Magnesium(I) Halide versus Magnesium Metal: Differences in Reaction Energy and Reactivity Monitored in Reduction Processes of P-Cl Bonds.

Magnesium(I) halides (MgI X; X=Cl, Br, I), as high temperature molecules, are trapped and finally stored at -80 °C in toluene/donor solutions. These solutions provide insights into the fundamental mechanism of reduction reactions using activated magnesium metal as a prototype for every base metal. The most important example of such a reaction is the preparation of Grignard reagents (RMgX). The details of this highly complex mechanism especially of intermediates between Mg metal and MgII (RMgX) remain unknown until today. The same is true for the reaction of bulk magnesium with Group 15 halide compounds that give biradicaloid species. We investigate the reduction of P-Cl bonds with solutions of [MgI Br(Nn Bu3 )]2 (1). The phosphanes [ClP(µ-NTer)]2 (2) and (Me3 Si)2 N-PCl2 (3), were chosen as they had successfully been reduced by Mg metal before. Furthermore, reactions of both 1 and Mg metal are compared with an MgI chelate complex L1 Mg-MgL1 containing a strong Mg-Mg σ-bond.

Realization of an Al≡Al Triple Bond in the Gas-Phase Na3 Al2 - Cluster via Double Electronic Transmutation.

The discovery of homodinuclear multiple bonds composed of Group 13 elements represents one of the most challenging frontiers in modern chemistry. A classical triple bond such as N≡N and HC≡CH contains one σ bond and two π bonds constructed from the p orbitals perpendicular to the σ bond. However, the traditional textbook triple bond between two Al atoms has remained elusive. Here we report an Al≡Al triple bond in the designer Na3 Al2 - cluster predicted in silico, which was subsequently generated by pulsed arc discharge followed by mass spectrometry and photoelectron spectroscopy characterizations. Being effectively Al2- due to the electron donation from Na, the Al atoms in Na3 Al2 - undergo a double electronic transmutation into Group 15 elements, thus the Al2- ≡Al2- kernel mimics the P≡P and N≡N molecules. We anticipate this work will stimulate more endeavors in discovering materials using Al2- ≡Al2- as a building block in the gas phase and in the solid state.

More Than 50 Years after Its Discovery in SiO2 Octahedral Coordination Has Also Been Established in SiS2 at High Pressure.

SiO2 exhibits a high-pressure-high-temperature polymorphism, leading to an increase in silicon coordination number and density. However, for the related compound SiS2 such pressure-induced behavior has not been observed with tetrahedral coordination yet. All four crystal structures of SiS2 known so far contain silicon with tetrahedral coordination. In the orthorhombic, ambient-pressure phase these tetrahedra share edges and achieve only low space filling and density. Up to 4 GPa and 1473 K, three phases can be quenched as metastable phases from high-pressure high-temperature to ambient conditions. Space occupancy and density are increased first by edge and corner sharing and then by corner sharing alone. The structural situation of SiS2 up to the current study resembles that of SiO2 in 1960: Then, in its polymorphs only Si-O4 tetrahedra were known. But in 1961, a polymorph with rutile structure was discovered: octahedral Si-O6 coordination was established. Now, 50 years later, we report here on the transition from 4-fold to 6-fold coordination in SiS2, the sulfur analogue of silica.

Synthesis, Structure, and Properties of Al((R)bpy)3 Complexes (R = t-Bu, Me): Homoleptic Main-Group Tris-bipyridyl Compounds.

The neutral homoleptic tris-bpy aluminum complexes Al((R)bpy)3, where R = tBu (1) or Me (2), have been synthesized from reactions between AlX precursors (X = Cl, Br) and neutral (R)bpy ligands through an aluminum disproportion process. The crystalline compounds have been characterized by single-crystal X-ray diffraction, electrochemical experiments, EPR, magnetic susceptibility, and density functional theory (DFT) studies. The collective data show that 1 and 2 contain Al(3+) metal centers coordinated by three bipyridine (bpy(•))(1-) monoanion radicals. Electrochemical studies show that six redox states are accessible from the neutral complexes, three oxidative and three reductive, that involve oxidation or reduction of the coordinated bpy ligands to give neutral (R)bpy or (R)bpy(2-) dianions, respectively. Magnetic susceptibility measurements (4-300 K) coupled with DFT studies show strong antiferromagnetic coupling of the three unpaired electrons located on the (R)bpy ligands to give S = (1)/2 ground states with low lying S = (3)/2 excited states that are populated above 110 K (1) and 80 K (2) in the solid-state. Complex 2 shows weak 3D magnetic interactions at 19 K, which is not observed in 1 or the related [Al(bpy)3] complex.

Low oxidation state aluminum-containing cluster anions: Cp(∗)AlnH(-), n = 1-3.

Three new, low oxidation state, aluminum-containing cluster anions, Cp*AlnH(-), n = 1-3, were prepared via reactions between aluminum hydride cluster anions, AlnHm (-), and Cp*H ligands. These were characterized by mass spectrometry, anion photoelectron spectroscopy, and density functional theory based calculations. Agreement between the experimentally and theoretically determined vertical detachment energies and adiabatic detachment energies validated the computed geometrical structures. Reactions between aluminum hydride cluster anions and ligands provide a new avenue for discovering low oxidation state, ligated aluminum clusters.

Two high-pressure phases of SiS2 as missing links between the extremes of only edge-sharing and only corner-sharing tetrahedra.

The ambient pressure phase of silicon disulfide (NP-SiS2), published in 1935, is orthorhombic and contains chains of distorted, edge-sharing SiS4 tetrahedra. The first high pressure phase, HP3-SiS2, published in 1965 and quenchable to ambient conditions, is tetragonal and contains distorted corner-sharing SiS4 tetrahedra. Here, we report on the crystal structures of two monoclinic phases, HP1-SiS2 and HP2-SiS2, which can be considered as missing links between the orthorhombic and the tetragonal phase. Both monoclinic phases contain edge- as well as corner-sharing SiS4 tetrahedra. With increasing pressure, the volume contraction (-ΔV/V) and the density, compared to the orthorhombic NP-phase, increase from only edge-sharing tetrahedra to only corner-sharing tetrahedra. The lattice and the positional parameters of NP-SiS2, HP1-SiS2, HP2-SiS2, and HP3-SiS2 were derived in good agreement with the experimental data from group-subgroup relationships with the CaF2 structure as aristotype. In addition, the Raman spectra of SiS2 show that the most intense bands of the new phases HP1-SiS2 and HP2-SiS2 (408 and 404 cm(-1), respectively) lie between those of NP-SiS2 (434 cm(-1)) and HP3-SiS2 (324 cm(-1)). Density functional theory (DFT) calculations confirm these observations.

The reaction rates of O2 with closed-shell and open-shell Al(x)⁻ and Ga(x)⁻ clusters under single-collision conditions: experimental and theoretical investigations toward a generally valid model for the hindered reactions of O2 with metal atom clusters.

In order to characterize the oxidation of metallic surfaces, the reactions of O2 with a number of Al(x)(-) and, for the first time, Ga(x)(-) clusters as molecular models have been investigated, and the results are presented here for x = 9-14. The rate coefficients were determined with FT-ICR mass spectrometry under single-collision conditions at O2 pressures of ~10(-8) mbar. In this way, the qualitatively known differences in the reactivities of the even- and odd-numbered clusters toward O2 could be quantified experimentally. To obtain information about the elementary steps, we additionally performed density functional theory calculations. The results show that for both even- and odd-numbered clusters the formation of the most stable dioxide species, [M(x)O2](-), proceeds via the less stable peroxo species, [M(x)(+)···O2(2-)](-), which contains M-O-O-M moieties. We conclude that the formation of these peroxo intermediates may be a reason for the decreased reactivity of the metal clusters toward O2. This could be one of the main reasons why O2 reactions with metal surfaces proceed more slowly than Cl2 reactions with such surfaces, even though O2 reactions with both Al metal and Al clusters are more exothermic than are reactions of Cl2 with them. Furthermore, our results indicate that the spin-forbidden reactions of (3)O2 with closed-shell clusters and the spin-allowed reactions with open-shell clusters to give singlet [M(x)(+)···O2(2-)](-) are the root cause for the observed even/odd differences in reactivity.

Aluminum Zintl anion moieties within sodium aluminum clusters.

Through a synergetic combination of anion photoelectron spectroscopy and density functional theory based calculations, we have established that aluminum moieties within selected sodium-aluminum clusters are Zintl anions. Sodium-aluminum cluster anions, Na(m)Al(n)(-), were generated in a pulsed arc discharge source. After mass selection, their photoelectron spectra were measured by a magnetic bottle, electron energy analyzer. Calculations on a select sub-set of stoichiometries provided geometric structures and full charge analyses for both cluster anions and their neutral cluster counterparts, as well as photodetachment transition energies (stick spectra), and fragment molecular orbital based correlation diagrams.

Nanoscale molecular silver cluster compounds in gram quantities.

All that's nano is not gold: Definite, crystalline molecular compounds of noble metals have been known for gold for the last few years. Now a simple synthesis of a nanoscale silver cluster compound containing [Ag44 (S-C6 H4 COOH)30 ](4-) ions has been found leading to gram scales quantities. Synthetic, structural, and bonding aspects of this novel compound are discussed in the context of metalloid clusters in general.

Photoelectron spectroscopy of the aluminum hydride anions: AlH2(-), AlH3(-), Al2H6(-), Al3H9(-), and Al4H12(-).

We report measurements of the negative ion photoelectron spectra of the simple aluminum hydride anions: AlH2(-), AlH3(-), Al2H6(-), Al3H9(-), and Al4H12(-). From these spectra, we measured the vertical detachment energies of the anions, and we estimated the electron affinities of their neutral counterparts. Our results for AlH2(-), AlH3(-), and Al2H6(-) were also compared with previous predictions by theory.

Monomeric group 13 metal(I) amides: enforcing one-coordination through extreme ligand steric bulk.

Reactions of the extremely bulky amido alkali metal complexes, [KL'(η(6)-toluene)], or in situ generated [LiL'] or [LiL″] {L'/ L″ = N(Ar*)(SiR(3)), where Ar* = C(6)H(2){C(H)Ph(2)}(2)Me-2,6,4 and R = Me (L') or Ph (L″)} with group 13 metal(I) halides have yielded a series of monomeric metal(I) amide complexes, [ML'] (M = Ga, In, or Tl) and [ML″] (M = Ga or Tl), all but one of which have been crystallographically characterized. The results of the crystallographic studies, in combination with computational analyses, reveal that the metal centers in these compounds are one coordinate and do not exhibit any significant intra- or intermolecular interactions, other than their N-M linkages. One of the complexes, [InL'], represents the first example of a one-coordinate indium(I) amide. Attempts to extend this study to the preparation of the analogous aluminum(I) amide, [AlL'], were not successful. Despite this, a range of novel and potentially synthetically useful aluminum(III) halide and hydride complexes were prepared en route to [AlL'], the majority of which were crystallographically characterized. These include the alkali metal aluminate complexes, [L'AlH(2)(µ-H)Li(OEt(2))(2)(THF)] and [{L'Al(µ-H)(3)K}(2)], the neutral amido-aluminum hydride complex, [{L'AlH(µ-H)}(2)], and the aluminum halide complexes, [L'AlBr(2)(THF)] and [L'AlI(2)]. Reaction of the latter two systems with a variety of reducing agents led only to intractable product mixtures.

Hunting for the magnesium(I) species: formation, structure, and reactivity of some donor-free Grignard compounds.

Magnesium bromide radicals have to be prepared as high-temperature molecules and trapped as a metastable solution because a seemingly simple reduction of donor-free Grignard compounds failed. However, the essential role of magnesium(I) species during the formation of Grignard compounds could be demonstrated experimentally.

The formal combination of three singlet biradicaloid entities to a singlet hexaradicaloid metalloid Ge14[Si(SiMe3)3]5[Li(THF)2]3 cluster.

The reaction of GeBr with LiSi(SiMe(3))(3) leads to the metalloid cluster compound [(THF)(2)Li](3)Ge(14)[Si(SiMe(3))(3)](5) (1). After the introduction of a first cluster of this type, in which 14 germanium atoms form an empty polyhedron, [(THF)(2)Li](3)Ge(14)[Ge(SiMe(3))(3)](5) (2), we present here further investigations on 1 to obtain preliminary insight into its chemical and bonding properties. The molecular structure of 1 is determined via X-ray crystal structure solution using synchrotron radiation. The electronic structure of the Ge(14) polyhedron is further examined by quantum chemical calculations, which indicate that three singlet biradicaloid entities formally combine to yield the singlet hexaradicaloid character of 1. Moreover, the initial reactions of 1 after elimination of the [Li(THF)(2)](+) groups by chelating ligands (e.g., TMEDA or 12-crown-4) are presented. Collision induced dissociation experiments in the gas phase, employing FT-ICR mass spectrometry, lead to the elimination of the singlet biradicaloid Ge(5)H(2)[Si(SiMe(3))(3)](2) cluster. The unique multiradicaloid bonding character of the metalloid cluster 1 might be used as a model for reactions and properties in the field of surface science and nanotechnology.

Synthesis of a pentasilapropellane. Exploring the nature of a stretched silicon-silicon bond in a nonclassical molecule.

We report on the successful synthesis of Si(5)Mes(6) (Mes = 2,4,6-trimethylphenyl), which consists of an archetypal [1.1.1] cluster core featuring two ligand-free, "inverted tetrahedral" bridgehead silicon atoms. The separation between the bridgehead Si atoms is much longer, and the bond strength much weaker, than usually observed for a regular Si-Si single bond. A detailed analysis of the electronic characteristics of Si(5)Mes(6) reveals a low-lying excited triplet state, indicative of some biradical(oid) character. Reactivity studies provide evidence for both closed-shell and radical-type reactivity, confirming the unusual nature of the stretched silicon-silicon bond in this "nonclassical" molecule.

Ga(8)Br(8).6NEt(3): formation and structure of donor-stabilized GaBr. A nanoscaled step on the way to beta-gallium?

Because of their thermodynamic instability, their sophisticated formation, and their high reactivity, only three textbook examples of Al(I)/Ga(I) subhalides have been crystallized so far: Al(4)Br(4), Al(4)I(4), and Ga(8)I(8). Here, we present the formation and structural characterization of molecular Ga(8)Br(8) species. The different structures of Ga(8)I(8) and Ga(8)Br(8) are discussed with regard to their different formation conditions and their different thermodynamic stability based on results from DFT calculations. Structural as well as thermodynamic properties of Ga(8)I(8) and Ga(8)Br(8) are strongly related to the low-temperature modifications beta-Ga and gamma-Ga. Therefore, our fruitful hypothesis about the fundamental relation between structure and energy of a number of metalloid clusters and the corresponding element modifications is now supported by two binary Ga(I)-halide compounds.

Al4(P(t)Bu2)6--a derivative of Al4H6--and other Al4 species: a challenge for bonding interpretation between Zintl ions and metalloid clusters.

The highly energetic molecule Al(4)H(6), with its distorted tetrahedral structure, was recently characterized via mass spectrometry and photoelectron spectroscopy investigations (Li, X.; et al. Science 2007, 315, 356). Here we present the preparation and structural investigation of the first analogous Al(4)R(6) cluster compound. In order to understand the bonding in this kind of Al(4) molecule, density functional theory and second-order Møller-Plesset perturbation theory calculations were performed. The results obtained are discussed in comparison with bonding in other Al(4) moieties, especially the aromatic bonding behavior in the dianionic planar Al(4)(2-) species (Li, X.; et al. Science 2001, 291, 859). Finally, on the basis of the results obtained for Al(4) species, a more general problem is discussed: the difference in bonding between Zintl ions and metalloid clusters.

[{(eta(5)-C(5)Me(5))(2)Sm}(4)P(8)]: a molecular polyphosphide of the rare-earth elements.

[{(eta(5)-C(5)Me(5))(2)Sm}(4)P(8)], a molecular polyphosphide of the rare-earth elements having a realgar core structure, was synthesized by a one-electron redox reaction of divalent samarocen and white phosphorus.

Metastable aluminum(I) compounds: experimental and quantum chemical investigations on aluminum(I) phosphanides--an alternative channel to the disproportionation reaction?

The well known thermodynamic instability of Al and Ga monohalides is caused by the favored disproportionation process to the bulk metal and the trihalides. During this highly complex process, a number of metalloid clusters that are intermediates on the way to the metal have been trapped. Therefore, all observations in the field of metalloid Al/Ga clusters have been traced to this favored disproportionation process. The failure to form phosphanide-substituted Al clusters, in contrast to the generation of similar Ga clusters and analogous Al amide clusters, was the starting point of this contribution. For aluminum(I) phosphanides, there exists a different decomposition route in which the salt-like bulk material AlP and not Al metal is the final product. The synthesis of two molecular "AlP" intermediate species, together with supporting DFT calculations, provide plausible arguments for this decomposition route, which is thermodynamically favored for many AlR/GaR species and which, surprisingly, has not been discussed before.

On the kinetics of the Al(13) (-)+Cl(2) reaction: Cluster degradation in consecutive steps.

The kinetics of the reaction system initiated by the Al(13) (-)+Cl(2) reaction was experimentally studied in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The Al(13) (-) clusters were produced by laser desorption/ionization of LiAlH(4), then transferred into the ICR cell, cooled by collisions with Ar, and exposed to an excess of Cl(2) with a concentration of approximately 10(8) cm(-3). Relative concentration-time profiles of Al(n) (-) clusters with n=13, 11, 9, and 7 as well as profiles of Cl(-) ions have been recorded. Other ionic species, besides traces of Al(12)Cl(-), were not found, which indicates a double-step degradation mechanism via the odd-numbered Al(n) (-) clusters. From a kinetic analysis of the experimental results, a rate coefficient of (5+/-2)x10(-10) cm(3) s(-1) for the Al(13) (-)+Cl(2) reaction was obtained. Furthermore, it is inferred from a simultaneous fit of all concentration-time profiles that the Al(n) (-)+Cl(2) reactions for n=13, 11, 9, and 7 occur with rate coefficients near the Langevin limit in the range k(bim) approximately (5+/-4)x10(-10) cm(3) s(-1). The branching ratios between the Al(n-2) (-)-producing and Cl(-)-producing channels of a given cluster Al(n)Cl(2) (-) indicate an increasing contribution of the Cl(-)-producing channels with decreasing cluster size. Statistical rate theory calculations on the basis of molecular data from quantum chemical calculations show that the experimental Al(n) (-) profiles are compatible with a sequence of association-elimination reactions proceeding via the formation of highly excited Al(n)Cl(2) (-) adducts followed by a sequential elimination of two AlCl molecules. Rate coefficients for these reactions were calculated, and the production of Cl(-) was shown probably not to proceed via these Al(n)Cl(2) (-) intermediates.

Reactivity of aluminum cluster anions with ammonia: selective etching of Al11(-) and Al12(-).

Reactivity of aluminum cluster anions toward ammonia was studied via mass spectrometry. Highly selective etching of Al(11)(-) and Al(12)(-) was observed at low concentrations of ammonia. However, at sufficiently high concentrations of ammonia, all other sizes of aluminum cluster anions, except for Al(13)(-), were also observed to deplete. The disappearance of Al(11)(-) and Al(12)(-) was accompanied by concurrent production of Al(11)NH(3)(-) and Al(12)NH(3)(-) species, respectively. Theoretical simulations of the photoelectron spectrum of Al(11)NH(3)(-) showed conclusively that its ammonia moiety is chemisorbed without dissociation, although in the case of Al(12)NH(3)(-), dissociation of the ammonia moiety could not be excluded. Moreover, since differences in calculated Al(n)(-) + NH(3) (n=9-12) reaction energies were not able to explain the observed selective etching of Al(11)(-) and Al(12)(-), we concluded that thermodynamics plays only a minor role in determining the observed reactivity pattern, and that kinetics is the more influential factor. In particular, the conversion from the physisorbed Al(n)(-)(NH(3)) to chemisorbed Al(n)NH(3)(-) species is proposed as the likely rate-limiting step.