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).

Cs2NaAl1−xCrxF6: A family of compounds presenting magnetocaloric effect

“…The values of S T at 3 K are summarized in Table 1. These results are comparable to the values obtained for other paramagnetic fluoride systems [16,17].…”

“…Common materials that display large MCEs below 80 K are intermetallic systems such as ErAl 2 and DyNi 2 , which show temperature changes of T ad ≈ 10 K (at T = 13 K) and T ad ≈ 14 K (at T = 21 K), under H = 50 kOe, [17] Gd(HCOO) 3 30 ∼50 ∼20 (at 1 K) [12] ErAl 2 50 37 10 (at 13 K) [46] DyNi 2 50 21 8 (at 21 K) [46] respectively [46]. Turning to the analysis of T ad results in Fig.…”

“…Other successful examples consist of perovskite-type oxides [11], metal-organic frameworks containing gadolinium [12], and molecular nanomagnets, such as Mn 32 [13] and Fe 14 [14]. However, very few studies have so far reported on caloric effects in fluoride systems [15][16][17], which is the aim of this paper.From the point of view of optical and structural properties, Cs 2 NaAl 1−x Cr x F 6 elpasolite single crystals, which crystallize in the R3m space group, have been thoroughly investigated [18][19][20][21][22][23][24][25][26][27]. The choice of the Cr 3+ ion as a doping impurity is justified by the fact that its 3d unfilled shell produces electronic transitions that increase the luminescent properties and the quantum yield of the material [20,[27][28][29].…”

“…Other successful examples consist of perovskite-type oxides [11], metal-organic frameworks containing gadolinium [12], and molecular nanomagnets, such as Mn 32 [13] and Fe 14 [14]. However, very few studies have so far reported on caloric effects in fluoride systems [15][16][17], which is the aim of this paper.…”

Cyano‐Bridged 4f‐3d Assemblies with Achiral Helical Chains: Syntheses, Structures, and Magnetic Properties

“…The intrachain Cr1···Nd1 distance across the cyanide bridge is 5.737 Å, and the Nd1 and Cr1 atoms in a pitch form a scarce irregular pentagonal channel viewed along the screw axis ( Figure S1, Supporting Information), different to the rhombus, rectangle, square, or ring observed in other helical chain compounds. [14] The experimental magnetization at 2.0 K increases monotonically and achieves a value of 3.07 at 70 kOe. In the crys-tal lattice of compounds 1-4, the crystallized water molecules are hydrogen-bonded to one coordinated water molecule and one non-bridging cyanide N atom and are adjacent to free water molecules.…”