Lecture 13a - UCLA

Lecture 13a - UCLA

Lecture 13a Metal Carbonyl Compounds Introduction The first metal carbonyl compound described was Ni(CO)4 (Ludwig Mond, ~1890), which was used to refine nickel metal (Mond Process) Metal carbonyls are used in many industrial processes aiming at carbonyl compounds i.e., Monsanto process (acetic acid), Fischer Tropsch process or Reppe carbonylation (vinyl esters) Vaskas complex (IrCl(CO)(PPh3)2) absorbs oxygen reversibly and serves as model for the oxygen absorption of myoglobin and hemoglobin Carbon Monoxide Carbon monoxide is a colorless, tasteless gas that is highly toxic because it strongly binds to the iron in hemoglobin It is generally described with a triple bond because the bond distance of d=113 pm is too short for a double bond i.e., formaldehyde (d=121 pm) The structure on the left is the major contributor because both atoms have an octet in this resonance structure, which means that the carbon atom is bearing the negative charge The lone pair of the carbon atom is located in a sp-orbital Carbon monoxide is isoelectronic with the nitrosyl cation (NO +) Bond Mode of CO to Metals The CO ligand usually binds via the carbon atom to the metal C s-bond O C

O p-backbond The lone pair on the carbon forms a s-bond with a suitable d-orbital of the metal The metal can form a p-back bond via the p*-orbital of the CO ligand Electron-rich metals i.e., late transition metals in low oxidation states are more likely to donate electrons for the back bonding A strong p-back bond results in a shorter the M-C bond and a longer the C-O bond due toOthe population ofC an anti-bonding orbital in M C M O the CO ligand (I) (II) Synthesis Some compounds can be obtained by direct carbonylation at room temperature or elevated temperatures 25 oC/1 atm Ni(CO)4 (CO)= 2057 cm -1 Fe(CO)5 (CO)= 2013, 2034 cm -1 CrCl3 + Al + 6 CO Cr(CO)6 + AlCl 3

(CO)= 2000 cm -1 Re2O 7 + 17 CO Re2(CO)10 + 7 CO 2 (CO)= 1983, 2013, 2044 cm -1 Ni + 4 CO Fe + 5 CO 2 Fe(CO)5 150 oC/100 atm CH3COOH Fe 2(CO)9 + CO (CO)= 1829, 2019, 2082 cm -1 UV-light In other cases, the metal has to be generated in-situ by reduction of a metal halide or metal oxide Many polynuclear metal carbonyl compounds can be obtained using photochemistry, which exploits the labile character of many M-CO bonds (bath tub chemistry) Structures I Three bond modes found in metal carbonyl compounds O O C

C M M O C M M M M terminal 2 3 The terminal mode is the most frequently one mode found exhibiting a carbon oxygen triple bond i.e., Ni(CO)4 The double or triply-bridged mode is found in many polynuclear metals carbonyl compounds with an electron deficiency i.e., Rh6(CO)16 (four triply bridged CO groups) Which modes are present in a given compound can often be determined by infrared spectroscopy Structures II Mononuclear compounds CO CO OC CO

OC M OC M CO CO CO M CO CO CO CO OC CO M(CO)6 (Oh) i.e., Cr(CO)6 M(CO)5 (D3h) i.e., Fe(CO)5 M(CO)4 (Td) i.e., Ni(CO)4 Dinuclear compounds CO CO OC OC

M OC CO OC M OC CO CO M2(CO)10 (D4d) i.e., Re2(CO)10 O C OC O C Fe OC OC CO Fe C O CO CO Fe2(CO)9 (D3h)

O C OC Co OC OC O C CO Co CO CO OC CO CO OC Co OC Co OC CO CO Co2(CO)8 Co2(CO)8 (solid state, C2v) (solution, D3d)

Infrared Spectroscopy Free CO: 2143 cm-1 Terminal CO groups: 1850-2120 cm-1 2-brigding CO groups: 1750-1850 cm-1 3-bridging CO groups: 1620-1730 cm-1 Compound (CO) (cm-1) Ni(CO)4 2057 Fe(CO)5 2013, 2034 Cr(CO)6 2000 Re2(CO)10 1976, 2014, 2070 Fe2(CO)9 1829, 2019, 2082 Rh6(CO)16 1800, 2026, 2073

Ag(CO)+ 2185 Non-classical metal carbonyl compounds can have (CO) greater than the one observed in free CO 13 C-NMR Spectroscopy Terminal CO: 180-220 ppm Bridging CO: 230-280 ppm Examples: M(CO)6: Cr: 211 ppm, Mo: 201.2 ppm, W: 193.1 ppm Fe(CO)5 Solid state: 208.1 ppm (equatorial) and 216 ppm (axial) in a 3:2-ratio Solution: 211.6 ppm (due to rapid axial-equatorial exchange) Fe2(CO)9 (solid state): 204.2 ppm (terminal), 236.4 ppm (bridging) Co2(CO)8 Solid state: 182 ppm (terminal), 234 ppm (bridging) Solution: 205.3 ppm Application I Fischer Tropsch Reaction/Process The reaction was discovered in 1923 The reaction employs hydrogen, carbon monoxide and a metal carbonyl catalyst to form alkanes, alcohols, etc. Ruhrchemie A.G. (1936) Used this process to convert synthesis gas into gasoline using a catalyst Co/ThO2/MgO/Silica gel at 170-200 oC at 1 atm The yield of gasoline was only ~50 % while about 25 % diesel oil and 25 % waxes were formed An improved process (Sasol) using iron oxides as catalyst, 320-340 oC and 25 atm pressure affords 70% gasoline

Application II Second generation catalyst are homogeneous i.e. [Rh6(CO)34]2 Union Carbide: ethylene glycol (antifreeze) is obtain at high pressures (3000 atm, 250 oC) O M CO M CO H2 M C H H2 M H2 M O CH2 CH3 M M OCH3 M H M

CH3 M CH2 CH3 CO M CH 4 COCH3 H2 H2 H2 CH 3OH CO M COCH2CH3 H Gasolines Production of long-chain alkanes is favored at a temperature around 220 oC and pressures of 1-30 atm Application III Monsanto Process (Acetic Acid) This process uses cis-[(CO)2RhI2]- as catalyst to convert methanol and carbon dioxide to acetic acid

The reaction is carried out at 180 oC and 30 atm pressure Oxidative Addition (+I to (+III) CH3OH H2O CO I I I CO COCH3 Rh Rh I CH3COOH CO Insertion CO Rh CH3I I HI CH3 I

CO I CO I CH3COI Reductive Elimination (+III) to(+) I COCH3 CO Rh I CO CO I CO Addition Two separate cycles that are combined with each other Application IV Hydroformylation It uses cobalt catalyst to convert an alkene, carbon monoxide and hydrogen has into an aldehyde The reaction is carried at moderate temperatures (90-150 oC) and high pressures (100-400 atm) HCo(CO)4

CO RCH2CH2CHO HCo(CO)3 RCH2CH 2COCo(H2)(CO)3 CH2=CHR HCo(CO)3(CH2=CHR) H2 RCH 2CH2COCo(CO)3 RCH2CH 2Co(CO)3 RCH 2CH 2Co(CO)4 CO Application V Reppe-Carbonylation Acetylene, carbon monoxide and alcohols are reacted in the presence of a catalyst like Ni(CO)4, HCo(CO)4 or Fe(CO)5 to yield acrylic acid esters The synthesis of ibuprofen uses a palladium catalyst on the last step to convert the secondary alcohol into a carboxylic acid CO, [Pd] H2, Raney Ni (CH3CO) 2O/HF O OH

COOH This process is much greener than the original process because the atom economy is 99+ % (after recycling) Application VI Vaskas Complex (1961) Originally synthesized from IrCl3, triphenylphosphine and various alcohols i.e., 2-methoxyethanol. Triphenylphosphine as a ligand and reductant in the reaction A more convenient synthesis uses N,N-dimethylformamide as the CO source (DMF decomposes to CO + HNMe2) Aniline is frequently used as an accelerant The resulting bright yellow complex is square planar (IrCl(CO)(PPh3)2) because Ir(I) exhibits d8-configuration The two triphenylphosphine ligands are in trans configuration due to the steric demand of the triphenylphosphine ligands CO Ph3P Ir Cl PPh3 Application VII Vaskas Complex (cont.) The carbonyl stretching mode in the complex is consistent with a strong p-backbonding ability (d(CO)= 116.1 pm (free CO, d= 113 pm)) The complex is a 16 VE system that reactants with broad variety of compounds under oxidative addition usually via a cis addition in which the Cl and the CO ligand fold back CO Ph3P Ir Cl PPh3

X- Y Y X Ph3P Ir Cl PPh3 CO Note that a molecule like oxygen is bonded side-on in the light orange complex: d(O-O)=147 pm (free oxygen: 121 pm, peroxide (O22-:149 pm)) (O-O)=856 cm-1 (free oxygen: 1556 cm-1, peroxide (O22-: 880 cm-1)) Note that the older literature reports a d(O-O)=130 pm, which is more consistent with a superoxide (O2-)! The addition of oxygen to Vaskas complex is reversible Application VIII Vaskas Complex (cont.) X-Y (CO) in cm-1 none 1967 H2 1983 O2 2015

HCl 2046 MeI 2047 I2 2067 Cl2 2075 The resulting products exhibit increased carbonyl stretching frequencies because the metal does less p-backbonding due to its higher oxidation state (Ir(III)) A similar trend is also found for the Ir-P bond length, which increases in length compared to the initial complex

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