Thermally accessible triplet state of π-nucleophiles does exist. Evidence from first principles study of ethylene interaction with copper species

Abstract

Three different models of ethylene interaction with copper species, namely, the Cu(100) surface, odd-numbered copper clusters C₂H₄/Cu_n (where n = 3, 7, 11, 15, 17, 19, 21, 25 and 27) and atomic copper C₂H₄/Cu were studied theoretically. It was found that the ethylene molecule possesses three different types of bonding depending on the presence of the unpaired spin on the reacting copper atom. These bonding structures demonstrate different types of band gap (bulk) or SOMO–LUMO gaps (cluster/atom), where SOMO stands for the singly occupied and LUMO means the lowest unoccupied molecular orbitals of the copper species. The obtained results are in good agreement with the previous experimental and computational results on the structural, spectral and energetic properties of the studied species. The bulk copper and sub-nanosized clusters (n > 7) build up the mono-π-bonded ground state complexes with ethylene where the latter species possesses the C_2v symmetry. The single-atom complex C₂H₄/Cu forms the C_S-symmetrical ground state ²A′ and the excited ²B₂ and ⁴B state complexes of the C_2v and C₂ symmetry, respectively. The ²A′ state complex is mono-σ-bonded and involves the singlet ethylene moiety. The more tightly bound excited ²B₂ complex has the di-σ-bonded structure and corresponds to the triplet ethylene. The adiabatic energy difference between the ²B₂ and ²A′ states is equal to 10.8 kcal mol⁻¹ and can be ascribed to the singlet–triplet splitting of the ethylene moiety interacting with copper. The QTAIM analysis supports the coordination type of the Cu–C bonds in all the studied complexes. Formation of the C₂H₄/Cu(100), C₂H₄/Cu_n and C₂H₄/Cu species is in accord with the well-known Dewar–Chatt–Duncanson model, in such a way that the opposing σ-donation step yields the ground state complex (²A′), while the subsequent more expensive supporting π*-back donation step provides the excited ²B₂ state complex. In the present paper we have developed a computational procedure to optimize the latter complex.