Design Principles and Theory of Paramagnetic Fluorine‐Labelled Lanthanide Complexes as Probes for 19F Magnetic Resonance: A Proof‐of‐Concept Study

The synthesis and spectroscopic properties of a series of CF₃‐labelled lanthanide(III) complexes (Ln=Gd, Tb, Dy, Ho, Er, Tm) with amide‐substituted ligands based on 1,4,7,10‐tetraazacyclododecane are described. The theoretical contributions of the ¹⁹F magnetic relaxation processes in these systems are critically assessed and selected volumetric plots are presented. These plots allow an accurate estimation of the increase in the rates of longitudinal and transverse relaxation as a function of the distance between the Ln^III ion and the fluorine nucleus, the applied magnetic field, and the re‐rotational correlation time of the complex, for a given Ln^III ion. Selected complexes exhibit pH‐dependent chemical shift behaviour, and a pK_a of 7.0 was determined in one example based on the holmium complex of an ortho‐cyano DO3A‐monoamide ligand, which allowed the pH to be assessed by measuring the difference in chemical shift (varying by over 14 ppm) between two ¹⁹F resonances. Relaxation analyses of variable‐temperature and variable‐field ¹⁹F, ¹⁷O and ¹H NMR spectroscopy experiments are reported, aided by identification of salient low‐energy conformers by using density functional theory. The study of fluorine relaxation rates, over a field range of 4.7 to 16.5 T allowed precise computation of the distance between the Ln^III ion and the CF₃ reporter group by using global fitting methods. The sensitivity benefits of using such paramagnetic fluorinated probes in ¹⁹F NMR spectroscopic studies are quantified in preliminary spectroscopic and imaging experiments with respect to a diamagnetic yttrium(III) analogue.     

Fluoride Binding and Crystal‐Field Analysis of Lanthanide Complexes of Tetrapicolyl‐Appended Cyclen

Lanthanide complexes of tetrapicolyl cyclen displayed remarkably high affinities for fluoride (log K≈5) in water, and were shown to form 1:1 complexes. The behaviour of these systems can be rationalised by changes to the magnitude of the crystal‐field parameter, . However, such changes are not invariably accompanied by a change in sign of this parameter: for early lanthanides, the N8 donor set with a coordinated axial water molecule ensures that the magnetic anisotropy has the opposite sense to that observed in the analogous dehydrated lanthanide complexes.     

High Magnetic Moments in Manganese‐Doped Silicon Clusters

We report on the structural, electronic, and magnetic properties of manganese‐doped silicon clusters cations, Si_nMn⁺ with n=6–10, 12–14, and 16, using mass spectrometry and infrared spectroscopy in combination with density functional theory computations. This combined experimental and theoretical study allows several structures to be identified. All the exohedral Si_nMn⁺ (n=6–10) clusters are found to be substitutive derivatives of the bare Si_n+1⁺ cations, while the endohedral Si_nMn⁺ (n=12–14 and 16) clusters adopt fullerene‐like structures. The hybrid B3P86 functional is shown to be appropriate in predicting the ground electronic states of the clusters and in reproducing their infrared spectra. The clusters turn out to have high magnetic moments localized on Mn. In particular the Mn atoms in the exohedral Si_nMn⁺ (n=6–10) clusters have local magnetic moments of 4 μ_B or 6 μ_B and can be considered as magnetic copies of the silicon atoms. Opposed to other 3d transition‐metal dopants, the local magnetic moment of the Mn atom is not completely quenched when encapsulated in a silicon cage.     

Synthesis,Isolation, and Spectroscopic Characterization of Holmium‐Based Mixed‐Metal Nitride Clusterfullerenes: HoxSc3−xN@C80 (x=1, 2)

The synthesis, isolation and spectroscopic characterization of holmium‐based mixed metal nitride clusterfullerenes Ho_xSc_3?xN@C₈₀ (x=1, 2) are reported. Two isomers of Ho_xSc_3?xN@C₈₀ (x=1, 2) were synthesized by the reactive gas atmosphere method and isolated by multistep recycling HPLC. The isomeric structures of Ho_xSc_3?xN@C₈₀ (x=1, 2) were characterized by laser‐desorption time‐of‐flight (LD‐TOF) mass spectrometry and UV/Vis/NIR, FTIR and Raman spectroscopy. A comparative study of M_xSc_3?xN@C₈₀ (M=Gd, Dy, Lu, Ho) demonstrates the dependence of their electronic and vibrational properties on the encaged metal. Despite the distinct perturbation induced by 4f¹⁰ electrons, we report the first paramagnetic ¹³C NMR study on Ho_xSc_3?xN@C₈₀ (I; x=1, 2) and confirm I_h‐symmetric cage structure. A ⁴⁵Sc NMR study on HoSc₂N@C₈₀ (I, II) revealed a temperature‐dependent chemical shift in the temperature range of 268–308 K.     

Magnetic Bistability of Isolated Giant‐Spin Centers in a Diamagnetic Crystalline Matrix

Polynuclear single‐molecule magnets (SMMs) were diluted in a diamagnetic crystal lattice to afford arrays of independent and iso‐oriented magnetic units. Crystalline solid solutions of an Fe₄ SMM and its Ga₄ analogue were prepared with no metal scrambling for Fe₄ molar fractions x down to 0.01. According to high‐frequency EPR and magnetic measurements, the guest SMM species have the same total spin (S=5), anisotropy, and high‐temperature spin dynamics found in the pure Fe₄ phase. However, suppression of intermolecular magnetic interactions affects magnetic relaxation at low temperature (40 mK), where quantum tunneling (QT) of the magnetization dominates. When a magnetic field is applied along the easy magnetic axis, both pure and diluted (x=0.01) phases display pronounced steps at evenly spaced field values in their hysteresis loops due to resonant QT. The pure Fe₄ phase exhibits additional steps which are firmly ascribed to two‐molecule QT transitions. Studies on the field‐dependent relaxation rate showed that the zero‐field resonance sharpens by a factor of five and shifts from about 8 mT to exactly zero field on dilution, in agreement with the calculated variation of dipolar interactions. The tunneling efficiency also changes significantly as a function of Fe₄ concentration: the zero‐field resonance is significantly enhanced on dilution, while tunneling at ±0.45 T becomes less efficient. These changes were rationalized on the basis of a dipolar shuffling mechanism and transverse dipolar fields, whose effect was analyzed by using a multispin model. Our findings directly prove the impact of intermolecular magnetic couplings on SMM behavior and disclose the magnetic response of truly isolated giant spins in a diamagnetic crystalline environment.     

A Linear 3d–4f Tetranuclear CoIII2DyIII2 Single‐Molecule Magnet: Synthesis,Structure, and Magnetic Properties

A linear tetranuclear 3d–4f Co₂Dy₂ cluster assembled from a polydentate Schiff base exhibits single‐molecule magnet (SMM) behavior with an anisotropic barrier of 33.8 K. Due to the presence of diamagnetic cobalt(III) ions, the tetranuclear cluster of 1 behaves magnetically like a dinuclear Dy₂ system. However, the diamagnetic segment might efficiently minimize undesirable intermolecular magnetic interactions, thereby improving the performance of the SMM behavior of 1 . This discrete complex presents us with a unique opportunity to study the magnetic properties and to probe the dynamics of magnetization in a magnetically isolated Dy₂ system.     

Partial‐Homogeneity‐Based Two‐Dimensional High‐Resolution Nuclear Magnetic Resonance Spectroscopy under Inhomogeneous Magnetic Fields

High‐resolution multidimensional nuclear magnetic resonance (NMR) spectroscopy serves as an irreplaceable and versatile tool in various chemical investigations. In this study, a method based on the concept of partial homogeneity is developed to offer two‐dimensional (2D) high‐resolution NMR spectra under inhomogeneous fields. Oscillating gradients are exerted to encode the high‐resolution information, and a field‐inhomogeneity correction algorithm based on pattern recognition is designed to recover high‐resolution spectra. Under fields where inhomogeneity primarily distributes along a single orientation, the proposed method will improve performances of 2D NMR spectroscopy without increasing the experimental duration or significant loss in sensitivity, and thus may open important perspectives for studies of inhomogeneous chemical systems.     

Peptoid‐Ligated Pentadecanuclear Yttrium and Dysprosium Hydroxy Clusters

A new family of pentadecanuclear coordination cluster compounds (from now on simply referred to as clusters) [{Ln₁₅(OH)₂₀(PepCO₂)₁₀(DBM)₁₀Cl}Cl₄] (PepCO₂=2‐[{3‐(((tert‐butoxycarbonyl)amino)methyl)benzyl}amino]acetate, DBM=dibenzoylmethanide) with Ln=Y and Dy was obtained by using the cell‐penetrating peptoid (CPPo) monomer PepCO₂H and dibenzoylmethane (DBMH) as supporting ligands. The combination of an inorganic cluster core with an organic cell‐penetrating peptoid in the coordination sphere resulted in a core component {Ln₁₅(μ₃‐OH)₂₀Cl}²⁴⁺ (Ln=Y, Dy), which consists of five vertex‐sharing heterocubane {Ln₄(μ₃‐OH)₄}⁸⁺ units that assemble to give a pentagonal cyclic structure with one Cl atom located in the middle of the pentagon. The solid‐state structures of both clusters were established by single‐crystal X‐ray crystallography. MS (ESI) experiments suggest that the cluster core is robust and maintained in solution. Pulsed gradient spin echo (PGSE) NMR diffusion measurements were carried out on the diamagnetic yttrium compound and confirmed the stability of the cluster in its dicationic form [{Y₁₅(μ₃‐OH)₂₀(PepCO₂)₁₀(DBM)₁₀Cl}Cl₂]²⁺. The investigation of both static (dc) and dynamic (ac) magnetic properties in the dysprosium cluster revealed a slow relaxation of magnetization, indicative of single‐molecule magnet (SMM) behavior below 8 K. Furthermore, the χT product as a function of temperature for the dysprosium cluster gave evidence that this is a ferromagnetically coupled compound below 11 K.     

Single‐Molecule‐Magnet Behavior in the Family of [Ln(OETAP)2] Double‐Decker Complexes (Ln=Lanthanide,OETAP=Octa(ethyl)tetraazaporphyrin)

Double‐decker complexes of lanthanide cations can be readily prepared with tetraazaporphyrins (porphyrazines). We have synthesized and characterized a series of neutral double‐decker complexes [Ln(OETAP)₂] (Ln=Tb³⁺, Dy³⁺, Gd³⁺, Y³⁺; OETAP=octa(ethyl)tetraazaporphyrin). Some of these complexes show analogous magnetic features to their phthalocyanine (P_c) counterparts. The Tb³⁺ and Dy³⁺ derivatives exhibit single‐molecule magnet (SMM) behavior with high blocking temperatures over 50 and 10 K, respectively. These results confirm that, in double‐decker complexes that involve Tb or Dy, the (N₄)₂ square antiprism coordination mode has an important role in inducing very large activation energies for magnetization reversal. In contrast with their P_c counterparts, the use of tetraazaporphyrin ligands endows the presented [Ln(OETAP)₂] complexes with extraordinary chemical versatility. The double‐decker complexes that exhibit SMM behavior are highly soluble in common organic solvents, and easily processable even through sublimation.     

A Spin‐Crossover Cluster of Iron(II) Exhibiting a Mixed‐Spin Structure and Synergy between Spin Transition and Magnetic Interaction

How low can you go? An Fe^II₄ square was prepared by self‐assembly and exhibits both thermally induced and photoinduced spin crossover from a system with four high‐spin (HS) centers to one with two high‐spin and two low‐spin (LS) centers. The spin‐crossover sites are located on the same side of the square, and the spin transition and magnetic interactions (see picture) are synergistically coupled.

     

Hydroxo‐Bridged Dimers of Oxo‐Centered Ruthenium(III) Triangle: Synthesis and Spectroscopic and Theoretical Investigations

The homometallic hexameric ruthenium cluster of the formula [Ru^III₆(μ₃‐O)₂(μ‐OH)₂((CH₃)₃CCO₂)₁₂(py)₂] ( 1 ) (py=pyridine) is solved by single‐crystal X‐ray diffraction. Magnetic susceptibility measurements performed on 1 suggest that the antiferromagnetic interaction between the Ru^III centers is dominant, and this is supported by theoretical studies. Theoretical calculations based on density functional methods yield eight different exchange interaction values for 1 : J₁=?737.6, J₂=+63.4, J₃=?187.6, J₄=+124.4, J₅=?376.4, J₆=?601.2, J₇=?657.0, and J₈=?800.6 cm^?1. Among all the computed J values, six are found to be antiferromagnetic. Four exchange values (J₁, J₆, J₇ and J₈) are computed to be extremely strong, with J₈, mediated through one μ‐hydroxo and a carboxylate bridge, being by far the largest exchange obtained for any transition‐metal cluster. The origin of these strong interactions is the orientation of the magnetic orbitals in the Ru^III centers, and the computed J values are rationalized by using molecular orbital and natural bond order analysis. Detailed NMR studies (¹H, ¹³C, HSQC, NOESY, and TOCSY) of 1 (in CDCl₃) confirm the existence of the solid‐state structure in solution. The observation of sharp NMR peaks and spin‐lattice time relaxation (T1 relaxation) experiments support the existence of strong intramolecular antiferromagnetic exchange interactions between the metal centers. A broad absorption peak around 600–1000 nm in the visible to near‐IR region is a characteristic signature of an intracluster charge‐transfer transition. Cyclic voltammetry experiments show that there are three reversible one‐electron redox couples at ?0.865, +0.186, and +1.159 V with respect to the Ag/AgCl reference electrode, which corresponds to two metal‐based one‐electron oxidations and one reduction process.     

On‐Line Monitoring of Chemical Reactions by using Bench‐Top Nuclear Magnetic Resonance Spectroscopy

Real‐time nuclear magnetic resonance (NMR) spectroscopy measurements carried out with a bench‐top system installed next to the reactor inside the fume hood of the chemistry laboratory are presented. To test the system for on‐line monitoring, a transfer hydrogenation reaction was studied by continuously pumping the reaction mixture from the reactor to the magnet and back in a closed loop. In addition to improving the time resolution provided by standard sampling methods, the use of such a flow setup eliminates the need for sample preparation. Owing to the progress in terms of field homogeneity and sensitivity now available with compact NMR spectrometers, small molecules dissolved at concentrations on the order of 1 mmol L^?1 can be characterized in single‐scan measurements with 1 Hz resolution. Owing to the reduced field strength of compact low‐field systems compared to that of conventional high‐field magnets, the overlap in the spectrum of different NMR signals is a typical situation. The data processing required to obtain concentrations in the presence of signal overlap are discussed in detail, methods such as plain integration and line‐fitting approaches are compared, and the accuracy of each method is determined. The kinetic rates measured for different catalytic concentrations show good agreement with those obtained with gas chromatography as a reference analytical method. Finally, as the measurements are performed under continuous flow conditions, the experimental setup and the flow parameters are optimized to maximize time resolution and signal‐to‐noise ratio.     

核磁共振技术(NMR)在组合化学中的应用

对科学产生最大影响的分析方法是核磁共振技术(NMR),它被广泛用于许多领域.本文结合作者的研究结果评述了NMR在组合化学中的应用,着重于NMR在固相合成的应用、液态NMR和NMR在高通量筛选中的应用.