Design and Characterization of Phosphine Iron Hydrides: Toward Hydrogen-Producing Catalysts

• Published in: Inorganic chemistry 2015-07, Vol.54 (14), p.6928-6937
• Main Authors: Weber, Katharina, Weyhermüller, Thomas, Bill, Eckhard, Erdem, Özlen F, Lubitz, Wolfgang
• Format: Article
• Language: English
• Subjects: Catalysis; Crystallography, X-Ray; Electrochemical Techniques; Halogenation; Hydrogen - chemistry; Hydrogenase - chemistry; Iron Compounds - chemistry; Models, Molecular; Oxidation-Reduction; Phosphines - chemistry; Spectroscopy, Fourier Transform Infrared; Spectroscopy, Mossbauer
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.5b00911

Diamagnetic iron chloro compounds [(PPh 2NPh 2)­FeCp*Cl] [1Cl] and [(PCy 2NPh 2)­FeCp*Cl] [2Cl] and the corresponding hydrido complexes [(PPh 2NPh 2)­FeCp*H] [1H] and [(PCy 2NPh 2)­FeCp*H] [2H] have been synthesized and characterized by NMR spectroscopy, electrochemical studies, electronic absorption, and 57Fe Mössbauer spectroscopy (PPh 2NPh 2 = 1,3,5,7-tetraphenyl-1,5-diphospha-3,7-diazacyclooctane, PCy 2NPh 2 = 1,5-dicyclohexyl-3,7-diphenyl-1,5-diphospha-3,7-diazacyclooctane, Cp* = pentamethylcyclopentadienyl). Molecular structures of [2Cl], [1H], and [2H], derived from single-crystal X-ray diffraction, revealed that these compounds have a typical piano-stool geometry. The results show that the electronic properties of the hydrido complexes are strongly influenced by the substituents at the phosphorus donor atoms of the PR 2NPh 2 ligand, whereas those of the chloro complexes are less affected. These results illustrate that the hydride is a strong-field ligand, as compared to chloride, and thus leads to a significant degree of covalent character of the iron hydride bonds. This is important in the context of possible catalytic intermediates of iron hydrido species, as proposed for the catalytic cycle of [FeFe] hydrogenases and other synthetic catalysts. Both hydrido compounds [1H] and [2H] show enhanced catalytic currents in cyclic voltammetry upon addition of the strong acid trifluoromethanesulfonimide [NHTf2] (pK a MeCN = 1.0). In contrast to the related complex [(P tBuNBn)2FeCpC6F5H], which was reported by Liu et al. (Nat. Chem. 2013, 5, 228–233) to be an electrocatalyst for hydrogen splitting, the here presented hydride complexes [1H] and [2H] show the tendency for electrocatalytic hydrogen production. Hence, the catalytic direction of this class of monoiron compounds can be reversed by specific ligand modifications.