A Cationic Metal Glue Strategy for Expanding Paramagnetic Hetero-Multinuclear Metal-Oxo Clusters within Polyoxometalate Ligands

Precise structural design of large hetero-multinuclear metal-oxo clusters is crucial for controlling their large spin ground states and multielectron redox properties for application as a single-molecule magnet (SMM), molecular magnetic refrigeration, and efficient redox catalyst. However, it is difficult to synthesize large hetero-multinuclear metal oxo clusters as designed because the final structures are unpredictable when employing conventional one-step condensation reaction of metal cations and ligands. Herein, we report a “cationic metal glue strategy” for increasing the size and nuclearity of hetero-multinuclear metal-oxo clusters by using lacunary-type anionic molecular metal oxides (polyoxometalates, POMs) as rigid multidentate ligands. The employed method enabled the synthesis of {(FeMn₄)Mn₂Ln₂(FeMn₄)} oxo clusters (Ln=Gd, Tb, Dy, and Lu), which are the largest among previously reported paramagnetic hetero-multinuclear metal-oxo clusters in POMs and showed unique SMM properties. These clusters were synthesized by conjugating {FeMn₄} oxo clusters with Mn and Ln cations as glues in a predictable way, indicating that the “cationic metal glue strategy” would be a powerful tool to construct desired large hetero-multinuclear metal clusters precisely and effectively.     

Theoretical study of the geometric and electronic structure of neutral and anionic doped silver clusters, Ag5X with X = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni

Density functional theory (DFT) has been applied to investigate the low-lying electronic states of neutral and anionic transition metal doped silver clusters Ag₅X^0,− with X = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni using the B3LYP functional with the Stuttgart SDD basis sets. The structural features, frontier orbital energy gaps (HOMO and LUMO), vertical detachment energies, and vertical and adiabatic electronic affinities are evaluated. For all doped silver clusters, both in neutral and anionic states, two-dimensional and three-dimensional low-energy isomers are found to coexist. For neutral clusters, dopant Sc, Ti, V, and Mn atoms largely decrease the frontier orbital energy gaps, while they are markedly increased by Sc and Fe atoms in the anionic clusters. A completely quenched dopant magnetic moment is found in Ag₅Sc, while high spin magnetic moments are located on the other dopant atoms in Ag₅X^0,−.     

Small binary iron-carbon clusters with persistent high magnetic moments. A theoretical characterization

Characterization of the structural and electronic properties of binary iron-carbon clusters composed by six iron atoms and with up to nine carbon atoms was carried out with density functional theory calculations. Neutral, cations (q = +1), and anions (q = −1), some of them experimentally detected, were studied. The formation of dimers and trimers of carbon atoms over the iron surface were preferred. Moreover, some large carbon chains, with up to five atoms, were determined. High spin states emerged for the ground states, with multiplicities above 16, for all clusters independently of the number of carbon atoms attached to the iron core. All neutral clusters were stable because fragmentation (into carbon chains), dissociation (of a single carbon atom), and detachment of all carbons need high amounts of energy. Reactive species were defined by small HOMO-LUMO gaps. Charge transfer, to the carbon atoms, increased as the carbon content increased, producing, for some cases, an even-odd behavior for the magnetic moment of the Fe₆C_n particles.     

Preparation and Reaction of Naked Metal Clusters for Catalysis and Genetic Materials

Metal clusters that contain a small number of atoms usually present unique properties with dramatic dependence on their sizes, geometric structures, and compositions. The studies of naked metal clusters are devoted to develop new catalysts and functional materials of atomic precision, and enable to improve the fundamental theory of structure chemistry and to understand the basic reactions and properties bridging the gap between atoms and bulk materials. In particular, some interesting superatom clusters have received reasonable research interest indicative of materials gene of clusters. Here in this review, we simply summarize the preparation, stability, and reactivity of naked metal clusters with a few examples displayed. Hopefully it serves as a modest spur to stimulate more interest of related investigations in this field.     

Dissociative and associative attachment of NO to iron clusters

Electronic and geometrical structures of iron clusters with associative (FeNO, Fe2NO, Fe3NO, Fe4NO, Fe5NO, and Fe6NO) and dissociative (OFeN, OFe2N, OFe3N, OFe4N, OFe5N, and OFe6N) attachments of NO, as well as the corresponding singly negatively and positively charged ions, are computed using density functional theory with generalized gradient corrections. Both types of isomers are found to be stable and no spontaneous dissociation was observed during the geometry optimizations. The ground states correspond to dissociative attachment of NO for all iron clusters Fe(n), except for Fe and Fe+. All of the OFe(n)N clusters have ferrimagnetic ground states, except for OFe2N, OFe2N-, OFe4N, and OFe4N-, which prefer the ferromagnetic coupling. In the ferrimagnetic states, the excess spin density at one iron atom couples antiferromagnetically to the excess spin densities of all other iron atoms. Relative to the high-spin Fe(n) ground state, the lowest energy ferrimagnetic state quenches the total magnetic moments of iron clusters by 7, which is to be compared with a reduction in the magnetic moment of one in the lowest energy ferromagnetic states. Dissociation of NO on the iron clusters has a pronounced impact on the energetics of reactions; the Fe(n)NO+CO-->Fe(n)N+CO2 channels are exothermic while the OFe6N+CO--> Fe6N+CO2 channels are nearly thermoneutral.     

Reductive Activation of Small Molecules by Anionic Coinage Metal Atoms and Clusters in the Gas Phase

Metal atoms and clusters exhibit chemical properties that are significantly different or totally absent in comparison to their bulk counterparts. Such peculiarity makes them potential building units for the generation of novel catalysts. Investigations of the gas‐phase reactions between size‐ and charge‐selected atoms/clusters and small molecules have provided fundamental insights into their intrinsic reactivity, thus leading to a guiding principle for the rational design of the single‐atom and cluster‐based catalysts. Especially, recent gas‐phase studies have elucidated that small molecules such as O₂, CO₂, and CH₃I can be catalytically activated by negatively‐charged atoms/clusters via donation of a partial electronic charge. This Minireview showcases typical examples of such “reductive activation” processes promoted by anions of metal atoms and clusters. Here, we focus on anionic atoms/clusters of coinage metals (Cu, Ag, and Au) owing to the simplicity of their electronic structures. The determination of a correlation between their activation modes and the electronic structures might be helpful for the future development of innovative coinage metal catalysts.     

Strong spin-orbit effects in small Pt clusters: geometric structure, magnetic isomers and anisotropy

Ab initio density functional calculations including spin-orbit coupling (SOC) have been performed for Pt(n), n = 2-6 clusters. The strong SOC tends to stabilize planar structures for n = 2-5, whereas for clusters consisting of six atoms, three-dimensional structures remain preferred. SOC leads to the formation of large orbital magnetic moments and to a mixing of different spin states. Due to the spin-mixing the total magnetic moment may be larger or smaller than the spin moment in the absence of SOC. Both spin and orbital moments are found to be anisotropic. Because of the strong SOC the energy differences between coexisting magnetic isomers can be comparable to or even smaller than their magnetic anisotropy energies. In this case the lowest barrier for magnetization reversal can be determined by a magnetic isomer which is different from the ground state configuration.     

簇合物Mo_2X_8~(4-)(X=Cl,Br)低能态电子光谱

应用间略微分重叠自洽场和组态作用(INDO/S-CI)方法,计算双核簇合物Mo_2X_8~(4-)(X=Cl,Br)基态~1A_(1g)至不同低能态的d电子自旋允许的跃迁能,得出Mo—Mo四重键均裂时所需的能量大小依次为键σ>π>>δ。对于Mo_2Cl_8~(4-)簇合物态~1A_(1g)→1~1A_(2u)(δ→δ~*)和~1A_(1g)→2~1E_g(π→δ~*)的电子跃迁能,理论值(15584和22587cm~(-1))与低温时测得光谱实验值(18083和22000cm~(-1))较为接近。此外,本文还提出了簇合物Mo_2X_8~(4-)的态相同而X不同时,电子跃迁能变化,主要决定于参与电子转移过程的分子轨道组成成份的看法。     

The local electronic and magnetic properties of Fe impurity in Al clusters

The local magnetic property,d electronic structure and the charge transfer effect of Fe impurity in Al clusters have been studied by using a tight-binding model Hamiltonian in the unrestricted Hartree-Fock approximation, which includes intra-atomic and interatomic Coulomb interactions. We have obtained that local magnetic moment of Fe impurity in FeAl _N clusters decreases with increasing cluster size and convergences to zero (that of bulk given by Anderson) withN larger than 12, meanwhile, the local magnetic moment for smaller clusters depends on the clusters size and it is a monotonous descent function of cluster size. We have also found that the spin splitting of the localizedd states decreases as the cluster size increases, which mainly results from the interaction between the localized electrons of Fe atom and the delocalized electrons of Al atoms.     

Magnetic Anisotropy in “Scorpionate” First‐Row Transition‐Metal Complexes: A Theoretical Investigation

In this work we have analyzed in detail the magnetic anisotropy in a series of hydrotris(pyrazolyl)borate (Tp^?) metal complexes, namely [VTpCl]⁺, [CrTpCl]⁺, [MnTpCl]⁺, [FeTpCl], [CoTpCl], and [NiTpCl], and their substituted methyl and tert‐butyl analogues with the goal of observing the effect of the ligand field on the magnetic properties. In the [VTpCl]⁺, [CrTpCl]⁺, [CoTpCl], and [NiTpCl] complexes, the magnetic anisotropy arises as a consequence of out‐of‐state spin–orbit coupling, and covalent changes induced by the substitution of hydrogen atoms on the pyrazolyl rings does not lead to drastic changes in the magnetic anisotropy. On the other hand, much larger magnetic anisotropies were predicted in complexes displaying a degenerate ground state, namely [MnTpCl]⁺ and [FeTpCl], due to in‐state spin–orbit coupling. The anisotropy in these systems was shown to be very sensitive to perturbations, for example, chemical substitution and distortions due to the Jahn–Teller effect. We found that by substituting the hydrogen atoms in [MnTpCl]⁺ and [FeTpCl] by methyl and tert‐butyl groups, certain covalent contributions to the magnetic anisotropy energy (MAE) could be controlled, thereby achieving higher values. Moreover, we showed that the selection of ion has important consequences for the symmetry of the ground spin–orbit term, opening the possibility of achieving zero magnetic tunneling even in non‐Kramers ions. We have also shown that substitution may also contribute to a quenching of the Jahn–Teller effect, which could significantly reduce the magnetic anisotropy of the complexes studied.     

Density functional theory study of small vanadium oxide clusters

Density functional theory is employed to study structure and stability of small neutral vanadium oxide clusters in the gas phase. BPW91/LANL2DZ level of theory is used to obtain structures of VOy (y=1-5), V2Oy (y=2-7), V3Oy (y=4-9), and V4Oy (y=7-12) clusters. Enthalpies of growth and fragmentation reactions of the lowest energy isomers of vanadium oxide molecules are also obtained to study the stability of neutral vanadium oxide species under oxygen saturated gas-phase conditions. Our results suggest that cyclic and cage-like structures are preferred for the lowest energy isomers of neutral vanadium oxide clusters, and oxygen-oxygen bonds are present for oxygen-rich clusters. Clusters with an odd number of vanadium atoms tend to have low spin ground states, while clusters with even number of vanadium atoms have a variety of spin multiplicities for their ground electronic state. VO2, V2O5, V3O7, and V4O10 are predicted to be the most stable neutral clusters under the oxygen saturated conditions. These results are in agreement with and complement previous gas-phase experimental studies of neutral vanadium oxide clusters.     

Acetylene cyclotrimerization by early second-row transition metals in the gas phase. A theoretical study

The acetylene cyclotrimerization reaction mediated by the left-hand-side bare transition metal atoms Y, Zr, Nb, and Mo has been studied theoretically, employing DFT in its B3LYP formulation. The complete reaction mechanism has been analyzed, identifying intermediates and transition states. Both the ground spin state and at least one low-lying excited state have been considered to establish whether possible spin crossings between surfaces of different multiplicity can occur. Our results show that the overall reaction is highly favorable from a thermodynamic point of view and ground state transition states lie always below the energy limit represented by ground state reactants. After the activation of two acetylene molecules and formation of a bis-ligated complex, the reaction proceeds to give a metallacycle intermediate, as the alternative formation of a cyclobutadiene complex is energetically disfavored. All the examined reaction paths involve formation of a metallacycloheptatriene intermediate that in turn generates a metal-benzene adduct from which finally benzene is released. Similarities and differences in the behaviors of the considered four metal atoms have been examined.     

Ground state, growth, and electronic properties of small lanthanum clusters

The DMol cluster method based on density-functional theory has been employed to study the structural stability and electronic structure of La(n) (n=2-14) clusters. The ground states have been found out for lanthanum clusters. The Jahn-Teller effect plays an important role in this process because there are many isomers near the ground state. The magnetism is not sensitive to interatomic spacing when the change of interatomic spacing is in a small range. Lanthanum clusters grow in an icosahedral pattern. The results of the mean binding energy, of the second derivative of binding energy, and of the formation energy show strong odd-even alternation and that 7- and 13-atom clusters are magic. Further, the HOMO-LUMO gap, the mean nearest bond lengths, and the mean magnetic moments suggest that the convergence to bulk is slow and it shows an oscillatory behavior for small lanthanum clusters.     

On the ground state of Pd13

First-principles electronic structure calculations within a gradient corrected density functional formalism have been carried out to investigate the electronic structure and magnetic properties of Pd(13) clusters. It is shown that a bilayer ground-state structure that can be regarded as a relaxed bulk fragment is most compatible with the experimental results from Stern-Gerlach measurements. An icosahedral structure, considered to be the ground state in numerous previous studies, is shown to be around 0.14 eV above the ground state. A detailed analysis of the molecular orbitals reveals the near degeneracy of the bilayer or icosahedral structures is rooted in the stabilization by p- or d-like cluster orbitals. The importance of low-lying spin states in controlling the electronic and magnetic properties of the cluster is highlighted.     

Rb and Cs oligomers in different spin configurations on helium nanodroplets

The study of small clusters is intended to fill the knowledge gap between single atoms and bulk material. He nanodroplets are an ideal matrix for preparing and investigating clusters in a superfluid environment. Alkali-metal atoms are only bound very weakly to their surface by van der Waals forces. Due to the formation process, high-spin states of alkali-metal clusters on He nanodroplets are favorably observed, which is in contrast to the abundance in other preparation processes. Until now, the prevailing opinion was that stable clusters of the heavy alkali-metal atoms, rubidium (Rb) and cesium (Cs) on He nanodroplets, are limited to 5 and 3 atoms, respectively (Schulz et al., Phys. Rev. Lett. 2004, 92, 13401). Here, we present stable complexes of Rb(n)? and Cs(n)? consisting of up to n = 30 atoms, with the detection of large alkali-metal clusters being strongly enhanced by one-photon ionization. Our results also suggest that we monitored both high-spin and low-spin state clusters created on nanodroplets. The van der Waals bound high-spin alkali-metal clusters should show strong magnetic behavior, while low-spin states are predicted to exhibit metallic characteristics. Alkali-metal clusters prepared in these two configurations appear to be ideal candidates for investigating nanosized particles with ferromagnetic or metallic properties.     

Analysis of the reactivity of small cobalt clusters

The electronic structures of small cobalt clusters have been calculated within the local spin density approximation using the LCAO method. The calculations were done for simple geometries with the optimized number of interatomic bonds, and both for the bond length of the cobalt dimer and the bulk metal. The Fermi energy is found to be smaller for Co _N clusters withN=3, 4, 5 andN>10 than for the other ones. The variation of the Fermi energy with the cluster size correlates in a striking way with the observed H₂ tendency for chemisorption as found for cobalt clusters in a supersonic beam. Furthermore, the magnetic moments are somewhat smaller for these active clusters. In addition the lowest unoccupied levels of majority spin appear close to the highest occupied levels of minority spin which is not the case for the inert clusters.     

Chromium-doped germanium clusters CrGen (n = 1-5): geometry, electronic structure, and topology of chemical bonding

The structure and properties of small neutral and cationic CrGen(0,+) clusters, with n from 1 to 5, were investigated using quantum chemical calculations at the CASSCF/CASPT2 and DFT/B3LYP levels. Smaller clusters prefer planar geometries, whereas the lowest-lying electronic states of the neutral CrGe4, CrGe5, and cationic CrGe5+ forms exhibit nonplanar geometries. Most of the clusters considered prefer structures with high-spin ground state and large magnetic moments. Relative to the values obtained for the pure Gen clusters, fragmentation energies of doped CrGen clusters are smaller when n is 3 and 4 and larger when n = 5. The averaged binding energy tends to increase with the increasing number of Ge atoms. For n = 5, the binding energies for Ge5, CrGe5, and CrGe5+ are similar to each other, amounting to approximately 2.5 eV. The Cr atom acts as a general electron donor in neutral CrGen clusters. Electron localization function (ELF) analyses suggest that the chemical bonding in chromium-doped germanium clusters differs from that of their pure or Li-doped counterparts and allow the origin of the inherent high-spin ground state to be understood. The differential DeltaELF picture, obtained in separating both alpha and beta electron components, is consistent with that derived from spin density calculations. For CrGen, n = 2 and 3, a small amount of d-pi back-donation is anticipated within the framework of the proposed bonding model.     

Structure, stability, electronic and magnetic properties of Ni4 clusters containing impurity atoms

Using a gradient-corrected density functional method, we studied computationally how single impurity atoms affect the structure and the properties of a Ni4 cluster. H and O atoms coordinate at a Ni-Ni bond, inducing small changes to the structure of bare Ni4 which is essentially a tetrahedron. For a C impurity, we found three stable structures at a Ni4 cluster. In the most stable geometry, the carbon atom cleaves a Ni-Ni bond of Ni4, binding to all Ni atoms. Inclusion of the impurity atom leads to a partial oxidation of the metal atoms and, in the most stable structures, reduces the spin polarization of the cluster compared to bare Ni4. An H impurity interacts mainly with the Ni 4s orbitals, whereas the Ni 3d orbitals participate strongly in the bonding with O and C impurity atoms. For these impurity atoms, Ni 3d contributions dominate the character of the HOMO of the ligated cluster, in contrast to the HOMO of bare Ni4 where Ni 4s orbitals prevail. We also discuss a simple model which relates the effect of a H impurity on the magnetic state of metal clusters to the spin character (minority or majority) of the LUMO or HOMO of the bare metal cluster.     

First-principles study of electronic and magnetic properties of Co(n)Mn(m) and Co(n)V(m) (m + n < or = 6) clusters

The electronic and magnetic properties of small Co(n)Mn(m) and Co(n)V(m) (m + n < or = 6) clusters are systematically studied using density functional theory. The results show that Co and V atoms prefer to aggregate in Co-Mn and Co-V clusters, respectively. Significant magnetic moment enhancement in Co-Mn clusters with Mn doping and reduction in Co-V clusters with V doping are found, consistent with experiment results for larger clusters [Phys. Rev. Lett. 2007, 98, 113401]. The results are discussed by analyzing the magnetic coupling type and local magnetic moment on each atoms. Density of states and vertical ionization potentials are calculated and show cluster size dependent behavior.     

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.