In
chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...
, compounds of palladium(III) feature the noble metal
palladium
Palladium is a chemical element; it has symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1802 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas (formally 2 Pallas), ...
in the unusual +3
oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...
(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
Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...
.
Mononuclear compounds
Pd(III) has a d
7 electronic configuration, which leads to a
Jahn–Teller distorted
octahedral
In geometry, an octahedron (: octahedra or octahedrons) is any polyhedron with eight faces. One special case is the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex. Many types of i ...
geometry. The geometry could also be viewed as being intermediate between
square-planar and octahedral. These complexes are low-spin and
paramagnetic
Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, ...
.
The first Pd(III) complex characterized by
X-ray crystallography
X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...
was reported in 1987. It was obtained by oxidation of the
1,4,7-trithiacyclononane (ttcn) complex
d(ttcn)2sup>3+. X-ray crystallography revealed the expected Jahn–Teller distorted octahedral geometry, in spite of the highly symmetric structure of the
ligand
In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...
.
The first
organometallic
Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...
Pd(III) complex characterized by X-ray crystallography was reported in 2010. Organopalladium complexes supported with a macrocyclic tetradentate ligand undergo single-electron oxidation to give Pd(III) species that is stabilized by the axially-positioned amine. The authors propose that while the axial nitrogen stabilize a distorted octahedral geometry, the
t-Bu group and the rigidity of the
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
bond. In contrast to the mononuclear Pd(III) complexes, the Pd(III)-Pd(III)
dimers are diamagnetic.
The first example of a dipalladium(III) complex was obtained by oxidation of dinuclear Pd(II) complex of
triazabicyclodecene.
The first organometallic dinuclear Pd(III) complexes were reported in 2006 by Cotton and coworkers as well. These complexes catalyze the diborylation of terminal olefins. 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.
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).
Dinuclear Pd species are involved in Pd-catalyzed C-H chlorination. 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. 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.
Acetoxylation of
2-phenylpyridine was also demonstrated to involve a dinuclear Pd(III) intermediate.
See also
*
Palladium compounds
Palladium forms a variety of ionic, coordination, and organopalladium compounds, typically with oxidation state Pd0 or Pd2+. Palladium(III) compounds have also been reported. Palladium compounds are frequently used as catalysts in cross-coupling ...
References
{{Chemical compounds by element
Palladium compounds
P