Synthesis, X-ray Crystal Structures, and Spectroscopic, Electrochemical, and Theoretical Studies of Mn~(III) Complexes of Pyridoxal Schiff Bases with Two Diamines

摘要

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~1H_2 or pydxen) or N,N'-bis(pyridoxylidene)- 1,3-propanediamine (L~2H_2 or pydxpn), X = Cl, N3, NCS, and Y = H_2O, MeOH, EtOH, and another Mn~(III) compound [Mn(L1)(H_2O)_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,Odonor ligand used as model, and solvation effects. In silico stepwise one and two electron oxidation of a model pydxentype 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.