Theoretical study of thermal spin transition between the singlet state and the quintet state in the [Fe(2-picolylamine)₃]²⁺ spin crossover system

The spin transition between the low-spin singlet state and the high-spin quintet state in the [Fe(2-pic)₃]²⁺ (2-pic: 2-picolylamine) complex is studied by using density functional theory (DFT) calculations. After careful comparison of density functionals BLYP, B3LYP, and B3LYP* (which has 15% Hartree-Fock exchange compared with 20% for B3LYP), we concluded that the spin-state splitting can be accurately reproduced by using the B3LYP* functional. The potential energy surfaces along minimum energy pathways of the three spin states were calculated at the B3LYP*/6- 311+G** level of theory to find minimum energy crossing points (MECPs). The MECPs between the singlet and quintet states (SQ_M) were found (E_SQ = 6.8 kcal/mol), as well as the MECPs between the triplet and singlet states (ST_M, E_ST = 12.9 kcal/mol) and the triplet and quintet states (TQ_M, E_TQ = 12.8 kcal/mol). Although the distortion leading to SQ_M from the singlet equilibrium geometry is mainly a symmetric expansion of the Fe-N bonds, the distortions leading to ST_M and SQ_M are asymmetric. Normal mode analysis demonstrates that these geometrical distortions contain a combination of several low-frequency normal modes, and therefore, these modes play a significant role in the intersystem crossing via the crossing seam.