Interplay between Metal⋅⋅⋅π Interactions and Hydrogen Bonds: Some Unusual Synergetic Effects of Coinage Metals and Substituents

The ternary systems of C(2)H(4) (C(2)H(2) or C(6)H(6))-MCN-HF (M=Cu, Ag, Au) and the respective binary systems were investigated to study the interplay between metal⋅⋅⋅π interactions and hydrogen bonds. The metal⋅⋅⋅π interactions in C(2)H(4)-MCN become stronger with the irregular order Ag<Cu<Au, while the hydrogen bonds in MCN-HF become weaker following the same order. The metal⋅⋅⋅π interactions are weakened as the H atoms in the π system are replaced with electron-withdrawing groups and enhanced by electron-donating groups. Type 1 of these ternary systems, in which MCN acts as Lewis base and acid simultaneously, is more stable than type 2, in which C(2)H(4) acts as a double Lewis base. Negative cooperativity is present in type 2 ternary systems with a weakening of the metal⋅⋅⋅π interactions and the hydrogen bonds. Positive cooperativity is found in type 1 ternary systems with an enhancement of the metal⋅⋅⋅π interactions and the hydrogen bonds, except for C(2)(CN)(4)-AuCN-HF-1. The weaker metal⋅⋅⋅π interaction in C(6)H(6)-AuCN has a greater enhancing effect on the hydrogen bond in AuCN-HF than those in C(2)H(4)-AuCN and C(2)H(2)-AuCN. These synergetic effects were analyzed with the natural bond orbital and energy decomposition.