We determine equilibrium structures, adiabatic electron affinities, and resonance widths of various temporary anions to benchmark the complex absorbing potential equation-of-motion coupled-cluster (CAP-EOM-CC) method. The second-order approximation to CAP-EOM-CC with singles and doubles (SD) excitations is found to yield slightly lower resonance positions and widths than full CAP-EOM-CCSD. The basis set dependence of adiabatic resonance positions and widths is similar to that of the vertical quantities. We demonstrate the usefulness of structure optimizations of temporary anions by two examples. For the anions of acrylonitrile and methacrylonitrile, we observe good agreement for the adiabatic electron affinities and structural changes between our theoretical results and two-dimensional electron-energy loss spectra. For the unsaturated hydrocarbons ethylene, 1,3-butadiene, and cis- and trans-1,3,5-hexatriene, the agreement between theory and electron transmission spectroscopy is good for the lower-lying π* resonances, while our results for the 3π* resonance of trans-hexatriene suggest a shortcoming of the method or reinterpretation of the corresponding electron transmission spectrum. The experimentally determined difference between the electron affinities of the 2π* resonances of the cis isomer and the trans isomer of hexatriene are reproduced well by CAP-EOM-EA-CCSD and CAP-EOM-EA-CCSD(2).

We have investigated the ability of Co– and Fe–Mabiq complexes (Mabiq = 2–4:6–8-bis­(3,3,4,4-tetramethyldihydropyrrolo)-10-15-(2,2′-biquinazolino)-[15]-1,3,5,8,10,14-hexaene1,3,7,9,11,14-N6) to act as electrocatalysts for CO2 reduction. We observed marked differences in activity when switching the metal center, as the Fe complex outperforms its Co-containing analogue, both in terms of overpotential (η) and faradaic efficiency (FE). [Fe­(Mabiq)2(MeCN)2]­PF6 ([2] + ) selectively reduces CO2 to CO with an overpotential requirement of 500 mV. We have synthesized and fully characterized the two-electron reduced Na­(OEt2)­[Fe­(Mabiq)] ([2] – ), which consists of an intermediate spin FeII center coupled to a ligand biradical and exhibits a unique S = 1 spin state. Both electrochemical and reactivity studies with [2] – point toward a protonated precatalytic intermediate (I PhOH ). The molecular structure of I PhOH indicates the diketiminate carbon as the site of protonation and the ability of the Mabiq ligand to engage in hydrogen bonding interactions. The noninnocent Mabiq ligand, therefore, acts not only as an electron reservoir but also as a proton storage site. Our ligand system uniquely combines two beneficial features, a redox-active unit and a proton donor site, that in combination with the metal ion reduces overpotentials and facilitates selective CO2 conversion.

Die Natur hat metallbasierte Katalysatoren hervorgebracht, die Reaktionen in einer Weise beeinflussen, von der Synthesechemiker nur träumen können. Um diese Verbindungen als Inspiration zu nutzen, müssen die Prozesse in der Natur verstanden sein.