Sulfur and Selenium Atom Transfer Reactions of Tin Porphyrins

Abstract

Treatment of (meso-tetraphenylporphyrinato)tin(IV) sulfide, (TPP)Sn=S, with (meso-tetra-p-tolylporphyrinato )tin(II), (TTP)Sn11, in toluene results in the reversible exchange (K = 1.21 ± 0.03 at -10 °C) of a sulfur ligand to form (TPP)Sn11 and (TTP)Sn=S. The net result is a formal two-electron redox process between Snii and Sn1v. This occurs with a second-order rate constant at 30 °C of 0.40 ± 0.05 M-1 s1 (AH* = 10.9 ± 0.9 kcaVmol, !l.S* = -24.1 ± 2.8 cal(mol·K)1). Similarly, treatment of (meso-tetraphenylporphyrinato)tin(IV) selenide, (TPP)Sn=Se, with (meso-tetra-p-tolylporphyrinato)tin(II), (TTP)Snn, in toluene results in the reversible exchange (K = 1.45 ± 0.13 at -10 oq of a selenium ligand to form (TPP)Sn11 and (TTP)Sn=Se. This reaction occurs with a second-order rate constant at 30 oc of 87.3 ± 8.06 M1 s1 (Ali* 9.3 ± 0.5 kcaVmol, !l.S* = -18.8 ± 1.5 cal(mol·K)1 ). Discussion of an inner sphere mechanism involving a ,u-sulfido or a ,u-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 (1> Br> Cl> F-). Atom transfer reactions continue to be an area of fundanlental importance. Numerous studies have focused on oxygen atom transfer due to its relevance in both biological systems and industrial or laboratory oxidation processes.2•3 While these have provided a large number of examples involving the transfer of an oxygen atom between a metal center and organic or nonmetal substrates, the related process of intermetal oxygen atom transfer reactions is still underdeveloped by comparison. The scope of intermetal oxygen atom transfer has recently been reviewed.4 Relatively few studies have been reported on sulfur or selenium atom transfer. Examples involving the transfer of a sulfur or selenium atom from a non-metal species (e.g. phosphine chalcogenides or ethylene sulfide) to a low-valent metal center have recently been utilized to prepare novel terminal sulfido and selenido complexes of the early transition metals.5 These reactions are unusual in that phosphines generally remove sulfur from metal complexes due to the strength of the phosphine sulfide bond (s.:92 kcaVmol).6 Intermetal atom transfer reactions utilizing Cpz TiS5 and Cp2 TiSe5 as chalcogen transfer reagents have provided a synthetic route to new terminal and perchalcogenido species.710 These reactions formally represent a ® Abstract published in Advance A CS Abstracts, January 15, 1995. (1) Presidential Young Investigator, 1990-1995; Camille and Henry Dreyfus Teacher-Scholar 1993-1998. (2) For lead references, see: Ostovic, D.; Bruice, T. Ace. Chern. Res. 1992, 25, 314. (3) (a) Holm, R. H. Chern. Rev. 1987, 87, 1401. (b) Holm, R. H.; Donahue, J. P. Polyhedron 1993, 12, 571. (c) Jorgenson, K. A. Chern. Rev. 1989, 89, 431. (4) Woo, L. K. Chern. Rev. 1993, 93, 1125. (5) (a) Woo, L. K.; Hays, J. A.; Young, V. G., Jr.; Day, C. L.; Caron, C.; D'Souza, F.; Kadish, K. M. Inorg. Chern. 1993, 32, 4186. (b) Hall, K. A.; Mayer, J. M. J. Am. Chern. Soc. 1992, 114, 10402 and references therein. (6) Chernick, C. L.; Pedley, J. B.; Skinner, H. A. J. Chern. Soc. 1957, 1851. (7) Bolinger, C. M.; Hoots, J. E.; Rauchfuss, T. B. Organometallics 1982, 1, 223. (8) (a) Guilard, R.; Ratti, C.; Tabard, A.; Richard, P.; Dubois, D.; Kadish, K. M. lnorg. Chern. 1990, 29,2532. (b) Ratti, C.; Richard, P.; Tabard, A.; Guilard, R. J. Chern. Soc., Chern. Commun. 1989, 69. (9) Guilard, R.; Ratti, C.; Barbe, J.-M.; Dubois, D.; Kadish, K. M. Inorg. Chern. 1991, 30, 1537. (10) Poncet, J. L.; Guilard, R.; Friant, P.; Goulon-Ginet, C.; Goulon J. Nouv. J. Chim. 1984, 8, 583. 0002-7863/95/1517-1314$09.00/0 secondary atom transfer process3• since reduction of the rP-X5 (X = S, Se) ligand has taken place.4 Metalloporphyrin complexes have been used to investigate a variety of innersphere redox processes involving intermetal halogen, 11 oxygen, 12 and nitrogen atom transfer reactions. 13•14 We recently reported the discovery of intermetal oxygen, sulfur, and selenium atom transfer reactions involving titanium porphyrin complexes (eq 1). 15·16 Equation 1 also represents a secondary atom transfer process in which X2is reduced to 2xz-.4 (OEP)Ti(X2) + (TTP)Ti(PhC=CPh) (OEP)Ti=X + (TTP)Ti-X + PhC=CPh (1)