Synthesis, Characterization, and Reactions of Functionalized Nickel–Iron Dithiolates Related to the Active Site of [NiFe]-Hydrogenases

Mononuclear Four-Coordinate Bis-Fluoride Bis-NHC Complexes of Chromium(II), Iron(II), and Cobalt(II)

The reaction between silylamido complexes of Cr(II), Fe(II), and Co(II) and IMes·2HF salt in the presence of IMes (IMes = 1,3-dimesitylimidazol-2-ylidene) led to isolation of Cr(IMes)2F2 (2-Cr), Fe(IMes)2F2 (2-Fe), and Co(IMes)2F2 (2-Co). X-ray structural studies revealed that 2-Cr adopts square planar geometry, while 2-Fe and 2-Co have distorted tetrahedral geometry. Magnetic susceptibility studies of 2-Cr, 2-Fe, and 2-Co were consistent with high-spin complexes, S = 2 for 2-Cr/2-Fe and S = 3/2 for 2-Co. We demonstrated that fluoride can be successfully exchanged for cyanide and azide using trimethylsilyl cyanide and trimethylsilyl azide (3-Fe and 4-Fe). DFT studies suggest that the preference of 2-Cr to adopt square planar geometry over tetrahedral is due to its d4 metal center, where four electrons fill the lower-lying d-orbitals.

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.

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.

Synthetic Designs and Structural Investigations of Biomimetic Ni-Fe Thiolates

Described are the syntheses of several Ni(μ-SR)₂Fe complexes, including hydride derivatives, in a search for improved models for the active site of [NiFe]-hydrogenases. The nickel(II) precursors include (i) nickel with tripodal ligands: Ni(PS₃)^- and Ni(NS₃)^- (PS₃^3- = tris(phenyl-2-thiolato)phosphine, NS₃^3- = tris(benzyl-2-thiolato)amine), (ii) traditional diphosphine-dithiolates, including chiral diphosphine R,R-DIPAMP, (iii) cationic Ni(phosphine-imine/amine) complexes, and (iv) organonickel precursors Ni( o-tolyl)Cl(tmeda) and Ni(C₆F₅)₂. The following new nickel precursor complexes were characterized: PPh₄[Ni(NS₃)] and the dimeric imino/amino-phosphine complexes [NiCl₂(PCH═N^An)]₂ and [NiCl₂(PCH₂NH^An)]₂ (P = Ph₂PC₆H₄-2-). The iron(II) reagents include [CpFe(CO)₂(thf)]BF₄, [Cp*Fe(CO)(MeCN)₂]BF₄, FeI₂(CO)₄, FeCl₂(diphos)(CO)₂, and Fe(pdt)(CO)₂(diphos) (diphos = chelating diphosphines). Reactions of the nickel and iron complexes gave the following new Ni-Fe compounds: Cp*Fe(CO)Ni(NS₃), [Cp(CO)Fe(μ-pdt)Ni(dppbz)]BF₄, [( R,R-DIPAMP)Ni(μ-pdt)(H)Fe(CO)₃]BAr^F₄, [(PCH═N^An)Ni(μ-pdt)(Cl)Fe(dppbz)(CO)]BF₄, [(PCH₂NH^An)Ni(μ-pdt)(Cl)Fe(dppbz)(CO)]BF₄, [(PCH═N^An)Ni(μ-pdt)(H)Fe(dppbz)(CO)]BF₄, [(dppv)(CO)Fe(μ-pdt)]₂Ni, {H[(dppv)(CO)Fe(μ-pdt)]₂Ni]}BF₄, and (C₆F₅)₂Ni(μ-pdt)Fe(CO)₂(dppv) (DIPAMP = (CH₂P(C₆H₄-2-OMe)₂)₂; BAr^F₄^- = [B(C₆H₃-3,5-(CF₃)₂]₄^-)) Within the context of Ni-(SR)₂-Fe complexes, these new complexes feature new microenvironments for the nickel center: tetrahedral Ni, chirality, imine, and amine coligands, and Ni-C bonds. In the case of {H[(dppv)(CO)Fe(μ-pdt)]₂Ni}⁺, four low-energy isomers are separated by ≤3 kcal/mol, one of which features a biomimetic HNi(SR)₄ site, as supported by density functional theory calculations.

Nickel-Iron Dithiolato Hydrides Derived from H2 Activation by Their μ-Hydroxo Ligand-Containing Analogues

Here, we describe the synthesis and characterization of the two μ-hydrido models of [NiFe]H₂ases prepared from a new type of H₂ activation by the corresponding two μ-hydroxo models. In addition, another μ-hydrido model prepared by the anionic exchange between one of the two μ-hydrido models and KPF₆ is also reported. Interestingly, the synthesis of the two μ-hydrido models from H₂ activation represents the first functional modeling of the H₂ activation catalyzed by the [NiFe]H₂ase (Ni-SIr)_I state to give the Ni-R state.

Crystal structure of cis-[1,2-bis-(di-phenyl-phosphan-yl)ethene-κ2P,P']di-chlorido-platinum(II) chloro-form disolvate: a new polymorph

The title compound, [PtCl2(C26H22P2)]·2CHCl3 (I), is the third monoclinic polymorph of this platinum(II) complex involving the bidentate ligand cis-1,2-bis-(di-phenyl-phosphan-yl)ethyl-ene (cis-dppe) [for the others, see: Oberhauser et al. (1998a ▸). Inorg. Chim. Acta, 274, 143-154, and Oberhauser et al. (1995 ▸). Inorg. Chim. Acta, 238, 35-43]. The structure of compound (I) was solved in the space group P21/c, with one complex mol-ecule in the asymmetric unit along with two solvate chloro-form mol-ecules. The PtII atom is ligated by two P and two Cl atoms in the equatorial plane and has a perfect square-planar coordination sphere. In the crystal, the complex mol-ecule is linked to the chloro-form solvate mol-ecules by C-H⋯Cl hydrogen bonds and face-on C-Cl⋯π inter-actions. There are also weak offset π-π inter-actions present [inter-centroid distances are 3.770 (6) and 4.096 (6) Å], linking the mol-ecules to form supra-molecular sheets that lie in the bc plane.