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Title:Metal complexes with ω-alkenyl ligands. synthesis, structure, reactivity, and applications in chemical vapor deposition (CVD) and hydrosilylation catalysis
Author(s):Liu, Sumeng
Director of Research:Girolami, Gregory S
Doctoral Committee Chair(s):Girolami, Gregory S
Doctoral Committee Member(s):Abelson, John R; Fout, Alison R; Murphy, Catherine J
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Metal Complexes with ω-Alkenyl Ligands
Chemical Vapor Deposition
Hydrosilylation Catalysis
Abstract:This thesis describes the synthesis and characterizations of several metal complexes of lithium, platinum, rhodium and iridium complexes bearing ω-alkenyl ligands, in which the atom beta to the metal center bears two methyl groups to prevent β-hydrogen elimination reaction. The general reactivities, fluxional behaviors, and the potential applications of these complexes in chemical vapor deposition (CVD) and catalysis are described. Chapter 1 is a brief overview of the synthesis, structure, fluctuational behaviors and general reactivities of complexes with η1-σ-η2-alkene chelating ligands. Chapter 2 describes the first crystallographically characterized example of a non-conjugated olefin bound in a simple dihapto fashion to a lithium center, as part of a study of two alkyllithium compounds that contain C=C double bonds at the alkyl chain terminus: (2,2-dimethylbut-3-en-1-yl)lithium and the related pentenyl compound (2,2-dimethylpent-4-en-1-yl)lithium. The Li-olefin interactions in the crystal structure of (2,2-dimethylpent-4-en-1-yl)lithium serve as a model for those proposed to be present in the [RLi∙∙∙olefin] intermediate in olefin carbolithiation reactions. As seen in other systems, the Li-olefin interaction is correlated with deshielding of the 1H NMR resonances of the olefinic hydrogen atoms. DOSY and NOE measurements show that (2,2-dimethylbut-3-en-1-yl)lithium and (2,2-dimethylpent-4-en-1-yl)lithium remain tetrameric in cyclohexane and that the lithium-olefin interactions persist in solution. Addition of a Lewis base such as THF to these alkenyllithium species has two effects: the THF displaces the lithium-olefin interactions while accelerating the rate of carbolithiation. A deuteration experiment shows that compound (2,2-dimethylpent-4-en-1-yl)lithium undergoes reversible carbolithiation to the corresponding cyclobutylmethyllithium species in the presence of Lewis bases, but this transformation is thermodynamically uphill owing to ring strain. In comparison, the longer chain hexenyl species (2,2-dimethylhex-5-en-1-yl)lithium is thermodynamically unstable with respect to the intramolecular carbolithiation product [(3,3-dimethylcyclopentyl)methyl]¬lithium. We suggest that rate determining step in carbolithiation reactions may not always be formation of the C-C bond, as is often assumed, but in some cases may be formation of the lithium-olefin complex; the coordination of the olefin to lithium may occur in a concerted fashion with disaggregation of lithium clusters. Finally, we point out that activation enthalpies can be obtained solely from NMR line shapes above the coalescence point. Chapter 3 describes the synthesis and characterization of three platinum(II) ω-alkenyl complexes of stoichiometry Pt[CH2CMe2(CH2)xCH=CH2]2 where x is 0, 1, or 2: cis-bis(η1,η2-2,2-dimethyl¬but-3-en-1-yl)platinum, cis-bis(η1,η2-2,2-dimethylpent-4-en-1-yl)platinum, and cis-bis(η1,η2-2,2-dimethylhex-5-en-1-yl)platinum, as well as some related platinum(II) compounds formed as byproducts during their synthesis. The ω-alkenyl ligands in all three complexes bind to Pt by means of a Pt-alkyl sigma bond at one end of the ligand chain, and a Pt-olefin pi interaction at the other; the olefins reversibly decomplex from the Pt centers in solution. The good volatility of cis-bis(η1,η2-2,2-dimethylpent-4-en-1-yl)platinum (10 mTorr at 20 °C), its ability to be stored for long periods without decomposition, and its stability toward air and moisture, make it an attractive platinum CVD precursor. CVD of thin films from cis-bis(η1,η2-2,2-dimethylpent-4-en-1-yl)platinum shows no nucleation delay on several different substrates (SiO2/Si, Al2O3, and VN) and gives films that are unusually smooth. At 330 °C in the absence of a reactive gas, the precursor deposits platinum containing 50% carbon, but in the presence of a remote oxygen plasma, the amount of carbon is reduced to below the RBS detection limit without affecting the film smoothness. Under hot wall CVD conditions at 250 °C in the absence of a co-reactant, 72% of the carbon atoms in cis-bis(η1,η2-2,2-dimethylpent-4-en-1-yl)platinum are released as hydrogenated products (largely 4,4-dimethylpentenes), 22% are released as dehydrogenated products (all of which are the result of skeletal rearrangements), and 6 % remain in the film. Some conclusions about the CVD mechanism are drawn from this product distribution. Chapter 4 describes the detailed mechanistic studies of the pathway by which 2 reacts upon being heated in solution. In various solvents between 90 and 130 °C, 2 decomposes to generate ~ 1 equiv. of 4,4-dimethylpentenes by addition of a hydrogen atom to the pentenyl ligands in 2. The “extra” hydrogen atoms arise by dehydrogenation of other pentenyl ligands; some of these dehydrogenated ligands are released as methyl-substituted methylenecyclobutanes and cyclobutenes. A combination of isotope labeling and kinetic studies suggests that 2 decomposes by C–H activation of both allylic and olefinic C–H bonds to give transient platinum hydride intermediates, followed by reductive elimination steps to form the pentene products, but that the exact mechanism is solvent dependent. In C6F6, solvent association occurs before C–H bond activation, and the rate-determining step for thermolysis is most likely the formation of a Pt σ-complex. In hydrocarbon solvents, solvent plays little role before C-H bond activation, and the rate-determining step is most likely the formation of a Pt σ-complex only for γ-C–H and ε-C–H bond activation, but cleavage or formation of a C–H bond for δ-C–H bond activation. A comparison of the thermolysis reactions under CVD conditions and in solution suggests that the high smoothness of the CVD-grown films is due in part to rapid nucleation (which is a consequence of the availability of low barrier C–H activation pathways) and in part to the formation of carbon-containing species that passivate the Pt surface. Chapter 5 describes the synthesis and characterization of four ƞ1,ƞ2-2,2-dimethylpent-4-en-1-yl complexes of stoichiometry M(CH2CMe2CH2CH=CH2)(diene), where M is Rh or Ir, and the diene is 1,5-cyclooctadiene (COD), norbornadiene (NBD), or dibenzo[a,e]cyclooctatetraene (DBCOT). We also have made binuclear complex [Ir(CH2CMe2CH2CH=CH2)(DBCOT)]2(C14H26), in which the iridium centers are also coordinated to a bridging diolefin ligand. In all five complexes, the alkenyl ligand binds to the metal center by means of a M-alkyl sigma bond at one end of the ligand chain, and a M-olefin pi interaction at the other. The C=C bond reversibly decomplexes in solution; for Rh(CH2CMe2CH2CH=CH2)(DBCOT), the activation parameters for olefin decomplexation are ΔH‡ = 19 ± 1 kcal∙mol-1 and ΔS‡ = 8 ± 4 cal∙mol-1 K-1. When this Rh compound is heated in solution, a hydrogen atom on the γ-carbon of the alkenyl ligand is transferred to the α-carbon to form a substituted ƞ3-allyl complex Rh(ƞ3-anti-CH2=CHCHCMe3)(DBCOT). This latter compound subsequently isomerizes in solution to form the thermodynamically more stable isomer Rh(ƞ3-syn-CH2=CHCHCMe3)(DBCOT). Chapter 6 describes the preparation of the cis-bis(η1,η2-2,2-dimethylpent-4-en-1-yl)rhodate(I) anion, cis-[Rh(CH2CMe2CH2CH=CH2)2]–, and the interaction of this species with Li+ both in solution and in the solid state. For the lithium(diethylether) salt [Li(Et2O)]-[Rh(CH2CMe2CH2CH=CH2)2], VT-NMR and 1H{7Li} NOE NMR studies in toluene-d8 show that the Li+ cation is in close proximity to the dz2 orbital of rhodium. In the solid state structure of the lithium(12-crown-4) salt [Li(12-crown-4)2][Li{Rh(CH2CMe2CH2CH=CH2)2}2], one lithium atom is surrounded by two [Rh(CH2CMe2CH2CH=CH2)2]– anions, and in this assembly there are two unusually short Rh–Li distances of 2.48 Å. DFT calculations and ETS-NOCV analysis suggest that there is a weak dative interaction between the 4dz2 orbitals on the two Rh centers and the 2pz orbital of the Li+ cation. Chapter 7 describes the synthesis and characterizations of triethyl(cyclopentadienyl)-platinum(IV), CpPtEt3, and its methylcyclopentadienyl and pentamethylcyclopentadienyl analogs Cp′PtEt3, and Cp*PtEt3. We also a new starting material for these complexes, (PtEt3I)4. The compounds CpPtEt3 and Cp′PtEt3 are air- and water-stable liquids under ambient conditions, whereas Cp*PtEt3 is a solid and somewhat water-sensitive. NMR data are reported for all the new compounds, and Cp*PtEt3 has been crystallographically characterized. All three compounds are volatile and are indefinitely stable at -20 °C. The high volatility and low melting points of CpPtEt3 and Cp′PtEt3 suggest that these compounds are potentially interesting as CVD precursors for the deposition of Pt metal. Chapter 8 describes describe the synthesis, characterization, and catalytic hydrosilylation activity of platinum(II) dialkenyl compounds of stoichiometry PtR2, where R = CH2SiMe2(vinyl) (1) or CH2SiMe2(allyl) (2), and their 1,5-cyclooctadiene adducts PtR2(COD), denoted 1-COD and 2-COD. We also report analogous studies of the related norbornadiene and dibenzo[a,e]cycloocta¬tetraene complexes 1-NBD, 1-DBCOT, and 2-DBCOT. The diolefin-free compounds 1 and 2, as well as 1-NBD and 2-DBCOT, are air- and water-sensitive, and decompose slowly at room temperature under argon; in contrast, both 1-COD and 2-COD are colorless liquids and 1-DBCOT is a colorless solid which are stable under ambient conditions. In 1 and 2, the ω-alkenyl ligands are bidentate chelates, whereas in the COD, NBD, and DBCOT adducts they are unidentate. At 20 °C and 0.5 × 10-6 – 5 × 10-6 mol% precatalyst loadings, 1-COD shows no hydrosilylation activity toward many olefin substrates even after several hours, but turnover numbers as high as 200,000 are seen after 4 h at 50 °C. The amounts of isomerized (i.e., internal) olefins, olefin hydrogenation products, and dehydrogenative silylation products are comparable to or smaller than those seen for platinum(0) carbene catalysts. Activation of the PtII precatalyst occurs via three steps: slow dissociation of COD from 1-COD to form 1, rapid oxidative addition of silane to 1, and rapid reductive elimination of both alkenyl ligands to form Pt0 species. We further show that addition of small amounts (0.1 mol% per silane) of a consumable olefin inhibitor such as norbornadiene (NBD) or 1,3-divinyl-1,1,3,3-tetramethyldisiloxane (DVTMS) significantly increases the latency while maintaining the fast rate of catalyst activation at the end of the latency period. The latent catalytic behavior, the high turnover number, and the high anti-Markovnikov selectivity is a result of 1-COD being a slow-release pre-catalyst: the slow release of 1 from 1-COD at room temperature generates little active Pt0 catalyst at early times, resulting in little activity for hours. The low concentration of Pt0 during the initial stages of the catalysis inhibits the formation of colloidal Pt, which is known to cause side reactions. Such a slow-release pre-catalyst strategy can be a useful method to improve the performance of homogeneous catalysis. The high turnover numbers and the latent reaction kinetics seen for 1-COD make this compound a potentially useful precatalyst for injection molding or solvent-free hydrosilylation applications.
Issue Date:2020-05-08
Rights Information:Copyright 2020 Sumeng Liu
Date Available in IDEALS:2021-09-17
Date Deposited:2020-05

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