Reduction of cationic free-base meso-tris-N-methylpyridinium-4-yl porphyrins in positive mode electrospray ionization mass spectrometry

Abstract

Reductions involving more than one electron with formation of the M ϩ and [Mϩ2H] ϩ ions were observed for electrosprayed meso-tris (N-methylpyridinium-4-yl)porphyrin iodides, MI 3 . These reductions were studied by using different solvents and flow rates. Formation of the [Mϩ2H] ϩ ions occurred only for protic solvents and to a larger extent at lower flow rates. The type of the fourth substituent does not seem to affect the reduction processes. , it is generally accepted that redox reactions do occur in the capillary sprayers and that they are responsible for charge balance [1,2]. Redox processes in electrospray mass spectrometry were comprehensively investigated by and, more recently, electrochemical aspects and applications of electrospray ionization were the focus of reviews [9,10].Ions formed by one electron reduction from nonmetallated dicationic species in solution were previously observed in positive-mode electrospray mass spectrometry for ionic compounds such as diquaternary pyridinium salts [11-13] and free-base N-alkylpyridiniumyl porphyrin salts [14]. One-and two-electron reductions of the cations of metallated N-alkylpyridiniumyl porphyrin salts were also reported, when electrospray was used in the positive mode [15][16][17].When acquiring the electrospray mass spectra of 5,10,15-tris(N-methyl-pyridinium-4-yl)-20-phenylporphyrin iodide, MI 3 , we observed, besides the expected M 3ϩ ion, ions such as M 2ϩ , M ϩ , and [Mϩ2H] ϩ , which may be the result of reductive processes. Reductions involving more than one electron were never reported before for electrosprayed free-base cationic porphyrins and, although formation of [MϩnH] ϩ ions (n Ͼ 1) in desorption ionization (fast atom or ion bombardment) [18 -20] and matrix assisted laser desorption (MALDI) [21] has been described, it was never reported as occurring in electrospray mass spectrometry (ESI-MS) until now. These facts prompted us to use ESI-MS to investigate a group of meso-tris(N-methylpyridinium-4-yl)porphyrin iodides, MI 3 , with a different fourth meso substituent (see Scheme 1) to gather more information on the species formed with respect to their formation reactions and to their relation with the ionization processes occurring in the electrospray interface. ExperimentalThe key neutral porphyrin derivatives were obtained from crossed-Rothemund reactions using the appropriate benzaldehydes and pyrrole in refluxing acetic acid and nitrobenzene [22][23][24]. The alkylation with methyl

Reduction and adduct formation from electrosprayed solutions of porphyrin salts

“…Contrary to aliphatic ammonium hypervalent radicals that are transient species,19 aromatic hypervalent radicals are stabilized by partial charge delocalization throughout the macrocycle and are relatively stable. Although similar one‐electron reductions have been reported for aromatic quaternary ammonium salts, since 1980,12 by using several ionization techniques,14, 16, 19, 22, 23 multielectron reductions are less common 11. In the present case, the formation of the reduced species is not dependent on the structure of the counter ions, as reduction occurs for TMPyP with both the I − and p ‐CH 3 C 6 H 4 SO 3 − counter ions.…”

“…Reduction of multiply‐charged free base porphyrins, with formation of M + and [M + 2H] + ions, when using ESI‐MS in the positive ion mode, was first observed by us for the iodide salts of meso ‐tris( N ‐methylpyridinium‐4‐yl)porphyrins, MI 3 11. We suggested that these new species could be hypervalent radicals formed by multielectron reduction or protonation plus multielectron reduction of the M 3+ porphyrin cations, through the participation of counter ion/solvent adducts 11…”

Imidazole and imidazolium porphyrins: gas‐phase chemistry of multicharged ions

“…As expected and similar to ( N ‐alkylpyridinium‐4‐yl)porphyrins, for compounds 1 to 5 , the most abundant product ions of the M 3+ precursors are formed by loss of one methyl radical, [M − 15] 3+ , which confirms their predominant hypervalent mono‐radical character. Other less abundant product ions are present, namely, the [M − 31] 3+ , [M − 42] 3+ , [M − 56] 3+ , and [M − 96] 3+ ions.…”

“…In contrast, the formation of the [M − 145] 2+ ion may be ascribed to a loss of the DCP substituent by scission of the C–C meso ‐bond, which is similar to the losses observed for other porphyrins containing meso ‐substituents with six‐membered rings . Thus, the [M − 227] 2+ = [M − 145 − 27 − 55] 2+ ion may be formed by loss of the DCP substituent and by losses of 27 and 55 Da, from the protonated and from one of the neutral substituents, respectively.…”

“…We have shown that reduction processes occur, when applying ESI‐MS, in the positive ion mode, to a group of free‐base meso ‐tetrakis( N ‐alkylpyridinium‐4‐yl)porphyrins . Formation of hypervalent radical species such as the M 3+ , M 2+ , M + , and [M + 2H] + ions was observed as a result of multielectron reduction.…”