Structure, bonding, and reactivity of Ti and Zr amidate complexes: DFT and X-ray crystallographic studies

Easily prepared and highly modular organic amide proligands have been used to synthesize a series of new bis(amidate)-bis(amido) Ti and Zr complexes via protonolysis. These complexes have been structurally characterized by NMR spectroscopy and X-ray crystallography. The solid-state molecular structures of these complexes indicate that the amidate ligands bind to the metal centers in an exclusively bidentate fashion, resulting in discrete monomeric species. Geometric isomerism in these species is highly dependent upon the steric characteristics of the proligands utilized in the synthesis. In solution, these complexes are observed to isomerize on the NMR time scale, with one isomer being predominant. Bonding in the bis(amidate)-bis(amido) complexes was investigated by DFT calculations. The geometric isomers predicted by theory matched the experimentally observed results, within experimental error. The orbitals associated with amidate-metal bonding are energetically well below the frontier orbitals. The HOMO in these complexes is a pi orbital associated with amido ligand-to-metal bonding character, while the LUMO in all cases is a vacant d orbital on the metal center.     

Dramatic effect of homoallylic substitution on the rate of palladium-catalyzed diene cycloisomerization

Cycloisomerization of 4,4-bis(acetoxymethyl)-1,6-heptadiene (5) catalyzed by [(phen)Pd(Me)CNCH₃]⁺ [BAr₄]⁻ [Ar=3,5-C₆H₃(CF₃)₂] (2) to form predominantly 3,3-bis(acetoxymethyl)-1,5-dimethylcyclopentene (6) was ∼400 times faster than was the cycloisomerization of dimethyl diallylmalonate (1) under identical conditions. Mechanistic studies performed in conjunction with density functional theory calculations attribute the large rate acceleration of the cycloisomerization of 5 relative to the cycloisomerization of 1 to the formation of a stable oxo chelate complex as an intermediate in the cycloisomerization of 1, but not in the cycloisomerization of 5.     

Unusual solid-state behavior in a neutral [2]catenane bearing a hydrolyzable component

A template-directed strategy to forming a bis(diimide) macrocycle through an intermediate asymmetric [2]catenane is reported. Saponification of the ester linkages within the crown ether component is much slower in the mechanically interlocked structure when compared to the free crown. The predominance of a single translational isomer leads to a dimeric structure, resulting in the generation of infinite channels within the crystal lattice. [structure: see text]     

Intramolecular hydroamination of unactived olefins with Ti(NMe2)4 as a precatalyst

Commercially available Ti(NMe(2))(4) has been used effectively as a precatalyst in a facile protocol for the intramolecular hydroamination of aminoalkenes to yield pyrrolidine and piperidine heterocyclic products with isolated yields up to 92%. Geminally substituted substrates display the highest reactivity. This precatalyst is also effective for the hydroamination of activated internal alkenes, providing access to more complex heterocyclic target molecules.     

Interaction of atrazine with Laurentian fulvic acid: binding and hydrolysis

Ultrafiltration fractionation and liquid chromatography (LC) have been applied to the study of the binding and hydrolysis of the polar herbicide atrazine on a stoichiometrically well characterized fulvic acid. Binding requires extensive carboxylate site protonation but the binding sites represent a very small fraction of total carboxylate. The data show that binding of atrazine is not competitive with binding of the hydrolytic product hydroxyatrazine. However, smaller molecular weight fractions of the fulvic acid mixture compete with atrazine for sites on the larger molecular weight fraction. Binding equilibrium is not rapid. A model of binding involving hydrogen bonding and/or charge-transfer complexing to specific sites created dynamically by the conformational equilibria of the higher molecular weight polymeric fulvic acid fractions is proposed as the best accommodation of the variety of observed facts.     

Wavelength-dependent photochemistry in Cr(CNPh)6: a study of photosubstitution and photoinduced electron transfer using time-resolved spectroscopy

The wavelength dependence of photosubstitution, photoinduced electron transfer, and the time-resolved spectra of Cr(CNPh)6, a compound having low-lying MLCT states, were investigated. Photosubstitution quantum yields increase with increasing excitation energy while photoinduced electron transfer quantum yields decrease with increasing excitation energy. At the lowest excitation energy used (532 nm, or 18,800 cm(-1)), the quantum yields for both electron transfer and photosubstitution reach the same maximum value, 0.29. Picosecond time-resolved absorption spectra at 355 and 532 nm excitation wavelengths show two features: a bleach signal centered at 400 nm and an excited state absorption (ESA) in the 600 nm region. The ESA signal is much weaker for 532 nm excitations than for 355 nm excitations. Following a 355 nm flash, the bleach and ESA decay exponentially with the same lifetime of 23 micros. This implies a simple ligand dissociation followed by recombination. Bleach recovery kinetics after a 532 nm flash are more complicated: two or three exponential components are required to fit the data. Cr(CNPh)6 exhibits two photochemical mechanisms: at high excitation energy, a simple charge neutral dissociation occurs; at low energy, it is proposed that a phenylisocyanide radical anion dissociates, forming a radical pair that is responsible for the observed substitution and electron transfer reactivity, and the complicated nanosecond kinetics. The primary processes for both reactions occur in less than 20 ps.     

Self-Assembly and X-Ray Crystal Structures of Novel Sn(IV)Porphyrin Phenolates

A series of complexes of the type [Sn(TTP)L₂] have been prepared by the condensation of [Sn(TTP)OH₂] (TTP = meso-tetratolylporphyrin) with a range of substituted phenols. The resulting complexes were characterised using ¹H NMR and single crystal X-ray diffraction techniques. In each case, the condensation of the phenols with the Sn(IV)porphyrin in CDCl₃ solution is slow (h) but essentially quantitative. The slow kinetics of the formation of the diaxial phenolate complexes allows for the identification, by ¹H NMR spectroscopy, of three independent complexes within this process, namely an outer-sphere (H-bonded) complex as well as two independent phenolate complexes. The rate of condensation is in the order phenol 4-methoxyphenol > 4-nitrophenol and suggests a steric rather than pKa dependency.     

Real‐time measurement of peroxyacetyl nitrate using selected ion flow tube mass spectrometry

The on‐line detection of gaseous peroxyacetyl nitrate (PAN) using selected ion flow tube mass spectrometry (SIFT‐MS) has been investigated using a synthetic sample of PAN in air at a humidity of ～30%. Using the H₃O⁺ reagent ion, signals due to PAN at m/z 122, 77 and 95 have been identified. These correspond to protonated PAN, protonated peractetic acid and its water cluster, respectively. These products and their energetics have been probed through quantum mechanical calculations. The rate coefficient of H₃O⁺ has been estimated to be 4.5 × 10^?9 cm³ s^?1, leading to a PAN sensitivity of 138 cps/ppbv. This gives a limit of detection of 20 pptv in 10 s using the [M+H]⁺ ion of PAN at m/z 122. Copyright © 2010 John Wiley & Sons, Ltd.     

Generation of hyperentangled photon pairs

We experimentally demonstrate the first quantum system entangled in every degree of freedom (hyperentangled). Using pairs of photons produced in spontaneous parametric down-conversion, we verify entanglement by observing a Bell-type inequality violation in each degree of freedom: polarization, spatial mode, and time energy. We also produce and characterize maximally hyperentangled states and novel states simultaneously exhibiting both quantum and classical correlations. Finally, we report the tomography of a 2 x 2 x 3 x 3 system (36-dimensional Hilbert space), which we believe is the first reported photonic entangled system of this size to be so characterized.