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Synthesis, X‐ray Crystal Structures, and Spectroscopic, Electrochemical, and Theoretical Studies of Mn III Complexes of Pyridoxal Schiff Bases with Two Diamines
Author(s) -
Naskar Sumita,
Naskar Subhendu,
Butcher Ray J.,
Corbella Montserrat,
Espinosa Ferao Arturo,
Chattopadhyay Shyamal Kumar
Publication year - 2013
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201300047
Subject(s) - chemistry , crystallography , ligand (biochemistry) , ethylenediamine , metal , oxidation state , schiff base , crystal structure , electrochemistry , redox , moiety , metal ions in aqueous solution , covalent bond , stereochemistry , inorganic chemistry , biochemistry , receptor , organic chemistry , electrode
Six Mn III complexes of general formula [Mn(L)XY], in which L is a dideprotonated Schiff base ligand N , N′ ‐bis(pyridoxylidene)ethylenediamine (L 1 H 2 or pydxen) or N , N′ ‐bis(pyridoxylidene)‐1,3‐propanediamine (L 2 H 2 or pydxpn), X = Cl, N 3 , NCS, and Y = H 2 O, MeOH, EtOH, and another Mn III compound [Mn(L 1 )(H 2 O) 2 ]Cl have been synthesized. The structures of five of the complexes were determined by single‐crystal X‐ray diffraction studies. The compounds show a quasireversible Mn III /Mn II couple at ca. 0 V (vs. Ag/AgCl) and two to three overlapping oxidations at 1.0–1.3 V, which are assigned to Mn III /Mn IV oxidation and ligand (phenolate) oxidation. The redox potential of the phenolate moiety reported here is very similar to the Y z /Y z · + potential of photosystem II (PS II, Y z = tyrosine). Spectrochemical studies and DFT calculations support this assignment. The DFT calculations also show that there is considerable covalence in the metal–ligand bonds and the covalence increases with the oxidation state of the central metal ion. The geometry of the metal ion is found to be dependent on the oxidation state as well as spin state of the metal ion, the nature of the N,O‐donor ligand used as model, and solvation effects. In silico stepwise one and two electron oxidation of a model pydxen ‐ type complex shows strengthening of the metal–ligands interactions, but three‐electron oxidation could significantly weaken one of the Mn–O bonds, which might trigger splitting into a diphenoxyl diradical species and a transient Mn IV complex, in agreement with the experimental results.