Recent Advances in Low Valent Thorium and Uranium Chemistry
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis is a very well written review of low valent Th and U complexes and their small molecule reactivity, with well over 10 molecules sited. The paper is rather thorough and covers a lot of ground.
My biggest critique of the review would be that there are numerous references within that are bearing redox non-innocent ligands, or arenes that are acting as redox non innocent ligands. Scheme 16 is a great example of this. I would suggest either the removal of such complexes from the manuscript, as they are truly not "low valent" or perhaps the inclusion of a section that specifically markets the coordination chemistry and reactivity of RAL-U/Th complexes in the review. Work from Bart has focused on this in particular and there is some really good seminal work from the bridging arene chemistry of Evans. This may also help you gauge some of the observed reactivity. Particularly I would say mentioning some of Karsten Meyers early work where there was a ligand based radical and the concerted 2 electron reactivity with the U(II) would be worthwhile. Especially to show that whether the electrons are ligand or metal based, the reactivity can be identical. U(II) synthons from Bart also show similar 2-electron reactivity for the formation of U(IV) multiple bonds.
Personally i am also not the biggest fan of including the "transients" in this paper, molecules like 96 and 97, which have not been proven to exist. Take this comment with a grain of salt, but if there is no confirmation for the intermediary oxidation state then they should not be included.
Smaller things:
Scheme 8: Identity of A should be listed
Scheme 10: there is an issue with the chemical formula of 36 at the bottom. h-5 rather than eta-5
Scheme 16: Oxidation state label on Th for consistency.
Scheme 19: perhaps use A(chelate) rather than Alkali-metal(chelate)? this would make that scheme far less busy with respect to the text.
Scheme 23: Im not seeing the difference between the two Th molecules on the right and no accompanying text to describe the proposed difference.
Really good job all!
Author Response
Comment 1: This is a very well written review of low valent Th and U complexes and their small molecule reactivity, with well over 10 molecules sited. The paper is rather thorough and covers a lot of ground.
Response 1: Thank you very much.
Comment 2: My biggest critique of the review would be that there are numerous references within that are bearing redox non-innocent ligands, or arenes that are acting as redox non innocent ligands. Scheme 16 is a great example of this.
Response 2: The examples we cite are mostly for the sake of comparison between U and Th or to highlight the delicate role the ligand plays (which we have highlighted further in our Conclusions Section). As a pertinent example is the [Th(IV)-(arene)2-] complex 62 (Scheme 18) vs its U(II) analogue (86) (Scheme 30). As such we believe that these examples do offer a role in the review. Specifically for Scheme 16, we have opted to put it in the section where we discuss the reactivity of Th(III) complexes, as there is evidence that a triplet state involving a Th(III) metal centre is present and along its surface the observed chemical reactivity in Scheme 16 takes place. We have expanded Scheme 16 to highlight this pictorially. Schemes 17 and 18 are both there to highlight how some of these inverted arene complexes can give similar reactivity. Especially with regards to Scheme 18 this offers a great comparison with the uranium complexes supported by the same ligand in Scheme 30, as mentioned above.
Comment 3: Particularly I would say mentioning some of Karsten Meyers early work where there was a ligand based radical and the concerted 2 electron reactivity with the U(II) would be worthwhile.
Response 3: We think that you are referring to the 1st paper of the two part publication of the U(II) complex, where the redox U(III)/U(II) couple is determined and which also shows how the ligand undergoes activation and the basal arene ligand can stabilise a radical. This is reference 208.
Comment 4: I would suggest either the removal of such complexes from the manuscript, as they are truly not "low valent" or perhaps the inclusion of a section that specifically markets the coordination chemistry and reactivity of RAL-U/Th complexes in the review. Work from Bart has focused on this in particular and there is some really good seminal work from the bridging arene chemistry of Evans.
Response 4: We believe that a section on RAL-U/Th would broaden the review beyond its scope. The use of specific examples in order to underline the shuttle differences between the two metal centres and the role of ligand environment even as part of a reactivity pattern of these low valent complexes (e.g. Scheme 30) is balanced. Furthermore, in the introduction of low valent U(II) complexes, we mention the inverted arene complexes by Cummins (references 175, 186) and Evans (reference 187) and point the reader to the review by Liddle (reference 188) and point out that they are not U(II) complexes. We have further edited the last sentence of the first paragraph of section 3.1.1 (lines 470 and 474) as follows:' Therefore, these complexes belong to the wider class of inverted uranium arene complexes [188] and each U centre adopts a ≥ +3 formal oxidation state depending on the degree of the bridging arene reduction which acts as a redox-active ligand (RAL).' Furthermore, we have added in the last paragraph of section 3.2 (lines 802-804) the following: 'The reactivity shown in Figure 31 underlines the role that redox-active ligands (RALs) can play in obfuscating the ‘true’ valency of the metal centre, and to a certain degree mimic the reactivity of low valent actinide complexes [223]', where reference 233 points to the work of Bart et al.
Comment 5: Personally i am also not the biggest fan of including the "transients" in this paper, molecules like 96 and 97, which have not been proven to exist. Take this comment with a grain of salt, but if there is no confirmation for the intermediary oxidation state then they should not be included.
Response 5: There are some spectroscopic and computational evidence about the intermediacy of U(II) complexes 96 and 97. Nonetheless, we understand the referee's point of view and that is why we opted to include it in the reactivity section of U(II) complexes, especially since no well defined U complexes in this formal oxidation state were isolated.
Comment 6: Scheme 8: Identity of A should be listed
Response 6: Thank you very much for pointing this out; we have added to the caption what A stands for as well as what the numbers for x, y are for K and Li. We have also added to the text that the reduction with Cs metal leads to a dark purple solution but no Th(III) complex could be isolated in this case. As such we have deleted Cs from the reaction arrow.
Comment 7: Scheme 10: there is an issue with the chemical formula of 36 at the bottom. h-5 rather than eta-5
Response 7: Corrected, thank you for spotting this
Comment 8: Scheme 16: Oxidation state label on Th for consistency.
Response 8: Added and expanded Scheme as per our response 2
Comment 9: Scheme 19: perhaps use A(chelate) rather than Alkali-metal(chelate)? this would make that scheme far less busy with respect to the text.
Response 9: Edited according to the referee's suggestion
Comment 10: Scheme 23: Im not seeing the difference between the two Th molecules on the right and no accompanying text to describe the proposed difference.
Response 10: There is no difference between the two; between the two structures we have used the symbol ≡ to show that they are equivalent, ie the chemdraw structure on the right shows the doubly reduced arene as an L2X2 ligand (according to CBC) as a visual aid aimed to the novices that might be reading this review and to demonstrate that the arene ligand is no longer aromatic and with the negative charges localised. We are describing the above at the caption of the figure as follows: '(the reduced arene is an L2X2 ligand (structure on the right) according to the CBC model [209] with negative charges localised; as such it is best described as a cyclohexa-1,4-diene-3,6 diyl ligand with loss of aromaticity)'
Comment 11: Really good job all!
Response 11: Thank you very much for your kind words.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe paper involving Thorium and Uranium is per se really fascinating because the image of those metals is that they do not have a clear role in catalysis, and particularly uranium is related to nuclear energy.
The piece of text in the Abstract “Although U(III) complexes are an important class of low valent uranium compounds they will only be discussed very briefly.” must be removed or clarified. It is strange that a “bad aspect” is included in the short Abstract.
How the sigma aromaticity in (26) is demonstrated? Put further detail.
In the reference there journal titles sometimes are abbreviated and sometimes not, like ref. 119.
I must recognize I enjoyed reading it and I guess that this comes from a PhD thesis dissertation. This is a guess, but if this is the case, please refer to it. I a nice piece of work, where maybe I have missed a paragraph or more to discuss, or predict any catalytic activity those complexes could have (or have yet). There are 2 references in the text about catalysis, but simply extend them, or summarize at the very end. And grab there the references accordingly.
For any person that wants to start something on thorium or uranium, for sure this text will be helpful.
Author Response
Comment 1: The piece of text in the Abstract “Although U(III) complexes are an important class of low valent uranium compounds they will only be discussed very briefly.” must be removed or clarified. It is strange that a “bad aspect” is included in the short Abstract.
Response 1: We have deleted the phrase from the Abstract
Comment 2: How the sigma aromaticity in (26) is demonstrated? Put further detail.
Response 2: Other than the experimental SQUID measurements pointing to a diamagnetic compound despite the blue color of the complex characteristic of a Th(III) containing species, the sigma-aromaticity of the Th3 core is mainly based on computational studies. There is no other evidence based on reactivity as the authors of the original report describe how the synthesis and isolation of (26) can be challenging.
The calculations supporting the sigma-aromaticity have been debated by three independent research groups. The finer details of these are beyond the scope of the review or the expertise of the authors. We have added the following in the manuscript which we hope might give the more expert reader an idea: 'This was challenged by Szczepanik , [36] suggesting that a multi-centre charge-shift bonding between the Th and Cl atoms is dominant (ie delocalisation along the Th-Cl bridges). A very recent detailed study by Kaltsoyiannis et al., [35] scrutinizing different basis sets and employing extended NBO (Natural Bond Order) analysis (specifically Adaptive Natural Density Partitioning – AdNDP) showed that delocalized electrons are associated unsurprisingly with the COT ligands but also the Th3 centres. Indeed, the latter showed that the bonding contribution in the Th3 core resembles the Kohn-Sham HOMO calculated using the most appropriate SARC-TZVPP basis set from the ones scrutinised. Furthermore, calculations examining the responses of electron delocalisation in an applied magnetic field further gave evidence for the aromaticity of the Th3 core. Lin and Mo have also arrived at the conclusion that (26) displays such σ-aromaticity [[34].'
Comment 3: In the reference there journal titles sometimes are abbreviated and sometimes not, like ref. 119.
Response 3: We are extremely thankful for pointing out these inconsistencies to us. We have double-checked our references, and we have now rectified other similar inconsistencies.
Comment 4: I must recognize I enjoyed reading it and I guess that this comes from a PhD thesis dissertation. This is a guess, but if this is the case, please refer to it.
Response 4: Thank you very much for your kind words. The review does not come from the introduction of a PhD thesis, but we will keep your suggestion in mind for the future (when we will hopefully have all the correct paperwork to try some actinide chemistry)
Comment 5: a nice piece of work, where maybe I have missed a paragraph or more to discuss, or predict any catalytic activity those complexes could have (or have yet). There are 2 references in the text about catalysis, but simply extend them, or summarize at the very end. And grab there the references accordingly.
Response 5: Thank you for your suggestion. At the moment catalytic activity with such low valent complexes is not known. We have added as a comment or maybe prediction the following paragraph in the Conclusions section: 'Although studies in the reactivity of these low valent complexes are limited compared to U(III) mediated transformations, small molecule activation and recently catalysis, [176] from the discussion above the potential for these extremely low valent complexes to promote new chemical transformations is clear, especially considering their high reduction potentials. Central to this is the role of the ligand and its ability to control and modulate multi-electron transfers to substrates as well as determine the ground state electronic configuration of the actinide. For effective catalysis though, cycling the ‘high’ valent oxidised Ann+ (An = Th, U; n ≥ 3) products to their more reactive low valent Ann+ (An = Th, U; n ≤ 3) reduced couples pauses challenges. One obvious solution would be to use electrochemical methods, but another plausible way to circumvent our way to effective catalysis is to take inspiration by the reports by Marks et al. proposing the formation of Th(III) complexes from the photochemical elimination of H2 from Th(IV) hydrides and alkyls [16,17], and use photo-reductants. For example, acridine photoreductants [223] have been recently synthesised with E1/2 redox potential of their excited state of -2.91 to -3.6 V vs SCE (ca -2.5 to -3.1 V vs Fc/Fc+), values that are in the range for the Ann+/An(n-1)+ (An = Th (n = 4, 3) U (n = 3)) redox couples shown in Table 3.'
Comment 6: For any person that wants to start something on thorium or uranium, for sure this text will be helpful.
Response 6: Again, thank you very much.
