Variable temperature 1H NMR studies of [M(COD)(μ-L)] 2 (M  Rh, Ir; L = substituted hydroxypyridinate). Facile intramolecular bridging ligand exchange and metal-metal bond cleavage

Abstract

The variable temperature 1H NMR spectra of [M(COD)(μ-L)] 2 (M  Rh, Ir: L = 2-hydroxypyridinate (μ-hp), 6-methyl-2-hydroxypyridinate (mhp)) in toluene- d 8 solutions are described. Our analysis of the 1H NMR spectra indicates that the binuclear hydroxypyridinate complexes undergo fluxional processes significantly different than previously studied mononuclear analogues. Two distinct exchange processes interconvert nonequivalent COD protons in [Rh(COD)(μ-hp)], but only the lower energy process is observed in [Ir(COD)(μ-hp)] 2. The resonances of the bridging ligands are essentially unaffected by temperature changes. The low energy process (process 1) interconverts COD hydrogens trans to N with those trans to O, but does not interconvert “inside” with “outside”ogens. The second process (process 2), observed for [Rh(COD)(μ-hp)] 2, interconverts inside with outside protons. Line shape analyses of the variable temperature spectra of the hp complexes yielded the rates and activation parameters of processes 1 and 2. Analogous behavior was observed for [Ir(COD)(μ-mhp)] 2 and [Rh(COD)(μ-mhp)] 2. No exchange behaviour was observed in the spectrum of the related complex, [Ir(COD)(μ-pz)] 2. The effects of concentration, added COD, and added nucleophile (acetonitrile) on the rates of exchange are discussed. “Cross-over” experiments, in which two different [M(COD)(μ-L)] 2 species were mixed together, demonstrate that process 1 is strictly intramolecular. The rotation and/or dissociation of COD ligands, the complete dissociation of the bridging ligands and the rupture of the M-M interaction are not involved with process 1. Process 1 must involve only motion of the bridging ligands, and does not involve the formation of mononuclear intermediates. For process 2, dissociation of COD was ruled out, but the rotation of the coordinated diene or the inversion of the 8-membered metallocyclic ring are viable mechanisms. Cross-over experiments show that at temperatures where the rate of process 2 is appreciable, metal-metal and bridging ligand exchange occurs. These results, and the large positive entropy of activation of process 2 in [Rh(COD)(μ-hp)] 2 imply a dissociative mechanism. Process 2 has a substantially higher activation barrier in [Ir(COD)(μ-hp)] 2 than in [Rh(COD)(μ-hp)] 2, consistent with the stronger M-M interaction expected for the Ir 2 complex. Both proposed pathways for process 2 involve substantial weakening of the M-M interaction.