NMR SPIN RELAXATION AS A PROBE OF THE FORMATION OF LONG-LIVED, GEOMETRICALLY WELL-DEFINED SOLUTE SOLVENT COMPLEXES - SOLUTIONS OF TRIS(ACETYLACETONATO)MANGANESE(III)

Solvent-solute outer sphere complex formation was studied in solutions of tris(acetylacetonato)manganese(III) (Mn[acac]3) with several solvents using nuclear magnetic relaxation techniques. The principal objective was to establish experimental criteria that are capable of distinguishing outer-sphere complexes which are structurally well-defined and which retain their internal geometry for times long compared to the time scale of molecular reorientation from the less specific effects of preferential solvation. To this end, the NMR paramagnetic relaxation enhancement (NMR-PRE) produced by Mn [acac]3 was studied in a range of magnetic field strengths where the zero-field splitting (zfs) term H(zfs) of the electron spin Hamiltonian H(S) is comparable to or larger than the electronic Zeeman term H(Z). In this situation, the precessional motion of the electron spin is quantized along molecule-fixed coordinate axes, and the NMR-PRE depends markedly on the orientation of the nuclear spin in the molecular coordinate frame. An analysis of the magnetic field- and temperature-dependence of the NMR-PRE in the vicinity of the zfs-limit (H(zfs) >> H(Z)) and in the intermediate regime (H(zfs) congruent-to H(Z)) in principle distinguishes true intramolecular spin relaxation, in which the nuclear position remains fixed relative to the molecular coordinate system during molecular reorientation, from intermolecular spin relaxation, in which the solute and solvent undergo independent translational and reorientation diffusion. Following this approach, geometrically well-defined solute-solvent complexes were identified in the Mn[acac]3/CHCl3 system and, to a lesser degree, in four other solvents. The outer-sphere Mn[acac]3/CHCl3 complex appears to involve two solvent molecules per Mn [acac]3, with the solvent molecules approaching the metal ion along the nominal 3-fold axis of the Mn [acac]3 complex, the CHCl3 hydrogen atom pointing inward and weakly hydrogen bonded to an oxygen atom of the Mn(III) coordination sphere. Three ketone solvents and cyclohexane solvent showed no evidence of outer-sphere complex formation. The Mn[acac]3/CHCl3 complex is surprisingly long-lived. At magnetic field strengths of several tesla, modulation of the CHCl3 proton contact shift by chemical exchange reactions produces a large, strongly field-dependent contribution to the proton T2, from which the complex lifetime was estimated to be ca. 20 mus.