3.1 The general fragmentation patterns of diaryliodonium cations
Firstly, we studied some representative diaryliodonium salts (1 ~4 , structures were shown inScheme 2 ). These diaryliodonium salts could be easily ionized by loss of OTf to give diaryliodonium cations [M –OTf]+ in the form of [Ar1-I+-Ar2] and their ESI-MS/MS were shown in Figure 1 . According to the previous reports and our studies of these spectra,13,36,40 we proposed two major fragmentation patterns of [Ar1-I+-Ar2] cations: 1) Reductive elimination10-13,36 to give diaryl coupling product ions [Ar1-Ar2]+• by loss of an iodine atom, followed by the generation of further fragment ions from these species. Such gas-phase reductive elimination rearrangement reaction was similar to our previous finding of the process from [CF3-I+-Ph] atm/z 273 to [PhCF3]+• atm/z 146 by loss of an iodine atom in ESI-MS/MS of [CF3-I+-Ph];362) The C–I bond cleavage to produce aryl cations [Ar1]+ or [Ar2]+, which might further give [Ar1+H2O]+ or [Ar2+H2O]+ and [Ar1+N2]+ or [Ar2+N2]+ adduct ions,41 along with their respective fragment ions. This type fragmentation has also be found in the in ESI-MS/MS of [CF3-I+-Ph], in which C–I bond cleavage of [CF3-I+-Ph] atm/z 273 gave rise to [Ph]+ at m/z 77 or [PhI]+• at m/z 204 by loss of CF3.36
Taking the MS/MS of [1 –OTf]+ as an illustrative example, we observed that the reductive elimination of [Ph-I+-Ph] gave diaryl coupling product ion biphenyl+• at m/z 154 rather than I+• at m/z 127. This divergence could be attributed to the significantly higher ionization energy of iodine (10.45 eV) compared to biphenyl (8.16 eV).42Meanwhile, C–I bond cleavage of [Ph-I+-Ph] produced [Ph]+ at m/z 77, along with [Ph+H2O]+ at m/z 95 and [Ph+N2]+ at m/z 105. Notably, we did not observe the formation of [PhI]+• at m/z 204, which could be attributed to the higher ionization energy of PhI (8.72 eV) in comparison to the phenyl radical (8.32 eV).42Therefore, [Ar1-I+-Ar2] predominantly formed diaryl coupling product ions and aryl cations in ESI-MS/MS. The two principal fragmentation pathways of [Ar1-I+-Ar2] originating from diaryliodonium salts (1 ~4 ) were depicted in Scheme 3 . The structural assignments for fragment ions were supported by accurate mass measurements (see in Table S1 ~S3 ).