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6 Organometallic Chemistry 9/26/10 W 4:00- 6:50 PM (08/23/10 – 12/13/10) 3615 Bayou Building Zerong Wang, 3531-6 Bayou, Tel. (281)-283-3795, Fax (281)-283-3709 Office Hours: TTH 1:00-2:30 PM Middle Term Test & Final Exam Oct. 13 Nov. 10 Dec. 8 1 9/26/10 Scores Item Points Total points Attendance 200 points 200 points Middle Term 200 points 400 points Final Exam 400 points 400 points Total Points 1000 points Grade Percentage A 92.0-100% B- 78.0-79.9% D+ 63.0-64.9% A- 89.0-91.9% C+ 75.0-77.9% D 58.0-62.9% B+ 86.0-88.9% C 68.0-74.9% D- 55.0-57.9% B 80.0-85.9% C- 65.0-67.9% F <55.0% 2 9/26/10 Attendance & Performance Each class for 20 points of attendance for the following dates Sept. 22 Ch. 5,6 Sept. 29 Ch. 7,8 Oct. 6 Ch. 9,10 Oct. 20 Ch. 11 Oct. 27 Ch. 12,13 Nov. 3 Handout Nov. 17 Handout In addition, performance will be counted into your total scores, things considered for the performance include coming to class on time (e.g., not late for more than 15 minutes), and not leaving the class before the lecture ends. Organometallic Chemistry • Definition of organometallic chemistry: transformations of organic compounds using metals. • Organometallic chemistry is at the interface between inorganic and organic chemistry. – Inorganic: subset of coordination chemistry – Organic: subset of synthetic methods • Other interdisciplinary areas – Bioorganometallic chemistry – Surface organometallic chemistry – Fullerene-metal complexes 6 3 9/26/10 Traditional Organometallic Compounds in Organic Chemistry Organometallic Compounds - Reagents with carbon- metal bonds H H C C - + H3CH2CH2CH2C-Li (H3C)2Cu Li H MgBr Butyllithium vinylmagnesium bromide Dimethylcopper lithium Carbon-Metal Bonds in Organometallic Compounds C X C MgX - + δ+ δ- δ δ _ C X C MgX C Alkyl halides: Carbanions: nucleophile electrophiles react with electrophile 4 9/26/10 Polarity of Bonds Metal Alkyls • General formula R-M (R = alkyl, M = metal) • The C-M bond is a covalent bond! • However, C--M+ or R--M+ bond tend to be polarized. • This is especially true for organometallic compounds conataining the more electropositive metals, i.e. alkali and alkaline earth metals. • Generally, the alkyl fragment of the organometallic compound is very reactive; however this depends on the metal, changing the metal alters the polarization of the R-M bond. • Thus different organometallic compounds are used in many different types of organic reactions. 5 9/26/10 Alkyl halides will react with some metals (M0) in ether or THF to form organometallic reagents Preparation of Organolithium Compounds 2 Li(0) R-X R-Li + LiX diethyl ether - + δ δ _ very strong bases C Li C very strong nucleophiles organolithium reagents are most commonly used as very strong bases and in reactions with carbonyl compounds M(0) H O R-X R-M 2 R-H + M-OH Preparation of Organomagnesium Compounds: Grignard Reagents Mg(0) R-X R-MgX (Grignard reagent) THF R-X can be an alkyl, vinyl, or aryl halide (chloride, bromide, or iodide) Solvent: diethyl ether (Et2O) or tetrahydrofuran (THF) H CH C CH CH 3 2 O 2 3 O diethyl ether (Et2O) tetrahydrofuran (THF) Alcoholic solvents and water are incompatible with Grignard reagents and organolithium reagents. Reactivity of the alkyl halide: -I > -Br > -Cl >> -F alkyl halides > vinyl or aryl halides 6 9/26/10 Most halides can be used directly to make an organometallic lithium or magnesium compound except alkynyl halides. These can be made, but special methods (later) are required. R-X + 2 Li R-Li + LiX R-X + Mg R-Mg-X alkyl sp3 -C-X -C-Li or -C-MgX halide vinyl sp2 =C-X =C-Li or =C-MgX halide sp2 aryl X Li or MgX halide alkynyl sp C-X doesn’t work directly * halide X R-CO-CH=CH-R HO-CH2CH2-R R1-CO-CH=CH2 epoxide R-COOH CO R1-CO-R CuI 2 R R1-X R1-COOR2 R1-R R-Li R1-C-OH R1-CO-R2 R1-C=CH R2 R-CO-Cl O 2 R1-CHO R-OH R-CO-R R1-C=C-Li R-CO-R1 7 9/26/10 Reactions of Grignard reagents The solvent or alkyl halides can not contain functional groups that are electrophilic or acidic. These are incompatible with the formation of the organomagnesium or organolithium reagent. Grignard reagents will deprotonate alcohols 0 _ Mg + H3O HO Br HO MgBr O H HO H _ BrMg Other incompatible groups: -CO2H, -OH, -SH, NH2, CONHR (amides) Reactive functional groups: aldehydes, ketones, esters, amides, halides, -NO2, -SO2R, nitriles 8 9/26/10 ( CARBON-METAL ) carbanion - + δ δ - Carbon-metal bonds : + are polar bonds that can be represented by a resonance hybrid of covalent and ionic covalent ionic structures. Carbon-metal bonds most reactive bond percent ionic strongest base are frequemtly classified as to how C-K 51 much “ionic character” they have, by using C-Na 47 Li and Mg an index called the C-Li 43 are the percent ionic character. C-Mg 35 metals used This index states the most often importance of the C-Zn 18 ionic resonance con- C-Cd 15 tributor relative to the covalent structure. C-Cu 9 best nucleophile least reactive Organocopper Compounds - + • Lithium Dialkylcopper (organocuprate ) [(R)2Cu] Li • Cuprates are less reactive than organolithium • R acts as a Nucleophile • Oxidation state of copper is Cu(I). • Nucleophile “R” will attack various organic electrophiles. • Organocuprates are used in cross-coupling reactions to form higher alkanes. • Cross-Coupling Reaction: coupling of two different alkyls R and R’ to yield a new alkane (R-R’). This type of reaction is used to make new C-C between alkyl groups. 9 9/26/10 Molecules in Organometallic Chemistry Involves at least one TRANSITION METAL that form more than two bonds with other groups (coordinates), known as organometallic complex. Have more than one coordinates that do not involve in the reactions but stablize the complex. Coordinates bind to the transition metal via s, p bonds, involving dsp or spd orbitals, and possibly with anti- bonding orbital participated. Coordinates can be as simple as hydrogen atom (hydride), H2, NO, N2, CO, CN, or complicated molecules such as alkene, carbene, phosphine, etc. Types of Metal-Ligand Interactions • Sigma (σ) donor ligands Pi (π) donor ligands Examples of donor and acceptor ligands • Pi (π) acceptor ligands Sigma donor Pi donor* Pi acceptor* - NH3 HO CO - - - H2O F , Cl CN - - H RCOO PR3 These ligands also act as σ donors. 10 9/26/10 Why 18 electrons? antibonding Electron Counting understanding structure and reactivity Cr = 6 Rh = 9 2*Bz = 12 + 3* P→ = 6 e-count 18 Cl = 1 + e-count 16 tot. charge 0 tot. charge 0 2*Bz - - ox state 0 3*P→ - Cl -1 - ox state +1 11 9/26/10 Why Count Electrons ? • Basic tool for understanding structure and reactivity. • Simple extension of Lewis structure rules. • Counting should be “automatic”. • Not always unambiguous ⇒ don’t just follow the rules, understand them! Predicting Reactivity 12 9/26/10 Predicting Reactivity Most likely associative: II 16-e Pd 16-e PdII 18-e PdII Predicting Reactivity 13 9/26/10 Predicting Reactivity Almost certainly dissociative: 16-e Cr(0) 18-e Cr(0) 18-e Cr(0) The Basis of Electron Counting • Every element has a certain number of valence orbitals: 1 { 1s } for H 4 { ns, 3´np } for main group elements 9 { ns, 3´np, 5´(n-1)d } for transition metals s px py pz dxy dxz dyz dx2-y2 dz2 14 9/26/10 The Basis of Electron Counting • Every orbital wants to be “used", i.e. contribute to binding an electron pair. • Therefore, every element wants to be surrounded by 1/4/9 electron pairs, or 2/8/18 electrons. – For main-group metals (8-e), this leads to the standard Lewis structure rules. – For transition metals, we get the 18-electron rule. • Structures which have this preferred count are called electron-precise. Compounds Are Not Always Electron-Precise ! The strength of the preference for electron-precise structures depends on the position of the element in the periodic table. • For very electropositive main-group elements, electron count often determined by steric factors. – How many ligands "fit" around the metal? – "Orbitals don't matter" for ionic compounds • Main-group elements of intermediate electronegativity (C, B) have a strong preference for 8-e structures. • For the heavier, electronegative main-group elements, there is 2- the usual ambiguity in writing Lewis structures (SO4 : 8-e or 12-e?). Stable, truly hypervalent molecules (for which every Lewis structure has > 8-e) are not that common (SF6, PF5). Structures with < 8-e are very rare. 15 9/26/10 Compounds Are Not Always Electron-Precise ! The strength of the preference for electron-precise structures depends on the position of the element in the periodic table. For early transition metals, 18-e is often unattainable for steric reasons The required number of ligands would not fit. For later transition metals, 16-e is often quite stable In particular for square-planar d8 complexes. For open-shell complexes, every valence orbital wants to be used for at least one electron More diverse possibilities, harder to predict. Prediction of Stable Complexes Cp Fe, ferrocene: 18-e 2 Cp Co, cobaltocene: 19-e Cp Ni, nickelocene: 20-e Very stable. 2 2 Chemically reactive, Behaves as an aromatic Strong reductant, reacts with air.
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