Synthesis and reactivity of dichloroboryl complexes of platinum(II)

The reaction between [Pt(nbe)3] (nbe=norbornene), two equivalents of the phosphines PPh3, PMePh2 or PMe2Ph and 1 equivalent of BCl3 affords the platinum dichloroboryl species [PtCl(BCl2)(PPh3)2], [PtCl(BCl2)(PMePh2)2] and [PtCl(BCl2)(PMe2Ph)2]. All three complexes were characterised by X-ray crystallography and reveal that the boryl group lies trans to the chloride. With PMe3 as the phosphine, the complex [PtCl(BCl2)(PMe3)2] is isolated in high yield as a white crystalline powder although crystals suitable for X-ray crystallography were not obtained. Crystals were obtained of a product shown by X-ray crystallography to be the unusual dinuclear species [Pt2(BCl2)2(PMe3)4(micro-Cl)][BCl4] which reveals an arrangement in which two square planar platinum(II) centres are linked by a single bridging chloride which is trans to a BCl2 group on each platinum centre. The reaction of [PtCl(BCl2)(PMe3)2] with NEt3 or pyridine (py) affords the adducts [PtCl{BCl2(NEt3)}(PMe3)2] and [PtCl{BCl2(py)}(PMe3)2], respectively, both characterised spectroscopically. The reaction between [PtCl(BCl2)(PMe3)2] and either 4 equivalents of NHEt2 or piperidine (pipH) results in the mono-substituted boryl species [PtCl{BCl(NEt2)}(PMe3)2] and [PtCl{BCl(pip)}(PMe3)2], respectively, the former characterised by X-ray crystallography. Treatment of either [PtCl(BCl2)(PMe3)2] (in the presence of excess NEt3) or [PtCl{BCl(NEt2)}(PMe3)2] with catechol affords the B(cat) (cat=catecholate) derivative [PtCl{B(cat)}(PMe3)2] which is also formed in the reaction between [Pt(PMe3)4] and ClB(cat) and also from the slow decomposition of [Pt{B(cat)}2(PMe3)2] in dichloromethane over a period of months. The compound [Pt{B(cat)}2(PMe3)2] was prepared from the reaction between [Pt(PMe3)4] and B2(cat)2.     

Bidentates versus monodentates in asymmetric hydrogenation catalysis: synergic effects on rate and allosteric effects on enantioselectivity

C 1-Symmetric phosphino/phosphonite ligands are prepared by the reactions of Ph 2P(CH 2) 2P(NMe 2) 2 with ( S)-1,1'-bi-2-naphthol (to give L A ) or ( S)-10,10'-bi-9-phenanthrol (to give L B ). Racemic 10,10'-bi-9-phenanthrol is synthesized in three steps from phenanthrene in 44% overall yield. The complexes [PdCl 2( L A,B )] ( 1a, b), [PtCl 2( L A,B )] ( 2a, b), [Rh(cod)( L A,B )]BF 4 ( 3a, b) and [Rh( L A,B ) 2]BF 4 ( 4a, b) are reported and the crystal structure of 1a has been determined. A (31)P NMR study shows that M, a 1:1 mixture of the monodentates, PMePh 2 and methyl monophosphonite L 1a (based on ( S)-1,1 '-bi-2-naphthol), reacts with 1 equiv of [Rh(cod) 2]BF 4 to give the heteroligand complex [Rh(cod)(PMePh 2)( L 1a )]BF 4 ( 5) and homoligand complexes [Rh(cod)(PMePh 2) 2]BF 4 ( 6) and [Rh(cod)( L 1a ) 2]BF 4 ( 7) in the ratio 2:1:1. The same mixture of 5- 7 is obtained upon mixing the isolated homoligand complexes 6 and 7 although the equilibrium is only established rapidly in the presence of an excess of PMePh 2. The predominant species 5 is a monodentate ligand complex analogue of the chelate 3a. When the mixture of 5- 7 is exposed to 5 atm H 2 for 1 h (the conditions used for catalyst preactivation in the asymmetric hydrogenation studies), the products are identified as the solvento species [Rh(PMePh 2)( L 1a )(S) 2]BF 4 ( 5'), [Rh(S) 2(PMePh 2) 2]BF 4 ( 6') and [Rh(S) 2( L 1a ) 2]BF 4 ( 7') and are formed in the same 2:1:1 ratio. The reaction of M with 0.5 equiv of [Rh(cod) 2]BF 4 gives exclusively the heteroligand complex cis-[Rh(PMePh 2) 2( L 1a ) 2]BF 4 ( 8), an analogue of 4a. The asymmetric hydrogenation of dehydroamino acid derivatives catalyzed by 3a, b is reported, and the enantioselectivities are compared with those obtained with (a) chelate catalysts derived from analogous diphosphonite ligands L 2a and L 2b , (b) catalysts based on methyl monophosphonites L 1a and L 1b , and (c) catalysts derived from mixture M. For the cinnamate and acrylate substrates studied, the catalysts derived from the phosphino/phosphonite bidentates L A,B generally give superior enantioselectivities to the analogous diphosphonites L 2a and L 2b ; these results are rationalized in terms of delta/lambda-chelate conformations and allosteric effects of the substrates. The rate of hydrogenation of acrylate substrate A with heterochelate 3a is significantly faster than with the homochelate analogues [Rh( L 2a )(cod)]BF 4 and [Rh(dppe)(cod)]BF 4. A synergic effect on the rate is also observed with the monodentate analogues: the rate of hydrogenation with the mixture containing predominantly heteroligand complex 5 is faster than with the monophosphine complex 6 or monophosphonite complex 7. Thus the hydrogenation catalysis carried out with M and [Rh(cod) 2]BF 4 is controlled by the dominant and most efficient heteroligand complex 5. In this study, the heterodiphos chelate 3a is shown to be more efficient and gives the opposite sense of optical induction to the heteromonophos analogue 5.     

Is restricted M–P rotation a common feature of enantioselective monophos catalysts? An example of restricted Rh–P rotation in a secondary phosphine complex

The enantioselective hydrogenation catalyst [Rh(α-CgPH)₂(cod)]BF₄, (CgPH = 6-phospha-2,4,8-trioxa-adamantane) exists in solution as a mixture of two slowly interconverting rotamers, one with C₂- and the other with C₁-symmetry.     

Aerosol characterization and apportionment using cascade impactors and activation analysis

The use of particle-size distribution measurements and techniques for aerosol physical characterization, together with composition data for 30 elements, has led to useful deductions about the sources of airborne particulates. Collections with cascade impactors operated at three locations in Toronto, Canada, were analyzed by INAA and photonuclear activation (IPAA). The efficiency and reliability of impactors for aerosol size-distribution characterization was studied by scanning electron microscopy and calculating equivalent aerodynamic diameters; values obtained in this way were in close agreement with predicted values. Toronto aerosols exhibited both expected and unusual particle-size distribution patterns with many elements exhibiting either size distributions biased toward large (e.g. Al, Sc, Ti, Fe) or small particles (e.g. Br, Cl, Cu). Other elements (viz. Pb, Zn, As, Sb) exhibited different and unusual size distributions, presumably from different sources. Factor analysis and chemical mass balance (CMB) applied to data for a suburban site allowed emission source identification and aeorosol apportionment with a fair degree of certainty, and, preliminary results using factor analysis for size-fractionated samples, indicate a potential for relatively accurate source apportionment in more complex airsheds.     

Double-diffusive Finger Convection in a Hele–Shaw Cell: An Experiment Exploring the Evolution of Concentration Fields,Length Scales and Mass Transfer

We present the results of an experiment conducted to explore the temporal and spatial development of double-diffusive finger convection in a Hele–Shaw cell. Two solutions each containing a different density affecting component were layered in a density stable configuration (sucrose solution over a more dense salt solution) with a nearly perturbation-free interface between. The mismatch of diffusive time scales for the two components leads to local density instabilities that generate upward and downward convecting fingers. Throughout the course of the experiment, a full-field quantitative light transmission technique was used to measure concentration fields of a dye tracer dissolved in the salt solution. Analysis of these fields yielded the temporal evolution of length scales associated with the vertical and horizontal finger structure as well as mass transfer. Distinct developmental stages are identified with strong correlation between all measures. These data provide a baseline that can be used to develop and evaluate both process-level models that simulate the full complexity of the evolving flow field and large-scale effective models that integrate over small-scale behavior.     

Multi-element correlations for airborne particulate source attribution

Photon and thermal neutron activation analysis were used to analyze airborne particulate matter obtained during the period January to March of 1984 for up to thirty elements. Elemental concentrations and meteorological data from the Toronto, Ontario area were subjected to factor analysis and the major sources of particulate deduced. Elemental profiles for six sources, viz. crustal soil, automobile exhaust, road salt, refuse incineration, oil combustion and an unidentified arsenic source, were used in a chemical element balance to quantitatively determine the contribution of each source to the ambient aerosol. Re-entrained soil contributed 67%, automobile exhaust contributed 15% and the other sources contributed lesser amounts to the inorganic particulate matter. Particle-size distributions for nine elements, viz. Al, Fe, Sc, As, Br, Cl, Zn, Mn, V, are presented as an argument for the development of receptor models involving particle-size distribution data.     

Applications of a travelling wave-based radio-frequency-only stacked ring ion guide

The use of radio-frequency (RF)-only ion guides for efficient transport of ions through regions of a mass spectrometer where the background gas pressure is relatively high is widespread in present instrumentation. Whilst multiple collisions between ions and the background gas can be beneficial, for example in inducing fragmentation and/or decreasing the spread in ion energies, the resultant reduction of ion axial velocity can be detrimental in modes of operation where a rapidly changing influx of ions to the gas-filled ion guide needs to be reproduced at the exit. In general, the RF-only ion guides presently in use are based on multipole rod sets. Here we report investigations into a new mode of ion propulsion within an RF ion guide based on a stack of ring electrodes. Ion propulsion is produced by superimposing a voltage pulse on the confining RF of an electrode and then moving the pulse to an adjacent electrode and so on along the guide to provide a travelling voltage wave on which the ions can surf. Through appropriate choice of the travelling wave pulse height, velocity and gas pressure it will be shown that the stacked ring ion guide with the travelling wave is effective as a collision cell in a tandem mass spectrometer where fast mass scanning or switching is required, as an ion mobility separator at pressures around 0.2 mbar, as an ion delivery device for enhancement of duty cycle on an orthogonal acceleration time-of-flight (oa-TOF) mass analyser, and as an ion fragmentation device at higher wave velocities.     

Bulky triarylarsines are effective ligands for palladium catalysed Heck olefination

The four arsines, As{C6H3(o-CH3)(p-Z)}3{Z=H (2a) or OMe (2b)} and As{C6H3(o-CHMe2)(p-Z)}3{Z=H (2c) or OMe (2d)} react with [PdCl2(NCPh)2] or [PtCl2(NCBu(t))2] to give trans-[MCl2L2] or trans-[M2Cl2(mu-Cl)2L2]. The crystal structures of trans-[PdCl2(2a)2] and [PtCl2(2d)2] have been determined, the latter as its dichloromethane solvate. The structures show that in these complexes, the ligands adopt gga type conformations as do all analogous tri-o-tolyl- and tri-o-isopropylphenylphosphines in square-planar and octahedral complexes. The variable-temperature NMR behaviour of the complexes shows that they are fluxional due to restricted As-C bond rotation. The rate of the fluxionality is more rapid than in the analogous phosphine complexes and this is associated with longer As-C and As-M bonds allowing more free movement. The catalytic activity of the palladium complexes of the arsines and their phosphine analogues for the reaction of 4-bromoacetophenone and n-butyl acrylate has been screened. The results show that the arsines are generally superior to the phosphines as ligands for this catalysis. Tri(o-isopropylphenyl)phosphine and tri(o-isopropylphenyl)arsine are superior to tri-o-tolylphosphine as ligands for this Heck reaction and a p-methoxy substituent improves the arsine catalyst but not the phosphine catalyst. The phosphine catalysts are superior to the arsine catalysts for the reaction of 4-chloroacetophenone and n-butyl acrylate. These observations are discussed in the context of ligand stereoelectronic effects, as measured by the Tolman electronic parameter, nuCO of the [NiL(CO)3]{L=AsAr3 or PAr3}.     

A Mössbauer effect study of Y2Fe17 and Y2Fe17N2.6

The Mössbauer effect spectra of Y₂Fe₁₇ and Y₂Fe₁₇N_2.6, obtained at 85 K, are analyzed with a model consistent with their crystallographic structures and band structure calculations of their magnetic moments. The increases in the individual site isomer shifts and hyperfine fields upon nitrogenation correlate well with the increases in both the individual Wigner-Seitz cell volumes and the magnetic moments. The spectral analysis, which requires ten sextets, indicates that the magnetization lies in a general direction in the hexagonal basal plane of these compounds.