Inorganic Mechanistic Case Studies Flashcards
If you add an alkane to a metal carbonyl you can have two intermediates
You can break the C-H bond and made a metal hydride and M-C bond
What is the structure of the alkyl hydride intermediate complex and how does the oxidation state change?
- Loss of a CO means oxidation state has increased
- So v(CO) bands have shifed up (less electron density on the metal = less e- density going into antibonding = strong CO bond)
If you add an alkane to a metal carbonyl you can have two intermediates
You can break the C-H bond and made a metal hydride and M-C bond
What is the structure of the alkane complex intermediate and how does the oxidation state change
- Coordination like this in a sideways fashion, producing sigma bonds
- The oxidation state doesnt change BUT the v(CO) bands shift down
- This is because the carbonyl removes electron density from the metal, therefore removing the carbonyl, results in more e- density on the metal, so the band moves down
If you add hydrogen to a metal carbonyl, you can have two intermediates
If we break the H-H bond, we made two new metal hydride bonds
What is the structure of the Dihydride complex formed and how does the oxidation state change?
- Oxidation state increases AND v(CO) band shifts up
- Loss of a CO so OS increases
- v(CO) shift up as less e- density on the metal
If you add hydrogen to a metal carbonyl, you can have two intermediates
If also can result in coordination of the dihydride
What is the structure of the new dihyride complex and does the oxidation state change?
- Oxidation state is not changed
- Removal of the pi-acceptor CO, resulting in the v(CO) shifting down
Xe can also be the ligand added on during this reaction
What happens to tthe oxidation state and the v(CO) shift
- Oxidation state is unchanged
- v(CO) shifts down due to more e- density on the metal due to the loss of the carbonyl
N₂ can also be the ligand added on during this reaction
What happens to the oxidation state and the v(CO) shift?
- Oxidation state unchanged
- v(CO) band shifted down (due to more e- density on the metal due to loss of carbonyl) BUT not as much as xenon or alkane complex
- This is because nitrogen is a pi acceptor
How could we find evidence that Xe is coordination to hydrogen shown in the way on the left or the right?
- Using ft-IR due to dipole on hydrogen
- Can further prove this by switching H to D and seeing this band shift
What is a Supercritical fluid?
- A supercritical fluid is a substance at a temperature and pressure above its critical point, where it exhibits properties of both a liquid and a gas. In this state the fluid has:
- Density similar to a liquid (allowing it to dissolve substances like a solvent)
- Viscosity and diffusivity similar to a gas (enabling it to penetrate materials easily)
- Easy to obtain for Xe and CO₂
Supercritical fluids can dissolve both…
gases and solids
(like organometallic compounds)
The solubility of a supercritical fluid increases with…
applied pressure (density)
For a given pressure of H₂, there is…
…a much higher concentration of H₂ in a supercritical fluid than in a conventional solvent
We can undertake the following reaction in a supercritical fluid allowing for a higher concentration of hydrogen
What does the FT-IR spectrum look like before and after this reaction
- New bands appear and they have shifted down in wavenumber
- (higher e- density)
- Characteristic of coordination of H₂
We can activate methane feedstock by making in a metal hydride
What will be the difference in the oxidation state and v(CO) from the intermediate to the product
What technique would we use to understand the chemistry of these species?
Time-resolved IR spectroscopy
The following number are second order rate constants
If the number is high what does that mean for the lifetime of a species
- If the number is high, it means it reacts with Co very rapidly - short lived
- If the number is low, it means it is long lived
- For the elements with lower numbers, we can cool them down which slows the rate, allowing an NMR e.g. to be run
How does the time-resolved IR change during this organometallic alkane & C-H activation
- Activation of an alkane can easily be monitored as v(CO) bands of the organometallic alkane shift down due to no change in oxidation state and removal of π-accepting CO ligand while v(CO) band of alkyl hyride shift up because the metal centre is oxidised
- Decay of the red band due to the loss of the [CpRh(CO)(alkane)] intermediate species
- Increase of the green band due to the production of the [CpRh(CO)(alky)H] species
- The rate at which these bands decay/form determine reaction kinetics
The type of alkane affect the rate of decay
How?
- The longer the alkane is, the slower the reaction is
- Reactivity is linear with carbon chain length for [CpRh(CO)(alkane)
- Rate is governed by the interplay between barrer to activation and barrier to alkane coordination to different “C-H” units by hopping between coordination modes i.e. walking of the alkane before activation - primary C-H which activates
UV photolysis of [CpMn(CO)₃] in Ar Matrix/ 12 K
UV photolysis resulted in loss of parent bands at 2033 and 1951 cm⁻¹ and the production of new bands at 1973 and 1903 cm⁻¹ together with an IR band indicating the production of free CO in the matrix
- Production of a free CO = loss of CO ligand from compound
- (although 3xCO in original molecule, symmetry means only 2xCO bands)
- Shift in v(CO) band consistent with loss of π-accepting CO
UV photolysis of [CpMn(CO)₃] in N₂ Matrix / 12 K
UV photolysis results in the loss of parent bands at 2033 and 1951 cm⁻¹ and the production of 3 new bands at 1987 and 1936 and 2073 cm⁻¹
- v(CO) bands at 1987 and 1936 cm⁻¹ similar to those assigned to [CpMn(CO)₂]
- band at 2073 cm⁻¹ - assigned to v(NN) band
UV photolysis of [CpMn(CO)₃]/Flash photolysis with IR detection of alkane solution with CO
UV photolysis resulted in loss of parent bands at 1987 and 1936 cm⁻¹ and the production of new species with bands at 1994 and 1962 cm⁻¹
- The transient (intermediate) reacted with CO to reform the parent
- Note: ROR with CO depends on nature of alkane
- Produced a similar species in Ar matrix, with v(CO) bands slightly higher
The following reaction was done again but with Nitrogen in solution
The rate of decay of [CpMn(CO)₂(alkane)] depended linearly on the concentration of N₂ in solution
Suggesting….
- Two bands decay due to the nitrogen species being formed - pseudo first order rate constant = excess of gas where rate in depended on nitrogen conc
- UV photolysis of CpMn(CO)₃ with H₂
- UV photolysis resulted in loss of parent bands at 2033 and 1951 cm⁻¹ and the production of [CpMn(CO)₂(alkane] with bands at 1994 and 1963 cm⁻¹
- Transient reacts with H₂ to form a species with bands at 1992 and 1933 cm⁻¹
- The rate of decay of [CpMn(CO)₂(alkane] depended linearly on the conc of H₂ in solution
Suggesting…
- Similarity of the v(CO) band position suggests no oxidation state change
- Indicating a dihydrogen complex
- UV photolysis of [CpRe(CO)₃] with H₂
- UV photolysis resulted in loss of parent bands at 2033 and 1951 cm⁻¹ and the production of [CpRe(CO)₂(alkane] with bands at 1994 and 1962 cm⁻¹
- This transient reacts with H₂ → a species with bands at 2028 and 1968 cm⁻¹
- The rate of decay of [CpMn(CO)₂(alkane] depended linearly on the conc of H₂ in solution
- Similarity of v(CO) band position suggests an oxidation state change
- Indicating the dihydride complex
- [Cr(CO)₆] matrix isolation studies shows Xe or Kr can bind to metal centres, BUT Xe doesn’t bind as stronly as alkanes
- This means Xe will not displace the alkane from M-alkane in alkane solution
- Can we still see organometallic Xe complexes in solution at room temperature?
- By reacting supercritical xenon solution at room temperature
- Displaces a CO
How can we test for the formation of this [W(CO)₅Xe]
- Using time-resolved IR spectra taken 100 ns after photolysis shows [W(CO)₅L] IR bands shifted
- Kinetics: transient has a longer lifetime showing preferential binding of Xe to [W(CO)₅ in the supercritical Kr solvent
[CpRe(CO)L] complexes are long-lived and experiements in supercritical Kr with Xe can allow…
the reaction of [CpRe(CO)₂Kr] + Xe to be monitored
How can we use NMR to monitor organometallic noble has complexes (i.e. Xe)
- If the number is high, it means it reacts with Co very rapidly - short lived
- If the number is low, it means it is long lived
- For the elements with lower numbers, we can cool them down which slows the rate, allowing an NMR e.g. to be run
- e.g. coupling can be seen between Xe and P/F for this complex
The reduction of CO₂ offers an alternative to fossil fuels for various organic industrial feedstocks and fuels. Consequently, efficient and scalable approaches for the reduction of CO₂ to products such as methane and methanol can generate value from its emissions
How can you do this?
Transition metal-based catalysts in the most efficient for activation of CO₂ owinf to their ability to interact with CO₂ molecules in various ways
CO₂ has been observed to have three different binding modes (carbon bound η¹-C, η¹-O and bidentate, η²-(C-O) bonding and η²-(O-O) bonding
What do they look like?
What does the following time-resolved IR tell us about the product formed in this reaction
- Shift down in frequency of v(CO) bands indicates η¹-O bound CO₂
- Reactivity very similar to corresponding W-Xe complex
What do the following graph tell us about how the Mn and Re complexes react with scCO₂
- Mn and Re have different binding to CO₂ coordination
- Mn η¹-O and Re η²-(C-O)
What does the following graph tell us about the way [CpMn(CO)₃] reacts
