Abstract
A protocol is developed for the coordination of the formate anion (HCOO(-)) to neutral metalloporphyrins (Pors) and -phthalocyanines (Pcs) containing divalent metals as a means to improve their ion formation in electrospray ionization (ESI). This method is particularly useful when the oxidation of the neutral metallomacrocycle fails. While focusing on Zn(II)Pors and Zn(II)Pcs, we show that formate is also readily attached to Mn(II), Mg(II) and Co(II)Pcs. However, for the Co(II)Pc secondary reactions can be observed. Upon collision-induced dissociation (CID), Zn(II)Por/Pc·formate supramolecular complexes can undergo the loss of CO2 in combination with transfer of a hydride anion (H(-)) to the zinc metal center. Further dissociation leads to electron transfer and hydrogen atom loss, generating a route to the radical anion of the Zn(II)Por/Pc without the need for electrochemical reduction, although the Zn(II)Por/Pc may have a too low electron affinity to allow electron transfer directly from the formate anion. In addition to single Por molecules, multi Por arrays were successfully analyzed by this method. In this case, multiple addition of formate occurs, giving rise to multiply charged species. In these multi Por arrays, complexation of the formate anion occurs by two surrounding Por units (sandwich). Therefore, the maximum attainment of formate anions in these arrays corresponds to the number of such sandwich complexes rather than the number of porphyrin moieties. The same bonding motif leads to dimers of the composition [(Zn(II)Por/Pc)2·HCOO](-). In these, the formate anion can act as a structural probe, allowing the distinction of isomeric ions with the formate bridging two macrocycles or being attached to a dimer of directly connected macrocycles.
Highlights
The characterization of new functional Por-related materials is closely related to the development of modern mass spectrometry
Since Pors are efficient chromophores, which strongly absorb in the UV region, they have even been analyzed by direct laser desorption/ionization (LDI) without the need for a matrix.[15,16,17]
The formate adduct anion 9·HCOO− shows an under collision-induced dissociation (CID) loss of CO2 accompanied by H− transfer to Zn(II)Pc 9, resulting in the species 9·H− at m/z 873.2 (Fig. 2c)
Summary
The characterization of new functional Por-related materials is closely related to the development of modern mass spectrometry. The EA of Zn(II)Pc 9 is unknown, but we assume that it is similar to the EA values established for other divalent M(II)Pors (EA = 1.5–2.1 eV).[47,48,49] The same scenario is observed for the addition of common monoatomic anions like Cl− (EA = 3.6 eV), Br− (EA = 3.4 eV) and I− (EA = 3.0 eV).[50] In this case, CID would result in the loss of the charge carrier without any observable analyte ions.
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