D. Michael P. Mingos

Dielectric parameters relevant to microwave dielectric heating

Microwave dielectric heating is rapidly becoming an established procedure in synthetic chemistry. This review summarises the basic theory underlying microwave dielectric heating and collates the dielectric data for a wide range of organic solvents which are commmonly used in microwave syntheses. The loss tangents of the solvents, which may be related to the ability of the solvent to absorb energy in a microwave cavity, depend on the relaxation times of the molecules. These relaxation times depend critically on the nature of the functional groups and the volume of the molecule. Functional groups capable of hydrogenbonding have a particularly strong influence on the relaxation times. The relaxation times of solvents decrease as the temperature of the solvent is increased. Loss tangent data at different microwave frequencies are also presented and discussed.

Tilden Lecture. Applications of microwave dielectric heating effects to synthetic problems in chemistry

This review has presented the principles underlying the dielectric heating effects observed for chemical compounds in solution and in the solid state. The applications of the technique to a wide range of chemical syntheses have also been indicated. The field is in its infancy and therefore much of the effort to date has been directed towards understanding the rate enhancements of known reactions. The next few years should see the isolation of new compounds using microwave heating techniques

Molecular orbital analysis of the bonding in low nuclearity gold and platinum tertiary phosphine complexes and the development of isolobal analogies for the M(PR3) fragment

Abstract The stoichiometries and geometries of gold and platinum tertiary phosphine cluster compounds of the type M n (PR 3 ) n x + ( n = 3–6) have been analysed using extended Huckel molecular orbital calculations. The isolobal nature of the M(PH 3 ) fragment depends critically on the nature of the metal atom M, and may be used to provide a basis for predicting a wide spectrum of homo- and hetero-nuclear metal cluster compounds.

Effects of microwave dielectric heating on heterogeneous catalysis

The effect of microwave dielectric heating on both endothermic and exothermic reactions was investigated. Apparent equilibrium shifts for both reactions were observed which were attributed to the formation of spatial hot spots in the catalyst bed. The possible location of remarkable temperature gradients was examined experimentally and theoretically.

Superheating effects associated with microwave dielectric heating

Fluoroptic temperature measurements have established that organic solvents in a microwave cavity superheat by 13–26 °C above their conventional boiling points at atmospheric pressure; this behaviour is interpreted using a model of nucleate boiling that emphasises the importance of the wetting properties of the solvents.

A historical perspective on Dewar's landmark contribution to organometallic chemistry

Abstract A historical background to Dewars development of a bonding model for metal–alkene complexes in 1951 is reviewed and its implications described. Chatt and Duncansons subsequent contribution is also reviewed.

Determination of the Tolman cone angle from crystallographic parameters and a statistical analysis using the crystallographic data base

SummaryA new method of evaluating the Tolman cone angle from X-ray structural data available from the Cambridge Crystallographic Data Base has been developed and a statistical analysis of the cone angles of the phosphines PPh3, PMe2Ph, PMePh2, PMe3, PEt3 and PCy3 (Cy = cyclohexyl) in transition metal complexes has been completed.

Synthesis, structural characterization and photophysical properties of ethyne-gold( I) complexes

Abstract A series of closely related ethynyl-gold(I) complexes was synthesized by reaction of Au(PR 3 )Cl with an alkaline solution of the ethyne. The molecular structures of the ethynediyl-digold complexes NpPh 2 PACCAuPNpPh 2 ·2CHCl 3 ( 1 ) (Np = naphthyl), Np 2 PhPAuCCAuPNp 2 Ph·6CHCl 3 ( 2 ) and Fc 2 PhPAu CCAuPFc 2 Ph·4EtOH ( 3 ) (Fc = ferrocenyl) and the phenylethynyl-gold complex Fc 2 PhPAuC CPh ( 4 ) were determined by single-crystal X-ray diffraction measurements. Variation of the phosphines does not have a significant influence on the bonding in the central PAuCC unit, the AuP and AuC distances being in the ranges 2.274(4)–2.289(5) and 1.983(8)–2.002(6) A, respectively. Although none of the compounds have short Au⋯Au contacts, compounds 1 and 2 do show novel CH⋯π interactions between the proton of CHCl 3 and the π-electron system of the CC bond. In 1 , pairs of CHCl 3 molecules are located with their protons 2.4 A from the centre of the CC bond with the C H bond directed orthogonally towards the centre of the ethyne bond. In 2 , two pairs of CHCl 3 molecules are located around the CC bond, with 2.5 A between the proton and the centre of the triple bond, resulting in a pseudo-octahedral arrangement around CC. In addition to the CH⋯π interactions, the structures of 1 and 2 also show a range of intermolecular aromatic-aromatic interactions. The first structural determination of naphthylphosphines resulted in estimates of their steric requirements. The UV-visible spectra of CH 2 Cl 2 solutions of the ethynediyl compounds exhibit intense absorption bands at ca. 300 nm assignable to intraligand transitions. Excitation of solid sample or fluid solution of complex 1 at λ > 330 nm resulted in intense long-lived luminescence. Excitation of a solution of 2 at 350 and 380 nm led to different types of photoluminescence.

Microwave assisted catalytic reduction of sulfur dioxide with methane over MoS2 catalysts

The catalytic reduction of sulfur dioxide with methane to form carbon dioxide and sulfur has been studied over MoS2/Al2O3 catalysts. The reaction has been found to occur with microwave (2.45 GHz) heating at recorded temperatures as much as 200 ◦ C lower than those required when conventional heating was used. An activation energy of 117 kJ mol −1 has been calculated for the conventionally heated reaction, but an Arrhenius analysis of the data obtained with microwave heating was not possible, probably because of temperature variations in the catalyst bed. The existence of hot spots in the catalysts heated by microwave radiation has been verified by the detection of -alumina at a recorded temperature some 200 ◦ C lower than the temperature at which the -t o-alumina phase transition is normally observed. Among four catalysts prepared in different ways, a mechanically mixed catalyst showed the highest conversion of SO2 and CH4 for microwave heating at a given temperature. Supported catalysts, sulfided either by conventional heating or under microwave conditions, showed little difference in the extent of SO2 and CH4 conversions. The highest conversions to carbon dioxide and sulfur, combined with low production of undesirable side products, was obtained when the molar ratio of SO2 to CH4 was equal to two, the stoichiometric ratio.

Molecular and crystal structure of 3,3-bis(triethylphosphine)-1,2-di-carba-3-platinadodecaborane(11), and molecular-orbital analysis of the ‘slip’ distortion in carbametallaboranes

Crystals of the title compound are monoclinic, space group P21/n with unit-cell dimensions a= 9.375(4), b= 15.985(3), c= 16.033(7)A, and β= 93.56(5)°. The structure has been solved using 6 068 observed reflections recorded at ca. 215 K on a four-circle diffractometer, and refined by least squares to R 0.047. The geometry of the cage is that of a highly distorted icosahedron in which the platin urn atom ‘slips’ towards B (8) and the metalbonded face ‘folds’ across B(4)… B(7). Molecular-orbital calculations based on the extended- Huckel approximation have accounted for the ‘slip’ and ‘fold’ distortions observed in this and related Carbametallaboranes.