Synthesis and N-H reductive elimination study of dinuclear ruthenium imido dihydride complexes

Article abstract of DOI:10.1021/ja3005682

Diruthenium imido dihydride complexes [(Cp*Ru)₂(μ-NAr) (μ-H)₂] (Ar = Ph (2a), p-MeOC₆H₄ (2b), p-ClC₆H₄ (2c), 2,6-Me₂C₆H ₃ (2d); Cp* = η⁵-C₅Me₅) have been synthesized by hydrogenation of the corresponding bis(amido) complexes [Cp*Ru(μ-NHAr)]₂ (1a-d). Reductive elimination of the N-H bond from 2a-c in the presence of arene yields the amido hydride complexes [(Cp*Ru)₂(μ-NHAr)(μ-H)(μ- η²: η²-arene)] containing a π-bound arene. The rate and kinetic isotope effect for this reaction are consistent with a mechanism involving initial rate-determining reductive elimination of an N-H bond to produce the coordinatively unsaturated amido hydride species {(Cp*Ru) ₂(μ-NHAr)(μ-H)} (A) followed by rapid trapping of this species by an arene. The existence of A is also supported by the reversible interconversion of [(Cp*Ru)₂(μ-NHPh)(μ-H)(μ- η²:η²-C₇H₈)] with the tetranuclear complex [(Cp*Ru)₄(μ₄-NHPh)(μ- NHPh)(μ-H)₂] (4), a dimerization product of A through a μ₄-NHPh bridge. DFT calculations provide structures of A and transition states for the N-H reductive elimination. Two distinct reaction pathways are found for the N-H reductive elimination, one of which involves direct migration of a μ-hydride to the μ-NAr ligand, and the other involves formation of a transient terminal hydride species.

Full text of DOI:10.1021/ja3005682

Article
pubs.acs.org/JACS
Synthesis and N−H Reductive Elimination Study of Dinuclear
Ruthenium Imido Dihydride Complexes
Shin Takemoto,* Yusuke Yamazaki, Takahiro Yamano, Daichi Mashima, and Hiroyuki Matsuzaka*
Department of Chemistry, Graduate School of Science, Osaka Prefecture University, Gakuen-cho 1-1, Naka-ku, Sakai,
Osaka 599-8531, Japan
S
* Supporting Information
ABSTRACT: Diruthenium imido dihydride complexes
[(Cp*Ru)2(μ-NAr)(μ-H)2] (Ar = Ph (2a), p-MeOC6H4
(2b), p-ClC6H4 (2c), 2,6-Me2C6H3 (2d); Cp* = η5-C5Me5)
have been synthesized by hydrogenation of the corresponding
bis(amido) complexes [Cp*Ru(μ-NHAr)]2 (1a−d). Reductive
elimination of the N−H bond from 2a−c in the presence of
arene yields the amido hydride complexes [(Cp*Ru)2(μ-NHAr)-
(μ-H)(μ-η2:η2-arene)] containing a π-bound arene. The rate and
kinetic isotope effect for this reaction are consistent with a
mechanism involving initial rate-determining reductive elimination of an N−H bond to produce the coordinatively unsaturated
amido hydride species {(Cp*Ru)2(μ-NHAr)(μ-H)} (A) followed by rapid trapping of this species by an arene. The existence of
A is also supported by the reversible interconversion of [(Cp*Ru)2(μ-NHPh)(μ-H)(μ-η2:η2-C7H8)] with the tetranuclear
complex [(Cp*Ru)4(μ4-NHPh)(μ-NHPh)(μ-H)2] (4), a dimerization product of A through a μ4-NHPh bridge. DFT cal-
culations provide structures of A and transition states for the N−H reductive elimination. Two distinct reaction pathways are
found for the N−H reductive elimination, one of which involves direct migration of a μ-hydride to the μ-NAr ligand, and the
other involves formation of a transient terminal hydride species.
complexes5 as well as nitride6,7 and dinitrogen8 complexes of
imido-like bonding has been well documented. In contrast, only
a small number of imido hydride complexes have been shown
INTRODUCTION
■
Reductive elimination of a N−H bond is a fundamental organo-
metallic process of considerable importance in understanding
to undergo N−H reductive elimination when treated with ex-
metal-mediated N−H bond-forming and -breaking processes.
ternal substrates such as CO,9 PMe3,10 and H2.11 In none of
One of such processes of great interest is the stepwise N + H
addition reaction in the catalytic synthesis of ammonia over
these cases has the mechanism of the N−H bond-forming step
solid-state iron and ruthenium catalysts.1 Although recent com-
been studied in considerable detail.
Ruthenium is one of the best elemental catalysts for N2
putational studies have provided a detailed picture of this N−H
reduction,12 and several polynuclear ruthenium complexes related
bond-forming process,2 potentially useful information will be
to metal-mediated N−H bond-forming and -breaking processes
obtained from the study of relevant N−H bond-forming
have been reported. Suzuki et al. studied a series of triruthenium
reactions in homogeneous transition metal complexes.
μ3-imido hydride clusters of the type [(Cp*Ru)3(μ3-NR)(μ-H)3]
Studies on the reductive elimination of N−H bond have
in their work on the activation of ammonia, hydrazines, and
mainly concerned amido hydride complexes of late transition
azobenzene by [(Cp*Ru)3(μ-H)3(μ3-H)2].13 Diruthenium
amido hydride complexes have been prepared by reduction of
organonitriles,14 hydrazine,15 or azide16 on diruthenium com-
plexes. To our knowledge, diruthenium imido hydride com-
plexes have not been reported, although a local Ru2(μ-NH)(μ-H)
structure has been proposed in a transition state of NH + H
addition reaction on a ruthenium surface.2a,b
metals.3 Trogler reported the reductive elimination of aniline
from trans-[PtH(NHPh)(PEt3)2], which was proposed to be
associatively induced by added substrates such as PEt3, CO, and
H2.3a Bergman studied the kinetics of PPh3-induced reductive
elimination of RNH2 from [Cp*Ir(NHR)H(PPh3)] and pro-
posed a mechanism involving initial rearrangement of η5-Cp*
to η3-Cp* followed by coordination of PPh3 and elimination of
RNH2.3b Hartwig and Goldman recently reported a direct
spontaneous reductive elimination of N−H bond from [(PCP)Ir-
(NH2)H] to give [(PCP)Ir(NH3)],3c and Brookhart reported a
similar chemistry in arylamido complexes of the type [(POCOP)-
Ir(NHAr)H].3d
We have recently developed a synthetic method for dinuclear
ruthenium imido complexes of the type [(Cp*Ru)2(μ-NPh)
(μ-L)] (L = CO, CH2, and related ligands).17 We now re-
port synthesis of imido dihydride complexes of the type
[(Cp*Ru)2(μ-NAr)(μ-H)2]. This article describes the synthesis
Transition metal imido complexes have attracted attention as
models for surface-bound metal imides and nitrides.4 Heterolytic
addition of H2 to metal−nitrogen multiple bonds of imido
Received: January 18, 2012
Published: September 13, 2012
© 2012 American Chemical Society
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and characterization of these dinuclear imido dihydride com-
plexes and the study of their N−H reductive coupling reactivity,
including characterization of amido hydride derivatives and kinetic
and computational studies of the N−H bond-forming step.
RESULTS AND DISCUSSION
■
Synthesis of Imido Dihydride Complexes. Treatment of
[Cp*Ru(μ-NHPh)]2 (1a)18 with 1 atm of H2 at −80 °C in
toluene followed by warming to room temperature with stirring
over 2 h afforded the imido dihydride complex [(Cp*Ru)2(μ-
demonstrates for the first time that α-abstraction from amido
complexes can be induced by oxidative addition of H2.
To probe the possibility of H−H bond formation in 2a−d, an
H/D exchange reaction was attempted. When a solution of 2a-d2
in C6D6 was stirred under 1 atm of H2 at room temperature for
20 h, no reaction took place. This lack of H/D exchange
indicates that 2a-d2 does not reductively eliminate D2 under the
conditions employed. In contrast, the tetradeuteride complex
[(Cp*Ru)2(μ-D)4] was reported to undergo an H/D exchange
reaction with 1 atm of H2 at room temperature to give
[(Cp*Ru)2(μ-H)4] quantitatively.20
1
NPh)(μ-H)2] (2a) in 71% yield as monitored by H NMR
spectroscopy (eq 1).19 Prolonged reaction of 2a with H2 (12 h)
Formation of Amido Hydride Complexes. The imido
dihydride complex 2a underwent N−H reductive elimination at
room temperature in toluene over the course of 12 h to yield
the amido hydride toluene complex 3a quantitatively (eq 3).
resulted in hydrogenolysis of 2a to [(Cp*Ru)2(μ-H)4]20 and
aniline. Since 2a underwent N−H reductive elimination even at
room temperature to give the amide hydride toluene complex
[(Cp*Ru)2(μ-NHPh)(μ-H)(μ-η2:η2-C7H8)] (3a; vide infra),
2a was characterized spectroscopically without isolation.
Substituted arylamido complexes [Cp*Ru(μ-NHAr)]2 (Ar =
p-MeOC6H4 (1b), p-ClC6H4 (1c), 2,6-Me2C6H3 (1d)) also
reacted with H2 to give the corresponding imido dihydride
complexes 2b−d (eq 1), which were thermally more robust
than 2a and could be isolated as analytically pure red crystalline
solids in 61%, 67%, and 74% yield, respectively.
Complexes 2b,c reacted similarly to give the corresponding
amido hydride toluene complexes 3b,c. In contrast, 2d re-
mained intact even after heating at 110 °C in toluene for 12 h.
When 2a was allowed to react in nonaromatic solvent (i.e.,
cyclohexane or THF), a complicated mixture of products
including the tetranuclear amido hydride complex 4 (vide infra)
was produced.
1
The H NMR spectrum of 2a in C6D6 showed the hydride
resonance at δ −11.25 in 2H intensity relative to the 30H signal
of C5Me5 at δ 1.83. Variable-temperature T1 measurement gave
the T1 minimum of 468 ms (500 MHz, −80 °C) for the
hydride resonance, a value consistent with a classical dihydride
formulation.21 The IR spectrum of 2a (Nujol) showed a weak
band at 1643 cm−1 assignable to a ν(Ru−H) stretch for the
bridging hydride ligands.22 2b−d also showed similar
spectroscopic features except that the ν(Ru−H) band for 2d
was too weak to be assigned unambiguously. The assignment of
ν(Ru−H) bands in 2a−c was confirmed by the disappearance
of these bands in the IR spectra of corresponding d2
isotopomers [(Cp*Ru)2(μ-NAr)(μ-D)2], although ν(Ru−D)
bands in these complexes were obscured by overlapping with
other peaks.
Complex 3a was isolated in 68% yield (based on 1a) as a red
crystalline solid and characterized by analytical and spectro-
1
scopic techniques. The H NMR spectrum of 3a in C6D6
showed signals assignable to the hydride and N−H protons at
δ −13.72 (s, 1H) and −1.38 (br, 1H), respectively, along with a
C5Me5 signal at δ 1.50 (s, 30H). The signals of the toluene unit
were observed as free molecule at 22 °C, indicating that toluene
was displaced by C6D6. However, at −80 °C in THF-d8 in the
presence of 10 equiv of toluene (added to suppress dissociation
of toluene from 3a), five distinct aryl protons for the μ-η2:η2-
toluene ligand (δ 5.67 (1H), 3.03 (1H), 2.91 (1H), 2.86−2.79
(2H)) and two inequivalent Cp* groups were observed in the
1H NMR spectrum of 3a, indicating that dissociation of toluene
was very slow at this temperature. The IR spectrum of 3a
showed weak bands at 3210 and 1597 cm−1 assignable to νNH
and νRuH, respectively, the latter being consistent with the
bridging hydride formulation.22,24 To provide additional
support for the structure of 3a, the μ-η2:η2-naphthalene analogue
[(Cp*Ru)2(μ-NHPh)(μ-H)(μ-η2:η2-C10H8)] (5a) was prepared
by ligand exchange from 3a and characterized by X-ray analysis
(Figure 1). In contrast to 3a, the signals of the bound naphthalene
moiety in 5a were identified in its 1H and 13C{1H} NMR spectra
even at room temperature. The chemical shifts of the μ-hydride,
To gain insight into the mechanism of the formation of 2a−
d, 1a was treated with D2 in toluene-d8. This produced the
imido dideuteride complex 2a-d2 (87%) and PhNH2 (100%) as
1
monitored by H NMR spectroscopy (eq 2). The reaction
proceeded similarly in nondeuterated toluene to give 2a-d2 and
PhNH2. These results suggest that 2a was formed by initial
oxidative addition of H2 to 1a followed by elimination of aniline
via α-abstraction (i.e., proton transfer from one anilido ligand
to the other). This type of addition−elimination sequence has
also been proposed for the reaction of 1a with dative two-
electron ligands such as CO.17a The α-abstraction from amido
complexes is an established route to transition metal imido
complexes and has been induced thermally or by addition of
dative ligands.23 To our knowledge, the synthesis of 2a−d,
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Figure 1. ORTEP drawing of 5a with 30% ellipsoids. Selected
interatomic distances (Å): Ru(1)···Ru(2) = 2.8428(9); Ru(1)−N(1) =
2.119(3); Ru(1)−C(1) = 2.216(4); Ru(1)−C(2) = 2.193(4); Ru(2)−
N(1) = 2.130(3); Ru(2)−C(3) = 2.181(4); Ru(2)−C(4) = 2.217(4);
N(1)−C(11) = 1.415(5); C(1)−C(2) = 1.420(6); C(2)−C(3) =
1.445(6); C(3)−C(4) = 1.420(6).
Figure 2. ORTEP drawing of 4 with 30% ellipsoids. Selected interatomic
distances (Å): Ru(1)···Ru(2) = 2.7304(14); Ru(1)−N(1) = 2.177(10);
Ru(1)−C(1) = 2.229(11); Ru(1)−C(6) = 2.239(11); Ru(2)−N(1) =
2.082(10); Ru(3)···Ru(4) = 2.8638(16); Ru(3)−N(2) = 2.128(9);
Ru(3)−C(2) = 2.225(12); Ru(3)−C(3) = 2.155(11); Ru(4)−N(2) =
2.108(8); Ru(4)−C(4) = 2.180(12); Ru(4)−C(5) = 2.246(12); N(1)−
C(1) = 1.356(14); N(2)−C(7) = 1.399(13); C(1)−C(2) = 1.440(15);
C(2)−C(3) = 1.408(15); C(3)−C(4) = 1.441(16); C(4)−C(5) =
1.442(15); C(5)−C(6) = 1.436(16); C(1)−C(6) = 1.388(17).
N−H, and Cp* protons in 5a (δ −13.44, −1.26, and 1.50,
respectively) were quite similar to those of 3a, indicating similar
solution structures for these compounds.
When 3a was dissolved in THF, the μ-η2:η2-toluene ligand
was easily liberated, and the tetranuclear complex 4 was pro-
duced quantitatively (eq 4). 4 regenerated 3a upon dissolution
Although 2d did not undergo N−H reductive elimination under
thermal conditions in toluene at 110 °C (vide supra), it smoothly
reacted with carbon monoxide at room temperature to give the N−H
bond-formed product 6 having two terminal CO ligands (eq 5).
6 was isolated in 70% yield as a red crystalline solid and char-
acterized by standard analytical and spectroscopic techniques.
in toluene. 4 was isolated in 79% yield as red block crys-
tals from THF−MeCN and characterized by X-ray diffrac-
tion (Figure 2). The tetranuclear structure consists of two
{(Cp*Ru)2(μ-NHPh)(μ-H)} units linked through η2:η2-coor-
dination of a phenyl group, resulting in an unprecedented μ4-
anilido ligand. The η2:η2-arene-bonded diruthenium unit
(Ru(3)−Ru(4)) has a structure analogous to that of 5a, while
the other diruthenium unit (Ru(1)−Ru(2)) has no additional
arene and contains a μ-κ1:η3-NHPh ligand. This type of μ-κ1:η3-
enamide structure is uncommon. The closest precedent is the
aza-π-allyl ligand in [(Cp*Ru)2(μ-H)(μ-NPriC3H5)].25
The 1H NMR spectrum of 4 in THF-d8 showed four
inequivalent Cp* signals at δ 1.78, 1.67, 1.56, and 1.46 in equal
intensity, indicating that the dimeric tetranuclear structure was
maintained in solution. The π-bound phenyl ring was clearly
identified by the characteristic upfield signals at δ 2.73 (3H),
2.50 (1H), and 1.96 (1H). The N−H resonances were found at
δ 0.66 and 5.10, the latter downfield signal being likely due to
the μ4-NHPh moiety. Two hydride resonances were observed
at δ −9.49 and −13.63.
Kinetic Study of N−H Reductive Elimination. To
investigate the mechanism of N−H reductive elimination in
2a−c, a kinetic study was examined. We devised a reaction
shown in eq 6, in which the isolable imido dihydride complex
2b was treated with excess C6D6 in cyclohexane-d12 to give the
amido hydride μ-η2:η2-C6D6 complex 3b′ (the same species
formed when 3b was dissolved in C6D6). The progress of the
reaction was followed by 1H NMR spectroscopy, and the
observed rate constants are summarized in Table 1.
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Table 1. Rate Constants for the Reaction of 2b with Excess L
in Cyclohexane-d12
[2b]0 (mol L−1
)
L
[L] (mol L−1
)
T (°C)
kobs (s−1
)
0.059
0.059
0.060
0.059
0.059
0.057
0.053
0.052
0.060
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
C10H8
1.2
1.2
1.2
1.2
1.2
1.7
2.1
2.6
1.2
25
30
35
40
45
35
35
35
35
2.2 × 10−5
4.0 × 10−5
6.5 × 10−5
1.3 × 10−4
2.3 × 10−4
6.0 × 10−5
6.4 × 10−5
5.9 × 10−5
6.5 × 10−5
The reaction was found to be cleanly first-order in [2b]
and showed zero-order dependence on [C6D6]. Rate measure-
ment at five different temperatures from 25 to 45 °C gave
the following activation parameters: Ea = 22(1) kcal mol−1,
ΔH⧧ = 22(1) kcal mol−1, and ΔS⧧ = 6(3) eu (Figure 3).
Figure 3. Arrhenius plot for the conversion of 2b to 3b′ at 25−45 °C.
The rate was also measured for the reaction of 2b with excess
naphthalene in cyclohexane-d12 to form [(Cp*Ru)2(μ-NHAr)-
(μ-H)(μ-η2:η2-C10H8)] (5b); the observed rate constant for
Figure 4. Optimized structures of local minima and transition states in
2a + C6H6 → 3a″ in C6H6 (M06, PCM). The optimized structure of A
(Ar = 2,6-Me2C6H3) is also shown. Hydrogen atoms on C5Me5 groups
and aryl rings are omitted for clarity. Interatomic distances are given in Å.
Scheme 1
this reaction (6.5 × 10−5 s−1 at 35 °C) was identical to that for
the reaction in eq 6 (6.2(3) × 10−5 s−1). These results suggest
that the entering arene molecule is not involved in the rate-
determining step.
To explore the nature of the rate-determining step, we
examined the kinetic isotope effect (KIE) by comparing the
rates of reactions of 2b and 2b-d2 in C6D6 (eq 7). We observed
a normal kinetic isotope effect of kH/kD = 1.7 (kH = 4.1 × 10−5 vs
kD = 2.4 × 10−5 s−1), which indicates that the rate-determining
step involves dissociation of a Ru−H bond. We also examined
KIE for 2c and 2c-d2 (eq 7) and obtained kH/kD = 2.1 (kH =
4.0 × 10−5 vs kD = 1.9 × 10−5 s−1). Interestingly, the rates of
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Figure 5. Gibbs free energy profile for 2a + C6H6 → 3a″ in C6H6 (M06, PCM). Energies (kcal mol−1) are relative to 2a + C6H6. Enthalpies are given
in parentheses.
reactions of 2b and 2c (and 2b-d2 and 2c-d2) are quite similar
despite the opposing electronic effect of the p-MeO and p-Cl
substituents. The electron-donating p-MeO group would
enhance the basicity of the imido nitrogen but might retard
the reductive elimination from ruthenium centers, whereas
electron-withdrawing p-Cl substituent would impart opposite
effects. This could be a possible reason for the similar kinetics
of the reactions of 2b and 2c.
distances (1.94 Å) are typical of μ-imido ligands of Ru−N
multiple bond character.17,27
As shown in Figure 5, 2a was found to give the amido
hydride complex A via the transition state TS1 or via the
transition state TS1′ with activation energies of 22.9 and 22.6
kcal mol−1, respectively. In TS1 the migrating hydrogen atom
bridges the Ru2N triangle, whereas in TS1′ the migrating
hydrogen atom bridges one of the Ru−N bonds. These results
suggest that the N−H reductive elimination from 2a may
proceed either by maintaining the bridging hydride structure or
by the formation of a terminal hydride ligand. This N−H
reductive elimination step is rate-determning in the overall
reaction. We could not find any stable imido dihydride benzene
adduct, which would be formed if the reaction of 2a and C6H6
proceeded via initial coordination of C6H6. Attempts to locate
such adducts led to collapse to 2a and free C6H6.
The optimized structure of A (Figure 4) shows a pyramidal
two-legged piano stool geometry at both Ru centers28 and a
folded central Ru2(N)(H) ring (the dihedral angle between two
N−Ru−H planes is 40.5°). A is 0.9 kcal mol−1 less stable than
2a and easily binds a benzene molecule with an activation free
energy of 5.9 kcal mol−1 to give the η2-C6H6 adduct B and then
the μ-η2:η2-C6H6 adduct 3a″. The benzene binding reaction
A + C6H6 → 3a″ is exergonic (ΔG = −2.8 kcal mol−1) and
exothermic (ΔH = −19.5 kcal mol−1). The small free energy
barrier and free energy change for A + C6H6 → 3a″ are
consistent with the facile reversible arene dissociation from 3a
observed experimentally. The endothermicity of C6H6 loss
from 3a″ indicates that this process is driven by entropy gain.
Next, we compared the behavior of 2a−c. Optimized struc-
tures were calculated at B3PW91 level for 2a−c, the corres-
ponding amido hydride complexes A, and the transition states
(TS1 and TS1′) between 2a−c and A. The relative free
energies of these species are listed in Table 2. The structures
Possible reactivity patterns of 2a−c that involve the Ru−H
bond dissociation would include (i) the reductive elimination
of an N−H bond, (ii) bridge-to-terminal rearrangement of a
hydride ligand, and (iii) reductive H−H coupling to form an η2-
dihydrogen complex. The activation energy for the conversion
of a Ru−H−Ru bridge to a terminal Ru−H and an unbridged
Ru−Ru bond has been estimated to be 10 kcal mol−1.26 Thus,
the bridge-to-terminal hydride rearrangement is unlikely to
dominate the overall rate of the present reaction whose Ea is
estimated to be 22 kcal mol−1. The formation of an η2-
dihydrogen complex is also unlikely given the absence of H/D
exchange between 2a-d2 and H2 (vide supra). Therefore, the
rate-determining dissociation of a Ru−H bond in 2a−c is most
likely attributed to the reductive elimination of an N−H bond.
The simplest mechanism consistent with these observations
involves initial rate-determining reductive elimination of an N−H
bond to form the unsaturated amido hydride intermediate
[(Cp*Ru)2(μ-NHAr)(μ-H)] (A) followed by rapid trapping of
this species by an arene molecule (Scheme 1). The facile
interconversion between 3a and 4 (eq 4) also strongly suggests
the existence of the intermediate A.
Computational Study. To explore the nature of the
unsaturated amido hydride intermediate and the transition state
of the N−H bond formation, we performed a computational
study based on density functional theory (DFT). Here we
mainly discuss the results obtained by calculations at the M06
level, but some data obtained at the B3PW91 level are also used.
We initially examined the reaction of 2a with benzene to give
[(Cp*Ru)2(μ-NHPh)(μ-H)(μ-η2:η2-C6H6)] (3a″). Optimized
structures on the potential energy surface of 2a + C6H6 → 3a″
are shown in Figure 4, and a Gibbs free energy profile for this
reaction is given in Figure 5.
Table 2. Gibbs Free Energies for the N−H Reductive
Elimination from [(Cp*Ru)2(μ-NAr)(μ-H)2] (2a−c) to
a
[(Cp*Ru)2(μ-NHAr)(μ-H)] (A) in C6H6 (B3PW91)
Ar
TS1
TS1′
A
Ph
23.8
24.0
23.6
24.7
8.3
8.4
9.2
The optimized structure of 2a has an approximate C2
symmetry with the two Cp* groups taking a staggered
orientation and the phenyl ring being tilted from the Ru2N
plane (dihedral angle 45.4°). The hydride ligands symmetrically
bridge the Ru atoms with Ru−H distances (1.81−1.83 Å)
similar to those in [(Cp*Ru)2(μ-H)4].20,26b The Ru−N
b
p-MeOC6H4
p-ClC6H4
25.9
24.5
a
Energies (kcal mol−1) are relative to 2a−c for each NAr series,
b
Calculated for PCM-optimized structures. Calculated for gas-phase
optimized structure with single-point PCM correction.
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of 2b−c, TS1, TS1′, and A are similar to the corresponding
species in the NPh series. The computed activation energies for
the N−H reductive elimination in 2b are in good agreement
with the experimentally observed activation energy for 2b
(22 kcal mol−1). Also, the similarity of computed activation
energies for the N−H reductive elimination in 2b and 2c is
consistent with the similar reaction kinetics for 2b and 2c. We
also computed kinetic isotope effect for 2b/2b-d2 and 2c/2c-d2
by using the approximation kH/kD = exp {(ΔG⧧ − ΔG⧧H)/
D
RT}.29 For the reaction pathway via TS1, we obtained kH/kD =
1.59 for 2b/2b-d2 and kH/kD = 1.64 for 2c/2c-d2. These values
are comparable to the experimentally determined KIEs. For the
reaction pathway via TS1′, smaller KIEs (1.27 for 2b/2b-d2 and
1.29 for 2c/2c-d2) were calculated. This may indicate that the
pathway via TS1 might predominate in the real reaction system.
Finally, we computationally examined the reactivity of 2d.
We calculated optimized structures of 2d, TS1, A (Ar = 2,6-
Me2C6H3), and 3d″; relative Gibbs free energies of these
species are shown in Figure 6. The optimized structure of 2d is
Figure 7. Proposed mechanism of NH + H → NH2 on Ru(0001)
surface.2a Relative energies are given in kcal mol−1.
with similar activation energies. The product is an edge-bridging
NH2 (Figure 7(c)), which is less stable than NH + H. Our
diruthenium imido dihydride system seems to provide a model
for these transition states and reproduce the relative instability
of NH2 species compared to the NH + H pair.
CONCLUSIONS
■
Oxidative addition of H2 to the bis(amido) complexes 1a−d
induced elimination of ArNH2 via α-H-abstraction to yield the
imido dihydride complexes 2a−d. Complexes 2a−c underwent
the reductive elimination of an N−H bond in the presence of
excess arene to give the amido hydride arene complexes 3a−c
and 3a′−c′. The rate and deuterium kinetic isotope effect were
determined for the formation of 3b′,c′, and it has been
proposed that the reaction proceeds with the rate-determining
N−H reductive elimination from 2a−c followed by trapping of
the unsaturated amido hydride species [(Cp*Ru)2(μ-NHAr)-
(μ-H)] (A) by an arene molecule. The dimer of A (complex 4),
which contains an unprecedented μ4-NHPh bridge, was isolated
and shown to interconvert reversibly with 3a.
The transition states and energetics for the N−H reductive
elimination in 2a−d was studied by DFT calculations. The
calculations indicated that the N−H reductive elimination in
2a−c can proceed either by maintaining the bridging hydride
structure or via the formation of a terminal hydride ligand. This
microscopic picture is very similar to that proposed for the
NH + H addition reaction on Ru surface.
Figure 6. Gibbs free energy profile for 2d + C6H6 → 3d″ in C6H6
(M06, PCM). Energies (kcal mol−1) are relative to 2d + C6H6.
Enthalpies are given in parentheses.
similar to those of 2a−c. The optimized structure of A (Ar =
2,6-Me2C6H3; Figure 4) contains a C−H agostic interaction
between a methyl group in the NHAr ligand and one of the Ru
centers (CH···Ru = 2.16 Å). Between 2d and A, we found a
transition state (TS1) that was analogous to the TS1s in the
other NAr series. The activation energy for the conversion of
2d to A is 23.9 kcal mol−1, which is comparable to that for the
conversion of 2a to A. Attempted optimization of TS1′ for this
Ar = 2,6-Me2C6H3 system led to convergence to TS1. On the
other hand, benzene binding to A derived from 2d is
energetically highly unfavorable; the free energy of the benzene
adduct 3d″ is calculated to be 14.9 kcal mol−1 relative to 2d +
C6H6. Thus, the apparent lack of reactivity of 2d under thermal
conditions (i.e., toluene, 110 °C) can be ascribed to the
instability of the amido hydride arene adduct, which would be
too sterically hindered. This view is consistent with the smooth
reaction of 2d with a smaller ligand, CO, at room temperature,
which gives the bis-CO adduct of A (i.e., complex 6; eq 5).
The reaction pathways deduced for the N−H reductive
elimination in 2a−d have considerable similarity with those
proposed for the NH + H addition reaction on Ru surface
(Figure 7).2 It has been shown that both NH and H occupy
the 3-fold hollow sites in their most stable states (Figure 7(a)).
In the transition states the NH group is proposed to move toward
a bridge site, and the H atom approaches to this group either
from a bridge site (Figure 7(b)) or from a top site (Figure 7(b′))
This work represents the first mechanistic study of imido−hydride
reductive elimination in a well-defined organometallic system.
EXPERIMENTAL SECTION
■
General Remarks. All manipulations were performed under an
atmosphere of nitrogen using standard Schlenk techniques unless otherwise
noted. Aniline and 2,6-dimethylaniline were degassed and stored over
activated 4-Å molecular sieves in the dark. Anhydrous solvents (THF,
hexane, toluene, diethyl ether, and acetonitrile) were purchased from
commercial vendors and degassed before use. Benzene-d6, cyclohexane-d12,
and THF-d8 were degassed by freeze−pump−thaw cycles and stored over
activated 4-Å molecular sieves. [Cp*Ru(μ3-Cl)]4 and [Cp*Ru(μ-NHPh)]2
(1a) were prepared according to the literature.30,17c Lithium amides
LiNHAr were prepared in situ by deprotonation of ArNH2 with n-BuLi in
THF at −80 °C. Other reagents were purchased from commercial vendors
and used without further purification unless otherwise noted.
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[Cp*Ru(μ-NHC6H4OMe-p)]2 (1b). A solution of LiNHC6H4OMe-
p (2.66 mmol) in 5 mL of THF was transferred via a cannula to a
slurry of [Cp*Ru(μ3-Cl)]4 (721 mg, 0.663 mmol) in 10 mL of THF at
−80 °C. The mixture was allowed to warm to room temperature over
3 h with stirring to give a dark blue solution. The solution was
evaporated to dryness, and the residue was extracted with toluene.
Recrystallization from toluene−hexane afforded 1b as dark blue
crystals. Yield 488 mg (0.680 mmol, 51%). Anal. Calcd for C34H46-
N2O2Ru2: C, 56.96; H, 6.47; N, 3.91. Found: C, 56.59; H, 6.56; N,
3.62. 1H NMR (400 MHz, C6D6): δ 7.50 (m, 4H, aryl), 7.04 (m, 4H,
aryl), 3.56 (s, 6H, OMe), 1.43 (s, 30H, Cp*), −1.09 (br, 2H, NH).
13C{1H} NMR (100 MHz, C6D6): δ 156.0, 154.5, 122.2, 113.4 (aryl),
73.3 (C5Me5), 55.3 (OMe), 11.6 (C5Me5). IR (Nujol, cm−1): 3213 (w, νNH).
[Cp*Ru(μ-NHC6H4Cl-p)]2 (1c). A solution of LiNHC6H4Cl (3.70
mmol) in 6 mL of THF was transferred via a cannula to a slurry of
[Cp*Ru(μ3-Cl)]4 (1.00 g, 0.922 mmol) in 8 mL of THF at −80 °C.
The mixture was allowed to warm to room temperature over 3 h with
stirring to give a dark blue solution. The solution was evaporated to
dryness, and the residue was extracted with toluene. Recrystallization
from toluene−hexane afforded 1c as a dark blue crystalline solid. Yield
966 mg (1.33 mmol, 72%). Anal. Calcd for C32H40N2Cl2Ru2: C, 52.96;
T1 min = 474 ms (−60 °C, 500 MHz)). 13C{1H} NMR (100 MHz,
C6D6): δ 165.0, 157.3, 120.0, 114.1 (aryl), 91.8 (C5Me5), 55.1 (OMe),
12.3 (C5Me5). IR (Nujol, cm−1): 1650 (νRuH).
[(Cp*Ru)2(μ-NC6H4Cl-p)(μ-H)2] (2c). In a Schlenk flask of ca. 50
mL inner volume, 1c (417 mg, 0.574 mmol) was dissolved in 20 mL of
toluene. After cooling to −80 °C, the flask was evacuated, filled with
H2 (1 atm), and sealed. The mixture was allowed to warm to room
temperature over 3 h with stirring to give a dark red solution. The
solution was evaporated under reduced pressure, and the residue was
recrystallized from THF−MeCN at −30 °C to give 2c as a red block
crystals. Yield 228 mg (0.379 mmol, 67%). Anal. Calcd for C26H36-
NClRu2: C, 52.03; H, 6.05; N, 2.33. Found: C, 51.55; H, 5.87; N, 2.38.
1H NMR (400 MHz, C6D6): δ 7.34 (d, J = 8.6 Hz, 2H, aryl), 7.25
(d, J = 8.6 Hz, 2H, aryl), 1.79 (s, 30H, Cp*), −11.28 (s, 2H, μ-H, T1
min = 467 ms (−60 °C, 500 MHz)). 13C{1H} NMR (100 MHz,
C6D6): δ 170.1, 128.7, 128.1, 118.7 (aryl), 92.0 (C5Me5), 12.1
(C5Me5). IR (Nujol, cm−1): 1590 (νRuH).
[(Cp*Ru)2(μ-NXy)(μ-H)2] (2d). In a Schlenk flask of ca. 100 mL
inner volume, 1d (729 mg, 1.02 mmol) was dissolved in 42 mL of
toluene. After cooling to −80 °C, the flask was evacuated, filled with
H2 (1 atm), and sealed. The solution was allowed to warm to room
temperature over 3 h with stirring to give a reddish brown solution.
The solution was concentrated under reduced pressure and layered
with acetonitrile to gave 2d as a red crystalline solid. Yield 448 mg
(0.755 mmol, 74%). Anal. Calcd for C28H41NRu2: C, 56.64; H, 6.96;
1
H, 5.56; N, 3.86. Found: C, 52.60; H, 5.38; N, 3.73. H NMR (400
MHz, C6D6): δ 7.33 (d, J = 7.9 Hz, 4H, aryl), 7.28 (br, 4H, aryl), 1.26
(s, 30H, Cp*), −1.29 (br, 2H, NH). 13C{1H} NMR (100 MHz,
C6D6): δ 161.4, 127.9, 124.7, 122.9 (aryl), 73.1 (C5Me5), 11.3 (C5Me5).
[Cp*Ru(μ-NHXy)]2 (1d). A solution of LiNHXy (4.0 mmol) in
6 mL of THF was transferred via a cannula to a slurry of [Cp*Ru(μ3-
Cl)]4 (1.054 g, 0.974 mmol) in 14 mL of THF at −80 °C. The mixture
was warmed to 0 °C over 2 h with stirring to give a dark violet
solution. The solution was evaporated to dryness, and the residue was
extracted with toluene chilled at −10 °C. Recrystallization from
toluene−hexane at −30 °C afforded 1d as a violet crystalline solid.
Yield 997 mg (1.40 mmol, 72%). Anal. Calcd for C36H50N2Ru2: C,
1
N, 2.36. Found: C, 56.62; H, 7.14; N, 2.32. H NMR (400 MHz,
C6D6): δ 7.23 (d, J = 7.4 Hz, 2H, aryl), 7.07 (t, J = 7.4 Hz, 1H, aryl),
1.83 (s, 6H, C6H3Me2), 1.74 (s, 30H, Cp*), −10.98 (s, 2H, μ-H, T1
min = 476 ms (−80 °C, 500 MHz)). 13C{1H} NMR (100 MHz,
C6D6): δ 170.8, 127.2, 121.6, 119.9 (aryl), 91.9 (C5Me5), 18.5
(C6H3Me2), 11.7 (C5Me5).
[(Cp*Ru)2(μ-NHPh)(μ-H)(μ-η2:η2-C7H8)] (3a). In a Schlenk flask
of ca. 100 mL inner volume, 1a (1.80 g, 2.74 mmol) was dissolved in
45 mL of toluene. After cooling to −80 °C, the flask was evacuated,
filled with H2 (1 atm), and sealed. The mixture was allowed to warm
to room temperature over 2 h with stirring to yield a dark red solution.
The H2 gas was removed by evacuation, and the solution was stirred
under N2 for additional 12 h at room temperature. The solution was
concentrated under reduced pressure and layered with acetonitrile.
After diffusion of the solvents was complete, 3a was isolated as a red
solid. Yield 1.23 g (1.87 mmol, 68%). Anal. Calcd for C33H45NRu2: C,
1
60.65; H, 7.07; N, 3.93. Found: C, 60.20; H, 7.07; N, 3.87. H NMR
(500 MHz, C6D6): δ 7.39 (d, J = 7.4 Hz, 2H, aryl), 7.27 (d, J = 7.4 Hz,
2H, aryl), 7.01 (t, J = 7.4 Hz, 2H, Ar), 3.00 (s, 6H, C6H3Me2), 2.32
(s, 6H, C6H3Me2), 1.08 (s, 30H, Cp*), −0.14 (br, 2H, NH).
[(Cp*Ru)2(μ-NPh)(μ-H)2] (2a). In a Schlenk flask of ca. 50 mL
inner volume, 1a (232 mg, 0.353 mmol) and 1,3,5-trimethoxybenzene
(59.3 mg, 0.353 mmol, internal standard) were dissolved in 15 mL of
toluene. After cooling to −80 °C, the flask was evacuated, filled with
H2 (1 atm), and sealed. The mixture was allowed to warm to room
temperature over 2 h with stirring to give a dark red solution. 1H NMR
analysis showed that 2a and 3a were formed in 71% and 26% yield,
respectively. 1H NMR for 2a (500 MHz, C6D6): δ 7.55 (d, J = 7.4 Hz,
2H, Ph), 7.34 (t, J = 7.4 Hz, 2H, Ph), 7.26 (t, J = 7.4 Hz, 1H, Ph), 1.83
(s, 30H, Cp*), −11.25 (s, 2H, μ-H, T1 min = 468 ms (−80 °C, 500
MHz)). 13C{1H} NMR for 2a (100 MHz, cyclohexane-d12): δ 164.1,
128.3, 122.9, 116.9 (Ph), 92.0 (C5Me5), 12.2 (C5Me5). IR (Nujol,
cm−1): 1643 (νRuH, assigned by comparison with D-isotopomer).
[(Cp*Ru)2(μ-NPh)(μ-D)2] (2a-d2). In a Schlenk flask of ca. 20 mL
inner volume, 1a (19.5 mg, 0.0297 mmol) and 1,3,5-trimethoxy-
benzene (5.5 mg, 0.0327 mmol, internal standard) were dissolved in
0.5 mL of toluene-d8. After cooling to −80 °C, the flask was evacuated,
filled with D2 (1 atm), and sealed. The mixture was allowed to warm
to room temperature over 3 h with stirring to give a dark red solution.
1H NMR analysis showed that 2a-d2 and PhNH2 were formed in 87%
and 100% yield, respectively. Similar experiment in nondeuterated
1
60.25; H, 6.89; N, 2.13. Found: C, 60.07; H, 7.08; N, 2.09. H NMR
(500 MHz, C6D6): δ 6.98 (br, 2H, Ph), 6.71 (t, J = 7.4 Hz, 1H, Ph),
6.65 (br, 2H, Ph), 1.50 (s, 30H, Cp*), −1.38 (br, 1H, NH), −13.72
(s, 1H, μ-H). 13C{1H} NMR (100 MHz, C6D6, 80 °C): δ 164.1, 127.8,
123.1, 119.3 (Ph), 89.9 (C5Me5), 9.9 (C5Me5). IR (Nujol, cm−1): 3210
(w, νNH), 1597 (w, νRuH); the assignment was confirmed by red shift in
3a-d2: νND = 2387 cm−1, νRuD obscured by other peaks. 1H NMR (500
MHz, THF-d8, −80 °C, in the presence of 10 equiv of toluene): δ 7.00
(t, 1H, J = 7.6 Hz, NHPh), 6.73 (t, 1H, J = 7.6 Hz, NHPh), 6.56 (t,
1H, J = 7.6 Hz, NHPh), 6.46 (d, 1H, J = 7.6 Hz, NHPh), 6.32 (t, 1H,
J = 7.6 Hz, NHPh), 5.67 (d, 1H, J = 5.5 Hz, C6H5Me), 3.03 (d, 1H, J =
5.5 Hz, C6H5Me), 2.91 (d, 1H, J = 6.7 Hz, C6H5Me), 2.86−2.79 (m,
2H, C6H5Me), 1.93 (s, 3H, C6H5Me), 1.56 (s, 15H, Cp*), 1.50 (s,
15H, Cp*), −1.83 (br, 1H, NH), −13.85 (s, 1H, μ-H). 13C{1H} NMR
(100 MHz, THF-d8, −80 °C, in the presence of 10 equiv of toluene):
δ 164.0, 130.9 (C6H4Me), 127.6, 127.5, 125.2, 120.1 (NHPh), 119.2
(C6H4Me), 116.6 (NHPh), 89.6, 89.0 (C5Me5), 56.7, 53.9, 51.2, 50.4
(C6H4Me), 21.3 (C6H4Me), 10.1, 9.9 (C5Me5).
1
toluene also gave 2a-d2 as confirmed by H NMR spectroscopy.
[(Cp*Ru)2(μ-NC6H4OMe-p)(μ-H)2] (2b). In a Schlenk flask of ca.
50 mL inner volume, 1b (347 mg, 0.484 mmol) was suspended in
20 mL of hexane. After cooling to −80 °C, the flask was evacuated,
filled with H2 (1 atm), and sealed. The mixture was allowed to warm
to room temperature over 3 h with stirring to give a dark red solution.
The solution was evaporated under reduced pressure, and the residue
was washed with acetonitrile to give 2b as a red crystalline solid. Yield
177 mg (0.296 mmol, 61%). Anal. Calcd for C27H39NORu2: C, 54.43; H,
6.60; N, 2.35. Found: C, 54.23; H, 6.69; N, 2.34. 1H NMR (400 MHz,
C6D6): δ 7.72 (d, J = 8.8 Hz, 2H, aryl), 6.95 (d, J = 8.8 Hz, 2H, aryl),
3.45 (s, 3H, OMe), 1.88 (s, 30H, Cp*), −11.40 (s, 2H, μ-H,
[(Cp*Ru)2(μ-NHC6H4OMe-p)(μ-H)(μ-η2:η2-C7H8)] (3b). 2b (63
mg, 0.106 mmol) was dissolved in 5 mL of toluene, and the solution
was stirred at 35 °C for 24 h. Removal of the solvent in vacuo yielded
1
3b as a red solid. Yield 72 mg (0.104 mmol, 98%). H NMR (400
MHz, C6D6): δ 6.61 (m, 4H, aryl), 3.37 (s, 3H, OMe), 1.52 (s, 30H,
Cp*), −1.36 (br, 1H, NH), −13.81 (s, 1H, μ-H). 13C{1H} NMR (100
MHz, C6D6): δ 157.3, 153.4, 121.2, 113.0 (aryl), 89.8 (C5Me5), 55.1
(OMe), 10.1 (C5Me5). IR (Nujol, cm−1): 3211 (w, νNH), 1607
(w, νRuH); the assignment was confirmed by red shift in 3b-d2: νND
=
2386 cm−1, νRuD obscured by other peaks.
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[(Cp*Ru)2(μ-NHC6H4Cl-p)(μ-H)(μ-η2:η2-C7H8)] (3c). 2c (43.2 mg,
0.072 mmol) was dissolved in 5 mL of toluene, and the solution was
stirred at 35 °C for 24 h. Removal of the solvent in vacuo yielded 3c as
difference Fourier method and were not included in the structure.
Other hydrogen atoms were placed at calculated positions and treated
as riding models. Thermal ellipsoid plots were drawn by ORTEP-3 for
Windows.34 Detailed crystallographic data are given in the Supporting
Information.
Computational Details. All calculations were performed with
Gaussian 09 program35 using the M0636 or B3PW9137 functionals
with LanL2DZ effective core pseudopotential and basis set38 for Ru
and 6-31G(d,p) basis set39 for all other atoms. Geometry optimiza-
tions were performed without any symmetry constraints and in the
presence of C6H6 solvent using the polarizable continuum model40
unless otherwise mentioned. Local minima and transition states were
checked by analytical frequency calculations, and intrinsic reaction
coordinate calculations and subsequent geometry optimizations were
used to confirm the connection of each transition state with cor-
responding local minima. Gibbs free energies are those in C6H6
solution and at 298.15 K and 1 atm.
1
a red solid. Yield 49.3 mg (0.071 mmol, 99%). H NMR (400 MHz,
C6D6): δ 6.92 (br, 2H, aryl), 6.92 (d, J = 6.4 Hz, 2H, aryl), 1.43
(s, 30H, Cp*), −1.65 (br, 1H, NH), −13.72 (s, 1H, μ-H). 13C{1H}
NMR (100 MHz, C6D6): δ 163.1, 128.2, 127.9, 123.2 (aryl), 89.6
(C5Me5), 10.0 (C5Me5). IR (Nujol, cm−1): 3207 (w, νNH), 1594 (w,
νRuH); the assignment was confirmed by red shift in 3c-d2: νND = 2385
cm−1, νRuD obscured by other peaks.
[(Cp*Ru)4(μ4-NHPh)(μ-NHPh)(μ-H)2] (4). 3a (130 mg, 0.198
mmol) was dissolved in THF (1 mL), and the solution was stirred for
5 min. Layering 3 mL of acetonitrile onto this solution gave dark
brown block crystals of 4. The crystals were used for the single-crystal
X-ray study. Yield 88 mg (0.0778 mmol, 79%). Anal. Calcd for
C52H74N2Ru4: C, 55.20; H, 6.59; N, 2.48. Found: C, 55.27; H, 6.60; N,
2.54. 1H NMR (400 MHz, THF-d8): δ 6.83 (br, 2H, Ph), 6.50 (m, 3H,
Ph), 5.10 (br, 1H, NH), 2.73 (m, 3H, μ4-NHPh), 2.50 (d, J = 7.2 Hz,
1H, μ4-NHPh), 1.96 (m, 1H, μ4-NHPh), 1.78, 1.67, 1.56, 1.46 (s, 15 H
each, Cp*), 0.66 (brs, 1H, NH), −9.49 (s, 1H, μ-H), −13.63 (s, 1H,
μ-H). 13C{1H} NMR (100 MHz, THF-d8, 60 °C): δ 165.4, 128.8,
124.2, 120.2 (Ph), 92.0, 90.6, 85.4, 74.1 (C5Me5), 114.8, 57.7, 54.3, 49.7,
48.5, 34.1 (μ4-NHPh), 13.9, 12.9, 11.2, 11.1 (C5Me5). IR (Nujol, cm−1):
3277, 3170 (w, νNH).
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data in CIF format; coordinates and energies
of calculated complexes; and complete ref 35. This material is
free of charge via the Internet at http://pubs.acs.org.
[(Cp*Ru)2(μ-NHPh)(μ-H)(μ-η2:η2-C10H8)] (5a). To a solution of
3a (59 mg, 0.090 mmol) in 4 mL of THF was added naphthalene
(14 mg, 0.107 mmol), and the solution was stirred for 1 h at room
temperature. Recrystallization from THF−MeCN gave 5a as red plate
crystals. Yield 92.6 mg (0.134 mmol, 100%). Anal. Calcd for
C36H45NRu2: C, 62.31; H, 6.54; N, 2.02. Found: C, 62.69; H, 6.67;
N, 2.06. 1H NMR (400 MHz, C6D6): δ 7.44, 7.19 (dd, J = 5.3, 3.2 Hz,
2H each, C10H8), 6.94 (br, 1H, Ph), 6.60 (m, 3H, Ph), 5.64 (br, 1H,
Ph), 3.71, 3.05 (dd, J = 5.3, 3.2 Hz, 2H each, μ-η2:η2-C10H8), 1.50
(s, 30H, Cp*), −1.26 (br, 1H, NH), −13.44 (s, 1H, μ-H). 13C{1H} NMR
(100 MHz, C6D6, 80 °C): δ 163.0, 136.9, 127.7, 125.3, 124.7, 122.4, 119.1
(aryl), 90.5 (C5Me5), 52.5, 49.6 (μ-η2:η2-C10H8), 10.0 (C5Me5). IR
(Nujol, cm−1): 3265 (w, νNH), 1652 (w, νRuH).
[(Cp*Ru)2(μ-NHXy)(μ-H)(CO)2] (6). A solution of 2d (130 mg,
0.213 mmol) in 10 mL of toluene was stirred under 1 atm of CO at
room temperature for 2 h. The resulting red solution was evaporated
to dryness, and the residue was recrystallized from hexane at −30 °C
to give 6 as red crystalline solid. Yield 101 mg (0.155 mmol, 73%).
Anal. Calcd for C30H41NO2Ru2: C, 55.45; H, 6.36; N, 2.16. Found: C,
55.42; H, 6.78; N, 2.07. 1H NMR (400 MHz, C6D6): δ 7.00 (t, J = 7.2
Hz, 2H, aryl), 6.85 (t, J = 7.2 Hz, 1H, aryl), 4.31 (br, 1H, NH), 2.77,
2.52 (s, 3H each, C6H3Me2), 1.71, 1.55 (s, 15H each, Cp*), −13.99
(s, 1H, μ-H). 13C{1H} NMR (100 MHz, C6D6): δ 206.5, 206.2 (CO),
152.6, 131.6, 131.3, 129.3, 128.8, 121.1 (aryl), 94.4, 94.3 (C5Me5),
23.0, 21.3 (C6H3Me2), 10.4, 10.0 (C5Me5). IR (Nujol, cm−1): 3330 (w,
νNH), 2009, 1891 (s, νCO), 1619 (w, νRuH).
AUTHOR INFORMATION
■
Corresponding Author
matuzaka@c.s.osakafu-u.ac.jp
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Scientific
Research (C) and a Grant-in-Aid for Scientific Research on
Innovative Areas “Molecular Activation Directed toward
Straightforward Synthesis” from the Ministry of Education,
Science, Sports, and Culture of Japan. We also thank financial
support from Toyota Motor Corporation.
REFERENCES
■
(1) Ertl, G. In Catalytic Ammonia Synthesis; Jennings, J. R., Ed.;
Plenum Press: New York, 1991; pp 109−131.
(2) (a) Zhang, C. J.; Lynch, M.; Hu, P. Surf. Sci. 2002, 496, 221.
(b) Honkala, K.; Hellman, A.; Remediakis, I. N.; Logadottir, A.;
Carlsson, A.; Dahl, S.; Christensen, C. H.; Nørskov, J. K. Science 2005,
307, 555. (c) McKay, H. L.; Jenkins, S. J.; Wales, D. J. J. Phys. Chem. C
2009, 113, 15274. (d) Macgregor, S. A. Organometallics 2001, 20,
1860.
Kinetic Experiments. The following operation is representative.
2b (19.4 mg, 0.0326 mmol) and 1,3,5-trimethoxybenzene (5.1 mg)
as internal standard were dissolved in a mixture of cyclohexane-d12
(0.50 mL) and benzene-d6 (144 μL, 1.63 mmol), and the solution was
transferred to an NMR tube by a syringe. After being flame-sealed, the
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17034
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Journal of the American Chemical Society
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