UC Berkeley

Chapter 1. Synthesis of the cyclometallated complexes Cp*Ru(IXy-H) (2; IXy = 1,3-bis(2,6-dimethylphenyl)imidazol-2-ylidene; IXy-H, the deprotonated form of IXy, 1-(2-CH2C6H3-6-methyl)-3-(2,6-dimethylphenyl)imidazol-2-ylidene-1-yl; Cp* = η5-C5Me5) and Cp*Ru(IXy-H)(N2) (3) was achieved by dehydrochlorination of Cp*Ru(IXy)Cl (1) with KCH2Ph. Complexes 2 and 3 activate primary silanes to afford the silyl complexes Cp*(IXy-H)(H)RuSiH2R (R = p-Tol (4), Mes (5), and Trip (6), respectively). DFT studies indicate that these complexes are close in energy to the corresponding, isomeric silylene species Cp*(IXy)(H)Ru=SiHR. Indeed, reactivity studies indicate that various reagents trap the silylene isomer of 6, Cp*(IXy)(H)Ru=SiHTrip (6a). Thus, benzaldehyde reacts with 6 to give a [2+2] cycloaddition product (7), while 4-bromoacetophenone reacts via C-H bond cleavage and formation of the enolate Cp*(IXy)(H)2RuSi(H)[OC(=CH2)C6H4Br]Trip (8). Addition of the O-H bond of 2,6-dimethylphenol across the Ru=Si bond of 6a gives Cp*(IXy)(H)2RuSiH(2,6-Me2C6H3O)Trip (9). Interestingly, CuOTf and AgOTf also react with 6 to provide unusual, Lewis acid-stabilized silylene complexes in which CuOTf or AgOTf bridges the Ru-Si bond. The AgOTf complex, which was crystallographically characterized, exhibits a structure similar to that of [Cp*(iPr3P)(H)2RuSiHMes]+, with a 3-center 2-electron Ru-Ag-Si interaction. NBO analysis of the MOTf complexes supports this type of bonding, and characterizes the donor interaction with Ag (or Cu) as involving a delocalized interaction with contributions from the carbene, silylene and hydride ligands of Ru.

Chapter 2. An N-heterocylic carbene (NHC) ring-opening reaction was observed upon treatment of the silyl complex Cp*(IXy-H)(H)RuSiH2Mes (1) with LiCH2SiMe3. This reaction results in formation of a novel, anionic Fischer-type carbene complex Cp*(H)Ru{κ2-CHN(Xyl)CH[Si(CH2SiMe3)Mes]N(Xyl)Li} (4). This ring-opening reaction demonstrates a new pathway for NHC degradation via the cooperation of ruthenium, silicon and lithium. Additionally, treatment of the dinitrogen complex Cp*(Xyl-H)Ru(N2) with LiCH2SiMe3 led to the doubly-metallated complex [Cp*(Xyl-2H)Ru]Li (5), which exhibits a solid-state polymeric structure and metallation of both xylyl groups of the Xyl ligand. Finally, removal of a hydride from 4, achieved with two equiv of B(C6F5)3, led to C-H activation of the Cp* ligand and formation of an unusual, formally dianionic η5-Me4C5CH2B(C6F5)3 ligand in a ruthenium carbene complex [η5-Me5C5CH2B(C6F5)3](H)Ru{κ3-CHN(Xyl)CH[SiH(CH2SiMe3)Mes]N(Xyl)} (7).

Chapter 3. An intramolecular 1,2(α)-H migration in a saturated ruthenium stannylene complex, to form a ruthenostannylene complex, involves a reversal of the role for a coordinated stannylene ligand, from that of an electron donor to an acceptor in the transition state. This change in the bonding properties for a stannylene group, with a simple molecular motion, lifts the usual requirement for generation of an unsaturated metal center in migration chemistry.

Chapter 4. Reactivity studies of the thermally stable ruthenostannylene Cp*(IXy)(H)2Ru-Sn-Trip (1) towards a variety of organic substrates are described. Complex 1 reacts with benzoin and an α,β-unsaturated ketone to undergo [1 + 4] cycloaddition reactions and afford Cp*(IXy)(H)2RuSn(κ2-O,O-OCPhCPhO)Trip (2) and Cp*(IXy)(H)2RuSn(κ2-O,C-OCPhCHCHPh)Trip (3), respectively. The reaction of 1 with ethyl diazoacetate resulted in a tin-substituted ketene complex Cp*(IXy)(H)2RuSn(OC2H5)(CHCO)Trip (4), likely a decomposition product from the putative ruthenium substituted stannene complex. The isolation of a ruthenium substituted stannene Cp*(IXy)(H)2RuSn(=Flu)Trip (5) or stannaimine Cp*(IXy)(H)2RuSn(κ2-N,O-NSO2C6H4Me)Trip (7) complexes was achieved by treatments of 1 with 9-diazofluorene or tosyl azide, respectively.

Chapter 5. Masked silylenes Cp*(IXy-H)(H)RuSiH2R (R = Mes and Trip) exhibit metallosilylene (LnM-Si-R) character upon reacting with organic substrates to afford unprecedented silaoxiranyl ([1 + 2] cycloadducts with non-enolizable ketones), oxasilacyclopentenyl ([1 + 4] cycloadducts with enones) and silaiminyl (with tosyl azide) complexes. These silicon-containing complexes are striking as they are derived from primary silanes MesSiH3 and TripSiH3 via activation of all three Si-H bonds. Theoretical studies currently in progress are investigating the the transformations that yield these complexes.

Chapter 6. An unusual cyclometalation reaction involving C-C bond activation of Cp*(IPr)RuCl results in Cp*(IPr')Ru(L) (L = propene or N2, 6a-c) [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; IPr' = 1-(6-isopropylphenyl)-3-(2,6-diisopropylphenyl)imidazol-2-ylidene] which features a NHC-C(sp2) chelating ligand, and this allowed for a comparison between the complexes bearing chelating NHC-C(sp2) and NHC-C(sp3) (complex 1a) ligands. The weaker CO stretching frequency found for Cp*(IXy-H)Ru(CO) relative to Cp*(IPr')Ru(CO) suggests the NHC-C(sp3) ligand is more electron donating than NHC-C(sp2). Both 1a and 6a-c activate H2 and MesSiH3 to afford the corresponding hydride and silyl hydride complexes. However, while complex 1a, with a weaker Ru-C(sp3) bond, undergoes C-H reductive elimination and a sequence of reactions in the presence of hydride ligands to afford a trihydride and the masked silylene complexes, such C-H bond reductive elimination was not observed in 6a-c, likely due to its stronger Ru-C(sp2) bond. Moreover, the reaction of 1a with B(C6F5)3 resulted in a zwitterionic complex via C-B bond formation. In contrast, B(C6F5)3 abstracted a hydride from 6a-c and led to C-C bond formation.