An Example of a Transition from a Closed-Shell Singlet to an Open-Shell Triplet Diradical Form: Stabilization of the...

A metal-ion coordination-promoted transition from a closed-shell singlet (CSS) form of an NNO donor ligand to an open-shell triplet (OST) and open-shell singlet (OSS) forms has been disclosed. Ruthenium(II) complexes of 2-(((2-((5-nitropyridin-2-yl)amino)phenyl)imino)methyl)phenol (L₁H₂), which is a source of both aminyl and phenoxyl radicals, have been explored. 2e^- oxidized [L₁^2--2e^-] form of L₁^2- is either a CSS quinoid (L₁^Q) or an open-shell diradical (L₁^dirad) incorporating aminyl and phenoxyl radicals. The OST form of L₁^dirad is presented by (L₁^dirad)_S=1, while the OSS form is abbreviated by (L₁^dirad)_S=0. Ruthenium(II) complexes cis-[Ru^II(L₁^dirad)(PPh₃)Cl₂] (1) and trans-[Ru^II(L₁^2-)(PPh₃)₂(CO)] (2) were successfully isolated. L₁^Q is the ground electronic state of the noncoordinated [L₁^2--2e^-] form, while in 1, L₁^dirad form has been stabilized. 1·CH₂Cl₂ is superparamagnetic at 2 K due to the (L₁^dirad)_S=1 form and diamagnetic at above 50 K stabilizing the (L₁^dirad)_S=0 form. Broken symmetry, BS (1,1) density functional theory calculations infer that OST with (L₁^dirad)_S=1 form, is the ground electronic state of 1, while OSS with (L₁^dirad)_S=0 form, is the excited state. The OST form of the frozen matrix of 1 at 5 K has been authenticated by the X-band EPR signals at g = 4.325-3.070 and 2.074-1.377. The powder spectrum at 5 K also displays signals at g = 3.516 and 2.385-1.840. 1 exhibits an NIR double excitation transition at 700-1100 nm. Similar to 1, 2²⁺ is a L₁^dirad complex of ruthenium(II) authenticated by BS DFT calculations and reveals a double excitation transition at 400-600 nm.