First-principles study on the stability of different coordination structures of manganese (II) -pyrimidine complexes

Abstract

The oriented design of transition metal complexes is a cutting-edge topic in modern coordination chemistry, among which manganese (II) [Mn (II)] pyrimidine complexes have attracted much attention in the development of novel luminescent materials due to their low cost and high luminescence quantum yield. In this study, the unique stability mechanism of the N1 coordination isomer in 2-amino-4-methylpyrimidine-Mn (II) complexes is revealed for the first time by multiscale calculations. Existing Mn (II) - pyrimidine systems mostly focus on the tetrahedral configuration and luminescence properties, whereas systematic quantification of the selectivity of the ligand sites has not been reported. In this study, the stability of three coordination isomers (N1, N2 and N3) of Mn (II) complexes was investigated using first-principles calculations. Geometry optimisation results show that the N1 isomer has the lowest total energy (−837.38 eV) and its planar ligand configuration (dihedral angle 176.177°) effectively maintains the planarity of the π-conjugated system. Hirshfeld surface analysis reveals that the stability of N1 is due to the synergistic effects of strong ionic bonding (Mn - Cl, 4.9 %), oriented π-π stacking (C···C, 4.6 %) and a dense hydrogen bonding network (N - H···N, 10.9 %). It is emphasised that in complex systems the overall structural integrity can easily outweigh the enthalpic contribution of individual stronger coordination bonds. Electronic structure calculations show that the indirect bandgap of N1 is 1.7975 eV (5.8 % deviation from the experimental value), the conduction band/valence band k-space separation is related to the 400 nm Stokes shift phenomenon, the Mn - 3d orbital spin splitting (1.8 eV) dominates the band-side state distribution, and the symmetric hybridisation of the C/N - 2p orbitals promotes the carrier precipitation. These findings establish the design principle of "ligand symmetry-orbital hybridization", which provides a new strategy for the development of highly efficient manganese-based phosphorescent materials; the multi-scale framework (structure optimization → surface analysis → electronic calculations) is universal in transition metal complex engineering, which provides a quantitative pathway for the development of manganese-based materials in fields such as bioimaging and optical communications.