The electronic structure and deexcitation pathways of an isolated metalloporphyrin ion resolved by metal L-edge spectroscopy

The local electronic structure of the metal-active site and the deexcitation pathways of metalloporphyrins are crucial for numerous applications but difficult to access by commonly employed techniques. Here, we applied near-edge X-ray absorption mass spectrometry and quantum-mechanical restricted active space calculations to investigate the electronic structure of the metal-active site of the isolated cobalt(iii) protoporphyrin IX cation (CoPPIX+) and its deexcitation pathways upon resonant absorption at the cobalt L-edge. The experiments were carried out in the gas phase, thus allowing for control over the chemical state and molecular environment of the metalloporphyrin. The obtained mass spectra reveal that resonant excitations of CoPPIX+ at the cobalt L3-edge lead predominantly to the formation of the intact radical dication and doubly charged fragments through losses of charged and neutral side chains from the macrocycle. The comparison between experiment and theory shows that CoPPIX+ is in a 3A2g triplet ground state and that competing excitations to metal-centred non-bonding and antibonding σ* molecular orbitals lead to distinct deexcitation pathways.


Experimental Part
The mass calibration was obtained from experiments carried out at the VUV U125-2_NIM beamline (6 -40 eV) of the BESSY II synchrotron (HZB, Berlin) using the home-built tandem mass spectrometer of the Groningen University group. 1 Similar to the Nanocluster trap end station, cobalt(III) protoporphyrin cations (CoPPIX + ) were produced with an electrospray ionisation source, mass-to charge selected and thermalised with helium buffer gas in a Paul trap at room temperature. The ions were accumulated and subsequently irradiated in the ion trap with the VUV photons. Cationic fragments were analysed using a time-of-flight spectrometer in reflectron mode. In contrast to the Nanocluster trap end station, the tandem-mass-spectrometer setup allowed CoPPIX + precursor ions (m/z 619.6) and cationic products at m/z ⪆ 80 to be detected at the same time and with a spectrometer resolution of m/∆m = 2000. With this spectrometer configuration, isotopic patterns of singly and doubly charged precursors ions and fragments were resolved. The mass calibration was based on the well-known photofragmentation mass spectrum of leucine enkephalin, including the parent ion peak (m/z 556.3), the b 4 fragment (m/z 425.2) and the phenylalanine and tyrosine immonium ions (m/z 120.1 and 136.1, respectively). 1,2 VUV photoabsorption of CoPPIX + (data not shown) leads to photoionisation and to formation of a number of photofragments similar to those observed upon cobalt L-edge photoabsorption of CoPPIX + . Therefore, based on this calibration, the oxidation state and photofragments of CoPPIX + were assigned (see main text) and a mass calibration for the Nanocluster trap end station was obtained.  Figure ESI-2 shows the mass spectrum of CoPPIX + ions obtained after resonant photoexcitation at the cobalt L 3 -edge. In contrast to the Nanocluster trap end station (see main text) and as explained earlier, the tandem mass spectrometer enables the trapping and detection of ions over a broad mass range covering the region where singly, doubly and triply charged fragment peaks are expected (m/z < 610, m/z < 310 and m/z < 206, respectively). Due to the data treatment the peak corresponding to the precursor ions at m/z 620 appears negative reflecting the depletion of precursor ions upon irradiation. The present data show no evidence for the production of singly charged fragments and we thus assume that singly charged fragments are not produced during the experiments performed at the Nanocluster trap end station.
The fragmentation pattern in Figure ESI-2 is similar as in Figure 2, although the mass resolution in Figure 2 allows a better differentiation of the fragment peaks (see Experimental section). The CoPPIX 2+ signal is about twice as high than the signal of fragments with 200 < m/z < 250 in Figure 2 compared to Figure ESI-2. One reason for the difference can be the trap and the spectrometer settings for the Nanocluster trap end station, which were set for a higher sensitivity to the high m/z range. Another reason can be the difference in temperature in both experiments. Data in Figure ESI-2 were collected at room temperature, whereas data in Figure 2 were collected at 18K i.e. with lower internal energy. Additionally, for the Nanocluster trap the helium buffer gas is constantly present during the irradiation such that irradiated precursor ions may cool down excess energy by collision with the helium which could prevent some fragmentation. This was not the case for the experiments presented in Figure ESI-2 where the helium injection in the trap is pulsed synchronously to the ion injection and pumped down before irradiation.

m/z
Losses of CoPPIX 2+/3+ 300.6 -w 291.6 -2w  Figure 4).   Figure 5). b-p. Measured PIY spectra for several product ions formed after inner-shell excitation of cobalt 2p electrons of CoPPIX + in the photon energy range 770-804 eV. The PIYs of these photofragments are partly affected by contributions resulting from overlapping peaks in the mass spectrum, multiple possible fragments of same m/z (see Table  ESI-1) and isotopic contributions from neighboring fragment peaks. For better comparison the PIY spectra are normalised to the maximum of peak C.

Computational Part
Figure ESI-5 Calculated structure of CoPPIX + after geometry optimisation using density functional theory (DFT) with the B3LYP functional and the 6-311g* basis set using  Figure ESI-7 Schematic representations of selected low-lying states in CoPPIX + which were used for X-ray spectra simulations. The relative energy is shown by taking lowest energy state 3 A 2g as reference.

Figure ESI-8
The dependence on the number of core excited states for each spin state for the 3 A 2g ground state.

Figure ESI-9
The XAS calculation with different initial states having different electronic configuration as presented in the Figure ESI-

Figure ESI-10
Comparison between the experimental TIY of CoPPIX + and calculated XAS spectra at the cobalt L-edge including the core-excited states contribution analysis.