Determination of the relative ligand‐binding strengths in heteroleptic IrIII complexes by ESI‐Q‐TOF tandem mass spectrometry

An electrospray ionization quadrupole time‐of‐flight mass spectrometer has been utilized to investigate the relative ligand‐binding strengths in a series of heteroleptic‐charged iridium(III) complexes of the general formula [(C^N)₂Ir^III(S‐tpy)](PF₆) by using variable collision energies. Collision‐induced dissociation experiments were performed in order to study the stability of the Ir^III complexes that are, for instance, suitable phosphors in light‐emitting electrochemical cells. The ratio of signal intensities belonging to the fragment and the undissociated complex depends on the collision energy applied for the tandem mass spectra (MS/MS) analysis. By defining the threshold collision energy and the point of complete complex dissociation, it is possible to estimate the relative complex stabilities depending on the nature of the coordinated ligands [i.e. type of cyclometalating ligand (C^N), substituents on the S‐shaped terpyridine (S‐tpy)]. The collision energy values differed as a function of the coordination sphere of the Ir^III centers. Copyright © 2011 John Wiley & Sons, Ltd.