Reactions of 29 Transition Metal Cations, in the Same Oxidation State and under the Same Gas-Phase Conditions, with Sulfur

A total of 29 transition metals (all except Tc), all as ions M(+), have been reacted with gaseous S(8). The reactivities and reaction products provide a unique set of comparative data on a fundamental reaction of the elements. The results underlie the interpretation of many other processes and compounds in condensed phases. Series of product ions [MS(y)()](+) are formed, with y generally starting at 4, and increasing with time through 8 up to 10, 12, 16, or 21 (for La(+)). A general mechanism is proposed, in which the first {MS(8)}(+) encounter complex is reactive and undergoes S-S bond scission and rearrangement around the metal, such that [MS(8)](+) is not an early product. The early transition metals react faster than later members of the series, and third row metals react about twice as fast as first row metals. The metals which are more chalcophilic in condensed-phase chemistry are apparently less so as M(+); Hg(+) does not form observable [HgS(y)()](+) (except for a very low yield of [HgS(3)](+)) and is remarkably less reactive with sulfur than most of the other metal ions. Simple electron transfer between M(+) and S(8) does not occur except possibly for Ir(+), but S(8)(+) is sometimes observed and is believed to be formed by electron transfer from S(8) to some [MS(y)()](+) complexes. Interpretation of the rates of reaction of the ions of groups 3, 4, and 5 with S(8) is complicated because they react with adventitious water in the cell forming oxo-species. The results are discussed in the context of condensed-phase metal polysulfide chemistry.     

A [2 + 2] cycloaddition route to dimethylaminomethylene vinamidinium salts

[reaction: see text] Trifluoropropanoic acid reacts with 1 equiv of POCl3 in DMF to generate the trifluoromethyl enamine (7). At this stage, two reaction manifolds are available. The expected reaction with additional POCl3 generates the 2-trifluoromethyl vinamidinium salt (3c). However, thermally driven loss of fluoride generates an iminium ion, which sets the stage for a [2 + 2] cycloaddition to ultimately generate the dimethylaminomethylene vinamidinium salt (1).     

Determination and characterization of a transition structure for water–formaldehyde addition

Quantum mechanical calculations of the geometric, energetic, electronic, and vibrational features of a transition structure for gas-phase water–formaldehyde addition (FW1?) are described, and a new transition-structure search algorithm is presented. Basis-set-dependent effects are assessed by comparisons of computed properties obtained from self-consistent field (SCF) molecular orbital (MO) calculations with STO-3G, 4-31G, and 6-31G^** basis sets in the absence of electron correlation. The results obtained suggest that STO-3G-level calculations may be sufficiently reliable for the prediction of the transition structure of FW1? and for the transition structures of related carbonyl addition reactions. Moreover, the calculated activation energy for formation of FW1? from water and formaldehyde (?44 kcal mol^?1) is very similar in all three basis sets. However, the energy of formaldehyde hydration predicted by STO-3G (? ?45 kcal mol^?1) is about three times larger than that predicted by the other two basis sets, with the activation energy for dihydroxymethane dehydration also being too large in STO-3G. Calculated force constants in all three basis sets are generally too large, leading to vibrational frequencies that are also too large. However, uniformly scaled force constants (in internal coordinates) give much better agreement with experimental frequencies, scaled 4-31G force constants being slightly superior to scaled STO-3G force constants.     

Pd-catalysed cross coupling of terminal alkynes to diynes in the absence of a stoichiometric additive

An efficient, room temperature procedure for the cross-coupling of a range of terminal alkynes, using standard Sonogashira cross-coupling conditions (Pd/Cu) is presented. At higher reaction temperatures, head-to-tail or head-to-head dimerisation affords 1,3- and 1,4-disubstituted enynes, respectively as minor products.     

Ammonium cyanate shows N-H...N hydrogen bonding, not N-H...O

The transformation of ammonium cyanate into urea, first studied over 170 years ago by W?hler and Liebig, has an important place in the history of chemistry. To understand the nature of this solid state reaction, knowledge of the crystal structure of ammonium cyanate is a prerequisite. Employing neutron powder diffraction, we demonstrate conclusively that, in the structure of ammonium cyanate, the NH(4)(+) cation forms N-H...N hydrogen bonds to four cyanate N atoms at alternate corners of a distorted cube, rather than our previously proposed alternative arrangement with N-H...O hydrogen bonds to cyanate O atoms at the other four corners.     

Direct functionalization of the cyclometalated 2-(2′-pyridyl)phenyl ligand bound to iridium(III)

Treatment of [Ir(ppy)₂(μ-Cl)]₂ and [Ir(ppy)₂(dtbpy)][OTf] (ppy = 2-(2′-pyridyl)phenyl; dtbpy = 4,4′-di-tert-butyl-2,2′-bipyridine; OTf = triflate) with pyridinium tribromide in the presence of Fe powder led to isolation of [Ir(4-Br-ppy)(μ-Br)]₂ (1) and [Ir(4-Br-ppy)₂(dtbpy)][OTf] (2), respectively. Pd-catalyzed cross-coupling of 2 with RB(OH)₂ afforded [Ir(4-R-ppy)₂(dtbpy)][OTf] (R = 4′-FC₆H₄ (3)), 4′-PhC₆H₄ (4), 2′-thienyl (5), 4′-C₆H₄CH₂OH (6). Treatment of 4 with B₂(pin)₂ (pin = pinacolate) afforded [Ir{4-(pin)B-ppy}₂(dtbpy)][OTf] (7). The alkynyl complexes [Ir(4-PhCC-ppy)₂(dtbpy)][OTf] (8) and [Ir{4-Me₂(OH)CC-ppy}(4-Br-ppy)(dtbpy)][OTf] (9) were prepared by cross-coupling of 2 with PhCCSnMe₃ and Me₂C(OH)CCH, respectively. Ethynylation of [Ir(fppy)₂(dtbpy)][OTf] (fppy = 5-formyl-2-(2′-pyridyl)phenyl) with Ohira’s reagent MeCOC(N₂)P(O)(OEt)₂ afforded [Ir{5-HCC-ppy}₂(dtbpy)][OTf] (10). The solid-state structures of 2, 5, 7, and 10 have been determined.     

Structural similarities and differences among mixed-metal clusters containing a single M(PPh3) (M = Cu, Ag or Au) fragment

The heteronuclear metal clusters [H₃MRu₄(CO)₁₂(PPh₃)] and [MFe₃(μ-COMe)(CO)₁₀(PPh₃)] (M = Cu, Ag or Au) have been prepared; the gold-tetraruthenium cluster has a different metal core geometry to that adopted by the copper and silver analogues, whereas all three iron-Group IB mixed-metal clusters have the same geometry.     

Donor-stabilized cations and imine transfer from N-silylphosphoranimines

A series of donor-stabilized N-silylphosphoranimine salts [DMAP.PCl2=NSiMe3]+X- (DMAP = 4-(dimethylamino)pyridine) were prepared by the reaction of Cl3P=NSiMe3 with DMAP in the presence of silver salts AgX (X = OSO2CF3, BF4, and SbF6). Repeating the reaction in the absence of AgX gave the chloride salt [DMAP.PCl2=NSiMe3]Cl which has been shown to be in equilibrium with free DMAP and Cl3P=NSiMe3. Attempts to stabilize a N-silylphosphoranimine cation with phosphine donors led to unexpected imine transfer chemistry. For example, Cl3P=NSiMe3 reacts with phosphines, R3P (R = nBu and Ph), to produce the metathesis products PCl3 and R3P=NSiMe3 which subsequently react together to afford the N-phosphinophosphoranimines R3P=N-PCl2 and ClSiMe3 as a byproduct.     

Applying Tikhonov regularization to process pendant droplet tensiometry data

The problem of obtaining the first and second derivatives of the profile of a pendant droplet is formulated as an integral equation of the first kind. This equation is solved by Tikhonov regularization in which the method of general cross validation is used to guide the selection of the regularization parameter. These derivatives are converted into mean curvature as a function of droplet height. Surface tension is then obtained by regression computation between the mean curvature and two possible algebraic expressions suggested by the Laplace-Young equation. This way of obtaining surface tension is demonstrated by applying it to a number of published droplet profiles. Some of the problems encountered are discussed and solutions suggested.     

"Exact" surface free energies of iron surfaces using a modified embedded atom method potential and lambda integration

Previously a new universal lambda-integration path and associated methodology was developed for the calculation of "exact" surface and interfacial free energies of solids. Such a method is in principle applicable to any intermolecular potential function, including those based on ab initio methods, but in previous work the method was only tested using a relatively simple embedded atom method iron potential. In this present work we apply the new methodology to the more sophisticated and more accurate modified embedded atom method (MEAM) iron potential, where application of other free- energy methods would be extremely difficult due to the complex many-body nature of the potential. We demonstrate that the new technique simplifies the process of obtaining "exact" surface free energies by calculating the complete set of these properties for the low index surface faces of bcc and fcc solid iron structures. By combining these data with further calculations of liquid surface tensions we obtain the first complete set of exact surface free energies for the solid and liquid phases of a realistic MEAM model system. We compare these predictions to various experimental and theoretical results.