ONO dianionic pincer-type ligand precursors for the synthesis of σ,π-cyclooctenyl iridium(III) complexes: Formation mechanism and coordination chemistry

Article abstract of DOI:10.1021/om400767d

The σ,π-cyclooctenyl iridium(III) pincer compounds [Ir(κ³-pydc-X)(1-κ-4,5-η-C₈H ₁₃)] (X = H (1), Cl, Br) have been prepared from [Ir(μ-OMe)(cod)] ₂ and the corresponding 4-substituted pyridine-2,6-dicarboxylic acids (H₂pydc-X) or, alternatively, from their lithium salts (X = H) and [Ir(cod)(CH₃CN)₂]PF₆. Deuterium labeling studies in combination with theoretical calculations have shown that formation of 1 involves a metal-mediated proton transfer in the reactive intermediate [Ir(κ²-Hpydc)(cod)], through the solvent-stabilized hydrido complex [IrH(κ³-pydc)(cod)(CH₃OH)], followed by olefin insertion. The formation of this hydrido intermediate results from solvent-assisted proton transfer through a hydrogen-bonding network, forming an eight-membered metallacycle. In contrast, reaction of [Ir(μ-OMe)(cod)] ₂ with iminodiacetic acid derivatives, RN(CH₂COOH) ₂, gave the stable iridium(I) mononuclear [Ir{κ²- MeN(CH₂COOH)(CH₂COO)}(cod)] (R = Me) complex having a free carboxymethyl group and the tetranuclear complex [Ir₄{κ ⁴-PhN(CH₂COO)₂}₂(cod)₄] (R = Ph) with doubly deprotonated ligands. The molecular structure of the related cyclooctene complex [Ir₄{κ⁴-PhN(CH ₂COO)₂}₂(coe)₈] has been determined by X-ray analysis. Reaction of 1 with monodentate N- and P-donor ligands gave the compounds [Ir(κ³-pydc)(1-κ-4,5-η-C ₈H₁₃)(L)] (L = py, BnNH₂, PPh₃, PMe₃). Reaction of 1 with the short-bite bis(diphenylphosphino) methane (dppm) afforded the mononuclear 1-dppm, with an uncoordinated P-donor atom, or the dinuclear complex 1₂-dppm as a function of the molar ratio used. Similarly, the dinuclear complexes 1₂-dppe and 1 ₂-dppp have been prepared using 1,2-bis(diphenylphosphino)ethane (dppe) and 1,3-bis(diphenylphosphino)propane (dppp) as bridging ligands. The diphosphine-bridged dinuclear assemblies have been obtained as two diastereoisomers in a 1:1 ratio due to the chirality of the mononuclear building block. The single-crystal X-ray structures of 1-py and 1-dppm are reported.

Full text of DOI:10.1021/om400767d

Article
pubs.acs.org/Organometallics
ONO Dianionic Pincer-Type Ligand Precursors for the Synthesis of
σ,π-Cyclooctenyl Iridium(III) Complexes: Formation Mechanism and
Coordination Chemistry
Duc Hanh Nguyen, Ingo Greger, Jesus J. Perez-Torrente,* M. Victoria Jimenez, F. Javier Modrego,
́
́
́
Fernando J. Lahoz, and Luis A. Oro*
́
́
́
Departamento de Quımica Inorgan
́
ica, Instituto de Sıntesis Quımica y Catal
́
isis Homogen
́
ea-ISQCH, Facultad de Ciencias,
Universidad de Zaragoza-CSIC, C/Pedro Cerbuna, 12, 50009 Zaragoza, Spain
S
* Supporting Information
ABSTRACT: The σ,π-cyclooctenyl iridium(III) pincer com-
pounds [Ir(κ3-pydc-X)(1-κ-4,5-η-C8H13)] (X = H (1), Cl, Br)
have been prepared from [Ir(μ-OMe)(cod)]2 and the correspond-
ing 4-substituted pyridine-2,6-dicarboxylic acids (H2pydc-X) or,
alternatively, from their lithium salts (X = H) and [Ir(cod)-
(CH3CN)2]PF6. Deuterium labeling studies in combination with
theoretical calculations have shown that formation of 1 involves a
metal-mediated proton transfer in the reactive intermediate [Ir(κ2-
Hpydc)(cod)], through the solvent-stabilized hydrido complex
[IrH(κ3-pydc)(cod)(CH3OH)], followed by olefin insertion. The
formation of this hydrido intermediate results from solvent-assisted
proton transfer through a hydrogen-bonding network, forming an
eight-membered metallacycle. In contrast, reaction of [Ir(μ-OMe)(cod)]2 with iminodiacetic acid derivatives, RN(CH2COOH)2,
gave the stable iridium(I) mononuclear [Ir{κ2-MeN(CH2COOH)(CH2COO)}(cod)] (R = Me) complex having a free
carboxymethyl group and the tetranuclear complex [Ir4{κ4-PhN(CH2COO)2}2(cod)4] (R = Ph) with doubly deprotonated
ligands. The molecular structure of the related cyclooctene complex [Ir4{κ4-PhN(CH2COO)2}2(coe)8] has been determined by
X-ray analysis. Reaction of 1 with monodentate N- and P-donor ligands gave the compounds [Ir(κ3-pydc)(1-κ-4,5-η-C8H13)(L)]
(L = py, BnNH2, PPh3, PMe3). Reaction of 1 with the short-bite bis(diphenylphosphino)methane (dppm) afforded the
mononuclear 1-dppm, with an uncoordinated P-donor atom, or the dinuclear complex 12-dppm as a function of the molar ratio
used. Similarly, the dinuclear complexes 12-dppe and 12-dppp have been prepared using 1,2-bis(diphenylphosphino)ethane
(dppe) and 1,3-bis(diphenylphosphino)propane (dppp) as bridging ligands. The diphosphine-bridged dinuclear assemblies have
been obtained as two diastereoisomers in a 1:1 ratio due to the chirality of the mononuclear building block. The single-crystal X-
ray structures of 1-py and 1-dppm are reported.
processes.6 Periana et al. have shown that the octahedral
INTRODUCTION
■
Ir(III) complexes [Ir(κ2-acac)2(R)(py)] (R = CH3, Ph),
bearing two acetylacetonate ligands (acac), one leaving alkyl
group, and one accessible coordination site, efficiently catalyzed
the hydroarylation of olefins, which likely proceeded via Ir(III)/
Ir(V) interconversion.7 O-donor ligands in these systems play
an important role, facilitating the access to high oxidation states
by modulating the electron density at the metal centers via a
hard/hard interaction or π-donating effects.8
The chemistry of pincer ligands, bearing three coordination
sites providing a rigid meridional environment to the metal
center, is one of the most dynamic areas in modern inorganic
chemistry, due to the impressive number of applications in
materials science and catalysis.1 In spite of the apparent
simplicity of the ligand framework, pincer ligands are valuable
tools for generating metal complexes with an adequate balance
of thermal stability and reactivity. Although a large number of
ligands bearing C-, N-, P-, or S-donor-based functionalities have
been synthesized,2 pincer ligands having O-donor fragments
have been much less studied.3 In this context, the ability of
OCO and ONO trianionic pincer ligands to support high-
oxidation-state metal complexes with vacant coordination sites,
which have been recently exploited for a range of catalytic
transformations, is remarkable.4,5
Recently, iridium complexes bearing O-based dianionic
pincer ligands have attracted considerable attention with regard
to C−H bond activation. The robustness and stability of the
ligand framework, with strongly electron donating hard oxygen
atoms, can provide more facile access to the relatively high
oxidation state of the metal center, thereby promoting the C−
Low-valent iridium complexes have been reported to show
Received: August 1, 2013
good activity for C−H activation and functionalization
© XXXX American Chemical Society
A
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Scheme 1. Synthetic Routes to [Ir(κ3-pydc)(1-κ-4,5-η-
H bond oxidative addition under mild conditions. Bercaw and
Labinger et al. reported the synthesis of Ir(I) and Ir(III)
complexes bearing a diphenolate−imidazolyl−carbene triden-
tate ligand (Chart 1(i)), but no C−H bond activation
C8H13)] (1) from 2,6-Pyridinedicarboxylic Acid and Its
a
Lithium Salts
Chart 1. Ligand Framework of O-Based Dianionic Pincer
t
Iridium Complexes (R = Bu)
a
Reactions were conducted in CH2Cl2/MeOH (3/1).
scarcely soluble in most organic solvents, including dichloro-
methane and methanol, although surprisingly the compounds
have an acceptable solubility in a 5/1 mixture of both solvents.
The σ,π-cyclooctenyl ligand in complexes 1−3 has been
unequivocally identified by NMR spectroscopy.13 For example,
the olefinic protons and carbons (CH) were observed as two
characteristic low-field multiplet resonances at 5.87 and 5.64
ppm in the 1H NMR spectrum of 3, which correlates with those
at 89.0 and 85.0 ppm in the 13C{1H} NMR spectrum. The Ir−
CH resonance was easily identified in the 13C APT spectrum at
14.6 ppm. Full assignment of the resonances and connectivity
in the σ,π-cyclooctenyl ligand was achieved with the help of 2D
chemistry was demonstrated.9 Recently, the same group has
described the synthesis of bis(phenolate)pyridine iridium(III)
pincer complexes (Chart 1(ii)) and their ability to smoothly
activate both intra- and intermolecular C−H bonds.10 On the
other hand, at the same time we reported the synthesis of
unsaturated iridium(III) pyridinedicarboxylate pincer com-
plexes (Chart 1(iii)), which exhibited a notable catalytic
activity in the borylation of arenes involving C−H bond
activation under thermal conditions.11 It is noteworthy that the
synthesis of some iridium and rhodium complexes containing
pyridine-2,6-dicarboxylate ligands has been described, but no
catalytic applications were reported.12
1
1H−1H COSY, H−13C HSQC, and HMBC experiments.
In the present contribution we report on the synthesis and
reactivity of unsaturated σ,π-cyclooctenyl complexes [Ir(κ3-
pydc-X)(1-κ-4,5-η-C8H13)] (X = H, Cl, Br). The straightfor-
ward synthesis of these unusual iridium(III) pincer complexes
from pyridine-2,6-dicarboxylic acids and standard dinuclear
iridium(I) starting materials, the simple ligand architecture of
these complexes, and their potential catalytic applications based
on C−H activation have prompted us to investigate their
formation mechanism. In addition, the scope of the synthetic
methodology for the preparation of iridium(III) pincer
complexes derived from related dicarboxylic compounds as
precursors has also been investigated.
The stability of 1 allows for alternative synthetic approaches
from cationic mononuclear iridium complexes and 2,6-
pyridinedicarboxylic acid lithium salts. Thus, treatment of
[Ir(cod)(CH3CN)2]PF6 with lithium 6-carboxypicolinate (HLi-
pydc) in CH2Cl2/MeOH gave 1 in 96% yield without the need
for chromatographic purification after removing the LiPF6 salt
(method B, Scheme 1). It is noteworthy that reaction of
[Ir(cod)(CH3CN)2]PF6 with lithium pyridine-2,6-dicarboxylate
(Li2pydc) also gave 1 in excellent yield instead of the expected
iridium(I) anionic complex Li[Ir(κ3-pydc)(cod)] (method C,
Scheme 1). In this case, the formation of 1 can be explained by
the presence of adventitious water in the organic solvents,
which plays an important role in providing the required proton
for the formation of the cyclooctenyl ligand. In sharp contrast,
only a very small amount of 1 was formed on starting from the
chlorido-bridged dimer [Ir(μ-Cl)(cod)]2 and pyridine-2,6-
dicarboxylate lithium salts, even under heating conditions.
Mechanistic Studies on the Formation of [Ir(κ3-
pydc)(1-κ-4,5-η-C8H13)]. The synthesis of [Ir(κ3-pydc)(1-κ-
4,5-η-C8H13)] (1) has been investigated by NMR. First, it is
reasonable to propose that the protonation of the methoxido
bridges in [Ir(μ-OMe)(cod)]2 by H2pydc results in the
formation of the mononuclear neutral iridium(I) intermediate
[Ir(κ2-Hpydc)(cod)] (4) having the monodeprotonated ligand
κ2-N,O coordinated and an uncoordinated carboxyl group
(Scheme 1, method A). This species should be also
straightforwardly generated by replacement of the labile
acetonitrile ligands in [Ir(cod)(CH3CN)2]+ by lithium 6-
carboxypicolinate (Scheme 1, method B). In fact, the clean
synthesis of 1 using this method further supports this proposal.
The intermediate compound 4, which is immediately formed
after mixing the reagents by both methods, has been
RESULTS AND DISSCUSION
■
Synthesis of Pyridinedicarboxylate Pincer Complexes
[Ir(κ3-pydc-X)(1-κ-4,5-η-C8H13)]. The σ,π-cyclooctenyl
iridium(III) compound [Ir(κ3-pydc)(1-κ-4,5-η-C8H13)] (1)
can be straightforwardly prepared by the reaction of 2,6-
pyridinedicarboxylic acid (H2pydc) with 0.5 molar equiv of the
dinuclear 1,5-cyclooctadiene iridium methoxy-bridged complex
[Ir(μ-OMe)(cod)]2 in CH2Cl2/MeOH at room temperature
(method A, Scheme 1). The compound was obtained as the
methanol solvate 1·MeOH, as was confirmed by a X-ray
analysis, and isolated as an air- and moisture-stable yellow solid
in 90% yield after chromatographic purification.11 This
synthetic method is applicable to the preparation of related
4-substituted pyridine-2,6-dicarboxylate pincer complexes.
Thus, reaction of the complex [Ir(μ-OMe)(cod)]2 with 4-
chloro- and 4-bromopyridine-2,6-dicarboxylic acids afforded the
corresponding complexes [Ir(κ3-pydc-Cl)(1-κ-4,5-η-C8H13)]
(2) and [Ir(κ3-pydc-Br)(1-κ-4,5-η-C8H13)] (3), which were
isolated as red solids in excellent yield. These complexes are
B
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characterized in solution. The H NMR spectrum of [Ir(κ2-
Hpydc)(cod)] (4) at 208 K (CD2Cl2/CD3OD) features a set
of four resonances at 4.26 and 3.77 ppm (CH protons) and
2.09 and 1.28 ppm (>CH2, exo and endo protons), which is in
full agreement with the integrity of the cyclooctadiene ligand.
Furthermore, the HR ESI-MS showed peaks at m/z 490.0633
(ESI+) and 466.0672 (ESI−) corresponding to the ions [M +
Na]+ and [M − H]−, respectively. Warming to room
temperature resulted in broad resonances, although no
evidence of proton transfer from the carboxyl group to the
cod ligand was immediately observed. Interestingly, the
formation of the related [Ir(κ2-Hpydc-Br)(cod)] species was
also observed under similar conditions.
A plausible mechanism for the proton transfer leading to the
cyclooctenyl ligand is a metal-mediated process. Thus, the
protonation of the iridium center in [Ir(κ2-Hpydc)(cod)] (4)
by the free −COOH arm of the tridentate ligand, formally an
oxidative addition, should result in the octahedral iridium(III)
hydrido complex [IrH(κ3-pydc)(cod)]. Subsequent insertion of
one of the CC bonds into the Ir−H bond would account for
the formation of 1 (Scheme 2). In this context, it is worth
mentioning that the oxidative addition of carboxylic acids to
iridium(I) complexes is a well-documented process.14
confirmed both in the 13C{1H} NMR, which showed the
1
expected deuterium-coupled triplet resonance at 34.73 ppm
(JC‑D = 19.0 Hz), and in the H NMR at 1.21 ppm. The endo
2
and exo protons (8-H) of the resulting >CH2 next to the Ir−C
bond in 1 have been identified at 0.92 (tt, JH−H = 13.9 and 2.2
Hz) and −0.01 ppm (ddd, JH−H = 13.9, 7.3, and 3.6 Hz),
respectively (Figure 1a).11 The triplet feature of the 8-H-endo
resonance is due to two similarly large couplings with 8-H-exo
and one of the adjacent H-7 protons with an axial−axial
relationship. This assignment is also consistent with the
observation that in cyclooctenyl ligands with a metallo-chair
conformation the low-shift proton of a geminal pair is located at
the face of the cyclooctenyl ligand opposite to the metal
fragment.16 Thus, the lack of a resonance at δ 0.92 ppm in the
1H NMR spectrum of 1-d1 (Figure 1b), assigned to the 8-H-
endo proton, supports the proposed hydride/insertion mech-
anism, as the migratory insertion proceeds with cis stereo-
chemistry.
Unfortunately, we have not been able to observe any hydrido
intermediate by monitoring the reactions leading to complexes
1−3. In addition, the stereochemistry of 1-d1 is also compatible
with the intramolecular direct protonation of the cycloctadiene
ligand by the carboxyl group. In fact, the direct protonation of
cycloctadiene on electron-rich nickel(0) complexes has been
demonstrated.16 On the other hand, the exo nucleophilic attack
by methoxide on cyclooctadiene palladium complexes to give
chlorido-bridged cyclooctenylmethoxy palladium dimers has
been also described.17
Scheme 2. Proposed Mechanism for the Formation of [Ir(κ3-
pydc)(1-κ-4,5-η-C8H13)(CH3OH)] (1·MeOH)
DFT Calculations on the Proton Transfer Mechanism
in [Ir(κ2-Hpydc)(cod)]. In order to ascertain the proton
transfer mechanism in [Ir(κ2-Hpydc)(cod)] (4) leading to the
formation of the σ,π-cyclooctenyl iridium(III) compound
[Ir(κ3-pydc)(1-κ-4,5-η-C8H13)] (1) as the methanol solvate,
1·MeOH, a detailed computational study using DFT
calculations has been carried out both in the gas phase and
in dichloromethane as solvent. In the following discussion
energy differences are expressed as ΔG for gas-phase
calculations and ΔE (CH2Cl2) for solution calculations.
In the gas phase, the optimized structure of the initial
intermediate [Ir(κ2-Hpydc)(cod)] (4) shows the expected
square-planar coordination environment for an Ir(I) center
which is bonded to cod and a κ2-N,O coordinated 6-
carboxypicolinato ligand. The remaining uncoordinated
carboxyl group is located out of the molecular plane and
directed away from the iridium center. A prominent feature of
the structure of the final Ir(III) reaction product, 1·MeOH, is
that the 2,6-pyridinedicarboxylato ligand plane runs eventually
parallel to the unchanged double bond of the σ,π-cyclooctenyl
ligand, in a 90° turn relative to the starting position in 4. This
fact, along with the coordination of a methanol molecule to the
final complex, suggests some complexity in the reaction
mechanism.
Assuming that the reorientation of the tridentate ligand could
occur before the proton transfer to the cyclooctadiene ligand,
rotation of the 6-carboxypicolinato ligand in 4 and coordination
of a MeOH molecule leads to 4a, which is just 6.8 kcal mol−1
above the starting mononuclear complex and has been shown
to be a minimum in the potential energy surface of the
molecule. The formation of the cyclooctenyl group could
proceed by either the formation of a hydrido intermediate
followed by insertion of the hydrido ligand into the double
bond or by a direct protonation of the double bond by the
acidic carboxyl group. Interestingly, the carboxyl group in 4a
The synthesis of 1 from Li2pydc, where traces of adventitious
water could act as proton source (Scheme 1, method C),
requires further comments. Although the reaction of Li2pydc
with H2O to give back LiHpydc is possible, most probably, the
protonation of the carboxylate free arm in the anionic complex
Li[Ir(κ2-pydc)(cod)] by H2O to give [Ir(κ2-Hpydc)(cod)] (4)
and LiOH is taking place. The direct protonation of the iridium
center by H2O in the strongly nucleophilic anionic complex to
give a hydrido intermediate is a less likely pathway.15
In order to shed light on the operating mechanism for the
proton transfer, we have prepared the deuterium-labeled
compound [Ir(κ3-pydc)(1-κ-4,5-η-C8H12D)] (1-d1) by two
different methods. Thus, reaction of [Ir(cod)(CH3CN)2]PF6
with lithium pyridine-2,6-dicarboxylate (Li2pydc) in the
presence of D2O resulted in the complete incorporation of
deuterium into the cyclooctenyl ligand (Scheme 1, method C).
Interestingly, 1-d1 can be also prepared from [Ir(μ-OMe)-
(cod)]2 and H2pydc in CD2Cl2/CD3OD (method A). The
presence of the C8H12D cyclooctenyl ligand in 1-d1 was
C
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1
Figure 1. Cyclooctenyl region of the H NMR spectra of (a) [Ir(κ3-pydc)(1-κ-4,5-η-C8H13)] (1) and (b) [Ir(κ3-pydc)(1-κ-4,5-η-C8H12D)] (1-d1).
Scheme 3. Gas-Phase DFT Calculated ΔG (in kcal mol−1) Energy Profile for the Proton Transfer Mechanisms in [Ir(κ2-
Hpydc)(cod)] (4) Leading to the Formation of the Methanol Solvate of [Ir(κ3-pydc)(1-κ-4,5-η-C8H13)] (1)
has the required orientation for both processes. Scanning of the
Ir−H distance from the carboxyl group in the search for a
potential hydrido is always uphill, in the gas phase, and any full
optimization reverts to 4a. On the other hand, scanning the
C(olefin)−H distance leads to the transition state TS4a‑1 at a
relative energy of 37.0 kcal mol−1, which connects with the final
complex 1·MeOH. Although this is a possible reaction pathway,
the overall activation energy of 43.8 kcal mol−1 from 4 makes it
inaccessible (Scheme 3, left side). In solution the computed
behavior is somewhat different. It is possible to detect, instead,
a transition state leading to the hydrido intermediate 4b (Figure
2), which eventually leads to the insertion product 1 (see
below). Interestingly, the hydrido intermediate 4b is unstable to
a reoptimization in the gas phase, as expected according to the
results shown above.
A possible pathway can be provided by participation of
alternative hydrido iridium(III) intermediates derived from 4.
The tridentate 2,6-pyridinedicarboxylate ligand can adopt, in
principle, either fac or a mer disposition in an octahedral
geometry. The geometry optimization, in the gas phase, of a fac
mononuclear hydrido isomer leads to the high-energy (25.2
kcal mol−1 relative to 4) and strongly distorted structure 4e,
D
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4e (11.8 kcal mol−1 relative to 4) and features a
pseudooctahedral coordination geometry with a methanol
molecule coordinated (Ir−O, 2.25 Å) trans to the hydrido
ligand and at hydrogen-bonding distance of an uncoordinated
carboxylate group (O···H, 1.45 Å) of the κ2-N,O coordinated
pyridine-2,6-dicarboxylate ligand. The coordination of meth-
anol and the hydrogen bond to the uncoordinated carboxylate
group provides relief to the strain observed for the tridentate
pyridine-2,6-dicarboxylate ligand in 4e. In support of the
stabilizing role of methanol, calculations of a similar structure
using a molecule of THF instead of methanol ends in
dissociation of the THF molecule and a hydrido complex
with a similar geometry to the strained 4e. In addition, the
formation of the final insertion product bearing a THF ligand
instead of methanol, 1-THF, is just 3.4 kcal mol−1 exergonic in
contrast to the 5.7 kcal mol−1 released in the formation of 1·
MeOH. This is consistent with the very long reaction times
required (5 days at room temperature) for the formation of 1
from [Ir(μ-OMe)(cod)]2 and H2pydc using THF as solvent.
The transition state TS4c‑1 located at an activation energy 14
kcal/mol above 4c (25.8 kcal mol−1 global) leads to the
insertion product 1·MeOH (Scheme 3, right side; Figure 3b).
Eventually, although this hydrido complex is somewhat less
stable than 4d, it opens a kinetically more feasible reaction
pathway. In any case, formation of the intermediate hydrido
complexes is endergonic, which agrees with the fact that they
remain unobserved during the reaction.
The formation of the hydrido intermediate 4c from 4 has
also been studied. Scanning both the direct and the reverse
processes in a search for the corresponding transition structure
ends up in the intermediate 4a. Although the stabilizing role of
methanol is required to outline a reasonable reaction pathway
to 1·MeOH, the search of a transition state from 4 to 4c using
just one methanol molecule has been fruitless. The reaction has
been studied with two molecules of methanol taking part in the
process, one required to account for the coordinated molecule
of methanol in the intermediate 4c and another one as an
intervening mediator in the proton transfer of the carboxylic
proton to the iridium atom. This has allowed us to find the
transition structure TS4−4c, which shows that the process for the
formation of the Ir(III) hydrido intermediate takes place by
simultaneous transfer of the carboxylic proton to a methanol
Figure 2. Gas-phase DFT computed energies (ΔG, kcal mol−1)
relative to [Ir(κ2-Hpydc)(cod)] (4) for a mer (4d), fac (4e), and
methanol stabilized (4c) isomers of the hydrido complex [IrH(κ3-
pydc)(cod)] (isomer 4b was found to be stable only in solution
calculations).
which shows a pseudooctahedral geometry where the 2,6-
pyridinedicarboxylate ligand spans roughly three fac-disposed
coordination sites in a very strained conformation (Figure 2).
This high energy suggests that 4e is not a viable intermediate.
More stable is the pseudooctahedral mer hydrido isomer 4d,
where the three donor atoms of the 2,6-pyridinedicarboxylate
ligand are coplanar with one of the double bonds of the
cyclooctadiene ligand, with the hydrido ligand trans to the other
double bond of the diolefin, which is very weakly coordinated.
In spite of this structure being very close in energy to the
starting compound (+3.1 kcal mol−1), it does not have the
required stereochemistry to produce the insertion final product.
The CC double bond lies perpendicular to the Ir−H bond,
and consequently, the insertion process requires a substantial
amount of activation energy, TS4d‑1 (33.5 kcal/mol), still too
high to be acceptable within the reaction mechanism.
In light of these findings in the gas phase, alternative reaction
pathways with explicit participation of methanol have been
explored. A more accessible pathway to the insertion process is
provided via the hydrido complex 4c, stabilized by a methanol
molecule (Figure 2). This intermediate has lower energy than
Figure 3. Structures for the transition states TS4−4c (a) and TS4c‑1 (b). Some hydrogen atoms have been omitted for clarity.
E
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Scheme 4. DFT Calculated ΔE (in kcal mol−1) Energy Profile in Dichloromethane Solution for the Proton Transfer
Mechanisms in [Ir(κ2-Hpydc)(cod)]·2MeOH (4·2MeOH) Leading to the Formation of the Methanol Solvate of [Ir(κ3-pydc)(1-
κ-4,5-η-C8H13)] (1)
molecule, which transfers its own proton to the iridium center
in a concerted way through an eight-membered metallacycle
(Figure 3a). The geometry of the transition state is quite
advanced toward the formation of the hydrido complex, as
there is only 9 kcal/mol of difference between them and its
formation is endergonic relative to the starting compound 4.
The distances of the transferred proton between the carboxyl
and the methanol are 1.50 Å to the carboxyl oxygen atom and
1.04 Å to the methanol oxygen atom. On the other hand, the
hydrido ligand is located 1.66 Å from the iridium atom (1.56 Å
in the final product) and 1.43 Å from the methanol oxygen
atom. The moderate activation energy (20.8 kcal mol−1) for the
proton transfer process along with that involved in the
transition from the hydrido complex to the final species (14
kcal mol−1) gives a global activation energy of 25.8 kcal mol−1
(Scheme 3, right side), which accounts for the long reaction
times required to drive the reaction to completion.
Optimization in solution, using dichloromethane as solvent
and SMD as solvation model, shows some differences but again
illustrates that this is the most accessible pathway to a hydrido
intermediate.
relative to 4a·2MeOH, which is reached after TS′4a‑4b at 27.6
kcal mol−1. As shown above, this hydrido species is a shallow
minimum unstable to a gas-phase reoptimization. It is followed
by TS′4b‑1 at 34.6 kcal mol−1 and eventually leads to 1, which is
slightly endoergic at this level of calculation (+1.7 kcal mol−1).
This transition state is only 1.5 kcal mol−1 above TS′4c‑1 in the
methanol-assisted pathway. The methanol-assisted processes
are qualitatively similar in the gas phase and in solution,
although the energies involved in the latter reflect the large
stabilization of 4a·2MeOH. The hydrido intermediate 4c·
MeOH located at 9.4 kcal mol−1 is reached through TS′4−4c
with an activation energy of 22.1 kcal mol−1 above 4a·2MeOH.
From the hydrido intermediate 4c·MeOH, after loss of the
hydrogen-bonded methanol molecule through TS′4c‑1 at 33.1
kcal mol−1 (slightly lower than TS′4b‑1), 1 is reached (see the
Supporting Information). As in the gas phase, the solvent-
assisted pathway shows the most feasible formation of the key
hydrido intermediate, although in fact the limiting step of the
reaction is the insertion step in both pathways (Scheme 4).
Interestingly, solvent-mediated proton transfer has been
recently reported to be involved in a series of processes such as
formation of hydrido intermediates,18 heterolytic hydrogen
splitting,19 alkyne to vinylidene isomerization,20 and addition
reactions21 or H/D exchange,22 among others. The role of
solvent as a proton shuttle for proton transfer reactions has also
been evidenced in several gold-catalyzed reactions.23
In order to trace the solution energy landscape, the TS4−4c
and both end products of the IRC calculation were reoptimized
in solution, including the two intervening methanol molecules.
These calculations have shown a remarkable stabilization in
solution of 4 by the two hydrogen-bonded methanol molecules
(4·2MeOH, 18.0 kcal mol−1 relative to 4 in dichloromethane
solution). In solution, this 4·2MeOH species was then used as a
reference for the energies and all results are expressed relative
to it and reported as ΔE (kcal mol−1) (Scheme 4).
Imino and Pyridinediacetic Acid Derivatives As
Potential ONO Pincer Ligand Precursors. In order to
investigate the key factors behind the synthesis of these unusual
unsaturated σ,π-cyclooctenyl iridium(III) complexes, we have
studied the potential of other dicarboxylic acids for the
preparation of related complexes having different pincer
ligands. In particular, we have used the iminodiacetic acid
In contrast with the gas-phase results, the protonation of the
metal by the carboxyl group in the 4a species renders the
hydrido intermediate 4b with an energy of +26.5 kcal mol−1
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derivatives RN(CH2COOH)2 (R = Me, Ph) as precursors for
dianionic tridentate pincer ONO ligands. In contrast to the
rigidity of 2,6-pyridinedicarboxylate, the remarkable flexibility
of these ligands allows for the accommodation of different
coordination environments, and in fact, both fac24 and mer25
dispositions have been found in octahedral transition-metal
complexes (R = Me).
COSY related resonances at 5.00/4.85 and 4.63/4.32 ppm
correspond to the CH protons of the double bonds trans to
the N and O atoms, respectively.27 Signals for the inequivalent
>CH2 of the 2-((carboxymethyl)(methyl)amino)acetate ligand
were observed at 71.2 and 69.0 ppm in the 13C{1H} NMR and
as two AB quartets centered at 3.83 and 3.68 ppm (JAB ≈ 16
Hz) in the 1H NMR spectra. Interestingly, two protons of each
AB system are mutually coupled (JH−H = 1.8 Hz), resulting in a
set of resonances with an unusual shape (Figure 4). The
carboxyl group in 5 was not observed, probably due to H/D
exchange with CD3OD.
Compound 5 is formally an analogue of the intermediate
species [Ir(κ2-Hpydc)(cod)] proposed in the formation of 1.
However, the intramolecular proton transfer to the iridium
center from the carboxyl group in 5 does not take place and the
formation of a related σ,π-cyclooctenyl iridium(III) complex
was not observed. A comparison of the acidic strengths of
H2pydc (pKa1 = 2.10 and pKa2 = 4.38)28 and MeN-
(CH2COOH)2 (pKa1 = 2.50, pKa2 = 9.50)29 evidenced the
less acidic character of N-methyliminodiacetic acid. Despite
having very similar first acid dissociation constants, Ka1, the
second acid dissociation constants, Ka2, differ by 5 orders of
magnitude. Thus, the acidity of the COOH proton appears to
play a key role in the proton transfer process, which is in full
agreement with the behavior observed in oxidative addition
reactions of carboxylic acids to [Ir(cod)(PMe3)3]Cl.14b
Interestingly, the related N-phenyliminodiacetic acid PhN-
(CH2COOH)2 has acidic properties very similar to those of
H2pydc (pKa1 = 2.40, pKa2 = 4.96),30 and thus, its reactivity has
been investigated.
The reaction of [Ir(μ-OMe)(cod)]2 with N-methyliminodi-
acetic acid in THF for 3 h at 50 °C gave an orange solution
from which the compound [Ir{κ2-MeN(CH2COOH)-
(CH2COO)}(cod)] (5) was isolated as an air-sensitive
yellow-orange solid in good yield. Compound 5 is silent
under ESI conditions, and satisfactory elemental analyses could
not be obtained because of its air sensitivity. However, the
mononuclear formulation relies on key features of the NMR
spectra. The integrity of the cod ligand in 5 became evident
1
both in the H and 13C{1H} NMR spectra (CD3OD), which
showed four well-defined resonances for the CH protons and
carbons, denoting the lack of symmetry in the molecule. Most
probably 5 is a neutral iridium(I) species having a
monodeprotonated κ2-N,O coordinated N-methyliminodiacetic
acid and a free carboxymethyl group (Figure 4).26 The 1H−1H
Addition of a methanol solution of PhN(CH2COOH)2 to a
solution of [Ir(μ-OMe)(cod)]2 in dichloromethane resulted in
the formation of a red solid, compound 6, irrespective of the
stoichiometric ratio (1:1 or 2:1). The infrared spectrum of 6
did not show any broad absorption in the −OH stretching
region, which confirms that both carboxyl groups have been
deprotonated. Unfortunately, the low solubility of this
compound precludes further characterization by NMR. In the
same way, reaction of the related cyclooctene iridium hydroxy-
Figure 4. Selected region of the 1H NMR spectrum of [Ir{κ2-
MeN(CH2COOH)(CH2COO)}(cod)] (5).
Figure 5. (a) Molecular structure of [Ir4{κ4-PhN(CH2COO)2}2(coe)8] (7). Primed atoms are related to the unprimed ones by the symmetry
transformation 1.5 − x, 0.5 − y, z. Hydrogen atoms have been omitted for clarity. (b) Schematic drawing showing the κ4 coordination of each N-
phenyliminodicarboxylate group in 7 (only the olefinic carbons of the cyclooctene ligands have been represented).
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Table 1. Selected Bond Distances (Å) and Angles (deg) for the Tetranuclear Complex [Ir4{κ4-PhN(CH2COO)2}2(coe)8] (7)
Bond Distances
Ir(1)−O(1)
Ir(1)−C(1)
Ir(1)−C(2)
Ir(2)−O(2)
Ir(2)−C(25)
Ir(2)−C(26)
C(1)−C(2)
C(25)−C(26)
2.105(2)
2.109(3)
2.101(3)
2.105(2)
2.106(3)
2.114(3)
1.415(5)
1.412(5)
Ir(1)−O(3)
Ir(1)−C(9)
Ir(1)−C(10)
Ir(3)−O(4)
Ir(3)−C(35)
Ir(3)−C(36)
C(9)−C(10)
C(35)−C(36)
2.112(2)
2.096(3)
2.122(3)
2.099(2)
2.120(3)
2.100(3)
1.407(5)
1.416(5)
a
Bond Angles
O(1)−Ir(1)−O(3)
O(1)−Ir(1)−M(1)
O(1)−Ir(1)−M(2)
O(2)−Ir(2)−O(2′)
O(2)−Ir(2)−M(3)
O(2)−Ir(2)−M(3′)
M(3)−Ir(2)−M(3′)
85.12(9)
91.08(9)
O(3)−Ir(1)−M(1)
O(3)−Ir(1)−M(2)
M(1)−Ir(1)−M(2)
O(4)−Ir(3)−O(4′)
O(4)−Ir(3)−M(4)
O(4)−Ir(3)−M(4′)
M(4)−Ir(3)−M(4′)
172.14(9)
91.11(9)
173.05(9)
84.54(13)
91.52(10)
174.47(10)
92.67(10)
93.35(10)
83.59(12)
92.11(10)
174.84(10)
92.31(10)
a
M(1), M(2), M(3), and M(4) represent the midpoints of the C(1)−C(2), C(9)−C(10), C(25)−C(26), and C(35)−C(36) olefinic double bonds,
respectively. Primed atoms are related to the unprimed atoms by the symmetry transformation 1.5 − x, 0.5 − y, z.
bridged complex [Ir(μ-OH)(coe)2]2 with PhN(CH2COOH)2
under the same conditions gave an insoluble yellow-orange
solid that has been characterized as the tetranuclear complex
[Ir4{κ4-PhN(CH2COO)2}2(coe)8] (7) by a single-crystal X-ray
diffraction study. The formation of 7 probably results from the
fast intermolecular deprotonation of the free −COOH arm by
[Ir(μ-OH)(coe)2]2 in the likely intermediate [Ir{κ2-PhN-
(CH2COOH)(CH2COO)}(coe)2]. The IR spectra of com-
pounds 6 and 7 are comparable, which allows us to propose a
similar tetranuclear formulation for [Ir4{κ4-PhN-
(CH2COO)2}2(cod)4] (6) with a closely related structure
derived from the replacement of coe ligands by cod. 6 and 7
were obtained in 80% and 90% yields, respectively, using the
correct stoichiometric ratio. Crystals of 7 suitable for X-ray
analysis were obtained by slow diffusion of solutions of both
reactants in the corresponding solvents. The molecular
structure of 7 is shown in Figure 5, and Table 1 collects the
most relevant bond parameters.
The complex is tetranuclear, but only half of the molecule is
crystallographically independent. The molecule exhibits C2
symmetry with an intramolecular crystallographic 2-fold axis
passing through the Ir(2) and Ir(3) atoms. According to this,
the four metal atoms conform a strictly planar, almost perfect
square, with the metal atoms separated well over 5 Å and with
each iridium atom bonded to the olefinic bonds of two
cyclooctene ligands. Each tetradentate iminodicarboxylate
ligand is linked to all four metals, with each pair of oxygens
of each carboxylate group bridging two contiguous metals, in an
alternate way at both sides of the metal square, having an
uncoordinated amine group (Figure 5b). All four metals exhibit
slightly distorted square-planar environments typical of Ir(I)
atoms; it is noteworthy to point out that while the Ir(1)
coordination plane is almost coplanar with the tetrametallic
square skeleton (dihedral angle 13.26(5)°), those of Ir(2) and
Ir(3) are nearly perpendicular (mean 83.57(5)°) to the
tetrametallic reference plane.
those reported in other related carboxylate-bridged dinuclear
Ir(I) complexes such as [Ir(μ-O2C8H15)(cod)]2 (O2C8H15 = 2-
ethylhexanoate), with a mean value of 2.097(3) Å,31 or in the
polymeric [IrAg(μ-O2CCF3)2(cod)]n, where the “Ir(μ-O-
carboxylate)2(olefin)2” units are also present (mean 2.095(5)
Å).32 The average olefinic CC bond length, 1.413(3) Å,
compares well with other bis-cyclooctene κ2-O,O iridium
complexes33 (mean 1.417(8) Å); this value is clearly longer
than the free olefinic CC double bond (mean 1.316(15) Å)34
and should be interpreted as the result of the electron-rich Ir(I)
engaging in a high degree of π back-bonding.
The results described above strongly suggest that the acidity
of the dicarboxylic compound is not the only factor that
controls the formation of the unsaturated σ,π-cyclooctenyl
iridium(III) complexes. In spite of its weaker Brønsted basicity,
the stronger N-coordination ability of the pyridine fragment
versus that of the aliphatic amines also should play an
important role in the protonation step. In order to test this
hypothesis, the reactivity of 2,2′-(pyridine-2,6-diyl)diacetic acid,
py(CH2COOH)2 (pKa1 = 3.08, pKa2 = 5.92), was studied.28
The reaction with standard dinuclear and mononuclear iridium
starting materials invariably led to the formation of scarcely
soluble yellow solids with poorly resolved NMR spectra
without any evidence for the formation of a σ,π-cyclooctenyl
iridium(III) complex. Thus, both the acidity and the rigidity of
the tridentate ligand are determinant factors in the formation of
σ−π-cyclooctenyl iridium(III) complexes. As it has been shown
before, the key transition state for the proton transfer requires a
rigid structure in order to allow the carboxylic group to
approach the iridium center. However, in the case of highly
flexible tridentate ligands the freedom of the carboxyl group
and its fast intermolecular deprotonation by iridium dinuclear
complexes probably determines the course of the reaction.
Reactivity of [Ir(κ3-pydc)(1-κ-4,5-η-C8H13)] with Mono-
dentate N- and P-Donor Ligands. Although iridium(III)
complexes are usually inert to substitution, the presence of
strong labilizing ligands can promote ligand replacement
reactions. The trans influence exerted by the iridium−alkyl
bond in the solvate [Ir(κ3-pydc)(1-κ-4,5-η-C8H13)(MeOH)]
(1·MeOH) points to a labile methanol ligand. Thus, complex 1
can be considered an unsaturated iridium(III) complex and,
The metal coordination bond distances, Ir−C and Ir−O,
show very similar values, with all of the figures within very
narrow ranges, 2.096−2.122(3) and 2.099−2.112(2) Å,
respectively, which substantiates the identical electronic nature
of all four metals. The Ir−O bond lengths are very similar to
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consequently, methanol replacement by more coordinating N-
and P-donor ligands could be possible.
Reaction of 1 with an excess of pyridine or benzylamine gave
the corresponding complexes [Ir(κ3-pydc)(1-κ-4,5-η-C8H13)-
(py)] (1-py) and [Ir(κ3-pydc)(1-κ-4,5-η-C8H13)(BnNH2)] (1-
BnNH2), which were isolated as yellow solids in excellent yields
(Scheme 5). Under similar conditions, 2-picoline did not react
Scheme 5. Mononuclear Complexes Derived from 1
Figure 6. Molecular structure of [Ir(κ3-pydc)(1-κ-4,5-η-C8H13)(py)]
(1-py). Bond distances (Å) and angles (deg): Ir−O(1) = 2.078(3),
Ir−O(4) = 2.076(3), Ir−N(1) = 1.981(3), Ir−N(2) = 2.209(3), Ir−
C(11) = 2.079(4), Ir−C(15) = 2.175(4), Ir−C(16) = 2.178(4),
C(15)−C(16) = 1.373(6); O(1)−Ir−O(4) = 157.62(11), O(1)−Ir−
N(1) = 78.80(12), O(4)−Ir−N(1) = 78.86(12), N(1)−Ir−M =
177.02(12), N(2)−Ir−C(11) = 176.30(15), C(11)−Ir−M =
84.64(15). M represents the midpoint of the olefinic C(15)−C(16)
double bond.
with 1, probably due to steric constraints. The complexes 1-py
and 1-BnNH2 were fully characterized by spectroscopic means
and elemental analysis. The coordination of the N-donor
ligands is confirmed in the 1H NMR spectra, where the
expected set of resonances for the σ,π-cyclooctenyl ligand was
also observed. A characteristic sign for the ligand exchange
reaction is the downfield shift by 2−3 ppm of the Ir−CH−
resonance (C-1) in the 13C{1H} NMR spectra upon the
coordination of the N-donor ligands in trans positions.
The molecular structure of 1-py has been determined by an
X-ray analysis, and it is shown in Figure 6. The iridium center
exhibits a distorted-octahedral environment with links to the
tridentate meridional O,N,O-coordinated pyridinedicarboxy-
late, to the formally bidentate σ,π-cyclooctenyl ligand, and to
the nitrogen of the pyridine group. The iridium configuration is
such that the olefinic bond is situated trans to the nitrogen of
the pydc ligand, while the alkylic cyclooctenyl carbon is also
positioned trans to the pyridine group. Major distortions from
the octahedral coordination arise from the chelating units
within the tridentate pydc group (mean N−Ir−O = 78.83(9)°)
and, in a minor extension, from the chelating nature of the σ,π-
bonded cyclooctenyl ligand (C(11)−Ir−M = 84.64(15)°). The
pydc ligand is strictly planar and defines a meridional metal
coordination plane where the olefin exhibits an in-plane
conformation (line-plane angle 1.5(3°)).
The most relevant structural feature of this molecule is the
long Ir−N(2) bond length, 2.209(3) Å, a clear consequence of
the high structural trans effect of the Ir−alkyl bond. Similar Ir−
N bond distances have been observed in Ir(III) complexes
where the pyridine molecule is coordinated to the iridium trans
to a high trans-influence ligand: this is the case in [Ir(acac)2R-
(py)], where R is the cyclohexyl group (Ir−N = 2.225(6) Å,
Ir−C = 2.060(7) Å)7c or a vinyl moiety (Ir−N = 2.209(14) Å,
Ir−C = 1.97(3) Å),35 and also when the pyridine group
occupies a position trans to a hydride ligand ([Ir(PCy3)-
(Py)3H2]+; Ir−N = 2.216(2) and 2.2101(19) Å)36 or to a
metal−metal bond such as in [Ir(μ-O2CCH3)(CO)Cl(py)]2
(Ir−N = 2.200(5) Å).37
Although no iridium structure has been reported including
the pincer tridentate pydc ligand, a vast piece of Ir(III)
structural chemistry has been described with different
substituted derivatives for the N,O-bidentate pyridine-2-
carboxylate group.38 It is remarkable that in all these complexes
the Ir−N interaction (2.014−2.215 Å) is surprisingly longer
than the distance observed in 1-py (1.981(3) Å). Additionally,
the Ir−O bonds in 1-py seem also to be stronger (2.078 and
2.076(3) Å) than the usual Ir−O interaction in the bidentate
pyridinecarboxylate complexes (range 2.033−2.194 Å).38 Both
facts are solid-state evidence of the reported special stabilization
associated with the pincer ligands, particularly for pydc.11 As a
consequence of the noted strong Ir−pydc interaction and also
of the higher oxidation state of the metal atom in 1-py (in
comparison to that observed in 6), the Ir−olefin bond seems to
be very weak, as evidenced by the relatively long Ir−C bond
lengths (2.175 and 2.178(4) Å) and by the short CC
distance of 1.373(6) Å (weak π-back-donation), only slightly
elongated from the ideal value of a free double bond
(1.316(15) Å).34
Compounds [Ir(κ3-pydc)(1-κ-4,5-η-C8H13)(PPh3)] (1-
PPh3) and [Ir(κ3-pydc)(1-κ-4,5-η-C8H13)(PMe3)] (1-PMe3)
(Scheme 4) were prepared by reacting 1 with 1 equiv of the
corresponding phosphine ligand and isolated as lemon yellow
solids in excellent yields. The 31P{1H} NMR of the complexes
showed the expected singlet resonances at −7.23 (1-PPh3) and
−38.98 ppm (1-PMe3). In addition, the coordination of the
PR3 ligands trans to the iridium−alkyl bond of the cyclooctenyl
ligand is supported by the large JC−P coupling of the Ir−CH−
resonance (C-1) in the 13C{1H} NMR spectra, which was
observed at 31.72 (JC−P = 85.8 Hz) and 31.29 ppm (JC−P = 91.5
Hz), respectively. It is noticeable that an excess of PR3 does not
induce the decoordination of either the σ,π-cyclooctenyl or the
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pincer ligands, which demonstrates the outstanding stability of
the rigid molecular framework in 1.
observed in 1-py (2.176(3) Å), although the CC bond
distance does not vary accordingly (1.387(13) vs 1.373(6) Å).
A remarkable difference of this molecule in comparison with
1-py is the deviation of the olefinic carbons from the strictly
planar meridional plane described by the three donor atoms of
the pyridinedicarboxylate group and the metal atom; while in 1-
py the olefin fits reasonably in this plane (N−Ir−M =
177.02(12)°), in 1-dppm the olefin is clearly out of this
plane by a mean value of 0.322(8) Å, with a closer N−Ir−M
bond angle of 171.3(3)°. Most probably this fact should be
associated with the bulkier character of dppm in comparison to
pyridine.
As noted for the pyridine in 1-py, the high structural trans
effect of the alkylic Ir−C bond elongates the Ir−P(1) bond
length to a value of 2.458(2) Å. As far as we know, this is the
longest Ir−P bond distance observed in a Ir(III) complex
containing a PR3 phosphine situated trans (C−Ir−P > 160°) to
an alkylic sp3 carbon (range 2.276−2.410 Å, mean 2.343(2)
Å).37 If the metal complex should contain a dppm ligand, the
longest Ir−P separation (trans to an alkylic carbon) described
so far is 2.382(1) Å in [IrMe2(dppm)2][OTf].40 All these facts
evidence a weak bonding interaction between the metal and the
monodentate diphosphine.
Reactivity of [Ir(κ3-pydc)(1-κ-4,5-η-C8H13)] with Biden-
tate P-Donor Ligands. In order to induce a possible change
in the coordination mode of the σ,π-cyclooctenyl moiety in
[Ir(κ3-pydc)(1-κ-4,5-η-C8H13)] (1), we have explored its
reactivity with chelating diphosphines. Reaction of 1 with 1
equiv of bis(diphenylphosphino)methane (dppm) gave [Ir(κ3-
pydc)(1-κ-4,5-η-C8H13)(κ1-dppm)] (1-dppm), which features a
monocoordinated dppm ligand (Scheme 5). The compound,
which was obtained as a lemon yellow solid in 87% yield, has
been fully characterized in solution by spectroscopic methods
and in the solid state by a single-crystal X-ray analysis. The
presence of a κ1-dppm coordinated ligand became evident in
the 31P{1H} NMR spectrum, which showed two strongly
coupled doublet resonances at −17.47 and −27.81 ppm (JP−P
=
70.7 Hz) corresponding to the coordinated and uncoordinated
P donor atoms, respectively.39 On the other hand, the
coordination of the dppm ligand trans to the Ir−CH− bond
of the σ,π-cyclooctenyl ligand was derived from the large JC−P
coupling of 86.2 Hz observed for the C-1 resonance at 31.57
ppm in the 13C{1H} NMR spectrum. In addition, the >CH2
resonance of the dppm ligand was observed as a doublet of
doublets at 23.96 ppm with JC−P couplings of 31.2 and 16.7 Hz.
The molecular structure of 1-dppm is shown in Figure 7.
This molecule resembles quite well that of 1-py, in which the
Reaction of 1 with 1 equiv of 1,2-bis(diphenylphosphino)-
ethane (dppe) gave a yellow solid that consists of roughly a 1/1
mixture of [Ir(κ3-pydc)(1-κ-4,5-η-C8H13)(κ1-dppe)] (1-dppe)
and the dinuclear compound [Ir(κ3-pydc)(1-κ-4,5-η-
C8H13)]2(μ-dppe) (12-dppe), having a dppe ligand bridging
two [Ir(κ3-pydc)(1-κ-4,5-η-C8H13)] fragments (Scheme 6).
Scheme 6. Dinuclear Complexes Derived from 1
Figure 7. Molecular structure of [Ir(κ3-pydc)(1-κ-4,5-η-C8H13)(κ1-
dppm)] (1-dppm). Bond distances (Å) and angles (deg): Ir−O(1) =
2.074(6), Ir−O(4) = 2.057(6), Ir−N(1) = 1.975(6), Ir−P(1) =
2.458(2), Ir−C(33A) = 2.108(14), Ir−C(37) = 2.225(9), Ir−C(38) =
2.204(8), C(37)−C(38) = 1.387(13); O(1)−Ir−O(4) = 157.1(2),
O(1)−Ir−N(1) = 78.3(3), O(4)−Ir−N(1) = 78.8(3), N(1)−Ir−M =
171.3(3), P(1)−Ir−C(33A) = 167.6(5), C(33A)−Ir−M 81.7(5). M
represents the midpoint of the olefinic C(37)−C(38) double bond;
bond parameters involving C(33) are only expressed for the
disordered carbon atom of higher occupancy.
The 31P{1H} NMR (121.49 MHz) spectrum of this mixture in
CD2Cl2 at room temperature showed a set of three broad
resonances, suggesting a fluxional behavior. However, the
spectrum at 223 K featured well-defined resonances. The two
coupled resonances at −12.12 and −14.17 ppm (JP−P = 32 Hz)
were assigned to 1-dppe and the two signals at −10.89 and
−11.09 ppm to the two diastereoisomers of 12-dppe (see
below). Most probably, both species are in a dppe-mediated
dynamic equilibrium41 that causes resonances to broaden at
room temperature:
pyridine group has been substituted by a monodentate P-
bonded dppm ligand. The complex exhibits an analogous
octahedral coordination, with the same stereodistribution of
ligands and statistically identical molecular parameters for the
pydc and for the σ,π-bonded cyclooctenyl ligands. Only the Ir−
C bond distances of the coordinated olefinic group are
significantly longer (mean 2.215(6) Å in 1-dppm) than those
2 1‐dppe ⇄ 12‐dppe + dppe
J
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The dinuclear complex 12-dppm has been isolated from the
solution obtained after dissolving a solid mixture of 1 and 1/2
equiv of dppm in CH2Cl2/MeOH (3/1). This method has also
revealed to be useful for the synthesis of the compounds 12-
dppe and 12-dppp using the corresponding diphosphines. The
complexes have been obtained as yellow solids in yields of over
80% and fully characterized by elemental analysis, mass spectra,
and multinuclear NMR spectroscopy. The ESI spectra of the
three compounds showed the molecular ions of the
mononuclear complexes [1-diphos]+ resulting from the loss
of one of the iridium fragments. However, the ESI spectrum of
12-dppe showed the molecular ion at m/z 1329.2 with the right
isotopic distribution.
The C-1 atom of the σ−π cyclooctenyl ligand in 1 is a
stereogenic center, and thus, compound 1 is chiral and exists as
a pair of enantiomers, namely (RC)-1 and (SC)-1. The assembly
of the dinuclear compounds 12-(μ-diphos) can produce three
stereoisomers: the enantiomeric pair [(RC)-1]2(μ-diphos)/
[(SC)-1]2(μ-diphos) (indistinguishable by NMR) and [(RC)-
1][(SC)-1](μ-diphos), with C2 and Cs symmetries, respectively
(Scheme 5). As expected, the dinuclear compounds exist as a
1/1 mixture of both diastereoisomers, which have been
observed in the 31P{1H} NMR spectra as two single resonances
(see the Experimental Section). In addition, the C-1 resonances
of the equivalent σ,π-cyclooctenyl ligands in both diaster-
eoisomers of 12-dppe were observed as doublets of doublets at
32.55 and 32.41 ppm in the 13C{1H} NMR spectrum with JC−P
coupling constants of 90.0 and 2.2 Hz, which further supports
the proposed structure.
bis(diphenylphosphino)methane ligand the nuclearity can be
modulated and a mononuclear complex having a κ1-dppm
ligand has been also prepared.
EXPERIMENTAL SECTION
■
General Considerations. All experiments were carried out under
an atmosphere of argon using Schlenk techniques or a glovebox.
Solvents were obtained from a Solvent Purification System (Innovative
Technologies). CD2Cl2 and DMSO-d6 (Euriso-top) were dried using
activated molecular sieves. Methanol-d4 (<0.02% D2O, Euriso-top)
was used as received. Elemental analyses were carried out with a
PerkinElmer 2400 CHNS/O analyzer. NMR spectra were recorded on
Bruker AV-400 and AV-300 spectrometers. Chemical shifts are
reported in ppm relative to tetramethylsilane and referenced to
partially deuterated solvent resonances. Coupling constants (J) are
given in hertz. Spectral assignments were achieved by a combination of
1H−1H COSY, 13C DEPT, APT, 1H−13C HSQC, and 1H−13C HMBC
experiments. The numbering scheme used in the NMR data of the
compounds is shown in Figure 1. Electrospray mass spectra (ESI-MS)
were recorded on a Bruker MicroTof-Q instrument using sodium
formate as reference. MALDI-TOF mass spectra were obtained on a
Bruker Microflex mass spectrometer using DCTB (trans-2-[3-(4-tert-
butylphenyl)-2-methyl-2-propenylidene]malononitrile) or dithranol as
the matrix. Standard literature procedures were used to prepare the
starting materials [Ir(μ-OMe)(cod)]2,42 [Ir(μ-OH)(coe)2]2,43 and
[Ir(cod)(NCCH3)2]PF6.44 Pyridine-2,6-dicarboxylic acid (H2pydc)
and N-methyliminodiacetic and N-phenyliminodiacetic acids were
obtained from Fluka, Aldrich, and Acros, respectively, and used as
received. 4-Bromopyridine-2,6-dicarboxylic acid (Hpydc-Br) was
prepared from chelidamic acid (4-hydroxypyridine-2,6-dicarboxylic
acid, Fluka) following the procedure described in the literature.45
Lithium Pyridine-2,6-dicarboxylate (Li2pydc). To a suspension
of 2,6-pyridinedicarboxylic acid (0.268 g, 1.60 mmol) in diethyl ether
(20 mL) was added dropwise a 1.6 M solution of n-BuLi in hexane
(2.0 mL, 3.2 mmol) with stirring at 195 K. The reaction mixture was
warmed to room temperature within 1 h. During this time, the
suspension turned from white to pale yellow. The resulting suspension
was concentrated, decanted, and filtered to afford an off-white solid,
which was washed with diethyl ether (3 × 10 mL) and dried in vacuo.
CONCLUSIONS
■
The synthesis of the unusual σ,π-cyclooctenyl iridium(III)
pincer compounds [Ir(κ3-pydc-X)(1-κ-4,5-η-C8H13)] can be
accomplished by several routes, involving the corresponding 4-
substituted 2,6-pyridinedicarboxylic acids or their lithium salts
and standard mono- or dinuclear iridium complexes. An
investigation of the mechanism of the reaction has shown the
initial formation of the species [Ir(κ2-Hpydc)(cod)] having a
κ2-N,O coordinated monodeprotonated ligand. Deuterium
labeling studies in combination with theoretical calculations at
the DFT level have shown that the formation of the σ,π-
cyclooctenyl iridium(III) complexes involves a metal-mediated
proton transfer to the 1,5-cyclooctadiene ligand through the
methanol-stabilized hydrido species [IrH(κ2-pydc)(cod)-
(CH3OH)]. Remarkably, the formation of this key hydrido
intermediate results from the solvent-assisted proton transfer
through a hydrogen-bonding network involving a carboxyl
group, a methanol molecule, and the iridium center, forming an
eight-membered metallacycle.
1
Yield: 0.279 g (97%). H NMR (400.16 MHz, 298 K, DMSO-d6): δ
8.24 (m, 2H), 8.18 (m, 1H).
Lithium 6-Carboxypicolinate (HLipydc). 2,6-Pyridinedicarbox-
ylic acid (0.268 g, 1.60 mmol) and n-BuLi in hexane (1.0 mL, 1.6
mmol) were reacted in diethyl ether (20 mL) at 195 K. Workup as
described above gave the salt as an off-white solid. Yield: 0.263 g
1
(95%). H NMR (400.16 MHz, 298 K, DMSO-d6): δ 13.38 (br, 1H,
−COOH), 8.23 (m, 2H), 8.17 (m, 1H).
[Ir(κ3-pydc)(1-κ-4,5-η-C8H13)] (1). Method A. [Ir(μ-OMe)(cod)]2
(0.132 g, 0.200 mmol) and H2pydc (0.068 g, 0.40 mmol) were reacted
in a CH2Cl2/MeOH mixture (3/1, 20 mL) for 14 h to give a bright
yellow solution. The solution was brought to dryness in vacuo and the
crude compound dissolved in a CH2Cl2/MeOH (20/1) mixture (2
mL) and then eluted through an alumina column (5 × 1.5 cm) to give
a yellow solution. Concentration of the solution to ca. 1 mL and slow
addition of diethyl ether gave the compound as a yellow solid, which
was filtered, washed with diethyl ether (2 × 3 mL), and dried in vacuo.
Yield: 0.170 g (85%).
The application of this synthetic methodology with related
dicarboxylic acid precursors to dianionic tridentate pincer ONO
complexes has shown a narrow scope. However, the reactivity
of some iminodiacetic acid derivatives, RN(CH2COOH)2 (R =
Me, Ph), has allowed us to identify the acidity (pKa) and the
rigidity of the potential tridentate ligand precursor as the key
factors leading to σ,π-cyclooctenyl iridium(III) complexes.
[Ir(κ3-pydc)(1-κ-4,5-η-C8H13)] behaves as an unsaturated
species, and coordination of some monodentate N- and P-
donor ligands has led to the preparation of octahedral
complexes. The stability of the rigid molecular framework has
allowed for the preparation of diphosphine-bridged dinuclear
assemblies that have been obtained as a mixture of two
diastereoisomers. Interestingly, in the case of the short-bite
Method B. A solid mixture of [Ir(cod)(CH3CN)2]PF6 (0.106 g,
0.200 mmol) and LiHpydc (0.035 g, 0.200 mmol) was dissolved in a
CH2Cl2/MeOH mixture (3/1, 20 mL). The resulting red solution was
stirred for 14 h to give a bright yellow solution. The solution was
filtered and then concentrated under vacuum to ca. 5 mL. Slow
addition of diethyl ether (20 mL) gave the compound as a yellow solid
that was filtered, washed with diethyl ether, and dried under vacuum.
Yield: 0.096 g (96%).
Method C. [Ir(cod)(CH3CN)2]PF6 (0.106 g, 0.200 mmol) and
Li2pydc (0.036 g, 0.200 mmol) were reacted in a CH2Cl2/MeOH
mixture (3/1, 20 mL) for 14 h to give a bright yellow solution.
Workup as described above (method B) afforded the compound as a
K
dx.doi.org/10.1021/om400767d | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
yellow solid. Yield: 0.090 g (90%). The analytical and spectroscopic
data evidenced that compound 1 was actually isolated as the methanol
solvate 1·MeOH.11 Anal. Found: C, 38.42; H, 3.98; N, 2.75. Calcd for
C15H16IrNO4.CH3OH: C, 38.54; H, 4.04; N, 2.81 (method B).
[Ir(κ3-pydc)(1-κ-4,5-η-C8H12D)] (1-d1). A solid mixture of [Ir-
(cod)(CH3CN)2]PF6 (0.053 g, 0.10 mmol) and [Li2pydc] (0.018 g,
0.10 mmol) was dissolved in CD2Cl2/CD3OD/D2O (4.2 mL, 3/1/0.1,
respectively). The solution stirred at room temperature for 14 h and
then directly analyzed by NMR. 1H NMR (400.16 MHz, 298 K,
CD2Cl2/CD3OD): δ 8.35 (t, JH−H = 7.8 Hz, 1H, 4-H), 8.18 (dd, JH−H
= 7.8 and 1.2 Hz, 1H, 3-H or 5-H), 8.12 (dd, JH−H = 7.8 and 1.2 Hz,
1H, 3-H or 5-H) (pydc), 5.81 (dt, JH−H = 9.1 and 3.0 Hz, 1H, 4-H),
5.59 (d, JH−H = 9.9 Hz, 1H, 5-H), 3.13 (d, JH−H = 15.9 Hz, 1H, 6-H),
2.35 (m, JH−H = 11.0 and 4.3 Hz, 1H, 3-H), 2.29−2.13 (bm, 2H, 3-H
for C84H130Ir4N2O8·2CH3OH: C, 48.51; H, 6.53; N, 1.32. Crystals
suitable for X-ray diffraction were obtained by layering a solution of
[Ir(μ-OH)(coe)2]2 in dichloromethane over a solution of N-
phenyliminodiacetic acid in methanol at room temperature.
[Ir(κ3-pydc)(1-κ-4,5-η-C8H13)(py)] (1-py). The compound was
prepared by refluxing a solution of [Ir(κ3-pydc)(1-κ-4,5-η-C8H13)] (1)
in neat pyridine and isolated as a yellow solid in 85% yield.11 Good
quality microcrystals for X-ray diffraction were grown by slow diffusion
of diethyl ether into a dichloromethane solution of 1-py at 258 K.
[Ir(κ3-pydc)(1-κ-4,5-η-C8H13)(BnNH2)] (1-BnNH2). Benzylamine
(2 mL) was added to a suspension of [Ir(κ3-pydc)(1-κ-4,5-η-C8H13)]
(1·MeOH; 0.100 g, 0.200 mmol) in CH2Cl2 (20 mL). The reaction
mixture was heated at 90 °C for 3 h with stirring to give a yellow
solution. The solution was cooled to room temperature and
evaporated to dryness in vacuo. Crystallization from CH2Cl2/Et2O
at low temperature afforded the compound as lemon yellow
microcrystals. Yield: 0.100 g (87%). Anal. Found: C, 46.23; H, 4.54;
N, 4.95. Calcd for C22H25IrN2O4: C, 46.06; H, 4.39; N, 4.88. 1H NMR
(400.16 MHz, 298 K, CD2Cl2): δ 8.08 (t, 1H, JH−H = 8.0 Hz, 4-H),
7.96 (dd, 1H, JH−H = 7.3 and 1.6 Hz), 7.88 (dd, 1H, JH−H = 7.3 and 1.6
Hz) (3-H and 5-H) (pydc), 7.32−7.22 (m, 4H, −NH2 and Bn), 7.06
(m, 2H, Bn), 6.93 (m, 1H, Bn) (BnNH2), 5.76 (td, 1H, JH−H = 9.5 and
3.6 Hz, 4-H), 5.41 (d, 1H, JH−H = 8.0 Hz, 5-H) (C8H13), 3.18 (m, 2H,
>CH2, Bn), 3.02 (d, 1H, JH−H = 16.1 Hz), 2.35 (m, 1H), 2.14−2.03
(m, 2H), 1.88−1.73 (m, 2H), 1.50 (d, 1H, JH−H = 13.2 Hz), 1.37 (m,
1H), 0.89 (t, 1H, JH−H = 13.9 Hz), 0.60 (dd, 1H, JH−H = 13.2 and 9.2
Hz), 0.27 (m, 1H) (C8H13). 13C{1H} NMR (100.62 MHz, 298 K,
CD2Cl2): δ 176.36, 176.06 (CO), 148.48, 147.46 (C-2 and C-6)
(pydc), 141.44 (Bn), 140.83 (C-4), 132.07, 131.72 (C-3 and C-5)
(pydc), 131.36 (Cm), 121.27 (Cp), 130.95 (Co) (Bn), 90.20 (C-4),
85.72 (C-5) (pydc), 45.89 (>CH2, Bn), 41.39 (C-2), 35.47 (C-8),
28.80 (C-6), 28.64 (C-3), 25.94 (C-7), 18.82 (C-1) (C8H13). MS
(MALDI-TOF, DCTB, CH2Cl2): m/z 467.9 (M+ − BnNH2).
[Ir(κ3-pydc)(1-κ-4,5-η-C8H13)(PR3)]. A solid mixture of [Ir(κ3-
pydc)(1-κ-4,5-η-C8H13)] (1·MeOH; 0.100 g, 0.200 mmol) and the
corresponding PR3 (0.200 mmol) was dissolved in CH2Cl2/MeOH
(3/1, 10 mL), and the solution was stirred at room temperature for 14
h. The resulting solution was evaporated to dryness. The residue was
subsequently dissolved in a minimum volume of dichloromethane, and
then diethyl ether was added to give the compounds as lemon yellow
solids, which were filtered, washed with diethyl ether (3 × 10 mL), and
dried under vacuum.
and 6-H), 2.01−1.82 (bm, 3H, 1-H, 2-H and 7-H), 1.59 (bd, JH−H
=
14.1 Hz, 1H, 7-H), 0.39 (dd, JH−H = 12.3 and 3.6 Hz, 1H, 2-H), 0.01
ppm (d, JH−H = 3.3 Hz, 1H, 8-H) (C8H12D). 13C{1H} NMR (75.48
MHz, 298 K, CD2Cl2/CD3OD): δ 175.55, 175.22 (CO), 148.31,
147.17 (C-2 and C-6), 141.30 (C4) (pydc), 89.53 (C-4), 85.08 (C-5),
40.69 (C-2), 34.73 (t, JC‑D = 19.0 Hz, C-8), 27.83 (C-6), 26.29 (C-3),
2
25.07 (C-7), 15.09 (C-1) (1-κ-4,5-η-C8H12D). H NMR (61.42 MHz,
298 K, CH2Cl2/MeOH): δ 1.21 (s, 1D, C8H12D). MS (MALDI-TOF,
DCTB, CH2Cl2/MeOH): m/z 467.9 (M+), 316.4 (M+ − pydc).
[Ir(κ3-pydc-Br)(1-κ-4,5-η-C8H13)] (3). H2pydc-Br (0.246 g, 1.00
mmol) and [Ir(μ-OMe)(cod)]2 (0.331 g, 0.500 mmol) were reacted in
CH2Cl2/MeOH (40 mL, 5:1) for 14 h at room temperature to give a
red solution. Work up as described above for 1 (method A) gave the
compound as red solid. Yield: 0.532 g (94%). Anal. Found: C, 32.05;
H, 2.92; N, 2.53. Calcd for C15H15BrIrNO4·H2O: C, 31.98; H, 3.04; N,
1
2.49. H NMR (400.16 MHz, 298 K, CD2Cl2/CD3OD): δ 8.28 (s,
1H), 8.24 (s, 1H) (3-H and 5-H, pydc-Br), 5.87 (td, JH−H = 8.8 and
3.7 Hz, 1H, 4-H), 5.64 (br, 1H, 5-H), 3.17 (d, JH−H = 15.9 Hz, 1H, 6-
H), 2.40−2.10 (m, 3H, 3-H and 6-H), 2.01 (m, 1H, 1-H), 1.97 (d,
JH−H = 9.4 Hz, 1H, 2-H), 1.85 (dt, JH−H = 14.1 and 4.6 Hz, 1H, 7-H),
1.63 (t, JH−H = 6.08 Hz, 1H, 7-H), 0.94 (tt, JH−H = 14.6 and 2.4 Hz,
1H, 8-H), 0.40 (dd, JH−H = 14.2 and 9.22, 1H, 2-H), 0.03 (dm, JH−H
=
13.8 Hz, 1H, 8-H) (C8H13). 13C{1H} NMR (75.48 MHz; 298 K;
CD2Cl2/CD3OD): δ 172.8 (CO), 147.7, 146.6 (C-2 and C-6), 133.1,
132.8 (C-3 and C-5), 135.6 (C-4) (pydc-Br), 89.0 (C-4), 85.0 (C-5),
39.6 (C-2), 34.1 (C-8), 26.8 (C-6), 25.3 (C-3), 24.0 (C-7), 14.6 (C-1)
(C8H13). MS (MALDI-TOF, DCTB, CH2Cl2): m/z 527.2 (M+).
[Ir{κ2-MeN(CH2COOH)(CH2COO)}(cod)] (5). A solid mixture of
[Ir(μ-OMe)(cod)]2 (0.041 g, 0.062 mmol) and N-methyliminodi-
acetic acid (0.018 g, 0.124 mmol) was dissolved in THF (15 mL) and
the solution stirred for 3 h at 50 °C. The solvent was removed under
vacuum, and the orange residue was washed with hexane (2 × 2 mL)
and then dried in vacuo. Yield: 0.047 g (85%). 1H NMR (400.16 MHz,
CD2Cl2/CD3OD, 298 K): δ 5.00 (m, 1H), 4.85 (m, 1H), 4.63 (m,
1H), 4.32 (m, 1H) (CH, cod), 3.83 (AB q, 2H, δA 3.90, δB 3.76, JAB
= 16.1 Hz, JAC = 1.8 Hz, >CH2), 3.68 (CD q, 2H, δC 3.79, δD 3.56, JCD
= 16.6 Hz, JAC = 1.8 Hz, >CH2), 2.89 (s, 3H, −CH3), 2.80−2.67 (m,
2H), 2.64−2.55 (m, 2H), 2.46 (m, 1H), 2.23 (m, 1H), 1.91 (m, 1H),
1.56 (m, 1H) (>CH2, cod). 13C{1H} NMR (75.48 MHz, CD2Cl2/
CD3OD, 298 K): δ 181.3, 180.4 (CO), 87.1, 82.9, 77.7, 76.4 (=CH,
cod), 71.2 (>CH2), 69.0 (>CH2), 56.4 (−CH3), 34.7, 31.5, 30.2, 26.1
(>CH2, cod).
[Ir(κ3-pydc)(1-κ-4,5-η-C8H13)(PPh3)] (1-PPh3). Yield: 0.137 g (96%).
Anal. Found: C, 54.47; H, 4.32; N 1.98. Calcd for C33H31IrNO4P: C,
1
54.38; H, 4.29; N 1.92. H NMR (400.16 MHz, 298 K, CD2Cl2): δ
7.81 (t, 1H, JH−H = 8.2 Hz, 4-H), 7.66 (dd, 1H, JH−H = 7.8 and 1.2
Hz), 7.57 (dd, 1H, JH−H = 7.8 and 1.2 Hz) (3-H and 5-H) (pydc), 7.32
(tq, 3H, JH−H = 7.3 and 1.5 Hz), 7.21 (td, 6H, JH−H = 7.3 and 2.0 Hz),
7.13 (tt, 6H, JH−H = 8.3 and 1.3 Hz) (PPh3), 5.70 (t, 1H, JH−H = 6.8
Hz, 4-H), 5.61 (t, 1H, JH−H = 8.6 Hz, 5-H), 2.95 (d, 1H, JH−H = 16.9
Hz, 6-H), 2.35−2.48 (m, 1H, 3-H), 2.20−2.30 (m, 2H, 3-H and 6-H),
2.11 (m, 1H, 2-H), 1.84 (dtt, 1H, JH−H = 18.1, 13.9, and 4.3 Hz, 7-H),
1.59 (d, 1H, JH−H = 13.9 Hz, 7-H), 1.46 (q, 1H, JH−H = 3.3 Hz, 1-H),
1.34 (td, 1H, JH−H = 3.3 and 13.9 Hz, 8-H), 0.89 (m, 1H, 2-H), 0.65
(m, 1H, 8-H) (C8H13). 13C{1H} NMR (75.479 MHz, 298 K, CD2Cl2):
δ 172.07, 171.48 (CO), 145.81, 144.63 (C-2 and C-6), 137.56 (C-4)
(pydc), 133.77 (d, JC−P = 10.5 Hz, Co), 130.47 (Cp) (PPh3), 129.40,
129.10 (C-3 and C-5, pydc), 128.82 (d, JC−P = 8.9 Hz, Cm), 128.63 (d,
JC−P = 60.0 Hz, Cipso) (PPh3), 90.67 (C-4), 82.15 (C-5), 37.59 (d, JC−P
[Ir4{κ4-PhN(CH2COO)2}2(cod)4] (6). A solution of N-phenyl-
iminodiacetic acid (0.035 g, 0.167 mmol) in methanol (3 mL) was
added to a solution of [Ir(μ-OMe)(cod)]2 (0.111 g, 0.167 mmol) in
dichloromethane (3 mL) to give a red solid in 1 min. The solution was
decanted, and the solid was washed with methanol (2 × 3 mL) and
dried under vacuum. Yield: 0.108 g (80%). Anal. Found: C, 38.46; H,
4.10; N, 1.76. Calcd for C52H66Ir4N2O8: C, 38.65; H, 4.12; N, 1.73.
[Ir4{κ4-PhN(CH2COO)2}2(coe)8] (7). A solution of N-phenyl-
iminodiacetic acid (0.014 g, 0.068 mmol) in methanol (2 mL) was
added to a solution of [Ir(μ-OH)(coe)2]2 (0.058 g, 0.068 mmol) in
dichloromethane (2 mL). After 1 min a yellow-orange solid
precipitated from the solution. The solution was decanted, and the
solid was washed with methanol (2 × 3 mL) and dried under vacuum.
Yield: 0.066 g (90%). Anal. Found: C, 48.86; H, 6.35; N, 1.35. Calcd
= 3.8 Hz), 32.46 (d, JC−P = 3.4 Hz) (C-2 and C-8), 31.72 (d, JC−P
=
85.8 Hz, C-1), 27.33 (d, JC−P = 7.2 Hz, C-7), 26.11, 23.39 (C-3 and C-
6) (C8H13). 31P{1H} NMR (161.99 MHz, 298 K, CD2Cl2): δ −7.23
(s). MS (MALDI-TOF, DCTB, CH2Cl2): m/z 467.9 (M+ − PPh3).
[Ir(κ3-pydc)(1-κ-4,5-η-C8H13)(PMe3)] (1-PMe3). Yield: 0.098 g
(90%). Anal. Found: C, 39.74; H, 4.69; N, 2.54. Calcd for
1
C18H25IrNO4P: C, 39.84; H, 4.64; N 2.58. H NMR (400.16 MHz,
298 K, CD2Cl2): δ 8.07 (t, 1H, JH−H = 7.5 Hz, 4-H), 8.02 (dd, 1H,
JH−H = 9.4 and 1.4 Hz), 7.97 (dd, 1H, JH−H = 9.4 and 1.4 Hz) (3-H
and 5-H) (pydc), 5.86 (t, 1H, JH−H = 8.8 Hz, 4-H), 5.55 (t, 1H, JH−H
=
8.8 Hz, 5-H), 3.00 (d, 1H, JH−H = 16.9 Hz), 2.53 (m, 1H), 2.25 (m,
L
dx.doi.org/10.1021/om400767d | Organometallics XXXX, XXX, XXX−XXX

Organometallics
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1H), 2.15−1.97 (m, 2H), 1.83 (dtt, 1H, JH−H = 13.9, 18.3, and 3.7
Hz), 1.56 (m, 1H), 1.29−1.15 (m, 2H), 0.97 (m, 1H) (C8H13), 0.87
(d, 9H, JH−P = 14.3 Hz, PMe3), 0.79 (m, 1H, C8H13). 13C{1H} NMR
(100.62 MHz, 298 K, CD2Cl2): δ 172.58 (d, JC−P = 2.2 Hz), 172.05 (d,
[Ir(κ3-pydc)(1-κ-4,5-η-C8H13)]2(μ-dppe) (12-dppe). Yield: 0.113 g
(85%). Anal. Found: C, 50.39; H, 3.98; N, 2.01. Calcd for
C56H56Ir2N2O8P2: C, 50.52; H, 4.24; N, 2.10. 1H NMR (400.16
MHz, 298 K, CD2Cl2): δ 7.81 (t, 1H, JH−H = 7.8 Hz), 7.79 (t, 1H, JH−H
JC−P = 2.9 Hz) (CO), 146.09, 145.13 (C-2 and C-6), 138.34 (d, JC−P
=
= 7.8 Hz), 7.69 (dd, 1H, JH−H = 7.8 and 1.3 Hz), 7.60 (dd, 1H, JH−H =
2.2 Hz, C-4), 129.82 (d, JC−P = 2.2 Hz), 129.46 (t, J = 2.9 Hz) (C-3
and C-5) (pydc), 87.93 (d, JC−P = 1.5 Hz, C-4), 82.44 (C-5), 37.17 (d,
JC−P = 4.4 Hz), 32.54 (d, JC−P = 3.7 Hz) (C-2 and C-8), 31.29 (d, JC−P
= 91.5 Hz, C-1), 28.42 (d, JC−P = 8.1 Hz, C-7), 26.64, 23.83 (C-3 and
C-6) (C8H13), 9.46 (d, J = 22.0 Hz, PMe3). 31P{1H} NMR (161.99
MHz, 298 K, CD2Cl2): δ −38.98 (s). MS (MALDI-TOF, DCTB,
CH2Cl2): m/z 467.9 (M+ − PMe3).
7.8 and 1.3 Hz), 7.57 (dd, 1H, JH−H = 7.8 and 1.3 Hz), 7.50 (dd, 1H,
JH−H = 7.8 and 1.3 Hz) (pydc), 7.40−7.10 (set of m, 12H), 6.97 (t,
2H, JH−H = 8.4 Hz), 6.87 (m, 4H), 6.79 (t, 2H, JH−H = 8.1 Hz) (Ph,
dppe), 5.76 (t, 1H, JH−H = 8.8 and 2.8 Hz), 5.66 (br m, 3H) (=CH,
C8H13), 2.99 (d, 2H, J = 16.2 Hz), 2.43 (m, 2H), 2.13 (m, 4H), 2.0
(m, 2H), 1.90−1.65 (set of m, 6H), 1.59 (m, 2H), 1.35−1.15 (m, 4H),
0.80 (m, 2H), 0.63 (m, 2H) (>CH2, C8H13 and dppe). 13C{1H} NMR
(100.63 MHz, 298 K, CD2Cl2): δ 171.18, 172.14, 171.63, 171.56
(CO), 145.72, 144.58 (C-2 and C-6), 138.16, 138.07 (C-4) (pydc),
133.0−132.23, 131.05, 130.91, 130.79, 130.60, 129.69−129.47 (Ph,
dppe), 129.47, 129.40, 129.19, 129.12 (C-3 and C-5, pydc), 127.50−
126.66 (Cipso, Ph, dppe), 90.13, 89.45, 82.06, 81.45 (C-4 and C-5,
C8H13), 37.47, 37.26 (>CH2), 32.55 (dd, JC−P = 90.0 and 2.2 Hz, C-1),
32.56, 32.39 (>CH2), 32.41 (dd, JC−P = 90.0 and 2.2 Hz, C-1)
(C8H13), 27.89 (t, JC−P = 3.6 Hz, >CH2), 27.73 (t, JC−P = 3.6 Hz,
>CH2) (dppe), 26.43, 26.33, 23.51, 23.49, 17.04 (d, JC−P = 7.3 Hz),
16.79 (d, JC−P = 6.6 Hz) (>CH2, C8H13). 31P{1H} NMR (161.99 MHz,
298 K, CD2Cl2): δ −13.68 (s), −13.82 (s). MS (MALDI-TOF, DCTB,
[Ir(κ3-pydc)(1-κ-4,5-η-C8H13)(κ1-dppm)] (1-dppm). A solution
of [Ir(κ3-pydc)(1-κ-4,5-η-C8H13)] (1·MeOH) (0.100 g, 0.200 mmol)
in CH2Cl2/MeOH (3/1, 10 mL) was slowly cannulated into a solution
of dppm (0.077 g, 0.20 mmol) in CH2Cl2 (5 mL). The reaction
mixture was stirred at room temperature for 14 h and then evaporated
to dryness. The residue was dissolved in the minimum volume of
dichloromethane, and then diethyl ether was added to give a lemon
yellow solid, which was filtered, washed with diethyl ether (3 × 10
mL), and dried under vacuum. Yield: 0.148 g (87%). Crystals suitable
for X-ray diffraction were grown by slow diffusion of diethyl ether into
a dichloromethane solution of 1-dppm at 258 K. Anal. Found: C,
56.62; H, 4.32; N, 1.42. Calcd for C40H38IrNO4P2: C, 56.46; H, 4.50;
N, 1.65. 1H NMR (400.16 MHz, 298 K, CD2Cl2): δ 7.65 (td, 1H, JH−H
= 7.7 and 1.3 Hz, 4-H), 7.58 (dd, 1H, JH−H = 7.8 and 1.3 Hz), 7.45
(dd, 1H, JH−H = 7.8 and 1.3 Hz) (3-H and 5-H) (pydc), 7.19−7.04 (m,
14H, Ph), 6.99 (m, 2H, Ph), 6.91 (m, 2H, Ph), 6.84 (m, 2H, Ph)
(dppm), 6.43 (t, 1H, JH−H = 7.8 Hz, 4-H), 6.16 (t, 1H, JH−H = 8.6 Hz,
5-H), 3.06 (d, 1H, JH−H = 20.0 Hz) (C8H13), 3.02 (d, 2H, JH−H = 6.8
Hz, >CH2, dppm), 2.48 (m, 1H), 2.30−2.18 (m, 2H), 2.05 (m, 1H),
1.87 (m, 1H), 1.61 (d, 1H, JH−H = 14.9 Hz), 1.38−1.25 (bm, 2H), 0.88
(m, 1H), 0.65 (m, 1H) (C8H13). 13C{1H} NMR (75.479 MHz, 298 K,
CD2Cl2): δ 172.30, 171.58 (CO), 145.74 (d, JC−P = 1.6 Hz), 144.30 (d,
JC−P = 1.6 Hz) (C-2 and C-6) (pydc), 138.12 (d, JC−P = 32.6 Hz),
138.04 (d, JC−P = 31.2 Hz), 137.94 (d, JC−P = 32.4 Hz), 137.85 (d, JC−P
= 30.9 Hz) (Cipso, dppm), 137.15 (d, JC−P = 1.6 Hz, C-4) (pydc),
133.72 (d, JC−P = 10.3 Hz), 133.70 (d, JC−P = 10.4 Hz), 133.07, 132.78,
132.40, 132.27 (d, JC−P = 10.0 Hz), 132.26 (d, JC−P = 9.9 Hz), 132.13
(dppm), 130.44 (d, JC−P = 2.0 Hz), 129.60 (d, JC−P = 2.0 Hz) (C-3 and
C-5) (pydc), 129.05, 128.68, 128.56 (d, JC−P = 7.8 Hz), 128.55, 128.40
(d, JC−P = 8.3 Hz), 128.39 (d, JC−P = 8.5 Hz), 128.23 (dppm), 89.64
(d, JC−P = 6.7 Hz, C-4), 81.72 (d, JC−P = 7.8 Hz, C-5) (pydc), 37.09 (d,
JC−P = 5.9 Hz), 32.24 (d, JC−P = 3.3 Hz) (C-2 and C-8), 31.57 (d, JC−P
= 86.2 Hz, C-1), 27.55 (d, JC−P = 7.3 Hz, C-7), 26.07 (C-3 or C-6)
(C8H13), 23.96 (dd, JC−P = 31.2 and 16.7 Hz, >CH2, dppm), 23.66 (C-
3 or C-6) (C8H13). 31P{1H} NMR (161.99 MHz, 298 K, CD2Cl2): δ
−17.47 (d, JP−P = 70.7 Hz), −27.81 (d, JP−P = 70.7 Hz). MS (MALDI-
+
CH2Cl2): m/z 989.2 [Ir(dppe)2 ], 699.1 [Ir(C8H13)(dppe)+]. MS
(ESI, CH3CN): m/z 1329.2 [M]+, 864.2 (M+ − 1).
[Ir(κ3-pydc)(1-κ-4,5-η-C8H13)]2(μ-dppp) (12-dppp). Yield: 0.112 g
(83%). Anal. Found: C, 50.45; H, 4.31; N, 2.21. Calcd for
C57H58Ir2N2O8P2: C, 50.88; H, 4.34; N, 2.08. 1H NMR (400.16
MHz, 298 K, CD2Cl2): δ 7.73 (t, 1H, JH−H = 7.6 Hz,), 7.72 (t, 1H,
JH−H = 7.6 Hz) (4-H, pydc), 7.64 (dd, 1H, JH−H = 7.8 and 1.3 Hz),
7.59 (dd, 1H, JH−H = 7.6 and 1.3 Hz), 7.46 (br d, 1H, JH−H = 7.6 Hz),
7.41 (br d, 1H, JH−H = 7.3 Hz) (H-3 and H-5, pydc), 7.28 (t, 2H, JH−H
= 7.3 Hz,), 7.19−7.13 (set of m, 6H), 7.11−7.01 (set of m, 8H), 6.91
(t, 2H, JH−H = 8.32 Hz), 6.79 (t, 2H, JH−H = 8.4 Hz) (Ph, dppp), 5.77
(m, 2H), 5.66 (br t, 1H, JH−H = 8.4 Hz), 5.58 (t, 1H, JH−H = 8.4 Hz)
(H-4 and H-5, pydc), 2.88 (d, 1H, JH−H = 16.7 Hz), 2.79 (d, 1H, JH−H
= 16.2 Hz), 2.42−2.37 (m, 3H), 2.28−2.10 (m, 6H), 2.05−1.90 (m,
4H), 1.88−1.70 (m, 3H), 1.51 (t, 2H, JH−H = 12.1 Hz), 1.27 (m, 4 H),
0.91−0.82 (m, 2H), 0.67−0.57 (m, 2H) (>CH2, dppp and C8H13).
31P{1H} NMR (161.99 MHz, 298 K, CD2Cl2): δ −17.12 (s), −18.10
(s). MS (ESI, CH3CN): m/z 879.2 (M+ − 1).
Theoretical Calculations. All computations were performed using
the Gaussian 09 (RevB.01) package.46 The structures were fully
optimized without geometrical constraints, and the stationary points
(minima and TS) were confirmed by frequency calculations. The
connections between the transition states and the minima were
checked by visual inspection of the negative frequency, and an
extensive IRC calculation in both directions was performed for the
transition state TS4‑4c. The calculations were carried out using the
B3LYP functional, and the basis sets used were LANL2DZ
supplemented with an f function and its associated ECP for iridium47
and 6-31G** for the rest of the atoms. Solution calculations were
performed in dichloromethane as solvent using the SMD continuum
model as implemented in Gaussian 09. Gas-phase energies are
reported as Gibbs energies and solution phase energies as E (solv,
CH2Cl2, SMD). The structures of the optimized molecules were
depicted with the CyLview program.48
Crystal Structure Determination. Data collection was performed
at low temperature (100(2) K) on a Bruker SMART APEX CCD (1-
py and 1-dppm) or on a Bruker APEX DUO (7) diffractometer
equipped with graphite-monochromated Mo Kα radiation (λ =
0.71073 Å) using narrow frames (0.3° in ω). Cell parameters were
refined from the observed setting angles and detector positions of
strong reflections (31744 reflections, 2θ ≤ 30.27° (7); 9435
reflections, 2θ ≤ 28.85° (1-py); 894 reflections, 2θ ≤ 16.57° (1-
dppm)). Data were corrected for Lorentz and polarization effects
using SAINT-PLUS,49 and a multiscan absorption correction was
applied with the SADABS program.50 The structure was solved by
Patterson or direct methods and completed by successive difference
Fourier syntheses (SHELXS-86).51 Refinement was carried out by full-
+
TOF, DCTB, CH2Cl2): m/z 961.2 [Ir(dppm)2 ], 684.1 (M+ − pydc).
[Ir(κ3-pydc)(1-κ-4,5-η-C8H13)]2(μ-diphosphine). A solid mixture
of [Ir(κ3-pydc)(1-κ-4,5-η-C8H13)] (1·MeOH; 0.100 g, 0.200 mmol)
and the corresponding diphosphine (0.100 mmol) was dissolved in
CH2Cl2/MeOH (3/1, 10 mL), and the solution was stirred at room
temperature for 14 h. The yellow solution was evaporated to dryness
to give a yellow solid. The solid was dissolved in the minimum volume
of dichloromethane, and then diethyl ether was added. The yellow
lemon solid that was obtained was filtered, washed with diethyl ether
(3 × 10 mL), and dried under vacuum.
[Ir(κ3-pydc)(1-κ-4,5-η-C8H13)]2(μ-dppm) (12-dppm). Yield: 0.110 g
(84%). Anal. Found: C, 49.81; H, 4.23; N, 2.20. Calcd for
C55H54Ir2N2O8P2: C, 50.14; H, 4.13; N, 2.13. 1H NMR (400.16
MHz, 298 K, CD2Cl2): δ 7.95−7.82 (m, 2H), 7.71 (d, 1H), 7.66−7.62
(m, 3H) (pydc), 7.21−7.10 (br m, 6H, Ph), 6.98−6.89 (br m, 6H, Ph),
6.80 (m, 4H, Ph), 6.64 (bd, 4H, Ph) (dppm), 5.46 (m, 1H), 5.24 (m,
1H), 5.17 (m, 2H) (CH, C8H13), 2.83 (m, 2H), 2.36 (m, 2H),
2.27−1.98 (set of m, 6H), 1.77 (m, 2H), 1.59 (set of m, 4H), 1.34 (m,
2H), 1.19 (m, 2H), 0.80 (m, 2H), 0.57 (m, 2H) (>CH2, C8H13 and
dppm). 31P{1H} NMR (161.99 MHz, 298 K, CD2Cl2): δ −10.96 (s),
−11.64 (s). MS (MALDI-TOF, DCTB, CH2Cl2): m/z 961.2
+
[Ir(dppm)2 ]. MS (ESI, CH3CN): m/z 850.2 (M+ − 1).
M
dx.doi.org/10.1021/om400767d | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
matrix least squares on F2 with SHELXL97,52 including isotropic and
subsequent anisotropic displacement parameters for all non-hydrogen
atoms. Treatment of hydrogen atoms and detected static disorder
problems are described below.
ACKNOWLEDGMENTS
■
́
Financial support from the Ministerio de Economıa y
Competitividad (MEC/FEDER) of Spain (Project CTQ2010-
́
́
15221), Diputacion General de Aragon (Group E07) and
Crystal data for 1-py: C20H21IrN2O4, Mr = 545.59, monoclinic,
space group P21/n, crystal size 0.221 × 0.203 × 0.023 mm, a =
9.6334(6) Å, b = 15.3382(9) Å, c = 13.2311(8) Å, β = 111.1380(10)°,
V = 1823.47(19) Å3, Z = 4, ρcalcd = 1.987 g cm−3, μ(Mo Kα) = 7.351
mm−1, minimum and maximum transmission factors 0.293 and 0.849,
22260 measured reflections (2.12 ≤ θ ≤ 28.86°), 4476 unique
reflections (Rint = 0.0409); 4476/0/327 data/restraints/parameters,
final R1 = 0.0276 (I > 2σ(I)), wR2 = 0.06373, S = 1.066 for all data;
maximum difference peak/hole 1.972/−1.383 e Å−3. Hydrogen atoms
of the carboxylate and pyridine ligands were included in observed
positions and refined as free isotropic atoms. Anomalous displacement
parameters revealed the presence of static disorder involving five
carbon atoms of the σ,π-cyclooctenyl ligand. Two fragments with
complementary occupancies were included in the refinement. Carbon
atoms with the lower occupancy were maintained isotropic, whereas
those of higher occupancy were refined anisotropically. Hydrogen
atoms were included in calculated positions for this disordered ligand.
Crystal data for 1-dppm: C40H38IrNO4P2, Mr = 850.85, triclinic,
Fondo Social Europeo, and CONSOLIDER INGENIO-2010,
under the Projects MULTICAT (CSD2009-00050) and
́
Factorıa de Crystalizacion (CSD2006-0015), is gratefully
acknowledged. I.G. thanks the Humboldt Foundation for a
research fellowship. We also thank Centro de Supercomputa-
́
́
cion de Galicia (CESGA) for access to their computational
facilities. This paper is dedicated to Professor Antonio Laguna
on the occasion of his 65th birthday.
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ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files giving X-ray crystallographic data for the structure
determinations of compounds 1-py, 1-dppm, and 7, tables
giving optimized coordinates for compounds 1 and 4, species
4a−e, and related transition states, and an animation of the
methanol-assisted proton transfer process. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*J.J.P.-T.: e-mail, perez@unizar.es; fax, 34 976761143; tel, 34
976762025.
Notes
The authors declare no competing financial interest.
N
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dx.doi.org/10.1021/om400767d | Organometallics XXXX, XXX, XXX−XXX