Iridium(III) and Rhodium(III) Half-Sandwich Coordination Compounds with 11H-Indeno[1,2-b]quinoxalin-11-one Oxime: A Case of Spontaneous Resolution of Rh(III) Complex

Round 1

Reviewer 1 Report

Two half-sandwich Ir (III) and Rh (III) complexes were synthesized by the reactions of IQ-1 and [M(Cp*)Cl₂]₂ (M=Ir, Rh) dimers. IR, H-NMR and XRD analyses were taken to determine the successful syntheses and structures of the complexes. Specifically, a spontaneous resolution of enantiomers was observed for Rh (III) complex. According to the XRD analysis, it was found that the intermolecular contacts CH…Cl and CH…C in Rh complex bound all ligands around the chiral metal center and a combination of CH…O and CH…C contacts lead to a homochiral supermolecular structure so that reinforced the spontaneous resolution in Rh complex. So I recommend its acceptance after resolving the following comments :

1. There are some writing and even grammer errors in this manuscript . For example, in line 14, there is a lack of a period behind the “DMF”.

2. The structural differences of Ir and Rh complexes should be organized and summarized carefully to clarify the structural factors leading to the spontaneous enantiomers resolution of Rh complex.

Author Response

1. There are some writing and even grammer errors in this manuscript . For example, in line 14, there is a lack of a period behind the “DMF”.

The manuscript was proof-read and the errors were corrected.

2. The structural differences of Ir and Rh complexes should be organized and summarized carefully to clarify the structural factors leading to the spontaneous enantiomers resolution of Rh complex.

The difference in crystal packing was summarized in the Conclusions section (lines 243-253).

Reviewer 2 Report

The paper submitted to Inorganics-1972819 by V.V. Matveevskaya, D.I. Pavlov, A.S. Potapov shows the detailed crystal structures of iridium(III) and rhodium(III) half-sandwich coordination compounds with 11H-indeno[1,2-b]quinoxalin-11-one oxime and pentamethylcycropendiene by means of single-crystal X-ray diffraction analysis, and discuss the chirality.

The Ir(III) complex is racemic and crystallizes in a centrosymmetric space group. However, the Rh(III) complex shows a chiral space group and the formation of racemic conglomerate from achiral precursors without chiral additive. These results are worthwhile for the publication of Inorganics-MDPI. The manuscript is well written and presented and the results are interesting. I think that the article can be acceptable for Inorganics-MDPI.

Author Response

The paper submitted to Inorganics-1972819 by V.V. Matveevskaya, D.I. Pavlov, A.S. Potapov shows the detailed crystal structures of iridium(III) and rhodium(III) half-sandwich coordination compounds with 11H-indeno[1,2-b]quinoxalin-11-one oxime and pentamethylcycropendiene by means of single-crystal X-ray diffraction analysis, and discuss the chirality.

The Ir(III) complex is racemic and crystallizes in a centrosymmetric space group. However, the Rh(III) complex shows a chiral space group and the formation of racemic conglomerate from achiral precursors without chiral additive. These results are worthwhile for the publication of Inorganics-MDPI. The manuscript is well written and presented and the results are interesting. I think that the article can be acceptable for Inorganics-MDPI.

We thank the reviewer for a positive feedback on our work.

Reviewer 3 Report

The paper of Andrei S. Potapov and co-authors is a fundamental work on synthesis two new heteroleptic Iridium(III) and Rhodium(III) complexes with Cp* and IQ-1. The authors obtained two compounds [Ir(Cp*)(IQ-1)Cl] and [Rh(Cp*)(IQ-1)Cl], isolated as single crystals, determined their structures and encountered an unexpected effect - cocrystallization of two isomers (R/S) in Rh(III) case.

I have one big question to the article, I will try to formulate it.

This concerns the synthesis and characterization of complex 2. In the experimental part, it is indicated that complex 2 can be obtained in methanol and dimethylformamide. Powder was isolated from methanol, and crystals were isolated from dimethylformamide, from which X-ray diffraction analysis was made. Usually, the solvent strongly influences the packing in the crystal. And based on the text of the article, the authors claim that DMF crystals are one and the same product with methanol phase. This fundamentally disagrees with the powder diffraction data for complex 2. The powder pattern for DMF crystals does not agree at all with the calculated one and with powder obtained from methanol - even the number of peaks is different.

It is absolutely clear that the authors have different phases of products from methanol and dimethylformamide. Although their composition is the same (which is confirmed by experimental data).

For clarity, the authors need to do XRPD for the mass of crystals from DMF to confirm the single phase. Diffractograms in their current form are unacceptable. It is also necessary to provide a detailed description of the methodology for analyzing diffraction patterns and provide References to sources.

Moreover, it seems to me that the authors missed the main idea - the co-crystallization of R and S isomers is not at all spontaneous, but the changing of the solvent led to this. Most likely, in methanol there is only a centrosymmetric phase, while in DMF there are two isomers - this is what is indirectly confirmed by XRD data. It was the crystallization from dimethylformamide that affected the packing of the crystals. Perhaps the authors should grow crystals for complex 1 from DMF, and if the effect repeats, then we can talk about chirality control. And the value of the work is much higher than stated by the authors (just luck).

Line 147: “Rh(III) complex 2 crystallized as fine orange needles in a chiral P212121 space group forming a racemic conglomerate. Each isomer from the enantiomeric pair (S-2 and R-2) was structurally characterized independently” - How are crystals different? How were they separated? Is it possible to estimate the ratio?

In the description of the structures, labels for carbon atoms are used, which are not in the figures. (C23H23, C21, C8 etc.)

Author Response

I have one big question to the article, I will try to formulate it.

This concerns the synthesis and characterization of complex 2. In the experimental part, it is indicated that complex 2 can be obtained in methanol and dimethylformamide. Powder was isolated from methanol, and crystals were isolated from dimethylformamide, from which X-ray diffraction analysis was made. Usually, the solvent strongly influences the packing in the crystal. And based on the text of the article, the authors claim that DMF crystals are one and the same product with methanol phase. This fundamentally disagrees with the powder diffraction data for complex 2. The powder pattern for DMF crystals does not agree at all with the calculated one and with powder obtained from methanol - even the number of peaks is different.

It is absolutely clear that the authors have different phases of products from methanol and dimethylformamide. Although their composition is the same (which is confirmed by experimental data).

We need to point out that the PXRD patterns presented in the manuscript are for the powder and crystalline samples obtained from methanol only, no data for DMF was included. Likewise, the single crystals used for the X-ray crystal structure determination of compound 2 were also obtained from methanol (and crystals with the same crystallographic parameters were also obtained from DMF). In the description of the synthesis of compound 2, we point out, that two phases are obtained – the initially formed powder and a crystalline precipitate formed upon cooling the filtrate. Both of these phases were analyzed by PXRD analysis and were indeed different. The PXRD pattern of the powder precipitate (red in Figure S2) does not correspond to the pure phase of the racemic conglomerate, this was pointed out in the manuscript (lines 78-79). Obviously, this precipitate contains a centrosymmetric phase in addition to the conglomerate. We note this in the manuscript and did not use this phase for further characterization (line 80). At the same time, we need to point out that the chemical composition of this “mixed phase” corresponds to the molecular formula of the compound 2, which was confirmed by NMR and elemental analysis (data not included in the manuscript to avoid misleading conclusions). The second diffractogram in Figure S2 (blue line) was obtained for the grinded mass of crystals precipitated from the methanol filtrate. It contains all of the peaks from the pattern calculated for the conglomerate phase and no unidentified peaks are present. The difference in intensity of the calculated and the experimental peaks is quite typical and may be a result of a preferred orientation of microcrystals in PXRD measurement cell. In the revised version, we include a newly recorded diffractogram with improved signal-to-noise ratio, this pattern contains a minor peak at about 14 degrees that was to weak to be visible in the initial pattern.

For clarity, the authors need to do XRPD for the mass of crystals from DMF to confirm the single phase. Diffractograms in their current form are unacceptable.

We are thankful to the reviewer for drawing the attention to the synthesis in DMF. After careful examination of PXRD patterns of the bulk product 2, we conclude that it cannot be considered as a pure single phase, despite the same chemical composition. Similarly to the precipitate from methanol, it is probably a mixture of two forms – conglomerate and centrosymmetric and in the absence of single-crystal diffraction data for the centrosymmetric phase, we decided to exclude the synthetic procedure using DMF as solvent to avoid inadvertently misleading the reader.

The PXRD for crystals does not agree at all with the calculated one and with powder obtained from methanol It is also necessary to provide a detailed description of the methodology for analyzing diffraction patterns and provide References to sources.

PXRD experimental details were added (lines 215-217).

Moreover, it seems to me that the authors missed the main idea - the co-crystallization of R and S isomers is not at all spontaneous, but the changing of the solvent led to this. Most likely, in methanol there is only a centrosymmetric phase, while in DMF there are two isomers - this is what is indirectly confirmed by XRD data. It was the crystallization from dimethylformamide that affected the packing of the crystals. Perhaps the authors should grow crystals for complex 1 from DMF, and if the effect repeats, then we can talk about chirality control. And the value of the work is much higher than stated by the authors (just luck).

The synthesis of compound 2 in DMF was addressed in the replies above. We have attempted the synthesis of compound 1 in DMF and a crystalline product was obtained, but the PXRD analysis revealed that this bulk material did not correspond to the single crystals of the centrosymmetric phase. Therefore, a partial resolution probably takes place, but in the absence of the crystal structure of the conglomerate phase (the crystals are of poor quality) and unoptimized synthesis procedure in DMF to obtain pure phases we do not include these results in the manuscript.

Line 147: “Rh(III) complex 2 crystallized as fine orange needles in a chiral P212121 space group forming a racemic conglomerate. Each isomer from the enantiomeric pair (S-2 and R-2) was structurally characterized independently” - How are crystals different? How were they separated? Is it possible to estimate the ratio?

The crystals of the enantiomers rotate the polarized light and may thus be discerned under the microscope with polarized lighting. A note on this was added to the manuscript (lines 149-150) and a photograph under the polarized light was added to the supplementary materials. As for the ratio of the enantiomer, visually it is 1:1 and no other ratio is to be expected, since there are no reasons for the enantioselectivity in the synthetic procedure.

In the description of the structures, labels for carbon atoms are used, which are not in the figures. (C23H23, C21, C8 etc.)

Figures 2 and 4 were updated accordingly.

Round 2

Reviewer 1 Report

This manuscript has been improved, so I suggest to accept it for publication.

Reviewer 3 Report

The authors answered almost all my questions and took into account my suggestions. Appropriate corrections have been made to the manuscript.

Despite the fact that the part with diffraction studies is still unclear to me, I propose to accept this manuscript as is for printing in Inorganics.

I wish the authors good luck in their future work and believe that the spontaneous separation of isomers will become not spontaneous at all, but controllable and predictable. And try to obtain a crystalline phase for compound 1 from dimethylformamide.