Abstract
The connection of a sterically constrained 3-methyl-pyrazine ring to a N-methyl-benzimidazole unit to give the unsymmetrical α,α’-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)2∙H2O (III), [Zn(L5)3](ClO4)2 (IV), [Ni(L5)3](BF4)2∙1.75CH3CN (V), and [Zn(L5)3](BF4)2∙1.5CH3CN (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 (∆oct) and nephelauxetic effects (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 effect, which had been not foreseen by the authors.
Highlights
In line with the formulation of the ligand field theory [1,2], or as it was originally called by Bethe, crystal-field theory [3], it was realized that an open-shell metal with at least two valence electrons in a specific chemical environment could exist with either high-spin or low-spin configuration [4].Following van Vleck’s approach to magnetism [5], Pauling perceptively recognized that it would be feasible to obtain systems in which two spin states could be present simultaneously, while their ratio should depend on the energy difference between them [6,7]
Compared with pyridine-carboxylic acids, which are activated via their transformation into acyl chloride with the help of thionyl chloride or oxalyl chloride [59], the electron-rich pyrazine analogue 1 produced only negligible yield (
The 3-methylpyrazine-2-carboxylic acid 1 was activated as its anhydride through reaction with either isobutyl chloroformate or pivaloyl chloride
Summary
In line with the formulation of the ligand field theory [1,2], or as it was originally called by Bethe, crystal-field theory [3], it was realized that an open-shell metal with at least two valence electrons in a specific chemical environment could exist with either high-spin or low-spin configuration [4]. Due to the ‘on–off’ switching of the magnetic properties accompanying the spin transition from the low-spin diamagnetic configuration (1A1 label in octahedral symmetry) to the high-spin paramagnetic form (5T2 label in octahedral symmetry) for d6 transition metals in pseudo-octahedral geometry (Scheme 1a), the ‘magic’ [FeIIN6] chromophores, where N is a heterocyclic nitrogen donor atom, have been intensively investigated [11,12,13,14,15,16,17,18,19,20] Various external stimulations such as changes in temperature [21,22], pressure [23,24], magnetic field [25] or light-irradiation [26,27] can be used for inducing the SCO processes, which makes these microscopic magneto-optical switches very attractive for their introduction into responsive macroscopic materials [12,13,16,28,29,30,31]. Scheme 3. (a) Chemical structure and synthesis of the didentate ligand L5 shown in its anti-conformation heme 3. (a) Chaenmd (bic) aaslsocsiattreud 1cHtuNrMeR sapnecdtrumsywniththnuemsbiserinogfschtehmee (CdDidCle3,n29t8aKte). ligand L5 shown in its ant nformation and (b) associated 1H NMR spectrum with numbering scheme (CDCl3, 298 K)
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