Chloride-Triggered Disproportionation of a Mononuclear Rh^II(nbd) Species to Rh^I(nbd) and Rh^III(η¹-norbornenyl) Complexes:  Possibilities for Wacker Type Mono-oxygenation of Norbornadiene to Norbornenone

The reactivity of the N₃-ligand rhodium nbd complex [(fac-κ³-Me-dpa)Rh^I(nbd)]⁺ ([1]⁺) toward H⁺ and the oxidants Ag⁺, FeCl₃, and ferrocenium (Fc⁺) has been investigated in detail (Me-dpa = N-(2-pyridylmethyl)-N-(6-methyl-2-pyridylmethyl)amine); nbd = norborna-2,5-diene). The oxidation state +II compound [(fac-κ³-Me-dpa)Rh^II(nbd)]²⁺ ([1]²⁺) was prepared by one-electron oxidation of [1]⁺ with Ag⁺ or Fc⁺. EPR spectroscopy and DFT geometry optimizations suggest that [1]²⁺ is a square pyramid with the two double bonds of norbornadiene and the two inequivalent pyridyl donors of the fac-Me-dpa ligand in the basal plane and the Me-dpa N^H_amine donor at the apical position. DFT calculations suggest that this conformation may be somewhat stabilized by an agostic interaction between a C−H bond of the Me-dpa methyl group and the Rh^II center. Rh^II(nbd) complex [1]²⁺ is stable in acetone at rt for several hours, but in the presence of water and Cl^- it disproportionates to a 1:1 mixture of [1]⁺ and the η¹-3-hydroxynorbornenyl dinuclear complex [{(mer-κ³-Me-dpa)Rh^III(η¹-3-hydroxynorborn-5-en-2-yl)(μ₂-Cl)}₂]²⁺ ([2]²⁺). This reaction most likely involves external (Wacker type) attack of OH^- to a coordinated double bond of nbd in [1]³⁺ or its Cl^- adduct. In MeOH, similar reactivity involving nucleophilic attack of MeO^- to Rh(nbd) is observed, yielding the η¹-3-methoxy-norbornenyl dinuclear complex [{(mer-κ³-Me-dpa)Rh^III(η¹-3-methoxynorborn-5-en-2-yl)(μ₂-Cl)}₂]²⁺ ([3]²⁺). In contrast to the above nucleophilic attack of OH^- or MeO^- to the coordinated nbd fragment, protonation of [1]⁺ initially occurs at a pyridine group of the Me-dpa ligand, yielding the corresponding square planar complex [(κ²-Me-dpaH)Rh^I(nbd)]²⁺ ([4]²⁺). In the absence of Cl^-, [4]²⁺ is stable, but the presence of Cl^- triggers migration of H⁺ from the MedpaH⁺ ligand to the metal and subsequent insertion of a double bond of nbd in the thus obtained Rh^III-hydride bond. This endo attack of H⁺ to the Rh^I(nbd) fragment of [1]⁺ results in the formation of the dinuclear η¹-norbornenyl complex [{(mer-κ³-Me-dpa)Rh^III(η¹-norborn-5-en-2-yl)(μ₂-Cl)}₂]²⁺ ([5]²⁺) and contrasts the exo-attack of OH^- or MeO^- upon formation of [2]²⁺ and [3]²⁺. Protonation or nucleophilic attack to other M(nbd) complexes (M = Pd^II, Rh^III) has invariably resulted in η³-coordinated norbornenyl derivatives. The (X-ray) structures of [2]²⁺ and [5]²⁺ are thus unique examples of η¹-coordinated norbonenyl derivatives with a noncoordinated double bond. As a result of this unusual coordination mode, the 3-hydroxynorborn-5-en-2-yl fragment in [2]²⁺ undergoes β-elimination to give norbornenone upon heating. In the presence of norbornadiene, [1]⁺ is recovered from this reaction. Thus, these systems offer new possibilities for catalytic oxidation of norbornadiene to norbornenone.