A labile and catalytically active imidazol-2-yl fragment system

Article abstract of DOI:10.1002/anie.201005100

More than an innocent bystander: Deprotonation of the N-H group in I with nBuLi/[D₈]THF gave a highly reactive {(thf)₂LiCl} adduct II of an imidazol-2-yl species. Compounds II and IV readily heterolyze H ₂ to form III, whereas iPrOH acts as a hydrogen source to generate III from II. Copyright

Full text of DOI:10.1002/anie.201005100

DOI: 10.1002/anie.201005100
N-Heterocyclic Carbenes
A Labile and Catalytically Active Imidazol-2-yl Fragment System**
Valentꢀn Miranda-Soto, Douglas B. Grotjahn,* Andrew L. Cooksy, James A. Golen,
Curtis E. Moore, and Arnold L. Rheingold
N-heterocyclic carbenes (NHCs)[1] and their complexes[2] are
excellent catalysts for a broad array of organic transforma-
tions, where the NHC ligands impart useful electronic and
steric properties to metal centers.[3] In these systems, with
commonly used ancilliary NHC ligands that are substituted at
nitrogen atom(s) by alkyl, aryl, or other groups,[2,3] all
catalytic transformations take place at the metal center,
which is stabilized and/or activated by the NHC ligand.
However, transformations that may possibly involve both the
metal center and at one ring nitrogen of the NHC ligand are
much less common,[4,5c,e–g] and are limited to protic NHC
imidazol-2-yl fragments and the metal center lead to ligand
exchange processes that involve A and B, hydrogen activation
(B!C!D), and ultimately, catalytic behavior (D + ketone!
E!B + alcohol). Key differences between results herein
from previous work[4] include: 1) greatly enhanced rates of
reaction (for example, ligand exchange within minutes instead
of days); 2) the ability to tune ligand exchange rates over
several orders of magnitude; and perhaps most importantly
3) catalytic behavior, which was not at all apparent before.
Moreover, we show that 15N chemical shift information on
natural-abundance samples gives valuable information on the
environment of the imidazol-2-yl or imidazolylidene ligand.
Reaction of 1[4] with [CpRuCl(cod)] (Cp = cyclopenta-
dienyl, cod = 1,5-cyclooctadiene) in THF at 1008C led to
phosphorus coordination and complete tautomerization of
the imidazole to carbene 2, which shows a low-field 1H NMR
signal (d = 10.28 ppm, NH) and a doublet in the 13C{1H} NMR
complexes[4–6] or their conjugated bases. Thus, the N H
ꢀ
function of a protic NHC complex (A or D; Scheme 1) could
2
(d = 184.1 ppm, JCP = 22.5 Hz, C2) that are consistent with
structure 2 (Scheme 2).
Scheme 1. Imidazol-2-yl and imidazol-2-ylidene fragment systems. The
square indicates a vacant site.
behave as Brønsted acid, whereas the basic nitrogen atom in
the imidazol-2-yl complex (B or C) may behave as a Brønsted
base. Moreover, reactivity of A with a base could lead to B, a
transient species with both a vacant metal coordination site
and a basic nitrogen atom, which could bind (C) and activate a
substrate (C to D). These reactivity patterns might also be
compatible with protic NHC complexes derived from other
NHC ligands.
Herein, we report a versatile organometallic system in
which the combined reactivity of the imidazol-2-ylidene or
[*] Dr. V. Miranda-Soto, Prof. D. B. Grotjahn, Prof. A. L. Cooksy
Department of Chemistry and Biochemistry
San Diego State University
5500 Campanile Drive San Diego, CA 92182-1030 (USA)
Fax: (+1)619-594-1620
E-mail: grotjahn@chemistry.sdsu.edu
Prof. J. A. Golen, Dr. C. E. Moore, Prof. A. L. Rheingold
Department of Chemistry and Biochemistry
University of California at San Diego
La Jolla, CA 92093-0385 (USA)
Scheme 2. Synthesis and reactivity of imidazol-2-yl and imidazol-2-
ylidene complexes (in [D8]THF at room temperature unless otherwise
specified). Yields are of isolated products unless otherwise specified.
LDA=lithium diisopropylamide.
[**] We thank the NSF for partial support of this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005100.
Angew. Chem. Int. Ed. 2011, 50, 631 –635
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
631

Communications
To illustrate reactivity of the protic NHC moiety toward
[D8]THF with KPF6 deepens the initial orange color to
slightly red, and when an amine is added, the reaction color
turns to yellow instantaneously and reactions go to comple-
tion within minutes to give protic NHC complexes 6a–d
bases, treatment of an orange-yellow solution of 2 in [D8]THF
with 1 equivalent of nBuLi (or lithium diisopropylamide,
LDA) at room temperature gives a reddish-orange solution.
The single organometallic product was at first tentatively
formulated as 3 because its 1H NMR spectrum showed
absence of an NH signal and the presence of a resonance at
d = 2.48 ppm in the 7Li spectrum; the 13C NMR spectrum
contained a doublet for the imidazolyl carbon (d = 175.0 ppm,
2JCP = 31.4 Hz).[7] Ultimately, a single crystal of this highly
reactive complex was analyzed by X-ray diffraction (see
below). In contrast, when iPrOH is used as solvent instead
THF, the reaction of 2 with nBuLi gave hydride 4 rather than
3; this result, along with conversion of preformed 3 into 4 by
iPrOH, is taken as evidence of conversions of type B!E.
Moreover, hydride 4 can also be synthesized by either
heterolysis of hydrogen gas by 3 (B!C!D) or in a one-pot
synthesis from 2 and NaH. The 1H NMR spectrum of 4
showed the presence of an NH (d = 9.13 ppm) and a hydride
1
(Scheme 4). In particular, the H spectrum of 6a showed the
Scheme 4. Synthesis of protic NHC complexes and their deprotonation
with strong bases.
2
ligand (d = ꢀ13.06 ppm, d, JHP = 36.5 Hz). Conversion of 2
into 4 involves deprotonation of the NH group by using strong
bases. When lithium-containing base nBuLi was used, the
lithiated ruthenium complex 3 was observed spectroscopi-
cally. In contrast, when NaH reacts with 2, the sodium
analogue (3Na) of 3 is not seen. Reaction of 2 deuterated at
the NH group (2ND) with NaH afforded 4 with at least 90%
H at the hydride position, suggesting NaCl release with rapid
transfer of the hydride to ruthenium.[8]
Further reactivity studies on 3 with ethylene afforded
neutral complex 5, whereas its reaction with 1 equivalent of
H2O regenerates 2 with precipitation of solid LiOH. Ethylene
complex 5 can also be synthesized in two steps, first by facile
ionization of 2 by saturating a solution in acetone or THF as
solvent[7] with ethylene, a process requiring 12 h at 708C, but
which is carried out in 5 min at ambient temperature with the
aid of KPF6. Subsequent deprotonation of 5·HCl by NaOMe
readily forms 5.
presence of an NH and the appearance of singlet for the
coordinated NH3 (d = 11.51 and 2.07 ppm, respectively),
whereas the bonded methylamine in complex 6b showed
diasterotopic NH2 protons with different chemical shifts.[7]
Interestingly, under the same conditions, diethylamine incom-
pletely formed the analogue 6d, whereas the larger amines
diisopropylamine and triethylamine did not form 6e,f,
probably because of steric factors. Deprotonation of protic
NHC complexes 6a–c with (Me3Si)2NNa gave imidazol-2-yl
complexes 7a–c, wherein 7a can also be obtained from 2 using
NaNH2. However, a surprising result, already shown in
Scheme 2, was that LDA reacted with 2 to form 3 instead
an analogue of 7 (with R3N = diisopropylamine) as NaNH2
did forming 7a.
X-ray structures[10] of 2, 5, 5·HCl, and 7a show different
hydrogen-bonding networks involving the imidazole ring and
confirm the presence of an NH group in 2 and 5·HCl and its
absence in 5 and 7a.[7] The intramolecular NH···Cl distance in
2 is large (4.47(2) ꢀ), whereas a closer mutual intermolecular
contact between two molecules with NH···Cl 2.56(3) ꢀ exists.
In contrast, 5·HCl showed intramolecular hydrogen bonding
between the chloride anion and the NH group (NH···Cl
On the basis of these findings, we tested 4 as catalyst for
transfer hydrogenation of acetophenone using iPrOH. This
reaction is significant because most but not all catalysts
require added base.[9] Thus, under the conditions of Scheme 3,
ꢀ
2.44(5) ꢀ; Figure 1). The structure of 5 had a Cimidazolyl Ru
ꢀ
bond of 2.071(2) ꢀ, whereas the Ccarbene Ru bond in 5·HCl is
shorter (2.061(4) ꢀ). Finally, the crystalline structure of 7a
showed intermolecular hydrogen bonding between protons of
the coordinated NH3 and the basic nitrogen atom of the
imidazol-2-yl fragment, with NH3···N 2.17(3) ꢀ. In contrast,
3[10] (Figure 2) shows clearly the attachment of a {(thf)2LiCl}
fragment to proposed species B.
Scheme 3. Catalytic transfer hydrogenation of acetophenone.
complex 4 catalyzes the reduction of acetophenone to afford
1-phenylethanol in 97% yield, a process that may proceed as
shown in E in Scheme 1. The conversion of acetophenone (2m
in iPrOH) into 1-phenylethanol using low catalyst loadings
(0.05%) can also be achieved by catalyst precursor 2 and base
(to generate 4 in situ) with quantitative yields.
Reactivity studies on 2 also showed that the chloride
ligand can be easily removed using KPF6 in presence of
suitable amines and ammonia. Treatment of a solution of 2 in
Single-crystal X-ray structures (Table 1) show that imida-
zol-2-yl ligands are characterized by small differences (0.029–
ꢀ
0.041 ꢀ) in N C bond lengths to the metal-bound carbon
atom, whereas protic NHC complexes show experimentally
indistinguishable distances. Despite this trend, a more general
technique applicable to solution-phase studies was sought.
Observation of 15N chemical shifts is an extremely useful tool
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Angew. Chem. Int. Ed. 2011, 50, 631 –635

change in 15N chemical shift of N3 is seen when the NH of a
protic NHC is deprotonated and converted into basic nitro-
gen of an imidazol-2-yl system, for example 7a (dN3-N1
=
89.5 ppm). In general, we observe that the 15N chemical
shift of the N3 in protic NHC complexes is shifted to low field
when they are converted into imidazol-2-yl complexes,
whereas the difference between N3-H and N1-R (R = meth-
ylene) in a protic NHC complex is quite small.[7]
Turning toward ligand exchange and reactivity studies on
complexes 7a–c and 3, we found that all of the complexes
react with ethylene and hydrogen, but with dramatically
different rates (Scheme 5): in reactions with ethylene, sub-
stitution on 3 was 40 to more than 500 times faster than ligand
substitution on 7a–c; in reactions with hydrogen, substitution
on 3 was approximately 20 to 60 times faster.
Figure 1. Molecular structure of protic N-heterocyclic carbene complex
ꢀ
5·HCl, in which a hydrogen bond is formed between the N H group
and the chloride anion. Thermal ellipsoids are set at 50% probability.
Figure 2. Molecular structure of 3. Selected bond lengths [ꢀ] and
angles [8]: Ru1–C6 2.0659(17), Ru1–Cl1 2.4629(5), Li1–Cl1 2.317(3),
Li1–N1 1.997(3), N1–C6 1.351(2); Ru1-C6-N1 130.41(13), Ru1-Cl1-Li1
89.468, Cl1-Ru1-C6 91.05(5), Cl1-Li1-N1 104.95(15), Li1-N1-C6
108.96(14). Thermal ellipsoids are set at 50% probability.
Scheme 5. Reactivity of 3 and 7a–c with hydrogen gas and ethylene.
Conditions: [D8]THF, room temperature.
Among amine complexes, reac-
Table 1: Differences in bond lengths [ꢀ] and 15N chemical shifts [ppm] show the nature of the bonding in
the heterocycle for 2, 3, 5, 5·HCl, and 7a.
tions of NH3 analogue 7a were the
slowest of all, which in the NH3 case
would be consistent with less relief
of steric strain in rate-determining
dissociation.[12] Consistent with this
observation, 6e,f could not be syn-
thesized, whereas 6a–d could. To
examine the feasibility of reaction
of 7 and 3 with hydrogen by bond
activation on C prompted by the
basic heterocyclic nitrogen of the
imidazol-2-yl fragment, we are cur-
rently performing calculations,
which so far predict that 4 (compare
Ru–C2
N1–C2
N3–C2
D[a]
dN1
dN3
dN3–dN1
2
3
5
2.047(2)
1.361(3)
1.3761(2)
1.383(2)
1.351(5)
1.374(3)
1.360(3)
1.351(2)
1.342(2)
1.361(4)
1.345(3)
0.001(6)
0.025(2)
0.041(4)
ꢀ198.6
ꢀ196.8
ꢀ200.8
ꢀ197.6
ꢀ199.9
ꢀ197.1
ꢀ122.1
1.5
74.7
nd
14.5
89.5
2.0659(17)
2.0715(17)
2.060(4)
[b]
–
5·HCl
7a
ꢀ0.010(09)
ꢀ183.1
2.045(2)
0.029(6)
ꢀ110.4
[a] D=(N1-C2)ꢀ(N3-C2). [b] Not observed.
to determine metal or hydrogen bonding or protonation of
imidazole nitrogen atoms.[11] Therefore, we obtained 15N
with D) is 11.5 kcalmolꢀ1 lower in energy than its tautomer C
in this system. Experimentally, exposure of 4 to D2[13] (1 atm,
[D8]THF, room temperature) led to appearance of deuterium
1
chemical shifts on natural abundance samples by 2D H–15N
1
correlations as a diagnostic method for either the basic
nitrogen atom or the NH function at the imidazole fragment
(both at N3). For example, the protic NHC complexes 2 and 4
at the NH and RuH positions. No HD was seen by H NMR
(estimated detection limit 5%), which is consistent with
pairwise loss of both reactive hydrogen atoms from 4 and
intermediacy of C and B in hydrogen activation.
showed two 15N peaks very close in chemical shift (2: dN3-N1
=
1.5 ppm), with the NH peak assigned using gHSQC (similar
trends were seen in 5·HCl and 6a–c). In contrast, a dramatic
In summary, we have synthesized a versatile protic NHC
complex 2 that can be easily transformed at room temper-
Angew. Chem. Int. Ed. 2011, 50, 631 –635
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www.angewandte.org
633

Communications
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(NaH, nBuLi, alkoxides, LDA, NaNH2) or ionizing agents
(such as KPF6) in presence of ethylene or amines. Ionization
of the chloride ligand in 2 in the presence of ethylene or
amines allowed formation of cationic complexes 5·HX and 6.
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ligand exchange of the chloride by ethylene to give 5·HCl,
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THFallowed formation of 7a, and surprisingly 3, respectively,
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exchange studies on 3 and 7a–c with ethylene and hydrogen
gas leads to formation of complex 5 and hydride 4, respec-
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rate formation of 5 and 4 increases in the order 3 @ 7c > 7a >
7b, showing a striking ability to tune reactivity. Notably, 4
alone or 2 with added base were excellent catalysts for the
transfer hydrogenation of acetophenone with low catalyst
loading (1% and 0.05%) and good yield, in complete contrast
to the previously reported {Cp*Ir} system, which was
disappointingly inactive as a catalyst.[4] Furthermore, our
structural studies on protic NHC complexes and imidazol-2-yl
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1
systems using 2D H–15N correlations, showed the profound
differences in the N3 nitrogen environment between the
proton-substituted N3 and the basic N3 in imidazol-2-yl
complexes, results which are expected to be a useful basis for
further studies of protic NHC complexes.
Taken together, the fundamental reactivity of protic NHC
carbene complex 2 in presence of ionizing agents or bases
allows its facile transformation, consistent with structures B–
E. Secondary interactions at N3 that involve either hydrogen
acceptance or hydrogen-bond donation together with ligand
exchange processes are highlighted by catalytic behavior of
hydride 4 and the useful reactivity of complexes 3–7a–c. Our
results add protic NHC complexes to a variety of other
diverse compound classes[14] capable of bifunctional activa-
tion of hydrogen or alcohols, but additional work will be
needed to adequately compare the promising chemistry of
protic NHC derivatives with previous work. Thus, further
synthetic, reactivity and catalysis studies on 2 and related
protic NHC complexes are a topic of active investigation.
Received: August 14, 2010
Revised: November 10, 2010
Published online: December 15, 2010
Keywords: homogeneous catalysis · hydrides ·
.
imidazol-2-yl complexes · N-heterocyclic carbenes · ruthenium
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[8] We thank a reviewer for suggesting this possible mechanism and
Laponnaz, H. Wadepohl, L. H. Gade, Chem. Eur. J. 2007, 13,
5994 – 6008.
ꢀ
the means to test it. Whereas the Ru H position of 4 was
[12] Dissociation of CH3CN ligands on [CpRu(CH3CN)3]+ has been
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ꢀ
occupied by protium, the N H position of the product 4 was only
approximately 50% deuterated, which is most likely a result of
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[13] We thank a reviewer for suggesting this experiment. After 1 day,
ꢀ
the N H position was occupied by only 26% protium, whereas
ꢀ
the Ru H position was occupied by 69%. This difference may
reflect the fact that after longer times, evidence for HD exchange
into the Cp ring was seen, a finding which will require further
investigation.
[10] CCDC 784993 (2), CCDC 796316 (3), CCDC 784994 (5·HCl),
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