Review Reports - Isocyanide π-Hole Interactions Supported by Aurophilic Forces

Round 1

Reviewer 1 Report

In this paper the authors report on the synthesis and characterization of a gold(I) complex with an isocyanide ligand of formula [AuCl(CNC6H3-4-Cl-2-I)]. The solid state structure of the complex is characterized by the presence of different types of weak interactions namely aurophilicity, and a more rare pi-holeCN···Cl interactions. Furthermore, the terminal I-atoms of the ligand of different complex molecules are involved in a three-center halogen bonding. The combinations of the above mentioned interactions forms a 2D extended ladder-type architecture. A theoretical analysis of the weak interactions present in the system has been carried out.

The manuscript is well written and scientifically sound. I recommend acceptance after major revisions.

Suggestions and comments:

- Comment: The crystal data have been collected with a Cu radiation even if the complex contains heavy atoms such as Au and I. This is in principle can lead to have too small adp if the absorption correction is not carried out properly. Was the use of Cu radiation the only option available or are there other reasons for this choice?

- Change the title of section 3. in “Results and discussion” instead of “Results”.

- Change the title of section 4. in “Conclusions” instead of “Discussion”.

- Figure 1 is not clear and in my opinion should be reprepared. The atoms labels don’t report the symmetry code. Symmetry generated atoms should be labeled properly e.g. Au’, with the full symmetry code reported in the figure caption (‘ = 1-X, -Y, 1-Z). Moreover from the chosen perspective it is difficult to understand which of the Au…Au contact is the shortest and which one is the longest, also because the gold atoms are all labelled Au1. In Figure 1 I would only represent the two molecules interacting in the head to tail fashion. The second aurophilic interaction is shown also in figure 2.

- Figure 2, showing the crystal packing is not clear and, in my opinion, should be reprepared. Cell axes should be included in the Figure and the direction towards which the 2D ladder type architecture develops should be indicated. I would say that the Ladder type architecture is developed along the b axis. Moreover as stated by the authors Molecules from different 2D-ladder arrays are linked to each other via I···I contacts. These types of contacts are developed along two crystallographic directions, namely b and c axes, affording the complete crystal packing. A view of the of the crystal packing along the a axis should offer the best perspective to appreciate all the weak interaction occurring in the crystal.

Author Response

The crystal data have been collected with a Cu radiation even if the complex contains heavy atoms such as Au and I. This is in principle can lead to have too small adp if the absorption correction is not carried out properly. Was the use of Cu radiation the only option available or are there other reasons for this choice?

We agree with the reviewer that, for example, Mo radiation would be better for structures bearing heavy atoms. In our case, however, rather small crystals weakly diffract with the Mo radiation, therefore we choose Cu-Kα. Absorption correction was made with CrysAlisPro 1.171.40.71a (Rigaku Oxford Diffraction 2020).

We added the following phrase in Experimental Section: “Empirical absorption correction was accounted using spherical harmonics implemented in SCALE3 ABSPACK scaling algorithm (CrysAlisPro, 1.171.40.71a, Rigaku Oxford Diffraction 2020).”

Change the title of section 3. in “Results and discussion” instead of “Results”.

Done

Change the title of section 4. in “Conclusions” instead of “Discussion”.

Done

Figure 1 is not clear and in my opinion should be reprepared. The atoms labels don’t report the symmetry code. Symmetry generated atoms should be labeled properly e.g. Au’, with the full symmetry code reported in the figure caption (‘ = 1-X, -Y, 1-Z). Moreover from the chosen perspective it is difficult to understand which of the Au…Au contact is the shortest and which one is the longest, also because the gold atoms are all labelled Au1. In Figure 1 I would only represent the two molecules interacting in the head to tail fashion. The second aurophilic interaction is shown also in figure 2.

We redraw Figure 1, added new atomic labels, and the description of symmetry code in the figure caption. We also slightly changed the perspective view to show that one of the Au···Au contacts is shorter. In the revised version of the manuscript, we placed this Figure in the supplementary materials and replaced the previous Figure 1 with the new one, demonstrating only the head-to-tail dimer.

Figure S1. Identified Au···Au and C_CN···Cl contacts in the crystal structure of 1. Symmetry operations which are used to generate corresponding atoms: ‘ is 1–х; 2–y; 1–z and “ is +х; 1+y; +z.

Figure 1. Identified Au···Au and C_CN···Cl contacts in the crystal structure of 1. Symmetry operation which is used to generate corresponding atoms:‘ is 1–x; 2–y; 1–z.

Figure 2, showing the crystal packing is not clear and, in my opinion, should be reprepared. Cell axes should be included in the Figure and the direction towards which the 2D ladder type architecture develops should be indicated. I would say that the Ladder type architecture is developed along the b axis. Moreover as stated by the authors Molecules from different 2D-ladder arrays are linked to each other via I···I contacts. These types of contacts are developed along two crystallographic directions, namely b and c axes, affording the complete crystal packing. A view of the of the crystal packing along the a axis should offer the best perspective to appreciate all the weak interaction occurring in the crystal.

We redraw Figure 2 and added cell axes. We also indicated in the text that the 2D-ladder type architecture passes along the b axis. We added new Figure S2 in the Supporting Information; the latter figure demonstrates simultaneous occurrence of Au···Au and I···I contacts along the a axis.

Figure 2. 2D-ladder type architecture occurred via different type noncovalent interactions.

Figure S2. A view of a fragment of the crystal packing (along the a axis) demonstrating 2D-ladder type architecture and I···I HaBs.

We also redraw Figure 3, added cell axis and symmetry operations codes.

Figure 3. I···I contacts in the crystal structure of 1. Symmetry operations which are used to generate corresponding atoms: ‘ is 1–x; ½+y; 3/2–z and “ is 1–x; –½+y; 3/2–z.

Reviewer 2 Report

The manuscript by Kukushkin and coll. deals with the discovery of weak intramolecular π-hole interactions between coordinated cyanide and chlorine ligand. While from metrical data the situation is borderline, the authors were able to confirm the occurrence of this bonding using AIM and NCI methods. These results are of interest for crystal engineering and physical chemistry since the strength of such unconventional bonding may be significantly increased for heavier halogens. I think that the manuscript worth to be accepted in Crystals after following small modifications.

1) I do not understand why the direct comparison of C...Cl distances (d4 in Table 1) with the sum of VdW radii between carbon and chlorine (3.5 Å) was not considered as additional parameter illustrating the existence of this bonding.

2) The values in carbon NMR should be rounded to decimals.

3) The style of references is not homogeneous and should be unified according to MDPI guidelines

Author Response

We added corresponding phrase in the text, p. 3: The Au···Au contact is involved in the system of noncovalent interactions, which includes also two antiparallel C_CN···Cl short separations (3.428(6) vs Bondi[54,55] S_vdW 3.45 Å).

We also discussed this contact in the text, just right after Table 1: “Other unconventional contacts in the crystal structure are C_CN···Cl (3.428(6) Å) and C_CN···Au (3.495(5) Å). For the C_CN···Cl separation, the C···Cl distance is smaller than Bondi S_vdWC + Cl 3.45 Å. Geometry considerations based on the data collected in Table 1 indicate that the Cl nucleophilic site is directed to the isocyano carbon. The availability of the π-hole_CN···LP_Cl interaction was confirmed in appropriate theoretical study (see later).”

2) The values in carbon NMR should be rounded to decimals.

Done as requested.

3) The style of references is not homogeneous and should be unified according to MDPI guidelines

Improved accordingly.

Reviewer 3 Report

This manuscript by prof. Kukushkin et al. reported synthesis, crystal structure and computational calculation of a gold(I) isocyanide complex. In the crystal structure, the authors found that the Au···Au and pi(hole)···Cl interactions made two-dimensional extended ladder-type architecture. The DFT calculation for the dimers clearly described the possibilities of aurophilic and pi(hole)···Cl or Au interactions. The discussion was clear and based on the experimental results. This reviewer recommends publication of this manuscript as it is.

Author Response

We thank the reviewer for his/her positive assessment of our study.

Round 2

Reviewer 1 Report

Congratulation to all the authors fo the very nice work.