Slow spin crossover in bis-meridional Fe²⁺ complexes through spin-state auto-adaptive N6/N8 coordination.

Area of Science:

• Coordination Chemistry
• Materials Science
• Supramolecular Chemistry

Background:

• Iron(II) spin crossover (SCO) complexes with long-lived excited high-spin (HS) states are crucial for molecular switch applications.
• Enhancing the kinetic stability of spin-state isomers is key to overcoming the limitations of cooperative SCO.
• Modifying ligand substituents offers a pathway to tune SCO properties and improve kinetic stability.

Purpose of the Study:

• To develop a novel approach for slowing down spin-state exchange in SCO complexes.
• To synthesize and characterize new N4 ligands and their corresponding Fe(II) complexes.
• To investigate the impact of ligand structural modifications on coordination geometry, dynamics, and spin-state lifetimes.

Main Methods:

• Synthesis of N4 ligand 6-(3-(pyridin-2-yl)-1H-pyrazol-1-yl)-2,2'-bipyridine (3b) and its N3 analogue (3a).
• Extended solid-state structure-chemical analysis and solution-state NMR studies.
• Density-functional theory (DFT) modeling to elucidate coordination behavior and SCO reaction pathways.

Main Results:

• The new ligand 3b exhibits variable coordination numbers (N3, N3(+1), N4) depending on the metal center and spin state.
• DFT calculations reveal hemi-labile coordination of additional donors in the HS state, leading to an autogenous switch in coordination number.
• The [Fe(3b)2]2+ complex shows significantly longer spin-state lifetimes (τ > 1 ms at 235 K) compared to the analogue [Fe(3a)2]2+ (150 ns).

Conclusions:

• Commutation of a phenyl to a pyridyl substituent in the ligand framework effectively slows down spin-state exchange.
• The spin-state adaptive coordination number in Fe(II) complexes with ligand 3b leads to increased activation barriers for SCO.
• This strategy provides a general method to design SCO complexes with long-lived HS excited states and enhanced kinetic stability.