Synthesis and characterization of a family of tetranuclear manganese(iii) phosphonate complexes

Abstract

Four tetranuclear manganese(III) complexes containing phosphonate ligands (RPO₃²⁻, R = Me, Et and n-Bu), [Mn₄O₂(MePO₃)₂(O₂CMe)₄(bpy)₂]·4MeCOOH (1), [Mn₄O₂(EtPO₃)₂(O₂CMe)₄(bpy)₂]·7H₂O (2), [Mn₄O₂(n-BuPO₃)₂(O₂CMe)₄(bpy)₂]·4H₂O·(COOH)₂ (3) and [Mn₄O₂(MePO₃)₂(O₂CPh)₄(phen)₂]·4PhCOOH·2MeCN (4), have been synthesized and characterized by IR spectroscopy, ESI-MS spectrometry, electrochemical analysis, magnetic measurements and TGA. The X-ray analysis shows that the four complexes all have one [Mn₄(μ₃-O)₂]⁸⁺ core with four coplanar Mn atoms disposed in an extended “butterfly-like” arrangement and two O atoms triply bridging each “wing”, and the peripheral ligation is provided by four μ₂-MeCO₂⁻ or four μ₂-PhCO₂⁻, two terminal bpy or phen (bpy = 2,2′-bipyridine, phen = 1,10-phenanthroline) groups at both ends of the molecule, and two bicapping μ₃-phosphonate ligands, which each binds to three manganese centers above and below the plane. The cyclic voltammetry of 1 and 3 each displays four reduction and one oxidation processes, of which the four reduction processes are possibly attributed to the reduction of four Mn^III of the compounds to Mn^II. The ESI-MS data indicate that the family of complexes are stable in solution. Magnetic susceptibility measurements reveal that compounds 1 and 3 display antiferromagnetic interactions between the adjacent Mn^III ions mediated through the μ₃-O, O–C–O and O–P–O bridges.