The Study of Rhodium and Ruthenium Complexes for the Activation and Functionalization of Non-Polar Bonds

BURGESS, SAMANTHA A. The Study of Rhodium and Ruthenium Complexes for the Activation and Functionalization of Non-Polar Bonds. (Under the direction of Professor T. Brent Gunnoe).

The increasing global demand for fossil resources has amplified the importance of more efficient catalytic processes to convert hydrocarbons into higher value chemicals. Thus, the activation and subsequent functionalization of covalent bonds (e.g., C–H, H–H, and Si–H) is a primary focus of the catalysis community. The activation of H–H, Si–H and C–H bonds by 1,2-addition across M–X (X = OR, NR2) bonds of d6 and d8 complexes has been reported, and these stoichiometric reactions are of interest for incorporation into catalytic cycles for hydrogenation, hydrosilylation or C–H functionalization. In this thesis, studies of 1,2-addition of Y–H bonds (Y = C, H, or Si) across a metal–alkoxide and amido bonds are reported.
The Rh(III) complexes [(tbpy)2Rh(OMe)(L)][X]n (tbpy = 4,4'-di-tert-butyl-2,2'-bipyridyl; L = MeOH, n = 2, X = OTf (OTf = trifluoromethanesulfonate) and TFA (TFA = trifluoroacetate); L = TFA, n = 1, X = OTf) have been shown to activate dihydrogen. Kinetic studies of this reaction reveal a first-order dependence on the Rh(III) methoxide complex and a dependence on dihydrogen that is between zero- and first-order. Combined experimental and computational studies have led to a proposed mechanism for hydrogen activation by [(tbpy)2Rh(OMe)(MeOH)][OTf][TFA] that involves MeOH dissociation, H2 coordination, and 1,2-addition of dihydrogen across a Rh methoxide bond. The analogous complexes bearing TFA counterions, [(tbpy)2Rh(OMe)(L)][TFA]n (L = MeOH, n = 2; L = TFA, n = 1), activate Si–H bonds of Et3SiH presumably by the 1,2-addition of a Si–H bond across a Rh methoxide bond.
The Rh(III) aniline complex [(MesNNN)Rh(Me)(NH2Ph)(OTf)][OTf] (MesNNN = 2,6-diacetylpyridinebis(2,4,6-trimethylaniline)) was synthesized. Deprotonation of coordinated aniline to give the corresponding anilido complex was not successful. For example, treatment of [(MesNNN)Rh(Me)(NH2Ph)(OTf)][OTf] with NHEt2 results in formation of the Rh(I) amido complex (MesNNN)Rh(NEt2).
The C–H activation of benzene and catalytic ethylene hydrophenylation using a cationic Ru(II) metal complex, [(HC(pz5)3)Ru(P(OCH2)3CEt)(NCMe)Ph][BAr'4] (HC(pz5)3 = tris(5-methyl-pyrazolyl)methane; BAr'4 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), has been pursued. Studies of our previously reported charge neutral Ru(II) ethylene hydrophenylation catalysts, TpRu(L)(NCMe)Ph [Tp = hydridotris(pyrazolyl)borate; L = CO, PMe3, P(OCH2)3CEt or ], suggest that accessing a less electron-rich cationic version of TpRu(P(OCH2)3CEt)(NCMe)Ph would give higher turnover numbers of ethylbenzene. Studies of catalytic ethylene hydrophenylation using the cationic Ru(II) complex [(HC(pz5)3)Ru(P(OCH2)3CEt)(NCMe)Ph][BAr'4] supports this hypothesis. The reaction of [(HC(pz5)3)Ru(P(OCH2)3CEt)(NCMe)Ph][BAr'4] (0.025 mol% relative to benzene) in benzene with C2H4 (15 psi) at 90 °C gives 565 turnovers of ethylbenzene after 131 hours. This corresponds to an approximate one-pass 95% yield based on ethylene, and is a 28-fold improvement compared to the charge neutral catalyst TpRu(P(OCH2)3CEt)(NCMe)Ph. Under identical conditions, [(HC(pz5)3)Ru(P(OCH2)3CEt)(NCMe)Ph][BAr'4] is only 1.3 times less active than TpRu(P(OCH2)3CEt)(NCMe)Ph, but the increased stability of the cationic Ru(II) catalyst allows higher temperatures (up to 175 °C) to be employed, which significantly enhance the rate by ~42-fold.