Valence Tautomerism in Chromium Half-Sandwich Triarylmethylium Dyads^†

Round 1

Reviewer 1 Report

This manuscript is a very nice piece of work and can be published as such. There are just minor points the authoer might  consider prior to the publication.

Page 2, top line: the impression CAr3n+ is a bit confsuing as it covers also neutral and anionic species. For example, a symbol z might look more clear than n+.

Page 3, below Sche 3. caption: "for -resp- five-step procedures" looks odd. What does it mean?

Page 4, Table 1: 1-H and L-H are mentioned in table, but I was wondering (until the relevan part in the discussion) what does thay stand for. Scheme 5 and figure 6 should be sited here.

Author Response

This manuscript is a very nice piece of work and can be published as such. There are just minor points the author might  consider prior to the publication.

Page 2, top line: the impression CAr3n+ is a bit confsuing as it covers also neutral and anionic species. For example, a symbol z might look more clear than n+.

This is a good suggestion, and we have changed accordingly.

Page 3, below Sche 3. caption: "for -resp- five-step procedures" looks odd. What does it mean?

This was meant to say that the synthesis of complex 1⁺ requires four synthesis steps, and that of complexes 2⁺ and 3⁺ five steps (the extra one being the substitution of one CO ligand for PR₃). We have changed the phrasing to “Complexes 1⁺ ‑ 3⁺ were synthesized in four (1⁺) or five steps (2⁺, 3⁺) as detailed in Scheme 3.”

Page 4, Table 1: 1-H and L-H are mentioned in table, but I was wondering (until the relevant part in the discussion) what does thay stand for. Scheme 5 and figure 6 should be cited here.

Good point. We are now referring to Scheme 5 to make things clear.

Reviewer 2 Report

R.F. Winter and coworkers designed three chromium half-sandwich complexes [Cr(CO)₂L(η⁶-C₆H₄(2-OMe)-C₆H₄-C⁺{-C₆H₄(4-CF₃)}₂] (L = CO, P(OPh)₃, PPh₃). It is hoped that valence tautomerism can be achieved by adjusting the oxidation potential of metal units. Cyclic voltammetry, IR spectroscopy and EPR spectroscopy were used to characterize the properties. The results show that the PPh₃-substituted complex has valence tautomer. The three complexes decompose easily, some of the decomposition products were also analyzed by NMR and IR spectroscopy, ESI-MS, single crystal X-ray analysis and cyclic voltammetry. This work involves a lot of work and has potential for publication in Inorganics, pending addressing the following points:

1. IR spectrum of 2⁺ hints at the presence of small amounts of the diradical valence tautomer. Are these small amounts of the diradical valence tautomer represented in cyclic voltammetry and EPR spectroscopy?

2. Is it possible to figure out why PPh₃-substituted compound has valence tautomer, P(OPh)₃-substituted compound has small amounts of the valence tautomer, whereas CO-substituted compound has no valence tautomer?

3. Page 11, “Still, 1⁺ decomposes within several hours to chromium half-sandwich complex 1-H and minor amounts of the free ligand L-H, as revealed by the NMR spectra in Figure 5”. However, ¹H-NMR spectra of 1⁺ shown in Figure 5 only shows the presence of 1-H, not L-H.

4. In the reference section the references need to be carefully checked and consistent in writing format.

no

Author Response

R.F. Winter and coworkers designed three chromium half-sandwich complexes [Cr(CO)₂L(η⁶-C₆H₄(2-OMe)-C₆H₄-C⁺{-C₆H₄(4-CF₃)}₂] (L = CO, P(OPh)₃, PPh₃). It is hoped that valence tautomerism can be achieved by adjusting the oxidation potential of metal units. Cyclic voltammetry, IR spectroscopy and EPR spectroscopy were used to characterize the properties. The results show that the PPh₃-substituted complex has valence tautomer. The three complexes decompose easily, some of the decomposition products were also analyzed by NMR and IR spectroscopy, ESI-MS, single crystal X-ray analysis and cyclic voltammetry. This work involves a lot of work and has potential for publication in Inorganics, pending addressing the following points:

1. IR spectrum of 2⁺hints at the presence of small amounts of the diradical valence tautomer. Are these small amounts of the diradical valence tautomer represented in cyclic voltammetry and EPR spectroscopy?

The CV does not show an individual wave (but 3⁺ does not show such one either, as the equilibrium establishes at a rate faster than cyclic voltammetry). For 2⁺, the extreme instability made it impossible to record meaningful EPR spectra. The reason is, that our preparative glove box does not allow for temperatures below -40 °C, and at this temperature we see a lot of decomposition of this sample, before we get it ready for EPR. As many of the decomposition products are also EPR active, we cannot make any scientifically safe statements.

2. Is it possible to figure out why PPh₃-substituted compound has valence tautomer, P(OPh)₃-substituted compound has small amounts of the valence tautomer, whereas CO-substituted compound has no valence tautomer?

The propensity of such systems to engage in valence tautomeric equilibria depends on the difference between the intrinsic redox potentials for metal-centered oxidation and tritylium-based reduction. This is exactly the ordering we find in this study. This message is provided is provided at several occasion throughout the manuscript, culminating in the quantitative assessment by calculating the equilibrium constant (and, in the case of 3⁺, even of its T-dependency). We have nevertheless amphasized this point once more in the Summary and Conclusions Section as follows: These compounds, symbolized as Cr(0)‑C₆H₄‑CAr₂⁺, were investigated with respect to their propensity to coexist with their paramagnetic redox isomers (valence tautomers) Cr(I)^+•‑C₆H₄‑CAr₂^•. The latter result from intramolecular electron transfer from the Cr(0) donor to the tritylium-type acceptor unit so that the equilibria between the two isomers depend on the difference between the half-wave potentials for metal-centered oxidation and ligand-based reduction. (the newly added part is marked in yellow).

3. Page 11, “Still, 1⁺decomposes within several hours to chromium half-sandwich complex 1-H and minor amounts of the free ligand L-H, as revealed by the NMR spectra in Figure 5”. However, ¹H-NMR spectra of 1⁺shown in Figure 5 only shows the presence of 1-H, not L-H.

The problem here is that most NMR resonance signals of 1-H and L-H are almost identical. The only ones that differ appreciably are those on the anisole ring, which coordinates to Cr in 1-H but is uncoordinated in L-H, and the singlets of the methoxy groups. We have modified Figure 5 so to make this easier to see in shading all resonances common to 1-H and L-H in gray, those specific to 1-H in blue, and those specific to L-H in violet color. This is now also explained in the legend.

4. In the reference section the references need to be carefully checked and consistent in writing format.

We have carefully checked the references and found indeed several inconsistencies (for which we apologize), in particular with respect to whether a doi is given or not, the citation style for book chapters, and an inverted ordering of given and family names in ref. 51. What remains are differences with respect to whether the title of the citations is written in capital letters for nouns or not. We however decided to stick with the formatting of the original publications, which are by definition heterogeneous.