Synthesis and self-assembly of spin-labile and redox-active manganese(iii) complexes

Abstract

New amphiphilic and spin-labile Mn^III complexes based on dianionic N₄O₂-hexadentate sal₂trien or sal₂bapen ligands, which contain OC₆H₁₃, OC₁₂H₂₅, or OC₁₈H₃₇ alkoxy substituents at different positions of the salicylidene unit were prepared (H₂sal₂trien = N,N′′′-bis(salicylidene)-1,4,7,10-tetraazadecane, H₂sal₂bapen = N,N′′′-bis(salicylidene)-1,5,8,12-tetraazadodecane). According to electrochemical measurements, these complexes undergo two (quasi)reversible redox processes. Temperature-dependent magnetic measurements revealed a high-spin configuration for all sal₂trien complexes (S = 2) and gradual spin crossover for sal₂bapen complexes from high to low spin (S = 1). The chain length strongly influences the spin crossover, as C₁₈-functionalization stabilizes the low spin state at much higher temperatures than shorter alkyl chains. Moreover, long alkyl chains allow for spontaneous self-assembly of the molecules, which was investigated in single crystals and in Langmuir-films at the air–water interface. Long alkyl chains (C₁₂ or C₁₈) as well as a mutual syn-orientation of these molecular recognition sites were required for the Langmuir monolayers to be stable.