I. Energetics of Carbon-Hydrogen Bond Activation of Functionalized hydrocarbons II. Carbon-Hydrogen and Carbon-Carbon Nitride Bond Activation of Acetonitrile and Benzonitrile.

摘要

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 DeltaGre≠ 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 CH activation to be kinetically favored by ∼2 kcal/mol, and show a large thermodynamic preference for C--C activation (DeltaG° ≈13 kcal/mol).;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.