Unexpected formation of [M]2+ from [M+CuCl+H]2+ ions under CID conditions, where M is a molecule of 3,5-bis(2,2’-bipyridin-4-ylethynyl)benzoic acid or its methyl ester

Abstract [M+CuCl+H]2+ ions were generated using electrospray ionization (ESI); where M is a molecule of 3,5-bis(2,2’-bipyridin-4-ylethynyl)benzoic acid or its methyl ester (1 and 2, respectively). The ions were subjected to CID-MS/MS analysis. It was found that their gas phase decomposition lead to the formation of rare di-cations [M]2+, namely [1]2+ and [2]2+ ions. The formation of [1]2+ ion from [3+H+CuCl]2+ ion in the second fragmentation, where 3 is ethyl ester of 3,5-bis(2,2-bipyridin-4-ylethynyl)benzoic acid, was also observed since in the first fragmentation step the loss of ethylene molecule from [3+H+CuCl]2+ ion took place. To the best of our knowledge, it is the first time that [M]2+ ions formation from respective metal complexes has been reported. It is also unusual that formation of [M]2+ ions is not accompanied by formation of [M]+∙ ions. Furthermore, as expected, theoretical calculation and electron ionization mass spectra show that 1 and 2 are not especially prone to form [M]2+ ions. Thus formation of [M]2+ ions under CID conditions is very surprising. Graphical Abstract

However, to the best of our knowledge, there are no examples of [M] 2+ ions formation from respective metal-M complexes. In this paper, we report the formation of [M] 2+ ions from [M+H+CuCl] 2+ ions, upon gas phase decompositions of the latter, where M is 3,5-bis(2,2bipyridin-4-ylethynyl)benzoic acid or its methyl ester, results obtained for ethyl ester are also included (1, 2 and 3, Fig. 1).
The sample solutions were infused into the ESI source by a syringe pump at a flow rate of 5 µl min -1 . The electrospray voltage was 2.7 kV and the cone voltage -30 or 50 V. Cone voltage has the most profound effect on the full scan mass spectra obtained. An increase in this parameter leads to the so called, "in-source" fragmentation/dissociation, but a too low cone voltage could cause a decrease in sensitivity. At higher cone voltage of 50 V, [M+H+Cu] 2+ ions were generated and then their CID-MS/MS spectra were obtained. The source temperature was 80°C and the desolvation temperature was 250°C. Nitrogen was used as the cone gas and desolvating gas at the flow-rates of 50 and 800 l h -1 , respectively. Argon was used as a collision gas at the flow-rate of 0.5 ml min -1 in the collision cell. The applied collision energy (CE, laboratory frame), the most important parameter for CID-MS/MS experiments, is indicated in each CID-MS/MS spectrum shown.
Accurate mass measurements of [1]  Electron ionization mass spectra (70 eV) were recorded on a Bruker 320-MS triple quadrupole mass spectrometer; the compounds were introduced by using a direct inlet probe.
Calculation of ionization potentials are as follows: Structures of the studied compounds neutral, bearing +1 and +2 charge were optimized with the latest functional from Head-Gordon and coworkers, which included empirical dispersion (wb97xd) [23] and 6-31++G(d,p) basis set, as wb97xd functional was indicated in recent studies "to be just right for conjugated systems at all chain lengths" [24]. In the excited states, doublet state of +1 cations, and both singlet and triplet states of +2 cations were considered. Ionization potential -as defined -is the amount of energy required to remove an electron from molecule. The suitable adiabatic ionization potentials were calculated as the difference between the energies of the appropriate cationic forms and neutral ones at their optimized geometries [25]. The vertical ionization potentials were calculated as the difference between the energies of the appropriate cation forms and neutral ones at optimized neutral geometries [25]. The calculations of all the structures were performed with Gaussian09 [26].
As expected, by adjusting M and CuCl 2 concentrations, it is possible to generate ions composed of M, proton and copper, namely [M+H+CuCl] 2+ ions. Formation of these ions is not especially interesting. On the other hand, we found that upon gas phase decompositions of these ions, unexpectedly, doubly charged [M] 2+ ions are formed. Fig. 2 2+ ions by the so-called dissociation "in-source" and then obtain their CID-MS/MS spectra. Fig. 3 shows a CID-MS/MS spectrum of [1+H+Cu] 2+ ion as an example.  Results obtained for 3 (ethyl ester of 1, Fig. 1) were different from the above discussed results obtained for 1 and 2. Namely, the decomposition of [3+H+CuCl] 2+ ion did not yield [3] 2+ ion, but led to the formation of fragment ion at m/z 239 which can be assigned as [1] 2+ ion, as shown in Fig. 5.  In the first fragmentation step, [3+H+CuCl] 2+ ion loses an ethylene molecule producing fragment ion at m/z 288.5 (ion can be assigned as [1+H+CuCl] 2+ ). In the second fragmentation step, the [1] 2+ ion is formed from the ion at m/z 288.5. The loss of the ethylene molecule occurs through the six-membered ring, as shown in Scheme 1, and can be regarded as the so-called McLafferty rearrangement [29].
It is interesting to check if, under EI conditions, the [M] 2+ ions can be formed from compounds 1-3. EI conditions are regarded as the so-called "hard" conditions, since electron beam has the energy of 70 eV (standard value). Fig. 6 shows EI mass spectra of 1-3.
As shown in Fig. 6