DFT Computational Design of a Ligand-Driven Light-Induced Mechanism for Spin-State Switching

A new ligand-driven light-induced spin-state switching mechanism has been designed based on B3LYP*/6-311++G(d,p) DFT calculations of a series of Co-II, Ni-II, and Cu-II complexes with photochromic 2H-chromene (2H-1-benzopyran) ligands 4-8 functionalized with N donor groups. The photoinduced electrocyclic rearrangements of the bis-chelate four-coordinate Ni-II complexes with the ring-closed forms of the ligands, all with high-spin ground electronic states, are predicted to lead to the formation of a mixture of approximately equal amounts of the diamagnetic square-planar and paramagnetic pseudo-tetrahedral isomers of these complexes containing ring-opened o-quinonoid forms of the ligands. Of the Co-II complexes, only that with the 2H-chromen-8-amine ligand 4 exhibits the properties required for manifestation of the photoinitiated spin-state switching. No local minima corresponding to four-coordinate copper complexes with ring-closed isomers of the functionalized 2H-chromenes were located on the respective potential energy surfaces. In their ring-closed forms, 2H-chromenes react with Cu-II ions to give two-coordinate Cu-I complexes, whereas ring-opened o-quinonoid isomers form four-coordinate bis-chelate Cu-II complexes. In the Ni-II complexes with isomeric forms of 2H-pyrano[3,2-h]quinoline ligand 8, the counterions Cl- and even BF4- enter into the coordination sphere of the central atom to form stable six-coordinate metal complexes with high-spin ground states. The predicted capacity of the Ni-II complexes for light-induced spin-state switching is retained with the bulkier BPh4- counterion. The low-spin state of the ion-pair formed by the complex with the ring-opened form of ligand 8 is 10.5 kcalmol(-1) more energetically favorable than the high-spin form.