Lezione 8

κ-carbides of varying composition, seemingly responsible for age hardening in high-Al steel alloys, have been detected to precipitate both at grain boundaries and in the bulk grain of steels. Herein we report the bulk-phase synthesis of “Mn₃AlC” by arc plasma sintering and rapid solidification. Single crystals have been found suitable for X-ray diffraction using Mo radiation and yield a lattice parameter of <i>a</i> = 3.875(2) Å. We find a mixed occupation of the 1<i>a</i> position by Al and Mn, which, together with the C position being fully occupied, leads to the actual composition Mn_3.1Al_0.9C. Additional energy-dispersive X-ray–scanning electron microscopy measurements support the composition and corroborate the homogeneity. SQUID data collected on the polycrystalline ferromagnetic sample exhibit a Curie temperature of about 295 ± 13 K and a soft magnetic behavior. The small but significant nonstoichiometry on 1<i>a</i> leads to a slightly larger lattice parameter, a higher electron count, and, thus, a lowered density of states at the Fermi level, indicative of increased phase stability

Tuning the Condensation Degree of {Fe^III_<i>n</i>} Oxo Clusters via Ligand Metathesis, Temperature, and Solvents

Trinuclear μ₃-oxo-centered iron­(III) isobutyrate clusters readily react with polyalcohol organic ligands under one-pot synthesis conditions. Depending on the ligand, solvent, and temperature, a range of hexa-, dodeca-, and doicosanuclear iron­(III) oxo-hydroxo condensation products, isolated as (mdeaH₃)₂[Fe₆O­(thme)₄Cl₆]·0.5­(MeCN)·0.5­(H₂O) (<b>1</b>), [Fe₁₂O₄(OH)₂(teda)₄(N₃)₄(MeO)₄]­N₃(NO₃)_0.5(MeO)_0.5·2.5­(H₂O) (<b>2</b>), [Fe₁₂O₆(teda)₄Cl₈]·6­(CHCl₃) (<b>3</b>), [Fe₂₂O₁₆(OH)₂(O₂CCHMe₂)₁₈(bdea)₆(EtO)₂(H₂O)₂]·2­(EtOH)·5­(MeCN)·6­(H₂O) (<b>4</b>), and [Fe₂₂O₁₄(OH)₄(O₂CCHMe₂)₁₈(mdea)₆(EtO)₂(H₂O)₂]­(NO₃)₂·EtOH·H₂O (<b>5</b>), where tedaH₄ = <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetrakis­(2-hydroxyethyl)­ethylenediamine; thmeH₃ = 1,1,1-tris­(hydroxymethyl)­ethane; mdeaH₂ = <i>N</i>-methyldiethanolamine; and bdeaH₂ = <i>N</i>-butyldiethanolamine. Complete carboxylate metathesis in the {Fe₃} precursor complexes by thme^3– or teda^4– and the agglomeration of the formed species under solvothermal conditions afforded carboxylate-free {Fe₆} product (<b>1</b>) in MeCN/CH₂Cl₂ or {Fe₁₂} complexes (<b>2</b> and <b>3</b>) in MeOH/EtOH and CHCl₃/thf, respectively (thf = tetrahydrofuran). Single-crystal X-ray diffraction analyses revealed that <b>1</b> contains a [Fe₆O­(thme)₄Cl₆]^2– cluster anion with a Lindqvist-type {Fe₆(μ₆-O)} core motif, charge-compensated by two protonated mdeaH₃⁺ cations. <b>2</b> comprises a [Fe₁₂O₄(OH)₂(teda)₄­(N₃)₄(MeO)₄]²⁺ cation with a {Fe₁₂O₄(OH)₂}²⁶⁺ core, whereas <b>3</b> contains a charge-neutral [Fe₁₂O₆(teda)₄(Cl)₈] complex with an {Fe₁₂O₆}²⁴⁺ core. Finally, employing flexible bdeaH₂ or mdeaH₂ ligands under soft reaction conditions afforded giant {Fe₂₂} oxo-hydroxo complexes (<b>4</b> and <b>5</b>) with a central {Fe₆} layer sandwiched between two outer {Fe₈} groups. Magnetic studies of <b>1</b>–<b>5</b> revealed strong antiferromagnetic coupling between the Fe^III spin centers in all clusters

Synthesis, Structure, and Magnetic Properties of a New Family of Tetra-nuclear {Mn₂^IIILn₂}(Ln = Dy, Gd, Tb, Ho) Clusters With an Arch-Type Topology: Single-Molecule Magnetism Behavior in the Dysprosium and Terbium Analogues

Sequential reaction of Mn­(II) and lanthanide­(III) salts with a new multidentate ligand, 2,2′-(2-hydroxy-3-methoxy-5-methylbenzylazanediyl)­diethanol (<b>LH</b>_<b>3</b>), containing two flexible ethanolic arms, one phenolic oxygen, and a methoxy group afforded heterometallic tetranuclear complexes [Mn₂Dy₂(LH)₄(μ-OAc)₂]­(NO₃)₂·2CH₃OH·3H₂O (<b>1</b>), [Mn₂Gd₂(LH)₄(μ-OAc)₂]­(NO₃)₂·2CH₃OH·3H₂O (<b>2</b>), [Mn₂Tb₂(LH)₄(μ-OAc)₂]­(NO₃)₂·2H₂O·2CH₃OH·Et₂O (<b>3</b>), and [Mn₂Ho₂(LH)₄(μ-OAc)₂]­Cl₂·5CH₃OH (<b>4</b>). All of these dicationic complexes possess an arch-like structural topology containing a central Mn^III–Ln–Ln–Mn^III core. The two central lanthanide ions are connected via two phenolate oxygen atoms. The remaining ligand manifold assists in linking the central lanthanide ions with the peripheral Mn­(III) ions. Four doubly deprotonated LH^2– chelating ligands are involved in stabilizing the tetranuclear assembly. A magnetochemical analysis reveals that single-ion effects dominate the observed susceptibility data for all compounds, with comparably weak Ln···Ln and very weak Ln···Mn­(III) couplings. The axial, approximately square-antiprismatic coordination environment of the Ln³⁺ ions in <b>1</b>–<b>4</b> causes pronounced zero-field splitting for Tb³⁺, Dy³⁺, and Ho³⁺. For <b>1</b> and <b>3</b>, the onset of a slowing down of the magnetic relaxation was observed at temperatures below approximately 5 K (<b>1</b>) and 13 K (<b>3</b>) in frequency-dependent alternating current (AC) susceptibility measurements, yielding effective relaxation energy barriers of Δ<i>E</i> = 16.8 cm^–1 (<b>1</b>) and 33.8 cm^–1 (<b>3</b>)

Synthesis, Structure, and Magnetic Properties of a New Family of Tetra-nuclear {Mn₂^IIILn₂}(Ln = Dy, Gd, Tb, Ho) Clusters With an Arch-Type Topology: Single-Molecule Magnetism Behavior in the Dysprosium and Terbium Analogues

Sequential reaction of Mn­(II) and lanthanide­(III) salts with a new multidentate ligand, 2,2′-(2-hydroxy-3-methoxy-5-methylbenzylazanediyl)­diethanol (<b>LH</b>_<b>3</b>), containing two flexible ethanolic arms, one phenolic oxygen, and a methoxy group afforded heterometallic tetranuclear complexes [Mn₂Dy₂(LH)₄(μ-OAc)₂]­(NO₃)₂·2CH₃OH·3H₂O (<b>1</b>), [Mn₂Gd₂(LH)₄(μ-OAc)₂]­(NO₃)₂·2CH₃OH·3H₂O (<b>2</b>), [Mn₂Tb₂(LH)₄(μ-OAc)₂]­(NO₃)₂·2H₂O·2CH₃OH·Et₂O (<b>3</b>), and [Mn₂Ho₂(LH)₄(μ-OAc)₂]­Cl₂·5CH₃OH (<b>4</b>). All of these dicationic complexes possess an arch-like structural topology containing a central Mn^III–Ln–Ln–Mn^III core. The two central lanthanide ions are connected via two phenolate oxygen atoms. The remaining ligand manifold assists in linking the central lanthanide ions with the peripheral Mn­(III) ions. Four doubly deprotonated LH^2– chelating ligands are involved in stabilizing the tetranuclear assembly. A magnetochemical analysis reveals that single-ion effects dominate the observed susceptibility data for all compounds, with comparably weak Ln···Ln and very weak Ln···Mn­(III) couplings. The axial, approximately square-antiprismatic coordination environment of the Ln³⁺ ions in <b>1</b>–<b>4</b> causes pronounced zero-field splitting for Tb³⁺, Dy³⁺, and Ho³⁺. For <b>1</b> and <b>3</b>, the onset of a slowing down of the magnetic relaxation was observed at temperatures below approximately 5 K (<b>1</b>) and 13 K (<b>3</b>) in frequency-dependent alternating current (AC) susceptibility measurements, yielding effective relaxation energy barriers of Δ<i>E</i> = 16.8 cm^–1 (<b>1</b>) and 33.8 cm^–1 (<b>3</b>)

Ultralarge 3d/4f Coordination Wheels: From Carboxylate/Amino Alcohol-Supported {Fe₄Ln₂} to {Fe₁₈Ln₆} Rings

A family of wheel-shaped charge-neutral heterometallic {Fe^III₄Ln^III₂}- and {Fe^III₁₈M^III₆}-type coordination clusters demonstrates the intricate interplay of solvent effects and structure-directing roles of semiflexible bridging ligands. The {Fe₄Ln₂}-type compounds [Fe₄Ln₂(O₂CCMe₃)₆­(N₃)₄(Htea)₄]·2­(EtOH), Ln = Dy (<b>1a</b>), Er (<b>1b</b>), Ho (<b>1c</b>); [Fe₄Tb₂(O₂CCMe₃)₆­(N₃)₄(Htea)₄] (<b>1d</b>); [Fe₄Ln₂(O₂CCMe₃)₆­(N₃)₄(Htea)₄]·2­(CH₂Cl₂), Ln = Dy (<b>2a</b>), Er (<b>2b</b>); [Fe₄Ln₂(O₂CCMe₃)₄­(N₃)₆(Htea)₄]·2­(EtOH)·2­(CH₂Cl₂), Ln = Dy (<b>3a</b>), Er (<b>3b</b>) and the {Fe₁₈M₆}-type compounds [Fe₁₈M₆(O₂CCHMe₂)₁₂­(Htea)₁₈(tea)₆(N₃)₆]·<i>n</i>(solvent), M = Dy (<b>4</b>, <b>4a</b>), Gd (<b>5</b>), Tb (<b>6</b>), Ho (<b>7</b>), Sm (<b>8</b>), Eu (<b>9</b>), and Y (<b>10</b>) form in ca. 20–40% yields in direct reaction of trinuclear Fe^III pivalate or isobutyrate clusters, lanthanide/yttrium nitrates, and bridging triethanolamine (H₃tea) and azide ligands in different solvents: EtOH for the smaller {Fe₄Ln₂} wheels and MeOH/MeCN or MeOH/EtOH for the larger {Fe₁₈M₆} wheels. Single-crystal X-ray diffraction analyses revealed that <b>1</b>–<b>3</b> consist of planar centrosymmetric hexanuclear clusters built from Fe^III and Ln^III ions linked by an array of bridging carboxylate, azide, and aminopolyalcoholato-based ligands into a cyclic structure with a cavity, and with distinct sets of crystal solvents (2 EtOH per formula unit in <b>1a</b>–<b>c</b>, 2 CH₂Cl₂ in <b>2</b>, and 2 EtOH and 2 CH₂Cl₂ in <b>3</b>). In <b>4</b>–<b>10</b>, the largest 3d/4f wheels currently known, nearly linear Fe₃ fragments are joined via mononuclear Ln/Y units by a set of isobutyrates and amino alcohol ligands into virtually planar rings. The magnetic properties of <b>1</b>–<b>10</b> reveal slow magnetization relaxation for {Fe₄Tb₂} (<b>1d</b>) and slow relaxation for {Fe₄Ho₂} (<b>1c</b>), {Fe₁₈Dy₆} (<b>4</b>), and {Fe₁₈Tb₆} (<b>6</b>)

A V₁₆-type Polyoxovanadate Structure with Intricate Electronic Distribution: Insights from Magnetochemistry

The black-green solid (NEt₄)₅­[V₁₆O₃₈(Br)]­·2H₂O (<b>1</b>) was synthesized by the pH-controlled reaction of a mixed-valence precursor (NH₄)₈­[H₉V^IV₁₂V^V₇O₅₀]­·11H₂O with Et₄NBr in water under aerobic conditions. Compound <b>1</b> crystallizes as pseudomerohedral three-domain twins with pronounced pseudosymmetry and very large voids accommodating the majority of the countercations and solvent water molecules. The central structural motif of <b>1</b> is represented by a spherical, mixed-valence, host–guest vanadium-oxo cluster [V^IV/V₁₆O₃₈(Br)]<i>^q</i> with <i>q</i> = 5–, 4–, or 6–, exhibiting dominant antiferromagnetic and weaker ferromagnetic exchange interactions. The intriguing valence-state and dependent magnetic behavior of this compound have been unraveled by weighted model Hamiltonian calculations combined with diffraction, quantum mechanical, spectroscopic, and spectrometric techniques. It appears that <b>1</b> features a hitherto not identified and initially not evident V^IV/V^V average ratio of 8:8 which corresponds to an average charge <i>q</i> = 5– of the polyoxovanadate. Our study makes a substantial contribution to the further development of methods improving the understanding of poorly soluble mixed-valence polyoxometalates with complex spin architectures

Assembly of Cerium(III) 2,2′-Bipyridine-5,5′-dicarboxylate-based Metal–Organic Frameworks by Solvent Tuning

Small changes to the reaction conditions differentiate between two metal–organic frameworks (MOFs), {[Ce₂(H₂O)­(bpdc)₃(dmf)₂]·2­(dmf)}_<i>n</i> (<b>1</b>) and {[Ce₄(H₂O)₅(bpdc)₆(dmf)]·<i>x</i>(dmf)}_<i>n</i> (<b>2</b>), that were solvothermally synthesized from cerium­(III) nitrate hexahydrate and 2,2′-bipyridine-5,5′-dicarboxylic acid (H₂bpdc) in dimethylformamide (dmf). The two compounds illustrate how the flexibility of the coordination geometry of Ce^III translates into MOFs, the formation of which readily adapts to different solvent environments

Covalent Co–O–V and Sb–N Bonds Enable Polyoxovanadate Charge Control

The formation of [{Co^II(teta)₂}­{Co^II₂(tren)­(teta)₂}­V^IV₁₅Sb^III₆O₄₂(H₂O)]·ca.9H₂O [teta = triethylenetetraamine; tren = tris­(2-aminoethyl)­amine] illustrates a strategy toward reducing the molecular charge of polyoxovanadates, a key challenge in their use as components in single-molecule electronics. Here, a V–O–Co bond to a binuclear Co²⁺-centered complex and a Sb–N bond to the terminal N atom of a teta ligand of a mononuclear Co²⁺ complex allow for full charge compensation of the archetypal molecular magnet [V₁₅Sb₆O₄₂(H₂O)]^6–. Density functional theory based electron localization function analysis demonstrates that the Sb–N bond has an electron density similar to that of a Sb–O bond. Magnetic exchange coupling between the V^IV and Co^II spin centers mediated via the Sb–N bridge is comparably weakly antiferromagnetic

Avoiding Magnetochemical Overparametrization, Exemplified by One-Dimensional Chains of Hexanuclear Iron(III) Pivalate Clusters

One-dimensional chain coordination polymers based on hexanuclear iron­(III) pivalate building blocks and 1,4-dioxane (diox) or 4,4′-bipyridine (4,4′-bpy) bridging ligands, [Fe₆O₂(O₂CH₂)­(O₂CCMe₃)₁₂(diox)]_<i>n</i> (<b>1</b>) and [Fe₆O₂(O₂CH₂)­(O₂CCMe₃)₁₂(4,4′-bpy)]_<i>n</i> (<b>2</b>), showcase the utility of the angular overlap model, implemented in the program <i>wxJFinder</i>, in the predictive identification of the relative role of intra- and intercluster coupling

Ammonothermal Synthesis, Crystal Structure, and Properties of the Ytterbium(II) and Ytterbium(III) Amides and the First Two Rare-Earth-Metal Guanidinates, YbC(NH)₃ and Yb(CN₃H₄)₃

We report the oxidation-controlled synthesis of the ytterbium amides Yb­(NH₂)₂ and Yb­(NH₂)₃ and the first rare-earth-metal guanidinates YbC­(NH)₃ and Yb­(CN₃H₄)₃ from liquid ammonia. For Yb­(NH₂)₂, we present experimental atomic displacement parameters from powder X-ray diffraction (PXRD) and density functional theory (DFT)-derived hydrogen positions for the first time. For Yb­(NH₂)₃, the indexing proposal based on PXRD arrives at <i>R</i>3̅, <i>a</i> = 6.2477(2) Å, <i>c</i> = 17.132(1) Å, <i>V</i> = 579.15(4) ų, and <i>Z</i> = 6. The oxidation-controlled synthesis was also applied to make the first rare-earth guanidinates, namely, the doubly deprotonated YbC­(NH)₃ and the singly deprotonated Yb­(CN₃H₄)₃. YbC­(NH)₃ is isostructural with SrC­(NH)₃, as derived from PXRD (<i>P</i>6₃/<i>m</i>, <i>a</i> = 5.2596(2) Å, <i>c</i> = 6.6704(2) Å, <i>V</i> = 159.81(1) ų, and <i>Z</i> = 2). Yb­(CN₃H₄)₃ crystallizes in a structure derived from the [ReO₃] type, as studied by powder neutron diffraction (<i>Pn</i>3̅, <i>a</i> = 13.5307(3) Å, <i>V</i> = 2477.22(8) ų, and <i>Z</i> = 8 at 10 K). Electrostatic and hydrogen-bonding interactions cooperate to stabilize the structure with wide and empty channels. The IR spectra of the guanidinates are compared with DFT-calculated phonon spectra to identify the vibrational modes. SQUID magnetometry shows that Yb­(CN₃H₄)₃ is a paramagnet with isolated Yb³⁺ (4f¹³) ions. A <i>CONDON 2.0</i> fit was used to extract all relevant parameters