Fluoride-bridged {Ln2Cr2} polynuclear complexes from semi-labile mer-[CrF3(py)3] and [Ln(hfac)3(H2O)2]

Abstract

The trifluorido complex mer-[CrF₃(py)₃] (py = pyridine) reacts with 1 equiv. of [Ln(hfac)₃(H₂O)₂] and depending on the solvent forms the tetranuclear clusters [Cr₂Ln₂(μ-F)₄(μ-OH)₂(py)₄(hfac)₆], 1Ln, and [Cr₂Ln₂(μ-F)₄F₂(py)₆(hfac)₆], 2Ln, in acetonitrile and 1,2-dichloroethane, respectively (Ln = Y, Gd, Tb, Dy, Ho, and Er; hfacH = 1,1,1,5,5,5-hexafluoroacetylacetone). Reaction with [Dy(hfac)₃(H₂O)₂] in dichloromethane produces the dinuclear cluster [CrDy(μ-F)F(OH₂)(py)₃(hfac)₄], 3Dy. All the clusters feature fluoride bridges between the chromium(III) and lanthanide(III) centres. Fits of susceptibility data for 1Gd and 2Gd reveal the fluoride-mediated chromium(III)–lanthanide(III) exchange interactions to be 0.43(5) cm⁻¹ and 0.57(7) cm⁻¹, respectively (in the convention). Heat capacity measurements on 2Gd reveal a moderate magneto-caloric effect (MCE) reaching −ΔS_m(T) = 11.4 J kg⁻¹ K⁻¹ for ΔB₀ = 9 T → 0 T at T = 4.1 K. Out-of-phase alternating-current susceptibility (χ′′) signals are observed for 1Dy, 2Dy and 2Tb, demonstrating slow relaxation of the magnetization.