Synthesis, spectroscopy and redox properties of mononuclear manganese(II) and manganese(IV) complexes with N-(aryl)-pyridine-2-aldimine (L) and its amide derivatives. X-ray structural characterization of [Mn(MeL)2(NCS)2] (MeL = N-(4-methylphenyl)-pyridine-2-aldimine)

Abstract

A series of mononuclear Mn II and Mn IV complexes of general formulae [MnL 2 (NCS) 2 ] (1a-1d) and [Mn(L¢) 2 (NCS) 2 ] (2a-2c) have been prepared where L are Schiff bases obtained by the condensation of pyridine-2-aldehyde with para-alkyl-substituted aniline, and L¢ are the corresponding amide ligands. The room temperature magnetic susceptibility data of (1a-1d) indicate that Mn II is in a high spin state. The cyclic voltammograms of (1a-1d) exhibit a one-electron quasi-reversible Mn II fiMn III oxidation. A linear correlation has been found when E 0 [Mn III /Mn II ] is plotted against Hammett r p parameters. X-ray crystallographic data of (1b) shows that the central Mn II ion adopts a distorted octahedral geometry with six different MnAN distances. Upon oxidation of Mn II complexes (1b-1d) by H 2 O 2 , the corresponding Mn IV complexes (2a-2c) were obtained, and the Schiff base ligands were oxidized to the corresponding amides. The lowest energy LMCT bands of these Mn IV complexes correlate linearly with Hammett r p parameters. The redox behavior of the Mn IV complexes has been investigated by cyclic voltammetry. E.p.r. spectra of the Mn II and Mn IV complexes are also reported.

Crystal structures and Hirshfeld surface analysis oftrans-bis(thiocyanato-κN)bis{2,4,6-trimethyl-N-[(pyridin-2-yl)methylidene]aniline-κ2N,N′}manganese(II) andtrans-bis(thiocyanato-κN)bis{2,4,6-trimethyl-N-[(pyridin-2-yl)methylidene]aniline-κ2N,N′}nickel(II))

“…Each metal(II) cation is located on a centre of inversion and adopts a distorted octahedral coordination environment with four N atoms from two symmetry-related PM-TMA ligands in the equatorial plane and two N atoms from symmetry-related NCS À anions in a trans axial arrangement. The M-N bond lengths [2.174 (2) to 2.312 (2) Å for (I) and 2.027 (3) to 2.184 (2) Å for (II)] and N-M-N bond angles [74.27 (6) to 180 for (I) and 78.4 (1) to 180 for (II)] for both complexes are all in the normal range for similar Schiff base complexes with Mn II (Chattopadhyay et al, 2002;Lucas et al, 2005) and Ni II (Guo, 2009;Layek et al, 2014) ions. Note that the Mn1-N1-C1 bond angle [164.3 (2) ] in (I) is somewhat more bent than the corresponding Ni1-N1-C1 bond angle [176.7 (3) ] in (II).…”