Novel charged [14]azolophanes: associative behaviour revealed by electrospray ionization

Abstract

Electrospray ionization mass spectrometry (ESI-MS) is a powerful analytical technique 1 which is also applied in supramolecular chemistry.2,3 For the characteriziation of a series of tetracationic catenanes, at both the molecular and supramolecular level, ESI-MS is better than other softionization mass spectrometric techniques (e.g., fast atom bombardment mass spectrometry (FAB-MS)).2 Developing the chemistry of heterocyclic betaines, we have reported 4,5 the first examples of multiply charged [1 4 ]heterophanes built up from imidazolium methylene s-triazolate subunits: the bis-betaines 1 and betaines 2ÁPF 6 . They have been characterized on the basis of 1 H and 13 C NMR spectroscopic data 4 and single-crystal X-ray diffraction analysis of 1a and 2aÁPF 6 .5 Herein, we report how the sensitivity of the electrospray ionization technique can be exploited to explore the behaviour of bis-betaines 1 and betaines 2ÁPF 6 and their immediate dicationic precursors 3Á2X in the gas phase. EXPERIMENTALPositive ion ESI-MS was performed using a VG-Quattro mass spectrometer (Micromass Instruments, Manchester, UK). The electrospray interface (pneumatically assisted) was operated under the following conditions: the nebulizing nitrogen gas flow was 10 L h À1 and the drying nitrogen 450 L h À1 , source temperature 80°C with a capillary voltage of 3.5 kV and the cone voltage was varied between 50 and 100 V depending on the experiment. The flow rate was 7 mL min À1 . Samples were dissolved in a 1:1 mixture of H 2 O/ CH 3 CN at a concentration of 250 pmol mL À1 . Negative ion ESI-MS experiments were conducted under the same conditions as positive ion ESI-MS experiments.Analyses by FAB-MS were recorded on the VG-Quattro spectrometer using a cesium atom beam operating at 10 kV. The source temperature was 25°C and the matrix used was 3-nitrobenzyl alcohol. RESULTS AND DISCUSSIONAll six macrocycles gave clean positive ion ESI mass spectra at a cone voltage of 50V; the same major peaks were formed and the base peak in each case corresponds to the doubly charged ion [MH 2 ] 2 , irrespective of the structural pattern of the p-rich subunits s-triazolate and/or s-triazole. Structure 1.

Selection of Betaine Building Blocks for the Construction of Quadrupolar Heterophane Frameworks

“…On the other hand, we have exploited the sensitivity of the electrospray ionization technique to examine the gas-phase behavior of multicharged [1 4 ]metaheterophanes containing either proton-ionizable 1,2,4-triazole or betaine units, the dications (MH 2 ·2X) and bis(betaines) (M) shown in Figure 1. [14] For macrocyclic bis(betaines) (M) and dicationic [1 4 ]metaheterophanes (MH 2 ·2X) (see Figure 1), the positive-ion ESI-MS data at 50 V showed the same major peaks, the base peak in both cases corresponding to the doubly charged ions [MH 2 ] 2ϩ . At higher cone voltage the base peak was the ion [M ϩ H] ϩ .…”

“…ESI responses were not observed in the negative-ion mode experiments, with the exception of MH 2 ·2PF 6 , which displayed a peak of a PF 6 Ϫ anion at m/z ϭ 145. [14] Here we utilized the sensitivity of electrospray ionization mass spectrometry to examine the betaine building blocks 1a, 2c, 4d, 5b, 5e, 6b, and 6e (B), together with their cationic counterparts 7a·Cl, 7a·PF 6 , 8c·Cl, 10d·Cl, 11b·Cl, 11e·Cl, 12b·Cl, and 12e·Cl (BH·X). Experiments were carried out in both the positive-and negative-ion modes and all fifteen neutral analytes underwent proton-mediated ion-molecule reactions at low cone voltage (Vc ϭ 55 V) while molecular fragmentation produced the most abundant species at 120 V (Tables 3 and 4, see Supporting Information).…”

Imidazolium-Based Dicationic Cyclophanes. Solid-State Aggregates with Unconventional (C–H)+···Cl− Hydrogen Bonding Revealed by X-ray Diffraction

“…The negative-ion ESI-MS experiments were first performed on a VG-Quattro mass spectrometer from Micromass Instruments, as previously described [7,13]. Then, negative-ion ESI-MS analyses were recorded on a ZQ mass spectrometer from Waters- Single-crystal X-ray diffraction analysis.…”

“…Positive-ion electrospray ionization mass spectrometry was used for characterizing imidazoliun-based frameworks, with prime examples derived from [1 4 ]metaheterophanes 1·2X (MH 2 ·2X), 2 and 4.2X (M·2X) [6b,9,13,14]. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 In the negative-ion mode ESI-MS experiments, we examined azolophanes 1·2X [13] together with a set of [{imidazolio}methyl] azolate betaines B and their corresponding salts BH·X [7]. The negative-ion ESI responses displayed peaks attributable to non-covalent self-aggregates: further study to confirm this was restricted to instrumental limitation.…”

“…The negative-ion ESI responses displayed peaks attributable to non-covalent self-aggregates: further study to confirm this was restricted to instrumental limitation. Under the same experimental conditions [7,13] [4,14,15a]. Thereafter, a variety of quaternary imidazolium-linked frameworks were reported for anion recognition driven by ( For the X-ray diffraction analysis of 4b·2Cl·3.5H 2 O·0.5CH 3 CN, many different crystals were used but crystals deteriorated so fast that, in consequence, the final refinement values were inappropriate for a detailed discussion of non-covalent interactions.…”