Synthesis and Characterization of Cyclometalated NHC Platinum Complexes with Chelating Carboxylate Ligands
Abstract
Platinum(II) complexes [Pt((CC)-C-*)(acac)], where (CC)-C-* = 3-R-1-(phenyl-C-2)-1H-benzimidazol-2-ylidene-C-2 (R = Me, Ph) and acac = pentane-2,4-dionate, reacted with functionalized acetic acids YCH2CO2H (Y = NMe2, SMe and PPh2) with elimination of acetylacetone to produce the bis-chelate complexes of the type [Pt((CC)-C-*)(YCH2CO2)]. 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 = NMe2 and PPh2.
These observations were well reproduced by different DFT functionals at the double- level of theory, which suggested rather small energy differences between the geometric isomers. The photophysical measurements showed that complexes [Pt((CC)-C-*)(YCH2CO2)] are emissive in the blue region ((em) 460-490 nm) with photoluminescence quantum yields up to 36% and emission lifetimes in the range of 3-5 mu s.