High-frequency EPR study of a new mononuclear manganese(III) complex:: [(terpy)Mn(N3)3] (terpy=2,2′:6′,2"-terpyridine)

The isolation and structural characterization of [(terpy)Mn-III(N-3)(3)], complex 1, is reported (terpy = 2,2':6',2"- terpyridine). Complex 1, a product of the reaction between the mixed-valence dimer [(terpy)(H2O) Mn-III(O)(2)- Mn-IV(OH2)(terpy)](NO3)(3) and NaN3, crystallizes in a triclinic system, space group P (1) over bar, a = 8.480(1) Angstrom, b = 8.9007(2) Angstrom, c = 12.109(2) Angstrom, alpha = 93.79(1)degrees, beta = 103.17(1)degrees, gamma = 103.11(1)degrees and Z = 2. Complex 1 exhibits a Jahn-Teller distortion of the octahedron characteristic of a six-coordinated high-spin Mn(III). A vibrational spectroscopic study was performed. The v(asym)(N-3) mode of complex 1 appears in the IR as a strong band at 2035 cm(-1) with a less intense feature at 2072 cm(-1), and in the FT-Raman as a strong band at 2071 cm(-1) with a weaker broad band at 2046 cm(-1). The electronic properties of complex 1 were investigated using a high-field and high frequency EPR study (190-475 GHz). The different spin Hamiltonian parameters have been determined (D = -3.23 (+/-0.01) cm(-1), E = 0.48 (+/-0.01) cm(-1), E' = 0.53 (+/-0.01) cm(-1), g(x) = 2.00 (+/-0.005), g(y) = 1.98 (+/-0.005); g(z) = 2.01 (+/-0.005)). These parameters are in agreement with the geometry of complex 1 observed in the crystal structure, a D < 0 related to the elongated distortion, and a value of fin close to 0.2 as expected from the highly distorted octahedron. The two values of the E-parameter are explained by the presence of two slightly different structural forms of complex 1 in the crystal lattice. A second hypothesis was explored to explain the experimental data. The calculation for the simulation was done taking into account that the g and D tensors are not collinear due to the low symmetry of complex 1. In that case, the spin Hamiltonian parameters found are D = -3.29 (<plus/minus>0.01) cm(-1), E = 0.51 (+/-0.01) cm(-1), g(x) = 2.00 (+/-0.005), g(y) = 1.98 (+/-0.005), and g(z) = 2.01 (+/-0.005).