Synthesis of a pyrazol-3-ylidene palladium complex, pyrazolium salts and mesomeric betaines of pyrazole as N-heterocyclic carbene precursors

Article abstract of DOI:10.1515/znb-2012-0410

The synthesis of a pyrazol-3-ylidene palladium complex is described, and results of a single-crystal X-ray diffraction study are presented. The properties of pyrazolium salts as well as of two types of heterocyclic mesomeric betaines, the pseudo-cross-con

Full text of DOI:10.1515/znb-2012-0410

Synthesis of a Pyrazol-3-ylidene Palladium Complex, Pyrazolium Salts
and Mesomeric Betaines of Pyrazole as N-Heterocyclic Carbene
Precursors
Andrij Dregera, Martin Niegerb, Martin Drafza, and Andreas Schmidta
a Clausthal University of Technology, Institute of Organic Chemistry, Leibnizstraße 6,
38678 Clausthal-Zellerfeld, Germany
b University of Helsinki, Laboratory of Inorganic Chemistry, 00014 University of Helsinki, Finland
Reprint requests to Prof. Dr. Andreas Schmidt. E-mail: schmidt@ioc.tu-clausthal.de
Z. Naturforsch. 2012, 67b, 359 – 366; received November 25, 2011
The synthesis of a pyrazol-3-ylidene palladium complex is described, and results of a single-crystal
X-ray diffraction study are presented. The properties of pyrazolium salts as well as of two types
of heterocyclic mesomeric betaines, the pseudo-cross-conjugated mesomeric betaine pyrazolium-3-
carboxylate and its cross-conjugated analog pyrazolium-4-carboxylate, are compared.
Key words: NHC, Pyrazol-4-ylidene, NHC Complex, Decarboxylation, X-Ray Structure, CCMB,
PCCMB
Introduction
enes such as carbon dioxide [16 – 18]. By contrast,
the prop-2-en-1-iminiummoiety III and the 2-(iminio-
methyl)acrylate partial structure IV are building blocks
of precursors of remote N-heterocyclic carbenes
(rNHC) [19 – 21] which are set apart from the class
of abnormal N-heterocyclic carbenes (aNHC) [22 –
24]. Deprotonations and decarboxylations from partial
structures III and IV, however, are seemingly limited
to a quite small number of suited substrates. Thus, only
few examples of deprotonations of substituted pyra-
zolium salts have been reported to date [25]. Imidazol-
4-ylidenes, members of the class of abnormal NHCs,
have been obtained by decarboxylation of imidazol-
ium-4-carboxylates [3].
The iminium as well as the iminium-2-carboxyl-
ate partial structures I and II of heterocycles and het-
eroaromatics can serve as N-heterocyclic carbene pre-
cursors, when deprotonation of I or decarboxylation of
II are performed, respectively (Fig. 1). Thus, imidaz-
olium- [1 – 4], pyridinium- [5, 6], quinolinium- [7, 8],
pyrazolium- [9 – 12], indazolium- [12 – 15], and other
hetarenium-carboxylates undergo decarboxylation to
N-heterocyclic carbenes which can be trapped in situ
or examined spectroscopically. Conversely, it is well-
known that NHCs can be trapped with heterocumul-
N-Heterocyclic carbenes of pyrazole and of its rel-
ative, indazole, have so far stood in the shadow of the
impressive development of pyrazole [26, 27] and inda-
zole chemistry [28] taking place during the last decade.
However, some very interesting structures and appli-
cations have been published recently. As outlined in
Fig. 2, two types of N-heterocyclic carbenes of pyra-
zole can be differentiated, the NHC pyrazol-3-ylid-
ene V – which can be represented by a zwitterionic
canonical formula and by an electron sextet structure –
Fig. 1.
Fig. 2.
c
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A. Dreger et al. · N-Heterocyclic Carbene Precursors
Table 1. Crystal structure data for the Pd complex 5 · CH-
Cl3 · 1/6 H2O.
and pyrazol-4-ylideneVI. The carbenoid C atom of the
latter is not adjacent to the N atoms, and it cannot be
represented by electron sextet structures. Therefore, it
has been referred to as remote N-heterocyclic carbene
(rNHC) [22 – 24], cyclic bent allene [25], or aromatic
zwitterion [29].
Several pyrazol-3-ylidene complexes have been de-
scribed, the structures of which are summarized in a
recent review on pyrazole chemistry [27]. Palladium
pyrazol-3-ylidene complexes have been obtained by
oxidative addition of 3-chlorotetramethylpyrazolium
chloride and 3-chloro-1,5-dimethyl-2,4-diphenylpyr-
azolium chloride to a palladium(0) complex [30]. Like-
wise, oxidative addition of suited ligand precursors to
Formula
C29H27Cl2N2PPd · CHCl3 · 1/6 H2O
Mr
734.17
Crystal size, mm3
T, K
0.24×0.16×0.08
123(2)
Crystal system
Space group
trigonal
R3 (no. 148)
38.704(3)
10.732(1)
13923(2)
18
1.58
1.1
6654
¯
˚
a = b, A
˚
c, A
3
˚
V, A
Z
Dcalcd, g cm−3
µ(MoKα ), mm−1
F(000), e
hkl range
2θmax, deg.
50, 50, 13
55
[Pd2(dba)3]/PPh3 led to PdII(pyrazol-4-ylidene) com- Refl. measured / unique / Rint 70724 / 7086 / 0.032
Param. refined
356
plexes [24, 31]. The ligand donor strength of pyrazol-
3-ylidene and pyrazol-4-ylidene has been determined
by 13C NMR spectroscopy [32]. In continuation of
our studies of heterocylic mesomeric betaines [33, 34],
zwitterions [35, 36], N-heterocyclic carbenes [37 – 39],
and the chemistry of organic polycations [40], we de-
scribe here the synthesis of a new pyrazol-3-ylidene
palladium complex, as well as new aspects of the syn-
theses and properties of pyrazolium salts and pyrazol-
ium-3- and -4-carboxylates as potential N-heterocyclic
carbene precursors.
R(F) (for I ≥ 2σ(I)) /
wR(F2) (all refl.)a,b
GoF (F2)c
0.051 / 0.141
1.122
−3
˚
∆ρfin (max / min), e A
1.983 (near Pd1) / −1.073
R = ΣꢀFo| − |Fcꢀ/Σ|Fo|; wR = [Σw(Fo − Fc2)2/Σw(Fo2)2]1/2
,
a
b
2
w = [σ2(Fo2)+(AP)2 +BP]−1, where P = (Max(Fo2,0)+2Fc2)/3;
GoF = [Σw(Fo −Fc2)2/(nobs −nparam)]1/2
.
c
2
Results and Discussion
Deprotonation of the pyrazolium salt 1, obtained
by methylation of pyrazole with methyl iodide in
THF according to known procedures [41, 42], gives
the N-heterocyclic carbene pyrazol-3-ylidene 2 in situ
(Scheme 1). This NHC can be detected as the sodium
adduct in electrospray ionization mass spectromet-
ric measurements at m/z = 195.1, [M+Na]+ [11, 12].
When KOtBu is employed as base, and toluene is
used as the solvent to remove the stabilizing water of
crystallization by azeotropic distillation, a sequence of
ring cleavage, 6π-electrocyclization of 3 and subse-
quent tautomerization occurs to result in the forma-
tion of 4-aminoquinoline 4 [10]. As already shown,
this reaction is applicable to a broad variety of pyra-
zolium salts to accomplish syntheses of new substi-
tuted quinolines [11]. When the salt was dissolved in
anhydrous acetonitrile and treated with silver oxide
Scheme 1. Carbene generation from a pyrazolium salt with
an acidic proton in position 3, followed by rearrangement or
complex formation.
plexes of 1,4-diphenyl-2,3-dimethyl-pyrazol-5-ylidene
and its 1,2,3,4-tetramethyl derivative [30]. Spraying
a sample of 5 from methanol gives a base peak at
m/z = 576.7 in the electrospray ionization mass spec-
trum which can be assigned to the complex minus one
chloride.
◦
at 20 C, followed by the addition of bis(triphenyl-
phosphine)palladium(II) chloride, the palladium com-
Single crystals of the palladium complex 5 were
plex 5 was isolated in 51 % yield. The carbene carbon obtained by slow evaporation of a concentrated solu-
atom appears at δ = 165.6 ppm in the 13C NMR spec- tion of the complex in chloroform and analyzed by
trum, slightly more downfield than that of the com- X-ray diffraction. The complex crystallizes as the sol-
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A. Dreger et al. · N-Heterocyclic Carbene Precursors
361
Scheme 2. Decarboxylation of pyrazol-
ium-3-carboxylate 6.
˚
Table 2. Selected bond lengths (A), angles (deg), and dihe-
dral angles (deg) for the palladium complex 5 with estimated
standard deviations in parentheses.
Pd1–Cl1
N1–N2
C2–C3
N1–C7
P1–C14
2.3621(1)
1.3457(1)
1.3913(1)
1.4381(1)
1.8237(1)
Pd1–Cl2
N1–N5
C3–C4
C4–C6
2.3523(1)
1.3726(1)
1.3687(1)
1.4989(1)
C2–N1–Pd1
P1–C20–Pd1
Pd1–C2–Cl1
Pd1–Cl2–Cl1
Pd1–P1–Cl2
123.057(5) C2–C3–Pd1
115.966(4) P1–C26–Pd1
87.469(3) Pd1–C2–Cl2
91.821(3) Pd1–P1–Cl1
87.799(3)
130.562(5)
115.817(4)
175.839(4)
176.794(4)
Cl1–Pd1–C2–C3
−77.4(4)
Cl1–Pd1–C2–N1
Cl1–Pd1–P1–C20
101.7(3)
93.7(7)
Cl1–Pd1–P1–Cl14 −24.6(8)
Cl1–Pd1–P1–C26 −146.2(7)
Cl2–Pd1–P1–C26 −62.9(2)
Cl2–Pd1–P1–C20
P1–Pd1–C2–N1
176.9(2)
−81.5(3)
Cl2–Pd1–P1–C14 58.71(14)
Fig. 4 (color online). Projection of the crystal structure of
5 · CHCl3 · 1/6 H2O as viewed down the z axis showing the
hydrophilic channals running parallel to the z axis. The dis-
ordered water molecules are located in these channals.
planar geometry around the palladium center. The car-
bene ring plane is twisted by approximately 10◦ from
a perpendicular geometry to the PdCCl2P coordination
plane. The Pd–C(carbene) bond length was determined
to be 199.30(1) pm and is comparable with those re-
ported for other standard NHC-Pd complexes [30] as
well as pyrazole-based rNHC-Pd complexes [24, 31].
The Pd–P bond has a length of 224.76(1) pm and is
slightly shorter than in the aforementioned NHC [30]
and rNHC complexes [24, 31].
The pyrazol-3-ylidene 2 can also be generated by
decarboxylation of the pyrazolium-3-carboxylate 6 to
induce aldol additions, Knoevenagel reactions, and re-
dox esterifications (Scheme 2) [9]. In addition, this
compound as well as its derivatives undergoes a sim-
ilar sequence of reactions yielding 4-aminoquinolines
as mentioned before. According to a DFT calculation,
the decarboxylation of 6 to 2 requires an activation en-
Fig. 3. Molecular structure of the palladium complex 5 in the
crystal including the co-crystallized chloroform molecule.
The displacement ellipsoids are drawn at the 50 % probabil-
ity level.
vate 5 · CHCl3 · 1/6 H2O. Results are given in Table 1
and are shown in Figs. 3 and 4; selected bond lengths,
bond angles and torsion angles are presented in Ta-
ergy of 80 kJ mol−1
.
For comparison, we prepared a cross-conjugated
ble 2. The complex crystallizes in the trigonal space analog of 6, the pyrazolium-4-carboxylate 9, which
¯
group R3 with Z = 18 and exhibits a cis configura- possesses the partial structure IV (Fig. 1). We started
tion of the chlorine ligands in an essentially square- by condensation of phenylhydrazine with ethyl 2-acet-
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362
A. Dreger et al. · N-Heterocyclic Carbene Precursors
Scheme 3. Preparation of pyrazolium-4-
carboxylate 9.
yl-3-oxobutanoate 7 to the pyrazole ester 8, which was the π electron system, when electron sextet struc-
first methylated with dimethyl sulfate in the presence tures are involved as shown (Fig. 5). In addition, this
of catalytic amounts of nitrobenzene and then saponi- class of compounds can be identified by characteris-
fied under acidic conditions to yield the target be- tic dipole types, as realized by Ollis, Stanforth and
taine 9 in good overall yield (Scheme 3). The ther- Ramsden [43]. Dipoles of PCCMB can be used in cy-
mal behavior of this pyrazolium-4-carboxylate is dif- cloadditions [44]. By contrast, pyrazolium-4-carbox-
ferent from that of the pyrazolium-3-carboxylate, as ylates belong to the class of cross-conjugated hetero-
heating in mesitylene at reflux temperature yielded the cyclic mesomeric betaines (CCMB). According to the
pyrazole ester 10 as result of an N-demethylation/O- mesomeric structures, the charges are strictly delocal-
methylation sequence. Betaine 9 gives the base peak ized in separated parts of the molecule. Characteristic
of the electrospray ionization mass spectrum at m/z = dipole types can also be dissected from this class of
253.1, [M+Na]+, measured from methanol as solvent compounds.
and spraying the sample at 0 V fragmentor voltage.
Fig. 6 displays the frontier orbital profile of both
At 100 V, the corresponding pyrazolium salt, formed types of mesomeric betaines. Characteristically for
on decarboxylation of the betaine 9 to the carbene 11 both of them, the highest occupied molecular orbitals
followed by protonation of the carbene by the sol- (HOMOs) and the lowest unoccupied molecular or-
vent can be detected as a prominent peak at m/z = bitals (LUMOs) are located in separated parts of the
187.1, [11+H]+. At vigorous measurement conditions common π electron system. As already recognized by
(250 V), the corresponding remote N-heterocyclic car- Potts, in both types of conjugation the anionic part is
bene can be detected at m/z = 209.1 as the sodium bound by a union bond through a nodal position of the
adduct [11+Na]+, and, in the presence of lithium car- HOMO to the cationic part of the molecule [45, 46].
bonate, at m/z = 193.1, [11+Li]+. These results are
The salt which possesses partial structure III
in agreement with calculations which predict a much (Fig. 1) was finally prepared starting from 1,2-diphen-
higher energy barrier (120 kJ mol−1) for the decarbox- ylhydrazine and 4-chloropentenone in chlorobenzene,
ylation of 9 to 11 in comparison to the decarboxylation followed by precipitation as perchlorate (Scheme 4).
of 6 to 2.
Attempts to convert this salt into the ester by de-
The betaines 6 and 9 belong to two distinct types of protonation with nBuLi, followed by the addition of
heterocyclic mesomeric betaines. Pyrazolium-3-carb- (MeO)2CO failed, presumably due to the low acidity
oxylate possesses partial structure II (Fig. 1) and is a of 4-H of the pyrazolium salt. It is also known from
member of the class of pseudo-cross-conjugated het- the literature, that the in situ deprotonation of 1,2,3,5-
erocyclic mesomeric betaines (PCCMB). This class tetrasubstituted pyrazolium salts with basic metal pre-
of compounds can be recognized by a closer inspec- cursors such as Pd(OAc)2 or Ag2O was unsuccess-
tion of the mesomeric structures. Positive as well as ful [28, 35]. In agreement with these observations, the
negative charges are distributed in common areas of remote N-heterocyclic carbene 14 could not be de-
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Fig. 5. Characteristic features
of PCCMB and CCMB.
Fig. 6 (color online). From
left to right: HOMO (6),
LUMO (6), HOMO (9),
and LUMO (9).
Scheme 4. Preparation of
a pyrazolium salt with an
acidic proton in position 4.
tected in electrospray ionization mass spectrometry, heterocyclic mesomeric betaine (CCMB) pyrazolium-
even under more vigorous reactions conditions as ap- 4-carboxylate 9 to the remote N-heterocyclic carbene
plied before, such as 250 V fragmentor voltage and the (rNHC) pyrazol-4-ylidene requires considerably more
presence of lithium carbonate in the solution.
energy in comparison to 6.
According to a DFT calculation, the deprotonation
of the pyrazolium salt 1 to the N-heterocyclic car-
bene 2 by methoxide under formation of methanol is
by 86.8 kJ mol−1 more favorable than the analogous
deprotonation of 14 to the remote N-heterocyclic car-
bene 15.
In summary, the pseudo-cross-conjugated hetero-
cyclic mesomeric betaine (PCCMB) pyrazolium-3-
carboxylate 6 is a suitable precursor for N-heterocyclic
carbenes (NHC), and so are pyrazolium salts such as 1
which possess a proton in position 3. Pyrazolium salt 1
was used to prepare a pyrazol-3-ylidene palladium
Experimental Section
Flash chromatography was performed with silica gel 60
(0.040 – 0.063 mm). Nuclear magnetic resonance (NMR)
spectra were obtained with a Bruker Avance III (600 MHz)
and a Bruker Avance (400 MHz) spectrometer. Spectra were
recorded with the solvent peak or tetramethylsilane as the
internal reference. Chemical shifts are given in ppm. Multi-
plicities are described by using the following abbreviations:
s = singlet, d = doublet, t = triplet, q = quartet, sept = septet,
and m = multiplet. FT-IR spectra were obtained on a Bruker
Vector 22 instrument in the range of 400 to 4000 cm−1. All
complex. The decarboxylation of the cross-conjugated substances were measured as pellets (2.5 %) in KBr. The
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364
A. Dreger et al. · N-Heterocyclic Carbene Precursors
mass spectra were measured with a Varian 320 MS Triple (1.5 mmol) of dimethyl sulfate. The mixture was heated to
Quad GC/MS/MS coupled with a Varian 450-GC. The ESI reflux temperature for 2 h. After cooling, the solvent was dis-
mass spectra were measured with an Agilent LCMSD Series tilled off in vacuo, and the residue was treated with 15 mL
HP1100 with APIES. Samples were sprayed from methanol. of 18 % hydrochloric acid. Then, the mixture was heated
Melting points are uncorrected and were determined in an for 7 h at reflux temperature. After neutralization with aque-
apparatus according to Dr. Tottoli (Bu¨chi). All yields are iso- ous sodium hydroxide, the water was distilled off, and the
lated yields. They are not optimized.
residue was extracted six times with portions of 10 mL of
ethanol. After distilling off the solvent, the resulting residue
was subjected to chromatography (MeOH). The mesomeric
betaine 9 was isolated as a colorless solid in 82 % yield,
dec. 240 ◦C. – 1H NMR (400 MHz, CD3OD): δ = 7.74 –
7.83 (m, 3 H, Ar-H), 7.61 – 7.64 (m, 2 H, Ar-H), 3.66 (s, 3 H,
2-CH3), 2.77 (s, 3 H, 5-CH3 oder 3-CH3), 2.44 (s, 3 H, 5-
CH3 oder 3-CH3). – 13C NMR (100 MHz, 21 ◦C, CD3OD):
δ = 167.8, 149.8, 149.4, 133.9, 132.7, 132.0, 130.1, 120.4,
34.7, 12.4, 11.7. – IR (KBr): ν = 3441, 1601, 1381, 1322,
1192, 809, 782, 705 cm−1. – MS ((+)-ESI): m/z = 231.1
(100) [M+H]+. – HRMS ((+)-ESI): m/z = 231.1134 (calcd.
231.1134 for C13H15N2O2, [M+H]+).
Complex of bis(triphenylposphine)palladium(II) chloride
and 2,5-dimethyl-1-phenylpyrazol-3-ylidene (5)
A sample of 0.447 g (1.49 mmol) of 2,5-dimethyl-1-phen-
ylpyrazolium iodide (1) was dissolved in 20 mL of anhydrous
MeCN. Then, 0.345 g (1.49 mmol) of silver(I) oxide was
added. The mixture was stirred for 2 h at r. t. and then fil-
tered through a plug of celite. The filtrate was then treated
with 0.526 g (0.75 mmol) of bis(triphenylposphine)pallad-
ium(II) chloride and stirred for additional 3 h. After filtration
through celite, the solvent was distilled off in vacuo, and the
residue was subjected to chromatography (acetone-CH2Cl2 =
1 : 2). Complex 5 was isolated as a pale-yellow solid (51 %),
m. p. 180 – 183 ◦C. – 1H NMR (400 MHz, CDCl3): δ =
7.72 – 7.77 (m, 6 H, PPh3-H), 7.53 – 7.61 (m, 3 H, Ph-H),
7.45 – 7.49 (m, 3 H, PPh3-H), 7.36 – 7.40 (m, 6 H, PPh3-H),
7.18 (d, 1 H, Ph-H, J = 6.8 Hz), 6.73 (d, 1 H, Ph-H, J =
6.8 Hz), 5.97 (s, 1 H, 4-H), 3.69 (s, 3 H, 2-CH3), 1.79 (s,
3 H, 5-CH3). – 13C NMR (100 MHz, CDCl3): δ = 165.6,
Methyl 3,5-dimethyl-1-phenylpyrazol-4-carboxylate (10)
A sample of 58 mg (0.25 mmol) of 2,3,5-trimethyl-1-
phenylpyrazolium-4-carboxylate 9 was suspended in 2 mL of
mesitylene and heated under reflux over a period of 2 h. After
cooling, the solvent was distilled off in vacuo, and the residue
was subjected to chromatography (petroleum ether-EtOAc =
5 : 1). The ester was isolated in 39 % yield. – 1H NMR
(400 MHz, CDCl3): δ = 7.38 – 7.50 (m, 5 H, Ph-H), 3.86
(s, 3 H, O-CH3), 2.52 (s, 3 H, 5-CH3 or 3-CH3), 2.50 (s, 3 H,
5-CH3 or 3-CH3). – All spectroscopic data are in agreement
with those reported in the literature [47].
144.1, 134.6 (d, JCP = 11.1 Hz), 132.4, 131.8, 130.9 (d, JCP
=
2.4 Hz), 130.8, 130.6 (d, JCP = 53.3 Hz), 130.4, 128.4, 128.3
(d, JCP = 11.0 Hz), 127.7, 114.2, 39.3, 12.0. – IR (KBr): ν =
3464, 3052, 1589, 1529, 1497, 1435, 1349, 1223, 1096, 788,
748, 695, 535, 513 cm−1. – MS ((+)-ESI): m/z (%) = 576.7
(100) [M–Cl]+.
3,5-Dimethyl-1,2-diphenylpyrazolium perchlorate (14)
Ethyl 3,5-dimethyl-1-phenyl-1H-pyrazole-4-carboxylate (8)
A solution of 20.1 g (0.2 mol) of acetylacetone in
200 mL of anhydrous diethyl ether was treated with 41.7 g
(0.2 mol) of PCl5. The reaction mixture was stirred at r. t.
for 1 h and then poured into 300 mL of water and ice. The
organic phase was separated and then treated with a slurry
of 9.0 g (0.09 mol) of calcium carbonate in 100 mL of wa-
ter. After stirring at r. t. over a period of 14 h, the organic
layer was concentrated and finally distilled in vacuo [48].
The resulting 4-chloropent-3-en-2-one (13) was isolated in
81 % yield. A sample of 0.652 g (5.5 mmol) of 13 in
50 mL of chlorobenzene was then treated with 0.921 g
(5.0 mmol) of hydrazobenzene. After heating to reflux tem-
perature for 5 h the solution was concentrated in vacuo to
approximately 80 % of its original volume. After cooling
to r. t., 100 mL of water was added, and the resulting mix-
ture was extracted three times with 50 mL of diethyl ether.
A sample of 1.0 g (5.81 mmol) of ethyl diacetoacetate 7
in 10 mL of ethanol was treated with 0.628 g (5.81 mmol)
of phenylhydrazine and 1 mL of concentrated hydrochloric
acid. The mixture was heated at reflux for 2 h. After cooling,
the solution was neutralized with aqueous NaOH and evap-
orated to dryness in vacuo. The residue was then subjected
to chromatography (petroleum ether-EtOAC = 5 : 1). The es-
ter 8 was obtained in 41 % yield. – 1H NMR (400 MHz,
CDCl3): δ = 7.38 – 7.50 (m, 5 H, Ar-H), 4.33 (q, 2 H,
CH3CH2O, J = 7.1 Hz), 2.52 (s, 3 H, CH3), 2.50 (s, 3 H,
CH3), 1.38 (t, 3 H, CH3CH2O, J = 7.1 Hz). – All spectro-
scopic data are in agreement with those reported in the liter-
ature [47].
2,3,5-Trimethyl-1-phenylpyrazolium-4-carboxylate (9)
A sample of 0.366 g (1.5 mmol) of ethyl 3,5-dimethyl- Finally, a saturated solution of LiClO4 was added to pre-
1-phenylpyrazol-4-carboxylate (8) in 10 mL of xylene con- cipitate the compound as the perchlorate, which was fil-
taining one drop of nitrobenzene was treated with 0.14 mL tered off and dried on air. The salt 14 was obtained as a
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A. Dreger et al. · N-Heterocyclic Carbene Precursors
365
pale-brownish solid in 56 % yield, m. p. 182 – 183 ◦C. – B3LYP density functional. All calculated structures were
1H NMR (400 MHz, [D6]DMSO): δ = 7.58 – 7.60 (m, 4 H, proven to be true minima by the absence of imaginary fre-
Ph-H), 7.46 – 7.53 (m, 6 H, Ph-H), 7.06 (s, 1 H, 4-H), quencies. Plots were obtained using MAESTRO 9.1.207, the
2.32 (s, 6 H, 3-CH3 und 5-CH3). – 13C NMR (100 MHz, graphical interface of JAGUAR.
[D6]DMSO): δ = 148.2, 131.8, 131.5, 129.8, 129.1, 108.1,
12.0. – IR (KBr): ν = 1556, 1497, 1415, 1088, 778, 695,
623 cm−1. – MS ((+)-ESI): m/z (%) = 249.1 (100). – HRMS
((+)-ESI): m/z = 249.1390 (calcd. 249.1392 for C17H17N2,
[M+H]+).
X-Ray structure determination
Data of 5 were collected on a Nonius Kappa-CCD diffrac-
tometer using graphite-monochromatized MoKα radiation
◦
˚
(λ = 0.71073 A) at T = −150 C, and the structure was
solved by Patterson methods and refined by full-matrix least-
squares on F2 [49]. A semi-empirical absorption correction
was applied. All non-hydrogen atoms were refined anisotrop-
ically, and hydrogen atoms were located from difference
Fourier maps and refined at idealized positions using a rid-
ing model. One water molecule is disordered about the 3-fold
axis (in 0, 0, z).
Calculations
All density-functional theory (DFT) calculations were
carried out by using the JAGUAR 7.7.107 software running
on Linux 2.6.18-238.el5 SMP (x86 64) on two AMD Phe-
nom II X6 1090T processor workstations (Beowulf-cluster)
parallelized with OpenMPI 1.3.4. MM2-optimized structures
were used as starting geometries. Complete geometry op-
timizations were carried out on the implemented LACVP*
basis set (Hay-Wadt effective core potential (ECP) basis on
heavy atoms, N31G6* for all other atoms) and with the
CCDC 855158 contains the supplementary crystallo-
graphic data for this paper. This data can be obtained free
of charge from The Cambridge Crystallographic Data Centre
(CCDC) via www.ccdc.cam.ac.uk/data request/cif.
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