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

Diamagnetic iron chloro compounds
[(PPh2NPh2)­FeCp*Cl] [1Cl] and [(PCy2NPh2)­FeCp*Cl] [2Cl] and the corresponding hydrido complexes [(PPh2NPh2)­FeCp*H] [1H] and [(PCy2NPh2)­FeCp*H] [2H]
have been synthesized and characterized by NMR spectroscopy, electrochemical
studies, electronic absorption, and 57Fe Mössbauer
spectroscopy (PPh2NPh2 = 1,3,5,7-tetraphenyl-1,5-diphospha-3,7-diazacyclooctane, PCy2NPh2 = 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 PR2NPh2 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] (pKaMeCN = 1.0). In contrast to the related complex [(PtBuNBn)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.