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

Abstract

Two bis(unsymmetrical diimine) complexes of (L(NO(2))(ϕ1))(L(NO(2))(ϕ2))M(II)Cl(2) family with M = Fe and Mn, are reported (L(NO(2))(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(II)Cl(2)·0.5H(2)O (1) and tcc-(L(NO2)(32.0))(L(NO2)(79.4))Mn(II)Cl(2)·0.5H(2)O (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(NO(2))(42))(L(NO(2))(61))Fe(II)Cl(2), 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, χ(2) = 1.3, τ(avg) = 0.57 ns; 2: λ(ext) = 368, λ(em) = 524 nm, χ(2) = 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))(2)Fe(II)Cl(2) (3), congener of 1 without -NO(2) substitution and non-emissive (bpy)(2)Fe(II)Cl(2) (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.

Synthesis and characterization of some water soluble Zn(ii) complexes with (E)-N-(pyridin-2-ylmethylene)arylamines that regulate tumour cell death by interacting with DNA

“…18 Recently, some transition metal (Fe(II), Mn(II), Zn(II), Cd(II) or Pd(II)) complexes of unsymmetrical iminopyridyl ligands were found to exhibit atropisomerism, π-π stacking and photoluminescence properties. 19,20 Zinc is abundant in eukaryotes and plays a variety of physiological roles. [21][22][23] In addition, Zn(II) complexes are often utilized in many fields such as radioprotection, 24 antidiabetic insulin-mimetism, 25,26 antibacterial or antimicrobial agents, [27][28][29] tumour photosensitizers 30 and binder complexes at DNA sites.…”

“…18 Recently, some transition metal (Fe(II), Mn(II), Zn(II), Cd(II) or Pd(II)) complexes of unsymmetrical iminopyridyl ligands were found to exhibit atropisomerism, π-π stacking and photoluminescence properties. 19,20 4…”

Unsymmetrical diimine chelation to M(ii) (M = Zn, Cd, Pd): atropisomerism, pi–pi stacking and photoluminescence

“…We failed to pre-isolate the ligands in absence of metal ions except the L OH which has been isolated by the reported procedure. 8 Unless otherwise stated all the compounds have been prepared by the same procedures. Details of the procedure have been outlined for the synthesis of 1a and it has been followed to synthesize all other type-I species.…”

“…Recently, significant features of photoluminescence of the coordinated unsymmetrical diimine with 14pe have been reported. 8 Unsymmetrical diimines (L R f ) with an N-Ar function coordinated to a redox innocent metal centre as shown in Chart 1, potentially can have atropisomers due to rotation of the Ar group around the single N-Ar bond. The isomers are defined by the dihedral angle, f (f = dihedral angle between the diimine unit including the pyridine ring and the phenyl ring planes).…”

“…It is to be noted that with f = 0, isomers having a maximum energy gap between the p diimine (HOMO) and p diimine * (LUMO) are more like conjugated 14pe systems (p iso-electronic with anthracene molecule), which display numerous photoactivities. 8 These features are absent in heterocyclic symmetrical rigid diimines, like 2,2¢-bipyridine or 1,10-phenanthroline incorporating 12pe only.…”