Two-step spin transition in a 1D Fe(II) 1,2,4-triazole chain compound

Area of Science:

• Materials Science
• Coordination Chemistry
• Solid-State Physics

Background:

• Spin crossover (SCO) polymers are materials that can switch between low-spin and high-spin states in response to external stimuli like temperature.
• 1,2,4-triazole-based iron(II) coordination polymers are of interest due to their potential for tunable SCO properties.

Purpose of the Study:

• To investigate the thermochromic spin crossover properties of a novel 1D coordination polymer, [Fe(βAlatrz)3](BF4)2⋅2 H2O.
• To elucidate the mechanism behind the observed two-step spin transition and the influence of hydration on SCO behavior.

Main Methods:

• Superconducting quantum interference device (SQUID) magnetometry
• (57)Fe Mössbauer spectroscopy
• Differential scanning calorimetry (DSC)
• Infrared (IR) and Raman spectroscopy
• Density Functional Theory (DFT) calculations

Main Results:

• A hysteretic, abrupt two-step spin crossover was observed for the first time in a 1,2,4-triazole-based Fe(II) 1D coordination polymer.
• The two-step SCO transition, with a 1:2 low-spin/high-spin ratio in the intermediate phase, is attributed to a distribution of chain lengths.
• Non-coordinated water molecules significantly influence the spin transition, while BF4(-) anions and coordinated water play a minor role in vibrational properties.

Conclusions:

• The hydrated SCO polymer exhibits a unique two-step transition, distinct from the one-step transition observed in its dehydrated counterpart.
• The presence and state of water molecules are critical factors in modulating the spin crossover mechanism in these 1D coordination polymers.