Synthesis and structure of an Amino-Linked N-Heterocyclic carbene and the reactivity of its aluminum adduct

Article abstract of DOI:10.1021/om800917j

Treatment of [HC{(MES)N(CHCH)N(CH₂CH₂NHtBu}]Br- HBr (1) with an excess of NaOH afforded the organic-soluble imidazolium salt 2, [HC{(MES)N(CHCH)N(CH₂CH₂NHtfBu}]Br, the structure of which was confirmed by X-ray crystallography. Deprotonation of 2 by NaN(SiMe ₃)₂ yielded the thermally unstable amino-linked free carbene, 3. The molecular structure of 3 was determined by single-crystal X-ray analysis, revealing an unexpectedly close intermolecular contact associated with the carbene and amine through an N-HC interaction. In contrast to compound 3, the reaction of 2 with LiN(SiMe₃)₂ gave the much more stable LiBr carbene adduct 3-LiBr, which serves as an effective carbene transfer agent for organoaluminum compounds to give the corresponding A1R ₃-NHC (4a, Me; 4b, Et) in high yield. X-ray diffraction studies of 4a and 4b confirmed the formation of monomeric distorted tetrahedral Al species, in which the NHC binds via conventional odonation of the lone pair to the electrophilic metal center. The reaction of benzaldehyde with 4a resulted in the quantitative formation of a zwitterionic species consisting of a distorted pseudotetrahedral aluminate center covalently linked to imidazolium, 5. The product 5 resulted from insertion of the carbonyl moiety into the Al-carbene group. A similar reactivity with benzaldehyde was observed in compound 6, which was independently synthesized from addition of l,3-bis(mesityl)imidazol-2- ylidene to AlMe₃.

Full text of DOI:10.1021/om800917j

1060
Organometallics 2009, 28, 1060–1067
Synthesis and Structure of an Amino-Linked N-Heterocyclic
Carbene and the Reactivity of its Aluminum Adduct
Wei-Chih Shih,† Chun-Han Wang,† Yang-Ting Chang,† Glenn P. A. Yap,‡ and
Tiow-Gan Ong*,†
Institute of Chemistry, Academia Sinica, Nangang, Taipei, Taiwan, Republic of China, and Department of
Chemistry and Biochemistry, UniVersity of Delaware, Newark, Delaware 19716
ReceiVed September 21, 2008
Treatment of [HC{(MES)N(CHCH)N(CH2CH2NHtBu}]Br · HBr (1) with an excess of NaOH afforded
the organic-soluble imidazolium salt 2, [HC{(MES)N(CHCH)N(CH2CH2NHtBu}]Br, the structure of
which was confirmed by X-ray crystallography. Deprotonation of 2 by NaN(SiMe3)2 yielded the thermally
unstable amino-linked free carbene, 3. The molecular structure of 3 was determined by single-crystal
X-ray analysis, revealing an unexpectedly close intermolecular contact associated with the carbene and
amine through an N-H · · · C interaction. In contrast to compound 3, the reaction of 2 with LiN(SiMe3)2
gave the much more stable LiBr carbene adduct 3-LiBr, which serves as an effective carbene transfer
agent for organoaluminum compounds to give the corresponding AlR3-NHC (4a, Me; 4b, Et) in high
yield. X-ray diffraction studies of 4a and 4b confirmed the formation of monomeric distorted tetrahedral
Al species, in which the NHC binds via conventional σ-donation of the lone pair to the electrophilic
metal center. The reaction of benzaldehyde with 4a resulted in the quantitative formation of a zwitterionic
species consisting of a distorted pseudotetrahedral aluminate center covalently linked to imidazolium, 5.
The product 5 resulted from insertion of the carbonyl moiety into the Al-carbene group. A similar reactivity
with benzaldehyde was observed in compound 6, which was independently synthesized from addition of
1,3-bis(mesityl)imidazol-2-ylidene to AlMe3.
donor NHC scaffold to provide stability for hard electropositive
metal centers, as well as the ability to afford a library of ligands
due to their relatively straightforward preparation. Unfortunately,
due to their lack of stability in the free form at room temperature,
these ligand motifs have found only limited use thus far.
Introduction
The remarkable discovery of N-heterocyclic carbenes (NHCs)
as neutral σ-donating ligand platforms prompted a massive
breakthrough in homogeneous transition metal catalysis.1 More
recently, hybrid ligand sets based on the presence of anionic
functional pendants linked to NHC, led by Arnold, Fryzuk, Shen,
and Kawaguchi, have received much attention.2 The interest in
these systems is fueled by the simultaneous use of anionic
σ-amino or alkoxide groups coupled with the neutral electron-
* To whom correspondence should be addressed. E-mail: tgong@
chem.sinica.edu.tw.
† Academia Sinica, Nangang.
In the case of group 13 elements such as Al, the use of NHCs
as good σ-donor molecules to stabilize the trivalent compounds
was established with the isolation of the first alane adduct of
an imidazol-2-ylidene, A.3 When compared to AlH3 · NMe3,
which decomposed above 100 °C,4 the stability of A is
surprisingly high, with a melting point of 246 °C. Therefore,
aluminum complexes supported by ligands bearing N-hetero-
cyclic carbene (NHC) groups may have promising and diverse
chemical reactivity with potential applications in catalysis. With
only a handful of literature references to date,5 this area has
been neglected, particularly when it comes to understanding the
nature of Al-NHC bonding and its reactivity.
It has been extensively documented that aluminum com-
pounds can serve as powerful nucleophilic alkylating reagents
in organic transformations.6 For example, Schneider et al.
reported that phosphine ligands could be used as catalytic Lewis
bases for the ring-opening alkylation of epoxides by alkylalu-
‡ University of Delaware.
(1) (a) Weskamp, T.; Schattenmann, W. C.; Spiegler, M.; Herrmann,
W. A. Angew. Chem., Int. Ed. 1998, 37, 2490–2493. (b) Herrmann, W. A.
Angew. Chem., Int. Ed. 2002, 41, 1291. (c) Romero, P. E.; Piers, W. E.;
Mcdonald, R. Angew. Chem., Int. Ed. 2004, 43, 6161–6165. (d) Sprengers,
J. W.; Wassenaar, J.; Clement, N. D.; Cavell, K. J.; Elsevier, C. J. Angew
Chem., Int. Ed 2005, 44, 2026–2029. (e) Shi, J. C.; Lei, N.; Tong, Q. S.;
Peng, Y. R.; Wei, J. F.; Jia, L. Eur. J. Inorg. Chem. 2007, 2221–2224. (f)
Navarro, O.; Kelly, R. A.; Nolan, S. P. J. Am. Chem. Soc. 2003, 125, 16194–
16195.
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Zhang, Y. J. Organomet. Chem. 2005, 690, 6227–6232. (b) Arnold, P. L.;
Mungur, S. A.; Blake, A. J.; Wilson, C. Angew. Chem., Int. Ed. 2003, 42,
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10.1021/om800917j CCC: $40.75
2009 American Chemical Society
Publication on Web 01/20/2009

Amino-Linked N-Heterocyclic Carbene
Organometallics, Vol. 28, No. 4, 2009 1061
minum reagents.6i,j Furthermore, studies by Nguyen6d and
Dorta6h on the use of bulky NHCs have demonstrated the
viability of a similar catalytic process. More importantly, the
concept of frustrated Lewis pairs, pioneered by Power and
Stephan, has generated tsunami-level hype in the use of Lewis
pairs of main group elements for hydrogen and other small
molecule activation.7 With this in mind, we have sought to (a)
improve the preparation of functional amino-pendant-linked
NHC species and (b) subsequently exploit the potential of this
ligand with respect to Al complexes and their reactivity. In this
contribution, we report the solid state structure of a thermally
unstable amino-carbene and its unusual intermolecular interac-
tion between the carbene center and the amide. We also describe
the peculiar example of an Al complex supported by an amino-
NHC and its corresponding reactivity with electrophiles.
Figure 1. Molecular diagram of 2 with thermal ellipsoids drawn
at the 30% probability level.
Scheme 1
Results and Discussion
Synthesis and Structural Features of the Amino-Linked
Carbene. Imidazolium salts containing a covalently linked
ammonium group (1) were first reported by the Arnold group
(Scheme 1).2b However, salts such as 1 contain two cationic
groups, leading to solubility problems in organic solvents and
therefore making it difficult to work with strong organic bases
like alkyl lithium. Thus, altering the ionic character in 1 may
have a desirable effect on the compound’s solubility. Indeed, 1
equiv of HBr can be removed from 1 with a slight excess of
NaOH to afford the amine-imidazolium bromide 2 with ease.
The identity of 2 is confirmed by NMR spectroscopy, elemental
analysis, and single-crystal X-ray diffraction. The solid state
1
structure of 2 is illustrated in Figure 1. The H and 13C NMR
chemical shifts of the N-CH-N fragment for compound 2 (δ
1H 9.70 ppm, δ 13C 141.0 ppm) are in the range observed for
imidazolium salts. The structural parameters within the cation
are unremarkable and consistent with those observed previously
in other salts,8 having a typical large internal N(1)-C(10)-N(2)
angle of 109.1(2)° and short carbenium-nitrogen distances of
1.337(3) and 1.325(3) Å. Of more interest is the close contact
between the ion pairs, associated through C-H · · · Br (2.915
Å, 3.786 Å, 155.1°) and N-H · · · Br(2.859 Å, 3.677 Å, 155.9°)
interactions.9
Compound 2 is much more soluble in THF than compound
1 and can be deprotonated by NaN(TMS)2 to afford free carbene
3 under mild conditions. The 13C NMR spectrum of 3 in C6D6
shows a carbenic signature peak at δ 216.3, and the structural
identity of 3 is unambiguously established by single-crystal
X-ray diffraction (Figure 2). Compound 3 represents a rare
example of a structurally characterized N-heterocyclic carbene
possessing a covalently linked functionalized pendant moiety.
Crystallographic data for this type of free carbene species remain
sparse. Recently, Fryzuk et al. have isolated the first stable free
carbene featuring a mesityl substituent on an amino pendant
arm, (mesityl)NHCH2CH2-[NC(CHCH)N(mesityl)]. Unfortu-
nately, details concerning its structural characterization and
stability are not fully discussed in the paper.10 The structural
parameters of 3 are comparable to both the free carbene iso-
lated by Arduengo,8 and Fryzuk’s (mesityl)NHCH2CH2-[NC-
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Org. Lett. 2001, 3, 2229–2231. (e) Biswas, K.; Prieto, O.; Goldsmith, P. J.;
Woodward, S. Angew. Chem., Int. Ed. 2005, 44, 2232–2234. (f) Sales, Z. S.;
Nassar, R.; Morris, J. J.; Henderson, K. W. J. Organomet. Chem. 2005,
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1062 Organometallics, Vol. 28, No. 4, 2009
Shih et al.
Figure 4. Molecular diagram of 3-LiBr with thermal ellipsoids
drawn at the 30% probability level, showing two symmetry-
independent, but chemically similar molecules. Hydrogen atoms
have been omitted for clarity. Selected bond distances (Å) and
angles (deg): Br(1)-Li(1) 2.514(7), Li(1)-O(1) 1.977(7), Li(1)-C(7)
2.125(7), Li(1)-N(1) 2.186(8), Br(2)-Li(2) 2.488(8), Li(2)-O(2)
1.975(8), Li(2)-C(29) 2.144(8), N(2)-C(7) 1.360(5), N(3)-C(7)
1.367(5),N(5)-C(29)1.355(5),N(6)-C(29)1.368(5),N(2)-C(7)-N(3)
101.8(3), N(5)-C(29)-N(6) 102.3(3).
Figure 2. Molecular diagram of 3 with thermal ellipsoids drawn
at the 30% probability level. Atom H3 was located from the
difference map and refined with a riding model. All other hydrogen
atoms are omitted for clarity. Selected bond lengths (Å) and angles
(deg): C(10)-N(1) 1.3702(14), C(10)-N(2) 1.3613(15), C(11)-C(12)
1.3421(17), N(2)-C(10)-N(1) 101.51(10).
ylidene and diphenylamine.12 The slightly longer interaction in
3 may be a consequence of the unfavorable steric crowding
incurred by the close proximity between the tert-butyl group
and mesityl sidearm. Regardless, this is an unusual circumstance
involving a nontraditional hydrogen bond between a more
electronegative nitrogen and a carbon atom. In this case, the
nonbonded tert-butyl amine of the carbene may donate its lone
pair into the empty pπ orbital of the carbenic center of another
molecule, which may further disrupt the rather weak aromatic
conjugation of 3 from the irregular π donation of Naryl and Nalkyl
,
Figure 3. View of a one-dimensional chain of NHC 3 formed by
two N(3)-H(3) · · · C(10) interactions (blue dotted lines) and two
π-π interactions between the imidazole moiety and the mesityl
ring (red dotted lines).
and hence it downgrades the overall stability of the compound.
Preparation of a Thermally Stable Lithium Bromide
Carbene Adduct as a Potential Carbene Transfer Agent. To
validate our thoughts with respect to the instability of 3, we
attempted to cap the tBu-amino pendant arm and prevent it from
interacting with another carbene molecule by generating a
lithium halide carbene adduct. Deprotonation of 2 with LiN-
(SiMe3)2 at 0 °C in THF afforded the carbene species 3-LiBr
in high yield (Scheme 1). The 1H NMR resonances demonstrate
that 3-LiBr is uniquely different from 3. Due to 13C-7Li spin
coupling, a rather broad signature peak for the carbene carbon
of 3-LiBr appears at 201 ppm in the 13C NMR spectrum,
signaling the possible formation of a lithium-carbene adduct.
A single-crystal X-ray diffraction study was undertaken to
determine the connectivity in 3-LiBr, and a structural model is
presented in Figure 4. There are two independent units in the
asymmetric unit, in which both lithium atoms exhibit pseudot-
etrahedral geometry by forming three dative bonds with NHC,
amine, and THF. The Li-C distances (Li(1)-C(7) 2.125(7) and
Li(2)-C(29) 2.144(8) Å) are the shortest reported for a lithium-
carbene adduct. For comparison, [Li{C5H2(SiMe3)3{C(NtBu-
CH)2}] has a Li-C bond length of 2.155 Å,13 while the dimer
[tBuNHCH2CH2[C{tBuN(CHCH)N}] · Li-Br]2, prepared by the
(CHCH)N(mesityl)]. Specifically, 3 possesses almost statistically
equal bond lengths for both carbene-nitrogen bonds (N(2)-C-
(10)alkyl ) 1.3613(1) Å, N(1)-C(10)aryl ) 1.370(1) Å, and
N(1)-C(10)-N(2) ) 101.51(10)°). The slightly different bond
lengths in the N(1)-C(10)-N(2) moiety may exemplify the
prospect of irregular π donation of Naryl and Nalkyl toward
carbenic center.
Notably, compound 3 is thermally unstable. Heating a solution
of 3 in C6D6 at 60 °C results in the product’s decomposition
within a few minutes. In addition, decomposition occurs slowly
at room temperature under a nitrogen atmosphere over several
hours to intractable products. Although the lack of sufficient
steric protection is a major contributor toward the overall
stability of carbene 3, we speculate that uneven aromatic
conjugation may further aggravate this situation.11 Careful
inspection of the crystal packing of 3 (Figure 3) reveals an
unexpectedly close intermolecular contact associated with the
carbenic carbon and the amine through a N-H · · · C interaction
(N(3)-H(3) · · · C(10), 2.507 Å, 3.404 Å, 170°).9 The intermo-
lecular interaction is somewhat longer than that reported in a
similar case between cocrystallized 1,3-dimethylimidazol-2-
(12) Cowan, J. A.; Clyburne, J. A. C.; Davidson, M. G.; Harris, R. L. W.;
Howard, J. A. K.; Kupper, P.; Leech, M. A.; Richards, S. P. Angew. Chem.,
Int. Ed. 2002, 41, 1432–1434.
(13) Arduengo, A. J.; Tamm, M.; Calabrese, J. C.; Davidson, F.;
Marshall, W. J. Chem. Lett. 1999, 1021–1022.
(11) (a) Kausamo, A; Tuononen, H. M.; Krahulic, K. E.; Roesler, R.
Inorg. Chem. 2008, 47, 1145–1154. (b) Diez-Gonzalez, S.; Nolan, S. P.
Coord. Chem. ReV. 2007, 251, 874–883.

Amino-Linked N-Heterocyclic Carbene
Organometallics, Vol. 28, No. 4, 2009 1063
Scheme 2
Arnold group, has a much longer Li-C distance of 2.196 Å.2b
The molecular structure of 3-LiBr also reveals that the amine
function is bonded to the Li+ center rather than demonstrating
any noticeable interaction with the carbene carbon and thus
preventing any possible interruption of π-conjugation within
the carbene ring system. 3-LiBr can be stored in its crystalline
state at room temperature for indefinite periods of time without
any signs of decomposition. To our delight, when a toluene
solution of 3-LiBr was heated to 100 °C for several hours, there
was also no indication of decomposition.
catalytic amounts of either NHC or imidazolium salts.6d The
authors believe NHC-AlR3 to be the active catalytic species for
this process, but no subsequent mechanistic studies concerning
this catalyst system have been reported. In addition, no specific
reactivity experiments have been undertaken to explore the
nature of the Al-carbene bonding, despite the significant focus
in recent chemistry on the use of boron-carbene or phosphine
Lewis base pairs with respect to small molecule activation.7b-h
On the basis of this finding, we should anticipate reactivity
between an electrophile and compound 4. Exposure of a solution
of 4a in C6D6 to benzaldehyde resulted in the quantitative
Preparation and Characterization of Organoaluminum
Complexes Supported by an Amino-Linked Carbene. The
excellent thermal stability displayed by 3-LiBr prompted an
examination of its effectiveness as a carbene transfer agent,
similar to the well-known silver NHC species. Amino-pendant-
linked NHC 3-LiBr was readily introduced to AlR3 (R ) Me
(a), Et (b)) in THF to produce 4a and 4b, respectively, in good
1
formation of a new species, 5, over a period of 5 min. The H
NMR spectrum of 5 indicates the incorporation of a single
equivalent of benzaldehyde, whereas the integration of the Al-
CH3 signal remains unchanged at 9H. The disappearance of the
carbenium carbon at δ 177.0 ppm in the 13C NMR spectrum of
5, corresponding to the rise of a new signal at 153.2 ppm,
strongly suggests that the NHC is acting as a noninnocent ligand.
An X-ray diffraction study of a white crystal of complex 5
confirmed a somewhat surprising outcome for this reaction, as
presented in Figure 6.
1
yield (Scheme 2). The H NMR spectroscopic features of 4a
are notably different than 3-LiBr, displaying a new singlet
integrating to 9 H at -0.48 ppm for the Al-Me protons and a
sharp triplet (3J ) 8.0 Hz) integrating to 1 H at 0.26 ppm for
the amine proton. 4b also exhibits a similar proton NMR pattern
to 4a, with one singlet for the amino-tert-butyl group at 0.85
ppm integrating to 9 protons and two more singlets for the
mesityl groups at 2.06 and 1.90 ppm integrating to a total of 9
protons. With the exception of the high-field resonances, the
aluminum ethyl groups of 4b are observed at 1.45 (t, 9H) and
0.09 ppm (q, 6H). The shift in the high-field 13C resonance at
177 and 176 ppm in 4a and 4b, respectively, compared to that
in 3-LiBr (207 ppm) and 3 (216 ppm), is attributed to the
depletion of the NHC’s electron density by the strong Lewis
acidity of the Al center.
Rather than undergoing alkyl addition, compound 5 displays
insertion of the carbonyl moiety into the Al-carbene group.
Complex 5 is a zwitterionic species consisting of a distorted
pseudotetrahedral aluminate center covalently linked to imida-
zolium. To the best of our knowledge, there are no reported
structures of imidazolium-linked aluminate species, the closest
example resembling 5 being the imidazolium tetraphenoxyalu-
minate salts [IMeMe · H][Al(OPh)4] (IMeMe ) tetramethylimi-
Single crystals of 4a suitable for X-ray analysis were grown
from a concentrated ether solution at -20 °C and represent the
first isolated organoaluminum complex supported by an amino-
pendant-linked NHC (Figure 5). The structure consists of a
tetrahedrally distorted Al center bearing a dangling nonbonded
amino sidearm with an Al(1)-N(3) distance of 5.40 Å. The
nonbonding nature of the amine may be due to the coordination
of the electron-rich carbene and its saturation of the Al center.
To a lesser extent, steric repulsion may also play a role in
discouraging such an Al-amino interaction. The Al-carbene
bond length is 2.074(2) Å, in line with reported AlMe3-1,3-
diisopropyl-4,5-dmethylimidazol-2-ylidene B (2.082(3) Å). How-
ever, the independent Al-Me bond distances in 4a (1.980(2),
1.985(3), and 1.995(2) Å) are slightly shorter than those in B
(2.062(7) and 1.940 (5) Å).5f The bond lengths (1.365(3) and
1.360(2)Å)andbondangle(103.13(18)°)intheN(1)-C(10)-N(2)
moiety of 4a are not significantly different from those of the
free carbene, 3.
Figure 5. Molecular diagram of 4a with thermal ellipsoids drawn
at the 30% probability level. Atom H3 was located from the
difference map and refined with a riding model. All other hydrogen
atoms are omitted for clarity. Selected bond distances (Å) and angles
(deg): Al-C(21) 1.980(2), Al-C(19) 1.985(3), Al-C(20) 1.995(2),
Al-C(10) 2.074(2), N(1)-C(10) 1.365(3), N(2)-C(10) 1.360(2),
C(21)-Al-C(19)111.37(12),C(21)-Al-C(20)111.50(12),C(19)-Al-C(20)
116.08(11),C(21)-Al-C(10)109.00(10),(19)-Al-C(10)108.34(11),
C(20)-Al-C(10) 99.70(10), N(2)-C(10)-N(1) 103.13(18).
Reactivity of Al-NHCs with Benzaldehyde. Nguyen et al.
reported the high-yield synthesis of alcohols from the ring-
opening reaction of epoxides with triethylaluminum using

1064 Organometallics, Vol. 28, No. 4, 2009
Shih et al.
In the presence of benzophenone, compound 6 undergoes a
similar chemical transformation to that of 5 to afford 7, as
1
observed by H NMR spectroscopy (Scheme 4). The atom
connectivity in 7 was further confirmed by single-crystal X-ray
diffraction (Figure 8). Although the free amino pincer arm did
play a role in the carbene stability of 3 (Vide supra), the control
experiment failed to support the hypothesis that the dangling
amino arm helps to mediate the insertion of carbonyl into the
Al-carbene bond. Thus, an alternative pathway to afford 5 and
7 has also been considered, involving intermolecular nucleo-
philic attack of the NHC ligand via a five-coordinate Al complex
(pathway B). Considering the fact that there are several cases
of isolated five-coordinate organoaluminum complexes sup-
ported by a structurally rigid multidentate framework,17 the latter
pathway is the more likely of the two.
Conclusion
Figure 6. Molecular diagram of 5 with thermal ellipsoids drawn
at the 30% probability level. Atom H3 was located from the
difference map and refined with a riding model. All other hydrogen
atoms have been omitted for clarity. Selected bond distances (Å)
and angles (deg): Al(1)-O(1) 1.8030(12), Al(1)-C(1) 1.988(2),
Al(1)-C(3) 1.9960(19), Al(1)-C(2) 1.999(2), N(1)-C(11) 1.348(2),
N(2)-C(11) 1.341(2), O(1)-C(10) 1.381(2), C(9)-C(10) 1.529(2),
C(10)-C(11)1.517(2),O(1)-Al(1)-C(1)104.16(8),O(1)-Al(1)-C(3)
107.52(7), C(1)-Al(1)-C(3) 111.26(9), O(1)-Al(1)-C(2) 108.81(7),
C(1)-Al(1)-C(2)111.78(9),C(3)-Al(1)-C(2)112.82(8),N(2)-C(11)-N(1)
107.16(14).
In conclusion, a soluble amino-linked imidazolium salt, 2,
has been synthesized. Deprotonation of 2 with NaN(TMS)2
produced the marginally stable compound 3, whose crystal
structure suggests that the instability is the consequence of a
detrimental effect arising from the interaction of the amino
sidearm with the free carbene center. The rare example of an
amino-linked NHC-Al complex, 4, was obtained, and its solid
state structure is consistent with the formation of monomeric
distorted tetrahedral Al species in which the NHC binds via
donation of the lone pair to the electrophilic metal center,
without coordinating to the dangling arm of tert-butylamine.
Benzaldehyde readily inserts into the Al-carbene bond of 4,
affording a zwitterionic complex consisting of a distorted
pseudotetrahedral aluminate center covalently linked to imida-
zolium, 5. A similar chemical transformation with benzaldehyde
was observed for compound 6, [C{(MES)N(CHCH)N-
(MES)}] · AlMe3, suggesting that the dangling amino arm of
NHC is not responsible for mediating the insertion of carbonyl
into the Al-carbene bond. We hope that these observations with
regard to amino-pendant-linked NHC and their Al complexes
will open up new avenues for chemical reactivity with other
main group elements. Further experimental and theoretical
studies on Al complexes supported by amino-linked NHC are
now underway.
dazol-2-ylidene).14 The Al-O distance of 1.803(1) Å in 5 is
slightly longer than those reported for the Al-OPh analogues
(1.730(3), 1.742(3), 1.709(3), 1.736(3) Å), whereas the average
Al-CH3 distance for 5 (∼1.994 Å) is very similar to that of
4a. The cationic component of 5 has intra-ring parameters
consistentwithtypicalimidazoliumsalts,wheretheN(1)-C(11)-N(2)
angle of 107.1(1)° is larger than its parent carbene 3 (101.51(10)°),
indicating a significant enhancement in electronic delocalization
about the heterocycle.
Recent literature shows that NHC-transition metal bonds are
not inert and NHC ligands are able to participate in various
inter- and intramolecular reactions.15,16 However, thus far, there
have been no reports of experimental work with regard to
the reactivity of Al-NHC. One of our hypotheses suggests the
ability of the dangling amino group on the Al complex to serve
as a directing group for the nucleophilic attack by the carbene
via intermolecular hydrogen bonding with the carbonyl group,
as shown in Scheme 3, pathway A. We sought additional
evidence for this pathway by further preparing the separate
compound AlMe3-NHC, 6, independently synthesized by the
addition of 1,3-bis(mesityl)imidazol-2-ylidene to AlMe3 (Scheme
4). Interestingly, the crystal structure determination of 6,
analogous to the reported AlMe3-1,3-diisopropyl-4,5-dimeth-
ylimidazol-2-ylidene, B, has not been reported to date in the
literature. The structure of product 6 is confirmed by single-
crystal X-ray diffraction (Figure 7). The bond distances of
Al-Cmethyl in 6 (1.975(2), 1.987 (2), 1.987(2) Å) are no different
from the amino-linked NHC of 4a. However, the presence of a
slightly longer Al-carbenic bond of 2.097(2) Å, compared to
4a, indicates that 1,3-bis(mesityl)imidazol-2-ylidene is a less
electron rich carbene than the amino-linked NHC of 4a.
Experimental Section
General Procedures. All air-sensitive manipulations were
performed under an atmosphere of nitrogen using Schlenk tech-
niques and/or a glovebox. Toluene, hexane, THF, and ether were
purified by passage through a column of activated alumina using a
solvent purification system purchased from Innovative Technology,
Inc. Deuterated benzene and toluene were dried by vacuum transfer
from activated molecular sieves. LiN(SiMe3)2, AlMe3 (2.0 M in
toluene), and AlEt3 (1.0 M in hexane) were purchased from Aldrich
and used without further purification. NaN(SiMe3)2 was obtained
from Alfa Aesar. 1H and 13C NMR spectra were recorded on Bruker
300 MHz, Bruker 400 MHz, and Bruker 500 MHz spectrometers
using the residual proton signal in the deuterated solvent for
reference.Compound1,2b[HC{(MES)N(CHCH)N(CH2CH2NHtBu}]-
HBr2, and 1,3-bis(mesityl)imidazol-2-ylidene18 were prepared as
previously reported.
(14) Cole, M. L.; Hibbs, D. E.; Jones, C.; Junk, P. C.; Smithies, N. A.
Inorg. Chim. Acta 2005, 358, 2455–2455.
(15) Becker, E.; Stingl, V.; Dazinger, G.; Puchberger, M.; Mereiter, K.;
Kirchner, K. J. Am. Chem. Soc. 2006, 128, 6572–6573.
(16) Fantasia, S.; Jacobsen, H.; Cavallo, L.; Nolan, S. P. Organometallics
2007, 26, 3286–3288.
(17) (a) Rutherford, D; Atwood, D. A. Organometallics 1996, 15, 4417–
4422. (b) Munoz-Hernandez, M.-A.; Sannigrahi, B.; Atwood, D. A. J. Am.
Chem. Soc. 1999, 121, 6747–6748. (c) Munoz-Hernandez, M.-A.; Keizer,
T. S.; Wei, P.; Parkin, S.; Atwood, D. A. Inorg. Chem. 2001, 40 (26), 6782–
6787.

Amino-Linked N-Heterocyclic Carbene
Organometallics, Vol. 28, No. 4, 2009 1065
Scheme 3
Table 1. Crystallographic Data and Refinement for 2, 3, and 3-LiBr
Scheme 4
2
3
3-LiBr
formula
fw
C18H28BrN3
366.34
C18H27N3
285.43
C22H35BrLiN3O
444.38
cryst syst
space group
a (Å)
b (Å)
c (Å)
monoclinic
P2(1)/n
8.7287(3)
17.7547(5)
12.5192(4)
90
105.4620(10) 88.846(2)
90
4
triclinic
P1
9.3823(3)
9.6495(4)
10.0070(4)
83.027(2)
triclinic
P1
j
j
12.1381(5)
14.1671(5)
14.4333(5)
91.415(2)
106.716(2)
95.938(2)
4
1.250
59612/8257
0.0462
R (deg)
ꢀ (deg)
γ (deg)
74.717(2)
2
1.093
14210/3903
0.0219
1.029
Z
Dcalcd (mg/m3)
reflns collected
indep reflns (Rint)
1.301
15687/3289
0.0358
Preparation of [HC{(MES)N(CHCH)N(CH2CH2NHtBu}]Br (2).
To a solution of 1.0 M sodium hydroxide was added compound 1
(4.000 g, 8.90 mmol). The mixture was stirred at room temperature
for 4 h. The resulting solution was extracted with CH2Cl2, and the
organic layer was dried over MgSO4. The solvent was removed in
Vacuo to afford a white residue, which was dissolved in 50 mL of
acetone/CH2Cl2 (50:50 by volume) and cooled to -10 °C to afford
pure white microcrystals. Yield: 85% (2.790 g). 1H NMR (CDCl3,
400 MHz, 25 °C): δ 9.70 (br, 1H, NCHN), 8.01 (s, 1H,CH), 7.05
(s, 1H, CH), 6.85 (s, 2H, C6H2), 4.64 (t, 3JH-H ) 6 Hz, 2H, CH2),
goodness-of-fit on F2 1.027
1.042
R1, wR2 [I > 2σ(I)] 0.0338/0.0759 0.0443/0.1261 0.0503/0.1294
28.9 (tBu), 20.9 (Ar-Me), 17.2 (Ar-Me). Anal. Calcd for
C18H28BrN3: C, 59.01; H, 7.70; N, 11.47. Found: C, 58.64; H, 8.05
N, 11.50.
Preparation of [C{(MES)N(CHCH)N(CH2CH2NHtBu}] (3). To
the suspension of 3 (3.680 g, 10.0 mmol) in 40 mL of THF was
added a THF solution of NaN(TMS)2 (1.830 g, 10.0 mmol) at -
3
2.88 (t, JH-H ) 6 Hz, 2H, CH2), 2.20 (s, 3H, CH3), 1.93 (s, 6H,
CH3), 0.85 (s, 9H, tBu). 13C NMR (CDCl3, 100 MHz, 20 °C): δ
141.0 (NCHN), 138.2 (Ar), 134.2 (Ar), 130.7 (Ar), 129.6 (Ar), 123.6
(NCH), 122.1 (NCH), 50.6 (NCH2), 49.9 (NCMe3), 42.1 (NCH2),
Figure 7. Molecular diagram of 6 with the thermal ellipsoids drawn
at the 30% probability level. All other hydrogen atoms are omitted
for clarity. Selected bond distances (Å) and angles (deg): Al-C(1)
1.9750(19), Al-C(2) 1.9870(15), Al-C(2A) 1.9870(15), Al-C(3)
2.0965(16),N(1)-C(3)1.359(2),N(2)-C(3)1.365(2),N(1)-C(3)-N(2)
103.44(13).
Figure 8. Molecular diagram of 7 with the thermal ellipsoids drawn
at the 30% probability level. All other hydrogen atoms are omitted
for clarity.

1066 Organometallics, Vol. 28, No. 4, 2009
Shih et al.
Table 2. Crystallographic Data and Refinement for 4a, 5, 6, and 7
4a
5
6
7
formula
fw
C21H36AlN3
357.51
C28H42AlN3O
463.63
C22H27AlCl2N2
417.34
C31H39AlN2O
482.62
cryst syst
space group
a (Å)
b (Å)
c (Å)
orthorhombic
Pbca
8.4077(17)
16.499(3)
32.626(7)
90
90
90
8
1.049
15 914/3972
0.0473
1.082
0.0534/0.1411
triclinic
P1
orthorhombic
Pnma
23.0321(9)
12.1449(5)
7.9660(3)
90
90
90
4
1.244
18 391/2577
0.0277
1.057
0.0737/0.2406
monoclinic
P2(1)/n
9.5070(2)
15.8593(4)
19.3497(5)
90
92.5880(10)
90
4
1.100
26 403/6678
0.0693
1.022
0.0549/0.1257
j
9.6323(3)
12.5350(4)
13.8141(7)
108.364(2)
108.548(2)
103.3880(10)
2
1.104
17 488/4881
0.0330
R (deg)
ꢀ (deg)
γ (deg)
Z
Dcalcd (mg/m3)
reflns collected
indep reflns (Rint)
goodness-of-fit on F2
R1, wR2 [I > 2σ(I)]
1.077
0.0439/0.1049
20 °C. After stirring for 10 min, the insoluble salt was removed
from the reaction mixture by filtering through Celite to afford a
light yellowish solution. The volatiles were removed under vacuum,
and the product was further purified by crystallization from a
Preparation of [C{(MES)N(CHCH)N(CH2CH2NHtBu}] · AlEt3
(4b). To a THF solution of 3-LiBr (10.0 mmol, 4.430 g) was added
1.0 M AlEt3 in hexane (10.0 mmol, 10 mL) at room temperature.
After stirring for 12 h, the solvent was removed in Vacuo to afford
a white powdered solid. The crude solid was extracted with toluene
(30 mL), and the white insoluble precipitate was removed by
filtration. The filtrate was dried in Vacuo to afford a white solid,
which was further purified by recrystallization from ether/hexane
1
concentrated ether solution at -30 °C. Yield: 67% (1.910 g). H
NMR (C6D6, 500 MHz, 25 °C): δ 6.75 (s, 2H, C6H2), 6.64 (d, 3JH-H
3
) 1.5 Hz, 1H, CH), 6.39 (d, JH-H ) 1.5 Hz, 1H, CH), 4.00 (t,
3JH-H ) 6.0 Hz, 2H, CH2), 2.83 (dt, 2H, CH2), 2.11 (s, 3H, CH3),
3
1
2.07 (s, 6H, CH3), 1.35 (t, JH-H ) 7.2 Hz, H, NH), 0.98 (s, 9H,
(50:50 volume) at -30 °C. Yield: 70% (2.80 g). H NMR (C6D6,
tBu). 13C NMR (C6D6, 125 MHz, 25 °C): 216.3 (Ccarbene), 139.3
(Ar), 137.0 (Ar), 135.4 (Ar), 129.0 (Ar), 120.2 (NCH), 119.5 (NCH),
52.3 (NCH2), 49.9 (NCMe3), 44.2 (NCH2), 29.2 (tBu), 20.9 (Ar-
Me), 18.1 (Ar-Me). HR-MS (FAB): m/z [(M + H)+] calculated for
C18H28N3 286.2278, found 286.2287.
500 MHz, 25 °C): δ 6.79 (d, JH-H ) 1.5 Hz, 1H, CH), 6.69 (s,
3
2H, C6H2), 5.90 (d, 3JH-H ) 1.5 Hz, 1H, CH), 4.04 (t, 3JH-H ) 5.5
Hz, 2H, CH2), 2.64 (m, 2H, CH2), 2.06 (s, 3H, CH3), 1.90 (s, 6 H,
3
CH3), 1.45 (t, JH-H ) 8 Hz, 9H, AlCH2CH3), 0.85 (s, 9H, tBu),
3
3
0.27 (t, JH-H ) 8.0 Hz, 1H, NH), 0.09 (q, JH-H ) 8 Hz, 6H,
AlCH2). 13C NMR (C6D6, 125 MHz, 25 °C): 176.1 (br, Ccarbene),
139.2 (Ar), 135.8 (Ar), 135.1 (Ar), 129.3 (Ar), 122.2 (NCH), 121.4
(NCH), 50.8 (NCH2), 50.1 (NCMe3), 43.6 (NCH2), 28.9 (tBu), 21.0
(Ar-Me), 17.5 (Ar-Me), 11.5 (CH2CH3), 0.8 (CH2). HR-MS (EI):
m/z [(M - Et)+] calculated for C22H37AlN3 370.2803, found
370.2798.
Preparation of [C{(MES)N(CHCH)N(CH2CH2NHtBu}] · LiBr-
(THF) (3-LiBr). To a suspension of 2 (3.680 g, 10.0 mmol) in 40
mL of THF was added a solution of LiN(TMS)2 (1.670 g, 10.0
mmol) in THF at -20 °C. After stirring for 10 min, the solvent
was removed in Vacuo to afford a white powdery solid, which was
further purified by crystallization from toluene/THF (70:30 by
volume) at -30 °C to afford clear colorless crystals. Yield: 83%
(3.100 g). 1H NMR (C6D6, 500 MHz, 25 °C): δ 6.77 (s, 2H, C6H2),
6.44 (s, 1H, CH), 6.19 (s, 1H, CH), 4.00 (br, 2H, CH2), 3.54 (t,
3JH-H ) 6.0 Hz, 4H, THF), 2.62 (br, 2H, CH2), 2.12 (s, 3H, CH3),
2.08 (s, 6H, CH3), 1.81 (br, 1H, NH), 1.39 (t, 3JH-H ) 6.0 Hz, 4H,
THF), 1.06 (s, 9H, tBu). 13C NMR (C6D6, 125 MHz, 25 °C): 206.5
(br, Ccarbene), 139.5 (Ar), 137.3 (Ar), 135.6 (Ar), 129.0 (Ar), 120.0
(NCH), 119.0 (NCH), 67.8 (THF), 51.5 (NCH2), 50.4 (NCMe3),
44.4 (NCH2), 29.1 (tBu), 25.7 (THF), 21.4 (Ar-Me), 18.2 (Ar-Me).
Mp: 122 °C. Anal. Calcd for C22H35BrLiN3O: C, 59.46; H, 7.94;
N, 9.46. Found: C, 59.45; H, 7.56; N, 9.40.
Preparation of (Me3Al-OCHPh)[C{(MES)N(CHCH)N(CH2CH2-
NHtBu}] (5). To a THF solution of 4 (0.28 mmol, 0.100 g) was
added benzaldehyde (0.28 mmol, 0.030 g) at room temperature.
After stirring for 2 h, the solvent was removed in Vacuo to afford
a yellow residue, which was further purified by recrystallization
from ether at -30 °C to afford colorless crystals. Yield: 61.8%
1
(0.073 g). H NMR (C6D6, 400 MHz, 25 °C): δ 7.26-6.99 (m,
5H, Ph), 6.64 (s, 1H, CH), 6.53 (s, 1H, C6H2), 6.50 (s, 1H, C6H2),
6.18 (s, 1H, CH), 5.62 (s, 1H, PhCH), 4.91 (m, 1H, CH2), 3.81 (m,
1H, CH2), 2.76 (m, 1H, CH2), 2.44 (m, 1H, CH2), 2.06 (s, 3H,
CH3), 1.98 (s, 3H, CH3), 1.35 (s, 3H, CH3), 0.88 (s, 9H, tBu), 0.63
3
(dd, JH-H ) 8.4 Hz, 1H, NH), -0.31 (s, 9H, AlMe3). 13C NMR
Preparation of [C{(MES)N(CHCH)N(CH2CH2NHtBu}] · AlMe3
(4a). To a THF solution of 3-LiBr (10.0 mmol, 4.430 g) was added
2.0 M AlMe3 in toluene (10.0 mmol, 5 mL) at room temperature.
After stirring for 12 h, the solvent was removed in Vacuo to afford
a white solid. The crude solid was extracted with toluene (30 mL),
and the white insoluble precipitate was removed by filtration. The
filtrate was dried in Vacuo to afford a white solid, which was further
purified by recrystallization from ether/hexane (50:50 volume) at
(C6D6, 100 MHz, 25 °C): 152.9 (NCN), 143.8 (Ar), 141.3 (Ar),
135.8 (Ar), 135.2 (Ar), 131.3 (Ar), 130.4 (Ar), 130.0 (Ar), 128.8
(Ar), 128.3 (Ar), 128.1 (PhCO), 127.7 (Ar), 123.2 (NCH), 119.8
(NCH), 72.0 (NCMe3), 50.5 (NCH2), 42.7 (NCH2), 29.4 (tBu), 21.3
(Ar-Me), 18.3 (Ar-Me), 17.6 (Ar-Me), -5.4 (AlMe3). Mp: 93 °C.
Anal. Calcd for C28H42AlN3O: C 72.54, H 9.13, N 9.06. Found: C
72.37, H 9.06, N 8.99.
1
Preparation of [C{(MES)N(CHCH)N(MES)}] · AlMe3 (6). To a
THF solution of 1,3-bis(mesityl)imidazol-2-ylidene (10 mmol,
3.04 g) was added AlMe3 in toluene (10.0 mmol, 5 mL) at room
temperature. After stirring for 12 h, the solvent was removed in
Vacuo to afford a white powdered solid, which was further purified
by recrystallization from toluene/THF (70:30 by volume) at -30
-30 °C. Yield: 75% (2.68 g). H NMR (C6D6, 400 MHz, 25 °C):
3
δ 6.69 (s, 2H, C6H2), 6.67 (d, JH-H ) 1.5 Hz, 1H, CH), 5.91 (d,
3JH-H ) 1.5 Hz, 1H, CH), 4.00 (t, 3JH-H ) 5.8 Hz, 2H, CH2), 2.62
(m, 2H, CH2), 2.05 (s, 3H, CH3), 1.90 (s, 6 H, CH3), 0.84 (s, 9H,
3
tBu), 0.26 (t, JH-H ) 8.0 Hz, 1H, NH), -0.48 (s, 9H, CH3). 13C
NMR (C6D6, 125 MHz, 25 °C): 177.0 (br, Ccarbene), 139.2 (Ar), 135.3
(Ar), 135.3 (Ar), 129.2 (Ar), 122.2 (NCH), 121.0 (NCH), 50.9
(NCH2), 50.1 (NCMe3), 43.7 (NCH2), 28.9 (tBu), 21.0 (Ar-Me),
15.5 (Ar-Me), -7.2 (AlMe3). Mp: 59 °C. Anal. Calcd for
C21H36AlN3: C, 70.55; H, 10.15; N, 11.75. Found: C, 70.12; H,
10.30; N, 11.59.
1
°C. Yield: 75% (2.82 g). H NMR (C6D6, 500 MHz, 25 °C): δ
(18) Arduengo, A. J.; Krafczyk, R.; Schmutzler, R.; Hugh, C. A.;
Goerlich, J. R.; Marshall, W. J.; Unverzagt, M. Tetrahedron 1999, 55,
14523–14534.

Amino-Linked N-Heterocyclic Carbene
Organometallics, Vol. 28, No. 4, 2009 1067
6.82 (s, 4H, C6H2), 6.06 (s, 2H, CH), 2.15 (s, 6H, CH3), 2.07 (s,
12H, CH3), -0.75 (s, 9H, Me). 13C NMR (C6D6, 125 MHz, 25
°C): 178.2 (br, Ccarbene), 139.4 (Ar), 135.5 (Ar), 135.3(Ar), 129.3
(Ar), 122.7 (NCH), 21.0 (Ar-Me), 17.6 (Ar-Me), -7.6 (Me).
Preparation of (Me3Al-OCHPh)[C{(MES)N(CHCH)N(MES)}]
(7). To a THF solution of 6 (1 mmol, 0.376 g) was added
benzaldehyde (1.1 mmol, 0.111 g) at room temperature. After
stirring for 2 h, the solvent was removed in Vacuo to afford a yellow
residue, which was further purified by recrystallization from toluene/
THF (70:30 by volume) at -30 °C to afford yellow crystals. Yield:
50% (0.241 g). Suitable crystals for crystallographic determination
were grown from a hexane diffusion into a THF solution. 1H NMR
(d8-THF, 500 MHz, 25 °C): δ 7.56 (s, 2H, C6H2), 7.15 (s, 2H, CH),
(PhCO), 21.3 (Ar-Me), 18.6 (Ar-Me), 17.7 (Ar-Me), -6.8 (Me).
HR-MS (EI): [(M - Me)+] calculated for C30H36AlN2O 467.2643;
found 467.2643.
Acknowledgment. We are grateful to the Taiwan National
Science Council (NSC: 96-2113-M-001-013-MY2) and Aca-
demia Sinica for their generous financial support. We would
also like to thank Prof. Darrin Richeson (University of
Ottawa, Ontario) and Prof. Jennifer Scott (Royal Military
College of Canada, Ontario) for helpful discussions and Prof.
Michael Fryzuk for supplying the crystal structure informa-
tion of (mesityl)-NHCH2CH2-[NC(CHCH)N(mesityl).
3
7.02 (dd, JH-H ) 7.0 Hz, 2H, Ph), 6.94-6.91 (m, 2H, Ph), 6.93
Supporting Information Available: Crystal data are also
available as CIF files for compounds 2, 3, 3-LiBr, 4a, 5, 6, and 7.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(s, 2H, C6H2), 6.78 (d, 3JH-H ) 7.5 Hz, 2H, Ph), 5.83 (s, 1H, PhCH),
2.38 (s, 6H, CH3), 2.30 (s, 6H, CH3), 1.71 (s, 6H, CH3), -1.47 (s,
9H, Me). 13C NMR (d8-THF, 125 MHz, 25 °C): 153.3 (NCN), 142.6
(Ar), 141.6 (Ar), 136.8 (Ar), 136.6 (Ar), 132.5 (Ar), 130.3 (NCH),
130.1 (Ar), 128.1 (Ar), 127.94 (Ar), 127.90 (Ar), 124.1 (Ar), 70.5
OM800917J