Characterization of supported and unsupported transition metal compounds by liquid secondary ion mass spectrometry

The positive ion liquid secondary ion mass spectra (LSIMS) of metal acetylacetonates (acacs) contain cluster ions that involve multiple metal centers. Several metal acacs form adduct ions with m-nitrobenzyl (m-NBA) alcohol used as the LSIMS support matrix. In these adduct ions, deprotonated m-NBA molecules act as bridges between metal centers. The LSIMS of a series of dirhodium carboxylates show that the electron-withdrawing strength of the carboxylate group controls whether the base peak in the mass spectrum is the molecular ion or an adduct ion formed between the molecule and a matrix molecule of sulfolane. However, no cluster ions are formed for these dirhodium compounds. Similarly, highly charged cationic compounds do not form cluster ions, nor do they form adduct ions with the LSIMS solvent. Instead, the counter ion is retained to maintain a single positive charge in the ion. LSIMS spectra of neutral and charged metallocene compounds are compared. Contrary to the assertions that these compounds cluster extensively in the gas phase, the LSIMS provide no evidence for this process. To support studies of surfaces prepared for catalytic studies, silica- and clay-supported transition metal acacs were characterized by LSIMS. Release of the supported transition metal compounds from the surface occurs during ion exchange.