Hydrogenation of Benzonitrile to Benzylamine Catalyzed by Ruthenium Hydride Complexes with P−NH−NH−P Tetradentate Ligands:  Evidence for a Hydridic−Protonic Outer Sphere Mechanism

The reaction of RuHCl(PPh 3 ) 3 with the tetradentate ligand [PPh 2 (( ortho -C 6 H 4 )CH 2 NHCH 2 - )] 2 {ethP 2 (NH) 2 } in THF produces the new complex trans -RuHCl{ethP 2 (NH) 2 } ( 1 ) as a mixture of two isomers. The complex RuHCl{ethP 2 (NH) 2 } ( 1 ) when activated with KO t Bu/KH is a very active catalyst for the hydrogenation of benzonitrile to benzylamine in toluene, more active than the known catalyst Ru(H 2 ) 2 H 2 (PCy 3 ) 2 ( 2 ). A mixture of 1 and 2 and base also results in efficient conversion of benzonitrile to benzylamine. The complex RuHCl{tmeP 2 (NH) 2 } ( 3 ) where tmeP 2 (NH) 2 is [PPh 2 (( ortho -C 6 H 4 )CH 2 NHCMe 2 - )] 2 is a less active catalyst for this reaction. These catalyst systems are air sensitive and extremely moisture sensitive. Experimental and theoretical (DFT) evidence is presented for a new mechanism for nitrile hydrogenation: the successive hydrogenation of the CN triple bond and then the CN double bond of the intermediate imine by H + /H - transfer from a trans dihydride active catalyst. The amido complex RuH{tmeP 2 N(NH)} ( 4 ) has similar activity to 3 /base for the base-free hydrogenation of benzonitrile and is moderately active for the catalytic hydration of benzonitrile. The new complex RuH(BH 4 ){ethP 2 (NH) 2 } ( 7 ) was prepared by reacting 1 with NaBH 4 but is found to be a poor catalyst for nitrile hydration.