What is the spin state of the ground state of an aromatic species? Can this be spin state be manipulated by charge? These questions are addressed by Borden, Kim, Sessler and coworkers¹ for the hexaphyrin 1. B3LYP/6-31G(d) optimization of 1 shows it to be a ground state singlet. This structure is shown in Figure 1.

1

13+

Protonation of the three pyrrole nitrogens creates 1³⁺, which has interesting frontier orbitals. The HOMO of 1³⁺, of a₁” symmetry, has nodes running through all six nitrogens. The next higher energy orbital, of a₂” symmetry, has a small π-contribution on each nitrogen. Protonation will therefore have no effect on the energy of the a₁” orbital, but the charge will stabilize the a₂” orbital. This will lower the energy gap between the two orbitals, suggesting that a ground state triplet might be possible. The lowest singlet and triplet states of 1³⁺ are also shown in Figure 1.

1
Singlet 13+	Triplet 13+

Figure 1. (U)B3LYP/6-31G(d) optimized structures of 1 and singlet and triplet 1³⁺.

This spin state change upon protonation was experimentally verified by synthesis of two analogues of 1, shown below. The triprotonated versions of both are observed to have triplet character in their EPR spectrum.

References

(1) Fukuzumi, S.; Ohkubo, K.; Ishida, M.; Preihs, C.; Chen, B.; Borden, W. T.; Kim, D.; Sessler, J. L. "Formation of Ground State Triplet Diradicals from Annulated Rosarin Derivatives by Triprotonation," J. Am. Chem. Soc. 2015, 137, 9780-9783, DOI: 10.1021/jacs.5b05309.

InChIs

1: InChI=1S/C45H24N6/c1-2-8-29-28(7-1)34-16-22-13-24-18-36-30-9-3-4-10-31(30)38-20-26(50-43(38)42(36)48-24)15-27-21-39-33-12-6-5-11-32(33)37-19-25(49-44(37)45(39)51-27)14-23-17-35(29)41(47-23)40(34)46-22/h1-21,46,49-50H/b22-13-,23-14-,24-13-,25-14-,26-15-,27-15-
InChIKey=UMEHBSBBAUKCTH-UIJINECUSA-N

Spisak, et al. treated sumanene 1 with excess potassium in THF.¹

They obtained an interesting structure, characterized by x-ray crystallography: a mixture of the dianion and trianion of 1 (well these are really conjugate di- and tribases of 1, but we’ll call them di- and trianions for simplicity’s sake). A fragment of the x-ray structure is shown in Figure 1, showing that there is one potassium cation on the concave face and six potassium ions on the convex face.

Figure 1. X-ray structure of 1 surrounded by six K⁺ ions on the convex face and one K⁺ on the concave face.

To help understand this structure, they performed RIJCOSX-PBE0/cc-pVTZ computations on the mono-, di-, and trianion of 1. The structure of 1 (which I optimized at ωB97X-D/6-311G(d)) and the trianion are displayed in Figure 2. The molecular electrostatic potential of the trianion shows highly negative regions in the 5-member ring regions, symmetrically distributed and prime for coordination with 6 cations.

1	trianion of 1

Figure 2. Optimized structure of 1 and its trianion.

References

(1) Spisak, S. N.; Wei, Z.; O’Neil, N. J.; Rogachev, A. Y.; Amaya, T.; Hirao, T.; Petrukhina, M. A. "Convex and Concave Encapsulation of Multiple Potassium Ions by Sumanenyl Anions," J. Am. Chem. Soc. 2015, 137, 9768-9771, DOI: 10.1021/jacs.5b06662.

InChIs

1: InChI=1S/C21H12/c1-2-11-8-13-5-6-15-9-14-4-3-12-7-10(1)16-17(11)19(13)21(15)20(14)18(12)16/h1-6H,7-9H2
InChIKey=WOYKPMSXBVTRKZ-UHFFFAOYSA-N

Trianion of 1: InChI=1S/C21H9/c1-2-11-8-13-5-6-15-9-14-4-3-12-7-10(1)16-17(11)19(13)21(15)20(14)18(12)16/h1-5,7-8H,9H2/q-3
InChIKey=IHJVIPHOCKVJDZ-UHFFFAOYSA-N