Katalytische Fixierung von Kohlenstoffdioxid mittels früher Übergangsmetallkomplexe
Final Report Abstract
This research project was directed at studying the capability of strong Lewis acids to mediate the (catalytic) fixation of the carbon oxides CO2 and CO. The strategy pursued was to combine a structurally defined, soluble Lewis acid component with a potentially reactive carrier of reducing equivalents, such as hydride. This was realized in the form of novel ion pairs of the general formula [L2Sc]n+[HB(C6F5)3]-n, n = 1, 2. Bulky η5-cyclopentadienides, e.g. [C5(CH3)5]-(Cp*), or bidentate β-diketiminato ligands stabilize the Lewis acidic Sc mono- and dications. Solution and solid-state structural analysis of the ion pairs revealed multiple dynamic dative Sc- F bonds that associate the ionic components to form a spatially confined reactive pocket. Partial Sc-F bond dissociation allows small molecule substrates COx to access the metal site. Chemically significant quantities of highly reactive Sc-hydrides are not present in solutions of the ion pairs. The capability of [HB(C6F5)3]- to affect formal B-H bond addition to one C-O double bond of CO2 was of relevance for this work. Associated with non Lewis-acidic [TMPH]+, TMP = 2,2,6,6-tetramethylpiperidine, driving the reaction of [HB(C6F5)3]- with CO2 to form formatoborate [HCO2B(C6F5)3]- requires temperatures >50°C. Formato-borate is subject to rapid B-O bond cleavage in the presence of catalytic quantities of reactive perfluoroarylborane-silane species to form a silylformate and regenerate [HB(C6F5)3]-. Stepwise deoxygenative hydrosilylation of the silylformate by-product proceeds rapidly, via a bissilyloxymethane intermediate, to form CH4 and disiloxane as the ultimate products. In clear contrast, [Cp*2Sc][HB(C6F5)3] rapidly affects CO2 fixation at ambient conditions affording the ion pair [Cp*2Sc][HCO2B(C6F5)3]. This result demonstrates the beneficial effect of the Lewis-acidic component on substrate fixation. Tandem B(C6F5)3-[Cp*2Sc][HB(C6F5)3] mediates the catalytic deoxygenative hydrosilylation of CO2 to CH4 at ambient conditions, employing Et3SiH as sacrificial reductant and oxygen acceptor. Mechanistic studies addressed the roles of hydrido- and formato-borate ion pairs in the reaction, and uncovered an accelerating effect of the bis(triethylsilyloxy)methane organic intermediate on catalysis. Formation of the separated ion pair [Cp*2Sc(κ2-(Et3SiO)2- CH2)][HCO2B(C6F5)3] promotes formato-borate dissociation from Sc, thereby accelerating the regeneration of [HB(C6F5)3]- by borane-silane species. Analogous studies of the tandem system B(C6F5)3-[(N^N)Sc][HB(C6F5)3]2 (N^N = [κ2-(ArNC(tBu)CH(tBu)CNAr)]-, Ar = 2,6-(iPr)2C6H3) were thwarted by the chemical non-innocence of the β-diketiminate framework toward CO2. An NMR spectroscopic study indicated the formation of the ion pair [{κ3-(N^N^C-(OB(C6F5)3)O)Sc}2-μ2- (OC(H)O)3][HCO2{B(C6F5)3}2]. The α-carboxylato-β-diketimine Sc fragment is prone to irreversibly rearrange into higher nuclearity, carboxylato-bridged clusters which do not sustain catalysis. Notably, isotopic labeling studies revealed that the nucleophilic attack of the βdiketiminato ligand at CO2 is reversible. Mechanistic insight into the Lewis acid promoted COx fixation reaction was obtained from a combined isotope labelling and computational study of the stoichiometric reaction of [Cp*2Sc][HB(C6F5)3] with CO. Formal CO insertion into both the B-H and B-Caryl bond of [HB(C6F5)3]- yields ion pairs of the scandocene cation and the unusual anions formyl-tris(perfluorphenyl)borate and 2,2,3-tris(perfluorophenyl)-1,2-oxaboriran-2-ide. Computations support circumstantial experimental evidence of the intermediacy of a reactive decamethylscandocinium-isocarbonyl species. κ-O bonding to strongly electrophilic Sc disturbs CO’s frontier orbitals such that it’s antibonding π*-orbitals, which are of significant C-AO parentage, become degenerate and energetically available for reaction with the [HB(C6F5)]- counterion. A similar mechanism may be evoked for CO2 fixation. The proximal spatial arrangement of the reactive components, mediated through multiple dynamic Sc-F interactions, allows the elementary steps of substrate fixation to be effectively coupled. In this sense, these reactions are a homogeneous analogs of other systems in which chemistry occurs in a physically confined environment, for example the pores of a zeolite, or the active site of an enzyme. This work laid the basis for future studies in the group of reactive ion pair combinations that incorporate redox active Lewis acid components.
Publications
- Tandem Frustrated Lewis Pair/Tris(pentafluorophenyl)borane-Catalyzed Deoxygenative Hydrosilylation of Carbon Dioxide, J. Am. Chem. Soc. 2010, 132, 10660–10661
Berkefeld, A.; Piers, W. E., Parvez, M.
- Reactivity of Scandium β- Diketiminate Alkyl Complexes with Carbon Dioxide, Organometallics 2012, 31, 810-818
LeBlanc, F. A.; Berkefeld, A.; Piers, W. E., Parvez, M.
