Lithium and aluminium complexes supported by chelating phosphaguanidinates

Diisopropylcarbodiimide, (i)PrN[double bond, length as m-dash]C[double bond, length as m-dash]N(i)Pr, inserts into the lithium-phosphorus bond of in situ prepared "Ph(2)PLi(THF)(n)" to afford the lithium salt, [Li(Ph(2)PC{N(i)Pr}(2))(THF)(n)](x)(2a); alternatively, this compound can be made by deprotonation of the neutral phosphaguanidine, Ph(2)PC{N(i)Pr}{NH(i)Pr}(1a) with (n)BuLi. Displacement of the THF solvate in 2a is readily achieved with TMEDA to afford Li(Ph(2)PC{N(i)Pr}(2))(TMEDA)(3a). X-Ray crystallographic analyses show that 2a exists as a dimer in the solid state with a folded ladder structure and an N,N' chelating phosphaguanidinate, while 3a is monomeric with N,P-coordination of the ligand to lithium. Compound 2a reacts via a transmetallation pathway with AlMe(2)Cl to afford the dimethylaluminium complex, Al(Ph(2)PC{N(i)Pr}(2))Me(2)(4a), which can also be prepared by protonation of a methyl group of AlMe(3) using 1a. The formation of a series of dialkylaluminium compounds has been investigated employing this latter pathway using both 1a and the N,N'-dicyclohexyl analogue, Ph(2)PC{NCy}{NHCy}(1b), affording Al(Ph(2)PC{NR}(2))Et(2)(5a,b), Al(Ph(2)PC{NR}(2))(i)Bu(2)(6a,b) and the diphenylaluminium compound Al(Ph(2)PC{N(i)Pr}(2))Ph(2)(7a). The oily nature of most of the dialkyl compounds and high sensitivity to oxygen and moisture lead to difficulty in manipulation and characterization; however, NMR spectroscopy indicated highly pure products (>95%) upon removal of the solvent. The molecular structures of the crystalline examples 4a and 7a are reported, showing monomeric aluminium species with symmetrically chelating phosphaguanidinate ligands. The series of aluminium compounds AlLCl(2){L=[EC{NiPr}(2)](-): A, E=Me; B, E=Me(2)N; C, E=(Me(3)Si)(2)N and D, E=Ph(2)P} were investigated using density functional theory. In the more simple cases A and B, the delocalized electron density of the metallacycle was represented by a combination of the HOMO and an orbital of lower energy (A, HOMO-5; B, HOMO-6). The HOMO-1 in B was pi-bonded across the Me(2)N-C bond suggesting delocalization of electron density into the metallacycle. In the more complex systems C and D, delocalization within the metallacycle was less extensive due to the (Me(3)Si)(2)N- and Ph(2)P-moieties. A number of occupied orbitals in D, however, display phosphorus 'lone-pair' characteristics, indicating that these species have the potential to behave as Lewis bases in the formation of poly(metallic) systems.     

Reactions of a stable silylene with halocarbons

An unusual product formation is observed for the insertion reaction of the thermally stable silylene Si[(NCH(2)Bu(t))(2)C(6)H(4)-1,2][abbrev. as Si(NN)] into the carbon-halogen bond of alkyl or aryl halides RHal (Hal=Cl, Br). In general, depending on the halogen, the reaction either results in a disilane of type (NN)Si(Hal)-(R)Si(NN) for Hal=Cl or a mixture of disilane and the monosilane (NN)Si(R)(Hal) for Hal=Br. The results are put into context to previously suggested mechanisms. The disilane (NN)Si(Hal)-(R)Si(NN)(Hal=Cl or Br) is thermally labile and mild thermolysis yields the corresponding monosilane (NN)Si(R)(Hal) and silylene 1. Additionally, strong evidence is presented for a radical pathway for the reaction of 1 and RHal.     

Stimulated raman pumping to H2 (v″=1, J″=1) and resonance-enhanced multiphoton ionization from it using a single laser

Light pulses from one ArF laser are used both a) to produce H₂ in v=1, J=1 of the X-state by means of stimulated Raman pumping (SRP) and b) to analyze for that state via (2+1) resonance-enhanced multiphoton ionization (REMPI). Both SRP and REMPI have been previously measured in H₂, but with a different laser for each process. Some of our laser light is Raman shifted in H₂. The resulting mixture of fundamental (193 nm) and first Stokes (210 nm) light is focused into low-pressure H₂ where the SRP and REMPI both occur. The SRP is efficient and it produces only v=1, J=1. As the laser is tuned, a REMPI spectrum occurs from excitation by two photons of 193 nm, by two photons of 210 nm, or by one photon each of 210 and 193 nm. The features of this approach are a) that the necessary temporal and spatial overlaps are automatically achieved, b) that the frequency difference generated in the Raman shifter is precisely that needed for SRP, c) that large pulse energies are available for the REMPI, and d) that only one laser is needed.     

Synthesis and conformational analysis of α,α-difluoroalkyl heteroaryl ethers

We report the efficient synthesis of difluoroalkyl aryl ethers from the rearrangement of heteroaryl ketones and aldehydes, mediated by xenon difluoride and HF/pyridine in methylene chloride at room temperature. Computational analysis of difluoroalkylethers shows that there is potential to allow access to conformational space not accessible to the hydrogenated parent.     

Ueber die Zersetzung des frisch gef?llten Silberchlorids durch Sonnenlicht

Ohne Zusammenfassung     

Optimization of analysis of soft X-ray spectromicroscopy at the Ca 2p edge

Absorption saturation frequently occurs in X-ray absorption spectroscopy whenever transmission detection is used and the sample is too thick. This can strongly distort X-ray absorption spectra, particularly for transitions which have very high intensity and narrow line width relative to the instrumental resolution. This problem is particularly severe at the strong 2p → 3d resonances at the Ca 2p edge, such that there are detectible absorption saturation effects for mineral samples as thin as 30 nm. We describe methods to identify such spectral distortion in scanning transmission X-ray microscopy (STXM) and avoid its effects when generating reference spectra by selecting spatial regions with undistorted spectra. We also show that taking absorption saturation into account is critical when STXM is applied to systems with thick regions. We outline an approach which can provide accurate thickness maps even in the regions where the thickness is such the absorption is saturated at the resonance peaks. Environmental samples such as biofilms, soil samples, sediments, and precipitates are typically heterogeneous in both composition and thickness. They often contain regions thicker than the saturation threshold, yet one would still like to analyse such samples correctly and quantitatively. By excluding the peak energies where absorption saturation distortion occurs, we show it is possible to achieve a meaningful quantitative analysis. We demonstrate the advantages of this approach in the context of mapping two different CaCO₃ species biomineralized within a natural river biofilm.     

Structural studies of group 1 metal 4-azapentalenyl complexes

The first structurally authenticated 4-azapentalenyl complexes are reported, along with DFT studies, that reveal tendencies for their cyclopentadienyl isostructural nature and an ability to exhibit binding modes of varying hapticity.     

Group 1 and 13 complexes of aryl-substituted bis(phosphinimino)methyls

The organolithium reagent [{HC(Ph₂PNC₆H₂Me₃-2,4,6)₂}Li(OEt₂)] was easily obtained by deprotonation of H₂C(Ph₂PNC₆H₂Me₃-2,4,6)₂ with nBuLi in diethyl ether solution. The crystal structure of [{HC(Ph₂PNC₆H₂Me₃-2,4,6)₂}Li(OEt₂)] has been determined and shown to consist a monomeric chelate structure that contains a distorted, trigonal planar lithium centre. The ligand precursor has also been deprotonated with both Me₃Al and Me₂AlCl to yield the tetrahedral organoaluminium complexes, [{HC(Ph₂PNC₆H₂Me₃-2,4,6)₂}AlMe₂] and [{HC(Ph₂PNC₆H₂Me₃-2,4,6)₂}Al(Cl)Me]. Reaction of [{HC(Ph₂PNC₆H₂Me₃-2,4,6)₂}Li(OEt₂)] with either AlX₃ (X=Cl, Br, I) or GaCl₃ yielded a series of dihalo derivatives [{HC(Ph₂PNC₆H₂Me₃-2,4,6)₂}MX₂] all of which have been shown to exist as similar monomeric species containing four-coordinate group 13 centres.