Synthetic frontiers of inorganic chemistry and ligand design

Question 1

Why are NMR spectra recorded at low temperatures? (Page 1)

Answer 1

To prevent fluxionality (atoms interchanging within the molecule)

Question 2

What is the problem with using NMR to analyse structure in solution? (Page 1)

Answer 2

Equilibria form between different aggregation states of the molecule. A tetramer has an equilibrium with 2 dimers and the 2 dimers have an equilibrium with 4 monomers. The change in aggregation state leads to a change in the NMR spectra. For example, there is a septet when the molecule is a tetramer but when it is two dimers there is only a quintet.

Question 3

What is the actual structure of a Grignard reagent in a solution? (Page 2)

Answer 3

An equilibrium is formed where two X atoms act as a bridge in a dimer with 2 ligands being lost/gained for the monomers/dimer.

Another equilibrium is formed with the two monomers (2RMgX) forming R2Mg + MgX2. These two molecules then form an equilibrium with a similar dimer to before with 2L gained/lost for the dimer/monomer.

Question 4

What is the primary tool for studying the Schlenk equilibrium? (Page 2)

Answer 4

1H and 25Mg NMR
Mass spectrometry studies (gas phase) can also give useful info

Question 5

What are turbo Grignard reagents? (Page 3)

Answer 5

RMgX.LiCl, these turbo Grignard reagents can lead to the formation of highly functionalised Grignard reagents

Question 6

What is the solid state structure of (TMP)MgCl.LiCl? (Page 4)

Answer 6

2 THF ligands connected to a Li centre
The Li centre and the Mg centre are bridged by 2 Cl atoms
The Mg is connected to a THF ligand and a TMP ligand

Question 7

What can DOSY tell us that normal NMR can’t? (Page 4)

Answer 7

Can identify different components/fragments of a molecule in solution and give some indication of their relative sizes. Can help to understand the specific chemistry of the molecule in solution.

Question 8

What is an inverse crown? (Page 5)

Answer 8

Lewis acidic metals in the centre of the ring, that bind anions

Question 9

What is a pre-inverse crown structure? (Page 5)

Answer 9

Used to prepare inverse crowns by deprotonation of substrates

Question 10

Is it normal for Mg to deprotonate and metallate napthalene? (Page 6)

Answer 10

No, it is an unusual feature observed in the specific inverse crown shown on page 6. Other metallating reagents can achieve this but with much poorer yields and regioselectivity

Question 11

How does Mg coordinate napthalene in the inverse crown structure shown? (Page 6)

Answer 11

Napthalene is deprotonated by the Mg nBu base of the pre inverse crown and then coordinated by Mg.

Question 12

What happens to reactivity when replacing K for Na in the pre-inverse crown structure? (Page 6)

Answer 12

A different ring structure is produced that is more reactive than K equivalent (Mg nBu will deprotonate napthalene faster in this structure)

Question 13

What is the problem with trying to form a stable Mg(I) compound? (Page 7)

Answer 13

The species is thermodynamically unstable with respect to disproportionation to form Mg(0) and Mg (II)

Question 14

What are the three examples of bulky ligands that can lend kinetic stability to molecules that would otherwise be highly unstable? (Page 8)

Answer 14

Priso, NAC NAC, Si(SiMe3)3

Question 15

Is the bond length of the Mg-Mg bond where both Mg atoms are in the +1 ox state shorter or longer than elemental Mg(0)? (Page 8)

Answer 15

Shorter, suggesting there is a degree of covalency in the Mg(I)-Mg(I) bond

Question 16

Is the HOMO of the L2Mg-MgL2 complex a sigma or pi orbital? (Page 9)

Answer 16

Sigma orbital

Question 17

Is the LUMO of the L2Mg-MgL2 complex a sigma or pi orbital? (Page 9)

Answer 17

Pi orbital

Question 18

What happens to the orbitals of Ferrocene when it undergoes one-electron oxidation? (Page 10)

Answer 18

The a1g orbital falls below the e2g orbitals and the electron is removed from the higher energy e2g orbital

Question 19

What happens to the orbitals of Ferrocene when it undergoes two-electron oxidation? (Page 10)

Answer 19

Nothing happens to the position of the orbitals, one electron is lost from the a1g orbital and one electron is lost from one of the e2g orbitals

Question 20

What is a frustrated Lewis pair? (Page 11)

Answer 20

When a Lewis acid-base adduct can’t form because of the structures of Lewis acid and base

Question 21

What are the limitations of FLP-based catalytic hydrogenation? (Page 15)

Question 22

Why are phosphines prone to oxidation? (Page 16)

Answer 22

Because P=O bond is very strong

Question 23

What is the Orpen-Connelly model? (Page 17)

Answer 23

σ-donation from the ligand to the metal and π-back bonding from the metal to an antibonding orbital on the ligand

Question 24

What determines if a ligand is a good π-acceptor? (Page 17)

Answer 24

The energy of the σ* orbital on the ligand, if it has a low energy then the ligand is a better π-acceptor

Question 25

What does the stretching frequency of the C=O bond tell us about how good a σ-donor the phosphine ligand is in an Ni(CO)3(PR3) complex? (Tolman electronic parameter) (Page 18)

Answer 25

If vCO is high, the phosphine is a worse sigma donor, this is because there will be less electron density on the metal available for backbonding to the CO ligand (backbonding weakens CO bond)

Question 26

What is the Tolman steric parameter? (Page 20)

Answer 26

Measurement of the cone angle to determine how bulky the R groups attached are. If cone angle is too large then it is favourable for one ligand to dissociate