Synthesis, Structure, and Magnetic and Redox Properties of Linear Bis-Dinuclear Complexes Afforded by Schiff Base Ligands Containing Catecholate and Pyridine or Imidazole Groups

Abstract

3-[(((2′-pyridyl)methyl)imino)methyl]benzene-1,2-diol, H 2 L 2 ) 3-[((2-(2′-pyridyl)ethyl)imino)methyl]benzene-1,2-diol, and H 3 L 3 ) 3-[((2-(4′-imidazolyl)ethyl)imino)methyl]benzene-1,2-diol, have been synthesized and studied. 1 crystallizes in the monoclinic system, space group P2 1 /c, Z ) 2, a ) 11.396(1) Å, b ) 18.583(2) Å, c ) 9.888(1) Å, and ) 103.51(1)°. The structure was solved by direct methods and refined to conventional agreement indices R ) 0.028 and R w ) 0.029. The molecular structure of 1 consists of discrete [Mn II 2 L 1 (CH 3 CO 2 ) 2 (CH 3 OH)] 2 molecules. 1 is a chainlike bis-dinuclear Mn(II) complex involving an unprecedented arrangement of pentacoordinated (N 2 O 3 ) and hexacoordinated (O 6 ) manganese ions as the outer and inner pairs of metal centers, respectively. The different Mn‚‚‚Mn distances result from significant differences in bridging moieties, two catecholato bridges for the shortest distance between inner manganese ions, and one catecholato and two acetato bridges for the longer distances between inner and outer manganese ions. The symmetry of the N 2 O 3 donor set around the outer Mn(1) is close to a trigonal bipyramid while the O 6 ligand environment of the inner Mn(2) can be described as a rhombically distorted octahedron. The synthesis, IR, EPR, and magnetic susceptibility of 1-3 suggest a similar bis-dinuclear structure for all three complexes. Variabletemperature magnetic susceptibility studies establish the presence of antiferromagnetic exchange interactions between Mn(II) centers of 1-3 with predominence of the interactions between inner and outer metal ions (-3.4 to -5 cm -1 ) over the interaction between inner manganese ions (-0.5 to 0.5 cm -1 ). EPR spectroscopy indicates that the bis-dinuclear arrangement of complexes 1-3 is retained in aprotic solvents. As shown by EPR, O 2 oxidation leads to the release of the constitutive Mn(II) dinuclear units, prior to formation of mononuclear Mn(II) and finally Mn(III) species. Observation of two irreversible two-electron oxidation waves in the CV profiles of 1-3 is consistent with a similar splitting of the bis-dinuclear complex molecules induced by electrochemical oxidation. These studies suggest that complexes resulting from the reaction of manganese with the fully oxidized form of such electroactive ligands may afford interesting new higher oxidation-state polynuclear Mn species.

The use of di-2-pyridyl ketone in manganese(II) benzoate chemistry: Two novel linkage isomers containing the ketone form of the ligand and a neutral cubane containing the ligand in its gem-diolate(-1) form

“…The broadness of this isotropic EPR absorption is consistent with the presence of magnetically interacting Mn II ions in the solid-state [30].…”

Synthesis, characterization, and formation constant of hexacoordinate iron(III) complexes

“…Our formation constants and Gibbs free energies follow the sequence: 5-OMe 4 5-H 4 5-NO 2 4 5-Br 6 3.61 (AE0.02) 9.31 (AE0.08) À20.59 (AE0.05) À53.1 (AE0.2) HL 7 3.91 (AE0.08) 9.77 (AE0.09) À22.3 (AE0.2) À55.7 (AE0.2) HL 8 3.33 (AE0.05) 9.45 (AE0.06) À19.0 (AE0.1) À53.9 (AE0.1) HL 9 3.54 (AE0.09) 9.60 (AE0.07) À20.2 (AE0.2) À54.7 (AE0.2) HL 10 4.11 (AE0.07) 10.09 (AE0.09) À23.4 (AE0.2) À57.6 (AE0.2) HL 11 3.99 (AE0.08) 9.8 (AE0.1) À22.7 (AE0.2) À56.0 (AE0.2) HL 12 3.78 (AE0.07) 9.68 (AE0.08) À21.6 (AE0.2) À55.2 (AE0. 2) Hexacoordinate iron(III) 133…”

“…Thermochromism and photochromism of the ligands [8], preparation, properties, and structural aspects of some Mn [9][10][11], Fe [12], Co [13][14][15], Cu [14][15][16][17], Ni [14,15], V [18], and Ga [12] complexes with these Schiff bases have been carried out. Comparatively little work is available on complexation and formation constants of Fe(III) compounds.…”

Binuclear lanthanide complexes of 2,3-dihydroxy-1,4-naphthoquinone

“…They have been found to possess variety of structural peculiarities forming mononuclear and polynuclear metal complexes with transition and lanthanide ions incorporating fully oxidized quinone; one electron reduced semiquinone and fully reduced catechol forms [1]. Even though, the coordination chemistry of transition metal-quinone complexes has been the subject of exploration for variety of purposes [2,3]; it is rather surprising that there are relatively few reports on the structural propensities of lanthanides-quinone chelates. For example, there are reports on the polymeric complexes with anthraquinone [4] and embelin [5], amino-naphthoquinone [6] and halolawsone-monoximes [7].…”