HEPTACOORDINATION OF MANGANESE(II) BY THE POLYAZACYCLOALKANE [21]ANEN7 - CRYSTAL-STRUCTURE OF THE [MN([21]ANEN7)](CLO4)2 SOLID COMPOUND AND THERMODYNAMICS OF COMPLEXATION IN WATER SOLUTION

The thermodynamic parameters for the equilibrium of complexation between manganese(II) and 1,4,7,10,13,16,19-heptaazacy-cloheneicosane ([21]aneN7) have been determined by potentiometry and microcalorimetry in 0.15 mol dm-3NaClO4at 25 °C. The [21]aneN7ligand forms with manganese(II), in solution, a very stable complex (log K = 9.79 (1)) that preserves the metal ion from air oxidation, even in alkaline solution. The thermodynamic stability of this complex in solution is mainly entropic in nature (ΔH° = -5.0 (5) kcal mol1-; TΔS° = 8.2 kcal mol1-). The absence of protonated complexes in solution suggests the heptacoordination of [21]aneN7to manganese(II). This is supported by the crystal structure of the complex [Mn([21]-aneN7)](C1O4)2. Crystals of this compound are orthorhombic, space group P22121, with a = 9.751 (3) Å, b = 15.040 (4) Å, c = 32.781 (5) Å, V= 4807 (2) Å3, Z = 8, dcalc= 1.53 g/cm3, μ = 7.44 cm1-, F(000) = 2327.44, and M = 555.32. The structure has been solved by using the heavy-atom method and F? and ΔF Fourier synthesis and was refined by least-squares methods to R = 0.085 (Rw= 0.069) for 1390 reflections having I >3σ(I). The asymmetric unit contains two independent [Mn([21]aneN7)]2+complexed molecules in which manganese(II) is similarly heptacoordinated by [21]aneN7, which disposes its nitrogen atoms at the vertices of an irregular polyhedron. The main difference between the two independent molecules is determined by the conformation of two chelate rings. © 1990, American Chemical Society. All rights reserved.