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 O 2 leads to the surprising discovery that at low temperature the initial reaction product is a highly labile peroxide complex cis-[Pd(IPr) 2 (η 2 -O 2 )]. At temperatures ≳ -40 °C, cis-[Pd(IPr) 2 (η 2 -O 2 )] adds a second O 2 o form trans-[Pd(IPr) 2 (η 1 -O 2 ) 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 O 2 , cis-[Pd(IPr) 2 (η 2 -O 2 )] reacts at low temperature with H 2 O 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 O 2 nor reaction with H 2 O occurs under comparable conditions for cis-[Pd(IMes) 2 (η 2 -O 2 )] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene). The increased reactivity of cis-[Pd(IPr) 2 (η 2 -O 2 )] is attributed to the enthalpy of binding of O 2 o [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 (η 2 -O 2 )]. Arene-arene interactions are more favorable and serve to further stabilize cis-[Pd(IMes) 2 (η 2 -O 2 )]. Inclusion of dispersion effects in DFT calculations leads to improved agreement between experimental and computational enthalpies of O 2 binding. A complete reaction diagram is constructed for formation of trans-[Pd(IPr) 2 (η 1 -O 2 ) 2 ] and leads to the conclusion that kinetic factors inhibit formation of trans-[Pd(IMes) 2 (η 1 -O 2 ) 2 ] at the low temperatures at which it is thermodynamically favored. Failure to detect the predicted T-shaped intermediate trans-[Pd(NHC) 2 (η 1 -O 2 )] for either NHC = IMes or IPr is attributed to dynamic effects. A partial potential energy diagram for initial binding of O 2 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.