Synthesis of a Palladium Dimer Supported by a C-Bound Trifluoroacetonate Bridge Formed by Cleavage of a Hexafluoroacetylacetonate Ligand.

Palladium(II) hexafluoroacetylacetonate (Pd(Hfacac)2) is known to form adducts of bases, such as lutidine (2,6-dimethylpyridine). When treated with approximately 3 equiv of lutidine, Pd(Hfacac)2 yields a 1:1 complex as reported in the literature, Pd(O,O-Hfacac)(C-Hfacac)(lutidine), 1. However, when the amount of excess lutidine is increased, a new complex, 2, is formed. A single-crystal X-ray structure of 2 proves it is a rare example of a dimeric palladium complex containing two Pd(Hfacac)(lutidine) fragments bridged by a dianionic trifluoroacetonate ligand, µ-CHC(O)CF3. The formation of 2 is accompanied by a white precipitate determined to be a mixture of trans-Pd(O2CCF3)2(lutidine)2 (3), confirming the fate of the missing trifluoroacetate fragment from the cleavage of the Hfacac ligand, and [lutidinium][Hfacac] (4). Subsequent experiments revealed the determinative role that water played in this reaction. The mechanism of cleavage of the Hfacac ligand was explored by DFT methods.

Electrochemical PINOylation of Methylarenes: Improving the Scope and Utility of Benzylic Oxidation through Mediated Electrolysis.

A mediated electrosynthetic method has been developed for selective benzylic oxidation of methylarenes. Phthalimide-N-oxyl (PINO) radical generated by proton-coupled electrochemical oxidation of N-hydroxypthalimide serves as a hydrogen atom-transfer (HAT) mediator and as a radical trap for the benzylic radicals generated in situ. This mediated electrolysis method operates at much lower anode potentials relative to direct electrolysis methods for benzylic oxidation initiated by single-electron transfer (SET). A direct comparison of SET and mediated-HAT electrolysis methods with a common set of substrates shows that the HAT reaction exhibits a significantly improved substrate scope and functional group compatibility. The PINOylated products are readily converted into the corresponding benzylic alcohol or benzaldehyde derivative under photochemical conditions, and the synthetic utility of this method is highlighted by the late-stage functionalization of the non-steroidal anti-inflammatory drug celecoxib.

Minimal Basis Set Hartree-Fock Corrected with Atom-Centered Potentials for Molecular Crystal Modeling and Crystal Structure Prediction.

Crystal structure prediction (CSP), determining the experimentally observable structure of a molecular crystal from the molecular diagram, is an important challenge with technologically relevant applications in materials manufacturing and drug design. For the purpose of screening the randomly generated candidate crystal structures, CSP protocols require energy ranking methods that are fast and can accurately capture the small energy differences between molecular crystals. In addition, a good ranking method should also produce accurate equilibrium geometries, both intramolecular and intermolecular. In this article, we explore the combination of minimal-basis-set Hartree-Fock (HF) with atom-centered potentials (ACPs) as a method for modeling the structure and energetics of molecular crystals. The ACPs are developed for the H, C, N, and O atoms and fitted to a set of reference data at the B86bPBE-XDM level in order to mitigate basis-set incompleteness and missing correlation. In particular, ACPs are developed in combination with two methods: HF-D3/MINIs and HF-3c. The application of ACPs greatly improves the performance of HF-D3/MINIs for lattice energies, crystal energy differences, energy-volume and energy-strain relations, and crystal geometries. In the case of HF-3c, the improvement in the crystal energy differences is much smaller than in HF-D3/MINIs, but lattice energies and particularly crystal geometries are considerably better when ACPs are used. The resulting methods may be useful for CSP but also for quick calculation of molecular crystal lattice energies and geometries.

Structural analysis of helicene molecules adsorbed on symmetric surfaces.

Helicenes are chiral polyaromatic hydrocarbon molecules which self-assemble into ordered monolayers on solid substrates, and are of current interest in the study of supramolecular systems and the development of smart materials. In this work we investigate the geometry of helicene monomers and stacked dimers on (111) facets of coinage metals. The geometry of the adsorbed molecules is shaped by the coupling of intermolecular dispersive forces, intramolecular steric repulsion between end rings and surface-molecule interactions. Thus, binding and stereospecificity outcomes vary broadly depending on the identity of molecule/surface pair. Overall, homochiral interactions are found to be more effective than heterochiral stacking, due to a better fit between the helical structures in like dimers. On a surface, this effect is enhanced by the flattening of surface-proximal molecular rings. However, our results show that the "sandwich" effect of the second molecular layer increases molecular footprints in the first layer, with potentially large implications in monolayer organization and surface commensuration.

Fundamental aspects in surface self-assembly: theoretical implications of molecular polarity and shape.

We investigate fundamental aspects of structure formation in molecular self-assembly, by examining the emergence of order upon adsorption of a series of model molecules. It is known that strongly polar diatomic molecules form three-dimensional crystals in the absence of a substrate. This tendency can be disrupted upon assembly on a solid surface, and various other types of order may arise. Depending on the relative strength of the interactions, disordered phases, two-dimensional crystals commensurate to the surface, and unmodified crystals were observed upon adsorption of simple dipoles in the present work. Introduction of steric features, in the form of a longer backbone or substituents external to the polar pair, led to even richer phase diagrams. The formation of two-dimensional phases with nematic (parallel) or antiparallel alignment was accomplished by altering the polarity of the end groups on needle-like molecules, whereas embedded charged groups made two-dimensional structure unstable for even very long molecules. These molecules preferred to align in long, often desorbed, molecular wires. The wealth of phases observed here parallel the results of experimental systematic or incidental studies of the relationships between molecular interactions and self-assembled patterns, and provide some insight into the molecular handles that self-assembly researchers can wield to guide the process towards a desired structural outcome.