Porphyrin-based multi-metallic complexes

In nature, the catalytically active sites of metalloenzymes are often rigidly fixed structures in an adaptive protein matrix which defines a spatial arrangement of the metal-containing ligands relative to each other. In such systems proximity of several metal cations is typically required to achieve catalytic function. Correspondingly, strongly interacting metal sites are the basis for the unique catalytic activity of many multinuclear metalloproteins such as hemocyanin[1],[2] hemerythrin[3], superoxide dismutase[4], carbon monoxide dehydrogenase[5] or cytochrome c oxidase[6]. To gain further understanding of the unique coordination chemistry of such complexes, one ideally needs a rigid molecular system, which allows one to tune the distances between the different metal centers without also influencing them by significantly changing the ligand field. Then it becomes possible to vary the interactions between metal centers, in particular those which give rise to cooperative catalytic effects, by slight variations of the rigid framework.

Figure 1: Possible coordination spheres and the general motive of monomeric porphyrins.

One system of interest in this context is covalently linked dimeric porphyrin metal complexes which can be tuned by systematic structural variation and whose cooperative properties can be compared to those of the corresponding constituent monomers.

Figure 2: Schematic representation porphyrin-based bimetallic complexes.

Porphyrins offer the opportunity to coordinate numerous different metal ions without changing ligands and can be synthesized in a tailor-made way.[7],[8] Especially stepwise syntheses are of great interest towards rationally accessing artificial made heterobimetallic active site analogs, e.g. of the above-mentioned enzymes. This makes dimeric porphyrins a perfect choice not only to model and understand elementary enzymatic reactivity – but also for studying cooperative magnetic, catalytic, and optical properties of two spatially close-lying metal ions. In principle, three different bisporphyrin topologies are possible: coplanar, tilted and cofacial. Cofacial orientation provides closer metal-metal separations and is therefore of most interest for cooperative effects.

Based on a paper of Naruta et al. published in 2017, where a stacked Fe(III)-porphyrin dimer was synthesized and used as catalyst for the reduction of CO2 to CO, we are investigating the synthesis of new/different porphyrin dimers with rigid backbones.[9] In the case of a homobimetallic Ni(II) complex we were able to grow signal crystals and could determine the molecular structure.

         

Figure 3: Crystal structure of a homobimetallic Ni(II) porphyrin complex.

Furthermore enforced aggregation of various metal ions or prosthetic groups often constitutes an essential instrument in biological systems for the arrangement of catalytically active sites.[10] Inspired by the three-dimensional formation of bacterial photosynthetic reactions centers from Rhodopseudomonas viridis containing six interacting tetrapyrroles, several porphyrin-based synthetic models have been developed to synthesize and investigate catalysts for photosynthetic charge separation.[11],[12] Hence one should consider at least three components in a precise aggregation model and therefore we are pursuing synthetic routs to covalently linked trimeric and even larger analogs of multimetallic porphyrin complexes.

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[7]              A. Treibs, Liebig Ann. Chem. 1969, 728, 115-143.

[8]              M. O. Senge, Chem. Commun. 2011, 47, 1943-1960.

[9]              E. A. Mohamed, Z. N. Zahran, Y. Naruta, Chem. Mat. 2017, 29, 7140-7150.

[10]           G. M. Dubowchik and A. D. Hamilton, Journal of the Chemical Society, Chemical Communications 1987, 293-295.

[11]           T. Nagata, A. Osuka and K. Maruyama, Journal of the American Chemical Society 1990, 112, 3054-3059.

[12]           M. R. Wasielewski, Chemical Reviews 1992, 92, 435-461.