The effects of an electron-withdrawing group on the organic chemistry of an η-bound benzene ring are explored using the complex TpW(NO)(PMe)(η-PhCF). This trifluorotoluene complex was found to undergo a highly regio- and stereoselective 1,2-addition reaction involving protonation of an ortho carbon followed by addition of a carbon nucleophile. The resulting 1,3-diene complexes can undergo a second protonation and nucleophilic addition with a range of nucleophiles including hydrides, amines, cyanide, and protected enolates. Interestingly, the addition of the second proton and nucleophile occurs in a 1,4-fashion, again with a high degree of regio- and stereocontrol. Oxidation of the metal allows for the isolation of highly substituted trifluoromethylcyclohexenes with as many as four stereocenters set by the metal. The ability to synthesize enantio-enriched organics was also demonstrated for a diene and a trisubstituted cyclohexene. Substitution from an enantio-enriched η-dimethoxybenzene complex in neat trifluorotoluene yielded enantio-enriched trifluorotoluene complex, which was elaborated into cyclohexadienes and cyclohexenes with ee's ranging from 92 to 99%.

Eleven 2,2′-bipyridine (bpy) ligands functionalized with attachment groups for covalent immobilization on silicon surfaces were prepared. Five of the ligands feature silatrane functional groups for attachment to metal oxide coatings on the silicon surfaces, while six contain either alkene or alkyne functional groups for attachment to hydrogen-terminated silicon surfaces. The bpy ligands were coordinated to Re(CO)5Cl to form complexes of the type Re(bpy)(CO)3Cl, which are related to known catalysts for CO2 reduction. Six of the new complexes were characterized using X-ray crystallography. As proof of principle, four molecular Re complexes were immobilized on either a thin layer of TiO2 on silicon or hydrogen-terminated silicon. The surface-immobilized complexes were characterized using X-ray photoelectron spectroscopy, IR spectroscopy, and cyclic voltammetry (CV) in the dark and for one representative example in the light. The CO stretching frequencies of the attached complexes were similar to those of the pure molecular complexes, but the CVs were less analogous. For two of the complexes, comparison of the electrocatalytic CO2 reduction performance showed lower CO Faradaic efficiencies for the immobilized complexes than the same complex in solution under similar conditions. In particular, a complex containing a silatrane linked to bpy with an amide linker showed poor catalytic performance and control experiments suggest that amide linkers in conjugation with a redox-active ligand are not stable under highly reducing conditions and alkyl linkers are more stable. A conclusion of this work is that understanding the behavior of molecular Re catalysts attached to semiconducting silicon is more complicated than related complexes, which have previously been immobilized on metallic electrodes.

The dihapto-coordination of benzene to the πbasic fragment {TpW(NO)(PMe 3 )} (Tp = hydridotris-(pyrazolyl)-borate) enhances the basicity of the arene ligand to the point that it can be protonated with a mild Brønsted acid (diphenylammonium triflate; pK a ∼ 1). The resulting η 2benzenium complex reacts with a wide range of nucleophiles including protected enolates, cyanide, amines, methoxide, and aromatic nucleophiles to form 5-substituted 3,4-η 2 -1,3-cyclohexadiene complexes in good yield (42−70%). These coordinated dienes were successfully taken through a second protonation and nucleophilic addition with a similar scope of nucleophiles (54− 80%). The resulting cis-3,4-and cis-3,6-disubstituted η 2 -cyclohexene complexes were prepared with high regio-and stereocontrol, as governed by the asymmetric nature of π-allyl intermediates. In some cases, a diene linkage isomerization from 3,4-η 2 to 1,2-η 2 could be effected with a redox catalyst, and reactions of the latter species led to cis-3,5-disubstituted cyclohexene products exclusively. Oxidative decomplexation afforded the free cyclohexene products in moderate yield (37−68%). Additionally, when a single enantiomer of the chiral dearomatization agent was used, the elaborated cyclohexenes were able to be synthesized in enantioenriched forms (86−90% enantiomeric excess). Full characterization of 40 new compounds is provided that includes two-dimensional NMR, IR, electrochemical and in some cases crystallographic data.

Multiple H2O2-forming mechanisms are accessible in Fe(porphyrin)-catalysed oxygen reduction, a key reaction in both fuel cell technologies and oxygen-utilizing enzymes.