Studies on electronic structures and related properties of complexes M(tap)32+ (M = Fe, Ru, Os) by the DFT method

Abstract

Studies on a series of complexes M(tap)₃²⁺ (M = Fe, Ru, Os; tap = 1,4,5,8-tetraazaphenanthrene) are carried out using the DFT method at the B3LYP/LanL2DZ level. The electronic structures and related chemical properties of complexes M(tap)₃²⁺, e.g. the energies and components of some frontier molecular orbitals, the spectral properties, and the net charge populations of some main atoms of the complexes, etc. have been investigated. In addition, computations on the complex Ru(phen)₃²⁺(phen = 1,10-phenanthroline) are also performed for comparison. The computational results show some very interesting trends in electronic structures and related properties of complexes M(tap)₃²⁺. First, the computational energies of some frontier molecular orbitals of Ru(tap)₃²⁺ are all lower than those of Ru(phen)₃²⁺. In particular, the energies of LUMOs of the complexes Os(tap)₃²⁺ and Ru(tap)₃²⁺ are all rather low and their electron affinities are rather high, leading to the prediction that an electron is very easily transferred from DNA guanine base to the LUMO of the complexes. This may be a major reason why these cations are excellent photoreagents for DNA. Second, the components of the HOMO of the four complexes all arise mainly from d orbitals of the central metal ions, and the components of the LUMO arise mainly from p orbitals of C and N atoms in the ligands, so their electronic ground state bands are theoretically all assigned to singlet metal-to-ligand charge-transfer (¹MLCT) transitions in absorption spectra, similar to those of Ru(bpy)₃²⁺. Third, for M(tap)₃²⁺, the most negative charges are populated on N1, with the next populated on N4. Fourth, the chemical stabilities (S) of the complexes are in order S_III > S_II > S_I, according to coordination energy computations. In addition, the central atom M has little effect on the coordination bond lengths and bond angles of the complexes. The obtained results should be important references for the synthesis of the complexes, mechanistic analysis of their photochemistry, electrochemistry and catalysis chemistry.