Conformational isomerization in Co(acac)₂via spin-state switch: a computational study

Conformational dynamics of ligands in transition metal complexes can give rise to interesting physical, chemical, spectroscopic, and magnetic properties of the complexes. The changing ligand environment often affects the d-orbital splitting pattern that allows multiple possible ways of electron arrangement in the frontier molecular orbitals, resulting in several closely spaced electronic states with different orbital and spin symmetries. The system can explore these states with either thermal or photophysical means. In the present work, we demonstrate the possibility of a spin transition in Co(acac)₂ assisted by a conformational rearrangement of the ligand. Electronic structure calculations show that the complex adopts a square-planar and tetrahedral geometry with low-spin and high-spin electronic configurations, respectively. A spin-conserved conformational change involves a larger energy barrier in both high- and low-spin states. On the other hand, a low-lying minimum-energy-crossing point exists between the two spin-states that provides a low-energy pathway for conformational isomerization between the two isomers. While the spin-assisted isomerization from a tetrahedral to square planar form requires crossing a 10 kcal mol^-1 barrier, the reverse barrier is only 2 kcal mol^-1. The calculation of the magnetic properties of the complex reveals a large magnetic anisotropy barrier of 57.6 cm^-1 for this complex in the high-spin state.