No abstract

With petroleum supplies dwindling, there is increasing interest in selective methods for transforming other carbon feedstocks into hydrocarbons suitable for transportation fuel. We report the development of highly productive, well-defined, tandem catalytic systems for the metathesis of n-alkanes. Each system comprises one molecular catalyst (a "pincer"-ligated iridium complex) that effects alkane dehydrogenation and olefin hydrogenation, plus a second catalyst (molecular or solid-phase) for olefin metathesis. The systems all show complete selectivity for linear (n-alkane) product. We report one example that achieves selectivity with respect to the distribution of product molecular weights, in which n-decane is the predominant high-molecular-weight product of the metathesis of two moles of n-hexane.

A significant increase in the C−O stretching force constant (F CO) and a decrease in C−O bond length (r CO) result upon coordination of carbon monoxide to various cationic species. We report a study designed to elucidate the factors responsible for this effect. In particular, we distinguish between an explanation based on electrostatic effects and one based on withdrawal of electron density from the 5σ orbital of CO, an orbital generally considered to have some antibonding character. Ab initio electronic structure calculations on CO in the presence of a positive point charge (located on the carbon side of the bond axis) reveal that a simple Coulombic field increases the C−O stretching force constant and decreases the bond length. Coordination of CO to a simple cationic Lewis acid such as H+ or CH3 + is calculated to increase F CO (and decrease r CO) to extents slightly less than those engendered by a point charge at the same distance from the carbonyl carbon. These results indicate that electron donation from the 5σ orbital has no intrinsic positive effect on the magnitude of F CO. Calculations were also conducted on several symmetrical, neutral, and cationic transition metal complexes, including some examples of the recently discovered homoleptic noble-metal carbonyls. It is found that F CO values can be quantitatively interpreted using a model which invokes only the effects of M−CO π-back-bonding and an electrostatic parameter. There is no correlation between the extent of σ-bonding (as measured by the depopulation of the CO σ orbitals) and F CO. Calculations on trigonal bipyramidal d8 metal pentacarbonyls permit a comparison between inequivalent ligands (axial and equatorial) which, being coordinated to the same metal center, must experience approximately the same electrostatic field. In the case of Ru(CO)5, π-back-bonding to the axial and equatorial carbonyls is of virtually equal magnitude, while σ-donation is much greater from the axial ligands than from the equatorial ligands. Nevertheless, the F CO and r CO values of the two ligand sets are essentially equal, confirming that the magnitude of σ-donation does not affect these parameters.

The insertion of an iridium complex into an N-H bond in ammonia leads to a stable monomeric amido hydride complex in solution at room temperature. This reaction advances the transition-metal coordination chemistry of ammonia beyond its role for more than a century as an ancillary ligand. The precursor for this insertion reaction is an iridium(I) olefin complex with an aliphatic ligand containing one carbon and two phosphorus donor atoms. Kinetic and isotopic labeling studies indicate that olefin dissociates to give a 14-electron iridium(I) fragment, which then reacts with ammonia. This cleavage of the N-H bond under neutral conditions provides a foundation on which to develop future mild catalytic transformations of ammonia, such as olefin hydroamination and arene oxidative amination.