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

A series of mononuclear manganese(iii) complexes [Mn(X-sal(2)-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 & PRIME;-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 T-1/2 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.

Automated summary

A series of mononuclear manganese(iii) complexes with various ligand substituents were synthesized and their spin-state switching behavior was investigated. The structural effect was found to play a more significant role in the spin-state switch than the electronic effect from different substituents. The transition temperature for the spin-state switch varied depending on the ligand substituent, with different complexes showing gradual or abrupt transitions.