Nature and Strength of the Inner-Core H⋅⋅⋅H Interactions in Porphyrinoids

Several recent publications have illustrated that electrostatic attraction is not solely responsible for strong hydrogen bonds. Even electropositive and less electronegative atoms such as Te and Se are capable of forming strong H-bonds. Herein, we provide evidence for intramolecular homopolar dihydrogen bonds [HOMO-DHBs; X-N(C)-H^δ+ ⋅⋅⋅^δ+ H-N(C)-Y] in porphyrins and related compounds for the first time; these bonds are revealed by careful Cambridge Structural Database (CSD) exploration, quantum theory of atoms in molecules, compliance constant calculations, and natural bond orbital and noncovalent interaction (NCI) analysis. A search of the CSD showed that the inner-core hydrogen atom distances were less than 2.5 Å (sum of the van der Waals radii of two hydrogen atoms is 2.4 Å) in porphyrinoids, i.e. about 75 % of the cases. This suggested an attractive interaction between hydrogen atoms carrying a positive charge, which was further supported by quantum-chemical calculations. The HOMO-DHB energy in some cases was found to be as much as around 20 kJ mol^-1 , which is comparable to that of any conventional H-bond energy such as for the NH₃ dimer. The interplay between hyperconjugative attraction and steric constraint favorably decided the strength of the HOMO-DHBs. We expect that HOMO-DHBs could be revealed in many more systems, such as corroles, phlorins, crown ethers, and constrained systems having hydrogen atoms in close contact, and could be an important noncovalent interaction to consider in supramolecular chemistry.