H₂ Binding, Splitting, and Net Hydrogen Atom Transfer at a Paramagnetic Iron Complex

Abstract

The reactivity of H2 with abundant transition metals is crucial for developing catalysts for energy storage in chemical bonds. While diamagnetic transition metal complexes that bind and split H2 have been extensively studied, paramagnetic complexes that exhibit this behavior remain rare. We describe the reactivity of a square planar S = ½ FeI(P4N2)+ cation (FeI+) that reversibly binds H2/D2 in solution, exhibiting an inverse equilibrium isotope effect of KH2/KD2 = 0.58(4) at-5.0 °C. In the presence of excess H2, the dihydrogen complex FeI(H2)+ cleaves H2 at 25 °C in a net hydrogen atom transfer reaction to give the dihydrogen-hydride cation trans-FeII(H)(H2)+. The proposed mechanism of H2 splitting involves both intraand intermolecular steps, resulting in a mixed firstand second-order rate law with respect to initial [FeI+]. The key intermediate is a paramagnetic dihydride complex, trans-FeIII(H)2+, whose weak FeIII-H bond dissociation free energy (calculated BDFE = 44 kcal/mol) leads to bimetallic H-H homolysis, generating trans-FeII(H)(H2)+. Reaction kinetics, thermodynamics, electrochemistry, EPR spectroscopy, and DFT calculations all support theproposed reaction mechanism. File list (2) download file view on ChemRxiv FeH2_ChemRxiv.pdf (1.03 MiB) download file view on ChemRxiv SI_FeH2comm_ChemRxiv.pdf (6.61 MiB)

Thermodynamic and kinetic studies of H2 and N2 binding to bimetallic nickel-group 13 complexes and neutron structure of a Ni(η2-H2) adduct

“…This is remarkable considering that the full range of literature Δ G ° values encompasses H 2 binding to various transition metals ( e.g. Cr, Mo, W, Re, Fe, Ru, Co, and Ir) in diverse ligand environments 10,11,13,14,39–45. In contrast, the NiML trio features an isostructural Ni site within the same ligand framework, where the primary difference is the identity of the group 13 metal.…”

“…Despite the ubiquity of H 2 binding as a key fundamental reaction step in catalysis,9 limited experimental data has been reported regarding the thermodynamic and kinetic favorability of H 2 binding to transition metals, especially for the first-row transition metals 6–8. To the best of our knowledge, thermodynamic H 2 -binding data have only been reported for a handful of first-row metal complexes: Cr(PCy 3 ) 2 (CO) 3 ,10,11 [Mn(CO)(dppe) 2 ] + ,12 [Fe(P 4 N 2 )] + ,13 and Co(TPB),14 where dppe is bis(diphenylphosphino)ethane, P 4 N 2 is a tetraphosphine with two pendant amines, and TPB is tris( o -diisopropylphosphinophenyl)borane. Moreover, without a series of similar M(η 2 -H 2 ) adducts with which to compare thermodynamic data, the understanding gained by quantifying H 2 binding to a single metal complex are relatively limited.…”

Tuning ligand field strength with pendent Lewis acids: access to high spin iron hydrides

“…Iron-dihydrides are intermediates in myriad homogenous catalytic reactions and the recent push to develop earth-abundant transition metal catalysts has fueled many research groups to explore their reactivity 1. Beyond relevance to homogenous catalysis, iron-dihydrides have been implicated in biological reduction sequences.…”

Synthesis and Structure of Iron (II) Complexes of Functionalized 1,5-Diaza-3,7-Diphosphacyclooctanes

“…The iron complexes of 1,5-diaza-3,7-diphosphacyclooctanes are less studied in spite of their promising activity in H 2 splitting and electrooxidation [3,[18][19][20] and N 2 binding and activation [5]. In particular, the set of substituents on phosphorus atoms of the investigated P 2 N 2 ligands is restricted and includes phenyl [21][22][23][24], tert-butyl [19,20,24,25], cyclohexyl [21,24], and benzyl groups [24], whereas the nitrogen atoms bear phenyl or benzyl groups.…”

“…The cyclopentadienyl-containing complexes draw the main interest due to their activity in hydrogen electrooxidation [3,19,20,25]. The only additionally functionalized diazadiphosphacyclooctane ligand, namely, 1,5-diphenyl-3,7-bis(2 -diphenylphosphinoethyl)-1,5-diaza-3,7-diphosphacyclooctane, has been used for the synthesis of unusual square planar iron complexes stabilized by the coordination with exocyclic phosphino groups, which are able to activate both hydrogen and nitrogen molecules [5,18].…”