Transmetalation of Chromocene by Lithium-Amide, -Phosphide, and -Arsenide Nucleophiles

The pnictogen-centered nucleophiles LiE­(SiMe₃)₂ (E = N, P, or As) substitute a cyclopentadienide ligand of chromocene (Cp₂Cr), with elimination of lithium cyclopentadienide, to give the series of pnictogen-bridged compounds [(μ:η²:η⁵-Cp)­Cr­{μ-N­(SiMe₃)₂}₂Li] (<b>1</b>) and [(η⁵-Cp)­Cr­{μ-E­(SiMe₃)₂}]₂, with E = P (<b>2</b>) or E = As (<b>3</b>). Whereas <b>1</b> is a heterobimetallic coordination polymer, <b>2</b> and <b>3</b> are homometallic dimers, with the differences being due to a structure-directing influence of the hard or soft character of the bridging group 15 atoms. For compound <b>1</b>, the experimental magnetic susceptibility data were accurately reproduced by a single-ion model based on high-spin chromium­(II) (<i>S</i> = 2), which gave a <i>g</i>-value of 1.93 and an axial zero-field splitting parameter of <i>D</i> = −1.83 cm^–1. Determinations of phosphorus- and arsenic-mediated magnetic exchange coupling constants, <i>J</i>, are rare: in the dimers <b>2</b> and <b>3</b>, variable-temperature magnetic susceptibility measurements identified strong antiferromagnetic exchange between the chromium­(II) centers, which was modeled using the spin Hamiltonian <i>H</i> = −2<i>J</i>(<i>S</i>_CrA·<i>S</i>_CrB), and produced large coupling constants of <i>J</i> = −166 cm^–1 for <b>2</b> and −77.5 cm^–1 for <b>3</b>

Systematic Study of a Family of Butterfly-Like {M₂Ln₂} Molecular Magnets (M = Mg^II, Mn^III, Co^II, Ni^II, and Cu^II; Ln = Y^III, Gd^III, Tb^III, Dy^III, Ho^III, and Er^III)

A family of 3d–4f [M^II₂Ln^III₂(μ₃-OH)₂(O₂C^<i>t</i>Bu)₁₀]^2– “butterflies” (where M^II = Mg, Co, Ni, and Cu; Ln^III = Y, Gd, Tb, Dy, Ho, and Er) and [Mn^III₂Ln^III₂(μ₃-O)₂(O₂C^<i>t</i>Bu)₁₀]^2– molecules (where Ln^III = Y, Gd, Tb, Dy, Ho, and Er) has been synthesized and characterized through single-crystal X-ray diffraction, SQUID magnetometry, and ab initio calculations. All dysprosium- and some erbium-containing tetramers showed frequency-dependent maxima in the out-of-phase component of the susceptibility associated with slow relaxation of magnetization, and hence, they are single-molecule magnets (SMMs). AC susceptibility measurements have shown that the SMM behavior is entirely intrinsic to the Dy and Er sites and the magnitude of the energy barrier is influenced by the interactions between the 4f and the 3d metal. A trend is observed between the strength of the 3d-4f exchange interaction between and the maximum observed in the χ″_M(<i>T</i>)

Iron Lanthanide Phosphonate Clusters: {Fe₆Ln₆P₆} WellsDawson-like Structures with <i>D</i>_3<i>d</i> Symmetry

Reaction of [Fe₃(μ₃-O)­(O₂C^<i>t</i>Bu)₆(HO₂C^<i>t</i>Bu)₃]­(O₂C^<i>t</i>Bu) and [Ln₂(O₂C^<i>t</i>Bu)₆(HO₂C^<i>t</i>Bu)₆] (Ln = lanthanide) with three different phosphonic acids produce a family of highly symmetrical {Fe₆Ln₆P₆} clusters with general formula [Fe₆Ln₆(μ₃-O)₂(CO₃)­(O₃PR)₆(O₂C^<i>t</i>Bu)₁₈], where R = methyl <b>1</b>, phenyl <b>2</b>, or <i>n</i>-hexyl <b>3</b>. All the clusters present an analogous metal frame to the previously reported {Ni₆Ln₆P₆} both being related to the well-known Wells–Dawson ion from polyoxometallate chemistry. These highly symmetrical clusters have, or approximate very closely to, <i>D</i>_3<i>d</i> point symmetry. Both Fe^III and Gd^III ions are magnetically isotropic and could thus exhibit promising magnetocaloric properties; hence we investigated the {Fe₆Gd₆P₆} compounds accordingly. Modeling the magnetic data of [Fe₆Gd₆(μ₃-O)₂(CO₃)­(O₃PPh)₆(O₂C^<i>t</i>Bu)₁₈] by the finite-temperature Lanczos method gave a strong antiferromagnetic Fe···Fe interaction (<i>J</i>_Fe–Fe = −30 cm^–1) and very weak Gd···Gd and Gd···Fe exchange interactions (|<i>J</i>| < 0.1 cm^–1). The strong antiferromagnetic Fe···Fe interaction could account for the relatively smaller −Δ<i>S</i>_m value observed, compared against the {Ni₆Gd₆P₆} analogues

Physicochemical properties of near-linear Ln(II) bis-silylamide complexes (Ln = Sm, Eu, Tm, Yb)

Following our report of the first near-linear lanthanide (Ln) complex, [Sm­(N^††)₂] (<b>1</b>), herein we present the synthesis of [Ln­(N^††)₂] [N^†† = {N­(Si^<i>i</i>Pr₃)₂}; Ln = Eu (<b>2</b>), Tm (<b>3</b>), Yb (<b>4</b>)], thus achieving approximate uniaxial geometries for a series of “traditional” Ln^II ions. Experimental evidence, together with calculations performed on a model of <b>4</b>, indicates that dispersion forces are important for stabilization of the near-linear geometries of <b>1</b>–<b>4</b>. The isolation of <b>3</b> under a dinitrogen atmosphere is noteworthy, given that “[Tm­(N″)­(μ-N″)]₂” (N″ = {N­(SiMe₃)₂}) has not previously been structurally authenticated and reacts rapidly with N₂(g) to give [{Tm­(N″)₂}₂(μ-η²:η²-N₂)]. Complexes <b>1</b>–<b>4</b> have been characterized as appropriate by single-crystal X-ray diffraction, magnetic measurements, electrochemistry, multinuclear NMR, electron paramagnetic resonance (EPR), and electronic spectroscopy, along with computational methods for <b>3</b> and <b>4</b>. The remarkable geometries of monomeric <b>1</b>–<b>4</b> lead to interesting physical properties, which complement and contrast with comparatively well understood dimeric [Ln­(N″)­(μ-N″)]₂ complexes. EPR spectroscopy of <b>3</b> shows that the near-linear geometry stabilizes <i>m</i>_<i>J</i> states with oblate spheroid electron density distributions, validating our previous suggestions. Cyclic voltammetry experiments carried out on <b>1</b>–<b>4</b> did not yield Ln^II reduction potentials, so a reactivity study of <b>1</b> was performed with selected substrates in order to benchmark the Sm^III → Sm^II couple. The separate reactions of <b>1</b> with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), azobenzene, and benzophenone gave crystals of [Sm­(N^††)₂(TEMPO)] (<b>5</b>), [Sm­(N^††)₂(N₂Ph₂)] (<b>6</b>), and [Sm­(N^††)­{μ-OPhC­(C₆H₅)­CPh₂O-κ<i>O</i>,<i>O</i>′}]₂ (<b>7</b>), respectively. The isolation of <b>5</b>–<b>7</b> shows that the Sm^II center in <b>1</b> is still accessible despite having two bulky N^†† moieties and that the N-donor atoms are able to deviate further from linearity or ligand scrambling occurs in order to accommodate another ligand in the Sm^III coordination spheres of the products

Making hybrid [n]-rotaxanes as supramolecular arrays of molecular electron spin qubits

Quantum information processing (QIP) would require that the individual units involved--qubits--communicate to other qubits while retaining their identity. In many ways this resembles the way supramolecular chemistry brings together individual molecules into interlocked structures, where the assembly has one identity but where the individual components are still recognizable. Here a fully modular supramolecular strategy has been to link hybrid organic-inorganic [2]- and [3]-rotaxanes into still larger [4]-, [5]- and [7]-rotaxanes. The ring components are heterometallic octanuclear [Cr7NiF8(O2C(t)Bu)16](-) coordination cages and the thread components template the formation of the ring about the organic axle, and are further functionalized to act as a ligand, which leads to large supramolecular arrays of these heterometallic rings. As the rings have been proposed as qubits for QIP, the strategy provides a possible route towards scalable molecular electron spin devices for QIP. Double electron-electron resonance experiments demonstrate inter-qubit interactions suitable for mediating two-qubit quantum logic gates