Manipulation of carbon-element bonds by Pincer ligatediridium complexes

Since the seminal contributions of Moulton and Shaw in the 1970’s, who synthesized the first “pincer” complexes, including tBuPCPIrHCl (tBuPCP = κ3-2,6-C6H3(CH2P(tBu)2)2), the chemistry of transition metal complexes ligated by tri-dentate, planar, pincer-type ligands has experienced vast development. Pincer-complexes of 1st, 2nd, and 3rd row transition metals are now known, and have been employed in a number of useful processes such as alkane, alcohol, and N-heterocycle dehydrogenation, Heck coupling, and C-H borylation reactions. In the Jones lab at the University of Rochester, we have found that the less bulky iso-propyl substituted analog (iPrPCP) of Moulton and Shaw’s original ligand facilitates greatly enhanced reactivity when bound to iridium in comparison to its tert-butyl counterpart. For example, the C-C cleavage of biphenylene by iPrPCPIr occurs in less than 30 minutes at room temperature, while the same reaction requires heating at 125 °C for 24 hours with tBuPCPIr. In some cases, novel reactions not possible with tBuPCPIr have been discovered. While iPrPCPIr was found to catalyze the dehydrogenative coupling of un-functionalized vinyl arenes to form 1,4-butadienes at relatively low catalyst loadings (1-5 mole%), tBuPCPIr did not produce any observable coupling products. Also, iPrPCPIr(PhC≡CPh) was found to undergo isomerization to a unique iridaindene product when heated at 100 °C, whereas only decomposition occurred with tBuPCPIr(PhC≡CPh). These transformations, and others, mediated by iPrPCPIr are discussed.