Main group systems for small molecule activation and catalysis

Question 1

Main group vs. transition metal compounds

Answer 1

Transition metal compounds:
Usually have partially occupied valence d orbitals that are often relatively close in energy
Often coloured due to small orbital energy separations
Often interact with small molecules e.g. CO, H2, C2H4
Often paramagnetic
Stereochemical electron pair character less pronounced
Antiferromagnetic coupling is common

Main group compounds:
Valence s/p orbitals are either fully occupied or empty
Valence s/p orbitals are far apart energetically
Usually colourless
Generally do not interact strongly with CO, H2 or C2H4
Usually diamagnetic
Have stereochemically active electron pairs which form the basis of VSEPR theory
Antiferromagnetic coupling in stable compounds is not common

Question 2

Antiferromagnetic coupling

Answer 2

Gives an equal number of magnetic fields in opposite directions

Question 3

Hydrogenation of alkene catalytic cycle

Answer 3

Flashcard

Question 4

Transition metal H2/alkene binding modes

Answer 4

Flashcard

Question 5

Stoichiometric and catalytic transformations similar to those seen with transition metal compounds can be accomplished with

Answer 5

1. ‘Frustrated’ Lewis pairs

2. Alkaline earth compounds (e.g of Mg, Ca, Sr, Ba)

Question 6

Frustrated Lewis pair

Answer 6

A compound/mixture containing a Lewis acid and Lewis base that, because of steric hindrance/repulsion caused by large substituents, cannot combine to to form a classical LB:–>LA adduct
This leaves a strong ‘unquenched’ Lewis acid and a strong ‘unquenched’ Lewis base in the mixture, resulting in unusual reactivity with small molecules

This concept of ‘frustration’ is not always correct - the LA and LB do not always need to be completely separate
As long as there is a degree of equilibration between the LB:–>LA adduct and the free LB and LA, heterolytic cleavage of H2 can occur

Question 7

First non-transition metal system for reversible H2 uptake and elimination

Answer 7

Draw (see flashcard)

Question 8

Examples of more simple/general FLPs

Answer 8

Intermolecular:
tBu3P + B(C6F5)3 –(H2)–> [tBu3PH]+[HB(C6F5)3]-
tBu3P + BPh3 –(H2)–> [tBu3PH]+[HBPh3]-
iPr2NH + B(C6F5)3 –H2,110C–> [iPr2NH2]+[HB(C6F5)3]-
(this rxn requires a higher temp because N is less sterically hindered by iPr groups)
+ see flashcard for carbene and intramolecular

Question 9

Current theories for the mechanism of H2 heterolytic cleavage by FLPs

Answer 9

1. Electric field model

2. Electron transfer model

Question 10

Electric field model for H2 heterolytic cleavage

Answer 10

FLPs activate the H-H bond by polarisation, owing to the electric field created by their donor/acceptor atoms
H-H bond polarised in a linear transition state
Draw

Question 11

Electron transfer model

Answer 11

FLPs activate the H-H bond by a synergistic interaction with the H2 sigma and sigma* orbitals - similar to that established for transition metals
The transition state is slightly skewed - H2 is at an angle to both the acid and base components
This enhances the orbital overlap between LB-sigma*(H2) and LA-sigma(H2)

Question 12

Other examples of small molecule activation by FLPs

Answer 12

See flashcard

Question 13

Reactivity of FLPs and H2 with multiple bonds

Answer 13

Double bonds can insert into the B-H bond
The ‘hydride’ attached to B ends up on the electrophilic portion of the double bond (i.e. the C in C=O or C=N)
Overall chemistry = FLP-catalysed hydrogenation of C=E multiple bonds (triple bonds too)

Question 14

Parallels between alkaline earth metals (i.e. group 2) and the lanthanides

Answer 14

Group 2 =
Single +2 oxidation state with no accessible redox behaviour
M-X bonding is non-directional and ionic

Lanthanides =
Only really 3+
Bonding very labile and no directionality

Basically, group 2 complexes can be used as d0 ‘lanthanide-mimetic’ reagents in catalysis

Question 15

Polarisability and electronegativity trends of alkaline earth metals

Answer 15

Electronegativity decreases down the group (i.e. the metals become more electropositive down the group) - this means the BaR interaction is more ionic than the MgR interaction
Polarisability increases down the group because ionic radius increases (but still a 2+ charge) - i.e. the metal becomes easier to manipulate, it’s easier to exchange its bonding interactions

Question 16

Typical types of reactions seen for alkaline earth compounds

Answer 16

1. Sigma-bond metathesis (2sigma-2sigma process)

2. Insertion into polarised multiple bonds (2sigma-2pi process)

Question 17

Sigma-bond metathesis

Answer 17

Draw

Question 18

Insertion into polarised multiple bonds

Answer 18

‘Multiple bond heterofunctionalisation with alkaline earth compounds’
Draw

Question 19

Rate determining step of insertion into multiple bonds

Answer 19

Formation of first 4 membered transition state i.e. insertion of Ae-N into double bond, double bond breaks

Question 20

Order of energy of transition state for RDS of insertion into multiple bonds

Answer 20

Highest energy = Mg > Ba > Ca > Sr

i.e. rate of reaction depends on identity of metal

Question 21

Rationalise the order of reactivity of the group 2 metals in the intermolecular hydroamination reactions

Answer 21

The reaction is dictated by the polarisation and polarisability of key moieties
C=C bond is initially completely non-polar
As the C=C bond begins to interact with the delta+ Ae and delta- N, a polarisation is imposed across the C=C pi system (it is the pi-system that’s polarised because it is the most polarisable electron density)
Therefore, you would predict that the greatest polarisation would be induced from the most ionic bond (Ba-N)
C=C bond will be more readily polarised if it comes into contact with a much more compact, “harder” region of electron density
Therefore, the Ba centre is too polarisable itself to induce the necessary polarisation across C=C
Although Mg is the smallest and most compact, the charge distribution across the Mg-N bond is insufficient to induce effective polarisation of C=C
Ca and Sr are ‘just right’ - they are sufficiently ionic enough to induce polarisation and sufficiently hard enough to cause the charge separation to take place
Overall, Sr out-performs Ca (as predicted)

Question 22

Kinetics for piperidine addition across C=C in styrene using Ca / Sr as the catalyst

Answer 22

Deduced activation energies for each catalytic reaction from Arrhenius plot
Although reaction with Sr was quicker, from the kinetics calculations, the activation energy for the Sr-catalysed process was significantly higher than for Ca
However, the DeltaS value for the Ca reaction is much more negative than for the Sr reaction
This is due to the entropy associated with the assembly of the transition state
Sr catalysis occurs via a much looser assembly of the substrates around the metal cation, probably because it is a larger cation i.e. access to the Sr centre is more facile than for Ca
There is an overall compromise between the size of the cation, the polarising ability of the cation-N interaction and the polarisability of the electron density within all the bonds as the transition state is formed

Question 23

Hydrogenation of alkenes with alkaline earth compounds

Answer 23

Occurs with no change to oxidation state of Ae centre
Just relies on the very polarised reaction pathway
Draw

Question 24

Why is the reactivity with group 2 complexes more dependent on the identity of the metal than with lanthanide complexes?

Answer 24

Due to the much greater change in M2+ radius and charge density down group 2 than there is across the period for Ln3+