Molybdenum complexes 1,2-dithiolene ligands

Lowe and Garner (1993a,b) have synthesized three new dithiolene ligands and formed complexes with a variety of transition metals (Fig. 23) including molybdenum [40]-[42], tungsten [43] and [44] and nickel [45]—[48]. The electrochemical properties of the complexes and free ligands were studied by... 

A combination of descent in symmetry (Ctv —> C3v —> Cs) and orbital overlap considerations have been utilized in order to assign the six lowest energy bands in the (L-/V )MoO(dithiolene) series. The inherently low oscillator strengths of the Mo ligand-field bands in oxo-molybdenum complexes (s < 100 M em ), coupled with the presence of low-energy LMCT excitations, result in ligand-field transitions that are difficult to observe in (L-/V3)MoO(dithiolene) compounds. As a result, LMCT bands are anticipated to dominate the absorption spectra of these complexes. 

For (L-N3)MoO(SR)2 complexes the two thiolate groups must be cis to one another, as is required by the proposed structure for molybdop-terin (1). Several (L-N3)MoO(SR)2 complexes containing both monoden-tate and chelating thiolate ligands have been prepared (91,151,153). Their EPR parameters are similar to those for the Mo(V) states of enzymes. To date, (L-N3)MoO(tdt) (tdt, toluenedithiolate) (41) is the only example of a model compound that contains both an [Mo O] group and a single dithiolene ligand, a structural feature postulated for the molybdenum(V) state of Mo-co (2). 

From the cofactor structure, we now know that the thiol sulfurs are not directly connected to the pterin ring rather, they are appended from the pyran ring. Therefore, formation of urothione from the molybdenum cofactor must involve a cyclization step, and a similar process may also be involved in the formation of form B (1). Such reactions have been modeled through oxidation of quinoxaline dithiolene ligand (Scheme 2.29) as well as in complexes (see Scheme 2.20 above). ... 

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