Pyrazole and Pyrazolate as Ligands in the Synthesis and Stabilization of New Palladium(II) and (III) Compounds

The versatility of pyrazole/pyrazolate
as ligands has allowed the synthesis and the structural characterization
of four different types of new orthometalated palladium compounds,
for which DFT calculations have been performed in order to investigate
their relative stabilities. [Pd2{μ-(C6H4)­PPh2}2{μ-(R,R′2pz)}2] (R = R′ = H, 2a; R =
Br, R′ = H, 2b; R = CH3, R′
= H, 2c; R = H, R′ = CH3, 2d; R = Br, R′ = CH3, 2e) compounds
with exo-bidentate pyrazolatos are the first paddlewheel
dinuclear palladium­(II) compounds with pyrazolato bridging ligands
described and characterized in the literature. In the process of the
synthesis of 2a, a new tetranuclear intermediate compound,
[Pd4{μ-(C6H4)­PPh2}4(μ-pz)2(μ–OH)2] (3a), has been isolated and structurally characterized.
Compounds of the general formula [Pd2{μ-(C6H4)­PPh2}2Br2(R,R′2pzH)2] (R = R′ = H, 4a; R =
Br; R′ = H, 4b; R = CH3; R′
= H, 4c; R = H; R′ = CH3, 4d; R = Br; R′ = CH3, 4e) with pyrazoles
as monodentate ligands have also been obtained, in which, according
to the QTAIM analysis, additional Br···HNpz weak interactions stabilize their structure. The tetranuclear Pd2Ag2 compounds, [Pd2{μ-(C6H4)­PPh2}2{μ-(R,R′2pz-Ag-R,R′2pz)}2] (R = R′
= H, 5a; R = Br; R′ = H, 5b; R =
CH3, R′ = H, 5c), showed a distorted
tetrahedron disposition of the metal atoms. The QTAIM analysis revealed
an enhanced stability because of additional metal–metal interactions.
New palladium­(III) compounds, [Pd2{μ-(C6H4)­PPh2}2{μ-(R,R′2pz)}2Cl2] (R = R′ = H, 6a; R = Br, R′ = H, 6b) were also synthesized
by oxidation of compounds 2 with PhICl2. DFT
calculations highlighted their greater stability compared to that
of similar compounds with N,N-donor ligands, such as formamidinatos
and triazenidos.