Linking Cr3 triangles through phosphonates and lanthanides: synthetic, structural, magnetic and EPR studies

Abstract

The preparation and structural characterisation of five 3d–4f mixed metal phosphonate cages with general formula [Cr^III₆Ln^III₂(μ₃-O)₂(H₂O)₂(O₃P^tBu)₄(O₂C^tBu)₁₂(HOⁱBu)₂(ⁱPrNH₂)₂] where Ln^III = La, 1; Tb, 3; Dy, 4; Ho, 5 and [Cr^III₆Gd^III₂(μ₃-O)₂(H₂O)₂(O₃P^tBu)₄(O₂C^tBu)₁₂(HOⁱBu)₄] (2) are reported. The structure contains two oxo-centred {Cr₃} triangles, bridged by phosphonates and lanthanides. The magnetic behaviour of 1 has been modelled as two non-interacting isosceles triangles, involving two antiferromagnetic interactions (J₁ = −8.8 cm⁻¹) with a smaller ferromagnetic interaction for the unique edge of the triangle (J₂ = +1.3 cm⁻¹) giving an isolated S = 3/2 ground state per triangle. The quartet ground state has been proven through simulation of electron paramagnetic resonance (EPR) spectra obtained at the X- and Q-band. EPR simulations have also resulted in the introduction of small single-ion Zero Field Splitting (ZFS) parameters D = ±0.19 cm⁻¹ and rhombic term E = ±0.02 cm⁻¹, which are consistent with strong exchange limit calculations for an isolated S = 3/2 (D = ±0.22 and E = ±0.018 cm⁻¹).