Selective C-C coupling of two nitriles affording rare-earth diazametallacyclopentadienes: synthesis, cooperative reactivity and mechanistic studies

While the reductive coupling of two nitriles mediated by transition metals has been widely studied in the past ten years, the analogous process promoted by rare-earth metals remains elusive. Herein, we report the selective C-C coupling of two nitriles by rare-earth metal reduced naphthalene complexes to assemble the first authentic rare-earth diazametallacyclopentadienes. Mechanistic studies reveal that the reaction proceeds through the insertion-release-insertion sequence, which is different from the known oxidative cyclometallation process for transition metals. This provides an alternative mechanism for metal-mediated reductive coupling of two nitriles. Furthermore, reactivity investigation indicates that diazascandacyclopentadiene exhibits distinct 1,4-hydrogenation and insertion reactions toward H-2, isothiocyanate, isocyanide, and W(CO)(6) to give diversified rare-earth metallacycles. These results indicate that rare-earth diazametallacyclopentadienes present unique chemistry, which is different from that of group 4 diazametallacyclopentadienes.