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Three for the Price of One: Concomitant I⋯N, I⋯O, and I⋯π Halogen Bonds in the Same Crystal Structure
oleh: Steven van Terwingen, Ruimin Wang, Ulli Englert
Format: | Article |
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Diterbitkan: | MDPI AG 2022-11-01 |
Deskripsi
The ditopic molecule 3-(1,3,5-trimethyl-1<i>H</i>-4-pyrazolyl)pentane-2,4-dione (HacacMePz) combines two different Lewis basic sites. It forms a crystalline adduct with the popular halogen bond (XB) donor 2,3,5,6-tetrafluoro-1,4-diiodobenzene (TFDIB) with a HacacMePz:TFDIB ratio of 2:3. In a simplified picture, the topology of the adduct corresponds to a <b>hcb</b> net. In addition to the expected acetylacetone keto O and pyrazole N acceptor sites, a third and less common short contact to a TFDIB iodine is observed: The acceptor site is again the most electron-rich site of the pyrazole π-system. This iminic N atom is thus engaged as the acceptor in two orthogonal halogen bonds. Evaluation of the geometric results and of a single-point calculation agree with respect to the strength of the intermolecular contacts: The conventional N⋯I XB is the shortest (2.909(4) Å) and associated with the highest electron density (0.150 <i>e</i>Å<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mrow><mo>−</mo><mn>3</mn></mrow></msup></semantics></math></inline-formula>) in the bond critical point (BCP), followed by the O⋯I contact (2.929(3) Å, 0.109 <i>e</i>Å<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mrow><mo>−</mo><mn>3</mn></mrow></msup></semantics></math></inline-formula>), and the π contact (3.2157(3) Å, 0.075 <i>e</i>Å<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mrow><mo>−</mo><mn>3</mn></mrow></msup></semantics></math></inline-formula>). If one accepts the idea of deducing interaction energies from energy densities at the BCP, the short contacts also follow this sequence. Two more criteria identify the short N⋯I contact as the most relevant: The associated C–I bond is significantly longer than the database average, and it is the only intermolecular interaction with a negative total energy density in the BCP.