ALBRECHT RESEARCH GROUP

We believe that a specific design of ancillary ligands plays a crucial role in imparting likewise unique properties to the metal center of a homogeneous catalyst. Thus, the ligands employed in our group are obtained by a modular synthetic approach, in order to conveniently screen both steric and electronic factors. The first category of interest includes triazolylidenes , a class of mesoionic N-heterocyclic carbenes (MICs) that we first discovered in 2008, [J. Am. Chem. Soc. 2008] whose precursors are easily obtained by the copper-catalyzed azide-alkyne cycloaddition reaction. In the past years, they have shown to provide for catalytic purposes an attractive alternative to soft, strongly sigma-donating ligands like phosphines. [J. Am. Chem. Soc. 2021; Green Chem. 2021; Angew. Chem. Int. Ed. 2010]. Moreover, they have often displayed superior performances comparing to those of systems bearing classical NHC ligands.
In 2017, we also started to investigate pyridylidene amines (PYEs) and pyridylidene amides (PYAs) ligands, which are as well readily afforded in a modular fashion via common synthetic steps. [ACS Catal. 2023] Thanks to the resonance effect, their bonding character is a mixed contribution of the neutral imine and the zwitterionic amide structures, which can be tuned by precise substitution of the molecule and subsequently induce different properties to the resulting catalyst. For example, we recently discovered that such systems could be employed for highly efficient oxidation processes [Angew. Chem. Int. Ed. 2020] and formic acid dehydrogenation [ACS Catal. 2022].

The synthesis of N-heterocycles is of paramount importance for the pharmaceutical industry. They are often synthesized through environmentally unfriendly methods, generating significant amount of waste. Recently, we investigated the activity of iron-based catalysts for the intramolecular C-H amination reaction to directly generate C-N bonds, using azides as nitrogen source. [J. Am. Chem. Soc. 2021; Chem. Sci. 2023, Cat. Sci. Technol. 2023] Among them, a triazolylidene-Fe complex was found to be the most active catalyst reported to date for this reaction. [J. Am. Chem. Soc. 2021] Furthermore, we focused our attention on the mechanistic description of this transformation through DFT calculations and other sophisticated techniques, such as Mössbauer spectroscopy and in cristallo measurements. (in collaboration with K. Meyer and S. Grabowsky groups)

Inspired by the structural similarity of the imidazole unit displayed by histidine and N-heterocyclic carbenes, we have been intrigued about the implications of such carbenes in metalloenzymes. In collaboration with Prof. Paradisi (University of Bern), we studied the case of blue copper proteins to investigate the possibility of NHC bonding to the Cu center, and to evaluate its effect on the redox properties of the enzyme. [Angew. Chem. Int. Ed. 2018]
Recently, we discovered that the hydrogenation process catalyzed by an iridium-PYE complex relies on the storage and release of one equivalent of hydride on the organic fragment of the ligand instead of as a more classical metal-hydride bond. [ACS Catal. 2023] This ability of mimicking the natural hydride-carrier cofactor NADH is currently under investigation, and we are evaluating a possible conjugation of such artificial cofactor with NADH-dependent oxidoreductases.

Our group is currently committed in developing strategies involving homogeneous catalysis to tackle the increasing energy demand and the necessity of alternative fuels. In the latter case, the current research is mostly focused on the use of hydrogen gas, which can be provided through various metal-assisted processes, such as water oxidation (WO) and transformation of liquid organic hydrogen carriers (LOHCs). Formic acid is one of the most attractive LOHCs since its dehydrogenation reaction only produces hydrogen gas and carbon dioxide. For this application, we reported an iridium-PYE catalyst which outperformed previous state-of-the-art systems (TON up to 3 million vs 2.4 million) and is relatively cheap due to the low cost and easy accessibility of the PYE ligand. [ACS Catal. 2022] The development of efficient homogeneous catalyst bearing also first- and second-row TMs is ongoing. [Cat. Sci. Technol. 2023]
At the same time, we are also involved in the metal-assisted regeneration of formic acid starting from cheap carbon feedstocks. Our group identified a class of nickel(II)-triazolylidene complexes active in the selective CO₂ electroreduction to formate, which reported the highest activity (faradaic efficiency up to 83%) among the Ni-based systems for this transformation. [Green Chem. 2021]
In collaboration with other research groups, we furthermore synthesized a family of iridium-Cp* complexes bearing triazolylidene ligands and applied them for cerium(IV)-assisted water oxidation catalysis. [ACS Catal. 2015; Angew. Chem. Int. Ed. 2010] In this study, we explored the correlation between activity and the different substitution of the triazolylidene ligand and the introduction of a chelating moiety (e.g. pyridine) to obtain bidentate ligands. Now we are mostly focused on the screening of Earth-abundant metal-based analogous complexes for such application.