Reactivity of Tantalum Hydride Aryloxide Complexes toward Organic Isocyanide Reagents

The tantalum monohydride compound [Ta(OC 6 H 3 Pr i 2 -2,6) 2 Cl 2 (H)(PMe 2 Ph) 2 ] ( 1 ) reacts with organic isocyanides RNC to produce the phosphine adduct η 2 -iminoformyl complexes [Ta(OC 6 H 3 Pr i 2 -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(OC 6 H 3 Pr i 2 -2,6) 2 Cl 2 {HC(PMe 3 )NC 6 H 3 Pr i 2 -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 η 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) Å. 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(OC 6 H 3 Pr i 2 -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(OC 6 H 3 Pr i 2 -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(OC 6 H 3 Pr i 2 -2,6) 2 Cl 2 {RN CHC( C NBu t )NBu t }] ( 6 ) (R = C 6 H 3 Pr i 2 -2,6). The tantalum dihydride compound [Ta(OC 6 H 3 Bu t 2 -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 t 2 -2,6)Cl 2 {N(R)CH 3 }] ( 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.