Solvation Structures and Kinetics of Solvent Exchange Reactions of the Manganese(II) Ion in Six Nitriles As Studied by X-ray Diffraction, EXAFS, and NMR Techniques

The solution structure of the manganese(II) ion in acetonitrile (methyl cyanide, AN) has been determined by an X-ray diffraction technique:  there are 6.0 ± 0.2 Mn−N bonds and the Mn−N bond length is 221 ± 1 pm. The solvation structures of the manganese(II) ion in propiononitrile (ethyl cyanide, PN), butyronitrile (propyl cyanide, BuN), isobutyronitrile (isopropyl cyanide, ⁱBuN), valeronitrile (butyl cyanide, VN), and benzonitrile (phenyl cyanide, BzN) have been determined by extended X-ray absorption fine structure spectroscopy. The manganese(II) ion in all of these nitriles is six-coordinate octahedral with an Mn−N bond length of 221 ± 1 pm. The activation parameters for solvent exchange on the manganese(II) ion in the six nitriles have been determined by the nuclear magnetic resonance line broadening technique at various temperatures and pressures. The values of the rate constant ( , s^-1), activation enthalpy (ΔH^⧧, kJ mol^-1), activation entropy (ΔS^⧧, J mol^-1 K^-1), and activation volume (ΔV^⧧, cm³ mol^-1) for the nitrile exchange are 1.3 × 10⁷, 28.6 ± 0.5, −13 ± 2, and −5.8 ± 0.2 in AN; 1.3 × 10⁷, 29.6 ± 0.7, −10 ± 3, and −2.1 ± 0.4 in PN; 9.9 × 10⁶, 31.3 ± 0.8, −6 ± 3, and −5.0 ± 0.8 in BuN; 1.1 × 10⁷, 40.0 ± 0.4, 24 ± 1, and −2.5 ± 0.7 in ⁱBuN; 9.3 × 10⁶, 35.6 ± 0.7, 8 ± 3, and +0.6 ± 0.6 in VN; 1.2 × 10⁷, 36.9 ± 0.8, 14 ± 3, and none in BzN, respectively. The values of ΔH^⧧ and ΔS^⧧ gradually increase with increasing bulkiness of the nitrile molecule, and the reaction mechanism is concluded to be less associative with increasing the bulkiness, because the Mn−N bond energy in the nitrile-solvated manganese(II) ion is similar in all nitriles. The change in the ΔV^⧧ value is discussed in terms of the change in the size of the inner sphere.