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Reports: UNI3 49424-UNI3: Rational Design and Synthesis of Structural Analog Complexes of the Active Site of Ni-Fe Hydrogenases
Introduction
We proposed to design and synthesize of analog complexes the active sites of Ni-Fe hydrogenases.
The synthesis start with fac-[Fe(CO)3X3]- (X = Br and I). We first managed to replace two halides by cyanide and obtained key intermediates fac-[Fe(CN)2(CO)3X]- (X = Br and I). Thiolate bridging Ni-Fe analog complexes [(dppe)Ni(m-SEt)2Fe(CN)2(CO)2]6 (dppe = 1,2-Bis(dipheylphosphino)ethane and SEt = ethanethiolate) and (dppe)Ni(m-pdt)Fe(CN)2(CO)2 (pdt = 1,3-propanedithiolate) were synthesized and structurally characterized. These results have been published. We are currently working on the synthesis of more Ni-Fe analog complexes with the general formula (L2)Ni(m-SR)2Fe(CN)2(CO)2 (L2 = bi-dentate supporting ligand) to study the effects of bridging thiolates and supporting ligands on the structural, stability and electronic properties of the analog complexes. Additionally, we are studying the reactions of intermediates fac-[Fe(CN)2(CO)3X]- (X = Br and I) with thiolates and other ligands.
Results
1. Synthesis of fac-[Fe(CO)3X3]1- (X = Br, I)
fac-[Fe(CO)3X3]1- (X = Br, I) were synthesized in high yield (Br (87%) and I (77%)). Both complexes were structurally characterized to confirm their geometry. These results have been published in Inorganic Chemistry Communications.
2. Synthesis of the Key Intermediate: fac-[Fe(CN)2(CO)3X]1- (X = Br, I)
fac-[Fe(CN)2(CO)3I]1- was synthesized by substitution reaction between 2 equivalents of cyanide and fac-[Fe(CO)3I3]1- (Scheme 1) (70% yield). However, fac-[Fe(CN)2(CO)3Br]1- was synthesized by the reaction of fac-[Fe(CN)2(CO)3I]1- with Br2 (Scheme 2) (90% yield). X-ray diffraction experiment has confirmed their structures (Figure 1). Part of these results has been published in Inorganic Chemistry.
Scheme 1: Synthesis of fac-[Fe(CN)2(CO)3I]1-
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Scheme 2: Synthesis of fac-[Fe(CN)2(CO)3Br]1-
Figure 1: ORTEP drawing of fac-[Fe(CN)2(CO)3I]1- at 50% probability level
3. Synthesis of Thiolate Bridging Ni-Fe analog complexes
Ni-Fe analog complexes [(dppe)Ni(m-SEt)2Fe(CN)2(CO)2]6, (dppe)Ni(m-pdt)Fe(CN)2(CO)2 and (dppp)Ni(m-pdt)Fe(CN)2(CO)2 (dppp = 1,3-Bis(diphenylphosphino)propane were synthesized and structurally characterized. Synthetic pathway is shown in Figure 2. Part of these results has been published in Inorganic Chemistry.
Figure 2: Synthesis of dithiolate bridging Ni-Fe complexes
In (dppe)Ni(m-pdt)Fe(CN)2(CO)2 and (dppp)Ni(m-pdt)Fe(CN)2(CO)2, the NiS2Fe rhomb are folded with dihedral angle close to 110o, in [(dppe)Ni(m-SEt)2Fe(CN)2(CO)2]6, the NiS2Fe rhomb are flat with dihedral angle close to 170o. As a result, the Ni-Fe distances are vastly different (2.8Å for folded structures vs. 3.4Å for flat structure). These results demonstrated that some changes in bridging thiolates have substantial effects on the structures of metal centers. On the contrary, the change of diphosphine supporting ligands (dppe or dppp) does not change the structures of the metal centers. We will study these effects in detail with different thiolates and supporting ligands. By choosing a series thiolates with different steric and electronic proprieties, we are hoping to figure out what are the factors to control the geometry of the resulting Ni-Fe dimmer. We will also study the system with supporting ligands which are less structural hindered to find out the effects from supporting ligands on the nickel side.
One of the unpredicting effects is the rearrangement of CN/CO ligands on iron during the preparation of Ni-Fe dimers. The structures of (dppe)Ni(m-pdt)Fe(CN)2(CO)2 and (dppp)Ni(m-pdt)Fe(CN)2(CO)2 showed that the 2 CN- ligands are trans- to one another. Since we started with the cis-CN intermediate fac-[Fe(CN)2(CO)3I]1-, a ligand rearrangement has occurred during its reaction with pdt, Ni(dppe)Cl2 or during crystallization. It is possible that an isomeric structure could also be present or could be generated under different reaction conditions (e.g. doing the reaction and crystallization in the dark). To illustrate the intrinsic reactivity of fac-[Fe(CN)2(CO)3X]1-, (X = Br, I) we are exploring the reaction of fac-[Fe(CN)2(CO)3X]1-, (X =Br, I) with many ligands such as thiolates, alkoxides, carboxylates, amines and phosphines. The reaction of fac-[Fe(CN)2(CO)3I]1- with triphenylphosphine (PPh3) was studied in detail.
4. Kinetics study of the reaction of fac-[Fe(CN)2(CO)3I]1- with PPh3
The reaction of fac-[Fe(CN)2(CO)3I]1- with PPh3 showed that one the CO trans- to iodide can be replaced. Kinetics study showed that the substitution reaction is first order with respect to fac-[Fe(CN)2(CO)3I]1- and PPh3. Transition state enthalpy and entropy changes are obtained. The transition state entropy for this reaction is positive suggesting a dissociative reaction pathway. IR spectra of the substitution reaction are shown in Figure 3. A manuscript describing these results was submitted to Inorganica Chimica Acta.
Figure 3: IR spectra of the reaction of 0.020 M fac-[Fe(CN)2(CO)3I]- with 0.60 M PPh3 in THF at 35.0oC. Spectra were collected every 2 minutes except for the last one, which was collected 2 hours after the mixing of reactants.
5. Alternative route to Ni-Fe analog complexes
Alternatively, we tried the reaction between fac-[Fe(CN)2(CO)3I]-, thiolates and Ni2+ in methanol solution in an attempt to make [(RS)2Ni(m-SR)2Fe(CN)2(CO)2]2-. This approach is still underway. However, we have isolated an Fe-Ni-Fe trinuclear complex [Ni[(m-SEt)2Fe(CN)2(CO)2]2]2- (Figure 4). In this complex, Nickel atom is coordinated to 4 thiolates. Addition of 2 equivalents of ethanethiolates may producing the desired [(EtS)2Ni(m-SEt)2Fe(CN)2(CO)2]2- (Scheme 3). This reaction is actively under investigation.
Figure 4: ORTEP drawing of [Ni[(m-SEt)2Fe(CN)2(CO)2]2]2- at 50% probability level
Scheme 3: Proposed reaction to make [(EtS)2Ni(m-SEt)2Fe(CN)2(CO)2]2-
Summary
Key intermediates fac-[Fe(CN)2(CO)3X]-, (X = Br, I) were synthesized. Thiolates-bridging Ni-Fe complexes as analog of the active sites of Ni-Fe hydrogenases were prepared. Effects on the metal center structures by bridging thiolates and terminal supporting ligands are studied. The proposed reactions were carried out smoothly. We are confident that we can achieve our goal in the near future.
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