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 ± 1.0 kcal/mol) that is approximately one-half that of [Pd­(IMes)2] (−27.9 ± 1.5 kcal/mol). Computational studies identify the cause as interligand repulsion forcing a wider C–Pd–C angle and tilting of the NHC plane in cis-[Pd­(IPr)2(η2-O2)]. Arene–arene interactions are more favorable and serve to further stabilize cis-[Pd­(IMes)2(η2-O2)]. Inclusion of dispersion effects in DFT calculations leads to improved agreement between experimental and computational enthalpies of O2 binding. A complete reaction diagram is constructed for formation of trans-[Pd­(IPr)2(η1-O2)2] and leads to the conclusion that kinetic factors inhibit formation of trans-[Pd­(IMes)2(η1-O2)2] at the low temperatures at which it is thermodynamically favored. Failure to detect the predicted T-shaped intermediate trans-[Pd­(NHC)2(η1-O2)] for either NHC = IMes or IPr is attributed to dynamic effects. A partial potential energy diagram for initial binding of O2 is constructed. A range of low-energy pathways at different angles of approach are present and blur the distinction between pure “side-on” or “end-on” trajectories for oxygen binding.