Determining the Ligand Properties of N-Heterocyclic Carbenes from 77Se NMR Parameters

Article abstract of DOI:10.1002/ejic.201500174

The ¹J_CSe coupling constants for a range of NHC-selenium adducts have been measured and used to establish a correlation with the σ-donor strength of the respective carbenes. For the subclass of amido-carbenes, the ¹J_C

Full text of DOI:10.1002/ejic.201500174

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DOI:10.1002/ejic.201500174
Determining the Ligand Properties of N-Heterocyclic
77
Carbenes from Se NMR Parameters
Kathrin Verlinden,[a] Hannes Buhl,[a] Walter Frank,[a] and
Christian Ganter*[a]
Keywords: Nitrogen heterocycles / Carbenes / Ligand properties / Rhodium / Selenium / NMR spectroscopy
1
The JCSe coupling constants for a range of NHC–selenium were more readily obtained and show the same trend as the
adducts have been measured and used to establish a corre-
lation with the σ-donor strength of the respective carbenes.
For the subclass of amido-carbenes, the 1JCSe values revealed
a high donor capacity, very much in contrast to what the
DFT-calculated HOMO energies suggest. The JCH coupling
constants for the C-2 atoms in azolium-type NHC precursors
1JCSe coupling constants. In addition, the use of 77Se chemi-
cal shifts to determine π-acidity has been extended to a
broader range of derivatives, namely 1·Se–22·Se. The supe-
rior resolution of the δ(77Se) method in comparison with the
Tolman electronic parameters derived from IR spectroscopy
is demonstrated for the caffeine-derived bis-carbene 19.
1
However, as the TEP reflects the overall ligand properties,
it does not provide an insight into the relative importance
of the σ-donor and π-acceptor properties of a particular
NHC. In contrast, NMR methods have recently been re-
ported that allow the determination of only the π-acceptor
strength of NHCs. While Bertrand[7] and Hudnall[8] and
their co-workers measured the 31P NMR chemical shifts of
NHC–phosphinidene adducts, we utilized the 77Se NMR
chemical shifts of easily available NHC–Se adducts to de-
termine the π-acceptor strengths of the NHCs within the
adducts.[9] This method has lately been applied to a broader
range of NHCs by Nolan and co-workers.[10] Both the
phosphinidene and selenium adducts can be represented by
two limiting canonical structures, a neutral hetero-olefin (B)
and a polarized structure (A) with a C–P/Se single bond
(for representations of these structures for the NHC–Se ad-
ducts, see Scheme 1). The 31P and 77Se NMR chemical
shifts are very sensitive to the relative position of a given
adduct within the two limiting structures: π-acidic NHCs
feature a higher degree of C–P/Se double-bond character
leading to NMR signals shifted to lower field in compari-
son with less π-acidic NHCs.[11]
Introduction
Since Arduengo et al.[1] discovered the first isolable N-
heterocyclic carbene more than 20 years ago, a wide range
of sterically and electronically diverse carbenes have been
reported.[2] Owing to their strong σ-donor character, NHCs
are suitable as ligands for transition-metal complexes and
are increasingly employed in homogeneous catalysis, rival-
ling the ubiquitous phosphorus ligands in organometallic
chemistry.[3] The potential π-back-bonding exerted by
NHCs has been the subject for debate for a long time.
Meanwhile, theoretical and experimental studies have pro-
vided clear evidence that the acceptor character of NHCs
can significantly contribute to the metal–NHC bonding in-
teraction, and a couple of strongly π-acidic NHCs have
been prepared.[4]
To select a carbene for a specific application, detailed
knowledge of its properties is of crucial importance. In par-
ticular, a couple of methods have been developed over the
past years to assess the electronic properties of ligands.[5]
For example, the Tolman electronic parameter (TEP) relies
on the measurement of the CO stretching frequencies in
complexes of the type [(NHC)Ni(CO)3] or [cis-(CO)2-
(NHC)MCl] (M = Rh or Ir);[6] thus, the electron density on
the metal as a result of donor and acceptor contributions
of the ligand is measured. Originally developed for phos-
phorus ligands, the TEP has become the most frequently
used parameter to assess the ligand properties of NHCs.
Scheme 1. Canonical structures of selenium–NHC adducts.
[a] Institut für Anorganische Chemie und Strukturchemie,
Heinrich-Heine-Universität,
It is well known that the coupling constants between di-
rectly bonded atoms arise mainly from the Fermi contact
between nuclear magnetic moments and the electron spins
40225 Düsseldorf, Germany
E-mail: christian.ganter@hhu.de
http://www.metallorganik.hhu.de/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201500174.
in s orbitals.[12] Several groups have shown that the JPSe
1
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coupling constants of phosphane–selenides are correlated
to the basicity of the corresponding phosphanes, increasing
basicity being associated with decreasing coupling con-
Results and Discussion
While the 77Se chemical shifts of easily available NHC–
stants.[13,14] According to Bent’s rule,[15] the coupling con- Se adducts have already been used to assess π-acidity, we
stant increases as the s character of the phosphorus orbital wondered whether the 1JCSe coupling constants of these de-
involved in bonding to the selenium atom increases as a rivatives might provide a quantitative measure of the σ-do-
consequence of more electron-withdrawing substituents on nor strength of the respective NHCs. Therefore we at-
1
phosphorus. This effect is also observed for other NMR- tempted to obtain the JCSe coupling constants from the
active nuclei such as 11B or 195Pt.[13,16]
selenium satellites in the 13C{1H} NMR spectra, however,
Herein we report our attempts to determine the σ-donor this proved to be difficult for two reasons. On one hand,
1
character of NHCs by measuring the JCSe coupling con- the relative abundances of 13C and 77Se are 1.1 and 7.5%,
stants of their corresponding selenium adducts. In addition, respectively, which means that there is a probability of
to assess the π-acceptor properties, 77Se chemical shift data 0.08% that a 13C atom is adjacent to a 77Se atom. On the
for a range of new NHC–Se adducts are presented and other hand, the signal of the carbene carbon has a very low
compared with the corresponding 31P NMR spectroscopic intensity as a consequence of a long relaxation time T1,
data of related PhP–NHC adducts. Also, the resolution li- which demands a long relaxation delay between pulses. This
mit of the δ(77Se) method is compared with that of the TEP obstacle could be partly alleviated by the use of [Cr(acac)3]
method.
as a relaxation enhancing reagent and by increasing the
Table 1. NMR spectroscopic data and TEP values for the compounds considered in this study.
1
[a] 77Se NMR spectra recorded in [D6]acetone, JCSe coupling constants determined in CDCl3. [b] 13C NMR spectra recorded in [D6]
DMSO. [c] Calculated by DFT methods. [d] Obtained in CDCl3. [e] Calculated as described below. [f] Dipp (2,6-diisopropylphenyl)
instead of Mes (mesityl). [g] Mes instead of Dipp.
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number of scans. Thus, the 13C{1H} NMR spectra of nearly relation of R2 = 0.935 for seven pairs of compounds for
saturated solutions of the selenium adducts in a 0.1 mol/L which both sets of data are available.
solution of [Cr(acac)3] in CDCl3 were recorded with around
With potential scales for both the σ-donor as well as the
70000 scans (2.5 d) on a 200 MHz (1H) NMR spectrometer. π-acceptor strengths of carbenes in hand, it was tempting
The use of a 600 MHz spectrometer allowed data acqui- to combine these measures to arrive at a calculated TEP
sition within 24 h, as demonstrated for one example.
value that gives an indication of the overall ligand proper-
The NHC–selenium adducts 3·Se, 4·Se, 6·Se, 7·Se, 11·Se, ties. Thus, we proposed Equation (1).
14·Se, 16·Se, 21·Se, and 22·Se were synthesized and their
TEP = aδ(77Se) + b1JCSe + c
(1)
1JCSe coupling constants determined (Table 1) by the pro-
1
cedure outlined above. The JCSe values cover a range of
Optimization of the weighting factors a, b, and c against
the experimental values of the NHCs 3, 4, 7, 11, 14, 16,
and 22 revealed R2 = 0.833 for the correlation between the
calculated and experimental TEP values. Apart from the
amido carbenes 14, 21, and 22, all the NHCs examined have
TEP values within a range of 5 cm–1 and a maximum differ-
ence of 4 cm–1 was observed between the calculated and
experimental TEP values. Thus, although the correlation is
not high enough to allow for a quantitative rationalization
of the TEP values on the basis of the NMR spectroscopic
data, the 77Se NMR chemical shifts and 1JCSe coupling con-
stants are suitable for giving qualitative insights into the
σ-donor and π-acceptor abilities of NHCs. However, with
25 Hz with the largest coupling constant of 239 Hz being
observed for the selenium adduct 7·Se, which suggests that
the NHC 7 should be the weakest σ-donor. This is in accord
with the strong –I effect exerted by the electronegative
chlorine atoms at the 4- and 5-positions. According to the
δ(77Se) value of 174 ppm for 7·Se, NHC 7 is only weakly π-
acidic. Thus, the TEP value of 2055 cm–1 for 7, indicative
of an overall quite poor donor ligand, is mainly due to its
poor σ-basicity.
According to its 1JCSe value of 214 Hz, the saturated six-
membered-ring NHC 11 is the best σ-donor, whereas the
saturated five-membered-ring carbene SIMes 6 is also a
good σ-donor. However, some of the findings were quite
surprising, in particular, the trend observed for the amido
carbenes does not follow chemical intuition. According to
the coupling constants, the donor strength seems to in-
crease significantly by the successive introduction of carb-
onyl groups, whereas the opposite trend would be expected
on the basis of the –I effect exerted by the electronegative
oxygen atoms. The coupling constants suggest that the six-
membered-ring diamido carbene 21 is almost as good a σ
donor as the best donor of the series, NHC 11. The high
TEP values of the amido carbenes 14, 21, and 22, which
indicate that these ligands are quite poor overall donors,
are thus a result of their high π-acceptor character, which
is evident from the 77Se NMR chemical shifts. This is in
particular the case for the oxalamide NHC 22, which has
by far the highest TEP value of all the carbenes included
1
regard to the donor capabilities, we will focus on the JCH
coupling constants in future work because they are much
more conveniently obtained from the proton-coupled 13C
NMR spectra of the carbene precursors.
The selenium adducts 1·Se–3·Se, 7·Se, 8·Se, 10·Se–12·Se,
14·Se–17·Se, 19·Se2, and 21·Se were additionally charac-
terized by 77Se NMR spectroscopy to probe the π-acceptor
character of the NHCs within the adduct, thus extending
the range of compounds reported in our previous study.[9]
The NMR data of all the compounds are compiled in
Table 1. Thirteen analogous NHC–phosphinidene adducts
have previously been characterized by 31P NMR spec-
troscopy by Bertrand and Hudnall and their co-workers.[7,8]
Plotting the 31P versus 77Se chemical shifts of the related
NHC adducts (Figure 1) confirmed the previously observed
linear correlation of both scales with a correlation coeffi-
cient of R2 = 0.905, which is only slightly worse than the
correlation obtained before for five pairs of related adducts
1
in the present study. Because the JCSe coupling constants
decrease for the series of selenium adducts 3·SeϾ 14·SeϾ
22·Se, the σ-donor and π-acceptor strengths would seem to
increase with the introduction of carbonyl groups into the
backbone.
Surprisingly, carbenes 3 and 4 are characterized as being
worse σ-donors than the amido carbenes. However, they are
also the poorest π-acceptors in the series, which collectively
leads to TEP values at the lower end of the range. With a
coupling constant of 233 Hz, the thiazolylidene 16 is a little
less σ-donating than IMes 3. The higher TEP value of the
former (2054 vs. 2050 cm–1 for 3) can be explained by the
increased π-acceptor character of NHC 16, as reflected by
the 77Se NMR shifts of 396 (16·Se) and 35 ppm (3·Se).
At a later stage of our investigations, we noticed that the
1JCH coupling constants for the C-2 atom in the cationic
1
carbene precursors show the same trend as the JCSe cou-
pling constants of the selenoureas (Table 1), covering the
range of 200–229 Hz. Indeed, plotting the JCH versus JCSe
Figure 1. Plot of 31P NMR chemical shifts against observed 77Se
coupling constants of related compounds resulted in a cor- NMR chemical shifts.
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(R2 = 0.955). The linear Equation (2) of the correlation en-
ables the determination of the selenium and phosphorus
chemical shifts from each other.
δ(31P) = 0.1563δ(77Se) – 37.1 ppm
(2)
The selenium adducts 1·Se–22·Se show the expected
trends in π acidity and are in accord with the observations
made by Bertrand and co-workers.[7] Thus, unsaturated five-
membered-ring diamino carbenes 1·Se–5·Se are the poorest
π-acceptors with the highest-field resonances (i.e.,
Ͻ100 ppm). A couple of chemical modifications that lead
to increased π-acceptor strength can be deduced from the
data in Table 1: Saturation of the backbone (8, 9), annu-
lation to aromatic rings (4, 10, 15), increase of the NCN
angle (11, 13, 15, 20, 21), replacement of nitrogen by the
weaker σ-donor sulfur (16), and introduction of electron-
withdrawing groups into the backbone (7, 14, 17–19, 21,
22).
Scheme 2. Synthesis of 10·Se–Me and 12·Se–Me and estimation of
the 77Se NMR chemical shift of 12·Se.
Evidently, the introduction of carbonyl groups adjacent
to the nitrogen atoms has the most pronounced effect, the
amide resonance involving the nitrogen lone pair at the ex-
Thus, the chemical shift for the cationic selenium adduct
pense of decreased stabilization of the carbene center, which 12·Se was estimated by this procedure to be 245 ppm, which
becomes a better acceptor. Thus, the monoamido (14, 17– is shifted to lower field by around 60 ppm compared with
19) and especially the diamido carbenes (21, 22) are the the neutral relative 10·Se, thus indicating the superior π-
strongest π-acceptors in the series, resonating in the low- acidity of the cationic species.
field region at up to 860 ppm.
As is clear from the data compiled in Table 1, the 77Se
Another interesting aspect was the investigation of the NMR chemical shifts of the carbenes discussed in this study
influence of cationic charge on the π-acceptor character. cover a wide range of around 850 ppm, whereas their TEP
Recently we reported the preparation of cationic NHCs by values lie within a much narrower range of roughly 24 cm–1
the η6 coordination of a [(C5Me5)Ru]+ fragment to the aro- (or even only 19 cm–1 if compound 22 is disregarded). Thus,
matic rings of carbenes 4 and 15.[4o,17e] Unfortunately, the the selenium NMR data clearly provide a much better reso-
synthesis of the corresponding selenium adducts of the cat- lution, which should allow the identification of two dif-
ionic carbenes was not successful. Moreover, the coordina- ferent carbenic centers within one molecule. This was
tion of the cationic fragment to the selenium adducts 4·Se demonstrated for the caffeine-derived bis-carbene 19. To
and 15·Se did not yield the desired compounds either.
obtain its corresponding carbonyl complex, the dication
As an alternative, alkylation of the pyridine nitrogen 19·[HOTf]2 was deprotonated with NaHMDS and then al-
atom of the 1,3-dineopentyl-4-azabenzimidazolylidene 10 lowed to react with [Rh(cod)Cl]2 (Scheme 3). The molecular
was carried out to give the cationic carbene 12,[4q] but the structure of the resulting complex 19·[Rh(cod)Cl]2 (23) was
synthesis of its selenium adduct 12·Se was again not suc- confirmed by X-ray diffraction (Figure 2).
cessful. Therefore methylation of compound 10·Se with Me-
However, although the positions of the two Rh(cod)Cl
OTf was attempted to assess the π-acceptor character of fragments are well defined, the bridging bis-carbene ligand
carbene 12. However, the selenium atom appeared to be appeared to be disordered by a superposition of two orien-
more nucleophilic than the pyridine nitrogen atom in the tations, thus preventing the diamino and the amino-amido
backbone and the Me–Se compound 10·Se–Me was ob- carbene moieties from being distinguished.[18] Complex 23
tained that displayed a 77Se NMR signal at δ = 158 ppm, was subsequently converted into the bis(dicarbonyl) com-
thus being shifted to a higher field by 25 ppm than that of plex 19·[Rh(CO)2Cl]2 (24) by exposure of a dichlorometh-
10·Se (Scheme 2).
ane solution of 23 to gaseous CO (Scheme 3). Although the
On the other hand, exposure of compound 10·Se to NMR spectra indicate a clean conversion to complex 24,
methyl trifluoromethanesulfonate under more forcing con- the IR spectrum shows only two strong CO stretching vi-
ditions (microwave irradiation) resulted in the double meth- brations at 2015 and 2091 cm–1 from which a TEP value of
ylation of the selenium and pyridine nitrogen atom leading 2063 cm–1 was calculated (Figure 3). Thus, IR spectroscopy
to the dicationic derivative 12·Se–Me, which has a 77Se does not allow differentiation between the electronic prop-
chemical shift of 220 ppm. To derive a rough estimate of erties of the two carbene centers within the molecule. One
the chemical shift of the monocationic derivative 12·Se, possibility for solving this problem could be the synthesis
25 ppm were added to the value for 12·Se–Me, assuming of the mixed iridium/rhodium bis(dicarbonyl) complex
that the effect of the additional methyl group bonded to [Ir(CO)2Cl]19[Rh(CO)2Cl] as the CO stretching vibrations
selenium was the same for both 10·Se–Me and 12·Se–Me for the Rh and Ir dicarbonyl complexes for a given NHC
(Scheme 2).
usually differ by around 15 cm–1.[2d] This difference should
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Scheme 3. Synthesis of compounds 23 and 24. Reagents and conditions: (a) NaHMDS, [Rh(cod)Cl]2, thf, 5 h, –80 °C to room temp.;
(b) CO, CH2Cl2, 45 min, room temp.
other hand, the carbene center of the five-membered ring is
in the range of other imidazolylidenes with little π-acceptor
character. Collectively, 77Se NMR spectroscopy provides a
much higher resolution than the TEP scale based on IR
spectroscopy.
Figure 2. Molecular structure of complex 23. The ellipsoids are
drawn at the 25% probability level. Hydrogen atoms and solvent
molecules have been omitted for clarity. Only one of the two disor-
dered positions of the bridging ligand is shown.
be sufficient to resolve the two carbene moieties. However,
the synthesis of the mixed complexes was not as trivial as
had been initially expected and all the attempts to synthe-
Figure 4. 77Se NMR spectrum of 19·(Se)2 in CDCl3 with KSeCN
in D2O as external standard.
size this complex carried out in our laboratory have so far
been unsuccessful.
With the aim of rationalizing the observed trends of the
77Se NMR chemical shifts and the 1JCSe coupling constants,
the HOMO and LUMO energies of a representative range
of carbenes were computed by DFT methods (B3LYP/def2-
TZVP). All the nitrogen substituents were replaced by
methyl groups to speed up the calculations. The results are
depicted in Figures 5 and 6. The order of δ(77Se) values is
qualitatively well reproduced by the calculated LUMO en-
ergies (R2 = 0.8969), which thus warrants a correlation be-
tween chemical shifts and π-acceptor character with the
amido carbenes being the strongest acceptors.[19] On the
other hand, the HOMO energies should reflect the σ-donor
properties of the carbenes. Indeed, with the exception of the
amido carbenes, the calculated HOMO energies parallel the
experimentally derived coupling constants (R2 = 0.9101).
Although according to their HOMO energies, the amido
Figure 3. IR spectrum of complex 24 in CH2Cl2.
derivatives 14, 21, and 22 are characterized as being poor
Fortunately, the characters of the two carbene centers in σ-donors, the coupling constants suggest a much higher do-
the bis-adduct 19·(Se)2 were easily discerned in the 77Se nor character. At this stage we do not have a convincing
NMR spectrum. Two well-separated signals were detected explanation for this contradictive finding. However, it
in CDCl3 solution at δ = 516 and 113 ppm (Figure 4). Ow- should be noted that the HOMO energies span a range of
ing to the electron-withdrawing amido group and the larger only 1.6 eV, whereas the LUMO energies cover a much
NCN angle, the carbene center of the six-membered ring broader range of 4.2 eV. Thus, structural modifications of
can be assigned to the lower-field signal. This high value the basic NHC motif affect the acceptor character to a
indicates significantly increased π-back-bonding. On the much higher extent than the donor properties.
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ranges of 135–137 pm for the N–C(carbene) bonds and
182–184 pm for the C–Se bonds.[10a,20–22] It should be noted
that the Se adducts of strongly π-acidic diamido NHCs fea-
ture significantly shorter C–Se bonds of around
178 pm.[4n,23] The most significant deviations from the ide-
alized five-membered-ring geometry are observed for the
thiazolylidene derivative 16·Se; as is usually observed, the S
atom prefers a small intra-ring angle of 93.18(7)°, which
leads to an increase in the SCN angle to 108.5(1)°, whereas
slightly smaller NCN angles in the range of 104–107° are
observed for the imidazole-derived compounds 2·Se and
10·Se. Compared with the neutral 10·Se, the doubly methyl-
ated dicationic derivative 12·Se–Me features a significantly
elongated C(carbene)–Se bond of 190.31(15) pm, thus ap-
proaching the value for a Se–C single bond,[24] and the Se–
Me bond has a length of 195.9(2) pm. These values are
close to those observed for a neutral 2-(methylselenyl)imid-
azole.[25] Selenoureas are known to form adducts with iod-
ine or to react to form Se–Se-bonded dicationic dimers sta-
bilized by interaction with triiodide anions.[26] Likewise, a
closely related compound, 2-(cyanoselenyl)imidazolium
iodide, forms dimeric aggregates in which two iodide anions
form bridges between the selenium atoms of two cations.[27]
A related aggregation is observed in the structure of 12·Se–
Me, with the oxygen atoms of two triflate anions in bridging
positions between the Se atoms of two dicationic moieties
Figure 5. Plot of calculated LUMO energies vs. observed 77Se
NMR chemical shifts. [a] LUMO+1 due to the required π sym-
metry. [b] LUMO+2 due to the required π symmetry.
Figure 6. Plot of calculated HOMO energies vs. observed 1JCSe cou-
pling constants.
Crystals of 2·Se, 9·Se, 10·Se, 16·Se, and 12·Se–Me suit-
able for X-ray diffraction analysis were obtained by solvent
diffusion methods. The molecular structures of the com-
pounds are depicted in Figures 7 and 8. The five-membered
rings in all the derivatives are entirely flat within the limit
of accuracy. For the selenoureas 2·Se, 9·Se, 10·Se, and 16·Se,
Figure 8. Left: Molecular structure of 12·Se–Me. Right: Dimeric
aggregate of two molecules of 12·Se–Me and two triflates. The el-
lipsoids are drawn at the 50% probability level. Hydrogen atoms
interatomic distances are observed within the usual narrow and OTf– counter ions have been omitted for clarity.
Figure 7. Molecular structures of 2·Se, 9·Se, 10·Se, and 16·Se. The ellipsoids are drawn at the 25 (9·Se) and 35% (2·Se, 10·Se, 16·Se)
probability levels. Hydrogen atoms have beeen omitted for clarity.
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General Preparation of Carbene Selenium Adducts: The desired
NHC precursor was suspended in thf with an excess selenium pow-
der. Sodium bis(trimethylsilyl)amide (NaHMDS, 2 m in thf,
1.2 equiv.) was diluted with thf (5 mL) and added dropwise to the
suspension at –78 °C. After stirring for 30 min at –78 °C, the mix-
ture was allowed to warm up to room temperature, while stirring
was continued overnight. The solvent was removed under high vac-
uum and the crude product was dissolved in dichloromethane. Af-
ter slow filtration through Celite the solution was either dried in
vacuo to obtain the product as a powdery solid (method a) or con-
centrated under high vacuum and n-hexane was added, which
(Figure 8). In the centrosymmetric four-membered rings,
each Se atom has one short [290(1) pm] and one long con-
tact [483(1) pm] to the triflate oxygens. The arrangement is
almost rectangular with angles slightly larger than 90° at
the oxygen atoms (95°) and slightly smaller at the selenium
atoms (85°).
Conclusions
NHC–selenium adducts have been characterized by 77Se caused precipitation of a powdery solid, which was isolated by fil-
1
tration, washed with n-hexane, and dried under high vacuum for
several hours (method b) or the oily residue obtained by evapora-
tion of the filtrate was dissolved in diethyl ether and evaporated
under high vacuum to obtain the product as a powdery solid
(method c).
NMR chemical shifts and, for the first time, by JCSe cou-
pling constants. The NMR parameters as well as DFT-de-
rived HOMO and LUMO energies were correlated to the
σ-donor and π-acceptor properties of the carbenes, respec-
tively. It has been found that the NMR parameters offer a
much better resolution to distinguish carbene centers than
the widely used TEP parameters, which rely on IR measure-
1,3-Diisopropylimidazole-2-selenone (1·Se): Reagents: 1,3-diiso-
propylimidazolium bromide (0.50 g, 2.14 mmol), NaHMDS
(1.18 mL, 2.36 mmol), selenium (0.51 g, 6.5 mmol). Purification
1
ments. The JCH coupling constants for the C-2 atom in
1
method a, yield 37% (0.18 g, 0.79 mmol), white powder. H NMR
the cationic NHC precursors are straightforwardly available
and show the same trend as the 1JCSe coupling constants of
the NHC–Se adducts.
(600 MHz, CDCl3): δ = 6.89 (br. s, 2 H, CH), 5.24 [sept,
3
3JHH = 6.8 Hz, 2 H, CH(CH3)2], 1.35 [d, JHH = 6.8 Hz, 12 H,
CH(CH3)2] ppm. 13C{1H} NMR (150 MHz, CDCl3): δ = 153.2 (s,
CSe), 115.3 (s, CH), 50.5 [s, CH(CH3)2], 22.1 [s, CH(CH3)2] ppm.
77Se NMR (114 MHz, [D6]acetone): δ = –3 (s, CSe) ppm. MS (EI):
m/z = 232 [M]·+, 190 [M – iPr]·+, 148 [M – 2 iPr]·+. C9H16N2Se
(231.20): calcd. C 46.76, H 6.98, N 12.12; found C 46.62, H 6.70,
N 12.12.
Experimental Section
General: Unless otherwise noted, all operations were carried out
under precautions to exclude air and moisture. Glassware was dried
at 120 °C in an oven for at least 12 h. All solvents were dried rigor-
ously, freshly distilled prior to use, and stored under nitrogen (di-
ethyl ether and tetrahydrofuran over Na/benzophenone, dichloro-
methane over CaH2, and n-hexane over Na). Acetone and aceto-
nitrile were obtained from a solvent purification system (Braun).
Alumina was heated at 200 °C for at least 12 h, cooled under high
1,3,4,5-Tetramethylimidazole-2-selenone (2·Se): Reagents: 1,3,4,5-
tetramethylimidazolium iodide (0.34 g, 1.40 mmol), NaHMDS
(0.77 mL, 1.54 mmol), selenium (0.17 g, 2.10 mmol). Purification
method b, yield 42% (0.12 g, 0.59 mmol), brown powder. 1H NMR
(600 MHz, CDCl3): δ = 3.64 (s, 6 H, CH3), 2.11 (s, 6 H, CH3) ppm.
13C{1H} NMR (150 MHz, CDCl3): δ = 153.8 (s, CSe), 123.1 (s,
Cq), 34.2 (s, CH3), 9.6 (s, CH3) ppm. 77Se NMR (114 MHz, [D6]-
acetone): δ = 3 (s, CSe) ppm. MS (EI): m/z = 204 [M]·+, 189 [M –
CH3]·+, 123 [M – Se]·+. C7H12N2Se (203.15): calcd. C 41.39, H 5.95,
N 13.79; found C 41.18, H 5.88, N 13.52.
1
vacuum, and deactivated with water (5% by weight). H, 13C{1H},
19F, and 77Se NMR spectra were recorded with a Bruker Avance
1
III 600 spectrometer. The JCSe coupling constants were recorded
with a Bruker Avance 200 spectrometer. Chemical shifts (δ) are
reported in parts per million (ppm) downfield from tetramethylsil-
ane using the solvent as internal standard. The 77Se NMR spectra
were referenced to external KSeCN in D2O at a specific concentra-
tion with a chemical shift of –316.5 ppm (4.0 mol/L) or –329.0 ppm
(0.25 mol/L). Coupling constants are given in Hz. A Shimadzu IR
Affinity-1 spectrometer was used to record IR spectra. Mass spec-
tra were recorded with a Finnigan LCQ Deca Thermo Quest (ESI),
Bruker Ultrafex TOF (MALDI), Finnigan TSQ 7000 (EI), Thermo
Finnigan Trace DSQ (GC–MS), or a UHR-QTOF maXis 4G
Bruker Daltonics (HRMS-ESI) spectrometer. A vario MICRO
cube was used for elemental analysis.
1,3-Dimesitylimidazole-2-selenone (3·Se): Reagents: 1,3-dimesit-
ylimidazolium chloride (0.50 g, 1.46 mmol), NaHMDS (0.80 mL,
1.6 mmol), selenium (0.17 g, 2.20 mmol). Purification method b,
1
yield 40% (0.23 g, 0.59 mmol), beige powder. H NMR (600 MHz,
CDCl3): δ = 7.02 (s, 4 H, CHMes), 6.96 (s, 2 H, CH), 2.34 (s, 6 H,
CH3), 2.13 (s, 12 H, CH3) ppm. 13C{1H} NMR (150 MHz, CDCl3):
δ = 157.81 (s, CSe), 139.58 (s, CMes), 135.57 (s, CMes), 134.40 (s,
CMes), 129.46 (CHMes), 120.30 (s, C-4, C-5), 21.38 (s, CH3), 18.21
(s, CH3) ppm. 77Se NMR (114 MHz, [D6]acetone): δ = 35 (s,
CSe) ppm. MS (EI): m/z = 384 [M]·+, 303 [M – Se]·+, 185 [M – Se –
Mes]·+, 119 [Mes]·+. C21H24N2Se (383.39): calcd. C 65.79, H 6.31,
N 7.31; found C 65.69, H 6.01, N 7.27.
Sodium bis(trimethylsilyl)amide (2 m in thf; Acros Organics, CAS:
1070-89-9), selenium powder (Strem Chemicals, 99.5%; CAS: 7782-
49-2), and methyl trifluoromethanesulfonate (Acros Organics,
CAS: 333-27-7) were purchased and used as received. Compounds
4·Se, 5·Se, 9·Se, 13·Se, 18·Se, 20·Se, 22·Se,[9] 7·Se,[10b] 11·Se,[17c]
15·Se,[17e] 17·Se, and 19·Se[17g] are known in the literature. N,NЈ-
1,3-Dimesitylimidazolidine-2-selenone (6·Se): Reagents: 1,3-dimesit-
ylimidazolidinium chloride (1.00 g, 2.92 mmol), NaHMDS
(1.89 mL, 3.8 mmol), selenium (0.35 g, 4.38 mmol). Purification
1
method a, yield 45% (0.50 g, 1.30 mmol), white powder. H NMR
(600 MHz, CDCl3): δ = 6.98 (s, 4 H, CHMes), 4.00 (s, 4 H, CH2),
2.32 (s, 12 H, CH3), 2.32 (s, 6 H, CH3) ppm. 13C{1H} NMR
(150 MHz, CDCl3): δ = 180.2 (s, CSe), 138.5 (s, CMes), 136.4 (s,
CMes), 135.1 (s, CMes), 129.6 (s, CHMes), 48.8 (s, C-4, C-5), 21.3 (s,
CH3), 18.0 (s, CH3) ppm. 77Se NMR (114 MHz, CDCl3): δ = 111
(s, CSe) ppm. 77Se NMR (114 MHz, [D6]acetone): δ = 116 ppm.
MS (EI): m/z = 386 [M]·+, 371 [M – CH3]·+, 305 [M – Se]·+, 267
1,4-Bis(4-fluorophenyl)-1,4-diaza-1,3-butadiene,[28]
1·HBr,
4·HBr,[29] 2·HI,[30] 3·HCl, 5·HCl,[31] 6·HCl,[32] 7·HCl,[33]
10·HPF6,[34] 11·HBr,[17c] 13·HBr,[35] 1,3-dimesityl-4-hydroxyimid-
azolium chloride,[36] 16·HClO4,[17f] 17·HNO3, 19·(HOTf)2,[17g]
18·HCl,[17h] and 21·HOTf[4i] were prepared according to the litera-
ture.
Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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/KAP1
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Pages: 11
www.eurjic.org
FULL PAPER
[M – Mes]·+, 119 [Mes]·+. C21H26N2Se (385.41): calcd. C 65.44, H
6.80, N 7.27; found C 65.15, H 6.86, N 7.10.
stirred overnight. The solvent was removed under high vacuum to
give a beige powder, which was washed several times with diethyl
ether, yield 67% (0.10 g, 0.20 mmol). 1H NMR (600 MHz, [D6]-
1,3-Bis(4-fluorophenyl)imidazolidinium
Hexafluorophosphate
3
4
acetone): δ = 8.82 (dd, JHH = 4.6, JHH = 1.4 Hz, 1 H, CH), 8.69
(8·HPF6): Sodium borohydride (1.40 g, 36.01 mmol) in portions of
0.2 g was added to a solution of N,NЈ-1,4-bis(4-fluorophenyl)-1,4-
diaza-1,3-butadiene (2.20 g, 9.00 mmol) in thf (40 mL) at 0 °C over
a period of 30 min. The mixture was warmed to room temperature,
stirred for 18 h, and subsequently heated at reflux for 2 h. Ice–water
(100 mL) was added to this reaction mixture over 30 min followed
by the cautious addition of 1 m hydrochloric acid (20 mL). A color-
less solid precipitated, which was collected by filtration and dried
in vacuo. The resulting N,NЈ-bis(4-fluorophenyl)ethane-1,2-di-
amine was used in the next step without any purification. The di-
amine (1.00 g, 4.03 mmol) was heated at reflux over 15 h with so-
dium hexafluorophosphate (0.69 g, 4.20 mmol) in triethyl orthofor-
mate (30 mL). A colorless solid precipitated that was collected by
filtration, washed several times with n-hexane (20 mL), and dried
in vacuo, yield 24% (0.86 g, 2.20 mmol). 1H NMR (300 MHz, [D6]-
DMSO): δ = 9.87 (s, 1 H, CH), 7.72–7.64 (m, 4 H, CHphenyl), 7.50–
7.40 (m, 4 H, CHphenyl), 4.58 (s, 4 H, CH2) ppm. 13C{1H} NMR
3
4
3
(dd, JHH = 8.5, JHH = 1.4 Hz, 1 H, CH), 7.80 (dd, JHH = 8.5,
3JHH = 4.6 Hz, 1 H, CH), 4.83 (s, 2 H, N-CH2), 4.77 (s, 2 H, N-
CH2), 2.78 (s, 3 H, Se-CH3), 1.16 [s, 9 H, C(CH3)3], 1.14 [s, (CH3)
3] ppm. 13C{1H} NMR (150 MHz, [D6]acetone): δ = 210.0 (s, CSe),
149.7 (s, CH), 146.1 (s, Cq), 128.4 (s, Cq), 125.1 (s, CH), 123.6 (s,
CH), 59.4 (s, CH2), 57.8 (s, CH2), 35.7 [s, C(CH3)3], 35.5 [s, C(CH3)
3], 28.8 [s, C(CH3)3], 28.5 [s, C(CH3)3], 12.5 (s, Se-CH3) ppm. 19F
NMR (282 MHz, [D6]acetone): δ = –78.84 (s, CF3) ppm. 77Se
NMR (114 MHz, [D6]acetone): δ = 158 (s, CSe) ppm. MS (ESI):
m/z = 354 [M]+, 260 [M – SeMe]·+. C18H28F3N3O3SSe (502.45):
calcd. C 43.03, H 5.62, N 8.36, S 6.38; found C 42.79, H 5.37, N
8.29, S 6.22.
4-Methyl-2-methylselanyl-1,3-dineopentylimidazolium[4,5-b]pyridine
Trifluoromethanesulfonate (12·Se–Me): In a microwave tube 1,3-di-
neopentylimidazo[4,5-b]pyridine-2-selenone (0.10 g, 0.27 mmol)
was suspended in methyl trifluoromethanesulfonate (1.2 mL,
10 mmol). The mixture was stirred in the microwave oven at 100 °C
for 5 min, shaken, and then again stirred in the microwave oven at
100 °C for 5 min. The resulting clear red solution was suspended
in diethyl ether (20 mL) and the mixture stirred for 30 min. After
this time the resulting beige precipitate was filtered off, washed with
thf (10 mL), and dried in vacuo, yield 46% (0.09 g, 0.14 mmol). 1H
1
(75 MHz, [D6]DMSO): δ = 160.5 (d, JCF = 244.6 Hz, CF), 152.3
4
3
(s, C-2), 132.6 (s, JCF = 2.7 Hz, CN), 120.8 (s, JCF = 8.5 Hz,
CH), 116.4 (s, JCF = 23.2 Hz, CH), 49.0 (s, CH2) ppm. 19F NMR
2
1
(282 MHz, [D6]DMSO): δ = –70.16 (d, JFP = 711.2 Hz, 6 F, PF6),
–114.86 (s, 3 F, Fphenyl) ppm. MS (ESI): m/z = 259 [M]+.
C15H13F8N2P (404.24): calcd. C 44.57, H 3.24, N 6.93; found C
44.60, H 3.36, N 6.93.
3
NMR (600 MHz, [D6]acetone): δ = 9.55 (dd, JHH = 8.7 Hz, 1 H,
CH), 9.37 (d, 3JHH = 6.1 Hz, 1 H, CH), 8.46 (dd, 3JHH = 8.7, 3JHH
= 6.1 Hz, 1 H, CH), 5.41 (s, 1 H, N-CH2), 5.20 (s, 1 H, N-CH2),
5.08 (s, 3 H, NCH3), 5.03 (s, 1 H, N-CH2), 4.95 (s, 1 H, N-CH2),
2.93 (s, 3 H, Se-CH3), 1.23 [s, 9 H, C(CH3)3], 1.20 [s, (CH3)3] ppm.
13C{1H} NMR (150 MHz, [D6]acetone): δ = 210.0 (s, CSe), 149.0
(s, CH), 140.4 (s, Cq), 135.2 (s, CH), 134.9 (s, Cq), 125.2 (s, CH),
1,3-Bis(4-fluorophenyl)imidazolidene-2-selenone (8·Se): Reagents:
1,3-bis(4-fluorophenyl)imidazolidinium
hexafluorophosphate
(0.25 g, 0.85 mmol), NaHMDS (0.5 mL, 1.00 mmol), selenium
(0.170 g, 2.14 mmol). Purification method b, yield 64% (0.18 g,
0.54 mmol), orange powder. 1H NMR (600 MHz, CDCl3): δ =
7.53–7.49 (m, 4 H, CH), 7.16–7.11 (m, 4 H, CH), 4.15 (s, 4 H,
CH2) ppm. 13C{1H} NMR (150 MHz, CDCl3): δ = 182.1 (s, CSe),
1
121.9 (q, JCF = 321.3 Hz, CF3), 62.1 (s, CH2), 61.3 (s, CH2), 48.6
(s, N-CH3), 36.2 [s, C(CH3)3], 35.9 [s, C(CH3)3], 28.3 [s, C(CH3)
3],28.2[s,C(CH3)3],14.8(s,Se-CH3) ppm.77SeNMR(114 MHz,[D6]-
acetone): δ = 220 (CSe) ppm. MS (ESI): m/z = 370 [M]+, 355 [M –
Me]·+, 298 [M – Np]·+. C20H31F6N3O6S2Se (666.55): calcd. C 36.04,
H 4.69, N 6.30, S 9.62; found C 35.78, H 4.61, N 6.27, S 9.72.
1
4
161.6 (d, JCF = 248 Hz, CF), 137.4 (s, JCF = 3.2 Hz, CN), 128.6
3
2
(s, JCF = 8.5 Hz, CH), 116.2 (s, JCF = 22.8 Hz, CH), 51.3 (s,
CH2) ppm. 19F NMR (564 MHz, CDCl3):
–113.52 (s,
Fphenyl) ppm. 77Se NMR (114 MHz, [D6]acetone): δ = 178 (s,
CSe) ppm. MS (EI): m/z
338 [M]·+, 257 [M Se]·+.
δ
=
=
–
1,3-Dimesityl-4-oxoimidazole-2-selenone (14·Se): 1,3-Dimesityl-4-
hydroxyimidazolium chloride (0.50 g, 1.40 mmol) and selenium
powder (0.13 g, 1.68 mmol) were suspended in thf (50 mL) and
cooled to 0 °C. NaHMDS (1.6 mL, 3.20 mmol) was added drop-
wise. After stirring for 20 min at 0 °C the mixture was warmed to
room temperature and stirring was continued for another 3 h. HCl
(3 m, 0.75 mL, 2.24 mmol, in MeOH) was added dropwise to the
resulting intense green solution at 0 °C. After stirring for 2 h at
room temperature, the solvent was evaporated in vacuo and the
black residue suspended in dichloromethane (20 mL). After fil-
tration through Celite the solution was dried in vacuo to obtain
the product as a grey powder, yield 54% (0.30 g, 0.75 mmol). 1H
NMR (600 MHz, CDCl3): δ = 7.03 (s, 2 H, CHMes), 7.02 (s, 2 H,
CHMes), 4.22 (s, 2 H, CH2), 2.34 (s, 3 H, CH3), 2.34 (s, 3 H, CH3),
2.27 (s, 6 H, CH3), 2.22 (s, 6 H, CH3) ppm. 13C{1H} NMR
(150 MHz, CDCl3): δ = 184.7 (s, CSe), 170.3 (s, CO), 140.1 (s,
CMes), 139.8 (s, CMes), 136.1 (s, CMes), 135.6 (s, CMes), 133.3 (s,
CMes), 130.0 (s, CHMes), 129.6 (s, CHMes), 54.4 (s, CH2), 21.4 (s,
CH3), 21.3 (s, CH3), 18.0 (s, CH3), 17.9 (s, CH3) ppm. 77Se NMR
(114 MHz, [D6]acetone): δ = 295 (s, CSe) ppm. MS (EI): m/z = 400
[M]·+, 385 [M – O]·+, 319 [M – Se]·+. C21H24N2OSe (399.39): calcd.
C 63.15, H 6.06, N 7.01; found C 62.90, H 5.86, N 6.91.
C15H12F2N2Se·1/6Se: calcd. C 51.42, H 3.45, N 7.99; found C
51.56, H 3.53, N 7.86.
1,3-Dineopentylimidazo[4,5-b]pyridine-2-selenone (10·Se): Reagents:
1,3-dineopentylimidazolium[4,5-b]pyridine hexafluorophosphate
(0.50 g, 1.20 mmol), NaHMDS (0.70 mL, 1.40 mmol), selenium
(0.27 g, 3.45 mmol). Purification method c, yield 59% (0.24 g,
1
0.71 mmol), white powder. H NMR (600 MHz, CDCl3): δ = 8.24
3
4
3
(dd, JHH = 4.9, JHH = 1.3 Hz, 1 H, 7-H), 7.52 (dd, JHH = 8.1,
4JHH = 1.3 Hz, 1 H, 5-H), 7.12 (dd, JHH = 8.0, JHH = 4.9 Hz,
1H, 6-H), 4.50 (s, 2 H, CH2), 4.39 (s, 2 H, CH2), 1.11 [s, 9 H,
C(CH3)3], 1.11 [s, 9 H, (CH3)3] ppm. 13C{1H} NMR (150 MHz,
CDCl3): δ = 173.6 (s, CSe), 147.2 (s, Cq), 143.1 (s, C-7), 127.5 (s,
Cq), 118.2 (s, C-6), 117.4 (s, C-5), 56.9 (s, N-CH2), 55.6 (s, N-CH2),
35.7 [s, C(CH3)3], 35.5 [s, C(CH3)3], 29.4 [s, C(CH3)3], 29.4 [s,
C(CH3)3] ppm. 77Se NMR (114 MHz, [D6]acetone): δ = 183 (s,
CSe) ppm. MS (EI): m/z = 339 [M]·+, 258 [M – Se]·+, 188 [M – Se –
Np]·+. C16H25N3Se (338.35): calcd. C 56.80, H 7.45, N 12.42; found
C 56.73, H 7.32, N 12.30.
3
3
2-Methylselanyl-1,3-dineopentylimidazolium[4,5-b]pyridine
Tri-
fluoromethanesulfonate (10·Se–Me): 1,3-Dineopentylimidazo-
[4,5-b]pyridine-2-selenone (0.10 g, 0.30 mmol) was dissolved in
dichloromethane (7 mL). Methyl trifluoromethanesulfonate
(40 μL, 0.33 mmol) was added dropwise and the mixture was
3-(2,6-Diisopropylphenyl)-4,5,6,7-tetrahydrobenzo[d]thiazole-2-
selenone (16·Se): Reagents: 3-(2,6-Diisopropylphenyl)-4,5,6,7-tetra-
Eur. J. Inorg. Chem. 0000, 0–0
8
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I50174
/KAP1
Date: 09-04-15 13:41:58
Pages: 11
www.eurjic.org
FULL PAPER
hydrobenzo[d]thiazolium perchlorate (0.50 g, 1.25 mmol),
NaHMDS (0.70 mL, 1.4 mmol), selenium (0.30 g, 3.75 mmol). Pu-
flate (0.50 g, 0.95 mmol), NaHMDS (0.57 mL, 1.14 mmol), selen-
ium (0.11 g, 1.43 mmol). Purification method a, yield 60% (0.26 g,
0.58 mmol), pink powder. H NMR (600 MHz, CDCl3): δ = 6.97
1
rification method a, yield 88% (0.41 g, 1.10 mmol), yellow powder.
3
1H NMR (600 MHz, CDCl3): δ = 7.49 (t, JHH = 7.8 Hz, 1 H, (s, 4 H, CHMes), 2.33 (s, 6 H, CH3,Mes), 2.14 (s, 12 H, CH3,Mes),
3
H
Dipp), 7.31 (d, JHH = 7.8 Hz, 2 H, HDipp), 2.58–2.54 (m, 2 H,
1.78 [s, 6 H, C(CH3)2] ppm. 13C{1H} NMR (150 MHz, CDCl3): δ
3
CH2), 2.45 [sept, JHH = 7.0 Hz, 2 H, CH(CH3)2], 2.05–2.01 (m, 2 = 182.8 (s, CSe), 169.8 (s, CO), 139.1 (s, CMes), 136.1 (s, CMes),
H, CH2), 1.88–1.83 (m, 2 H, CH2), 1.76–1.71 (m, 2 H, CH2), 1.32
134.7 (s, CMes), 129.8 (s, CMes), 49.3 [s, C(CH3)2], 24.9 [s, C(CH3)
3
3
[d, JHH = 7.0 Hz, 6 H, CH(CH3)2], 1.15 [d, JHH = 7.0 Hz, 6 H,
2], 21.4 (s, CH3), 17.9 (s, CH3) ppm. 77Se NMR (114 MHz, [D6]-
CH(CH3)2] ppm. 13C{1H} NMR (150 MHz, CDCl3): δ = 181.0 (s, acetone): δ = 847 (s, CSe) ppm. MS (EI): m/z = 456 [M]·+, 441 [M –
CSe), 145.9 (s, Cq), 140.4 (s, Cq), 133.9 (s, Cq), 130.6 (s, CHDipp),
Me]·+, 375 [M – Se]·+. HRMS (ESI+, CH3CN/H2O): calcd. for
125.6 (s, Cq), 124.9 (s, CHDipp), 29.0 [s, CH(CH3)2], 25.6 (s, CH2), C24H29N2O2Se 456.4665 [M + H]+; found 457.1390.
24.8 [s, CH(CH3)2], 24.0 [s, CH(CH3)2], 23.6 (s, CH2), 22.9 (s, CH2),
Crystal Structure Determinations: Details concerning the X-ray
21.9 (s, CH2) ppm. 77Se NMR (114 MHz, [D6]acetone): δ = 396 (s,
structure determinations can be found in the Supporting Infor-
CSe) ppm. MS (EI): m/z = 379 [M]·+, 298 [M – Se]·+. C19H25NSSe
mation.
(378.43): calcd. C 60.30, H 6.66, N 3.70, S 8.47; found C 60.19, H
CCDC-1042142 (for 2·Se), -1042143 (for 9·Se), -1042144 (for
10·Se), -1042145 (for 16·Se), -1042146 (12·Se–Me), and -1042128
(for 23) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
6.45, N 3.79, S 8.69.
19·[Rh(cod)Cl]2 (23): 1,3,7,9-Tetramethyl-6-oxopurinediium bis(tri-
fluoromethanesulfonate) (0.40 g, 0.81 mmol) and [Rh(cod)Cl]2
(0.40 g, 0.81 mmol) were suspended in thf (30 mL) and cooled to
–78 °C. NaHMDS (1.05 mL, 2.10 mmol) was then added dropwise.
After stirring for 30 min at low temperature, the mixture was
warmed to room temperature and stirred for another 5 h. The sol-
vent was removed in vacuo and the product purified by column
chromatography over alumina using dichloromethane/thf (3:1) as
eluent, yield 32 % (0.18 g, 0.26 mmol). 1H NMR (600 MHz,
CDCl3): δ = 5.17–5.07 (m, 4 H, CHcod), 5.06 (s, 3 H, N-CH3), 4.64
(s, 3 H, N-CH3), 4.50 (s, 3 H, N-CH3), 4.39 (s, 3 H, N-CH3), 3.36–
3.22 (m, 4 H, CHcod), 2.49–2.33 (m, 8 H, CH2,cod), 2.06–1.94 (m,
8 H, CH2,cod) ppm. 13C{1H} NMR (150 MHz, CDCl3): δ = 215.7
(d, 1JCRh = 50.8 Hz, CRh), 194.7 (d, 1JCRh = 50.8 Hz, CRh), 151.1
(s, CO), 140.4 (s, Cq), 114.9 (s, Cq), 101.5 (d, 1JCRh = 6.5 Hz, Ccod),
Acknowledgments
The authors thank Maria Beuer and Dr. Klaus Schaper for the
configuration of 77Se NMR measurements and their excellent sup-
port. Annika Liske is thanked for valuable assistance with the ex-
perimental work. Computational support and infrastructure were
provided by the Center for Information and Media Technology
(ZIM) at the Heinrich Heine University Düsseldorf (Germany).
1
1
101.2 (d, JCRh = 6.5 Hz, Ccod), 100.4 (d, JCRh = 6.5 Hz, Ccod),
1
1
[1] A. J. Arduengo III, R. L. Harlow, M. Kline, J. Am. Chem. Soc.
1991, 113, 361–363.
100.2 (d, JCRh = 6.5 Hz, Ccod), 71.2 (d, JCRh = 14.4 Hz, Ccod),
1
1
70.8 (d, JCRh = 14.4 Hz, Ccod), 69.5 (d, JCRh = 14.4 Hz, Ccod),
1
[2] For leading reviews, see, for example: a) D. Bourissou, O.
Guerret, F. P. Gabbai, G. Bertrand, Chem. Rev. 2000, 100, 39–
92; b) F. E. Hahn, M. C. Jahnke, Angew. Chem. Int. Ed. 2008,
47, 3122–3172; Angew. Chem. 2008, 120, 3166–3216; c) M. Me-
laimi, M. Soleilhavoup, G. Bertrand, Angew. Chem. Int. Ed.
2010, 49, 8810–8849; Angew. Chem. 2010, 122, 8992–9032; d)
T. Dröge, F. Glorius, Angew. Chem. Int. Ed. 2010, 49, 6940–
6952; Angew. Chem. 2010, 122, 7094–7107; e) L. Benhamou,
E. Chardon, G. Lavigne, S. Bellemin-Laponnaz, V. César,
Chem. Rev. 2011, 111, 2705–2733; f) D. Martin, M. Melaimi,
M. Soleihavoup, G. Bertrand, Organometallics 2011, 30, 5304–
5313; g) D. J. Nelson, P. Nolan, Chem. Soc. Rev. 2013, 42,
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69.4 (d, JCRh = 14.4 Hz, Ccod), 44.5 (s, CH3), 39.8 (s, CH3), 39.4
(s, CH3), 37.5 (s, CH3), 33.1 (s, CH2,cod), 32.8 (s, CH2,cod), 32.6 (s,
CH2), 32.2 (s, CH2), 29.0 (s, CH2), 28.9 (s, CH2), 28.7 (s, CH2) ppm.
MS (MALDI-TOF): m/z = 649 [M – Cl]·+. C25H36Cl2N4ORh2
(685.30): calcd. C 43.82, H 5.30, N 8.18; found C 44.12, H 5.30, N
7.88.
19·[Rh(CO)2Cl]2 (24): The complex 19[Rh(cod)Cl]2 (70 mg,
0.102 mmol) was dissolved in dichloromethane. A stream of CO
was bubbled through the solution whilst stirring for 45 min. During
this time, the bright-yellow solution became bleached, which indi-
cates complete conversion. The IR spectrum of the resulting bis-
dicarbonyl-NHC compound was recorded from the reaction mix-
ture. The solution was concentrated under high vacuum and n-
hexane was added, which caused precipitation of a powdery solid.
This solid was isolated by filtration, washed with n-hexane, and
dried under high vacuum for several hours. The liberated cyclo-
octadiene could not be completely removed, yield 73 % (44 mg,
0.075 mmol). 1H NMR (600 MHz, CDCl3): δ = 4.75 (s, 3 H, CH3),
4.46 (s, 3 H, CH3), 4.30 (s, 3 H, CH3), 4.19 (s, 3 H, CH3) ppm.
13C{1H} NMR (150 MHz, CDCl3): δ = 205.8 (d, JCRh = 43.9 Hz,
1
CRh), 186.9 (d, 1JCRh = 45.6 Hz, CRh), 184.8 (d, 1JCRh = 55.8 Hz,
1
1
COtrans), 184.4 (d, JCRh = 55.8 Hz, COtrans), 181.4 (d, JCRh
=
1
55.8 Hz, COcis), 181.0 (d, JCRh = 55.8 Hz, COcis), 151.0 (s, CO),
140.0 (s, Cq), 115.8 (s, Cq), 45.7 (s, CH3), 40.7 (s, CH3), 40.3 (s,
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Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Date: 09-04-15 13:41:58
Pages: 11
www.eurjic.org
FULL PAPER
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Received: February 18, 2015
Published Online: ᭿
Eur. J. Inorg. Chem. 0000, 0–0
10
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I50174
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Date: 09-04-15 13:41:58
Pages: 11
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FULL PAPER
N-Heterocyclic Carbenes
K. Verlinden, H. Buhl, W. Frank,
C. Ganter* ...................................... 1–11
Determining the Ligand Properties of N-
Heterocyclic Carbenes from 77Se NMR
Parameters
The electronic properties of a range of N-
heterocyclic carbenes have been evaluated
by using the 77Se NMR chemical shifts of
NHC–Se adducts, to map the π-acceptor
which provide insights into the σ-donor
ability of the respective carbene. DFT cal-
culations were conducted to rationalize the
data. The NMR-derived parameters have
been compared with TEP values.
Keywords: Nitrogen heterocycles / Carb-
enes / Ligand properties / Rhodium / Sel-
enium / NMR spectroscopy
1
character, and JCSe coupling constants,
Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim