SYNTHESIS, STRUCTURES, AND KINETICS AND MECHANISM OF DECOMPOSITION OFTERMINAL METAL AZIDE COMPLEXES - ISOLATED INTERMEDIATES IN THE FORMATION OF IMIDOMETAL COMPLEXES FROM ORGANIC AZIDES
G. Proulx et Rg. Bergman, SYNTHESIS, STRUCTURES, AND KINETICS AND MECHANISM OF DECOMPOSITION OFTERMINAL METAL AZIDE COMPLEXES - ISOLATED INTERMEDIATES IN THE FORMATION OF IMIDOMETAL COMPLEXES FROM ORGANIC AZIDES, Organometallics, 15(2), 1996, pp. 684-692
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 = 13.4
491(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(C
H3)(N-3-p-NMe(2)-C6H4) (1b) crystallizes in the space group Pna2(1) wi
th the following parameters: a = 23.275(5) Angstrom, b = 6.497(2) Angs
trom, 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 comple
xes to 70 degrees C for several hours or subjecting them to UV irradia
tion for minutes affords dinitrogen and the previously characterized i
mido species Cp(2)Ta(CH3)(= NAr) in quantitative yield. Treatment of t
he 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 at
om 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 C
p(2)Ta(CH3)((15)NPh) and Cp(3)Ta((CH3)-C-13)(NPh). The rate of decompo
sition 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)(= NC6
H5) was found to be first order in [Cp(2)Ta(CH3)(N3C6H5)] at all conce
ntrations and temperatures examined. Rates measured over a 40 degrees
C temperature range gave Eyring parameters Delta H(n)ot equal = 25.4 k
cal/mol and Delta S double dagger = -0.7 eu. A Hammett sigma/p investi
gation 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 implication
s of these results are discussed.