Prototropic rearrangements in cycloheptatrienyl PCP pincer iridium complexes

Abstract

When the cycloheptatriene iridium(iii) pincer complex (PCP)Ir(CO)(H)(Cl) (3) (PCP = 2,7-(CH(2)P(t)Bu(2))(2)C(7)H(5)) is treated with the bases NaH, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and lithium 2,2,6,6-tetramethylpiperidide (LiTMP) under various conditions different products are obtained. At elevated temperatures and with DBU or LiTMP as a base the trans dihydride (PCP')Ir(CO)(H)(2) (PCP' = 2-(CHP(t)Bu(2))-7-(CH(2)P(t)Bu(2))C(7)H(4)) (5) is formed where the pi-system extends into one of the phosphine bridges. This compound loses H(2) to give the square-planar iridium(I) carbonyl complex (PCP'IrCO). The dihydride 5 can also rearrange to the new isomeric iridium(I) carbonyl 6 (PCP''IrCO, PCP'' = 2,7-(CH(2)P(t)Bu(2))(2)C(7)H(5)). Thus the two hydrides have moved into the ligand backbone creating a methylene group in the 3-position of the cycloheptatriene ring. Alternatively, 6 is formed by a rearrangement from 6a which differs from 6 by having the methylene group in the 4-position of the cycloheptatriene ring. The iridium(I) carbonyl 6a in turn is made from 3 by treatment with DBU at room temperature. Interestingly, when compound is heated to reflux in THF the hydrogen bound at the metal carbon is shifted to a carbon atom in the cycloheptatriene ring generating a ring methylene group (3a). From this complex HCl is eliminated upon chromatography forming 6 as the final product. Quantum chemical calculations at various levels of theory illustrate the relative energetic stabilities of all iridium complexes.