Proton NMR Study of Oxo-Bridged Dimanganese(III) Complexes: Solution State Structures and an Isotropic Shift/Magnetic Exchange Correlation

The (1)H NMR spectra of a series of manganese-oxo aggregates have been examined, and a characteristic signature was found for each complex. For the dimanganese(III,III) complexes [Mn(2)O(OAc)(2)(HB(pz)(3))(2)], [Mn(2)O(OAc)(2)(tacn)(2)](2+), [Mn(2)O(OAc)(2)(H(2)O)(2)(bpy)(2)](2+), and [Mn(2)O(OAc)(2)(bpta)(2)](2+) (HB(pz)(3) = hydrotris(pyrazol-1-yl)borate; tacn = 1,4,7-triazacyclononane; bpy = 2,2'-bipyridine, and bpta = N,N-bis(2-pyridylmethyl)-tert-butylamine), the (1)H NMR spectra reveal a resonance associated with acetate, found downfield between 58 and 80 ppm, and a generally well resolved set of terminal ligand resonances which can be divided into two classes: those resonances associated with pyridyl or pyrazolyl ring protons and those of methylene groups. A number of the pyridine ring resonances have been unambiguously assigned by the examination of methyl-substituted derivatives. Data for these derivatives also support a coordination geometry-dependent pathway for spin delocalization. Moreover, interpretation of the (1)H NMR spectra leads to the conclusion that the solution-state structures of all members of the series are the same as the reported solid-state structures. A strong linear correlation between the magnetic coupling constant (J) and the isotropic shift of the acetate resonance was observed within this series of {Mn(2)O(OAc)(2)}(2+) core complexes. Furthermore, comparisons of the acetate proton isotropic shift ratio (DeltaH(Mn)/DeltaH(Fe)) to the ratio of the squared effective magnetic moments &mgr;(eff)(2)(Mn)/ &mgr;(eff)(2)(Fe) for complexes with the {M(2)O(OAc)(2)}(2+) core (where M = Mn(3+) or Fe(3+)) revealed excellent agreement (within 10%) between these two quantities.