Dihydrogen-catalyzed reversible carbon-hydrogen and nitrogen-hydrogen bond formation in organometallic iridium complexes

Article abstract of DOI:10.1002/anie.201201811

Dihydrogen at work! H₂ catalyzes with high efficiency a prototropic rearrangement of aminopyridinate ligands bound to a {(η⁵-C₅Me₅)Ir^III} unit. The catalytic isomerization implies reversible formation and cleavage of H-H, C-H, and N-H bonds. Copyright

Full text of DOI:10.1002/anie.201201811

Angewandte
Chemie
DOI: 10.1002/anie.201201811
Dihydrogen Catalysis
Dihydrogen-Catalyzed Reversible Carbon–Hydrogen and Nitrogen–
Hydrogen Bond Formation in Organometallic Iridium Complexes**
Josꢀ E. V. Valpuesta, Nuria Rendꢁn, Joaquꢂn Lꢁpez-Serrano, Manuel L. Poveda, Luis Sꢃnchez,
Eleuterio ꢄlvarez, and Ernesto Carmona*
The molecule dihydrogen plays a key role in nature.[1]
Although in recent times it has become the ideal energy
carrier,[2] its generation in vast quantities by environmentally
clean methods is still a major scientific and technical chal-
lenge.[3] Nonetheless, H2 is an essential reagent for many highly
important heterogeneous and catalytic processes.[4]
Scheme 1. The anionic aminopyridinate ligands employed in this work
Homogeneous hydrogenations were originally thought to
entail oxidative addition and reductive elimination at a metal
center,[4b] but the pioneer discovery by Kubas and co-workers
of the first sigma-complex of H2[5] led to a new paradigm that
emphasizes the key role of such species in this and other
transformations.[6,7] Direct hydrogen transfer from metal–H2
complexes may occur, an important step in catalytic hydro-
genations by electrophilic compounds being heterolytic H2
(R=iPr, complexes 1 and 2; R=CH3, compounds 3 and 4).
bound aminopyridinate ligands (Scheme 1) in an organome-
tallic system.
Similarly to somewhat related complexes,[12,21] the amino-
pyridinate iridium compound 1 (Scheme 2) that contains an
{(h5-C5Me5)IrIII} unit and an aminopyridinate group in which
the amido functionality acts as a s- and p-donor ligand,
reacted with H2 (CH2Cl2, 1 atm) to yield a known dinuclear
trihydride,[22] along with an equimolecular mixture of the free
and protonated aminopyridine, HAp and [H2Ap]BArF,
respectively (BArFꢀ = B[3,5-(CF3)2C6H3]4ꢀ). Before reaching
completion, NMR studies of the reaction mixture revealed
the presence of small amounts of unreacted 1 and of a third
metal-containing product, 2, which was subsequently isolated
and characterized as an isomer of 1 with the structure shown
in Figure 1. As can be seen, formation of complex 2 requires
activation.[8] H H heterolysis is relevant to the function of
ꢀ
hydrogenases[9–13] and may proceed intramolecularly, with
formal proton transfer to a cis sulfur, nitrogen, or oxygen
donor ligand.[2,9,14–16] In the last years, activation of dihydro-
gen by compounds of main-group elements has also been
demonstrated.[17,18]
In view of the myriad of stoichiometric and catalytic
hydrogenations known, it is plausible that in addition to being
a key reagent, the molecule of H2 could act as a catalyst of
important transformations such as the formation and cleavage
ꢀ
activation of a benzylic C H bond of 1, with formal hydrogen
ꢀ
of H X bonds in coordination compounds (where X repre-
transfer to the amido nitrogen. An eighteen-electron config-
uration is achieved in the latter species by means of
pseudoallylic coordination of the activated benzylic unit.
Complexes 1 and 2 have been characterized by single
crystal X-ray diffraction (Figure 1B). In solution they exhibit
characteristic 1H and 13C{1H} NMR spectra that are in accord
with their structures (see the Supporting Information). Thus,
whereas in 1 the two methyl groups of the pyridine aryl
substituent (namely 2,6-Me2C6H3) appear in the 1H NMR
spectrum as a singlet with d 2.28 ppm (relative intensity 6H),
in isomer 2 this signal is replaced by a singlet at 2.48 ppm
(3H) plus two doublets at 3.68 (1H) and 2.07 (1H) ppm
sents for example C, O, N, or S). However, while commonly
used ligands, such as N-heterocyclic carbenes, have been
shown to catalyze sophisticated organometallic rearrange-
ments,[19] information on catalysis by H2 is scarce,[20] and to our
knowledge it has never been disclosed in a homogeneous
system. Herein we show that H2 catalyzes with high efficiency
ꢀ
ꢀ
the formation and rupture of C H and N H bonds of iridium-
[*] Dr. J. E. V. Valpuesta, Dr. N. Rendꢀn, Dr. J. Lꢀpez-Serrano,
Prof. M. L. Poveda, Dr. L. Sꢁnchez, Dr. E. ꢂlvarez, Prof. E. Carmona
Departamento de Quꢃmica Inorgꢁnica-Instituto de Investigaciones
Quꢃmicas (IIQ), Universidad de Sevilla-Consejo Superior de
Investigaciones Cientꢃficas
(2JHH = 4.5 Hz). The latter two signals are due to the Ir CH2
protons. Furthermore, a new signal attributable to the NH
ꢀ
Avda. Amꢄrico Vespucio 49, Isla de la Cartuja, 41092 Sevilla (Spain)
E-mail: guzman@us.es
[**] Financial support (FEDER contribution) from the Spanish Ministry
of Science (Projects CTQ2010-17476 and Consolider-Ingenio 2010
CSD2007-00006) and the Junta de Andalucꢃa (Grant FQM-119 and
Project P09-FQM-4832) is gratefully acknowledged. The use of
computational facilities of the Consejo Superior de Investigaciones
Cientꢃficas (CSIC, Cluster Trueno) and the Centre of Supercomput-
ing of Galicia (CESGA) is also acknowledged. N.R. and J.L.S. thank
the MICINN for Ramꢀn y Cajal contracts.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201811.
Scheme 2. Reaction of compound 1 with an excess of hydrogen.
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before that happens, complexes 1 and 2 abruptly react with H2
to give the dinuclear trihydride of Scheme 2, in a reaction with
an ill-defined and as yet unclear kinetic law.
Interconversion of compounds 1 and 2 entails reversible
ꢀ
formation and cleavage of an amido N H bond and of
ꢀ
a benzylic C H bond of the pyridine 2,6-Me2C6H3 substituent.
ꢀ
Others have previously observed reversible C H bond
activation reactivity in transition metal-coordinated phos-
phine, N-heterocyclic carbene, and other ligands.[25–27] To gain
mechanistic insight, further experimental and theoretical
work was performed. First, a kinetic isotope effect kH/kD of
1.3 was measured. This value is nearly identical to that
computed (see the Supporting Information) according to the
mechanistic route shown in Scheme 3. KIE values close to
unity are often observed in reactions of H2 with unsaturated
metal complexes.[24,28] In the reaction of 1 with D2, deuterium
incorporation at the nitrogen and all benzylic sites of the two
compounds was observed, although not unexpectedly, deut-
eration of 1 occurred with a slower rate than for 2.
Second, the carbonyl adducts 1·CO and 2·CO were
ꢀ
isolated (Supporting Information, Scheme S1) with n(C O)
values of 2050 and 2030 cmꢀ1, respectively, that demonstrate
the electrophilicity of the iridium centers in parent compounds
1 and 2.[9] Finally, complexes 3 and 4, closely related to 1 and 2,
respectively, but having a 2,6-Me2C6H3 substituent at the amido
(or amine) nitrogen atom (Scheme 1) in place of the bulkier
2,6-iPr2C6H3, were also studied. The release of steric pressure
at the iridium center caused by this structural modification
resulted in a faster dihydrogen catalysis (t1/2 ca. 5 min; H2
concentration ca. 7 mol% with respect to 1) with no detectable
variation in the equilibrium constant (Keq =4ꢁ0.5).
Figure 1. A) H2 catalyzes the reversible isomerization of complexes
1 and 2. B) The solid-state molecular structures of complexes 1 and 2.
Ellipsoids set at 30% probability; H atoms (except on C12 in 2+) and
anions are omitted for clarity.
proton of 2 can be found with d 5.94 ppm. In the 13C NMR
spectrum the iridium-bound methylene carbon appears at
34.9 ppm, with a one-bond 13C–1H NMR coupling constant
1JCH = 155 Hz.
Stirring compound 1 in the absence of H2 at 208C (or at 50–
608C) for several days did not lead to any reaction, meaning
that the prototropic rearrangement (Figure 1A) that permits
interconversion of the two compounds is promoted by dihy-
drogen. Further studies on this system revealed that in the
presence of H2 complexes 1 and 2 are in dynamic equilibrium.
The dihydrogen-induced isomerization occurs between two
non-hydride species, and in this regard, it may be recalled that
many transition-metal polyhydrides isomerize by consecutive
elimination and addition of dihydrogen.[23] Recently, Sola and
co-workers have reported that dihydrogen catalyzes the syn/anti
isomerization of five-coordinate iridium monohydride com-
On the basis of these results, it seems probable that
iridium dihydrogen complexes resulting from coordination of
ꢀ
plexes of a pincer PSiP ligand, without involving Ir H/H2 atom
exchange.[24] We found that starting from 1, a H2 concentration
of about 6 mol% with respect to 1 (as determined by solution
1H NMR in CD2Cl2) led at 208C to an equilibrium mixture of
2:1 of about 4 (Keq =4ꢁ0.5), in a reaction characterized by
a half-life t1/2 of about 2.5 h (see the Supporting Information).
The same equilibrium mixture was reached starting from pure
complex 2, although longer reaction times were required. The
rate of the 1!2 isomerization is dependent on the H2
concentration and, for example, the t1/2 for this transformation
under one atmosphere of H2 (208C, CD2Cl2, 32 mol% with
respect to 1) is about 30 minutes. A series of experiments
showed that the possible action of adventitious water and of
small amounts of acids or bases could be discounted. A
heterogeneous process catalyzed by iridium colloidal particles
was equally disproved (see the Supporting Information).
Unfortunately, once the equilibrium is attained, and sometimes
Scheme 3. Proposed catalytic cycle for the H2-mediated interconversion
of complexes 1 and 2.
2
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H2 to the electrophilic iridium centers of 1 and 2 are key
intermediates in their H2-catalyzed interconversion (and by
extension in that of 3 and 4). These species (1·H2 and 2·H2 in
Scheme 3) are structurally analogous to the already discussed
carbonyl adducts, 1·CO and 2·CO, respectively. Accordingly,
their formation requires only relatively facile changes in the
coordination mode of the aminopyridinate ligands in the
starting complexes 1 and 2. In the former case, H2 coordina-
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ꢀ
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h3 to h1. As for the isomerization of compound 1 into 2,
ꢀ
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2
ꢀ
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ꢀ
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ꢀ
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ꢀ
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Dihydrogen has always been viewed as an essential
reactant for industrial and laboratory syntheses, and for
nearly thirty years as an important ligand in coordination and
organometallic chemistry. Our reaction system demonstrates
that hydrogen is also capable of efficiently and reversibly
catalyzing the formation and rupture of the bonds that forms
with other common elements. Future research could disclose
ꢀ
ꢀ
related catalytic processes involving not only C H and N H
ꢀ
ꢀ
bonds, but also others, such as O H or S H bonds.
Received: March 6, 2012
Revised: May 2, 2012
Published online: && &&, &&&&
ꢀ
Keywords: C H activation · dihydrogen catalysis · iridium ·
.
ꢀ
ligand isomerization · N H activation
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Communications
Communications
Dihydrogen Catalysis
J. E. V. Valpuesta, N. Rendꢁn,
J. Lꢁpez-Serrano, M. L. Poveda,
L. Sꢂnchez, E. ꢃlvarez,
E. Carmona*
&&&& — &&&&
Dihydrogen-Catalyzed Reversible
Carbon–Hydrogen and Nitrogen–
Hydrogen Bond Formation in
Organometallic Iridium Complexes
Dihydrogen at work! H2 catalyzes with
high efficiency a prototropic rearrange-
ment of aminopyridinate ligands bound
to a {(h5-C5Me5)IrIII} unit. The catalytic
isomerization implies reversible forma-
ꢀ
ꢀ
ꢀ
tion and cleavage of H H, C H, and N
H bonds.
4
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 4
These are not the final page numbers!