Nicola Casati

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.

Anagostic Interactions under Pressure: Attractive or Repulsive?

Square-planar d(8)-ML4 complexes might display subtle but noticeable local Lewis acidic sites in axial direction in the valence shell of the metal atom. These sites of local charge depletion provide the electronic prerequisites to establish weakly attractive 3c-2e M⋅⋅⋅H-C agostic interactions, in contrast to earlier assumptions. Furthermore, we show that the use of the sign of the (1)H NMR shifts as major criterion to classify M⋅⋅⋅H-C interactions as attractive (agostic) or repulsive (anagostic) can be dubious. We therefore suggest a new characterization method to probe the response of these M⋅⋅⋅H-C interactions under pressure by combined high pressure IR and diffraction studies.

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.

Slow relaxation of the magnetization in non-linear optical active layered mixed metal oxalate chains.

New Co(II) members of the family of multifunctional materials of general formula [DAMS](4)[M(2)Co(C(2)O(4))(6)]·2DAMBA·2H(2)O (M(III) = Rh, Fe, Cr; DAMBA = para-dimethylaminobenzaldehyde and [DAMS(+)] = trans-4-(4-dimethylaminostyryl)-1-methylpyridinium) have been isolated and characterized. Such new hybrid mixed metal oxalates are isostructural with the previously investigated containing Zn(II), Mn(II), and Ni(II). This allows to preserve the exceptional second harmonic generation (SHG) activity, due to both the large molecular quadratic hyperpolarizability of [DAMS(+)] and the efficiency of the crystalline network which organizes [DAMS(+)] into head-to-tail arranged J-type aggregates, and to further tune the magnetic properties. In particular, the magnetic data of the Rh(III) derivative demonstrate that high spin octacoordinated Co(II) centers behave very similarly to the hexacoordinated Co(II) ones, being dominated by a large orbital contribution. The Cr(III) derivative is characterized by ferromagnetic Cr(III)-Co(II) interactions. Most relevantly, the Fe(III) compound is characterized by a moderate antiferromagnetic interaction between Fe(III) and Co(II), resulting in a ferrimagnetic like structure. Its low temperature dynamic magnetic properties were found to follow a thermally activated behavior (τ(0) = 8.6 × 10(-11) s and ΔE = 21.4 K) and make this a candidate for the second oxalate-based single chain magnet (SCM) reported up to date, a property which in this case is coupled to the second order non linear optical (NLO) ones.

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.

Solid-State Reversible Nucleophilic Addition in a Highly Flexible MOF

A flexible and porous metal-organic framework, based on Co(II) connectors and benzotriazolide-5-carboxylato linkers, is shown to selectively react with guest molecules trapped in the channels during the sample preparation or after an exchange process. Stimulated by a small crystal shrinking, upon compression or cooling, the system undergoes a reversible, nonoxidative nucleophilic addition of the guest molecules to the metal ion. With dimethylformamide, only part of the penta-coordinated Co atoms transform into hexa-coordinated, whereas with the smaller methanol all of them stepwise increase their coordination, preserving the crystallinity of the solid at all stages. This extraordinary example of chemisorption has enormous implications for catalysis, storage, or selective sieving.

The α and β forms of oxalic acid di-hydrate at high pressure: a theoretical simulation and a neutron diffraction study

The high pressure structure of α-oxalic acid di-hydrate shows a prototypical behaviour of acid to base proton transfer reaction. The solid state transformation has been characterized by neutron and X-ray diffraction as well as by quantum chemical calculations with periodic boundary conditions. The two resonant configurations playing the most important role in the hydrogen bond mechanism, i.e. a neutral and a charge transfer configuration, are interchanged by modifying the pressure on the system. A different behaviour is predicted for the less common β form, despite an apparently similar packing motif. Implications for the characterization of hydrogen bonds in the solid state are discussed as well as the possibility of estimating internal energy variations from experimental measurements.

Chiral cyclopropylamines in the synthesis of new ligands; first asymmetric Alkyl-BIAN compounds

The synthesis of new pinene-derived chiral cyclopropylamines and their use in the synthesis of the first asymmetric Alkyl-BIAN ligands (Alkyl-BIAN = bis-(alkylimino)acenaphthenequinone) are reported.

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.