Piero Macchi

Chemical bonding in transition metal carbonyl clusters: complementary analysis of theoretical and experimental electron densities

Abstract In the last few years, the quantum theory of atoms in molecules has become the paradigm for interpreting theoretical and experimental electron density distributions. Within this framework, the link between bonding modes and topological properties has been fully achieved for ‘light atom’ molecules. However, the derived correspondence rules cannot be extended straightforwardly to organometallic compounds since bonds to a transition metal display a different and much narrower spectrum of topological indexes. The complementary usage of theoretical computations on a set of prototype transition metal molecules and experimental determinations of the electron density in transition metal carbonyl clusters are discussed. Since these compounds are characterised by weakly bound metal cages and fluxional carbonyl ligands, the focus is on the nature of metal–metal and metal–carbonyl interactions as well as on the evolution of three-centre-four-electron M(μ-CO)M bonds along the (CO)M–M↔M(μ-CO)M↔M–M(CO) conversion path. The interpretation of the electron density distribution here proposed could be extended reasonably to a wider class of organometallic compounds.

Relativistic analytical wave functions and scattering factors for neutral atoms beyond Kr and for all chemically important ions up to I

Relativistic wave functions for elements with Z = 37-54 [Su & Coppens (1998). Acta Cryst. A54, 646-652] have been fitted with a linear combination of Slater-type functions as defined by Bunge, Barrientos & Bunge [At. Data Nucl. Data Tables (1993), 53, 113-162], for use in charge-density analysis and other applications. In addition, numerical relativistic wave functions have been calculated for all chemically relevant ions up to Z = 54, and corresponding analytical expressions have been derived. X-ray scattering factors calculated from the numerical wave functions are parameterized [in the sin(straight theta)/lambda ranges 0.0-2.0, 2.0-4.0 and 4.0-6.0 A(-1)] with six Gaussian functions, using the same method applied previously by Su & Coppens [Acta Cryst. (1997), A53, 749-762].

Mechanistic Study of the Palladium–Phenanthroline Catalyzed Carbonylation of Nitroarenes and Amines: Palladium–Carbonyl Intermediates and Bifunctional Effects

Palladium-phenanthroline complexes catalyze both the nitroarene carbonylation reaction and the amine oxidative carbonylation reaction to give, depending on the conditions, carbamates and ureas. There is evidence that the key step in both processes is the amine carbonylation. Here, we show that when the reaction is run in methanol key intermediate compounds have the general formula [Pd(RPhen)(COOMe)(2)] (1) (RPhen = 1,10-phenanthroline or one of its substituted derivatives). The kinetics of the reaction of 1 with toluidine in the presence of a carboxylic or phosphorus acid is first-order with respect to complex, acid, and toluidine. A CO atmosphere is also required for the reaction to proceed. Acid dimerization was shown not to be influential under the concentration conditions examined, but reaction between the acid and toluidine is not negligible and a correction has to be applied. Diphenylphosphinic acid is more effective than any carboxylic acid in promoting this reaction, as also observed under catalytic conditions. A series of equilibria and an irreversible acid-assisted proton transfer explain the observed data. Formation of an adduct between complexes of the kind 1 and CO was spectroscopically observed when RPhen = 2,9-Me(2)Phen. Several analogous complexes were also spectroscopically characterized and the X-ray structure of [Pd(2,9-Me(2)Phen)Cl(2)(CO)] was solved. This shows an asymmetric coordination of the nitrogen ligand. Kinetic measurements were also conducted under catalytic conditions. An Eyring plot shows that the effect of the acidic promoter is to decrease the DeltaS(double dagger) value, whereas no positive effect is observed on DeltaH(double dagger). A temperature-dependent correction for the reaction between the acid and aniline and phenanthroline present under the reaction conditions has to be applied. Comparison of the results obtained under stoichiometric and catalytic conditions strongly supports the view that 1 is involved even in the latter and that the acid is acting as a bifunctional promoter.

[Pt19(CO)21(NO)]3− and [Pt38(CO)44]2−: Nitrosyl Bending through Intramolecular Electron Transfer as an Intermediate Step in the Nucleation Process from Polydecker to ccp Platinum Carbonyl Clusters

The intermediacy of CO/NO substitution in the condensation of [Pt19(CO)22]4− into [Pt38(CO)44]2− (structure shown) has been demonstrated. Two high-nuclearity carbonyl metal clusters, including one with an unprecedented nitrosyl ligand, have been synthesized and structurally characterized.

Hydrogen migration in oxalic acid di-hydrate at high pressure?

Single crystal X-ray diffraction of oxalic acid dihydrate H(2)C(2)O(4).2H(2)O (1), at pressures up to 5.3 GPa, shows significant changes in the molecular geometries, probably accompanied by a proton migration from H(2)C(2)O(4) to H(2)O and formation of ionic species, as predicted by periodic DFT calculations.

Molecular Crystals Under High Pressure: Theoretical and Experimental Investigations of the Solid–Solid Phase Transitions in [Co2(CO)6(XPh3)2] (X=P, As)

An accurate analysis of the solid-state structural transformations occurring in species of the general formula [Co(2)(CO)(6)(XPh(3))(2)] (X = P, As) by experimental X-ray diffraction under non-ambient conditions (on variation of pressure or temperature) and by ab initio theoretical modelling is presented. After a crystal-to-crystal phase transition, the conformation of carbonyl ligands about the Co-Co bond changes from staggered to (almost) eclipsed. The analysis of high-pressure and low-temperature structures sheds light on the peculiar behaviour of the metal-metal bond, the most flexible in the molecule, which is initially compressed, then elongated due to increased intra-carbonyl repulsion and eventually compressed again. Theoretical calculations in the solid state allow the thermodynamic quantities of the transformation to be computed and prediction of the distortions produced by the external stress. The change in molecular symmetry is induced by the increase in the high internal energy associated with the staggered carbonyl conformation, which does not allow efficient packing if the crystalline volume is reduced. The eclipsed conformer, though much less stable in isolation, guarantees better entanglement between molecules.

Stability-inducing strain: application to the synthesis of alkyl-BIAN ligands (alkyl-BIAN = bis(alkyl)acenaphthenequinonediimine)

Ring strain is normally associated with increased reactivity and decreased stability of the strained molecule. However, we report here some examples in which the presence of a strained ring causes a stabilization of the molecule, allowing the isolation of some members of a class of otherwise unstable compounds. Alkyl-BIAN (alkyl-BIAN = bis(alkyl)acenaphthenequinonediimine) ligands have been elusive for 70 years. We have investigated the reason for earlier failures and identified it as an isomerization of the initially formed CN double bond. This isomerization is driven by a release of ring strain in the five-membered ring of the acenaphthene moiety. The use of amines in which the –NH2 group is bound to a quaternary carbon atom cannot be employed to avoid the isomerization because these amines are too sterically encumbered to react at all. However, the use of amines in which the amino group is bound to a strained ring solves the problem, because the isomerization would cause an even larger strain than the one that is released. Cyclopropylamine (Cypr-NH2) is the ideal amine, no isomerization being observed at all. Cyclobutylamine (Cybu-NH2) can also be employed, as well as amines in which the strain derives from the presence of a bi- o tri-cyclic system: 2-amino-exo-norbornane (Norb-NH2) and 2-aminoadamantane (Ad-NH2). The best synthetic procedure involves a transimination reaction from a [ZnCl2(Ar-BIAN)] complex, where Ar contains electron-withdrawing groups, but the direct synthesis from acenaphthenequinone and the amine is also possible in the case of Cypr-BIAN. The structure of [Pd(Cypr-BIAN)(η3-CH2C(CH3)CH2)][PF6], [ZnCl2(Cybu-BIAN)], [ZnCl2(Norb-BIAN)] and [NiBr2(Ad-BIAN)], has been determined by X-ray diffraction. Preliminary data indicate that Cypr-BIAN is a much stronger ligand than any Ar-BIAN compound.

Synthesis, crystal structure and crystal packing of diaza[5]helicenes

Analysis of the crystalline packing of known aza[5]helicenes is presented together with some newly synthesized diaza[5]helicenes species, characterized by NMR spectroscopy and X-ray diffraction. The comparison with carbo[5]helicenes and monoaza[5]helicenes emphasizes the importance of the “shape factor” (responsible of the isomorphism between some species in the series) as well as the lack of a unique supramolecular effect organizing the molecules in the crystals (responsible for the polymorphism observed for some compounds).

Improving the quality of diamond anvil cell data collected on an area detector by shading individual diamond overlaps

Crystallographic data collected on a CCD using a diamond anvil cell may present overlapping diamond and sample reflections. These false intensities may not be rejected by common data processing programs and may affect the final hkl list. A utility was developed that shades these reflections during the integration, enabling their elimination before the solution or refinement. Moreover, this procedure avoids the error propagation to other reflections, caused by overlapping spots, through the profile fitting procedure of an integration run.

The acentric nature of trans-stilbazole crystals and the origin of its NLO response

The X-ray crystal structure and the second order nonlinear optical response of trans-stilbazole are discussed. Given that diffraction intensities are completely insensitive to any small distortion from centrosymmetry, stilbazole X-ray diffraction data are better modelled in the centrosymmetric P21/c space group and cannot be used to distinguish between alternative acentric options. However, second harmonic generation, notable in itself, sheds light on the acentric nature of the crystals and allows the most reasonable structural model to be selected. A previously unobserved low energy photoemission, probably due to through-space charge transfers, is also reported.