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 {Cr-III-F-Ln(III)} units dictates the cluster topology. Specifically, we show that reaction of cis-[(CrF2)-F-III(NN)(2)]NO3 (NN = 1,10-phenanthroline (phen) or 2,2'-bipyridine (bpy)) with Ln(NO3)(3)center dot xH(2)O produces isostructural series of molecular {Ln(2)Cr(2)} squares (1-9) with linear fluoride bridges. In a parallel fashion, fac-[(CrF3L)-F-III], where L = N,N',N ''-trimethyl-1,4,7-triazacyclononane (Me(3)tacn), reacts with Nd(NO3)(3)center dot 6H(2)O to form a fluoride-centered penta-nuclear complex and fac-[(CrF3L)-F-III'], with L' = 1,1,1-tris((methylamino)methylethane) (Me(3)tame), reacts with [Ln(hfac)(3)(H2O)(2)] (hfacH = 1,1,1,5,5,5-hexafluoroacetylacetone) to yield an isostnictural series of {Ln(3)Cr(2)} (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 LnF(3). 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) ((H) over cap = J(12)(S) over cap (1).(S) over cap (2) formalism) and vanishingly small J(Gd-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 -Delta S-m = 28.7 J kg(-1) K-1 (mu H-0 = 90 to 0 kOe).