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

Abstract

AbstractA 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 CoII, NiII, and CuII 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 NiII 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 CoII 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 CuII ions to give two‐coordinate CuI complexes, whereas ring‐opened o‐quinonoid isomers form four‐coordinate bis‐chelate CuII complexes. In the NiII 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 NiII 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 kcal mol–1 more energetically favorable than the high‐spin form.