Molecules of the year 2025: Benzene-busting inverted sandwich.

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.

The first example of such an inverted sandwich was reported in 1983 (CAMZAP^‡)[2] and this made me wonder how many examples have subsequently been discovered. A search of the CSD (crystal structure database) using the following query was undertaken.

This query places a centroid to the central benzene ring and measures its distances to the two metal ions, along with the angle subtended at the centroid. A C-C distance is also defined. The results (82 hits, increasing to 182 for the more general CR, R=C or H above) are shown below (Figure 1).

Figure 1a. Metal to centroid distances and angle at the benzene centroid.

Figure 1b. Benzene CC ring distances.

Figure 1c. Publication year for benzene as sandwich filler.

Figure 1d. Publication year for structures including substituted benzenes as sandwich filler.

An outlier in Figure 1c that dates from 1975 also corresponds to the outlier in Figure 1a seen as a blue dot and is actually an example of TWO metal Pd atoms sandwiched by benzene rings (inadvertently captured by the search definition above). There has been an explosive growth in the reports of crystal structures of such complexes since 2024, which suggests that this is currently an area of intense activity.

The molecule of the year referred to above goes by the CSD name BAFHUQ[3] and its structure is shown below (Figure 2, M-centroid length 1.856Å). This molecule has a calculated[4] (D4) dispersion stabilisation of 62.8 kcal/mol, deriving in large measure from interactions of the isopropyl groups on the P ligand.

Figure 2. BAFHUG

HOQNEK[5],[6] with Ti as metal (Figure 3) is an example having the shortest M-benzene centroid length (1.69Å). The dispersion contribution in this case is 46.2 kcal/mol.

Figure 3. HOQNEK

CALNIP[7],[8] has the shortest distance to the ring centroid of 1.56Å, with toluene as the sandwich filler.

Figure 4. CALNIP (Fe-centroid 1.56Å, K-centroid 2.96Å).

The trends above suggest that a new area of the reactivity of aromatic molecules such as benzene when sandwiched between two metal atoms may be emerging.

^‡Actually a “triple decker” sandwich containing both ligand-metal-ligand and the inverted metal-ligand-metal motifs.

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.
2. A.W. Duff, K. Jonas, R. Goddard, H.J. Kraus, and C. Krueger, “The first triple-decker sandwich with a bridging benzene ring”, Journal of the American Chemical Society, vol. 105, pp. 5479-5480, 1983.
3. Zhang, Leiyang., Jiang, Ziang., Zhang, Cuijuan., Cheng, Kehang., Li, Songyang., Gao, Yueze., Wang, Xiaotai., and Chu, Jiaxiang., “CCDC 2417331: Experimental Crystal Structure Determination”, 2025.
4. H. Rzepa, “[Embargoed]”, 2025.
5. E. Álvarez-Ruiz, I. Sancho, M. Navarro, I. Fernández, C. Santamaría, and A. Hernán-Gómez, “A Mixed-Valence Ti(II)/Ti(III) Inverted Sandwich Compound as a Regioselective Catalyst for the Uncommon 1,3,5-Alkyne Cyclotrimerization”, Inorganic Chemistry, vol. 63, pp. 8642-8653, 2024.
6. Álvarez-Ruiz, Elena., Sancho, Ignacio., Navarro, Marta., Fernández, Israel., Santamaría, Cristina., and Hernán-Gómez, Alberto., “CCDC 2324418: Experimental Crystal Structure Determination”, 2024.
7. A.A. Danopoulos, P. Braunstein, K.Y. Monakhov, J. van Leusen, P. Kögerler, M. Clémancey, J. Latour, A. Benayad, M. Tromp, E. Rezabal, and G. Frison, “Heteroleptic, two-coordinate [M(NHC){N(SiMe₃)₂}] (M = Co, Fe) complexes: synthesis, reactivity and magnetism rationalized by an unexpected metal oxidation state”, Dalton Transactions, vol. 46, pp. 1163-1171, 2017.
8. Danopoulos, Andreas A.., Braunstein, Pierre., Monakhov, Kirill Yu.., van Leusen, Jan., Kögerler, Paul., Clémancey, Martin., Latour, Jean-Marc., Benayad, Anass., Tromp, Moniek., Rezabal, Elixabete., and Frison, Gilles., “CCDC 1469268: Experimental Crystal Structure Determination”, 2017.