Manganese-Schiff base complexes as catalysts for water photolysis

Abstract

Four manganese(III)-Schiff base complexes (1-4) of formula [MnL n (H 2 O) 2 ] 2 (ClO 4 ) 2 ÁmH 2 O (n = 1-4; m = 0, 1) have been prepared. The multidentate H 2 L n Schiff base ligands consist of 3R,5R-substituted N,N 0 -bis(salicylidene)-1,2-diimino-2,2-dimethylethane, where R = OEt, OMe, Br or Cl. The complexes have been thoroughly characterized by elemental analysis, mass spectrometry, magnetic susceptibility measurements, IR, UV, paramagnetic 1 H NMR and EPR spectroscopies. Other properties, including redox studies and molar conductivity measurements, have also been assessed. The crystal structure of 1 was solved by X-ray diffraction, which revealed the dimeric nature of the compound through m-aqua bridges. The ability of these complexes to split water has been studied by water photolysis experiments, with the oxygen evolution measured in aqueous media in the presence of a hydrogen acceptor (p-benzoquinone), the reduction of which was followed by UV-spectroscopy. The discussion of the photolytic behaviour includes advances in the knowledge of the structural motifs and the chemical activity of this type of complex, as revealed by the development of several characterization techniques in the last decade. Parallel-mode Mn III EPR shows that complexes 1-4 not only mimic reactivity but also share some structural characteristics from partially assembled natural OEC clusters.

Syntheses, structures and catecholase activity of two cobalt(III) complexes derived from N,N′-ethylenebis(3-ethoxysalicylaldiimine): A special host–guest system from a special ligand

“…[26,64,65,69], or methanol [25,26], Y is an anion such as nitrate [26,64,65] or perchlorate [25,26,69] and X is an anionic ligand such as acetate [24,61,64,70], azide [24,25,68], chloride [24,25,63,67], thiocyanate [24,62] and selenocyanate [24]. A common feature in these latter two types of compounds is that at least one water molecule is coordinated with the M III center (which occupies N 2 O 2 compartment) and that water molecule interacts with the O 4 compartment of the neighboring symmetry related molecule to form dimer-ofmononuclear type structure [24][25][26][61][62][63][64][65][67][68][69][70].…”

“…As already mentioned, a number of mononuclear M III compounds are known from H 2 L OEt ligands [1,19,[24][25][26]57,58,[61][62][63][64][65][66][67][68][69][70]. Fig.…”

“…Another unique aspect is the presence of two apical anionic ligands. Several mononuclear M III compounds (M = Mn III , Fe III , Co III ) having the metal ion in the N(imine) 2 O(phenoxo) 2 compartment are known derived from double-compartment acyclic Schiff base ligands (both 3-ethoxysalicylaldehyde-diamine and 3-methoxysalicylaldehyde-diamine) [1,[24][25][26]57,58,[61][62][63][64][65][66][67][68][69][70]. In none of those compounds, the M III center is coordinated with two apical anionic ligands as in 1.…”

Biomimetic Catalysts for Oxidation of Veratryl Alcohol, a Lignin Model Compound

“…are essential to establish a supramolecular network that approximates the manganese ions to distances below 5 Å, which is considerably shorter than the 10 Å for monomeric compounds [26]. …”

Synthetic Route to Novel Asymmetric Tetradentate Ligands Containing Both Amino and Imino Groups

“…Biomimetic modeling of metal complexes has denoted a better catalytic behaviour when the complex is able to easily coordinate the substrate molecule, and this is favoured when the catalyst has either a vacancy in the coordination sphere or a labile ligand [2][3]. To achieve this, a careful design of the ligand is necessary.…”