title: 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 cis-[Cr(III)F(2)(NN)(2)]NO(3) (NN = 1,10-phenanthroline ("phen") or 2,2'-bipyridine ("bpy")) with Ln(NO(3))(3)·xH(2)O produces isostructural series of molecular {Ln(2)Cr(2)} squares (1-9) with linear fluoride bridges. In a parallel fashion, fac-[Cr(III)F(3)L], where L = N,N',N″-trimethyl-1,4,7-triazacyclononane ("Me(3)tacn"), reacts with Nd(NO(3))(3)·6H(2)O to form a fluoride-centered penta-nuclear complex and fac-[Cr(III)F(3)L'], with L' = 1,1,1-tris-((methylamino)methylethane) ("Me(3)tame"), reacts with [Ln(hfac)(3)(H(2)O)(2)] (hfacH = 1,1,1,5,5,5-hexafluoroacetylacetone) to yield an isostructural 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) (Ĥ = J(12)Ŝ(1)·Ŝ(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 {Gd(3)Cr(2)} (11) cluster give rise to a very large magneto-caloric effect of -ΔS(m) = 28.7 J kg(-1) K(-1) (μ(0)H = 90 to 0 kOe).
authors.affiliation: Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark.
authors.forename: Torben
authors.initials: T
authors.lastname: Birk
authors.forename: Kasper S
authors.initials: KS
authors.lastname: Pedersen
authors.forename: Christian Aa
authors.initials: CA
authors.lastname: Thuesen
authors.forename: Thomas
authors.initials: T
authors.lastname: Weyhermüller
authors.forename: Magnus
authors.initials: M
authors.lastname: Schau-Magnussen
authors.forename: Stergios
authors.initials: S
authors.lastname: Piligkos
authors.forename: Högni
authors.initials: H
authors.lastname: Weihe
authors.forename: Susanne
authors.initials: S
authors.lastname: Mossin
authors.forename: Marco
authors.initials: M
authors.lastname: Evangelisti
authors.forename: Jesper
authors.initials: J
authors.lastname: Bendix
mesh_terms: D002857:Chromium / Q000737:chemistry; D005459:Fluorides / Q000737:chemistry*; D007700:Kinetics; D028581:Lanthanoid Series Elements / Q000737:chemistry; D060328:Magnetic Phenomena; D008958:Models, Molecular; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000737:chemistry*; D013696:Temperature
chemical_list: D028581:Lanthanoid Series Elements; D009942:Organometallic Compounds; D002857:Chromium; D005459:Fluorides