Stabilization of the hydroperoxido ligand: A 1κO,2κO′ dimetallic coordination mode

Article abstract of DOI:10.1002/anie.200702559

(Chemical Equation Presented) A novel coordination mode of the hydroperoxido ligand in which each oxygen atom is bonded to a distinct metal center is present in a rare thermally stable example of this otherwise elusive family of complexes, which was obtained by protonating a bis-peroxido-bridged Rh2 complex. The OOH ligand is displaced by a chloride ion to give the corresponding chlorido-bridged complex (see scheme, L₃ = tris(methylenediphenylphosphane)-phenylborate).

Full text of DOI:10.1002/anie.200702559

Angewandte
Chemie
DOI: 10.1002/anie.200702559
Stable Hydroperoxido Complex
Stabilization of the Hydroperoxido Ligand: A 1kO,2kO’ Dimetallic
Coordination Mode**
Cristina Tejel,* Miguel A. Ciriano, Sonia JimØnez, Vincenzo Passarelli, and JosØ A. López
Recent advances in homogeneous catalytic oxidation of
organic compounds have focused on the use of environ-
mentally benign oxidants, ideally oxygen.[1] Nevertheless, the
ways in which transition-metal complexes activate oxygen
and the mechanisms by which the oxidation reactions occurs
ne)phenylborate) was obtained as an orange crystalline
solid by placing a solution of [Rh(PhBP3)(cod)] (2; cod =
1,5-cyclooctadiene) in dichloromethane under an oxygen
atmosphere at 358C for 10 h. The oxygenation reaction occurs
cleanly at the metal centers without oxidation of the
remain a complicated puzzle. Typically, low-valent metal phosphane arms of the ligand. Figure 1 shows the X-ray
centers oxidatively add oxygen to give peroxido (or super-
oxido) complexes, but this essentially nucleophilic peroxido
ligand requires further activation to form a more electrophilic
species for effective oxygenation of organic substrates, as
pointed out by Akita, Moro-oka et al.[2] Possibilities for such
structure of 1,[11] in which two rhodium atoms are k3-
coordinated to a fac-[PhBP3]À ligand and are joined together
by two peroxido ligands that bridge the metal centers through
a single oxygen atom. In the Rh(h2-O2) moiety, the peroxido
À
ligand is noticeably nonsymmetric (Rh(1) O(2) 2.013(2),
À
À
further activation could be weakening of the O O bond by its
Rh(1) O(1) 2.151(2) Š), whereby the longer distance corre-
bridging two metal centers[3] and protonation of the peroxido
ligand. Thus, metal hydroperoxido complexes have been
proposed as intermediates in both transition-metal-catalyzed
and enzymatic oxidations with molecular oxygen, for exam-
ple, in Pd-catalyzed aerobic oxidation of alcohols[4] and
biological oxidation achieved by copper enzymes such as
galactose oxidase[5] and superoxide dismutase.[6] Hydroper-
oxido iron species play an important role in oxidations
catalyzed by cytochrome P450.[7] In spite of this interest, such
complexes appear in the literature as very elusive species with
low thermal stability and short lifetimes, and very few
transition-metal hydroperoxido complexes have been struc-
turally characterized so far.[8,9] As part of our ongoing efforts
to understand oxygenation reactions,[10] we investigated the
reactivity of rhodium complexes with oxygen. Herein we
describe the isolation and full characterization of a thermally
stable dinuclear complex of rhodium containing the first
bridging hydrogen(peroxido) (hydroperoxido) ligand coordi-
nated in a m-1kO,2kO’-OOH mode and a bridging peroxido
(peroxo) ligand, as well as related compounds.
sponds to the bridging oxygen atom, which in turn is closer to
À
the other rhodium atom (Rh(1’) O(1) 2.105(2) Š). The
À
À
O(1) O(2) bond length of 1.463(3) Š is in the range of O
O single bonds. Two precedents of this unusual bridging mode
of a peroxido ligand have been reported to date for late
transition metals: the related complex [{Rh(Cl)(PPh3)2}2(m-
k1:h2-O2)2],[12] and the recently described [{Pd(k2-TpiPr)}2(m-
k1:h2-O2)(py)] (TpiPr = hydridotris(3,5-diisopropylpyrazolyl)-
borato).[8d] The dinuclear nature of 1 is maintained in solution
according to NMR diffusion (DOSY) experiments.[13] For
selected NMR spectroscopic data of the new complexes, see
the Supporting Information.
The nucleophilic character of the peroxido ligands in 1
was further confirmed by reaction with one molar equivalent
of HBF4. Protonation of one of the peroxido ligands of 1 leads
to formation of peroxido/hydroperoxido complex [{Rh-
(PhBP3)}2(m-h2:h2-O2)(m-1,2-OOH)]BF4 ([3]BF4). In the
structure of the cation [3]+ in the solid state (Figure 2),[11]
two Rh(PhBP3) moieties are bridged by one hydroperoxido
ligand in a m-1kO,2kO’-OOH coordination mode and a
peroxido ligand bonded in a m-1h2:2h2-O2 fashion. The
location of the hydroperoxido proton on the O(1)/O(2)
atoms was established from NMR spectroscopy measure-
ments (see below). The hydroperoxido ligand is quite sym-
Peroxido-bridged dirhodium complex [{Rh(PhBP3)}2(m-
k1:h2-O2)2] (1; [PhBP3]À = tris(methylenediphenylphospha-
[*] Dr. C. Tejel, Prof. M. A. Ciriano, S. JimØnez, Dr. V. Passarelli,
Dr. J. A. López
À
metrically situated between the rhodium atoms, with Rh(1)
À
O(1) and Rh(2) O(2) distances of 2.135(3) and 2.046(3) Š,
Departamento de Química Inorgµnica, Instituto de Ciencia de
Materiales de Aragón, C.S.I.C.-Universidad de Zaragoza
Pedro Cerbuna 12, 50009 Zaragoza (Spain)
Fax: (+34)976-761-187
À
respectively. The O OH distance of 1.450(4), Š and the Rh-
O-O-Rh angles centered on the oxygen atoms of 108.2(2) and
106.3(2)8 indicate an end-on coordination mode to each metal
center. Furthermore, the coordination of the peroxido ligand
O(3)O(4) in 3 is also remarkable.
E-mail: ctejel@unizar.es
[**] The generous financial support from MEC/FEDER (project
CTQ2005-06807) and DGA (PIP 019/2005) is gratefully acknowl-
edged. S.J. thanks DGA (Diputación General de Aragón) for a
fellowship. We also thank Dr. J. M. AndrØs and Dr. M. C. Mayoral
from the Instituto de Carboquímica (CSIC) and Dr. M. L. Sanjuµn
from the Instituto de Ciencia de Materiales de Aragón for recording
the Raman spectra
Crystallographically characterized precedents of this
unusual m-h2:h2-O2 coordination mode of the peroxido
ligand to transition metals have only been reported for
copper complexes that are models of oxyhemocyanin.[14]
However, the related CuO2Cu moieties are systematically
planar,[14] while the RhO2Rh framework has a butterfly
conformation with a folding angle of 120.6(2)8. Moreover,
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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indicated in Scheme 1: tautomerism
arising from a 1,2-hydrogen shift
from one oxygen atom to the other
in the hydroperoxido ligand
(Scheme 1, a), and twisting of this
ligand (Scheme 1, b) to generate a
symmetry plane bisecting both rho-
dium atoms. Accordingly, the
31P{1H} NMR spectrum shows two
signals (in 1:2 intensity ratio). This
motion is expected to have a low
energy barrier, as can be inferred
from the experimental facts.
Another plausible mechanism pro-
ducing the observed spectra is inter-
conversion of the m-1,1-OOH and m-
1,2-OOH coordination modes on
the rhodium atoms. However, this
should be a higher-energy process
since it involves cleavage and for-
Figure 1. ORTEP drawing of 1 (primed atoms are related to unprimed atoms by an inversion center).
À
mation of Rh O bonds. The OOH
proton, which gives a sharp signal at
1
d = 4.44 ppm in the H NMR spec-
trum, is located on the O(1)/O(2) atoms (Figure 2), because it
is coupled to the trans P(1)/P(4) atoms and reveals a NOE
signal enhancement of the ortho protons of the phenyl groups
bonded to P(2),P(3)/P(5),P(6) on saturation of the hydro-
peroxido signal (see the Supporting Information).
Hydroperoxido ligands acting as bridges are quite rare,
with only one example in the chemistry of Co, Cu and Mo
crystallographically characterized up to date.[9] Moreover, in
these three compounds the OOH moiety was found to be
coordinated in a m-1,1-OOH fashion, while complex 3
represents the first example of a m-1,2-OOH hydroperoxido
ligand, that is, each oxygen atom is bonded to only one metal
center.
The structural changes in the core of the complex after
protonation of one of the peroxido ligands on route from 1 to
3 are shown in Scheme 2. The interaction of the proton with
one oxygen atom, probably O(2), leads to opening of the
Figure 2. ORTEP drawing of 3. For phenyl substituents, only ipso
À
carbon atoms are depicted. Selected bond lengths [Š]: Rh(1) O(3)
À
À
À
2.125(3), Rh(1) O(4) 2.144(3), Rh(2) O(3) 2.151(3), Rh(2) O(4)
2.132(3).
À
RhO2 metallacycle by selectively breaking the Rh O(1)
À
the O O distance in the peroxido ligand (1.504(4) Š) is
bond. This generates a coordination vacancy on this rhodium
atom that is immediately occupied by the neighboring oxygen
longer than that of the hydroperoxido ligand in 3 (1.450(4) Š)
and that of 1 (1.463(3) Š) but similar to those of related m-
h2:h2-O2 Cu complexes with hexapyridine dinucleating
ligands (1.491(5), 1.485(8) Š).[14] These elongated peroxido
2
2
À
atom to produce a m-h :h -peroxido ligand with a longer O O
bond than in the starting complex. The alternative pathway to
À
ligands are on the way to cleavage of the O O bond leading to
a bis(m-oxo) complex.[15]
Complex 3, like 1, was found to be dinuclear in solution by
DOSY experiments,[13] while the NMR spectra (see the
Supporting Information) are consistent with averaged C2h
fluxional
species.
Indeed,
variable-temperature
31P{1H} NMR spectra in dichloromethane show that the
process is frozen below À758C. At this temperature the
spectrum consists of six broad signals which indicate that all
phosphorus nuclei are inequivalent. This rules out a static
structure with a m-1,1-hydroperoxo ligand, which would also
give a symmetric spectrum. Rationalization of the observed
spectra entails simultaneous occurrence of the two processes
Scheme 1.
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À
the Rh Rh distance of 3.425 Š is too large to propose the bis-
+
À
oxido isomer with an Rh Rh bond. The cation [4] provides a
second example of a rhodium complex with a peroxido ligand
in m-h2:h2-O2 coordination mode.
Resonance Raman spectra of complexes 1, 3, and 4
obtained with laser excitation at 785 nm showed bands at 810,
802, and 801 cmÀ1, respectively, which shift to 764, 757, and
755 cmÀ1, respectively, for the 18O-labeled compounds. These
À
bands were assigned to the O O stretch in the three
complexes on the basis of frequency and isotopic shift
(46 cmÀ1). Complex 3, which contains both peroxido and
hydroperoxido ligands in different coordination modes,
À
should give two distinct n(O O) bands in this region, but
Scheme 2.
only one broad band was observed under these conditions.
Irradiation of the solid with polarized laser light at 488 nm
and observation of the crossed component revealed two
bands at 790 and 806 cmÀ1 (see the Supporting Information).
Although lower frequencies could be expected for the
complexes 3–4 with the m-h2:h2-coordinated peroxide ligand,
À
the m-1,1-OOH moiety by breaking of the Rh O(2) bond
does not occur.
Another interesting feature of the hydroperoxido com-
plex 3 is its remarkable stability in the absence of coordinating
anions. Indeed, [3]BF4 remains unaltered in solution at room
temperature for days, which is quite surprising, since hydro-
peroxido complexes have been evoked as quite unstable
species requiring isolation at low temperature or additional
modes of stabilization such as hydrogen bonds. However,
addition of chloride ions, in the form of bis(triphenylphos-
phine)iminium chloride (PPNCl), to a solution of [3]BF4 in
dichloromethane triggers the release of the hydroperoxido
ligand with formation of [{Rh(PhBP3)}2(m-h2:h2-O2)(m-
Cl)]BF4 ([4]BF4) along with the bis-peroxido complex 2 and
mononuclear bis-chlorido RhIII complex [Rh(PhBP3)Cl2] (5).
Moreover, complex 5 results from the reaction of 2 with four
molar equivalents of dry HCl, since protonation of the
peroxido ligand to H2O2 is combined with ligand replacement
by chloride. However, if only two molar equivalents of
HCl(aq) are used, complex [4]Cl is the major product of the
reaction, with 5 and unconsumed 2 as minor components of
the reaction mixture. Figure 3 shows the structure of the
cation [4]+,[11] in which the peroxido ligand bridges the metal
bending of the m-h2:h2-Rh2O2 core could be responsible for
the O O stretch moving to higher energies.
[16]
À
In summary, we have isolated and fully characterized the
first complex with a bridging hydroperoxido ligand in a m-1,2-
OOH coordination mode, which is remarkably stable at room
temperature. This hydroperoxido ligand coexists with a
bridging peroxido ligand in the same compound. Moreover,
two examples of a novel m-h2:h2-O2 coordination mode of the
peroxido ligand in rhodium chemistry are reported. They
provide a new insight into the modes of activation of dioxygen
by two metal centers.
Received: June 12, 2007
Revised: December 11, 2007
Published online: February 1, 2008
Keywords: coordination modes · O ligands · oxygenation ·
.
peroxo complexes · rhodium
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