Ligand-Directed Reactivity in Dioxygen and Water Binding to cis-[Pd(NHC)2(η2-O2)]

Reaction of [Pd(IPr)2] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and O2 leads to the surprising discovery that at low temperature the initial reaction product is a highly labile peroxide complex cis-[Pd(IPr)2(η2-O2)]. At temperatures ≳ −40 °C, cis-[Pd(IPr)2(η2-O2)] adds a second O2 to form trans-[Pd(IPr)2(η1-O2)2]. Squid magnetometry and EPR studies yield data that are consistent with a singlet diradical ground state with a thermally accessible triplet state for this unique bis-superoxide complex. In addition to reaction with O2, cis-[Pd(IPr)2(η2-O2)] reacts at low temperature with H2O in methanol/ether solution to form trans-[Pd(IPr)2(OH)(OOH)]. The crystal structure of trans-[Pd(IPr)2(OOH)(OH)] is reported. Neither reaction with O2 nor reaction with H2O occurs under comparable conditions for cis-[Pd(IMes)2(η2-O2)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene). The increased reactivity of cis-[Pd(IPr)2(η2-O2)] is attributed to the enthalpy of binding of O2 to [Pd(IPr)2] (−14.5 ...