1. Ligand-directed reactivity in dioxygen and water binding to cis-[Pd(NHC)2(η2-O2)]
- Author
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Steven P. Nolan, Leonardo F. Serafim, Miguel Ángel Fernández-González, Manuel Temprado, Burjor Captain, Catherine S. J. Cazin, Carl D. Hoff, Taryn D. Palluccio, Subhojit Majumdar, Elena V. Rybak-Akimova, Karl Wieghardt, Neil C. Tomson, and Xiaochen Cai
- Subjects
Stereochemistry ,Ether ,Crystal structure ,COORDINATED PALLADIUM(0) ,010402 general chemistry ,01 natural sciences ,Biochemistry ,Medicinal chemistry ,Catalysis ,SUPEROXIDE ,chemistry.chemical_compound ,Colloid and Surface Chemistry ,Reactivity (chemistry) ,Singlet state ,Organometallic chemistry ,MOLECULAR-OXYGEN ,010405 organic chemistry ,Diradical ,Ligand ,AEROBIC OXIDATION ,General Chemistry ,HETEROCYCLIC CARBENE LIGANDS ,TRANSITION-METAL-COMPLEXES ,0104 chemical sciences ,O-2 ,ORGANOMETALLIC CHEMISTRY ,Chemistry ,chemistry ,LIQUIDS ,Water binding ,BOND - Abstract
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)(eta(2)-O-2)]. At temperatures greater than or similar to -40 degrees C, cis-[Pd(IPr)(2)(eta(2)-O-2)] adds a second O-2 to form trans-[Pd(IPr)(2)(eta(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)(eta(2)-O-2)] 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 O-2 nor reaction with H2O occurs under comparable conditions for cis-[Pd(IMes)(2)(eta(2)-O-2)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene). The increased reactivity of cis-[Pd(IPr)(2)(eta(2)-O-2)] is attributed to the enthalpy of binding of O-2 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)(eta(2)-O-2)]. Arene-arene interactions are more favorable and serve to further stabilize. cis-[Pd(IMes)(2)(eta(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)(eta(1)-O-2)(2)] and leads to the conclusion that kinetic factors inhibit formation of trans-[Pd(IMes)(2)(eta(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)(eta(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.
- Published
- 2018