Steric Quenching of Mn(III) Thermal Spin Crossover: Dilution of Spin Centers in Immobilized Solutions

Spin-state switching: chemical modulation and the impact of intermolecular interactions in manganese(III) complexes

A series of mononuclear manganese(III) complexes [Mn(X-sal2-323)](ReO4) (X = 5 Cl, 1; X = 5 Br, 2; X = 3,5 Cl, 3; X = 3,5 Br, 4; and X = 5 NO2, 5), containing hexadentate ligands prepared using the condensation of N,N'-bis(3-aminopropyl)ethylenediamine and 5- or 3,5-substituted salicylaldehyde, has been synthesized. Variable temperature single-crystal X-ray diffraction, magnetic, spectroscopic, electrochemical, and spectroelectrochemical analyses, and theoretical calculations have been used to explore the role of various ligand substituents in the spin-state switching behavior of the prepared manganese(III) complexes. All five complexes consist of an analogous distorted octahedral monocationic MnN4O2 surrounding offered by the flexible hexadentate ligand and ReO4- as the counter anion. However, a disordered water molecule was detected in complex 4. Complexes 1 (X = 5 Cl) and 5 (X = 5 NO2) show gradual and complete spin-state switching between the high-spin (HS) (S = 2) and the low-spin (LS) (S = 1) state with T1/2 values of 146 and 115 K respectively, while an abrupt and complete transition at 95 K was observed for complex 2 (X = 5 Br). Alternatively, complex 3 (X = 3, 5 Cl) exhibits an incomplete and sharp transition between the HS and LS states at 104 K, while complex 4 (X = 3, 5 Br) (desolvated) remains almost LS up to 300 K and then displays gradual and incomplete SCO at a higher temperature. The nature of the spin-state switch and transition temperature suggest that the structural effect (cooperativity) plays a more significant role in comparison with the electronic effect coming from various substituents (Cl, Br, and NO2), which is further supported by the detailed structural, electrochemical, and theoretical studies.

Spontaneous Chiral Resolution of a MnIII Spin-Crossover Complex with High Temperature 80 K Hysteresis

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, [MnIII (sal2 323)]SCN ⋅ EtOH (1), into Δ and Λ forms, without the use of chiral reagents, where sal2 323 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.

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.