Do Sitting-Atop Metalloporphyrin Complexes Exist? Observation of N−H- - -π Bonding in Arene Solvates of a Diprotonated Porphyrin Dication

The existence of sitting-atop metalloporphyrin complexes, the
proposed intermediates in the metalation of free-base porphyrins,
has been explored via the microscopic reverse reaction, i.e.,
protonation of metalloporphyrins with a strong acid. The reaction
of M(Tp-TP) (M = Zn, Cu, Ni; Tp-TP = dianion of tetra-p-tolylporphyrin) with mesitylenium carborane salts produced only
the demetalated porphyrin in its diprotonated form, [H4Tp-TP][carborane]2 (carborane = CHB11H5Cl6-, CHB11H5Br6-, CHB11Cl11-).
In arene solvents, the H4Tp-TP2+ dication shows an unusually
upfield shifted 1H NMR resonance at ca. −6 ppm, which X-ray
crystallography reveals to arise from N−H- - -π hydrogen bonding
of the acidic protons to arene solvent molecules.