Protophilicity, electrochemical property, and desulfurization of diiron dithiolate complexes containing a functionalized C2 bridge with two vicinal basic sites

doi:10.1016/j.poly.2009.01.006

Polyhedron

Volume 28, Issue 6, 23 April 2009, Pages 1138-1144

https://doi.org/10.1016/j.poly.2009.01.006 Get rights and content

Abstract

Two diiron dithiolate complexes [{μ-SC(NBn)CH(NHBn)S-μ}Fe₂(CO)₅L] (L = PPh₃, 2; P(Pyr)₃, 3) containing a functionalized C₂ bridge with two vicinal basic sites were prepared and characterized as models of the FeFe-hydrogenase active site. The molecular structures of 2 and its N-protonated form [(2H_N)(OTf)] were determined by X-ray analyses of single crystals. In the solid state of [(2H_N)(OTf)]. Each asymmetric unit contains a molecule of [(2H_N)(OTf)] and a molecule of water. The molecule of water is close to the iron atom of the [Fe(CO)₃] unit (Fe···O(H₂O), 4.199 Å). The complexes 2 and 3 are relatively protophilic. ³¹P NMR spectra and cyclic voltammograms show that they can be protonated by the mild acids CCl₃COOH and CF₃COOH. Electrochemical studies show that the first reduction peak of 3 at −1.51 V versus Fc⁺/Fc is 110 mV more positive than that (−1.62 V) found for the analogous diiron azadithiolate complex [{(μ-SCH₂)₂N(CH₂C₆H₅)}Fe₂(CO)₅P(Pyr)₃] (7). Protonation of 2 and 3 leads to the anodic shifts of 610–650 mV for the Fe^IFe^I/Fe^IFe⁰ reduction potentials. The shifts are apparently larger than that (450 mV) for protonation of 7. The reaction of the all-carbonyl complex [{μ-SC(NBn)CH(NHBn)S-μ}Fe₂(CO)₅L] with two equivalents of bis(diphenylphosphino)methane (dppm) in refluxing toluene affords a desulfurized complex [(μ-S)(μ-dppm)₂Fe₂(CO)₄] (6). The reaction process was studied. A dppm mono-dentate intermediate [{μ-SC(NBn)CH(NHBn)S-μ}Fe₂(CO)₅(κ¹-dppm)] (4) and a dppm μ-bridging species [{μ-SC(NBn)CH(NHBn)S-μ}Fe₂(CO)₄(μ-dppm)] (5) have been isolated and spectroscopically characterized.

Graphical abstract

Diiron dithiolate complexes containing a functionalized C₂ bridge with two vicinal basic sites were prepared as models of the FeFe-hydrogenase active site. Protonation of these complexes is instant and reversible, which leads to a large anodic shift (610–650 mV) for the Fe^IFe^I/Fe^IFe⁰ reduction potentials. Desulfurization reaction was found for the C₂-bridged diiron complex.

Introduction

The chemistry of diiron dithiolate complexes has attracted intensive attention in recent years since their close resemblance in structure to the FeFe-hydrogenase active site (H-cluster) [1], [2]. The DFT investigations suggest that the central atom in the dithiolate bridge of the H-cluster is a nitrogen (SCH₂NHCH₂S) or an oxygen atom (SCH₂OCH₂S) [3], [4], [5]. Both the theoretical calculations and the studies on the synthetic model complexes have shown that the internal base may act as a proton transfer relay in the [FeFe]-H₂ase active site (H-cluster) [6], [7], [8], [9]. The protophilicity and the electrochemical property of diiron dithiolate complexes containing different internal bases have been extensively reported. Protonation of diiron azadithiolate complexes at the bridging-nitrogen atom has been verified spectroscopically and crystallographically [8], [9], [10], [11], [12], [13], which results in considerable anodic shifts (370–420 mV) of the Fe^IFe^I-to-Fe^IFe⁰ reduction potentials for the diiron complexes. Double protonated forms of diiron azadithiolate complexes, both at the bridging-nitrogen atom and the iron center, have also been spectroscopically characterized in situ [14], [15], [16]. Recently, we have reported the crystal structure of a proton-hydride diiron complex [(μ-H)(μ-pdt){Fe(CO)₃}{Fe(CO)(κ²p,p′-PNHP)}](OTf)₂ (PNP = Ph₂PCH₂N(n-Pr)CH₂PPh₂) [17]. In addition, there is one report on the double protonation at the σ-donor ligand (CN⁻) and the iron center, resulting in an unstable species [18]. The pyridyl group has been tethered to the central carbon of the propane-1,3-dithiolato (pdt) bridge of the diiron complex as an internal base and a hemi-labile ligand [19]. The protonation and deprotonation of the pyridyl group can control the extent of carbonylation of the diiron model complex.

The afore-mentioned internal bases of the FeFe-Hase mimics, namely, the nitrogen atoms in the azapropanedithiolato bridge, the CN⁻ ligand, and the pyridyl group, can be protonated in organic solvents only in the presence of strong acids [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Studies on the diiron complexes featuring an internal basic site, which can be readily protonated by weak acids with a large influence on the Fe^IFe^I/Fe^IFe⁰ reduction potential, are of interest for design of iron-based electrochemical and photochemical catalysts for proton reduction to dihydrogen. In this work, we prepared and characterized two diiron dithiolate complexes [{μ-SC(NBn)CH(NHBn)S-μ}Fe₂(CO)₅L] (L = PPh₃, 2; P(Pyr)₃, 3) containing a functionalized C₂ bridge with two vicinal basic sites. We found that the reaction of the complex [{μ-SC(NBn)CH(NHBn)S-μ}Fe₂(CO)₆] (1) with two equivalents of bis(diphenylphosphino)methane (dppm) in refluxing toluene afforded a desulfurized complex [(μ-S)(μ-dppm)₂Fe₂(CO)₄] (6) via a dppm mono-dentate intermediate [{μ-SC(NBn)CH(NHBn)S-μ}Fe₂(CO)₅(κ¹-dppm)] (4) and a dppm μ-bridging species [{μ-SC(NBn)CH(NHBn)S-μ}Fe₂(CO)₄(μ-dppm)] (5). The complexes 2 and 3 can be protonated by mild acids CCl₃COOH and CF₃COOH in acetonitrile, resulting in a 610–650 mV anodic shift of the Fe^IFe^I/Fe^IFe⁰ reduction potentials. Here, we report the preparation and characterization of 2–6, the molecular structures of 2, its protonated species [(2H_N)(OTf)], and 6, the protophilicity of 2 and 3, as well as the electrochemical properties of 2 and 3 compared with the analogous diiron azadithiolate complex [{(μ-SCH₂)₂N(CH₂C₆H₅)}Fe₂(CO)₅P(Pyr)₃] (7).

Section snippets

General procedures and materials

All reactions and operations related to organometallic complexes were carried out under dry, oxygen-free dinitrogen with standard Schlenk techniques. Solvents were dried and distilled prior to use according to the standard methods. Commercially available chemicals, Me₃NO · 2H₂O, HOTf, CCl₃COOH, CF₃COOH, PPh₃, and dppm, were of reagent grade and used as received. Ligand P(Pyr)₃, diiron carbonyl complexes [{μ-SC(NBn)CH(NHBn)S-μ}Fe₂(CO)₆] (1) and [{(μ-SCH₂)₂N(CH₂C₆H₅)}Fe₂(CO)₅P(Pyr)₃] (7) were

Preparation and spectroscopic characterization of the complexes 2, 3, and [(2H_N)(OTf)]

The parent complex [{μ-SC(NBn)CH(NHBn)S-μ}Fe₂(CO)₆] (1) was prepared according to the literature method [20], which is an oil and very sensitive to air and moisture. More stable diiron complexes [{μ-SC(NBn)CH(NHBn)S-μ}Fe₂(CO)₅PPh₃] (2) and [{μ-SC(NBn)CH(NHBn)S-μ}Fe₂(CO)₅P(Pyr)₃] (3) were obtained by treating 1 with a phosphine ligand in the presence of the CO-removing reagent Me₃NO · 2H₂O in CH₃CN. The complexes 2 and 3 are stable in the solid state and in the solution. They were characterized by

Conclusion

The ³¹P NMR spectra and CVs show that the complexes 2 and 3 with a C₂ bridge containing two basic sites are more protophilic than other internal bases in the FeFe-Hase mimics reported so far. The omplexes 2 and 3 can be protonated by CCl₃COOH and CF₃COOH in organic solvent. Protonation and deprotonation processes between 2 and [(2H_N)(OTf)], as well as 3 and [(3H_N)(OTf)], are instant and reversible. The protonated species [(2H_N)(OTf)] is relatively stable because of the proper distance for

Acknowledgments

We are grateful to the National Natural Science Foundation of China (Grant No. 20633020), the National Basic Research Program of China (Grant No. 2009CB220009), the Program for Changjiang Scholars and Innovative Research Team in University (IRT0711), the Swedish Energy Agency, the Swedish Research Council, and K&A Wallenberg Foundation for financial support of this work.

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Protophilicity, electrochemical property, and desulfurization of diiron dithiolate complexes containing a functionalized C2 bridge with two vicinal basic sites

Abstract

Graphical abstract

Introduction

Section snippets

General procedures and materials

Preparation and spectroscopic characterization of the complexes 2, 3, and [(2HN)(OTf)]

Conclusion

Acknowledgments

Structure

Trends Biochem. Sci.

Inorg. Chim. Acta

J. Organomet. Chem.

Polyhedron

Science

J. Am. Chem. Soc.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Inorg. Chem.

J. Am. Chem. Soc.

Inorg. Chem.

Angew. Chem., Int. Ed.

Angew. Chem., Int. Ed.

Inorg. Chem.

Protophilicity, electrochemical property, and desulfurization of diiron dithiolate complexes containing a functionalized C₂ bridge with two vicinal basic sites

Preparation and spectroscopic characterization of the complexes 2, 3, and [(2H_N)(OTf)]