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

The new diiodine basicity scale pK(BI2) 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 approximate to Se>S>I approximate to O> Br>Cl>F for the iodine-bond acceptor atomic site and SbO approximate to NO approximate to 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 pi bases and of aromatic amines, thioethers and selenoethers as N, S and Se bases, respectively. The pK(BI2) values of nBu(3)N(+) -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 I-2. The family independence can be restored by the addition of an electrostatic parameter, either the experimental pK(BHX) hydrogen-bond basicity scale or the computed minimum electrostatic potential.