Aggregation of tetranuclear Mn building blocks with alkali ion. Syntheses, crystal structures and chemical behaviors of monomer, dimer and polymer containing [Mn4(µ3-O)2]8+ units

Abstract

Aggregation of tetranuclear Mn₄O₂ building blocks with alkali ion was studied. Several Mn(III) complexes containing [Mn₄O₂(AcO)₇(pyz)₂]⁻ (pyz = pyrazinate) anion(s) were obtained from an assembly system containing Mn(II), MnO₄⁻, HOAc and Hpyz (Napyz or Kpyz). These [Mn₄O₂]⁸⁺ complexes have monomeric (1 and 2), dimeric (4 and 5) and one-dimensional chain (3) structures of which alkali metal ion connects the Mn ions of adjacent [Mn₄O₂]⁸⁺ units through µ_1,1- and µ_1,3-carboxylate bridges. Complexes 2 or 3 were converted into [Mn₁₂O₁₂(AcO)₁₆(H₂O)₄] in EtOH solution in the presence of HOAc. However, in MeOH solution, a coordination polymer [Mn₂(HCOO)₄(H₂O)₄]_n was obtained accompanying the oxidation of MeOH to become HCHO and HCOOH. Tracing the ¹H NMR spectra of 2 or 3 in CD₃OD, the disappearance of the resonance signals in 3 h indicated the decomposition of the [Mn₄O₂]⁸⁺ cores. Complex 2 exhibits its proton NMR signals in CDCl₃ which are similar to those of its pic analogue but accompany downfield shift to various extents for all the corresponding signals. Variable-temperature magnetic susceptibilities of complexes 2–5 in the range 5–300 K were recorded and were fitted for an Mn₄O₂ butterfly core, giving the fitting parameters J_bb = −2.67 to −3.76 cm⁻¹ and J_wb = −1.16 to −3.14 cm⁻¹. Small J values indicate weak antiferromagnetic coupling interactions of the Mn(III) sites and the spin ground states are considered as S_T = 0 based on the J_bb/J_wb ratio ≈1 for these complexes. The ESR spectra were recorded for complex 2 in dual-mode at liquid-helium temperatures and no obvious signal could be found. The addition of p-cresol gives rise to the reduction of the [Mn₄O₂]⁸⁺, resulting in observable signals.