The use of phosphonates for constructing 3d–4fclusters at high oxidation states: synthesis and characterization of two unusual heterometallic CeMn complexes

Abstract

The use of phosphonic acids in the synthesis of mixed-metal CeMn complexes has led to the formation of two phosphonate complexes with unusual topologies: [Ce₂Mn₆O₆(OH)₅(t-BuPO₃)₆(O₂CMe)₃]·53H₂O (1·53H₂O) and [Ce₂₂Mn₁₂O₃₄(MePO₃)₁₂(O₂CMe)₃₃(OMe)₆(NO₃)(H₂O)₁₂]_n (2). The two mixed-metal CeMn complexes were both prepared from a reaction system containing Mn(O₂CMe)₂ and (NH₄)₂[Ce(NO₃)₆] with similar procedures except for using different phosphonic acids (tert-butylphosphonic acid and methylphosphonic acid, respectively) as coligands. Both complexes possess rare topology of triangular type, with compound 1 being a 0D discrete cluster, whereas, compound 2 is a 1D polymer. The octanuclear core of complex 1 is composed of three symmetry equivalent distorted cubanes {Ce^IV₂Mn^IV₂O₂(OH)₂} sharing a trigonal-bipyramidal unit {Ce^IV₂(OH)₃} in the centre. Compound 2 is a one-dimensional chain polymer of identical Ce₂₂Mn₁₂O₃₄ units linked together by NO₃⁻ and MeCO₂⁻ groups, while the Ce₂₂Mn₁₂O₃₄ unit is constituted by two centrosymmetric Ce₉^IVCe₂^IIIMn^IV₆O₁₇ subunits, which features three identical distorted cubanes {Ce^IV₂Mn^IV₂O₄} connecting to a central trigonal-bipyramidal unit {Ce^IV₃O₂}, and two additional Ce^III ions capping the top and bottom of the central trigonal bipyramid by six MePO₃²⁻ ligands. Complexes 1 and 2 are the first high-nuclearity Mn/Ln aggregates reported to date using phosphonates as ligands. Magnetic susceptibility measurements reveal that compound 1 displays dominant ferromagnetic interactions between the adjacent metal ions with the best fit parameters for the exchanges are J₁ = 6.186 cm⁻¹, J₂ = 4.172 cm⁻¹, and with a result of S = 9 ground state confirmed by the M versus HT⁻¹ data, which indicates the spins of all the six Mn^IV ions in the cluster are parallel to each other. In contrast, the data for 2 reveals overall antiferromagnetic couplings within the cluster and a resulting S = 6 ground state. Both the in-phase signal χ′_MT and out-of-phase signal χ′′_M of the two complexes exhibit frequency-dependent below approximate 3 K.