John L. Spencer

The coordination chemistry of pentafluorophenylphosphino pincer ligands to platinum and palladium.

The synthesis of electron-poor PCP pincer ligands 1,3-((C6F5)2PO)2C6H4, 1,3-((C6F5)2PCH2)2C6H4, and 1-((C6F5)2PO)-3-(tBu2PCH2)C6H4, and their coordination chemistry to platinum and palladium is described. The most electron-poor ligand 1,3-((C6F5)2PO)2C6H4 (POCOPH) reacts with Group 10 metal chloride precursors to form a range of unusual cis, trans-dimers of the type κ(2)-P,P-[(POCOPH)MCl(L)]2 (M = Pt, Pd; L = Cl, Me), which undergo metallation to form [(POCOP)MCl] pincer complexes only under prolonged thermolysis. The formation of such cis,trans-dimers during pincer complex formation can be mitigated through the use of starting materials with more strongly binding ancillary ligands, improving the overall rate of ligand metallation. Carbonyl complexes of the type [(PCP)M(CO)](+) were synthesised from the pincer chloride complexes by halide abstraction, and displayed large ν(C-O) values, from 2170-2111 cm(-1), confirming the electron-poor nature of the compounds. The [(PCP)Pd(CO)](+) complexes also demonstrated the ability to reversibly bind carbon monoxide both in solution and the solid state, with the rate of decarbonylation increasing with increasing wavenumber for the C-O stretch.

Some problems in understanding the electronic transport properties of carbon nanotube ropes

Abstract The resistance of single-wall carbon nanotube (SWCN) ropes or mats, and some individual tubes, typically shows a crossover from non-metallic to metallic temperature dependence as temperature increases. This systematic pattern is consistent with a series heterogeneous model involving metallic resistance and tunnelling through barriers such as defects and inter-rope contacts. The metallic resistivity term increases linearly with temperature for the ropes or mats, but faster for the individual nanotubes. In contrast to the almost vanishing thermoelectric power expected from electronic band structure calculations, the measured values for mats or films (including recent measurements in a vacuum) are even larger than for typical metals. The thermopower increases with temperature as for metals, but has a characteristic non-linear shape. This temperature dependence can be modelled, for example, with parallel conduction in metallic and semiconducting tubes, but the size of the metallic thermopower required is anomalously large.

Sidechain engineering in anthracene derivatives: towards photofunctional liquid crystals

A series of anthracene derivatives were synthesised to explore how their sidechain configurations influenced their phase behaviour and thereby guiding the design of photofunctional liquid crystalline materials. In the case of 9,10-diphenylanthracene derivatives, longer and more alkyl sidechains resulted in lower melting temperatures, yet liquid crystallinity was not observed. A novel room-temperature molecular liquid was synthesised based on 9,10-diphenylanthracene, the optical properties of which may be exploited in photonic applications. Liquid crystallinity was observed in one of the derivatives of 9,10-bis(phenylethynyl)anthracene, forming a nematic phase at around 210°C. These results highlight the potential opportunities for photofunctional materials with enhanced properties if liquid crystalline anthracenes can be found with lower phase transition temperatures.

Transition metal complexes of the pyridylphosphine ligand o-C6H4(CH2PPy2)2

The synthesis and coordination behaviour of the pyridylphosphine ligand o-C6H4(CH2PPy2)2 (Py = 2-pyridyl) are reported. The phosphine selenide was synthesised and the 1JPSe value of 738 Hz indicates the phosphorus atoms have a similar basicity to PPh3. The ligand reacts with platinum(ii) and palladium(ii) complexes to give simple diphosphine complexes of the type [MX2(PP)] (M = Pt, X = Cl, I, Me, Et; M = Pd, X = Cl, Me). When the ligand is reacted with chloromethyl(hexa-1,5-diene)platinum the [PtClMe(PP)] complex results, from which a series of unsymmetrical platinum complexes of the type [PtMeL(PP)]+ (L = PPh3, PTA, SEt2 and pyridine) can be made. This enabled the comparison of the cis and trans influences of a range of ligands. The following cis influence series was compiled based on 31P NMR data of these complexes: Py ≈ Cl > SEt2 > PTA > PPh3. Reaction of [PtClMe(PP)] with NaCH(SO2CF3)2 and carbon monoxide slowly formed an acyl complex, where the CO had inserted in the Pt-Me bond. Attempts to achieve P,P,N chelation, through abstracting the chloride ligand in [PtClMe(PP)], were unsuccessful. When the ligand reacted with platinum(0), palladium(0) and silver(i) complexes the bis-chelated complexes [M(PP)2] (M = Pt, Pd) and [Ag(PP)2]+ were formed respectively. Reaction of the ligand with [Ir(COD)(μ-Cl)]2 formed [IrCl(PP)(COD)]. When the chloride ligand was abstracted, the pyridyl nitrogens were able to interact with the iridium centre facilitating the isomerisation of the 1,2,5,6-η4-COD ligand to a 1-κ-4,5,6-η3-C8H12 ligand. The X-ray crystal structure of [Ir(1-κ-4,5,6-η3-C8H12)(PPN)]BPh4 confirmed the P,P,N chelation mode of the ligand. In solution, this complex displayed hemilabile behaviour, with the pyridyl nitrogens exchanging at a rate faster than the NMR time scale at room temperature.

Crystal structure of bis­[1,3-bis­(di­phenyl­phosphan­yl)propane-κ2P,P′]platinum(II) dichloride chloro­form penta­solvate

In the title compound, [Pt{Ph2P(CH2)3PPh2}2]Cl2·5CHCl3, the PtII cations, located on a centre of inversion, is coordinated by two chelating diphosphane ligands in a geometry which is close to square-planar. The chelate rings adopt a chair conformation. The PtII cations are arranged in layers separated by Cl− anions as well as CHCl3 solvent molecules. While this complex has been reported previously [Anderson et al. (1983 ▸). Inorg. Chim. Acta, 76, L251–L252], this is the first time a structure has been determined.

The Synthesis of Carbon Nanotubes from Metal Nanoparticles

Iron, nickel and cobalt nanoparticles on the order of 2–5 nm in diameter have been synthesised and deposited on oxidised silicon wafers. These wafers have then been subjected to plasma enhanced chemical vapour deposition using ethanol as a carbon source gas. Preliminary results show the presence of long, narrow carbonaceous structures at temperatures as low as 275° C.

Manipulation and purification of multi-walled carbon nanotubes

Carbon nanotubes (CNTs) consist of one or more rolled-up sheets of graphitic structure. Some preparative methods produce single-walled cylinders, others give tubes with several nested cylinders arranged concentrically (multiwalled carbon nanotubes, or MWCNTs). Electrically, they can behave as semiconductors or metals, as also predicted theoretically. The wide range of conductivity should permit a purification and fractionation of the tubes by means of the force exerted on them in an electric field. We describe micro-scale concentration and orientation of MWCNTs, and also their characterisation using Raman spectroscopy. This is used to monitor their purification using medium-scale apparatus incorporating a flow cell.