Tetrel Bonding Interactions Involving Carbon at Work: Recent Advances in Crystal Engineering and Catalysis
Round 1
Reviewer 1 Report
MS: carbon-936288
Title: Tetrel bonding interactions involving carbon at work: recent advances in crystal engineering and catalysis
Author: Antonio Frontera, Departament de Química, Universitat de les Illes Balears, Spain
The author presents in this feature paper an overview of terel bonding as formed in crystalline solids involving sp3-hybridized carbon and describes the utilization of two allotropic forms of carbon (C60 and carbon nanotubes) as supramolecular catalysts discussed on the basis of σ- and π-hole interactions. The topic is of interest, the article is well structured and presented, therefore this referee recommend publication after considering the following two points.
1) Although the discussion of weak intermolecular interactions in terms of σ-and π--holes has become very popular one caveat is appropriate. The σ-and π--hole model is electrostatic in nature. However, there is also the kinetic aspect of chemical bonding and weak interactions, as pointed out in Ruedenberg’s seminal work, where he shows that a pure electrostatic model may underestimate the important role of the lowering of the kinetic energy associated with electron delocalization upon bond formation. This important point cannot be neglected in a feature article and has to be discussed. For example, the work of Sethio et al. on tetrel bonding (Molecules, 23, 2763, 2018) clearly demonstrates that focusing just on the σ-hole description can only lead to an incomplete picture and other important facets of tetrel bonding are missed. The same holds also for π-hole interactions, as for example recently discussed by Yannacone et al. (Crystals, 10, 1556, 2020).
2) For crystal engineering computational studies are most relevant, which discuss bonding in the crystal environment. However, the cited computational studies were not performed in the solid state. Periodicity is necessary even for molecular crystals in order to account for the crystal packing effect, which is not covered by a molecular cluster model. Recent work of Tao et. al. (Molecules, 25, 1589, 2020) clearly shows this important fact for crystalline halogen bonding, which holds in the same way for crystalline tetrel bonding.
These two aspects should be included in the article and the corresponding literature should be cited. Giving a broader view instead of a more narrow description would increase the value of this feature article and could open new fruitful discussions or even collaborations with colleagues in the field, which is in the spirit of this journal.
Author Response
The author presents in this feature paper an overview of terel bonding as formed in crystalline solids involving sp3-hybridized carbon and describes the utilization of two allotropic forms of carbon (C60 and carbon nanotubes) as supramolecular catalysts discussed on the basis of σ- and π-hole interactions. The topic is of interest, the article is well structured and presented, therefore this referee recommend publication after considering the following two points.
1) Although the discussion of weak intermolecular interactions in terms of σ-and π--holes has become very popular one caveat is appropriate. The σ-and π--hole model is electrostatic in nature. However, there is also the kinetic aspect of chemical bonding and weak interactions, as pointed out in Ruedenberg’s seminal work, where he shows that a pure electrostatic model may underestimate the important role of the lowering of the kinetic energy associated with electron delocalization upon bond formation. This important point cannot be neglected in a feature article and has to be discussed. For example, the work of Sethio et al. on tetrel bonding (Molecules, 23, 2763, 2018) clearly demonstrates that focusing just on the σ-hole description can only lead to an incomplete picture and other important facets of tetrel bonding are missed. The same holds also for π-hole interactions, as for example recently discussed by Yannacone et al. (Crystals, 10, 1556, 2020).
Reply: I thank the referee for taking these important manuscripts into my attention. These works have been included in the manuscript and commented in the main text. See lines 89-97 and references 55-60
2) For crystal engineering computational studies are most relevant, which discuss bonding in the crystal environment. However, the cited computational studies were not performed in the solid state. Periodicity is necessary even for molecular crystals in order to account for the crystal packing effect, which is not covered by a molecular cluster model. Recent work of Tao et. al. (Molecules, 25, 1589, 2020) clearly shows this important fact for crystalline halogen bonding, which holds in the same way for crystalline tetrel bonding.
Reply: I totally agree. This has been mentioned in the review (see lines 98-101) and reference 61.
Reviewer 2 Report
The manuscript entitled "Tetrel bonding interactions involving carbon at work: recent advances in crystal engineering and catalysis" by A. Frontera provides a concise review on σ- and π-hole interactions involving tetrels and their applications in catalysis. I recommend publication of this manuscript after minor corrections.
General comments on Figures:
The lengths of interactions are missing a unit (Angstrom)
Figure 1: "This work" is difficult to read because there are too many solid colors. It may be better to "de-color" the periodic table to make carbon more easily visible.
Page 5 lines 126-127 - How is this different from solvent coordination?
Page 9 Line 259 - "un" should be in
General comments on section 3.2. π-hole catalysis with carbon allotropes
Page 8 Lines 220-224: "In case of heteroaromatic rings like electron deficient azines the interaction cannot be easily termed since atoms belonging to two different groups of the Periodic Table participate as electron acceptors." - This statement is okay for azines but this section of the manuscript only discusses anion-π catalysis. The anion-π interactions are well known in the literature so is it necessary to introduce another label such as π-hole interaction?
Author Response
General comments on Figures:
The lengths of interactions are missing a unit (Angstrom)
Reply: This information is given in the legends to the figures
Figure 1: "This work" is difficult to read because there are too many solid colors. It may be better to "de-color" the periodic table to make carbon more easily visible.
Reply: Figure 1 has been improved
Page 5 lines 126-127 - How is this different from solvent coordination?
Reply: In the coordinated solvent, the sigma hole is more intense and the noncovalent distances are usually shorter than in non-coordinated molecules
Page 9 Line 259 - "un" should be in
Reply: Fixed
General comments on section 3.2. π-hole catalysis with carbon allotropes
Page 8 Lines 220-224: "In case of heteroaromatic rings like electron deficient azines the interaction cannot be easily termed since atoms belonging to two different groups of the Periodic Table participate as electron acceptors." - This statement is okay for azines but this section of the manuscript only discusses anion-π catalysis. The anion-π interactions are well known in the literature so is it necessary to introduce another label such as π-hole interaction?
Reply: I agree that anion–π is a very well-known and recognized term used to define the interaction of anions with π-systems. However, the interaction also fits in the definition of pi-hole tetrel bond, since a tetrel atom is accepting charge from an electron rich atom. In any case, I have used the term anion–pi as a primary one in section 2 and the term π-hole has been maintained in parenthesis
