C O M M U N I C A T I O N S
Scheme 1
Though the putative zirconium-imido complex could not be isolated
from this reaction, in the presence of 1,2-diphenylhydrazine it
reacted further to release (p-tolyl)NH2 and azobenzene.
A ZrdNPh intermediate could be formed in two ways (step c).
After coordination of 1,2-diphenylhydrazine to 1, an R,â-NH
elimination of aniline could form the imide directly. This path is
analogous to the heterolytic activation of a peroxide O-O bond
by a transition metal.13 Alternatively, the imide could be formed
by N-N oxidative addition14 followed by 1,2-NH elimination from
the resulting zirconium bis(amide) complex.15 We do not favor this
path because we have seen no evidence for N-N oxidative addition
reactivity with hydrazines such as Me2N-NMe2 and 1-dimethyl-
aminopyrrole; however, we cannot rule out a reversible oxidative
addition followed by fast 1,2-NH elimination to generate the
zirconium imido species.
Complex 1 is the first example of a d0 metal complex that
catalyzes a multielectron reaction through the use of ligand-based
valence changes. While the disproportionation of 1,2-diphenylhy-
drazine is thermodynamically favorable, there are few examples
of catalysts for this reaction.16 In the case of 1, the electrophilic
properties of the zirconium center coupled with the redox properties
of the ligand enable catalytic turnover in the system. Further studies
are required to determine if auxiliary reagents can be used to realize
selective N-N oxidations or reductions.
of 1,2-diphenylhydrazine to a yellow solution of 1a resulted in an
immediate color change to dark green. Upon completion of the
reaction the color change lightened to orange. The 1H NMR
spectrum of the final mixture revealed 1 equiv of azobenzene, 2
equiv of aniline, and [N2O2red]ZrL3 (1b, L ) NH2Ph). GC-MS
confirmed the identity and quantities of the organic products.
Increased equivalents of 1,2-diphenylhydrazine led to longer
reaction times but yielded the same product ratios. Thus, 10 equiv
of hydrazine yielded 5 equiv of azobenzene and 10 equiv of aniline
after 1 day. Reactions with 100 equiv of 1,2-diphenylhydrazine were
completed in 6 days.
Acknowledgment. The authors thank Dr. Mason Haneline for
assistance with X-ray data collection. This work was supported by
the NSF-CAREER program (CHE-0645685).
Supporting Information Available: Detailed experimental pro-
cedures, characterization data for 1a and 2, X-ray diffraction data, and
further discussion of reactive oxidizing species. This material is
2PhHN-NHPh 9
18 PhNdNPh + 2NH2Ph
References
(1) Schrock, R. R.; Glassman, T. E.; Vale, M. G. J. Am. Chem. Soc. 1991,
113, 725-726.
Preliminary experiments to elucidate the mechanism of the 1,2-
diphenylhydrazine reaction have been conducted. Concentration-
dependence studies suggest that the reaction rate is first order in 1
and first order in 1,2-diphenylhydrazine. Reactions carried out at
(2) (a) Ichimura, K. Chem. ReV. 2000, 100, 1847-1875. (b) Yu, Y.; Nakano,
M.; Ikeda, T. Nature 2003, 425, 145-145.
(3) Cole, L. G.; Gilbert, E. C. J. Am. Chem. Soc. 1951, 73, 5423-5427.
(4) (a) Ohki, Y.; Takikawa, Y.; Hatanaka, T.; Tatsumi, K. Organometallics
2006, 25, 3111-3113. (b) Peters, R. G.; Warner, B. P.; Burns, C. J. J.
Am. Chem. Soc. 1999, 121, 5585-5586.
0.04 M 1 gave a pseudo-first-order kobs of 4.5(7) × 10-5 s-1
.
Whereas reactions carried out under an H2 atmosphere did not
change the ratio of azobenzene to aniline products, reactions carried
out in the presence of excess 9,10-dihydroanthracene afforded
aniline and anthracene with minimal azobenzene. These results
suggest that azobenzene is formed by H-atom abstraction rather
than by H2 elimination; however, reactions carried out with 1,2-
diphenylhydrazine-d2 yielded only a small kinetic isotope effect of
1.25. Finally, the addition of excess aniline did not measurably
inhibit the disproportionation reaction, but no reaction occurred if
THF was used as the reaction solvent or in the presence of pyridine.
On the basis of the mechanistic studies above and the well-
established H-atom abstraction chemistry of metal imidos and metal
oxos,11 we propose the catalytic cycle shown in Scheme 1, the key
species being a [N2O2ox]Zr(dNPh)L2 oxidant formed in step c.12
This proposal is supported by the development of a dark-green
solution and a UV-vis absorbance at λmax ) 739 nm during the
reaction. Furthermore, 1a reacted rapidly with the nitrene transfer
reagent (p-tolyl)N3 to release N2 and give a similar green solution.
(5) Blackmore, K. J.; Ziller, J. W.; Heyduk, A. F. Inorg. Chem. 2005, 16,
5559-5561.
(6) Stanciu, C.; Jones, M. E.; Fanwick, P. E.; Abu-Omar, M. M. J. Am. Chem.
Soc. 2007, 129, 12400-12401.
(7) Haneline, M. R.; Heyduk, A. F. J. Am. Chem. Soc. 2006, 26, 8410-
8411.
(8) Blackmore, K. J.; Ziller, J. W.; Heyduk, A. F. Inorg. Chem. 2008, 47,
265-273.
(9) Chaudhuri, P.; Hess, M.; Mu¨ller, J.; Hildenbrand, K.; Bill, E.; Weyher-
mu¨ller, T.; Wieghardt, K. J. Am. Chem. Soc. 1999, 121, 9599-9610.
(10) Control reactions using ZrCl4(THF)2, Zr(OtBu)4, and Zr(NMePh)4 did not
result in disproportionation of 1,2-diphenylhydrazine.
(11) (a) Zidilla, M. J.; Dexheimer, J. L.; Abu-Omar, M. M. J. Am. Chem. Soc.
2007, 129, 11505-11511. (b) Bakac, A. J. Am. Chem. Soc. 2000, 122,
1092-1097.
(12) Other oxidizing species were considered. See Supporting Information for
further discussion.
(13) Holleman, A. F.; Wiberg, E. Inorganic Chemistry; Wiberg, N., Aylett, B.
J., Eds.; Academic Press: San Diego, CA, 2001; p 499.
(14) Hoover, J. M.; DiPasquale, A.; Mayer, J. M.; Michael, F. E. Organome-
tallics 2007, 26, 3297-3305.
(15) Walsh, P. J.; Baranger, A. M.; Bergman, R. G. J. Am. Chem. Soc. 1992,
114, 1708-1719.
(16) Nakajima, Y.; Suzuki, H. Organometallics 2005, 24, 1860-1866.
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