The Diiodine Basicity Scale: Toward a General Halogen-Bond Basicity Scale

• Published in: Chemistry : a European journal Vol. 17; no. 37; pp. 10431 - 10444
• Main Authors: Laurence, Christian, Graton, Jérôme, Berthelot, Michel, El Ghomari, Mohamed J.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 05-09-2011; WILEY‐VCH Verlag; Wiley Subscription Services, Inc; Wiley-VCH Verlag
• Subjects: Chemical Sciences; Chemistry; halogen bonds; halogenated compounds; Iodine; Lewis acids; Lewis bases; noncovalent interactions; Lewis acids; halogen bonds; noncovalent interactions; halo- genated compounds; Lewis bases
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201101071

The new diiodine basicity scale pKBI2 is quasi‐orthogonal to most known Lewis basicity scales (hydrogen‐bond, dative‐bond and cation basicity scales). The diiodine basicity falls in the sequence N>P≈Se>S>I≈O>Br>Cl>F for the iodine‐bond acceptor atomic site and SbO≈NO≈AsO>SeO>PO>SO>CO>O>SO2 or PS≫S>CS≫NCS for the functionality of oxygen or sulfur bases. Substituent effects are quantified through linear free energy relationships, which allow the calculation of individual complexation constants for each site of polybases and thus the classification of aromatic ethers as carbon π bases and of aromatic amines, thioethers and selenoethers as N, S and Se bases, respectively. The pKBI2 values of nBu3N+‐N−C≡N, 2‐aminopyridine and 1,10‐phenanthroline reveal a superbasic nitrile, a hydrogen‐bond‐assisted iodine bond and a two‐centre iodine bond, respectively. The diiodine basicity scale is a general inorganic but family‐dependent organic halogen‐bond basicity scale because organic halogen‐bond donors such as IC≡N and ICF3 have a stronger electrostatic character than I2. The family independence can be restored by the addition of an electrostatic parameter, either the experimental pKBHX hydrogen‐bond basicity scale or the computed minimum electrostatic potential.
Basic instinct: Halogen‐bond basicity is the thermodynamic tendency of a substance to accept a halogen bond and is measured using a dual‐parameter equation in which the diiodine basicity scale is combined with an electrostatic parameter, either a hydrogen‐bond basicity scale or the minimum electrostatic potential (see figure). This equation takes into account the greater electrostatic character of organic halogen‐bond donors than inorganic ones.