Synthesis, structures, and kinetics and mechanism of decomposition of terminal metal azide complexes: Isolated intermediates in the formation of imidometal complexes from organic azides

Treatment of Cp(2)Ta(CH3)(PMe(3)) (Cp = eta(5)-C5H5) with aryl azides ArN3 (Ar = C6H5, p-CF3C6H4, P-NMe(2)C(6)H(4)) affords rare examples of terminal phenylazido complexes Cp(2)Ta(CH3)(N3Ar) and free phosphine in >85% yield. CP2Ta(CH3)(N3C6H5) (la) crystallizes in the space group P2(1) with the following parameters: a = 6.6838(13 Angstrom, b = 13.4491(17) Angstrom, c = 20.082(3) Angstrom, beta = 98.714(14)degrees, V = 1784.4(9) Angstrom(3), Z = 4, R = 2.36%, and R(w) = 2.82%. Cp(2)Ta(CH3)(N-3-p-NMe(2)-C6H4) (1b) crystallizes in the space group Pna2(1) with the following parameters: a = 23.275(5) Angstrom, b = 6.497(2) Angstrom, c = 11.621(2) Angstrom, orthorhombic, V= 1757.3(12) Angstrom(3), Z = 4, R = 1.53%, and R(w) = 1.70%. Heating the terminal azide complexes to 70 degrees C for several hours or subjecting them to UV irradiation for minutes affords dinitrogen and the previously characterized imido species Cp(2)Ta(CH3)(= NAr) in quantitative yield. Treatment of the alpha-N-15-labeled azide Ph-N-15 = N = N with Cp(2)Ta(PMe(3))(CH3) gives Cp(2)Ta(NN(15)NPh)(CH3), and thermolysis of this material leads to Cp(2)Ta(CH3)((15)NPh). This shows conclusively that the nitrogen atom that is originally bound to the Ta center is incorporated into the free N-2 product. A double-labeling experiment was also undertaken in which equimolar amounts of Cp(2)Ta((CH3)-C-13)(NNNPh) and Cp(2)Ta(CH3)(NN(15)NPh) were shown to give only the singly labeled imido species Cp(2)Ta(CH3)((15)NPh) and Cp(3)Ta((CH3)-C-13)(NPh). The rate of decomposition of the azide complexes was strongly affected by traces of air. However, in the presence of the powerful O-2 and H2O scavenger Cp(2)Zr(CH3)(2), the conversion of Cp(2)Ta(CH3)(N3C6H5) to Cp(2)Ta(CH3)(= NC6H5) was found to be first order in [Cp(2)Ta(CH3)(N3C6H5)] at all concentrations and temperatures examined. Rates measured over a 40 degrees C temperature range gave Eyring parameters Delta H(n)ot equal = 25.4 kcal/mol and Delta S double dagger = -0.7 eu. A Hammett sigma/p investigation using para-substituted phenylazido complexes demonstrated that the rate of dinitrogen loss is accelerated by the presence of electron-withdrawing groups on the azide nitrogen. The mechanistic implications of these results are discussed.