Stabilities of immonium ions derived from N-heterocyclic carbenes probed by collision-induced dissociation mass spectrometry

Article abstract of DOI:10.1002/ejoc.201200386

We report efficient methods for the synthesis of both symmetrically and non-symmetrically substituted bis(imidazol-2-yliden)ammonium ions and a new family of bis(imidazol-2-ylidene)formamidines. The fragmentation pathways of these cations were studied in detail by electrospray ionization combined with tandem mass spectrometry. Interestingly, in these cations direct C-N bond cleavage leading to loss of alkyl radicals can compete with rearrangement reactions leading to closed-shell fragments. The experimental results for the major fragmentation channels are compared with those derived from DFT calculations. Inter alia, a novel rearrangement of the cationic species involving specific alkyl migrations between the two imidazole subunits has been found. ESI combined with tandem mass spectrometry and DFT have been used for investigation of the fragmentation pathways of substituted bis(imidazol-2- yliden)ammonium ions and bis(imidazol-2-ylidene)formamidinium ions. Interestingly, in these cations direct C-N bond cleavage leading to loss of alkyl radicals can compete with rearrangements leading to closed-shell fragments. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Full text of DOI:10.1002/ejoc.201200386

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FULL PAPER
DOI: 10.1002/ejoc.201200386
Stabilities of Immonium Ions Derived from N-Heterocyclic Carbenes Probed by
Collision-Induced Dissociation Mass Spectrometry
Svetlana M. Polyakova,[a] Roman A. Kunetskiy,[a] and Detlef Schröder*[a]
In memoriam Dr. Ilya M. Lyapkalo
Keywords: Carbenes / Cations / Collision-induced dissociation / Density functional calculations / Electrospray ionization /
Lipophilic cations / Mass spectrometry / Phase-transfer catalysis
We report efficient methods for the synthesis of both symmet- icals can compete with rearrangement reactions leading to
rically and non-symmetrically substituted bis(imidazol-2-
yliden)ammonium ions and a new family of bis(imidazol-2-
ylidene)formamidines. The fragmentation pathways of these
closed-shell fragments. The experimental results for the
major fragmentation channels are compared with those de-
rived from DFT calculations. Inter alia, a novel rearrange-
cations were studied in detail by electrospray ionization com- ment of the cationic species involving specific alkyl mi-
bined with tandem mass spectrometry. Interestingly, in these
cations direct C–N bond cleavage leading to loss of alkyl rad-
grations between the two imidazole subunits has been found.
lysts and were shown to withstand hydrolysis under drasti-
cally alkaline conditions thanks to the efficient delocaliza-
tion of charge over the entire molecular systems and to the
protection of centers by sterically bulky N-alkyl substitu-
ents and of the imidazolium rings by the methyl groups, all
serving to prevent undesirable processes. Although the 1,3-
dialkyl-4,5-dimethylimidazol-2-ylidenimine system is quite
new as a structural fragment in synthetic methodology,[8,9]
it has already proved itself to be a highly electron-donating
group. It offers a challenge for the design of new classes of
compounds with predictable properties, which might be bet-
ter understood by detailed studies of their constituent frag-
ments.
Here we report a mass spectrometric study of selected
delocalized lipophilic cations directed towards deepening
understanding of the fragmentation mechanisms of these
compounds, which might in turn assist in improving their
thermal and chemical robustness. In particular, use is made
of collision-induced dissociation (CID), a technique in
which the ionic species of interest are first mass-selected
and then subjected to energizing collisions. CID has unique
advantages for elucidation of the structural features of com-
pounds and for study of the mechanisms of rearrangement
reactions occurring in the gas phase, through provision of
detailed fragmentation data.[10] For our study we chose a set
of both symmetrically and non-symmetrically substituted
BIMA cations together with a new family of bis(imidazol-
2-ylidene)formamidines (BIF), which were examined by
electrospray ionization (ESI) combined with tandem mass
spectrometry.
Introduction
Phase-transfer catalysis with the aid of lipophilic organic
cations capable of maintaining efficient concentrations of
reactive anion species in organic media has been recognized
as a convenient and powerful methodology in organic
chemistry.[1] However, there is still growing interest in
charge-delocalized, lipophilic cations, due to their stability
and high performance in organic phase-transfer reactions
under extreme conditions.[2] Low costs, operational sim-
plicity, and environmental benefits are key features of
phase-transfer catalysts for industrial applications.[3] In this
respect, robust phosphazenium cations have been evaluated
for the production of high-quality polymeric materials[4]
and as catalysts for halogen-exchange reactions.[5] Chiral
sp2-quaternized ammonium salts have been used effectively
in enantioselective phase-transfer catalysis of conjugate ad-
dition reactions.[6]
Recently, an efficient approach for the synthesis of delo-
calized lipophilic cations – formally bis(imidazol-2-ylidene)-
substituted ammonium ions (BIMA) – was discovered.[7]
These BIMA compounds are potential phase-transfer cata-
[a] Institute of Organic Chemistry and Biochemistry, Academy of
Sciences of the Czech Republic,
Flemingovo n. 2, 16610 Prague 6, Czech Republic
Fax: +420-220-183-462
E-mail: schroeder@uochb.cas.cz
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201200386.
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FULL PAPER
Scheme 1. Synthesis of symmetrically and non-symmetrically substituted BIMA derivatives and synthesis of BIF salts. a) and b) KF,
–
MeCN, room temp., 1–2 d, then aq. NaBF4. BF4 was used as a counterion in all cases but is omitted in the scheme (iPr = isopropyl,
Cy = cyclohexyl, Np = neopentyl).
All reactions proceeded very cleanly and no chromato-
graphic purification was required at any stage. The final
Results and Discussion
Synthesis
products were transformed into tetrafluoroborate salts with
aqueous NaBF4 and purified by recrystallization.
The syntheses and the structures of bis(1,3-dialkyl-4,5-
dimethylimidazol-2-yliden)amides 1 (BIMA) and bis(1,3-di-
alkyl-4,5-dimethylimidazol-2-ylidene)formamidines 2 (BIF)
studied in this work are shown in Scheme 1. The 2-chloro-
and 2-iminoimidazolium salts 3 and 4 used as precursors
can easily be synthesized by literature procedures.[7]
Mass Spectrometric Experiments
Both types of conjugated cationic heterocycles – BIMA
The symmetrically N-alkyl-substituted BIMA derivatives 1 and BIF 2 – were studied by ESI combined with tandem
1a–c were prepared as described earlier.[7] KF-mediated mass spectrometry. In the lower mass range, all samples
coupling of the appropriate 2-chloroimidazolium salt 3 and gave the intact quasi molecular ions [M]+ almost exclusively
2-iminoimidazolium salt 4 was shown to be effective for the under soft ionization conditions. In the high-mass range,
–
preparation of the non-symmetrically N-alkyl-substituted ion pairs of the type [M+·BF4 ·M+] were observed, their
BIMA 1d, which was isolated in good yield (Scheme 1, abundances increasing with concentration of the sample
path a).
and their fragmentation exclusively leading to [M]+ con-
The bis(imidazol-2-ylidene)formamidinium ions (BIF) 2, comitant with the loss of the neutral ion pairs
–
analogues of the BIMA cations with extended conjugated [M+·BF4 ].[13,14] The monomeric cations [M]+ then served
π-systems and thus more highly delocalized charges, were as precursor ions for fragmentation studies. To this end, the
also designed.[11] The BIF ions were prepared in good yields ion of interest, [M]+, was mass-selected in the ion trap (i.e.,
through KF-mediated coupling of the corresponding 2- all other ions were ejected) and then subjected to a radiofre-
chloroimidazolium ions 3 with formamidinium chloride quency pulse to increase the kinetic energy of the parent
(Scheme 1, path b); the latter reagent was obtained by treat- ion. Collisions with the helium buffer gas present in the
ment of formamidinium acetate[12] with acetyl chloride in trap convert the kinetic energy into internal energy, thereby
methanol.
causing ion fragmentation.
Table 1. Primary and secondary ionic fragments formed upon CID of mass-selected BIMA cations 1 and BIF cations 2.[a]
Primary ionic fragments, m/z (rel. int.)
NCE[b] [M]+ [M – R + H]+ [M – R]+·
Secondary ionic fragments, m/z (rel. int.)
[M – 2R + 2H]+
[M – 2R + H]+· [M – 2R]+
[M – R – CH3]+ [M – C5H7N3R2]+
1a 43
1b 43
1c 55
1d 46
374 (7)
534 (10)
486 (8)
454 (13)
332 (44)
452 (100)
416 (5)
412 (58)
372 (86)
359 (100)
479 (100)
331 (100)
451 (55)
415 (100)
411 (100)
371 (82)
358 (18)
478 (7)
290 (3)
289 (5)
369 (52)
345 (2)
329 (84)
288 (2)
368 (4)
344 (5)
328 (6)
317 (2)
179 (3)
235 (3)
259[c] (5)
219 (8)
370 (27)
346 (Ͻ1)
330 (22)
400 (33)
356 (4)
2a 45
2b 43
401 (7)
561 (5)
317 (6)
397 (10)
316 (1)
396 (Ͻ1)
206 (22)
286 (8)
[a] Data normalized to the most intense signal (100). [b] Normalized collision energy (NCE, given as % value) used in the experiments.
The NCE values were selected in order to provide good intensities of the fragment ions, while keeping a significant signal for the precursor
ion. A rough conversion into absolute collision energies is given as Ecoll = [0.088·NCE(%)] eV. [c] Additional fragments of notable abun-
dance correspond to [M – R – C4H8]+· (m/z 359, 9%) and [M – R – C4H9]+ (m/z 358, 9%).
2
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Immonium Ions Derived from N-Heterocyclic Carbenes
In the fragmentation of the BIMA cations 1 (Table 1), lized cation radical [M – R]+·. Alternatively, the cleavage of
one can distinguish the primary reactions occurring at low- the C–N bond can be accompanied by hydrogen migration
est collision energies and due to the fragmentation of a sin- from the alkyl group to the core (probably to one of the
gle substituent from secondary processes at elevated colli- nitrogen atoms) by a stepwise mechanism or via a cyclic
sion energies involving two (or more) substituents.[15,16] In transition structure to afford the closed-shell cation [M – R
every case the initial fragmentation is associated with the + H]+. We note in passing that no deuterium incorporation
loss of an N-alkyl group either as a radical R· or as the into parent or daughter ions was observed when the electro-
corresponding olefin (denoted as “– R + H”), in which one spray measurements were performed in deuterated solvents.
of the hydrogen atoms in R remains in the ionic species.
The fragmentations of the corresponding BIF cations 2
Subsequently, these primary fragments can again lose either are perfectly analogous in that losses of the radicals R· and
an olefin or a radical, leading to three secondary fragments of the corresponding olefins compete with each other in the
from the two primary ones; for non-symmetrical substitu- initial fragmentation, followed by further degradation of
tion (R϶ RЈ) further channels evolve from the correspond- the N-alkyl groups as was observed for the BIMA cations
ing combinations.
1a–1d (Table 1).
As an illustration, let us consider the case of the tetraiso-
Next, let us consider the energy dependence of the frag-
propyl BIMA derivative 1a. Upon CID, the parent cation mentations with 1b as an example (Figure 2). Up to a nor-
·
with m/z 374 can either lose a propyl radical (C3H7 ) to malized collision energy of NCE = 30%, no fragmentation
·
afford a fragment ion with m/z 331 or the substituent can is observed, and then the losses of C6H10 and C6H11 to
be lost as propene (C3H6) while one hydrogen atom stays afford ions at m/z 452 and m/z 451, respectively, begin to
in the ionic product, which hence appears at m/z 332. Be- rise with more or less the same thresholds. After having
cause the precursor cations are closed-shell species, the frag- reached maxima slightly above NCE = 40%, both channels
ment with m/z 331 accordingly corresponds to a cation radi- gently decline, which is ascribed to their feeding into the
cal, whereas the ion at m/z 332 is a closed-shell species. At secondary dissociation routes to the consecutive fragments
elevated collision energies, the larger energy deposited in with m/z 368–370. Breakage of the BIMA core to afford a
the parent ion leads to sequential fragmentations. The fragment at m/z 259 with approximately the same energy
closed-shell ion (m/z 332) and the cation radical (m/z 331) behavior as the secondary C–N bond cleavages, is also ob-
undergo losses of both C3H6 and C3H7· to afford fragments served. At about NCE = 50%, the parent ion is no longer
with m/z 290 and 289 for the former ion and with m/z 289 present and at larger collision energies the abundances of
and 288 for the latter. An additional, minor fragmentation the fragment ions change only gradually, as is expected for
pathway involves loss of methyl radical with formation of the mass-selective excitation of the parent ion in an ion-trap
an ion at m/z 316 and cleavage of the central C–N linkage to experiment.
afford an ion with m/z 179 (Figure 1). These fragmentation
pathways are fully confirmed by the corresponding MSn ex-
periments (see below).
Figure 2. Evolution of the major fragment ions upon CID of mass-
selected 1b (m/z 534) at variable collision energies: ᭿) m/z 534,
᭹) m/z 452, ᭺) m/z 369, ♦) m/z 451, ᭛) m/z 370, ᭡) m/z 259 (4ϫ),
Δ) m/z 368.
Figure 1. Collision-induced dissociation spectrum of the mass-se-
lected ion at m/z 374 generated by ESI-MS/MS of a dilute acetoni-
trile solution of 1a (NCE = 36%). The inset shows the low-mass
fragment ions with m/z 288–290 at expanded scale.
The behavior of the primary daughter ions [M – R + H]+
derived from 1 and 2 does not differ topologically from that
of the parent cations [M]+ when probed in MS3 experiments
With regard to the fact that the molecules studied each (see Table S1 in the Supporting Information). In such MS3
bear a charge delocalized over the conjugated system, one experiments, the quasi-molecular cations are mass-selected
can assume that the fragmentation of these cations will in and subjected to CID, and after fragmentation the desired
the first instance affect the weakest bonds (i.e., the C–N fragments are mass-selected again and themselves subjected
bonds of the N-alkyl groups). Homolytic C–N bond cleav- to CID. With this procedure even complicated consecutive
age accordingly leads to the formation of a resonance-stabi- pathways in ion chemistry can be elucidated.[10,17] In the
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FULL PAPER
case of the non-symmetrically substituted cation 1d, it is quent expulsion of R· + CH3 involves both imidazole sub-
firmly established that the second C–N bond cleavage in- units. As a feasible explanation of these reactions, in
volves the second imidazolium ring. The secondary frag- Scheme 3 we suggest cyclization reactions to form either
·
ments thus only appear at m/z 328–330, corresponding to six- or seven-membered rings.
·
the involvement of both N-isopropyl and N-cyclohexyl sub-
In order to verify that loss of CH3 involves the N-alkyl
stituents, but no peaks around m/z 370 or 290, representing substituents, the deuterated analogue [D12]-1a, in which the
either two N-cyclohexyl or two N-isopropyl substituents, methyl groups in the 4- and 5-positions of the imidazole
respectively, were detected. Also notable is that secondary rings are fully labeled, was prepared. In the ESI-MS spectra
fragmentation of this type is much more pronounced for the of the labeled compound, the quasi-molecular peak [D12
N-cyclohexyl-containing substrates than for the compounds M]+ was observed at m/z 386 (see the Supporting Infor-
with acyclic substituents (Table 1). mation) instead of the signal at m/z 374 for 1a. Fragmenta-
–
The major fragmentation pathways of the cation radical tion of the labeled compound gave both cation and cation-
species [M – R]+· derived from cations 1 and 2 were also radical species twelve mass units heavier than those de-
elucidated under MS3 conditions (see Table S2 in the Sup- scribed above for 1a (i.e., with m/z 344 and m/z 343, respec-
porting Information). Common to all BIMA derivatives 1 tively). Mass selection of the signal at m/z 343, correspond-
is the further degradation of the N-alkyl substituents, giving ing to the deuterated cation-radical species [D12 – M –
rise to loss either of the corresponding radical R· or of the C3H7]+·, and subsequent CID afforded a signal at m/z 328
corresponding olefin (R – H). Most illuminating with re- corresponding to a loss of 15 mass units. These observa-
spect to the mechanisms of ion fragmentation are the data tions fully confirm the above suggestion that the losses of
for the unsymmetrically substituted precursor 1d. The cor- methyl radicals do not involve the methyl substituents of
responding [M – C3H7]+· ion (m/z 411) undergoes compet- the imidazolium rings in the BIMA cations, but rather in-
·
ing losses of C6H10 and C6H11 upon CID to afford frag- volve the N-alkyl groups. We note in passing that all other
ment ions with m/z 329 and m/z 328. Analogously, the com- fragmentation products of the deuterated sample are also
plementary [M – C6H11]+· ion (m/z 371) undergoes compet- in agreement with the proposed structures.
·
ing losses of C3H6 and C3H7 also to afford fragments with
The low-mass peaks in the MS3 spectra (last column in
m/z 329 and m/z 328. Interestingly, however, the substitu- Table 1) in the m/z 179–259 range for the BIMA com-
tion pattern demonstrates that the loss of R· from one of pounds obviously each exclude only one imidazole unit.
the imidazole rings is followed by loss of RЈ· (or the corre- Formally, these signals correspond to the cations derived
sponding olefin) from the other imidazole unit. This reac- from the corresponding N-heterocyclic carbenes by loss of
tion sequence can be explained in terms of the formation of hydride: [NHC – H]+. The formation of these fragments can
a resonance-stabilized nitrenium ion as shown in Scheme 2. be attributed to an either stepwise or simultaneous re-
·
Further, eliminations of CH3 radicals are observed for arrangement consisting of i) H-migration, and ii) C–N
the radical cations derived from the BIMA compounds pos- bond cleavage (Scheme 4). Formation of more stable iso-
sessing either isopropyl or neopentyl groups, but not from meric ions with m/z 179 can proceed through iii) 1,2-alkyl
those with the cyclopropyl substituent. Moreover, the MS3 shift followed by iv) H-migration.
experiments with the unsymmetrically substituted precursor
The occurrence of yet another rearrangement, 1,5-alkyl
·
1d show an expulsion of CH3 from the [M – C6H11]+· frag- migration from one imidazolium ring to the other, in the
ment with m/z 371, whereas this particular fragmentation BIMA molecules is implied in the case of 1d. To our sur-
pathway does not occur for the corresponding [M – prise, both cation radicals [M – C3H7]+· at m/z 411 and [M –
C3H7]+· cation radical with m/z 411 (see the Supporting In- C6H11]+· at m/z 371 gave fragmentation products at m/z 219
formation). Accordingly we also conclude that the subse- upon CID, rather than forming the peaks with m/z 179 and
Scheme 2. Plausible sequence for the consecutive losses of radicals R and RЈ from each of the imidazole subunits in the non-symmetrically
substituted cation 1d.
4
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Immonium Ions Derived from N-Heterocyclic Carbenes
Scheme 3. Suggested cyclization reactions associated with the losses of CH3· radicals from the cation radicals with isopropyl and neopentyl
substituents derived from 1a and 1c.
Scheme 4. Proposed mechanism of the formation of the [NHC – H]+ species for the example of 1a and computed energetics.
m/z 259 as were obtained for the symmetrically substituted including all secondary fragmentations are summarized.
1a and 1b, respectively. Although the fragment ion with m/z The branching ratios (BRs) of cations relative to cation-
219 is formed in either way, MS4 experiments reveal struc- radicals as well as the measured and calculated appearance
tural differences for the ions formed through losses of R energies (AE) are shown in Table 2.
and RЈ. The ion with m/z 219 derived from [M – C3H7]+·
Inspection of the branching ratios in Table 2 reveals sev-
undergoes loss of C3H6 in MS4 to give 177, whereas the eral interesting trends. When going from the N-isopropyl
peak at m/z 219 derived from [M – C6H11]+· shows loss of to N-cyclohexyl substituents, the ratio between the cationic
C6H10 to give a peak at m/z 137. These results unambigu- closed-shell fragments [M – R + H]+ and the cation radicals
ously demonstrate that isomeric ions are formed in the con- [M – R]+· changes in the case of the BIMA cations from
secutive fragmentations, and for these we accordingly sug- 31:69 for 1a to 58:42 for 1b, suggesting a more facile loss
gest the corresponding immonium ions as depicted in of the isopropyl radical than of the olefin. For the BIF cat-
Scheme 5.
ions, the olefin losses are generally more pronounced (i.e.,
The corresponding cation-radicals [M – R]+· derived 71:29 for 2a and 88:12 for 2a), but the tendency to en-
from the BIF cations 2 differ from those obtained from the hanced loss of an isopropyl radical is maintained. To a first
BIMA derivatives in that no losses either of olefins or of approximation, the preference for formation of cation over
alkyl radicals are observed in the corresponding MS3 ex- cation-radical for both families upon changing the N-alkyl
periments (see Table S2 in the Supporting Information). In- groups from isopropyl to cyclohexyl can be attributed to
stead, cleavages of the linkage between the two imidazole the more favorable formation of a disubstituted double
subunits lead to ions with the general structure shown in bond in the latter case.[18] Comparison of BIMA and BIF
Scheme 6. Unfortunately, the synthesis of non-symmetri- cations with the same N-alkyl substituents reveals increas-
cally substituted BIF compounds is more difficult because ing amounts of the [M – R + H]+ fragments, which can be
the first imidazolination step is rate-determining, whereas attributed to the enhanced basicity of the nitrogen atoms in
the second step is very fast due to precipitation of the prod- the more extended and hence more charge-delocalized BIF
uct from the reaction mixture. Therefore, it was not possible cations 2. Interestingly, the appearance energies are very
to investigate non-symmetrically substituted BIF derivatives similar for all compounds and are also almost equal for
in order to probe the occurrence of similar alkyl migrations both processes – the loss of the olefin and that of the alkyl
in the BIF cations.
radical. Although this might appear surprising with respect
For comparison of the various compounds, the ratios of to the so-called “even-electron rule” in mass spectrometry,
the primary product ions [M – R + H]+ and [M – R]+· which predicts that closed-shell ions preferentially eliminate
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Scheme 5. Plausible pathways for the formation of isomeric immonium ions after losses of radicals R and RЈ from the non-symmetrically
substituted cation 1d.
Scheme 6. Proposed fragmentation of the cation radical [M – C3H7]+· derived from BIF cation 2a.
Table 2. Branching ratios (BRs)[a] of product ions observed in MS/MS experiment and the measured and calculated appearance energies
(AE).
AE[b] [eV]
AEcalc [eV]
AEcalc [eV]
[c]
[c]
Product
m/z
BR [%]
B3LYP/6-31g(d)
(B3LYP/cc-pVTZ)
B97D/6-31g(d)
(B97D/cc-pVTZ)
1a
[M – R + H]+
[M – R]+·
332
331
31
69
2.77
2.79
2.05 (1.93)[d]
2.78 (2.41)[e]
in: 1.89 (1.73)
out: 2.47 (2.29)
1.90 (1.79)[d]
3.10 (2.76)[e]
in: 2.29 (2.14)
out: 2.73 (2.57)
1b
1c
1d
[M – R + H]+
[M – R]+·
452
451
58
42
2.92
2.95
in: 1.95 (1.78)
out: 2.51 (2.32)
[M – R + H]+
[M – R]+·
416
415
5
95
3.30
3.27
in: 2.05 (1.89)
out: 2.55 (2.34)
[M – R + H]+
[M – R]+·
412
411
32
68
2.95
2.97
in: 1.88 (1.72)
out: 2.45 (2.26)
[M – RЈ + H]+
[M – RЈ]+·
372
371
50
50
2.92
2.94
in: 1.95 (1.78)
out: 2.53 (2.34)
2.04 (1.92)[d]
2.16 (2.00)[f]
2a
2b
[M – R + H]+
[M – R]+·
359
358
479
478
71
29
88
12
2.97
2.99
3.01
3.00
1.81 (1.70)[d]
2.49 (2.34)[f]
[M – R + H]+
[M – R]+·
2.19 (2.02)[f]
[a] BR normalized to a sum of 100. In the analysis, all secondary fragmentations are summed into the primary channels. [b] Appearance
energies (AE) were obtained from CID fitting of the thresholds and converted to eV scale. The relative error of the AE values with regard
to the quality of the fits is below Ϯ0.05 eV, whereas the absolute error is estimated as Ϯ0.2 eV. [c] Calculated relative energies at 0 K. For
the in/out distinction relating to the positions of the alkyl substituents in the BIMA compounds, see the computational results. [d] Values
corresponding to the barrier associated with olefin elimination. [e] Values corresponding to the barrier for 1,3-H migration after olefin
loss. [f] The BIF cations have C1 symmetry and so the bond strengths differ slightly for all four substituents. The value given refers to
the most facile loss of R; for the others, see the Supporting Information.
closed-shell fragments and only radical cations expel open- competing radical pathways are in fact quite common in N-
shell fragments, exceptions to this rule through efficiently alkylated ammonium and immonium ions.[19] In some cases,
6
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Immonium Ions Derived from N-Heterocyclic Carbenes
radical losses can even dominate the fragmentation routes mally considered as carbene-coordinated N+ species.[25] The
of closed-shell ions.[20]
angle between the two imidazolium rings is close to 70°,
and the structure is bent with a C–N–C angle of 128.5°.
Because of the sterically demanding substituents, rotations
are significantly hindered, such that two types of isopropyl
groups can be distinguished in the molecule: either the alkyl
groups are located inside the bent angle with the hydrogen
atoms of CH groups facing towards one another (Figure 3,
in) or these hydrogen atoms are directed away from core,
being located in positions opposite one another (Figure 3,
out).
Computational Results
The processes described above are discussed here in the
context of gas-phase DFT calculations. The electronic
structures of the BIMA cations 1 and BIF cations 2 were
examined by use of the B3LYP[21] hybrid DFT functional
or the B97D[22] functional in combination with the 6-31g(d)
and cc-pVTZ basis sets, implemented in the Gaussian 09
suite of programs.[23] The use of two different functionals is
because of the widespread application of B3LYP on the one
hand, and the coverage of dispersion effects by B97D on
the other. The geometry optimizations were performed with
use of the coordinates from X-ray analysis as starting
points. For both functionals, the computed structures are
in good agreement with the solid-state structures.[7] How-
ever, the energetic data show a significant spread with re-
spect to the basis sets and the functionals, with differences
of up to 0.4 eV between the B3LYP and B97D results and
of about 0.2 eV between the values obtained with the large
and the small basis sets. Although the computed energy de-
mands for ion fragmentation are generally lower with the
larger basis set, no systematic trends are obvious for the
differences between B3LYP and B97D. The computed
thresholds are generally lower than the experimentally mea-
Figure 3. Structure of 1a optimized by use of B3LYP/6-31g(d) with
in/out designations of the isopropyl groups. Colour code: gray for
C, black for N, white for H. Angle C–N–C: 128.5°; dihedral angle
N–C–C–N: 66.9°.
The losses of an alkyl radical and of an olefin were com-
sured AE values, by as much as 1 eV in energy, which could puted to proceed with different energies depending on the
either be due to the limited theoretical description or could site of C–N bond cleavage. A schematic presentation of the
be caused by the operation of kinetic shifts in ion dissoci- potential energy surface for the two pathways is depicted in
ation.[24] Notwithstanding these uncertainties, the theoreti- Figure 4. The loss of an isopropyl radical from the in region
cal data provide some useful understanding about the struc- of 1a requires 1.73 eV and is thus 0.56 eV less endothermic
tures involved and the qualitative energetics.
than that from the out position (Figure 4, left). The loss of
Figure 3 shows the structure of the optimized closed- an olefin from the out position in cation 1a can take place
shell cation 1a. The calculated structure of 1a has pseudo- by initial cleavage of the C–N bond concomitant with mi-
C2 symmetry, and the BIMA derivatives may thus be for- gration of a hydrogen atom to the central nitrogen atom via
Figure 4. Computed reaction coordinate for loss of a radical (left) or of an olefin (right) from the gaseous immonium ion 1a with use of
B3LYP/cc-pVTZ//B3LYP/6-31g* (bold) and B97D/cc-pVTZ//B97D/6-31g*.
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S. M. Polyakova, R. A. Kunetskiy, D. Schröder
FULL PAPER
Figure 5. Optimized structures [B3LYP/6-31g(d)] of TS1 for loss of propene from a) 1a and b) 2a. The substituents on the imidazolium
rings are omitted for clarity. Selected bond lengths are given in Å.
TS1. Subsequent 1,3-H migration via TS2 to the imidazole
nitrogen atom that has lost the substituent leads to more
energetically favorable tautomer (Figure 4, right). Although
the loss of propene from the in region is energetically more
favorable, no reasonable formation pathways from cation 1a
could be found, unless one were to speculate about H-trans-
fer from the cleaved radical in an ion-dipole complex[26] or
substantial conformational changes. Interestingly, the en-
ergy barrier associated with reaching TS1 of the olefin loss
from the out position is close to the energy requirement for
the radical loss from the in position.
In the transition structure leading to olefin elimination,
the C–N bond has already been cleaved, whereas hydrogen
transfer to the central nitrogen atom is not yet complete
(Figure 5a); a similar transition structure for propene loss
is found for the BIF 2a (Figure 5b). In combination with
their energy demands, these structures support the implica-
tion derived from the almost identical appearance energies
of these channels in the experiments that the olefin and
Figure 6. Computed [B3LYP/6-31g(d)] energetics and structures for
alkyl radical losses occur in a similar fashion.
losses of isopropyl radicals from the in and out conformers for a
In a more general context, the computed homolytic C–
BIMA model with three N-methyl substituents and one N-iso-
propyl substituent. The dihedral angle between the two imidazole
N bond energies of the title compounds are surprisingly low
in relation to a general strength of about 3 eV for covalent rings, which is a measure for the planarization of the conjugated
C–N bonds.[27] The possible effects of steric strain on the
bond energies were therefore computationally investigated
π-system, is given below each structure. Atom labels: carbon (dark
gray), nitrogen (black), hydrogen (light gray).
by replacement of three of the four isopropyl groups in 1a
by methyl substituents (Figure 6). For the remaining iso-
propyl moiety, B3LYP/6-31g(d) calculations predict homo-
Conclusions
lytic bond energies of 1.99 eV (in) and 2.61 eV (out), which
A series of bis(1,3-dialkyl-4,5-dimethylimidazol-2-ylid-
are quite similar to the corresponding values of 2.05 eV (in) ene)amines 1 (BIMA) and bis(1,3-dialkyl-4,5-dimethylimid-
and 2.78 eV (out) for 1a. Inspection of the computed struc- azol-2-ylidene)formamidines 2 (BIF) with different N-alkyl
tures reveals that the imidazole rings, which are strongly substituents were synthesized from 2-chloroimidazolium
tilted relative to one another in the in conformer (ΦNCCN
=
salts and studied by electrospray ionization combined with
65°), approach planarity much more closely upon loss of tandem mass spectrometry. The primary fragmentations of
the in-isopropyl group (ΦNCCN = 39°). In contrast, upon the quasi-molecular ions involve competitive losses either
loss of isopropyl from the out position the dihedral angle of alkyl radicals or of neutral olefins, both originating from
remains almost unchanged. Because a decrease in ΦNCCN the N-alkyl substituents of the imidazolium rings. The
increases the conjugation of the π-systems of the two imid- branching ratios of these two fragmentation pathways, lead-
azole subunits, the difference in the bond energies, and in ing to the closed-shell ions [M – R + H]+ and the cation
particular the low bond energy of the in isomer, can thus radicals [M – R]+·, respectively, depend on the nature of the
be attributed to the energy gain upon relaxation of the π- N-alkyl substituents in both families. Interestingly, however,
system with loss of an in-substituent, whereas the overall both channels efficiently compete for all compounds investi-
steric hindrance is of minor importance.
gated, which is primarily due to the weakening of the C–N
8
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Immonium Ions Derived from N-Heterocyclic Carbenes
2929 cm–1 (w). MS (ESI+): m/z (%) = 454.4 (100) [M]+. HRMS
bonds as a result of the steric strain in the title compounds.
Changes in the branching ratios of ion fragmentation occur
on going from BIMA to BIF, with the N-substituents kept
the same. The investigation of a non-symmetrically N-alkyl-
substituted BIMA cation reveals the occurrence of several
(ESI+): calcd. for C28H48N5 454.3904 [M]+; found 454.3903.
+
C28H48BF4N5 (541.52): calcd. C 62.10, H 8.93, N 12.93; found C
61.92, H 8.51, N 12.74.
2-{[1,3-Diisopropyl-4,5-dimethyl-1H-imidazol-2(3H)-ylidenamino]-
new rearrangements, including specific alkyl migrations be- methylenamino}-1,3-diisopropyl-4,5-dimethyl-1H-imidazolium Tet-
rafluoroborate (2a): AcCl (1.57 g, 20.0 mmol) was added dropwise
at 5 °C to cold MeOH (20 mL). After the system had been stirred
for 10 min at 5 °C, formamidinium acetate (0.42 g, 4.0 mmol) was
added and the reaction mixture was allowed to warm to room tem-
perature. The solvent was evaporated, and the residue was diluted
with EtOAc (2 mL), concentrated, and dried in high vacuum. KF
(3.70 g, 64.0 mmol), salt 3a (2.42 g, 8.0 mmol), and MeCN (12 mL)
were added to the flask, and the mixture was stirred at room tem-
tween the two imidazole subunits. Common to all cations
investigated here is the high stability of the backbone, in
that all observed major dissociation pathways originate
from the N-alkyl substituents. Moreover, even in these frag-
mentation reactions direct C–N bond cleavages compete
with hydrogen-rearrangement pathways. Thus, the gas-
phase data indeed provide an explanation for the high ther-
mal stabilities of the title compounds in the condensed
phase.
1
perature for 2 d. After completion of the reaction, indicated by H
NMR, a small portion of chloroform was added, the suspension
was filtered through a Schott glass filter directly into a separation
funnel, and the solid residue was washed a few times with small
portions of chloroform. A dilute aq. solution of NaBF4 (8.8 g,
80.0 mmol) was added to the solution in the separation funnel, fol-
lowed by vigorous shaking. The organic phase was separated, and
the aqueous phase was extracted once with a small portion of chlo-
roform. The combined chloroform fractions were dried (MgSO4),
filtered, and concentrated at reduced pressure. The residue was dis-
solved in EtOAc (20 mL), and the solid product was precipitated
by addition of Et2O (10 mL) with stirring, filtered, and dried in
vacuo. Yield 1.54 g (79 %) of cream crystalline compound; m.p.
113–114 °C. 1H NMR (400 MHz, CDCl3): δ = 1.50 (d, 3J = 7.1 Hz,
24 H, CHMe2), 2.27 (s, 12 H, Me), 4.52 (sept, 3J = 7.1 Hz, 4 H,
CHMe2), 7.64 (s, 1 H, CH) ppm. 13C NMR (101 MHz, CDCl3): δ
= 9.7 (Me), 21.1 (CHMe2), 48.6 (CHMe2), 121.1 (C=C), 147.9 (C–
Experimental Section
General: NMR spectra were recorded with a Bruker Avance I™ 400
1
(400 MHz for H, 100.6 MHz for 13C) spectrometer in CDCl3 un-
less stated otherwise. Chemical shifts are reported on the δ scale in
1
ppm relative to SiMe4 (δ = 0 ppm) as an internal standard for H
NMR and to the corresponding deuterated solvent signals[28] for
13C NMR spectroscopy. Multiplicity is indicated as follows: s (sing-
let), brs (broad singlet), d (doublet), t (triplet), q (quartet), sept
(septet), oct (octet), m (multiplet). Coupling constants (J) are given
in Hz. IR spectra of the neat compounds were recorded with a
Bruker ALPHA-P instrument with a Platinum ATR module. High-
resolution mass spectra (HRMS) were recorded with a LTQ Or-
bitrap XL instrument by electrospray ionization (ESI). Melting
points were measured with a SMP10 melting point apparatus. Ele-
mental analyses were obtained with a Perkin–Elmer PE 2400
Series II CHNS analyzer.
N), 160.2 (CH) ppm. IR (neat): ν = 910 (w), 993 (m), 1033 (vs),
˜
1048 (vs), 1092 (m), 1209 (w), 1286 (w), 1374 (m), 1436 (s), 1469
(s), 1491 (s), 1537 (m), 1587 (m), 2881 (w), 2942 (w), 2978 (w) cm–1.
MS (ESI+): m/z (%) = 401.4 (100) [M]+. HRMS (ESI+), m/z: calcd.
for C23H41N6+: 401.3387 [M]+; found 401.3388.
All reactions were carried out under dry argon in dried (heat-gun)
glassware containing magnetic stirring bars. KF, pre-dried as de-
scribed[7] prior to each experiment, was strongly heated in vacuo in
a reaction flask with a heat gun for a few minutes to remove traces
of moisture and allowed to cool to room temperature under argon.
MeCN was distilled from CaH2 prior to use. Compounds 1a–c,
the 2-chloro- and 2-aminoimidazolium salts,[7] and formamidinium
acetate[12] were prepared by literature procedures.
1,3-Dicyclohexyl-2-{[1,3-dicyclohexyl-4,5-dimethyl-1H-imidazol-
2(3H)-ylidenamino]methylenamino}-4,5-dimethyl-1H-imidazolium
Tetrafluoroborate (2b): The compound was prepared by the pro-
cedure described above for 2a, starting from salt 3b (1.53 g,
4 mmol). Complete conversion of the starting material was ob-
served after stirring at room temperature for 24 h. Yield 1.08 g
(83%) of a pale yellow solid; m.p. 110 °C. 1H NMR (400 MHz,
CDCl3): δ = 1.09–1.42 (m, 12 H, CH2), 1.61–2.12 (m, 28 H, CH2),
2-[1,3-Dicyclohexyl-4,5-dimethyl-1H-imidazol-2(3H)-ylidenamino]-
1,3-diisopropyl-4,5-dimethyl-1H-imidazolium Tetrafluoroborate (1d):
KF (0.465 g, 8.0 mmol), the 2-chloroimidazolium salt 3b (0.383 g,
1.0 mmol), and the 2-aminoimidazolium salt 4 (0.283 g, 1.0 mmol)
were mixed in MeCN (2 mL) under argon and stirred at room tem-
perature for 24 h. After completion of the reaction, the mixture
was diluted with CHCl3 (10 mL) and filtered through a Schott glass
filter directly into a separation funnel. The chloroform layer was
shaken with diluted aq. NaBF4 (0.55 g, 5.0 mmol), separated, dried
(MgSO4), and concentrated at reduced pressure. Recrystallization
from EtOAc gave the title compound (0.384 g, 71%) as a colorless
3
2.28 (s, 12 H, Me), 4.05 (t, J = 11.9 Hz, 4 H, CH), 7.72 (s, 1 H,
CH) ppm. 13C NMR (101 MHz, CDCl3): δ = 10.4 (Me), 25.1
(CH2), 26.2 (CH2), 31.4 (CH2), 57.4 (CH), 121.4 (C=C), 148.5
(CN), 160.4 (CH) ppm. IR (neat): ν = 895 (m), 1033 (vs), 1051 (vs),
˜
1345 (m), 1380 (s), 1438 (s), 1492 (vs), 1540 (m), 1569 (m), 2856
(w), 2930 (m) cm–1. MS (ESI+): m/z (%) = 561.5 (100) [M]+. HRMS
(ESI+) calcd. for C35H57N6 561.4639 [M]+; found 561.4638.
+
Mass Spectrometry: The sample solutions (1ϫ 10–6 m in CH3CN)
were investigated by use of a Finnigan LCQ ion-trap mass spec-
trometer with an electrospray ionization (ESI) source.[20c] Nitrogen
solid; m.p. 160 °C. 1H NMR (400 MHz, CDCl3): δ = 0.98–1.25 (m, was used as nebulizer gas, and the samples were introduced into
3
6 H, CH2), 1.38 (d, J = 7.0 Hz, 12 H, CHMe2), 1.56–1.99 (m, 14 the ESI source through a needle at a flow rate of 5 μLmin–1. The
H, CH2), 2.14 (s, 12 H, Me), 3.55–3.73 (m, 2 H, CH), 4.12 (sept,
MSn spectra were obtained by collision-induced dissociation (CID)
3J = 6.9 Hz, 2 H, CHMe2) ppm. 13C NMR (101 MHz, CDCl3): δ with helium buffer gas present in the ion trap after isolation of the
= 10.2 (Me), 10.5 (Me), 21.2 (CHMe2), 25.2 (CH), 26.3 (CH), 31.3 precursor ions of interest. Unless otherwise mentioned, the op-
(CH), 48.1 (CHMe2), 57.0 (CH), 119.3 (C=), 119.8 (C=), 144.6
erating conditions used were as follows: sheath gas flow rate 50
arbitrary units, aux. gas flow rate 5 arbitrary units, spray voltage
4.5 kV, heated capillary temperature 200 °C, capillary voltage and
(NCN) ppm. IR (neat): ν = 894 (w), 1033 (vs), 1047 (vs), 1092 (m),
˜
1234 (w), 1261 (w), 1379 (m), 1426 (m), 1586 (s), 2858 (w),
Eur. J. Org. Chem. 0000, 0–0
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S. M. Polyakova, R. A. Kunetskiy, D. Schröder
FULL PAPER
[10] a) K. Levsen, H. Schwarz, Mass Spectrom. Rev. 1983, 2, 77–
148; b) C. Cheng, M. L. Gross, Mass Spectrom. Rev. 2000, 19,
398–420.
[11] The stabilities of BIF cations were tested under heterogeneous
drastic basic conditions by analogy with ref.[7] Two degradation
products were observed in the reaction mixture, with a half-life
for 2b of ca. 1 h.
tube lens offset were automatically adjusted for the highest inten-
sity of studied ion. The following parameters were used: the isola-
tion width was fixed at m/z 2 for MS2 and at m/z 1 for MS3 and
higher to avoid interferences from isotopic species, three microscans
were carried out with a maximum injection time of 200 ms. The
CID spectra were recorded with a qz value of 0.25 and an activation
time of 20 ms, for which a phenomenological calibration of the
NCE values into an absolute collision energy scale was recently
introduced.[29,30] The actual calibration factor during the measure-
ments was established by CID of benzylpyridinium ions leading to
Ecoll = [0.088·NCE(%)]eV. Mass spectra were recorded from m/z 50
to 2000 in positive-ion mode and about 100 scans were accumu-
lated.
[12] Formamidinium acetate was synthesized from trimethyl ortho-
formate with NH4OAc by a literature procedure, see: E. C. Tay-
lor, W. A. Ehrhart, J. Am. Chem. Soc. 1960, 82, 3138–3141. In
attempted BIF synthesis, use of formamidinium acetate led to
nucleophilic substitution of chloride in 3 by acetate anion.
[13] D. Schröder, L. Duchácˇková, I. Jusˇinski, M. Eckert-Maksic´, J.
Heyda, L. Tu˚ma, P. Jungwirth, Chem. Phys. Lett. 2010, 490,
14–18.
Computational Chemistry: The calculations were carried out with
the Gaussian 09 program package[23] by use of DFT with either the
hybrid functional B3LYP or B97D, which includes a correction of
dispersion. The initial calculations with 6-31g(d) basis sets were
repeated as single-point energy calculations by use of cc-pVTZ. For
the optimized structures, vibrational frequencies were computed
with use of B3LYP/6-31g(d) in order to confirm that the stationary
points corresponded to true minima and to calculate zero-point
vibrational energy (ZPVE) corrections. The reported energies all
include ZPVE and refer to 0 K.
[14] A. Deyko, K. R. J. Lovelock, P. Licence, R. G. Jones, Phys.
Chem. Chem. Phys. 2011, 13, 16841–16850.
[15] In order to avoid confusion, we point out that ion excitation in
IT-MS occurs mass-selectively. The secondary fragmentations
observed are thus not due to excitation of the fragment ions,
but to the direct deposition of a large amount of internal en-
ergy into the parent ion, which allows the amount of dissoci-
ation to be effectively controlled, see: R. G. Cooks, R. E. Kai-
ser Jr, Acc. Chem. Res. 1990, 23, 213–219.
Supporting Information (see footnote on the first page of this arti-
cle): Additional MS/MS experiments, labeling data, details of the
computations and complete NMR spectra.
[16] For consecutive dissociations, see also: V. H. Wysocki, H. I.
Kenttämaa, R. G. Cooks, Int. J. Mass Spectrom. Ion Processes
1987, 75, 181–208.
Acknowledgments
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This work was supported by the Czech Academy of Sciences
(RVO 61388963), the European Research Council (AdG HORI-
ZOMS), and the IOCB postdoctoral project (to S. P.). The kind
assistance of Dr. Christopher Shaffer and Dr. Jana Roithová in the
interpretation of the computational data is gratefully acknowl-
edged.
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Immonium Ions Derived from N-Heterocyclic Carbenes
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Received: March 27, 2012
Published Online: ᭿
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11

Job/Unit: O20386
/KAP1
Date: 30-05-12 16:50:35
Pages: 12
S. M. Polyakova, R. A. Kunetskiy, D. Schröder
FULL PAPER
Lipophilic Cations
ESI combined with tandem mass spec-
trometry and DFT have been used for
investigation of the fragmentation path-
ways of substituted bis(imidazol-2-yliden)-
ammonium ions and bis(imidazol-2-ylid-
ene)formamidinium ions. Interestingly, in
these cations direct C–N bond cleavage
leading to loss of alkyl radicals can com-
pete with rearrangements leading to closed-
shell fragments.
S. M. Polyakova, R. A. Kunetskiy,
D. Schröder* ................................... 1–12
Stabilities of Immonium Ions Derived from
N-Heterocyclic Carbenes Probed by Colli-
sion-Induced Dissociation Mass Spec-
trometry
Keywords: Carbenes / Cations / Collision-
induced dissociation / Density functional
calculations
/ Electrospray ionization /
Lipophilic cations / Mass spectrometry /
Phase-transfer catalysis
12
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Eur. J. Org. Chem. 0000, 0–0