Electrophilic Substitution of Nitrogen Heterocycles by Molybdenum Sulfide Complexes

Abstract

The electrophilic tetranuclear complex [(Cp‘Mo(μ-S))2(S2CH2)]2(BF4)2 (1: Cp‘ = C5H5 (1a), C5H4Me (1b)), in which two dinuclear complexes are joined by a sulfur−sulfur bond, reacts with the electron-rich nitrogen heterocycles pyrroles and indoles at room temperature. The reactions involve heterolytic scission of the disulfide bond by the nucleophiles followed by proton transfer, and the products are [(Cp‘Mo)2(S2CH2)(μ-S)(μ-SH)]BF4 (2) and [(Cp‘MoS)2(S2CH2)(μ-S)(μ-S-pyrrolyl)]BF4 (3−6). The reactions have been characterized for pyrrole, 1-methylpyrrole, 1,2,5-trimethylpyrrole, and 1-methylindole. The regiochemistry of the reactions depends on pyrrole substituents. Complex 1 also reacts readily with the coordinated pyrrolyl ligand in (PMe2Ph)3Cl2Re(NC4H4). The electrophilic substitution occurs on the 3-position of the heterocycle to form [(Cp‘Mo)2(S2CH2)(μ-S)(μ-SC4H3N)ReCl2(PMe2Ph)3]BF4 (7). Complex 7 undergoes a further reaction with 1 to give the hydrogen abstraction product [(Cp‘Mo)2(S2CH2)(μ-S2C4H2N)ReCl2(PMe2Ph)3]BF4 (8). An X-ray diffraction study of 8b confirms that the 3- and 4-carbons of the pyrrolyl ligand are coordinated to the μ-sulfido ligands of the molybdenum dimer. The new products suggest that there are a number of ways in which electron-rich heterocycles might interact with a sulfided molybdenum catalyst in the hydrodenitrogenation reactions. Reactions of 1 with other nucleophiles have also been surveyed.