'Metal complexes as ligands' for the synthesis of coordination polymers: A MnIII monomer as a building block for the preparation of an unprecedented 1-D {MnIIMnIII}n linear chain

Abstract

A relatively unexplored synthetic route in redox-active Mn(II/III) coordination chemistry has been employed toward the preparation of a new mixed-valence MnII/III 1-D linear chain from the reaction of [MnIII(sacb)2]-precursor with a MnII source, where sacbH2 is the Schiff base ligand N-salicylidene-2-amino-5-chlorobenzoic acid. The mononuclear (Pr2NH2)[MnIII(sacb)2] (1) compound was obtained in excellent yields (>85%) from the 1:2:3 reaction of Mn(O2CMe)2·4H2O, sacbH2 and Pr2NH, respectively. In 1, the two doubly deprotonated sacb2- ligands act as Ocarboxylate, Nimine, Ophenoxide-tridentate chelates, while the second carboxylate O atom of sacb2- is dangling and H-bonded to the Pr2NH2+ countercation. Complex 1 was subsequently used as a 'ligand' to react stoichiometrically with the 'metal' Mn(NO3)2· 4H2O thus leading to the 1-D coordination polymer ([MnIIMnIII(sacb)2(H2O)2(MeOH)2](NO3))n (2) in good yields (~50%). The removal of Pr2NH2+ from the vicinity of the [MnIII(sacb)2]-metalloligand has rendered possible (vide infra) the coordination of the second Ocarboxylate of sacb2- to neighboring (MnII(H2O)2(MeOH)2)2+ units, and consequently the formation of the 1-D polymer 2. Direct-current (dc) magnetic susceptibility studies revealed the presence of very weak antiferromagnetic exchange interactions between alternating MnIII and MnII atoms with a coupling constant of J =-0.08 cm-1 for g = 2.00. The combined results demonstrate the potential of the 'metal complexes as ligands' approach to yield new mixed-valence Mn(II/III.)coordination polymers with interesting structural motifs and physicochemical properties.