Electrophilic Substitution of Nitrogen Heterocycles by Molybdenum Sulfide Complexes

The electrophilic tetranuclear complex [(Cp‘Mo(μ-S)) 2 (S 2 CH 2 )] 2 (BF 4 ) 2 ( 1: Cp‘ = C 5 H 5 ( 1a ), C 5 H 4 Me ( 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 (S 2 CH 2 )(μ-S)(μ-SH)]BF 4 ( 2 ) and [(Cp‘MoS) 2 (S 2 CH 2 )(μ-S)(μ-S-pyrrolyl)]BF 4 ( 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 (PMe 2 Ph) 3 Cl 2 Re(NC 4 H 4 ). The electrophilic substitution occurs on the 3-position of the heterocycle to form [(Cp‘Mo) 2 (S 2 CH 2 )(μ-S)(μ-SC 4 H 3 N)ReCl 2 (PMe 2 Ph) 3 ]BF 4 ( 7 ). Complex 7 undergoes a further reaction with 1 to give the hydrogen abstraction product [(Cp‘Mo) 2 (S 2 CH 2 )(μ-S 2 C 4 H 2 N)ReCl 2 (PMe 2 Ph) 3 ]BF 4 ( 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.