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

Abstract

Diamagnetic iron chloro compounds [(P(Ph)2N(Ph)2)FeCp*Cl] [1Cl] and [(P(Cy)2N(Ph)2)FeCp*Cl] [2Cl] and the corresponding hydrido complexes [(P(Ph)2N(Ph)2)FeCp*H] [1H] and [(P(Cy)2N(Ph)2)FeCp*H] [2H] have been synthesized and characterized by NMR spectroscopy, electrochemical studies, electronic absorption, and (57)Fe Mössbauer spectroscopy (P(Ph)2N(Ph)2 = 1,3,5,7-tetraphenyl-1,5-diphospha-3,7-diazacyclooctane, P(Cy)2N(Ph)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 P(R)2N(Ph)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] (pKa(MeCN) = 1.0). In contrast to the related complex [(P(tBu)N(Bn))2FeCp(C6F5)H], 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.

Fragmentation and transferability in Hirshfeld atom refinement

“…APPENDIX A References to experimental data used in this work Sorted alphabetically by CSD refcodes: ACUCIN (Nikonova et al, 2018), ADOWUO (Hermann et al, 2017), AHUKIZ (Weber et al, 2015), AMEPOS (Janicki & Starynowicz, 2010), ARAQUH (Hemgesberg et al, 2016), BECFAT (Veronelli et al, 2017), CAMVES (Dittrich et al, 2002), DAYCEO (Uppal et al, 2017), DNEDAM01 (Gruhne et al, 2021), DUBWAB (Nugrahani et al, 2019), DUCMAQ (Grabowsky et al, 2009), EDAWIP (Evans & Gilardi, 2001), EGUFIY (Malassis et al, 2019), GLYALB08 (Capelli et al, 2014), IYAQOP02 (Marshall et al, 2016), KOVPIX (Thanzeel et al, 2019), LETHIE (Chen et al, 2018), LOKPOT (Ballmann et al, 2019), LOSLEL (Johnas et al, 2009), MANMUJ08 (Jorgensen et al, 2014), MOSCOP (Clegg, 2019), MOVPIZ (Lapointe et al, 2019), MUHBOI (Butschke et al, 2015), OXACDH46 (Kamin ´ski et al, 2014), PCYPOL04 (Mohanraj et al, 2018), PPHCHN11…”

Protonation of Ferrocene: A Low‐Temperature X‐ray Diffraction Study of [Cp2FeH](PF6) Reveals an Iron‐Bound Hydrido Ligand

“…This phase transition resulted in the lower symmetry space group P3 1 (No.144) with minimal changes in the lattice parameters,m ostly owing to the lower temperature.The observed symmetry reduction affected the twofold rotation axis along the short diagonal of the unit cell, which now had to be accounted for by an additional twin law. [28,29] Thes tructures determined from the two data sets recorded at 30 Ka nd 100 Kr evealed slightly bent ferrocene backbones (see Table 1). To avoid variations in the CÀHbond lengths of the cyclopentadienyl rings,t hese hydrogen atoms were placed in positions of optimized geometry.T he ironbound hydride hydrogen atom H11, however,was left on the position where it was first located from ad ifference Fourier synthesis,and was allowed to ride on Fe1, resulting in d(Fe1-H11) = 1.62 .T he Fe-H distances observed at 100 K (1.49(4) )a nd 30 K( 1.62 )a gree well with the FeÀH bond lengths of non-bridging hydrides as established by neutron diffraction experiments on iron hydride compounds, which range from 1.513 in dihydrido(dihydrogen-H,H')tris(ethyldiphenylphosphine)iron [23] to 1.615 in an FeH 6 4À anion.…”

“…[24] Iron-hydride distances determined by X-ray diffraction are,b yn ature,s horter.Asearch in the CSD (ConQuest 1.19, 2017) [25] database of related Cp iron hydrides revealed Fe-H distances ranging from 1.34 [26,27] to 1.59 . [28,29] Thes tructures determined from the two data sets recorded at 30 Ka nd 100 Kr evealed slightly bent ferrocene backbones (see Table 1). Thetwo cyclopentadienyl rings are tilted against each other as indicated by the interplanar angles of 14.48 8 (at 30 K) and 11.88 8 (at 100 K) between the two cyclopentadienyl rings.This opens the space to accommodate the hydride ligand.…”

Protonation of Ferrocene: A Low‐Temperature X‐ray Diffraction Study of [Cp2FeH](PF6) Reveals an Iron‐Bound Hydrido Ligand

“…Iron–hydride distances determined by X‐ray diffraction are, by nature, shorter. A search in the CSD (ConQuest 1.19, 2017) database of related Cp iron hydrides revealed Fe–H distances ranging from 1.34 Å to 1.59 Å . The structures determined from the two data sets recorded at 30 K and 100 K revealed slightly bent ferrocene backbones (see Table ).…”