Unique Reactivity Characteristics of Mo-coordinated !&- and S$- Ligands

The synthesis of the new dithiolene complexes, [{(MeOOC),C,S2}2Mo(2-S)]; and [OMo(S2C2(COOMe)2)2]2, is reported. These complexes are obtained by the reaction of dicarbomethoxyacetylene (DMA) with either [(S~)MO(S)(~~-S)~MO(S)(S~)]~or [(CS~)MO(S)(~~-S)~MO(S)(CS~)]~and [OMO(S~)~]~-, respectively. The reaction of [(S4)Mo(0)(~2-S)2Mo(O)(S2)]2with DMA results in the new dithiolene complex [{(MeOOC)2C2S2}M~(0)(~2-S)]~, which is the isomeric form of the vinyl disulfide complex obtained in the reaction of the [(S2)Mo(0)(~2-S)2Mo(O)(S2)]2complex with DMA. The difference in reactivity between the two complexes that contain the same [Mo202S212+ core is attributed to the intrinsically different reactivity characteristics of the Sgand S:ligands. As a result of Mo-S d,-p, bonding an alternation in the S-S bond lengths is observed in virtually all of the structurally characterized Mo-S4 units. The consequent weakening of the S-S bonds adjacent to the Mo-S bonds allows for the ready dissociation of $3 from the MO-coordinated Siligands. This weakening also accounts for the facile formation of dithiolenes in cycloaddition reactions of alkynes with the Mo-S, units. By comparison, the S-S bond in side-on MO-coordinated Stligands is strengthened as a result of depopulation of the ligand x*-orbitals. Reactions of the latter with alkynes do not proceed by cycloaddition. Instead, insertion into the Mo-S bond has been reported for at least one such reaction. The importance of activated polysulfide ligands in the hydrodesulfurization reaction is discussed. The extensive chemistry of molecular binary molybdenum sulfide compounds presents a unique opportunity for the study of units which may be structurally and electronically similar to the Mo-S active sites in the catalysts employed in the hydrodesulfurization (HDS) reaction. The wide variety of binary molybdenum sulfides that have emerged in recent years derives primarily from the reactivity of the MoSidianion toward sulfur reagents. The complex equilibria and interconversions between MO thioanions that prevail in MoSi-+NH.,),S, or MoS:--Ss solutions, in aqueous or non-aqueous media, have been described.’ Depending on the conditions employed and the nature of the counterions salts of thioanions, such as [Mo2(S2),]2-,2 (Mo$,)~-,’ * Author to whom correspondence should be addressed. CMo2(S)4(S2)(S4)12 -,1*3 CMo2(S),(S.4)212 -,I [(MoS,),MOS]~-~ (Mo~S~~)~-’ and (Mo,S,)~-, can be isolated from these solutions in crystalline form and their structures have been determined (Fig. 1). The reaction chemistry of the MO thioanions with electrophilic reagents is extensive. With electrophiles such as CS2 and dicarboalcoxyacetylenes the (MO&,)‘and [Mo~(S),(S,),]~anions react to form the [(CS,),MoS]‘-,’ [Mo~(S),(CSJ,]~’ and {Mo[S,C,(COOR),],)~8 complexes. The CS:and dithiolene ligands in these compounds possibly are generated by cycloaddition reactions to the coordinated S, 2ligands. The [Mo~S~O~]~anion displays unique reactivity in its reaction with dicarbomethoxyacetylene and forms the very interesting {Mo202S2[SSC2(COOMe)2]2~2~ complex.g In the latter, insertion of the alkyne into the 350 D. COUCOUVANIS et al.