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

Abstract

The use of kinetically robust chromium­(III) fluorido complexes as synthons for mixed 3d-4f clusters is reported. The tendency toward linear {Cr^III–F–Ln^III} units dictates the cluster topology. Specifically, we show that reaction of <i>cis</i>-[Cr^IIIF₂(NN)₂]­NO₃ (NN = 1,10-phenanthroline (“phen”) or 2,2′-bipyridine (“bpy”)) with Ln­(NO₃)₃·<i>x</i>H₂O produces isostructural series of molecular {Ln₂Cr₂} squares (<b>1</b>–<b>9</b>) with linear fluoride bridges. In a parallel fashion, <i>fac</i>-[Cr^IIIF₃L], where L = <i>N</i>,<i>N′</i>,<i>N</i>″-trimethyl-1,4,7-triazacyclononane (“Me₃tacn”), reacts with Nd­(NO₃)₃·6H₂O to form a fluoride-centered penta-nuclear complex and <i>fac</i>-[Cr^IIIF₃L′], 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} (<b>10</b>–<b>14</b>) 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 (<i>Ĥ</i> = <i>J</i>₁₂<b>Ŝ</b>₁·<b>Ŝ</b>₂ formalism) and vanishingly small <i>J</i>_Gd–Gd of 0.06(0) cm^–1. The large spin and small anisotropy together with weak exchange interactions in the {Gd₃Cr₂} (<b>11</b>) cluster give rise to a very large magneto-caloric effect of −Δ<i>S</i>_m = 28.7 J kg^–1 K^–1 (μ₀<i>H</i> = 90 to 0 kOe)