Insertion reactions of tantalum(V) carborane alkyl and aryl complexes with nitriles and isonitriles. Thermal and photochemical isomerization of eta(2)-iminoacyl isomers

Tantalum carborane complexes (Et2C2B4H4)CpTaR2 (R = Me, Ph) undergo insertion reactions with nitriles and isonitriles under conditions that differ from formally isoelectronic metallocene compounds of group 4 metals. The dimethyltantalum system requires UV-vis irradiation, whereas the diphenyltantalum complex generally reacts at elevated temperatures. Although insertions of isonitriles into metal-alkyl bonds have been reported in a wide variety of cases, there are very few examples of stable isomeric eta(2)-iminoacyl intermediates that can be isolated and completely converted from one form into the other. Two such cases are reported here, including the X-ray crystallographic analysis of both isomers of (Et2C2B4H4)CpTaCH3(eta(2)-C,N-C{=N-t-Bu}CH3) (5a,5b) and one isomer of (Et2C2B4H4)CpTaCH3(eta(2)-C,N-C{=N(2,6-Me2C6H3)}C6H5)(7a). Photochemical isomerization of 7a, followed by exposure to an additional equivalent of isocyanide, resulted in a novel cage-insertion reaction to give [(Et2C2B4H3(C6H5)(CHN(2,6-Me2C6H3))]CpTa(eta(2)-C,N-C{=N-(2,6-Me2C6H3)}C6H5) (8), the crystal structure of which was also determined. Stereoelectronic and steric factors that may contribute to the thermodynamic stability of N-out isomers of eta(2)-iminoacyl complexes are suggested, and data indicating the relatively electron-rich nature of the (carborane)(Cp))(TaR2)-R-V fragment is identified.