Palladium(II) Complexes Bearing an Indazole-Derived N‑Heterocyclic Carbene and Phosphine Coligands as Catalysts for the Sonogashira Coupling and the Hydroamination of Alkynes

Indazolin-3-ylidenes (indy) are among the most strongly donating N-heterocyclic carbenes, but the structural diversity of their complexes is still limited. Two dimeric palladium­(II) complexes, [PdBr₂(indy-5)]₂ (<b>2a</b>) and [PdBr₂(indy-6)]₂ (<b>2b</b>) (indy-5 = 2,3-dihydro-1<i>H</i>-pyrazolo­[1,2-<i>a</i>]­indazolin-3-ylidene, indy-6 = 6,7,8,9-tetrahydropyridazino­[1,2-<i>a</i>]­indazolin-3-ylidene], bearing indazolin-3-ylidene ligands with different sizes of the fused aliphatic ring can be obtained by silver carbene transfer. The reaction of these dimers with pyridine yielded <i>trans</i>-[PdBr₂(indy)­(pyridine)] complexes (<b>3a</b>,<b>b</b>), while the poorly soluble monophosphine complexes <i>cis</i>-[PdBr₂(indy)­(PPh₃)] (<b>4a</b>,<b>b</b>) were obtained by reaction with triphenylphosphine. Ligand substitution of the latter with silver trifluoroacetate afforded <i>cis</i>-[Pd­(O₂CCF₃)₂(indy)­(PPh₃)] complexes (<b>5a</b>,<b>b</b>) with improved solubilities, allowing for their detailed characterizations. In the presence of sodium tetrafluoroborate, cationic bis­(phosphine) complexes <i>trans</i>-[PdBr­(PPh₃)₂]­[BF₄] (<b>6a</b>,<b>b</b>) could be obtained. Similarly, <i>cis</i>-[PdBr­(dppe)]­[BF₄] (<b>7a</b>,<b>b</b>) and <i>cis</i>-[PdBr­(dppp)]­[BF₄] (<b>8a</b>,<b>b</b>) were obtained (dppe = bis­(diphenylphosphino)­ethane; dppp = bis­(diphenylphosphino)­propane) with the respective chelating diphosphines. A preliminary catalytic study revealed that the complexes incorporating monodentate phosphine ligands are good catalysts for the Sonogashira cross-coupling, while moderate to good yields were achieved with all complexes for the hydroamination of carbon–carbon triple bonds.