Insights into dioxygen binding on metal centers: an ab initio multireference electronic structure analysis

Area of Science:

• Computational Chemistry
• Inorganic Chemistry
• Quantum Chemistry

Background:

• Dioxygen (O2) binding to metal centers is crucial for biological and industrial processes.
• The initial step often involves single-electron transfer, forming superoxo species, not direct O2 adducts.
• Understanding this electron transfer is key to controlling O2 reactivity.

Purpose of the Study:

• To investigate the electronic structure changes during O2 association with Fe(II) and Co(II) complexes.
• To elucidate the mechanism behind the inevitable single-electron transfer to O2.
• To compare theoretical interpretations of metal-ligand interactions.

Main Methods:

• Highly correlated wavefunction-based ab initio calculations using CASSCF/NEVPT2.
• Exploration of electronic-structure evolution for O2 association on Fe(II)(BDPP) and Co(II)(BDPP).
• Formulation of two-fragment spin eigenfunctions using Clebsch-Gordan coefficients (CGCs) for analysis.

Main Results:

• O2 binding to Fe(II) and Co(II) complexes leads to the formation of M(III)-superoxo species.
• An avoided crossing between diabatic curves (M(II)-O2 triplet and M(III)-superoxo) drives the electron transfer.
• CGC analysis of spin populations complements traditional valence bonding (VB) analyses.

Conclusions:

• The electronic structure of O2 binding to Fe(II) and Co(II) complexes inherently favors single-electron transfer.
• Avoided crossings are critical in dictating the outcome of O2 ligation.
• DFT methods show limitations in accurately describing metal-ligand magnetic couplings in such systems.