Ligand-Directed Reactivity in Dioxygen and Water Binding to cis-[Pd(NHC)22-O2)]

Taryn D. Palluccio, Xiaochen Cai, Subhojit Majumdar, Leonardo F. Serafim, Neil C. Tomson, Karl Wieghardt, Catherine S.J. Cazin, Steven P. Nolan, Elena V. Rybak-Akimova, Miguel Ángel Fernández-González, Manuel Temprado, Burjor Captain, Carl D. Hoff

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Abstract

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)22-O2)]. At temperatures ≳-40 °C, cis-[Pd(IPr)22-O2)] adds a second O2 to form trans-[Pd(IPr)21-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)22-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)22-O2)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene). The increased reactivity of cis-[Pd(IPr)22-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)22-O2)]. Arene-arene interactions are more favorable and serve to further stabilize cis-[Pd(IMes)22-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)21-O2)2] and leads to the conclusion that kinetic factors inhibit formation of trans-[Pd(IMes)21-O2)2] at the low temperatures at which it is thermodynamically favored. Failure to detect the predicted T-shaped intermediate trans-[Pd(NHC)21-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.

Original languageEnglish (US)
Pages (from-to)264-276
Number of pages13
JournalJournal of the American Chemical Society
Volume140
Issue number1
DOIs
StatePublished - Jan 10 2018

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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