Mass spectrometric detection of ion pairs containing rigid copper clusters and weakly coordinating counter ions using liquid injection field desorption/ionisation.

A comparative mass spectrometric investigation using electrospray ionisation (ESI) and liquid injection field desorption/ionisation (LIFDI) techniques is reported for the highly luminescent and cationic copper cluster [(PCP)${}_3$Cu${}_4$]${}^{+}$ (1[Formula: see text], PCP = [1,3-(Ph${}_2$P)${}_2$C${}_6$H${}_3$]${}^{-}$). Depending on the available counter ion X${}^{-}$, ion pairs consisting of the original or a modified cluster cation and the weakly coordinating counter ion can be detected by LIFDI-high-resolution-mass spectrometry in addition to the cluster cation. Notably, only large counter ions with an extremely low tendency for metal coordination give rise to the observation of ion pairs, whereas smaller ions such as BF${}_4^{-}$ do not show peaks corresponding to ion pairs in their mass spectra. In principle, two pathways were identified for the formation of positively charged ion pairs: (i) association of a generated Cu${}^{+}$ ion to the neutral ion pair [(PCP)${}_3$Cu${}_4$]X (1+X, X${}^{-}$ = BAr${}_{20}^F$, BAr${}_{24}^F$) and (ii) abstraction of an electron from the neutral ion pair [(PCP)${}_3$Cu${}_4$]X (1+X), leading to the oxidised ion pair [1+X][Formula: see text] (X${}^{-}$ = Al(OR${}^F$)${}_4$).