Models of the iron-only hydrogenase: a comparison of chelate and bridge isomers of Fe2(CO)4{Ph2PN(R)PPh2}(μ-pdt) as proton-reduction catalysts

Abstract

Reactions of Fe2(CO)6(μ-pdt) (pdt = SCH2CH2CH2S) with aminodiphosphines Ph2PN(R)PPh2 (R = allyl, (i)Pr, (i)Bu, p-tolyl, H) have been carried out under different conditions. At room temperature in MeCN with added Me3NO·2H2O, dibasal chelate complexes Fe2(CO)4{κ(2)-Ph2PN(R)PPh2}(μ-pdt) are formed, while in refluxing toluene bridge isomers Fe2(CO)4{μ-Ph2PN(R)PPh2}(μ-pdt) are the major products. Separate studies have shown that chelate complexes convert to the bridge isomers at higher temperatures. Two pairs of bridge and chelate isomers (R = allyl, (i)Pr) have been crystallographically characterised together with Fe2(CO)4{μ-Ph2PN(H)PPh2}(μ-pdt). Chelate complexes adopt the dibasal diphosphine arrangement in the solid state and exhibit very small P-Fe-P bite-angles, while the bridge complexes adopt the expected cisoid dibasal geometry. Density functional calculations have been carried out on the chelate and bridge isomers of the model compound Fe2(CO)4{Ph2PN(Me)PPh2}(μ-pdt) and reveal that the bridge isomer is thermodynamically favourable relative to the chelate isomers that are isoenergetic. The HOMO in each of the three isomers exhibits significant metal-metal bonding character, supporting a site-specific protonation of the iron-iron bond upon treatment with acid. Addition of HBF4·Et2O to the Fe2(CO)4{κ(2)-Ph2PN(allyl)PPh2}(μ-pdt) results in the clean formation of the corresponding dibasal hydride complex [Fe2(CO)4{κ(2)-Ph2PN(allyl)PPh2}(μ-H)(μ-pdt)][BF4], with spectroscopic measurements revealing the intermediate formation of a basal-apical isomer. A crystallographic study reveals that there are only very small metric changes upon protonation. In contrast, the bridge isomers react more slowly to form unstable species that cannot be isolated. Electrochemical and electrocatalysis studies have been carried out on the isomers of Fe2(CO)4{Ph2PN(allyl)PPh2}(μ-pdt). Electron accession is predicted to occur at an orbital that is anti-bonding with respect to the two metal centres based on the DFT calculations. The LUMO in the isomeric model compounds is similar in nature and is best described as an antibonding Fe-Fe interaction that contains differing amounts of aryl π* contributions from the ancillary PNP ligand. The proton reduction catalysis observed under electrochemical conditions at ca. -1.55 V is discussed as a function of the initial isomer and a mechanism that involves an initial protonation step involving the iron-iron bond. The measured CV currents were higher at this potential for the chelating complex, indicating faster turnover. Digital simulations showed that the faster rate of catalysis of the chelating complex can be traced to its greater propensity for protonation. This supports the theory that asymmetric distribution of electron density along the iron-iron bond leads to faster catalysis for models of the Fe-Fe hydrogenase active site.

Synthesis, spectroscopic characterization, and crystal structures of diiron pentacarbonyl complexes with monosubstituted tris(3-chlorophenyl)phosphine or cyclohexyldiphenylphosphine ligands

“…2 and 3, the molecular structures of both complexes contain a butterfly diiron cluster coordinated by propanedithiolate, five terminal carbonyls and a monophosphine ligand. The [15], but shorter than those found in natural enzymes (2.55-2.62 Å ) [2,3] and other diiron complexes [25][26][27][28]. The average Fe-C bond distances of the substituted Fe (1.768 Å for 2 and 1.764 Å for 3) are slightly shorter than those of the unsubstituted Fe (1.796 Å for 2 and 1.781 Å for 3) due to the monophosphine having stronger electron-donating properties than CO [29,30].…”

Electrochemical and Computational Study of the Reactivity of a Diiron Azadithiolate Complex towards Protons in the Presence of Coordinating Anions

“…[1][2][3][4][5][6] Two key features have been identified and targeted: [7][8][9][10][11][12][13][14][15][16][17] (1) a rotated geometry along the Fe-Fe axis favoring the formation of a terminal hydride intermediate and (2) an amino group that can be protonated, in the dithiolate bridge, serving as a proton shuttle to the metal center (Scheme 1). On that basis, Rauchfuss and co-workers synthesized [Fe 2 (μ-adt H )(CO) 2 -(κ 2 -dppv) 2 ] {adt H = (SCH 2 ) 2 NH, dppv = (PPh 2 ) 2 C 2 H 2 }, a FeFe-H 2 ase model that catalyzes the electrochemical reduction of a weak acid (pK a ≈ 15 in MeCN) to H 2 at an unprecedented TOF value of 58,000 s -1 with an overpotential of 0.5 V.…”

Reactions of Fe2(μ‐odt)(CO)6 (odt = 1, 3‐oxadithiolate) with small bite‐angle diphosphines to afford the monodentate, chelate, and bridge diiron complexes: Selective substitution, structures, protonation, and electrocatalytic proton reduction

“…Meanwhile, Sun and co‐workers found that a toluene solution of diiron adt complex Fe 2 { μ ‐adt N(Pr‐ n ) }(CO) 6 with dppm was refluxed to afford only a bridge complex Fe 2 { μ ‐adt N(Pr‐ n ) }(CO) 4 ( μ ‐dppm) . Moreover, Hogarth, Song, and we reported that a series of PNP‐chelate diiron complexes Fe 2 ( μ ‐xdt)(CO) 4 { κ 2 ‐(Ph 2 P) 2 NR} were prepared by the carbonyl substitutions of Fe 2 ( μ ‐xdt)(CO) 6 (xdt = pdt, edt, adt) with ligands (Ph 2 P) 2 NR (PNP) using Me 3 NO assistance and UV irradiation …”