Pseudotetrahedral high-spin manganese(II)-complexes with (S or R)-N-1-(Ar)ethyl-salicylaldimine: Chiroptical property, chirality induction at-metal, paramagnetism, redox-potential, PXRD structures and DFT/TDDFT

Abstract

Pseudotetrahedral high-spin bis[(S or R)-N-1-(Ar)ethyl-salicylaldiminato-κ2N,O]-Λ/Δ-Mn(II) {Ar = C6H5 (MnSL1 or MnRL1), p-MeOC6H4 (MnSL2 or MnRL2) and p-BrC6H4 (MnSL3 or MnRL3)} are synthesized from reactions between enantiopure Schiff base ligands (S or R)-N-1-(Ar)ethyl-salicylaldimine and manganese(II) chloride with induction of Λ/Δ-chirality-at-metal. ESI mass spectra show the parent ion peak for the mononuclear species at m/z 504 (MnRL1 or MnSL1) and 662 (MnSL3 or MnRL3). The molar conductance values imply mononuclear and non-electrolyte nature of the complexes with metal-to-ligand 1:2 stoichiometry in dimethylformamide (DMF). Electronic circular dichroism (ECD) spectra exhibit mirror-image relationships, and hence confirm enantiopurity or enantiomeric excess of the R- or S-ligated complexes in solution. It suggests the diastereomer or diastereomeric excess of Δ-MnRL or Λ-MnSL in solution combined with the analyses on experimental and simulated ECD spectra by Density Functional Theory/Time-dependant Density Functional Theory (DFT/TDDFT), resulting from diastereoselectively induced Δ- or Λ-chiralityat-metal. In the contrast, DFT optimized structures in gas-phase disclose Λ-MnRL or Δ-MnSL diastereomer slightly more stable than Δ-MnRL or Λ-MnSL by 2–4 kcal/mol. Variable temperature magnetic moment values (μeff. = 5.25−5.55 μB at 263−321 K) correspond to the high-spin state of Mn(II)-complexes in methanol. Cyclic voltammograms demonstrate two one electron charge transfer processes for MnIII/II and MnII/I couples in acetonitrile, respectively. Powder XRD patterns reveal a well-defined crystalline nature and allow structure determination for the MnRL3, using the Expo-2014 program followed by Rietveld refinement. PXRD structure suggests the formation of four-coordinated mononuclear manganese(II)-complex by the two phenolate-oxygen and two imine-nitrogen atoms from two molecules of Schiff base ligands in a pseudotetrahedral geometry.