Multi-electron activation of dioxygen on zirconium(IV) to give an unprecedented bisperoxo complex

Article abstract of DOI:10.1021/ja075396u

Although long proposed as key intermediates in different industrial and biological catalytic processes, bis(peroxides) of heavier transition metals have been elusive species for bulk manipulations. We report on the synthesis and structural characterization of the first such bisperoxo molecule of Zr(IV) as well as on its bis(diamido) precursor. Reduction of the glyoxal-bis(2,6-diisopropylphenyl)imine with magnesium powder in ether followed by reaction with ZrCl4 and extraction in hexane afforded the Zr(IV) bis(diamido) species 1. Further reaction with dry dioxygen produces the Zr(IV) bisperoxo compound 2, isolated as stable orange-brown crystals in 30% yield. X-ray diffraction data revealed an unprecedented quasi-orthogonal orientation of the two peroxide groups, which also have very long O-O bond distances of 1.512(4) and 1.504(4) A. Copyright

Full text of DOI:10.1021/ja075396u

Published on Web 09/21/2007
Multi-electron Activation of Dioxygen on Zirconium(IV) to Give an
Unprecedented Bisperoxo Complex
Corneliu Stanciu, Mary E. Jones, Phillip E. Fanwick, and Mahdi M. Abu-Omar*
Brown Laboratory, Department of Chemistry, Purdue UniVersity, 560 OVal DriVe, West Lafayette, Indiana 47907
Received July 19, 2007; E-mail: mabuomar@purdue.edu
In nature multi-electron redox processes are carried out by
complex multicomponent reaction centers as in cytochrome c
oxidase (O2 fixation) and photosystem II (O2 evolution). Chemists
often harness the diversity of transition metals to carry out multi-
electron reactions.1 In this study, we report the synthesis and
structural characterization of an unprecedented η2-bisperoxo ZrIV
coordination complex supported by two bulky bidentate diimine
ligands. The source of the peroxo ligands is oxygen, which we
propose is activated on ZrIV with the necessary four electrons (two
per O2) originating from the ligand. The recent work of Wieghardt,
Chirik, and Heyduk on redox active ligands is worthy of special
mention.2,3
As often-postulated oxidants in industrial and biological pro-
cesses, transition metal peroxo complexes are of special interest,
particularly when formed from molecular oxygen because of the
push for “green” oxidations.4 Formation of peroxo complexes from
Figure 1. Crystal structure of 1: 50% ellipsoids, hydrogens omitted for
reaction of early transition metals in low oxidation states with
dioxygen is rare because their oxo complexes are stable and tend
to form readily.5 Despite the growing number of heterogeneous
zirconium catalysts that utilize hydrogen peroxide in selective
oxidations,6 virtually nothing is known about the peroxo complexes
of zirconium.7-9
clarity. Selected bond lengths in angstrom and angles in degrees: Zr-N21
) 2.0485(19), Zr-N24 ) 2.091(2), Zr-C22 ) 2.512(2), Zr-C23 ) 2.532-
(3), N21-C22 ) 1.405(3), C22-C23 ) 1.359(4), N21-Zr-N11 ) 117.08-
(7), N21-Zr-N24 ) 86.77(8), and N21-C22-C23 ) 122.1(2).
A recent report on the reduction of glyoxal-bis(2,6-diisopropy-
lphenyl)imine to afford the first boryl anion10 prompted us to
investigate this diamido ligand for the preparation and potential
stabilization of low valent ZrII.11 Reaction of reduced diamido ligand
with ZrCl4(THF)2 afforded the tetrahedral bis(diamido) ZrIV cleanly
(Scheme 1). The molecular structure of 1 (Figure 1) shows a
tetrahedral geometry around zirconium ( N(21)-Zr-N(14) )
116.02°), Zr-Nav distance of 2.068 Å, Cbridge-Nav ) 1.405 Å,
C(12)-C(13) ) 1.37 Å, and C(22)-C(23) ) 1.36 Å. The olefinic
π-bonds appear to exert some electron donation into zirconium.
The average Zr-C(sp2) distance is 2.513 Å, which is shorter than
van der Waals interactions. Another explanation may involve π
donation from the nitrogen lone pair, which folds the ligand forcing
the CdC double bond closer to the metal.12 Nonetheless, the crystal
structure is in full agreement with a Zr4+ bis(diamido) formulation.13
Compound 1 reacts with dry dioxygen to give the η2-bisperoxo
ZrIV complex 2 (Scheme 1). The crystal structure of 2 is shown in
Figure 2. The geometry around zirconium is octahedral with each
peroxo ligand counted as occupying a single coordination site. For
example, the angles N(11)-Zr-center of O(11)-O(12) ) 80.89°,
N(11)-Zr-N(14) ) 68.82°, center of O(11)-O(12)-Zr-center
Figure 2. Crystal structure of η2-bisperoxo zirconium(IV), 2: 50%
ellipsoids, hydrogens and solvent molecules omitted. Selected bond lengths
in angstrom and angles in degrees: Zr-O21 ) 2.038(3), Zr-O22 ) 2.034-
(3), Zr-N24 ) 2.433(4), Zr-N21 ) 2.438(4), O21-O22 ) 1.504(4), N24-
C23 ) 1.271(6), C22-C23 ) 1.478(6), N11-Zr-N24 ) 114.01(12), O21-
Zr-N24 ) 79.57(13), and N24-C23-C22 ) 119.6(4).
of O(21)-O(22) ) 179.55°. The key bond lengths of the molecule
are as follows: O-O ) 1.508(av) Å, Zr-Operoxo ) 2.034(av) Å,
Zr-Nav ) 2.44 Å, N-Cbridge ) 1.265(av) Å, C(12)-C(13) ) 1.47
Scheme 1
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J. AM. CHEM. SOC. 2007, 129, 12400-12401
10.1021/ja075396u CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Å, and C(22)-C(23) ) 1.48 Å.14 All of this data is in agreement
with a ZrIV bisperoxo bis(diimine) complex. Hence, dioxygen is
reduced to peroxo and the ligand is oxidized from diamido to
diimine, a net four-electron process! The reaction of 1 with O2 to
give 2 is quite remarkable because it constitutes a rare activation
of dioxygen by high valent d0 transition metal.15 While 2 is the
first structurally characterized bisperoxo complex of Zr, bisperoxo
complexes of other heavy transition metals including Mo, W, and
Re are plentiful.16
The two peroxo ligands in compound 2 do not lie in the same
plane. Instead they are virtually orthogonal (perpendicular) toward
each other with a torsion angle O(11)-O(12)-O(21)-O(22) of
78°, constituting the first example of this geometry for any
bisperoxo metal complex. The principal interaction of a η2-peroxo
(or oxygen) ligand with transition-metal d-orbitals involves the
degenerate π* orbitals on peroxo and the metal π-orbitals.17 The
orthogonal geometry of the peroxo ligands on zirconium in 2
reduces the competition for metal d-orbitals and thus maximizes
the bonding (Supporting Information). It is remarkable that complex
2 is stable enough to be isolated and characterized because it
contains strictly σ-donor diimine ligands. Most likely the bulky
isopropylphenyl substiuents on the ligand provide significant
shielding which protects the peroxo groups.
(2,6-diisopropylphenyl)imine ligand for generation of other amido
species of heavy transition metals.
Acknowledgment. We thank the Chemical Sciences Division,
Office of Basic Energy Sciences, U.S. Department of Energy
(Grants No. DE-FG02-06ER15794 and DE-FG-02-03ER15466) for
support of this work. M.M.A.O. is grateful to Professor Karl
Wieghardt for valuable discussions.
Supporting Information Available: Materials and methods, sche-
matic MO diagram, and X-ray tables and figures, complete X-ray data
for 1 and 2 (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
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1
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doublets centered at 5.86 and 5.88 ppm (due to the dissymmetry
of the molecule). These correspond to the protons of the two
3
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ethylenic bridges. A coupling constant JH-H of about 15 Hz is in
good agreement with a cis-orientation of the protons. In 13C NMR
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character gained by the aromatic ligand, as opposed to the cis-
ethylenic features present in 1.18 The infrared spectra provide
additional evidence for the diimine character of the two aromatic
ligands in the bisperoxo compound 2. A very characteristic medium
to strong intensity absorption band at 1618 cm-1 proves the presence
of a CdN group in the molecule. This absorption band is absent
from the IR spectrum of 1. Instead the characteristic CdC stretching
vibration at 1670 cm-1, along with the HCd bending at 800 cm-1
and the cis HCdCH out-of-plane bending at 680 cm-1 is observed
for the bis(diamido) complex 1.
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of zirconium are known to bind/activate dinitrogen,19 1 does not
react with N2, as all manipulations were carried out under N2
atmosphere. The electron deficient and highly oxophilic Zr in 1
may form a weak adduct with dioxygen. Subsequently O2 on
zirconium is reduced by electron transfer from the diamido ligand
to afford the peroxo complex. An alternative mechanism would be
the interaction of O2 directly with the ligands followed by peroxide
coordination to Zr.20 Attempts to prepare the potential monoperoxo
intermediate under limiting O2 were not successful. The only
tractable product was bisperoxo 2. Under inert atmosphere in the
glovebox, bisperoxo 2 in the solid-state reverts back to 1 over a
period of 7-10 days.21
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(21) Also when a solution of 2 is degassed under vacuum, it reverts to complex
1. The details of this reaction to yield O2 and complex 1 in the solid state
as well as in solution will be reported elsewhere.
Further spectroscopic, reactivity, and kinetic studies aimed at
better understanding the formation of ZrIV bis(peroxide) are in
progress in our laboratory. Also, we are testing the glyoxal-bis-
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