Synthesis and reactivity of iridium complexes of a macrocyclic PNP pincer ligand

Abstract

Having recently reported on the synthesis and rhodium complexes of the novel macrocyclic pincer ligand PNP-14, which is derived from lutidine and features terminal phosphine donors trans-substituted with a tetradecamethylene linker (Dalton Trans., 2020, 49, 2077–2086 and Dalton Trans., 2020, 49, 16649–16652), we herein describe our findings critically examining the chemistry of iridium homologues. The five-coordinate iridium(I) and iridium(III) complexes [Ir(PNP-14)(η²:η²-cyclooctadiene)][BAr^F₄] and [Ir(PNP-14)(2,2′-biphenyl)][BAr^F₄] are readily prepared and shown to be effective precursors for the generation of iridium(III) dihydride dihydrogen, iridium(I) bis(ethylene), and iridium(I) carbonyl derivatives that highlight important periodic trends by comparison to rhodium counterparts. Reaction of [Ir(PNP-14)H₂(H₂)][BAr^F₄] with 3,3-dimethylbutene induced triple C–H bond activation of the methylene chain, yielding an iridium(III) allyl hydride derivative [Ir(PNP-14*)H][BAr^F₄], whilst catalytic homocoupling of 3,3-dimethylbutyne into Z-tBuCCCHCHtBu could be promoted at RT by [Ir(PNP-14)(η²:η²-cyclooctadiene)][BAr^F₄] (TOF_initial = 28 h⁻¹). The mechanism of the latter is proposed to involve formation and direct reaction of a vinylidene derivative with HCCtBu outside of the macrocyclic ring and this suggestion is supported experimentally by isolation and crystallographic characterisation of a catalyst deactivation product.