Thermochemistry on the Solid State Coordination Compounds M(Nic)₂ · H₂O(s) (M = Mn and Mg, Nic = Nicotinate)

Abstract

Two novel solid state coordination compounds – manganese nicotinate monohydrate and magnesium nicotinate monohydrate were synthesized by the method of room temperature solid phase synthesis and ball grinder. FTIR, chemical and elemental analyses and X-ray powder diffraction techniques were applied to characterize the structure and composition of these complexes. In accordance with Hess’ law, hermochemical cycles were designed, the enthalpy changes of the solid phase reactions of nicotinic acid with manganese acetate tetrahydrate and magnesium acetate tetrahydrate were determined as Δ_rH⁰_m,l = (39.66±0.36) kJ · mol^–1 and Δ_rH⁰_m,2 = (40.73±0.17) kJ · mol^–1 by use of an isoperibol solution-reaction calorimeter, respectively. The standard molar enthalpy of formation of the title complexes M(Nic)₂ · H₂O(s) were calculated as Δ_fH⁰_{Mn(Nic)2·H2O} = –(1165.01±2.01) kJ · mol^–1 and Δ_fH⁰_{Mg(Nic)2·H2O} = –(1390.49±1.99) kJ · mol^–1 by use of the enthalpies of dissolution and other auxiliary thermodynamic data. In addition, the values of the standard molar enthalpies of formation of M(Nic)₂·H₂O(s) (M = Mg, Mn, Zn) were compared, a law of change of [–Δ_fH⁰_{Zn(Nic)2·H2O}] < [–Δ_fH⁰_{Mn(Nic)2·H2O}] < [–Δ_fH⁰_{Mg(Nic)2·H2O}] can be obtained, and was in relation with the stability of the three monohydrated nicotinates.