Self-assembled decanuclear NaI2MnII4MnIII4 complexes: from discrete clusters to 1-D and 2-D structures, with the MnII4MnIII4 unit displaying a large spin ground state and probable SMM behaviour

Abstract

The synthesis, magnetic characterization and X-ray crystal structures are reported for five new manganese compounds, [Mn^III(teaH₂)(sal)]·¹/₂H₂O (1), [Na^I₂Mn^II₄Mn^III₄(teaH)₆(sal)₄(N₃)₂(MeOH)₄]·6MeOH (2), [Na^I₂Mn^II₄Mn^III₄(teaH)₆(sal)₄(N₃)₂(MeOH)₂]_n·7MeOH (3), [Na^I₂Mn^II₄Mn^III₄(teaH)₆(sal)₄(N₃)₂(MeOH)₂]_n·2MeOH·Et₂O (4) and [K^I₂Mn^II₄Mn^III₄(teaH)₆(sal)₄(N₃)₂(H₂O)₂]_n·5MeOH (5). Complex 1 is a mononuclear compound, formed via the reaction of Mn(NO₃)₂·4H₂O, triethanolamine (teaH₃) and salicylic acid (salH₂) in a basic methanolic solution. Compound 2 is a mixed-valent hetero-metallic cluster made up of a Mn₈Na₂ decanuclear core and is formed via the reaction of sodium azide (NaN₃) with 1. Compounds 3–5 are isolated as 1- or 2-D coordination polymers, each containing the decanuclear Mn₈M₂ (M = Na⁺ or K⁺) core building block as the repeating unit. Compound 3 is isolated when 1 is reacted with NaN₃ over a very short reaction time and forms a 1-D coordination polymer. Each unit displays inter-cluster bridges via the O-atoms of teaH²⁻ ligands bonding to the sodium ions of an adjacent cluster. Increasing the reaction time appears to drive the formation of 4 which forms 2-D polymeric sheets and is a packing polymorph of 3. The addition of KMnO₄ and NaN₃ to 1 resulted in compound 5, which also forms a 1-D coordination polymer of the decanuclear core unit. The 1-D chains are now linked via inter-cluster potassium and salicylate bridges. Solid state DC susceptibility measurements were performed on compounds 1–5. The data for 1 are as expected for an S = 2 Mn^III ion, with the isothermal M vs. H data being fitted by matrix diagonalization methods to give values of g and the axial (D) and rhombic (E) zero field splitting parameters of 2.02, −2.70 cm⁻¹ and 0.36 cm⁻¹ respectively. The data for 2–5, each with an identical Mn^II₄Mn^III₄ metallic core, indicates large spin ground states, with likely values of S = 16 (±1) for each. Solid state AC susceptibility measurements confirm the large spin ground state values and is also suggestive of SMM behaviour for 2–5 as observed via the onset of frequency dependent out-of-phase peaks.