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

Area of Science:

• Coordination Chemistry
• Materials Science
• Spectroscopy

Background:

• NNO donor ligands can exhibit diverse electronic states, including closed-shell singlet (CSS) and open-shell radical forms.
• Metal-ion coordination can influence and stabilize these electronic transitions.
• Ruthenium complexes are of interest due to their versatile coordination chemistry and potential applications.

Purpose of the Study:

• To explore the metal-ion coordination-promoted transition of an NNO donor ligand.
• To synthesize and characterize ruthenium(II) complexes of a specific NNO ligand (L1H2).
• To investigate the electronic states and magnetic properties of the resulting complexes.

Main Methods:

• Synthesis of ruthenium(II) complexes with the NNO ligand.
• Magnetic susceptibility measurements to determine spin states.
• X-band Electron Paramagnetic Resonance (EPR) spectroscopy for radical characterization.
• Broken symmetry (BS) density functional theory (DFT) calculations for electronic structure analysis.
• UV-Vis-NIR spectroscopy to study electronic transitions.

Main Results:

• Two ruthenium(II) complexes, cis-[RuII(L1dirad)(PPh3)Cl2] (1) and trans-[RuII(L12-)(PPh3)2(CO)] (2), were successfully isolated.
• Complex 1 exhibits superparamagnetism at low temperatures due to the stabilized open-shell triplet (OST) diradical form (L1dirad)S=1, transitioning to a diamagnetic state (OSS form, (L1dirad)S=0) at higher temperatures.
• EPR spectroscopy confirmed the OST form of the diradical in complex 1 at 5 K.
• DFT calculations indicated the OST form as the ground electronic state for complex 1.
• Complex 1 shows a near-infrared (NIR) double excitation transition, while complex 2 displays a similar transition in the visible region.

Conclusions:

• Metal-ion coordination successfully promotes a transition from a closed-shell singlet to open-shell radical forms of the NNO ligand.
• The ruthenium(II) complex 1 stabilizes the open-shell diradical form, with the OST state being the ground electronic state.
• The study demonstrates the utility of magnetic and spectroscopic techniques, alongside DFT calculations, in characterizing radical species in coordination complexes.