Well‐Defined,Nanometer‐Sized LiH Cluster Compounds Stabilized by Pyrazolate Ligands

The assembly of well‐defined large cluster compounds of ionic light metal hydrides is a synthetic challenge and of importance for synthesis, catalysis, and hydrogen storage. The synthesis and characterization of a series of neutral and anionic pyrazolate‐stabilized lithium hydride clusters with inorganic cores in the nanometer region is now reported. These complexes were prepared in a bottom‐up approach using alkyl lithium and lithium pyrazolate mixtures with silanes in hydrocarbon solutions. Structural characterization using synchrotron radiation revealed isolated cubic clusters that contain up to 37 Li⁺ cations and 26 H^? ions. Substituted pyrazolate ligands were found to occupy all corners and some edges for the anionic positions.     

Nanoscale Organolanthanum Clusters: Nuclearity-Directing Role of Cyclopentadienyl and Halogenido Ligands

Tetramethylaluminato/halogenido(X) ligand exchange reactions in half-sandwich complexes [Cp^RLa(AlMe₄)₂] are feasible in non-coordinating solvents and provide access to large coordination clusters of the type [Cp^RLaX₂]_x. Incomplete exchange reactions generate the hexalanthanum clusters [Cp^R₆La₆X₈(AlMe₄)₄] (Cp^R=Cp*=C₅Me₅, X=I; Cp^R=Cp′=C₅H₄SiMe₃, X=Br, I). Treatment of [Cp*La(AlMe₄)₂] with two equivalents Me₃SiI gave the nonalanthanum cluster [Cp*LaI₂]₉, while the exhaustive reaction of [Cp′La(AlMe₄)₂] with the halogenido transfer reagents Me₃GeX and Me₃SiX (X=I, Br, Cl) produced a series of monocyclopentadienyl rare-earth-metal clusters with distinct nuclearity. Depending on the halogenido ion size the homometallic clusters [Cp′LaCl₂]₁₀ and [Cp′LaX₂]₁₂ (X=Br, I) could be isolated, whereas different crystallization techniques led to the aggregation of clusters of distinct structural motifs, including the desilylated cyclopentadienyl-bridged cluster [(μ-Cp)₂Cp′₈La₈I₁₄] and the heteroaluminato derivative [Cp′₁₀La₁₀Br₁₈(AlBr₂Me₂)₂]. The use of the Cp′ ancillary ligand facilitates cluster characterization by means of NMR spectroscopy.     

Discrete Magnesium Hydride Aggregates: A Cationic Mg13H18 Cluster Stabilized by NNNN‐Type Macrocycles

Large magnesium hydride aggregates [Mg₁₃(Me₃TACD)₆(μ₂‐H₁₂)(μ₃‐H₆)][A]₂ ((Me₃TACD)H=1,4,7‐trimethyl‐1,4,7,10‐tetraazacyclododecane; A=AlEt₄, AlnBu₄, B{3,5‐(CF₃)₂C₆H₃}₄) were synthesized stepwise from alkyl complexes [Mg₂(Me₃TACD)R₃] (R=Et, nBu) and phenylsilane in the presence of additional Mg^II ions. The central magnesium atom is octahedrally coordinated by six hydrides as in solid α‐MgH₂ of the rutile type. Further coordination to six magnesium atoms leads to a substructure of seven edge‐sharing octahedra as found in the hexagonal layer of brucite (Mg(OH)₂). Upon protonolysis in the presence of 1,2‐dimethoxyethane (DME), this cluster was degraded into a tetranuclear dication [Mg₂(Me₃TACD)(μ‐H)₂(DME)]₂[A]₂.     

Synthesis and structures of the C5Me4SiMe3-supported polyhydride complexes over the full size range of the rare earth series

The acid-base reaction of [Ln(CH(2)SiMe(3))(3)(thf)(2)] with Cp'H gave the corresponding half-sandwich rare earth dialkyl complexes [(Cp')Ln(CH(2)SiMe(3))(2)(thf)] (1-Ln: Ln=Sc, Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; Cp'=C(5)Me(4)SiMe(3)) in 62-90% isolated yields. X-ray crystallographic studies revealed that all of these complexes adopt a similar overall structure, in spite of large difference in metal-ion size. In most cases, the hydrogenolysis of the dialkyl complexes in toluene gave the tetranuclear octahydride complexes [{(Cp')Ln(μ-H)(2)}(4)(thf)(x)] (2-Ln: Ln=Sc, x=0; Y, x=1; Er, x=1; Tm, x=1; Gd, x=1; Dy, x=1; Ho, x=1) as the only isolable product. However, in the case of Lu, a trinuclear pentahydride [(Cp')(2)Lu(3)(μ-H)(5)(μ-CH(2)SiMe(2)C(5)Me(4))(thf)(2)] (3), in which the C-H activation of a methyl group of the Me(3)Si unit on a Cp' ligand took place, was obtained as a major product (66% yield), in addition to the tetranuclear octahydride [{(Cp')Lu(μ-H)(2)}(4)(thf)] (2-Lu, 34%). The use of hexane instead of toluene as a solvent for the hydrogenolysis of 1-Lu led to formation of 2-Lu as a major product (85%), while a similar reaction in THF yielded 3 predominantly (90%). The tetranuclear octahydride complexes of early (larger) lanthanide metals [{Cp'Ln(μ-H)(2)}(4)(thf)(2)] (2, Ln=La, Ce, Pr, Nd, Sm) were obtained in 38-57% isolated yields by hydrogenolysis of the bis(aminobenzyl) species [Cp'Ln(CH(2)C(6)H(4)NMe(2)-o)(2)], which were generated in-situ by reaction of [Ln(CH(2)C(6)H(4)NMe(2)-o)(3)] with one equivalent of Cp'H. X-ray crystallographic studies showed that the fine structures of these hydride clusters are dependent on the size of the metal ions.     

Extended Open-Chain Polyenides as Versatile Delocalized Anion Ligands for Metal Chain Clusters

Although small cyclic- and open-chain unsaturated hydrocarbon anions such as cyclopentadienide and open-chain pentadienide are used as the strongly electron-donating auxiliary ligands for metal complexes, more extended π-conjugated unsaturated hydrocarbon anions have rarely been used in coordination chemistry, despite their potential ability to serve as the multiply bridging π-ligands for metal clusters. This work reports isolation of metal chain clusters bearing the multi-dentate, open-chain extended unsaturated hydrocarbon anion ligands. The extended open-chain π-conjugated polyenyl ligands could effectively stabilize oxidized palladium chains, including an unprecedented [Pd₄]⁴⁺ chain.     

Titanium-Oxo Clusters with Bi- and Tridentate Organic Ligands: Gradual Evolution of the Structures from Small to Big

Homometallic titanium oxo clusters are one of the most important groups of metal oxo clusters, with more than 300 examples characterized by X-ray structure analyses. Most of them are uncharged and are obtained by partial hydrolysis and condensation of titanium alkoxo derivatives. The cluster cores, ranging from 3 to >50 titanium atoms, are stabilized by organic ligands. Apart from residual OR groups, carboxylato and phosphonato ligands are most frequent. The article critically reviews and categorizes the known structures and works out basic construction principles by comparing the different cluster types.     

Molecular Rare‐Earth‐Metal Hydrides in Non‐Cyclopentadienyl Environments

Molecular hydrides of the rare‐earth metals play an important role as homogeneous catalysts and as counterparts of solid‐state interstitial hydrides. Structurally well‐characterized non‐metallocene‐type hydride complexes allow the study of elementary reactions that occur at rare‐earth‐metal centers and of catalytic reactions involving bonds between rare‐earth metals and hydrides. In addition to neutral hydrides, cationic derivatives have now become available.     

New Transition Metal Clusters with Ligands from Main Groups Five and Six

In both physics and chemistry, increased attention is being paid to metal clusters. One reason for this attitude is furnished by the surprising results that have been obtained from studies of the preparation, structural characterization and physical and chemical properties of the clusters. Whereas investigations of cluster reactivity are at present generally limited to three- or four-membered clusters, successful syntheses of clusters with many more metal atoms have recently been designed. These substances occupy an intermediate position between solid state chemistry and the chemistry of metal complexes. This review presents a versatile method for synthesizing metal clusters: the reaction of complexes of transition metal halides with silylated compounds such as E(SiMe₃)₂ (E = S, Se, Te) and E′R(SiMe₃)₂ (R = Ph, Me, Et; E′ = P, As, Sb). Although some of the compounds thus formed have already been prepared by other routes, the method affords ready access to both small and large transition metal clusters with unusual structures and valence electron concentrations; a variety of reactions in the ligand sphere are also possible.     

Well‐Defined Molecular Magnesium Hydride Clusters: Relationship between Size and Hydrogen‐Elimination Temperature

A new tetranuclear magnesium hydride cluster, [{ NN ‐(MgH)₂}₂], which was based on a N? N‐coupled bis‐β‐diketiminate ligand ( NN ^2?), was obtained from the reaction of [{ NN ‐(MgnBu)₂}₂] with PhSiH₃. Its crystal structure reveals an almost‐tetrahedral arrangement of Mg atoms and two different sets of hydride ions, which give rise to a coupling in the NMR spectrum (J=8.5 Hz). To shed light on the relationship between the cluster size and H₂ release, the thermal decomposition of [{ NN ‐(MgH)₂}₂] and two closely related systems that were based on similar ligands, that is, an octanuclear magnesium hydride cluster and a dimeric magnesium hydride species, have been investigated in detail. A lowering of the H₂‐desorption temperature with decreasing cluster size is observed, in line with previously reported theoretical predictions on (MgH₂)_n model systems. Deuterium‐labeling studies further demonstrate that the released H₂ solely originates from the oxidative coupling of two hydride ligands and not from other hydrogen sources, such as the β‐diketiminate ligands. Analysis of the DFT‐computed electron density in [{ NN ‐(MgH)₂}₂] reveals a counterintuitive interaction between two formally closed‐shell H^? ligands that are separated by 3.106 Å. This weak interaction could play an important role in H₂ desorption. Although the molecular product after H₂ release could not be characterized experimentally, DFT calculations on the proposed decomposition product, that is, the low‐valence tetranuclear Mg(I) cluster [( NN ‐Mg₂)₂], predict a structure with two almost‐parallel, localized Mg? Mg bonds. As in a previously reported β‐diketiminate Mg^I dimer, the Mg? Mg bond is not characterized by a bond critical point, but instead displays a local maximum of electron density midway between the atoms, that is, a non‐nuclear attractor (NNA). Interestingly, both of the NNAs in [( NN ‐Mg₂)₂] are connected through a bond path that suggests that there is bonding between all four Mg^I atoms.     

Alkali Metal Triphenyl- and Trihydridosilanides Stabilized by a Macrocyclic Polyamine Ligand

Potassium silanide [KSiH₃]_∞ contains 4.2 wt % of hydrogen and has been intensely studied as hydrogen storage material. The macrocyclic ligand Me₄TACD (1,4,7,10-tetramethyl-1,4,7,10-tetraaminocyclododecane, L ) stabilizes the full range of triphenylsilyl complexes [( L )MSiPh₃]_n (M=Li–Cs), which react with H₂ or PhSiH₃ to form molecular [( L )MSiH₃]_n that can be isolated in soluble form and fully characterized.     

Unprecedented Efficient Structure Controlled Phosphorescence of Silver(I) Clusters Stabilized by Carba‐closo‐dodecaboranylethynyl Ligands

{Ag₂(12‐C≡C‐closo‐1‐CB₁₁H₁₁)}_n and selected pyridine ligands have been used for the synthesis of photostable Ag^I clusters that, with one exception, exhibit for Ag^I compounds unusual room‐temperature phosphorescence. Extraordinarily intense phosphorescence was observed for a distorted pentagonal bipyramidal Ag^I₇ cluster that shows an unprecedented quantum yield of Φ=0.76 for Ag^I clusters. The luminescence properties correlate with the structures of the central Ag^I_n motifs as shown by comparison of the emission properties of the clusters with different numbers of Ag^I ions, different charges, and electronically different pyridine ligands.     

Reactions of Thorium Oxide Clusters with Water: The Effects of Oxygen Content

Thorium oxide has many important applications in industry. In this article, theoretical calculations have been carried out to explore the hydrolysis reactions of the ThO_n (n=1–3) clusters. The reaction mechanisms of the O-deficient ThO and the O-rich ThO₃ are compared with the stoichiometric ThO₂. The theoretical results show good agreement with the prior experiments. It is shown that the hydrolysis mainly occurred on the singlet potential surface. The overall reactions consist of two hydrolysis steps which are all favourable in energy. The effects of oxygen content on the hydrolysis are elucidated. Interestingly, among them, the peroxo group O₂²⁻ in ThO₃ is converted to the HOO− ligand, behaving like the terminal O²⁻ in the hydrolysis which is transformed into the HO− groups. In addition, natural bond orbital (NBO) analyses were employed to further understand the bonding of the pertinent species and to interpret the differences in hydrolysis.     

Molecular Calcium Hydride: Dicalcium Trihydride Cation Stabilized by a Neutral NNNN‐Type Macrocyclic Ligand

Hydrogenolysis of bis(triphenylsilyl)calcium containing the neutral NNNN‐type macrocyclic amine ligand Me₄TACD [Ca(Me₄TACD)(SiPh₃)₂] ( 2 ), gave the cationic dinuclear calcium hydride [Ca₂H₃(Me₄TACD)₂](SiPh₃) ( 3 ), characterized by NMR spectroscopy, single‐crystal X‐ray analysis, and DFT calculations. Compound 3 reacted with deuterium to give the deuteride [D₃]‐ 3 .     

Expanding the Chemistry of Molecular U2+ Complexes: Synthesis,Characterization, and Reactivity of the {[C5H3(SiMe3)2]3U}− Anion

The synthesis of new molecular complexes of U²⁺ has been pursued to make comparisons in structure, physical properties, and reactivity with the first U²⁺ complex, [K(2.2.2‐cryptand)][Cp′₃U], 1 (Cp′=C₅H₄SiMe₃). Reduction of Cp′′₃U [Cp′′=C₅H₃(SiMe₃)₂] with KC₈ in the presence of 2.2.2‐cryptand or 18‐crown‐6 generates [K(2.2.2‐cryptand)][Cp′′₃U], 2‐K(crypt) , or [K(18‐crown‐6)(THF)₂][Cp′′₃U], 2‐K(18c6) , respectively. The UV/Vis spectra of 2‐K and 1 are similar, and they are much more intense than those of U³⁺ analogues. Variable temperature magnetic susceptibility data for 1 and 2‐K(crypt) reveal lower room temperature χ_MT values relative to the experimental values for the 5f³ U³⁺ precursors. Stability studies monitored by UV/Vis spectroscopy show that 2‐K(crypt) and 2‐K(18c6) have t_1/2 values of 20 and 15 h at room temperature, respectively, vs. 1.5 h for 1 . Complex 2‐K(18c6) reacts with H₂ or PhSiH₃ to form the uranium hydride, [K(18‐crown‐6)(THF)₂][Cp′′₃UH], 3 . Complexes 1 and 2‐K(18c6) both reduce cyclooctatetraene to form uranocene, (C₈H₈)₂U, as well as the U³⁺ byproducts [K(2.2.2‐cryptand)][Cp′₄U], 4 , and Cp′′₃U, respectively.     

Synthesis of Planar Five‐Connected Nodal Ligands

The fiveway junction : The synthesis of ligands with five symmetrically diverging binding sites is described and their assembly into large spherical structures (such as that shown here) investigated.

     

New Molecular Cage Clusters of Pb by Encapsulation of Mg

Endohedral clusters formed from the Zintl ions Pb₁₀^2? and Pb₁₂^2? are particularly stable and therefore suitable for the assembly of larger aggregates. We therefore investigate the formation of Mg‐doped lead clusters in the gas phase, and demonstrate that a whole series of new molecular cage clusters of lead can be generated by encapsulation of magnesium. Mass spectrometry reveals that some of the cluster compounds, with one and two Mg atoms attached to the lead clusters, display large intensities compared to the pure lead clusters, which indicates that the compound clusters are particularly stable. The magnesium‐doped lead‐cluster assemblies were further analyzed within a molecular‐beam electric deflection experiment. Almost vanishing permanent dipole moments for MgPb_10–16 support the idea that a single Mg atom could be encapsulated within a highly symmetric lead cage, which results in structures with not only enhanced stability but also increased symmetry compared to the pure lead clusters Pb_N.     

Conformational States of Triosmium Clusters with Aminoacid Ligands: A Theoretical Study

The conformational state of triosmium clusters with aminoacid ligands (-H)Os₃( -NHCH(R)CO₂Et)(CO)₁₀ (R = H, CH₃) has been studied theoretically. The conformational analysis was performed using the MERA model. The potential curves of the internal rotation of organic ligands in the cluster complexes relative to the HN–CH bond were determined both for individual substances and their solutions. The structures of possible conformers are considered, reasons for their stability are revealed, and variations in the potential curves of internal rotation in various solvents are discussed. The barrier to rotation relative to the HN–CH bond of the alanine derivative is 76.8-78.7 kJ/mole in different media, which hinders free rotation. For the glycine derivative, the same barrier is 37.5-37.9 kJ/mole, which allows for substantial rotational vibrations.