Unusual bi- and tetra-nuclear cobalt(II) bis(salamo)-based complex: Preparation, crystal structure and theoretical calculations and fluorescence properties

Two unusual bis(salamo)-based Co(II) complexes, [Co-2(L)(OMe)].2MeOH (1) and [Co-4(L-1)(4)(MeOH)(4)] (2) were designed and successfully prepared, and characterized structurally. The crystal structure analysis showed that complex 1 is composed of one fully deprotonated ligand (L)(3-) moity, two divalent Co(II) atoms and one bridgecoordinated methanol molecule, while complex 2 is an asymmetric homotetranuclear structure made up of four Co(II) atoms, four fully deprotonated (L-1)(2-) units and four coordinated methanol molecules. It is noteworthy that the reaction of cobalt(II) nitrate with H3L produces complex 2. This complex is not obtained from the original ligand H3L but from a new ligand H2L1. Therefore, the phenomenon can be used to prepare metal(II/III) complexes with excellent structure. The molecular frontier orbitals HOMO and LUMO of H3L and complexes 1 and 2 were analyzed by DFT calculations, and the results showed that the energy gaps of the complexes were reduced due to the combination of the orbitals compared to the ligand. The reaction sites of H3L with Co(II) atoms were analyzed by ESP studies, and the results showed that the crystal structures of the complexes are consistent with the theoretical predictions. Hirshfeld surface analyses indicated the different interactions in the complexes, and revealed that the main contributions to the crystal stacking were from H-H, C-H and H-O interactions. The interactions in the complexes were further explored by IRI research, the results exhibited that there are abundant interactions in complexes 1 and 2, which are important for their construction of supramolecules. The fluorescence strengths of complexes 1 and 2 are significantly lower compared to H3L, which can be attributed to the ligand to metal charge transition (LMCT).