Mild, reversible reaction of iridium(III) amido complexes with carbon dioxide

Article abstract of DOI:10.1021/ic300923c

Unlike some other Ir(III) hydrides, the aminopyridine complex [(2-NH ₂-C₅NH₄)IrH₃(PPh₃) ₂] (1-PPh₃) does not insert CO₂ into the Ir-H bond. Instead 1-PPh₃ loses H₂ to form the cyclometalated species [(κ²-N,N-2-NH-C₅NH₄)IrH ₂(PPh₃)₂] (2-PPh₃), which subsequently reacts with CO₂ to form the carbamato species [(κ²-O,N-2-OC(O)NH-C₅NH₄)IrH ₂(PPh₃)₂] (10-PPh₃). To study the insertion of CO₂ into the Ir-N bond of the cyclometalated species, a family of compounds of the type [(κ²-N,N-2-NR-C ₅NH₄)IrH₂(PR′₃)₂] (R = H, R′ = Ph (2-PPh₃); R = H, R′ = Cy (2-PCy ₃); R = Me, R′ = Ph (4-PPh₃); R = Ph, R′ = Ph (5-PPh₃); R = Ph, R′ = Cy (5-PCy₃)) and the pyrimidine complex [(κ²-N,N-2-NH-C₄N ₂H₃)IrH₂(PPh₃)₂] (6-PPh₃) were prepared. The rate of CO₂ insertion is faster for the more nucleophilic amides. DFT studies suggest that the mechanism of insertion involves initial nucleophilic attack of the nitrogen lone pair of the amide on CO₂ to form an N-bound carbamato complex, followed by rearrangement to the O-bound species. CO₂ insertion into 1-PPh ₃ is reversible in the presence of H₂ and treatment of 10-PPh₃ with H₂ regenerates 1-PPh₃, along with Ir(PPh₃)₂H₅.

Full text of DOI:10.1021/ic300923c

Article
pubs.acs.org/IC
Mild, Reversible Reaction of Iridium(III) Amido Complexes with
Carbon Dioxide
Graham E. Dobereiner,† Jianguo Wu,† Michael G. Manas,† Nathan D. Schley,† Michael K. Takase,†
Robert H. Crabtree,*,† Nilay Hazari,*,† Feliu Maseras,‡,§ and Ainara Nova*,‡
†Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut, 06520, United States
‡Institute of Chemical Research of Catalonia (ICIQ), Ave Països Catalans, 16, 43007 Tarragona, Spain
§Departament de Química, Universitat Auton
̀
oma de Barcelona, 08193 Bellaterra, Spain
S
* Supporting Information
ABSTRACT: Unlike some other Ir(III) hydrides, the amino-
pyridine complex [(2-NH2−C5NH4)IrH3(PPh3)2] (1-PPh3)
does not insert CO2 into the Ir−H bond. Instead 1-PPh3 loses
H2 to form the cyclometalated species [(κ2-N,N-2-NH-
C5NH4)IrH2(PPh3)2] (2-PPh3), which subsequently reacts
with CO2 to form the carbamato species [(κ2-O,N-2-
OC(O)NH-C5NH4)IrH2(PPh3)2] (10-PPh3). To study the
insertion of CO2 into the Ir−N bond of the cyclometalated species, a family of compounds of the type [(κ2-N,N-2-NR-
C5NH4)IrH2(PR′3)2] (R = H, R′ = Ph (2-PPh3); R = H, R′ = Cy (2-PCy3); R = Me, R′ = Ph (4-PPh3); R = Ph, R′ = Ph (5-
PPh3); R = Ph, R′ = Cy (5-PCy3)) and the pyrimidine complex [(κ2-N,N-2-NH-C4N2H3)IrH2(PPh3)2] (6-PPh3) were prepared.
The rate of CO2 insertion is faster for the more nucleophilic amides. DFT studies suggest that the mechanism of insertion
involves initial nucleophilic attack of the nitrogen lone pair of the amide on CO2 to form an N-bound carbamato complex,
followed by rearrangement to the O-bound species. CO2 insertion into 1-PPh3 is reversible in the presence of H2 and treatment
of 10-PPh3 with H2 regenerates 1-PPh3, along with Ir(PPh3)2H5.
resulting compounds and few monosubstituted complexes are
known.9
INTRODUCTION
■
The catalytic conversion of CO2 to useful products is attractive
owing to its widespread availability, low cost, and nontoxic
nature.1 Desirable products include liquid fuels,2 cyclic
carbonates,3 and formic acid.4 CO2 is reactive toward strong
nucleophiles (e.g., RMgBr and RLi) but in general reactions
with weaker nucleophiles require harsh conditions and more
effective catalysts still need to be developed. The insertion of
CO2 into M−X bonds is a crucial step in many catalytic cycles
for CO2 conversion and has been studied in various contexts,
including CO2 hydrogenation4 and C−C bond formation.5
These examples of organometallic reactivity differ considerably
from those demonstrated in the organic literature.
One well-studied organic example is the reversible reaction of
CO2 with amines to form carbamic acids, where the resulting
acids must be “trapped” with base as the carbamate anion in
order to isolate any product.6 Indeed, amines find use in
current industrial applications for the “scrubbing” or removal of
CO2 from gaseous waste streams.7 Far less studied are the
analogous reactions with amido complexes to form products
containing carbamato ligands. Reactions with metal amido
species to form metal carbamato complexes are known,8
particularly for the early transition metals, but there are only
scattered reports for the platinum group metals, and few
mechanistic details have been determined for these reactions.
Furthermore, the majority of these examples feature N,N-
dialkylcarbamato ligands because of the relative solubility of the
A rare example involving a monosubstituted amido and a
platinum group metal was reported by Bergman and
Andersen,10 who found that a ruthenium amido complex
underwent facile reaction with CO2 to form a carbamato
complex (Scheme 1a). This reaction occurred without prior
ligand dissociation, and based on observations from low
temperature NMR spectroscopy, it was postulated that the
reaction involved direct electrophilic attack of CO2 by the
nitrogen atom to form an N-bound carbamato species, which
would rearrange to form the final O-bound product. The
feasibility of this mechanism was established when Roundhill
demonstrated that an isolated Pt N-bound carbamato complex,
generated from CO2 and a Pt amido species, readily rearranged
to the O-bound product in a polar solvent (Scheme 1b).11 Early
in 2012, we demonstrated that a Ni amido species supported by
a PCP pincer ligand (PCP = bis-2,6-ditert-butylphosphinome-
thylbenzene) also undergoes CO2 insertion via a mechanism
that involves nucleophilic attack of the amide on CO2 followed
by rearrangement from an N-bound carbamato complex to an
O-bound carbamato complex (Scheme 1c).12
In a recent computational study13 we compared the relative
hydricity of iridium(III) hydrides and used this information to
Received: May 6, 2012
Published: August 28, 2012
© 2012 American Chemical Society
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Scheme 1. (a) Reaction of a Ru Amido Complex with CO2,10 (b) Reaction of a Pt Amido Complex with CO2,11 (c) Reaction of a
12
Ni Amido Complex with CO2
build a model to predict the relative thermodynamic favorability
of insertion of CO2 into the Ir−H bond, a proposed
mechanistic step in the hydrogenation of CO2.14 We developed
a highly active CO2 hydrogenation catalyst with an N−H
hydrogen bond donor in the secondary coordination sphere
(Figure 1a); this N−H improved the thermodynamic
bonding between an Ir−H and the N−H proton of the 2-
aminopyridine ligand. This complex was found to lose an
equivalent of H2 thermally to generate an iridium amido
species. Here, we report that 1-PPh3 reacts with CO2 to form a
carbamato complex, rather than the expected formate complex.
The reaction proceeds via initial H2 loss, followed by insertion
of CO2 into the Ir−N bond of the cyclometalated species.
Moreover, in some cases the CO2 can be liberated upon
treatment of the carbamato complex with H2 to regenerate the
iridium starting material. We have investigated the CO2
insertion in detail from DFT calculations and a structure−
activity study using a family of related Ir complexes.
RESULTS AND DISCUSSION
■
Figure 1. (a) Our prior catalyst for CO2 hydrogenation.13 (b) A
Synthesis of Iridium Amido Complexes. Our previous
study of 1-PPh3 revealed that spontaneous loss of H2 under
ambient conditions generated the iridium amido complex, 2-
PPh3;15 however, this product was not isolated or fully
characterized at that time. We have now developed a viable
synthesis for 2-PPh3 (Scheme 2) and have fully characterized
this species. Complex 3-PPh3, formed by treatment of
related pendant amine compound.15.
favorability of CO2 insertion. As a continuation of this
investigation we were interested in the reactivity of CO2 with
other iridium(III) hydrides having H-bond donors in the
secondary coordination sphere.
16
We previously reported a mer iridium trihydride complex 1-
[IrH2(THF)2(PPh3)2]BF4 with 2-aminopyridine, can be
PPh3 (Figure 1b)15 in the context of the H···H dihydrogen
deprotonated by 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)
Scheme 2. Synthesis of 1-PPh3 or 2-PPh3 from a Single Cationic Intermediate 3-PPh3
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to generate 2-PPh3. If the deprotonation takes place under an
H2 atmosphere, 1-PPh3 is formed, along with Ir(PPh3)2H5.17 In
a typical experiment using 1 atm of H2 only around 16% of 1-
PPh3 is formed but the selectivity is highly dependent on the
exact temperature and pressure.
As part of this study the X-ray structure of 1-PPh3 was
elucidated (Figure 2). There is disorder in the position of the
terized examples of 2-aminopyridine ligands κ2-coordinated to
Ir, although there are many examples both for other transition
metals and the lanthanides.18 The two complexes have very
similar overall geometries. For example, 2-PPh3 features a 4-
membered chelating ring with an N(1)−Ir(1)−N(2) angle of
59.3(2)°, while the corresponding angle in 3-PPh3 is 60.8(3)°.
Unsurprisingly the Ir(1)−N(2) bond length in 2-PPh3,
2.190(5) Å, is significantly shorter than the Ir(1)−N(2) bond
length in 3-PPh3, 2.284(8) Å, consistent with the ligand
changing from neutral L type in 3-PPh3 to anionic X type in 2-
PPh3. However, comparison of the Ir−N bond lengths in 2-
PPh3 reveals that the Ir−Npyridine bond length (Ir(1)−N(1)
2.158(4) Å) is shorter than the Ir−Namide bond length (Ir(1)−
N(2) 2.190(5) Å). The bond length difference can be
rationalized in terms of a contribution from a resonance form
in which the negative charge is localized on the pyridyl nitrogen
(Figure 4a). This is confirmed by analyzing the bond lengths in
Figure 2. ORTEP of major component of 1-PPh3 at 30% probability.
Selected bond lengths (Å) and angles (deg): Ir(1)−P(1) 2.2693(8),
Ir(1)−P(2) 2.2638(8), Ir(1)−N(1) 2.208(9), N(1)−C(1) 1.45(4),
N(2)−C(1) 1.122(15), C(1)−C(2) 1.387(19), C(2)−C(3)
1.365(16), C(3)−C(4) 1.322(15), C(4)−C(5) 1.403(19); P(1)−
Ir(1)−P(2) 171.38(3), P(1)−Ir(1)−N(1) 92.7(11), P(2)−Ir(1)−
N(1) 95.8(11). The phenyl groups of PPh3 and the disorder in the 2-
aminopyridine ligand are hidden for clarity.
Figure 4. Two limiting resonance forms for deprotonated (a) 2-
aminopyridines and (b) 2-hydroxypyridines.
the pyridine ring. Two of the C−C bond lengths (C(2)−C(3)
1.322(8) Å and C(4)−C(5) 1.342(9) Å) are significantly
shorter than the other C−C bond lengths (C(1)−C(2)
1.392(7) Å and C(3)−C(4) 1.387(9) Å), in agreement with
some contribution coming from the iminopyridinato resonance
form (see Figure 4). Recently, Kempe and co-workers have
speculated that the iminopyridinato resonance form is favored
for late transition metals, especially for the heavier homologues
of the triads,18 and our results are consistent with this
hypothesis. In 3-PPh3, where the 2-aminopyridine ligand is
neutral, the C−C bond lengths in the pyridine ring are identical
(within error) and the Ir−Npyridine bond length (Ir−N1
2.184(8) Å) is significantly shorter than the Ir−Namine bond
length (Ir−N1 2.282(8) Å). The anion in 3-PPh3 appears to be
pyridyl ligand but it was possible to locate the position of the
N−H protons of the major component. One of the N−H
protons appears to be forming an H-bond with an Ir−H.
Although the hydrides on Ir were found in a difference Fourier
map, an unambiguous assignment is impossible due to disorder
in the ligands and residual electron density around the Ir center.
Nevertheless the crystal structure clearly shows that the
coordination sphere around Ir contains two trans PPh3 ligands,
three hydrides, and a κ1-bound aminopyridine.
The complexes 2-PPh3 (Figure 3a) and 3-PPh3 (Figure 3b)
were also characterized by X-ray crystallography. To the best of
our knowledge they are the first crystallographically charac-
Figure 3. (a) ORTEP of 2-PPh3 at 50% probability. Selected bond lengths (Å) and angles (deg): Ir(1)−P(1) 2.243(1), Ir(1)−P(2) 2.255(1), Ir(1)−
N(1) 2.158(4), Ir(1)−N(2) 2.190(5), N(1)−C(1) 1.342(7), N(2)−C(1) 1.303(6), C(1)−C(2) 1.392(7), C(2)−C(3) 1.322(8), C(3)−C(4)
1.387(9), C(4)−C(5) 1.342(9); P(1)−Ir(1)−P(2) 166.51(4), P(1)−Ir(1)−N(1) 92.8(1), P(1)−Ir(1)−N(2) 103.3(1), P(2)−Ir(1)−N(1) 98.0(1),
P(2)−Ir(1)−N(2) 89.2(1), N(1)−Ir(1)−N(2) 59.3(2), Ir(1)−N(1)−C(1) 95.9(3). The phenyl groups of PPh3 are hidden for clarity. (b) ORTEP
of 3-PPh3 at 50% probability. Selected bond lengths (Å) and angles (deg): Ir(1)−P(1) 2.295(2), Ir(1)−N(1) 2.184(8), Ir(1)−N(2) 2.282(8),
N(1)−C(1) 1.324(12), N(2)−C(1) 1.475(12), C(1)−C(2) 1.364(14), C(2)−C(3) 1.367(19), C(3)−C(4) 1.361(20), C(4)−C(5) 1.366(16);
P(1)−Ir(1)−N(1) 93.82(4), P(1)−Ir(1)−N(2) 96.14(4), N(1)−Ir(1)−N(2) 61.1(3), Ir(1)−N(1)−C(1) 99.5(6). The phenyl groups of PPh3 and
the cocrystallized solvent molecules are hidden for clarity.
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closely associated with the cationic iridium fragment; 3 exhibits
two short BF4···H−N interactions, with a N···F distance of 2.98
Å. A similar interaction has been observed between a BF4 anion
and an NH proton in an Ir(III) complex by Morris and co-
workers, although in that case only one H-bond was present.19
To assess which factors control reactivity with CO2, a variety
of complexes related to 1-PPh3 and 2-PPh3 were prepared
(Figure 5). The PCy3 analogue of 2-PPh3, 2-PCy3, was
Figure 6. ORTEP of dihydrido(2-hydroxypyridine-κ2-O,N)bis-
(triphenylphosphine)iridium(III) (7-PPh3) at 50% probability.
Selected bond lengths (Å) and angles (deg): Ir(1)−P(1) 2.281(1),
Ir(1)−P(2) 2.284(1), Ir(1)−O(1) 2.254(4), Ir(1)−N(1) 2.169(6),
O(1)−C(1) 1.283(7), N(1)−C(1) 1.337(7), C(1)−C(2) 1.404(10),
C(2)−C(3) 1.358(10), C(3)−C(4) 1.369(9), C(4)−C(5) 1.362(10);
P(1)−Ir(1)−P(2) 168.19(5), P(1)−Ir(1)−O(1) 93.5(1), P(1)−
Ir(1)−N(1) 99.1(1), P(2)−Ir(1)−O(1) 96.0(1), P(2)−Ir(1)−N(1)
91.8(1), O(1)−Ir(1)−N(1) 59.2(2), Ir(1)−O(1)−C(1) 92.5(3). The
phenyl groups of PPh3 are hidden for clarity.
Figure 5. Complexes prepared for screening with CO2.
Scheme 3. Proposed Pathway for the Generation of the
Carbamato Complex 10-PPh3
synthesized using a different route than the PPh3 compound,
because steric effects preclude the generation of [(PCy3)2(cod)-
21
Ir]+.20 Instead, the neutral polyhydride Ir(PCy3)2H5 was
treated with 2-aminopyridine at reflux in toluene. This route
was also used for the preparation of 5-PCy3, which contains a
2-N-phenylaminopyridine ligand. Similarly, 4-PPh3, 5-PPh3, 6-
PPh3, and the previously synthesized compound 7-PPh322 were
17
prepared through the treatment of Ir(PPh3)2H5 with 2-N-
methylaminopyridine, 2-N-phenylaminopyridine, 2-aminopyr-
imidine, and 2-hydroxypyridine respectively. The pyridine
supported trihydride, 8-PPh3, which cannot cyclometallate,
was prepared through the reaction of pyridine with Ir-
(PPh3)2H5, while the benzo[h]quinoline-2-amine complex 9-
PPh3 was prepared through literature methods.23 All of the new
complexes prepared as part of this work were fully
characterized.
7-PPh3. This is presumably because there is significantly less
strain associated with the 6-membered metallacyclic ring in 10-
PPh3, compared with the 4-membered metallacyclic ring in 7-
PPh3. The phosphine ligands are trans to one another, and the
1
hydride ligands are mutually cis, consistent with the H NMR
couplings observed for the hydride resonances (triplets of
doublets at δ −20.67, J = 16.8, 7.1 Hz, and δ −26.41 J = 17.1,
7.4 Hz). The crystal lattice of 10-PPh3 reveals an
intermolecular hydrogen bonding interaction that involves a
six membered ring composed of two N−H···O hydrogen bonds
with an N···O distance of 2.82 Å (Figure 7b).
The complex 7-PPh3 (Figure 6) was characterized by X-ray
crystallography. The structure has a geometry nearly identical
to that of 2-PPh3 (e.g., N1−Ir1−O1 bond angle of 7-PPh3 is
59.2(2)°, N1−Ir1−N2 angle of 2-PPh3 is 59.3(2)°). The
Ir(1)−N(1), 2.169(6), and Ir(1)−O(1), 2.254(4), bond
lengths are consistent with those observed in other Ir(III)
complexes supported by 2-hydroxypyridyl ligands24 and suggest
some contribution from both the pyridinolato and ketopyr-
idonato resonance form (see Figure 4b). This is again
confirmed by analyzing the C−C bond lengths in the pyridine
ring. There are two short C−C bonds (C(2)−C(3) 1.358(10)
Å and C(4)−C(5) 1.362(10) Å) and two long C−C bonds
(C(1)−C(2) 1.404(10) Å and C(3)−C(4) 1.369(9) Å),
although the difference is not as pronounced as in 2-PPh3.
CO2 Incorporation. The reaction of the trihydride 1-PPh3
with CO2 in DCM was slow at ambient temperature. Over 24
h, a precipitate formed that was insoluble in most common
solvents but sparingly soluble in DCM (Scheme 3). X-ray
crystallographic analysis showed that this is the neutral pyridyl
carbamato complex, 10-PPh3 (Figure 7a). The carbamato
ligand is O-bound with an Ir(1)−O(1) bond length of 2.165(3)
Å. The Ir(1)−N(1) bond length is 2.126(4) Å, which means
that both the Ir−N and Ir−O bonds are shorter than those in
A net loss of H2 must occur in order to form 10-PPh3 from
1-PPh3. Given that spontaneous loss of H2 from 1-PPh3 to 2-
PPh3 has previously been observed,15 we hypothesized that 2-
PPh3 is the true active intermediate which reacts with CO2
(Scheme 3). The reaction of a DCM solution of isolated 2-
PPh3 with CO2 resulted in the immediate formation of a
precipitate identified as 10-PPh3, and the reaction appeared to
be complete in less than 5 min at room temperature. This
indicates that 2-PPh3 is a plausible intermediate. It also suggests
that if 2-PPh3 is an intermediate, then the rate determining step
in the reaction of 1-PPh3 with CO2 is the loss of H2, which is
consistent with the observation that 2-PPh3 is not seen as an
intermediate by 1H or 31P NMR spectroscopy in the reaction of
1-PPh3 with CO2. Further support for an initial cyclometalation
with formation of an Ir amide came from the lack of reactivity
of CO2 with 9-PPh3. Due to geometric constraints 9-PPh3
cannot cyclometallate25 and even at elevated temperatures no
reaction was observed between 9-PPh3 and CO2. An alternative
pathway for the formation of 10-PPh3 from 1-PPh3 and CO2
involves initial insertion of CO2 into a hydride of 1-PPh3,
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Figure 7. (a) ORTEP of 10-PPh3 at 50% probability. Selected bond lengths (Å) and angles (deg): Ir(1)−P(1) 2.275(2), Ir(1)−P(2) 2.271(2),
Ir(1)−O(1) 2.165(3), Ir(1)−N(1) 2.126(4), O(1)−C(6) 1.254(5), O(2)−C(6) 1.231(7), N(2)−C(6) 1.391(7); P(1)−Ir(1)−P(2) 167.38(5),
P(1)−Ir(1)−O(1) 90.4(1), P(1)−Ir(1)−N(1) 95.1(1), P(2)−Ir(1)−O(1) 98.6(1), P(2)−Ir(1)−N(1) 94.2(1), O(1)−Ir(1)−N(1) 87.1(2), O(1)−
C(6)−O(2) 124.0(5), O(1)−C(6)−N(2) 119.2(5), O(2)−C(6)−N(2) 116.8(5). Selected hydrogen atoms and the phenyl groups of PPh3 are
hidden for clarity. (b) Solid state hydrogen bonding interaction in 10-PPh3 between two units in the crystal lattice. The N(2)−O(2) bond length is
2.815 Å. Selected hydrogen atoms and the phenyl groups of PPh3 have been removed for clarity.
followed by H2 loss. Given our previous studies on the difficulty
of CO2 insertion into Ir(III) hydrides13 and the fact that the
control compound 8-PPh3 (which cannot form a carbamato
complex) does not react with CO2, this route seems unlikely.
To further understand the factors controlling the reactivity of
the amido species with CO2, all of the cyclometalated
complexes prepared as part of this work were exposed to
CO2 in DCM. The results are summarized in Table 1. Several
we suggest that this reaction is slower as a result of electronic
effects.
Other complexes tried were completely unreactive under
ambient conditions. The N-phenyl analogues 5-PPh3 and 5-
PCy3 are totally unreactive toward CO2, as is the charged
complex 3-PPh3, which contains a cyclometalated amine ligand
rather than a cyclometalated amide ligand. The 2-hydroxypyr-
idyl supported species 7-PPh3 also does not react with CO2,
despite its structural similarities with 2-PPh3.
The complexes 10-PCy3 and 11-PPh3 were characterized by
X-ray crystallography (Figure 8). The structure of 10-PCy3 is
Table 1. Reactivity of Cyclometalated Ir Amides with 1 atm
CO2 in DCM
compound
reactivity with CO2
2-PPh3
2-PCy3
3-PPh3
4-PPh3
instant reaction at room temperature (product 10-PPh3)
instant reaction at room temperature (product 10-PCy3)
no reaction
Figure 8. (a) ORTEP of 10-PCy3 at 50% probability. Selected bond
lengths (Å) and angles (deg): Ir(1)−P(1) 2.3123(16), Ir(1)−P(2)
2.3259(16), Ir(1)−O(1) 2.198(3), Ir(1)−N(2) 2.189(5), O(1)−C(1)
1.253(7), O(2)−C(1) 1.251(6), N(2)−C(1) 1.401(8); P(1)−Ir(1)−
P(2) 164.83(6), P(1)−Ir(1)−O(1) 88.25(11), P(1)−Ir(1)−N(1)
97.08(14), P(2)−Ir(1)−O(1) 98.42(11), P(2)−Ir(1)−N(1)
97.15(14), O(1)−Ir(1)−N(1) 84.24(15), O(1)−C(1)−O(2)
124.7(6), O(1)−C(1)−N(2) 120.8(4), O(2)−C(1)−N(2) 114.5(5).
Selected hydrogen atoms and the cyclohexyl groups of PCy3 are
hidden for clarity. (b) ORTEP of 11-PPh3 at 50% probability. Ir(1)−
P(1) 2.271(2), Ir(1)−P(2) 2.265(2), Ir(1)−O(1) 2.168(5), Ir(1)−
N(1) 2.123(4), O(1)−C(6) 1.259(10), O(2)−C(6) 1.222(10),
N(2)−C(6) 1.396(8); P(1)−Ir(1)−P(2) 162.37(5), P(1)−Ir(1)−
O(1) 98.56(16), P(1)−Ir(1)−N(1) 101.07(18), P(2)−Ir(1)−O(1)
88.05(16), P(2)−Ir(1)−N(1) 96.09(19), O(1)−Ir(1)−N(1)
80.75(17), O(1)−C(6)−O(2) 124.2(6), O(1)−C(6)−N(2)
119.3(7), O(2)−C(6)−N(2) 116.5(7). Selected hydrogen atoms
and the phenyl groups of PPh3 are hidden for clarity.
instant reaction at room temperature to give a 9:1 mixture of
product and starting material (product 11-PPh3)
5-PPh3
5-PCy3
6-PPh3
7-PPh3
no reaction
no reaction
slow reaction at room temperature (product 12-PPh3)
no reaction
of our amido complexes react with CO2 to form carbamato
complexes, analogous to 2-PPh3. The PCy3 complex, 2-PCy3,
and a N−Me species, 4-PPh3, undergo facile reaction with CO2
under ambient conditions to form 10-PCy3 and 11-PPh3,
respectively. The reactivity of 4-PPh3 demonstrates that CO2
incorporation is not dependent on the presence of an N−H
proton. However, in this case complete conversion of 4-PPh3
to 11-PPh3 was not observed and under 1 atm of CO2 at room
temperature a 9:1 mixture of product to starting material was
present. Furthermore, the product 11-PPh3 could not be
isolated on a reasonable scale, as exposure to vacuum resulted
in the loss of CO2 and the regeneration of 4-PPh3. It was
possible to grow crystals of 11-PPh3 for X-ray diffraction under
an atmosphere of CO2 (vide infra) but exposure of these
crystals to vacuum resulted in loss of CO2. In contrast to these
rapid reactions, the reaction between the pyrimidine complex
6-PPh3 and CO2 took several hours at room temperature, and
analogous to 10-PPh3, although presumably due to steric
factors, the Ir−P bond lengths are longer in 10-PCy3 (Ir(1)−
P(1) = 2.3123(16) and Ir(1)−P(2) = 2.3259(16)) compared
with 10-PPh3 (Ir(1)−P(1) = 2.275(2) and Ir(1)−P(2) =
2.271(2)). In a comparable fashion to 10-PPh3 two intra-
molecular N−H···O hydrogen bonding interactions are present
in 10-PCy3 to form a dimer (as shown for 10-PPh3 in Figure
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7b). The N···O bond distance is 2.89 Å. In contrast 11-PPh3
does not contain any N−H bonds, and no solid state
intramolecular hydrogen bonding is apparent. This may explain
why CO2 insertion into 4-PPh3 does not appear to be as
thermodynamically favorable as insertion into 2-PPh3 or 2-
PCy3. In other regards the structure of 11-PPh3 is similar to 10-
PPh3.
with our experimental results). Two different pathways were
obtained for the insertion: one concerted and the other
stepwise (Figure 10). In the concerted mechanism the Ir−O
Computational Studies of the Mechanism of CO2
Insertion into 1-PPh3 and Cyclometalated Ir Species.
To understand the pathway for CO2 insertion into 1-PPh3, a
computational study was performed. A model system where
PPh3 was replaced by PH3 was utilized. The validity of this
model was confirmed by performing selected calculations with
PPh3 and these are reported in the Supporting Information.
The DCM solvent used in the experiments was included in the
calculations using a continuum model. Two possibilities were
considered: direct reaction of CO2 with 1-PH3, and reaction of
CO2 with the intermediate 2-PH3, which is obtained by H2
elimination of 1-PH3.26
CO2 insertion has previously been observed with the
complex (PNP)IrH3 (PNP = HN(iPr2PC2H4)2) shown in
Figure 1a and the insertion of CO2 into an Ir−H bond of 1-
PH3 was calculated (see Figure 9).13 The Gibbs free energy
Figure 10. ΔGDCM energy profiles, in kcal mol−1, for the CO2 insertion
into the Ir-NH bond of 2-PH3. PH3 ligands have been omitted for
clarity.
and N−C bonds are formed simultaneously in TS2−10-PH3
(Ir···O=2.87 and N···C=2.50 Å), while in the stepwise pathway
this happens in two steps. The N−C bond is formed first in
TS2−16-PH3 (N···C=1.95 Å) yielding the N-bound carbamato
species 16-PH3. Subsequently, this species undergoes a
rearrangement from the N-bound to the O-bound complex
via TS16−10-PH3 (Ir···N = 2.85 Å, Ir···O = 3.10 Å) to give the
final product 10-PH3.
Comparing the energy barriers for the concerted and
stepwise processes, the latter is preferred by 9.1 kcal mol−1.
Hence, the stepwise mechanism seems to be the one
responsible for the insertion of CO2 in 2-PH3. The low
Gibbs free energy obtained for the transition states of this
process, of 16.9 and 17.3 kcal mol−1, is consistent with a fast
reaction of CO2 with 2-PH3. This picture also explains why
intermediate 2-PPh3 is not observed in the reaction of 1-PPh3
with CO2, as suggested in Scheme 3. The almost thermoneutral
process, with 10-PH3 being only 0.2 kcal mol−1 below the
reactants (2-PH3 + CO2), is in agreement with a reversible
insertion of CO2 in the absence of hydrogen bonding.
Calculations on this mechanism (see Supporting Information)
using PPh3 instead of PH3 as spectator ligand lead to similar
relative energies.
Calculations of the reaction of CO2 with 4-, 5-, and 6-PH3
were also performed in order to check the experimental
observations shown in Table 2. The trend observed in the
energy barriers of these systems (ΔG⧧DCM = 21.0, 31.1, and 22.3
kcal mol−1, respectively) matches quite well with the
experimental results. For instance, the lack of CO2 insertion
with 5-PH3, using the Py-NPh ligand, is in agreement with a
relatively high energy barrier of 31.1 kcal mol−1. In this case the
transition state of the stepwise process could not be found, and
only a transition state analogous to TS2−10-PH3 was obtained.
This is probably due to the low nucleophilicity of NPh
compared to NH and NMe present in the other compounds.
Figure 9. ΔGDCM energy profiles, in kcal mol−1, for CO2 insertion into
Ir−H bond of 1-PH3. PH3 ligands have been omitted for clarity.
barrier for CO2 insertion into one of the trans hydrides of 1-
PH3 is 15.7 kcal mol−1, which is similar to the energy barrier
observed with (PNP)IrH3 (12.4 kcal mol−1). However the
reaction with 1-PH3 is slightly endothermic (ΔGDCM = 3.8 kcal
mol−1), while it is exothermic (ΔGTHF = −4.8 kcal mol−1) using
(PNP)IrH3. This is probably a consequence of the lower acidity
of the NH2 group compared with the metal bound NH group
−
of (PNP)IrH3, which stabilized the HCO2 anion far more
effectively through hydrogen bonding. This hypothesis is
−
supported by the higher energy of 14-PH3 + HCO2
(ΔGDCM = 14.4 kcal mol−1) compared with the analogous
complex with the PNP ligand (ΔGTHF = 6.3 kcal mol−1). After
formation of 15-PH3, elimination of H2 should occur in order
to form the final product 10-PH3. However all the pathways
tried were unsuccessful. From 1-PH3, a pathway where
nucleophilic attack of NH2 to CO2 is concurrent to the H2
formation was also calculated but it has high energy barriers
(see Supporting Information).
The insertion of CO2 into the NH group of 2-PH3 appears
to be a more likely process according to calculation (consistent
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Table 2. Crystal and Refinement Data for Complexes 1-PPh3, 2-PPh3, 3-PPh3, 7-PPh3, 10-PPh3, 10-PCy3, and 11-PPh3
1-PPh3
2-PPh3
3-PPh3
7-PPh3
empirical formula
formula weight
temperature (K)
a (Å)
C41H39IrN2P2
813.88
C41H37IrP2N2
811.92
IrP2N2BF4C41H38·1.21 (CH2Cl2)
1002.50
223
C41H36IrP2ON
812.91
223
223
223
11.5834(16)
12.2221(16)
13.2838(18)
90.472(6)
106.509(7)
102.999(7)
1751.6(4)
2
10.760(7)
12.160(7)
13.764(9)
101.791(11)
94.401(10)
111.133(10)
1621.9(17)
2
19.2265(14)
15.8767(12)
15.0485(11)
90
9.111(4)
11.977(6)
16.669(7)
70.480(11)
81.580(10)
73.712(13)
1642.7(13)
2
b (Å)
c (Å)
α (deg)
β (deg)
90
γ (deg)
volume (Å3)
90
4593.6(6)
4
Z
crystal system
space group
dcalc (Mg/m3)
θ range (deg)
triclinic
triclinic
orthorhombic
Pnma (#62)
1.449
triclinic
P1 (#2)
1.543
P1 (#2)
1.866
P1 (#2)
1.643
3.21−30.51
3.934
0.997−27.49
4.260
0.996−27.10
3.171
0.990−27.48
4.208
μ (mm−1
)
abs. correction
GOF
semiempirical
1.092
semiempirical
1.095
semiempirical
1.184
semiempirical
1.106
a
b
R1, wR2 [I > 2σ(I)]
0.0415, 0.0624
0.0411, 0.0657
0.0589, 0.1174
10-PCy3
0.0477, 0.0759
10-PPh3
11-PPh3
empirical formula
formula weight
temperature (K)
a (Å)
IrP2O2N2C42H37·2(CH2Cl2)
1025.80
IrC43H73O2N2P2·(CH2Cl2)
977.15
IrP2O2N2C43H39·2.5(CH2Cl2)
1082.29
223
223
223
11.729(4)
12.900(5)
15.653(6)
96.745(9)
101.430(8)
109.434(8)
2145.7(14)
2
10.9796(7)
12.2163(7)
18.3145(13)
96.882(7)
105.104(7)
102.724(7)
2272.1(3)
2
13.269(9)
13.481(8)
14.591(11)
66.345(20)
71.92(2)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
volume (Å3)
78.945(19)
2145.7(14)
2
Z
crystal system
space group
dcalc (Mg/m3)
θ range (deg)
triclinic
triclinic
triclinic
P1 (#2)
1.588
P1 (#2)
1.586
P1 (#2)
1.586
3.25−27.98
3.482
3.08−27.16
3.170
3.15−27.79
3.359
μ (mm−1
)
abs. correction
GOF
semiempirical
1.089
semiempirical
1.040
semiempirical
1.055
a
b
R1, wR2 [I > 2σ(I)]
0.0552, 0.0812
0.0541, 0.0922
0.0559, 0.1150
a
b
2
2
R1 = Σ∥Fo| −|Fc∥/Σ|Fo|. wR2 = [Σ[w(Fo − Fc2)2]/Σ[w(Fo )2]1/2
.
Calculations on 7-PH3, a model for the hydroxypyridyl complex
7-PPh3, reveals that insertion into the Ir−O bond is
thermodynamically uphill (ΔG = 17.9 kcal mol−1), which
explains the observed lack of reactivity.
Reversibility of CO2 Insertion into 2-PPh3. Treatment of
a DCM solution of the carbamato complex 10-PPh3 with 13C-
labeled CO2 produces a significant quantity of the 13C-labeled
product over 24 h at room temperature as determined by 13C
NMR spectroscopy (Scheme 4). This indicates that the CO2
insertion is likely a reversible process, and depending on
conditions, CO2 could be liberated.
Overall, the stepwise mechanism for the CO2 insertion in 2-
PH3 resembles the insertion mechanism observed with the Ru
complex shown in Scheme 1a.10 This process is also similar to
the mechanism proposed by our group for the insertion of CO2
into the Ni−N bound of complex [(PCP)Ni-NH2].12 However,
it should be noted that Chisholm and Extine observed that CO2
insertion into the W amide bond of W(NMe2)6 and
W2(NEt2)4Me2 was catalyzed by fortuitous amine.27 Elemental
analysis suggests that our complexes are pure and do not
contain free amine, and the reactions are performed in the
absence of water. In addition, the fact that 3-PPh3, which
contains a cyclometalated amine ligand, does not react with
CO2 strongly suggests that a reaction involving a free amine is
not occurring.
Scheme 4. Isotopic Exchange of the Carbon Atom in the
Carbamato Ligand in 10-PPh3 with 13CO2
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Though complex 10-PPh3 does not lose CO2 readily at room
temperature under vacuum in the solid state (presumably due
to the hydrogen bonding), we found that it does react with H2
even at 1 atm. The reaction of the carbamato complex 10-PPh3
with H2 at 1 atm and room temperature results in the
Ir(PPh3)2H5 species within a few hours (Scheme 5). The single
nucleophilic amides giving faster rates. The reaction with CO2
is reversible and addition of H2 results in the release of CO2
and the formation of the iridium trihydride. Further work will
be needed to find an entry into a potential catalytic cycle that
incorporates CO2 into organic reactants via carboxylation.
EXPERIMENTAL SECTION
■
Scheme 5. Loss of CO2 from 10-PPh3 under an H2
Atmosphere to Form Ir(PPh3)2H5
General. The syntheses of the metal complexes were conducted
using standard Schlenk or drybox techniques under a dinitrogen
atmosphere using dry, degassed solvents unless otherwise indicated.
The solvents for air- and moisture-sensitive reactions were dried by
passage through a column of activated alumina followed by storage
under dinitrogen. Dihydridobis(tetrahydrofuran) bis-
(triphenylphosphine)iridium(III) tetrafluoroborate,16 trihydrido(2-
aminopyridine) bis(triphenylphosphine)iridium(III),15
pentahydridobis(triphenylphosphine)iridium(III),17 and
pentahydridobis(tricyclohexylphosphine)iridium(III)17 were synthe-
sized according to literature procedures. 2-Aminopyridine was
sublimed under high vacuum at room temperature prior to use. All
other reagents were purchased commercially from Sigma-Aldrich,
Cambridge Isotope Laboratories, Airgas, Alfa-Aesar, or Strem
Chemicals and used as received unless otherwise indicated. NMR
spectra were recorded at room temperature on a 400 or 500 MHz
Bruker or Varian spectrometer. Chemical shifts are reported in ppm
with respect to residual internal protio solvent for H and 13C{1H}
1
NMR spectra and to an external standard for 31P{1H} spectra (85%
H3PO4 in H2O at δ 0.0 ppm). Coupling constants are reported in Hz.
IR spectra were measured using diamond smart orbit ATR on a
Nicolet 6700 FT-IR instrument unless noted. Elemental analyses were
performed by Robertson Microlit Laboratories (Madison, NJ). The 1H
and 31P NMR spectra are provided in the Supporting Information for
any compounds that did not have satisfactory elemental analyses.
Synthesis and Characterization of Compounds. Dihydrido(2-
amidopyridine-κ2-N,N′)bis(triphenylphosphine)iridium(III) (2-PPh3).
To a flame-dried Schlenk flask was added dihydrido(2-aminopyridine-
κ2-N,N′)bis(triphenylphosphine)iridium(III) tetrafluoroborate (3-
PPh3, 200.0 mg, 0.222 mmol, 1.0 equiv) (for preparation of 3-PPh3
see below). The flask was evacuated and backfilled with dinitrogen
three times, and 20 mL of dried degassed DCM was added. The
reaction was cooled to −30 °C, and a 1 M solution of 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) in tetrahydrofuran (222 μL,
0.222 mmol, 1.0 equiv) was added dropwise. The reaction was stirred
for 30 min, during which time it was allowed to warm to −10 °C and
then the solvent was removed under reduced pressure. Ten mL of
dried, degassed toluene was added and the resulting suspension was
filtered through Celite in vacuo. The filtered solution was evaporated
under reduced pressure leaving a light brown residue. This material
was recrystallized by layering a concentrated DCM solution with
pentane under dinitrogen atmosphere. The solution was decanted off
and the solid was dried overnight under vacuum. Crystals suitable for
X-ray diffraction were obtained by recrystallization (DCM/pentane,
hydridic resonance (δ −11.28, triplet, J = 12.0 Hz) present in
1
the H NMR spectrum of the reaction mixture indicates that
this is the only iridium hydride-containing product.
Reaction of complex 1-PPh3 or 2-PPh3 with H2 at room
temperature also results in the formation of Ir(PPh3)2H5.
Before all of 2-PPh3 is consumed, hydride resonances
1
corresponding to 1-PPh3 are also visible by H and 31P NMR
spectroscopy, suggesting that 1-PPh3 is an intermediate en
route to Ir(PPh3)2H5. Peaks corresponding to uncoordinated 2-
aminopyridine are also present, confirming the dissociation of
the ligand from iridium. Given the reversibility of the CO2
insertion reaction seen by isotopic labeling, we suggest that 10-
PPh3 loses CO2 to form the Ir amide 2-PPh3, followed by
reaction with two equivalents of H2 to form Ir(PPh3)2H5
(Scheme 5). The hydrogenolysis of 2-PPh3 is likely only
possible if the CO2 incorporation by 10-PPh3 is reversible. H2
has no obvious role in the release of CO2, and instead we
propose that H2 serves to trap 2-PPh3 and drive the reaction to
completion by the formation of Ir(PPh3)2H5, a thermodynamic
sink.
1
CONCLUSIONS
layering, under dinitrogen at RT). Yield 51 mg, 28%. H NMR (500
■
MHz, CD2Cl2): δ 7.61−7.55 (m, 12H, PPh3), 7.35−7.23 (m, 18H,
PPh3), 6.65 (d, J = 5.3 Hz, 1H, pyridine C−H), 6.52 (t, J = 7.6 Hz, 1H,
pyridine C−H), 5.36 (t, J = 6.1 Hz, pyridine C−H), 5.07 (d, J = 8.6
Hz, 1H, pyridine C−H), 3.27 (br s, 1H, −NH), −22.45 to −22.70 (m,
2H, Ir−H). 31P{1H} NMR (202 MHz, CD2Cl2): δ 19.42. 13C{1H}
NMR (126 MHz, CD2Cl2): δ 174.63 (s), 147.32 (s), 136.12 (t, J =
25.4 Hz), 134.77 (t, J = 6.2 Hz), 133.42 (s), 129.82 (s), 128.11 (t, J =
4.8 Hz), 110.23 (s), 103.59 (s). Anal. Calcd for C41H37IrN2P2: C,
60.65; H, 4.59; N, 3.45. Found: C, 60.41; H, 4.20; N, 3.37.
CO2 reacts with an Ir(III) trihydride supported by an 2-
aminopyridine ligand, but unlike previous hydrides we have
studied,13 CO2 does not insert into an Ir−H bond. Instead, the
complex undergoes cyclometalation, releasing H2 and generat-
ing a species with an Ir amide bond. This Ir−N bond rapidly
inserts CO2 to form an O-bound carbamato complex. The
reaction of late transition metal amide bonds with CO2 has not
been extensively studied10−12 and the factors that promote
insertion are still unclear. We have performed an experimental
structure−activity study and used calculations to propose a
reaction pathway. The reaction proceeds via initial nucleophilic
attack of the coordinated amide on CO2 to form an N-bound
carbamato species. Ligand rearrangement then occurs to
generate the O-bound carbamato complex. The rate of reaction
depends on the nucleophilicity of the amide, with more
Dihydrido(2-amidopyridine-κ2-N,N′)bis(tricyclohexylphosphine)-
iridium(III) (2-PCy3). To a suspension of IrH5(PCy3)2 (47 mg, 0.06
mmol) in 10 mL of dry C6H6 in a 50-mL flame-dried Schlenk flask, 2-
aminopyridine (34 mg, 0.36 mmol) was added. The mixture was
heated at 75 °C for 37 h. The volatiles were removed under vacuum.
The resulting residue was triturated with C6H6 (2 × 0.5 mL) to give 2-
PCy3 as a white solid, which was dried under vacuum. Yield: 32 mg,
62%. 1H NMR (400 MHz, CD2Cl2): δ 7.50 (d, J = 8 Hz, 1H, pyridine
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C−H), 6.73 (t, J = 8 Hz, 1H, pyridine C−H), 5.68 (t, J = 8 Hz,
pyridine C−H), 5.39 (d, J = 8 Hz, 1H, pyridine C−H), 3.43 (br s, 1H,
−NH), 1.80 (br s, 18H) and 1.63 (br s, 12H) and 1.53 (br s, 6H) and
1.28 (m, 12H) and 1.08 (m, 18H) (Cy), −25.37 to −25.66 (m, 2H,
Ir−H). 31P{1H} NMR (202 MHz, CD2Cl2) δ 22.31. 13C{1H} NMR
(126 MHz, CD2Cl2): δ 173.09 (s), 146.69 (s), 133.40 (s), 133.42 (s),
109.55 (s), 100.60 (s), 36.90 (t, J = 13 Hz), 30.59 (br s), 30.51 (br s),
28.47 (br s), 27.48 (s). Anal. Calcd for C41H73IrN2P2·CH2Cl2: C,
54.06; H, 8.10; N, 3.00. Found: C, 55.10; H, 8.02; N, 3.17.
IrH5(PPh3)2 (70 mg, 0.10 mmol) in 10 mL of dry C6H6 in a 50-mL
flame-dried Schlenk flask, 2-phenylaminopyridine (34 mg, 0.20 mmol)
was added. The mixture was heated at 65 °C for 25 h. The volatiles
were removed under vacuum. The resulting residue was triturated with
ether and washed with benzene and dried under vacuum to give 5-
PPh3 as a white solid. Yield: 59 mg, 66%. 1H NMR (500 MHz, C6D6):
δ 7.79−7.75 (m, 12H, Ar), 7.01−6.86 (m, 23H, Ar), 6.69 (t, J = 8 Hz,
1H, pyridine C−H), 6.36 (d, J = 10 Hz, 1H, pyridine C−H), 6.19 (d, J
= 10 Hz, pyridine C−H), 5.51 (t, J = 6 Hz, 1H, pyridine C−H),
−21.61 (td, JPH = 20 Hz, JHH = 10 Hz, 1H, Ir−H), −22.84 (td, JPH = 20
Hz, JHH = 10 Hz, 1H, Ir−H). 31P{1H} NMR (202 MHz, C6D6): δ
20.67 (t, J = 13 Hz). 13C{1H} NMR (126 MHz, C6D6) 167.38 (s),
147.81 (s), 147.68 (s), 136.63 (t, J = 25 Hz), 134.63 (t, J = 6 Hz),
129.50 (s), 128.55 (s), 122.47 (s), 121.78 (s), 118.65 (s), 108.66 (s),
106.51 (s). Anal. Calcd for C47H41IrN2P2: C, 63.57; H, 4.65; N, 3.15.
Found: C, 63.32; H, 4.69; N, 3.10.
Dihydrido(2-aminopyridine-κ2-N,N′)bis(triphenylphosphine)-
iridium(III) tetrafluoroborate (3-PPh3). To a 100-mL flame-dried
Schlenk flask was added dihydridobis(tetrahydrofuran) bis-
(triphenylphosphine)iridium( III) tetrafluoroborate
([IrH2(THF)2(PPh3)2]BF4, 914.0 mg, 0.962 mmol). The flask was
evacuated and backfilled with dinitrogen three times, and 5 mL of
dried degassed DCM was added. A dry degassed solution of 2-
aminopyridine (90.4 mg, 0.962 mmol) in DCM (20 mL) was added
dropwise over 5 min via cannula transfer. The reaction was allowed to
stir for 1.5 h. The yellow solution was concentrated under reduced
pressure, resulting in partial precipitation of the product. Fifteen mL of
dried degassed diethyl ether was added, and the solution was decanted
off the precipitated solid. The solid was washed with diethyl ether (2 ×
5 mL) and dried overnight under vacuum. Crystals suitable for X-ray
diffraction were grown from DCM/diethyl ether (vapor diffusion,
D i h y d r i d o ( 2 - p h e n y l a m i d o p y r i d i n e - κ 2 - N , N ′ ) b i s -
(tricyclohexylphosphine)iridium(III) (5-PCy3). To a suspension of
IrH5(PCy3)2 (163 mg, 0.21 mmol) in 65 mL of dry CH2Cl2 in a 100-
mL flame-dried Schlenk flask, 2-phenylaminopyridine (37 mg, 0.21
mmol) was added. The mixture was stirred at RT for 22 h. The
volatiles were removed under vacuum. The residue was dissolved in 2
mL of C6H6 and a precipitate formed after 10 min. After 1 h the
precipitate was collected by filtration, washed with cold CH2Cl2, and
dried under vacuum. Yield: 89 mg, 46%. 1H NMR (500 MHz,
CD2Cl2): δ 7.84 (d, J = 5 Hz, 1H, pyridine C−H), 7.13−7.07 (m, 5H,
Ar), 6.64 (t, J = 7 Hz, 1H, pyridine C−H), 6.59 (d, J = 9 Hz, 1H,
pyridine C−H), 6.11 (t, J = 7 Hz, 1H, pyridine C−H), 1.84−1.01(m,
66H, Cy), −24.89 (td, JPH = 17 Hz, JHH = 8 Hz, 1H, Ir−H), −25.32(td,
JPH = 17 Hz, JHH = 8 Hz, 1H, Ir−H). 31P{1H} NMR (202 MHz,
CD2Cl2): δ 20.22 (s). 13C{1H} NMR (100 MHz, CD2Cl2): 128.70 (s),
121.90 (s), 100.88 (s), 36.94 (t, J = 13 Hz), 30.47 (s), 30.30 (s), 28.30
(t, J = 6 Hz), 27.39. (Not all carbon peaks were observed because of
the low solubility). Anal. Calcd for C47H77IrN2P2: C, 61.07; H, 8.40;
N, 3.03. Found: C, 59.94; H, 7.91; N, 2.71.
1
RT). Yield 659.9 mg, 76%. H NMR (500 MHz, CD2Cl2): δ 7.59−
7.45 (m, 12H, Ar), 7.43−6.98 (m, 20H, Ar), 6.88−6.82 (m, 1H,
pyridine C−H), 6.61 (d, J = 8.0 Hz, 1H, pyridine C−H), 3.81 (s, 1H,
−NH2), −23.49 (td, JPH = 15.6 Hz, JHH = 8.7 Hz, 1H, Ir−H), −25.32
(td, JPH = 15.8 Hz, JHH = 8.7 Hz, 1H, Ir−H). 31P{1H} NMR (202
MHz, CD2Cl2): δ 22.48. 13C{1H} NMR (126 MHz, CD2Cl2): δ
158.03 (s), 148.10 (s), 137.86 (s), 133.37 (t, J = 6.3 Hz), 132.64 (t, J =
26.7 Hz), 130.54 (s), 128.64 (t, J = 5.0 Hz), 124.91 (s), 123.50 (s).
Anal. Calcd for C41H38BF4IrN2P2·CH2Cl2: C, 51.23; H, 4.09; N, 2.84.
Found: C, 51.65; H, 3.91; N, 2.62.
Trihydrido(2-aminopyridine)bis(triphenylphosphine)iridium(III).
An Alternative Preparation from 3. To a flame-dried Schlenk flask
was add ed dihydrido(2-aminopyridine- κ2 -N, N′)bis
(triphenylphosphine)iridium(III) tetrafluoroborate (93.7 mg, 0.104
mmol, 1 equiv). After purge and backfill with dinitrogen, a 1 M
tetrahydrofuran solution of DBU (115 μL, 0.115 mmol, 1.1 equiv),
and 3 mL of DCM were added. The flask was cooled to 0 °C. H2 was
introduced by bubbling and the reaction was stirred for 5 min. The
DCM was removed under reduced pressure and 2 mL of toluene was
added. The suspension was filtered through Celite under dinitrogen.
Degassed pentanes were layered on top of the filtrate and the reaction
was stored at −20 °C overnight. The solid was dried in vacuo at 0 °C
for 15 min. Yield 13.4 mg, 16%. The spectra of the obtained yellow-
white solid were consistent with the previously reported data.15
Ir(PPh3)2H5 is a major byproduct of this reaction and is removed in
the filtration step.
Dihydrido(2-amidopyrimidine-κ2-N,N′)bis(triphenylphosphine)-
iridium(III) (6-PPh3). To a suspension of IrH5(PPh3)2 (117 mg, 0.16
mmol) in 10 mL of dry C6H6 in a 50-mL flame-dried Schlenk flask, 2-
aminopyrimidine (19 mg, 0.20 mmol) was added. The mixture was
heated at 70 °C for 20 h. The volatiles were removed under vacuum
for 4 h. The resulting residue was redissolved in ∼1 mL of benzene
and 1 mL of pentane was added. The resulting precipitate was
decanted off the solution and washed with pentane and dried under
1
vacuum to give 6-PPh3 as an off-white solid. Yield: 92 mg, 70%. H
NMR (500 MHz, CD2Cl2): δ 7.62−7.58 (m, 12H, PPh3), 7.36−7.28
(m, 18H, PPh3), 7.42 (m, 1H, pyrimidine C−H), 7.06 (m, 1H,
pyrimidine C−H), 6.72 (m, pyrimidine C−H), 3.89 (s, 1H, −NH),
−22.21(td, JPH = 20 Hz, JHH = 10 Hz, 1H, Ir−H), −22.84 (m, 1H, Ir−
H). 31P{1H} NMR (202 MHz, CD2Cl2): δ 19.31(t, J = 14 Hz).
13C{1H} NMR (126 MHz, CD2Cl2): δ 171.17 (s), 155.24 (s), 155.08
(s), 135.11 (t, J = 25 Hz), 134.03 (t, J = 6 Hz), 129.40 (s), 128.25 (s),
127.64 (t, J = 5 Hz), 102.09 (s). Anal. Calcd for C40H36IrN3P2: C,
59.10; H, 4.46; N, 5.17. Found: C, 60.12; H, 4.54; N, 4.85.
Trihydrido(pyridine)bis(triphenylphosphine)iridium(III) (8-PPh3).
To a suspension of IrH5(PPh3)2 (70 mg, 0.10 mmol) in 10 mL of
dry C6H6 in a 50-mL flame-dried Schlenk flask, pyridine (25 μL, 0.20
mmol) was added. The mixture was heated at 45 °C for 19 h. The
volatiles were removed under vacuum for 4 h. The resulting residue
was dissolved in ∼1 mL of benzene and 1 mL of pentane was added.
The resulting precipitate was decanted off the solution and washed
with pentane and dried under vacuum to give 8-PPh3 as pale yellow
solid. Yield: 51 mg, 63%. This compound has previously been prepared
using an alternative method.28 The IR spectrum of our sample
matched that previously reported. The NMR data for 8-PPh3 is
reported here for future reference. 1H NMR (500 MHz, C6D6): δ 8.36
(d, J = 6 Hz, 2H, pyridine C−H), 8.12−8.09 (m, 12H, PPh3), 7.02−
6.94 (m, 18H, PPh3), 6.32 (m, 1H, pyridine C−H), 5.59 (dd, J = 7, 6
Hz, 2H, pyridine C−H), −8.73 (td, JPH = 20 Hz, JHH = 5 Hz, 2H, Ir−
H), −21.98 (td, JPH = 20 Hz, JHH = 5 Hz, 1H, Ir−H). 31P{1H} NMR
(202 MHz, C6D6): δ 33.50 (d, J = 16 Hz). 13C{1H} NMR (126 MHz,
D i h y d r i d o ( 2 - m e t h y l a m i d o p y r i d i n e - κ 2 - N , N ′ ) b i s -
(triphenylphosphine)iridium(III) (4-PPh3). To a suspension of
IrH5(PPh3)2 (108 mg, 0.15 mmol) in 6 mL of dry C6H6 in a 50-mL
flame-dried Schlenk flask, 2-methylaminopyridine (16 mg, 0.15 mmol)
was added. The mixture was heated at 65 °C for 21 h. The volatiles
were removed under vacuum. The resulting residue was recrystallized
from a mixture of C6H6/pentane (1:3) to give 4-PPh3 as a pale yellow
1
solid, which was dried under vacuum. Yield: 92 mg, 74%. H NMR
(400 MHz, CD2Cl2): δ 7.60−7.55 (m, 12H, Ar), 7.36−7.26 (m, 18H,
Ar), 6.74 (d, J = 4 Hz, 1H, pyridine C−H), 6.66 (t, J = 8 Hz, 1H,
pyridine C−H), 5.25 (m, pyridine C−H), 5.02 (d, J = 8 Hz, 1H,
pyridine C−H), 1.80 (s, 3H, −NMe), −22.22 (td, JPH = 16 Hz, JHH = 8
Hz, 1H, Ir−H), −22.75 (td, JPH = 16 Hz, JHH = 8 Hz, 1H, Ir−H).
31P{1H} NMR (202 MHz, CD2Cl2): δ 20.84 (t, J = 15 Hz). 13C{1H}
NMR (126 MHz, CD2Cl2): δ 171.60 (s), 147.40 (s), 136.14 (t, J = 25
Hz), 133.72 (s), 129.79 (s), 128.88 (s), 128.17 (t, J = 5 Hz), 104.48
(s), 101.55 (s), 32.78 (s). Anal. Calcd for C42H39IrN2P2: C, 61.08; H,
4.76; N, 3.39. Found: C, 61.33; H, 4.89; N, 3.12.
D i h y d r i d o ( 2 - p h e n y l a m i d o p y r i d i n e - κ 2 - N , N ′ ) b i s -
(triphenylphosphine)iridium(III) (5-PPh3). To a suspension of
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Inorganic Chemistry
Article
C6D6): δ 160.77 (s), 138.50 (t, J = 25 Hz), 135.07 (t, J = 6 Hz), 128.83
(s), 128.59 (s), 127.57 (t, J = 4 Hz), 123.70 (s).
1H, Ir−H). 31P{1H} NMR (202 MHz, CD2Cl2): δ 21.93(m). 13C{1H}
NMR (126 MHz, CD2Cl2): δ 165.03 (s), 158.35 (s), 157.31 (s),
153.68 (s), 134.46 (t, J = 6 Hz), 134.96(s), 133.96(s), 130.36, 128.85
(s), 128.58 (t, J = 5 Hz), 125.41(s), 113.43 (s). IR (cm−1) 1674
(νCO), 1293 (νCO). Anal. Calcd for C41H36IrN3O2P2: C, 57.47; H,
4.23; N, 4.90. Found: C, 56.10; H, 4.32; N, 4.85.
X-ray Crystallography. Crystal samples were mounted in
MiTeGen polyimide loops with immersion oil. The diffraction
experiments were carried out on a Rigaku SCXmini CCD detector
using filtered MoKα radiation (λ = 0.71075 Å) at a temperature of
−50 °C. The data frames were processed using Rigaku CrystalClear29
and corrected for Lorentz and polarization effects. The structures were
solved by direct methods30 or Patterson methods31 and expanded
using Fourier techniques.32 Non-hydrogen atoms were refined
anisotropically and hydrogen atoms were typically treated as idealized
contributions. Further details of the refinement are given in Table 2
and the Supporting Information.
Computational Details. All calculations were performed with the
Gaussian09 package33 of programs with the hybrid B3LYP func-
tional.34 The basis set was the ECP-adapted SDDALL35 with a set of
polarization functions for Ir36 and P,36 the all-electron 6-31G(d,p)37
for N, H, C, and the 6-31+G(d,p)38 for O of CO2.39 Full optimization
of geometry was performed without any symmetry constraints,
followed by analytical computation of the Hessian matrix to identify
the nature of the located extrema as minima or transition states. Each
transition state was relaxed toward reactant and product using the
vibrational data to confirm its nature. The zero-point, thermal, and
entropy corrections were evaluated to compute enthalpies and Gibbs
free energies (T = 298 K, P = 1 atm). The effect of DCM solvent (ε =
8.93) was included by using the continuum SMD model40 with single
point 6-311+G** calculations.41
D i h y d r i d o ( N - ( 2 - p y r i d y l ) c a r b a m a t o - κ 2 - N ′ , O ) b i s -
(triphenylphosphine)iridium(III) (10-PPh3). To a 25-mL round-
bottomed flask was added trihydrido(2-aminopyridine)bis-
(triphenylphosphine)iridium(III) (45.0 mg, 0.055 mmol) and DCM
(3 mL). The flask was purged with CO2 gas, which was bubbled
through the solution for 10 min (alternatively the solution could be
degassed using three freeze−pump−thaw cycles and then placed under
a static atmosphere of CO2). The reaction was allowed to stir for 24 h,
during which time a white crystalline solid began to precipitate. The
solid was filtered off under air, washed with DCM, cooled to 0 °C, and
dried under vacuum. The precipitated crystals were suitable for X-ray
diffraction. Yield 33.1 mg, 70%. 1H NMR (500 MHz, CD2Cl2): δ 7.59
(m, 12H), 7.36 (d, J = 6.0 Hz, 1H), 7.27 (m, 18H), 6.93 (t, J = 7.8 Hz,
1H), 5.87 (d, J = 8.3 Hz, 1H), 5.68 (t, J = 6.0 Hz, 1H), δ −20.67 (td, J
= 16.8, 7.1 Hz, 1H), −26.41 (td, J = 17.1, 7.4 Hz, 1H). 13C{1H} NMR:
The full carbon spectrum of this compound could not be recorded due
to its insolubility in all common solvents. 31P{1H} NMR (162 MHz,
CD2Cl2): δ 21.30 (d, J = 15.3 Hz). IR (nujol, cm−1): 1667.5 (νCO),
1329.1 (νCO). Anal. Calcd for C42H37IrN2O2P2: C, 58.94; H, 4.36; N,
3.27. Found: C, 58.89; H, 4.31; N, 3.28.
An isotopologue of 10-PPh3 with the carbon of the carbamato
group 13C-labeled was prepared by exposing a degassed solution of 10-
PPh3 in DCM to 13C-labeled CO2. Exchange occurred over a period of
24 h at RT. 13C{1H} NMR (126 MHz, CD2Cl2): 154.28 (13CO).
D i h y d r i d o ( N - ( 2 - p y r i d y l ) c a r b a m a t o - κ 2 - N ′ , O ) b i s -
(tricyclohexylphosphine)iridium(III) (10-PCy3). Excess CO2 at 1 atm
was added via a dual-manifold Schlenk line to a degassed and shaken
solution of 2-PCy3 (6 mg, 0.007 mmol) in 0.8 mL of CD2Cl2 in a J.
Young tube at RT. After 0.5 h the mixture was evaporated to dryness
to give 10-PCy3 as a white solid. Crystals suitable for X-ray diffraction
1
were grown from CH2Cl2 at −35 °C. Yield: 5.8 mg, 92%. H NMR
ASSOCIATED CONTENT
■
(500 MHz, CD2Cl2): δ 8.42 (d, J = 5 Hz, 1H, pyridine C−H), 7.50 (br
s, 1H, −NH), 7.43 (m, 1H, pyridine C−H), 6.46 (m, 2H, pyridine C−
H), 1.96−1.05 (m, 66H, Cy), −23.64 (td, JPH = 20 Hz, JHH = 10 Hz,
1H, Ir−H), −28.83 (td, JPH = 20 Hz, JHH = 10 Hz, 1H, Ir−H). 31P{1H}
NMR (202 MHz, CD2Cl2): δ 22.38. 13C{1H} NMR (126 MHz,
CD2Cl2): δ 30.00 (s), 29.07 (s), 27.68 (br s), 26.80 (s), 26.30 (s). The
full carbon spectrum of this compound could not be recorded due to
its insolubility in all common solvents. IR (cm−1) 1662 (νCO), 1311
(νCO). Anal. Calcd for C42H73IrN2O2P2·CH2Cl2: C, 52.85; H, 7.74;
N, 2.87. Found: C, 52.44; H, 7.28; N, 2.84.
S
* Supporting Information
1
Selected H and 31P NMR spectra, X-ray information for 1-
PPh3, 2-PPh3, 3-PPh3, 7-PPh3, 10-PPh3, 10-PCy3, and 11-
PPh3, the computed energy profile for the direct reaction of
CO2 with 1-PH3, the computed energy profile for the reaction
between CO2 and 2-PPh3, and Cartesian coordinates and
energies for optimized structures. This material is available free
of charge via the Internet at http://pubs.acs.org.
Dihydrido(N-(2-methylpyridyl)carbamato-κ2-N′,O)bis-
(triphenylphosphine)iridium(III) (11-PPh3). Excess CO2 at 1 atm was
added via a dual-manifold Schlenk line to a degassed and shaken
solution of 4-PPh3 (8.4 mg, 0.01 mmol) in 0.5 mL of CD2Cl2 in a J.
AUTHOR INFORMATION
Corresponding Author
*E-mail: robert.crabtree@yale.edu (R.H.C.), nilay.hazari@yale.
edu (N.H.), anova@iciq.es (A.N.).
■
1
Young tube at RT. After 0.5 h H and 31P spectra of the solution
showed that it was a mixture containing 91% 10-PPh3 and 9% 4-PPh3.
The mixture was dried under vacuum for 2 h. 1H and 31P spectra of the
resulting residue in CD2Cl2 showed that it was a mixture containing
27% 10-PPh3 and 73% 4-PPh3. As a result no pure 10-PPh3 was
isolated. Crystals suitable for X-ray diffraction were grown from
CH2Cl2/pentane under CO2 at −35 °C. 1H NMR (500 MHz,
CD2Cl2): δ 7.69 (d, J = 5 Hz, 1H, pyridine C−H), 7.55−7.51 (m, 12H,
Ar), 7.32−7.25 (m overlapping with 4-PPh3, 18H, Ar), 7.13 (m, 1H,
pyridine C−H), 6.28 (d, J = 10 Hz, 1H, pyridine C−H), 5.83 (m, 1H,
pyridine C−H), 2.38 (s, 3H, −NMe), −20.69 (m, 1H, Ir−H), −26.55
(m, 1H, Ir−H). 31P{1H} NMR (202 MHz, CD2Cl2): δ 20.22 (m).
Dihydrido(N-(2-pyrimidyl)carbamato-κ2 -N′,O)bis-
(triphenylphosphine)iridium(III) (12-PPh3). Excess CO2 at 1 atm was
added via a dual-manifold Schlenk line to a degassed and shaken
solution of 6-PPh3 (28 mg, 0.034 mmol) in 1 mL of CH2Cl2 in a J.
Young tube at RT. After 3 h the mixture was evaporated to dryness to
give crude product. Pure 12-PPh3 was obtained by recrystallization
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
G.E.D., N.D.S. and R.H.C. acknowledge funding from the
Division of Chemical Sciences, Geosciences, and Biosciences,
Office of Basic Energy Sciences of the U.S. Department of
Energy through Grant DE-FG02-84ER13297. Financial support
from the ICIQ foundation and the Spanish MICINN (projects
CTQ2011-27033, Consolider Ingenio 2010 CSD2006-0003
and a Juan de la Cierva contract for A.N.) is acknowledged.
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