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

Kaja Schubert,Christine Bülow,J Tobias Lau,Vicente Zamudio-Bayer,Lucas Schwob,Xin Wang,Simon Dörner,Martin Timm,Sadia Bari,Meiyuan Guo,Stephan Klumpp,Piter S Miedema,Bernd Von Issendorff,Simone Techert,Thomas Schlathölter,Kaan Atak

doi:10.1039/d0sc06591a

Abstract

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.

Highlights

Metalloporphyrins are very common organometallic molecules that play key roles in nature: heme is the prosthetic group in hemoglobin and is responsible for oxygen transport,[1] chlorophyll A is involved in the photosynthesis of plants[2] and cobalt protoporphyrin IX is an inducer of hemeoxygenase-1, an enzyme that catalyses the heme catabolism.[3]
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 s* molecular orbitals lead to distinct deexcitation pathways
In the energy region at the cobalt L-edge studied here (770–804 eV), we observe the strongest total ion yield (TIY) at 780.5 eV and we discuss the photofragmentation based on the mass spectrum at this energy

Summary

Introduction

Metalloporphyrins are very common organometallic molecules that play key roles in nature: heme (iron protoporphyrin IX) is the prosthetic group in hemoglobin and is responsible for oxygen transport,[1] chlorophyll A (a magnesium-containing porphyrin) is involved in the photosynthesis of plants[2] and cobalt protoporphyrin IX is an inducer of hemeoxygenase-1, an enzyme that catalyses the heme catabolism.[3]. Metalloporphyrins consist of a metal ion coordinated by four nitrogen atoms of a porphyrin macrocycle (Fig. 1). They show a strong absorption in the UV/vis, caused by resonant p–p* transitions in the porphyrin macrocycle, namely the Soret and Q bands.[12] The electronic structure of metalloporphyrins is crucial for their functionality and is determined by e.g. the axial ligands.

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Conclusion