Synthesis,Characterization, and Theoretical Investigation of Two‐Coordinate Palladium(0) and Platinum(0) Complexes Utilizing π‐Accepting Carbenes

An elegant general synthesis route for the preparation of two coordinate palladium(0) and platinum(0) complexes was developed by reacting commercially available tetrakis(triphenylphosphine)palladium/platinum with π‐accepting cyclic alkyl(amino) carbenes (cAACs). The complexes are characterized by NMR spectroscopy, mass spectrometry, and single‐crystal X‐ray diffraction. The palladium complexes exhibit sharp color changes (crystallochromism) from dark maroon to bright green if the C‐Pd‐C bond angle is sharpened by approximately 6°, which is chemically feasible by elimination of one lattice THF solvent molecule. The analogous dark orange‐colored platinum complexes are more rigid and thus do not show this phenomenon. Additionally, [(cAAC)₂Pd/Pt] complexes can be quasi‐reversibly oxidized to their corresponding [(cAAC)₂Pd/Pt]⁺ cations, as evidenced by cyclic voltammetry measurements. The bonding and stability are studied by theoretical calculations.     

Dative versus electron-sharing bonding in N-imides and phosphane imides R3ENX and relative energies of the R2EN(X)R isomers (E = N,P; R = H,Cl, Me,Ph; X = H,F, Cl)*

ABSTRACT

Quantum chemical calculations using density functional theory BP86 and M06-2X functionals in conjunction with def2-TZVPP basis sets have been carried out on the title molecules. The calculation results reveal that the N-imides R₃NNX are always clearly higher in energy than the imine isomers R₂NN(X)R. In the case of phosphane imides R₃PNX and the isomers R₂PN(X)R, the substituent R plays a critical role in determining their relative stabilities. When R is hydrogen or phenyl group, R₃PNX are always higher in energy than R₂PN(X)R but the former are more stable than the latter when R = Cl. Interestingly, the Me₃PNX and Me₂PN(X)Me are quite close in energy. The energy decomposition analysis suggests that the P–N bond in the phosphane imides R₃PNX (R = H, Cl, Me, Ph; X = H, F, Cl) should be described in terms of an electron-sharing single bond between two charged fragments R₃P⁺-NX^? that is supported by (R₃P)⁺←(NX)^? π-backdonation. The π-bond contributes 14–21% of the total orbital interactions while the σ-bond provides 60–68% of ΔE_orb.     

A Modular Access to Divinyldiphosphenes with a Strikingly Small HOMO–LUMO Energy Gap

The olefinic C−H bond functionalization of (NHC)CHPh (NHC=IPr=C{(NAr)CH}₂ 1 ; SIPr=C{(NAr)CH₂}₂ 2 ; Ar=2,6-iPr₂C₆H₃), derived from classical N-heterocyclic carbenes (NHCs), with PCl₃ affords the dichlorovinylphosphanes {(NHC)C(Ph)}PCl₂ (NHC=IPr 3 , SIPr 4 ). Two-electron reduction of 3 and 4 with magnesium leads to the formation of the divinyldiphosphenes [{(NHC)C(Ph)}P]₂ (NHC=IPr 5 , SIPr 6 ) as crystalline solids. Unlike literature-known diphosphenes, which are mostly yellow or orange, 5 is a green whereas 6 is a purple solid. Although the P=P bond lengths of 5 (2.062(1)) and 6 (2.055(1) Å) are comparable to those of the known diphosphenes (2.02–2.08 Å), the C−P bond lengths of 5 (1.785(1)) and 6 (1.797(1) Å) are, however, considerably shorter than a C −P single bond length (1.85 Å), indicating a considerable π-conjugation between C=C and P=P moieties. The HOMO–LUMO energy gap for 5 (4.15) and 6 (4.52 eV) is strikingly small and thus the narrowest among the diphosphenes (>4.93 eV) reported as yet. Consequently, 5 readily undergoes P=P bond cleavage at room temperature on treatment with sulfur to form the unique dithiophosphorane {(IPr)C(Ph)}P(S)₂ 7 . Interestingly, reaction of 5 with selenium gives the selenadiphosphirane [{(IPr)C(Ph)}P]₂Se 8 with an intact P−P bond.     

Preorganized Anion Traps for Exploiting Anion–π Interactions: An Experimental and Computational Study

1,3‐Bis(pentafluorophenyl‐imino)isoindoline (A^F) and 3,6‐di‐tert‐butyl‐1,8‐bis(pentafluorophenyl)‐9H‐carbazole (B^F) have been designed as preorganized anion receptors that exploit anion–π interactions, and their ability to bind chloride and bromide in various solvents has been evaluated. Both receptors A^F and B^F are neutral but provide a central NH hydrogen bond that directs the halide anion into a preorganized clamp of the two electron‐deficient appended arenes. Crystal structures of host–guest complexes of A^F with DMSO, Cl^?, or Br^? (A^F:DMSO, A^F:Cl^?, and ${{\rm A}{{{\rm F}\hfill \atop 2\hfill}}}$ :Br^?) reveal that in all cases the guest is located in the cleft between the perfluorinated flaps, but NMR spectroscopy shows a more complex situation in solution because of E,Z/Z,Z isomerism of the host. In the case of the more rigid receptor B^F, Job plots evidence 1:1 complex formation with Cl^? and Br^?, and association constants up to 960 M ^?1 have been determined depending on the solvent. Crystal structures of B^F and B^F:DMSO visualize the distinct preorganization of the host for anion–π interactions. The reference compounds 1,3‐bis(2‐pyrimidylimino)isoindoline (A^N) and 3,6‐di‐tert‐butyl‐1,8‐diphenyl‐9H‐carbazole (B^H), which lack the perfluorinated flaps, do not show any indication of anion binding under the same conditions. A detailed computational analysis of the receptors A^F and B^F and their host–guest complexes with Cl^? or Br^? was carried out to quantify the interactions in play. Local correlation methods were applied, allowing for a decomposition of the ring–anion interactions. The latter were found to contribute significantly to the stabilization of these complexes (about half of the total energy). Compounds A^F and B^F represent rare examples of neutral receptors that are well preorganized for exploiting anion–π interactions, and rare examples of receptors for which the individual contributions to the binding energy have been quantified.     

Development of an integrated flow injection system for the electro-oxidative leaching of uranium from geological samples and its spectrophotometric determination with Arsenazo III

This work proposes a new procedure for on-line electro-oxidative leaching and spectrophotometric determination of uranium in ore samples. By associating a conventional flow injection system, used for uranium determination with Arsenazo III, with an on-line system for electro-oxidative leaching, a fully integrated system was assembled. The systems were integrated after achieving optimum conditions for uranium determination and leaching. According to the results obtained in the present work, a current density of 280 mA cm⁻² generated enough hypochlorite ions in the electrolyte solution (3.6 mol L⁻¹ HCl + 2% (w/v) NaCl) to promote quantitative oxidation of U(IV) to U(VI) thus improving the extraction efficiency. The slurry density did not significantly affect the performance of the system and the increasing temperature resulted in a decrease in extraction efficiency. This methodology was applied in the determination of U₃O₈ in four ore samples and the results obtained agreed with those obtained by ICP-MS after conventional wet acid digestion of the samples.     

Optical guiding of a radially polarized laser beam for inverseCherenkov acceleration in a plasma channel

In a conventional inverse Cherenkov accelerator (ICA), the background neutral gas provides the necessary dispersion to maintain the synchronism between the drive laser and the accelerated electrons. A laser-driven ICA is susceptible to diffraction, and the acceleration length is limited to approximately a Rayleigh range (for a Gaussian beam). In this paper, an ICA configuration is proposed that avoids the laser diffraction limitation by employing a preformed plasma channel. It is shown that a radially polarized laser beam can be optically guided if the plasma density increases with radius-like r². Expressions for the guided axial and radial components of the laser field are derived, and a numerical example is discussed