Azadithiolate

Chiang and coworkers synthesized a dimer of compound 26 in which two diiron subunits are linked by two azadithiolate ligands as a model of the active site for the [FeFeJ-hydrogenase [203]. Protonation of 26 afforded the p-hydride complex [26-2H 2H ] via the initially protonated spieces [26-2H ] (Scheme 62). These three complexes were also characterized by the X-ray diffraction analyses. H2-generation was observed by electrochemical reduction of protons catalyzed by 26 in the presence of HBF4 as a proton source. It was experimentally ascertained that [26-2H 2H ] was converted into 26 by four irreversible reduction steps in the absence of HBF4. 

A light-driven compound containing a photosensitizing tetraphenylporphyrin group linked to a diiron azadithiolate moiety (25) has been synthesized as a model compound of iron-only hydrogenases. This compound reduces protons photochemically to hydrogen.392... 

Li, H. and Rauschfuss, B. 2002. Iron carbonyl sulfides, formaldehyde, and amines condense to given the proposed azadithiolate cofactor of the Fe-only hydrogenaes. J. Am. Chem. Soc. 124, 726-727. 

The two iron atoms are bridged by a novel dithiolate cofactor wherein the two sulfur atoms are connected via a three-atom chain. This three-atom chain has been postulated to be -CH2NHCH2- [15]. Heteroatom X (especially an amine nitrogen) is within hydrogen-bonding distance to a nearby cysteine-SH. For example, X and the S of cys 331 are separated in DdH by only 3.1 A, a distance that is appropriate for a hydrogen bond [15]. An azadithiolate cofactor could participate in H2 formation (Eq. 12.2). 

It is well known from classical organometallic chemistry that non-coordinating amine bases affect the behavior of H2 complexes [44]. The azadithiolate motif has been exploited in the design of new catalysts for hydrogen oxidation, as described in the next section on [NiFe] hydrogenases [45]. 

The azadithiolate-containing hexacarbonyl Fe2[(SCH2)2NCH2CgH4Br)](CO)i5 also catalyzes hydrogen evolution [43]. Due to the presence of six CO ligands, this complex is more easily reduced than the substituted derivatives, operating at 0.9 V vs. AgjAgCl (Fig. 12.6). 

Fig. 12.6 Proposed cycle for H2 production catalyzed by azadithiolate-modified diiron hexacarbonyl.

In recent work, Ott efal. have prepared the biomimetic complex 174 in which the diiron unit is covalently linked to a [Ru(terpy)2] photosensitizer, in a first attempt toward making a light-driven proton reduction system. IR spectra of 174 are identical to the parent azadithiolate complex, suggesting that the metal centers are electronically isolated. The observed excited-state lifetime is 6.5 ns, being substantially shorter than related non-iron-containing compounds, and combined with electrochemical measurements this shows that electron transfer from the photogenerated [Ru(terpy)2] excited state to the diiron center is uphill by 0.59 eV. ... 

The azadithiolate-bridged diiron compounds that have been developed as stmctural model systems for Fe-only hydrogenase are reviewed in this article. The functionalized diiron complexes which show some ability to generate hydrogen are surveyed, with emphasis on the synthesis and the electrocatalytic properties. The electrocatalytic properties of all complexes investigated by cyclic voltammetry in the presence and absence of acid are described. In addition, the application of electrochemical and IR spectroelectrochemical (SEC) techniques to the elucidation of the details of the electrocatalytic proton reduction is described. The functional mechanistic proposals are discussed from these work. 

One of the main directions of study, among many others, of the bioinoiganic chemist is to prepare diiron azadithiolate complexes which have been considered as relevant to the diiron subsite chemistry. So, in this article, we will present a short non-exhaustive overview of diiron azadithiolate model complexes related to the Fe-only hydrogenase active site. Several diiron complexes reported by our laboratory will be discussed, including the electrocatalytic properties. 

Although few detailed electroehemieal studies have been reported until now, several diiron azadithiolate complexes have been used as catalysts for the electrochemical reduction of protons to H2, including protonation and reduction behavior. The electrochemistry data cited in this article all have been converted to values relative to the Fc /Fc. 

Lawrence, J. D. Li, H. Rauehfuss, T. B. Benard, M. Rohmer, M.-M. Diiron azadithiolates as models for the iron-only hydrogenase active site synthesis, structure, and stereoeleetronies. Angew. Chem. Int. Ed. Engl. 2001, 40, 1768-1771. 

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