A bimetallic iridium(ii) catalyst: [{Ir(IDipp)(H)}2][BF4]2 (IDipp = 1,3-bis(2,6-diisopropylphenylimidazol-2-ylidene))

Article abstract of DOI:10.1039/c5cc03296b

A new Ir(ii) complex [{Ir(μ-κC_NHC,η⁶_Dipp-IDipp)(H)}₂][BF₄]₂ has been prepared and fully characterised. This complex acts as a catalyst for the hydroalkynylation of imines according to an un

Full text of DOI:10.1039/c5cc03296b

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DOI: 10.1039/C5CC03296B
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A bimetallic iridium(II) catalyst: [{Ir(IDipp)(H)}2][BF4]2 (IDipp = 1,3-
bis(2,6-diisopropylphenylimidazol-2-ylidene))
Received 00th January 20xx,
Accepted 00th January 20xx
Laura Rubio-Pérez,a Manuel Iglesias,a* Julen Munárriz,b Victor Polo,b Pablo J. Sanz Miguel,a Jesús J.
Pérez-Torrentea and Luis A. Oroa,c*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The new Ir(II) complex [{Ir(-CNHC,6Dipp-IDipp)(H)}2][BF4]2 has in acetone, Ir(I)···Ir(III) complex
D
is formed by deprotonation
been prepared and fully characterised. This complex acts as a of
catalyst for the hydroalkynylation of imines according to a
unprecedented diiridium-mediated mechanism.
B and concomitant formation of CF3SO3H.
The existence of Ir(II) complexes has been known since the
middle of the last century, especially in the form of dinuclear
entities featuring Ir–Ir bonds.1,2 However, their application as
catalysts has been scarcely described. A precedent is the
system reported by Wakatsuki and co-workers,3 where the
Ir(II) catalyst ([Ir(C5Me5)(-H)]2) plays the role of a base for the
reversible deprotonation of acidic organic compounds.
Studies on iridium-catalysed hydrogenation reactions have
proposed the formation of Ir(II) dimeric species as
a
Scheme 1 Examples of the activation of phenylacetylene by an Ir(II) dinuclear complex
(A) in CH2Cl2 (a) and acetone (b).
deactivation pathway for Ir(I) mononuclear catalysts, which
illustrates the presumed low catalytic activity of this type of
complexes.4 The lack of examples of iridium(II) catalysts
sharply contrasts with the widespread application of
rhodium(II) complexes in catalysis, especially remarkable are
the dinuclear catalysts reported by Doyle and co-workers.5
The reactivity of bimetallic iridium complexes with substrates
such as alkynes, molecular hydrogen, halogens, or halocarbons
very often diverges from that of their mononuclear
analogues.6 In the particular case of bimetallic Ir(II) complexes
the activation of the substrate may occur by single-site
oxidative addition at one of the metal centres.7 Scheme 1
shows an example of a solvent-dependent alkyne activation by
Herein, we report the preparation and full characterisation of
a bimetallic Ir(II) complex containing two terminal hydrides
and two bridging CNHC,
6Dipp-IDipp ligands. This Ir(II) complex
acts as a catalyst for the hydroakylnylation of imines that
operates by an unprecedented mechanism, which has been
proposed based on experimental data and theoretical
calculations at the DFT level.
At the outset of this work we aimed at the preparation of
unsaturated Ir(I) complexes that would allow the synthesis of
dinuclear or polynuclear species with metal-metal
interactions.8 In order avoid the formation of chloro-bridged
complexes9 the chloro ligand in [IrCl(COD)(IDipp)]10
(
1
) (COD =
1,5-cyclooctadiene) was abstracted with AgBF4 to give the
acetone adduct [Ir(acetone)(COD)(IDipp)]BF4 ) as a yellow
solid in 59% yield (Scheme 2). Hydrogenation of in acetone
under an atmospheric pressure of H2 afforded Ir(II) complex
[{Ir( CNHC ). The complex was
6Dipp-IDipp)(H)}2][BF4]2
dinuclear complex A 7b
oxidative addition of phenylacetylene to give
migration of the alkynyl ligand to the second iridium atom
affords Ir(III)···Ir(III) complex in dichloromethane. Conversely,
.
One of the iridium centres undergoes
. Subsequently,
B
(
2
2
C

-
,
(3
a. Departamento Química Inorgánica – ISQCH
isolated as a red-orange solid in 53% yield after evaporation of
the solvent and subsequent washing with small amounts of
diethyl ether (Scheme 2). The formation of the dinuclear
Universidad de Zaragoza – CSIC, 50009 Zaragoza (Spain)
E-mail: miglesia@unizar.es; oro@unizar.es.
b. Departamento Química Física – Instituto de Biocomputación y Física de Sistemas
Complejos (BIFI)
c. King Fahd University of Petroleum & Minerals (KFUPM)
Dhahran 31261 (Saudi Arabia).
iridium(II) complex (3) probably occurs via an unsaturated
iridium species resulting from the hydrogenation of the COD
Universidad de Zaragoza, 50009 Zaragoza (Spain).
†Electronic Supplementary Information (ESI) available: Synthesis, characterisation,
and experimental details. See DOI: 10.1039/x0xx00000x
ligand (formation of cyclooctane was observed by 1H NMR).11
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With the intention of exploring the catalytic activity of
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waDsOI: 1t0e.1s0t3e9d/C5CiCn0329th6Be
dinuclear Ir(II) complexes,
3
hydroalkynylation of imines.12 The experiments were
performed in NMR tubes at 80 ºC using acetone-d6 as solvent.
The great variety of propargylamines thus prepared shows a
good tolerance of the catalytic system for different alkyne and
imine substitution (Table 1).
Scheme 2 Synthesis of complexes 2 and 3. Reaction conditions: (i) AgBF4 (1 eq) in
acetone, 30 min at room temperature; (ii) H2 (1 atm) in acetone, 30 min at room
temperature.
Table 1 Hydroalkynylation of imines.
Crystals of [{Ir(IDipp)(
slow diffusion of diethyl ether into a saturated CH2Cl2 solution.
The global CNHC connectivity pattern of the
6
centrosymmetric cation was confirmed by single crystal X-ray
-H)}2][BF4]2·2CH2Cl2 (3) were grown by

,
Dipp
Entry
1
2
3
4
5
6
7
8
Alkyne (R3)
SiiPr3
SiEt3
SiEt3
SiMe3
Ph
Ph
p-MeC6H4
p-tBuC6H4
p-OMeC6H4
m-OMeC6H4
2,4,6-MeC6H2
CH2C6H5
(CH2)4CH3
(CH2)6CH3
C(CH3)(=CH2)
Cyclopropyl
R1
R2
Yield (%)
97
3
diffraction.‡ It is worth mentioning (i) the relatively short
intermetallic distance (Ir1–Ir1', 2.6482(6) Å), which suggests
the existence of a Ir–Ir single bond; (ii) the terminal hydrido
ligands, which were observed in the difference Fourier map
(Ir1–H1, 1.70(9) Å); and (iii) that the distortion exhibited by the
2,6-diisopropylphenyl units, clearly affected by metal
coordination, remains in solution (see NMR studies).
Ph
2-C4H4S
Ph
2-Py
2-C4H4S
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
p-tolyl
Ph
Ph
p-MeC6H4
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
88
80
94
64
88
73
74
75
85
96
77
74
68
59
89
9
10
11
12
13
14
15
16
General Conditions: Alkyne (0.15 mmol), imine (0.15 mmol), 3 (5 mol%) in 0.5
mL of acetone-d6, 16 h at 80 oC.
In the search for an understanding of the mechanisms involved
in this unique case of Ir(II) catalysis, we attempted the
isolation of possible intermediates of the catalytic cycle. To our
Fig. 1 View of cation 3 exhibiting a CNHC,6Dipp connectivity pattern.
1H NMR spectra of
3 in CD2Cl2 suggest the existence of an
surprise,
80 ºC in acetone overnight with or without the imine.
Conversely, alkynes (1 equivalent) do react with at 80 ºC in
3 does not undergo any noticeable transformation at
inversion centre as well as a symmetry plane defined by the
para carbons of the phenyl rings. The protons of the NHC’s
3
backbone appear as two doublets at
while the CH’s of the four isopropyl moieties emerge as two
septuplets at 2.38 and 2.39 ppm. A high field resonance
observed at –15.51 ppm can be assigned to the two hydrido
 7.87 and 7.42 ppm
acetone to give a new hydride species in conversions of ca. 10
%, even after long reaction times, which could not be isolated.

Addition of excess alkyne (5 equivalents) to
of polymerisation, dimerisation and cyclotrimerisation
products while the structure of remains unchanged. When
both substrates, imine and alkyne, were added together (1
equivalent each) to a solution of in deuterated acetone at
3 gives a mixture

ligands. Remarkably, the aromatic protons of the 2,6-
diisopropylphenyl groups come about as two different sets of
peaks. The para and meta hydrogen atoms of the non-
coordinated phenyl rings appear as a triplet and a doublet (JH-H
3
3
room temperature no reaction occurred; however, at 80 ºC
the corresponding propargylamine and the unchanged catalyst
= 7.9 Hz) at
 7.80 and 7.56 ppm, respectively. The meta and
para protons of the 6 coordinated phenyl rings, however,
3
were obtained.
Previous reports8b suggest that the new species obtained by
addition of 1 equivalent of alkyne to could be originated by
show a doublet and a triplet (JH-H = 6.5 Hz) at higher field,

6.27 and 4.05 ppm, respectively. The 13C{1H} NMR spectra in
CD2Cl2 presents as most representative peaks those belonging
to the coordinated and non-coordinated N–C carbon atoms of
the 2,6-diisopropylphenyl, which appear as two different
3
formation of an alkynyl complex with concomitant liberation
of H+ (analogously to the reaction described in Scheme 1b),
which would be in equilibrium with the starting materials. In
fact, addition of 1 equivalent of the imine, which can act as a
base, immediately leads to total consumption of the alkyne at
80 ºC.
resonances at
 93.5 and 145.8 ppm, respectively. This pattern
is maintained for all the aromatic carbons, those
corresponding to the 6 coordinated ring appearing at higher
fields. High-resolution ESI-MS showed a main peak at m/z =
1163.5090 (calculated 1163.5153) which supports the
proposed dimeric structure of the complex.
The experimental and literature data prompted us to propose
a catalytic cycle that would entail as first elementary step the
2 | J. Name., 2012, 00, 1-3
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reaction of with the alkyne to give an Ir-alkynyl complex with
concomitant protonation of the imine, as previously observed
for other dinuclear Ir complexes.13 Coordination of the latter
to the iridium centre, followed by insertion into the Ir–
C(alkynyl) bond and dissociation of the Ir–N bond, would
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3
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DOI: 10.1039/C5CC03296B
afford the corresponding propargylamine and 3. An alternative
pathway that would involve the deprotonation of the alkyne
by the hydrido ligand3 has been discarded as no reaction was
observed when
3 was treated with more acidic substrates,
such as HBF4 or MeOD.
Theoretical calculations at the DFT level on a model catalyst
(Fig. 2), using the B3LYP-D3 method including solvent
corrections, were carried out for substantiation of the
proposed mechanism. The energetic profile for the
hydroalkynylation of imines mediated by a model complex of
Scheme 3 Proposed catalytic cycle for the hydroalkynylation of imines catalysed
by complex 3.
the catalyst (
cation (Ir–Ir, 2.628 Å; Ir–H, 1.550 Å) deviates slightly when
compared to the X-ray crystal structure of
3) is shown in Figure 2. Geometry optimisation of
Subsequent oxidative addition of the C–H bond yields
features a bridging hydride and a 1-coordination by the ortho
carbon. Deprotonation of
leads again to a 2-coordination
mode in . Subsequently, in order to generate the gap
required for the approach of the protonated imine, a 1
coordination mode by the ortho carbon is adopted in . In TSE/F
the aromatic ring moves even further from the metal centre to
give way to the formation of the metallacyclobutane, an
agostic interaction by the ortho C–H bond being the only bond
between the ring and the Ir centre where the reaction takes
C, which
A
3.
C
Coordination of the imine to one of the Ir(II) centres in
A
D
stabilises the complex 2.5 kcal mol–1, slightly more than the
alkyne (–1.7 kcal mol–1) or the reaction solvent (–2.1 kcal mol–
1). Substitution of the imine ligand in A-imine by the alkyne to
-
E
give
intermediate
via TSB/C, the highest energy barrier (
give Ir(III)···Ir(III) intermediate . Deprotonation of
imine yields alkynyl intermediate (thus restoring the Ir(II)–
B
has an energy cost of 0.8 kcal mol–1. Subsequently,
B
undergoes oxidative addition of the C–H bond
G = 21.4 kcal mol–1), to
by the

C
C
place (this conformation is maintained in F).
D
It is worth mentioning that also the second aromatic ring
undergoes changes of hapticity throughout the catalytic cycle,
notwithstanding, the interaction of this aromatic ring with the
second Ir centre helps the maintain dimeric nature of the
catalyst. Moreover, the Ir–Ir bond and the likely switching
between terminal and bridging positions of the hydrides also
sustain the diiridium core. Especially significant is the case of
Ir(II) core) and the protonated imine, which destabilises the
system 9.3 kcal mol–1. Coordination of the latter to the same
Ir(II) centre affords
Complex undergoes insertion of the imine into the Ir–
E
, stabilising the system –10.0 kcal mol–1.
E
C(alkynyl) bond by means of transition state TSE/F, with an
energetic barrier of 11.8 kcal·mol–1. Finally, dissociation of the
propargylamine from F restarts the catalytic cycle (Scheme 3).
intermediate B, while the first ring is not interacting with the Ir
The 6 coordination of the 2,6-diisopropylphenyl groups of the
NHC ligands is not maintained throughout the catalytic cycle,
instead, hapticity changes take place in order to accommodate
centre where the reaction occurs, the second shows a 4
interaction with the other metal centre. This hapticity is
maintained in TSB/C, whereas in
coordination mode. In species and
2- and a 4-coordination, respectively; while a 5- and 6
coordination is adopted in TSE/F and , respectively (see
C -
the ring adopts a 6
the substrates. The hapticity shifts from 6 in
ipso and ortho carbon atoms) in A-amine, while in
A
to 2 (by the
this
aromatic ring is totally decoordinated. The formation of TSB/C
results in a 2-coordination analogous to that in
D
E the phenyl ring presents
B
a
-
F
A
.
Supporting Information). In this regard, examples of labile
-
arene coordination to iridium have been already reported.14
Fig. 2 DFT calculated Gibbs free energy profile (in kcal·mol–1, relative to the catalyst and the isolated reactants) for the hydroalkynylation of imines.
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J. Am. Chem. Soc., 1985, 107, 279; (b) M.A. Ciriano, S.
Sebastián, L.A. Oro, A. Tiripicchio, M. Tiripicchio-Camellini
and F.J. Lahoz. Angew. Chem. Int. Ed. Engl., 1988, 27, 402; (c)
C. Tejel, M.A. Ciriano, J.A. López, F.J. Lahoz, and L.A. Oro,
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Rahaman, M. Majumdar, A. Sinha and J. K. Bera, Chem.
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Z. Hou, T.-A Koizumi, A. Fujita, H. Yamazaki and Y. Wakatsuki,
J. Am. Chem. Soc., 2001, 123, 5812.
In conclusion, we have prepared and fully characterised an
NHC-stabilised bimetallic Ir(II) complex that efficiently
catalyses the hydroalkynylation of imines. Experimental
studies and DFT calculations support an unprecedented Ir(II)-
based mechanism that implies: (i) oxidative addition of the
alkyne’s C–H bond at one of the Ir centres; (ii) end-on
coordination of the alkynyl anion and concomitant protonation
of the imine; (iii) coordination of the protonated imine
followed by migratory insertion into the Ir–C(alkynyl) bond; (iv)
finally, dissociation of the propargyl amine affords the organic
compound and the bimetallic catalyst. Remarkably, the flexible
coordination of the arene ligand in this catalytic system
provides the vacant coordination sites required for the
catalysis to take place while, concomitantly, holds together the
bimetallic entity.
3
4
5
6
E. L. Kolychev, S. Kronig, K. Brandhorst, M. Freytag, P. G.
Jones and M. Tamm, J. Am. Chem. Soc., 2013, 135, 12448.
X. Xu and M. P. Doyle, Acc. Chem. Res., 2014, 47, 1396 and
references therein.
(a) L. A. Oro and E. Sola In Recent Advances in Hydride
Chemistry; (Eds. R. Poli, M. Peruzzini), Elsevier, Oxford, 2001,
pp. 299; (b) M. V. Jiménez, E. Sola, J. A. López, F. J. Lahoz and
L. A. Oro, Chem. Eur. J., 1998, 4, 1398; (c) M. V. Jiménez, E.
Acknowledgements
Sola, M. A. Egea, A. Huet, A. C. Francisco, F. J. Lahoz and L. A.
Oro, Inorg. Chem., 2000, 39, 4868.
7
8
(a) L. A. Oro, E. Sola, J. A. López, F. Torres, A. Elduque and F.
This work was supported by the Spanish Ministry of Economy
and Competitiveness (MINECO/FEDER) (CONSOLIDER INGENIO
CSD2009-0050, CTQ2011-27593 and CTQ2012-35665 projects)
and the DGA/FSE-E07. L.R.-P thanks to CONACyT for a
postdoctoral fellowship (204033). The support from KFUPM-
University of Zaragoza research agreement and the Center of
Research Excellence in Petroleum Refining & KFUPM is
gratefully acknowledged. V. P. and J. M. thankfully
acknowledge the resources from the supercomputer
"Memento", technical expertise and assistance provided by
BIFI-ZCAM (Universidad de Zaragoza).
J. Lahoz, Inorg. Chem. Commun., 1998,
1, 64; (b) M. V.
Jiménez, E. Sola, A. P. Martínez, F. J. Lahoz and L. A. Oro,
Organometallics, 1999, 18, 1125.
(a) V. Lavallo, Y. Canac, A. DeHope, B. Donnadieu and G.
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,
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P. G. Jones and M. Tamm, J. Am. Chem. Soc., 2013, 135
,
12448.
9
C. Y. Tang, J. Lednik, D. Vidovic, A. L.Thompson and S.
Aldridge, Chem. Commun., 2011, 47, 2523.
10 For preparation of complex
1 see: R. A. Kelly III, H. Clavier, S.
Giudice, N. M. Scott, E. D. Stevens, J. Bordner, I. Samardjiev,
C. D. Hoff, L. Cavallo and S. P. Nolan, Organometallics, 2008,
27, 202. Crystal data of a different polymorph of
1 (CCDC
Notes and references
1049930) is provided in the S. I..
11 O. Torres, M. Martın and E. Sola, Organometallics, 2009, 28,
́
‡ Crystal data. Compound
3: [C56H78B2Cl4F8Ir2N4], monoclinic,
863.
C2/c, a = 19.338(2) Å, b = 16.601(2) Å, c = 18.949(2) Å, β =
95.101(2)º, Z = 4, Mr = 1507.04, V = 6059.0(13) Å3, Dcalcd = 1.652
g cm–3, λ(Mo Kα) = 0.71073 Å, T = 100 K, µ = 4.628 mm–1, 27196
12 For examples of hydroalkynylation see: (a) T. Zeng, L. Yang,
R. Hudson, G. Song, A. R. Moores and C.-J. Li, Org. Lett.,
2011, 13, 442; (b) B. Marciniec and I. Kownacki in Iridium
Complexes in Organic Synthesis, Chapter 14, (Eds. L. A. Oro,
C. Claver), Wiley-VCH, Weinheim, 2009, pp. 345; (c) S.
Sakaguchi, T. Mizuta and Y. Ishii, Chem. Commun., 2004,
reflections collected, 7154 unique (Rint = 0.0496), 5661 observed,
2
R1(Fo) = 0.0485 [I > 2σ(I)], wR2 (Fo ) = 0.1148 (all data), GOF =
1.009. CCDC 1049931.
1
For examples of mononuclear Ir(II) complexes see: (a) R.
Mason, K. M. Thomas, H. D. Empsall, S. R. Fletcher, P. N.
Heys, E. M. Hyde, C. E. Jones and B. L. Shaw, J. Chem. Soc.
Chem. Commun., 1974, 612; (b) G. Pilloni, G. Schiavon, G.
Zotti and S. Zecchin, J. Organomet. Chem., 1977, 134, 305; (c)
M. P. Garcia, M. V. Jiménez, L. A. Oro, F. J. Lahoz and P. J.
Alonso, Angew. Chem. Int. Ed. Engl. 1992, 31, 1527; (d) M. P.
García, M. V. Jiménez, L. A. Oro and F. J. Lahoz,
Organometallics, 1993, 12, 4660; (e) H. Zhai, A. Bunn and B.
Wayland, Chem. Commun., 2001, 1294.
1638; (d) C. Wei and C-J. Li, J. Am. Chem. Soc. 2003, 125
,
9584; (e) S. Sakaguchi, T. Kubo and Y. Ishii, Angew. Chem. Int.
Ed., 2001, 40, 2534; (f) C. Fisherand E. M. Carreira, Org. Lett.,
2001, 3, 4319.
13 M. Martín, E. Sola, S. Tejero, J.-A. López and L. A. Oro, Chem.
Eur. J., 2006, 12, 4057.
14 F. Torres, E. Sola, M. Martín, J. A. López, F. J. Lahoz and L. A.
Oro, J. Am. Chem. Soc., 1999, 121, 10632.
2
For examples of di- and polynuclear Ir(II) complexes see: (a)
P. G. Rasmussen, J. E. Anderson, O. H. Bailey and M. Tamres,
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