Compressed and Expanded Lattices - Barriers to Spin-State Switching in Mn³⁺ Complexes

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.