Theoretical Investigation Of Reaction Mechanisms Of First-Row Transition Metal Ions With Aliphatic Amines

Density functional theory (DFT) has been employed to survey the gas-phase reactions of amines with first-row transition metal ions. For the hydride abstraction of methylamine with Cu⁺(¹S), geometries for all the stationary points involved are fully optimized at the B3LYP/6-311++G(d,p) and B3LYP/6-311++G(3df,2p) levels and the reaction is analyzed in terms of the topology of potential energy surface. Approach of copper cations towards methylamine molecules results in two encounter isomers, Cu+NH2CH3 and (Cu+H)CH2NH2, which could transfer into each other via two parallel routes, concerted metal movement and stepwise C-H activation-rearrangement. The hydride abstraction for producing CuH + CH₂NH₂⁺ arises from nonreactive dissociation of the CuH-NH₂CH₂⁺ bond following a charge-transfer step from (Cu+H)CH2NH2. In the reaction of Co⁺ with ethylamine, geometries and energies for all the stationary points involved are investigated at the B3LYP/6-311++G(2df,2pd) level. Six different"classical"N and"nonclassical"ethyl-H attached isomers are found for the Co+-ethylamine complexes. The classical complexes are much more stable than the nonclassical ones which have the complexation energies close to the Co+ complexes with small alkanes. Extensive conversions could occur readily between these encounter complexes. All conceivable reaction pathways from each encounter complex to the products observed experimentally are carefully surveyed and the most possible reaction mechanisms are derived. Activation of the Cα-H bond of ethylamine by Co+ through both the classical and nonclassical complexes leads to not only the H2 loss but also the hydride abstraction. The loss of ethylene arises from Co+ insertion into the polar C-N bond in the classical complexes as well as from Cβ-H activation through the nonclassical methyl-H attached complex of Co⁺-gauche-ethylamine. CH4 only forms via C-C activation from the nonclassical complex which has the metal binding two Hs from the different carbons. Initial N-H insertion is unlikely to be important. It is the reactions of the nonclassical complexes that closely parallel with the Co⁺ + alkane reactions. The theoretical work sheds new light on the title reactions and can be considered as a new approach to the reaction mechanisms of transition metal ions with primary amines.