Strong Electronic and Magnetic Coupling in M4 (M = Ni, Cu) Clusters via Direct Orbital Interactions between Low-Coordinate Metal Centers

Magneto-Luminescent Two-Dimensional Nanosheets of Gadolinium and Gold Nanocluster Assemblies with Surface Molecular Functionalization for White Light Emission

We report the formation of photoluminescent two-dimensional (2D) crystalline nanosheet assemblies of gadolinium ions and ligand-stabilized gold nanoclusters (Gd-Au NCs). Transmission electron microscopy, selected area electron diffraction in conjunction with atomic force microscopy, and field-emission scanning electron microscopy analyses substantiated the 2D nature of Gd-Au NC nanosheets. The optical and magnetic properties of the nanosheets were investigated by photoluminescence measurements and vibrating-sample magnetometry analyses. The so-formed crystalline product was further utilized to generate a synchronous tricolor (orange, green, and blue) emission from a single excitation wavelength through an inorganic surface complexation reaction. The independent emissions were tunable after ligand functionalization by acetylsalicylic acid and fluorescein on the Gd-Au NC assembly. Interestingly, the assembled superstructure with augmented quantum yield led to white light emission at λexc ≈ 325 nm with CIE of (0.34, 0.33) and CRI value of >85 in the liquid phase. Furthermore, the ability to modulate the luminescence properties through the surface complexation of the 2D nanosheets of Au NCs may bring about new avenues toward applications in light-emitting devices, sensing, and biomedical imaging.

Measuring Metal-Metal Communication in a Series of Ketimide-Bridged [Fe2]6+ Complexes

Reaction of Fe(acac)3 with 3 equiv of Li[N═C(R)Ph] (R = Ph, tBu) results in the formation of the [Fe2]6+ complexes, [Fe2(μ-N═C(R)Ph)2(N═C(R)Ph)4] (R = Ph, 1; tBu, 2), in low to moderate yields. Reaction of FeCl2 with 6 equiv of Li(N═C13H8) (HN═C13H8 = 9-fluorenone imine) results in the formation of [Li(THF)2]2[Fe(N═C13H8)4] (3) in good yield. Subsequent oxidation of 3 with ca. 0.8 equiv of I2 generates the [Fe2]6+ complex, [Fe2(μ-N═C13H8)2(N═C13H8)4] (4), along with free fluorenyl ketazine. Complexes 1, 2, and 4 were characterized by 1H NMR spectroscopy, X-ray crystallography, 57Fe Mössbauer spectroscopy, and SQUID magnetometry. The Fe-Fe distances in 1, 2, and 4 range from 2.803(7) to 2.925(1) Å, indicating that no direct Fe-Fe interaction is present in these complexes. The 57Fe Mössbauer spectra for complexes 1, 2, and 4 are all consistent with the presence of symmetry-equivalent high-spin Fe3+ centers. Finally, all three complexes exhibit a similar degree of antiferromagnetic coupling between the metal centers (J = -26 to -30 cm-1), as ascertained by SQUID magnetometry.

A Trinuclear Gadolinium Cluster with a Three-Center One-Electron Bond and an S = 11 Ground State

The recent discovery of metal-metal bonding and valence delocalization in the dilanthanide complexes (CpiPr5)2Ln2I3 (CpiPr5 = pentaisopropylcyclopentadienyl; Ln = Y, Gd, Tb, Dy) opened up the prospect of harnessing the 4fn5dz21 electron configurations of non-traditional divalent lanthanide ions to access molecules with novel bonding motifs and magnetism. Here, we report the trinuclear mixed-valence clusters (CpiPr5)3Ln3H3I2 (1-Ln, Ln = Y, Gd), which were synthesized via potassium graphite reduction of the trivalent clusters (CpiPr5)3Ln3H3I3. Structural, computational, and spectroscopic analyses support valence delocalization in 1-Ln resulting from a three-center, one-electron σ bond formed from the 4dz2 and 5dz2 orbitals on Y and Gd, respectively. Dc magnetic susceptibility data obtained for 1-Gd reveal that valence delocalization engenders strong parallel alignment of the σ-bonding electron and the 4f electrons of each gadolinium center to afford a high-spin ground state of S = 11. Notably, this represents the first clear instance of metal-metal bonding in a molecular trilanthanide complex, and the large spin-spin exchange constant of J = 168(1) cm-1 determined for 1-Gd is only the second largest coupling constant characterized to date for a molecular lanthanide compound.

Two-Electron Mixed Valency in a Heterotrimetallic Nickel-Vanadium-Nickel Complex

Mixed-valence complexes represent an enticing class of coordination compounds to interrogate electron transfer confined within a molecular framework. The diamagnetic heterotrimetallic anion, [V(SNS)₂{Ni(dppe)}₂]^-, was prepared by reducing (dppe)NiCl₂ in the presence of the chelating metalloligand [V(SNS)₂]^- [dppe = bis(diphenylphosphino)ethane; (SNS)^3- = bis(2-thiolato-4-methylphenyl)amide]. Vanadium-nickel bonds span the heterotrimetallic core in the structure of [V(SNS)₂{Ni(dppe)}₂]^-, with V-Ni bond lengths of 2.78 and 2.79 Å. One-electron oxidation of monoanionic [V(SNS)₂{Ni(dppe)}₂]^- yielded neutral, paramagnetic V(SNS)₂{Ni(dppe)}₂. The solid-state structure of V(SNS)₂{Ni(dppe)}₂ revealed that the two nickel ions occupy unique coordination environments: one nickel is in a square-planar S₂P₂ coordination environment (τ₄ = 0.19), with a long Ni···V distance of 3.45 Å; the other nickel is in a tetrahedral S₂P₂ coordination environment (τ₄ = 0.84) with a short Ni-V distance of 2.60 Å, consistent with a formal metal-metal bond. Continuous-wave X-band electron paramagnetic resonance spectroscopy, electrochemical investigations, and density functional theory computations indicated that the unpaired electron in the neutral V(SNS)₂{Ni(dppe)}₂ cluster is localized on the bridging [V(SNS)₂] metalloligand, and as a result, V(SNS)₂{Ni(dppe)}₂ is best described as a two-electron mixed-valence complex. These results demonstrate the important role that metal-metal interactions and flexible coordination geometries play in enabling multiple, reversible electron transfer processes in small cluster complexes.

The Potential of Gadolinium Ascorbate Nanoparticles as a Safer Contrast Agent

There have been health concerns raised against the use of gadolinium (Gd)-based magnetic resonance imaging contrast agents. The primary observation is that Gd ions are prone to leaking into the bloodstream, causing nephrogenic systemic fibrosis as one of the side effects. In addition, such leakage of the ions inhibits easy clearance from the body. Herein we propose that Gd-ascorbate nanoparticles could be one of the safer choices as they are rather stable in aqueous dispersion and they do not get affected by Zn or Fe ions in the medium. The magnetic properties of the ions are preserved in the nanoparticles, and particles when sufficiently small may be amenable to renal clearance from the human body. Thus, when an aqueous solution of Gd-acetate and ascorbic acid was left to evolve with time, a Gd-ascorbate complex was formed that led to the formation of nanoparticles with time. The sizes of the nanoparticles increased with time, and when the particles were sufficiently large, they precipitated out of the medium. In addition, smaller nanoparticles were consistently present at all times of observations. UV-vis, photoluminescence and FTIR spectroscopy, mass spectrometry, and transmission electron microscopy analyses confirmed the formation of nanoparticles of Gd-ascorbate complex. In addition, magnetic measurements confirmed the high relaxivity of the nanoparticles as compared to the parent salt, indicating the effectiveness of the nanoparticles as contrast agents. Density functional theory-based calculations of the molecular complex-based nanoparticles accounted for the experimental observations.

Electron Density Analysis of Metal-Metal Bonding in a Ni4 Cluster Featuring Ferromagnetic Exchange

We present a combined experimental and theoretical study of the nature of the proposed metal-metal bonding in the tetranuclear cluster Ni₄(NP^tBu₃)₄, which features four nickel(I) centers engaged in strong ferromagnetic coupling. High-resolution single-crystal synchrotron X-ray diffraction data collected at 25 K provide an accurate geometrical structure and a multipole model electron density description. Topological analysis of the electron density in the Ni₄N₄ core using the quantum theory of atoms in molecules clearly identifies the bonding as an eight-membered ring of type [Ni-N-]₄ without direct Ni-Ni bonding, and this result is generally corroborated by an analysis of the energy density distribution. In contrast, the calculated bond delocalization index of ∼0.6 between neighboring Ni atoms is larger than what has been found for other bridged metal-metal bonds and implies direct Ni-Ni bonding. Similar support for the presence of direct Ni-Ni bonding is found in the interacting quantum atom approach, an energy decomposition scheme, which suggests the presence of stabilizing Ni-Ni bonding interactions with an exchange-correlation energy contribution approximately 50% of that of the Ni-N interactions. Altogether, while the direct interactions between neighboring Ni centers are too weak and sterically constrained to bear the signature of a topological bond critical point, other continuous measures clearly indicate significant Ni-Ni bonding. These metal-metal bonding interactions likely mediate direct ferromagnetic exchange, giving rise to the high-spin ground state of the molecule.

Metal-Metal-Bonded Fe4 Clusters with Slow Magnetic Relaxation

Reaction of FeBr₂ with Li(N═C^tBu₂) (0.5 equiv) and Zn⁰ (2 equiv) results in the formation of the formally mixed-valent cluster [Fe₄Br₂(N═C^tBu₂)₄] (1) in moderate yield. The subsequent reaction of 1 with Na(N═C^tBu₂) results in formation of [Fe₄Br(N═C^tBu₂)₅] (2), also in moderate yield. Both 1 and 2 were characterized by zero-field ⁵⁷Fe Mössbauer spectroscopy, X-ray crystallography, and superconducting quantum interference device magnetometry. Their tetrahedral [Fe₄]⁶⁺ cores feature short Fe-Fe interactions (ca. 2.50 Å). Additionally, both 1 and 2 display S = 7 ground states at room temperature and slow magnetic relaxation with zero-field relaxation barriers of U_eff = 14.7(4) and 15.6(7) cm^-1, respectively. Moreover, AC magnetic susceptibility measurements were well modeled by assuming an Orbach relaxation process.

Synthesis, structure and magnetism of a novel series of trinuclear nickel(II) clusters

Five novel trinuclear nickel(II) clusters have been successfully synthesized, namely Ni3(fshz)2(L)4·n(sol) (H3fshz = N-formylsalicylhydrazide; L = pyridine, n = 0, 1; L = 4-methylpyridine, n = 1, sol = CH3CN,...

Modulating the relaxation dynamics of the Na2Mn3 system via an auxiliary anion change

This paper reports two closely related heteropentanuclear manganese complexes, namely, {Na₂Mn₃(opch)₃(μ₄-O)(μ₂-N₃) (μ₂-AcO)(μ₂-MeO)}·6CH₃OH·0.5H₂O (1) and {Na₂Mn₃(opch)₃(μ₄-O)(μ₂-N₃)₂(μ₂-AcO)}·2.5CH₃OH·2H₂O (2), where H₂opch is (E)-N'-(2-hydroxy-3-methoxybenzylidene)pyrazine-2-carbohydrazide. Single-crystal X-ray diffraction analysis reveals that the trigonal bipyramidal skeletons in both complexes are comparable, where a perfect triangular Mn₃ motif occupies the equatorial plane. Magnetic investigations suggest that overall antiferromagnetic coupling is present within the triangles of 1 and 2. However, their dynamic magnetic properties are drastically distinct. Indeed, complexes 1 and 2 show two kinds of dual slow magnetic relaxation processes that correspond to anisotropy barriers (Δ) of 9.2 cm^-1 (11.4 cm^-1 for 2) and 12.8 cm^-1 (30.0 cm^-1 for 2) for the low- and high-frequency domains, respectively. More importantly, a further comparative study of the structure and magnetism indicates that the coordination sphere of these two model complexes with the homologous hydrazone-based coordination sites undergoes an alteration from methoxide-O to azide-N upon a subtle change of the auxiliary anion accompanied by modulating octahedron geometries, leading to a further influence on different relaxation dynamics.

High Spin Cobalt Complexes Supported by a Trigonal Tris(Phosphinimide) Ligand

Terminal, π-basic moieties occupy a prominent position in the stabilization of unusual or reactive inorganic species. The electron-releasing, π-basic properties of phosphinimides (PN) have been employed to stabilize electron-deficient early transition metals and lanthanides. In principle, a ligand field comprised of terminal PN groups should enable access to high-valent states of late first row transition metals. Herein, we report a new class of multidentate phosphinimide ligands to logically explore this hypothesis. Access to such ligands is made possible by a new procedure for the electrophilic amination of rigid, sterically encumbering, multidentate phosphines. Such frameworks facilitate terminal PN coordination to cobalt as demonstrated by the synthesis of a trinuclear Co^II₃ complex and a homoleptic, three-coordinate Co^III complex. Interestingly, the Co^III complex exhibits an exceedingly rare S = 2 ground state. Combined XRD, magnetic susceptibility, and DFT studies highlight that terminally bound PNs engage in strong dπ-pπ interactions that present a weak ligand field appropriate to stabilize high-spin states of late transition metals.

Chalcogen-atom abstraction reactions of a Di-iron imidophosphorane complex

Reaction of the complexes [Fe2(μ2-NP(pip)3)2(NP(pip)3)2] (1-Fe) and [Co2(μ2-NP(pip)3)2(NP(pip)3)2] (1-Co), where [NP(pip)3]1- is tris(piperidinyl)imidophosphorane, with nitrous oxide, S8, or Se0 results in divergent reactivity. With nitrous oxide, 1-Fe forms [Fe2(μ2-O)(μ2-NP(pip)3)2(NP(pip)3)2] (2-Fe), with a very short Fe3+-Fe3+ distance. Reactions of 1-Fe with S8 or Se0 result in the bridging, side-on coordination (μ-κ1:κ1-E22-) of the heavy chalcogens in complexes [Fe2(μ-κ1:κ1-E2)(μ2-NP(pip)3)2(NP(pip)3)2] (E = S, 3-Fe, or Se, 4-Fe). In all cases, the complex 1-Co is inert.

Spectroscopic Investigation of a Metal-Metal-Bonded Fe6 Single-Molecule Magnet with an Isolated S = 19/2 Giant-Spin Ground State

The metal-metal-bonded molecule [Bu₄N][(^HL)₂Fe₆(dmf)₂] (Fe₆) was previously shown to possess a thermally isolated spin S = ¹⁹/₂ ground state and found to exhibit slow magnetization relaxation below a blocking temperature of ∼5 K [J. Am. Chem. Soc. 2015, 137, 13949-13956]. Here, we present a comprehensive spectroscopic investigation of this unique single-molecule magnet (SMM), combining ultrawideband field-swept high-field electron paramagnetic resonance (EPR) with frequency-domain Fourier-transform terahertz EPR to accurately quantify the spin Hamiltonian parameters of Fe₆. Of particular importance is the near absence of a 4th-order axial zero-field splitting term, which is known to arise because of quantum mechanical mixing of spin states on account of the relatively weak spin-spin (superexchange) interactions in traditional polynuclear SMMs such as the celebrated Mn₁₂-acetate. The combined high-resolution measurements on both powder samples and an oriented single crystal provide a quantitative measure of the isolated nature of the spin ground state in the Fe₆ molecule, as well as additional microscopic insights into factors that govern the quantum tunneling of its magnetization. This work suggests strategies for improving the performance of polynuclear SMMs featuring direct metal-metal bonds and strong ferromagnetic spin-spin (exchange) interactions.

Carboxylic acid-tuned nickel(ii) clusters: syntheses, structures, solution behaviours and magnetic properties

Three novel cicada-like nickel(ii) clusters, formulated as [Ni₆(bdped)₂(mba)₆(Hdmpz)₂(NO₃)₂(H₂O)₂]·4MeCN (SD/Ni6b), [Ni₅(bdped)₂(tca)₆(Hdmpz)(MeOH)₂(H₂O)]·MeOH (SD/Ni5a) and [Ni₄(Hbdped)₂(ba)₄(Hdmpz)₂]·2NO₃·2MeCN (SD/Ni4a), were obtained by tuning the auxiliary carboxylic acids, where H₂bdped = 1,2-bis-(3,5-dimethylpyrazol-1-yl)-ethane-1,2-diol; Hmba = 2-methylbenzoic acid; Hdmpz = 3,5-dimethyl-1H-pyrazole; Htca = 3-thiophenecarboxylic acid; and Hba = benzoic acid. The structures of SD/Ni6b, SD/Ni5a and SD/Ni4a are built from a central Ni₄O₄ opened cube, appending two to zero NiNO₅ octahedra. The solution behaviours of SD/Ni6b, SD/Ni5a and SD/Ni4a were studied in detail via an ESI-MS technique and their solution stabilities were confirmed. Magnetic analysis indicated the presence of Ising-type anisotropy: D = -13, -10, and -11 cm^-1 for SD/Ni6b, SD/Ni5a, and SD/Ni4a, respectively; moreover, dominantly ferromagnetic interactions were found between magnetic centers: J₁ = 6.5 cm^-1, J₂ = -0.44 cm^-1 and J₁ = 5.9 cm^-1, J₂ = 2.6 cm^-1 for SD/Ni5a and SD/Ni4a, respectively. Besides, the photocurrent signals were observed and they reached the maximum very quickly for these three nickel(ii) clusters and then their current intensities remained almost constant, which provide a possibility to be used for light-harvesting and photo-related catalysis.

Constructing an unprecedented {MnII38} matryoshka doll with a [Mn18(CO3)9] inorganic core and magnetocaloric effect

An unprecedented inner [Mn₁₈(CO₃)₉] inorganic core and [Mn₂₀] metal-organic periphery compose a high-nuclearity homometallic single-valent {Mn^II₃₈} molecular aggregate with a [Mn₆] ⊂ [Mn₁₂] ⊂ [Mn₈] ⊂ [Mn₁₂] matryoshka doll-like skeleton that displays a significant magnetocaloric effect (MCE).