Aggregation of tetranuclear Mn building blocks with alkali ion. Syntheses, crystal structures and chemical behaviors of monomer, dimer and polymer containing [Mn4(mu(3)-O)(2)](8+) units

摘要

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 mu(1,1)-and mu(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 [ Mn-2( HCOO)(4)( H2O)(4)](n) was obtained accompanying the oxidation of MeOH to become HCHO and HCOOH. Tracing the H-1 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 approximate to 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.