t))NBu(t)}] (5). Spectroscopic data on 4 and 5 and a single-crystal X-ray diffraction study of 5 show both molecules contain 2,5-diazacyclopent-2-ene rings with either ylide (4) or keteneimine (5) groups attached to the C-4 position. Addition of Bu(t)NC to 2a produces the complex [Ta(OC6H3Pr2i-2,6)(2)Cl-2{RN=CHC(=C=NBu(t))NBu(t)}] (6) (R=C6H3Pr2i-2,6). The tantalum dihydride compound [Ta(OC(6)H(3)Bu(2)(t)-2,6)(2)Cl-2(H)(PMePh(2))] (7) reacts with RNC to produce the monocyclometalated derivatives [Ta(OC(6)H(3)Bu(t)-CMe(2)CH(2))(OC(6)H(3)Bu(2)(t)-2,6)Cl-2{N(R)CH3}] (8) (a, R=C(6)H(3)Me(2)-2,6; b, R=Bu(t)), in which a total of three hydride groups have been transferred to the isocyanide substrate to produce the dialkylamido ligand.