Generation of Stable Ruthenium(IV) Ketimido Complexes by Oxidative Addition of Oxime Esters to Ruthenium(II): Reactivity Studies Based on Electronic Properties of the Ru?N Bond

Article abstract of DOI:10.1002/chem.201704102

The reaction of an oxime ester with [Ru(PPh₃)₃X₂] proceeded smoothly at room temperature to afford a stable Ru^IV ketimido complex as oxidative adduct. The structure of the complex was unambiguously determined by X-ray crystallographic analysis, which showed an almost linear Ru?N?C array. The electronic properties of the nitrogen atom were estimated by DFT calculations, and the results suggested double-bond character of the Ru?N bond. Kinetic studies and consideration of the substituent effect on the oxime ester led to the proposal of a reaction mechanism involving oxidative addition, which could proceed by N,O-chelating coordination to the Ru center prior to N?O bond cleavage. The obtained Ru ketimido complex could be transformed into a ruthenacycle by C?H activation by a concerted metalation–deprotonation mechanism in dichloromethane/methanol. Ru ketimido complexes with a tethered alkyne or alkene moiety underwent chloroamination of unsaturated C?C bonds followed by C?H activation, which resulted in the formation of a ruthenacycle. Considering the LUMO of an isolated Ru ketimido complex, the chloroamination should proceed by a synchronous 1,3-dipolar cycloaddition-type mechanism. Insight into the character and reactivity of Ru ketimido complexes will be helpful for developments in the catalytic transformation of oxime esters.

Full text of DOI:10.1002/chem.201704102

A Journal of
Accepted Article
Title: Generation of Stable Ruthenium(IV)-Ketimido Complexes via
Oxidative Addition of Oxime Esters to Ruthenium(II): Reactivity
Studies Based on Electronic Properties of the Ru-N Bond
Authors: Takuya Shimbayashi, Kazuhiro Okamoto, and Kouichi Ohe
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Generation of Stable Ruthenium(IV)–Ketimido Complexes via
Oxidative Addition of Oxime Esters to Ruthenium(II): Reactivity
Studies Based on Electronic Properties of the Ru–N Bond
Takuya Shimbayashi, Kazuhiro Okamoto,* and Kouichi Ohe*[a]
Abstract: The reaction of an oxime ester with RuX2(PPh3)3
proceeded smoothly at room temperature to afford a stable Ru(IV)–
ketimido complex as an oxidative adduct. The structure of the
complex was unambiguously determined by X-ray crystallographic
analysis, which showed an almost linear Ru–N–C array. The
electronic properties of the nitrogen atom were estimated by density
functional theory (DFT) calculations, and results suggested the
double-bond character of the Ru–N bond. Kinetic studies combined
with consideration of the substituent effect on the oxime ester led to
proposing the reaction mechanism involving oxidative addition,
which could proceed via N,O-chelate coordination to the Ru center
prior to N–O bond cleavage. The obtained Ru–ketimido complex
could be transformed into a ruthenacycle by C–H activation via the
Pd8) (Scheme 1) are still relatively scarce compared with the
well-documented access to the ketimido complexes by
stoichiometric reactions such as nucleophilic attack to metal–
nitrile complexes (M = W,9a,b Re,9c Ru9d), migratory insertion of
M–R to nitrile10a (M = Th,10b U,10b Ti,10c,d Zr,10e W,10f Fe,10g Rh10h),
and deprotonation–metalation starting from ketimines H–
N=CRR (M = Zr,11a Cr,11b Mo,11b Fe11c).12 Therefore, a better
understanding of the inherent nature of ketimido complexes
prepared from oxime derivatives is significant for further
developments in efficient catalytic reactions of oxime derivatives.
concerted
metalation–deprotonation
mechanism
in
dichloromethane/methanol mixed solvent. The Ru–ketimido complex
exhibited another reactivity with a tethered alkyne or alkene moiety
to undergo chloroamination of unsaturated C–C bonds, followed by
C–H activation, resulting in the formation of an isolated ruthenacycle.
Considering the LUMO of an isolated Ru–ketimido complex, the
chloroamination should proceed via a synchronous 1,3-dipolar
cycloaddition-type mechanism. Insight into the character and
reactivity of Ru–ketimido complexes will be helpful for developments
in the catalytic transformation of oxime esters.
Scheme 1. Oxidative addition of oxime derivatives giving
ketimido-metal complexes.
Introduction
Scheme 2. Rh- or Ru-mediated oxidative cleavage of oxime
esters.
Transition-metal-catalyzed processes involving N–O bond
cleavage of oxime derivatives have recently been of interest due
to their synthetic utility, especially for the efficient synthesis of N-
heterocyclic compounds.1 Many examples of such reactions
have been reported since the pioneering work of Narasaka on a
Pd-catalyzed intramolecular amino-Heck-type reaction.2 Metal–
ketimido complexes3 are considered key intermediates that can
undergo 1,2-insertion to C–C unsaturated bonds4 or N–C
reductive elimination from high-valent metal complexes5 to afford
nitrogen-containing heterocycles involving a N(sp2)–C bond.6
However, mechanistic studies based on isolation of the ketimido
complexes that are generated by oxidative addition of the oxime
derivatives to low-valent metal species (M = Ti,7a Re,7b Os7c,
During our concurrent studies on the transition-metal-catalyzed
transformation of cyclic oxime esters involving the oxidative
addition of a N–O bond to low-valent metal species,13 we found
interesting reactivity of oxime esters with Rh(I) species that
resulted in the formation of rhodacycles via oxidative addition
followed by C–H activation (Scheme 2; M = Rh).14 The Rh(III)–
ketimido complexes were assumed to have a bent C–N–Rh
structure because they could not be isolated or observed due to
the fast C–H activation.
Here, we report on a new type of Ru(IV)–ketimido complexes
having a linear Ru–N–C array formed via the oxidative addition
of oxime esters to Ru(II) precursors (Scheme 2; M = Ru). X-ray
crystallographic analysis and density functional theory (DFT)
calculations revealed the electronic properties, orbital
interactions, and bonding characters of the ketimido complexes.
The effect of the substituents on the aromatic rings of the oxime
esters provides a deeper mechanistic insight into the oxidative
addition. We found that ketimido complexes further underwent
intramolecular C–H activation and haloamination of the internal
unsaturated C–C bonds to afford new ruthenacycles under
ambient conditions.
[a]
Takuya Shimbayashi, Dr. Kazuhiro Okamoto, and Prof., Dr. Kouichi
Ohe
Department of Energy and Hydrocarbon Chemistry, Graduate
School of Engineering, Kyoto University
Nishikyo-ku, Kyoto 615-8510, Japan
E-mail: kokamoto@scl.kyoto-u.ac.jp; ohe@scl.kyoto-u.ac.jp
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201704102
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Results and Discussion
atom are largest. This calculation is consistent with the
elongated Ru–Cl2 bond compared with the Ru–Cl1 bond. In
contrast, the LUMO mainly contains an antibonding orbital on
the N–Ru–Cl1 three-atom unit (Figure 2). Hence this unit might
undergo synchronous cycloaddition with nucleophiles rather
than electrophiles. Moreover, NBO analysis supports the
existence of the double-bond character of the Ru–N bond (Table
1), which is consistent with the relatively shorter Ru–N bond
(bond length; 1.79 Å) as well as the linear array of the Ru–N–C1
structure around the nitrogen atom (bond angle; 173°). The sp
hybridization of the nitrogen atom is what distinguishes it from
the other late transition metals such as Pd and Rh.
Isolation of Ru–ketimido complexes
A stoichiometric reaction of oxime ester 1a with RuCl2(PPh3)3 in
C6D6 at room temperature afforded 79% yield of iminide complex
2a as a green precipitate via oxidative addition (Scheme 3). The
reaction of 1a with the bromo analogue RuBr2(PPh3)3 similarly
afforded complex 3a as a brown precipitate. Fine crystals of 2a
and
3a,
obtained
by
recrystallization
from
dichloromethane/hexane or benzene/hexane, were subjected to
X-ray crystallographic analysis.15 Results clearly showed that a
pivalate ligand coordinated to the Ru(IV) centers in 2 fashion.
The Ru–N bond in complex 2a is 1.790 Å in length, which is
similar to the bond length in the congener Os–ketimido complex
(Os–N bond: 1.777 Å).7c The length of the Ru–N bond is much
shorter than the bond in second-row late-transition-metal–
ketimido complexes (Rh–N bond: 2.039 Å, Pd–N bond: 1.999
Å),8a,12h which implies stronger interaction between the nitrogen
atom and the Ru center. The observed Ru–N–C1 angles of 169
(for 2a) and 173 (for 3a) indicated that both nitrogen atoms of
these complexes have sp hybridization (Figure 1). This is the
first example of mononuclear Ru–ketimido complexes having an
almost linear Ru–N–C array confirmed by X-ray crystallographic
analysis.16,17 Compared with the bent structure of ketimido
complexes composed of second-row late transition metals such
as Rh and Pd,8a,12h the obvious structural difference from the
Ru–ketimido complexes is of interest.
Figure 2. Illustration of HOMO (left) and LUMO (right) of 2a.
Calculation by B3LYP/LANL2DZ (for Ru) and 6-31G(d,p) (for
other atoms). Isovalue is 0.045.
Table 1. NBO analysis of 2a.
Type
BD
Bond
Ru–N
Ru–N
Coefficient
Occupancy
1.93
0.5113 Ru, 0.8594 N
0.6807 Ru, 0.7326 N
BD
1.87
Effect of substituents on the reaction rate of oxidative
addition
Scheme 3. Isolation of oxidative adducts 2a and 3a.
Steric hindrance of the t-Bu group on the ester moiety in 1
enabled easy isolation of the oxidative adducts due to stability
and low solubility in benzene. When oxime esters with other
substituents such as an isopropyl and ethyl group on the ester
moiety were used, oxidative adducts 2b and 2c also formed, but
no precipitates were observed. By adding hexane to the reaction
mixture after 2 h, complexes 2b and 2c were successfully
obtained as a green precipitate (Scheme 4a). Substituents on
the carboxylate moiety did not significantly affect the structural
properties of the Ru–ketimido complex.18 However, a variety of
carboxylates had influence on the reaction rate of the oxidative
addition. The rate constants became markedly smaller when the
substituent was a less hindered isopropyl or ethyl group
(Scheme 4b). These results imply a relationship between
conformations of the oxime ester and the mechanism of
oxidative addition. When the substituent on the ester moiety is
the bulky t-Bu group, the oxime ester favors a conformation
where the t-Bu group is far from the ketimine moiety. Such
conformation can readily interact with the Ru(II) complex by
N,O-chelation (Scheme 4c).
Figure 1. ORTEP illustrations of 2a (left) and 3a (right).
Hydrogen atoms and phenyl groups on phosphorus atoms are
omitted for clarity. Selected bond lengths [Å] and angles [] for
2a: Ru–N 1.790(4), N–C1 1.254(6), Ru–Cl1 2.297(1), Ru–Cl2
2.436(1), Ru–P 2.329(1), Ru–O1 2.123(3), Ru–O2 2.090(3), Ru–
N–C1 168.6(4); and for 3a: Ru–N 1.786(6), N–C1 1.273(9), Ru–
Br1 2.440(1), Ru–Br2 2.555(1), Ru–P 2.334(2), Ru–O1 2.131(5),
Ru–O2 2.086(4), Ru–N–C1 172.9(5).
Electronic properties of complex 2a were estimated by DFT
calculations using the B3LYP functional and LANL2DZ/6-
31G(d,p) basis sets. The HOMO mainly contains the Ru–Cl2
antibonding orbital in which the orbital coefficients of the chlorine
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oxygen atom, which may contribute to a lower  value. Overall,
the oxidative addition would proceed through the N,O-chelating
transition state that bears the slightly nucleophilic Ru(II) center
(Scheme 4c).
Scheme 5. Electronic influence on the reaction rate.
Figure 3. Hammett plot from reaction of 1 with RuCl2(PPh3)3.
Calculated  value is 0.194.
Formation of ruthenacycle by C–H activation
During our investigations into the reactivity of ketimido complex
2a, we found that 2a reacted in
a
1:1 mixture of
dichloromethane/methanol at room temperature to afford the
chloro-bridged ruthenacycle dimer 4 quantitatively (Scheme 6).
The crystal structure of 4, obtained by recrystallization from
dichloromethane/hexane, was successfully determined by X-ray
diffraction analysis (Figure 4).19 Due to the long distance
between the Ru center and the aromatic ortho-C(sp2)–H bond of
a ketimine moiety, linear complex 2a changes to bent ketimido
complex 2a prior to C–H activation (Scheme 7). We presume
that complex 2a undergoes intramolecular C–H activation via
the concerted metalation–deprotonation (CMD) mechanism to
Scheme 4. a) Ruthenium-ketimido complexes with various
carboxylate ligands. b) Substituent effect on the rate of oxidative
addition. c) Conformational consideration on the mechanism for
oxidative addition
To clarify the electronic effect of substituents on the reaction rate
of oxidative addition, we employed various oxime esters 1
bearing substituents at the para position of the aromatic ring.
The reaction rates observed in the oxidative addition to afford
ketimido complexes 2 were very different, depending on the
electronic nature of the para substituents (Scheme 5).18 When
oxime esters having an electron-donating group such as NMe2
and OMe were used, the rate constant k became lower than that
of oxime 1a (R = H). In contrast, a larger rate constant k was
observed in an oxime ester having a nitro group as the strong
form
a Ru(IV) ruthenacycle A. Dimerization, followed by
reduction with methanol, then leads to the formation of Ru(III)
dimeric complex 4 as a paramagnetic complex.20
electron-withdrawing group.
A chloro substituent gave a
marginal difference in the reaction rate compared with H
substitution. Correlation of Hammett  and k values provides
linear fit with a  value of +0.194 calculated from its slope
(Figure 3). This positive  value is rather small compared with
the usual oxidative addition step using Pd(0) and aryl halides.
The oxidative addition of N–O bonds to Ru(II) may indeed be
triggered by nucleophilic attack of the Ru(II) center to the
nitrogen atom. Simultaneously, the N–O bond may also be
activated by the Lewis acidic effect of the Ru center toward an
Scheme 6. Formation of a ruthenacycle via C–H activation.
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Figure 5. ORTEP illustration of ruthenacycle 6a (left) and 6b
(right). Hydrogen atoms and phenyl groups on phosphorus
atoms are omitted for clarity. Selected bond lengths [Å] and
angles [] for 6a: Ru–N 2.007(3), N–C2 1.324(6), Ru–C1
1.989(4), Ru–P1 2.362(1), Ru–P2 2.350(1), Ru–Cl1 2.446(1),
Ru–Cl2 2.857(1), Ru–N–C2 118.9(3), N–Ru–C1 78.9(1); and for
6b: Ru–N 2.003(3), N–C2 1.310(5), Ru–C1 2.017(4), Ru–P1
2.375(1), Ru–P2 2.344(1), Ru–Cl1 2.452(1), Ru–Cl2 2.747(1),
Ru–N–C2 119.9(3), N–Ru–C1 78.2(2).
Figure 4. ORTEP illustration of 4. Hydrogen atoms and phenyl
groups on phosphorus atoms are omitted for clarity. Selected
bond lengths [Å] and angles [] for 4: Ru–N 2.051(6), N–C2
1.320(1), Ru–C1 2.037(7), Ru–P 2.285(3), Ru–Cl1 2.349(2),
Ru–Cl2 2.481(2), Ru–Cl2 2.488(3), Ru–N–C2 120.8(5), N–Ru–
C1 77.2(3).
Based on the above observations, a plausible mechanism for
the formation of ruthenacycle 6 is shown in Scheme 9. First, a
Ru(II) species undergoes the oxidative addition of oxime ester 5
to generate a Ru(IV)–ketimido complex C as an intermediate.
Then, the 1,3-dipolar type of cycloaddition of the alkyne moiety
with the Ru–N–Cl unit might occur synchronously to afford the
syn-chloroamination product D. Then intramolecular C–H
activation via the CMD mechanism would occur more favorably
in intermediate D than in the prior complex C due to the
proximity of the C–H bond to the Ru center. Finally, the
coordination of another PPh3 to complex E, with the release of
pivalic acid, affords ruthenacycle complex 6.
Scheme 7. Plausible mechanism for the formation of complex 4.
Ru-mediated tandem reaction of oxime esters bearing a C–C
multiple bond
Studies on the reactivity of Ru–ketimido complexes toward
unsaturated molecules such as alkenes and alkynes are of
interest, especially in organic synthesis.21 The stoichiometric
reaction of alkyne-tethered oxime ester 5 with RuCl2(PPh3)3 in
C6D6 at room temperature proceeded smoothly within 18 h,
affording cyclic Ru(II) complex 6 in high yield (Scheme 8). X-ray
diffraction analysis of complexes 6a and 6b clearly
demonstrated the cis geometry of the nitrogen and chlorine
atoms in the olefinic moiety (Figure 5), which indicates the
formation of these complexes via syn chloroamination.22 Both
complexes 6a and 6b have a vacant coordination site at the
trans position to the aromatic ligand. Distribution of the LUMO in
ketimido complex 2a as shown in Figure 2 predictively suggests
the 1,3-dipolar nature of the N–Ru–Cl unit.
Scheme 9. Plausible mechanism for the formation of complex 6.
Similarly, alkene-tethered oxime ester 7 also reacted smoothly
with the Ru(II) complex to afford ruthenacycle 8 in moderate
yield (Scheme 10). X-ray diffraction analysis of a single crystal of
ruthenacycle
8
obtained by slow evaporation from
dichloromethane/benzene demonstrates that, in contrast to
ruthenacycle 6, this complex has a Ru(III) center (Figure 6).23
Complex 8 would also be formed via chloroamination of the
alkene moiety in 7 followed by C–H activation. Compared with
Ru(II) complex 6, Ru(III) complex 8 has one more chloro ligand
at the trans position to the ipso carbon atom of the aryl group,
which is vacant in complex 6. Complex 8 might be formed via
comproportionation between Ru(IV) and Ru(II) owing to the
vacant coordination site accessible for the chlorine atom.
Scheme 8. Intramolecular chloroamination of alkynes.
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Scheme 10. Intramolecular chloroamination of an alkene.
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Figure 6. ORTEP illustration of ruthenacycle 8. Hydrogen atoms
and phenyl groups on phosphorus atoms are omitted for clarity.
Selected bond lengths [Å] and angles []: Ru–N 2.100(3), N–C2
1.303(5), Ru–C1 2.056(3), Ru–P1 2.405(1), Ru–P2 2.399(1),
Ru–Cl1 2.332(1), Ru–Cl2 2.463(1), Ru–N–C2 117.7(2), N–Ru–
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Conclusions
A new type of Ru(IV)–ketimido complexes has been obtained by
the oxidative addition of oxime esters to Ru(II). X-ray
crystallographic analysis unambiguously revealed their linear
Ru–ketimido structure. DFT calculations also supported the sp
hybridization character of the nitrogen atoms and the double-
bond character of the Ru–N bond, reflecting the linear Ru–N–C
structure. The ketimido complexes readily underwent CMD-type
C–H activation to form stable ruthenacycle complexes.
Furthermore, the ketimido complexes underwent intramolecular
syn chloroamination with an alkyne or an alkene moiety, as
predicted by DFT calculations. The observations and
calculations of the electronic properties of Ru–ketimido
complexes, especially concerning Ru–N bonding, should be
helpful for further investigations of catalytic transformation
involving metal–ketimido intermediates.
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Acknowledgement
This study has been financially supported by Grant-in-aid for
JSPS Research Fellow (T.S.)(Grant No. 17J09627) and JSPS
KAKENHI (a Scientific Research (C); Grant No. 17K05861).
Keywords: Ruthenium • N ligands • Coordination modes • C–H
activation • Amination
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K. Okamoto, T. Shimbayashi, M. Yoshida, A. Nanya, K. Ohe,
Angew. Chem. Int. Ed. 2016, 55, 7199; e) K. Okamoto, K.
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(18) See the Supporting Information. CCDC 1574834 (2b), 1574835
(2c) and 1574841 (2e).
(15) CCDC 1574833 (2a) and 1574836 (3a) contain the
supplementary crystallographic data. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre.
(19) CCDC 1574839 (4).
(20) Complex 2a did not undergo ruthenacycle formation in the
absence of methanol, which suggests that methanol may assist
facile isomerization from linear ketimido complex to bent
ketimido complex probably due to protic interaction. Similar
effect was reported on another ruthenium system but the
reason is still unclear. C. Bohanna, M. A. Esteruelas, A. M.
López, L. A. Oro, J. Organomet. Chem. 1996, 526, 73.
(21) Intermolecular cycloaddition of ruthenacycle 3a with terminal
alkynes also afforded isoquinoline as cycloadduct, albeit in low
yield. See Supporting Information for details.
(16) Ru-aza-allenylidene, Ru3 cluster and Ru2 cluster were reported.
a) M. I. Bruce, M. A. Buntine, K. Costuas, B. G. Ellis, J.-F. Halet,
P. J. Low, B. W. Skelton, A. H. White, J. Organomet. Chem.
2004, 689, 3308; b) C. Bois, J. A. Cabeza, R. J. Franco, V.
Riera, E. Saborit, J. Organomet. Chem. 1998, 564, 201; c) J. A.
Cabeza, I. del Río, F. Grepioni, M. Moreno, V. Riera, M. Suárez,
Organometallics 2001, 20, 4190.
(17) Congener Os-ketimido complexes were reported: a) H. Werner,
W. Knaup, M. Dziallas, Angew. Chem. Int. Ed. 1987, 26, 248; b)
R. Castarlenas, M. A. Esteruelas, E. Gutiérrez-Puebla, E.
Oñate, Organometallics 2001, 20, 1545; c) R. Castarlenas, M.
A. Esteruelas, E. Oñate, Organometallics 2001, 20, 3283; d) R.
(22) CCDC 1574837 (6a) and 1574840 (6b).
(23) CCDC 1574838 (8).
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10.1002/chem.201704102
Chemistry - A European Journal
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T. Shimbayashi, K. Okamoto,* K. Ohe*
Page No. – Page No.
Generation of Stable Ruthenium(IV)–
Ketimido Complexes via Oxidative
Addition of Oxime Esters to
Ruthenium(II): Reactivity Studies
Based on Electronic Properties of the
Ru–N Bond
It’s clearly linear: Stable ruthenium(IV)-ketimido complexes were obtained via
oxidative addition of oxime esters to ruthenium(II). X-ray diffraction revealed their
linear Ru–N–C arrays. DFT calculation supports the electronic nature of Ru–N
bond. The ketimido complex could be an intermediate for chloroamination of an
alkyne or alkene.
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