Dinuclear Palladium Complexes of Pyrazole-Bridged Bis(NHC) Ligands: A Delicate Balance between Normal and Abnormal Carbene Coordination

Pyrazole-bridged bis­(imidazolium)
salts [H3LR]­(PF6)2 (R = Et, nBu, C6H2Me3-2,4,6, and tBu), which are precursors
to the corresponding pyrazole-bridged bis­(N-heterocyclic carbene)
ligands, were found to give, upon reaction with Pd­(OAc)2, dinuclear palladium complexes with either normal or abnormal binding
modes of the two NHC groups: via C2 in [LR2Pd2]­(PF6)2 (except
R = tBu) and via C4/5 in [aLR2Pd2]­(PF6)2. It has been shown that the course
of the reaction crucially depends on the amount of NH4OAc
added, suggesting an acetate-assisted pathway leading to [LR2Pd2]­(PF6)2. Further reaction of [LR2Pd2]­(PF6)2 and
[aLR2Pd2]­(PF6)2 with PdCl2 and NEt4Cl gave the corresponding neutral dinuclear complexes LEtPd2Cl3 and aLEtPd2Cl3 selectively, without any normal/abnormal rearrangement occurring
during transmetalation. Only aLtBuPd2Cl3 is accessible directly
from [H4LtBu]­Cl3 and Pd­(OAc)2. All
complexes have been characterized by NMR spectroscopy and elemental
analysis, and several of them have also been characterized by ESI
mass spectrometry and single-crystal X-ray diffraction. The observed
binding modes and structural features have been rationalized by density
functional theory calculations, which evidence that for a given complex
the thermodynamically favored conformer is found in the solid state.