Ubergangsmetallkomplexe mit Schwefelliganden, CXVI. Carbonylkomplexe mit [MoS4]-Zentren in mittleren und hohen Oxidationsstufen / Transition Metal Complexes with Sulfur Ligands, CXVI. Carbonyl Complexes with [MoS4] Centers in Medium and High Oxidation States

Abstract

In the quest of suitable precursors for the synthesis of Mo(IV) carbonyl complexes the compounds [Mo(CO)2(PCy3)(′buS4′)] (3) and [Mo(CO)2(PCy3)(′S4′)] (4) [′buS4′2- = 1,2-bis(2-mercapto-3,5-di-t-butyl-phenylthio)ethane(2-); ′S4′2- = 1,2-bis(2-mercaptophenylthio)ethane(2-)] were synthesized via CO substitution from [Mo(CO)3(′buS4′)] (13) and [Mo(CO)3(′S4′)](14). 4 was characterized by X -ray structure analysis and exhibits, due to the sterically demanding PCy3, a strongly compressed [Mo(′S4′)] core. Intramolecular redox reactions of 3, 4 and, in addition, [Mo(CO)2(PMe3)(′S4′)] (15) leading to thioether dealkylation via loss of C2H4 from the ′RS4′2- ligands and Mo(II)→ Mo(IV) oxidation yielded the Mo(IV) complexes [Mo(CO)(PCy3)(′buS2′)2] (6), [Mo(CO)-(PCy3)(′S2′)2] (7) and [Mo(CO)(PMe3)(′S2′)2] (8) [′buS2′)2]= 3,5-di-t-butyl-1,2-benzenedithiolate(2-), ′S2′2- = 1,2-benzenedithiolate(2-)]. Depending on the nature of the phosphines and the R substituents of the ′RS4′2- ligands, the syntheses required very specific reaction conditions ranging from thermolysis in vacuo to heating in boiling THF. For instance, [Mo(CO)2(PCy3)(′S4′)] (4) decomposes in CH2Cl2 solution even at ambient temperature yielding the binuclear Mo(II) complex [Mo(CO)(μ-′S4′)]2 (10), which was characterized by an X-ray structure determination. Thermolysis of 4 in vacuo at 80 °C , however, results in the formation of the Mo(IV) complex 7. By optimizing the reaction conditions preparative amounts of the prototype [Mo(CO)2(′buS2′)2] (1) have now also become accessible. These findings can be rationalized in part on the basis of results obtained from cyclic voltammetry. The redox potentials of the Mo(IV) complexes are drastically shifted by about -0.5 V per substitution of CO by PR3, and the substitution of ′S2′2- by ′buS2′2- ligands causes a negative shift in the range from 0.03 V to 0.18 V. The results of IR spectroscopical m onitoring the substitution of CO by PMe3 in [Mo(CO)(PMe3)(′RS2′)2] yielding [Mo(PMe3)2(′RS2′)2] suggested an associative substitution mechanism via the intermediate [Mo(CO)(PMe3)2(′RS2′)2]. The formation of [Mo(CO)-(PMe3)(′buS2′)2] (12) from [Mo(PMe3)2(′buS2′)2] (9) and CO under standard conditions further shows that the direct coordination of CO to Mo(IV ) centers is also possible if these centers are surrounded by thiolate donors. The results are discussed with respect to the reactivity of the Mo centers of the FeMo cofactors in nitrogenases.