Oxidative-addition reactions of diiodine to dinuclear rhodium pyrazolate complexes

The pyrazolato (Pz) rhodium(I) complexes [{Rh(mu-Pz)(CO)(L)}(2)] (L = CNBut , P(OMe)(3), PMe2Ph, P(OPh)(3), P(p-tolyl)(3)) result from me reaction of [ {Rh(mu-Pz)(CO)(2)}(2)] with the appropriate L ligand in a trans:cis ratio r anging from 60:40 (L-CNBut) to 95:5 (L = P(p-tolyl)(3)). The pure trans iso mers add 1 molar equiv of diiodine to give the dirhodium(II) complexes [{Rh (mu-Pz)(I)(CO)(L)}(2)] (L = CNBut (6), P(OMe)(3) (7), PMe2Ph (8), P(OPh)(3) (9)). These complexes incorporate two iodide Ligands trans to the rhodium- rhodium bond, as substantiated by the X-ray structure for 7, while the comp lex [(P{p-tolyl}(3))(CO)(I)Rh(mu-Pz)(2)(mu-CO)Rh(I)(P{p-tolyl}(3))] (10) co ntains a bridging ketonic CO ligand, due to the insertion of a terminal CO into the metal-metal bond. The metal-metal bond formation involves a 2e oxi dation, since identical compounds (6-9) are obtained by oxidation with [Fe( Cp)(2)](PF6) followed by addition of potassium iodide. Further reactions of the dirhodium(II) complexes 6-9 with diiodine leading to the metal-metal r upture are electrophilic additions, as exemplified by the reactions with th e positive iodine complex [I(Py)(2)](+). They start at the "endo site" (the metal-metal bond) if it is sterically accessible to the electrophile, to g ive directly the dirhodium(III) complexes [{Rh(mu-Pz)(I)(CO)(L)}(2)(mu-I)]( +) (L = CNBut, CO). Otherwise, as for the complexes with P-donor ligands, a bstraction of a iodide ligand trans to the metal-metal bond (the "exo site" ) occurs first, to give the dirhodium(II) cationic complexes [(PR3)(CO)(I)R h(mu-Pz)(2)Rh(CO)(PR3)](+) and triiodide. These react again with diiodine t o give dirhodium(III) complexes [{Rh(mu-Pz)(I)(CO)(PR3)}(2)(mu-I)](+) simil ar to those described above, but with triiodide or pentaiodide as counterio n, as substantiated by the X-ray structure of [{Rh(mu-Pz)(I)(CO)(PMe2Ph)}(2 )(mu-I)]I-5 (18). The diiridium(II) complexes [{Ir(mu-Pz)(I)(CO)(PR3)}(2)] (PR3 = P(OPh)(3), PMe2Ph) also react with diiodine to give the cationic dii ridium(III) complexes [{Ir(mu-Pz)(I)(CO)(PR3)}(2)(mu-I)]I-3 through a react ion pathway involving the "exo site", while no reaction is observed for [{I r(mu-Pz)(I)(CO)(2)}(2)]. Finally, replacement of a carbonyl ligand in [{Rh( mu-Pz)(I)(CO)(L)}(2)(mu-I)](+) (L = CNBut, CO) by iodide gives the, compoun ds [(CO)(L)(I)Rh(mu-Pz)(2)(mu-I)Rh(I)(2)(L)].