Ligand-accelerated activation of strong C-H bonds of alkanes by a (salen)ruthenium(VI)-nitrido complex

Article abstract of DOI:10.1002/anie.201204136

Kinetic and mechanistic studies on the intermolecular activation of strong C-H bonds of alkanes by a (salen)ruthenium(VI) nitride were performed. The initial, rate-limiting step, the hydrogen atom transfer (HAT) from the alkane to Ru^VI≡N, generates Ru^V=NH and RC^.HCH ₂R. The following steps involve N-rebound and desaturation. Copyright

Full text of DOI:10.1002/anie.201204136

Angewandte
Chemie
DOI: 10.1002/anie.201204136
CꢀH Activation
ꢀ
Ligand-Accelerated Activation of Strong C H Bonds of Alkanes by
a (Salen)ruthenium(VI)–Nitrido Complex**
Wai-Lun Man, William W. Y. Lam, Hoi-Ki Kwong, Shek-Man Yiu, and Tai-Chu Lau*
Selective functionalization of CꢀH bonds continues to be of
However, in the presence of 0.1m pyridine (py), the oxidation
of DHA by 1 is accelerated by over two orders of magnitude
great interest to chemists. Inspired by enzymes, such as
cytochrome P450 and methane monooxygenase (MMO), the
use of metal–oxo complexes (M = O) for CꢀH bond activa-
ꢀ
1
ꢀ1
(k ’ = 1.59m s ). More significantly, the system 1/py can
2
now activate CꢀH bonds as strong as those in cyclohexane
[
1]
ꢀ1 [10]
tion has been extensively studied. Analogous reactions with
(95.4 kcalmol ). We have previously reported that 1 can
[
2]
[11]
metal–imido (M = NR) species are also known. However,
there are only few reports of CꢀH bond activation by metal–
be activated by py to undergo aziridination of alkenes.
However, py also activates 1 toward bimolecular coupling of
[3]
[12]
nitrido (MꢁN) complexes. For instance, thermolysis of
the nitrido ligands. Hence, we studied the reaction of 1/py
[
4]
[5]
diruthenium–azido
and iridium–azido
complexes are
with alkanes under conditions of high dilution using a 50 mm
cuvette in order to minimize this coupling reaction. Figure 1
reported to give the corresponding nitrides, which then
undergo insertion into an aryl CꢀH bond to give amido
[6]
complexes. Photolysis of azido complexes of cobalt and
[
7]
uranium also generate transient nitrido species that undergo
insertion into the ligand CꢀH bond. Collisional activation of
an iron–azido complex by means of electrospray ionization
affords an intermediate iron(V)–nitrido species, which acti-
[8]
vates the CꢀH bond of the chelating ligand. In the above
examples, the nitrido species are putative or transient species
which undergo intramolecular CꢀH bond activation. We
report herein an example of intermolecular CꢀH bond
activation of a number of alkanes by a well-characterized
VI
(
salen)ruthenium(VI)–nitrido
complex,
[Ru (N)(L)-
(
MeOH)]PF (1) (L = N,N’-bis(salicylidene)-o-cyclohexyldi-
6
[
9]
amine dianion).
Complex 1 reacts readily with xanthene in CH CN at
3
room temperature, as monitored by UV/vis spectrophotom-
etry (see Figure S1 in the Supporting Information). The
reaction follows clean pseudo-first-order kinetics and the
Figure 1. Spectral changes at 200 s intervals for the reaction of
1 (5.00ꢀ10 m) with cyclohexane (1.00m) and py (0.1m) in (CH Cl)
2 2
at 296.0 K (a: before mixing, b: 5 s after mixing). Inset shows the time
trace at 702 nm and fit of the data to a first-order equation.
ꢀ6
pseudo-first-order rate constant (kobs), is independent of
VI
[
Ru N], but depends linearly on [xanthene], with a second-
ꢀ
1
ꢀ1 ꢀ1
order rate constant (k ) of (1.95 ꢂ 0.06) ꢀ 10 m
s
at
2
2
98.0 K. Complex 1 reacts similarly with 9,10-dihydroanthra-
shows the spectrophotometric changes for the reaction of
cene (DHA), but with a smaller k value of (4.15 ꢂ 0.26) ꢀ
1 with cyclohexane in the presence of excess py in (CH Cl) .
2
2
2
ꢀ3
ꢀ1 ꢀ1
1
0
m
s , in accordance with the stronger CꢀH bond of
The reaction follows clean pseudo-first-order kinetics for over
three half-lives. The pseudo-first-order rate constant kobs
depends linearly on [cyclohexane], and the reaction exhibits
saturation behavior with respect to [py] (Figure 2). The rate
law of the reaction is shown in Equation (1). The equilibrium
constant K for the reversible binding of py to 1 is (16.6 ꢂ
DHA compared with xanthene (Figure S2). A kinetic isotopic
effect (KIE) of (3.3 ꢂ 0.3) was obtained for DHA/[D ]-DHA.
4
Complex 1 does not react with alkanes with larger CꢀH bond
ꢀ
1
[10]
dissociation energies (BDEs) than DHA (78.0 kcalmol ).
ꢀ
1
1
.4) m , which agrees well with the value obtained from
ꢀ1
[11]
[*] Dr. W. L. Man, Dr. W. W. Y. Lam, Dr. H. K. Kwong, Dr. S. M. Yiu,
kinetic studies of alkene aziridination (15.6 ꢂ 1.1m ).
Prof. T. C. Lau
Â
Ã
ꢀ
ꢁ
Institute of Molecular Functional Materials and Department of
Biology and Chemistry, City University of Hong Kong
Tat Chee Avenue, Kowloon Tong, Hong Kong (China)
E-mail: bhtclau@cityu.edu.hk
VI
ꢀ
d Ru ðNÞ
Â
Ã
K½pyꢃ
VI
¼ kpy Ru ðNÞ ½alkaneꢃ
ð1Þ
dt
1 þ K½pyꢃ
The second-order rate constant (k ) was found to be
py
[
**] The work described in this communication was supported by the
Research Grants Council of Hong Kong (CityU 101810) and Hong
Kong University Grants Committee (AoE/P-03-08).
ꢀ
3
ꢀ1 ꢀ1
(
1.68 ꢂ 0.05) ꢀ 10
m
s
at 296.0 K. The oxidation of
[D ]cyclohexane occurs much more slowly with a KIE of
1
2
(
8.4 ꢂ 0.2). The rate constants for CꢀH activation of other
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201204136.
alkanes by 1 in the presence of 0.1m py were also determined
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Figure 2. a) Plots of kobs against [C H ] (solid circle) and [C D ] (open
6
12
6
12
circle) in (CH Cl) at 296.0 K. b) Plot of k against [py]. Inset shows
2
2
obs
the corresponding double reciprocal plot.
ꢀ5
ꢀ2
Figure 4. ESI/MS of 1 (3.62ꢀ10 m) with xanthene (4.82ꢀ10 m) in
CH CN. Insets show the expanded (top) and calculated (bottom)
isotopic patterns.
(
see Table S1 and Figures S3–S9 in the Supporting Informa-
3
tion). Large KIE values were also obtained in the oxidation of
toluene (k(C H CH )/k(C H CD ) = (10.1 ꢂ 0.2); Figure S9)
6
5
3
6
5
3
and fluorene (k(C H )/k(C D ) = (4.4 ꢂ 0.5); Figure S5). A
9
10
9
10
linear correlation between the rate constant and the CꢀH
Similar results were obtained for DHA (Figure S11–S12).
Complex 2 has been isolated as the green crystalline PF6 salt.
[
13–15]
ꢀ
BDE of the alkanes is found (Figure 3).
On the basis of
this linear correlation and large KIE values, we conclude that
The ESI/MS analysis of 2 in CH CN shows the parent ion at
3
the reaction between 1/py and alkanes occurs by an initial
m/z = 658. Compound 2-PF has a magnetic moment of m =
6
e
f
f
1
.92 m (Guoy method) at room temperature, consistent with
B
5
III
its formulation as a d Ru complex. The IR spectrum shows
ꢀ1
a n(NꢀH) stretch at 3292 cm . The structure of 2-PF has
6
been determined by X-ray crystallography (Figure 5). The
cation consists of a six-coordinate ruthenium center bound by
a salen ligand, an acetonitrile, and an imine ligand (N(H) =
xanthene(
) that is derived formally from insertion of the
ꢀ2H)
nitrido ligand of 1 into an alkyl CꢀH bond of xanthene,
together with the loss of a hydrogen atom from the a-carbon
center. The newly formed imine ligand is disordered. The N3–
C23 distance of 1.313(10) ꢁ (N3A–C23A = 1.292(11) ꢁ) is
typical of C=N bonds. The Ru1–N3 distance of 2.083(10) ꢁ
(
Ru1–N3A = 2.068(11) ꢁ) is comparable to the Ru–NH
3
[17]
III
distance (2.083(4) ꢁ) in [Ru (NH )(L)(NCCH )]PF .
3
3
6
Figure 3. Plot of log k ’’ (second-order rate constants per active hydro-
2
gen atom in 0.1m py) against CꢀH BDE of alkanes in (CH Cl) at
2
2
2
96.0 K.
rate-limiting hydrogen atom transfer (HAT) step. In the
oxidation of para-substituted toluenes, both electron-donat-
ing and electron-withdrawing substituents have an accelerat-
ing effect (Table S2), and a linear correlation was also
observed between the reaction rate and the a-H hyperfine
coupling constant (s8) of para-substituted toluenes (Fig-
ure S10), which is consistent with a benzyl radical intermedi-
[
16]
ate produced by HAT.
We then investigated the fate of the alkyl radical produced
in the initial HAT step. The ESI mass spectrum (ESI/MS,
Figure 4) of the green solution from reaction of 1 with
xanthene in CH CN shows the presence of two predominant
3
species, the peak at m/z = 504 is assigned to [Ru(L)-
+
(
[
NCCH ) ] , while the peak at m/z = 658 is assigned to
3
2
+
Ru(HN=xanthene(
ꢀ2H) 3
)(L)(NCCH )] (2). In the presence
Figure 5. Molecular structure of 2. Thermal ellipsoids are drawn at
30% probability. Hydrogen atoms (except N3 H) are omitted for
clarity.
+
of py in (CH Cl) , the observed species are [Ru(L)(py) ] (m/
z = 580) and [Ru(HN=xanthene(
ꢀ
2
2
2
+
)(L)(py)] (m/z = 696).
ꢀ2H)
2
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Complex 2 is most likely formed by an N-rebound mechanism
The accelerating effects of py on CꢀH bond activation by
VI +
VI
after the initial HAT from xanthene to Ru ꢁN. A similar
[Ru (N)(L)] may be due to a lowering of the reorganization
energy and/or an increase in hydrogen atom affinity of the
mechanism should also occur with DHA. However, in the
VI
reaction of 1 with alkanes with stronger CꢀH bonds, such as
Ru –nitrido species. Mayer has treated HAT reactions using
[13a,d]
ethylbenzene, toluene, cyclooctane, or cyclohexane, similar
N-rebound ruthenium products were not observed by ESI/
MS. For these substrates the predominant ruthenium species
a Marcus theory approach.
According to the Marcus
theory, the activation energy for HAT depends on the
reorganization energy as well as the driving force. Kochi has
+
V
+
is [Ru(L)(py)2] (m/z = 580). We then tried to detect any
reported that the epoxidation of alkenes by [Cr (salen)(O)]
[20]
organic products formed in these reactions by GC and GC–
is accelerated by pyridine N-oxide. In the five-coordinate
V
+
MS. When 1 was reacted with excess cyclohexane in (CH Cl)2
[Cr (salen)(O)] , the Cr atom is 0.53 ꢁ above the salen plane,
however it is pulled back to 0.26 ꢁ upon axial ligation with
pyridine N-oxide, and this is accompanied by a weakening of
the Cr=O bond. It is likely that similar geometrical changes
2
in the presence of 0.1m py, 8% (100% ꢀ mol of product/mol of
1) of cyclohexylamine was found (Figure S13). Moreover,
when Br was added to the reaction mixture, 10% of 1,2-
2
VI
dibromocyclohexane could be detected, thus indicating that
cyclohexene was originally present (Figure S14). No cyclo-
hexylamine and 1,2-dibromocyclohexane were detected in the
absence of 1. These results suggest that in the reaction of
occur upon coordination of py to Ru ꢁN, thus making the
complex more product-like, and hence lowering the reorgan-
ization energy for HAT. Trans axial ligands have also been
[
21]
found to affect Fe=O bond strengths. On the other hand, py
1
with cyclohexane, both N-rebound (formation of cyclo-
coordination may also enhance the hydrogen atom affinity of
VI
hexylamine) and desaturation (formation of cyclohexene)
reactions occur after the initial formation of the cyclohexyl
radical. When cyclooctane was used as substrate, 35% of
cyclooctene was found (Figure S15), but no cyclooctylamine
could be detected, thus indicating that in this case desatura-
Ru ꢁN, which would increase the driving force for HAT.
Goldberg and de Visser have reported that HAT reactions of
V
a Mn (O) species are accelerated by anionic axial ligands,
which is due to an enhancement of its hydrogen atom affinity
IV
[22]
as a result of an increase in basicity of the Mn (O) species.
[
18]
tion predominates over N-rebound. A general mechanism
We are currently doing DFT calculations in order to gain
for the reaction of 1 with alkanes is shown in Scheme 1.
more insight into the mechanisms of CꢀH activation by the 1/
py system.
In summary, we have pro-
vided definitive evidence for
intermolecular CꢀH activation
of alkanes with CꢀH bonds as
strong as those in cyclohexane
by
a
(salen)ruthenium(VI)–
nitrido species. The initial rate-
limiting step involves HAT to
form an alkyl radical, which
then undergoes N-rebound and
desaturation pathways that are
reminiscent of CꢀH activation
by cytochrome P450 and metal–
[
1,23]
oxo species.
To the best of
Scheme 1. Proposed mechanism for the reaction of 1 with alkanes.
our knowledge, desaturation of
alkanes by a metal–nitrido spe-
cies is unprecedented. Our find-
The initial step involves rate-limiting HAT from the
ings should provide insights into CꢀH functionalization using
V
alkane (RCH CH R) to 1 to produce Ru =NH (intermediate
metal nitrides.
2
2
A) and the radical RCCHCH R, which then undergo parallel
2
N-rebound and desaturation (through a second HAT) reac-
Received: May 28, 2012
Published online: && &&, &&&&
[
19]
tions. RCCHCH R probably also undergoes other radical
2
reactions to give undetected products. The N-rebound
ruthenium product (intermediate B) could undergo HAT
Keywords: alkanes · CꢀH activation · desaturation ·
.
hydrogen transfer · nitride complexes
(
presumably to complex 1) from an a-CꢀH bond to generate
a ruthenium(III) imine product (compound D), as in the case
of xanthene. Intermediate B could also be reduced to
a ruthenium(III) complex (intermediate E), presumably by
abstracting a hydrogen atom from a second substrate
molecule. Intermediate E could then undergo substitution
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CꢀH Activation
W. L. Man, W. W. Y. Lam, H. K. Kwong,
S. M. Yiu, T. C. Lau*
&&&& — &&&&
Ligand-Accelerated Activation of Strong
CꢀH Bonds of Alkanes by
a (Salen)ruthenium(VI)–Nitrido Complex
Kinetic and mechanistic studies on the
intermolecular activation of strong CꢀH
atom transfer (HAT) from the alkane to
VI
V
Ru ꢁN, generates Ru =NH and
bonds of alkanes by a (salen)ruthe-
nium(VI) nitride were performed. The
initial, rate-limiting step, the hydrogen
RCCHCH R. The following steps involve
2
N-rebound and desaturation.
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
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These are not the final page numbers!