Dear Colleagues,

Although tetrahedral tetracoordination is the fundamental paradigm of organic chemistry, the identification of methanium ion, CH₅⁺, in the laboratory through mass spectroscopic measurements challenged the way that one use to think about the coordination tendencies of carbon. While these experiments were carried out in 1950 and officially published in 1952 [1], it took another 47 years to record the infrared (IR) spectrum of this simple protonated methane [2]. Theoretical studies of two of the lithium carbides, CLi₅ and CLi₆, further motivated the interest in hypercoordinate carbon compounds. It is noted here that, perhaps, though these compounds are sometimes described as “hypervalent” in the literature, the extra electrons in lithium carbides beyond the usual “octet” are involved with the Li-Li bonding rather than with the Li-C bonding [3]. Therefore, these lithium carbides are hypercoordinate carbon molecules rather than hypervalent carbon molecules. Surprisingly, CLi₆ was detected through mass spectroscopic measurements [4] in 1992, though the theoretical studies were carried out in 1983. Theoretical work on Si₂(CH₃)₇⁺ and C(CH₃)₅⁺, and experimental works on [CCH₃]₆²⁺, HC[Au(PPh3)]₄⁺, [(C₆H₅)₃PAu₅C]⁺, [(Ph₃PAu)₆C]²⁺, C₆[CH₃]₆²⁺, etc., further motivated the interest in hypercoordinate carbon molecules [5-12].  Using anthracene moieties, Akiba and co-workers have shown penta and hexa coordinate organic molecules through X-ray crystallography and theoretical calculations [13-14]. Even in biological systems, such as the iron–molybdenum nitrogenease cofactor existing in diazotrphs, hexacoordinate carbon is predicted to be present [15].

The idea of making hypercoordinate carbon or silicon, or first-row or second-row elements in general, is not only to examine the coordination behavior of different elements but also to develop new materials. The main objective of this Special Issue is to collect some recent trends in this subject area, as the field is continuously emerging. Therefore, we wholeheartedly welcome contributions from both the experimental and theoretical scientific communities working in this intriguing field.

References:

[1] Tal'rose, V. L.; Lyubimova, A. K. Secondary Processes in the Ion Source of the Mass Spectrometer. Dokl. Akad. Nauk SSSR 1952, 86, 909 – 912.

[2] White, E. T.; Tang, J.; Oka, T. CH₅⁺: The Infrared Spectrum Observed. Science 1999, 284, 135 – 137.

[3] Schleyer, P. v. R.; Würthwein, E.-U.; Kaufmann, E.; Clark, T.; Pople, J. A. Effectively Hypervalent Molecules. 2. Lithium Carbide (CLi₅), Lithium Carbide (CLi₆), and the Related Effectively Hypervalent First Row Molecules, CLi5-nHn and CLi6-nHn. J. Am. Chem. Soc. 1983, 105, 5930.

[4] Kudo, H. Observation of Hypervalent CLi₆ by Knudsen-Effusion Mass Spectrometry. Nature 1992, 355, 432.

[5] Dávalos, J. A.; Herrero, R.; Abboud, J.-L. M.; Mó, O.; Yáñez, M. How Can a Carbon Atom Be Covalently Bound to Five Ligands? The Case of Si₂(CH₃)₇⁺. Angew. Chem., Int. Ed. 2007, 46, 381-385.

[6] McKee, W. C.; Agarwal, J.; Schaefer III, H. F.; Schleyer, P. v. R. Covalent Hypercoordination: Can Carbon Bind Five Methyl Ligands? Angew. Chem., Int. Ed. 2014, 53, 7875 – 7878.

[7] Hogeveen, H.; Kwant, P. Direct Observation of a Remarkably Stable Dication of Unusual Structure: [CCH₃]₆²⁺. Tetrahedron Lett. 1973, 14, 1665 – 1670.

[9] Schmidbaur, H.; Gabbaï, F. P.; Schier, A.; Riede, J. Hypercoordinate Carbon in Protonated Tetraauriomethane Molecules, Organometallics, 1995, 14, 4969 – 4971.

[10] Scherbaum, F.; Grohmann, A.; Müller, G.; Schmidbaur, H. Synthesis, Structure, and Bonding of the Cation [(C₆H₅)₃PAu₅C]⁺. Angew. Chem., Int. Ed. 1989, 28, 463 – 465.

[11] Scherbaum, F.; Grohmann, A.; Huber, B.; Krüger, C.; Schmidbaur, H. Aurophilicity as a Consequence of Relativistic Effects: The Hexakis(triphenylphosphaneaurio)methane Dication [(Ph₃PAu)₆C]²⁺. Angew. Chem., Int. Ed. 1988, 27, 1544 – 1546.

[12] Malischewski, M.; Seppelt, K. Crystal Structure Determination of the Pentagonal-Pyramidal Hexamethylbenzene Dication C₆(CH₃)₆²⁺. Angew. Chem., Int. Ed. 2017, 56, 368 – 370.

[13] Akiba, K.-y.; Yamashita, M.; Yamamoto, Y.; Nagase, S. Synthesis and Isolation of Stable Hypervalent Carbon Compound (10-C-5) Bearing a 1,8-Dimethoxyanthracene Ligand. J. Am. Chem. Soc. 1999, 121, 10644 – 10645.

[14] Yamaguchi, T.; Yamamoto, Y.; Kinoshita, D.; Akiba, K.-y.; Zhang, Y.; Reed, C. A.; Hashizume, D.; Iwasaki, F. Synthesis and Structure of a Hexacoordinate Carbon Compound. J. Am. Chem. Soc. 2008, 130, 6894 – 6895.

[15] Lancaster, K. M.; Roemelt, M.; Ettenhuber, P.;  Hu, Y.; Ribbe, M. W.; Neese, F.; Bergmann, U.; S. DeBeer, S. X-ray Emission Spectroscopy Evidences a Central Carbon in the Nitrogenase Iron-Molybdenum Cofactor. Science 2011, 334, 974 – 977.

Dr. Venkatesan S. Thimmakondu
Guest Editor

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