Fig. 4. Monitoring the etching process.

a Time-course UV-vis absorption spectra of the etching reaction of SS-1^NHC (c = 5 × 10^–5 M, 293 K) using cis-depe in dichloromethane: stage A, 1 s intervals; stage B, 5 min intervals; insets: photographs taken at the beginning of the reaction and after 70 min under ambient light, and a colour scale corresponding to the absorbance changes of the reaction. b The changes in absorbance at 345 nm and 421 nm as a function of time (corresponding to the spectra in a), suggesting that SS-2^NHC was rapidly formed. c ¹H NMR spectrum (d₆-acetone, 300 K), showing signals of SS-1^NHC (purple) and internal standard (IS, 1,3,5-trimethoxybenzene, blue-labelled). d Signals from SS-2^NHC (pink-labelled, 53% yield) measured immediately after adding cis-depe. e After 0.5 h, SS-2^NHC was formed in 98% yield. f A proposed etching mechanism with two intermediates Int1^L and Int2^L (L = NHC, TPP), with Int1^TPP and Int2^TPP detected by ESI-MS spectrometry (Suppl. Fig. 32). Colour code: Au, yellow; C, grey; L, cyan; P, orange. Source data are provided as a Source Data file.