Electronic structure-chemical property relationship of binuclear 3d-metal structures.
Final Report Abstract
Two- and multi-electron redox processes play a fundamental role in the conversion of chemical energy into electrical energy, e.g. in the conversion of hydrogen into electricity. As the central research question, this project assessed the essential elements for designing two-electron redox capable catalyst systems of the 3d metals for chemical-electrical energy conversion. By selectively combining electronically varied donor/acceptor ligand, the mechanisms of electronic coupling between two metal centers were investigated, and compared with the reactivity in H2 formation and oxidation. Specifically, two nickel centers were embedded in a framework environment consisting of two thiophenols and a bridging 6-ring π-system, whose combined binding modes stabilize a series of three redox forms of the Ni2S2 system. The electronic properties of the thiophenols are modulated by substituents in 4-position (F, tBu and NMe2), forming electronically symmetric and non-symmetric binding pockets in combination with the π-system. The mechanisms and strength of the electronic coupling in the three redox states differ considerably and are also variable. The ability of the binuclear complexes to form or oxidize H2 depending on the redox or charge state enables the linking of reactivity and electronic properties. The first key finding is that cooperative reactivity results from weak electronic coupling of the metal centers. Only then do the nickel centers show active-passive bipolar reactivity, i.e. one nickel center mediates the reaction while the other assists. In the case of H2 formation, the two electrons of the binuclear system are combined to form a Ni-H fragment, which does not occur for structures with strong electronic coupling in the same redox state and framework system. The S donors act as proton acceptors and carriers. During H2 oxidation, heterolytic H2 bond cleavage occurs at a [Ni(II)-S]+ fragment, forming a [Ni(0)-H]+ fragment supported only through bonding to the π-system. Presumably, the [Ni(0)-H]+ eventually dissociates H+ by H-bonding to the second Ni(II)-S fragment, forming the reduced redox state selectively. The cooperative mechanism contrasts with the reactivity of structurally identical mononuclear complexes that react irreversibly forming the thiophenol ligand and clusters of low-valent nickel. The second key finding is that the strength of the electronic coupling and the structural similarity of the S-bridges in M2S2 fragments are mutually dependent. This means that a structural distortion caused by the binding environment modulates the properties of M2S2 cores. This finding is probably reflected in the enhanced H2 oxidation reactivity of [NiFe]-H2ase, which in contrast to its [2Fe]-H2ase congener features structural non-equivalence of its S donors in certain reactive states. This project shows that charge transfer from strong σ-donors attenuates electronic coupling in M2S2 cores.
Publications
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Modulating Effect of Ligand Charge on the Electronic Properties of 2Ni–2S Structures and Implications for Biological 2M–2S Sites. Inorganic Chemistry, 59(23), 17234-17243.
Berkefeld, Andreas; Roemelt, Michael; Römelt, Christina; Schubert, Hartmut & Jeschke, Gunnar
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Broadly versus Barely Variable Complex Chromophores of Planar Nickel(II) from κ3-N,N′,C and κ3-N,N′,O Donor Platforms. Organometallics, 40(8), 1163-1177.
Alrefai, Riyadh; Hörner, Gerald; Schubert, Hartmut & Berkefeld, Andreas
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A Photoreactive Iron(II) Complex Luminophore. Journal of the American Chemical Society, 144(3), 1169-1173.
Leis, Wolfgang; Argüello Cordero, Miguel A.; Lochbrunner, Stefan; Schubert, Hartmut & Berkefeld, Andreas
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Chemiedozententagung 2022, Saarbrücken, A Photoreactive Iron(II) Complex Luminophore
Berkefeld, A.
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KCT 2022, Jena, Taking Cu(II) salan complexes for a swim – Proligand design, metalation and redox chemistry in aqueous solution
Stetter, A.
