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 (2a) and [PdBr₂(indy-6)]₂ (2b) (indy-5 = 2,3-dihydro-1H-pyrazolo­[1,2-a]­indazolin-3-ylidene, indy-6 = 6,7,8,9-tetrahydropyridazino­[1,2-a]­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 trans-[PdBr₂(indy)­(pyridine)] complexes (3a,b), while the poorly soluble monophosphine complexes cis-[PdBr₂(indy)­(PPh₃)] (4a,b) were obtained by reaction with triphenylphosphine. Ligand substitution of the latter with silver trifluoroacetate afforded cis-[Pd­(O₂CCF₃)₂(indy)­(PPh₃)] complexes (5a,b) with improved solubilities, allowing for their detailed characterizations. In the presence of sodium tetrafluoroborate, cationic bis­(phosphine) complexes trans-[PdBr­(PPh₃)₂]­[BF₄] (6a,b) could be obtained. Similarly, cis-[PdBr­(dppe)]­[BF₄] (7a,b) and cis-[PdBr­(dppp)]­[BF₄] (8a,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.