High-Frequency and -Field EPR Spectroscopy of Tris(2,4-pentanedionato)manganese(III): Investigation of Solid-State versus Solution Jahn-Teller Effects

High-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy of a classical coordination complex, Mn(acac)3 (Hacac = 2,4-pentanedione), has been performed on both solid powder and frozen solution (in CH2Cl2/toluene, 3:2 v/v) samples. Parallel mode detection X-band EPR spectra exhibiting resolved 55Mn hyperfine coupling were additionally obtained for frozen solutions. Magnetic susceptibility and field-dependent magnetization measurements were also made on powder samples. Analysis of the entire EPR data set for the frozen solution allowed extraction of the relevant spin Hamiltonian parameters: D = -4.52(2); E = 0.25(2) cm^-1; giso = 1.99(1). The somewhat lower quality solid-state HFEPR data and the magnetic measurements confirmed these parameters. These parameters are compared to those for other complexes of Mn(III) and to previous studies on Mn(acac)3 using X-ray crystallography, solution electronic absorption spectroscopy, and powder magnetic susceptibility. Crystal structures have been reported for Mn(acac)3 and show tetragonal distortion, as expected for this Jahn-Teller ion (Mn3+, 3d4). However, in one case, the molecule exhibits axial compression and, in another, axial elongation. The current HFEPR studies clearly show the negative sign of D, which corresponds to an axial (tetragonal) elongation in frozen solution. The correspondence among solution and solid-state HFEPR data, solid-state magnetic measurements, and an HFEPR study by others on a related complex indicates that the form of Mn(acac)3 studied here exhibits axial elongation in all cases. Such tetragonal elongation has been found for Mn3+ and Cr2+ complexes with homoleptic pseudooctahedral geometry as well as for Mn3+ in square pyramidal geometry. This taken together with the results obtained here for Mn(acac)3 in frozen solution indicates that axial elongation could be considered the "natural" form of Jahn-Teller distortion for octahedral high-spin 3d4 ions. The previous electronic absorption data together with current HFEPR and magnetic data allow estimation of ligand-field parameters for Mn(acac)3.