Influence of Radicals on Magnetization Relaxation Dynamics of Pseudo-Octahedral Lanthanide Iminopyridyl Complexes

Controlling quantum tunneling of magnetization (QTM) is a persistent challenge in lanthanide-based single-molecule magnets. As the exchange interaction is one of the key factors in controlling the QTM, we targeted lanthanide complexes with an increased number of radicals around the lanthanide ion. On the basis of our targeted approach, a family of pseudo-octahedral lanthanide/transition-metal complexes were isolated with the general molecular formula of [M­(L^•–)₃] (M = Gd (<b>1</b>), Dy (<b>2</b>), Er (<b>3</b>), Y (<b>4</b>)) using the redox-active iminopyridyl (L^•–) ligand exclusively, which possess the highest ratio of radicals to lanthanide reported for discrete metal complexes. Direct current magnetic susceptibility studies suggest that dominant antiferromagnetic interactions exist between the radical and lanthanide ions in all of the complexes, which is strongly corroborated by magnetic data fitting using a Heisenberg–Dirac–Van Vleck (HDVV) Hamiltonian (−2<i>J</i> Hamiltonian). A good agreement between the fit and the experimental magnetic data obtained using <i>g</i> = 2, <i>J</i>_rad‑rad = −111.9 cm^–1 for <b>4</b> and <i>g</i> = 1.99, <i>J</i>_rad‑rad = −111.9 cm^–1, <i>J</i>_Gd‑rad = −1.85 cm^–1 for <b>1</b>. Complex <b>2</b> shows frequency-dependent slow magnetization relaxation dynamics in the absence of an external magnetic field, while <b>3</b> shows field-induced frequency-dependent χ_M′′ signals. An ideal octahedral geometry around the lanthanide ion is predicted to be unsuitable for the design of a single-molecule magnet (SMM); nevertheless, complex <b>2</b> exhibits slow relaxation of magnetization with a record high anisotropy barrier for a six-coordinate Dy­(III) complex. A rationale for this unusual behavior is detailed and reveals the strength of the synthetic methodology developed

Mechanistic Investigation of Well-Defined Cobalt Catalyzed Formal <i>E</i>‑Selective Hydrophosphination of Alkynes

A formal <i>E</i>-selective hydrophosphination of terminal and internal alkynes catalyzed by a well-defined [Co­(PMe₃)₄] (<b>A</b>) complex is achieved under mild conditions in good-to-excellent yield. The reaction does not require any additives and/or external base for an efficient hydrophosphination reaction. The reaction provided excellent scope and good functional tolerance. Detailed spectroscopic analysis (NMR, EPR, and UV–vis) revealed that the low valent cobalt(0) complex undergoes oxidative addition with diphenylphosphine, followed by hydrometalation with alkyne, and subsequent reductive elimination led to the expected product. The detailed spectroscopic analyses along with the isotopic labeled experiments facilitate to intercept the active intermediates that are involved in the catalytic cycle, which are detailed. It was revealed that the suprafacial (<i>vide infra</i>) delivery of H and phosphorus to π-alkynes in a <i>syn</i>-fashion led to formal <i>E</i>-vinyl phosphine

Exploiting the Synergism of a Carbon–Catalyst Interface to Achieve Magneto-Electrocatalytic Overall Water Splitting at 2.197 V

The desire to electrolyze water at low energy and high kinetics for achieving rapid H2 production forms the holy grail for the paradigm shift to a sustainable H2-driven economy. While alkaline electrolysis is preferred due to the use of earth-abundant catalysts, its sluggish kinetics and high overpotential are the persistent challenges. Addressing this, we demonstrate the coupling of an externally applied magnetic field (Hext) to a synergistically designed interface of nanostructured carbon floret with antiferromagnetic NiO nanoflakes that act in unison to achieve rapid hydrogen generation (6.3 N m3 h–1 W–1) that is comparable with existing technologies. Specifically, the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) overpotentials are simultaneously reduced by 10 and 7%, respectively, under the influence of a weak fridge magnet (Hext = 200 mT). Consequently, ∼11% improvement in the energy efficiency is observed with a 21% reduced cell voltage for overall water splitting. The stability of the system is demonstrated over a prolonged lifetime of ∼95 h. This performance enhancement with Hext for both HER and OER is explained in terms of improved kinetic facility for the reaction and lower resistance of charge transfer pathway. Moreover, the electrocatalyst is seen to retain the improved performance for prolonged usage (∼3 h) even after the removal of the Hext, and hence, it provides an energy-efficient hydrogen and oxygen generation pathway

Electrocatalytic Hydrogen Evolution from Water by a Series of Iron Carbonyl Clusters

The development of efficient hydrogen evolving electrocatalysts that operate near neutral pH in aqueous solution remains of significant interest. A series of low-valent iron clusters have been investigated to provide insight into the structure–function relationships affecting their ability to promote formation of cluster-hydride intermediates and to promote electrocatalytic hydrogen evolution from water. Each of the metal carbonyl anions, [Fe₄N­(CO)₁₂]⁻ (<b>1</b>^–), [Fe₄C­(CO)₁₂]^2– (<b>2</b>^2–), [Fe₅C­(CO)₁₅]^2– (<b>3</b>^2–), and [Fe₆C­(CO)₁₈]^2– (<b>4</b>^2–) were isolated as their sodium salt to provide the necessary solubility in water. At pH 5 and −1.25 V vs SCE the clusters afford hydrogen with Faradaic efficiencies ranging from 53–98%. pH dependent cyclic voltammetry measurements provide insight into catalytic intermediates. Both of the butterfly shaped clusters, <b>1</b>^– and <b>2</b>^2–, stabilize protonated adducts and are effective catalysts. Initial reduction of butterfly shaped <b>1</b>^– is pH-independent and subsequently, successive protonation events afford H<b>1</b>^–, and then hydrogen. In contrast, butterfly shaped <b>2</b>^2– undergoes two successive proton coupled electron transfer events to form H₂<b>2</b>^2– which then liberates hydrogen. The higher nuclearity clusters, <b>3</b>^2– and <b>4</b>^2–, do not display the same ability to associate with protons, and accordingly, they produce hydrogen less efficiently

Influence of the Ligand Field on the Slow Relaxation of Magnetization of Unsymmetrical Monomeric Lanthanide Complexes: Synthesis and Theoretical Studies

A series of monomeric lanthanide Schiff base complexes with the molecular formulas [Ce­(HL)₃(NO₃)₃] (<b>1</b>) and [Ln­(HL)₂(NO₃)₃], where Ln^III = Tb (<b>2</b>), Ho (<b>3</b>), Er (<b>4</b>), and Lu (<b>5</b>), were isolated and characterized by single-crystal X-ray diffraction (XRD). Single-crystal XRD reveals that, except for <b>1</b>, all complexes possess two crystallographically distinct molecules within the unit cell. Both of these crystallographically distinct molecules possess the same molecular formula, but the orientation of the coordinating ligand distinctly differs from those in complexes <b>2</b>–<b>5</b>. Alternating-current magnetic susceptibility measurement reveals that complexes <b>1</b>–<b>3</b> exhibit slow relaxation of magnetization in the presence of an optimum external magnetic field. In contrast to <b>1</b>–<b>3</b>, complex <b>4</b> shows a blockade of magnetization in the absence of an external magnetic field, a signature characteristic of a single-ion magnet (SIM). The distinct magnetic behavior observed in <b>4</b> compared to other complexes is correlated to the suitable ligand field around a prolate Er^III ion. Although the ligand field stabilizes an easy axis of anisotropy, quantum tunnelling of magnetization (QTM) is still predominant in <b>4</b> because of the low symmetry of the complex. The combination of low symmetry and an unsuitable ligand-field environment in complexes <b>1</b>–<b>3</b> triggers faster magnetization relaxation; hence, these complexes exhibit field-induced SIM behavior. In order to understand the electronic structures of complexes <b>1</b>–<b>4</b> and the distinct magnetic behavior observed, ab initio calculations were performed. Using the crystal structure of the complexes, magnetic susceptibility data were computed for all of the complexes. The computed susceptibility and magnetization are in good agreement with the experimental magnetic data [χ_M<i>T</i>(<i>T</i>) and <i>M</i>(<i>H</i>)] and this offers confidence on the reliability of the extracted parameters. A tentative mechanism of magnetization relaxation observed in these complexes is also discussed in detail

Substituted versus Naked Thiourea Ligand Containing Pseudotetrahedral Cobalt(II) Complexes: A Comparative Study on Its Magnetization Relaxation Dynamics Phenomenon

A series of mononuclear tetrahedral cobalt­(II) complexes with the general molecular formula [Co­(L₁)₂X₂] [where L₁ = tetramethylthiourea ([(CH₃)₂N]₂CS) and X = Cl (<b>1</b>), Br (<b>2</b>), and I (<b>3</b>)] were isolated, and their structures were characterized by single-crystal X-ray diffraction. The experimental direct-current magnetic data are excellently reproduced by fitting both χ_M<i>T</i>(<i>T</i>) and <i>M</i>(<i>H</i>) simultaneously using the spin Hamiltonian (SH) parameters <i>D</i>_<b>1</b> = −18.1 cm^–1 and <i>g</i>_<b>1</b>,iso = 2.26, <i>D</i>_<b>2</b> = −16.4 cm^–1 and <i>g</i>_<b>2</b>,iso = 2.33, and <i>D</i>_<b>3</b> = −22 cm^–1 and <i>g</i>_<b>3</b>,iso = 2.4 for <b>1</b>–<b>3</b>, respectively, and the sign of <i>D</i> was unambiguously confirmed from X-band electron paramagnetic resonance measurements. The effective energy barrier extracted for the magnetically diluted complexes <b>1</b>–<b>3</b> (10%) is larger than the barrier observed for the pure samples and implies a nonzero contribution of dipolar interaction to the magnetization relaxation dynamics. The SH parameters extracted for the three complexes drastically differ from their respective parent complexes that possess the general molecular formula [Co­(L)₂X₂] [where L = thiourea [(NH₂)₂CS] and X = Cl (<b>1a</b>), Br (<b>2a</b>), and I (<b>3a</b>)], which is rationalized by detailed ab initio calculations. An exhaustive theoretical study reveals that both the ground and excited states are not pure but rather multideterminental in nature (<b>1</b>–<b>3</b>). Noticeably, the substitution of L by L₁ induces structural distortion in <b>1</b>–<b>3</b> on the level of the secondary coordination sphere compared to <b>1a</b>–<b>3a</b>. This distortion leads to an overall reduction in |<i>E</i>/<i>D</i>| of <b>1</b>–<b>3</b> compared to <b>1a</b>–<b>3a</b>. This may be one of the reasons for the origin of the slower relaxation times of <b>1</b>–<b>3</b> compared to <b>1a</b>–<b>3a</b>

Lanthanide-Based Porous Coordination Polymers: Syntheses, Slow Relaxation of Magnetization, and Magnetocaloric Effect

Two lanthanide-containing structurally analogous porous coordination polymers (PCPs) have been isolated with the general molecular formula [Ln₂(L₁)₂(H₂O)₄(ox)]_<i>n</i>.4<i>n</i>H₂O (where L₁ = fumarate, ox = oxalate; Ln = Dy (<b>1</b>), Gd (<b>2</b>)). Thermogravimetric analysis (TGA) and TG-MS measurements performed on <b>1</b> and <b>2</b> suggest that not only the solvated water molecules in the crystal lattice but also the four coordinated water molecules on the respective lanthanides in <b>1</b> and <b>2</b> are removed upon activation. Due to the removal of the waters, <b>1</b> and <b>2</b> lost their crystallinity and became amorphous, as confirmed by powder X-ray diffraction (PXRD). We propose the molecular formula [Ln₂(L₁)₂(ox)]_<i>n</i> for the amorphous phase of <b>1</b> and <b>2</b> (where Ln = Dy (<b>1′</b>), Gd (<b>2′</b>)) on the basis of XANES, EXAFS, and other experimental investigations. Magnetization relaxation dynamics probed on <b>1</b> and <b>1′</b> reveal two different relaxation processes with effective energy barriers of 53.5 and 7.0 cm^–1 for <b>1</b> and 45.1 and 6.4 cm^–1 for <b>1′</b>, which have been rationalized by detailed ab initio calculations. For the isotropic lanthanide complexes <b>2</b> and <b>2′</b>, magnetocaloric effect (MCE) efficiency was estimated through detailed magnetization measurements. We have estimated −Δ<i>S</i>_<i>m</i> values of 52.48 and 41.62 J kg^1– K^–1 for <b>2′</b> and <b>2</b>, respectively, which are one of the largest values reported for an extended structure. In addition, a 26% increase in −Δ<i>S</i>_m value in <b>2′</b> in comparison to <b>2</b> is achieved by simply removing the passively contributing (for MCE) solvated water molecule in the lattice and coordinated water molecules