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

Abstract

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 Pt(0) and Ni(0) is more exothermic than to Pd(0), and Ni(0) is predicted to react with a much lower barrier than either Pt(0) or Pd(0). 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 M(0) 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(PPh(3))(2) and Pt(PCy(3))(2) affords [PtH(dmiy)(PPh(3))(2)]BF(4) (10) and [PtH(dmiy)(PCy(3))(2)]BF(4) (11), while reaction between 3,4-dimethylthiazolium tetrafluoroborate and Pt(PCy(3))(2) yields [PtH(dmty)(PCy(3))(2)]BF(4) (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(PPh(3))(4) or Pd(dcype)(dba) yields [PtI(tmiy)(PPh(3))(2)]BF(4) (9) and [PdI(tmiy)(dcype)]BF(4) (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.

Platinum(ii) N-heterocyclic carbene complexes: coordination and cyclometallation.

“…Other interesting methods include the utilisation of an electron-reservoir complex, together with air, to generate NHCs, 9 or the use of imidazolium-2-carboxylates which lose CO 2 to generate NHCs, 10 though, increasingly, direct reaction of the imidazolium salt with the metal has found favour. [11][12][13][14] More recently, NHC ligands have been shown not to be innocent spectators, but reactive intermediates themselves, with a number of new products being formed. 15,16 Whilst most the studies on catalytically active systems have their emphasis on palladium based chemistry, 17 platinum complexes are also of interest.…”

Ionic Liquids, 3. Synthesis and Utilisation of Protic Imidazolium Salts in Homogeneous Catalysis

“…[14] Moreover, 13 C NMR monitoring of the reactions mixtures indicates no evidence for the formation of N-heterocarbene complexes via oxidative addition of the metal centre at C-2. [15,16] Nevertheless, the beneficial use of this proton reservoir cannot be generalised: for example, the use of [Rh(C 5 Me 5 )(C 2 H 4 ) 2 ], which is reported to be very efficient upon protonation for this reaction, [17] provides, in comparison, much lower results (Table 1). An explanation for these poorer results may be a strong coordination of the imidazole to the rhodium centre, which is partly overcome by higher concentration of acid.…”

Thiazolium and Benzothiazolium Ionic Liquids

“…The thiazoles are known in chemistry mainly due to their presence in thiamine (Vitamin B 1 ) in the form of substituted thiazolium salt (Mcguinness et al 2001). Thiazolium salts can be obtained successfully by a modification of the Hanztsch's thiazole synthsis.…”