Evidence for d-orbital aromaticity in Sn- and Pb-based clusters: is Sn12(2-) aromatic?

The electronic structures and stabilities of pure M(12)- and M(12)(2-) were systematically investigated within density functional theory. The nucleus-independent chemical shifts (NICSs) of I(h) Sn(12)(2-) and Pb(12)(2-) are -5.0 and -20.7 ppm, respectively, based on B3LYP/aug-cc-pVDZ-PP predictions, whereas the NICS of Sn(12)(2-) is predicted to be 1.1 ppm by B3LYP/LanL2DZ. A startling conclusion is that the NICS4d of Sn(12)(2-) and NICS(5d) of Pb(12)(2-) are -5.0 and -7.5 ppm, respectively, suggesting the significant contribution of the inner d orbitals to the total NICS values. This provides the first quantitative evidence for the existence of "d-orbital aromaticity" in Sn- and Pb-based clusters with three-dimensional structures. The d orbitals also contribute to the total NICSs of the K-coordinated clusters. The NICS predictions suggest that larger basis sets including d-orbitals are needed to analyze the aromaticity of some main-group-metal-based clusters (e.g., Sn- and Pb-based clusters) to obtain accurate predictions.     

X-ray crystallography and biological metal centers: is seeing believing?

Metalloenzyme crystal structures have a major impact on our understanding of biological metal centers. They are often the starting point for mechanistic and computational studies and inspire synthetic modeling chemistry. The strengths and limitations of X-ray crystallography in determining properties of biological metal centers and their corresponding ligand spheres are explored through examples, including ribonucleotide reductase R2 and particulate methane monooxygenase. Protein crystal structures locate metal ions within a protein fold and reveal the identities and coordination geometries of amino acid ligands. Data collection strategies that exploit the anomalous scattering effect of metal ions can establish metal ion identity. The quality of crystallographic data, particularly the resolution, determines the level of detail that can be extracted from a protein crystal structure. Complementary spectroscopic techniques can provide crucial information regarding the redox state of the metal center as well as the presence, type, and protonation state of exogenous ligands. The final result of the crystallographic characterization of a metalloenzyme is a model based on crystallographic data, supported by information from biophysical and modeling studies, influenced by sample handling, and interpreted carefully by the crystallographer.     

“Blue silver”: transformation of clusters and metal coagulation

The formation and transformations of blue silver ( _max 700 nm) during -irradiation of a weakly alkaline (pH 9) aqueous solution containing AgClO₄, polyacrylic acid (PAA), and isopropanol were studied. We believe that blue silver is a linear silver cluster stabilized on a polymeric molecule. During radiation-chemical reduction the cluster is transformed into new clusters ( _max = 365 and 460 nm). When all of the Ag⁺ ions present in the solution have been reduced, clusters coalesce and a new phase,i.e., colloidal silver particles, forms. The mechanism of the radiation-chemical transformations is discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 35–37, January, 1995.     

Which electron count rules are needed for four-center three-dimensional aromaticity?

A series of charged and neutral four-center n-electron (4c-ne, n = 1-4) molecules based on the adamantane framework, but which include combinations of boron, nitrogen, and phosphorus atoms at bridgehead positions, were studied computationally at the B3LYP/6-31G* level of density functional theory (DFT). The three-dimensional aromaticity, observed earlier for the 1,3,5,7-bisdehydroadamantane dication (1), is found to be general for 4c-2e electron systems. The degree of electron delocalization, evaluated by energetic, geometric, and various magnetic criteria, is quite independent of the molecular symmetry (point groups vary from Td to Cs), the degeneracy of the orbitals, the molecular charges, and the nature of the atoms participating in the delocalized bonding. Although the multiple positive (e.g., in 1 and some of the heteroatom systems) and multiple negative charges are strongly repulsive, the rigid adamantane frameworks help hold the bridgehead atoms within bonding distances with the fewer available electrons. The corresponding 4c-1e doublets are approximately half as aromatic as the 4c-2e singlets based on the same criteria. However, the three-electron systems may either adopt distorted but still four-center delocalized structures, or alternative 3c-2e two-dimensional arrangements in which the fourth bridgehead atom is more distant. There is no need to derive special rules for each point group for 4c-ne systems. Although the three-dimensional stabilization is computed to be quite appreciable, ranging between 10 and 50 kcalmol(-1), this delocalization energy is generally not sufficient to overcome distortion due to strain in higher homologues of 1 and in analogous noncage systems. Among the various 4c-2e homoadamantanedehydro dications studied, only the 1,8-dehydrohomoadamandiyl-3,6-dication forms a three-dimensional aromatic system.     

Can an electron-shell closing model explain the structure and stability of ligand-stabilized metal clusters?

We investigated the structure and stability of several aluminum hydride complexes to understand the essence of "superatom chemistry" and to gain a right perspective on the ligand (L)-stabilized metal (M) clusters. We successfully interpret the structure and stability using molecular orbital analysis, which clearly shows the failure of an electron-shell closing model (or a superatom model) to explain it. The structure and stability of Al(m)H(n) are closely associated with the molecular orbital stabilization owing to the effective orbital overlap between Al(m) (M(m)) and nH (nL). The importance of retaining the electronic structural integrity of M(m) in M(m)L(n)-within an electron-shell closing model-has been underestimated or even disregarded, and this has created the current controversies in the scientific community.     

Is nucleus-independent chemical shift scan a reliable aromaticity index for planar and neutral A2B2 clusters?

Nucleus-independent chemical shift (NICS) scan has been employed in order to explain the aromaticity in eleven planar, neutral and tetrameric mixed clusters of trans-A₂B₂ type. The results suggest that for seven A₂B₂ clusters, although very high negative NICS values are obtained while performing the NICS scan, yet the typical minimum is not present in the scanning curve. The NICS scanning procedure thus fails to account for the aromatic behavior in those four-membered A₂B₂ clusters. Amongst them, interestingly it has been observed that for four such clusters the scan shape looks like the mirror image of a typical scan of an antiaromatic system. Based on the NICS scan, at the most, those four clusters can be said to possess nonaromatic behavior. In case of the remaining four clusters, the NICS scan shape passes through a distinct minimum and successfully explains their aromatic behavior. Hence, the overall investigation indicates that NICS scan alone cannot be used as a reliable aromaticity index for planar and neutral A₂B₂ clusters.     

Which NICS aromaticity index for planar pi rings is best?

[STRUCTURE: SEE TEXT] Five increasingly sophisticated aromaticity indexes, based on nucleus-independent chemical shifts (NICS), were evaluated against a uniform set of aromatic stabilization energies (ASE) for 75 mono- and polyheterocyclic five-membered rings. While acceptable statistical correlations were given by all of the NICS methods, the most fundamentally grounded index, NICS(0)pizz (based on the pi contribution to the out-of-plane zz tensor component), performed best statistically (cc=0.980) and in practice. The easily computable NICS(1)zz index is a useful alternative (cc=0.968).     

Preparation and characterization of two three-dimensional metal–organic photoluminescent supramolecular networks

Two novel photoluminescent coordination compounds of the formula [Cd(atpt)phen(H₂O)] · H₂O (1) and [Zn₂(atpt)₂(bipy)₂(H₂O)₂] (2) (H₂atpt = 2-aminoterephthalic acid, bipy = 2,2′-bipyridine and phen = 1,10-phenanthroline) were synthesized through the self-assembly of H₂atpt and N-containing ligands (bipy for 1 and phen for 2) with metal(II) ions in the presence of NaOH, and were characterized by FT-IR spectroscopy, thermogravimetric analysis (TG), X-ray analysis and photoluminescence spectra in the solid state. Compound 1 is the first structurally characterized Cd(II) complex with the atpt ligand. The coordination mode of the atpt ligand in 2 is novel and is first reported in this presentation. X-ray crystallographic studies reveal that compound 1 shows a 1D architecture. Compound 1 further assembles into a 3D supramolecular network via interchain hydrogen bonds and π–π stacking interactions. Compound 2 exhibits a binuclear structure with intramolecular π–π stacking interactions, which is further extended into a 3D supramolecular framework via intermolecular hydrogen bonds and C–H?π interactions. Compounds 1 and 2 exhibit green photoluminescence in the solid state at room temperature.     

Nanotubes: number of Kekulé structures and aromaticity

Carbon nanotubes (CNTs) are composed of cylindrical graphite sheets consisting of sp(2) carbons. Due to their structure CNTs are considered to be aromatic systems. In this work the number of Kekulé structures (K) in "armchair" CNTs was estimated by using the transfer matrix technique. All Kekulé structures of the cyclic variants of naphthalene and benzo[c]phenanthrene have been generated and the basic patterns have been obtained. From this information the elements of the transfer matrix were derived. The results obtained indicate that K (and the resonance energy) is greater if tubulenes are extended in the vertical than in the horizontal direction. Tubulenes are therefore more stabile than cyclic strips. An illustration, obtained by using scanning probe microscope, has been attached to affirm the existence of thin CNTs.     

How many metal atoms are needed to dehydrogenate an ethylene molecule on metal clusters?: Correlation between reactivity and electronic structures of Fen+, Con+, and Nin+

The absolute cross section for dehydrogenation of an ethylene molecule on Mn+ [Fen+ (n = 2-28), Con+ (n = 8-29), and Nin+ (n = 3-30)] was measured as a function of the cluster size n in a gas-beam geometry at a collision energy of 0.4 eV in the center-of-mass frame in an apparatus equipped with a tandem-type mass spectrometer. It is found that (1) the dehydrogenation cross section increases rapidly above a cluster size of approximately 18 on Fen+, approximately 13 and approximately 18 on Con+, and approximately 10 on Nin+ and (2) the rapid increase of the cross section for Mn+ occurs at a cluster size where the 3d electrons start to contribute to the highest occupied levels of Mn+. These findings lead us to conclude that the 3d electrons of Mn+ play a central role in the dehydrogenation on Mn+.     

Magnetic criteria of aromaticity in a benzene cation and anion: how does the Jahn–Teller effect influence the aromaticity?

The aromatic/antiaromatic behavior of the Jahn–Teller (JT) active benzene cation and anion has been investigated using Density Functional Theory (DFT) calculations of Nuclear Independent Chemical Shifts (NICS) and magnetic susceptibility. NICS parameters have been scanned along the Intrinsic Distortion Path (IDP) for the benzene cation showing antiaromaticity which decreases with increasing deviation from D_6h to D_2h symmetry. Changes in NICS values along the IDP from D_6h to C_2v in the benzene anion revealed non-aromatic character.     

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.     

Detaching thiolates from copper and gold clusters: which bonds to break?

The interaction of alkanethiolates with small coinage metal clusters of copper and gold was studied based on density functional theory with a focus on the metal-thiolate junction. Calculation of fragmentation energies indicate that for Cu(n)-thiolate (n = 1,3,5,7, and 9) there is a progressive lowering in energy for the fragmentation of the S-C bond in the thiolate from a value of 2.9 eV for n = 1 to 1.4 eV for n = 9. The detailed electronic origins of this specific weakening are attributed to a polarization of electron density in the S-C bond as induced by bonding with the Cu(n) cluster. For the gold analogues, this effect is not observed and fragmentation at the S-C bond experiences only a slight 10% destabilization as n increases from 3 to 9. The relativistic origin of this difference between Cu and Au is discussed, and an analysis of bonding considerations is presented.     

Manganese carbonyl fluorides: are they viable molecules?

The mononuclear Mn(CO)(5)X and binuclear Mn(2)(CO)(8)(μ-X)(2) manganese carbonyl halides have long been known for the halogens Cl, Br, and I. However, the corresponding manganese carbonyl fluorides (X = F) remain unknown. The structures and thermochemistry of such manganese carbonyl fluorides and their decarbonylation products have now been investigated using density functional theory. In all cases singlet structures were found to have lower energies than the corresponding triplet structures. The expected octahedral structure is predicted for Mn(CO)(5)F. Decarbonylation of Mn(CO)(5)F is predicted to give trigonal bipyramidal Mn(CO)(4)F with equatorial fluorine. Further, decarbonylation gives tetrahedral Mn(CO)(3)F. All of the binuclear Mn(2)(CO)(n)F(2) structures (n = 8, 7, 6) are predicted to have a central Mn(2)F(2) unit with two bridging F atoms, a non-bonding Mn···Mn distance of ~3.1 ?, and exclusively terminal CO groups. The thermochemistry of these manganese carbonyl fluorides indicates that they are viable species. This suggests that the failure to date to synthesize the simple manganese carbonyl fluorides arises from a lack of a suitable synthetic method rather than from the instability of the desired products.     

Cycloaromatization of 1,4-pentadiynes: a viable possibility?

[structure: see text] The effects of several mostly sigma-withdrawing, pi-donating substituents X on the hitherto unknown Bergman-like cyclizations of 3-substituted 1,4-pentadiynes were studied at the BLYP/6-311+G//BLYP/6-31G level of theory. As the cyclization with X = OH(+) has the lowest barrier and is about thermoneutral, we predict that the title reaction is viable, for instance, through activation of derivatives with X = O with Lewis acids.     

Can we understand the different coordinations and structures of closed‐shell metal cation‐water clusters?

We present a model potential for studying M(q+)(H(2)O)(n=1,9) clusters where M stands for either Na(+), Cs(+), Ca(2+), Ba(2+), or La(3+). The potential energy surfaces (PES) are explored by the Monte Carlo growth method. The results for the most significant equilibrium structures of the PES as well as for energetics are favorably compared to the best ab initio calculations found in the literature and to experimental results. Most of these complexes have a different coordination number in cluster compared to experimental results in solution or solid phase. An interpretation of the coordination number in clusters is given. In order to well describe the transition between the first hydration sphere and the second one we show that an autocoherent treatment of the electric field is necessary to correctly deal with polarization effects. We also explore the influence of the cation properties (charge, size, and polarizability) on both structures and coordination number in clusters, as well as the meaning of the second hydration sphere. Such an approach shows that the leading term in the interaction energy for a molecule in the second hydration sphere is an electrostatic attraction to the cation and not a hydrogen bond with the water molecules in the first hydration sphere.     

What kinds of three-dimensional nets are possible with tris-chelated metal complexes as building blocks?

Tris-chelated metal complexes with octahedral geometry are sometimes used as building blocks for "self assembly" and "crystal engineering". These versatile building blocks easily form honey-comb type 2D nets. However, in this Perspective we discuss the different types of 3D nets that can be formed with these starting materials using Wells classification, and concentrate on the (10,3) nets. We show that several of these, and not only the (10,3)-a net, are possible by analysing the geometrical requirements of each net. We note that each possible net implies a specific assembly order of Delta or Lambda chirality of the building blocks.