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 D3h symmetry) and the Ray-Dutt
twist (conversion to C2v 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)3]2+ (bpy = 2,2′-bipyridine). We find that
the energies of the D3h and C2v 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 C3h geometry, which is considerably lower
in energy than the idealized D3h 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.