Ray-Dutt and Bailar Twists in Fe(II)-Tris(2,2'-bipyridine): Spin States, Sterics, and Fe-N Bond Strengths

Twisting motions in six-coordinate trischelate transition-metal complexes have long been recognized as a potential reaction coordinate for nondissociative racemization by changing the coordination geometry from octahedral to trigonal prismatic in the transition state. These pathways have been previously established as the Bailar twist (conversion to D symmetry) and the Ray-Dutt twist (conversion to C symmetry). Twisting motions have been shown to be associated with changes in spin state and are therefore of relevance not only to thermal isomerization pathways but also to spin-crossover (SCO) and intersystem crossing mechanisms. In this work, density functional theory and complete active space self-consistent field calculations are used to probe the structural and energetic features of idealized Bailar and Ray-Dutt twisting mechanisms for a model Fe(II) polypyridine complex, [Fe(bpy)] (bpy = 2,2'-bipyridine). We find that the energies of the D and C trigonal prismatic structures are strongly dependent on spin state, with thermally accessible species only being possible on the quintet surface, enforcing the necessary relationship between SCO and torsional motion. The Ray-Dutt twist on the quintet surface is calculated to proceed with a low barrier, and is likely the preferable twisting mechanism for this complex. We additionally identify a new distorted Bailar twist of C geometry, which is considerably lower in energy than the idealized D structure due to a combination of both steric and electronic factors. The computational analysis presented herein offers insight into how Fe-N bond strength, interligand steric repulsion, and ligand flexibility can be exploited to influence the rates of different twisting mechanisms and the critical motions involved.