Manganese metal organic frame work assisted magnetic ionic liquid single crystal

Abstract

This research focuses on the synthesis and characterization of a Metal-Organic Framework Assisted Ionic Liquid (MOFIL) with the chemical formula of [Mn(Cy)(R)₂]R·2H₂O; where, Cy is cyclam- (1,4,8,11-tetraazacyclotetradecane) and R is 4-tetradecyloxy benzoate- (4-CH₃(CH₂)₁₃OC₆H₄COO^-). This novel ionic liquid crystal is synthesized in a multistep process. The first precursor compound ethyl 4-tetradecyloxy benzoate, 4-CH₃(CH₂)₁₃OC₆H₄COOC₂H₅ (A) is obtained from a reaction in between the ethyl 4‑hydroxy benzoate, 4-HOC₆H₄COOC₂H₅ and 1-bromotetradecane, CH₃(CH₂)₁₃Br. Precursor A is further reacted with potassium hydroxide, KOH to yield the 2nd precursor a potassium salt of ethyl 4-tetradecyloxy benzoate, 4-CH₃(CH₂)₁₃OC₆H₄COOK (B). Then the precursor B is reacted with manganese chloride tetrahydrate, MnCl₂·4H₂O to synthesize the dinuclear Manganese (II) carboxylate intermediate compound, [Mn₂(R)₄(H₂O)₄] (1). Finally, the addition of cyclam to compound (1) results in the targeted mononuclear MOFIL compound with cyclam, [Mn(Cy)(R)₂]R·2H₂O (2). This article reports the synthetic procedure and structural elucidation of this MOFIL compound (2) along with its precursors (A and B) and intermediate (1) compound by Fourier Transform Infra-Red (FTIR) spectroscopy, Ultraviolet- Visible (UV- Vis) spectroscopy, and single crystal X-ray crystallography. In addition, the magnetic, thermal, and optical studies of the final MOFIL (2) and its dinuclear intermediate compound (1) are also presented. The dinuclear intermediate, (1) having a higher melting temperature around 150 °C and no development of optical texture as observed by Optical Polarizing Microscope (OPM) and Differential Scanning Calorimetric (DSC) data; doesn't fulfil the criteria of being a MOFIL. In contrast, the mononuclear final compound, (2) with cyclam, is crystallized in triclinic system, showing distorted octahedral N4O2 geometry around a mononuclear d⁴ Mn(III) centre; that is further evidenced by it's µ_eff value of 4.6 BM., calculated from room temperature magnetic susceptibility measurement. Compound (2) is thermally stable (T_dec, 242 °C) according to Thermogravimetric Analysis (TGA). It exhibits mesomorphism and behaves as ionic liquid with a melting temperature <110 °C as observed by OPM and DSC data.