Solvation and coordination chemistry of manganese(II) in some solvents. A transfer thermodynamic, complex formation, EXAFS spectroscopic and crystallographic study

The solvation and coordination chemistry of the manganese(II) ion in water, methanol, dimethylsulfoxide (dmso), N,N’-dimethylpropylene urea (dmpu), acetonitrile and N,N-dimethylthioformamide (dmtf) have been studied from solvation thermodynamic, structural and complex formation ability point of view by calorimetry and EXAFS spectroscopy. The heats of transfer from water to methanol, dmso, acetonitrile and dmtf have been determined to −37.7, −71.6, –23.1 and −36.3 kJ mol−1, respectively, from the heats dissolution of anhydrous manganese(II) trifluoromethanesulfonate by ampoule calorimetry in the neat solvents. Refinement of the EXAFS data of the hydrated and methanol and dmso solvated manganese(II) ions in solution show octahedral configuration and mean Mn-O bond distances of 2.165(3), 2.161(2) and 2.157(3) Å, respectively. The Mn-O bond distance is significantly shorter in dmpu solution, 2.087(3) Å. As dmpu is a space-demanding solvent upon coordination, the manganese(II) ion is only allowed to bind five dmpu molecules in the solvate complex. The acetonitrile solvated manganese(II) ion is six-coordinate in octahedral fashion in solution with a mean Mn-N bond distance of 2.193(3) Å. The relative permittivity of pyridine is too low to allow dissociation of salts of divalent metal ions and neutral pyridine solvated complexes are formed in pyridine solution. In concentrated manganese(II) trifluoromethanesulfonate dmtf solution a contact ion-pair is formed with one trifluoromethanesulfonate ion. In this complex manganese(II) binds five dmtf molecules and a trifluoromethanesulfonate oxygen with Mn-S and Mn-O bond distances of 2.598(6) and 2.16(1) Å, respectively, in an octahedral complex. Complex formation studies performed by titration calorimetry of the manganese(II)-bromide system show significantly more stable complexes in dmpu than in solvents where the solvated manganese(II) ion has regular octahedral symmetry. The dmpu solvated MnBr2 complex in dmpu solution is five coordinate with mean Mn-Br and Mn-O bond distances of 2.494(3) and 2.076(3) Å, respectively. The structure of the [MnBr2(dmpu)2] complex in solid state has distorted tetrahedral geometry with mean Mn-Br and Mn-O bond distances of 2.473(3) and 2.014(3) Å, respectively, and a Br-Mn-Br bond angle of 116.3(1) degrees.