Critical evaluation of a crystal structure of nitrogenase with bound N2 ligands

Recently, a 1.83 Å crystallographic structure of nitrogenase was suggested to show N2-derived ligands at three sites in the catalytic FeMo cluster, replacing the three $$\mu_{2}$$ μ 2 bridging sulfide ligands (two in one subunit and the third in the other subunit) (Kang et al. in Science 368: 1381–1385, 2020). Naturally, such a structure is sensational, having strong bearings on the reaction mechanism of the enzyme. Therefore, it is highly important to ensure that the interpretation of the structure is correct. Here, we use standard crystallographic refinement and quantum refinement to evaluate the structure. We show that the original crystallographic raw data are strongly anisotropic, with a much lower resolution in certain directions than others. This, together with the questionable use of anisotropic B factors, give atoms an elongated shape, which may look like diatomic atoms. In terms of standard electron-density maps and real-space Z scores, a resting-state structure with no dissociated sulfide ligands fits the raw data better than the interpretation suggested by the crystallographers. The anomalous electron density at 7100 eV is weaker for the putative N2 ligands, but not lower than for several of the $${\mu }_{3}$$ μ 3 bridging sulfide ions and not lower than what can be expected from a statistical analysis of the densities. Therefore, we find no convincing evidence for any N2 binding to the FeMo cluster. Instead, a standard resting state without any dissociated ligands seems to be the most likely interpretation of the structure. Likewise, we find no support that the homocitrate ligand should show monodentate binding. Graphic abstract

Syntheses, structures, and catalytic properties of two arene-ruthenium(II) complexes bearing N-(2-pyridinyl)aminodiphenylphosphine sulfide ligands

Treatment of [(arene)Ru(μ-Cl)Cl]2with Ph2P(S)NH(2-py) in the presence or absence of base gave two arene-ruthenium(II) complexes [(η6-p-cymene)Ru{κ2-N,N-Ph2P(S)N(2-py)}Cl] (1) and [(η6-benzene)Ru{κ1-N-Ph2P(S)NH(2-py)}Cl2] (2), which have been characterized by infrared, nuclear magnetic resonance spectroscopies, and mass spectrometry along with microanalyses. Crystal structures of Ph2P(S)NH(2-py) · ¼C6H14,1and2· ½CH2Cl2were determined by single-crystal X-ray diffraction. Two arene-ruthenium(II) complexes were tested as precatalysts for the transfer hydrogenation of acetophenone to give 1-phenyl ethanol.