Henry Rzepa's Blog

Sandwich compounds are the colloquial term used for molecules where a metal atom such as an iron dication is “sandwiched” between two carbon-based rings as ligands, most commonly cyclopentadienyl anion (the “bread”) as in e.g. Ferrocene – a molecule first discovered in 1951. An “inverted” sandwich is where the carbon ring is itself sandwiched between two metal ions and one such was reported this year [1] containing benzene in the middle with scandium as the metal. The novelty of the subsequent four-electron reduction of the benzene “filler” and its ring opening to a linear hexadiene unit resulted in this being selected as one “molecule of the year” for 2025.

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References

1. L. Zhang, Z. Jiang, C. Zhang, K. Cheng, S. Li, Y. Gao, X. Wang, and J. Chu, "Room Temperature Ring Opening of Benzene by Four-Electron Reduction and Carbonylation", Journal of the American Chemical Society, vol. 147, pp. 25017-25023, 2025. https://doi.org/10.1021/jacs.5c08414

The annual “Molecules of the Year” selections are available for the year 2025. A theme was elemental allotropes and one such was carbon in the form of C₄₈ stabilised by formation of a catenane C₄₈.M3 (M = red ligand below)[1] – it was not possible however to crystallise C₄₈.M3. When “unmasked” by removal of the M ligand, the true allotrope C₄₈ had a solution half-life of about 1 hour at 20°C. This follows the reports from 2019 onwards of a series of smaller cyclo[n]carbon allotropes, (n=6,10,12,13,14,16,18,20,26)[2],[3] which were only characterised on a solid surface and not in solution.

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References

1. Y. Gao, P. Gupta, I. Rončević, C. Mycroft, P.J. Gates, A.W. Parker, and H.L. Anderson, "Solution-phase stabilization of a cyclocarbon by catenane formation", Science, vol. 389, pp. 708-710, 2025. https://doi.org/10.1126/science.ady6054
2. K. Kaiser, L.M. Scriven, F. Schulz, P. Gawel, L. Gross, and H.L. Anderson, "An sp-hybridized molecular carbon allotrope, cyclo[18]carbon", Science, vol. 365, pp. 1299-1301, 2019. https://doi.org/10.1126/science.aay1914
3. H. Rzepa, "Cyclo[18]carbon: The Kekulé vibration calculated and hence a mystery!", 2019. https://doi.org/10.59350/jdy16-7rv58

In the previous post,[1] I was commenting that the transition state for borohydride reduction of a ketone contained some close contacts between the hydrogen of the borohydride and the hydrogen of water. A systematic search of the CSD reveals a modest number of such contacts have been observed in crystal structures (Table).  Since it is always good to have a reality check for constructed transition states, here I take a look at some of compounds showing the closest H…H contacts in the experimental database of structures.

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References

1. H. Rzepa, "The mechanism of borohydride reductions. Part 2: 4-t-butyl-cyclohexanone – Dispersion induced stereochemistry.", 2025. https://doi.org/10.59350/x5k75-t2m40

In the previous post[1] I mooted the possibility that a high energy form of the dimer of nitric oxide 1 might nonetheless be able to be detected using suitable traps (such as hydrogenation or cycloaddition). However, an interesting alternative is that this species could be trapped by nitric oxide itself. According to [2] in an article entitled “Decomposition of nitric oxide at elevated pressures” the rate of this termolecular reaction 3NO → N₂O + NO₂ are said to obey third order kinetics. One plausible mechanism for this process is shown below.

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References

1. H. Rzepa, "Hydrogenating the even more mysterious N≡N triple bond in a nitric oxide dimer.", 2025. https://doi.org/10.59350/rzepa.29626
2. T. Melia, "Decomposition of nitric oxide at elevated pressures", Journal of Inorganic and Nuclear Chemistry, vol. 27, pp. 95-98, 1965. https://doi.org/10.1016/0022-1902(65)80196-8

Previously[1] I looked at some of the properties of the mysterious dimer of nitric oxide  1 – not the known weak dimer but a higher energy form with a “triple” N≡N bond. This valence bond isomer of the weak dimer was some 24 kcal/mol higher in free energy than the two nitric oxide molecules it would be formed from. An energy decomposition analysis (NEDA) of 1 revealed an interaction energy[2] of +4.5 kcal/mol for the two radical fragments, compared to eg -27 kcal/mol for the equivalent analysis of the N=N double bond in nitrosobenzene dimer[3] So here I take a look at another property of N≡N bonds via their hydrogenation energy (Scheme), mindful that the dinitrogen molecule requires forcing conditions to hydrogenate, in part because of the unfavourable entropy terms (See Wiki and also here^‡ for a calculation of ΔG₂₉₈).

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References

1. H. Rzepa, "The even more mysterious N≡N triple bond in a nitric oxide dimer.", 2025. https://doi.org/10.59350/rzepa.29429
2. H. Rzepa, "N2O2 as strong dimer? bent NEDA 0 1 0 2 0 -2 Total Interaction (E) : 4.520 Wiberg NN bond index 1.0072 NN stretch 2604 cm-1", 2025. https://doi.org/10.14469/hpc/15468
3. H. Rzepa, "Nitrosobenzene dimer NEDA=2, 0,1 0,1 0,1 Total Interaction (E) : -27.564", 2025. https://doi.org/10.14469/hpc/15444

I started adding citations to my blog posts around 2012 using the Kcite plugin.^‡ Rogue Scholar is a service that monitors registered blog sources and adds all sorts of value to the original post, including identifying such citations and creating a list of them.

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