Oxidative-Addition Reactions of Diiodine to Dinuclear Rhodium Pyrazolate Complexes

The pyrazolato (Pz) rhodium(I) complexes [{Rh(μ-Pz)(CO)(L)}2] (L = CNBut, P(OMe)3, PMe2Ph, P(OPh)3, P(p-tolyl)3) result from the reaction of [{Rh(μ-Pz)(CO)2}2] with the appropriate L ligand in a trans:cis ratio ranging
from 60:40 (L = CNBut) 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(μ-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 complex [(P{p-tolyl}3)(CO)(I)Rh(μ-Pz)2(μ-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](PF6) 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(μ-Pz)(I)(CO)(L)}2(μ-I)]+ (L =
CNBut, 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 [(PR3)(CO)(I)Rh(μ-Pz)2Rh(CO)(PR3)]+ and triiodide. These react again with diiodine to give dirhodium(III) complexes [{Rh(μ-Pz)(I)(CO)(PR3)}2(μ-I)]+ similar to those described above, but with triiodide or pentaiodide as counterion, as
substantiated by the X-ray structure of [{Rh(μ-Pz)(I)(CO)(PMe2Ph)}2(μ-I)]I5 (18). The diiridium(II) complexes
[{Ir(μ-Pz)(I)(CO)(PR3)}2] (PR3 = P(OPh)3, PMe2Ph) also react with diiodine to give the cationic diiridium(III)
complexes [{Ir(μ-Pz)(I)(CO)(PR3)}2(μ-I)]I3 through a reaction pathway involving the “exo site”, while no reaction
is observed for [{Ir(μ-Pz)(I)(CO)2}2]. Finally, replacement of a carbonyl ligand in [{Rh(μ-Pz)(I)(CO)(L)}2(μ-I)]+ (L = CNBut, CO) by iodide gives the compounds [(CO)(L)(I)Rh(μ-Pz)2(μ-I)Rh(I)2(L)].