Catalytic reactivity of a zirconium(IV) redox-active ligand complex with 1,2-diphenylhydrazine

Article abstract of DOI:10.1021/ja710611v

Two-electron reactivity of [N₂O₂red]ZrL₃ (1a, N₂O₂red = N,N″-bis(3,5-di-tert-butyl-2-phenoxy)-1,2-phenylenediamide, L = THF) was explored with halogens and 1,2-diphenylhydrazine. Despite a formal d0 zirconium(IV) metal center, halogen oxidative addition occurred to form [N₂O₂^ox]ZrCl₂(THF) (2) with two-electron oxidation of the ligand. This ligand redox activity allows catalytic reactivity with 1,2-diphenylhydrazine resulting in disproportionation to form aniline and azobenzene via a putative zirconium-imide intermediate. Copyright

Full text of DOI:10.1021/ja710611v

Published on Web 02/12/2008
Catalytic Reactivity of a Zirconium(IV) Redox-Active Ligand Complex with
1,2-Diphenylhydrazine
Karen J. Blackmore, Neetu Lal, Joseph W. Ziller, and Alan F. Heyduk*
Department of Chemistry, UniVersity of California, IrVine, California 92697
Received November 26, 2007; E-mail: aheyduk@uci.edu
Hydrazine reactions with transition metals have implications for
industrially important organic synthesis applications and can be
informative substrates for modeling the metal-mediated reduction
of nitrogen to ammonia.1 Under oxidizing conditions, hydrazines
can be converted into azo molecules with an NdN bond that is
useful in electronic and materials applications.2 Azobenzene forma-
tion via disproportionation of 1,2-diphenylhydrazine is favored
thermodynamically (-14.6 kcal mol-1);3 however, a metal catalyst
with two-electron valence states is typically required to promote
the reaction. Although this reaction can be viewed as a transfer
hydrogenation, metal imides are often implied or even observed as
intermediates in the catalytic cycle, which proceeds via a hydrogen-
atom abstraction step.4
This Communication reports a new type of catalyst for the two-
electron disproportionation of diphenylhydrazine to azobenzene and
aniline. The results presented here are noteworthy because the metal
catalyst relies on two-electron valence changes that occur at a redox-
active ligand rather than at the metal center. It has been shown
that redox-active amidophenolate ligands can provide the reducing
equivalents for the two-electron oxidative addition of halogens to
tetravalent zirconium centers with a formal d0 electron count.5
Similar zirconium complexes with redox-active diamide ligands
participate in four-electron reactivity with oxygen.6 Ligand-enabled,
two-electron reductive elimination of C-C bonds has also been
demonstrated for redox-active amidophenolate ligands;7 however,
further development of these reactions has been thwarted by
complicated ligand exchange processes.8 Herein, we report the
zirconium chemistry of a tetradentate, tetraanionic, bis(amidophe-
nolate) ligand, N,N′-bis(3,5-di-tert-butyl-2-phenoxy)-1,2-phenylene-
diamide, [N2O2 ]
red 4-.9 Halogen oxidative-addition reactivity estab-
Figure 1. Synthesis and ORTEPs for [N2O2red]ZrL3 (1a, L ) THF) and
[N2O2ox]ZrCl2(THF) (2). Thermal ellipsoids are shown at 50% probability.
Solvent molecules and hydrogen atoms have been removed for clarity.
molecule in the equatorial plane in addition to the two axial THF
molecules. NMR spectroscopy indicates that the solid-state structure
is largely preserved in solution. Sharp ligand resonances, consistent
with C2V symmetry, were observed in the 1H NMR spectrum along
with broad resonances for the coordinated THF molecules.
Oxidation of 1a with PhICl2 occurred rapidly at -35 °C, resulting
in an immediate color change from pale yellow to intense forest
green. Crystals of [N2O2ox]ZrCl2(THF), 2, suitable for X-ray
crystallography were grown from ether/pentane solutions of the
complex. As shown in Figure 1, the two axial positions are occupied
by chloride ligands introduced upon oxidation. The tetradentate
ligand and one THF molecule comprise the equatorial plane of the
zirconium. The ligand oxidation state can be determined from
metrical parameters.9 Notably, a long C(7)-C(12) distance of 1.455-
(9) Å and short C-N distances of 1.347(11) and 1.333(12) Å are
indicative of the cyclohexadiene-diimine oxidation state depicted
lished the two-electron redox reactivity of [N2O2red]ZrL3 (1a, L )
THF). Complex 1a also was found to catalyze the disproportionation
of 1,2-diphenylhydrazine to aniline and azobenzene. A two-electron
oxidized [N2O2ox]Zr-imido species is implicated as an intermediate
in the disproportionation reaction.
Metalation of [N2O2 ]
red 4- with ZrCl4(THF)2 proceeded smoothly
to form 1a according to the strategy summarized in Figure 1.
Zirconium complex 1a was obtained as a bright-yellow, micro-
crystalline solid. X-ray quality single crystals were obtained from
diethylether solutions chilled to -35 °C and yielded the seven-
coordinate structure shown as an ORTEP in Figure 1. The
tetradentate ligand occupies four equatorial positions of a pentagonal
bipyramid. The bite angles within the ligand are small (∼72°),
leaving the oxygen-zirconium-oxygen angle large at 143.28(14)°.
This open angle provides room for the coordination of one THF
for [N2O2 ]
ox 2-. The ligand backbone 1H NMR resonances for 2 in
C6D6 shifted upfield by 0.5 ppm to 7.52 and 6.35 ppm, consistent
with a cyclohexadiene-diimine oxidation state. A broad and intense
absorbance at λmax ) 945 nm (ꢀ ) 20,800 M-1 cm-1) in the UV-
vis spectrum of 2 is responsible for the dark-green color observed
for the complex.9
Complex 1a reacted with 1,2-diphenylhydrazine at room tem-
perature to afford azobenzene and aniline.10 Addition of 2 equiv
9
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J. AM. CHEM. SOC. 2008, 130, 2728-2729
10.1021/ja710611v CCC: $40.75 © 2008 American Chemical Society

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
available free of charge via the Internet at http://pubs.acs.org.
2PhHN-NHPh 9
18 PhNdNPh + 2NH2Ph
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