Smart Ligands for Efficient 3d-, 4d- and 5d-Metal Single-Molecule Magnets and Single-Ion Magnets†
, Diamantoula Maniaki 1, Evangelos Pilichos 1, Eugenia Katsoulakou 1,* and Spyros P. Perlepes 1,2,*Round 1
Reviewer 1 Report
The review inorganics-790485 deals with nd (n = 3 to 5) metal mononuclear or polynuclear Single-Molecule Magnets. The authors treated this review in a chemical point of view highlighting the crucial role of the ligand in the establishment of the required electronic structure and properties to observe the slow relaxation of the magnetization. Such review is useful for scientists working in molecular magnetism field and I really enjoyed to read it. I would like to make two suggestions before publication. Can the authors reinforce the interest of the nd metal ions compared to nf metal ions in the introduction? It could be great if a part of the introduction would be devoted to a remember of the “most famous” classes of ligands used to design SMM and then to explain to the readers why the authors have selected (in a chemical point of view) the ligands exposed in this review. Such work should be added in the main introduction in addition to the introductions of each “chapter”.
Author Response
We thank Reviewer 1 for her/his positive comments.
Concerning the two points she/he raised, we are glad because we were able to address both. In detail:
“Can the authors reinforce the interest of the nd metal ions compared to nf metal ions in the introduction?”
The comment is useful. We have added a new paragraph (“Closing this introductory section…polynuclear molecular magnetic species”) in which we reinforce the interest of the nd metal ions in SMM/SIM chemistry compared to nf metal ions.
“It could be great if a part of the introduction would be devoted to a remember of the “most famous” classes of ligands used to design SMM and then to explain to the readers why the authors have selected (in a chemical point of view) the ligands exposed in this review. Such work should be added in the main introduction in addition to the introductions of each “chapter”.”
This comment is also valuable for the review. We have added new text “Concerning transition-metal ion polynuclear SMMs,…,including tris(methyl)methanide” in the first paragraph of section 3 (“Scope and Organization of this Review”), where we briefly summarize the most popular and efficient ligands in transition-metal SMM and SIM chemistry. For the better flow of the text, we considered it more appropriate to add this general information in section 3 rather than in section 1 (suggested by the reviewer).
We thank Reviewer 1 for her/his time to read our review and for the valuable comments provided.
Reviewer 2 Report
This is an interesting review on the applications of synthetic inorganic chemistry in the design of molecular magnets, particularly SMMs. The content strikes a balance between transition metals and lanthanides, classical coordination chemistry and organometallic chemistry. The ‘personal’ aspect of the account is also very nice and shines the spotlight on the authors’ own interesting contributions.
The requested revisions are minor. Subject to their implementation, the manuscript can be recommended for publication.
The only major aspect that the authors should consider is the quality and quantity of the diagrams, which I’m afraid is not good. In some cases, chemdraw schemes have been used to depict very complicated cluster compounds. It would be much better to use the crystal structure. In other cases, very complicated molecular structures are described only with text. Some specific cases are mentioned below. The authors are advised to enhance the visual appeal of the manuscript with better images, and more of them. Some magnetism graphs (especially AC susceptibility) could also be used to bring the manuscript to life.
Throughout: ‘inorganic chemistry’, ‘coordination chemistry’ and ‘molecular magnetism’ do not need to be capitalized. Also, there is no need to use a superscript number to denote the oxidation state of alkali metals since only one value is chemically reasonable (i.e. the oxidation state is always obvious).
All Ueff values are stated without standard deviations. It would better to include these, where possible.
Abstract
The distinction between SMM and SIM is arbitrary. It’s better to refer to the entire family as SMMs, including monometallic SMMs.
The statement that the ligands play a decisive role is a statement of the obvious. (the similar text starting on line 206 should also be deleted).
Ferromagnetic exchange is not a pre-requisite is polymetallic compounds. AF coupling can also be found, e.g. the Long/Evans radical-bridged compounds. (see also line 208, where the same error is made).
Strong axial CFs are only appropriate for metal ions with oblate electron density (Tb3+, Dy3+). Equatorial CFs lead to SMM behaviour in complexes based on prolate electron density (e.g. Er3+).
Cp is also discussed as a ligand. The reference to ‘soft (HSAB)’ is too generic.
The statement ‘we are doing a parenthesis here’ doesn’t make sense in English. (also line 239) Parenthesis?
Change ‘discover new, e.g. quantum, behaviors’ to ‘discover, e.g., new quantum behavior’.
Regarding the ‘Miller and Epstein groups broke the temperature barrier record’, it would be useful to know which property and associated temperature we are talking about here.
From line 88, the description of the background theory is applicable only to transition metal SMMs. This should be mentioned.
Line 124. Molecular nanomagnets is better since the deposition of single atoms on surfaces is the smallest imaginable nanomagnet.
Line 138. Refer to dysprosium rather than ‘lanthanide’.
Line 146. The citation of ref [27] is not relevant to MOF materials.
Figure 2 and the associated text are not appropriate for a frontier review article and must be removed. This material belongs in undergraduate textbooks.
The ChemDraw picture in Figure 3 needs to be improved.
Line 260: absolute minimum.
Fig. 7: the molecular structure this compound would be better represented through the crystallographically determined structure. The schematic is not clear. Also, we need some insight into the crystallographic symmetry of the molecule, particularly the relationship between the Fe centres, which can be achieved with an image of the crystal structure.
Line 400: the formula of the compound seems strange with only Harris notation. The azide ligands should also appear.
The discussion of the SMM properties on lines 411-418 seems curious. The presence of two relaxation process in the solvated material seems reasonable, but the switch to one process in the dried material is odd given that the central Fe atom is still distinct from the other six. A comment on this is merited.
The discussion of the heptanuclear azido complexes is interesting but could be brought to life by including selected AC graphs to show the SMM properties. This is true for other sections of the manuscript. In its current form, this aspect of the review is a little too simple for the experts and but also not accessible to non-experts.
The schematic of MnRe compound in Fig. 8 needs to be replaced with the crystal structure.
Line 582: the barrier is stated as negative.
Lines 608-608: the relative energies of the d-orbitals as proposed does not consider the possibility of s-dz2 mixing, which is known to lower the energy of dz2 below that of dxz and dyz (as is mentioned on the next page).
Line 708: the carbon atom is missing from the ligand.
Lines 812-823: no mention of the geometry of complexes 20-22 is provided so it is hard to discern how the various parameters relate to the molecular structure.
Line 857: the structure of the Gd/Cu complex should be included.
I’m not sure that a figure showing the basic click reactions is merited in a review of this nature, particularly when the structures of several, more important molecules are not included. A simple reference to important click papers will suffice.
Line 888: in order for the...
Line 932: the type and strength of the exchange should be stated.
Line 940-941; the non-expert reader will require some more information on why these compounds show promise as SMM-based quantum computers. This is not obvious from the text as written.
From line 1022: as with other compounds, I found it very surprising that the authors tried to describe the structure of an Mn31 cluster with text alone. We need a picture.
Line 1069: is this intended to imply that azide deprotonates the alcohol (pro-ligand, not ligand)? The alcohol probably has quite a high pKa so would azide be able to do this?
Line 1133: as drawn, this is not an oximate ligand, it is isocyanate. Oximate should be written as R2CNO.
Line 1236/1237: it seems more likely that an sp3 carbon in the ligand backbone gives a greater degree of conformational flexibility rather than a lower degree.
Conclusions: the comment about strong coupling being exclusive to transition metals should be revised in light of the (admittedly small) number of radical-bridged Ln SMMs.
Author Response
We are grateful to Reviewer 2 for her/his warm comments on the ms. She/he raised many points (all correct) and we were able to address almost all of them. She/he also discovered mistakes and we have corrected them. The critical “eye” on the review is excellent and I have met such a detailed report only a few times in my academic life (>40 years). In detail:
“The only major aspect that the authors should consider is the quality and quantity of the diagrams, which I’m afraid is not good. In some cases, chemdraw schemes have been used to depict very complicated cluster compounds. It would be much better to use the crystal structure. In other cases, very complicated molecular structures are described only with text. Some specific cases are mentioned below. The authors are advised to enhance the visual appeal of the manuscript with better images, and more of them. Some magnetism graphs (especially AC susceptibility) could also be used to bring the manuscript to life.”
This point was also raised by Reviewer 4. We have replaced several chemdraw diagrams [Figures 1, 3 (2 in the revised ms), 6 (5 in the revised ms), 7 (6 in the revised ms), 8b and 19 (20 in the revised ms)] of the initially submitted ms with figures derived from crystal structures. We have also added two new crystral structure figures (Figures 15 and 18 in the revised ms) and a new figure (Figure 7 in the revised ms with magnetic data).
“Throughout: ‘inorganic chemistry’, ‘coordination chemistry’ and ‘molecular magnetism’ do not need to be capitalized. Also, there is no need to use a superscript number to denote the oxidation state of alkali metals since only one value is chemically reasonable (i.e. the oxidation state is always obvious).”
We have strictly conformed to her/his suggestions throughout the entire ms.
“All Ueff values are stated without standard deviations. It would better to include these, where possible.”
We came back to the relevant literature sources and incorporated the requested standard deviations. In the few cases we have not added the standard deviations, those did not exist in the literature sources.
“The distinction between SMM and SIM is arbitrary. It’s better to refer to the entire family as SMMs, including monometallic SMMs.”
In the abstract we have given the alternative name (monometallic SMMs). In the text we continue to use the term “SIM”. However, to show the we fully respect the reviewer’s comment, we have added three sentences (“In the literature, SIMs…probably more awkward”) in section 1 in the text below Figure 1, where we clarify this point.
“The statement that the ligands play a decisive role is a statement of the obvious. (the similar text starting on line 206 should also be deleted).”
We have removed the statement from “Abstract”. We have also shortened the relevant paragraph in section 2.
“Ferromagnetic exchange is not a pre-requisite is polymetallic compounds. AF coupling can also be found, e.g. the Long/Evans radical-bridged compounds. (see also line 208, where the same error is made).”
We have added this (scientifically correct) statement in “Abstract” and removed our initial statement from the text.
“Strong axial CFs are only appropriate for metal ions with oblate electron density (Tb3+, Dy3+). Equatorial CFs lead to SMM behaviour in complexes based on prolate electron density (e.g. Er3+).”
We have incorporates this clarification in “Abstract”.
“Cp is also discussed as a ligand. The reference to ‘soft (HSAB)’ is too generic.”
Cp has been added in “Abstract”. As far as the “soft(HSAB)” term, we have explained the initials of this model in “Abstract”. More details had also been provided in section 7.
“The statement ‘we are doing a parenthesis here’ doesn’t make sense in English. (also line 239) Parenthesis?”
Since English are not our mother language, we apologize for this mistake. We have rephrased the relevant sentences in the text.
“Change ‘discover new, e.g. quantum, behaviors’ to ‘discover, e.g., new quantum behavior’.”
We have incorporated the correction in the text.
“Regarding the ‘Miller and Epstein groups broke the temperature barrier record’, it would be useful to know which property and associated temperature we are talking about here.”
We have incorporated the relevant information about the critical temperature value of this remarkable molecular material.
“From line 88, the description of the background theory is applicable only to transition metal SMMs. This should be mentioned.”
We have explicitly mentioned in the revised ms that the theory is applicable only to transition-metal SMMs.
“Line 124. Molecular nanomagnets is better since the deposition of single atoms on surfaces is the smallest imaginable nanomagnet.”
We have incorporated this correction.
“Line 138. Refer to dysprosium rather than ‘lanthanide’.”
We refer to “dysprosium” in the revised ms.
“Line 146. The citation of ref [27] is not relevant to MOF materials.”
This is absolutely correct. In order to keep the original reference list, we have rephrased the relevant sentence to “see information provided in ref. [27]”.
“Figure 2 and the associated text are not appropriate for a frontier review article and must be removed. This material belongs in undergraduate textbooks.”
As requested, we have removed Figure 2 of the initially submitted ms and shortened the relevant text. The numbering scheme of figures has been modified accordingly.
“The ChemDraw picture in Figure 3 needs to be improved.”
The figure (Figure 2 in the revised ms) has been improved replacing the chemdraw scheme with a figure derived from the crystal structure.
“Line 260: absolute minimum.”
The mistake has been corrected.
“Fig. 7: the molecular structure this compound would be better represented through the crystallographically determined structure. The schematic is not clear. Also, we need some insight into the crystallographic symmetry of the molecule, particularly the relationship between the Fe centres, which can be achieved with an image of the crystal structure.”
In the new figure (Figure 7 in the revised ms), the molecular structure is represented through the crystallographically determinded structure; the schematic is now clear. We have also added some information about the crystallographic symmetry of the molecule.
“Line 400: the formula of the compound seems strange with only Harris notation. The azide ligands should also appear.”
This was a typographic error. The correct representation of the core with the azido ligands has been incorporated into the revised ms.
“The discussion of the SMM properties on lines 411-418 seems curious. The presence of two relaxation process in the solvated material seems reasonable, but the switch to one process in the dried material is odd given that the central Fe atom is still distinct from the other six. A comment on this is merited.”
In the initially submitted review, we had stated the opposite in accordance with the original literature sources (refs. [70] and [71]), i.e., that the solvated material is characterized by one relaxation process, whereas the dried one by two relaxation processes. A possible explanation of this, admittedly unusual, behavior had been commented (as given by the authors in the original literature sources) on our initial submission.
“The discussion of the heptanuclear azido complexes is interesting but could be brought to life by including selected AC graphs to show the SMM properties. This is true for other sections of the manuscript. In its current form, this aspect of the review is a little too simple for the experts and but also not accessible to non-experts.”
We have partly satisfied the reviewer’s requirements. A new figure (Figure 7 in the revised ms) provides the reader with some ac data on sample 7a. Since the content of the review is purely chemical, we have avoided to add other magnetic figures. We are asking Reviewer’s 2 and your indulgence to retain the figures as they are.
“The schematic of MnRe compound in Fig. 8 needs to be replaced with the crystal structure.”
We have replaced the schematic of the MnRe compound with the crystal structure (Figure 8b in the revised ms).
“Line 582: the barrier is stated as negative.”
This typographic error has been corrected.
“Lines 608-608: the relative energies of the d-orbitals as proposed does not consider the possibility of s-dz2 mixing, which is known to lower the energy of dz2 below that of dxz and dyz (as is mentioned on the next page).”
Yes, indeed! We have added a clarification that these relative energies of the d orbitals do not consider the possibility of s-dz2 mixing; this is general energy scheme. Concerning the Fe(I) complex, theory provided strong evidence for this mixing and this is clearly reflected on Figure 10.
“Line 708: the carbon atom is missing from the ligand.”
This typographic error has been corrected and the carbon atom is now in the formula of the ligand.
“Lines 812-823: no mention of the geometry of complexes 20-22 is provided so it is hard to discern how the various parameters relate to the molecular structure.”
The pseudotetrahedral geometry of the three complexes is now mentioned in the text.
“Line 857: the structure of the Gd/Cu complex should be included.”
We have conformed to the reviewer’s suggestion and added a new figure (Figure 15) in the revised ms.
“I’m not sure that a figure showing the basic click reactions is merited in a review of this nature, particularly when the structures of several, more important molecules are not included. A simple reference to important click papers will suffice.”
Following the reviewer’s comment, we have removed the old Figure 15 keeping only the relevant reference.
“Line 888: in order for the...”
This grammatical mistake has been corrected.
“Line 932: the type and strength of the exchange should be stated.”
The antiferromagnetic nature of the interactions is now mentioned in the revised ms.
“Line 940-941; the non-expert reader will require some more information on why these compounds show promise as SMM-based quantum computers. This is not obvious from the text as written.”
We have added this new information (“; the latter…by means of chemical methods.”) in section 8.
“From line 1022: as with other compounds, I found it very surprising that the authors tried to describe the structure of an Mn31 cluster with text alone. We need a picture.”
We have added a new multiple crystal structure figure (Figure 18 in the revised ms) and now the readers can fully see and understand details of the Mn31 cluster.
“Line 1069: is this intended to imply that azide deprotonates the alcohol (pro-ligand, not ligand)? The alcohol probably has quite a high pKa so would azide be able to do this?”
The question is absolutely logical. We have added a clarification note (“; however, the main sources for ligand deprotonation…pyridine molecules”.) addressing this point. All the initial information was from the initial literature source (ref. [139]).
“Line 1133: as drawn, this is not an oximate ligand, it is isocyanate. Oximate should be written as R2CNO.”
Yes, indeed! We have performed the correction.
“Line 1236/1237: it seems more likely that an sp3 carbon in the ligand backbone gives a greater degree of conformational flexibility rather than a lower degree.”
We looked carefully at this point. What the authors of the original reference [174] mean is that the conformation flexibility of H2dpd is lower than that of a ligand containing two neighboring sp3 carbon atoms in the backbone. We have slightly rephrased the sentence and we believe that the meaning is now clear.
“Conclusions: the comment about strong coupling being exclusive to transition metals should be revised in light of the (admittedly small) number of radical-bridged Ln SMMs.”
The comment is absolutely correct. We have included the relevant statement [“(with the exception of radical-bridged 4f-metal SMMs [176])”] in section 12 and added a new reference (reg. [176]) in the revised ms.
Again, we thank Reviewer 2 for her/his time to read in such a detail our review and for the valuable comments/suggestions provided. Without her/his detailed comments, mistakes would appear in the published article. Moreover, all the revision comments help us to improve the quality and do better the readability of the review.
Reviewer 3 Report
This very interesting and well-written review has to be published as it is, except some typos I found. Nevertheless, due to the impossibility to print the document and since it is much more convenient for me to read on paper, I may not have seen all typos.
It was not easy to access to references, nevertheless it seems to me adequate.
Below the few typos I noted:
- line 50: I would write "in-depth" instead of "in depth"
- line 103: I think the authors mean at "higher temperature" instead at "lower temperature"
- line 116: may be a reference would be usefull
- at least 2 other typos in the same paragraph (I did not saved the last pdf version where I highligted the typos) that the electronic corrector will find.
Author Response
We thank Reviewer 3 for her/his positive comments.
Concerning the points she/he raised, we have addressed them. In detail:
“Below the few typos I noted:
- line 50: I would write "in-depth" instead of "in depth"
- line 103: I think the authors mean at "higher temperature" instead at "lower temperature"
- line 116: may be a reference would be usefull
- at least 2 other typos in the same paragraph (I did not saved the last pdf version where I highligted the typos) that the electronic corrector will find.”
The mistakes/typographic errors have been corrected and the requested reference was incorporated.
We thank Reviewer 3 for her/his time to study our review and for the useful corrections provided.
Reviewer 4 Report
The authors have reported a review focusing on the synthesis of SIM and SMM for 3d, 4d, and 5d metals. In terms of attention to synthesis, it is significant, unlike the previously reported reviews of SIM and SMM for transition metal ions for magnetic properties. In order to accept this review in Inorganics, it seems that the following modifications are necessary.
It is very difficult for the reader to distinguish the freehand drawing of the complex molecule, so this reviewer recommends replacing it with the crystal structure drawing.
In addition, the energy diagrams in Figures 10, 12 and 13 also guarantee accuracy, so this reviewer is advised to quote from the original paper.
Author Response
We thank Reviewer 4 for her/his positive comments.
Concerning the two points she/he raised, we are glad because we were able to address both. In detail:
“It is very difficult for the reader to distinguish the freehand drawing of the complex molecule, so this reviewer recommends replacing it with the crystal structure drawing.”
This point was also raised by Reviewer 2. The comment is correct. We have replaced several schematic drawings [Figures 1, 3 (2 in the revised ms), 6 (5 in the revised ms), 7 (6 in the revised ms), 8b and 19 (20 in the revised ms)] of the initially submitted ms with figures derived from crystal structures. We do believe that the corresponding structural representations are now more clear and attractive to the readers.
“In addition, the energy diagrams in Figures 10, 12 and 13 also guarantee accuracy, so this reviewer is advised to quote from the original paper.”
We fully agree that Figures 10, 12 and 13 guarantee more accuracy. Following the advice by the reviewer, we have modified them taking information from the corresponding original papers.
We thank Reviewer 4 for her/his time to study our review and for the valuable comments/suggestions provided
