Structures of Metalloporphyrin–Oligomer Multianions: Cofacial versus Coplanar Motifs as Resolved by Ion Mobility Spectrometry

Abstract

ABSTRACT:We have combined ion mobility mass spectrometry with quantum chemical calculations to investigate the gasphase structures of multiply negatively charged oligomers of mesotetra(4-sulfonatophenyl)metalloporphyrins comprising the divalent metal centers Zn II , Cu II , and Pd II . Sets of candidate structures were obtained by geometry optimizations based on calculations at both the semiempirical PM7 and density functional theory (DFT) levels. The corresponding theoretical cross sections were calculated with the projection approximation and also with the trajectory method. By comparing these collision cross sections with the respective experimental values we were able to assign oligomer structures up to the tetramer. In most cases the cross sections of the lowest energy isomers predicted by theory were found to agree with the measurements to within the experimental uncertainty (2%). Specifically, we find that for a given oligomer size the structures are independent of the metal center but depend strongly on the charge state. Oligomers in low charge states with a correspondingly larger number of sodium counterions tend to form stacked, cofacial structures reminiscent of H-aggregate motifs observed in solution. By contrast, in higher charge states, the stack opens to form coplanar structures. ■ INTRODUCTIONMetalloporphyrins constitute the reactive centers in a large number of biochemical systems such as hemoglobin, myoglobin, and chlorophyll. As a consequence, they are wellstudied in condensed phase. 1 It is also well-known that porphyrins and metalloporphyrins can self-assemble into larger oligomers in solution. This aggregation has attracted considerable interest, especially for porphyrins functionalized with highly polar groups such as sulfonic acids. 2−9 Much of this effort has been focused on water-soluble meso-tetra(4-sulfonatophenyl)porphyrin (TPPS). Ribóet al. 6 investigated aqueous solutions thereof spectroscopically. In studies performed at various concentrations and pH values, they found evidence for the formation of small J (edge-to-edge) and H (face-to-face) aggregates. The light scattering measurements of Micali et al. 10 have in fact suggested that even larger mesoscopicself-similar clusters of TPPS J-aggregates may form in solution. More recently, Hollingsworth et al. 9 have investigated the aggregation of TPPS in aqueous solution using UV−vis and fluorescence spectroscopy, small-angle X-ray scattering, analytical ultracentrifugation, and transmission electron microscopy. They inferred stacks of ring structures containing typically 25 monomeric units.TPPS oligomers as well as aggregates of the corresponding metalloporphyrins, MTPPS, can also be prepared and studied in gas phase, i.e., in the absence of solvent molecules. For example, multiply negatively charged aggregates of MTPPS (also containing sodium counterions) can be generated by electrospray ionization (ESI) and detected with mass spectrometry. To what extent such isolated aggregates can also provide an indirect measure of aggregatio...

A Synthetic Strategy for Cofacial Porphyrin‐Based Homo‐ and Heterobimetallic Complexes

“…Finally, we compared its calculated CCS with the measured CCS. The structures of noncovalently linked TPP dimers are published [38] . We thus determined a scaling factor of 0.94.…”

Photoelectron spectroscopy of the protoporphyrin IX dianion

“…Porphyrins have been studied in the gas phase using mass 6 and ion mobility 7,8 spectrometry, as well as infrared, 9 photodissociation 10 and photoelectron spectroscopy. [11][12][13][14] Protoporphyrin IX is a D 2h porphyrin with two propionic acid groups as substituents on the macrocycle, which can form a carboxylate monoanion or dicarboxylate dianion following the removal of one or two protons.…”

Reaction Monitoring and Structural Characterisation of Coordination Driven Self-Assembled Systems by Ion Mobility-Mass Spectrometry

“…The observed changes in Ω exp were compared to Ω calc values obtained from trajectory method applied to carefully optimised DFT structures. This built on similar earlier work observing the change in oligomeric metalloporphyrin geometry as the coordinating metal was changed ( Brendle et al, 2016 ). It was found that for M II containing oligomers, the charge state of the oligomer, and the corresponding number of sodium counterions, dictated if the structure formed would be “open” or “closed.” Interestingly, M III containing oligomers did not exhibit the same pattern, as the sulfonic groups of the porphyrins could readily bind to the central metals.…”