Synthesis and oxidation of Cp*IrIII compounds: functionalization of a Cp* methyl group

Abstract

[Cp*IrCl₂]₂ (1) and new Cp*Ir^III(L–L)X complexes (L–L = N–O or C–N chelating ligands; X = Cl, I, Me) have been prepared and their reactivity with two-electron chemical oxidants explored. Reaction of 1 with PhI(OAc)₂ in wet solvents yields a new chloro-bridged dimer in which each of the Cp* ligands has been singly acetoxylated to form [Cp^OAcIr^IIICl₂]₂ (2) (Cp^OAc = η⁵-C₅Me₄CH₂OAc). Complex 2 and related carboxy- and alkoxy-functionalized Cp^OR complexes can also be prepared from 1 plus (PhIO)_n and ROH. [Cp^OAcIr^IIICl₂]₂ (2) and the methoxy analogue [Cp^OMeIr^IIICl₂]₂ (3) have been structurally characterized. Treatment of [Cp*IrCl₂]₂ (1) with 2-phenylpyridine yields Cp*Ir^III(ppy)Cl (4Cl) (ppy = cyclometallated 2-phenylpyridyl) which is readily converted to its iodide and methyl analogues Cp*Ir^III(ppy)I (4I) and Cp*Ir^III(ppy)Me (4Me). Cp*Ir^III complexes were also prepared with N–O chelating ligands derived from anthranilic acid (2-aminobenzoic acid) and α-aminoisobutyric acid (H₂NCMe₂COOH), ligands chosen to be relatively oxidation resistant. These complexes and 1 were reacted with potential two-electron oxidants including PhI(OAc)₂, hexachlorocyclohexadienone (C₆Cl₆O), N-fluoro-2,4,6-trimethylpyridinium (Me₃pyF⁺), [Me₃O]BF₄ and MeOTf (OTf = triflate, CF₃SO₃). Iridium(V) complexes were not observed or implicated in these reactions, despite the similarity of the potential products to known Cp*Ir^V species. The carbon electrophiles [Me₃O]BF₄ and MeOTf appear to react preferentially at the N–O ligands, to give methyl esters in some cases. Overall, the results indicate that Cp* is not inert under oxidizing conditions and is therefore not a good supporting ligand for oxidizing organometallic complexes.