How does the tin(IV)-insertion to porphyrins proceed in water at ambient temperature?: Re-investigation by time dependent 1H NMR and detection of intermediates

Water soluble tin(IV)-porphyrins, trans-dihydroxy-5,10,15,20-tetra(N-methyl-4′-pyridiniumyl)porphyrinate tin (IV) (Sn(IV)TMPyP(OH)2) was successfully synthesized in high yield (90%) through one-pot process in water at room temperature by treating the parent free base (H2TMPyP) with Sn(II)Cl2. Re-investigation by the detailed kinetic analysis of the time dependent 1H NMR signals indicated that the tin(IV)-insertion reaction reversibly proceeded through three steps to start from free base dimer, [H2TMPyP]2, through Sn(II)TMPyP. New intermediate, an association complex between SnCl2 and H2TMPyP, which should be a precursor of Sn(II)TMPyP, was clearly observed in UV–vis and 1H NMR for the first time. Sn(II)TMPyP was also detected at around 500 nm in UV–vis spectroscopy, while it was silent against 1H NMR, which was supported by DFT calculation to indicate that Sn(II)TMPyP has a paramagnetic nature through substantial population on triplet spin states. To explore possible one-pot process for other porphyrins, such as neutral tetra-meso-substituted-tolylporphyrin (TTP), tetra-meso-substituted-mesitylporphyrin (TMP), and anionic tetra-meso-substituted-carboxyphenylporphyrin (TCPP), the reactions in water, binary-phase system of water/dichloromethane, and methanol were examined. The anionic TCPP suffered the tin(IV)-insertion at ambient temperature only in methanol with good yield (90%).