Spin State in Homoleptic Iron(II) Terpyridine Complexes Influences Mixed Valency and Electrocatalytic CO₂ Reduction

Area of Science:

• Organometallic Chemistry
• Electrochemistry
• Spectroscopy
• Catalysis

Background:

• Investigating spin-state dependent properties of transition metal complexes is crucial for understanding their reactivity.
• Homoleptic iron(II) complexes with terpyridine ligands offer a tunable platform for exploring spin crossover phenomena.
• The interplay between electronic structure and catalytic activity remains a key area of research.

Purpose of the Study:

• To elucidate the relationship between spin state and electrochemical/spectroscopic behavior in homoleptic Fe(II) complexes.
• To determine the influence of spin state on the catalytic activity of these complexes in CO2 reduction and proton reduction.

Main Methods:

• Synthesis and characterization of two homoleptic Fe(II) complexes with superbasic terpyridine derivatives.
• Electrochemical techniques (cyclic voltammetry) to probe redox potentials and reactivity.
• Spectroscopic methods to analyze electronic transitions and confirm spin states.
• Electrocatalytic studies to evaluate catalytic efficiency and product selectivity.

Main Results:

• Antiferromagnetic coupling in the high-spin species causes an anodic shift in reduction potential and forms a mixed-valence species with an intervalence-charge-transfer band.
• The low-spin Fe(II) complex acts as an effective precatalyst for electrocatalytic CO2 reduction, yielding CO with 37% Faradaic efficiency.
• The high-spin Fe(II) complex preferentially catalyzes proton reduction with approximately 20% Faradaic efficiency.

Conclusions:

• Spin state is a critical determinant of electrochemical and catalytic properties in these Fe(II) complexes.
• The low-spin state facilitates selective CO2 reduction, highlighting potential for carbon capture and utilization technologies.
• The high-spin state favors proton reduction, suggesting applications in hydrogen evolution.