Simulation Strategies for Unusual EPR Spectra of Binuclear Mixed-Valence Manganese Complexes: Synthesis, Properties, and X-ray Structures of the MnIIMnIII Complexes (ClO4)2*H2O and (ClO4)2*CH2Cl2

Article abstract of DOI:10.1021/ja00196a013

The preparation, single-crystal X-ray structures, variable-temperature magnetic susceptibility, and novel EPR data are presented for two mixed-valence Mn^IIMn^III complexes. (ClO4)2*H2O (1), where bpmp(1-) is the monoanion of 2,6-bis-4-methylphenol, crystallizes in the monoclinic space group P2₁/n with a=12.493 (1) Angstroem, b=21.583 (2) Angstroem, c=16.631 (2) Angstroem, β=95.31 (1) deg, Z=4, R_F=0.061 and R_wF=0.085 for 4298 unique reflections 3?(I)> at 297 (1) K. (ClO4)2*CH2Cl2 (2),where bcmp(1-) is the monoanion of 2,6-bis(1,4,7-triazacyclonon-1-ylmethy)-4-methylphenol, crystallizes in the orthorhombic space group Pca2₁, with a=19.273 (1) Angstroem, b=15.155 (1) Angstroem, c=13.749 (3) Angstroem, Z=4; R_F=0.074 and R_wF=0.085 for 2324 unique reflections 3?(I)> at 296 (1) K.Trapped-valence Mn^IIMn^III descriptions are appropriate for both 1 and 2, where in each case the two Mn atoms are bridged by two bidentate acetate ions and by the phenoxide O atom of the bpmp(1-) or bcmp(1-) ligand.In the case of either 1 or 2 both Mn ions show pseudooctahedral coordination, and the Mn^III ion is Jahn-Teller distorted along the "z" direction.Two one-electron quasi-reversible cyclic voltammogram waves (Mn^IIMn^III --> Mn^II2 and Mn^IIMn^III <-- Mn^III2, respectively) are seen for both 1 (+0.067 and +0.597 V vs Fc(1+)/Fc) and 2 (-0.133 and +0.347 V).Moderately well-resolved (1)H NMR signals are seen at room temperature for 1 in CD2Cl2 and 2 in CD3CN.Magnetic susceptibility data are presented for polycrystalline samples of 1 in the 2.2-298 K range and 2 in the range 5-301.2 K.Fitting these data to the theoretical susceptibility equation for an isotropic exchange interaction (H=-2JS₁*S₂) gave exchange parameters of -6.0 and -7.7 cm^-1 for 1 and 2, respectively.X-band EPR spectra for polycrystalline 1 and 2 in the 7.5-90 K range are illustrated and discussed.In addition to several other transitions the signals at g ca.4 and g ca.2 are associated with complexes in the S=3/2 excited and S=1/2 ground states, respectively.In a frozen glass (2:1:2 acetone/toluene/ethanol) at temperatures below ca.15 K the X-band spectra of 1 and 2 exhibit a g ca.2 signal structured with a 29-line manganese hyperfine pattern.It was necessary to run ca.20 K Q-band EPR spectra of glasses of 1 and 2 to understand these hyperfine patterns.Through detailed computer simulation it was concluded that only the manganese hyperfine structure for the z component of the g tensor is resolved in the X- and Q-band spectra and that hyperfine structure for the g_x and g_y components is broad to the point of not contributing significantly to the observed EPR spectra.The absence of resolved structure from the g_x and g_y components is attributed to some combination of strain effects (i.e., distributions) associated with the g, A, or J values.