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

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• 作者：Katharina Weber ; Thomas Weyherm眉ller ; Eckhard Bill ; 脰zlen F. Erdem ; Wolfgang Lubitz
• 刊名：Inorganic Chemistry
• 出版年：2015
• 出版时间：July 20, 2015
• 年：2015
• 卷：54
• 期：14
• 页码：6928-6937
• 全文大小：592K
• ISSN：1520-510X

文摘

Diamagnetic iron chloro compounds [(P^Ph₂N^Ph₂)FeCp*Cl] [1Cl] and [(P^Cy₂N^Ph₂)FeCp*Cl] [2Cl] and the corresponding hydrido complexes [(P^Ph₂N^Ph₂)FeCp*H] [1H] and [(P^Cy₂N^Ph₂)FeCp*H] [2H] have been synthesized and characterized by NMR spectroscopy, electrochemical studies, electronic absorption, and ⁵⁷Fe M枚ssbauer spectroscopy (P^Ph₂N^Ph₂ = 1,3,5,7-tetraphenyl-1,5-diphospha-3,7-diazacyclooctane, P^Cy₂N^Ph₂ = 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 P^R₂N^Ph₂ 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^tBuN^Bn)₂FeCp^C6F5H], which was reported by Liu et al. (Nat. Chem. 2013, 5, 228鈥?33) 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.