Sulfur and selenium atom transfer reactions of tin porphyrins

Article abstract of DOI:10.1021/ja00109a015

Treatment of (meso-tetraphenylporphyrinato)tin(IV) sulfide, (TPP)Sn=S, with (meso-tetra-p-toylporphyrinato)tin(II), (TTP)Sn^II, in toluene results in the reversible exchange (K = 1.21 ± 0.03 at -10°C) of a sulfur ligand to form (TPP)Sn^II and (TTP)Sn=S. The net result is a formal two-electron redox process between Sn^II and Sn^IV. This occurs with a second-order rate constant at 30°C of 0.40 ± 0.05 M^-1 s^-1 (ΔH^? = 10.9 ± 0.9 kcal/mol, ΔS^? = -24.1 ± 2.8 cal(mol·K)^-1). Similarly, treatment of (meso-tetraphenylporphynnato)tin(IV) selenide, (TPP)Sn=Se, with (meso-tetra-p-tolylporphyrinato)tin(II), (TTP)Sn^II, in toluene results in the reversible exchange (K = 1.45 ± 0.13 at -10°C) of a selenium ligand to form (TPP)Sn^II and (TTP)Sn=Se. This reaction occurs with a second-order rate constant at 30°C of 87.3 ± 8.06 M^-1 s^-1 (ΔH^? 9.3 ± 0.5 kcal/mol, ΔS^? = -18.8 ± 1.5 cal(mol·K)^-1). Discussion of an inner sphere mechanism involving a μ-sulfido or a μ-selenido bridged intermediate is presented. The rate ratio of selenium to sulfur atom transfer is 218:1 at 30°C. This rate behavior follows the normal trend as observed for the analogous halogen transfer reactions (I^- > Br^- > Cl^- > F^-).