Palladium(III) compounds

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In chemistry, compounds of palladium(III) feature the noble metal palladium in the unusual +3 oxidation state (in most of its compounds, palladium has the oxidation state II). Compounds of Pd(III) occur in mononuclear and dinuclear forms. Palladium(III) is most often invoked, not observed in mechanistic organometallic chemistry.[1][2]

Mononuclear compounds

Pd(III) has a d7

paramagnetic
.

The first Pd(III) complex characterized by

1,4,7-trithiacyclononane (ttcn) complex [Pd(ttcn)2]3+. X-ray crystallography revealed the expected Jahn–Teller distorted octahedral geometry, in spite of the highly symmetric structure of the ligand
.

The first

macrocyclic
structure inhibits the oxidation to a more conventional octahedral Pd(IV).

Dinuclear compounds

Structure

Pairs of Pd(III) centers can couple, giving rise to a Pd–Pd

dimers
are diamagnetic.

The first example of a dipalladium(III) complex was obtained by oxidation of dinuclear Pd(II) complex of triazabicyclodecene.[5]

The first organometallic dinuclear Pd(III) complexes were reported in 2006 by Cotton and coworkers as well.[6] These complexes catalyze the diborylation of terminal olefins.[7] Due to the facile reduction of these complexes to Pd(II) species by diborane, the authors proposed that the dinuclear Pd(III) complexes serve as precatalysts for active Pd(II) catalysts.

Dipalladium(III) complexes derived from ortho-metallated triphenylphosphine (Ph groups omitted for clarity).

Reactivity

The reactivity of dinuclear Pd(III) species as active catalytic intermediate is mostly discussed in the context of

C-H activation. While it was proposed that Pd-catalyzed oxidative C-H functionalization reactions involve a Pd(IV) intermediate, Ritter and coworkers first postulated that these oxidative reactions could involve a dinuclear Pd(III) intermediate instead of Pd(IV).[8]

Dinuclear Pd species are involved in Pd-catalyzed C-H chlorination.[9] Through X-ray crystallography, Ritter unambiguously showed that dinuclear Pd(III) complex is formed when the palladacycle is treated with two-electron oxidant, and such dinuclear complex undergoes C-Cl reductive elimination under ambient temperature. Both experimental and computational data was consistent with a concerted 1,1-reductive elimination mechanism for the C-Cl forming step.[10][11] The authors show that such bimetallic participation of redox event lowers the activation barrier for reductive elimination step by ~30 kcal/mol compared to a monometallic pathway.

2-phenylpyridine was also demonstrated to involve a dinuclear Pd(III) intermediate.[12]

See also

References

  1. doi:10.1016/j.ccr.2012.04.030
    .
  2. PMID 21461129
    .
  3. ISSN 0022-4936
    .
  4. PMID 20462195
    .
  5. ISSN 0002-7863
    .
  6. PMID 17044680
    .
  7. PMID 18932177
    .
  8. PMID 22029861
    .
  9. PMID 21500602
    .
  10. PMID 20858006
    .
  11. PMID 20873835
    .
  12. S2CID 207147534
    .
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