[tert-Butylcyanide · Lithium Bis(trimethylsilyl)amide]2, a Model of the Intermediate RCN. MR′ Complex Formed in Reactions of Cyanides RCN with Organometallic Compounds R′M

✍ Scribed by Prof. Dr. Gernot Boche; Ira Langlotz; Michael Marsch; Dr. Klaus Harms; Prof. Dr. Gernot Frenking

• Publisher: John Wiley and Sons
• Year: 1993
• Tongue: English
• Weight: 411 KB
• Volume: 32
• Category: Article
• ISSN: 0044-8249
• DOI: 10.1002/anie.199311711

✦ Synopsis

center of the cavity, and the mean distance between the guest and the arene carbon atoms of the host is 4.5 A; about 20% larger than the sum of the van der Waals radii. This separation is almost exactly that required to reach the maximum of attractive forces in a van der Waals interaction potential. [14] It is obvious from Figure that the relatively large stability difference between the CH, complex and the CFC complexes of 2 is not related to size parameters. We cannot yet give a good interpretation for this observation. The CFCs are much more soluble in (CDCl,), than methane, which may indicate the existence of favorable solvent -guest interactions, and may disfavor their inclusion in the cavity of 2.

At the microscopic level, the fact that methane and related small molecules can be so easily captured by a cryptophane to form stable supermolecules raises some questions about the state ofmatter in the interior of the host cavity. For one molecule of chloroform in host 2, which has a spherical cavity of 81.5 A3, the occupancy factor['51 e is 0.886, which would correspond to a very closely packed crystal. In this case, the magnitudes of A e (-8.2 kcalmol-') and of AS: (-16 calmol-' K-') are actually quite comparable with the heat and entropy of crystallization of organic compounds, and the description of this complex as an ordered pseudocrystal, in which host and guest are at contact distance, may seem relevant. For one molecule of methane in the same cavity, the occupancy factor is 0.348; by comparison, the magnitude of e is 0.77 x in the gas state (1 atm and 298 K), 0.173 at the critical point (45.6 atm and 190.6 K), and 0.67 in the solid at 0 K. It follows that the best description of the interior of the complex would be a supercritical fluid, in which the methane molecule behaves like a small sphere colliding with the walls of the cavity of 2 for a few ps (at 298 K) before escaping. It is amazing to consider that, translated to the macroscopic level, one molecule of methane in the cavity of 2 is equivalent to one mole of methane reduced to a volume of 49mL and exerting a pressure of 610 atm at 298 K![l6I Even though our knowledge of the guest behavior in the interior of such supermolecules is still very imprecise, we suggest that a description of these species in terms of equivalent macroscopic states of matter is soundly based, and may provide an interesting clue to understanding their properties. Such information may also be of great interest for the design and synthesis of new host systems.