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CHEM-E1130 Catalysis

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CHEM-E1130 Catalysis CHEM-E1130 Catalysis Introduction to Homogeneous Catalysis Jan Deska Laboratory of Organic Chemistry 11.2.2019 What is Homogeneous Catalysis? Catalytic reaction in which both catalyst & reactant interact in the same phase (generally in the liquid phase, either in solution of in liquid reactant) Nature of catalysts widespread in structure and complexity § acid or base catalysis (inorganic acids of bases) § biocatalysis (biomolecules i.e. proteins & cofactors) § organocatalysis (organic molecules e.g. amines, carbenes, organic acids, amphiphiles,...) § organometallic catalysis (transition metal complexes with organic and/or inorganic ligands) Supporting literature, e.g.: Organotransition Metal Chemistry, J. F. Hartwig, University Science Books, 2010 The Organometallic Chemistry of Transition Metals, R. H. Crabtree, 3rd ed, Wiley, 2001 What is Homogeneous Catalysis? Catalytic reaction in which both catalyst & reactant interact in the same phase (generally in the liquid phase, either in solution of in liquid reactant) Nature of catalysts widespread in structure and complexity § acid or base catalysis (inorganic acids of bases) § biocatalysis (biomolecules i.e. proteins & cofactors) § organocatalysis (organic molecules e.g. amines, carbenes, organic acids, amphiphiles,...) § organometallic catalysis (transition metal complexes with organic and/or inorganic ligands) topic of today's lecture Supporting literature, e.g.: Organotransition Metal Chemistry, J. F. Hartwig, University Science Books, 2010 The Organometallic Chemistry of Transition Metals, R. H. Crabtree, 3rd ed, Wiley, 2001 Scientific & Technological Impact Multiple Nobel prizes awarded to research on homogeneous catalysis 1963 Ziegler, Natta Metal-catalyzed polymerizations (e.g. PE, PP) 1973 Fischer, Wilkinson Sandwich organometallics (e.g. industrial hydrogenation catalysts) 1975 Cornforth Stereochemistry of enzyme reactions 2001 Knowles, Sharpless, Noyori Chirally catalyzed redox reactions (e.g. industrial asymmetric production of optically active amino acids) 2005 Chauvin, Grubbs, Schrock Olefin metathesis (e.g. ring-opening polymerization) 2010 Heck, Negishi, Suzuki Palladium-catalyzed cross coupling (e.g. key in pharma synthesis) 2018 Arnold Directed evolution of proteins (e.g. enzyme optimization) Heterogeneous vs Homogeneous Catalysis Homogeneous Catalysis Heterogeneous Catalysis activity high varies selectivity high (also stereoselectivity) varies reaction conditions limited to low temperatures high temperature & pressure lifetime varies, potentially challenging long catalyst poisoning low high diffusion issues none often significant catalyst separation challenging, expensive not required What is Organometallic Chemistry? § synthesis and reactions of molecular entities featuring carbon-metal bonds Classical Organometallic Chemistry § main group metal-carbon compounds § Grignard reagents, Barbier-type organoaluminiums & -zincs, organolithium § no (subtle) redox chemistry, one redox state clearly prefered § characteristics: mainly highly reactive, used in stoichiometric transformations Organotransition Metal Chemistry § transition metals as center of organometallic species § complex architecture, ligand structures and electronics crucial parameters § facile change of redox states often basis of catalysis § somewhere between organic chemistry and inorganic coordination chemistry Transition Metals – Structure & Bonding The Transition Metals d electrons in group 3 are easily removed via ionization, those in group 12 are stable and form part of the core electron configuration Transition Metals – Structure & Bonding Valence electron count for free (gas phase) transition metals: (n+1)s is Fe 2 6 4s 3d below (n)d in energy CO CO for complexed transition metals: the (n)d levels are 8 OC Fe 3d below the (n+1)s and thus get filled first. CO OC for oxidized metal centres, subtract the oxidation N 3d6 N Fe N state from the group # Mes Mes Cl Cl Transition Metals – Structure & Bonding Valence orbitals § 9 Valence Orbitals: upper limit of 9 bonds may be formed. In most cases a maximum of 6 σ bonds are formed and the remaining d orbitals are non-bonding. It's these non-bonding d orbitals that give TM complexes many of their unique properties. § 18 electron rule: upper limit of 18 e- can be accommodated without using antibonding molecular orbitals (MO's). § Saturation: complexes with a filled valence shell, i.e. (n)d + (n+1)s + (n+1)p = 18 e–, are called saturated, complexes with less than 18 valence electrons are unsaturated Transition Metals – Coordination Geometries Common geometries § governed by sterics § to a first approximation, geometry of TM complexes is determined by steric factors (VSEPR, valence shell electron pair repulsion). The M-L bonds are arranged to have the maximum possible separation around the metal centre. 180° L 120° 109.5° L M L M L L M L L L L coordination number: 2 coordination number: 3 coordination number: 4 linear trigonal planar tetrahedral L L L L L L M 120° 180° M 90° L L L 90° L L coordination number: 5 coordination number: 6 trigonal bipyramidal octahedral Transition Metals – Coordination Geometries Common geometries § governed by electronics § d electron count combined with the complex electron count must be considered when predicting geometries for TM complexes with non-bonding d electrons. Often this leads to sterically less favorable geometries for electronic reasons (e.g. coordination number 4, d8, 16 e- strongly prefers square planar geometry) . 180° 90-100° L L L L L L M L M 90° M 90° L L L 90° L L coordination number: 3 coordination number: 4 coordination number: 5 T-shaped square planar square pyramidal Transition Metals – Electronic Configurations Electron counting the electronic configuration and the degree of saturation determine stability and reactivity of organometallic complexes § step 1: determine the oxidation state of the central metal ü To do this, balance the ligand charges with an equal opposite charge on the metal. This is the metal's formal oxidation state. ü To determine ligand charges, create an ionic model by assigning each M-L electron pair to the more electronegative atom (L). This should result in stable ligand species or ones known as reaction intermediates in solution. Transition Metals – Electronic Configurations Electron counting the electronic configuration and the degree of saturation determine stability and reactivity of organometallic complexes § step 2: determine the d electron count § step 3: determine the electron count of ü To do this, subtract the metal's the entire complex oxidation state from its group # ü To do this, add the number of electrons donated by each ligand to the metal's d electron count Transition Metals – Electronic Configurations Hapticity (hn) the number of atoms (n) of the ligand directly involved in binding to the metal h ligands Proposed intermediates in VO(acac)2 catalyzed directed epoxidation of allylic alcohols (Sharpless epoxidation). µ ligands Transition Metals – Electronic Configurations – anionic (2 e–) neutral (2 e ) Electron counting exercise ligands = 4 x 2e–: 8 e– metal = d8: 8 e– Wilkinson's catalyst complex: 16 e– (unsaturated) Transition Metals – Electronic Configurations Electron counting exercise Noyori hydrogenation catalyst Transition Metals – Electronic Configurations Electron counting exercise Noyori hydrogenation catalyst ligands = 6 x 2e–: 12 e– metal = d6: 6 e– complex: 18 e– (saturated) Transition Metals – Electronic Configurations Electron counting exercise "Palladium tetrakis" (Tetrakis(triphenylphosphino)palladium) cross coupling catalysts Transition Metals – Electronic Configurations Electron counting exercise ligands = 4 x 2e–: 8 e– metal = d10: 10 e– "Palladium tetrakis" complex: 18 e– (saturated) (Tetrakis(triphenylphosphino)palladium) cross coupling catalysts Transition Metals – Electronic Configurations Electron counting exercise olefin hydroxylation catalyst Transition Metals – Electronic Configurations Electron counting exercise olefin hydroxylation catalyst ligands = 6 x 2e–: 12 e– metal = d6: 6 e– complex: 18 e– (saturated) Transition Metals – Electronic Configurations Hapticity (hn) – part 2 unsaturated ligands offer higher degree coordination modes as anionic ligands M M M h1-aryl h1-vinyl h1-alkynyl formal charge: –1 formal charge: –1 formal charge: –1 e donated: 2 e donated: 2 e donated: 2 as neutral ligands M M M h6-arene h2-alkene h2-alkyne formal charge: 0 formal charge: 0 formal charge: 0 e donated: 6 e donated: 2 e donated: 2 Transition Metals – Electronic Configurations Hapticity (hn) – part 2 unsaturated ligands offer higher degree coordination modes as h1 ligand H O M M M O h1-acetate h1-allyl h1-cyclopentadienyl formal charge: –1 formal charge: –1 formal charge: –1 e donated: 2 e donated: 2 e donated: 2 as hn ligand O M M M O h3-acetate h3-allyl h5-cyclopentadienyl formal charge: –1 formal charge: –1 formal charge: –1 e donated: 4 e donated: 4 e donated: 6 Transition Metals – Electronic Configurations neutral (2 e–) Electron counting exercise – part 2 – anionic (2 e–) anionic (4 e ) ligands = 4 x 2 e– + 1 x 4 e–: 12 e– metal = d6: 6 e– Crabtree's hydrogenation catalyst complex: 18 e– (saturated) Transition Metals – Electronic Configurations Electron counting exercise – part 2 polymerization catalyst Transition Metals – Electronic Configurations Electron counting exercise – part 2 polymerization catalyst ligands = 2 x 2 e– + 2 x 6 e–: 16 e– metal = d0: 0 e– complex: 16 e– (unsaturated) Transition Metals – Electronic Configurations
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