Abstract:
The electronic structures of two mixed-valence MnMn complexes, [LMnClBr]·HO (2) and [LMnBr]·1/2CHCl (3), and the Mn complex LMnCl (4) are probed. The L ligand is the dianion of the Schiff base condensation of 2 mol of 1,3-diaminopropane and 2 mol of 2,6-diformyl-4-tert-butylphenol. The X-ray structure of complex 4 has been determined by using direct methods to give discrepancy factors of R = 0.0501 and R = 0.0697 for 1884 observed (|F| > 6.0σ|F|) reflections. Complex 4 crystallizes in the triclinic space group P1 with one molecule in a unit cell with dimensions a = 7.270 (1) Å, b = 9.457 (2) Å, c = 12.316 (2) Å, α = 108.36 (1)°, β = 94.92 (1)°, and γ = 99.58 (1)°. The observed and calculated densities are 1.39 (2) and 1.41 g cm, respectively. The L ligand in LMnCl is essentially planar, with the Mn ions displaced by 0.69 Å on either side of the ligand plane. The Mn⋯Mn distance is 3.168 (3) Å. The X-ray structure of complex 2 has been determined by using direct methods to give R = 0.0711 and R = 0.0974 for 1732 observed (|F| > 6.0σ|F|) reflections. Complex 2 crystallizes in the monoclinic space group C2/c with 8 molecules in a unit cell with dimensions a = 21.064 (7) Å, b = 13.559 (4) Å, c = 27.652 (6) Å, and β = 105.51 (2)°. The observed and calculated densities are 1.38 (2) and 1.34 g cm. The structural results for [LMnClBr]·HO show that this complex is valence trapped. The Mn ion has an average Mn-O length of 1.936 (10) Å, while the Mn ion exhibits Mn-O lengths of 2.129 (10) and 2.386 (11) Å. The Mn ion is coordinated axially to two halide ions and is 0.15 Å out of the ligand-atom plane. The Mn ion is displaced 1.25 A from the ligand-atom plane on the opposite side and interacts with one of the halide ions coordinated to the Mn ion. Electrochemical results are presented for LMnX (X = Cl or Br) to show that these complexes have two one-electron oxidation waves at ca. +0.6 and ca. +1.2 V vs NHE. Variable-temperature magnetic susceptibility data are fit to give magnetic exchange parameters of +0.24, -2, and -1 cm for complexes 4, 2, and 3, respectively. Similar EPR data are found for the two mixed-valence complexes. At 7.5 K a glass of 3 gives an X-band spectrum with a broad g = 2 signal and apparent fine structure signals at g = 29.0, 7.4, 5.4, and 4.1. A combination of Orbach relaxation processes involving low-lying spin states and modulation of the g, g values for the ground doublet by strain effects on the magnitude of the isotropic exchange and/or single-ion zero-field splitting are suggested as the origin of the absence of resolution of manganese hyperfine structure in the g = 2 signal. Relatively weak antiferromagnetic interactions are seen for the five known binuclear MnMn complexes, whereas, strong antiferromagnetic interactions are found for the five known binuclear MnMn complexes. The possible causes of this difference in magnitude of exchange interactions are discussed.