Kinetic and structural studies on 'tethered' Ru(II) arene ketone reduction catalysts.

A series of kinetic and structural investigations on ruthenium-based catalysts for asymmetric transfer hydrogenation (ATH) of ketones are reported. A method is reported for monitoring the formation of ruthenium hydride species in real time using (1)H NMR spectroscopy.

Solvents for ring-closing metathesis reactions.

A study of the influence of eight diverse solvents on a Grubbs II-catalysed ring-closing metathesis (RCM) reaction reveals a complex dependence of the different reaction steps on the solvent and suggests acetic acid as a useful solvent for RCM reactions.

Catalytic alkene cyclohydroamination via an imido mechanism.

Chiral-at-metal half-sandwich diamide complexes catalyse enantioselective cyclohydroamination of aminoalkenes at unexpectedly high rates given their high coordination number and steric bulk; substantial evidence is presented which argues against the established sigma-bond insertion process and is strongly indicative of an imido [2+2] cycloaddition mechanism.

Umbrella motion in aziridines: use of simple chemical inputs to reversibly control the rate of pyramidal inversion.

The molecular motion associated with atomic inversion at an aziridine nitrogen can be essentially halted by metal complexation; addition of a second chemical input that decomplexes the metal from the aziridine restores fast inversion (k=40 s(-1) at 303 K).

Chiral metal architectures in aminopyridinato complexes of zirconium.

Optically pure 2-alkylaminopyridines (HL) are synthesised readily from bromopyridines and chiral amines [(S)-1,2,3,4-tetrahydro-1-naphthylamine and (S)-(-)-alpha-methylbenzylamine] using palladium-catalysed amination. Protonolysis reactions of these proligands with ZrX(4)(X = NMe(2), CH(2)Ph, CH(2)Bu(t)) yield zirconium aminopyridinates, usually of the type [ML(2)X(2)], some of which have been characterised by X-ray crystallography. Control of absolute configuration at the metal centre is pursued by investigation of the effects of chiral amine substituent, substitution at the pyridine rings and the identity of co-ligands. Surprisingly the conformationally flexible [small alpha]-methylbenzyl based aminopyridinato ligands promote much better control of chirality-at-zirconium than do the cyclic tetrahydronaphthyl analogues. One complex of the former class displays complete control of stereochemistry at 193 K; only one diastereomer out of eight possible structures is observed. It is found that there is an excellent correlation between observed selectivities and calculated diastereomer energy differences from DFT. All the complexes studied are in dynamic exchange between diastereomers. The rate of these processes (DeltaH ca. 40 kJ mol(-1)) as studied by Selective Polarisation Transfer-Selective Inversion Recovery experiments (SPT-SIR) and lineshape analyses are significantly faster than those for aminopyridines containing bulkier amido substituents (DeltaH ca. 70 kJ mol(-1)). This type of dependence on steric effects, and the impact of the trans effect, is consistent with an N-dissociative mechanism, i.e. conversion from six- to five-coordinate structure followed by rapid intramolecular scrambling.

Using noncovalent intra-strand and inter-strand interactions to prescribe helix formation within a metallo-supramolecular system.

The effect of inter-strand and intra-strand interactions is explored in a metallo-supramolecular system in which the metal-ligand coordination requirements may be satisfied by more than one different supramolecular architecture. This is achieved by introducing alkyl substituents onto the spacers of readily prepared bis(pyridylimine) ligands. The alkyl substituents induce twisting within the ligand strand and this intra-strand effect favours formation of helical architectures. The alkyl substituents also introduce inter-strand CH.pi interactions into the system. For the smaller methyl group these are most effectively accommodated in a trinuclear circular helicate architecture. A solution mixture of dinuclear double-helicate and trinuclear circular helicate results from which, for copper(I), the trinuclear circular helicate crystallises. The CH.pi interactions endow the circular helicate with a bowl-shaped conformation and the triangular unit aggregates into a tetrahedral ball-shaped array. Low-temperature NMR studies indicate that the CH.pi interactions also confer a bowl-shaped conformation on the triangle in solution. The larger ethyl groups can sustain intra-strand CH.pi interactions in the lower nuclearity double-helical system and this is the unique architecture for that ligand system in both solution and the solid state. Crystal structures are described for both the copper(I) and silver(I) complexes. Thus we show that intra-strand interactions may be used to induce helicity within this system, while the nuclearity of the array can be prescribed by the inter-strand interactions.

Silver-phosphine complexes of the highly methylated carborane monoanion [closo-1-H-CB11Me11]-.

The synthesis of the silver(I) salt of the highly methylated carborane anion [closo-1-H-CB(11)Me(11)](-) is described, Ag[closo-1-H-CB(11)Me(11)] 1, which in the solid state shows close intermolecular Ag...H(3)C contacts. Addition of various monodentate phosphines to 1 results in the formation of the complexes (R(3)P)Ag[closo-1-H-CB(11)Me(11)] [R = Ph, 2; cyclohexyl (C(6)H(11)), 3; (3,5-Me(2)-C(6)H(3)), 4]. All these complexes show close intermolecular Ag.H(3)C contacts in the solid state that are considerably shorter than the sum of the van der Waals radius of methyl (2.00 A) and the ionic radius of silver(I) (1.29 A). For 2 and 3 there are other close intermolecular Ag...H(3)C contacts in the solid state, arising from proximate carborane anions in the crystal lattice. Addition of methyl groups to the periphery of the phosphine ligand (complex 4) switches off the majority of these interactions, leaving essentially a single cage interacting with the cationic silver-phosphine fragment through three CH(3) groups. In solution (CD(2)Cl(2)) Ag...H(3)C contacts remain, as evidenced by both the downfield chemical shift change and the significant line-broadening observed for the cage methyl signals. These studies also show that the metal fragment is fluxional over the surface of the cage. The Ag...H(3)C interactions in solution may be switched off by addition of a stronger Lewis base than [closo-1-H-CB(11)Me(11)](-). Thus, addition of [NBu(4)][closo-1-H-CB(11)H(5)Br(6)] to 2 affords (Ph(3)P)Ag[closo-1-H-CB(11)H(5)Br(6)], while adding Et(2)O or PPh(3) affords the well-separated ion-pairs [(Ph(3)P)(L)Ag][closo-1-H-CB(11)Me(11)] (L = OEt(2) 5, PPh(3) 6,) both of which have been crystallographically characterized. DFT calculations on 2 (at the B3LYP/DZVP level) show small energy differences between the possible coordination isomers of this compound, with the favored geometry being one in which the [(Ph(3)P)Ag](+) fragment interacts with three of the [BCH(3)] vertices on the lower surface of the cage, similar to the experimentally observed structure of 4.

Efficient predetermination of chirality-at-zirconium.

Only one of eight possible diastereomers of the organometallic chiral-at-metal complex [ZrL2(CH2Ph)2] (L = a bidentate, chiral non-racemic pyrdine alcoholate) is observed by NMR spectroscopy in the slow exchange regime.

Problems and solutions for alkene polymerisation catalysts incorporating Schiff-bases; migratory insertion and radical mechanisms of catalyst deactivation.

Steric blocking of an intramolecular 1,2-migratory insertion reaction of a zirconium salicylaldiminato complex leads to a long-lived catalyst for ethene polymerisation, but promotes a new radical catalyst decomposition mechanism in certain instances; kinetic and thermodynamic parameters for both pathways have been established.

Spacer Control of Directionality in Supramolecular Helicates Using an Inexpensive Approach.

Careful selection of the spacer group used to separate the metal-binding domains allows control of the directionality in a helix; self-assembly leads uniquely to a double-helical cation with a head-to-tail (HT) configuration (shown schematically) in both the solid state and solution.