Tuning spin-crossover transition temperatures in non-symmetrical homoleptic meridional/facial [Fe(didentate)3]2+complexes: what for and who cares about it?

Abstract

The [FeN6] chromophores found in [Fe(didentate)3]2+complexes, where didentate is a non-symmetrical 2-(6-membered-heterocyclic ring)-benzimidazole ligand (Lk), exist as mixtures of two geometricalmer(C1-symmetry) andfac(C3-symmetry) isomers. Specific alkyl-substituted six-membered heterocyclic rings connected to the benzimidazole unit (pyridines in ligandsL1-L3, pyrazines inL4-L5and pyrimidines inL6-L7) control the ligand field strength and the electron delocalization so that [FeII(Lk)3]2+display tunable thermally-induced spin transitions in solution. Thermodynamic, spectroscopic (UV-Vis, NMR) and magnetic studies in solution demonstrate that [Fe(L6)3]2+(L6= 1-methyl-2-(pyrimidin-2-yl)-1H-benzo[d]imidazole) exhibits a close to room temperature spin transition (T1/2= 273(3) K) combined with a high stability formation constant n acetonitrile), which makes this complex suitable for the potential modulation of lanthanide-based luminescence in polymetallic helicates. A novel method is proposed for assigning specific thermodynamic spin crossover parameters tofac-[Fe(L6)3]2+andmer-[Fe(L6)3]2+isomers in solution. The observed difference relies mainly on the entropic content ΔSmerSCO− ΔSfacSCO= 11(1) J mol−1K−1, which favors the spin transition for the meridional isomer. Intermolecular interactions occurring in the crystalline state largely overcome minor thermodynamic trends operating in diluted solutions and a single configurational isomer is usually observed in the solid state. Among the thirteen solved crystal structures1-13containing the [M(Lk)3]2+cations (M = Fe, Ni, Zn,Lk=L6-L7), pure meridional isomers are observed six times, pure facial isomers also six times and a mixture (44%merand 56%fac) is detected only once. Solid-state magnetic data recorded for the FeIIcomplexes show the operation of slightly cooperative spin transitions in7(fac-[Fe(L6)3]2+) and12(mer-[Fe(L7)3]2+). For the meridional isomer in6, a two-step spin state transition curve, associated with two phase transitions, is detected.