Title

Manipulation of the ligand sphere of ruthenium metathesis catalysts for the synthesis of organometallic molecular wires

Date of Award

2008

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor(s)

Michael Sponsler

Keywords

Olefin metathesis, Molecular wires, Ruthenium, Metathesis catalysts

Subject Categories

Chemistry | Inorganic Chemistry | Organic Chemistry | Physical Sciences and Mathematics

Abstract

The Grubbs first- and second-generation catalysts were used as the building blocks toward organometallic molecular wires. Two main strategies were employed to modify the catalysts: olefin metathesis and ancillary ligand exchange.

Two equivalents of the first-generation catalyst were reacted with 1,3,5,7-octatetraene to give the new diruthenium compound [(PCy 3 ) 2 Cl 2 Ru] 2 (μ-CHCH=CHCH=CHCH). This stoichiometric reaction was named olefin metathesis for metal incorporation (OMMI) describing the metathesis replacement of the terminal CH 2 groups with a metal center. Structural characterization of the complex was done using NMR spectroscopy and NMR simulation. The electronic characterization using UV-vis and CV/DPV indicated that the new complex does exhibit intermetal communication.

Several attempts were made to produce aurophilic catalysts as precursors for molecules to be studied using an STM break junction method. This study utilized both ancillary ligand exchange (using pyrizine, dimethylaminopyridine, bipyridine, and mercaptopyridine) and OMMI (using vinylpyridine and vinylimidazole) to alter the ligand sphere around ruthenium. The catalysts with the new pyridinyl ligands were not suitable for the synthesis of diruthenium complexes using OMMI as their metathesis activity was too low.

The metathesis activity of the 3-bromopyridine analog of the first-generation catalyst was studied and found to be almost identical to the Grubbs first-generation catalyst in initiation but inferior when it came to stability during a reaction. While the usefulness of this complex as a metathesis catalyst is limited, it was found to be useful for ancillary ligand exchange. Ligand exchange using PBu 3 was also successfully done using the first-generation complexes to give a mixture of two new metathesis inactive complexes in situ. The new complexes could not be isolated but were characterized using 1 H and 31 P NMR.

Phosphine exchange using PMe 3 and PBu 3 was performed to the synthesize second-generation complexes (H 2 IMes)RuCl 2 (=CHPh)(PMe 3 ) and (H 2 IMes)RuCl 2 (=CHPh)(PBu 3 ) with reduced metathesis activity. The rates of OMMI with ethyl vinyl ether using phosphine-exchanged products were over two orders of magnitude less than that of Grubbs second generation catalyst. The PMe 3 complex was also shown to undergo an unprecedented associative phosphine exchange mechanism. The reaction of PBu 3 with the PMe 3 complex was faster than metathesis, depended on the concentration of incoming phosphine, gave second order kinetic plots, and through Eyring analysis showed a negative entropy of activation.

The PMe 3 complex, (H 2 IMes)RuCl 2 (=CHPh)(PMe 3 ), possessed an open coordination site that was more available than in other phosphine containing Grubbs-type catalysts. Carbon monoxide (CO) was stoichiometrically added to give a new saturated complex where a CO ligand filled the open coordination site. Excess CO led to a product that had two CO ligands and the benzylidene inserted into a mesityl group on the imidazolylidine. Phosphine exchange was also performed on diruthenium complexes in an attempt to make the bimetallic complexes more stable. Two stable diruthenium complexes were prepared but purification was not completely successful. The synthesis of the new diruthenium complexes along with the work on aurophilic complexes has paved the way for future work toward diruthenium organometallic molecular wires.

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