The Tyranny of Arm-Wrestling Methyls on Iron(II) Spin State in Pseudo-Octahedral [Fe(didentate)3] Complexes

The connection of a sterically constrained 3-methyl-pyrazine ring to a
N-methyl-benzimidazole unit to give the unsymmetrical , ’-diimine ligand L5 has been
programmed for the design of pseudo-octahedral spin-crossover [Fe(L5)3]2+ units, the transition
temperature (T1/2) of which occurs in between those reported for related facial tris-didentate
iron chromophores fitted with 3-methyl-pyridine-benzimidazole in a LaFe helicate (T1/2 ~ 50 K)
and with 5-methyl-pyrazine-benzimidazole L2 ligands (T1/2 ~350 K). A thorough crystallographic
analysis of [Fe(L5)3](ClO4)2 (I), [Ni(L5)3](ClO4)2 (II), [Ni(L5)3](BF4)2H2O (III), [Zn(L5)3](ClO4)2 (IV),
[Ni(L5)3](BF4)21.75CH3CN (V), and [Zn(L5)3](BF4)21.5CH3CN (VI) shows the selective formation
of pure facial [M(L5)3]2+ cations in the solvated crystals of the tetrafluoroborate salts and alternative
meridional isomers in the perchlorate salts. Except for a slightly larger intra-strand interannular
twist between the aromatic heterocycles in L5, the metric parameters measured in [Zn(L5)3]2+ are
comparable to those reported for [Zn(L2)3]2+, where L2 is the related unconstrained ligand. This
similitude is reinforced by comparable ligand-field strengths (Doct) and nephelauxetic e ects (as
measured by the Racah parameters B and C) extracted from the electronic absorption spectra recorded
for [Ni(L5)3]2+ and [Ni(L2)3]2+. In this context, the strictly high-spin behavior observed for [Fe(L5)3]2+
within the 5–300 K range contrasts with the close to room-temperature spin-crossover behavior of
[Fe(L2)3]2+ (T1/2 = 349(5) K in acetonitrile). This can be unambiguously assigned to an intraligand
arm wrestling match operating in bound L5, which prevents the contraction of the coordination
sphere required for accommodating low-spin FeII. Since the analogous 3-methyl-pyridine ring in
[Fe(L3)3]2+ derivatives are sometimes compatible with spin-crossover properties, the consequences
of repulsive intra-strand methyl–methyl interactions are found to be amplified in [Fe(L5)3]2+ because
of the much lower basicity of the 3-methyl-pyrazine ring and the resulting weaker thermodynamic
compensation. The decrease of the stability constants by five orders of magnitude observed in going
from [M(L2)3]2+ to [M(L5)3]2+ (M = NiII and ZnII) is diagnostic for the operation of this e ect, which
had been not foreseen by the authors.