Fundamental Investigations Into Dinitrogen Fixation by Group 4 Metals

Abstract

Addition of terminal or internal alkynes to a base free titanocene oxide results in synthesis of the corresponding oxometallocyclobutene. With appropriate cyclopentadienyl substitution, these compounds undergo reversible C-C reductive elimination offering a unique approach to cyclopentadienyl modification. Subsequent reactivity demonstrates the complete scission of the Ti=O multiple bond.

Cycloaddition of monosubstituted allenes with a monomeric, base free titanocene oxide resulted in isolation and crystallographic characterization of the corresponding oxatitanacyclobutanes. In solution these compounds are a mixture of (E) and (Z) isomers and interconvert by mechanisms that are dependent on the specific substitution of the allene. Facile carbonylation of the oxatitanacyclobutanes was also observed to yield rare examples of structurally characterized oxatitanacyclopentanones. These studies highlight the new chemistry available from synthesis of base free titanocene oxide compounds enabled by appropriate cyclopentadienyl substitution.

The hydrogenolysis of titanium nitrogen bonds in a family of bis(cyclopentadienyl) titanium amides, hydrazides, and imides via proton coupled electron transfer (PCET) is demonstrated. (η5-C5Me5)(py-Ph)Rh-H (py-Ph = 2-pyridylphenyl, [Rh]-H) and (η5- C5R5)(CO)3CrH ([Cr]R-H, R= H, Me) were used as catalysts for homolytic H2 activation followed by PCET to the nitrogen-containing fragment. Detailed mechanistic studies and an analysis of the underlying thermochemistry are employed to explain the decreased catalytic efficiency of [Cr]R-H compared to [Rh]-H. The N-H bond dissociation free energies (BDFEs) in 12 structurally similar compounds were determined through a combination of experimental and computational methods, providing a foundation for the use of N-H BDFEs as a metric to enable NH3 synthesis from H2 and N2 at a well-defined metal center.

Combination of the readily available a-diimine ligand, ((ArN=C(Me))2 Ar = 2,6-iPr2- C6H3), (iPrDI) with air-stable nickel(II) bis(carboxylates) generated a highly active catalyst exhibiting anti-Markovnikov selectivity for the hydrosilylation of alkenes with (EtO)3SiH. The exclusive selectivity for formation of terminal alkyl silanes was also observed with internal alkenes via a tandem isomerization-hydrosilylation pathway. The hydrosilylation of 1-octene with triethoxysilane, a reaction performed commercially in the silicones industry on a scale of > 12,000,000 lbs/year, was performed on a 10 g scale with 96 % yield and >98 % selectivity for the desired product.