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

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 [Cr(2)Ln(2)(mu-F)(4)(mu-OH)(2)(py)(4)(hfac)(6)], 1Ln, and [Cr(2)Ln(2)(mu-F)(4)F-2(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(mu-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 (H) over cap = J (S) over cap (1) . (S) over cap (2) convention). Heat capacity measurements on 2Gd reveal a moderate magneto-caloric effect (MCE) reaching -Delta S-m(T) = 11.4 J kg(-1) K-1 for Delta B-0 = 9 T -> 0 T at T = 4.1 K. Out-of-phase alternating-current susceptibility (chi '') signals are observed for 1Dy, 2Dy and 2Tb, demonstrating slow relaxation of the magnetization.