Unsymmetrical diimine complexes of iron(ii) and manganese(ii): synthesis, structure and photoluminescence of an isomer

Abstract

Two bis(unsymmetrical diimine) complexes of (L_NO2^ϕ1)(L_NO2^ϕ2)M^IICl₂ family with M = Fe and Mn, are reported (L_NO2^ϕ = (E)-3-nitro-N-(pyridine-2-ylmethylene)aniline; ϕ = dihedral angle between the diimine unit including pyridine ring and the phenyl ring planes). Pure tcc-(L_NO2^33.6)(L_NO2^79.3)Fe^IICl₂·0.5H₂O (1) and tcc-(L_NO2^32.0)(L_NO2^79.4)Mn^IICl₂·0.5H₂O (2) isomers have been successfully isolated in high yields and characterized by elemental analyses, variable temperature magnetic susceptibility measurements, IR, mass, UV-vis and Mössbauer spectra including the single-crystal X-ray structure determinations that identified strong intermolecular non-bonding interactions in lattice (tcc refers to trans–cis–cis positions with respect to pyridine N-imine N–Cl donors). Geometries optimizations of all possible tcc, ttt, ctc, ccc and cct isomers of iron at the B3LYP/DFT level in gas-phase have shown that the tcc-isomer incorporating two non-equivalent ligands as in (L_NO2⁴²)(L_NO2⁶¹)Fe^IICl₂, 1 (g), is stabilized by 6–20 kJ mol⁻¹ compared to other isomers where two ligands are equivalent. The frozen methanol glasses of 1 and 2 are luminescent at 77 K (1: λ_ext = 370, λ_em = 521 nm, χ² = 1.3, τ_avg = 0.57 ns; 2: λ_ext = 368, λ_em = 524 nm, χ² = 1.1, τ_avg = 0.90 ns). The DFT calculations have identified four closely spaced localized π^* orbitals comprising of two non-equivalent ligands as UPMOs. The features contrast the tcc-isomer of (L^ϕ)₂Fe^IICl₂ (3), congener of 1 without –NO₂ substitution and non-emissive (bpy)₂Fe^IICl₂ (4) where two ligands are equivalent. TD-DFT calculations have assigned intra-ligand (IL) and ligand to ligand charge transfer (LLCT) dominated excited states as the origin of luminescence of 1 and 2.