Dual Routes toward Observation of a trans-H2/Hydride Complex in an Iridium Pincer System and Hydrogenation Catalytic Activity

Abstract

ion of chloride from a six-coordinate complex, trans-[Ir(H)(Cl)(iPr)4(POCOP)(PPh3)] (1) [(iPr)4(POCOP) = 2,6-bis(di-iso-propylphosphinito)benzene, κ3-C6H3-1,3-[OP(iPr)2]2], using NaBAr4f leads to the generation of a dinitrogen complex, trans-[Ir(H)(N2)(iPr)4(POCOP)(PPh3)][BAr4f] (6). Addition of H2 to [Ir(H)(N2)(iPr)4(POCOP)(PPh3)]+ (6) under extremely mild conditions (1 bar, 298 K) resulted in the reversible coordination of H2 to generate the cis-[Ir(H)(η2-H2)(iPr)4(POCOP)(PPh3)]+ (cis-11) complex. The cis-[Ir(H)(η2-H2)(iPr)4(POCOP)(PPh3)]+ complex (cis-11) isomerized to a trans isomer, trans-[Ir(H)(η2-H2)(iPr)4(POCOP)(PPh3)]+ (trans-11), at 253 K. The isomerization process has been studied and supported by computations. Employing an alternative route, protonation of the trans-[Ir(H)2(iPr)4(POCOP)(PPh3)] complex (2) at 183 K resulted in the formation of an identical, trans-[Ir(H)(η2-H2)(iPr)4(POCOP)(PPh3)]+ complex (trans-11); upon warming the sample, the trans-H2/hydride complex isomerized to the cis isomer above 253 K. Two independent routes to obtain trans-[Ir(H)(η2-H2)(iPr)4(POCOP)(PPh3)]+ (trans-11) species have been established. Reaction of trans-[Ir(H)(N2)(iPr)4(POCOP)(PPh3)][BAr4f] (6) with C2H4 gave the cis-[Ir(H)(η2-C2H4)(iPr)4(POCOP)(PPh3)]+ complex (cis-15), which also undergoes isomerization to yield an equilibrium mixture of cis/trans-[Ir(H)(η2-C2H4)(iPr)4(POCOP)(PPh3)]+ complexes (cis-15 and trans-15) at 253 K. The trans-[Ir(H)(N2)(iPr)4(POCOP)(PPh3)]+ complex (6) is an excellent catalyst for hydrogenation of ethylene to ethane under very mild conditions.