This was the presentation given at the RSC Dalton's DYME meeting on 29/06/2021. The background of the presentation is using Group 13 Salphen catalysts for the cycloaddition between terminal epoxides and CO2 to form cyclic carbonates.

1. Beyond Al: Group 13 Salphen catalysts
for efficient CO2 Utilisation. Insight from
DFT calculations.
Ryan Lewis
1st Year PhD Student
Sheffield Hallam University

2. Reaction of Interest
Why are we interested in CO2
utilisation?
- Cheap readily available source of
carbon.
- Common waste product from
many chemical reactions.
Why are we interested in this reaction?
- 100% atom economy.
- Solvent free.
- High yields.
- Mild reaction conditions.
Fulfil a number of the green chemistry criteria.1
1. P. Anastas, N. Eghbali. Chem. Soc. Rev. 2010, 39, 301-312.

3. Cyclic Carbonates
• Properties – High boiling point, low odor, low toxicity, high
biodegradability, high solubility.
• Uses – Lubricants, pharmaceutical drug precursor, Li-Ion battery
technology and polymerisation reactions.
E. J. C. Lopes, A. P. C. Ribeiro and L. M. D. R. S. Martins, Catalysts, 2020, 10, 479

4. The Literature
- The Kleij, North, Wang and Qin catalysts are
all highly effective catalysts for the
cycloaddition reaction.
- However, there has been little research done
beyond Al.
J. Rintjema and A. W. Kleij, ChemSusChem, 2017, 10, 1274-1282.
- Wu et al. computationally studied the
Salens however discounted studying Ga
in further detail due to its binding energy
to the oxygen of the substrate.
- Al was also discounted due to the amount
of already conducted on Al-Salens.
T. Wu, T. Wang, L. Sun, K. Deng, W. Deng and R. Lu, ChemistrySelect,
2017, 2, 4533-4537.

5. The Salphens
F. Castro-Gómez, G. Salassa, A. W. Kleij and C. Bo, Chem. Eur. J., 2013, 19, 6289-6298.
• The salphen ligand remains one of the lesser studied catalyst structures.
• Kleij and Bo studied the Zn(II)-Salphen catalyst and found it to be an
effective catalyst for the cycloaddition reaction.
• Group 13 metal-salphens have received limited attention up to this point.
• Both North and Kleij have commented on solubility issues surrounding Al-
Salphen.
• In collaboration with the Whiteoak group we decided to revisit the
salphens but explore beyond Al.

6. Experimental Work
Co-Catalyst:
Catalyst:
Optimal Conditions:
All experimental work at
0.5 mol% Co-catalyst, 0.1
mol% catalyst and 8 bar
CO2
M = Al, Ga, In
Metal Centre Co-Catalyst Temperature (K) Yield %
In TBACl 318 35
In TBABr 318 67
In TBAI 318 88
Metal Centre Co-Catalyst Temperature (K) Yield %
Catalyst Free TBAI 328 5
Al TBAI 328 55
Ga TBAI 328 64
In TBAI 328 >99
Metal Centre Co-Catalyst Temperature (K) Yield %
In TBAI 298 41
In TBAI 318 88
In TBAI 328 >99
Ga TBAI 338 90
Al TBAI 338 64

7. Aims and Objectives
• To elucidate the full reaction mechanism for each Al-, Ga- and In-salphen
using DFT.
• To compare and contrast the observed mechanistic pathways and use this
to explain experimental findings.
• To use Quantum Theory of Atoms In Molecules (QTAIM) to explain key
mechanistic steps.
DFT methodology -
Optimisation and Frequency Terms: B97-D3 Def2-svp Def2/j
Solvated Energies: ωB97M-D3BJ def2-tzvpp def2/j
All computational values have been concentration corrected.

8. Outline of Al-Salphen Mechanism

9. Epoxide Ring Opening CO2 Insertion
Cyclic Carbonate Ring
Closing
FCR1
TS3R1
Int-1
TS1 Int-2
TS2

10. Outline of Ga-Salphen Mechanism

11. Epoxide Ring Opening CO2 Insertion
Cyclic Carbonate Ring
Closing
FCR2
TS4R2
Int-1
TS1 Int-2
TS2

12. Outline of In-Salphen Mechanism

13. Epoxide Ring Opening CO2 Insertion
Cyclic Carbonate Ring
Closing
FCR2
TS4R2
Int-1
TS1 Int-2
TS2

14. Key Points:
• In-Salphen is the most active catalyst energetically.
• Ga-Salphen is showing as the least active catalyst,
disregarding solubility effects.
• Al-Salphen has the highest transition state barrier.
• Al-Salphen clearly deviates from the other two mechanisms
with regards to the Cl abstraction.

15. Cl Abstraction
• The table compares the energetic terms for the abstraction of Cl at Int-1 for
each catalyst giving the relative stability of the di-chloro-catalyst complex
and the TBACl.
• In all cases, it is favourable to abstract the Cl using the catalyst.
• The stability of the abstractions are di-chloro Al-Salphen>Ga-Salphen>In-
Salphen.
• The bigger the metal, the less favourable it is to abstract.
• Di-chloro-Al-Salphen is significantly more stable than the other two metal
salphens indicating that it could be this precipitating out.
Abstracted by Gsolv(kcal mol-1)
Al-Salphen -6.33
Ga-Salphen -3.40
In-Salphen -1.00
TBA+ (Al Salphen Pathway) -2.11
TBA+ (Ga Salphen Pathway) -1.35
TBA+ (In Salphen Pathway) 8.21

16. QTAIM Analysis
• Topological analysis based on the electron density at bonding critical
points (BCP).
• Use different descriptors to identify the type of bond/interaction present
and then quantify the bond/interaction strength.
P. Popelier, in The Chemical Bond: Fundamental Aspects of Chemical Bonding, ed. G. Frenking and S. Shaik, Wiley-VCH, Germany, 1st
edn, 2014, ch. 8, pp. 271-308.

17. QTAIM Comparisons
Complex Bond Al-Salphen Ga-Salphen In-Salphen
IC
M-Cl -1.02E-02 -2.46E-02 -1.09E-02
M-O(A) 6.30E-03 -9.03E-03 -6.17E-04
M-O(B) 5.94E-03 -9.44E-03 -6.07E-04
M-N(A) -1.02E-03 -1.80E-02 -5.69E-03
M-N(B) -1.14E-03 -1.76E-02 -5.58E-03
M-O(Substrate) -3.05E-05 -5.71E-04 -6.87E-04
TS2
M-O(A) 6.21E-03 -9.03E-03 -8.21E-04
M-O(B) 6.06E-03 -9.75E-03 -7.81E-04
M-N(A) -1.51E-03 -1.80E-02 -6.04E-03
M-N(B) -1.62E-03 -1.80E-02 -6.29E-03
C(CO2)-O(epoxide) -2.52E-02 -4.56E-02 -2.60E-02
M-O(CO2) -4.26E-03 -5.05E-03 -1.41E-03
M-O(alkoxide) 5.17E-03 -9.99E-03 -1.66E-03
B
A B
A
• For Al-salphen the bonds bonding with the oxygen of
the Salphen all shows positive for the energy density
H(rb).
• However, for In- and Ga-salphen these values are
negative.
• This suggests an ionic type bond for Aluminum
compared to Gallium and Indium showing a covalent
type bond.

18. Conclusions Drawn
• The reaction mechanisms for Al-, Ga- and In-salphen for the cycloaddition
reaction between propylene oxide and CO2 have been fully elucidated.
• It is preferable for the abstraction of Cl by a second catalyst for all three
M-salphen catalysts.
• We have been able to justify the experimental observations seen by the
Whiteoak group theoretically.

19. Future Work
• Identify the exact species which is leading to solubility issues in the Al-
salphen pathway.
• Explore how the energies of the mechanism change if the halide bound to
the catalyst is changed.
• Explore the effects of changing the R groups on the catalyst on the
energies of the mechanism.

20. Acknowledgements
Dr Alex Hamilton
SHU
PhD Supervisor
Dr Christopher J.
Whiteoak
University of Alcalá
Experimental
Collaborator
Diego J. Cabrera
University of Alcalá
Experimental
Collaborator
Thank you for listening.
Please feel free to ask any questions
Ryan.D.Lewis@student.shu.ac.uk