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

Pyrazole-bridged bis­(imidazolium) salts [H₃<b>L</b>^<b>R</b>]­(PF₆)₂ (R = Et, ^<i>n</i>Bu, C₆H₂Me₃-2,4,6, and ^<i>t</i>Bu), 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 [<b>L</b>^<b>R</b>₂Pd₂]­(PF₆)₂ (except R = ^<i>t</i>Bu) and via C4/5 in [^a<b>L</b>^<b>R</b>₂Pd₂]­(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 [<b>L</b>^<b>R</b>₂Pd₂]­(PF₆)₂. Further reaction of [<b>L</b>^<b>R</b>₂Pd₂]­(PF₆)₂ and [^a<b>L</b>^<b>R</b>₂Pd₂]­(PF₆)₂ with PdCl₂ and NEt₄Cl gave the corresponding neutral dinuclear complexes <b>L</b>^<b>Et</b>Pd₂Cl₃ and ^a<b>L</b>^<b>Et</b>Pd₂Cl₃ selectively, without any normal/abnormal rearrangement occurring during transmetalation. Only ^a<b>L</b>^<b>tBu</b>Pd₂Cl₃ is accessible directly from [H₄<b>L</b>^{<i><b>t</b></i><b>Bu</b>}]­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