C-H bond activation of the methyl group of the supporting ligand in an osmium(III) complex upon reaction with H2O2: Formation of an organometallic osmium(IV) complex

Article abstract of DOI:10.1021/ic302169k

Oxidation of the hydroxoosmium(III) complex resulted in C-H bond activation of the methyl group of the supporting ligand (N,N′-dimethyl-2,11-diaza[3. 3](2,6)pyridinophane). The product was an osmium(IV) complex exhibiting a seven-coordinate structure with an additional Os-CH₂ bond.

Full text of DOI:10.1021/ic302169k

Communication
pubs.acs.org/IC
C−H Bond Activation of the Methyl Group of the Supporting Ligand
in an Osmium(III) Complex upon Reaction with H2O2: Formation of an
Organometallic Osmium(IV) Complex
Hideki Sugimoto,* Kenji Ashikari, and Shinobu Itoh*
Department of Materials and Life Science, Division of Advanced Science and Biotechnology, Graduate School of Engineering, Osaka
University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
S
* Supporting Information
ABSTRACT: Oxidation of the hydroxoosmium(III)
complex resulted in C−H bond activation of the methyl
group of the supporting ligand (N,N′-dimethyl-2,11-
diaza[3.3](2,6)pyridinophane). The product was an
osmium(IV) complex exhibiting a seven-coordinate
structure with an additional Os−CH2 bond.
etal−oxo species, especially those of the group 8
Figure 1. Crystal structures of the cationic parts of 1 (a) and 2 (b)
shown in 50% ellipsoids. The hydrogen atoms were omitted for clarity.
Selected bond lengths (Å) and angles (deg) for 1: Os1−Cl1 2.362(3),
Os1−Cl2 2.360(2); Cl1−Os1−N4 175.22(19), Cl2−Os1−N6
173.5(2), N1−Os1−N3 157.1(3). Selected bond lengths (Å) and
angles (deg) for 2: Os1−O1 2.051(4), Os1−O2 2.069(4); O1−Os1−
N4 171.57(16), O3−Os1−N2 172.09(14), N1−Os1−N3 154.65(15).
M
elements, play key roles in the oxidation processes of
organic synthesis1−5 as well as in the catalytic cycles of several
metalloenzymes in the case of iron.6−9 Among the metal−oxo
complexes of the group 8 elements, those of ruthenium have
been studied most extensively because the synthetic pathway has
been well established.3−5 Recently, a number of iron−oxo
complexes have also been reported, providing important insights
into the catalytic mechanism of iron monooxygenases.6−9 On the
other hand, osmium−oxo complexes have been considered to be
less reactive compared with the iron− and ruthenium−oxo
complexes because of the low-energy 5d orbitals, stabilizing the
oxo complexes by a strong interaction with the p orbitals of the
oxygen atom. Thus, little attention has been focused on the
chloride ions (Cl1 and Cl2). The N1−Os1−N3 angle of
157.1(3)° is significantly smaller than the regular angle of 180° in
a octahedron, which indicates that the octahedral geometry of the
Os1 center is highly distorted. The Os1 atom of 2 also has a
highly distorted octahedron, as indicated by the small N1−Os1−
N3 angle of 154.65(15)°.
reactivity of the osmium−oxo complexes,10 even though the
Hydrolysis of 1 and 2 was then examined to obtain
osmium(III)−aqua and/or −hydroxo complexes. An aqueous
solution of 1 showed an electrospray ionization mass spectrum
(ESI-MS) only due to 1, indicating that the chloride ligands were
not replaced by the solvent water molecules. On the other hand,
as shown in Figure 2, the ESI-MS spectrum of an aqueous
solution of 2 showed a peak cluster at m/z 598. The isotopic
distribution pattern is consistent with that of [OsIII(OH)(OBz)-
L]+ (3), indicating that one of the benzoate ligands of 2 was
replaced by a hydroxide ion. Then, the acid−base chemistry of 3
was investigated.23 Figure S1a in the SI shows the UV−visible
spectral change of 3 in the pH titration, where the absorption
band at 354 nm due to 3 increases with a contaminant decrease of
the absorption bands around 300 nm due to 3H ([OsIII(OH2)-
(OBz)L]2+, the aqua form of 3; eq 1) with isosbestic points at 267
and 335 nm as the pH of the solution is raised from 2 to 7. A plot
of the absorbance at 354 nm against pH gave a sigmoidal curve
(Figure S1a in the SI, inset) ascribable to the acid−base
equilibrium between the OsIIIOH2 in 3H and the OsIIIOH in 3
(eq 1). The pKa value was calculated as 4.5 from the plot of
11,12
inorganic osmium(VIII) compounds such as OsO4
and
13,14
[OsN(O)3]−
have been widely utilized in alkene dihydrox-
ylation and alkane hydroxylation. So far, a number of osmium−
oxo complexes with simple (poly)pyridyl ligands were
synthesized, but the research target was limited in proton-
coupled electron-transfer reactivity generating osmium−hy-
droxo and osmium−aqua species or photochemical proper-
ties.15−22
In this study, we examine the reaction of an osmium(III)
complex supported by a macrocyclic tetraaza ligand L (=N,N′-
dimethyl-2,11-diaza[3.3](2,6)pyridinophane) and H2O2 to find
aliphatic C−H bond activation, giving an osmium(IV) organo-
metallic complex. The reaction may involve an osmium(V)−oxo
species as a key reactive intermediate.
First, dichloro- and dibenzoatoosmium(III) complexes with
ligand L, [OsIIICl2L]PF6 (1) and [OsIII(OBz)2L]PF6 (2), were
synthesized as precursors [ChemDraw structures (Chart S1 in
the Supporting Information, SI), and synthetic procedures are
provided in the SI]. Crystal structures of 1 and 2 are shown in
Figure 1, and the crystal graphical parameters are summarized in
Tables S1 and S2 in the SI. The Os1 atom of 1 is coordinated
with the four nitrogen atoms (N1−N4) from L and the two
Received: October 4, 2012
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ic302169k | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
z 597.1 was consistent with that estimated for [OsIV(O)(OBz)-
L]+ or [OsIV(OH)(OBz)(L−H)]+ (L−H denotes a ligand losing
one H). Both compounds can be formally generated by one-
electron and one-proton loss from 3. On the other hand, the
isotopic distribution pattern of the dicationic peak cluster can be
assignable to [OsIV(OBz)(L−H)]2+. When the dicationic peak
cluster is assumed to be a fragment of the parent peak cluster, it
may be reasonable to assign the generated complex as
[OsIV(OH)(OBz)(L−H)]+ (4). Although a single crystal of 4
itself has yet to be obtained, the dibenzoate derivative
[OsIV(OBz)2(L−H)]+ (5) was successfully crystallized by
concentrating a solution of 4.
Figure 4 shows the crystal structure of 5. The Os1 atom is
coordinated by the four nitrogen atoms N1, N2, N2*, and N3 of
Figure 2. Positive ESI-MS of 3 generated by the treatment of 2 with
H2O. Inset: expanded spectrum around m/z 598 (obsd) and calculated
isotopic distribution pattern for 3.
log(A0 − A)/(A∞ − A) versus the pH (Figure S1b in the SI and
eq 1). The value is somewhat larger than that of [OsIII(OH2)-
(trpy)(bpy)]3+ (pKa = 2.0; trpy = 2,2′:6′,2″-terpyridine; bpy =
2,2′-bipyridine),15 reflecting the lower Lewis acidity of the
osmium(III) center coordinated by the anionic benzoate ligand
than that by the noncharged trpy and bpy ligands.
Ka
[OsIII(OH2)(OBz)L]2+ ⇌ [OsIII(OH2)(OBz)L]+ + H+
3H
3
(1)
Figure 4. Crystal structure of the cationic part of 5 shown in 50%
ellipsoids. The hydrogen atoms were omitted for clarity. Selected bond
lengths (Å) and angles (deg): Os1−O1 2.075(5), Os1−C1 2.101(9);
O1−Os1−N2* 167.77(18), N3−Os1−C1 169.9(3), N1−Os1−N3
151.4(3).
An aqueous solution of 3 was then treated with H2O2 under a
dinitrogen atmosphere at 70 °C.23 Figure 3a shows the UV−
L−H and the two oxygen atoms O1 and O1* of the two benzoate
ligands. Notably, a direct coordination of C1 to Os1 was
observed, which made a seven-coordinate osmium center [Os1−
C1 bond distance = 2.101(9) Å]. Similar intramolecular bond
formation between a metal center and a carbon atom of a tertiary
amine macrocyclic ligand was observed in some cobalt(III)
complexes, where the Co−C bond length was 1.950(10) Å.24,25
The bond angles for O1*−Os1−N2 (=O1−Os1−N2) and N3−
Os1−C1 [167.77(18) and 169.9(3)°, respectively] are signifi-
cantly larger than that of N1−Os1−N3 [151.4(3)°]. Thus, the
geometry of the seven-coordinate Os1 atom is best described as a
capped octahedron, where N1 acts as the capping ligand. In the
1H NMR spectrum of 5 in CD3CN, sharp signals were observed
in the range from 3.10 to 7.92 ppm (see the SI), indicating that
the spin state of complex 5 is S = 0 (singlet). Also, the presence of
−
one PF6 counteranion per one complex cation supports the
OsIV oxidation state in 5. Because complex 5 was generated just
by a ligand exchange reaction from OH− in 4 to OBz− in 5, it can
be concluded that complex 4 has a similar OsIV−CH2 bond.
A possible mechanism for the conversion of 2 to 5 is presented
in Scheme S1 in the SI. Because formation of the organometallic
OsIV−CH2 bond was not observed in the reaction of the
dichloroosmium(III) complex 1 with H2O2 in H2O and 1 hardly
underwent hydrolysis, the osmium(III) hydroxo complex 3
formed by hydrolysis of the osmium(III) dibenzoate complex 2 is
a starting complex for the C−H bond activation of the methyl
group of L. Then, the reaction of 3 and H2O2 may provide an
osmium(IV) oxo intermediate (I) through electron-transfer
oxidation followed by deprotonation or via O−O bond
homolytic cleavage of a dinuclear osmium-μ-peroxo-type
intermediate (not shown in Scheme S1 in the SI). Both reaction
pathways are consistent with the observed stoichiometry of
Figure 3. (a) UV−visible spectral changes observed upon addition of n/
10 equiv of H2O2 to 3 (n = 1−7; [3] = 0.1 mM) in H2O. Inset: plot of
absorbance at 275 nm against the molar ratio of [H2O2]/[3]. (b)
Positive ESI-MS spectrum of a solution measured after the reaction.
visible spectral changes for the titration of 3 with H2O2, where
the absorption bands around 280 and 370−470 nm decreased.
The plot of absorbance at 275 nm against the molar ratio of
[H2O2]/[3] (inset) clearly indicates the stoichiometry of 3:H2O2
= 1:0.5. As shown in Figure 3b, the ESI-MS spectrum measured
after the titration showed two peak clusters at m/z 597.1 as a
monocationic pattern and at m/z 290.1 as a dicationic pattern.
The isotopic distribution pattern of the parent peak cluster at m/
B
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Inorganic Chemistry
Communication
3:H2O2 = 1:0.5. The possible involvement of intermediate I is
strongly supported by the fact that the treatment of 3 with 1
equiv of (NH4)2CeIV(NO3)6 (CAN) or 0.5 equiv of
(NH4)2(S2O8) gave UV−visible and ESI-MS spectra nearly
identical with those of 4 produced by the reaction with H2O2. In
these cases, one-electron oxidation of 3 gives an osmium(IV)
complex, from which deprotonation of the hydroxo ligand takes
place to give osmium(IV) oxo intermediate I; the pKa value of the
hydroxo ligand of the osmium(IV) complexes supported by
pyridine-based ligands was reported to be smaller than 0.29,30
From intermediate I, C−H bond activation occurs to yield 4 via a
radical-type mechanism (hydrogen-atom abstraction concom-
itant with Os−C bond formation, path A) or an ionic mechanism
(proton abstraction and carbanion rebound, path B). It has been
reported that C−H bond activation is facilitated by an agnostic
interaction in cobalt(III) complexes with a macrocyclic ligand.24
The relatively short distances between Os1 and the methyl
carbon atoms as 3.052(7) and 3.069(7) Å in 2 suggest that there
is such an agnostic interaction between the osmium center and
the methyl group to facilitate Os−C bond formation.31,32 Ligand
substitution of the OH group in 4 with the benzoate ion gave the
crystallographically characterized 5. It should be noted that the
presence of a metal center having both −CH2− and −OH groups
as found in 4 has yet to be reported in other metal complexes
because the OH− ligand attached on a high-valent metal(IV)
center inserts into the MIV−C bond to yield a hydroxylated MII−
OCH2-type product.1−7
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ASSOCIATED CONTENT
■
S
* Supporting Information
(23) Compound 3 could not be isolated because of the high solubility
in H2O. The concentration of an aqueous solution of 3 resulted in
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and C(1) and ligation of the resulting iminium group (R2N+CH2) to
the osmium center in 5 are unlikely because the supporting ligand L is a
tertiary amine derivative.
X-ray crystallographic data in CIF format, experimental details
and characterization data, ChemDraw structures of the
complexes (Chart S1), proposed formation mechanism of 4
(Scheme S1), UV−visible spectral changes of 3 and the titration
plot (Figure S1), crystallographic data of 1, 2, and 5 (Table S1),
and selected bond lengths and angles for 1, 2, and 5 (Tables S2
and S3). This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
(26) Lay, P. A.; Sargeson, A. M.; Skelton, B. W.; White, A. H. J. Am.
*E-mail: sugimoto@mls.eng.osaka-u.ac.jp (H.S.), shinobu@mls.
eng.osaka-u.ac.jp (S.I.).
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Author Contributions
All authors have given approval to the final version of the
manuscript.
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Notes
(30) Cyclic voltammograms of 3 measured in water at various pH
values (Britton−Robbinson buffer solutions) gave an irreversible
oxidation wave. Information on the electrochemical generation of the
OsIVO intermediate was not obtained.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
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This work was partly supported by Grant 23350027 (to H.S.) for
Scientific Research (B) from the Japan Society for Promotion of
Science and Grants 24109015 (to H.S.) and 22105007 (to S.I.)
for Scientific Research on Innovative Areas from MEXT of Japan.
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dx.doi.org/10.1021/ic302169k | Inorg. Chem. XXXX, XXX, XXX−XXX