The Mechanism And Thermodynamics Of Alkane And Arene Carbon Hydrogen Bond Activation By Pentamethylcyclopentadienyl Rhodium Compounds

Author : Meagan Elizabeth Evans
Publisher :
ISBN 13 :
Total Pages : 452 pages
Book Rating : 4.:/5 (756 download)

DOWNLOAD NOW!

Download or read book I. Energetics of C-H Bond Activation of Functionalized Hydrocarbons written by Meagan Elizabeth Evans and published by . This book was released on 2011 with total page 452 pages. Available in PDF, EPUB and Kindle. Book excerpt: "Several transition-metal systems have been used to establish correlations between metal-carbon and carbon-hydrogen bonds. In the following studies, the [Tp'RhL] fragment where Tp' = tris(3,5-dimethylpyrazolyl)borate and L = neopentyl isocyanide, is used to investigate C-H bond activation of molecules with strong C-H bonds. The first study (Chapter 2) examines C-H bond activation of fluorinated aromatic hydrocarbons. Photolysis of the precursor, Tp'RhL(carbodiimide) in neat fluoroarene resulted in C-H activation products of the type, Tp'RhL(arylF)H. Both the kinetic and thermodynamic selectivities of rhodium for different C-H bonds on the fluorinated aromatic were examined. A strong thermodynamic preference was observed for C-H activation ortho to two fluorine atoms as opposed to one. Competition experiments along with [...characters removed] values allow for the determination of relative Rh-Caryl bond strengths and illustrate the large ortho fluorine effect on the strength of the Rh-Caryl bond. This study was the first to quantize experimentally the effect of an ortho fluorine on the strength of the metal-carbon bond. In a similar study, the [Tp'RhL] fragment was used to investigate C-H bond activation of a series of linear alkylnitriles and chloroalkanes (Chapter 3). The selectivity of [Tp'RhL] for C-H bonds of alkylnitriles and chloroalkanes has been previously studied, but the Rh-C bond strengths could not be determined. New experiments and insight allowed for the determination of relative Rh-C bond strengths of these C-H activation complexes. It is found that the CN and Cl substituents dramatically strengthen the M-C bond more than anticipated if in the [alpha]-position, with the effect on bond strength diminishing substantially as these groups move further from the metal (i.e, [beta], [gamma], [delta]). Examination of M-C vs. C-H bond strengths allows for the quantization of resonance stabilization on metal-carbon bonds. The work in Chapter 4 represents the first example of C-C cleavage by oxidative addition of the C-CN bond to a Rh(I) center. Here, the selectivity of the [Cp*Rh(PMe3)] fragment for both C-H and C-C bonds of acetonitrile is examined. Low temperature photolysis of Cp*Rh(PMe3)H2 in neat acetonitrile gives only the C-H activation product. Upon heating, this product is completely converted to the C-CN activation product. Density functional theory (DFT) was used to identify the transition states for C-H and C-C activation along with the ground state energies for all possible products. DFT calculations are in excellent agreement with the experimental observations and predict C-H activation to be kinetically favored by ~2 kcal/mol, and show a large thermodynamic preference for C-C activation [...characters removed]. Recent results in our group have shown the importance of the ancillary ligand on the ability of rhodium to cleave a C-CN bond. A rhodium metal fragment with a [pi]-acceptor ligand in place of a [sigma]-donating ligand shows exclusive selectivity for C-H activation, and does not show the same ability to cleave a C-CN bond. DFT calculations were utilized to study the energetics for C-H and C-CN bond activation of acetonitrile by four different metal fragments, [(dmpe)Ni], [TpRh(PMe3)], [TpRh(CNMe)] and [(C5Me5)Rh(CNMe)] (Chapter 5). The metal fragments with the [pi]-acceptor ligand (isocyanide) had much larger barriers to C-CN cleavage than the fragments with the [sigma]-donating ligand (phosphine). The ancillary phosphine ligand plays a crucial role in stabilizing the transition state to C-CN bond activation."--Leaves vii-viii.