Metalloid Al- and Ga-clusters: a novel dimension in organometallic chemistry linking the molecular and the solid-state areas?

Formation and fragmentation of metal-metal bonds on the way between stable metal compounds in which the metal atoms are oxidised (e.g. isolated species in solution or metal salts in bulk) and the bulk metal are the fundamental steps to understand this process in which formation and chemical behaviour of metalloid Al and Ga clusters as intermediates are essential. Many examples of metalloid Al and Ga clusters show that their formation reflects a high degree of complexity like that of the simple seeming formation of the bulk metal itself: starting from metastable Al(i) and Ga(i) solutions containing small molecular entities, metalloid clusters grow during many self-organization steps including aggregation as well as irreversible redox cascades. This novel class of clusters seems to open a new dimension in chemistry between the molecular and the solid-state area, because, for the first time, it is shown that under well selected conditions definite molecular species, i.e. metalloid clusters, grow via the formation of additional metal-metal bonds and that the solid metal represents the final step.     

Quantum chemical study of butane isomerization on clusters of aluminum and cobalt chlorides. Mixed complexes

Activated complexes and routes of the model catalytic process, viz., butane isomerization by the aluminum and cobalt chloride complexes, were calculated by the DFT/PBE/TZ2p quantum chemical method. Alkanes are activated via the alkyl mechanism to form binuclear bimetallic alkyl clusters, where the Co atoms are linked by the metal-metal bonds. The revealed binuclear complexes can transform into bimetallic alkyl clusters with similar energy in which the transition metal atoms are linked by bridges of the Cl atoms. The full model of the catalytic cycle was developed for the maximum multiplicity (7), and particular key regions related to the cleavage and formation of the C-C bonds were calculated with a lowered multiplicity (5 and 3). The sequence of mutual rearrangements of the polynuclear complexes provides the possibility of C-C bond cleavage in alkanes and formation of the metal-carbon bonds. The calculated energy barriers of particular stages of the cyclic catalytic process of butane isomerization are not higher than 29 kcal mol^?1 for multiplicity 7 and by ～10 kcal mol^?1 lower for a lower multiplicity.     

簇电荷对金属—金属键影响的量子化学研究

在探讨过渡金属原子簇化合物的金属——金属键的本质时,簇电荷的影响已引起人们的注意。簇电荷对金属——金属键的作用比较复杂,其中有价电子的成键效应和金属原子氧化数变化所产生的电荷效应。键长与簇电荷之间很难找到简单的关系。Cotton等曾对此问题做过初步讨论,但尚缺定量或半定量的理论计算依据,本文采用改进的电荷自洽EHMO程序(MAD—SCCO-EHMO)计算一系列Mo,Tc,Ru,Rh,和Re等二核簇的电子结构,根据M(?)lliken重迭集居分析,讨论簇电荷对金属——金属键的影响。     

Metal clusters as ligands. Substitution of fe ions in Fe/Mo/S clusters by thiophilic CuI ions

The reactivity of Fe/S and Fe/Mo/S clusters, similar or analogous to those occurring in biological systems, with thiophilic metal ions has not been explored. In this Communication, we demonstrate that synthesis of heteropolynuclear clusters with different coordination geometries for different metals at different sites is possible by metal substitution or by metal addition reactions. The two clusters we report herein ([(Cl4-cat)2Mo2Cu5Fe4S9(PnPr3)7(SPnPr3)2]PF6 and [(Cl4-cat)2Mo2Cu6Fe4S10(PnPr3)8]) contain Fe, Mo, and Cu, which display pseudotetrahedral, pseudooctahedral, and pseudotrigonal geometries, respectively. The synthesis of these clusters is achieved by the addition of appropriate amounts of [Cu(CH3CN)4]+ to [(Cl4-cat)2Mo2Fe8(PnPr3)6]. The formation of the different products is temperature- and solvent-dependent. The Cu(I) units incorporated into the metal cluster framework, either bind to available lone pairs of the already bridging S ligands or displace the less thiophilic Fe atoms. Among the essential features of these new molecules are recognizable Fe/S fragments including an Fe6S9 core in the first cluster and the pentlandite Fe4Cu4S6 core in the second cluster.     

Computational study of multiple-decker sandwich and rice-ball structures of neutral titanium-benzene clusters

Density functional theory calculations were applied to systematically and directly compare the relative energetic stability of multiple-decker sandwich and rice-ball structures for a variety of neutral Ti(m)Bz(n) clusters (m = 1-4, n = 1-5). Almost all structures favored the multiple-decker sandwich structure, as observed experimentally for early transition metals. The strength of each metal-benzene interaction averages 37 kcal/mol and remains relatively constant for sandwiches with three or more Ti atoms. The most stable smaller rice-ball structures did not have eta(6)-Bz bound to a single metal atom. Instead, the preferred coordination was having the plane of the benzene molecule parallel to a Ti(2) bond or a Ti(3) face, leading to some distortion of the benzene ring. The larger rice-ball structures, on the other hand, preferred to weaken the metal-metal bonds and retain eta(6)-Bz bound to a single metal atom, a structural motif shared with sandwiches.     

Metalloid aluminum and gallium clusters: element modifications on the molecular scale?

As members of the same group in the periodic table, the industrially significant elements aluminum and gallium exhibit strong similarities in the majority of their compounds. In contrast there are significant differences in the structures of the two elemental forms: Aluminum forms a typical closest-packed metallic structure whereas gallium demonstrates a diversity of molecular bonding principles in its seven structural modifications. It can therefore be expected that differences between Al and Ga compounds will arise when, as for the elemental forms, many metal-metal bonds are formed. To synthesize such cluster compounds, we have developed the following synthesis procedure: Starting from gaseous monohalides at around 1000 degrees C, metastable solutions are generated from which the elements ultimately precipitate by means of a disproportionation reaction at room temperature. On the way to the elemental forms, molecular Al and Ga cluster compounds can be obtained by selection of suitable ligands (protecting groups), in which a core of Al or Ga atoms are protected from the formation of the solid element by a ligand shell. Since the arrangement of atoms in such clusters corresponds to that in the elements, we have designated these clusters as metalloid or elementoid. In accordance with the Greek word [see text] (ideal, prototype), the atomic arrangement in metalloid clusters represents the prototypic or ideal atomic arrangement in the elements at the molecular level. The largest clusters of this type contain 77 Al or 84 Ga atoms and have diameters of up to two nanometers. They hold the world record with respect to the naked metal-atom core for structurally characterized metalloid clusters.     

Origin of the size-dependence of the polarizability per atom in heterogeneous clusters: The case of AlP clusters

An analysis of the atomic polarizabilities α in stoichiometric aluminum phosphide clusters, computed at the MP2 and density functional theory (DFT) levels, the latter using the B3LYP functional, and partitioned using the classic and iterative versions of the Hirshfeld method, is presented. Two sets of clusters are examined: the ground-state Al(n)P(n) clusters (n=2-9) and the prolate clusters (Al(2)P(2))(N) and (Al(3)P(3))(N) (N≤6). In the ground-state clusters, the mean polarizability per atom, i.e., α/2n, decreases with the cluster size but shows peaks at n=5 and at n=7. We demonstrate that these peaks can be explained by a large polarizability of the Al atoms and by a low polarizability of the P atoms in Al(5)P(5) and Al(7)P(7) due to the presence of homopolar bonds in these clusters. We show indeed that the polarizability of an atom within an Al(n)P(n) cluster depends on the cluster size and the heteropolarity of the bonds it forms within the cluster, i.e., on the charges of the atoms. The polarizabilities of the fragments Al(2)P(2) and Al(3)P(3) in the prolate clusters were found to depend mainly on their location within the cluster. Finally, we show that the iterative Hirshfeld method is more suitable than the classic Hirshfeld method for describing the atomic polarizabilities and the atomic charges in clusters with heteropolar bonds, although both versions of the Hirshfeld method lead to similar conclusions.     

Formation, structure and bonding of metalloid Al and Ga clusters. A challenge for chemical efforts in nanosciences

The renaissance of Al and Ga cluster chemistry is presented in three steps: on the grounds of boron hydride chemistry and the Wade concept, the first step starts in the early nineties of the last century with the formation of single Al-Al and Ga-Ga bonds in molecular entities, obtained by different synthetic approaches. The special method via reaction of high-temperature molecules like AlCl and its disproportionation to Al metal and AlX(3) leads to the second step which started about 10 years ago: the formation of nanoscaled metalloid Al and Ga clusters as intermediates on the way to the metal. Based on the structure of several recent examples, bonding is discussed with respect to the structure of the elements and the generation of naked metal atom clusters. After discussion of the individual metalloid clusters including experiments of the gaseous species and discussion about the jellium model, the third step and main part of this review starts only a few years ago. This latest period hardly can be called a renaissance period as, so far, interactions of nanoscaled metal atom clusters in a perfect 1-, 2- or 3-dimensional arrangement of a crystal have never been investigated before. The most remarkable result in this perspective is the superconducting behaviour of a Ga(84) cluster compound in the crystalline state which had never been observed in metal atom clusters before. However, these experiments show that superconductivity is only observed if the clusters in the crystal are perfectly orientated: as a cluster arrangement of this type can hardly be fabricated by physical methods, these results, which have been predicted by theory, may be called a disillusionment for nanosciences; for chemistry, however, these conclusions pose a challenge.     

Pt3Ru6 clusters supported on gamma-Al2O3: synthesis from Pt3Ru6(CO)21(mu3-H)(mu-H)3, structural characterization, and catalysis of ethylene hydrogenation and n-butane hydrogenolysis

The supported clusters Pt-Ru/gamma-Al2O3 were prepared by adsorption of the bimetallic precursor Pt3Ru6(CO)21(mu3-H)(mu-H)3 from CH2Cl2 solution onto gamma-Al2O3 followed by decarbonylation in He at 300 degrees C. The resultant supported clusters were characterized by infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies and as catalysts for ethylene hydrogenation and n-butane hydrogenolysis. After adsorption, the nu(CO) peaks characterizing the precursor shifted to lower wavenumbers, and some of the hydroxyl bands of the support disappeared or changed, indicating that the CO ligands of the precursor interacted with support hydroxyl groups. The EXAFS results show that the metal core of the precursor remained essentially unchanged upon adsorption, but there were distortions of the metal core indicated by changes in the metal-metal distances. After decarbonylation of the supported clusters, the EXAFS data indicated that Pt and Ru atoms interacted with support oxygen atoms and that about half of the Pt-Ru bonds were maintained, with the composition of the metal frame remaining almost unchanged. The decarbonylated supported bimetallic clusters reported here are the first having essentially the same metal core composition as that of a precursor metal carbonyl, and they appear to be the best-defined supported bimetallic clusters. The material was found to be an active catalyst for ethylene hydrogenation and n-butane hydrogenolysis under conditions mild enough to prevent substantial cluster disruption.     

Cooperative effects and strengths of hydrogen bonds in open-chain cis-triaziridine clusters (n = 2-8): a DFT investigation

We employ DFT/B3LYP method to investigate linear open-chain clusters (n = 2-8) of the cis-triaziridine molecule that is a candidate molecule for high energy density materials (HEDM). Our calculations indicate that the pervasive phenomena of cooperative effects are observed in the clusters of n = 3-8, which are reflected in changes in lengths of N...H hydrogen bonds, stretching frequencies, and intensities of N-H bonds, dipole moments, and charge transfers as cluster size increases. The n(N) --> sigma*(N-H) interactions, i.e., the charge transfers from lone pairs (n(N)) of the N atoms into antibonds (sigma*) of the N-H bonds acting as H-donors, can be used to explain the observed cooperative phenomena. The approaches based upon natural bond orbital (NBO) method and theory of atoms in molecule (AIM) to evaluating N...H strengths are found to be equivalent. In the process of N...H bonding, cooperative nature of n(N) --> sigma*(N-H) interactions promotes formation of stronger N...H bonds as reflected in increases in the capacities of cis-triaziridine clusters to concentrate electrons at the bond critical points of N...H bonds. The calculated nonadditive energies also show that the cooperative effects due to n(N) --> sigma*(N-H) interactions indeed provide additional stabilities for the clusters.     

Synthesis and characterization of an Al(69)(3-) cluster with 51 naked Al atoms: analogies and differences to the previously characterized Al(77)(2-) cluster

A disproportionation process of a metastable AlCl solution with a simultaneous ligand exchange-Cl is substituted by N(SiMe(3))(2)-leads to a [Al(69)[N(SiMe(3))(2)](18)](3-) cluster compound that can be regarded as an intermediate on the way to bulk metal formation. The cluster was characterized by an X-ray crystal structural analysis. Regarding its structure and the packing within the crystal, this metalloid cluster with 4 times more Al atoms than ligands is compared to the [Al(77)N(SiMe(3))(2)](20)](2-) cluster that has been published four years ago. Although there is a similar packing density of the Al atoms in both clusters as well as in Al metal, the X-ray structural analysis shows significant differences in topology and distance proportions. The differences between these-at a first glance almost identical-Al clusters demonstrate that results of physical measuring, e.g., of nanostructured surfaces which carry supposedly identical cluster species, have to be interpreted with great caution.     

Electrochemical synthesis, structural characterization, and decomposition of rhenium oxoethoxide, Re4O4(OEt)12. Ligand influence on the structure and bonding in the high-valent tetranuclear planar rhenium alkoxide clusters

Anodic oxidation of rhenium in ethanol in the presence of LiCl as a conductive additive results with high yield in formation of a new oxoethoxide cluster, Re(4)O(4)(OEt)(12). The structure of the planar centrosymmetric metal-oxygen core of this molecule is composed of four edge-sharing Re(V)O(6) octahedra. Eight electrons are available for the formation of metal-metal bonds indicated by five relatively short Re-Re distances within the Re 4-rhombus, a "planar butterfly" type cluster. The theoretical calculations are indicating relatively low contribution of metal-metal bonding in the stability of the core. The stability of the +V-oxidation state, unusual for rhenium alkoxides can be at least partially attributed to the size effects in the packing of ligands. The X-ray powder study indicates that treatment of Re(4)O(4)(OEt)(12). in ambient atmosphere rapidly transforms it into a mixed-valence derivative Re(4)O(6)(OEt)(10) with a structure related to the earlier investigated cluster Re(4)O(6)(O(i)Pr)(10). Thermal decomposition of the latter rhenium oxoethoxide results in reduction to rhenium metal at as low temperatures as 380 degrees C, producing aggregates of metal nanoparticles with the average size of 3 nm.     

Chemical structures from the analysis of domain-averaged Fermi holes: multiple metal-metal bonding in transition metal compounds

The recently proposed approach based on the analysis of domain-averaged Fermi holes was applied to the study of the nature of metal-metal bonding in transition metal complexes and clusters. The main emphasis was put on the scrutiny of the systems assumed to contain direct multiple metal-metal bonds. The studied systems involve: (1) systems of the type M(2)X(6) (M = Mo, W, X = CH(3)) anticipated to contain metal-metal triple bonds; (2) the molecule of W(2)Cl(8) ((4-)) as the representative of the systems with quadruple metal-metal bonding; (3) diatomic molecules Mo(2) and V(2) considered as the potential candidates for higher than quadruple metal-metal bonding. Although the resulting picture of bonding has been usually shown to agree with the original expectations based on early simple MO models, some examples were also found in which the conclusions of the reported analysis display dramatic sensitivity to the quality of the wave function used for the generation of the Fermi holes. In addition to this we also report some examples where the original theoretical predictions of multiplicity of metal-metal bonds have to be corrected.     

Two metalloid Ga22 clusters containing a novel Ga22 core with an icosahedral Ga12 center

In addition to the two so far known types of metalloid Ga(22) clusters a new type is presented in two compounds containing the anions [Ga(22)Br[N(SiMe(3))(2)](10)Br(10)](3-) (1) and [Ga(22)Br(2)[N(SiMe(3))(2)](10)Br(10)](2-) (2). In both anions 10 Ga atoms of the icosahedral Ga(12) core are directly connected to further Ga atoms. The two remaining Ga atoms (top and bottom) of the Ga(12) icosahedron are bonded to one (1) and two Br atoms (2), respectively. The formation and structure of both compounds containing a slightly different average oxidation number of the Ga atoms is discussed and compared especially with regard to the Ga(84) cluster compound and similar metalloid Al(n) clusters. Finally, the consequences arising from the presence of two very similar but not identical Ga(22) cluster compounds are discussed and special consideration is given to the so far not understood physical properties (metallic conductivity and superconductivity) of the Ga(84) cluster compound.     

The nonmetallicity of molybdenum clusters

Molybdenum clusters consisting of 2-55 atoms were investigated using density functional theory calculations with a plane-wave basis set. The results show that the linear and planar molybdenum clusters have a strong tendency to form dimers. This tendency results in the formation of alternate short and long bonds within a linear cluster, in which the strength of these short bonds is covalent. Therefore, the linear and planar Mo clusters exhibit significant nonmetallic characteristics. Furthermore, the linear and planar Mo clusters show a strong even-odd effect in binding energy with the even-numbered clusters being more stable than their neighboring odd-numbered clusters. On the other hand, the even-odd effect in the energy gap between the highest occupied and the lowest unoccupied molecular orbitals, i.e., the HOMO-LUMO energy gap, for the linear and the planar clusters is different. The odd-numbered linear clusters and even-numbered planar clusters have larger HOMO-LUMO energy gaps than their corresponding neighboring clusters.     

Glucose interaction with Au,Ag, and Cu clusters: Theoretical investigation

Interactions of α‐D ‐glucose with gold, silver, and copper metal clusters are studied theoretically at the density functional theory (CAM‐B3LYP) and MP2 levels of theory, using trimer clusters as simple catalytic models for metal particles as well as investigating the effect of cluster charge by studying the interactions of cationic and anionic gold clusters with glucose. The bonding between α‐D ‐glucose and metal clusters occurs by two major bonding factors; the anchoring of M atoms (M = Cu, Ag, and Au) to the O atoms, and the unconventional M…H? O hydrogen bond. Depending on the charge of metal clusters, each of these bonds contributes significantly to the complexation. Binding energy calculations indicate that the silver cluster has the lowest and gold cluster has the highest affinity to interact with glucose. Natural bond orbital analysis is performed to calculate natural population analysis and charge transfers in the complexes. Quantum theory of atoms in molecules was also applied to interpret the nature of bonds. © 2012 Wiley Periodicals, Inc.     

Stable clusters in the condensed state and some possibilities for gas phase clusters

The classical naked cluster ions of the post-transition elements that are stable in solid compounds and their lower charged analogues observed in mixed metal beams reflect the reduced number of good bonding orbitals. New cluster ions of indium that are hypoelectronic (fewer than 2n+2 skeletal bonding electrons) because of distortions or the bonding of heterometal atoms within the clusters are described. A large family of new, orbital-rich clusters of the group III and IV transition metals sheathed by halide are all centered by a wide variety of heteroatoms. Factors in their stability, possible analogous naked cluster targets, and some calculations are considered.     

Cluster size selectivity in the product distribution of ethene dehydrogenation on niobium clusters

Ethene reactions with niobium atoms and clusters containing up to 25 constituent atoms have been studied in a fast-flow metal cluster reactor. The clusters react with ethene at about the gas-kinetic collision rate, indicating a barrierless association process as the cluster removal step. Exceptions are Nb8 and Nb10, for which a significantly diminished rate is observed, reflecting some cluster size selectivity. Analysis of the experimental primary product masses indicates dehydrogenation of ethene for all clusters save Nb10, yielding either Nb(n)C2H2 or Nb(n)C2. Over the range Nb-Nb6, the extent of dehydrogenation increases with cluster size, then decreases for larger clusters. For many clusters, secondary and tertiary product masses are also observed, showing varying degrees of dehydrogenation corresponding to net addition of C2H4, C2H2, or C2. With Nb atoms and several small clusters, formal addition of at least six ethene molecules is observed, suggesting a polymerization process may be active. Kinetic analysis of the Nb atom and several Nb(n) cluster reactions with ethene shows that the process is consistent with sequential addition of ethene units at rates corresponding approximately to the gas-kinetic collision frequency for several consecutive reacting ethene molecules. Some variation in the rate of ethene pick up is found, which likely reflects small energy barriers or steric constraints associated with individual mechanistic steps. Density functional calculations of structures of Nb clusters up to Nb(6), and the reaction products Nb(n)C2H2 and Nb(n)C2 (n = 1...6) are presented. Investigation of the thermochemistry for the dehydrogenation of ethene to form molecular hydrogen, for the Nb atom and clusters up to Nb6, demonstrates that the exergonicity of the formation of Nb(n)C2 species increases with cluster size over this range, which supports the proposal that the extent of dehydrogenation is determined primarily by thermodynamic constraints. Analysis of the structural variations present in the cluster species studied shows an increase in C-H bond lengths with cluster size that closely correlates with the increased thermodynamic drive to full dehydrogenation. This correlation strongly suggests that all steps in the reaction are barrierless, and that weakening of the C-H bonds is directly reflected in the thermodynamics of the overall dehydrogenation process. It is also demonstrated that reaction exergonicity in the initial partial dehydrogenation step must be carried through as excess internal energy into the second dehydrogenation step.     

Negative ions of transition metal-halogen clusters

A systematic density functional theory based study of the structure and spectroscopic properties of neutral and negatively charged MX(n) clusters formed by a transition metal atom M (M=Sc,Ti,V) and up to seven halogen atoms X (X=F,Cl,Br) has revealed a number of interesting features: (1) Halogen atoms are bound chemically to Sc, Ti, and V for n≤n(max), where the maximal valence n(max) equals to 3, 4, and 5 for Sc, Ti, and V, respectively. For n>n(max), two halogen atoms became dimerized in the neutral species, while dimerization begins at n=5, 6, and 7 for negatively charged clusters containing Sc, Ti, and V. (2) Magnetic moments of the transition metal atoms depend strongly on the number of halogen atoms in a cluster and the cluster charge. (3) The number of halogen atoms that can be attached to a metal atom exceeds the maximal formal valence of the metal atom. (4) The electron affinities of the neutral clusters abruptly rise at n=n(max), reaching values as high as 7 eV. The corresponding anions could be used in the synthesis of new salts, once appropriate counterions are identified.