Brian F. G. Johnson

Structural studies of trigonal lamellar particles of gold and silver

The results of detailed structural studies of trigonal lamellar particles of both gold and silver are presented. The particles have been characterized both in sol by means of optical spectroscopy and powder X-ray diffraction and ex sol using high resolution electron microscopy in both plan view and profile imaging modes. The results of these studies have indicated that the particles have a trigonal outline and are shortened along a ≺111≻ direction to give a plate-like morphology. The presence of small numbers of parallel {111} twin planes has also been confirmed and used to explain the presence of the formally forbidden ⅓{422} reflections observed in plan view. The precise structural requirements for the observation of such reflections has also been confirmed using multislice calculations. Possible growth mechanisms for these particles are also discussed.

The synthesis, characterization and molecular structures of two mixed metal octahedral carbido clusters, Ru5RhC(CO)14(η5-C5Me5) and Ru5RhC(CO)9(η5-C5Me5)(η5-C5H5)2

Abstract Ionic coupling between the dianionic ruthenium cluster [Ru 5 C(CO 14 ] 2− and the dicationic rhodium species [Rh(C 5 Me 5 )MeCN) 3 ] 2+ in refluxing CH 2 Cl 2 yields the hexanuclear mixed metal carbido cluster Ru 5 RhC(CO) 14 (η 5 -C 5 Me 5 ) ( 1 ). Activation of ( 1 ) towards reaction with C 5 H 6 or C 6 H 8 by trimethylamine- N -oxide (Me 3 NO) results in the formation of Ru 5 RhC (CO) 9 (η 5 -C 5 Me 5 )(η 5 C 5 H 5 ) 2 ( 2 ) and Ru 5 RhC(CO) 11 (η 5 -C 5 Me 5 (μ 3 -η 2 :η 2 :η 2 -C 6 H 6 ) 3 respectively. Complexes 1 and 2 have been structurally characterized by X-ray diffraction.

Arene-cluster compounds

The synthesis and full characterization of a wide and diverse range of arene-clusters based on M 3 , M 4 , M 5 and M 6 metal-cluster units are described and their relevance to the chemisorption of arenes on to a metal surface is critically reviewed. Emphasis is placed on the cluster-sandwich compounds which are clearly related to Fischers classic example bis(benzene)chromium. The molecular organization in crystals of a variety of arene clusters has been investigated and good evidence for substantial intermolecular interaction between adjacent arene moieties established. More recent studies of the interaction of a range of larger polycyclic systems with similar cluster precursors are also described, as are attempts to provide suitable monomeric arene-cluster subtrates for polymer formation. Finally, attempts to produce Os 3 (CO) 6 (μ 3 -C 6 H 6 ) 2 , an attractive analogue of Cr(C 6 H 6 ) 2 , are reported

The coordination of benzene in clusters : the face-capping mode

Abstract The use of discrete metal cluster complexes as models of chemisorption systems in surface chemistry is an attractive hypothesis and has been emphasised by numerous workers in recent years. In this paper we shall examine possible boundary conditions to such a cluster-surface analogy, drawing extensively on the interfacial chemistry of benzene. The motivation for this work is provided by the molecular chemistry of a new class of arene clusters in which the arene molecule is bonded to an M3 triangulo-face within the cluster. The parent compounds in these studies, [M3(CO)9(μ3:η2:η2:η2-C6H6)] (M = Ru or Os), contain benzene in this important new face-capping bonding mode that accurately models benzene adsorption at a threefold site on the surface of a close-packed metal lattice. X-ray diffraction studies combined with NMR (1H and13C) and infrared spectroscopic studies lead to the conclusion that the benzene is coordinated to the metal triangle and is best viewed as a bond-localised cyclohexa-1,3,5-triene.

A new mechanism for the rearrangement of the icosahedral carboranes

Abstract A new mechanism which involves the anticubeoctahedron as the complementary geometry is postulated to rationalise previously reported data on the rearrangement of various icosahedral carboranes.

The preparation, characterisation and low temperature X-ray structure of Ru6(η2-μ4-CO)2(CO)13(η6-C6Me6)☆

Abstract The new organometallic cluster (η2-μ4-CO)2(CO)13(η6-C6Me6) has been prepared by the thermolysis of Ru3(CO)12 with hexamethylbenzene in octane and characterised by a single crystal X-ray diffraction study. It is isostructural with the known cluster Ru6(η2-μ4-CO)2(CO)13(η6-C6H3Me3) and the metal core constitutnts the same tetrahedral Ru4 unit with two edge-bridging Ru atoms. The mesitylene derivative has been shown to undergo rearrangement to afford the octahedral carbido cluster Ru6C(CO)14(η6-C6H3Me3), but this conversion is not observed for the new hexamethylbenzene derivative.

THE FORMATION OF CARBIDES IN HEXARUTHENIUM CARBONYL CLUSTERS

Abstract The introduction of carbido atoms into hexaruthenium clusters is discussed in terms of the cleavage of susceptible carbonyl ligands coordinated to non-carbido precursors. The isolation of an intermediate containing dihapto-carbonyl ligands in the well established synthesis of [Ru 6 C(CO) 14 (η 6 -arene)] species by thermolysis of [Ru 3 (CO) 12 ] in the presence of the arene, and its demonstrated conversion to the carbide a second hexaruthenium product and carbon dioxide, is interpreted in terms of a bimolecular mechanism involving cleavage of a dihapto-carbonyl ligand. With reference to this mechanism, the virtually quantitative conversion of [Ru 6 (CO) 18 ] 2− ( 5 ) into [Ru 6 C(CO) 16 ] 2− ( 6 ) and carbon dioxide which occurs at high temperature in solution is discussed, and an autocatalytic mechanism involving a bimolecular CO cleavage step is suggested. Finally, the conversion of [Ru 6 (CO) 18 ] 2− ( 5 ) into the neutral carbide [Ru 6 C(CO) 17 ] ( 1 ) by carbonyl ligand Cue5f8O cleavage induced by reaction with trifluoromethanesulfonic or trifiuoroethanoic anhydrides is discussed.

The mechanism of carbonyl substitution reactions of Ir4(CO)12−nLn (n=0−3)

Abstract A mechanism in which the heterolytic fission of the metal-metal bond is considered to be the key initial step in the substitution reactions of the tetranuclear clusters Ir4(CO)12−nLn (n=0–3) is described.

The synthesis, molecular and crystal structure of the bis(arene) hexaruthenium carbido-carbonyl isomers Ru6C(CO)11(C6H4Me2-1,3)(C6H5Me)☆

Abstract The reaction of Ru6C(CO)14(η6C6H4Me2-1,3) (1) with dihydrotoluene (C6H7Me) and Me3NO in dichloromethane yields Ru6C(CO)12(η6C6H4Me2-1,3)(μ2-C6H7Me) (2), Ru6C(CO)11(η6C6H4Me2-1,3)(μ3-C6H5Me) (3) and cis-Ru6C(CO)11(η6C6H4Me2-1,3)(η6-C6H5Me) (4). Reaction of 1 with toluene and Me3NO in dichloromethane-acetone affords trans-Ru6C(CO)11(η6C6H4Me2-1,3)(η6-C6H5Me) (5). On standing at room temperature 3 isomerises spontaneously to 4. The molecular structures of 4 and 5 have been established by X-ray crystallography.

Arene migration in ruthenium clusters: a kinetic study of the isomerisation of Ru5C(CO)12(μ3-η2:η2:η2-C6H6) to Ru5C(CO)12(η6-C6H6)

The migration of benzene in Ru5C(CO)12(C6H6) from a μ3-η2:η2:η2 position to an η6 site has been monitored by 1H NMR spectroscopy at 313, 323, 333, 343 and 356 K. From this data a detailed kinetic analysis has been undertaken. Possible transition state species and thus a mechanism for this isomerisation process are postulated, and the results discussed in the broader context of benzene surface phenomena, which is re-evaluated where appropriate.