Structure and stability of organometallic complexes studied by IMS and CID MS/MS

Organometallic Ru-based compounds are widely used as catalysts, treatment against cancer, for solar energy conversion and for many other applications. In this work, a series of ruthenium dicarboxylates with increasing ligand size have been chosen to study their structure and their stability.
These compounds were dissolved in acetonitrile. After the ionization we didn't see the full complex with two carboxylates, so we worked with the complex containing one carboxylate and one acetonitrile ligand.
Experiments were conducted with the Synapt G2 which mainly consists of an electrospray ionization source, a linear quadrupole to select the required mass, a mobility cell were ions of different shapes would get different drift times, a transfer cell were we can activate the collisional induced dissociation and a time of flight analyzer to analyze masses. With the introduction of ion mobility, an additional dimension can be given to the experiment by measuring the collisional cross section (CCS) of the various species. The CCS represents the shape of the complexes, which can be influenced by the nature of the ligands.
If we compare the arrival time distribution for the different complexes, we can see an increase in the arrival time when the size of the carboxylate increases. But within the limits of resolution, we can’t see any effect linked to the isomerism of the butyrate and the isobutyrate, despite a small difference of arrival time for the free acids. Arrival time must be translated into collision cross section to make a correlation with a molecular parameter.
The change of the CCS follows the expected trend based on the change of the carboxylate CCS. That means that the CCS is directly correlated with the size of the carboxylates. This would imply that the steric hindrance within the complex is low. If we compare CCS of complexes and free acids, we notice that free acids’CCS take a good part of the complexes’CCS. So we can understand that the addition of the acid to the complex does not induce a simple addition of volume.
While increasing collisional energy during the transfer, the bond of the acetonitrile is broken. The 50% survival of the precursor ion remains quasi constant, once corrected for the mass or the number of vibration modes. This would imply that the steric hindrance within the complex is low and does not affect the binding of the ligands to the central cation.