Manganese(III)-carboxylate binding: Chemistry and structure of a hydrated pyridinedicarboxylate

Abstract

The reaction of Mn(CH3COO)3 2H2O with the carboxyl-rich ligand pyridine-2,6-dicarboxylic acid (H2L) in methanol affords a high-spin (S = 2) hydrated bis-complex. Structure determination has revealed the solid to be [MnIII(H2 L)(L)] [MnIIIL2] 5H2 O: space group P−1; Z = 2; a = 7.527(3)Å3, b= 14.260(4)Å, c = 16.080(6)Å, α = 91.08(3)°, β = 103.58(3)°, γ= 105.41(3)° and V= 1611.2(10)Å3. Each ligand is planar and is bonded in the tridentate O2N fashion. The MnO4N2 coordination spheres show large distortions from octahedral symmetry. The lattice is stabilised by an extensive network of O…O hydrogen-bonding involving water molecules and carboxyl functions. Upon dissolution in water, protic redistribution occurs and the complex acts as the mono-basic acid Mn(HL)(L) (pK, 4.3 ±0.05). The deprotonated complex displays high metal reduction potentials: MnIVL2-MnIIIL(Formula presented.), 1.05V; MnIIIL(Formula presented.)MnIIL(Formula presented.)-, 0.28V vs. SCE © 1992, Indian Academy of Sciences. All rights reserved.