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(N₃)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 P1, a = 8.480(1) Å b = 8.9007(2) Å, c = 12.109(2) Å, α = 93.79(1)°, β = 103.17(1)°, γ = 103.11(1)° 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(N₃) 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 = -3.29 (±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 E/D 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 (±0.01) cm^-1, E = 0.51 (±0.01) cm^-1, g_x = 2.00 (±0.005), g_y = L98 (±0.005), and g, = 2.01 (±0.005).