SYNTHESIS, CHARACTERIZATION, AND REACTIONS OF A MONONUCLEAR TANTALUM-BENZYNE COMPLEX, TA(ETA(5)-C(5)ME(5))(ETA(4)-C4H6)(ETA(2)-C6H4)

The preparation and crystal structure of the tantalum-benzyne complex TaCp*(eta(4)-buta-1,3-diene)(eta(2)-C6H4) (8) (Cp* = eta(5)-pentamethylcyclopentadienyl) together with its reactions were studied. The benzyne complex 8 was prepared by thermolysis (70 degrees C, 21 h) of the methyl phenyl complex Ta(CH3)(C6H5)CP*(eta(4)-buta-1,3-diene) (7), which was synthesized by successive treatment of TaCl(2)Cp*(eta 4-buta-1,3-diene) (5) with 1 equiv of PhMgI and with 1 equiv of MeMgI in THF. The rate of thermolysis of 7 in C6D6 measured at the temperature range between 50 and 75 degrees C gave the thermodynamic data Delta G double dagger(55 degrees C) = 27.2 +/- 0.5 kcal/ mol. In the C-13 NMR spectrum of 8, a signal due to the ipso carbon atoms of benzyne was observed at 203.0 ppm. The monomeric structures of 7 and 8 were confirmed by single-crystal X-ray analysis. Complex 8 is the first example of the parent benzyne complex bearing a metallocene-like MCp(diene) fragment and thus expected to react with a wide range of organic substrates. Ethylene and 2-butyne inserted into the tantalum benzyne bond of 8 to form the metallacycles TaCp*(C6H4CH2CH2)(eta(4)-C4H6) (10) and TaCp*(C6H4CH2CH2)(eta(4)-C4H6) (11), respectively. Acetonitrile and carbon dioxide inserted also to give TaCp*{C6H4C=(Me)=N}(eta(4)-C4H6) (12) and TaCp*(C6H4C(-O)O)(eta(4)-C4H6) (13), respectively. Complex 8 was protonated by methanol and 3,3-dimethyl-1-butyne to form phenyl complexes Ta(OMe)(Ph)-CP*(eta(4)-C4H6) (14) and Ta(C=CCMe(3))(Ph)Cp*(eta(4)-C4H6) (15), respectively. The structures of 11, 12, 14, and 15 were elucidated by single-crystal X-ray analysis.