Michael J. Page

Hemilabile and bimetallic coordination in Rh and Ir complexes of NCN pincer ligands.

Two new pincer ligands have been developed that contain a central N-heterocyclic carbene (NHC) moiety linked to two pendant pyrazole groups by either a methylene (NCN(me)) or ethylene (NCN(et)) chain. The coordination of these two ligands to rhodium and iridium resulted in a variety of binding modes. Tridentate coordination of the ligands was observed in the complexes [Rh(NCN(me))(COD)]BPh4 (8), [Ir(NCN(me))(COD)]BPh4 (10), [Rh(NCN(et))(CO)2]BPh4 (13), and [Ir(NCN(me))(CO)2]BPh4 (14), and monodentate NHC coordination was observed for [Ir(NCN(me))2(COD)]BPh4 (11) and [Ir(NCN(et))2(COD)]BPh4 (12). Both tridentate and bidentate coordination modes were characterized for [Rh(NCN(et))(COD)]BPh4 (9) in the solution and solid state, respectively, while an unusual bridging mode was observed for the bimetallic complex [Rh(μ-NCN(me))(CO)]2(BPh4)2 (15). The impact of this diverse coordination chemistry on the efficiency of the complexes as catalysts for the addition of NH, OH, and SiH bonds to alkynes was explored.

Pyrazolyl-N-heterocyclic carbene complexes of rhodium as hydrogenation catalysts: The influence of ligand steric bulk on catalyst activity

A series of bidentate 1-(1-pyrazolylmethyl)-substituted NHC ligands (13a-c, 14a-c and 15a-c) were synthesised with substituents of varying steric bulk incorporated adjacent to the donor atoms. These ligands were coordinated to rhodium(I) to give a series of complexes of the general formula [Rh(L)(COD)]BPh4 (where L = a mixed-donor pyrazolyl-NHC ligand and COD = 1,5-cyclooctadiene). The solid state structures of [Rh(13b)(COD)]BPh4 (16b), [Rh(13c)(COD)]BPh4 (16c), [Rh(14a)(COD)]BPh4 (17a), [Rh(14b)(COD)]BPh4 (17b), [Rh(15a)2(COD)]BPh4 (18a), and [Rh(15b)(COD)]BPh4 (18b) were determined by single crystal X-ray diffraction. The complex [Rh(15a)2(COD)]BPh4 (18a) is unusual in that two of the pyrazolyl-NHC ligands (15a) are coordinated to the metal through the NHC donor instead of one ligand forming the expected chelate. These complexes (with the exception of 18a) were found to be effective catalysts for the hydrogenation of styrene. The catalytic activity was correlated with complex structure, and it was found that the greater the steric bulk of the metal bound ligand, the slower the rate of the hydrogenation.

Rhodium(I) and iridium(I) complexes of pyrazolyl-N-heterocyclic carbene ligands

Several Rh(I) and Ir(I) complexes containing an N-heterocyclic carbene-pyrazolyl chelate ligand have been synthesised. Determination of the single-crystal X-ray structure of the Ir(I) complex showed a novel binding mode with the iridium centre coordinated to two ligands via two carbene donors in preference to one ligand forming the entropically favoured chelate. The hydrogenation activity of several of these complexes was investigated along with that of previously synthesised Rh(I) and Ir(I) complexes containing an analogous phosphine-pyrazolyl chelate.

Polymorphs and pseudo-polymorphs: nine crystals containing [Fe(phen)3]2+ associated with [HgI4]2−

Nine different solvated crystals of [Fe(phen)3]2+ [HgI4]2− are described. They occur in six different crystal lattices (denoted A–F) and have the compositions A1-(acetone)(H2O), A2-(acetone)(H2O), A-(dmso)(H2O), B-(acetone)2, B-(dmf)2, C-acetone, D-(H2O)1.5, E-(dmf)2 and F-(ethanol)(CH3CN). No unsolvated crystals were found in many crystallisation experiments. This set of nine solvates includes two sets of isomorphous crystals (A ×3, B ×2), and two pairs of strict dimorphs (B-(dmf)2 and E-(dmf)2, and A1-(acetone)(H2O) and A2-(acetone)(H2O)): the difference between A1-(acetone)(H2O) and A2-(acetone)(H2O) is subtle. Analyses of the crystal packing, in relation to crystallisation conditions, lead to the general interpretation that electrostatic energies between the doubly charged ions are the dominant influence, causing the ion arrays to be fairly regular, but varied by the local motifs between [Fe(phen)3]2+ and [HgI4]2−, which are influenced by the invariant flanged shapes of [Fe(phen)3]2+ and [HgI4]2− and use I⋯phen-face and I⋯H–C phen-edge interactions. The known concerted embrace motifs between [Fe(phen)3]2+ complexes do not occur in these crystals. The geometrical requirements of the motifs between [Fe(phen)3]2+ and [HgI4]2− generate space in the crystals which is occupied by the solvent molecules, and it appears that the solvents best able to occupy these spaces form the more stable crystals.

Alkyne Activation Using Bimetallic Catalysts

Bimetallic catalysts are capable of activating alkynes to undergo a diverse array of reactions. The unique electronic structure of alkynes enables them to coordinate to two metals in a variety of different arrangements. A number of well-characterised bimetallic complexes have been discovered that utilise the versatile coordination modes of alkynes to enhance the rate of a bimetallic catalysed process. Yet, for many other bimetallic catalyst systems, which have achieved incredible improvements to a reactions rate and selectivity, the mechanism of alkyne activation remains unknown. This chapter summarises the many different approaches that bimetallic catalysts may be utilised to achieve cooperative activation of the alkyne triple bond.