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(PPh3)3 with the tetradentate ligand [PPh2((ortho-C6H4)CH2NHCH2-)]2 {ethP2(NH)2} in THF produces the new complex trans-RuHCl{ethP2(NH)2} (1) as a mixture of two isomers.
The complex RuHCl{ethP2(NH)2} (1) when activated with KOtBu/KH is a very active catalyst for the
hydrogenation of benzonitrile to benzylamine in toluene, more active than the known catalyst Ru(H2)2H2(PCy3)2 (2). A mixture of 1 and 2 and base also results in efficient conversion of benzonitrile to
benzylamine. The complex RuHCl{tmeP2(NH)2} (3) where tmeP2(NH)2 is [PPh2((ortho-C6H4)CH2NHCMe2-)]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{tmeP2N(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(BH4){ethP2(NH)2} (7) was prepared by reacting 1 with NaBH4 but is found to be a poor catalyst for nitrile hydration.