Oxidative-Addition Reactions of Diiodine to Dinuclear Rhodium Pyrazolate Complexes

Abstract

The pyrazolato (Pz) rhodium(I) complexes [{Rh(&mgr;-Pz)(CO)(L)}(2)] (L = CNBu(t), P(OMe)(3), PMe(2)Ph, P(OPh)(3), P(p-tolyl)(3)) result from the reaction of [{Rh(&mgr;-Pz)(CO)(2)}(2)] with the appropriate L ligand in a trans:cis ratio ranging from 60:40 (L = CNBu(t)) to 95:5 (L = P(p-tolyl)(3)). The pure trans isomers add 1 molar equiv of diiodine to give the dirhodium(II) complexes [{Rh(&mgr;-Pz)(I)(CO)(L)}(2)] (L = CNBu(t) (6), P(OMe)(3) (7), PMe(2)Ph (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 complex [(P{p-tolyl}(3))(CO)(I)Rh(&mgr;-Pz)(2)(&mgr;-CO)Rh(I)(P{p-tolyl}(3))] (10) contains 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 oxidation, since identical compounds (6-9) are obtained by oxidation with [Fe(Cp)(2)](PF(6)) followed by addition of potassium iodide. Further reactions of the dirhodium(II) complexes 6-9 with diiodine leading to the metal-metal rupture are electrophilic additions, as exemplified by the reactions with the 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 give directly the dirhodium(III) complexes [{Rh(&mgr;-Pz)(I)(CO)(L)}(2)(&mgr;-I)](+) (L = CNBu(t), CO). Otherwise, as for the complexes with P-donor ligands, abstraction of a iodide ligand trans to the metal-metal bond (the "exo site") occurs first, to give the dirhodium(II) cationic complexes [(PR(3))(CO)(I)Rh(&mgr;-Pz)(2)Rh(CO)(PR(3))](+) and triiodide. These react again with diiodine to give dirhodium(III) complexes [{Rh(&mgr;-Pz)(I)(CO)(PR(3))}(2)(&mgr;-I)](+) similar to those described above, but with triiodide or pentaiodide as counterion, as substantiated by the X-ray structure of [{Rh(&mgr;-Pz)(I)(CO)(PMe(2)Ph)}(2)(&mgr;-I)]I(5) (18). The diiridium(II) complexes [{Ir(&mgr;-Pz)(I)(CO)(PR(3))}(2)] (PR(3) = P(OPh)(3), PMe(2)Ph) also react with diiodine to give the cationic diiridium(III) complexes [{Ir(&mgr;-Pz)(I)(CO)(PR(3))}(2)(&mgr;-I)]I(3) through a reaction pathway involving the "exo site", while no reaction is observed for [{Ir(&mgr;-Pz)(I)(CO)(2)}(2)]. Finally, replacement of a carbonyl ligand in [{Rh(&mgr;-Pz)(I)(CO)(L)}(2)(&mgr;-I)](+) (L = CNBu(t), CO) by iodide gives the compounds [(CO)(L)(I)Rh(&mgr;-Pz)(2)(&mgr;-I)Rh(I)(2)(L)].