Oxidative Addition of the Imidazolium Cation to Zerovalent Ni, Pd, and Pt: A Combined Density Functional and Experimental Study

Oxidative addition of different imidazolium cations to zerovalent group 10 metals, to afford
heterocyclic carbene complexes, has been investigated by both density functional theory (DFT) and experimental
studies. The theoretical analysis shows that addition of imidazoliums to Pt0 and Ni0 is more exothermic than
to Pd0, and Ni0 is predicted to react with a much lower barrier than either Pt0 or Pd0. Strongly basic supporting
ligands on the metal, as well as cis-chelating ligands, increase the exothermicity of the reaction and also lower
the activation barrier. The addition of 2-H imidazoliums is easier and more exothermic than addition of
2-alkylimidazoliums, and a halo-imidazolium is expected to further lower the barrier to oxidative addition and
increase the exothermicity. The DFT results show that all three of the metals should be able to oxidatively add
imidazolium cations under appropriate conditions. Experimental studies confirmed that oxidative addition is
possible, and a number of Pt− and Pd−carbene complexes were prepared via oxidative addition of imidazolium
salts to M0 precursors. Most significantly, oxidative addition of 2-H azolium salts was found to readily occur,
and the reaction of 1,3-dimethylimidazolium tetrafluoroborate with Pt(PPh3)2 and Pt(PCy3)2 affords
[PtH(dmiy)(PPh3)2]BF4 (10) and [PtH(dmiy)(PCy3)2]BF4 (11), while reaction between 3,4-dimethylthiazolium
tetrafluoroborate and Pt(PCy3)2 yields [PtH(dmty)(PCy3)2]BF4 (12) (dmiy = 1,3-dimethylimidazolin-2-ylidene,
dmty = 3,4-dimethylthiazolin-2-ylidene). Addition of 2-iodo-1,3,4,5-tetramethylimidazolium tetrafluoroborate
to Pt(PPh3)4 or Pd(dcype)(dba) yields [PtI(tmiy)(PPh3)2]BF4 (9) and [PdI(tmiy)(dcype)]BF4 (14), respectively
(tmiy = 1,3,4,5-tetramethylimidazolin-2-ylidene, dcype = 1,3-bis(dicyclohexylphosphino)ethane)). X-ray crystal
structures are reported for complexes 9 and 11 (cis and trans). These studies clearly show for the first time
that oxidative addition of imidazolium and thiazolium cations is possible, and the results are discussed in
terms of the ramifications for catalysis in imidazolium-based ionic liquids with both carbene-based and non-carbene-based complexes.