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) hydratedbis-complex. Structure determination has revealed the solid to be [MnIII(H₂ L)(L)] [Mn^IIIL₂] 5H₂ O: space group P-1;Z = 2;a = 7.527(3)ų,b= 14.260(4)Å,c = 16.080(6)Å,α = 91.08(3)°,β = 103.58(3)°,λ= 105.41(3)° and V= 1611.2(10)ų. 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: Mn^IVL2^-Mn^IIIL 2^- , 1.05V; Mn^IIIL 2 ^- Mn^IIL 2 ^2- -, 0.28V vs. SCE.