SULFUR AND SELENIUM ATOM-TRANSFER REACTIONS OF TIN PORPHYRINS

Treatment of (meso-tetraphenylporphyrinato)tin(IV) sulfide, (TPP)Sn=S, with (meso-tetra-p-tolylporphyrinato)tin(II), (TTP)Sn-II, in toluene results in the reversible exchange (K = 1.21 +/- 0.03 at -10 degrees 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 o ccurs with a second-order rate constant at 30 degrees C of 0.40 +/- 0. 05 M(-1) s(-1) (Delta H-double dagger = 10.9 +/- 0.9 kcal/mol, Delta.S -double dagger = -24.1 +/- 2.8 cal(mol K)(-1)). Similarly, treatment o f (meso-tetraphenylporphyrinato)tin(IV) selenide, (TPP)Sn=Se, with (me so-tetra-p-tolylporphyrinato)tin(II), (TTP)Sn-II, in toluene results i n the reversible exchange (K = 1.45 +/- 0.13 at -10 degrees C) of a se lenium ligand to form (TPP)Sn-II and (TTP)Sn=Se. This reaction occurs with a second-order rate constant at 30 degrees C of 87.3 +/- 8.06 M(- 1) s(-1) (Delta H-double dagger 9.3 +/- 0.5 kcal/mol, Delta S-double d agger = -18.8 +/- 1.5 cal(mol.K)(-1)). Discussion of an inner sphere m echanism involving a mu-sulfido or a mu-selenido bridged intermediate is presented. The rate ratio of selenium to sulfur atom transfer is 21 8:1 at 30 degrees C. This rate behavior follows the ''normal'' trend a s observed for the analogous halogen transfer reactions (I- > Br- > Cl - > F-).