Shengda Ding

Interplay of hemilability and redox activity in models of hydrogenase active sites

Significance Segmentation of the bimetallic electrocatalysts under investigation into a metallodithiolate, bidentate S-donor ligand, and a receiver metal is effective for understanding the proton and electron uptake in H2-evolution reactions. Coexisting actor/reaction-involved ligands, i.e., electron-buffering NO and hemilabile, chelating metallodithiolate, subtly cooperate to control electrocatalytic H2-production mechanisms. Two mechanisms emerge in a single catalyst to yield H2: protonation of a hydride or reductive elimination from a metal dihydride. A Lewis acid–base pair appears by cleaving the hemilabile thiolate from the metal and serves as the reactive centers to process electrons and protons; a protonation or a reduction on the Lewis pair modulates their electron densities and protects these reactive centers from converting back to a dative bond. The hydrogen evolution reaction, as catalyzed by two electrocatalysts [M(N2S2)·Fe(NO)2]+, [Fe-Fe]+ (M = Fe(NO)) and [Ni-Fe]+ (M = Ni) was investigated by computational chemistry. As nominal models of hydrogenase active sites, these bimetallics feature two kinds of actor ligands: Hemilabile, MN2S2 ligands and redox-active, nitrosyl ligands, whose interplay guides the H2 production mechanism. The requisite base and metal open site are masked in the resting state but revealed within the catalytic cycle by cleavage of the MS–Fe(NO)2 bond from the hemilabile metallodithiolate ligand. Introducing two electrons and two protons to [Ni-Fe]+ produces H2 from coupling a hydride temporarily stored on Fe(NO)2 (Lewis acid) and a proton accommodated on the exposed sulfur of the MN2S2 thiolate (Lewis base). This Lewis acid–base pair is initiated and preserved by disrupting the dative donation through protonation on the thiolate or reduction on the thiolate-bound metal. Either manipulation modulates the electron density of the pair to prevent it from reestablishing the dative bond. The electron-buffering nitrosyl’s role is subtler as a bifunctional electron reservoir. With more nitrosyls as in [Fe-Fe]+, accumulated electronic space in the nitrosyls’ π*-orbitals makes reductions easier, but redirects the protonation and reduction to sites that postpone the actuation of the hemilability. Additionally, two electrons donated from two nitrosyl-buffered irons, along with two external electrons, reduce two protons into two hydrides, from which reductive elimination generates H2.

A reduced 2Fe2S cluster probe of sulfur-hydrogen versus sulfur-gold interactions.

The Ph3 PAu(+) cation, renowned as an isolobal analogue of H(+) , was found to serve as a proton surrogate and form a stable Au2 Fe2 complex, [(μ-SAuPPh3 )2 {Fe(CO)3 }2 ], analogous to the highly reactive dihydrosulfide [(μ-SH)2 {Fe(CO)3 }2 ]. Solid-state X-ray diffraction analysis found the two SAuPPh3 and SH bridges in anti configurations. VT NMR studies, supported by DFT computations, confirmed substantial barriers of approximately 25 kcal mol(-1) to intramolecular interconversion between the three stereoisomers of [(μ-SH)2 {Fe(CO)3 }2 ]. In contrast, the largely dative SAu bond in μ-SAuPPh3 facilitates inversion at S and accounts for the facile equilibration of the SAuPPh3 units, with an energy barrier half that of the SH analogue. The reactivity of the gold-protected sulfur atoms of [(μ-SAuPPh3 )2 {Fe(CO)3 }2 ] was accessed by release of the gold ligand with a strong acid to generate the [(μ-SH)2 {Fe(CO)3 }2 ] precursor of the [FeFe]H2 ase-active-site biomimetic [(μ2 -SCH2 (NR)CH2 S){Fe(CO)3 }2 ].

The Rich Structural Chemistry Displayed by the Carbon Monoxide as a Ligand to Metal Complexes

The diatomic CO molecule is a very important ligand in organometallic chemistry. The bond between the carbonyl and a metal is moderately strong and consists of a sigma bond, formed by donation of electron density to the metal from the carbonyl’s highest occupied molecular orbital (HOMO, the 5σ), and π bonds, formed by donation of electron density from the metal to the carbonyl’s lowest unoccupied molecular orbital (LUMO, the 2π). The carbonyl may also serve as a bridging ligand connecting two or more metal atoms. Depending on the relative orientation between the carbonyl and metals, one may classify a bridging carbonyl as symmetric bridging, bent semibridging, linear semibridging, face bridging, and bridging isocarbonyls. The rich structural chemistry displayed arises from a complex interplay between the metal’s electronic structure and the carbonyl’s 5σ and 2π. In addition, the carbonyl’s occupied 1π and 4σ orbitals may in certain cases donate electrons when it binds to electron-deficient metals, further complicating the electronic structure. Such complexity in the carbonyl–metal interaction raises challenges to the simple applications of Lewis bonding ideas and electron counting rules. Therefore, theoretical analyses have been applied, largely in a case-by-case pattern, to investigate the rationales behind the CO’s rich structural chemistry.

Structural and Electronic Responses to the Three Redox Levels of Fe(NO)N2S2-Fe(NO)2

The nitrosylated diiron complexes, Fe2 (NO)3 , of this study are interpreted as a mono-nitrosyl Fe(NO) unit, MNIU, within an N2 S2 ligand field that serves as a metallodithiolate ligand to a dinitrosyl iron unit, DNIU. The cationic Fe(NO)N2 S2 ⋅Fe(NO)2 + complex, 1+ , of Enemark-Feltham electronic notation {Fe(NO)}7 -{Fe(NO)2 }9 , is readily obtained via myriad synthetic routes, and shown to be spin coupled and diamagnetic. Its singly and doubly reduced forms, {Fe(NO)}7 -{Fe(NO)2 }10 , 10 , and {Fe(NO)}8 -{Fe(NO)2 }10 , 1- , were isolated and characterized. While structural parameters of the DNIU are largely unaffected by redox levels, the MNIU readily responds; the neutral, S= 1 / 2 , complex, 10 , finds the extra electron density added into the DNIU affects the adjacent MNIU as seen by the decrease its Fe-N-O angle (from 171° to 149°). In contrast, addition of the second electron, now into the MNIU, returns the Fe-N-O angle to 171° in 1- . Compensating shifts in FeMNIU distances from the N2 S2 plane (from 0.518 to 0.551 to 0.851 Å) contribute to the stability of the bimetallic complex. These features are addressed by computational studies which indicate that the MNIU in 1- is a triplet-state {Fe(NO)}8 with strong spin polarization in the more linear FeNO unit. Magnetic susceptibility and parallel mode EPR results are consistent with the triplet state assignment.