Protophilicity, electrochemical property, and desulfurization of diiron dithiolate complexes containing a functionalized C2 bridge with two vicinal basic sites
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 FeIFeI/FeIFe0 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 FeIFeI-to-FeIFe0 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 FeIFeI/FeIFe0 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 FeIFeI/FeIFe0 reduction potentials. Here, we report the preparation and characterization of 2–6, the molecular structures of 2, its protonated species [(2HN)(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 [(2HN)(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 [(2HN)(OTf)], as well as 3 and [(3HN)(OTf)], are instant and reversible. The protonated species [(2HN)(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.
References (30)
- et al.
Structure
(1999) - et al.
Trends Biochem. Sci.
(2000) - et al.
Inorg. Chim. Acta
(2002) - et al.
J. Organomet. Chem.
(2007) - et al.
Polyhedron
(1993) - et al.
Science
(1998) - et al.
J. Am. Chem. Soc.
(2001) - et al.
J. Am. Chem. Soc.
(2008) - et al.
J. Am. Chem. Soc.
(2001) - et al.
Inorg. Chem.
(2004)
J. Am. Chem. Soc.
Inorg. Chem.
Angew. Chem., Int. Ed.
Angew. Chem., Int. Ed.
Inorg. Chem.
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