A POCO type pincer complex of iridium: Synthesis, characterization, and catalysis

Abstract

A convenient synthesis of a new POCO-type pincer ligand, “tBuPOCOH”, (3-(di-tert-butylphosphinito)acetophenone is reported. Metallation using [Ir(COD)Cl]2 provides the dimeric species (μ-Cl-[tBuPOCOIrHCl]2) (1a) as a major product, along with isomer 1b. Though not fully characterized, 1b is shown to be chemically equivalent to 1a by a series of experiments with AgOTf and CO which lead to formation of a single product, tBuPOCOIr(CO)HOTf (3). The 1a/b mixture gives inferior results to iPrPCPIr when used as pre-catalyst in the dehydrogenative coupling of vinyl arenes, though olefin isomerization activity is enhanced. This system was also evaluated in the Geurbet conversion of ethanol to n-butanol and higher alcohols. The CO adducts of 1a/b, cis/trans-2, were found to give the best results as pre-catalyst, achieving a 33% yield of n-butanol and an overall 47% yield of n-alcohols with a catalyst loading of 0.5% when heated at 150 °C for 4 hours. This represents the first example of a pincer ligated iridium complex as catalyst in the Geurbet reaction of ethanol.