Triazenides as Suitable Ligands in the Synthesis of Palladium Compounds in Three Different Oxidation States: I, II, and III

New orthometalated dinuclear triazenide
palladium­(II) compounds
of the general formula Pd2[(C6H4)­PPh2]2[R–N–N–N–R]2 (R = C6H5, 3a; o-BrC6H4, o-3b; o-MeOC6H4, o-3c; o-MeC6H4, o-3d ; p-BrC6H4, p-3b; p-MeOC6H4, p-3c; p-MeC6H4, p-3d) have been synthesized and structurally
characterized. The characteristics of these compounds were compared
with the isoelectronic formamidinate derivatives. These triazenide
compounds have been suitable starting products in the synthesis of
new not so common dinuclear palladium­(I) compounds and new unusual
palladium­(III) ones. In the presence of an excess of the triazenide
ligand, compounds o-3b and o-3c underwent a reduction process giving dinuclear palladium­(I)
compounds, Pd2[R–N–N–N–R]2 (R = o-BrC6H4, o-4b; R = o-MeOC6H4, o-4c). DFT calculations
verified the importance of the mostly noncovalent Pd···Br
or Pd···OMe axial interactions on the stability of
these compounds. Under cyclic voltammetric conditions, compounds 3 with the only exception of compound o-3b, were found to undergo a first reversible wave that is
assigned as the Pd25+/Pd24+ pair. Compounds p-3b-d also showed at more positive potentials a second reversible wave,
Pd25+/Pd26+ pair. When
the electrochemical oxidation was performed in the presence of chloride,
the Cl–Pd­(III)–Pd­(III)–Cl species were detected.
By chemical oxidation of the palladium­(II) complexes with PhICl2, two new relatively stable dinuclear palladium­(III) compounds,
Pd2[(C6H4)­PPh2]2[R–N–N–N–R]2Cl2 (R = C6H5, 5a; R = p-BrC6H4, p-5b), were synthesized and spectroscopically characterized at low temperature.
DFT calculations have been performed to study the stability of all
the palladium complexes. The isolated palladium­(III) and -(II) compounds
have been tested as precatalysts at room temperature in the catalytic
2-phenylation of indole with [Ph2I]­PF6. With 3a the reaction was complete in the shortest reaction time,
7 h, with a 88% isolated yield. The highest yield (99%) but with higher
reaction time, 24 h, was obtained with o-3d when CH2Cl2 was added to the reaction medium
with the aim of solving the palladium complex. These catalytic results
were compared with those obtained with other orthometalated palladium
compounds with isoelectronic ligands: formamidinate and carboxylates.
In the search for a possible first step in this catalytic oxidation
process we have also performed DFT calculations exploring the potential
Pd­(III) intermediate formed by axial Pd–Ph interactions.