Zwitterionic-to-cationic ruthenium carbazole complexes: structure and selective hydroboration of quinolines and pyridines

We report the synthesis of the Ru(ii) zwitterionic complexes [MeRu(η6-carbazole-N-EMe2Cl)(PPh3)2] (E = Al, Ru-1; Ga, Ru-2) via a one-pot reaction of RuCl2(PPh3)3 with carbazole in the presence of excess group 13 trialkyl reagents (AlMe3 or GaMe3). Structural characterization confirms η6-coordination of ruthenium to a single fused aryl ring of the carbazole framework. The zwitterionic character of Ru-1 and Ru-2 results from deprotonation of the carbazole N–H bond to form an anionic amido fragment coordinated to a group 13 center, while the cationic Ru(ii) center is supported by two phosphines, a methyl ligand, and an η6-bound carbazole-derived ligand. Treatment of Ru-1 or Ru-2 with Brookhart's acid [H(OEt2)2][BArF4] reprotonates the amido nitrogen to afford the complex cation [MeRu(η6-carbazole)(PPh3)2]+ (Ru-3). While the zwitterionic complexes Ru-1 and Ru-2 show modest catalytic activity, protonation generates the highly active ion pair Ru-3, which exhibits superior activity and regioselectivity in the hydroboration of N-heterocycles, enabling either 1,4-reduction of pyridines or 1,2-reduction of quinolines under mild, solvent-free conditions without additional additives. Turnover numbers (TONs) of up to 874 for quinoline and 1000 for pyridine are achieved in gram scale reactions. These values compare favorably with those reported for related Ru, Rh, and Ni systems. Overall, this work provides a rare example of a group 13-stabilized zwitterionic ruthenium–arene system derived from carbazole activation and demonstrates its conversion into a highly active cationic catalyst, expanding the scope of ruthenium–arene chemistry for the selective functionalization of N-heterocycles.