Gabriel Aullón

Dihydrogen contacts in alkanes are subtle but not faint

Alkane molecules are held together in the crystal state by purportedly weak homonuclear R–H···H–R dihydrogen interactions. In an apparent contradiction, the high melting points and vaporization enthalpies of polyhedranes in condensed phases require quite strong intermolecular interactions. Two questions arise: ‘How strong can a weak C–H···H–C bond be?’ and ‘How do the size and topology of the carbon skeleton affect these bonding interactions?’ A systematic computational study of intermolecular interactions in dimers of n-alkanes and polyhedranes, such as tetrahedrane, cubane, octahedrane or dodecahedrane, showed that attractive C–H···H–C interactions are stronger than usually thought. We identified factors that account for the strength of these interactions, including the tertiary nature of the carbon atoms and their low pyramidality. An alkane with a bowl shape was designed in the search for stronger dihydrogen intermolecular bonding, and a dissociation energy as high as 12 kJ mol−1 is predicted by our calculations. Intermolecular non-polar H···H interactions between polyhedrane molecules may be as attractive as classical hydrogen bonds. A theoretical study identifies the chemical and structural factors that favour such attractive interactions.

Edge‐Sharing Binuclear d8 Complexes with XR Bridges: Theoretical and Structural Database Study of their Molecular Conformation

Coplanar or bent? Binuclear complexes of d8 transition metal ions appear in a variety of molecular conformations in which the coordination planes around the two metal atoms can be coplanar or bent, and the substituents at the bridging atoms may adopt different orientations (see figure). Ab initio theoretical studies together with a structural database analysis provide a rationale for the experimentally observed structures.

Synthesis and study of trinuclear Pd(II) and Pt(II) complexes with 2-mercaptonicotinic acid: Crystal and molecular structure of [Pd3(mercaptonicotinic acid)3Cl3]

Pd(II) and Pt(II) complexes of 2-mercaptonicotinic acid of formulae (M (mercaptonicotinic acid) Cl ) were synthesized and 33 3 characterized by the usual spectroscopic techniques including mass spectrometry. The crystal structure was obtained for the palladium complex. The molecule has a ternary symmetry, and mercaptonicotinic acid coordinates in a bidentate (N,S) mode to each palladium ion. The sulfur atom acts as a bridge between two palladium atoms. The fourth coordination site in the plane square geometry of the Pd(II) is occupied by a chloride ion. Identical molecular structure is proposed for the platinum compound in agreement with the spectroscopic and theoretical results.

Through-ring bonding in edge-sharing dimers of square planar complexes

Abstract A qualitative molecular orbital study on the title compounds has allowed us to establish the factors which determine the possibility of through-ring bonding. Orbital interaction diagrams and the resulting relationship between framework electron count (FEC) and through-ring bonding are analyzed for XR 2 bridging groups, as well as for XR 3 and other isolobal bridges. The influence of the electronegativity of the bridging atom X, and of the bulk of the terminal ligands has also been studied. The predictions made from the qualitative bonding model developed are consistent with existing structural data.

Ligand orientation effects on metal–metal, ligand–ligand and metal–ligand interactions

Abstract A number of chemical and physical properties of transition metal complexes can be explained by fundamental chemical criteria such as bond order, electron configuration, ligand or metal electronegativity, or the idealized coordination polyhedron. However, variations in the properties of similar compounds are associated with subtle differences in the spatial arrangement of the ligands, as revealed by combined structural database and molecular orbital studies. Combined theoretical and structural database studies show how ligand orientation affects the weak magnetic exchange coupling interactions in ligand-bridged binuclear complexes. Metal–ligand and ligand–ligand bonds in M 2 X 2 cores of double-bridged binuclear complexes are also sensitive to the relative position of the ligands as seen, among other cases, in the relative stabilities of the oxo and peroxo isomers of the synthetic analogues of oxy-hemocyanin.

Regioselective Orthopalladation of (Z)-2-Aryl-4-Arylidene-5(4H)-Oxazolones: Scope, Kinetico-Mechanistic, and Density Functional Theory Studies of the C–H Bond Activation

Orthopalladated complexes derived from (Z)-2-aryl-4-arylidene-5(4H)-oxazolones have been prepared by reaction of the oxazolone with palladium acetate in acidic medium. The reaction is regioselective, only the ortho C-H bond of the arylidene ring being activated, producing a six-membered ring. The scope and reaction conditions of the orthopalladation are dependent on the acidity of the solvent. In CF(3)CO(2)H a large number of oxazolones can be metalated under mild conditions. As acidity decreases a lesser number of oxazolones can be efficiently palladated and harsher conditions must be used to achieve similar yields. The C-H bond activation in acidic medium agrees with an ambiphilic mechanism, as determined from kinetic measurements at variable temperature and pressure for different oxazolones substituted at the arylidene ring. The mechanism has been confirmed by density functional theory (DFT) calculations, where the formation of the six-membered ring is shown to be favored from both a kinetic and a thermodynamic perspective. In addition, the dependence of the reaction rate on the acidity of the medium has also been accounted for via a fine-tuning between the C-H agostic precoordination and the proton abstraction reaction in the overall process occurring on coordinatively saturated [Pd(κ(N)-oxazolone)(RCO(2)H)(3)](2+).

Organometallic gold complexes of carborane. Theoretical comparative analysis of ortho, meta, and para derivatives and luminescence studies

The synthesis and X-ray analysis of complexes [(micro-1,12-C2B10H10){Au(PPh3)}2] and [(micro-1,2-C2B10H10){Au(PMe3)}2] have provided the experimental data needed to analyse two points. The first point is the use of these data to carry out a computational study with the aim of comparing the electronic structures and relative stabilities of the organometallic isomers [(micro-1,n-C2B10H10){Au(PR3)}2] (n=2, 7, 12; R=Ph, Me) with those of the parent carborane clusters ortho-, meta- and para-carborane and the influence of the monophosphine substituents. The second point is focused in the influence of the steric demand of the monophosphine in the presence or not of aurophilic interactions in the ortho derivatives [(micro-1,2-C2B10H10){Au(PR3)}2] (R=Me, Ph). The photoluminescent behaviour of both the carboranes and the organometallic complexes is presented.

The Structural Diversity Triggered by Intermolecular Interactions between AuIS2 Groups: Aurophilia and Beyond

The present study is aimed at elucidating the factors that direct the assembly of a specific family of Au(I) species. The assembly of Au(I) centers and dithiocarboxylato or xanthato ligands results in a surprising structural diversity observed by single-crystal X-ray diffraction. However, in solution, just evidences for discrete bimetallic [Au(2)L(2)] species have been observed. Interestingly, when dithiocarboxylato ligands have been used, a reversible supramolecular assembly has been observed forming the supramolecules of formulae [Au(2)L(2)](2) and [Au(2)L(2)](3). Initial studies on luminescent properties have been carried out at variable temperature. All the compounds show red emissions in the solid state at very similar energies, suggesting that the intramolecular interactions play a more relevant role in the luminescent properties than the intermolecular ones. The computational studies indicate that not only Au···Au interactions, but also Au···S and S···S ones play a role in the structure and energetic of the supramolecular species, as well as for the choice between supramolecular association or intramolecular oligomerization.

Theoretical Clues to the Mechanism of Dioxygen Formation at the Oxygen‐Evolving Complex of Photosystem II

The mechanism of the generation of dioxygen at the oxygen-evolving complex (OEC) of photosystem II (PSII), a crucial step in photosynthesis, is still under debate. The simplest unit present in the OEC that can produce O2 is a dinuclear oxo-bridge manganese complex within the tetranuclear Mn4 cluster. In this paper we report a theoretical study of the model complexes [Mn2(mu-O)2(NH3)6(H2O)2]n+ (n = 2-5), for which density functional calculations have been carried out for several electronic configurations. The molecular orbital picture deduced from the calculations indicates that one-electron oxidation of the Mn2IV,IV/(O2-)2 complex (n = 4) mostly affects the oxygen atoms, thus ruling out the existence of a MnV oxidation state in this context, while the incipient formation of an O-O bond in the O2(3-) transient species evolves exothermally toward the dissociation of dioxygen and a Mn2II,III couple. These results identify the electronic features that could be needed to enable an intramolecular mechanism of oxygen-oxygen bond formation to exist at the OEC during photosynthesis.

Copper versus thioether-centered oxidation: mechanistic insights into the non-innocent redox behavior of tripodal benzimidazolylaminothioether ligands.

A series of Cu(+) complexes with ligands that feature varying numbers of benzimidazole/thioether donors and methylene or ethylene linkers between the central nitrogen atom and the thioether sulfur atoms have been spectroscopically and electrochemically characterized. Cyclic voltammetry measurements indicated that the highest Cu(2+)/Cu(+) redox potentials correspond to sulfur-rich coordination environments, with values decreasing as the thioether donors are replaced by nitrogen-donating benzimidazoles. Both Cu(2+) and Cu(+) complexes were studied by DFT. Their electronic properties were determined by analyzing their frontier orbitals, relative energies, and the contributions to the orbitals involved in redox processes, which revealed that the HOMOs of the more sulfur-rich copper complexes, particularly those with methylene linkers (-N-CH2-S-), show significant aromatic thioether character. Thus, the theoretically predicted initial oxidation at the sulfur atom of the methylene-bridged ligands agrees with the experimentally determined oxidation waves in the voltammograms of the NS3- and N2S2-type ligands as being ligand-based, as opposed to the copper-based processes of the ethylene-bridged Cu(+) complexes. The electrochemical and theoretical results are consistent with our previously reported mechanistic proposal for Cu(2+)-promoted oxidative C-S bond cleavage, which in this work resulted in the isolation and complete characterization (including by X-ray crystallography) of the decomposition products of two ligands employed, further supporting the novel reactivity pathway invoked. The combined results raise the possibility that the reactions of copper-thioether complexes in chemical and biochemical systems occur with redox participation of the sulfur atom.