Seminar assignments - Big problem and solution file - master chem 423 ! PDF

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Big problem and solution file - Master chem 423 !...

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423/523 Organometallic Chemistry Problem set 2 1. Assign the oxidation state of each M. Assuming the 18-electron rule applies, identify the 1st row transition metal and sketch the complex: (a) (b) (c) (d) (e)

M(CO)(CS)(PPh3)2Br [M(CO)7]+ [(η3-C3Ph3)(η4-C4H4)M(NH3)2]+ [(η5-C5H5)(η4-C5H6)M]+ [(η3-C3H5)M(CN)4]2–

2. Assign the oxidation state of each M. Identify the 2nd row transition metal and sketch the complex: (a) (b) (c) (d) (e) (f)

(η5-C5H5)(CO)3M–M(CO)3(η5-C5H5) (η5-C5H5)(CO)2M=M(CO)2(η5-C5H5) [M(CO)3(NO)]– (η4-C8H8)M(CO)3 [M(CO)3(PMe3)]– (η5-C5H5)(η1-C3H5)(η3-C3H5)2M

(linear NO)

(16-electron complex)

3. What charge, z, would be necessary for the following to obey the 18-electron rule? (a) (b) (c) (d) (e)

[Ru(CO)4(SiMe3)]z [(η3-C3H5)V(CNMe)5]z [(η5-C6H7)Fe(CO)3]z [(η6-C6H6)2Ru]z [W(CO)5(SnPh3)]z

4. A complex has the empirical formula Re(CO)3Cl. How could it attain the 18-electron configuration without requiring any additional ligands? 5. Predict the hapticity (i.e. what is n in ηn) of each Cp ring in Cp2W(CO)2, and of each “triphos” ligand in [Pd{PPh2CH2CH2)3CPh}2]2+. 6. Comment on the observation that the ν(CO) band in [Fe(CO)6]2+ appears at 2203 cm-1 (compare with free CO). 7. When heated at low pressure, (η5-C5Me5)Rh(CO)2 reacts to give a gas and another product having a single peak in the 1H NMR and a single band near 1850 cm-1 in the infrared. Suggest a structure for this product. 8. Predict the distribution of products when carbon monoxide is lost from cis-Mn(COMe)(CO)4(13CO) assuming the reaction proceeds by deinsertion of CO (as opposed to Me migration, i.e. it is CO that moves to the vacant coordination site, not Me).

423/523 Organometallic Chemistry Problem set 3 1. Rationalise the following observations: (a) On going from Fe(CO)5 to Fe(CO)3(PPh3)2, absorptions in the IR spectrum at 2025 and 2000 cm-1 are replaced by bands at 1944, 1886 and 1881 cm-1. (b) On forming IrBr(CO){η2-C2(CN)4}(PPh3)2, the unique C-C bond in C2(CN)4 lengthens from 135 to 151 pm. (c) The Tolman cone angles of PPh3 and P(p-MeC6H4)3 are 145°, but that of P(o-MeC6H4)3 is 194°. (d) A single ν(CO) band is observed for the ion [Co(CO)3(PPh3)2]+. 2. The reaction of [(C6Me6)RuCl2]2 (A) with C6Me6 in the presence of AgBF4 gives [(C6Me6)2Ru][BF4]2 containing cation B. Treatment of this compound with Na in liquid NH3 yields a neutral Ru0 complex, C. Suggest structures for A, B and C. 3. List the following in order of decreasing reactivity towards trimethylamine oxide: Mo(CO)6, [Mn(CO)6]+, Mo(CO)2(Ph2PCH2CH2PPh2)2, [Mo(CO)5]2–, Mo(CO)4(Ph2PCH2CH2PPh2), Mo(CO)3(NO)2. What physical data would you choose to measure as an aid to ordering these complexes? 4. NO+ is isoelectronic with CO and often replaces CO in substitution reactions, so it might seem the reaction below is favourable. Comment on whether the process is likely. Mo(CO)6 + NOBF4 → [Mo(NO)6][BF4]6 + 6CO 5. A complex with the empirical formula Mn3(C5H5)3(NO)4 has infrared absorptions at 1320 and 1495 cm-1 and a single peak in the 1H NMR spectrum. Draw a plausible structure. 6. The product of reaction between PtCl2 and CO at high pressure and 200 °C has a molecular weight of 322. Find the formula and suggest possible isomers. Comment on the probable relative MC and CO bond lengths in these isomers. Can vibrational spectroscopy be used to distinguish between isomers?

423/523 Organometallic Chemistry Problem set 4 1. Strained alkenes such as cyclopropene or norbornene bind unusually strongly to metals. Suggest a reason why. 2. Alkynes readily bridge M-M bonds, in which case they act as 2e donors to each metal. Sketch the product of the reaction below, indicating the hybridization of the C atoms. PhCCPh + Co2(CO)8 → (µ 2-PhCCPh)Co2(CO)6 + 2CO 3. Draw as many bonding modes for cyclooctatetraene as you can think of. 4. Sketch the three π-MOs of the allyl anion, [C3H5]–. Draw metal d-orbitals that can interact with these MOs, and name the type of bonding (e.g. M→L π-acceptor). 5. The M-P distance in (η5-C5H5)Co(PEt3)2 is 221.8 pm and the P-C distance is 184.6 pm. The corresponding distances in [(η5-C5H5)Co(PEt3)2]+ are 223 pm and 182.9 pm. Account for the changes in these distances as the former complex is oxidised. 6. Predict the product of the reactions between: (a) [Ru(η5-C7H9)(η6-C7H8)]+ and H– (b) [W(η5-C5H5)2(η3-C3H5)]+ and H–. 7. Ligands of type X–Y only give 3c-2e “agostic” bonds to transition metals if X = H and Y lacks lone pairs. Why do you think this is so (consider alternative structures if X and Y are not H)? 8. [IrH2(H2O)2(PPh3)2]+ reacts with indene to give [Ir(C9H10)(PPh3)2]+ (A). On heating, this species rearranges with H2 loss to give [IrH(C9H7)(PPh3)2]+ (B). Only A reacts with ligands such as CO to displace C9H7. What do you think are the structures of A and B?

indene

423/523 Organometallic Chemistry Problem set 5 1. F3CI has a δ– CF3 group and a δ+ I. Because of this, to make trifluoromethyl complexes of transition metals, F3CC(O)Cl is often used. How does this approach work? 2. Metal alkoxides, like metal alkyls, can also β-eliminate. With this in mind: (a) explain why –OtBu is a common ligand in metal alkoxide chemistry. (b) what are the products of decomposition of primary and secondary alkoxide ligands? (c) how can alcohols, in the presence of a base, be used as reducing agents for metal complexes? 3. Mo(CO)6 undergoes substitution reactions with phosphine ligands, but the reaction never proceeds further than the Mo(CO)3(PR3)3 stage. If the phosphines are very bulky, the phosphines are arranged mer, but otherwise are always fac. Explain these two observations. 4. CpRe(NO)(CO)Me reacts with two equivalents of PMe3 to provide a product in which six ligands are bound to Re. The reaction has a large negative ΔS‡. Draw the product and suggest a plausible mechanism. 5. In the substitution of V(CO)6, the rate of reaction changes with respect to phosphine nucleophile according to the order PMe3 > PBu3 > P(OMe)3 > PPh3. What does this suggest about the mechanism? 6. In the following reaction scheme, name the reaction(s) occurring at each step, and work out the oxidation state and electron count of all metal complexes.

H

H –L

L2Pt

+L LPt

LPt H

L2Pt

+ CMe4

423/523 Organometallic Chemistry Problem set 6 1. Sketch the transition state for the first step in the oxidative addition of a benzyl halide and a square planar complex ML4. 2. Explain the following. The cis isomer of L2Pd(Et)2 decomposes immediately to give butane, but the trans isomer produces a 1:1 mixture of ethene and ethane. 3. The reaction of L2Pd(Me)2 with PhC*HDBr produced PhC*HDMe. What is the other product, and do you expect retention or inversion at the chiral carbon? 4. SO2 can insert into an LnM-CR3 bond. The reaction is thought to proceed by an SE2 pathway to form an ion pair, [LnM]+[OS(O)R]–. Collapse of the ion pair generates the O-sulfinate (formally a 1,2-insertion of SO2), which can rearrange to the S-sulfinate (formally a 1,1-insertion of SO2). Draw the transition state, the ion pair, and the O- and S-sulfinates. 5. Provide a mechanism for the reaction: LnZr–H + E–2-butene → LnZr–CH2CH2CH2CH3 6. Binding of Cr(CO)3 to an achiral arene: X Y

gives a chiral complex. Illustrate this. What about for meta- and para- substituted arenes? How might this be useful?

423/523 Organometallic Chemistry Problem set 7 1. In 1968, the Shell Oil Company reported that adding tertiary phosphine such as PBu3 to the oxo reaction resulted in hydroformylation taking place at less than 100 atm. Though it the new catalyst is a less active hydroformylation catalyst than HCo(CO)3, it is a better hydrogenation catalyst. Separation of catalyst from products is improved. The ratio of linear to branched product is as high as 9 to 1. Studies of the reaction failed to observe alkyl or acyl-containing intermediates. (a) Explain the greater selectivity of the new catalyst by drawing an appropriate transition state. (b) What are the advantages and disadvantages of the new catalyst being a good hydrogenation catalyst? (c) What does the failure to observe intermediates tell you about the likely rate-determining step? (d) Why is it easier to remove the products from the catalyst? 2. Draw a catalytic cycle for phosphine-rhodium-catalysed hydroformylation. The catalyst precursor is H(CO)Rh(PPh3)3. 3. Explain why increasing the concentration of phosphine in the phosphine-rhodium cycle slows the reaction rate, but also raises the linear/branched product ratio. 4. Rate of Rh/HI-catalysed carbonylation of 2-propanol to give a mixture of butanoic and 2-methylpropanoic acids is actually a little faster than that for carbonylation of ethanol and up to seven times more rapid than that for 1-propanol. If OA is still the rate-determining step in the pathway, what can be said about the mechanism of that step when 2-propanol is the starting material?

423/523 Organometallic Chemistry Problem set 8 1. Propose a mechanism for: R (OC)5W=CR(OR) OR

2. In principle, cyclopentene might metathesise to 1,6-cyclodecadiene (cdd). In fact, a polymer is formed. What is the structure of the polymer, and how does its formation, rather than that of cdd, relate to the question of pairwise versus non-pairwise mechanisms? 3. In some TiCl3-based polymerization catalysts, a small amount of NiCl 2 is added to shorten the chain length of the polymer. What is the role of the Ni? What is the structure of the end group when the polymer dissociates from the Ni-doped catalyst? What might be the effect of the additives FeCl3, HgCl2 and VCl5? 4. Two chemically inequivalent hydrides, HA and HB, in a metal dihydride complex at 50°C, resonate at δ –5 and –10 ppm, respectively, and are exchanging so that each resonance shows an initial broadening of 10 Hz at a field corresponding to 500 MHz. What is the rate of exchange? At 80°C we observe coalescence; what is the new rate of exchange? 5. Allyl complexes are characteristically fluxional, the principle pathway being an η3–η1–η3 process. Illustrate this process, and sketch the 1H spectra for the low and high T limits of the complex. 6. The variable temperature 13C NMR of (cyclooctatetraene)Ru(CO)3 is shown below. Suggest a reason for the changes in the spectrum with T, and assign the low T spectrum as best you can (free cyclooctatetraene shows a single resonance at 133 ppm).

423/523 Organometallic Chemistry Problem set 2 1. Assign the oxidation state of each M. Assuming the 18-electron rule applies, identify the 1st row transition metal and sketch the complex: (a) (b) (c) (d) (e)

CoI(CO)(CS)(PPh3)2Br [VI(CO)7]+ [(η3-C3Ph3)(η4-C4H4)FeII(NH3)2]+ [(η5-C5H5)(η4-C5H6)NiII]+ [(η3-C3H5)CoIII(CN)4]2–

2. Assign the oxidation state of each M. Identify the 2nd row transition metal and sketch the complex: (a) (b) (c) (d) (e) (f)

(η5-C5H5)(CO)3MoI–MoI(CO)3(η5-C5H5) (η5-C5H5)(CO)2TcI=TcI(CO)2(η5-C5H5) [Ru–II(CO)3(NO)]– (η4-C8H8)Ru0(CO)3 [Rh–I(CO)3(PMe3)]– (η5-C5H5)(η1-C3H5)(η3-C3H5)2ZrIV

(linear NO)

(16-electron molecule)

3. What charge, z, would be necessary for the following to obey the 18-electron rule? (a) (b) (c) (d) (e)

[Ru(CO)4(SiMe3)]–1 [(η3-C3H5)V(CNMe)5]0 [(η5-C6H7)Fe(CO)3]+1 [(η6-C6H6)2Ru]+2 [W(CO)5(SnPh3)]–1

4. A complex has the empirical formula Re(CO)3Cl. How could it attain the 18-electron configuration without requiring any additional ligands? Cl Re(CO)3

(OC)3Re Cl

(or triangular cluster, etc with single bonds)

5. Predict the hapticity (i.e. what is n in ηn) of each Cp ring in (η5-C5H5)(η3-C5H5)W(CO)2, and the n in κn of κ2-PPh2CH2CH2)3CPh}2]2+ (if 16e; 18e requires 1 κ3-, 1 κ2-). each “triphos” ligand in [Pd{κ 6. Comment on the observation that the ν(CO) band in [Fe(CO)6]2+ appears at 2203 cm-1 (compare with free CO). Higher than free CO, so no back-bonding; electrons donated from slightly anti-bonding electron pair on CO strengthens the CO bond; +ve charge on iron lowers energy of d-orbitals & ability to back-bond. 7. When heated at low pressure, (η5-C5Me5)Rh(CO)2 reacts to give a gas and another product having a single peak in the 1H NMR and a single band near 1850 cm-1 in the infrared. Suggest a structure for this product. See right:

CO Rh OC

8. Predict the distribution of products when carbon monoxide is lost from cisMn(COMe)(CO)4(13CO) assuming the reaction proceeds by deinsertion of CO (as opposed to Me migration, i.e. it is CO that moves to the vacant coordination site, not Me). 25% Mn(CO)5Me, 75% cis-Mn(CO)4(13CO)Me

Rh Rh

CO

423/523 Organometallic Chemistry Problem set 3 1. Rationalise the following observations: (a) On going from Fe(CO)5 to Fe(CO)3(PPh3)2, absorptions in the IR spectrum at 2025 and 2000 cm-1 are replaced by bands at 1944, 1886 and 1881 cm-1. PPh3 better σ-donor but poorer π-acceptor than CO, so the electron density pushed on to Fe by the phosphines is off-loaded on to the remaining COs, diminishing the C-O bond order. (b) On forming IrBr(CO){η2-C2(CN)4}(PPh3)2, the unique C-C bond in C2(CN)4 lengthens from 135 to 151 pm. Electron-withdrawing groups on alkene make it a relatively good π-acceptor, so metallacyclopropane approximation good for this complex; C-C bond order lowered. (c) The Tolman cone angles of PPh3 and P(p-MeC6H4)3 are 145°, but that of P(o-MeC6H4)3 is 194°. Para-Me doesn’t affect geometry near the P, but ortho-Me has large effect (make a model). (d) A single ν(CO) band is observed for the ion [Co(CO)3(PPh3)2]+. PPh3 groups must be trans on a TBP complex. 2. The reaction of [(C6Me6)RuCl]2 (A) with C6Me6 in the presence of AgBF4 gives [(C6Me6)2Ru][BF4]2 containing cation B. Treatment of this compound with Na in liquid NH3 yields a neutral Ru0 complex, C. Suggest structures for A, B and C.

3. List the following in order of decreasing reactivity towards trimethylamine oxide: Mo(CO)6, [Mn(CO)6]+, Mo(CO)2(Ph2PCH2CH2PPh2)2, [Mo(CO)5]2–, Mo(CO)4(Ph2PCH2CH2PPh2), Mo(CO)3(NO)2. What physical data would you choose to measure as an aid to ordering these complexes? The more δ+ the CO carbon, the easier the reaction, so the order is [Mn(CO)6]+ > Mo(CO)3(NO)2 > Mo(CO)6 > Mo(CO)4(Ph2PCH2CH2PPh2) > Mo(CO)2(Ph2PCH2CH2PPh2)2 > [Mo(CO)5]2– (cations > neutrals > anions, and within each class, better π-acceptor ligands > less good π-acceptor ligands. IR. 4. NO+ is isoelectronic with CO and often replaces CO in substitution reactions, so it might seem the reaction below is favourable. Comment on whether the process is likely. Mo(CO)6 + NOBF4 → [Mo(NO)6][BF4]6 + 6CO Net charge of > ±2 on complexes is rare, 6+ ridiculous. Mo would be incapable of π-donation in this complex.

5. A complex with the empirical formula Mn3(C5H5)3(NO)4 has infrared absorptions at 1320 and 1495 cm-1 and a single peak in the 1H NMR spectrum. Draw a plausible structure.

6. The product of reaction between PtCl2 and CO at high pressure and 200 °C has a molecular weight of 322. Find the formula and suggest possible isomers. Comment on the probable relative MC and CO bond lengths in these isomers. Can vibrational spectroscopy be used to distinguish between isomers? PtCl2(CO)2. 16e, d8 complex, most likely square-planar with cis and trans isomers. The cis isomer is likely to have the shortest M-C and longest C-O bonds, as the CO ligands in this complex are competing with the Cl ligands (π-donors) for π-electron density rather than with each other. Yes; the cis isomer will have 2 CO stretches, the trans just 1.

423/523 Organometallic Chemistry Problem set 4 1. Strained alkenes such as cyclopropene or norbornene bind unusually strongly to metals. Suggest a reason why. Rehybridisation (sp2 → sp3) upon binding leads to relief of strain. 2. Alkynes readily bridge M-M bonds, in which case they act as 2e donors to each metal. Sketch the product of the reaction below, indicating the hybridization of the C atoms. PhCCPh + Co2(CO)8 → (µ2-PhCCPh)Co2(CO)6 + 2CO sp3

Ph

Ph C

C Co(CO)3

(OC) 3Co

3. Draw as many bonding modes for cyclooctatetraene as you can think of. Lots! Don’t forget bridging modes… 4. Sketch the three π-MOs of the allyl anion, [C3H5]–. Draw metal d-orbitals that can interact with these MOs, and name the type of bonding (e.g. M→L π-acceptor).

M to L π-acceptor

L to M π-donor

L to M σ-donor

5. The M-P distance in (η5-C5H5)Co(PEt3)2 is 221.8 pm and the P-C distance is 184.6 pm. The corresponding distances in [(η5-C5H5)Co(PEt3)2]+ are 223 pm and 182.9 pm. Account for the changes in these distances as the former complex is oxidised. The oxidised complex is less able to donate electron density into σ* PEt3 antibonding orbitals, so P-C bonds become shorter. Weakening of π-back bonding will also lengthen the Co-P bond. However... the better energy match between metal and ligand orbitals upon oxidation of the metal would make the M-P bond become shorter due to stronger σ-bonding, and given that σbonding is much more important for phosphines than π-back bonding, I would have thought it would be the more important effect. Apparently not… 6. Predict the product of the reactions between: (a) [Ru(η5-C7H9)(η6-C7H8)]+ and H–. (b) [W(η5-C5H5)2(η3-C3H5)]+ and H–.

Work out the structures and then use DGM rules. Ru(η5-C7H9)2 W(η5-C5H5)2(η2-cyclo-CH2CH2CH2)

We’ll cover this material later in the term. 7. Ligands of type X–Y only give 3c-2e “agostic” bonds to transition metals if X = H and Y lacks lone pairs. Why do you think this is so (consider alternative structures if X and Y are not H)? Lone pairs always compete better than an agostic bond (both donate a pair of electrons to the metal). If X ≠ H, the σ* orbital is no longer spherical (it is, for example, a combination of two sp3 orbitals out of phase rather than an sp3 and a H 1s) and is much less accessible to a metal dorbital. 8. [IrH2(H2O)2(PPh3)2]+ reacts with indene to give [Ir(C9H10)(PPh3)2]+ (A). On heating, this species rearranges with H2 loss to give [IrH(C9H7)(PPh3)2]+ (B). Only A reacts with ligands such as CO to displace C9H7. What do you think are the structures of A and B?

indene

+

+

Ir

Ir PPh3

Ph3P

A

Ph3P

H PPh3

B

These structures obey the 18e rule and fit the empirical data. The reactivity described should probably read “Only A reacts with ligands such as CO to displace C9H10; B does not react to displace C9H7”. However, you don’t really need this to solve the structural question. The Cp-like indenyl ligand is difficult to displace as it is formally anionic, whereas the neutral arene can be replaced relatively easily by 3 COs.

423/523 Organometallic Chemistry Problem set 5 1. F3CI has a δ– CF3 group and a δ+ I. Because of this, to make trifluoromethyl complexes of transition metals, F3CC(O)Cl is often used. How does this approach work? Transmetallation requires the transfer of R+, so doesn’t work for F3CI. However, the acyl complex forms without difficulty and can lose the CO by alkyl migration. 2. Metal alkoxides, like metal alkyls, can also β-eliminate. With this in mind: (a) explain why –OtBu is a common ligand in metal alkoxide chemistry. No β-hydrogens. (b) what are the products of decomposition of primary and secondary alkoxide ligands? Aldehydes and ketones. (c) why are alcohols, in the presence of a base, used as reducing agents for metal complexes? The base converts the alcohol to an alkoxide, which after coordinating can β-eliminate an aldehyde (or ketone) to give M-H. 3. Mo(CO)6 undergoes substitution reactions with phosphine ligands, but the reaction never proceeds further than the Mo(CO)3(PR3)3 stage. If the phosphines are very bulky, the phosphines are arranged mer, but otherwise are always fac. Explain these two observations. CO is strongly trans-directing, so fac- is favoured. O...