Synthesis and Comparative Studies of Ru(II) Complexes for Metal-mediated C-H Activation and Olefin Hydroarylation Catalysis

Abstract

TpRu(CO)(NCMe)Ph catalyzes the addition of aromatic C-H bonds across double bonds (i.e., olefin hydroarylation). Second generation TpRu(L)(NCMe)R {L = PMe3, P(OCH2)3CEt or P(pyr)3; pyr = pyrollyl; R = Me or Ph} complexes were synthesized to compare olefin hydroarylation activity versus the parent CO analog (Chapters 2 through 4). Experimental and computational studies indicate that TpRu(L)(NCMe)Ph initiates ethylene hydrophenylation by dissociation of NCMe, coordination of ethylene, insertion of ethylene into the Ru-Ph bond, coordination of benzene and C-H activation of benzene to release the olefin hydroarylation product ethyl benzene. Previously, kinetic isotope effect studies indicated that the C-H activation step is the rate determining step for the overall catalytic cycle. Kinetic studies indicate the stronger donating PMe3 and P(OCH2)3CEt ligands increase the rate of degenerate benzene C-H activation for TpRu(L)(NCMe)Ph systems relative to TpRu(CO)(NCMe)Ph. Hammett studies and kinetic isotope effect studies are consistent with a C-H activation mechanism which proceeds by a sigma-bond metathesis pathway. Increasing the acidity of the C-H hydrogen, and the basicity of the ligand receiving the hydrogen, accelerates C-H activation. In benzene and ethylene mixtures, TpRu(L)(NCMe)Ph systems have decreased catalytic activity for ethylene hydrophenylation with the PMe3 and P(OCH2)3CEt systems decomposing to TpRu(L)(ï ¨3-C4H7) analogs. Increased metal electron density raises the activation barrier to olefin insertion allowing ethylene C-H activation to become competitive with ethylene insertion leading to TpRu(L)(ï ¨3-C4H7) formation. The P(pyr)3 ligand is electronically similar to CO, but its large steric bulk makes ethylene coordination endergonic, inhibiting entry into a catalytic ethylene hydrophenylation cycle. In Chapter 5, TpRu(PMe3)(NCMe)Me stoichiometrically activates the sp3 bonds of acetonitrile, acetone and nitromethane to form TpRu(PMe3)(NCMe)CH2CN, TpRu(PMe3){κ2-O,N-OC(Me)C(H)C(Me)NH} and TpRu(PMe3){κ2-O,N-N(O)C(H)(NO2)}, respectively. Experimental and computational studies suggest that C-H activation is promoted by thermodynamically favourable coordination via the heteroatomic functionality, increased basicity of the ligand receiving the hydrogen and substrate acidity. Additionally, TpRu(PMe3)(NCMe)Me mediates subsequent C-C/C-N bond-forming reactions with acetonitrile and acetone. In Chapter 6, a series of cationic [EpRu(L)(L’)R][A-] and [C(pz)4Ru(L)(L’)R][A-] [Ep = tris(pyrazolyl)ethane; L = PMe3, P(OCH2)3CEt, or CO; L’ = PPh3, NCMe or THF; R = Me or Ph; A- = BAr’4, BPh4 or OTf; BAr’4 = {tetrakis(3,5-trifluoromethyl)phenyl}borate; pz = pyrazolyl] complexes were synthesized and tested for olefin hydroarylation activity. [EpRu(CO)(NCMe)Ph][A-] (A- = BAr’4 or BPh4) was found to catalyze a 2 – 3 turnovers of ethyl benzene in ethylene and benzene mixtures. Cyclic voltammetry gave strongly positive irreversible oxidative potentials suggesting poor catalysis is linked to the greater electron deficiency of the complexes, relative to TpRu(CO)(NCMe)Ph. A successful cationic Ru olefin hydroarylation catalyst will likely exhibit a reversible Ru(III/II) redox potential near 1 V. TpaRu(CO)(NCMe)Ph {Tpa = allyl-tris(pyrazolyl)borate} was synthesized with the intention to attach it to the surface of mesoporous silica nanoparticles (MSN) in Chapter 7. In collaboration with another research group a prototype was developed. Initial catalytic studies showed no activity for ethylene hydrophenylation. Characterization of the new prototype {TpRu(CO)(NCMe)Ph-MSN} is challenging. IR spectroscopy suggests the Ru ligand structure was adversely altered by the MSN attachment process. New reaction schemes are proposed for the synthesis of TpRu(CO)(NCMe)Ph-MSN. Finally in Chapter 8, the detection and isolation of the low yield contaminant ClTpRu(PPh3)2H {ClTp = chlorotris(pyrazolyl)borate} from the known synthesis of TpRu(PPh3)2Cl is reported. The mechanism of the H/Cl metathesis at the Tp boron is suspected to form from intermediates leading to the formation of TpRu(PPh3)2Cl. ClTpRu(PPh3)2H was found to react with CH2Cl2 and CHCl3 to form ClTpRu(PPh3)2Cl. Both complexes were characterized with single crystal X-ray diffraction studies and 11B NMR spectroscopy. This report serves as an important warning that careful characterization for purity of TpRu(PPh3)2Cl, a common catalyst precursor, must be carried out following its synthesis.