Issues Relevant to C-H Activation at Platinum(II): Comparative Studies between Cationic, Zwitterionic, and Neutral Platinum(II) Compounds in Benzene Solution

Cationic late metal systems are being highly scrutinized due to their propensity to mediate so-called electrophilic C-H activation reactions. This contribution compares the reactivity of highly reactive cationic platinum(II) systems with structurally related but neutral species. Our experimental design exploits isostructural neutral and cationic complexes supported by bis(phosphine) ligands amenable to mechanistic examination in benzene solution. The data presented herein collectively suggests that neutral platinum complexes can be equally if not more reactive towards benzene than their cationic counter-parts. Moreover, a number of unexpected mechanistic distinctions between the two systems arise that help to explain their respective reactivity

Low-metallicity star formation: Relative impact of metals and magnetic fields

Low-metallicity star formation poses a central problem of cosmology, as it determines the characteristic mass scale and distribution for the first and second generations of stars forming in our Universe. Here, we present a comprehensive investigation assessing the relative impact of metals and magnetic fields, which may both be present during low-metallicity star formation. We show that the presence of magnetic fields generated via the small-scale dynamo stabilises the protostellar disc and provides some degree of support against fragmentation. In the absence of magnetic fields, the fragmentation timescale in our model decreases by a factor of ~10 at the transition from Z=0 to Z>0, with subsequently only a weak dependence on metallicity. Similarly, the accretion timescale of the cluster is set by the large-scale dynamics rather than the local thermodynamics. In the presence of magnetic fields, the primordial disc can become completely stable, therefore forming only one central fragment. At Z>0, the number of fragments is somewhat reduced in the presence of magnetic fields, though the shape of the mass spectrum is not strongly affected in the limits of the statistical uncertainties. The fragmentation timescale, however, increases by roughly a factor of 3 in the presence of magnetic fields. Indeed, our results indicate comparable fragmentation timescales in primordial runs without magnetic fields and Z>0 runs with magnetic fields.Comment: MNRAS in pres

Bis(phosphino)borates: A New Family of Monoanionic Chelating Phosphine Ligands

The reaction of dimethyldiaryltin reagents Me_(2)SnR_(2) (R ) Ph (1), p-MePh (2), m,m-Me_(2)Ph (3), p-tBuPh (4), p-MeOPh (5), p-CF_(3)Ph (6)) with BCl_(3) provided a high-yielding, simple preparative route to the corresponding diarylchloroboranes R_(2)BCl (R ) Ph (10), p-MePh (11), m,m-Me2Ph (12), p-tBuPh (13), p-MeOPh (14), p-CF_(3)Ph (15)). In some cases, the desired diarylchloroborane was not formed from an appropriate tin reagent Me_(2)SnR_(2) (R ) o-MeOPh (7), o,o-(MeO)_(2)Ph (8), o-CF_(3)Ph (9)). The reaction of lithiated methyldiaryl- or methyldialkylphosphines with diarylchloroboranes or dialkylchloroboranes is discussed. Specifically, several new monoanionic bis(phosphino)borates are detailed: [Ph_(2)B(CH_(2)PPh_(2))_(2)] (25); [(p-MePh)_(2)B(CH_(2)PPh_(2))_(2)] (26); [(p-^(t)BuPh)_(2)B(CH_(2)PPh_(2))_(2)] (27); [(p-MeOPh)_(2)B- (CH_(2)PPh_(2))_(2)] (28); [(p-CF_(3)Ph)_(2)B(CH_(2)PPh_(2))_(2)] (29); [Cy_(2)B(CH_(2)PPh_(2))_(2)] (30); [Ph_(2)B(CH_(2)P{p-^(t)BuPh}_(2))_(2)] (31); [(p-MeOPh)_(2)B- (CH_(2)P{p-^(t)BuPh}_(2))_(2)] (32); [Ph_(2)B(CH_(2)P{p-CF_(3)Ph}_(2))_(2)] (33); [Ph_(2)B(CH_(2)P(BH_(3))(Me)_(2))_(2)] (34); [Ph_(2)B(CH_(2)P(S)(Me)_(2))_(2)] (35); [Ph_(2)B(CH_(2)PiPr_(2))_(2)] (36); [Ph_(2)B(CH_(2)P^(t)Bu_(2))_(2)] (37); [(m,m-Me_(2)Ph)_(2)B(CH_(2)P^(t)Bu_(2))_(2)] (38). The chelation of diarylphosphine derivatives 25-33 and 36 to platinum was examined by generation of a series of platinum dimethyl complexes. The electronic effects of substituted bis(phosphino)borates on the carbonyl stretching frequency of neutral platinum alkyl carbonyl complexes were studied by infrared spectroscopy. Substituents remote from the metal center (i.e. on boron) have minimal effect on the electronic nature of the metal center, whereas substitution close to the metal center (on phosphorus) has a greater effect on the electronic nature of the metal center

Benzene C−H Activation at a Charge Neutral Zwitterionic Platinum(II) Complex

Cationic, coordinatively unsaturated metal centers exhibit a wide range of both stoichiometric and catalytic transformations. Such species are frequently generated by methide abstraction with a strong Lewis acid, or alternatively by protonation with an acid whose conjugate base is noncoordinating or weakly coordinating.3 We are targeting charge neutral, zwitterionic complexes that display reaction chemistry reminiscent of these reactive metal centers. Of particular interest is the development of species that exhibit transformations at X-H bonds, where an X-H bond refers generally to a C-H or other robust σ bond. A conceptual diagram illustrating this strategy is shown in Scheme 1

Space physics analysis network node directory (The Yellow Pages): Fourth edition

The Space Physics Analysis Network (SPAN) is a component of the global DECnet Internet, which has over 17,000 host computers. The growth of SPAN from its implementation in 1981 to its present size of well over 2,500 registered SPAN host computers, has created a need for users to acquire timely information about the network through a central source. The SPAN Network Information Center (SPAN-NIC) an online facility managed by the National Space Science Data Center (NSSDC) was developed to meet this need for SPAN-wide information. The remote node descriptive information in this document is not currently contained in the SPAN-NIC database, but will be incorporated in the near future. Access to this information is also available to non-DECnet users over a variety of networks such as Telenet, the NASA Packet Switched System (NPSS), and the TCP/IP Internet. This publication serves as the Yellow Pages for SPAN node information. The document also provides key information concerning other computer networks connected to SPAN, nodes associated with each SPAN routing center, science discipline nodes, contacts for primary SPAN nodes, and SPAN reference information. A section on DECnet Internetworking discusses SPAN connections with other wide-area DECnet networks (many with thousands of nodes each). Another section lists node names and their disciplines, countries, and institutions in the SPAN Network Information Center Online Data Base System. All remote sites connected to US-SPAN and European-SPAN (E-SPAN) are indexed. Also provided is information on the SPAN tail circuits, i.e., those remote nodes connected directly to a SPAN routing center, which is the local point of contact for resolving SPAN-related problems. Reference material is included for those who wish to know more about SPAN. Because of the rapid growth of SPAN, the SPAN Yellow Pages is reissued periodically

Zwitterionic and Cationic Bis(phosphine) Platinum(II) Complexes: Structural, Electronic, and Mechanistic Comparisons Relevant to Ligand Exchange and Benzene C−H Activation Processes

Structurally similar but charge-differentiated platinum complexes have been prepared using the bidentate phosphine ligands [Ph_(2)B(CH_(2)PPh_(2))_(2)], ([Ph_(2)BP_(2)], [1]), Ph_(2)Si(CH_(2)PPh_(2))_(2), (Ph_(2)SiP_(2), 2), and H_(2)C(CH_(2)PPh_(2))_(2), (dppp, 3). The relative electronic impact of each ligand with respect to a coordinated metal center's electron-richness has been examined using comparative molybdenum and platinum model carbonyl and alkyl complexes. Complexes supported by anionic [1] are shown to be more electron-rich than those supported by 2 and 3. A study of the temperature and THF dependence of the rate of THF self-exchange between neutral, formally zwitterionic [Ph_(2)BP_(2)]Pt(Me)(THF) (13) and its cationic relative [(Ph_(2)SiP_(2))Pt(Me)(THF)][B(C_(6)F_(5))_(4)] (14) demonstrates that different exchange mechanisms are operative for the two systems. Whereas cationic 14 displays THF-dependent, associative THF exchange in benzene, the mechanism of THF exchange for neutral 13 appears to be a THF independent, ligand-assisted process involving an anchimeric, η3-binding mode of the [Ph_(2)BP_(2)] ligand. The methyl solvento species 13, 14, and [(dppp)Pt(Me)(THF)][B(C_(6)F_(5))_(4)] (15), each undergo a C−H bond activation reaction with benzene that generates their corresponding phenyl solvento complexes [Ph_(2)BP_(2)]Pt(Ph)(THF) (16), [(Ph_(2)SiP_(2))Pt(Ph)(THF)][B(C_(6)F_(5))_(4)] (17), and [(dppp)Pt(Ph)(THF)][B(C_(6)F_(5))_(4)] (18). Examination of the kinetics of each C−H bond activation process shows that neutral 13 reacts faster than both of the cations 14 and 15. The magnitude of the primary kinetic isotope effect measured for the neutral versus the cationic systems also differs markedly (k(C6H6)/k(C6D6):  13 = 1.26; 14 = 6.52; 15 6). THF inhibits the rate of the thermolysis reaction in all three cases. Extended thermolysis of 17 and 18 results in an aryl coupling process that produces the dicationic, biphenyl-bridged platinum dimers [{(Ph_(2)SiP_(2))Pt}2(μ-η3:η3-biphenyl)][B(C6F5)4]2 (19) and [{(dppp)Pt}2(μ-η^(3):η^(3)-biphenyl)][B(C_(6)F_(5))_(4)]_(2) (20). Extended thermolysis of neutral [Ph_(2)BP_(2)]Pt(Ph)(THF) (16) results primarily in a disproportionation into the complex molecular salt {[Ph_(2)BP_(2)]PtPh_(2)}-{[Ph_(2)BP_(2)]Pt(THF)_(2)}+. The bulky phosphine adducts [Ph_(2)BP_(2)]Pt(Me){P(C_(6)F_(5))_(3)} (25) and [(Ph_(2)SiP_(2))Pt(Me){P(C_(6)F_(5))_(3)}][B(C_(6)F_(5))_(4)] (29) also undergo thermolysis in benzene to produce their respective phenyl complexes, but at a much slower rate than for 13−15. Inspection of the methane byproducts from thermolysis of 13, 14, 15, 25, and 29 in benzene-d6 shows only CH_(4) and CH3D. Whereas CH_(3)D is the dominant byproduct for 14, 15, 25, and 29, CH_(4) is the dominant byproduct for 13. Solution NMR data obtained for 13, its 13C-labeled derivative [Ph_(2)BP_(2)]Pt(^(13)CH_(3))(THF) (13-^(13)CH_(3)), and its deuterium-labeled derivative [Ph_(2)B(CH_(2)P(C_(6)D5)_(2))_(2)]Pt(Me)(THF) (13-d20), establish that reversible [Ph_(2)BP_(2)]-metalation processes are operative in benzene solution. Comparison of the rate of first-order decay of 13 versus the decay of d_(20)-labeled 13-d_(20) in benzene-d_(6) affords k_(13)/k_(13-d20) ~ 3. The NMR data obtained for 13, 13-^(13)CH_3, and 13-d_20 suggest that ligand metalation processes involve both the diphenylborate and the arylphosphine positions of the [Ph_(2)BP_(2)] auxiliary. The former type leads to a moderately stable and spectroscopically detectable platinum(IV) intermediate. All of these data provide a mechanistic outline of the benzene solution chemistries for the zwitterionic and the cationic systems that highlights their key similarities and differences

A homoleptic phosphine adduct of Tl(I)

A homoleptic phosphine adduct of thallium(I) supported by a tris(phosphino)borate ligand has been isolated and structurally characterized

Floating Point Geometric Algorithms for Topologically Correct Scientific Visualization

The unresolved subtleties of floating point computations in geometric modeling become considerably more difficult in animations and scientific visualizations. Some emerging solutions based upon topological considerations will be presented. A novel geometric seeding algorithm for Newton\u27s method was used in experiments to determine feasible support for these visualization applications

What affects authors' and editors' use of reporting guidelines? Findings from an online survey and qualitative interviews.

OBJECTIVES: To identify and understand, through data from multiple sources, some of the factors that affect authors' and editors' decisions to use reporting guidelines in the publication of health research. DESIGN: Mixed methods study comprising an online survey and semi-structured interviews with a sample of authors (online survey: n = 56; response rate = 32%; semi-structured interviews: n = 5) and journal editors (online survey: n = 43; response rate = 27%; semi-structured interviews: n = 6) involved in publishing health and medical research. Participants were recruited from an earlier study examining the effectiveness of the TREND reporting guideline. RESULTS: Four types of factors interacted to affect authors' and editors' likelihood of reporting guideline use; individual (e.g., having multiple reasons for use of reporting guidelines); the professional culture in which people work; environmental (e.g., policies of journals); and, practical (e.g., having time to use reporting guidelines). Having multiple reasons for using reporting guidelines was a particularly salient factor in facilitating reporting guidelines use for both groups of participants. CONCLUSIONS: Improving the completeness and consistency of reporting of research studies is critical to the integrity and synthesis of health research. The use of reporting guidelines offers one potentially efficient and effective means for achieving this, but decisions to use (or not use) reporting guidelines take many factors into account. These findings could be used to inform future studies that might, for example, test the factors that we have identified within a wider theoretical framework for understanding changes in professional practices. The use of reporting guidelines by senior professionals appears to shape the expectations of what constitutes best practice and can be assimilated into the culture of a field or discipline. Without evidence of effectiveness of reporting guidelines, and sustained, multifaceted efforts to improve reporting, little progress seems likely to be made.National Institute for Health Research (NIHR) Collaboration for Leadership in Applied Health Research and Care (CLAHRC) for the South West Peninsula. (http://clahrc-peninsula.nihr.ac.uk/)