Structural, magnetic and spectroscopic data on four new spin triplet Mn ³⁺ complexes reveals a large magnetic anisotropy. Spin state is sensitive to lattice contents as the spin quintet ground state is stabilized on co-crystallization with ethanol.

Reversible proton‐induced spin state switching of an Fe^III complex in solution is observed at room temperature. A reversible magnetic response was detected in the complex, [Fe^III(sal₂323)]ClO₄ (1), using Evans’ method ¹H NMR spectroscopy which indicated cumulative switching from low‐spin to high‐spin upon addition of one and two equivalents of acid. Infrared spectroscopy suggests a coordination‐induced spin state switching (CISSS) effect, whereby protonation displaces the metal‐phenoxo donors. The analogous complex, [Fe^III(4‐NEt₂‐sal₂323)]ClO₄ (2), with a diethylamino group on the ligand, was used to combine the magnetic change with a colorimetric response. Comparison of the protonation responses of 1 and 2 reveals that the magnetic switching is caused by perturbation of the immediate coordination sphere of the complex. These complexes constitute a new class of analyte sensor which operate by magneto‐modulation, and in the case of 2, also yield a colorimetric response.

Structural and magnetic data on two iron (III) complexes with a hexadentate Schiff base chelating ligand and Cl⁻ or BPh₄⁻ counterions are reported. In the solid state, the Cl⁻ complex [Fe(5F-sal₂333)]Cl, 1, is high spin between 5–300 K while the BPh₄⁻ analogue [Fe(5F-sal₂333)]BPh₄, 2, is low spin between 5–250 K, with onset of a gradual and incomplete spin crossover on warming to room temperature. Structural investigation reveals different orientations of the hydrogen atoms on the secondary amine donors in the two salts of the [Fe(5F-sal₂333)]⁺ cation: high spin complex [Fe(5F-sal₂333)]Cl, 1, crystallizes with non-meso orientations while the spin crossover complex [Fe(5F-sal₂333)]BPh₄, 2, crystallizes with a combination of meso and non-meso orientations disordered over one crystallographic site. Variable temperature electronic absorption spectroscopy of methanolic solutions of 1 and 2 suggests that both are capable of spin state switching in the solution.

Non‐centrosymmetric spin‐switchable systems are of interest for their prospective applications as magnetically active non‐linear optical materials and in multiferroic devices. Chiral resolution of simple spin‐crossover chelate complexes into the Δ and Λ forms offers a facile route to homochiral magnetic switches, which could be easily enantiomerically enriched. Here, we report the spontaneous resolution of a new hysteretic spin‐crossover complex, [Mn^III(sal₂323)]SCN ⋅ EtOH (1), into Δ and Λ forms, without the use of chiral reagents, where sal₂323 is a Schiff base resulting from condensation of 1,2‐bis(3‐aminopropylamino)ethane with 2‐hydroxybenzaldehyde. The enantiopurity of the Δ and Λ isomers was confirmed by single crystal X‐ray diffraction and circular dichroism. Quantum chemistry calculations were used to investigate the electronic structure. The opening of a wide 80 K thermal hysteresis window at high temperature highlights the potential for good magneto‐optical function at ambient temperature for materials of this type.

We report the structural and magnetic properties of two new Mn³⁺ complex cations in the spin crossover (SCO) [Mn(R-sal₂323)]⁺ series, in lattices with seven different counterions in each case. We investigate the effect on the Mn³⁺ spin state of appending electron-withdrawing and electron-donating groups on the phenolate donors of the ligand. This was achieved by substitution of the ortho and para positions on the phenolate donors with nitro and methoxy substituents in both possible geometric isomeric forms. Using this design paradigm, the [MnL1]⁺ (a) and [MnL2]⁺ (b) complex cations were prepared by complexation of Mn³⁺ to the hexadentate Schiff base ligands with 3-nitro-5-methoxy-phenolate or 3-methoxy-5-nitro-phenolate substituents, respectively. A clear trend emerges with adoption of the spin triplet form in complexes 1a-7a, with the 3-nitro-5-methoxy-phenolate donors, and spin triplet, spin quintet and thermal SCO in complexes 1b-7b with the 3-methoxy-5-nitro-phenolate ligand isomer. The outcomes are discussed in terms of geometric and steric factors in the 14 new compounds and by a wider analysis of electronic choices of Mn³⁺ with related ligands by comparison of bond length and angular distortion data of previously reported analogues in the [Mn(R-sal₂323)]⁺ family. The structural and magnetic data published to date suggest a barrier to switching may exist for high spin forms of Mn³⁺ in those complexes with the longest bond lengths and highest distortion parameters. A barrier to switching from low spin to high spin is less clear but may operate in the seven [Mn(3-NO₂-5-OMe-sal₂323)]⁺ complexes 1a-7a reported here which were all low spin in the solid state at room temperature.