Fluoride Bridges as Structure-Directing Motifs in 3d-4f Cluster Chemistry

The use of kinetically robust chromium­(III) fluorido complexes as synthons for mixed 3d-4f clusters is reported. The tendency toward linear {CrIII–F–LnIII} units dictates the cluster topology. Specifically, we show that reaction of cis-[CrIIIF2(NN)2]­NO3 (NN = 1,10-phenanthroline (“phen”) or 2,2′-bipyridine (“bpy”)) with Ln­(NO3)3·xH2O produces isostructural series of molecular {Ln2Cr2} squares (1–9) with linear fluoride bridges. In a parallel fashion, fac-[CrIIIF3L], where L = N,N′,N″-trimethyl-1,4,7-triazacyclononane (“Me3tacn”), reacts with Nd­(NO3)3·6H2O to form a fluoride-centered penta-nuclear complex and fac-[CrIIIF3L′], with L′ = 1,1,1-tris-((methylamino)­methylethane) (“Me3tame”), reacts with [Ln­(hfac)3(H2O)2] (hfacH = 1,1,1,5,5,5-hexafluoroacetylacetone) to yield an isostructural series of {Ln3Cr2} (10–14) trigonal bipyramids with no central ligand. The formation of the latter is accompanied by a partial solvolysis of the Cr­(III) precursor but without formation of insoluble LnF3. The magnetic properties of the gadolinium containing clusters allow quantification of fluoride-mediated, antiferromagnetic Gd–Cr exchange interactions of magnitude between 0.14 cm–1 and 0.71 cm–1 (Ĥ = J12Ŝ1·Ŝ2 formalism) and vanishingly small JGd–Gd of 0.06(0) cm–1. The large spin and small anisotropy together with weak exchange interactions in the {Gd3Cr2} (11) cluster give rise to a very large magneto-caloric effect of −ΔSm = 28.7 J kg–1 K–1 (μ0H = 90 to 0 kOe).