Development of 7-membered N-heterocyclic carbene ligands for transition metals

Article abstract of DOI:10.1016/j.jorganchem.2005.08.022

We recently reported the first example of a seven-membered N-heterocyclic carbene (NHC) ligand for transition metals. These ligands are attractive because the heterocyclic framework, derived from 2,2′-diaminobiphenyl, exhibits a torsional twist that results in a chiral, C₂-symmetric structure. The present report outlines the synthetic efforts that led to the development of these ligands together with the synthesis and structural characterization of metal complexes bearing seven-membered NHCs as ancillary ligands. The identity of nitrogen substituent, neopentyl versus 2-adamantyl, influences the synthetic accessibility and stability of the seven-membered amidinium salts and the NHC-metal complexes obtained via in situ deprotonation/metallation. Computational analysis of the seven-membered ring structures reveals the Hückel antiaromatic 8π electron system achieves significant M?bius aromatic stabilization upon undergoing torsional distortion of the heterocyclic ring.

Full text of DOI:10.1016/j.jorganchem.2005.08.022

Journal of Organometallic Chemistry 690 (2005) 6143–6155
www.elsevier.com/locate/jorganchem
Development of 7-membered N-heterocyclic carbene ligands
for transition metals
*
Christopher C. Scarborough, Brian V. Popp, Ilia A. Guzei, Shannon S. Stahl
Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706, United States
Received 1 June 2005; received in revised form 12 August 2005; accepted 12 August 2005
Available online 22 September 2005
Abstract
We recently reported the first example of a seven-membered N-heterocyclic carbene (NHC) ligand for transition metals. These ligands
are attractive because the heterocyclic framework, derived from 2,20-diaminobiphenyl, exhibits a torsional twist that results in a chiral,
C2-symmetric structure. The present report outlines the synthetic efforts that led to the development of these ligands together with the
synthesis and structural characterization of metal complexes bearing seven-membered NHCs as ancillary ligands. The identity of nitro-
gen substituent, neopentyl versus 2-adamantyl, influences the synthetic accessibility and stability of the seven-membered amidinium salts
and the NHC–metal complexes obtained via in situ deprotonation/metallation. Computational analysis of the seven-membered ring
structures reveals the Huckel antiaromatic 8p electron system achieves significant Mo¨bius aromatic stabilization upon undergoing tor-
¨
sional distortion of the heterocyclic ring.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: N-heterocyclic carbene; Palladium; X-ray structure
1. Introduction
the spatial orientation of the substituents. Some success
has been achieved with chiral chelating NHC ligands [2c],
but the identification of new frameworks for the prepara-
tion of chiral NHCs could have important implications
for asymmetric catalysis. With this goal in mind, we have
prepared transition metal complexes possessing the first
examples of seven-membered NHC ligands (7, Fig. 1) de-
rived from 2,20-diaminobiphenyl (8H) [11,12]. These ligands
exhibit a torsional twist that results in an axially chiral
structure. Herein, we outline the synthetic efforts that led
to the development of this new class of NHC ligands, to-
gether with solid state structures of a silver(I) and several
palladium(II) complexes bearing these ligands.
N-heterocyclic carbenes (NHCs) continue to emerge as a
powerful class of ligands for transition metals [1–3].
Although most NHCs are derived from 5-membered het-
erocycles [4] (2–4, Fig. 1), 4- and 6-membered analogs have
also been reported recently (1, 5 and 6, Fig. 1) [5,6]. In
addition, transition metal complexes possessing acyclic car-
bene ligands have been described [7]. NHC ligands are
attractive because they exhibit electronic properties similar
to phosphines but tend to be less substitutionally labile
[2,8] and have greater stability under oxidative conditions
[2b,9,10]. Despite these advantages, successful applications
of NHCs in asymmetric catalysis still lag significantly be-
hind phosphines, and the development of new chiral
NHC ligands is an area of active interest [2c]. A key limita-
tion of most known NHCs is the presence of a planar or
nearly planar heterocyclic framework, which constrains
2. Results and discussion
2.1. Attempted synthesis of seven-membered-ring amidinium
salts with N-aryl substituents
*
Sterically encumbered aryl groups such as 2,6-diisopro-
pylphenyl and 2,4,6-trimethylphenyl (mesityl), are among
Corresponding author. Tel.: +1 608 265 6288; fax: +1 608 262 6143.
E-mail address: stahl@chem.wisc.edu (S.S. Stahl).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.08.022

6144
C.C. Scarborough et al. / Journal of Organometallic Chemistry 690 (2005) 6143–6155
iPr
:
R
:
Ar/R
Ar/R
Ar/R
Ar/R
:
Ar
N
N
N
N
N
N
N
N
N
N
N
(iPr)2N
P
:
:
:
:
N
N
N
N
iPr
Ar/R
Ar
Ar/R
Ar/R
Ar/R
R
4
5
7
2
6
1
3
Fig. 1. NHC frameworks currently in the literature.
the most common substituents that have been used in the
preparation of stable NHCs and NHC–metal complexes.
We therefore targeted variants of 7 bearing N-mesityl
groups (Fig. 2). Mesitylation of 8H was very successful un-
der previously reported conditions [13], yielding the dime-
sitylated product 8Mes in 99% yield. Attempts to install
the carbene carbon by using a number of previously re-
ported protocols [14], however, were unsuccessful. This
lack of success may reflect the weak nucleophilicity of the
diaryl-substituted nitrogen atoms, and therefore, we shifted
our attention to N-alkylated variants of 8H, for which the
nitrogen centers should be more nucleophilic.
2.2. Synthesis of seven-membered-ring amidinium salts with
N-alkyl substituents
N-alkylated NHC-HBF4 species were easily accessed via
cyclization of N,N0-dialkyl diamines in the presence of
orthoesters under acidic conditions. Neopentyl (Np)
groups were installed by adding pivaloyl chloride to 8H fol-
lowed by reduction with lithium aluminum hydride (LAH)
[15], and the diamine 8Np was obtained in 91% overall yield.
Complex 8Np was heated in neat triethyl orthoformate in
the presence of NH4BF4 [14] to produce pure NHC-
HBF4 9Np in 93% yield without need for further purifica-
tion (Fig. 3). The structure of 9Np was unambiguously
assigned by single-crystal X-ray diffraction analysis [16]
(Fig. 4).
Fig. 4. Solid-state molecular structure of 9Np, showing BF4ꢀ counterion
and solvent molecule. Hydrogen atoms are omitted for clarity. Thermal
˚
ellipsoids are shown at 50% probability. Selected bond lengths (A) and
angles (ꢁ): C23–N1 1.3252(19), C23–N2 1.3154(19), N1–C17 1.4515(18),
N2–C6 1.4523(19), C17–C12 1.398(2), C6–C11 1.395(2), C11–C12
1.483(2); N1–C23–N2 127.07(13), C23–N1–C18 115.52(12), C23–N2–C5
116.27(12), C6–C11–C12-C17 40.6(2).
simple deprotonation of rigorously dried 9Np with carbon-
ate, hydride (with and without catalytic alkoxide), alkoxide,
amide, or carbanion bases. Decomposed starting material
was obtained in all cases. In all but one case, the amidinium
Amidinium salts are common precursors to stable NHCs
[1,2]; however, the free carbene, 7Np, was not accessible by
MesBr, NaOtBu
5% (rac)-BINAP
Mes
Formate
Equivalent
H2
2 cat. Pd/C
EtOAc
Mes
N
+
N
4% Pd2dba3
xylenes
99%
NO
NH2
N
H
H
H
O2N
H2N
N
X-
Mes
100%
Mes
8H
8Mes
Fig. 2. Attempted synthesis of a 7-membered NHC-precursor amidinium salt bearing aryl nitrogen substituents.
HC(OEt)3
NH4BF4
100 ˚C, 20h
93%
1) PivCl, Et3N
THF
2) LiAlH4, THF
91%
Np
N
+
N
N
H
H
( )-
H
BF4
8H
N
-
Np
9Np
8Np
Fig. 3. Synthesis of a 7-membered NHC-precursor amidinium salt bearing alkyl nitrogen substituents.

C.C. Scarborough et al. / Journal of Organometallic Chemistry 690 (2005) 6143–6155
6145
salt, 9Np, also proved to be an ineffective precursor for
1) Pd(OAc)2 ,NaI
KOtBu,THF
2) AgOAc, CH2Cl2
Np
in situ preparation of an NHC–metal complex by deproto-
nation in the presence of a transition metal (vide infra).
The variant of 9Np derived from enantiomerically re-
solved 2,20-diamino-1,10-binaphthyl was also prepared.
The salt, 10Np, was isolated in reasonable yield by using
the same procedure as that used for 9Np (Fig. 5) [15].
Unfortunately, attempts to obtain the free carbene derived
from 10Np were similarly unsuccessful.
Reasoning that increased steric bulk of the N-alkyl
group could enhance the stability of the free carbene, we
targeted more-sterically encumbered derivatives of 9H
(Fig. 6). Access to diamine 82-Ad was achieved in quantita-
tive yield by condensation of 8H with 2-adamantanone fol-
lowed by reduction with LAH. The reaction of 82-Ad with
triethylorthoformate produced 92-Ad [16], albeit in dimin-
ished yield compared to 9Np. The somewhat lower yield
in this reaction may reflect the increase in steric bulk of
the 2-adamantyl groups. To date, we have been unable to
prepare the enantiomerically resolved binaphthyl deriva-
tive of 92-Ad starting from 2,20-diamino-1,10-binaphthyl.
The difficulty in preparing this derivative may arise from
the fact that contraction of the biaryl torsional angle,
which is necessary to achieve condensation with the for-
mate reagent, is more restricted for binaphthyl than
biphenyl.
N
N
9Np
Ag OAc
18%
(two steps)
Np
11
Fig. 7. Synthesis of NHC-Ag(I) complex 11.
[(NHC)Pd(I)2]2 complex, however, yielded a semi-solid
1
product, and crude H NMR spectroscopic data revealed
the disappearance of the amidinium proton resonance
and was consistent with the presence of a carbene-contain-
ing product. After several unsuccessful attempts to isolate
the adduct, we added AgOAc to a solution of the crude
product with the hope of preparing an (NHC)Pd(OAc)2
complex. Instead, we obtained the (NHC)AgOAc complex,
11, from this reaction (Fig. 7). MALDI-TOF mass spec-
trometry revealed the presence of an [AgI(NHC)2]+ cation,
whereas X-ray analysis [16] (Fig. 8) revealed the linear
(NHC)AgI(j1-OAc) complex. These and previously re-
ported results [18] supports the presence of an equilibrium
in solution between the disproportionation salt, [(NHC)2-
AgI] [AgI(OAc)2], and the (NHC)AgI(OAc) complex
(Fig. 9), where (NHC)AgI(OAc) is the least soluble of the
two species in hydrocarbon solvents. As further evidence
of this equilibrium in solution, the carbene carbon reso-
nance does not show coupling to the silver(I) nucleus [18].
Silver(I)–NHC complexes have been shown to act as
carbene transfer agents to other transition metals, includ-
ing PdII salts [19]. Interestingly, these data suggest the
putative [(NHC)Pd(I)2]2 complex effects carbene transfer
to AgI. Nonetheless, because AgI complexes of NHCs have
been shown to act as carbene transfer agents to other tran-
sition metals, we attempted numerous alternative rational
syntheses of 11. Unfortunately, all such attempts failed.
In addition, the preparation of metal complexes from the
binaphthyl analog, 10Np, have been similarly unsuccessful.
Based on these observations, we conjecture that the steric
protection imparted by the relatively small neopentyl sub-
stituents may be insufficient to stabilize a free carbene
and to prepare stable NHC–metal complexes from these
amidinium precursors.
2.3. Preparation of metal complexes possessing
seven-membered NHC ligands
Our interest in palladium-catalyzed reactions prompted
us to target the synthesis of NHC–Pd complexes derived
from 9Np. Unfortunately, standard procedures for the prep-
aration of NHC-coordinated palladium complexes [1a,1b]
were unsuccessful when [Pd(allyl)Cl]2, Pd(OAc)2, and
PdCl2 were used as the palladium reagent. A procedure
outlined by Herrmann et al. [17] for the synthesis of an
1) PivCl, Et3N, THF
2) LiAlH4, THF
2 3) HC(OEt)3, NH4BF4
Np
N
+
N
NH
H
BF4
100 ˚C, 42%
NH2
-
In contrast to the difficulties we encountered in synthe-
sizing metal complexes from 9Np and 10Np, 92-Ad afforded
ready access to palladium(II) complexes. Synthesis of
(NHC)Pd(allyl)Cl (12) [16] by in situ deprotonation of
Np
10Np
Fig. 5. Synthesis of an enantiomerically resolved amidinium salt.
1) 2-adamantanone
toluene
2) LiAlH4, THF
HC(OEt)3
NH4BF4
100 ˚C, 65%
N
+
N
N
H
-
8H
H BF4
H
( )-
N
100%
82-Ad
92-Ad
Fig. 6. Synthesis of a sterically encumbered 7-membered NHC-precursor amidinium salt bearing 2-adamantyl substituents attached to nitrogen.

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C.C. Scarborough et al. / Journal of Organometallic Chemistry 690 (2005) 6143–6155
9
2-Ad in the presence of [Pd(allyl)Cl]2 [20] proceeded in high
yield (Fig. 10). The stability of this complex is demon-
strated by its purification via silica gel chromatography
under ambient conditions. [(NHC)Pd(Cl)2]2 (13) [16] was
prepared in quantitative yield by protonolysis of the allyl
ligand from (NHC)Pd(allyl)Cl complex 12 [20]. Subjection
of 13 to AgOAc or AgOCOCF3 in wet dichloromethane re-
sulted in formation of the (NHC)Pd(carboxylate)2 com-
pounds, 14 and 15, respectively (Fig. 10) [10a], each of
which was characterized by single-crystal X-ray crystallog-
raphy [16,21] (Figs. 11 and 12). Such species are attractive
because (NHC)Pd(O2CR)2(OH2) complexes have found
utility as catalysts both for alcohol oxidation [10] and Wac-
ker-type oxidative cyclization [22]. Interestingly, both 14
and 15 exhibit intermolecular hydrogen bonding between
the protons on the aqua ligand and the carbonyl oxygens
of the carboxylate ligands in the solid state (Fig. 13). This
solid-state structure contrasts the previously reported
(NHC)Pd(carboxylate)2(OH2) complexes, which exhibit
intramolecular hydrogen bonding between the protons on
the water ligand the carboxylate carbonyl oxygen atoms
[10,23].
Fig. 8. Solid-state molecular structure of 11. Hydrogen atoms and solvent
are omitted for clarity. Thermal ellipsoids are shown at 50% probability.
˚
2.4. Identification and reactivity studies of NHC–alcohol
adducts
Selected bond lengths (A) and angles (ꢁ): C1–Ag 2.0904(18), O1–Ag
2.1017(13), C1–N1 1.346(2), C1–N2 1.354(2), N1–C2 1.450(2), N2–C13
1.456(2), C2–C7 1.391(3), C13–C8 1.397(3), C7–C8 1.473(3); N1–C1–N2
117.74(16), C1–Ag–O1 179.35(6), C2–C7–C8–C13 41.1(3).
Attempts to isolate the free carbene 162-Ad by deproto-
nation of rigorously dried 92-Ad with carbonate, hydride
(with and without catalytic alkoxide), carbanion, or amide
bases were again unsuccessful, producing only decomposi-
tion products. When alkoxide bases were employed, alco-
hol adducts of the free carbene were observed by 1H
NMR spectroscopy [24]. Stirring of the tertiary butanol
adduct of 162-Ad in THF in the presence of [Pd(allyl)Cl]2
did not result in formation of 12. Similarly, the methanol
NHC
Ag
NHC
2
NHC Ag OAc
AcO
Ag
OAc
Fig. 9. Proposed equilibrium mixture of [(NHC)2AgI][AgI(OAc)2] and
(NHC)AgI(OAc) (11) in solution.
N
N
2-Ad
2-Ad
0.5 equiv
[Pd(allyl)Cl]2
KOtBu,T HF
Cl Pd Cl
Cl Pd Cl
HCl, Et2O
100%
92-Ad
( )-
N
N
N
N
AgOAc
CH2Cl2 100%
2-Ad
2-Ad
93%
AcO Pd OAc
Pd
2-Ad
2-Ad
N
N
OH2
14
Cl
12
13
AgO2CCF3
CH2Cl2 100%
N
N
2-Ad
2-Ad
F3CCO2 Pd O2CCF3
OH2
15
Fig. 10. Synthesis of four Pd(II) complexes bearing sterically hindered 7-membered NHC ligands.

C.C. Scarborough et al. / Journal of Organometallic Chemistry 690 (2005) 6143–6155
6147
Fig. 11. Solid-state molecular structure of 14. All H atoms except those on
the water ligand have been omitted for clarity. Thermal ellipsoids are
˚
shown at 50% probability. Selected bond lengths (A) and angles (ꢁ): Pd–
C5 1.932(2), Pd–O2 2.0189(17), Pd–O3 2.0215(17), Pd–O5 2.1207(16); C5–
Pd–O2 89.73(8), C5–Pd–O3 89.31(8), C5–Pd–O5 172.09(8), O2–Pd–O3
175.08(7).
Fig. 13. Intermolecular hydrogen bonding in the solid state for 14 (a) and
15 (b).
strain associated with the seven-membered ring framework
and provides Mo¨bius stabilization of the formally antiaro-
matic 8p-electron system (vide infra). We have evaluated
metrics derived from crystal structures to assess the degree
of this torsional twist, and we have defined two parameters,
a and b. The former corresponds to the dihedral angle (a)
between the two aryl rings of the biphenyl backbone. The
latter describes the torsional angle (b) between the two
N–CR bonds (i.e., C–Nꢁ ꢁ ꢁN–C) [12] (Table 1); this param-
eter reflects the spacial disposition of the nitrogen substitu-
ents directed into the metal coordination sphere.
Amidinium salt 92-Ad exhibits an angle a greater than that
of 9Np by 5.5ꢁ, possibly as a result of the larger steric bulk
of 2-adamantyl groups over neopentyl groups. The larger b
angle in 92-Ad compared to 9Np is likely also a result of the
larger steric bulk of 2-adamantyl compared to neopentyl.
Interestingly, the amidinium salt, 9Np, and the linear
(NHC)AgOAc complex, 11, exhibit very similar torsional
parameters. In contrast, significant differences in the b
parameter are observed for the amidinium salt and palla-
dium complexes that possess 2-adamantyl substituents.
The magnitude of b seems to increase with increasing size
and/or number of ligands in the coordination sphere. These
observations suggest that the cis substituents in the palla-
dium complexes exert significant influence on the distortion
of the carbene ligand.
Fig. 12. Solid-state molecular structure of 15. 50% probability ellipsoids
are shown for atoms refined anisotropically (see Section 4, [21]). All H
atoms except those on the water ligand are omitted for clarity. Only the
preferred orientation of the carbene ligand is shown. Selected bond lengths
(A) and angles (ꢁ): Pd–C5 1.955(7), Pd–O1 2.037(5), Pd–O3 2.024(5), Pd–
O5 2.141(5); C5–Pd–O1 87.4(2), C5–Pd–O3 89.9(2), C5–Pd–O5 173.1(2),
˚
O1–Pd–O3 176.1(2).
adduct of 162-Ad does not react with [Pd(allyl)Cl]2 to form
12 (Fig. 14). These observations suggest that in the synthe-
sis of the NHC–palladium adduct, 12, in situ deprotona-
tion of 92-Ad forms the free carbene 162-Ad, which reacts
with [Pd(allyl)Cl]2.
2.5. Torsional distortion of the seven-membered heterocyclic
framework
A unique aspect of this NHC framework is the presence
of a torsionally twisted structure. This twist alleviates ring

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C.C. Scarborough et al. / Journal of Organometallic Chemistry 690 (2005) 6143–6155
Carbonate,
Hydride,
Carbanion, or
KOtBu, THF
Amide
or
Bases
[Pd(allyl)Cl]2
N
N
N
N
NaOMe, MeOH
OR
H
92-Ad
( )-
:
( )-
12
162-Ad
R =tBu, Me
Fig. 14. Reactivity of amidinium salts with various bases.
Table 1
In order to probe the aromaticity of the 7-membered
ring, we used the method of ‘‘nucleus independent chemical
shifts’’ (NICSs) developed by Schleyer et al. [27]. Initial cal-
culation of the NICS value at the 7-membered ring center-
of-mass (COM) of 7CH3 led to a value of 6.7 ppm that
agrees well with the previously computed NICS value re-
ported for 7CH3 by Rzepa and co-workers [25] of
6.9 ppm. The positive magnitude of the COM NICS value
suggests that the free carbene possesses a formal antiaro-
matic electronic configuration.
Comparison of the tortional angles, a and b (defined in text), for
amidinium salts and metal complexes of the 7-membered NHC
Structure
a
b
Amidinium salt, 9Np
Amidinium salt, 92-Ad
NHC–Ag–OAc, 11
NHC–Pd(allyl)Cl, 12
[NHC–Pd(Cl)2]2, 13
NHC–Pd(OAc)2(OH2), 14
40.6(2)ꢁ
46.1(4)ꢁ
41.1(3)ꢁ
41.9(5)ꢁ
46.4(8)ꢁ
39.3(3)ꢁ
62.3(2)ꢁ
56.2(5)ꢁ
61.9(3)ꢁ
72.3(8)ꢁ
79.7(13)ꢁ
91.9(4)ꢁ
Rzepa and co-workers [25] examined the effects of com-
plexing 7CH3 to a palladium center and their calculations
suggest that the barrier to thermal racemization of bound
7CH3 is greater than 20 kcal/mol. We wanted to evaluate
this barrier in the free carbene with the aim to assess the
extent to which the Mo¨bius ring twist contributes to an
overall decrease in antiaromaticity in the free carbene.
The geometry of a flat 7-membered ring corresponding
to 7CH3 was optimized to a transition state structure
(7CH3-TS) with exactly one negative vibrational mode
(Fig. 15). 7CH3-TS lies 12.7 kcal/mol higher in energy and
if this value models the atropisomeric inversion barrier, it
suggests that thermal racemization of the free carbene will
be facile at room temperature. The NICS value computed
at the COM for 7CH3-TS, 11.7 ppm, is significantly higher
than that of the twisted structure and highlights the stabil-
2.6. Electronic structure analysis of seven-membered
N-heterocyclic carbenes
Seven-membered N-heterocyclic carbenes were unprece-
dented prior to our initial report [12]; however, Rzepa and
co-workers [25] had published an earlier computational
analysis of related structures, including 7CH3. Their study
probed the contribution of Mo¨bius aromaticity [26] to sta-
bilization of the 8p-electron seven-membered ring. Our
own DFT calculations of 7CH3 reproduces the twisted con-
formation of the carbene (Fig. 15) and reveals a biphenyl
dihedral angle (a) of 44.7ꢁ that agrees well with our exper-
imentally determined geometries (11, Fig. 8). The b value,
however, is only 23.6ꢁ, possibly reflecting the small size
of the methyl groups.
ization associated with a Mo¨bius ring twist in 7CH3
.
The use of NICS values to measure the diatropic (aro-
matic) versus paratropic (antiaromatic) induced ring cur-
rents at the ring COM has become a well-established
computational technique. Shortly after reporting the
COM NICS technique, Schleyer analyzed the total NICS
value based on the contributions from the diatropic and
paratropic components and suggested that the NICS value
˚
be calculated at 1 A above the ring COM in order to min-
imize local electronic effects [28]. This suggestion prompted
us to reevaluate the NICS value as a function of distance
Table 2
NICS value (ppm) as a function of distance above the ring center
˚
˚
˚
˚
˚
0 A (COM)
0.5 A
1 A
2 A
3 A
7CH3
7CH3-TS
6.7
11.7
4.2
10.1
1.2
7.1
0.3
2.3
0
0.6
Fig. 15. Computationally-derived ground- and transition-state structures
of a 7-membered NHC.

C.C. Scarborough et al. / Journal of Organometallic Chemistry 690 (2005) 6143–6155
6149
above the 7-membered ring center of 7CH3 and 7CH3-TS (Ta-
ca. 10 mg was dissolved in minimal diethyl ether, the vol-
ume of ether was doubled, and the solution was cooled
to ꢀ20 ꢁC. Warm n-heptane was layered onto the ꢀ20 ꢁC
ethereal solution. Overnight diffusion of the liquids at
ꢀ20 ꢁC afforded small yellow crystals.
ble 2). The resulting NICS value of 1.2 ppm (essentially
non-aromatic) for 7CH3 at 1 A above the ring center sug-
˚
gests that the torsional twist effectively cancels the antiaro-
matic contribution to the overall electronic structure. A
decrease in the magnitude of the NICS value is also ob-
See Table 3. A colorless (9Np and 11) or yellow (14 and
15) crystal was selected under oil under ambient conditions
and attached to the tip of a nylon loop. The crystal was
served for 7CH3-TS at 1 A relative to the COM calculation
˚
(Table 2); however, the positive value of 7.1 ppm reveals
that significant antiaromatic character remains.
This computational analysis indicates that evaluating
the true magnitude of the antiaromatic character of 7CH3
and 7CH3-TS is non-trivial; however, it is clear that Mo¨bius
aromatic stabilization occurs in structures with the frame-
work of 7. Further analysis of the individual NICS compo-
nents will be necessary in order to define the total NICS
value and consequently the antiaromaticity of the ring
systems.
mounted in a stream of cold nitrogen at 100(2) K (9Np
,
14, and 15) or 200(2) K (11) and centered in the X-ray
beam by using a video camera.
The crystal evaluation and data collection were per-
formed on a Bruker CCD-1000 diffractometer with Mo
˚
Ka (k = 0.71073 A) radiation and the diffractometer to
crystal distance of 4.9 cm.
The initial cell constants were obtained from three series
of x scans at different starting angles. Each series consisted
of 20 frames collected at intervals of 0.3ꢁ in a 6ꢁ range
about x with the exposure time of 10 (9Np, 14, and 15)
or 15 (11) seconds per frame. A total of 121 (9Np and
15), 206 (11), or 76 (14) reflections were obtained. The
reflections were successfully indexed by an automated
indexing routine built in the SMART program. The final cell
constants were calculated from a set of 8632 (9Np), 10,935
(11), 11,009 (14), or 9536 (15) strong reflections from the
actual data collection.
3. Conclusions
The studies outlined above demonstrate that expansion
of the heterocyclic core of an NHC ligand to encompass
a seven-membered ring causes the structure to deviate sig-
nificantly from a planar geometry. The torsional twist in
the seven-membered amidinium salts and corresponding
metal-coordinated carbenes results in axially chiral, C2-
symmetric structures that point toward their utility in
asymmetric catalysis. Toward this end, our current studies
are focusing on the preparation of enantiomerically re-
solved analogs of these compounds. Another important fo-
cus of our ongoing studies is the preparation of more stable
derivatives that will enable the isolation of stable carbenes,
a goal that has, thus far, been elusive.
The data were collected by using the hemisphere (9Np
and 11) data collection routine or the full sphere (14 and
15) data collection routine to survey the reciprocal space
˚
to the extent of a full sphere to a resolution of 0.80 A. A
total of 11,013 (9Np), 24,283 (11), 30,455 (14), or 27,822
(15) data were harvested by collecting three sets of frames
with 0.3ꢁ (9Np, 14, and 15) or 0.25ꢁ (11) scans in x with an
exposure time 24 s (9Np), 25 s (11), or 36 s (14 and 15) per
frame. These highly redundant datasets were corrected
for Lorentz and polarization effects. The absorption cor-
rection was based on fitting a function to the empirical
transmission surface as sampled by multiple equivalent
measurements [29].
4. Experimental
4.1. General
All manipulations were performed under an inert nitro-
gen atmosphere unless otherwise specified. Dry, oxygen-
A successful solution by the direct methods provided
most non-hydrogen atoms from the E-map. The remaining
non-hydrogen atoms were located in an alternating series
of least-squares cycles and difference Fourier maps. All
non-hydrogen atoms were refined with anisotropic dis-
placement coefficients unless specified otherwise. All hydro-
gen atoms were included in the structure factor calculation
at idealized positions and were allowed to ride on the
neighboring atoms with relative isotropic displacement
coefficients.
1
free solvents were employed. H and 13C NMR spectra
were recorded on a Bruker Phoenix-250, Bruker Homer-
300, Varian Mercury-300, Varian Unity-500, or a Varian
1
Inova-500 NMR spectrometer. H chemical shifts are re-
ported in ppm relative to Me4Si as an external standard,
while 13C chemical shifts are reported in ppm relative to
solvent.
4.2. Structural determination of 9Np, 11, 14, and 15
In the case of 9Np, there is also a solvent molecule of
chloroform in the asymmetric unit.
In the case of 11, there is a toluene solvent molecule
which is equally disordered over two positions and was re-
fined with soft restraints and constraints.
In the case of 14, there were two solvent molecules of
dichloromethane and/or ether present in the asymmetric
unit. A significant amount of time was invested in identifying
Crystals suitable for X-ray diffraction were obtained as
follows: 9Np:9Np (ca. 10 mg) was dissolved in minimal chlo-
roform. Vapor diffusion of n-pentane onto the chloroform
solution afforded crystals after 1 day. 11:11 (ca. 10 mg) was
taken up in minimal toluene. The volume of toluene was
then doubled. Vapor diffusion of n-pentane onto the tolu-
ene solution afforded crystals after 1-2 days. 14 and 15:

Table 3
Crystallographic data for reported complexes
Complex
9Np
11
14
15
Empirical formula
Formula weight
Temperature (K)
[C23H31N2][BF4] Æ CHCl3
541.68
273(2)
AgC25H34N2O2 Æ C7H8
593.54
200(2)
C37H46N2O5Pd Æ 2CH2Cl2
875.01
100(2)
C37H40F6N2O5Pd
813.11
100(2)
˚
Wavelength (A)
0.71073
Monoclinic
P21
0.71073
Monoclinic
P21/n
0.71073
Monoclinic
C2/c
0.71073
Monoclinic
P21/n
Crystal system
Space group
Unit cell dimensions
˚
a (A)
8.9030(4)
14.4914(7)
10.6132(5)
90
106.0570(10)
90
1315.86(11)
2
1.367
0.392
12.2161(18)
13.603(2)
18.526(3)
28.5870(15)
14.6415(8)
19.8079(11)
90
115.736(1)
90
7468.3(7)
8
1.556
0.831
3616
0.36 · 0.25 · 0.18
1.60–26.40
ꢀ35 6 h 6 35;
ꢀ18 6 k 6 17;
ꢀ24 6 l 6 24
30,455
10.6966(7)
˚
b (A)
18.7371(12)
17.0145(11)
90
95.650(1)
90
3393.5(4)
4
1.591
0.627
1664
0.47 · 0.26 · 0.23
2.16–26.39
ꢀ13 6 h 6 13;
ꢀ23 6 k 6 23;
ꢀ21 6 l 6 21
27,822
˚
c (A)
a(ꢁ)
b(ꢁ)
c(ꢁ)
90
102.401(2)
90
3006.7(8)
4
1.311
0.699
1240
0.29 · 0.23 · 0.19
2.24–26.44
ꢀ15 6 h 6 15;
ꢀ16 6 k 6 17;
ꢀ23 6 l 6 23
24,283
3
˚
Volume (A )
Z
Density (calculated) (Mg/m3)
Absorption coefficient (mmꢀ1
F(000)
)
564
Crystal size (mm3)
0.37 · 0.36 · 0.33
2.38–26.40
ꢀ11 6 h 6 11;
ꢀ18 6 k 6 18;
ꢀ13 6 l 6 13
11,013
h Range for data collection (ꢁ)
Index ranges
Reflections collected
Independent reflections (Rint
)
5288 (0.0186)
99.8 (26.40)
0.8814 and 0.8684
Full-matrix least-squares on F2
5288/1/313
1.041
6155 (0.0296)
99.5 (26.44)
0.8786 and 0.8229
Full-matrix least-squares on F2
6155/12/383
1.042
7613 (0.0365)
99.2 (26.40)
0.8649 and 0.7542
Full-matrix least-squares on F2
7613/3/414
1.048
6950 (0.0246)
99.8 (26.39)
0.8692 and 0.7570
Full-matrix least-squares on F2
6950/135/320
1.042
Completeness to theta, % (ꢁ)
Maximum and minimum transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R indices (all data)
Largest difference in peak and hole (e A
R1 = 0.0250; wR2 = 0.0630
R1 = 0.0258; wR2 = 0.0638
0.478 and ꢀ0.335
R1 = 0.0273; wR2 = 0.0684
R1 = 0.0335; wR2 = 0.0719
0.449 and ꢀ0.182
R1 = 0.0346; wR2 = 0.0916
R1 = 0.0436; wR2 = 0.0960
0.888 and ꢀ0.287
R1 = 0.1036; wR2 = 0.2624
R1 = 0.1078; wR2 = 0.2659
3.179 and ꢀ2.338
ꢀ3
˚
)

C.C. Scarborough et al. / Journal of Organometallic Chemistry 690 (2005) 6143–6155
6151
and refining the disordered molecules. Bond length
restraints were applied to model the molecules but the
resulting isotropic displacement coefficients suggested the
molecules were mobile. In addition, the refinement was
computationally unstable. Option SQUEEZE of program
PLATON [30] was used to correct the diffraction data for dif-
fuse scattering effects and to identify the solvate molecule.
PLATON calculated the upper limit of volume that can be
and TEA (15.2 mL, 109 mmol). Pivaloyl chloride
(11.3 mL, 91.7 mmol) was added dropwise via syringe to
the above stirred solution, yielding a white precipitate. Sys-
tem was heated to reflux (ca. 80 ꢁC) for 3 h under a N2
atmosphere. The precipitate was filtered and washed with
THF. Filtrate and washes were combined and liquids re-
moved on a rotary evaporator giving pure 8Piv as a white
powder in 100% yield. 1H NMR (CDCl3, 297 K,
300 MHz): d 1.00 (s, 18H), d 7.18 (s, 2H), d 7.24 (m, 4H),
d 7.46 (m, 2H), d 8.32 (d, 2H, J = 8.2 Hz). 13C NMR
(CDCl3, 297 K, 250 MHz): d 27.33, d 39.80, d 122.15, d
124.79, d 128.36, d 129.76, d 129.94, d 136.25, d 176.90.
HRMS (m/z): calculated 375.2048, measured 375.2060.
3
˚
occupied by the solvent to be 1558 A , or 21% of the unit
cell volume. The program calculated 689 electrons in the
unit cell for the diffuse species. This approximately corre-
sponds to two molecules of dichloromethane per Pd com-
plex in the asymmetric unit (672 electrons). It is very
likely that these solvate molecules are disordered over sev-
eral positions. Note that all derived results in the following
table are based on the known contents. No data are given
for the diffusely scattering species.
4.5. 2,20-Bis(neopentylamino)-1,10-biphenyl 8Np
Compound 8Np was synthesized by an adaptation of a
literature procedure [15]. An oven-dried 500 mL rbf
equipped with a stir bar was charged with LAH (8.03 g,
211 mmol) in a dry box. Reaction vessel was capped with
a septum, and 300 mL dry THF was added via syringe
(outside of dry box). In a 100 mL oven-dried rbf, 8Piv
(12.44 g, 35.3 mmol) was dissolved in 200 mL dry THF un-
der N2, which was then slowly cannula transferred to the
stirred LAH suspension. To the reaction vessel was at-
tached an oven-dried septum-capped condenser with a N2
inlet. The suspension was heated to reflux (ca. 80 ꢁC) for
3.5 days, followed by careful quenching with H2O until
fizzing ceased to be visible. 2 M NaOH(aq) was added until
a clear THF layer could be seen. The slurry was filtered and
the solid washed with THF. The layers were separated and
the aqueous layer was washed twice with 200 mL THF. Or-
ganic layers were combined, dried over MgSO4, and sol-
vent removed to yield a mixture primarily consisting of
starting material and product. The product was isolated
as a clear oil in 91% yield by column chromatography
(SiO2, 7.5% EtOAc in hexanes). 1H NMR (CDCl3,
297 K, 300 MHz): d 0.82 (s, 18H), d 2.84 (m, 4H), d 3.68
(t, 2H, J = 5.1 Hz), d 6.73 (m, 4H), d 7.09 (m, 2H), d
7.24 (m, 2H). 13C NMR (CDCl3, 297 K, 250 MHz): d
27.8, d 32.1, d 55.8, d 110.2, d 116.6, d 123.7, d 129.2, d
130.8, d 146.8. HRMS (m/z): calculated 325.2643, mea-
sured 345.2656.
In the case of 15, the carbene ligand is disordered over
two positions in a 63:37 ratio and was refined with an ide-
alized geometry. All atoms of the carbene ligand were re-
fined isotropically. The ligand disorder resulted in
multiple positions of the adamantly groups. Two positions
for each group were identified and refined. The residual
peaks of electron density suggested that a third orientation
for each adamantyl group could be present. After consider-
able effort was spent on identifying and refining the third
locations it was decided that these orientations of the ada-
mantyl group did not clearly correspond to chemically rea-
sonable positions and their incorporation into the model
resulted in only slight overall improvement of the struc-
tural refinement. Thus, the possible third positions were ig-
nored and only two positions for each adamantyl
substituent are presented.
4.3. 2,20-Diaminobiphenyl 8H
2,20-Dinitrobiphenyl 1 (102.0 g, 417.5 mmol) and 10%
Pd/C (16.4 g) were combined with 300 mL EtOAc in a
hydrogenation vessel. The vessel was pressurized to 40 psi
H2 for 3.5 h (when H2 was no longer being consumed).
The slurry was filtered through a plug of celite. Rotary
evaporation followed by drying on a vacuum line gave pure
1
product as a light yellow/orange powder in 100% yield. H
NMR (CDCl3, 297 K, 300 MHz): d 3.71 (s, 4H), d 6.79 (dd,
2H, J = 8.0, 1.2 Hz), d 6.84 (td, 2H, J = 7.5, 1,2 Hz), d 7.12
(dd, 2H, J = 8.1, 1.5 Hz), d 7.19 (ddd, 2H, J = 7.2, 1.5,
0.6 Hz). 13C NMR (CDCl3, 297 K, 300 MHz): d 115.58,
d 118.71, d 124.62, d 128.81, d 131.08, d 144.22. HRMS
(m/z): calculated 185.1078, measured 185.1073.
4.6. Amidinium tetrafluoroborate salt 9Np
Compound 8Np (10.3 g, 32.0 mmol) and NH4BF4
(3.36 g, 32.1 mmol) were combined in a 500 mL rbf
equipped with a stir bar and topped with a condenser.
20 mL of triethyl orthoformate was added and the system
was heated to ca. 100 ꢁC for 20 h. After cooling to room
temperature, pentane was added (ca. 100 mL). The suspen-
sion was filtered, and the crystals washed with pentane to
give 9Np as a white powdery solid in 93% yield. Crystals
suitable for X-ray analysis were achieved by vapor diffu-
4.4. 2,20-Bis(pivaloylamino)-1,10-biphenyl 8Piv
Compound 8Piv was synthesized by an adaptation of a
literature procedure [15]. 8H (6.50 g, 35.3 mmol) was
weighed into an oven-dried 100 mL rbf equipped with a
stir bar and a septum-capped condenser, followed by N2
purging. The system was charged with 70 mL dry THF
sion of n-pentane onto a CHCl3 solution of 9Np
NMR (CDCl3, 297 K, 300 MHz): d 0.77 (s, 18H), d 4.00
.
1H

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C.C. Scarborough et al. / Journal of Organometallic Chemistry 690 (2005) 6143–6155
(s, 4H), d 7.38 (m, 2H), d 7.51 (m, 6H), d 8.83 (s, 1H). 13C
NMR (CDCl3, 297 K, 300 MHz): d 27.1, d 34.0, d 66.4, d
122.9, d 130.0, d 130.4, d 130.6, d 132.8, d 146.3, d 171.3.
HRMS (m/z): calculated 335.2487, measured 335.2495.
bath, followed by a careful quenching with ca. 100 mL
water and 10 mL sat. NH4BF4. The resulting slurry was fil-
tered through a plug of celite, and the plug washed with
CH2Cl2. The aqueous layer was washed once with CH2Cl2
(ca. 100 mL). The organic layers were combined, dried over
MgSO4, filtered, and the solvent was removed, yielding
pure 82-Ad in 100% yield as an off-white powder. 1H
NMR (CDCl3, 297 K, 300 MHz): d 1.38–1.97 (m, 28H),
d 3.54 (s, 2H), d 4.05 (s, 2H), d 6.71 (m, 4H), d 7.11 (m,
4.7. (R)-2,20-bis(pivaloylamino)-1,10-binaphthyl
(R)-2,20-bis(pivaloylamino)-1,10-binaphthyl was synthe-
sized according to a literature procedure [15]. An oven-
dried 100 mL rbf fitted with a reflux condenser was charged
with 500 mg (1.8 mmol) (R)-(+)-2,20-diaminobinaphthyl
and 760 lL (5.45 mmol) TEA. 10 mL of dry THF was
cannula transferred into the reaction vessel, followed by
syringe addition of 560 lL (4.6 mmol) pivaloyl chloride.
The reaction mixture was refluxed 2 h, then filtered. The
volatiles were removed leaving pure product in 100% yield
as an off-white powder. 1H NMR (CDCl3, 297 K,
300 MHz): d 0.75 (s, 18H), d 7.14 (br s, 2H), d 7.18 (d,
J = 8.4 Hz, 2H), d 7.32 (ddd, J = 8.4, 6.9, 1.2 Hz, 2H), d
7.46 (ddd, J = 8.4, 6.9, 1.2 Hz, 2H), d 7.95 (d, J = 8.4 Hz,
2H), d 8.05 (d, J = 9.0 Hz, 2H), d 8.48 (d, J = 9.0 Hz, 2H).
2H),
d
7.22 (m, 2H).13C NMR (CDCl3, 297 K,
300 MHz): d 27.4, d 27.6, d 31.3, d 31.7, d 31.8, d 32.2, d
37.4, d 37.8, d 37.9, d 56.7, d 111.1, d 116.3, d 124.0, d
129.1,
d 130.9, d 145.3. HRMS (m/z): calculated
453.3269, measured 453.3247.
4.11. Amidinium tetrafluoroborate salt 92-Ad
5.76 g (12.7 mmol) 82-Ad and 1.3 g (12.7 mmol) NH4BF4
were combined under nitrogen in a 500 mL rbf, and ca.
200 mL triethyl orthoformate was added. The reaction
was heated to 100 ꢁC for 16 h, after which time the product
had crashed out of solution as a white powder. The reac-
tion cooled to room temperature, was filtered and the solid
washed with diethyl ether followed by pentane to give the
amidinium salt 92-Ad in 65% yield as a light fluffy white
powder without further purification. 1H NMR (CDCl3,
297 K, 300 MHz): d 0.97 (d, 2H, J = 13.0 Hz), d 1.14 (d,
2H, J = 13.0 Hz), d 1.47–2.09 (m, 22H), d 2.66 (s, 2H), d
4.77 (s, 2H), d 7.43 (m, 2H), d 7.51 (m, 6H), d 8.84 (s,
1H). 13C NMR (CDCl3, 297 K, 300 MHz): d 26.39, d
26.74, d 30.00, d 30.11, d 30.36, d 31.71, d 36.28, d 36.38,
d 37.01, d 65.55, d 124.29, d 129.48, d 129.51, d 129.56, d
134.09, d 143.97, d 175.42. HRMS (m/z): calculated
463.3113, measured 463.3099.
4.8. (R)-2,20-bis(neopentylamino)-1,10-binaphthyl
(R)-2,20-bis(neopentylamino)-1,10-binaphthyl was syn-
thesized according to a literature procedure [15]. 64% yield
as an off-white powder. 1H NMR (CDCl3, 297 K,
300 MHz): d 0.67 (s, 18H), d 2.94 (AMX pattern, 4H), d
3.73 (t, J = 6.0 Hz, 2H), d 7.00-7.03 (m, 2H), d 7.11-7.17
(m, 4H), d 7.25 (d, J = 9.3 Hz, 2H), d 7.75 (m, 2H), d
7.85 (d, J = 9.0 Hz).
4.9. Amidinium Salt 10Np
See synthesis of 9Np. Product did not crash out of reac-
tion, but was purified by removing excess orthoester under
vacuum, dissolving in minimal CH2Cl2, and crashing out
with pentane to give 10Np in 65% yield as a light brown
4.12. NHC–Ag–OAc (11)
Compound 9Np (524 mg, 1.24 mmol), NaI (778 mg,
5.19 mmol), KOtBu (186 mg, 1.66 mmol), and Pd(OAc)2
(278 mg, 1.24 mmol) were combined in a schlenk flask un-
der a nitrogen atmosphere. After cooling to ꢀ78 ꢁC,
15 mL dry THF was added, and the solution was stirred
overnight as the temperature slowly warmed to room tem-
perature. Solvent was removed in a dry box, and the residue
was taken up in CH2Cl2. Filtration through a plug of celite,
followed by washing the plug with a small amount of
CH2Cl2 gave a yellow/brown solution. Addition of AgOAc
and stirring for 24 h gave a brown suspension with white
ppt present. Filtration through celite, removal of solvent
in vacuo, taking up in minimal CH2Cl2 and addition of ex-
cess hexanes gave a semi-cloudy light brown solution. Re-
moval of small amount of solvent in vacuo promoted
precipitation of an off-white solid, which was collected by
filtration as pure 6 by NMR as a light tan powder in 18%
yield. Crystals suitable for X-ray analysis were achieved
by vapor diffusion of n-pentane onto a toluene solution of
1
powder. H NMR (CDCl3, 297 K, 300 MHz): d 0.64 (s,
18H), d 4.01 (d, J = 13.8 Hz, 2H), d 4.20 (d, J = 13.8 Hz,
2H), d 7.07 (d, J = 8.4 Hz, 2H), d 7.34 (t, J = 7.2 Hz,
2H), d 7.62 (m, 4H), d 8.00 (d, J = 8.1 Hz, 2H), d 8.12 (d,
J = 9.0 Hz), d 8.85 (s, 1H). 13C NMR (CDCl3, 297 K,
300 MHz): d 27.0, d 33.9, d 65.8, d 120.2, d 124.2, d
126.4, d 127.8, d 128.1, d 128.8, d 131.5, d 131.8, d 132.9,
d 147.5, d 173.1. HRMS (m/z): calculated 435.2800, mea-
sured 435.2794.
4.10. 2,20-bis(2-adamantylamino)-1,10-biphenyl 82-Ad
614 mg (3.33 mmol) of 8H and 1 g (6.65 mmol) 2-ada-
mantanone were combined with 1% (per amine functional-
ity) pTsOH in a Dean-Stark apparatus. The reagents were
dissolved in ca. 200 mL toluene and refluxed for 72 h. The
solvent was removed and 122 mg (3.22 mmol) LAH
was added, followed by ca. 200 mL THF. The reaction
flask was heated to 50 ꢁC for 2 h then cooled on an ice
1
11. H NMR (CDCl3, 297 K, 300 MHz): d 0.58 (s, 18H),

C.C. Scarborough et al. / Journal of Organometallic Chemistry 690 (2005) 6143–6155
6153
d 2.03 (s, 3H), d 3.90 (d, 2H, J = 13.4 Hz), d 4.20 (d, 2H,
4.15. NHC–Pd(OAc)2(OH2) (14)
J = 13.4 Hz), d 7.13 (m, 2H), d 7.30 (m, 6H). 13C NMR
(CDCl3, 297 K, 500 MHz): d 27.8, d 33.9, d 71.9, d 123.3,
d 127.8, d 128.8, d 129.0, d 135.4, d 149.5, d 179.3 (s, car-
bene). MALDI-TOF (m/z): 335.3 ((NHC–H)+), 441.0
(NHC–Ag+), 774.9 ((NHC)2Ag+).
Compound 13 (1 equivalent) was combined with 4.1
equivalents of AgOAc in wet CH2Cl2. After stirring under
ambient conditions for 1.5 h, the solution had changed
from bright orange-yellow to bright yellow. The suspension
was filtered over a plug of celite, and volatiles removed in
vacuo, yielding 14 in quantitative yield as a yellow solid.
Single crystals suitable for X-ray analysis were obtained
by layering n-heptane onto a ꢀ20 ꢁC ethereal solution of
14. 1H NMR (CDCl3, 297 K, 300 MHz): d 0.70–2.36
(m, 26H), d 4.6 (broad rise in baseline, 2H, OH2), d 4.81
(s, 2H), d 4.91 (s, 2H), d 7.09 (dd, J = 7.8, 1.2 Hz, 2H),
d 7.32 (td, J = 7.8, 1.8 Hz, 2H), d 7.38 (td, J = 7.5,
1.6 Hz, 2H), d 7.49 (dd, J = 7.5, 1.6 Hz, 2H).13C NMR
(CDCl3, 297 K, 300 MHz): d 23.77, d 26.62, d 26.84,
d 27.73, d 30.03, d 30.12, d 31.03, d 36.83, d 37.28, d
37.55, d 66.87, d 126.09, d 127.26, d 127.50, d 127.70, d
135.83, d 145.73, d 181.55 (carbene carbon not detected).
ESI-MS and MALDI-TOF (m/z) (highest intensity peaks
listed): 627.4 ([NHC–Pd(COOCH3)]+). Aqua ligands were
not observed by ESI or MALDI-TOF.
4.13. NHC–Pd(allyl)Cl (12)
200 mg (0.363 mmol) of aminidinium salt 92-Ad and
78 mg (0.213 mmol) [Pd(allyl)Cl]2 was combined with
1.2eq of KOtBu under nitrogen. The reaction was stirred
in THF for 12 h, filtered through celite, and purified on a
column under ambient conditions (SiO2, 1:1 Ether:Hex-
anes) to give the NHC–Pd(Allyl)Cl complex 11 in
93% yield as a light yellow-tan solid. Crystals suitable
for X-ray analysis were achieved by layering n-pentane
1
onto a ꢀ20 ꢁC toluene solution of 12. H NMR (CDCl3,
297 K, 500 MHz): d 0.79–2.11 (m, 67.2H), d 2.66 (s,
1.4H), d 2.76 (d, 1.4H, J = 11.0 Hz), d 3.16–3.31 (m,
4.8H), d 3.52 (s, 1H), d 3.76 (d, 1H, J = 6.0 Hz, 1H), d
3.99–4.21 (m, 3.8H), d 5.16 (m, 2.8H), d 5.45 (m, 2H), d
6.95 (m, 2.4H), d 7.28–7.56 (m, 16.8H). 13C NMR
(CDCl3, 297 K, 500 MHz): d 26.5, d 26.6, d 26.7, d 26.8,
d 26.9, d 29.9, d 30.0, d 30.09, d 30.11, d 30.2, d 30.4, d
30.5, d 31.1, d 31.2, d 36.1, d 36.8, d 37.0, d 37.1,
d 37.2, d 37.4, d 37.5, d 43.3, d 46.1, d 66.5, d 66.6, d
66.7, d 66.9, d 71.2, d 73.1, d 115.2, d 124.7, d 125.0, d
126.7, d 126.8, d 126.9, d 127.0, d 127.1, d 127.2,
d 127.6, d 136.3, d 136.4, d 137.1, d 146.2, d 146.4, d
146.6, d 147.1 (carbene carbons not detected). ESI-MS
(m/z) (highest intensity peaks listed): 609.1 (NHC–Pd(al-
lyl)+), 650.2 (NHC–Pd(allyl)Cl–Li+), 667.2 (NHC–Pd(al-
lyl)Cl–Na+), 1255.3 (([NHC–Pd(allyl)]2Cl)+).
4.16. NHC–Pd(OCOCF3)2(OH2) (15)
Compound 13 (1 equivalent) was combined with 4.1
equivalents of AgOCOCF3 in wet CH2Cl2. After stirring
under ambient conditions for 1.5 h, the solution had
changed from bright orange-yellow to bright yellow.
The suspension was filtered over a plug of celite, and
volatiles removed in vacuo, yielding 15 in quantitative
yield as a yellow solid. Single crystals suitable for
X-ray analysis were obtained by layering n-heptane onto
a ꢀ20 ꢁC ethereal solution of 15. 1H NMR (CDCl3,
297 K, 300 MHz): d 0.66–2.35 (m, 26H), d 4.85 (br s,
4H), d 7.10 (br d, 2H), d 7.35 (td, J = 7.5, 1.9 Hz,
2H), d 7.43 (td, J = 7.5, 0.9 Hz, 2H), d 7.51 (dd, J =
7.5, 1.8 Hz, 2H) (OH2 protons not observed). 13C
NMR (CDCl3, 297 K, 500 MHz): d 26.54, d 26.77, d
29.98, d 30.17, d 30.92, d 37.06, d 37.20, d 37.42 (exhibits
small downfield shoulder), d 67.59, d 115.24 (apparent
quartet, though outer lines not observed, J = 289 Hz), d
126.03, d 127.77, d 128.00, d 128.42, d 135.15, d 145.41
(carbonyl and carbene carbons not observed). ESI-MS
(m/z) (highest intensity peak listed): 681.4 ([NHC–
Pd(COOCF3)]+). Aqua ligands were not observed by
ESI or MALDI-TOF.
4.14. [NHC–Pd(Cl)2]2 (13)
A 50 mL rbf was charged with 100 mg (0.16 mmol) 12
and 2.0 mL 2.0 M ethereal HCl. The color instantly chan-
ged to bright yellow-orange. 8.0 mL ether was added and
the resultant suspension stirred 1 h. Volatiles were removed
in vacuo leaving pure 13 as a bright yellow-orange powder
in quantitative yield. 13 could be recrystalized by taking up
in a small amount of toluene and crashing out with excess
n-pentane (87%). Crystals suitable for X-ray analysis were
achieved by vapor diffusion of n-pentane onto a CH2Cl2
solution of 13. 1H NMR (CDCl3, 297 K, 500 MHz):
d 0.67 (d, J = 12.5 Hz, 2H), d 0.94 (d, J = 12.5 Hz, 2H),
d 1.26–2.37 (m, 20H), d 4.5 (br s, 1H), d 4.8–5.7 (br m,
3H), d 7.27 (br s, 2H), d 7.34 (t, J = 7.5 Hz, 4H), d 7.45
(dd, J = 7.5, 1.5 Hz, 2H). 13C NMR (CDCl3, 297 K,
500 MHz): d 26.66, d 26.98, d 30.13, d 30.49, d 31.25,
d 35.81, d 36.86, d 37.24 (br), d 37.52, d 68.10 (br),
d 127.15, d 127.28, d 127.42, d 127.79, d 135.47, d 145.78,
d 201.41. MS (ESI): m/z = (highest intensity peaks listed):
463.4 ([NHC + H]+), 663.2 ([NHC–Pd(Cl)2 + Na]+),
1303.6 ([M + Na]+), 1380.7.
4.17. Computational analysis
Density functional theory (DFT) calculations using the
B3 [31]-LYP [32] functional with the 6-31G(d) basis set
were employed with the GAUSSIAN 98 [33] suite of programs
to perform gas phase geometry optimizations on 7CH3 and
7
CH3-TS. Optimized structures were verified to be at a local
minimum or a saddle point by calculation of the harmonic
vibrational frequencies. ‘‘Nucleus independent chemical

6154
C.C. Scarborough et al. / Journal of Organometallic Chemistry 690 (2005) 6143–6155
[15] Procedure adapted from C. Drost, P.B. Hitchcock, M.F. Lappert, J.
Chem. Soc., Dalton Trans. (1996) 3595.
shifts’’ (NICS) values were calculated using the GIAO-SCF
method with a larger 6-311++G(d,p) basis set [27].
[16] Crystallographic data for the structural analyses have been deposited
with the Cambridge Crystallographic Data Centre, CCDC Nos.
273345 (9Np), 272584 (92-Ad), 273346 (11), 272585 (12), 272586 (13),
273348 (14), and 273347 (15). Copies of this information may be
obtained free of charge from: The Director, CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK, fax. (int code) +44 1223 336 033 or email:
deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk.
[17] W.A. Herrmann, V.P.W. Bo¨hm, C.W.K. Gsto¨ttmayr, M. Grosche,
C.-P. Reisinger, T. Weskamp, J. Organomet. Chem. 617–618 (2001)
616.
Acknowledgments
We appreciate financial support from the National Insti-
tutes of Health (RO1 GM67173-01) and the Dreyfus Foun-
dation (SSS-Teacher-Scholar Award).
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