Self-assembling Properties of an N-Heterocyclic Carbene-based Metallosurfactant: Pd-Coordination Induced Formation of Reactive Interfaces in Water

In this study, an N-heterocyclic carbene (NHC)-based metallosurfactant (MS), NHC-PdMS, was synthesized, where Pd(II) was bound to the NHC framework via a robust Pd carbene bond with NEt3 as a co-ligand. Surface tension measurements revealed that the critical micelle concentration (CMC) of NHC-PdMS (1.8 x 10(-4) M) was one order of magnitude lower than that of its MS precursor (imidazolium bromide). Coordination of the MS precursor and NEt3 to Pd(II) also influenced micelle size; the hydrodynamic diameters of NHC-PdMS and the MS precursor were observed to be 25.8 +/- 5.6 nm and 2.5 +/- 0.3 nm, respectively. Furthermore, small angle X-ray scattering measurements indicated that NHC-PdMS exhibited liquid crystalline behavior above 26 wt%, with a spacing ratio of 1:2:3 for the first, second, and third Bragg peaks. To understand the role of the reactive interface, NHC-PdMS was also applied to aqueous catalytic reactions. Owing to its low CMC value, a catalytic amount of NHC-PdMS (3 mol%) provided the reactive interface, which facilitated the aqueous Mizoroki-Heck reaction of various aryl iodides and styrene in good yields (72-95%). These results suggest that MS formation results in a drastic change in self-assembling properties, which are important for the development of highly reactive chemical interfaces in water.