Synthesis and Characterization of Cyclometalated NHC Platinum Complexes with Chelating Carboxylate Ligands

Abstract

Platinum(II) complexes [Pt(C ^ C*)(acac)], where C ^ C* = 3‐R‐1‐(phenyl‐κ C 2 )‐1 H ‐benzimidazol‐2‐ylidene‐κ C 2 (R = Me, Ph) and acac = pentane‐2,4‐dionate, reacted with functionalized acetic acids YCH 2 CO 2 H (Y = NMe 2 , SMe and PPh 2 ) with elimination of acetylacetone to produce the bis‐chelate complexes of the type [Pt(C ^ C*)(YCH 2 CO 2 )]. Whereas the Pt(II) precursor with the less bulky methyl substituent (R) gave rise to single products having trans ‐C(NHC),Y geometry, its phenyl congener reacted less selectively, affording exclusively the product with trans ‐S,C(Ph) configuration for Y = SMe and mixtures of cis and trans isomers for Y = NMe 2 and PPh 2 . These observations were well reproduced by different DFT functionals at the double‐ξ level of theory, which suggested rather small energy differences between the geometric isomers. The photophysical measurements showed that complexes [Pt(C ^ C*)(YCH 2 CO 2 )] are emissive in the blue region ( λ em 460–490 nm) with photoluminescence quantum yields up to 36 % and emission lifetimes in the range of 3–5 µs.