‘Metal Complexes as Ligands’ for the Synthesis of Coordination Polymers: A Mn(III) Monomer as a Building Block for the Preparation of an Unprecedented 1-D {Mn(II)Mn(III)}(n) Linear Chain

A relatively unexplored synthetic route in redox-active Mn(II/III) coordination chemistry has been employed toward the preparation of a new mixed-valence Mn(II/III) 1-D linear chain from the reaction of [Mn(III)(sacb)(2)](−) precursor with a Mn(II) source, where sacbH(2) is the Schiff base ligand N-salicylidene-2-amino-5-chlorobenzoic acid. The mononuclear (Pr(2)NH(2))[Mn(III)(sacb)(2)] (1) compound was obtained in excellent yields (>85%) from the 1:2:3 reaction of Mn(O(2)CMe)(2)∙4H(2)O, sacbH(2) and Pr(2)NH, respectively. In 1, the two doubly deprotonated sacb(2−) ligands act as O(carboxylate),N(imine),O(phenoxide)-tridentate chelates, while the second carboxylate O atom of sacb(2−) is dangling and H-bonded to the Pr(2)NH(2)(+) countercation. Complex 1 was subsequently used as a ‘ligand’ to react stoichiometrically with the ‘metal’ Mn(NO(3))(2)∙4H(2)O, thus leading to the 1-D coordination polymer {[Mn(II)Mn(III)(sacb)(2)(H(2)O)(2)(MeOH)(2)](NO(3))}(n) (2) in good yields (~50%). The removal of Pr(2)NH(2)(+) from the vicinity of the [Mn(III)(sacb)(2)](−) metalloligand has rendered possible (vide infra) the coordination of the second O(carboxylate) of sacb(2−) to neighboring {Mn(II)(H(2)O)(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 Mn(III) and Mn(II) 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.