ESR and NMR of Mn(II) Complexes in Methanol

Abstract

ESR spectra of Mn2+ in methanol solutions of Mn(ClO4)2 containing various amounts of LiCl, NaNCS, LiBr, LiNO3, and LiClO4 were studied. In all solutions, variations in the integrated intensity of the ESR signal upon addition of the electrolyte were observed. This effect was interpreted in terms of the formation of asymmetrically substituted octahedral Mn2+ complexes, in many of which the zero-field splitting interaction causes fast electronic relaxation and therefore an ESR signal too broad to observe. Some of the mixed solvent-anion complexes do however yield resolved ESR signals; in particular, a well-defined spectrum due to [Mn(MeOH)4(NCS)2] could be characterized. From the decrease of the ESR signal the equilibrium constants for the formation of monosubstituted octahedral complexes, [Mn(MeOH)5X]+, were estimated. The equilibrium constants for Cl−, NCS−, Br−, NO3−, and ClO4− are respectively >100, >100, 10, 5, and 1M−1. At high LiCl concentrations tetrahedral [MnCl4]2−, exhibiting resolved ESR lines, is formed. In the ESR spectrum of the system Mn(ClO4)2–LiCl–methanol there is, in addition to the intense sextuplet, also a very broad background signal which is attributed to the monochloro complex. From the over-all width of this signal an estimate of the zero-field splitting energy in this complex was made. Proton magnetic resonance measurements were made in methanol solutions of Mn(ClO4)2 containing various amounts of LiCl. There is a strong dependence of the proton linewidth on temperature and on LiCl concentration. The results were interpreted in terms of exchange of methanol molecules in and out of the solvation shells of [Mn(MeOH)6]2+ and the various mixed chloro complexes, but only for [Mn(MeOH)6]2+could a quantitative analysis be made. The kinetic parameters for the exchange reaction of this complex are 1 / τM(25°C) = 9.5 × 105sec−1 and ΔE‡ = 7.4 kcal mole−1. In the monochloro complex the exchange rate is about twice as high.