Publications

The reactivity of the amino(cyano)borane ⁱPr₂NBH(CN) (2) towards [RuH₂(η²-H₂)₂(PCy₃)₂] (3) was investigated. Our study reveals that the formation of the bis(isocyanoborane) ruthenium complex [RuH(CN)(CN-BHNⁱPr₂)₂(PCy₃)₂] (6) occurs via the intermediacy of the bis(σ-B–H) ruthenium complex [RuH(CN)(H₂BNⁱPr₂)(PCy₃)₂] (5). This transformation involves BCN linkage isomerisation in 2. The diisopropylaminoborane ligand in 5 can be displaced to ultimately produce the bis(σ-borane) complex [RuH₂(η²:η²-H₂BNⁱPr₂)(PCy₃)₂] (4) as a by-product.

Density functional theory (DFT, PBE0, and range separated DFT, RSH + MP2) and coupled-cluster with single and double and perturbative triple excitations (CCSD(T)) calculations have been used to probe the structural preference of d⁴ MH₃X^q (M = Ru, Os, Rh⁺, Ir⁺, and Re^–; X = H, F, CH₃, CF₃, SiH₃, and SiF₃) and of MX₄ (M = Ru; X = H, F, CH₃, CF₃, SiH₃, and SiF₃). Landis et al. have shown that complexes in which the metal is sd³ hybridized have tetrahedral and non-tetrahedral structures with shapes of an umbrella or a 4-legged piano stool. In this article, the influence of the metal and ligands on the energies of the three isomeric structures of d⁴ MH₃X and MX₄ is established and rationalized. Fluoride and alkyl ligands stabilize the tetrahedral relative to non-tetrahedral structures while hydride and silyl ligands stabilize the non-tetrahedral structures. For given ligands and charge, 4d metal favors more the non-tetrahedral structures than 5d metals. A positive charge increases the preference for the non-tetrahedral structures while a negative charge increases the preference for the tetrahedral structure. The factors that determine these energy patterns are discussed by means of a molecular orbital analysis, based on Extended Hückel (EHT) calculations, and by means of Natural Bond Orbital (NBO) analyses of charges and resonance structures (NRT analysis). These analyses show the presence of through-space interactions in the non-tetrahedral structures that can be sufficiently stabilizing, for specific metals and ligands, to stabilize the non-tetrahedral structures relative to the tetrahedral isomer.

The ²⁹Si chemical shifts in a series of closely related Ru(II) silyl complexes have been calculated by DFT methods and compared to the experimental values. The factors that lead to possible discrepancies between experimental and calculated values have been identified. It is shown that it is necessary to include the spin-orbit coupling associated with the relativistic effects of the heavy atoms for quantitative agreement with observed chemical shifts but trends are reasonably reproduced when the calculations do not include this correction. An NBO analysis of the NMR contributions from the bonds to Si and the Si core shows the greater importance of the former and a fine tuning originating from the latter.

Density functional theory calculations have been employed to model ethene hydroarylation using an [Ir(κ²-OAc)(PMe₃)Cp]⁺ catalyst, 1. The reaction proceeds via: (i) an acetate-assisted C–H activation of benzene via an AMLA-6 transition state; (ii) rate-limiting insertion of ethene into the Ir–Ph bond; and (iii) protonolysis of the β-phenylethyl species by HOAc. A range of competing processes are assessed, the most important of which are the C–H activation of ethene at 1 and trapping of the β-phenylethyl intermediate with ethene. The former process gives rise to Ir–vinyl species which can then access further ethene insertion to give stable allyl by-products. A comparison with other ethene hydroarylation catalysts reported in the literature is presented.

The results of a joint computational and experimental study of the cyclometallation reactions of dimethylbenzylamine (DMBA-H) with [IrCl₂Cp*]₂ and a range of chelating bases are presented. With acetate, density functional theory calculations on the key intermediate, [Ir(DMBA-H)(κ²-OAc)Cp]⁺, define a two-step C–H activation process involving initial κ²–κ¹ displacement of base to give an intermediate that is stabilized by internal H-bonding. Facile C–H bond cleavage then occurs via ‘ambiphilic metal ligand activation’ (AMLA). A similar pattern is computed for other carboxylates and bicarbonate, and in each case the ease of C–H activation is governed by the accessibility of the κ²–κ¹ base displacement step; thus, more weakly coordinating bases promote C–H activation. For triflate, [Ir(DMBA-H)(κ¹-CF₃SO₃)Cp]⁺ is more stable than its κ²-isomer and C–H activation proceeds with a barrier of only 3.8 kcal mol⁻¹. Experimental studies confirm that a range of carboxylates and triflate can effect cyclometallation; however, reactivity patterns are not consistent with the computed C–H activation barriers. Instead, the role of base in opening the [IrCl₂Cp*]₂ dimer and subsequent formation of the [Ir(DMBA-H)(base)Cp*]⁺ intermediates appears crucial. Calculations indicate these processes are far more favourable for acetate than for triflate.

Recent computational studies of C–H bond activation at late transition metal systems are discussed and processes where lone pair assistance via heteroatom co-ligands or carboxylates are highlighted as a particularly promising means of cleaving C–H bonds. The term ‘ambiphilic metal ligand activation’ (AMLA) is introduced to describe such reactions.

A well-characterised 14-electron rhodium phosphine complex, [Rh(PⁱPr₃)₃][BAr^F₄], which contains a β-CH agostic interaction, is observed to undergo spontaneous dehydrogenation to afford [Rh(PⁱPr₃)₂(PⁱPr₂(C₃H₅))][BAr^F₄]; calculations on a model system show that while C–H activation is equally accessible from the β-CH agostic species or an alternative γ-CH agostic isomer, subsequent β-H-transfer can only be achieved along pathways originating from the β-CH agostic form.

Pauson–Khand reactions of functionalized allenes with different alkynes give cyclopentenones with generally high regio- and stereoselectivities. The allenes react through their external double bonds, giving cyclopentenones with exocyclic double bond at their β positions and predominantly with E stereochemistry. Some intramolecular reactions with allenynes connected through aromatic rings are described. These give the corresponding heterocycles with moderate to good yields.

Intermolecular Pauson–Khand reactions are still quite limited in scope and yields. There are also few reports using allenes as the olefinic part in this version. We report herein new regio and stereoselective reactions of allenamides with several alkynes. These reactions give functionalized cyclopentenones bearing an exocyclic enamide, which can be useful synthetic intermediates.