A new class of remote N-heterocyclic carbenes with exceptionally strong σ-donor properties: Introducing benzo[c]quinolin-6-ylidene

Article abstract of DOI:10.1002/chem.201203294

We make the case for benzo[c]quinolin-6-ylidene (1) as a strongly electron-donating carbene ligand. The facile synthesis of 6- trifluoromethanesulfonylbenzo[c]quinolizinium trifluoromethanesulfonate (2) gives straightforward access to a useful precursor for oxidative addition to low-valent metals, to yield the desired carbene complexes. This concept has been achieved in the case of [Mn(benzo[c]quinolin-6-ylidene)(CO)₅] ⁺ (15) and [Pd(benzo[c]quinolin-6-ylidene)(PPh₃) ₂(L)]²⁺ L=THF (21), OTf (22) or pyridine (23). Attempts to coordinate to nickel result in coupling products from two carbene precursor fragments. The CO IR-stretching-frequency data for the manganese compound suggests benzo[c]quinolin-6-ylidene is at least as strong a donor as any heteroatom-stabilised carbene ligand reported. Copyright

Full text of DOI:10.1002/chem.201203294

FULL PAPER
DOI: 10.1002/chem.201203294
A New Class of Remote N-Heterocyclic Carbenes with Exceptionally Strong
s-Donor Properties: Introducing Benzo[c]quinolin-6-ylidene
Ulrich F. J. Mayer, Elliot Murphy, Mairi F. Haddow, Michael Green, Roger W. Alder, and
Duncan F. Wass*[a]
Abstract: We make the case for ben-
zo[c]quinolin-6-ylidene (1) as a strong-
ly electron-donating carbene ligand.
The facile synthesis of 6-trifluorome-
thanesulfonylbenzo[c]quinolizinium tri-
achieved in the case of [Mn-
AHCTUNGTRENNUNG
(benzo[c]quinolin-6-ylidene)(CO)5]+
coordinate to nickel result in coupling
products from two carbene precursor
fragments. The CO IR-stretching-fre-
quency data for the manganese com-
pound suggests benzo[c]quinolin-6-yli-
dene is at least as strong a donor as
any heteroatom-stabilised carbene
ligand reported.
(15)
ylidene)AHCUTNGTERNNUNG
and
[PdACTHNGUTERNNU(G benzo[c]quinolin-6-
(PPh3)2(L)]2+ L=THF (21),
OTf (22) or pyridine (23). Attempts to
fluoromethanesulfonate
(2)
gives
straightforward access to a useful pre-
cursor for oxidative addition to low-
valent metals, to yield the desired car-
bene complexes. This concept has been
Keywords: N-heterocyclic car-
benes · structure elucidation · tran-
sition metals
Introduction
can be replaced with structural elements of appropriate or-
bital situation and electron density. For example, in amino-
N-Heterocyclic carbenes (NHCs) are now widely studied as
examples of relatively stable free-singlet carbenes and as
supporting ligands for coordination compounds, especially
those with applications in catalysis. With the many variants
on the NHC theme that have been developed as ligands for
transition metals, it has become clear that the most stable
carbenes do not necessarily make the most useful ligands in
terms of catalytic applications. The strong s-donor ability of
these ligands makes them pre-eminent in many applications
in which an electron-rich catalyst metal centre is important,
such as metathesis, cross-coupling and carbon–carbon multi-
ple bond cyclisation reactions.[1a,b,e,2,3] The facile access to a
diverse range of structurally different NHCs allows easy
fine-tuning of steric and electronic parameters, an attractive
feature for such applications.[4]
The stability of such NHC fragments arises both from the
steric demand of the substituents on the nitrogen atoms of
the heterocyclic ring, which helps to prevent the carbene
from dimerising[5–7] and, perhaps more importantly, electron-
ic factors such as the beneficial overlap of the lone pair on
the nitrogen atoms with the empty p orbital of the carbene-
carbon centre.[1] However, the electronic stabilisation is nei-
ther confined to nitrogen atoms nor to their a-position rela-
tive to the carbene-carbon atom. Stabilising nitrogen atoms
ACHTUNGTREN(NUNG ylidene)carbenes (AYCs) stabilisation is induced through a
dipolar p system.[8] So-called “remote NHCs” (rNHC) incor-
porate an enamine p system into a cyclic arrangement to
provide the necessary aromatic stabilisation.[9a,10] This stabil-
ising effect of the enamine is not confined to a cyclic ar-
rangement, as demonstrated most recently by Fꢀrstner, who
reported remarkably strong electron-donating carbenes sta-
bilised by lateral enamines.[11] Their strong electron-donating
properties in connection with the protective encumbrance
from the sterically demanding N,N’-substituents result in fa-
vorably strong metal–carbene bonds.[12,13] Structures based
on “abnormal” carbenes or mesoionic carbenes have also at-
tracted recent attention.[14]
The many synthetic procedures for the generation of
NHC–metal complexes often rely on simple ligand exchange
and therefore the ability of the carbene ligand to exist in a
free or, at least, metastable state in order to react with the
transition-metal fragment. By contrast, the isolation of
rNHCs in their free form has not yet been achieved, but
nickel, palladium and platinum complexes of pyridinylidene,
quinolinylidene, isoquinolinylidene and acrylidenylidene by
oxidative addition of low-valent transition-metal fragments
into the carbon–halogen bond of suitable precursors have
been reported.[9b,c,15,16] Some palladium(II) complexes of the
type [PdACHTNURTGNENG(U carbene)HCATUNGTREN(NUGN PPh3)2(X)][A] (X=Cl or Br; A=BF4,
CF3SO3) have been reported to perform as highly efficient
[9c]
[a] Dr. U. F. J. Mayer, E. Murphy, Dr. M. F. Haddow, Prof. M. Green,
Prof. R. W. Alder, Prof. D. F. Wass
À
catalysts in Heck and Suzuki type C C coupling reactions,
whereas some nickel(II) complexes of the type [Ni-
AHCTUNGTREUNG(NN carbene)ACHUTNRTGE(GNNNU PPh3)2(Cl)]ACHTGUNTREN[NUNG BF4] have been reported to exhibit
high efficiency in Kumada–Corriu type cross-coupling reac-
School of Chemistry, University of Bristol
Cantockꢁs Close, Bristol, BS8 1TS (UK)
E-mail: Duncan.Wass@bristol.ac.uk
tions.[16]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203294.
Chem. Eur. J. 2013, 19, 4287 – 4299
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4287

Manganese pentacarbonyl complexes of an isoquinolin-4-
ylidene precursor have also been obtained from the oxida-
tive addition of the corresponding chloride precursors to
Na[Mn(CO)5] in THF.[15] Transmetallation of carbenes first
generated by oxidative addition to a late-transition metal is
a complementary synthetic approach to deliver transition-
metal complexes of rNHCs. For example, the chromium(0)-
pentacarbonyl complex of simple rNHC ligand 1-methylpyr-
Results and Discussion
Synthesis: Our general strategy to obtain complexes of ben-
zo[c]quinolin-6-ylidene was to generate dihalide or dipseu-
dohalide precursor compounds for this carbene of the type
illustrated in Scheme 1, which can be used to synthesise the
idine-4ACHTUNGTRENNUNG(1H)-ylidene can be prepared from 4-chloro-1-meth-
ylpyridinium triflate and Na2[Cr(CO)5] and subsequently
used to generate the corresponding gold(I) chloride com-
plex.[17,18] The carbonyl stretching frequencies of such rNHC
complexes were found to be significantly lower relative to
those of corresponding imidazole-2-ylidene metal complex-
es,[19,20b] indicating a significantly larger sd/pa ratio for pyri-
din-4-ylidenes relative to NHCs, which was corroborated by
DFT calculations.[10b]
Given the great success of NHCs as ligands, it seems sur-
prising that other carbenes, such as remotely stabilised and
carbocyclic carbenes, are underdeveloped, particularly when
one considers the experimental and calculated evidence for
the extremely strong s- donor properties of these systems.
With this in mind, proton affinities (PAs) and [Ni(CO)3(L)]
CO stretching frequencies were calculated for a selection of
known and potentially new carbenes using DFT (see
Table 1).[21] As expected, species with higher PAs have lower
ꢀ
Table 1. DFT-calculated proton affinities (PAs) and Ni(CO)3 C O
stretching frequencies.
Scheme 1. Structure of benzo[c]quinolin-6-ylidene carbene 1, synthetic
routes, and an example complex.
Carbene
PA
n of Ni(CO)3
[cmÀ1
[kJmolÀ1
]
]
2,3-diphenylcycloprop-2-en-1-ylidene
cycloheptatrienylidene
dimethyldihydroimidazolidene
dimethylimidazolidene
N-Me-2-pyridylcarbene
bis(dimethylamino)carbene
N-Me-4-pyridylcarbene
1104
1104
1113
1113
1156
1150
1225
1258
2068, 2074, 2130
2067, 2069, 2123
2062, 2068, 2129
2062, 2066, 2128
2063, 2066, 2126
2060, 2062, 2124
2056, 2060, 2118
2052, 2057, 2114
desired metal complexes in a final oxidative addition step.
This bypasses the risks associated with trying to isolate
“free” carbene species of an unknown type. The synthesis of
a variety of 6-hydroxy-benzo[c]quinolizinium halide salts
has been reported in the literature by the groups of Vier-
fond,[22,23] Ziegler[24] and Volovenko,[25,26] and the choice of
the appropriate method depends on the desired substituent
pattern present on the benzo[c]quinolizinium frame. Initial-
ly, we followed a modification of Vierfondꢁs synthetic proto-
col (Scheme 2) that enabled us to obtain 2-(2-chloropheny-
l)acylpyridinium chloride salt (2) in high yield (81%). The
condensation reaction of 2-lithiopicoline and 2-chlorobenzo-
nitrile was followed by the abstraction of HCl and a subse-
quent thermally induced intramolecular ring-closure, to give
benzo[c]quinolin-6-ylidene (1)
CO stretching frequencies, and these results are very much
in line with experimental data where these exist. Of specific
interest, the calculations identified carbene 1 as an excep-
tionally strong electron donor. This seems a doubly attrac-
tive target, since general and modular synthetic routes can
be envisaged. In particular, the two positions adjacent to the
carbene carbon are differently oriented with respect to this
carbene centre; substitution at these positions could lead to
unique control of reactivity, which is difficult to achieve
with more traditional carbene ligands. This report presents
the design and synthesis of precursors to and transition-
metal complexes of benzo[c]quinolin-6-ylidene (1). These
complexes have allowed us to verify the exceptional donor
abilities of this ligand.
Scheme 2. Synthesis of 6-hydroxybenzo[c]quinolizinium chloride 3. Reac-
tion conditions: i) THF, 16 h, À788C to RT. ii) H2SO4/H2O (pH 2), 408C,
1 h. iii) K2CO3 (pH 8). iv) HCl/EtOH (pH 2). v) MeOH/H2O (1:1 vol%),
K2CO3 (pH 8). vi) Argon, neat, 1808C.
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A New Class of Remote N-Heterocyclic Carbenes
FULL PAPER
6-hydroxybenzo[c]quinolizinium chloride 3 in good overall
yield (65%). The structures of compounds 2 and 3·H2O
have been determined by X-ray crystallography and are
very much as expected. The structures are described in the
Supporting Information.
In order to increase kinetic protection of the carbene
centre, we tried to introduce steric bulk in positions 5 and 7
of the benzo[c]quinolizinium moiety.[27] Unfortunately, when
applying the same synthetic strategy to 2-lithiopicoline/2,6-
dichlorobenzonitrile and 2-lithiobenzylpyridine/2-chloroben-
zonitrile, we found the synthetic protocol not to be com-
pletely transferrable. The condensation of 2-lithiopicoline
and 2,6-dichlorobenzonitrile, resulted in the formation of
the desired 6-hydroxy-8-chlorobenzo[c]quinolizinium chlo-
and 2-chlorobenzonitrile were subjected to similar reaction
conditions, unwanted benzo[b]quinolizinium chloride 5 was
obtained.
The formation of 5 under the applied reaction conditions
suggested that nucleophilic attack of 2-lithiobenzylpyridine
at the chloro substituent of the benzene ring is favored
almost exclusively with respect to reaction of 2-lithiobenzyl-
pyridine with the nitrile group to form the corresponding
lithio–imide 6 (see Scheme 4). The solubility of 2-lithioben-
zylpyridine was observed to be poor in THF, especially at
temperatures below À58C. It is feasible to assume that sig-
nificant amounts of 2-lithiobenzylpyridine are therefore only
available at temperatures above À58C, at which tempera-
ture formation of 5 is favoured with respect to 6 (Scheme 4).
The increased steric encumbrance of this secondary lithium
salt or the increased stability of the dibenzyl-type anion,
may also have a role.
ACHTUNGTRENNUNGride (4; Scheme 3). However, when 2-lithiobenzylpyridine
Subsequent reaction of 3 with a saturated aqueous solu-
tion of K2CO3 resulted in the abstraction of HCl and the for-
mation of benzo[c]quinazolinone 7, which was isolated as a
yellow solid in high yields (85%) after vacuum sublimation
(Scheme 5).
In order to transform the keto group in 7 into a leaving
group, several different synthetic strategies were applied.
Firstly, we attempted reactions with either SOX2 or COX2
(X=Cl, Br) to form the corresponding 6-halobenzo[c]quino-
lizinium halides. These reactions resulted in the formation
of white solids in almost quantitative yields, but the analyti-
cal characterisation of the obtained reaction products was
severely hampered by their virtual insolubility in all
common organic solvents. Furthermore, the extremely low
solubility excluded the reaction with transition-metal com-
pounds or attempted anion-exchange reactions.
Secondly, we attempted the transformation of a,b-unsatu-
rated ketone 7 into the corresponding 6-trifluoromethane-
Scheme 3. Reaction products from the addition of 2-lithiopicoline onto
2,6-dichlorobenzonitrile and 2-lithiobenzylpyridine onto 2-chlorobenzoni-
trile. i) THF, 16 h, À788C to RT. ii) H2SO4/H2O (pH 2), 408C, 1 h.
iii) K2CO3 (pH 8). iv) HCl/EtOH (pH 2). v) MeOH/H2O (1:1 vol%),
K2CO3 (pH 8). vi) Argon, neat, 1808C. .
sulfonylbenzo[c]quinolizinium
trifluoromethanesulfonate
(8), which was accomplished by the use of trifluoromethane-
Scheme 4. Mechanistic aspects for the reaction of 2-lithiobenzylpyridine with 2-chlorobenzonitrile. i) MeOH/H2O (1:1 vol%), K2CO3 (pH 8). ii) Argon,
neat, 1808C.
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4289

D. F. Wass et al.
even prolonged exposure of 8 in acetonitrile at elevated
temperatures did not lead to decomposition. In contrast, re-
action of 8 with pyridine resulted in the formation of 6-pyri-
diniumbenzo[c]quinolizinium
ditrifluoromethanesulfonate
(12). Similarly, reaction of 8 with one equivalent of PPh3 in
acetonitrile resulted in the formation of 6-triphenylphos-
AHCTUNGTREGpNNUN honiumbenzo[c]quinolizinium ditrifluoromethanesulfonate
(13) and reaction of 8 with one equivalent of 7 in acetoni-
trile resulted in the almost quantitative formation of 14
(Scheme 6). The structures of compounds 12 and 14 were
determined by X-ray crystallography and described in the
Supporting Information.
Scheme 5. Synthesis of 7–11. Reaction conditions: i) MeOH/H2O (1:1
vol%), K2CO3 (pH 8). ii) (CF3SO2)2O/CH2Cl2, À78 to 208C, 1 h. iii)
MeOSO2CF3, CH2Cl2, À78 to 208C, 1 h. iv) [R3O]
for 10, Et for 11), À78 to 208C, 1 h.
ACHTNUGTNER[NUGN BF4]/CH2Cl2 (R=Me
sulfonyl anhydride ((CF3SO2)2O). The potential of the tri-
fluoromethanesulfonate a good leaving group in subsequent
organometallic reactions made this particularly attractive.
The conversion of a,b-unsaturated ketones into vinyl tri-
fluoromethanesulfonates using (CF3SO2)2O in the presence
of a scavenger base, such as 2,6-di-tert-butyl-4-methylpyri-
dine, has been reported by Stang and co-workers, as have
similar reactions with simple amides and vinylogous
amides.[28,29] The addition of auxiliary base was unnecessary
in case of 7, since a comparable pyridine moiety is already
incorporated in the parent ketone. When methyl trifluoro-
methansulfonate was used instead of (CF3SO2)2O, 6-meth-
Scheme 6. Reactivity of 8 towards Lewis bases. Reaction conditions: i)
C5H5N, 20 8C, 16 h. ii) PPh3 in CH3CN, 208C, 16 h. iii) 7 in CH3CN, 208C,
16 h.
Synthesis of metal complexes by oxidative addition
A
6-trifluoromethanesulfonyl benzo[c]quinolizinium trifluoro-
obtained in moderate yield (43%). Alkylation of the keto
ACHTUNGTRENUNmNG ethanesulfonate (8) to low valent metals
group in 8 can alternatively be readily accomplished by the
use of Meerwein salts [R3O]
6-alkoxybenzo[c]quinolizinium tetrafluoroborates 10 and 11
were obtained from the reaction of 7 and [R3O][BF4] (R=
Me, Et) in CH2Cl2 in very good yields (90 and 77% respec-
tively). The structures of compounds 5, 7, 8 and 11 have
been determined by X-ray crystallography and are described
in the supplementary information.
G
Synthesis of [(benzo[c]quinolin-6-ylidene)Mn(CO)5] (15):
Our initial aim of this study was to verify experimentally the
strong s-donor nature of benzo[c]quinolin-6-ylidene suggest-
ed by our calculations. With this is mind, we targeted a
metal–carbonyl complex for which corresponding data for
other carbene complexes exists. The usual complexes of the
type [Ni(CO)3(L)] (as in our calculations) or [Rh(CO)2(L)]2
are inaccessible by the methods we have in hand, since both
rely on generating free or metastable carbenes rather than
the oxidative-addition route that proved successful for the
palladium complex.
Solubility of 8 and reactivity towards Lewis bases: Com-
pound 8 was obtained as a white solid after almost quantita-
tive precipitation from CH2Cl2 at room temperature. Purifi-
cation of 8 was achieved by washing with THF and subse-
quent recrystallisation from diethyl ether and acetonitrile at
room temperature. Reaction or decomposition of 8 in aceto-
nitrile at room temperature was not observed. In fact, 8 dis-
played remarkably high stability towards acetonitrile and
However, a modification of the method of Raubenheim-
er,[15] for the synthesis of a manganese(I)pentacarbonyl(2-
methoxy-N-methyl,4-dihydroquinolin-4-ylidene)
triflate
complex, in which Na[Mn(CO)5] is added to 8 at low
(À788C) temperature provided a successful route to a com-
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A New Class of Remote N-Heterocyclic Carbenes
FULL PAPER
ꢀ
Table 2. C=Mn bond lengths and C O stretching frequencies of Mn–
NHC complexes.
ꢀ
Complex
C=M Bond
length [ꢄ]
C O Stretching
frequencies [cmÀ1
]
1
2
3
mean
2065
Scheme 7. Synthesis of 15.
2.104
2128
2069
1997
no data
2129
2078
2036
2081
2.090
2134
2130
2049
2075
2045
2025
2076
2076
no data
product of the reductive dimerisation of two carbene frag-
ments from a NiII transition-metal centre, that is, [Ni-
AHCTUNGTERNN(GUN carbene)2(L)2] (L=MeCN) ;17) under formation of
[Ni(L)n] (n=2–4). In order to obtain cis/trans-17, from
which the reductive dimerisation could potentially proceed,
a ligand exchange between two molecules of 18 must occur
(Scheme 8).[32]
Figure 1. Molecular structure of 15 with thermal ellipsoids drawn at the
30% probability level. Counterion and solvent of crystallisation omitted
À
À
for clarity. Selected bond lengths [ꢄ] and angles [8]: Mn C1 2.104, Mn
Single crystals of 16 suitable for X-ray-diffraction analysis
were obtained from diffusion of diethyl ether into a saturat-
ed solution of 16 in acetonitrile at ambient temperatures
and the molecular structure of 16 is shown in Figure 2.
In order to further probe the transmission effects and the
electrochemical reversibility of the dication/radical-cation
couple in 16/19, the redox behaviour of 16 was investigated
by cyclic voltammetry. Figure 2 shows the cyclic voltammo-
gram of 16 obtained from a 1.0ꢃ10À3 mÀ1 solution in acetoni-
À
À
À
C18 1.845, C1 C2 1.372, C2 C3 1.423, N1 C3 1.362; C1-Mn-C18 177.58,
C14-Mn-C16 175.66, C15-Mn-C17 174.97, Mn-C1-C2 117.32, Mn-C1-C13
128.08.
plex of the type [MnACHTNUGTRNENUG(carbene)(CO)5]ACHTUNTGREN[NGUN OTf] (15) (Scheme 7).
This route yielded complex 15 as single crystals suitable for
X-ray crystallography (Figure 1). As with the palladium
complex, (see below, Figure 3) some deviation from planari-
ty is observed in the aromatic rings, due to the loss of aro-
maticity, resulting from the metal–carbene resonance contri-
bution. Steric crowding between the benzo-fused ring and
carbonyl ligands also causes slight rotation of the carbene,
in the plane of the rings.
Crucially, the CO bond-stretching frequencies for 15,
when compared to analogous data for related NHC and
other carbene complexes (Table 2), prove the benzo[c]qui-
nolin-6-ylidene ligand to be at least as strong a donor ligand
as the best reported to date.[15,30,31] These data support both
the validity of our calculation methodology in this case and
also augur well for the further development of these ligands
for catalysis.
trile with 0.1mÀ1 in [nBu4N]
ACTHNUTRGENUN[G PF6] as the supporting electro-
lyte. The voltammogram was obtained at room temperature
with a scan rate of 50 mVsÀ1 and is referenced to the SCE
at 0.00 V. Under these conditions 16 exhibited a reversible
(peak–peak separation of 43 mV) reduction at À0.76 V. In-
terestingly, only half-wave reduction potential was observed
for 16 down to reduction potentials of À2.00 V. In order to
ascertain a one or two-electron transfer process, simulations
for one and two-electron reduction processes were carried
out and supported a one-electron reduction process.
Dication 16 could potentially accommodate two electrons,
but it is reasonable to assume considerable stability for radi-
cal cation 19, due to the delocalisation of the free radical
over the extended conjugated p system. A subsequent sec-
ondary reduction of radical cation 19 to form neutral 20
would require substantially stronger reduction potentials,
due to the intrinsic strain induced by the steric repulsion be-
Reaction of 8 with [Ni
The reaction between 8 and an equimolar amount of [Ni-
(cod)2] in acetonitrile resulted in the formation of 16, the
ACHTUNGRTEN(NUNG cod)2] (COD=1,5-cyclooctadiene):
ACHTUNGTRENNUNG
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4291

D. F. Wass et al.
Compound 20 can be syn-
thesised in low yields (ca. 10%)
under very forceful conditions
and we were able to indirectly
prove its existence by coupling
two molecules of 7 using low-
valent titanium species and sub-
sequently oxidising 20 with two
equivalents of ferrocenium
hexafluorophosphate
Scheme 10).
(see
Synthesis of palladium com-
plexes [Pd(benzo[c]quinolin-6-
ylidene)
(PPh3)2(L)]2+ L=THF
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
(21), OTf (22) or pyridine (23):
The wide utility of palladium
compounds in catalysis made
such complexes attractive tar-
gets to evaluate the potential of
benzo[c]quinoline-6-ylidene as
a supporting ligand in catalysis.
Reaction of
8
with [Pd-
AHCTUNTGREG(NUNN PPh3)4] in THF led to the for-
mation of 21 in good yield
(72%; Scheme 11). During the
À
Scheme 8. Oxidative addition of [NiACTHNUGTRENUNG(cod)2] into the C O bond in 8, subsequent ligand exchange in 18 and re-
ductive dimerisation from trans-17 to give 16.
course of the reaction, a clear
orange solution is obtained ini-
tially at ambient temperatures, followed by an almost imme-
diate precipitation of 21, which is indicative of a very limited
solubility of 21 in THF. The structure of 21 was confirmed
1
by H and 13C{1H} NMR spectroscopy and elemental analy-
sis. The mass spectrum for 21 shows the [Pd-
(PPh3)2]+ fragment at
AHCTUNGTRE(GNNUN benzo[c]quinolin-6-ylidene)AHCTNURTGENGN(NU CF3SO3)ACHTGUNTRENNUGN
m/z=958. Attempted crystallisation of 21 from CH2Cl2/pen-
tane mixtures at room temperature led to the almost quanti-
tative isolation of 22·2CH2Cl2. The crystals obtained were
used to perform single-crystal X-ray diffraction analysis and
the solid-state structure of 22·2CH2Cl2 is presented in
Figure 3. The carbene moiety is disordered over two posi-
tions with relative occupancies of 1:1 and allows only limited
conclusions as to estimated carbene–palladium bond lengths.
However, the complex connectivity for 22 can be confirmed
and supports the proposed mechanism of formation (see
Scheme 11).
Figure 2. The cyclic voltammogram of a CH3CN solution of 16 (solid
line). Simulated electrochemical one-electron (dotted line) and two-elec-
tron reduction processes (dashed line) are shown. Inset: Molecular struc-
ture of 16 with thermal ellipsoids drawn at the 30% probability level.
À
Two CF3SO3 ions have been omitted for clarity. Selected bond lengths
The corresponding reaction of 8 with [Pd
dine generated 23 in good yield (71%). The mass spectrum
for 23 showed only the [Pd(benzo[c]quinolin-6-ylidene)-
(CF3SO3) (benzo[c]quinolin-6-ylidene)-
(PPh3)2]+ and [Pd
(CF3SO3)
(PPh3)]+ fragments at m/z=958 and 696 respec-
ACHTUNGTREN(UNNG PPh3)4] in pyri-
À
À
À
[ꢄ] and angles [8]: N1 C1 1.379(2), N1 C5 1.374(2), N1 C13 1.426(2),
À
À
À
À
À
C1 C2 1.358(2), C2 C3 1.403(2), C3 C4 1.367(2), C4 C5 1.406(2), C5
AHCTUNGTRENNUNG
À
À
À
C6 1.420(2), C6 C7 1.349(2), C7 C8 1.438(2), C7 C7 1.491(2); C5-N1-
C13-C8 À4.1(2), C6-C7-C7-C6 À79.0(2).
A
R
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
tively. Single crystals suitable for X-ray diffraction analysis
were obtained from diffusion of diethyl ether into a saturat-
ed solution of 23 in acetonitrile at ambient temperature.
The solid-state structure of 23 is shown in Figure 4.
It is interesting that the carbene moiety in 23 shows signif-
icant deviation from co-planarity for the three aromatic
rings. The dihedral angle C3-N1-C8-C13 in 23 is 12(2)8, rela-
tween hydrogen atoms in position C6 and C9 of the ben-
zo[c]quinon-6-ylidene moiety resulting from the co-planarity
of the p system (see Figure 2), supporting the assumption,
that only one electron is transferred at the relatively moder-
ate reduction potential of À0.76 V to form 19 (see
Scheme 9).
4292
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Chem. Eur. J. 2013, 19, 4287 – 4299

A New Class of Remote N-Heterocyclic Carbenes
FULL PAPER
Scheme 9. Electrochemical reduction of dication 16 to generate radical cation 19 and attempted subsequent electrochemical reduction to generate neu-
tral 20.
Scheme 10. Reductive dimerisation of 7 to give 20 and subsequent oxida-
tion using [Fc]ACHTUNGTRENNUNG
[PF6À].
tive to 0.5(3) (8), 0.1(3) (11), 4.1(2) (12), 3.0(2) (14) and
4.1(2)8 (16) in the corresponding benzo[c]quinolizinium
compounds supporting a carbene-type bond situation with a
C=C double bond in 23.[33]
Two sharp singlets at 20.4 and 22.8 ppm in the 31P{1H}
NMR spectrum for 23 and 21, respectively, are characteristic
for only one chemically and magnetically distinct phospho-
rous environment and are indicative of a trans coordination
for the two molecules of PPh3. Both compounds appear to
be stable in solution with respect to cis/trans isomerisation
and no evidence for an isomeric exchange can be observed
by 31P{H} NMR spectrum, for example, 23 in CD2Cl2 even
after one week at ambient temperature. As evident from
Figure 3. ORTEP representation of 22·ACTHNUTRGNE(UNG CH2Cl2)2 with thermal ellipsoids
drawn at the 30% probability level. All hydrogen atoms and solvent mol-
ecules have been omitted for clarity. Only selected atoms have been la-
À
À
belled. Selected bond lengths [ꢄ] and angles [8]: Pd1 O1 2.157(3), Pd1
À
À
À
1
31P{1H} and H NMR analysis, compound 21 can be trans-
C7a 1.96(4), Pd1 C7b 1.99(3), Pd1 P1 2.3385(10), Pd1 P2 2.3583(10);
C7a-Pd1-P1 88.4(15), C7b-Pd1-P1 90.3(16), P1-Pd1-P2 174.70(4), C7a-
Pd1-O1 174.4(9), C7b-Pd1-O1 178.3(10), P1-Pd1-C7a-C8a À100(4), P1-
Pd1-C7b-C8b À99(4), C6a-C7a-C8a 119(3), C6b-C7b-C8b 119(2), C5A-
N1A-C13 A-C8A À4(4), C5B-N1B-C13B-C8B À11(4).
formed quantitatively into 23 by dissolution in pyridine at
room temperature.
Preliminary catalyst screening for a range of C<-C>C
À
and C N coupling reactions and studies to confirm the in-
Scheme 11. Synthesis of palladium complexes 21–23.
Chem. Eur. J. 2013, 19, 4287 – 4299
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4293

D. F. Wass et al.
Experimental Section
Methods and materials: Where necessary, reactions were carried out
under an atmosphere of dry argon using either standard Schlenk line
techniques or an MBraun Labmaster glove box. NMR spectra were re-
corded on a Varian 400 (1H: 399.8 MHz, 13C: 100.5 MHz) or a Varian 500
(1H: 499.9 MHz, 13C: 125.7 MHz) spectrometer at ambient temperatures,
with all resonances referenced to residual NMR solvent resonances. All
elemental analyses were carried out by the Laboratory for Microanalysis
of the University of Bristol. All mass spectrometry analyses were carried
out on a VG AutoSpec by the Mass Spectrometry Service of the Univer-
sity of Bristol.
Dry N2-saturated diethyl ether, toluene, hexane, acetonitrile and methyl-
ene chloride were collected from a Grubbs-type solvent system using fil-
tration through an alumina column impregnated with deoxygenated cata-
lysts. THF was distilled from freshly cut sodium and benzophenone. Pen-
tane and pyridine were distilled from freshly ground CaH2 and degassed
prior to use. ACS grade methanol and acetone were purchased from
Fisher Scientific and used as received.
Silica (60 A, 35–70 mm particle size) and Al2O3 (activated, neutral, 507C,
150 mm particle size) were purchased from Fisher Scientific and Sigma–
Aldrich respectively and used as received. Potassium carbonate was pur-
chased from Fisher Scientific and used as received.
Figure 4. Molecular structure of 23 with thermal ellipsoids drawn at the
30% probability level. All hydrogen atoms have been omitted for clarity
and only selected atoms have been labeled. Selected bond lengths [ꢄ]
The compounds 2-chlorobenzonitrile and 2,6-dichlorobenzonitrile were
purchased from Sigma-Aldrich and purified prior to use by column chro-
matography (Silica) with diethyl ether for the former and THF for the
latter as eluents. The compounds 2-methylpyridine (2-picoline) and 2-
benzylpyridine were purchased from Sigma-Aldrich and distilled from
CaH2 prior to use. A 1.6m solution of nBuLi in hexane was purchased
from Acros and used as received, Methyl trifluoromethanesulfonate, tri-
À
À
À
À
and angles [8]: Pd N2 2.119(17), Pd P1 2.348(5), Pd P2 2.359(5), Pd C1
À
À
À
1.998(18), C5 C6 1.41(3), C6 C7 1.35(3), N1 C7 1.39(2); C1-Pd1-N2
177.3(7), P1-Pd1-P2 170.2(2), C1-Pd1-P1 87.1(5), N2-Pd1-P1 93.6(5), C1-
Pd1-P2 88.9(5), N2-Pd1-P2 90.0(5), C2-C1-C13 117.4(2), P1-Pd1-C1-C2
96.6(14), C3-N1-C8-C13 À12(2).
fluoromethanesulfonic anhydride, [NiACHTUNGRTENNN(GU cod)2], [PdACHTUNRGTEG(NNUN PPh3)4], [PtACHTUNGTRENNUNG(PPh3)4]
and PPh3 were purchased from Sigma-Aldrich and used as received.
2,4,6-Trimethylaniline was purchased from Sigma-Aldrich and distilled
from CaH2 prior to use. Potassium hydride was purchased as a 30 wt%
suspension in mineral oil. Prior to use, it was washed with generous por-
tions of dry toluene and pentane before all volatiles were removed under
reduced pressure.
tegrity of the metal–carbene fragment during catalysis are
underway, and will be reported in due course.
Conclusion
The compounds 2-(2-chlorophenacyl)pyridinium chloride (1) and 6-hy-
droxybenzo[c]quinolizinium chloride (2) were prepared by a modification
of the synthetic protocol as described by Vierfond and co-workers.[22] The
detailed synthetic procedure we used is outlined below.
We introduce benzo[c]quinolin-6-ylidene (1) as a new class
of strongly electron-donating carbene ligand. Useful precur-
sors to 1 are readily assembled from simple pyridine and
benzene components by methods that should permit the in-
troduction of a range of structural diversity. This potentially
useful facet is likely to require judicious choice of substitu-
ents to avoid complication in some cases by unwanted side
reactions. We continue to explore along these lines and
progress will be reported in due course. The facile synthesis
of 6-trifluoromethanesulfonylbenzo[c]quinolizinium trifluor-
omethanesulfonate (2) gives straightforward access to a
useful precursor for oxidative addition to low-valent metals,
Synthesis of 2-(2-chlorophenacyl)pyridinium chloride (2): A solution of
2-picoline (3.95 mL, 40 mmol) in THF (40 mL) was cooled to À788C and
nBuLi (25 mL, 1.6m in hexane, 40 mmol, 1 equiv) was added in small por-
tions. The reaction mixture was stirred for 10 min at À788C until an
orange precipitate formed. Warming to 08C resulted in the formation of
a clear orange solution that was added dropwise to a solution of 2-chloro-
benzonitrile (5.5 g, 40 mmol, 1 equiv) in THF (40 mL) at À788C. The re-
action mixture was stirred for 16 h, whilst being allowed to warm to
room temperature. All moisture sensitive compounds were quenched by
careful addition of water (100 mL) and all organic volatiles were re-
moved under reduced pressure. After adjusting pH 2 (H2SO4/H2O), the
reaction mixture was heated to 408C for 1 h. Addition of a saturated
aqueous solution of K2CO3 until pH 8 resulted in a phase separation be-
tween a colourless aqueous phase and an orange oil. The aqueous phase
was extracted with pentane (2ꢃ100 mL) and the combined organic
phases were washed with water (2ꢃ100 mL), brine (100 mL) and dried
over MgSO4. All solvents were removed under reduced pressure and the
viscous orange residue was dissolved in ethanol (10 mL). After addition
of 6m HCl (10 mL) and stirring for 10 min, the reaction mixture was
evaporated to dryness to give a white to yellow solid. The solid residue
was recrystallised from hot ethanol (ca. 200 mL). Pale beige crystals were
obtained at 58C and were washed with diethyl ether (2ꢃ20 mL), pentane
(2ꢃ20 mL) and dried in vacuo. The crystals were dissolved in methanol
(10 mL) and precipitated by drop-wise addition to rapidly stirred diethyl
ether (200 mL) to yield 8.7 g (32.4 mmol, 81.1%) of the target compound
as a bright yellow powder. Single crystals suitable for X-ray diffraction
to yield carbene complexes such as [Mn
ylidene)(CO)5]+ (15) and [Pd
(benzo[c]quinolin-6-ylidene)-
ACHTUNGTRENNUNG
(PPh3)2(L)]2+ L=THF (21), OTf (22) or pyridine (23). CO
ACHTUNGERTN(NUNG benzo[c]quinolin-6-
AHCTUNGTRENNUNG
stretching-frequency data for the manganese compound sug-
gests benzo[c]quinolin-6-ylidene is at least as strong a donor
as any heteroatom-stabilised carbene ligand reported. With
the synthetic organometallic chemistry of this ligand on cat-
alytically relevant metals, such as palladium, in hand, we are
now starting to explore its potential in catalysis for reactions
in which electron-rich metal centres are believed to be pre-
ferred.
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Chem. Eur. J. 2013, 19, 4287 – 4299

A New Class of Remote N-Heterocyclic Carbenes
FULL PAPER
analysis were obtained by diffusion of diethyl ether into a saturated solu-
tion of 1 in methanol. H NMR: (3.99.8 MHz, [D4]methanol)—this shows
7.62 ppm (ddd, J=13.5, 6.36, 2.3 Hz, 1H; ArH); 13C NMR ([D6]DMSO,
100.5 MHz): d=163.44, 145.90, 139.05, 137.96, 133.07, 132.95, 132.89,
132.15, 125.99, 120.35, 119.26, 118.23, 104.45 ppm; MS (EI, 70 eV): m/z
(%): 229.0 (80) [M]+, 201.0 (100) [MÀCO]+, 166.1 (45) [MÀCOCl]+.
1
a mixture of keto and enol tautomers, in the ratio of 1.4:1 by relative in-
tegration; integrations are given relative to the other peaks of that tauto-
mer, with the keto peaks denoted as *: d=8.92 (ddd, 5JHH =0.7 Hz,
4JHH =1.5 Hz, 3JHH =5.9 Hz, 1H), 8.65 (td, 4JHH =1.71 Hz, 3JHH =7.8 Hz,
1H), 8.57* (ddd, 5JHH =0.7 Hz, 4JHH =1.7 Hz, 3JHH =6.1 Hz, 1H), 8.40*
Synthesis of 5: Identical synthetic protocol as for synthesis of 2, from 2-
benzylpyridine (10.54 g, 60.2 mmol) and 2-chlorobenzonitrile (8.28 g,
60.2 mmol). Purified by crystallisation from either, hot ethanol or slow
diffusion of diethyl ether into a solution of 5 in methanol. Yield after
crystallisation from hot ethanol: 2.86 g (15%); yellow solid. 1H NMR
(399.8 MHz, [D6]DMSO): d=10.31 (s, 1H; NH2), 9.17–9.10 (m, 1H; N=
CH), 9.07 (d, J=8.56 Hz, 1H), 7.93–7.86 (m, 1H), 7.84–7.76 (m, 1H),
7.67–7.54 (m, 3H), 7.44–7.22 ppm (m, 6H); 13C NMR ([D6]DMSO,
100.5 MHz, 208C): d=150.24, 134.56, 134.50, 133.63, 131.66, 129.84,
129.25, 128.42, 127.09, 126.95, 126.17, 126.04, 125.31, 121.12, 118.05,
114.62 ppm; MS (EI, 70 eV): m/z (%): 270.1 (100) [MÀHCl]+.
5
4
3
(ddd, JHH =1.7 Hz, JHH =7.6 Hz, JHH =8.3 Hz, 1H), 8.18–8.09* (m, 2H),
8.09–8.04 (m, 1H), 7.92–7.98 (m, 1H), 7.67 (ddd, 5JHH =1.2 Hz, JHH
=
4
6.1 Hz, 3JHH =7.6 Hz, 1H), 7.62–7.43 ppm (m, 4H+5H*); 13C NMR
(101 MHz, [D4]methanol)—this shows a mixture of keto and enol tauto-
mers: d=196.11, 164.71, 151.33, 150.09, 146.07, 144.29, 142.26, 140.90,
136.52, 133.76, 131.75, 131.34, 130.74, 130.30, 129.41, 127.85, 125.99,
124.66, 121.69, 96.47, 46.48 ppm; MS (EI, 70 eV): m/z (%): 231.1 (35)
[M]+, 196.1 (100) [MÀCl]+, 168. 1 (78) [MÀClÀCO]+; elemental analysis
calcd (%) for C13H11Cl2NO: C 58.23, H 4.13, N 5.22, found: C 58.06, H
4.05, N 5.33.
Synthesis of benzo[c]quinazolinon
ACHUTGTNRNEUG(N pyridoACHTGNUTRENNUNG
Hydroxybenzo[c]quinolizinium chloride
3
Synthesis of 6-hydroxybenzo[c]quinolizinium chloride (3): Compound 2
(1.5 g, 5.59 mmol) was dissolved in water (5 mL) to give a clear pale
yellow solution and a saturated aqueous solution of K2CO3 was added
until pH 8 was reached. Phase separation was observed and the yellow to
orange organic phase was extracted from the aqueous phase with diethyl
ether (3ꢃ20 mL). The combined ether fractions were dried over Na2SO4,
filtered and all solvents were removed under reduced pressure to yield
an orange oil. Under an atmosphere of dinitrogen the orange oil was
heated for 2 h to 2008C resulting in the formation of a beige to grey pre-
cipitate. After cooling the reaction products to 258C, they were dissolved
in ethanol (5 mL) and precipitated into large quantities of ethyl acetate
(100 mL) to precipitate an off-white powder (crude yield: 0.91 g,
3.92 mmol, 70%). The powder was dissolved in methanol and slow diffu-
sion of diethyl ether at 58C resulted in the formation of pale yellow crys-
tals of 3·H2O suitable for X-ray diffraction analysis. Analytically pure
samples of 3 were obtained from the precipitation of a methanolic solu-
tion (5 mL) of these crystals into rapidly stirred diethyl ether (30 mL), fil-
solved in a mixture of methanol and water (30 mL, 1:1 vol%). A saturat-
ed aqueous solution of K2CO3 was added dropwise until the aqueous
phase was almost discoloured and an orange oil was separated. The aque-
ous phase was extracted with methylene chloride (6ꢃ100 mL) and the
combined organic fractions were dried over MgSO4. All volatiles were re-
moved under reduced pressure to leave a bright yellow solid (1.13 g,
5.81 mmol, 90%). Satisfactory elemental analysis for the yellow solid was
obtained after recrystallisation from hot acetone, followed by vacuum
sublimation (1508C, 5.0ꢃ10À3 mbar). Crystals suitable for single-crystal
X-ray diffraction analysis were obtained by crystallisation of 7 from hot
3
acetone. 1H NMR ([D6]DMSO, 399.8 MHz, 208C): d=8.97 (d, JHH
=
7.5 Hz, 1H; H1), 8.49 (d, 3JHH =9.0 Hz, 1H; H10), 8.36 (dd, 4JHH =1.7 Hz,
3JHH =8.0 Hz, 1H; H7), 7.81 (m, 1H; H9), 7.62 (t, 3JHH =7.2 Hz, 1H; H8),
7.29 (m, 2H; H3 and H4), 6.74 (m, 1H; H2), 6.28 ppm (s, 1H; H5);
13C NMR ([D6]DMSO, 125.7 MHz, 208C): d=172.6, 146.3, 137.1, 132.6
(C9), 130.4 (C1), 132.1 and 125.3 (C3 and C4), 130.4 (C1), 127.2 (C8), 126.8
(C7), 117.7 (C10), 112.3 (C2), 104.5 ppm (C5); MS (EI, 70 eV): m/z (%):
195.1 (92) [M]+, 167.1 (100%) [MÀCO]+; elemental analysis calcd (%)
for C13H9NO: C 79.98; H 4.65; N 7.17; found: C 79.70, H 4.80, N 7.20.
tration of the white solid and subsequent drying under vacuum (0.84 g,
3
3.64 mmol, 65%). 1H NMR (399.8 MHz, [D6]DMSO): d=9.83 (d, JHH
=
7.2 Hz, 1H; H1), 8.87 (d, 3JHH =8.9 Hz, 1H; H10), 8.47 (m, 1H; H7), 8.18
(m, 3H; H3, H4, H9), 7.96 (t, J=8.0 Hz, 1H; H8), 7.79 (dt, J=7.0, 1.8 Hz,
1H; H2), 7.35 ppm (s, 1H; H5); 13C NMR ([D6]DMSO, 100.5 MHz,
208C): d=161.1, 146.4, 137.9 (C9), 135.8, 133.5 (C1), 131.8 and 129.6 (C3
and C4), 126.2 (C8), 124.4 (C7), 121.6, 120.0 (C10), 117.2 (C2), 101.4 ppm
Synthesis of 6-trifluoromethanesulfonylbenzo[c]quinolizinium trifluoro-
methanesulfonate (8) A stirred solution of 7 (0.300 g, 1.54 mmol) in
CH2Cl2 (10 mL) was cooled to À788C. At this temperature, (CF3SO2)2O
(0.280 mL, 1.69 mmol, 1.1 equiv) was added dropwise by syringe and stir-
ring was continued for 1 h at À788C. The cooling bath was removed and
the reaction mixture was allowed to warm to room temperature while
stirring was continued for 16 h. The formation of a white precipitate in a
clear pink supernatant was observed. All volatiles were removed under
reduced pressure and the remaining white solid was washed with THF
5
+
+
À
(C ); MS (EI, 70 eV): m/z (%): 195.1 (92) [M] , 167.1 (100) [M CO] ;
elemental analysis calcd (%) for C13H12ClNO2: C 62.53, H 4.84, N 5.61;
found: C 62.60, H 4.76, N 5.60.
Synthesis of 2-(2,4-dichlorophenacyl)pyridinium chloride: An analogous
procedure to that employed for synthesis of 2 was followed for synthesis
of 2-(2,4-dichlorophenacyl)pyridinium chloride. 1H NMR (399.8 MHz,
[D6]DMSO): d=8.60 (d, J=5.9 Hz, 1H; ArH), 8.37 (t, J=7.5 Hz, 1H;
ArH), 8.29 (d, J=8.3 Hz, 1H; ArH), 7.70–7.50 (m, 4H; ArH), 5.87 ppm
(s, 1H; C=CH); 13C NMR ([D6]DMSO, 100.5 MHz): d=162.18, 150.74,
144.68, 140.54, 135.75, 133.58, 132.25, 128.82, 128.60, 124.76, 121.74,
96.32 ppm; MS (EI, 70 eV): m/z (%): 265.0 (15) [M]+, 230.0 (70)
[MÀCl]+, 202.0 (70) [MÀCOCl]+, 172.9 (100) [MÀPyCH2]+.
(3ꢃ10 mL). After drying in vacuo, analytically pure
8 (0.700 mg,
1.46 mmol, 95%) was obtained. Single crystals of 8 were obtained from
the slow diffusion of diethyl ether into a saturated solution of 8 in aceto-
nitrile at 208C. 1H NMR ([D3]acetonitrile, 399.8 MHz, 208C): d=10.14
(d, 3JHH =7.0 Hz, 1H), 9.03 (d, 3JHH =9.1 Hz, 1H), 8.69–8.67 (m, 2H),
8.43–8.40 (m, 2H), 8.35–8.30 (m, 2H), 8.22–8.18 ppm (m, 1H); 13C NMR
([D3]acetonitrile, 125.7 MHz, 208C): d=149.6, 144.3, 142.3, 136.7, 135.3,
135.1, 132.2, 129.7, 126.0, 123.8, 121.2, 120.5, 119.0, 117.3, 114.1 ppm; MS
(EI, 70 eV): m/z (%): 328 (18) [M]+, 195 (80) [MÀCF3SO3]+, 167 (100)
[195ÀCO]; HRMS (ESI): m/z calcd for C14H9N2O3F3S: 328.0255 [M]+;
found: 328.0242; elemental analysis calcd (%) for C15H9F6NO6S2: C
37.74, H 1.90, N 2.93; found: C 37.76, H 1.96, N 3.04.
Synthesis of 7-chloro-6-hydroxypyridoACTHNUTRGNEUGN[1,2-a]quinolin-11-ium chloride (4):
2-(2,4-Dichlorophenacyl)pyridinium chloride (4.0 g, 13.4 mmol) was dis-
solved in methanol (10 mL). This yellow solution was treated with excess
saturated aqueous potassium carbonate solution (200 mL), in a separat-
ing funnel and the resulting mixture was extracted with THF (5ꢃ50 mL).
The combined extracts were dried over MgSO4, filtered and the solvents
removed in vacuo. Toluene was added and then removed by rotary evap-
orator, in order to remove remaining traces of water by azeotropic codis-
tillation. The resulting oil was then dried under high vacuum for 30 mins
at room temperature before being heated to 2008C, under nitrogen, for
2.5 h. The oil crystallised upon cooling and the crystals were washed with
CH2Cl2, to remove any impurities. The resulting product could be recrys-
Synthesis of 6-methoxybenzo[c]quinolizinium trifluoromethanesulfonate
(9): Ketone 7 (0.200 g, 1.02 mmol) was dissolved in CH2Cl2 (10 mL) and
cooled to À788C, before CH3OSO2CF3 (0.128 mL, 1.13 mmol) was added
dropwise by syringe. The reaction mixture was stirred for 15 min at
À788C, after which the cooling bath was removed and the reaction mix-
ture was allowed to warm slowly to room temperature. Stirring was con-
tinued at this temperature for 15 min. The yellow precipitate obtained
was filtered, washed with CH2Cl2 (3ꢃ5 mL) and dried in vacuo (0.156 g,
43%). 1H NMR ([D3]acetonitrile, 399.8 MHz, 208C): d=9.68–9.65 (m,
1
tallised from hot ethanol to give 1.8 g (45%) of yellow crystals. H NMR
(399.8 MHz, [D6]DMSO): d=9.80 (d, J=7.3 Hz, 1H; ArH), 8.96 (dd, J=
8.3, 1.71 Hz, 1H; ArH), 8.13 (m, 2H; ArH), 8.00 (m, 2H; ArH),
1H), 8.75–8.72 (m, 1H), 8.51 (ddd, 3JHH =0.5 Hz, JHH =1.6 Hz, JHH
8.2 Hz, 1H), 8.31–8.22 (m, 2H), 8.18–8.14 (m, 1H), 8.02–7.98 (m, 1H),
=
Chem. Eur. J. 2013, 19, 4287 – 4299
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4295

D. F. Wass et al.
7.85 (dt, JHH =1.9 Hz, JHH =14.1 Hz, 1H), 7.54 (s, 1H), 4.28 ppm (s, 3H);
13C NMR ([D3]acetonitrile, 125.7 MHz, 208C): d=162.0, 140.4, 134.6,
133.6, 131.5, 128.1, 125.3, 123.8, 122.7, 122.3, 118.7, 100.7, 58.8 ppm; MS
(EI, 70 eV): m/z (%): 210 (10) [M]+, 195 (90) [MÀCH3]+, 167 (100)
[MÀCOMe]+; elemental analysis calcd (%) for C15H12F3NO4S: C 50.14,
H 3.37, N 3.90; found: C 50.25, H 3.56, N 3.91.
(m, 2H), 8.00–7.95 (m, 3H), 7.87–7.69 ppm (m, 14H); 13C NMR
([D3]acetonitrile, 125.7 MHz, 208C): d=150.0, 146.1, 143.3, 136.0, 135.2,
132.5, 130.6, 129.8, 127.3, 124.9, 122.7, 122.4, 119.2 ppm; 31P NMR
(299.9 MHz, [D3]acetonitrile): d=23.0 ppm (s); MS (ESI): m/z (%): 590.1
(100) [M+CF3SO3]+, 220.6 (30) [M]2+
; HRMS (EI): m/z calcd for
C31H24N1P1+: 220.5818 [M]2+; found: 220.5820; elemental analysis calcd
(%) for C33H24F6NO6PS2: C 53.59, H 3.27, N 1.89; found: C 54.03, H
3.61, N 2.28.
Synthesis of 6-methoxybenzo[c]quinolizinium tetrafluoroborate (10):
Ketone 7 (0.233 g, 1.19 mmol) and [Me3O]ACTHNUTRGNEUNG[BF4] (0.206 g, 1.40 mmol)
were mixed in the absence of solvent and cooled to À788C before
CH2Cl2 (10 mL) was added. The reaction mixture was allowed to slowly
warm to ambient temperature over the course of 16 h. All volatiles were
removed under reduced pressure, and the white residue was washed with
MeOH (2ꢃ5 mL) and dried in vacuo (0.320 g, 1.08 mmol, 90% yield).
1H NMR ([D6]DMSO, 400.2 MHz, 228C): d=10.12 (d, 3JHH =7.0 Hz,
1H,), 9.11 (d, 3JHH =9.0 Hz, 1H), 8.50 (dd, 3JHH =8.1 Hz, 4JHH =1.5 Hz,
1H), 8.46–8.42 (m, 1H), 8.25–8.20 (m, 1H), 8.08–8.04 (m, 1H), 7.99–7.95
(m, 1H), 7.92 (s, 1H), 4.29 ppm (s, 3H); 13C NMR ([D6]DMSO,
100.6 MHz, 238C): d=160.5, 146.6, 140.0, 139.2, 135.8, 133.9, 130.8, 127.5,
124.3, 121.8, 121.6, 118.9, 100.6, 58.5 ppm; MS (EI, 70 eV): m/z (%): 210
(10) [M]+, 195 (70) [MÀCH3]+, 167 (100) [MÀCOCH3]+; HRMS (EI):
m/z calcd for C14H12NO: 210.0919 [M]+; found: 210.0916; elemental anal-
ysis calcd (%) for C14H12BF4NO: C 56.61, H 4.07, N 4.72; found: C 56.48,
H 4.32, N 4.59.
Synthesis of 14: A solution of triflate 8 (0.100 g, 0.210 mmol) in acetoni-
trile (2 mL) was treated dropwise with a solution of ketone 7 (41.0 mg,
0.210 mmol) in acetonitrile (2 mL). The clear yellow solution was stirred
for 16 h at ambient temperature to precipitate a pale beige solid. The re-
action mixture was warmed briefly to 608C until all the residue was dis-
solved, filtered and allowed to cool slowly to room temperature. Color-
less crystals (0.131 g, 0.195 mmol, 93%) of the target compound were ob-
tained after cooling to 58C. 1H NMR ([D3]acetonitrile, 399.8 MHz,
258C): d=9.98 (d, 3JHH =6.6 Hz, 1H), 8.97 (d, 3JHH =9.3 Hz, 1H), 8.68
(dd, 3JHH =8.1 Hz, 4JHH =1.5 Hz, 1H), 8.50–8.46 (m, 1H), 8.35–8.29 (m,
2H), 8.16–8.12 (m, 2H), 7.94 ppm (s, 1H); 13C NMR ([D3]acetonitrile,
100.5 MHz, 258C): d=157.3, 146.3, 141.7, 135.4, 134.8, 132.2, 129.2, 125.2,
124.7, 122.3, 119.2, 110.2, 100.9 ppm; MS (ESI): m/z (%): 523.1 (100)
[MÀCF3SO3]+, 373.1 (20) [MÀH]2+
; HRMS (ESI): m/z calcd for
C27H18F3N2O4S+: 523.0934 [MÀCF3SO3]+; found: 523.0924; elemental
analysis calcd (%) for C28H18F6N2O7S2: C 50.00, H 2.70, N 4.17; found: C
49.96, H 2.61, N 4.25.
Synthesis of 6-ethoxybenzo[c]quinolizinium tetrafluoroborate (11):
Ketone 7 (0.233 g, 1.19 mmol) and [Et3O]ACHTNURGTNEUNG[BF4] (0.265 g, 1.40 mmol) were
mixed in the absence of solvent and cooled to À788C before CH2Cl2
(10 mL) was added. The reaction mixture was allowed to slowly warm to
ambient temperature over the course of 16 h. All volatiles were removed
under reduced pressure, and the white residue was dissolved in CH3CN
(5 mL), filtered and precipitated by dropwise addition into rapidly stirred
Synthesis of 15: Sodium metal (20 mg, 0.89 mmol) was added to mercury
(0.37 mL) in small pieces and stirred for approximately 15 mins, until all
amalgamated. [Mn2(CO)10] (137 mg, 0.35 mmol) was added to the result-
ing amalgam in THF (6 mL). This mixture was stirred for 2 h before fil-
tering through celite, directly into a flask containing solid bis triflate 8
(200 mg, 0.42 mmol). The resulting red suspension was stirred overnight
and an off-white solid filtered off. This solid was dried under vacuum and
extracted with CH2Cl2 (3ꢃ2 mL). The resulting solution was concentrat-
ed to approximately 2 mL and cooled to À208C to give 15 as very pale
yellow prisms (22 mg, 0.044 mmol, 10% yield). 1H NMR ([D6]DMSO,
399.8 MHz): d=10.12 (d, J=7.1 Hz, 1H), 9.08 (d, J=8.1 Hz, 1H), 8.76 (s,
1H), 8.61 (dd, J=7.3, 1.96 Hz, 1H), 8.43–8.52 (m, 2H), 8.13–8.06 (m,
2H) 8.03–7.96 ppm (m, 1H); 13C NMR ([D6]DMSO, 100.4 MHz): d=
209.79, 179.08, 140.60, 139.64, 137.37, 137.22, 136.20, 133.21, 132.37,
132.25, 130.21, 126.80, 122.89, 119.60 ppm; FTIR (solid state): n˜ =2127.9
(C=O stretch), 2069.2 (C=O stretch), 1997.0 (C=O stretch), 1634.3,
1604.0, 1579.3, 1570.8, 1524.6, 1488.9, 1453.4, 1438.8, 1422.7, 1256.3,
diethyl ether (25 mL) to give a white powder (0.285 g, 0.916 mmol,
3
77%). 1H NMR ([D3]acetonitrile, 399.8 MHz, 258C): d=9.61(d, JHH
=
4
7.1 Hz, 1H), 8.69 (d, 3JHH =8.6 Hz, 1H), 8.51 (dd, 3JHH =8.3 Hz, JHH
=
1.7 Hz, 1H), 8.29–8.25 (m, 1H), 8.21–8.18 (m, 1H), 8.15–8.11 (m, 1H),
8.00–7.96 (m, 1H), 7.83 (dt, 3JHH =7.1 Hz, 4JHH =1.7 Hz, 1H), 7.47 (s,
1H), 4.53 (q, 3JHH =7.1 Hz, 2H), 1.62 ppm (t, 3JHH =7.1 Hz, 3H);
13C NMR ([D3]acetonitrile, 125.7 MHz, 258C): d=161.5, 147.9, 140.5,
136.7, 134.9, 133.6, 131.7, 128.4, 125.6, 123.0, 122.5, 118.8 (overlapping
with NMR solvent at 118.7), 101.4, 101.3, 68.3, 14.8 ppm; MS (EI, 70 eV):
m/z (%): 224 (80) [M]+, 195 (95) [MÀC2H5]+, 167 (100) [MÀCOC2H5]+;
HRMS (EI): m/z calcd for C15H14NO: 224.1075 [M]+; found: 224.1073; el-
emental analysis calcd (%) for C15H12F3NO4S: C 50.14, H 3.37, N 3.90;
found: C 50.25, H 3.56, N 3.91.
1225.3, 1210.8, 1150.2, 1028.0, 914.0, 902.1, 842.7, 770.9, 732.9, 649.5 cmÀ1
;
MS (ESI): m/z (%): 345.99 (100) [MÀCO]+, 373.98 (55) [M]+.
Synthesis of 6-pyridiniumbenzo[c]quinolizinium ditriflate (12): A sample
of 8 (0.100 g, 0.210 mmol) was dissolved in pyridine (5 mL) and stirred at
208C for 16 h. All volatiles were removed under reduced pressure to
yield a colourless crystalline solid. Purification was achieved by recrystal-
lisation from the slow diffusion of diethyl ether into a saturated solution
of the product in CH3CN at 208C. Colorless crystals (0.115 g,
0.207 mmol, 99% yield) suitable for single crystal X-ray diffraction analy-
Synthesis of 16: In the absence of solvent, 8 (0.100 g, 0.210 mmol) and
[NiACHTNUTRGNENG(U cod)2] (0.057 g, 0.207 mmol) were mixed in the absence of solvent.
Acetonitrile (2.0 mL) was added slowly at 208C and stirring was contin-
ued for 16 h, after which a yellow solution was obtained. All volatiles
were removed under reduced pressure and the grey residues were dis-
solved in acetonitrile (1 mL) and filtered. Diffusion of diethyl ether
vapour into this solution resulted in the formation of colorless crystals
(0.065 g, 0.099 mmol, 94%). An analytically pure sample of 16 was ob-
tained after column chromatography (Al2O3, MeCN) and crystallisation
from an acetonitrile/diethyl ether mixture. 1H NMR ([D3]acetonitrile,
sis were obtained. 1H NMR ([D3]acetonitrile, 399.8 MHz, 208C): d=
4
10.24 (d, 3JHH =7.0 Hz, 1H), 9.07–9.10 (m, 3H), 8.69–8.67 (tt, JHH
=
2.5 Hz, 3JHH =16.0 Hz, 1H), 8.82–8.73 (m, 2H), 8.66 (s, 1H), 8.48–8.45
(m, 3H), 8.39–8.34 (m, 1H), 8.11 (t, 3JHH =15.5 Hz, 1H), 7.70 ppm (dd,
4JHH =1.3 Hz, 3JHH =8.3 Hz, 1H); 13C NMR ([D3]acetonitrile, 125.7 MHz,
208C): d=150.0, 146.1, 143.3, 136.0, 135.2, 132.5, 130.6, 129.8, 127.3,
124.9, 122.7, 122.4, 119.2 ppm.
3
399.8 MHz, 208C): d=10.18 (d, JHH =7.2 Hz, 2H), 9.04 (d, 3JHH =9.0 Hz,
4
3
2H,), 8.73–8.69 (m, 2H), 8.64 (dd, JHH =1.8 Hz, JHH =8.3 Hz, 2H), 8.39–
8.35 (m, 4H), 8.28–8.24 (m, 2H), 7.93–7.89 (m, 2H), 7.79–7.76 ppm (dd,
4JHH =1.47 Hz, 3JHH =8.19 Hz, 2H); 13C NMR ([D3]acetonitrile,
125.7 MHz, 208C): d=143.3, 142.2, 141.9, 135.0, 134.7, 134.1, 131.5, 129.6,
128.7, 126.6, 125.8, 124.8, 118.6 ppm; MS (ESI): m/z (%): 507.1 (100)
Synthesis of 6-triphenylphosphoniumbenzo[c]quinolizinium ditriflate
(13): Triphenylphosphine (0.100 g, 0.210 mmol) and
8
(54.9 mg,
0.210 mmol) were mixed in the absence of solvent. Acetonitrile (2 mL)
was added and a clear yellowish solution was obtained instantly. After
stirring for 16 h, all volatiles were removed under reduced pressure. The
foamy yellow residue was washed consecutively with CH2Cl2 (2ꢃ3 mL)
and THF (3ꢃ5 mL) to precipitate a white solid, which was filtered and
dried under reduced pressure to yield the target compound (0.140 g,
[(C13H9N)
for C27H18N2F3O3S: 507.0985 [(C13H9N) CTHUNGTRENNUNG
(CF3SO3)]+, 179.1 (36) [(C13H9N)2]2+; HRMS (ESI): m/z calcd
2A
(CF3SO3)]+; found: 507.0977; ele-
mental analysis calcd (%) for C13H9NO: C 79.98, H 4.65, N 7.17; found:
C 79.70, H 4.80, N 7.20.
Synthesis of 21: Compound 8 (0.100 g, 0.210 mmol) and [PdACHTUNGTRENNUNG(PPh3)4]
0.189 mmol, 90%). 1H NMR ([D3]acetonitrile, 399.8 MHz, 208C): d=
(0.242 g, 0.210 mmol) were mixed in the absence of solvent before THF
(3 mL) was added at À788C. Stirring was continued for 1 h, before the
reaction mixture was allowed to warm to 08C, after which stirring was
3
10.24 (d, 3JHH =6.9 Hz, 1H), 9.01 (d, 3JHH =8.9 Hz, 1H), 8.77 (t, JHH
=
15.6 Hz, 1H), 8.62 (d, 3JHH =7.6 Hz, 1H), 8.51–8.47 (m, 1H), 8.20–8.16
4296
www.chemeurj.org
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 4287 – 4299

A New Class of Remote N-Heterocyclic Carbenes
FULL PAPER
continued for 2 h. A pale beige precipitate formed rapidly, which was iso-
lated by filtration, washed with toluene (3ꢃ5 mL), pentane (3ꢃ5 mL)
and dried in vaccuo (0.178 g, 0.151 mmol, 72% yield). 1H NMR
([D2]methylenechloride, 399.8 MHz, 258C): d=9.65 (d, 3JHH =7.1 Hz,
1H), 9.02 (dd, 3JHH =8.1 Hz, 4JHH =1.2 Hz, 1H), 8.44 (d, 3JHH =8.8 Hz,
R1 =0.060 [I>2s(I)], wR2 =0.156, S=1.107. All esds (except the esd in
the dihedral angle between two least squares. planes) are estimated using
the full covariance matrix.
X-ray crystal structure analysis for 16: Only half a molecule of 16 is con-
tained in the asymmetric C2/c unit cell. The full molecule is generated by
3
3
1H,), 8.12 (t, JHH =7.8 Hz, 1H), 7.90–7.85 (m, 2H), 7.72 (t, JHH =7.6 Hz,
1H), 7.52–7.47 (m, 14H), 7.35–7.31 (m, 6H), 7.24–7.21 (m, 13H), 3.70–
3.65 (m, 4H), 1.84–1.80 ppm (m, 4H); 13C NMR ([D2]methylenechloride,
75.6 MHz, 218C): d=139.3, 139.0, 134.8, 134.7, 134.6, 133.9, 132.8, 132.0,
131.8, 131.7, 131.6, 130.4, 129.5, 129.1, 129.0, 128.9, 128.7, 128.6, 128.2,
127.9, 126.0, 123.1, 118.1, 68.3, 26.1 ppm; 31P{H} NMR ([D2]methylene
chloride, 121.4 MHz, 218C): d=22.8 ppm (s); MS (ESI): m/z (%): 958.1
a
crystallographic XYZ element; C53H43Cl4F6NO6P2PdS2, Mw =
656.56 gmolÀ1
pale yellow blocks, crystal size 0.17ꢃ0.17ꢃ0.08 mm,
monoclinic, space group C2/c, a=21.4143(5), b=11.2527(3), c=
12.2863(3) ꢄ, a=g=90.00, b=110.8520(10)8, V=2766.70(12) ꢄ3, T=
100 K, Z=4, l(MoKa)=0.71073 ꢄ, empirical absorption correction (min/
,
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
max transmission=0.68/0.75), Bruker Apex II CCD diffractometer,
2.04<q<27.59, 24192 measured reflections, 3205 independent reflec-
tions, 2801 reflections with I>2s(I). Structure solved by direct methods
and refined by full-matrix least squares against F2 to R1 =0.037 [I>
2s(I)], wR2 =0.086, S=1.044. All esds (except the esd in the dihedral
angle between two least squares. planes) are estimated using the full co-
variance matrix.
(100) [(C13H9N)
(CF3SO3)2]+, 696.0 (20) [(C13H9N)
z calcd for C50H39F3NO3P2PdS: 958.1107 [(C13H9N)
(PPh3)2Pd
E
ACHTUNGRTEN(NUNG PPh3)Pd-
E
U
ACHTUNGTRENNUNG
N
ACHTUNGTRENNUNG
found: 958.1131; elemental analysis calcd (%) for C55H47F6NO7P2PdS2: C
55.96, H 4.01, N 1.19; found: C 56.53, H 4.25, N 1.29.
Synthesis of 22: Pentane vapor was allowed to slowly diffuse into a solu-
tion of 21 in CH2Cl2, to give single crystals of 22. NMR data was identical
to that of 21 as both were recorded in [D2]methylene chloride, in which
the triflate counterion displaces the previously coordinated THF mole-
cule. Elemental analysis calcd (%) for C51H39F6NO6P2PdS2: C 55.27, H
3.55, N 1.26; found: C 56.49, H 4.26, N 1.29.
X-ray crystal structure analysis for 23: C56H44F6N2O6P2PdS2, Mw =
1187.39 gmolÀ1, colorless blocks, crystal size 0.07ꢃ0.08ꢃ0.08 mm, mono-
clinic, space group C2/c, a=33.0544(12), b=14.1897(5), c=22.5278(8) ꢄ,
a=g=90.00, b=99.418(2)8, V=10423.8(6) ꢄ3, T=100 K, Z=8,
l(MoKa)=0.71073 ꢄ, empirical absorption correction (min/max transmis-
sion=0.64/0.75), Bruker Apex II CCD diffractometer, 1.86<q<27.50,
80172 measured reflections, 11954 independent reflections, 8826 reflec-
tions with I>2s(I). Structure solved by direct methods and refined by
full-matrix least squares against F2 to R1 =0.071 [I>2s(I)], wR2 =0.119,
S=5.934. All esds (except the esd in the dihedral angle between two
least squares. planes) are estimated using the full covariance matrix.
Synthesis of 23: In the absence of solvent, [PdACTHUNRTGNEUNG(PPh3)4] (0.210 mmol,
0.242 g) and 8 (0.100 g, 0.210 mmol) were mixed, before dry pyridine
(5 mL) was added at 08C. The resulting dark brown solution was stirred
for 16 h. After removal of all volatiles under reduced pressure and wash-
ing with THF (3ꢃ5 mL), toluene (3ꢃ5 mL) and pentane (5 mL), the
target compound was isolated as a grey solid and dried in vacuo (0.255 g,
0.150 mmol, 71% yield). Single crystals suitable for X-ray diffraction
analysis were obtained from a diffusion of diethyl ether into a saturated
solution of the target compound in acetonitrile. 1H NMR
([D2]methylenechloride, 399.8 MHz, 258C): d=9.36 (dd, 3JHH =8.1 Hz,
X-ray crystal structure analysis for 22: C53H43Cl4F6NO6P2PdS2, Mw =
1278.14 gmolÀ1, yellow blocks, crystal size 0.32ꢃ0.18ꢃ0.12 mm, mono-
clinic, space group P21/c, a=13.1041(4), b=19.4984(5), c=22.3558(6) ꢄ,
a=g=90.00, b=97.688(2)8, V=5660.8(3) ꢄ3, T=200 K, Z=4,
l(MoKa)=0.71073 ꢄ, empirical absorption correction ((min/max trans-
mission=0.60/0.75), Bruker Apex II CCD diffractometer, 1.39<q<
27.48, 92844 measured reflections, 12984 independent reflections, 9837
reflections with I>2s(I). Structure solved by direct methods and refined
by full-matrix least squares against F2 to R1 =0.087 [I>2s(I)], wR2 =
0.145, S=1.163. All esds (except the esd in the dihedral angle between
two least squares. planes) are estimated using the full covariance matrix.
4JHH =1.5 Hz, 1H), 9.30 (d, 3JHH =7.1 Hz, 1H), 8.22–8.18 (m, 2H), 8.10–
3
8.09 (m, 2H), 8.08–7.97 (m, 3H), 7.83–7.79 (m, 1H), 7.69 (dt, JHH
=
4
6.9 Hz, JHH =2.0 Hz, 1H), 7.39–7.34 (m, 12H), 7.24–7.13 (m, 19H), 6.69–
6.65 (m, 2H), 2.28 (s, 3H), 2.02 ppm (s, 6H); 13C NMR
([D2]methylenechloride, 100.5 MHz, 258C): d=178.2, 151.3, 138.6, 137.9,
137.8, 136.5, 133.8, 133.7, 133.6, 133.0, 132.4, 131.2, 130.9, 130.6, 130.3,
129.7, 129.0, 128.9, 128.8, 127.5, 127.2, 126.9, 126.7, 125.2, 122.9, 121.9,
119.7, 116.0 ppm; 31P NMR ([D2]methylene chloride, 121.4 MHz, 258C):
X-ray crystal structure analysis for 15: C20H11Cl2F3MnNO8S, Mw =
608.21 gmolÀ1
mono
a=g=90.00, b=103.4108, V=2436.02 ꢄ3, T=200 K, Z=6, l
0.71073 ꢄ, empirical absorption correction (min/max transmission
,
pale yellow prisms, crystal size 0.28ꢃ0.20ꢃ0.15 mm,
clinic, space group P21/n, a=12.1911, b=16.2847, c=12.6143 ꢄ,
(MoKa)=
d=20.4 ppm (s); MS (ESI): m/z (%): 958.1 (2) [Pd
ACHTUGNTRENGUN(C13H9N)ACHTUGNTERN(NUGN PPh3)2-
AHCTUNGTRENNUNG
A
E
N
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
sis calcd (%) for C56H44F6N2O6P2PdS2: C 56.64, H 3.73, N 2.36; found: C
56.81, H 4.00, N 2.57.
=
0.60/0.75), Bruker Apex II CCD diffractometer, 2.08<q<27.56, 92844
measured reflections, 5602 independent reflections, 4545 reflections with
I>2s(I). Structure solved by direct methods and refined by full-matrix
least squares against F2 to R1 =0.1308 [I>2s(I)], wR2 =0.4166, S=2.110.
All esds (except the esd in the dihedral angle between two least squares.
planes) are estimated using the full covariance matrix.
Crystallographic data
X-ray crystal structure analysis for 33: The nitrogen atom N2 in the pyri-
dinium moiety in 33 is disordered over two positions C2/N2 with relative
occupancies approximating 1:1. X-ray Crystal Structure Analysis for 33:
C19H15ClN2, Mw =306.78 gmolÀ1, yellow blocks, crystal size 0.22ꢃ0.18ꢃ
0.18 mm, monoclinic, space group C2/c, a=15.6994(4), b=10.2109(2), c=
20.7769(6) ꢄ, a=g=90.00, b=111.2620(10)8, V=3103.93(13) ꢄ3, T=
Theoretical methods: All structures were optimised with Gaussian 03,[34]
using the standard B3LYP density functional as implemented in Gaussi-
an[35] and the standard 6–31G
ACTHUNTGRENNG(U d,p) basis set with 5 d functions on all
100 K, Z=8, lACHTUNGTRENNUNG(MoKa)=0.71073 ꢄ, empirical absorption correction (min/
atoms except for the metal centre, where the Stuttgart/Dresden effective
core potential (SDD) was used, specifying the default number of core
electrons (MDF10 for Ni).[36] After geometry optimisation with default
settings, minima were verified by frequency calculations.
max transmission=0.68/0.75 ), Bruker Apex II CCD diffractometer,
2.43<q<27.52, 13578 measured reflections, 3576 independent reflec-
tions, 3104 reflections with I>2s(I). Structure solved by direct methods
and refined by full-matrix least squares against F2 to R1 =0.036 [I>
2s(I)], wR2 =0.087, S=1.043. All esds (except the esd in the dihedral
angle between two least squares. planes) are estimated using the full co-
variance matrix.
X-ray crystal structure analysis for 7: C13H9N2O, Mw =195.21 gmolÀ1
brown laths, crystal size 0.35ꢃ0.12ꢃ0.06 mm, orthorhombic, space group
,
Acknowledgements
We thank Dr. Natalie Fey (Bristol) for assistance with the computation
of the carbonyl stretching frequencies. The EPSRC is thanked for fund-
ing of E.M. via the Bristol Synthesis Doctoral Training Centre.
Pbca, a=9.6438(2), b=14.2266(4), c=26.8589(7) ꢄ, a=b=g=90.008,
V=3685.00(16) ꢄ3, T=100 K, Z=16, l
ACTHNUGTRENNUG(MoKa)=0.71073 ꢄ, empirical ab-
sorption correction (min/max transmission=0.89/1.00), Bruker Apex II
CCD diffractometer, 2.60<q<27.51, 30616 measured reflections, 4237
independent reflections, 3445 reflections with I>2s(I). Structure solved
by direct methods and refined by full-matrix least squares against F2 to
Chem. Eur. J. 2013, 19, 4287 – 4299
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4297

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4298
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Chem. Eur. J. 2013, 19, 4287 – 4299

A New Class of Remote N-Heterocyclic Carbenes
FULL PAPER
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G
Received: September 14, 2012
aryl bromide and the subsequent reductive dimerisation to yield bi-
Published online: February 1, 2013
Chem. Eur. J. 2013, 19, 4287 – 4299
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4299