Molecular Engineering of Metalloporphyrins for High‐Performance Energy Storage: Central Metal Matters

Abstract Porphyrin derivatives represent an emerging class of redox‐active materials for sustainable electrochemical energy storage. However, their structure–performance relationship is poorly understood, which confines their rational design and thus limits access to their full potential. To gain such understanding, we here focus on the role of the metal ion within porphyrin molecules. The A2B2‐type porphyrin 5,15‐bis(ethynyl)‐10,20‐diphenylporphyrin and its first‐row transition metal complexes from Co to Zn are used as models to investigate the relationships between structure and electrochemical performance. It turned out that the choice of central metal atom has a profound influence on the practical voltage window and discharge capacity. The results of DFT calculations suggest that the choice of central metal atom triggers the degree of planarity of the porphyrin. Single crystal diffraction studies illustrate the consequences on the intramolecular rearrangement and packing of metalloporphyrins. Besides the direct effect of the metal choice on the undesired solubility, efficient packing and crystallinity are found to dictate the rate capability and the ion diffusion along with the porosity. Such findings open up a vast space of compositions and morphologies to accelerate the practical application of resource‐friendly cathode materials to satisfy the rapidly increasing need for efficient electrical energy storage.


Synthesis:
The synthesis of A2B2-porphyrins is straightforward and consists of three synthesis steps (Scheme 1). In step one, we synthesize the appropriate meso-dipyrromethane from pyrrole and an aldehyde with the intended substituent with the electron-withdrawing group. In step two, the ring-closing reaction takes place, flowing the Macdonald condensation. The condensation takes place between the meso-dipyrromethane and (trimethylsilyl)-propiolaldehyde. The metalation of the free-base porphyrin is unproblematic -conditions depend on the used metal. Synthesis of 5-phenylpyrromethane (1). [2] A mixture of pyrrole (140 mL, 2 mol) and benzaldehyde (10.2 mL, 0.1 mol) was bubbled 15 min with Argon. The reaction mixture was cooled with an ice bath and trifluoroacetic acid (0.78 mL, 0.01 mol) was added dropwise. After that, the reaction mixture was extracted 3x with ethyl acetate. The combined organic phase was extracted with water and dried over sodium sulphate. After column chromatography (SiO2, hexane:ethyl acetate, 2:1) , a yellow solid was obtained with 9% (2.0 g) yield. 1  (2). [3] A mixture of ( General procedure for deprotection of TMS-group. [4] The porphyrin (0.05 mmol) was dissolved in 20 mL dry THF and 1 mL of 1M solution of TBAF in THF was added. The mixture was stirred overnight under an argon atmosphere. The reaction was quenched by adding 50 mL of water. THF was removed under reduced pressure. The precipitate was filtrated and dried overnight at 100 °C and 2.0•10 -2 mbar.

Supporting Figures and Tables
Matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI ToF MS) is frequently applied to analyze macrocycles and their metal complexes. All MALDI mass spectra were obtained by solvent-based sample preparation methods. About 0.1 mg of the analyte was dissolved or suspended in 2 ml of MeOH. A small amount (0.1-2.5 µL) of the solution was put on the stainless steel substrate and dried in air.
The MALDI-TOF mass spectra of newly synthesized 4, 5, 9, 10 showed appropriate signals for the molecular ions and proved their desired nature. The molecular ion was the most abundant high mass ion with a distinct isotopic distribution in all cases. The relative abundances of the isotopic ions are in good agreement with the simulated spectra, as reported in Figure S3-S6, where every spectral result and calculated values are summarised. In all cases except NiDEPP (10), mass spectra were detected in the positive ion mode. In addition, acidic compounds (such as alkynes) can also be detected as single negatively charged ions in the negative ion MALDI ToF mass spectra.
It is common for small ions like Ni 2+ that insertion into porphyrins suffers from the fact that Ni 2+ is too small to perfectly fit into the square planar cavity formed by the four pyrrole nitrogen atoms (ionic radii Figure S1) Ni II -porphyrins show a rich conformational behavior: a (dz2) 2 electronic configuration and small ionic radius (0.69 Å) of Ni II favor relatively short equilibrium Ni-N bond distances. This results in nonplanar ruffled Ni-porphyrin conformations, [7] in which individual pyrrole rings are twisted about the Ni-N axes and significant alternating displacements of the Cm sites above and below the mean molecular plane take place. [8][9][10] (Figure 2b-e). Figure S14. The rate capability of DEPP electrode with an increase in the charge-discharge rate from 100 mA g -1 to 10 A g -1 and then a decrease to 500 mA g -1 (a) and selected voltage profile (b).  Figure S15. The rate capability of CoDEPP electrode with an increase in the charge-discharge rate from 100 mA g -1 to 10 A g -1 and then a decrease to 500 mA g -1 . Figure S16. The rate capability of NiDEPP electrode with an increase in the charge-discharge rate from 100 mA g -1 to 10 A g -1 and then a decrease to 500 mA g -1 . Coulombic efficiency (%) Figure S17. The rate capability of CuDEPP electrode with an increase in the charge-discharge rate from 100 mA g -1 to 10 A g -1 and then a decrease to 500 mA g -1 .
Figure S18. The rate capability of ZnDEPP electrode with an increase in the charge-discharge rate from 100 mA g -1 to 10 A g -1 and then a decrease to 500 mA g -1 .    Table S4: Crystal data and structure refinement.