Bridging Fluoride Research Articles

One-dimensional coordination polymers of [Co3(dpa)4](2+) and [MF6](2-) (M = Re(IV), Zr(IV) and Sn(IV)).

One-dimensional coordination polymers of alternating metal-metal bonded trinuclear [Co3(dpa)4](2+) (dpa = the anion of 2,2'-dipyridylamine) building blocks and [ReF6](2-) (1), [ZrF6](2-) (2) or [SnF6](2-) (3) linkers have been self-assembled and crystallographically characterized. Magnetic measurements reveal a significant ferromagnetic coupling (J/k(B) = +9.9 K) between S = 1/2 {Co3(6+)} and S = 3/2 Re(IV) magnetic sites through a single, unsupported fluoride bridge in 1.

The preparation and structure of Ge3F8 - a new mixed-valence fluoride of germanium, a convenient source of GeF2.

The new binary mixed-valence fluoride of germanium, Ge3F8, has been obtained by heating GeF4 with powdered Ge in an autoclave (390 K/4 bar/48 h). The structure contains pyramidal Ge(II)F3 and octahedral Ge(IV)F6 units, linked by fluoride bridges. The new compound is the missing member of the series (GeF2)n·GeF4 (n = 2, 4, or 6). Sublimation of (GeF2)n·GeF4in vacuo provides a convenient source of GeF2 in ca. 30% overall yield.

Dinuclear μ-fluoro cations of copper, silver and gold

Terminal fluoride complexes of the monovalent group 11 metals, supported by N-heterocyclic carbene (NHC) ligands, react with one-half equivalent of triphenylmethyl cation to form fluoride-bridged dinuclear cations. An improved preparation of the starting terminal fluorides is introduced. The crystal structures of the cations display bent MFM arrangements with large intermetallic separations. The bridging fluorides are highly labile, undergoing facile hydrolysis. The fluoride-bridged digold complex adds across an allene CC bond to form a diaurated allylic fluoride.

Inside Cover: Fluoride-Bridged {GdIII3MIII2} (M=Cr, Fe, Ga) Molecular Magnetic Refrigerants (Angew. Chem. Int. Ed. 9/2014)

Molecular refrigerators consisting of unusual fourth-row metal-ion–lanthanide complexes with bridging fluoride ions are presented by M. Evangelisti and J. Bendix in their Communication on page 2394 ff. The combination of lightweight auxiliary ligands and tunable interactions by choice of metal ion makes these systems interesting modules for low-temperature cooling applications.

Chemical equilibria in the UO22+–H2O2–F−/OH− systems and possible solution precursors for the formation of [Na6(OH2)8]@[UO2(O2)F]2418− and [Na6(OH2)8]@[UO2(O2)OH]2418− clusters

The focus of this study is on the relationship between uranyl(VI) poly-peroxo clusters in the solid state and their possible precursors in solution. For this purpose, the complex formation in the ternary U(VI)-H2O2-F(-) system has been studied by potentiometric titrations, measuring p[H(+)] and p[F(-)], revealing that significant amounts of ternary uranyl(VI)-peroxide-fluoride complexes are formed. Based on the analysis of these data we find that there are two models consistent with structure data and previous speciation in the uranyl(VI)-peroxide-carbonate system (Dalton. Trans., 2012, 41, 11635-11641). One model contains ternary complexes (UO2)4(O2)4F(-) and (UO2)4(O2)4F2(2-) and the other (UO2)4(O2)4F(-) and (UO2)5(O2)5F3(3-); we have chosen the second model as the one most consistent with available information. We suggest that (UO2)4(O2)4F(-) is a building block in the U-24 cluster, [Na6(OH2)8]@[UO2(O2)F]24(18-) identified in a single-crystal X-ray diffraction study of the solid phase that slowly precipitates from the slightly acidic test solutions. At p[H(+)] ≈ 9.5, a new solid phase is formed that contains the cluster [Na6(OH2)8]@[UO2(O2)OH]24(18-), also identified from an X-ray structure. Both structures contain η(2)-η(2) bridging peroxide and η(2) bridging fluoride or hydroxide ions, respectively. As fluoride bridges are unknown in solution coordination chemistry, it is unlikely that the U-24 fluoride cluster is formed in solution. We suggest that both the solid state fluoride and hydroxide clusters are formed in the crystallization from smaller precursors identified in solution. The study illustrates the importance of accurate control of the solution chemistry when preparing poly-peroxo-metallate clusters and also that the mechanism of their formation is still an open field of research.

Dinuclear Complexes Containing Linear M–F–M [M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II)] Bridges: Trends in Structures, Antiferromagnetic Superexchange Interactions, and Spectroscopic Properties

The reaction of M(BF(4))(2)·xH(2)O, where M is Fe(II), Co(II), Ni(II), Cu(II), Zn(II), and Cd(II), with the new ditopic ligand m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene (L(m)*) leads to the formation of monofluoride-bridged dinuclear metallacycles of the formula [M(2)(μ-F)(μ-L(m)*)(2)](BF(4))(3). The analogous manganese(II) species, [Mn(2)(μ-F)(μ-L(m)*)(2)](ClO(4))(3), was isolated starting with Mn(ClO(4))(2)·6H(2)O using NaBF(4) as the source of the bridging fluoride. In all of these complexes, the geometry around the metal centers is trigonal bipyramidal, and the fluoride bridges are linear. The (1)H, (13)C, and (19)F NMR spectra of the zinc(II) and cadmium(II) compounds and the (113)Cd NMR of the cadmium(II) compound indicate that the metallacycles retain their structure in acetonitrile and acetone solution. The compounds with M = Mn(II), Fe(II), Co(II), Ni(II), and Cu(II) are antiferromagnetically coupled, although the magnitude of the coupling increases dramatically with the metal as one moves to the right across the periodic table: Mn(II) (-6.7 cm(-1)) < Fe(II) (-16.3 cm(-1)) < Co(II) (-24.1 cm(-1)) < Ni(II) (-39.0 cm(-1)) ≪ Cu(II) (-322 cm(-1)). High-field EPR spectra of the copper(II) complexes were interpreted using the coupled-spin Hamiltonian with g(x) = 2.150, g(y) = 2.329, g(z) = 2.010, D = 0.173 cm(-1), and E = 0.089 cm(-1). Interpretation of the EPR spectra of the iron(II) and manganese(II) complexes required the spin Hamiltonian using the noncoupled spin operators of two metal ions. The values g(x) = 2.26, g(y) = 2.29, g(z) = 1.99, J = -16.0 cm(-1), D(1) = -9.89 cm(-1), and D(12) = -0.065 cm(-1) were obtained for the iron(II) complex and g(x) = g(y) = g(z) = 2.00, D(1) = -0.3254 cm(-1), E(1) = -0.0153, J = -6.7 cm(-1), and D(12) = 0.0302 cm(-1) were found for the manganese(II) complex. Density functional theory (DFT) calculations of the exchange integrals and the zero-field splitting on manganese(II) and iron(II) ions were performed using the hybrid B3LYP functional in association with the TZVPP basis set, resulting in reasonable agreement with experiment.

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-[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).

Direct observation of a ferri-to-ferromagnetic transition in a fluoride-bridged 3d–4f molecular cluster

We report on the synthesis, crystal structure and magnetic characterisation of the trinuclear, fluoride-bridged, molecular nanomagnet [Dy(hfac)3(H2O)–CrF2(py)4–Dy(hfac)3(NO3)] (1) (hfacH = 1,1,1,5,5,5-hexafluoroacetylacetone, py = pyridine) and a closely related dinuclear species [Dy(hfac)4–CrF2(py)4]·½CHCl3 (2). Element-specific magnetisation curves obtained on 1 by X-ray magnetic circular dichroism (XMCD) allow us to directly observe the field-induced transition from a ferrimagnetic to a ferromagnetic arrangement of the Dy and Cr magnetic moments. By fitting a spin-Hamiltonian model to the XMCD data we extract a weak antiferromagnetic exchange coupling of j = −0.18 cm−1 between the DyIII and CrIII ions. The value found from XMCD is consistent with SQUID magnetometry and inelastic neutron scattering measurements. Furthermore, alternating current susceptibility and muon-spin relaxation measurements reveal that 1 shows thermally activated relaxation of magnetisation with a small effective barrier for magnetisation reversal of Δeff = 3 cm−1. Density-functional theory calculations show that the Dy–Cr couplings originate from superexchange via the fluoride bridges.

Cobalt(ii) fluorophosphate frameworks

Nine new cobalt fluorophosphates frameworks have been synthesised hydrothermally in a fluoride-rich medium, through the use of CoF(3) and HPF(6) as reactants in combination with Group I/II metal hydroxides and organic cations as templating species. Products have been structurally characterised using single crystal X-ray diffraction. Reaction in fluoride-rich conditions produces chain, layer and three dimensional framework structures containing new and unusual structural features based on the linking of PO(3)F, PO(2)(OH,F)(2) and Co(O,F)(n) polyhedra (n = 5-6) including μ(2), μ(3) and μ(4) bridging fluoride. Structures exhibiting inter-layer spaces and channels frequently have these lined by terminal fluoride anions of the PO(3)F, PO(2)(OH,F)(2) and Co(O,F)(n) polyhedra.

Fluoride-bridged {Ln2Cr2} polynuclear complexes from semi-labile mer-[CrF3(py)3] and [Ln(hfac)3(H2O)2

The trifluorido complex mer-[CrF(3)(py)(3)] (py = pyridine) reacts with 1 equiv. of [Ln(hfac)(3)(H(2)O)(2)] and depending on the solvent forms the tetranuclear clusters [Cr(2)Ln(2)(μ-F)(4)(μ-OH)(2)(py)(4)(hfac)(6)], 1Ln, and [Cr(2)Ln(2)(μ-F)(4)F(2)(py)(6)(hfac)(6)], 2Ln, in acetonitrile and 1,2-dichloroethane, respectively (Ln = Y, Gd, Tb, Dy, Ho, and Er; hfacH = 1,1,1,5,5,5-hexafluoroacetylacetone). Reaction with [Dy(hfac)(3)(H(2)O)(2)] in dichloromethane produces the dinuclear cluster [CrDy(μ-F)F(OH(2))(py)(3)(hfac)(4)], 3Dy. All the clusters feature fluoride bridges between the chromium(III) and lanthanide(III) centres. Fits of susceptibility data for 1Gd and 2Gd reveal the fluoride-mediated chromium(III)-lanthanide(III) exchange interactions to be 0.43(5) cm(-1) and 0.57(7) cm(-1), respectively (in the Ĥ = JŜ1· Ŝ2 convention). Heat capacity measurements on 2Gd reveal a moderate magneto-caloric effect (MCE) reaching -ΔS(m)(T) = 11.4 J kg(-1) K(-1) for ΔB(0) = 9 T → 0 T at T = 4.1 K. Out-of-phase alternating-current susceptibility (χ'') signals are observed for 1Dy, 2Dy and 2Tb, demonstrating slow relaxation of the magnetization.

Positively Charged Lanthanide Complexes with Cyclen-Based Ligands: Synthesis, Solid-State and Solution Structure, and Fluoride Interaction

The syntheses of a new cyclen-based ligand L(2) containing four N-[2-(2-hydroxyethoxy)ethyl]acetamide pendant arms and of its lanthanide(III) complexes [LnL(2)(H(2)O)]Cl(3) (Ln = La, Eu, Tb, Yb, or Lu) are reported, together with a comparison with some Ln(III) complexes of a previously reported analogue L(1) in which two opposite amide arms have been replaced by coordinating pyridyl units. The structure and dynamics of the La(III), Lu(III), and Yb(III) complexes in solution were studied by using multinuclear NMR investigations and density functional theory calculations. Luminescence lifetime measurements in H(2)O and D(2)O solutions of the [Ln(L(2))(H(2)O)](3+) complexes (Ln = Eu or Tb) were used to investigate the number of H(2)O molecules coordinated to the metal ion, pointing to the presence of an inner-sphere H(2)O molecule in a buffered aqueous solution. Fluoride binding to the latter complexes was investigated using a combination of absorption spectroscopy and steady-state and time-resolved luminescence spectroscopy, pointing to a surprisingly weak interaction in the case of L(2) (log K = 1.4 ± 0.1). In contrast to the results in solution, the X-ray crystal structure of the lanthanide complex showed the ninth coordination position occupied by a chloride anion. In the case of L(1), the X-ray structure of the [(EuL(1))(2)F] complex features a bridging fluoride donor with an uncommon linear Eu-F-Eu entity connecting two almost identical [Eu(L(1))](3+) units. Encapsulation of the F(-) anion within the two complexes is assisted by π-π stacking between the pyridyl rings of two complexes and C-H···F hydrogen-bonding interactions involving the anion and the pyridyl units.

Cyclo-Tetra-μ-fluorido-1:2κ2F;2:3κ2F;3:4κ2F;1:4κ2F-octanitrato-1κ8O,O′;3κ8O,O′-tetrakis(1,10-phenanthroline)-2κ4N,N′;4κ4N,N′-2,4-dichromium(III)-1,3-dineodymium(III) methanol tetrasolvate monohydrate

In the title compound, [Cr2Nd2F4(NO2)8(C12H8N2)4]·4CH3OH·H2O, two cis-difluoridobis(1,10-phenanthroline)chromium(III) fragments containing octa­hedrally coordinated chromium(III) bridge via fluoride ions to two tetra­nitratoneodymate(III) fragments, forming an uncharged tetra­nuclear square-like core. The fluoride bridges are fairly linear, with Cr—F—Nd angles of 168.74 (8)°. Cr—F bond lengths are 1.8815 (15) Å, slightly elongated compared to those of the parent chromium(III) complex, which has bond lengths ranging from 1.8444 (10) to 1.8621 (10) Å. The tetra­nuclear complex is centered at a fourfold rotoinversion axis, with the Cr and Nd atoms situated on two perpendicular twofold rotation axes. The uncoordinated water mol­ecule resides on a fourfold rotation axis. The four methanol solvent mol­ecules are located around this axis, forming a cyclic hydrogen-bonded arrangement. The title compound is the first structurally characterized example of unsupported fluoride bridges between lanthanide and transition metal ions.

Magnetic Properties of a Manganese(III) Chain with Monoatomic Bridges: catena-MnF(salen)

In the solid state, MnF(salen) forms chains wherein fairly linear fluoride bridges between high-spin Mn(III) centers are observed. We interpret the magnetic properties of these chains by use of the classical Fisher model and by use of the high-temperature expansion approach, as well as by exact matrix diagonalization of the spin Hamiltonian, of model rings. In solution, electron paramagnetic resonance shows the chains to be symmetrically cleaved to monomeric MnF(salen).

Fluoride-Rich, Hydrofluorothermal Routes to Functional Transition Metal (Mn, Fe, Co, Cu) Fluorophosphates

Hydrofluorothermal methods are shown to offer a facile route to a very large family of mid-late first row, transition metal fluorophosphates with 50 new compounds identified to date for manganese(III), iron(III), cobalt(II), and copper(II). Reactions of a transition metal fluoride with a phosphate source in a very low-water, high-fluoride content system and in the presence of framework charge balancing metal cations or template molecular cations, lead to materials with structures formed from linked M(O,F)(n) and P(O,F)(n) polyhedra. The structures of these materials, which contain a variety of 1, 2, and 3-dimensional motifs with a level of framework termination dependent upon their fluoride content, show numerous useful characteristics for functionality and applications. The new and unusual features of these fluorophosphate materials include interlayer spaces or channels lined with fluoride ions, metal polyhedra, M(O,F)(n), linked through vertex, edge, or face-sharing, and μ(2), μ(3), and μ(4) bridging fluoride ions. Preliminary characterization of the properties of some of these metal fluorophosphates is reported, including reductive lithium ion insertion, of interest for Li-ion battery positive electrodes, ion exchange reactions, the formation of nanoporous material derivatives through template destruction, and magnetic susceptibility behaviors.

In Situ Experimental Evidence for a Nonmonotonous Structural Evolution with Composition in the Molten LiF−ZrF4 System

We propose in this paper an original approach to study the structure of the molten LiF-ZrF(4) system up to 50 mol % ZrF(4), combining high-temperature nuclear magnetic resonance (NMR) and extended X-ray absorption fine structure (EXAFS) experiments with molecular dynamics (MD) calculations. (91)Zr high-temperature NMR experiments give an average coordination of 7 for the zirconium ion on all domains of composition. MD simulations, in agreement with EXAFS experiments at the K-edge of Zr, provide evidence for the coexistence of three different Zr-based complexes, [ZrF(6)](2-), [ZrF(7)](3-), and [ZrF(8)](4-), in the melt; the evolution of the concentration of these species upon addition of ZrF(4) is quantified. Smooth variations are observed, apart from a given composition at 35 mol % ZrF(4), for which an anomalous point is observed. Concerning the anion coordination, we observe a predominance of free fluorides at low concentrations in ZrF(4), and an increase of the number of bridging fluoride ions between complexes with addition of ZrF(4).

Preparation and structure of [formula omitted] [TPyA = tris(2-pyridylmethyl)amine] possessing terminal and bridging fluorides

Reaction of [Cr III(NCMe) 6](BF 4) 3 with tris(2-pyridylmethyl)amine (TPyA) in MeCN results in a fluoride abstraction from BF 4 - yielding the formation of the unexpected [ Cr 2 III F 3 ( TPyA ) 2 ] ( BF 4 ) 3 that has a linear, symmetric F − bridging two Cr III ions with each Cr III ion also having a terminal fluoride. Antiferromagnetic coupling is observed between the two Cr III ions with a J/ k B of −19 K (−13 cm −1) ( H = −2 JS a · S b), and this is an order of magnitude stronger than expected for two bent fluoride linkages.

Technetium Fluoride Trioxide, TcO3F, Preparation and Properties.

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Technetium Fluoride Trioxide, TcO3F, Preparation and Properties

TcO4- in HF solution reacts to form Tc3O9F4- along with some TcO3F. Pure TcO3F is obtained if a mixture of HF/BiF5 is applied. TcO3F dimerizes in the solid state via fluoride bridges, similar to the structures of CrO2F2 and VOF3. TcO3F reacts in HF with AsF5 or SbF5 under formation of TcO2F2+As(Sb)F6-.

Structure and properties of oxofluoroniobate glasses based on MnNbOF5

The MnNbOF5-BaF2 and MnNbOF5-PbF2 systems have a wider glass region than the binary system NbO2F-BaF2. According to thermogravimetric analysis data, the glasses in these systems are characterized by lower glass transition temperatures than the similar binary glass 50NbO2F·50BaF2. The thermal stability range and the degree of thermal stability of the glasses depend on the modifying cation. As demonstrated by IR and Raman spectroscopy, the glass structure is built from Nb(O,F)6 polyhedra linked into a glass network by oxygen and fluoride bridges. This is explained by the fact that the starting component MnNbOF5·4H2O decomposes to NbO2F upon melting. The manganese cations form their own polyhedra, which are incorporated into the glass network. The modifying cations exert a considerable effect on the glass network structure and on short-and medium-range order domains.

A theoretical study of the fluoride exchange between UO2F+(aq) and UO22+(aq)

Experimental data on the thermodynamics and reaction mechanism of the inner-sphere fluoride exchange reaction U17O2(2+) + UO2F+ <==> U17O2F+ + UO2(2+) have been compared with different intimate reaction mechanisms using quantum chemical methods. Two models have been tested that start from the outer sphere complexes, (H2O)[U(A)O2F(OH2)4+]...[U(B)O2(OH2)5(2+)] and [U(A)O2F(OH2)4+]...[U(B)O2(OH2)5(2+)]; the geometry and energies of the intermediates and transition states along possible reaction pathways have been calculated using different ab initio methods, SCF, B3LYP and MP2. Both the experimental data and the theoretical results suggest that the fluoride exchange takes place via the formation and breaking of a U-F-U bridge that is the rate determining step. The calculated activation enthalpy DeltaH( not equal) = 30.9 kJ mol(-1) is virtually identical to the experimental value 31 kJ mol(-1); however this agreement may be a coincidence as we do not expect a larger accuracy than 10 kJ mol(-1) with the methods used. The calculations show that the fluoride bridge is formed as an insertion of U(A)O2)F(OH2)4+ into U(B)O2(OH2)5(2+) followed by a subsequent transfer of water from the first to the second coordination sphere of U(B).