Deciphering the Influence of Meridional versus Facial Isomers in Spin Crossover Complexes

Chelate coordination of non-symmetrical didentate
pyrazine-benzimidazole (L1) or pyridine-benzimidazole
(L2) N-donor ligands around divalent iron in acetonitrile produces
stable homoleptic triple-helical spin crossover
[Fe(Lk)3]2+ complexes existing as mixtures of meridional (C1-
symmetry) and facial (C3-symmetry) isomers in slow exchange
on the NMR timescale. The speciation deviates from
the expected statistical ratio mer/fac=3:1, a trend assigned
to the thermodynamic trans-influence, combined with solvation
effects. Consequently, the observed spin state FeII
lowspin
$FeII
high-spin equilibria occurring in [Fe(Lk)3]2+ refer to mixtures
of complexes in solution, an issue usually not considered
in this field, but which limits rational structure-properties
correlations. Taking advantage of the selective and
quantitative formation of isostructural facial isomers in nonconstrained
related spin crossover d-f helicates (HHH)-
[LnFe(Lk)3]5+ (Ln is a trivalent lanthanide, Lk=L5, L6), we
propose a novel strategy for assigning pertinent thermodynamic
driving forces to each spin crossover triple-helical
isomer. The different enthalpic contributions to the spin
state equilibrium found in mer-[Fe(Lk)3]2+ and fac-[Fe(Lk)3]2+
reflect the Fe@N bond strengths dictated by the trans-influence,
whereas a concomitant solvent-based entropic contribution
reinforces the latter effect and results in systematic
shifts of the spin crossover transitions toward higher temperature
in the facial isomers.