Synthesis, Structure and Stability of New Pt^II–Bis(N‐Heterocyclic Carbene) Complexes

Abstract

The use of metal acetates has allowed the synthesis of NHC complexes without isolation of the free carbene. This route is very useful in cases where the metal acetates are readily available and has been used, for example, for the synthesis of bis(N‐heterocyclic carbene) complexes of palladium(II). In the case of platinum, however, the corresponding acetate is not commercially available and is difficult to synthesize. Previously published syntheses require either an external base (e.g. NaOAc or KOtBu) or a transmetalation step. Here we present a new general synthetic pathway for platinum–bis(carbene) complexes using platinum(II) acetylacetonate as a commercially available metal precursor. Aromatic substituents greatly enhance the stability of the metal complexes against acidic media. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Chiral bisimidazolium salts derived from amino acids and their palladium(II)- and platinum(II)-biscarbene complexes

“…We modified the published procedure for the preparation of the imidazoles 1a-j (without previous esterification of the L-amino acid) by substituting HCl/MeOH by SOCl 2 /ROH, which is more convenient and turned out to be superior to the one-pot imidazole synthesis [12,53] where glyoxal and paraformaldehyde react with the amino acid ester hydrochloride in methanol solution.…”

“…The additional stability of bidentate ligands and their palladium(II)-and platinum(II)-biscarbene complexes was described before [17,[52][53][54][55][56][57][58][59][60][61][62] and we therefore aimed at combining both concepts by using methylene bridged bisimidazolium salts derived from natural L-amino acids as precursors for the metal complex synthesis of palladium(II)-and platinum(II)-biscarbene complexes.…”

The Role of NHC Ligands in Oxidation Catalysis

“…The advantage is that it allows one to deprotonate the imidazolium salts in the presence of groups which are labile towards stronger bases. We found Pd(OAc) 2 [54][55][56] and Pt(acac) 2 [57] especially useful in that regard. In cases where no basic metal precursor is available it is often successful to add NaOAc as an external base [58,59].…”

“…We recently succeeded in synthesizing platinum-NHC complexes which are stable under the reaction conditions [57]. Introduction of aromatic substituents (Scheme 24, R= NO 2 , Cl, Br, OCH 3 , COOEt) in the imidazole changed the properties of the complexes quite significantly.…”

“…As soon as the carbene complex is formed it shows extraordinary stability, even against strong acids and oxidizing agents like peroxodisulfate [55][56][57]59]. Therefore, these complexes can be used in catalytic processes which have not been possible before with the phosphine ligands, e.g.…”