Dithiolene ligands, synthesis

The dithiolene ligands offer a variety of synthetic and structural possibilities and choices. The following description introduces in an intentionally general fashion (which some might consider to be superficial) the various classes and describes the methods used in their synthesis, while a more detailed discussion of their properties will be given in the subsequent parts of this section. 

Since several molybdoenzymes feature a single dithiolene ligand active site (30), the synthesis of monodithiolene complexes has been of interest. Two general approaches can be envisioned. Stepwise installation of dithiolene... 

Information on the synthesis of the dithiolene ligands and their complexes may be found in Chapter 1 of this volume (39). Likewise, information on the preparation of the dithiolene complex-based compounds described in this chapter may be found in previous articles and reviews (9, 10, 40-46) and will not be discussed here. 

A close structural relationship with I IF derivatives, especially BEDT-TTF, is exhibited by dddt metal complexes [dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate (63)]. The most interesting feature of this dithiolene ligand is the ability of its metal complexes to form not only anionic salts like dmit, but also cationic salts like TTF derivatives [89], to afford non-stoichiometric IR salts of type [M(dddt)2]mX . Thus the cyclic voltam-mogram of [Bu4N][Ni(dddt)2], after its initial oxidation, exhibits the reduction of neutral [Ni(ddt)2]° to anion [Ni(ddt)2]" at 0 V, and its further reduction to the dianion [Ni(dddt)2]2 , as well as the oxidation of [Ni(dddt)2]° to the cation [Ni(dddt)2] + at 0.8 V (MeCN versus Ag/Ag/Cl). The feasible synthesis of conducting donor-acceptor complexes involving dddt metal derivatives as donors and dmit metal derivatives as acceptors has also been demonstrated [90]. 

Consistent with the dithiolene structure proposed for the oxidized derivative of molybdopterin (3), no nonexchangeable H resonances are observed in the region expected for the CH protons of a dithiolate structure (5b). The mass spectrum of 3 is also consistent with the proposed dithiolene structure (19, 34). Resonance Raman spectra of DMSO reductase show bands at 1575 cm (oxidized form) and 1568 cm (reduced form) that are assigned to the C=C stretch of the dithiolene unit of 2 (40). However, the delocalized electronic structure of dithiolene ligands makes it difficult to assign the C=C stretch with certainty. As Rajagopalan notes (19), ultimate proof of the structure [of molybdopterin] will have to await either X-ray studies on a molyb-doenzyme or unequivocal chemical synthesis of the molecule. ... 

Synthesis of Dithiolene Ligands and Complexes from Protected Dithiolene Precursors... 

Scheme 2.25 Synthesis of cobalt complex coordinated by a single dithiolene ligand.

Scheme 2.32 Bradshaw et al. synthesis of reduced pyranopterin dithiolene ligand in a highly protected form.

The synthesis of dithiolene complexes has been exhaustively reviewed by Rauchfuss. The most common methods used to prepare Mo enzyme model compounds are the reactions of alkynes with polysulfido-Mo complexes and of protected or free dithiolene ligands with Mo complexes. Dithiolene-Mo complexes fall into two broad classes, those containing arene-l,2-dithio-lates (pseudo-dithiolenes) and those containing true dithiolenes both are accorded equal status here. These synthetic methods and structural types all feature in the excellent, dithiolene-based models now available for enzymes of the SO and DMSOR families and biologically relevant mixed-ligand dithiolene and pterindithiolene complexes the reader is referred to... 

The suppression of the thermodynamically favoured decomposition of Mo enzyme active sites is achieved by the stabilization and steric protection afforded by the apoprotein. Co-ligand sterie protection also plays an important role in stabilizing known MoHMs (Seetion 7.4). Unfortunately, the synthesis of sterically encumbered dithiolene ligands, an obvious strategy for... 

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