Determination of intrinsic binding modes by mass spectrometry: Gas-phase behavior of adamantylated bisimidazolium guests complexed to cucurbiturils

Article abstract of DOI:10.1002/chem.201201444

Adamantylated bisimidazolium cations exhibit a distinct fragmentation pathway in contrast to their cucurbit[7]uril (CB7) complexes (see scheme). The observed alternative fragmentation of the guest molecule in a complex clearly correlates to the supposed s

Full text of DOI:10.1002/chem.201201444

COMMUNICATION
DOI: 10.1002/chem.201201444
Determination of Intrinsic Binding Modes by Mass Spectrometry:
Gas-Phase Behavior of Adamantylated Bisimidazolium Guests
Complexed to Cucurbiturils
[a, b]
Petra Brannꢀ,[a] Michal Rouchal,[a] Petr Kulhꢀnek,[c]
ˇ
Jarmila Cernochovꢀ,
Ivo Kurˇitka,[b] and Robert Vꢁcha*[a]
The chemistry of cucurbit[n]urils (CBs) inclusion com-
plexes with various ligands has been extensively studied in
the condensed phase.[1] The joint development of mass spec-
trometry (MS) techniques has provided insight into the in-
trinsic binding behavior of these host–guest systems readily
available in the gas phase.[2] These MS data have been used
to examine self-sorting pseudorotaxane aggregates[3] or dis-
tinguish a pseudorotaxane arrangement for the studied com-
plexes. Depending on the initial concentrations, the forma-
tion of a 1:1 or 2:1 aggregates of butane-1,4-diamine and
CB6 have been observed in the gas phase. Whereas the 1:1
complex adopted a pseudorotaxane arrangement that result-
ed in loss of guest and simultaneous extensive fragmentation
of the CB6 cage under collision-induced dissociation (CID)
conditions, the 2:1 portal-bound aggregate solely underwent
facile loss of one guest.[4] Correspondingly, the aggregates of
CB6 with phenylenediamine isomers have been examined to
show that o- and m-isomers preferentially bound on the ex-
terior, whereas p-isomer was buried in the CB6 cavity, in a
pseudorotaxane fashion.[5] Additionally, the suppression of
fragmentation channels of the ethylenediamine Ni complex
after its inclusion into the CB8 cavity has been observed
under CID conditions.[6] Furthermore, the CID was used to
distinguish rotaxane with phenolic axel and tetralactam
macrocyclic wheel, in which the axel centerpiece was
cleaved, from nonspecific unthreaded aggregate which de-
composed to the wheel and axel.[7]
In some cases of multiply charged aggregates, the CID led
to the electrostatic repulsion-driven cleavage of an axel with
retention of pseudorotaxane arrangement of host and guest
residue as it has been demonstrated for the supramolecular
aggregates of molecular tweezers containing dendritic violo-
gen dications[8a,b] or crown-ether/ammonium rotaxanes.[8c]
The selective binding of CB6 to lysine residues and the sub-
sequent removal of a lysine fragment-CB6 complex was re-
cently introduced as a probe to determine the structure of
small proteins.[9]
Formation of inclusion complexes of various adamantane
derivatives with CBs in solution has been well documented.
Almost ideally sphere-shaped adamantane cage perfectly
fits the hydrophobic interior cavity of CB7 to form highly
stable complexes. Typical values of binding constants (e.g.,
1.7·1014 mÀ1 for 1-adamantylammonium with CB7 or
5·1015 mÀ1 for dication of N-(2-aminoethyl)-1-adamantyla-
mine with CB7) allow such adamantane derivatives to be
classed as ultrahigh affinity guests for CB7 along with
bicycloACHTUNGTRNEUNG
[2.2.2]octane and ferrocene derivatives.[10] As the CBs
cavity is a magnetic shielding region, the considerable up-
field shifts of resonances for the adamantane protons indi-
cate the preferable positioning of the adamantane cage
inside the cavity.[1c,11] On the other hand, CB8 with larger
cavity binds adamantane derivatives with reduced affinity (a
typical binding constant for CB8 with 1-adamantylamonium
is 8.2·108 mÀ1) and the lower homologue CB6 does not in-
clude adamantane cage.[1c] Nevertheless, this specificity
allows to design an interesting self-sorting multi-component
systems[11] or utilize the adamantane derivatives as competi-
tive controllers of aggregation processes.[12]
ˇ
[a] J. Cernochovꢀ, P. Brannꢀ, Dr. M. Rouchal, Dr. R. Vꢁcha
Department of Chemistry, Faculty of Technology
Tomas Bata University in Zlꢁn
Imidazolium-based ionic liquids with various length of
alkyl chains demonstrate different stoichiometry (1:1 or 2:1)
with CB6 as well as two distinct binding modes differing in
the imidazolium unit penetration depth.[13] Bisimidazolium
(BIM) salts with short terminal methyl substituents and p-
xylylene spacer between imidazolium rings form a 1:1 aggre-
gates with CB7 or CB8 in water with binding constants of
about 2–3·106 mÀ1. In these complexes, the aromatic spacer
occupies the interior of CB cavity and charged imidazolium
rings cap both CB portals.[14] It has been described, that
complexation of a,a’-bis(3-(1-methylimidazolium))-p-xylene
dication inside the CB7 cavity significantly decrease the H/
ˇ
Nꢀmestꢁ T. G. Masaryka 275, 762 72 Zlꢁn (Czech Republic)
Fax : (+420)576-031-560
E-mail: rvicha@ft.utb.cz
ˇ
ˇ
[b] J. Cernochovꢀ, Dr. I. Kuritka
Polymer Centre, Faculty of Technology
Tomas Bata University in Zlꢁn
ˇ
Nꢀmestꢁ T. G. Masaryka 275, 762 72 Zlꢁn (Czech Republic)
[c] Dr. P. Kulhꢀnek
Central European Institute of Technology (CEITEC)
Masaryk University, Kamenice 5, 625 00 Brno (Czech Republic)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201444.
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Figure 1. Positive-ion mode ESI mass spectra (full scan) of compound 2a
in a MeOH/H2O (1:1, v:v) solution. a) First-order mass spectra, b) MS2
of m/z 223. The assignments for the observed signals are shown in brack-
ets. The ion being fragmented in the tandem mass spectra is marked with
a bold, downward arrow.
was observed based on the 79Br and 81Br isotopologue ion
pair at m/z 525 and 527. This association loses a neutral HBr
molecule to produce the sole singly charged N-heterocyclic
carbene[18] observed at m/z 445 (Figure S7d in the Support-
ing Information).
Scheme 1. Structures and dimensions[17] of the bisimidazolium salts and
cucurbiturils used in this work.
À
À
D exchange rate for the N CH N protons due to H-bond-
ing to the CB7 portal carbonyl oxygen atoms.[15] Additional-
ly, the positioning of the CB7 unit on the initially unfavored
dicationic binding site of the BIM 2d (for the structure, see
Scheme 1) was observed as a result of a cooperative supra-
molecular interaction between CB7 and b-cyclodextrin units
in the ternary complex.[16a] Upon the ESI-MS data, the simi-
lar dicationic binding site was suggested to be complexed to
the CB7 due to high initial CB7 concentration.[16b]
We subsequently analyzed 25 mm mixtures of the BIM li-
gands and CBs (Scheme 1) in a 50 mm solution of NaCl in
water. In the spectra of the BIM/CB7 mixtures, we initially
focused on the fragmentation of the doubly charged ions
from the 1:1 complexes [BIM·CB]2+. Figure 2c and d show
that, for compound 1c with CB7, the fragmentation of the
1:1 complex at m/z 930 forms three ions at m/z 549, 1163,
and 1311. Based on the corresponding MS spectra for the in-
dividual BIM and CB7 compounds, we assigned these sig-
nals to the singly charged residue of 1c arising from “149”
fragmentation, the singly charged association of CB7·H+
and the singly charged CB7·AdCH2+, respectively. The ion
at m/z 1311 loses a neutral AdCH fragment and forms
CB7·H+ at m/z 1163 after further fragmentation (MS3).
Therefore, the observed fragmentation for BIM 1c com-
plexed to CB7 is essentially the identical to that of free BIM
(compare Figure S16b in the Supporting Information and
Figure 2d).
Herein, we describe the consecutive fragmentation of CB-
complexed BIM guest dications, that is, cleavage of the co-
À
À
valent C N and C C bonds, while retaining the supramolec-
ular interactions between the charged guest residue and the
CB molecule. In addition to the unprecedented gas-phase
reactivity (to the best of our knowledge), we demonstrate
that analysis of the ESI-MS spectra of host–guest systems
may serve as a tool for estimating the ability of CBs to slip
over the molecular axel.
We first analyzed MeOH/H2O (1:1, v:v) solutions contain-
ing only BIM dibromides (Scheme 1). Figure 1a shows that
five significant ions were observed in the positive-ion mode
ESI-MS spectra for compound 2a and assigned through a
detailed investigation using tandem mass spectrometry and
CID experiments. The fragmentation of the ion at m/z 223
(MS2) created two product ions at m/z 297 and 149 (Fig-
ure 1b). Further fragmentation of m/z 297 (MS3) led to the
parallel neutral loss of imidazole, diazine, or diimidazoylme-
thane to yield the ions at m/z 229, 217, or 149, respectively
(Figure S7c in the Supporting Information). We therefore
assigned the ion at m/z 223 to the doubly charged molecular
BIM ion and the ions at m/z 297 and 149 to its singly charg-
ed fragments that form during the ESI process. The loss of
the singly charged adamantylmethyl cation (m/z 149) will
hereinafter be referred to as “149” fragmentation. In addi-
tion, the supramolecular associate between BIM and BrÀ
In contrast, we obtained completely different results for
the mixture of 2a and CB7 (Figure 2a and b). In the MS2
spectrum for this complex, we observed two ions at m/z 730
and 656 formed from the parent ion [BIM·CB7]2+ at
m/z 804. Subsequent fragmentation of the ion at m/z 730
(MS3) led to the sole formation of the ion at m/z 656. This
may be rationalized as the sequential loss of neutral 1-ada-
mantylcarbene fragment (or its isomer)[19] from both ends of
the BIM scaffold to yield the doubly charged aggregate of
the CB7 and BIM residue. This release of neutral AdCH
fragments (exact mass of m/z 148) will be hereinafter refer-
red to as the “148” fragmentation.
All other examined adamantylated BIMs displayed at
least one (but usually exactly one) of these fragmentation
pathways and no additional significant signals were ob-
served. Little variations in signal intensities can be reasona-
bly attributed to the different axel bulkiness which influen-
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Determination of Intrinsic Binding Modes by Mass Spectrometry
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successively complexed to the CB7 interior cavity to form
the 1:1 or 1:2 aggregates depending on the CB7 concentra-
tion. In contrast, the downfield shift of signals of methylene
or p-xylylene bridges suggested the location of the central
part of BIM close to the CB7 portal. Considering previously
discussed binding constants for similar structure motifs with
CBs, we assume that binding of CB7 at adamantane site is
strongly favored and observation of any other potential ar-
rangements is disabled using NMR. However, this finding
does not contradict the formation of pseudorotaxane com-
plexes with different geometry under treatment in the gas
phase.
We propose that the aggregates evidenced in solution are
transferred to the gas phase using ESI to observe corre-
sponding signals in the first-order mass spectra. We believe
that the CB host cannot reach the opposite end of the guest
skeleton by walking “outside” of the guest molecule without
decomposing the supramolecular complex. Therefore, the
presence of substituents with varying bulk results in three
distinct situations (Figure 3), as indicated via further frag-
Figure 2. Positive-ion mode ESI mass spectra (full scan) of aqueous solu-
tions of 2a·CB7 and 1c·CB7. a) First-order mass spectra of 2a·CB7,
b) MS2 of m/z 804, c) first-order mass spectra of 1c·CB7, d) MS2 of
m/z 930. The assignments for the observed signals are shown in the
brackets. The fragmented ions in the tandem mass spectra are marked
with bold, downward arrows.
Figure 3. Schematic drawing of the slipping modes of the CB unit over
the BIM dication.
ces the activation barriers to the slipping of CB7 between
particular binding sites. Thus, the intensity of the second
“148” fragmentation product signals was rather lowered in
the case of BIM 2b, 1e, or 2e and completely disappeared
in the case of BIM 1a or 1b. Additionally, the CID (MS2) of
doubly charged aggregates of the BIM 1a or 1b with CB7
resulted in products of “149” fragmentation accompanied by
products of one “148” fragmentation. Comparing the inten-
sities of the corresponding signals (Figure S40 and S41 in
the Supporting Information), we may see that the 1a·CB7
complex was predominantly cleaved via the “148” fragmen-
tation in contrast to the 1b·CB7. The complexes of CB7 and
BIM 1e or 2e (Figure S43 and S47 in the Supporting Infor-
mation) displayed fragmentation pattern similar to those ob-
served for the corresponding methylene-bridged BIMs and
the additional hypothetical binding site on the aromatic
bridge did not affect fragmentation pattern.
mentation analyses: 1) The CB is bound to the end of the
BIM molecule and cannot slip over the imidazolium unit.
Only the products of “149” fragmentation are observed.
Typical examples of such BIM salts are 1c or 2c (Figure 2d
and Figure S45 in the Supporting Information). 2) The CB
may slip over the imidazolium unit once bound to the BIM
but cannot easily reach the opposite end of the BIM. In
such cases we observed products from both “149” and “148”
fragmentations. However, the “148” fragmentation occurred
only once. In other words, only one adamantylmethyl sub-
stituent can be released in this manner. Typical examples of
such BIM salts are 1a or 1b (Figures S40 and S41 in the
Supporting Information). 3) The CB can freely slip over the
BIM skeleton, and the products of “148” fragmentation
were predominantly observed. Typical examples of such
BIM salts are 2a, 1e, 2b, or 2e (Figure 2b and Figures S43,
S44, and S47 in the Supporting Information).
1
The H NMR spectra recorded for the mixtures of BIMs
with CB7 in D2O indicated formation of inclusion com-
plexes with pseudorotaxane geometry (Figure S60 in the
Supporting Information). The considerable upfield shift of
all adamantane signals hand in hand with diminishing of sig-
nals of free BIM when the CB7 molar fraction reached of
0.7 evidenced that both terminal adamantane cages were
We next focused on the fragmentation analysis of 2:1 ag-
gregates of CB with BIM. With the exception of compounds
1a and 2a, the signals obtained for the 2:1 aggregates were
not observed for the equimolar aqueous solutions of CB and
BIM in the presence of NaCl. The transfer of the 2:1 aggre-
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R. Vꢁcha et al.
gates from solution to the gas phase was supported by ex-
cluding NaCl from the tested mixture and enhancing the
mole fraction of CB in the solution. The 2:1 aggregates
were typically observed as either doubly charged ions or
their triply charged Na+ associations. A further CID experi-
ment led to the loss of the CB unit to form the doubly
charged 1:1 complex with nearly identical fragmentation
patterns to those previously described (Figures S48–S57 in
the Supporting Information). However, for BIMs 2a and
2b, the fragmentation of the doubly charged 2·CB7·BIM at
m/z 1385 and m/z 1399 yielded an unexpected pair of ions at
m/z 1311 and 1237 and m/z 1325 and 1251, respectively (Fig-
ures S53 and S54 in the Supporting Information). We as-
signed these ions to the doubly charged products of the se-
quential “148” fragmentation complexed to two CB7 units.
The 2:1 aggregates of two CB7 and doubly protonated dii-
midazoylmethane with signals at m/z 1237 and 1251 further
fragmented (MS4) to yield signals at m/z 1311 and 1339, re-
spectively. In both cases, these ions were accompanied with
a signal at m/z 1163. We interpreted this as an electrostatic
repulsion-driven cleavage of the 2:1 complex to form a
[CB7·H]+ ion at m/z 1163 and the singly charged aggregates
of the CB7 and corresponding singly protonated diimida-
zoylmethane at m/z 1311 and 1339, respectively.
Finally, we would like to note the unusual forms observed
of the CBs. In some cases, the complex between the CB7
and the charged BIM fragments decompose to neutral BIM
residues and a CB ion at m/z 1161. Similarly, the fragment
at m/z 1079 was observed when mixtures containing
Me6CB6 and a suitable ligand were analyzed. Figure 4
shows the obtained tandem mass spectrum for the mixture
of CB7 with 1a. The ion at m/z 1559 was obtained through
sequential MS2 experiments beginning with the ion at
m/z 855 (doubly charged CB7·BIM), which produced the
minor ion at m/z 1559 under CID conditions (Figure S40b
in the Supporting Information). We assigned this ion to
+
+
[CB7+BIM-AdCH2
]
and a further fragmentation (Fig-
ure S40c in the Supporting Information) led to the loss of a
neutral dibenzimidazoylmethane unit to yield [CB7+
+
+
AdCH2
]
at m/z 1311. The parallel fragmentation of
m/z 1559 led to the loss of neutral adamantylmethylbenzimi-
dazole and the ion at m/z 1293. Subsequent fragmentation
yielded the ion at m/z 1161 (Figure S40d in the Supporting
Information). The suggested pathway was supported by the
further isolation and fragmentation analyses of the ions at
m/z 1311 and 1293. Using these data, we were able to assign
the ions at m/z 1293 and 1161 to the singly charged complex
of CB7 and benzimidazolylmethyl and the singly charged
CB7 without a single hydrogen atom, respectively. We can
speculate that latter ion is possibly formed by a hydride
transfer from the neutral CB7 molecule to the benzimidazo-
lylmethyl cation. To support this idea, the structure and for-
mation of the [CB7-HÀ]+ cation was further studied using a
computational chemistry framework (for details and analy-
sis, see the Supporting information). Density functional cal-
culations indicated that the reaction depicted in Figure 4 is
favorable towards the products by À160 kJmolÀ1. It can be
expected that the transfer probably occurs within the
CB7·benzimidazolylmethyl cation complex. Indeed, its pre-
dicted structure shows reasonable mutual orientation of
both donor and acceptor atoms of hydride ion with atom-to-
atom distance of 3.5 ꢃ.
To examine the scope of the described phenomenon, we
analyzed mixtures of the benzylated BIMs 1d and 2d with
CB7 and Me6CB6. Typically, in the MS spectrum of 2d we
observed ions at m/z 409/411, 329, 239, and 91, which we as-
signed to [BIM+BrÀ]+, [BIM-H+]+, [BIM-Bn+]+ and Bn+
(Figure S25 in the Supporting Information), respectively.
Under the same experimental conditions, we observed two
additional signals at m/z 746 and 505 when analyzing the
equimolar mixture of 2d and CB7. Upon further fragmenta-
tion analysis, we assigned these signals to the [CB7·BIM]2+
and [CB7·BIM·Na]3+ aggregates, respectively. The former
ion decomposed into the fragments observed at m/z 253,
707, 1161, and 1401 (Figure S46b in the Supporting Informa-
tion). The assignment of the ion at m/z 1161 is discussed
below and the rest were assigned to [BIM-77+]+, [CB7+
BIM-78]2+, and [CB7+BIM-Bn+]+, respectively. As can be
clearly observed, the presence of CB7 promoted alternative
fragmentation pathways involving the loss of a neutral frag-
ment. We can conclude that CB7 clearly changed the frag-
mentation mechanism and supported the charge retention
on the axel residue during the fragmentation process. With a
view to extension of our work to other CB homologue, we
analyzed mixtures of BIM salts 1d and 2d with Me6CB6, a
water-soluble analogue of CB6. Signals for [BIM+
Me6CB6]2+ were clearly observed and fragmented (Figur-
es S58 and S59 in the Supporting Information) to yield ions
In conclusion, we have demonstrated that CBs may dra-
matically change the fragmentation pathways of axel mole-
assigned
to
[BIM-Bn+]+,
[Me6CB6+BIM-Bn+]+,
[Me6CB6+Bn+]+ at m/z 1171 and the ion at m/z 1079 (see
below). These results indicate that Me6CB6 can form supra-
molecular complexes with benzylated BIM but cannot slip
over the BIM skeleton to provide the alternative fragmenta-
tions contrary to CB7, which has a larger internal cavity di-
ameter.
Figure 4. Tandem mass spectra (MS3) of m/z 1559 (top) and a schematic
representation of the final step in the proposed pathway toward m/z 1161
product ion (bottom).
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Determination of Intrinsic Binding Modes by Mass Spectrometry
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tions decompose to two singly charged fragments in accord-
ance with electrostatic repulsion, some complexed dications
release neutral fragments to form doubly charged complex
of CB and axel residue. Furthermore, latter fragmentation
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ˇ
ˇ
We would like to thank to Dr. Vladimꢁr Sindelꢀr (Department of Chem-
istry, Faculty of Science, Masaryk University, Brno, Czech Republic) for
the kind gift of the CBs used in this study. This work was financially sup-
ported by the Internal Founding Agency of Tomas Bata University in
Zlꢁn, project No. IGA/FT/2012/016 and project CZ.1.05/1.1.00/02.0068 fi-
nanced by the European Regional Development Fund at the CEITEC -
Central European Institute of Technology. Access to the MetaCentrum
computing facilities was provided under the program “Projects of Large
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Keywords: adamantane · cucurbituril · host–guest systems ·
imidazolium salts · supramolecular chemistry
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Received: April 26, 2012
Revised: August 3, 2012
Published online: && &&, 2012
Chem. Eur. J. 2012, 00, 0 – 0
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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These are not the final page numbers!

R. Vꢁcha et al.
Adamantylated bisimidazolium cations
exhibit a distinct fragmentation path-
way in contrast to their cucurbit[7]uril
(CB7) complexes (see scheme). The
observed alternative fragmentation of
the guest molecule in a complex
clearly correlates to the supposed steri-
cally hindered or allowed slippage of
the macrocycle over the axel molecule.
Gas-Phase Reactions
ˇ
J. Cernochovꢀ, P. Brannꢀ, M. Rouchal,
ˇ
P. Kulhꢀnek, I. Kuritka,
R. Vꢁcha* ...................... &&&& — &&&&
Determination of Intrinsic Binding
Modes by Mass Spectrometry: Gas-
Phase Behavior of Adamantylated
Bisimidazolium Guests Complexed to
Cucurbiturils
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www.chemeurj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!