Mono- and Bimetallic Aluminum Alkyl, Aryl, and Hydride Complexes of a Bulky Dipyrromethene Ligand

Reactions of an appropriate organoaluminum reagent with a sterically demanding dipyrromethene (<b>2</b>) produced the monomeric complexes (<i>N</i>,<i>N</i>)­AlR₂ (where (<i>N</i>,<i>N</i>) is deprotonated dipyrromethene; R = Me (<b>3</b>), ^tBu (<b>4</b>), Ph (<b>5</b>), H (<b>6</b>)). The alkyl and aryl derivatives <b>3</b>–<b>5</b> are surprisingly stable and tolerant to exposure to air, moisture, and silica gel. Dihydride <b>6</b> is less robust and reacts with water to form the oxo-bridged aluminoxane [(<i>N</i>,<i>N</i>)­AlH]₂O (<b>7</b>). The presence of terminal hydrides in <b>6</b> and <b>7</b> was confirmed by NMR and IR spectroscopy. Furthermore, observed Al–H stretching frequencies agree well with those predicted by DFT calculations. We also report the addition of ^tBuLi to the meso position of <b>2</b>, affording 5-<i>tert</i>-butyl-5-phenyl-2,2′-dimesityldipyrromethane (<b>8</b>). In addition to NMR (¹H, ¹³C) and IR spectroscopy, compounds <b>2</b>–<b>8</b> were further characterized by X-ray crystallography. Solid-state structures show that the aluminum and dipyrromethene ligand are nearly planar when the coligands at aluminum are slender

Bonding in Uranium(V) Hexafluoride Based on the Experimental Electron Density Distribution Measured at 20 K.

The electron density distribution of [PPh4][UF6] was obtained from high-resolution X-ray diffraction data measured at 20 K. The electron density was modeled with an augmented Hansen-Coppens multipolar formalism. Topological analysis reveals that the U-F bond is of incipient covalent nature. Theoretical calculations add further support to the bonding description gleaned from the experimental model. The impact of the uranium anomalous dispersion terms on the refinement is also discussed

Experimental Charge-Density Study of the Intra- and Intermolecular Bonding in TKX-50

The intra- and intermolecular bonding in the known phase of dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate, TKX-50, has been analyzed on the basis of the experimentally determined charge density distribution from high-resolution X-ray diffraction data obtained at 20 K. This was compared to the charge density obtained from DFT calculations with periodic boundary conditions using both direct calculations and derived structure factors. Results of topological analysis of the electron density corroborate that TKX-50 is best described as a layered structure linked primarily by a number of hydrogen bonds as well as by a variety of other interactions. Additional bonding interactions were identified, including a pair of equivalent 1,5-type intramolecular closed-shell interactions in the dianion. Refinement of anharmonic motion was shown to be essential for obtaining an adequate model, despite the low temperature of the study. Although generally unusual, the implementation of anharmonic refinement provided a significant improvement compared to harmonic refinement of both traditional and split-core multipole models

Experimental and Theoretical Electron Density Determination for Two Norbornene Derivatives: Topological Analysis Provides Insights on Reactivity

The electron density distribution of two substituted norbornene derivatives (<i>cis</i>-5-norbornene-<i>endo</i>-2,3-dicarboxylic anhydride (<b>1</b>) and 7-oxabicylo[2.2.1]­hept-5-ene-<i>exo</i>-2,3-dicarboxylic anhydride (<b>2</b>) has been determined from low-temperature (20 K) X-ray diffraction data and from DFT calculations with periodic boundary conditions. Topological analysis of the electron density is discussed with respect to <i>exo</i>-selective additions, the partial <i>retro</i>-Diels–Alder (rDA) character of the ground state, and intermolecular interaction energies

Bonding in Uranium(V) Hexafluoride Based on the Experimental Electron Density Distribution Measured at 20 K

The electron density distribution of [PPh₄]­[UF₆] was obtained from high-resolution X-ray diffraction data measured at 20 K. The electron density was modeled with an augmented Hansen–Coppens multipolar formalism. Topological analysis reveals that the U–F bond is of incipient covalent nature. Theoretical calculations add further support to the bonding description gleaned from the experimental model. The impact of the uranium anomalous dispersion terms on the refinement is also discussed

Accessibility and External versus Intercalative Binding to DNA As Assessed by Oxygen-Induced Quenching of the Palladium(II)-Containing Cationic Porphyrins Pd(T4) and Pd(<i>t</i>D4)

Studies reveal that it is possible to design a palladium­(II)-containing porphyrin to bind exclusively by intercalation to double-stranded DNA while simultaneously enhancing the ability to sensitize the formation of singlet oxygen. The comparisons revolve around the cations [5,10,15,20-tetra­(<i>N</i>-methylpyridinium-4-yl)­porphyrin]­palladium­(II), or Pd­(T4), and [5,15-di­(<i>N</i>-methylpyridinium-4-yl)­porphyrin]­palladium­(II), or Pd­(<i>t</i>D4), in conjunction with AT and GC rich DNA binding sequences. Methods employed include X-ray crystallography of the ligands as well as absorbance, circular dichroism, and emission spectroscopies of the adducts and the emission from singlet oxygen in solution. In the case of the bulky Pd­(T4) system, external binding is almost as effective as intercalation in slowing the rate of oxygen-induced quenching of the porphyrin’s triplet excited state. The fractional efficiency of quenching by oxygen nevertheless approaches 1 for intercalated forms of Pd­(<i>t</i>D4), because of intrinsically long triplet lifetimes. The intensity of the sensitized, steady-state emission signal varies with the system and depends on many factors, but the Pd­(<i>t</i>D4) system is impressive. Intercalated forms of Pd­(<i>t</i>D4) produce higher sensitized emission yields than Pd­(T4) is capable of in the absence of DNA