American Chemical Society
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Chloride-Triggered Disproportionation of a Mononuclear RhII(nbd) Species to RhI(nbd) and RhIII1-norbornenyl) Complexes:  Possibilities for Wacker Type Mono-oxygenation of Norbornadiene to Norbornenone

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posted on 2004-08-30, 00:00 authored by Dennis G. H. Hetterscheid, Jan M. M. Smits, Bas de Bruin
The reactivity of the N3-ligand rhodium nbd complex [(fac3-Me-dpa)RhI(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 [(fac3-Me-dpa)RhII(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 NHamine 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 RhII center. RhII(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 [{(mer3-Me-dpa)RhIII1-3-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 [{(mer3-Me-dpa)RhIII1-3-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)RhI(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 RhIII-hydride bond. This endo attack of H+ to the RhI(nbd) fragment of [1]+ results in the formation of the dinuclear η1-norbornenyl complex [{(mer3-Me-dpa)RhIII1-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 = PdII, RhIII) 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.