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Synthesis of a low-valent Al4+ cluster cation salt


Low-valent aluminium compounds are very reactive main-group species and have therefore been widely investigated. Since the isolation of a stable molecular Al(I) compound in 1991, [(AlCp*)4] (Cp* = [C5Me5]), a variety of highly reactive neutral or anionic low-valent aluminium complexes have been developed. By contrast, their cationic counterparts have remained difficult to access. Here, we report the synthesis of [Al(AlCp*)3]+[Al(ORF)4] (RF = C(CF3)3) through a simple metathesis reaction between [(AlCp*)4] and Li[Al(ORF)4]. Unexpectedly, the [Al(AlCp*)3]+ salt forms a dimer in the solid state and concentrated solutions. Addition of Lewis bases results in monomerization and coordination to the unique formal Al+ atom, giving [(L)xAl(AlCp*)3]+ salts where L is hexaphenylcarbodiphosphorane (x = 1), tetramethylethylenediamine (x = 1) or 4-dimethylaminopyridine (x = 3). The Al+–AlCp* bonds in the resulting [(L)xAl(AlCp*)3]+ cluster cations can be finely tuned between very strong (with no ligand L) to very weak and approaching isolated [Al(L)3]+ ions (when L is dimethylaminopyridine).

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Fig. 1: Selected examples of Al(I) complexes.
Fig. 2: Synthesis and characterization of [Al(AlCp*)3]+[pf] (1).
Fig. 3: Decomposition of [Al(AlCp*)3]+[pf] (1).
Fig. 4: Reactivity towards Lewis bases.
Fig. 5: Bonding analysis of the metalloid clusters.

Data availability

X-ray crystallographic data are available free of charge from the Cambridge Crystallographic Data Centre (CCDC) under the reference numbers 2123471 (1), 2124880 (2), 2123536 (3), 2123485 (4) and 2123538 (5) via All other data supporting the findings are contained in the main text or the Supplementary Information. Source data are provided with this paper.


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We thank the Fonds of the Chemical Industry for a fellowship for P.D., the German Research Foundation for the funding of project KR2046/35-1 and the Albert Ludwig University of Freiburg for supporting the work. We thank M. Schmitt and B. Butschke for fruitful dialogue and advice on quantum-chemical calculations and single-crystal X-ray diffraction crystallography. We acknowledge T. Ludwig and M. Daub for measurement of powder X-ray diffraction and H. Scherer and F. Bitgül for measurement of NMR spectra. Furthermore, we acknowledge support by the state of Baden-Württemberg through bwHPC and the German Research Foundation through grant no. INST 40/575-1 FUGG (JUSTUS 2 cluster).

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Authors and Affiliations



P.D. and J.W. planned and carried out all experiments and analysed the data. P.D. carried out the single-crystal X-ray diffraction measurements and the mass spectrometry measurements and conducted the computational investigations. P.D. and I.K. wrote the manuscript.

Corresponding author

Correspondence to Ingo Krossing.

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The authors declare no competing interests.

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Peer review information

Nature Chemistry thanks Sakya Sen and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–94, Tables 1–40, Discussion, all coordinates of optimized structures listed along with vibrational frequencies and refs. 1–56.

Supplementary Data 1

Crystallographic data for compound 1, CCDC 2123471.

Supplementary Data 2

Crystallographic data for compound 2, CCDC 2124880.

Supplementary Data 3

Crystallographic data for compound 3, CCDC 2123536.

Supplementary Data 4

Crystallographic data for compound 4, CCDC 2123485.

Supplementary Data 5

Crystallographic data for compound 5, CCDC 2123538.

Supplementary Data 6

Source data of Supplementary Fig. 76.

Supplementary Data 7

Source data of Supplementary Fig. 77.

Source data

Source Data Fig. 2c

NMR data for Fig. 2c.

Source Data Fig. 2f

UVVIS data for figure 2f.

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Dabringhaus, P., Willrett, J. & Krossing, I. Synthesis of a low-valent Al4+ cluster cation salt. Nat. Chem. 14, 1151–1157 (2022).

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