Anion-exchange-triggered 1,3-Shift of an NH proton to iridium in protic N-heterocyclic carbenes: Hydrogen-bonding and Ion-pairing effects

Article abstract of DOI:10.1002/anie.200905691

Proton release: A series of five-coordinate iridium(l) phosphine complexes with protic N-heterocyclic carbene ligands have been prepared which display NH-Cl hydrogen bonding (see scheme; cod = 1,5-cyclooctadiene). Exchange of the chloride for less coordinating anions triggers the reversible 1,3-shift of the NH proton to the iridium, which is thought to proceed by a novel water-assisted protonrelay mechanism.

Full text of DOI:10.1002/anie.200905691

Communications
DOI: 10.1002/anie.200905691
N-Heterocyclic Carbenes
Anion-Exchange-Triggered 1,3-Shift of an NH Proton to Iridium in
Protic N-Heterocyclic Carbenes: Hydrogen-Bonding and Ion-Pairing
Effects**
Guoyong Song, Yan Su, Roy A. Periana, Robert H. Crabtree, Keli Han,* Hongjie Zhang,* and
Xingwei Li*
The chemistry of N-heterocyclic carbenes (NHCs) has
become one of the most active and exciting topics in synthesis
and catalysis.[1] NHCs bearing N-alkyl or -aryl wingtips are
predominant, and there are only limited, although increasing,
literature reports on NHC systems with hydrogen wingtips
ligated on transition metals such as iridium,[2] rhodium,[3]
osmium,[4] ruthenium,[4a–c,5] rhenium,[6] manganese,[7] chro-
mium,[8] and platinum (Figure 1).[9] In general, these NH-
protic NHC complexes can be synthesized by 1) protonation
of 2- or 4-pyridyl or 2-imidazolyl complexes;[6,10] 2) cleavage
[2c,5a]
[9]
À
À
of N C
or N Si bonds within NHC units; 3) intra-
molecular attack of a ligated isonitrile by a pendent NH2 or
OH group generated in situ;[5c,8] and 4) metal-mediated
À
tautomerization of N-heterocycles, a process involving C H
activation.[2–4,5a,b,11] Among these methods, metal-mediated
tautomerization is most intriguing in that N-heterocycle to
NHC tautomerization is important not only in biological
[3]
process[12] but also in C C bond formation, where rhodium
À
and ruthenium[5b] protic NHC complexes are established as
active catalysts.
However, almost all the literature reports in this area are
limited to the synthetic aspects, reactivity, catalytic applica-
tions of such complexes, and the energetics of N-bound and C-
Figure 1. Examples of protic NHC complexes.
TpMe2 =tris(dimethylpyrazolyl)borate.
[*] Y. Su,[+] Prof. Dr. K. Han
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Dalian 116023 (China)
E-mail: klhan@dicp.ac.cn
Dr. G. Song,[+] Prof. Dr. R. A. Periana, Prof. Dr. X. Li[+]
The Scripps Research Institute, Scripps Florida
Jupiter, FL 33458 (USA)
bound tautomers.[13] Mechanistic studies on this tautomeriza-
tion process are rare and only two reports are known.[3a,4e]
Furthermore, few experimental studies have been carried out
À
on the the most important N H bond-formation step that
generates NHCs (or the microscopic-reverse process). Berg-
man, Ellman, and co-workers reported the first studies on the
tautomerization of 3-methyl-3,4-dihydroquinazoline via a
rhodium(III) hydride intermediate, which, on the basis of
theoretical studies, undergoes b-hydride insertion to give the
NHC product.[3a] We feel that controllable interconversions
between the protic NHC and the metal hydride precursor can
greatly alter the electronic effect of metal center, an
important feature in catalysis and molecular recognition.
Thus it is highly desirable that the thermodynamics between
protic NHC complexes and the metal hydride precursors can
be readily tuned. We now report the synthesis of a series of
rare 18-electron iridium(I) protic NHC complexes, which
upon anion exchange can undergo 1,3-shift of the NH proton
to iridium to give iridium(III) hydrides. Significantly, the
thermodynamics of this process can be readily tuned by the
counteranion, the solvent, and the phosphine coligand.
E-mail: xli@scripps.edu
Prof. Dr. R. H. Crabtree
Chemistry Department, Yale University
New Haven, CT 06520 (USA)
Prof. Dr. H. Zhang
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences
Changchun 130022 (China)
[+] G.S. and Y.S. contributed equally to this work. X.L. conceived and
designed the experiments.
[**] X.L. and R.A.P. thank the Scripps Research Institute for support.
K.H. thanks NKBRSF (No. 2007CB815202) and NSFC (No.
20833008) for support. The synthesis and crystallographic analysis
(Dr. Yongxin Li) of complex 5b-Cl were performed at Nanyang
Technological University. We thank Dr. Bolin Lin (Stanford Univer-
sity) for helpful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200905691.
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We have recently reported the tautomerization of amide-
functionalized 2,3-bipyridyl (1)[2d] to give protic NHC com-
plex 2 (Scheme 1), during which process a hydride species
(d(1H) = À14.9 ppm) has been detected by 1H NMR spectros-
Indeed, it reacted with the less reactive [{Ir(cod)Cl}2] in
CH2Cl2 to give a yellow precipitate (Scheme 2). Unfortu-
nately, this product is essentially insoluble in any common
organic solvent and defies NMR analysis. In the IR spectrum,
À
no Ir H stretching band was evident. Instead, a broad NH
(3480 cmÀ1) resonance peak was observed. Thus we tenta-
tively assign this compound to complex 4. Addition of
triarylphosphines to a suspension of 4 (CH2Cl2) gave red
solutions, from which analytically pure 18-electron iridium(I)
NHC phosphine complexes 5a-Cl–5e-Cl could be isolated
(54–65%) and spectroscopically characterized. In particular,
in the 13C NMR spectra (CD2Cl2) the Ir Ccarbene of 5a-Cl–5e-
À
Cl resonates characteristically as a doublet signal within a
narrow range of d(13C) = 193.5–196 (2JPC = 8.4–8.8 Hz), in line
with typical values reported for pyridylidene complex-
2
es.[2d,4a,14] The small JPC(carbene) coupling constant suggests the
cis relationship between the phosphine and the NHC. In the
1H NMR spectra of 5a-Cl–5e-Cl, a characteristic broad low-
field signal (d(1H) = 14.74 to 15.63) was observed for the NH
proton.
Scheme 1. Hydrogen-bonding-assisted tautomerization of
2,3-bipyridyls. cod=cycloocta-1,5-diene.
X-ray crystallographic analysis of 5b-Cl unambiguously
confirmed its 18-electron, distorted trigonal-bipyramidal
structure (Figure 2).[15] The Ir–C9 distance (2.0052(19) ꢀ) is
within the normal range expected for pyridylidene complex-
copy.[2d] In contrast, switching 1 to its amide-free analogue
leads to a mixture of a protic NHC complex and a hydride
species (5:1 ratio) under the same conditions. It is likely that
the amide group in 1 facilitates the 1,3-migration of the
hydride to the pyridyl nitrogen, resulting in an enhanced
selectivity of the IrI carbene over the IrIII hydride. In this work
we focus on the microscopic-reverse 1,3-hydrogen-shift
process.
es.[1f,2d,14] Consistent with previous observations,[2d,4a,14] the C
À
C bond lengths in the pyridylidene ring show large variations,
with the C10–C11 bond (1.333(3) ꢀ) being significantly short.
The H···Cl distance (2.267 ꢀ) is smaller than the sum of the
van der Waals radii, suggestive of hydrogen bonding.[4a,13]
In general, 18-electron, five-coordinate IrI complexes are
less common,[16] and we reason that the NH hydrogen in 5a-
We reason that 1,9-phenanthroline (3) is a more reactive
cyclometalating reagent owing to its structural rigidity.
Scheme 2. Synthesis and subsequent conversion of IrI complexes having with protic NHC ligands.
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CD2Cl2. The Keq can be tuned to range from < 0.02 (lowest
limit of NMR detection) to 1.17, as a result of the hydrogen-
bonding and/or ion-pairing effects. The largest Keq was
observed for the least coordinating BArF À, while for halide
4
counteranions only the iridium(I) NHC complexes were
observed in the 1H NMR spectra. More coordinating or
interfering anions help to stabilize the carbene complexes
likely through NH···X interactions.[17] DFT studies further
revealed that the conversion of 5a-Cl to the hypothetical 6a-
Cl is endergonic by 8.3 kcalmolÀ1, consistent with the
unidirectional formation of complex 5a-Cl in experiments.
Counteranion effects have been reported in catalysts; switch-
ing counteranions can change the catalytic activity or lead to
different selectivity as in [Au(PR3)X]-catalyzed (X = OTf or
OTs) cyclization reactions.[18] However, well-characterized
counteranion effects in carbene complexes are rare.[17,19]
Solvents are also shown to strongly influence the equilib-
rium between 5a-BArF and 6a-BArF . In solvents that are
Figure 2. X-ray crystal structure of 5b-Cl shown with 50% thermal
ellipsoids.
4
4
Cl–5e-Cl might migrate to the iridium to yield rather common
18-electron iridium(III) hydride aryl complexes if the H···Cl
hydrogen bond is properly disrupted. Indeed, a hydride
species was observed by 1H NMR spectroscopy when
potentially hydrogen-bonding acceptors such as CD3CN,
[D6]acetone, [D8]THF, and [D4]MeOH, only 5a-BArF was
4
observed in the 1H NMR spectra, and these solvents serve to
“lock” the NH species by means of hydrogen bonding.
Changing the solvent from CD2Cl2 to CDCl3 resulted in an
increase of the Keq from 1.17 to 2.80. This is probably because
of the enhanced acidity of CDCl3 such that it acts as a weak
proton donor[20] and can stabilize the pyridyl N atom in 6a-
NaBArF (equimolar) was added to a CD2Cl2 solution of 5a-
4
Cl, to give an equilibrium system between 5a-BArF and 6a-
4
BArF (Scheme 2). The same equilibrium mixture can be
4
alternatively synthesized from the reaction of [Ir(cod)-
(PPh3)Cl], 1,9-phenanthroline, and NaBArF (equimolar) in
BArF . However, the difference in solvent polarity might be
4
4
CD2Cl2 by a C–H activation approach (Scheme 2). The
also responsible. We also noted that addition of D2O or
equilibrium constant Keq was determined to be 1.17
CD3OD to an equilibrium mixture of 5a-BArF4 and 6a-BArF
4
(DG298 = À0.09 kcalmolÀ1) for the conversion of 5a-BArF to
in CD2Cl2 caused the disappearance of both the NH and the
IrH signals in the 1H NMR spectrum, indicative of the
exchange between labile NH, OD, and IrH protons/deute-
riums.
4
6a-BArF (CD2Cl2), and the DH was estimated to be
4
À1.3 kcalmolÀ1 based on a van’t Hoff plot (À10 to 378C).
The hydride of 6a-BArF resonates characteristically at
4
d(1H) = À13.58 ppm (d, 2JHP = 10.8 Hz, À508C) in CD2Cl2.
In addition, the Ir–Caryl carbon resonates as a doublet
(d(13C) = 161.6 ppm, 2JPC = 10.2 Hz) in the 13C{1H} NMR
spectrum. These small coupling constants suggest that both
the hydride and the Caryl are cis to the phosphine ligand.
The electronic effects of isosteric phosphines can further
tune this equilibrium (CD2Cl2), where a more donating
phosphine ligand leads to a larger Keq (Table 2). This trend
Table 2: The electronic effects of isosteric phosphine ligands on the
Although the structure of 6a-BArF could not be elucidated
equilibrium between BArF complexes in Scheme 2.
4
4
unambiguously, DFT was employed to further support this
proposed structure. The calculated DG298 value of À0.8 kcal
molÀ1 (CH2Cl2) between 5a+ and 6a+ agrees well with the
experimental value (À0.09 kcalmolÀ1), while the calculated
free energy (298 K, CH2Cl2) of other possible cationic cis
hydride phosphine complexes is at least 6.8 kcalmolÀ1 higher
than that of 5a+ (see the Supporting Information). The
6a+/5a+
6c+/5c+
6d+/5d+
6e+/5e+
Keq
1.17
2.73
5.61
4.57
0.37
1.03
0.074
À1.2
+
pKa of HPAr3
essentially non-coordinating nature of BArF À should provide
is expected for this formal IrI to IrIII oxidation process, and a
metal in a higher oxidation state is stabilized by an electron-
rich phosphine. Quantitative analyses indicate that the lnKeq
4
the best scenario for DFT modeling where the anion is
ignored.
[21]
Pronounced counteranion effects (Table 1) have been
observed for the equilibra between PPh3 complexes 5a-X and
correlates well (R2 = 0.999) with the pKa of the phosphine
and with the sp value of the para substituent in substituted
triphenylphosphines (see Table 2 and the Supporting Infor-
mation). Thus the basicity of the phosphine affects the DG of
this equilibrium by imparting its electron density directly to
the metal. To obtain the unidirectional formation of the
iridium(III) hydride product, a more electron-rich phosphine
6a-X (XÀ = BArF À, PF6À, BF4À, IÀ, carboraneÀ, and ClÀ) in
4
Table 1: Effects of the counteranion on the equilibrium [5a-X Q 6a-X].
BArF
Carborane
(CB11H12)
PF6
BF4
Cl or I
4
is necessary. Thus hydride complex 7-BArF was obtained as
4
the only isomer in CD2Cl2 from equimolar 1,9-phenanthro-
Keq
1.17
0.82
0.30
0.15
<0.02
line, [Ir(cod)(PEt3)Cl], and NaBArF [Eq. (1)]. Here the
4
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unidirectionality might originate from a combination of
electronic (pKa of HPEt3+ = 8.69) and steric effects of PEt3.
We further explored the electronic effects of the chelating
ligand on this equilibrium. Analogous to the synthesis of 7-
BArF , 2,3’-bipyridine was allowed to react with [Ir(cod)-
4
(PPh3)Cl] and NaBArF4 (equimolar) in CD2Cl2. Interestingly,
when the reaction was conducted under relatively mild
conditions (468C, 30 h), essentially only the hydride complex
8-BArF was observed by NMR spectroscopy (Scheme 3).
4
Prolonged heating (528C, 60 h) of the reaction mixture or a
solution of isolated 8-BArF4 produced an equilibrium mixture
of hydrides 8-BArF4 and 9-BArF4 in 3.9:1 ratio at 298 K. In all
cases, no isomeric protic NHC complexes were observed.
Thus the migration of the hydride to the nitrogen is probably
thermodynamically unfavorable as a result of the electronic
effects of this chelating ligand.
Scheme 3. Cyclometalation of 2,3’-bipyridine.
mechanism is applicable if the IrI center acts as a reasonable
proton acceptor. Thus these two pathways were analyzed by
DFT methods (Figure 3). In the b-hydrogen insertion path-
À
Mechanistic studies of the 1,3-H-shift process were
conducted both experimentially and theoretically. As was
proposed by Bergman, Ellman et al.,[3a] a b-hydrogen inser-
tion mechanism is a possible pathway. However, the four-
membered-ring transition state may be less accessible owing
to the tethering effect of the pyridine ring. In addition to this
pathway, we note that water as a catalyst can facilitate the
classical metal-free NH to NH tautomerization of adenine,[22]
where a cyclic transition state has been established. This
way a transition state (TS1) is located (d(Ir H) = 1.82 ꢀ and
°
À
d(N H) = 1.40 ꢀ), and the large calculated DG298 (33.5 kcal
molÀ1 in CH2Cl2) for the forward reaction suggests that it is
unlikely at room temperature. In the water-assisted 1,3-
hydrogen-shift mechanism, water is bound to NH moiety to
give an intermediate (5a+···H2O) with a calculated DG298 of
6.6 kcalmolÀ1 for hydration. Subsequently 5a+···H2O under-
goes facile proton transfer via a six-membered metallacyclic
transition state (TS2). This proton-transfer process has a low
Figure 3. Free-energy diagram of two possible pathways for the conversion of 5a+ to 6a+. Important distances [ꢀ] are given.
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Communications
barrier (DG°298 = 11.8 kcalmolÀ1), and an overall barrier of
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18.4 kcalmolÀ1 is located for this pathway. Thus it can be
predicted that the actual kinetics may depend on the
concentration of water in CD2Cl2.
The kinetics of the conversion of 5a-BArF to 6b-BArF
4
4
has been experimentally evaluated at two different water
concentrations. The observed rate kobs was measured to be
3.0 ꢁ 10À4 sÀ1 at 298 K for the forward reaction (2.5 ꢁ 10À4 sÀ1
for the reverse reaction) in CaH2-dried CD2Cl2, which
typically contains 20 ppm water.[23] In contrast, the equilibri-
um was achieved within only seven minutes (time needed to
acquire an NMR spectrum) when water-saturated CD2Cl2 was
used ([H2O] = 0.14m), where the Keq is slightly changed to
1.03. The strongly water-dependent kinetics suggest that the
water-assisted 1,3-hydrogen-shift mechanism is more likely. In
particular, the estimated experimental value of DG°298 is
18.3 kcalmolÀ1 for the forward reaction using kobs = k[H2O],
where [H2O] = 1.5 mm (20 ppm) (see the Supporting Infor-
mation), which matches well with that calculated for the
water-assisted mechanism (18.4 kcalmolÀ1). Although the
probability of having a water dimer is low at low concen-
trations of water, we further examined the mechanism of a
water-dimer-assisted formal 1,3-proton shift; this is kinetically
unlikely with a calculated DG°298 of 29.8 kcalmolÀ1 (see the
Supporting Information). We noted the report of water
dependence in the synthesis of a related protic pyridylidene
complex,[2g] but in this case no conclusion could be drawn on
the intermediacy of metal hydride and on the role of water in
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À
metal H bond cleavage or formation.
In summary, we have synthesized a series of 18-electron
iridium(I) protic NHC complexes by the C–H activation of
1,9-phenanthroline. The protic NHC ligand is stabilized by
NH···Cl hydrogen bonds, disruption of which by anion
exchange leads to an equilibrium between iridium(I) protic
NHC complexes and the corresponding cationic iridium(III)
hydride aryl complexes, as a result of a reversible 1,3-
hydrogen shift from the nitrogen to the iridium center. The
effects of the solvent, counteranion, chelating ligand, and
phosphine have been studied. A combination of DFT and
experimental studies strongly supports a novel water-assisted
proton-relay mechanism in this transformation. The tuning of
the energies of protic NHC complexes and their hydride
precursors by readily controllable parameters should make it
possible for each species to perform its role to full capacity in
catalytic systems. Further studies on the reactivity and
catalytic properties of iridium protic NHC complexes are in
progress.
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´
´
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Received: October 9, 2009
Published online: December 28, 2009
[14] a) G. Song, Y. Zhang, Y. Su, W. Deng, K. L. Han, X. Li,
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[15] Crystal data of 5b-Cl were collected on a Bruker X8 Kappa
CCD diffractometer at 173 K using graphite-monochromated
MoKa radiation (l = 0.71073 ꢀ). The APEX2 Software Suite
(Bruker, 2005) was used for data acquisition, structure solution,
and refinement. Absorption corrections were applied using
SADABS. The structure was solved by direct methods and
refined by full-matrix least squares method on F2 using X-shell.
Crystal data: C38H32ClF3IrN2P, red, 832.28 gmolÀ1, F(000) =
À
Keywords: C H activation · iridium · N-heterocyclic carbenes ·
tautomerism
.
[1] For reviews on N-heterocyclic carbenes, see a) W. A. Herrmann,
Angew. Chem. 2002, 114, 1342 – 1363; Angew. Chem. Int. Ed.
2002, 41, 1290 – 1309; b) N-Heterocyclic Carbenes in Transition
Metal Catalysis (Ed.: F. Glorius), Springer, Heidelberg, 2007;
c) F. E. Hahn, M. C. Jahnke, Angew. Chem. 2008, 120, 3166 –
916
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Angewandte
Chemie
1640, orthorhombic, space group P212121, a = 11.7727(5), b =
15.5461(7), c = 17.4063(8) ꢀ, V= 3185.7(2) ꢀ3, Z = 4, d =
1.735 gcmÀ3, m = 3.374 mmÀ1, no. data collected = 158967, R-
(int) = 0.0375, goodness-of-fit on F2 = 1.050, R(all data) =
0.0227, Rw = 0.0363, largest peak and hole 0.590 and
À0.406 eꢀÀ3. CCDC 743784 contains the supplementary crys-
tallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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[23] The traces of water in the CD2Cl2 solvent used can be quantified.
However, it is difficult to quantify the traces of water in the 5a-
Cl and NaBArF solids and in the NMR tube. Thus it is hard to
4
measure with reasonable accuracy the reaction kinetics under
different water concentrations.
Angew. Chem. Int. Ed. 2010, 49, 912 –917
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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