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

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 (L 1 H 2 ), which is a source of both aminyl and phenoxyl radicals, have been explored. 2e – oxidized [L 1 2– -2e – ] form of L 1 2– is either a CSS quinoid (L 1 Q ) or an open-shell diradical (L 1 dirad ) incorporating aminyl and phenoxyl radicals. The OST form of L 1 dirad is presented by (L 1 dirad ) S=1, while the OSS form is abbreviated by (L 1 dirad ) S=0 . Ruthenium(II) complexes cis -[Ru II (L 1 dirad )(PPh 3 )Cl 2 ] ( 1 ) and trans -[Ru II (L 1 2– )(PPh 3 ) 2 (CO)] ( 2 ) were successfully isolated. L 1 Q is the ground electronic state of the noncoordinated [L 1 2– -2e – ] form, while in 1, L 1 dirad form has been stabilized. 1 ·CH 2 Cl 2 is superparamagnetic at 2 K due to the (L 1 dirad ) S=1 form and diamagnetic at above 50 K stabilizing the (L 1 dirad ) S=0 form. Broken symmetry, BS (1,1) density functional theory calculations infer that OST with (L 1 dirad ) S=1 form, is the ground electronic state of 1, while OSS with (L 1 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, 2 2+ is a L 1 dirad complex of ruthenium(II) authenticated by BS DFT calculations and reveals a double excitation transition at 400–600 nm.