Dithiolato- and Diselenolato-Bridged Nickel–Iron Biomimetics for the Active Site of [NiFe]Hydrogenases

Functionalized nickel(II)-iron(II) dithiolates as biomimetic models of [NiFe]-H2ases

To develop the structural and functional modeling chemistry of [NiFe]-H₂ases, a series of new biomimetics for the active site of [NiFe]-H₂ases have been prepared by various synthetic methods. Treatment of the mononuclear Ni complex (pnp)NiCl₂ (pnp = (Ph₂PCH₂)₂NPh) with (dppv)Fe(CO)₂(pdt) (dppv = 1,2-(Ph₂P)₂C₂H₂, pdt = 1,3-propanedithiolate) and KPF₆ gave the dicarbonyl complex [(pnp)Ni(pdt)Fe(CO)₂(dppv)](PF₆)₂ ([1](PF₆)₂). Further treatment of [1](PF₆)₂ and [(dppe)Ni(pdt)Fe(CO)₂(dppv)](BF₄)₂ (dppe = 1,2-(Ph₂P)₂C₂H₄) with the decarbonylation agent Me₃NO and pyridine afforded the novel sp³ C-Fe bond-containing complexes [(pnp)Ni(SCH₂CH₂CHS)Fe(CO)(dppv)]PF₆ ([2]PF₆) and [(dppe)Ni(SCH₂CH₂CHS)Fe(CO)(dppv)]BF₄ ([3]BF₄). More interestingly, the first t-carboxylato complexes [(pnp)Ni(pdt)Fe(CO)(t-O₂CR)(dppv)]PF₆ ([4]PF₆, R = H; [5]PF₆, R = Me; [6]PF₆, R = Ph) could be prepared by reactions of [1]PF₆ with the corresponding carboxylic acids RCO₂H in the presence of Me₃NO, whereas further reactions of [4]PF₆-[6]PF₆ with aqueous HPF₆ and 1.5 MPa H₂ gave rise to the μ-hydride complex [(pnp)Ni(pdt)Fe(CO)(μ-H)(dppv)]PF₆ ([7]PF₆). Except for H₂ activation by t-carboxylato complexes [4]PF₆-[6]PF₆ to give a μ-hydride complex ([7]PF₆), the sp³ C-Fe bond-containing complex [2]PF₆ was found to be a catalyst for proton reduction to H₂ under CV conditions. Furthermore, the chemical reactivity of the μ-hydride complex [7]PF₆ displayed in the e^- transfer reaction with FcPF₆ in the presence of CO, the H₂ evolution reaction with the protonic acid HCl, and the H^- transfer reaction with N-methylacridinium hexafluorophosphate ([NMA]PF₆) was systematically studied. As a result, a series of the expected products such as H₂, ferrocene, the dicarbonyl complex [1](PF₆)₂, the μ-chloro complex [(pnp)Ni(pdt)Fe(CO)(μ-Cl)(dppv)]PF₆ ([8]PF₆), the t-MeCN-coordinated complex [(pnp)Ni(pdt)Fe(CO)(t-MeCN)(dppv)](PF₆)₂ ([9](PF₆)₂) and the H^- transfer product AcrH₂ were produced. While all the newly prepared model complexes were structurally characterized by spectroscopic methods, the molecular structures of some of their representatives were confirmed by X-ray crystallography.

Synthesis, Structures and Chemical Reactivity of Dithiolato-Bridged Ni-Fe Complexes as Biomimetics for the Active Site of [NiFe]-Hydrogenases

To develop the structural and functional modeling chemistry of [NiFe]-H2ases, we have carried out a study regarding the synthesis, structural characterization and reactivity of a new series of [NiFe]-H2ase model complexes. Thus, treatment of diphosphine dppb-chelated Ni complex (dppb)NiCl2 (dppb = 1,2-(Ph2P)2C6H4) with (dppv)Fe(CO)2(pdt) (dppv = 1,2-(Ph2P)2C2H2, pdt = 1,3-propanedithiolate) and NaBF4 gave dicarbonyl complex [(dppb)Ni(pdt)Fe(CO)2(dppv)](BF4)2 ([A](BF4)2). Further treatment of [A](BF4)2 with Me3NO and Bu4NCN or KSCN afforded t-cyanido and t-isothiocyanato complexes [(dppb)Ni(pdt)Fe(CO)(t-R)(dppv)]BF4 ([1]BF4, R = CN; [2]BF4, R = NCS), respectively. While azadiphosphine MeN(CH2PPh2)2-chelated t-hydride complex [MeN(CH2PPh2)2Ni(pdt)Fe(CO)(t-H)(dppv)]BF4 ([3]BF4) was prepared by treatment of dicarbonyl complex [MeN(CH2PPh2)2Ni(pdt)Fe(CO)2(dppv)](BF4)2 ([B](BF4)2) with Me3NO and 1.5 MPa of H2, treatment of dicarbonyl complex [B](BF4)2 with Me3NO (without H2) in pyridine resulted in formation of a novel monocarbonyl complex [MeN(CH2PPh2)2Ni(SCHCH2CH2S)Fe(CO)(dppv)]BF4 ([4]BF4) via the unexpected sp3 C-H bond activation reaction. Furthermore, azadiphosphine PhN(CH2PPh2)2-chelated µ-mercapto complex [PhN(CH2PPh2)2Ni(pdt)Fe(CO)(µ-SH)(dppv)]BF4 ([5]BF4) was prepared by treatment of dicarbonyl complex [PhN(CH2PPh2)2Ni(pdt)Fe(CO)2(dppv)](BF4)2 ([C](BF4)2) with Me3NO and H2S gas, whereas treatment of azadiphosphine Ph2CHN(CH2PPh2)2-chelated dicarbonyl complex [Ph2CHN(CH2PPh2)2Ni(pdt)Fe(CO)2(dppe)](BF4)2 ([D](BF4)2, dppe = 1,2-(Ph2P)2C2H4) with Me3NO⋅2H2O gave rise to µ-hydroxo complex [Ph2CHN(CH2PPh2)2Ni(pdt)Fe(CO)(µ-OH)(dppe)]BF4 ([6]BF4). All the possible pathways for formation of the new model complexes are briefly discussed, and their structures were fully characterized by various spectroscopic techniques and for six of them by X-ray crystallography.

Cysteine residue-bridged dinuclear Ni–Fe complexes related to [NiFe]-H2ases

Cysteine residue-containing [NiFe]-H2ase models 1–6 have been prepared for the first time and some of them were found to be catalysts for H2 production from HOAc under CV conditions.

Biochemical and artificial pathways for the reduction of carbon dioxide, nitrite and the competing proton reduction: effect of 2nd sphere interactions in catalysis

Reduction of oxides and oxoanions of carbon and nitrogen are of great contemporary importance as they are crucial for a sustainable environment.

Heterodinuclear nickel(ii)-iron(ii) azadithiolates as structural and functional models for the active site of [NiFe]-hydrogenases

To develop the biomimetic chemistry of [NiFe]-H₂ases, the first azadithiolato-bridged NiFe model complexes [CpNi{(μ-SCH₂)₂NR}Fe(CO)(diphos)]BF₄ (5, R = Ph, diphos = dppv; 6, 4-ClC₆H₄, dppv; 7, 4-MeC₆H₄, dppv; 8, CO₂CH₂Ph, dppe; 9, H, dppe) have been synthesized via well-designed synthetic routes. Thus, treatment of RN[CH₂S(O)CMe]₂ with t-BuONa followed by reaction of the resulting intermediates RN(CH₂SNa)₂ with (dppv)Fe(CO)₂Cl₂ or (dppe)Fe(CO)₂Cl₂ gave the N-substituted azadithiolato-chelated Fe complexes [RN(CH₂S)₂]Fe(CO)₂(diphos) (1, R = Ph, diphos = dppv; 2, 4-ClC₆H₄, dppv; 3, 4-MeC₆H₄, dppv; 4, CO₂CH₂Ph, dppe). Further treatment of 1–4 with nickelocene in the presence of HBF₄·Et₂O afforded the corresponding N-substituted azadithiolato-bridged NiFe model complexes 5–8, while treatment of 8 with HBF₄·Et₂O resulted in formation of the parent azadithiolato-bridged model complex 9. While all the new complexes 1–9 were characterized by elemental analysis and spectroscopy, the molecular structures of model complexes 6–8 were confirmed by X-ray crystallographic study. In addition, model complexes 7 and 9 were found to be catalysts for H₂ production with moderate i_cat/i_p and overpotential values from TFA under CV conditions.

The first azadithiolato-bridged NiFe model complexes with a general formula [CpNi{(μ-SCH₂)₂NR}Fe(CO)(diphos)]BF₄ have been synthesized, characterized, and for some of them found to be catalysts for proton reduction to H₂ under CV conditions.

Two heterodinuclear NiFe-based sulfenate complexes mimicking an S-oxygenated intermediate of an O2-tolerant [NiFe]-H2ase: synthesis, structures, and reactivity

Two biomimetic models for an O₂-tolerant [NiFe]-H₂ase are successfully prepared by reactions of sulfenate complex 2 with Fe₂(CO)₉ and CpFe(CO)₂BF₄.

Crystal structure of NiFe(CO)5[tris(pyridyl-meth-yl)aza-phosphatrane]: a synthetic mimic of the NiFe hydrogenase active site incorporating a pendant pyridine base

The synthesis and the crystal structure of a NiFe complex containing an aza­phosphatrane ligand is discussed. The NiFe(TPAP)(CO)₅ complex contains a pendant pyridine base in close proximity to the NiFe metal center.

The reaction of Ni(TPAP)(COD) {where TPAP = [(NC₅H₄)CH₂]₃P(NC₂H₄)₃N} with Fe(CO)₅ resulted in the isolation of the title heterobimetallic NiFe(TPAP)(CO)₅ complex di-μ-carbonyl-tricarbon­yl[2,8,9-tris­(pyridin-2-yl­meth­yl)-2,5,8,9-tetra­aza-1-phosphabi­cyclo­[3.3.3]undeca­ne]ironnickel, [FeNi(C₂₄H₃₀N₇P)(CO)₅]. Characterization of the complex by ¹H and ³¹P NMR as well as IR spectroscopy are presented. The structure of NiFe(TPAP)(CO)₅ reveals three terminally bound CO mol­ecules on Fe⁰, two bridging CO mol­ecules between Ni⁰ and Fe⁰, and TPAP coordinated to Ni⁰. The Ni—Fe bond length is 2.4828 (4) Å, similar to that of the reduced form of the active site of NiFe hydrogenase (∼2.5 Å). Additionally, a proximal pendant base from one of the non-coordinating pyridine groups of TPAP is also present. Although involvement of a pendant base has been cited in the mechanism of NiFe hydrogenase, this moiety has yet to be incorporated in a structurally characterized synthetic mimic with key structural motifs (terminally bound CO or CN ligands on Fe). Thus, the title complex NiFe(TPAP)(CO)₅ is an unique synthetic model for NiFe hydrogenase. In the crystal, the complex mol­ecules are linked by C—H⋯O hydrogen bonds, forming undulating layers parallel to (100). Within the layers, there are offset π–π [inter­centroid distance = 3.2739 (5) Å] and C—H⋯π inter­actions present. The layers are linked by further C—H⋯π inter­actions, forming a supra­molecular framework.

Dithiolato- and halogenido-bridged nickel–iron complexes related to the active site of [NiFe]-H2ases: preparation, structures, and electrocatalytic H2 production

A new series of [NiFe]-H₂ase mimics (5a,b–7a,b) has been prepared and structurally characterized; particularly, they have been found to be pre-catalysts for H₂ production from Cl₂CHCO₂H under CV conditions.

Complexes of Ni(i): a “rare” oxidation state of growing importance

The synthesis and diverse structures, reactivity (small molecule activation and catalysis) and magnetic properties of Ni(i) complexes are summarized.

Synthetic [NiFe] models with a fluxional CO ligand

A [NiFe] complex [(dppe)Ni(pdt)FeCp*(CO)]BF₄ was characterized as two isomers, and their interconversions were established by thermal process and electrochemistry.

Nickel–ruthenium-based complexes as biomimetic models of [NiFe] and [NiFeSe] hydrogenases for dihydrogen evolution

Electrocatalytic proton reduction was studied using nickel–ruthenium complexes that were developed as models for [NiFe] and [NiFeSe] hydrogenases.