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Electrospray ionization study of tricarbonyl fac-[Re(CO)3 (PO)(X)]-type complexes: influence of ancillary co-ligands in the release of carbon monoxide.

RATIONALE
fac-[Re(CO)3 (PO)(X)]-type complexes (PO = chelated bidentate tertiary phosphine (1-), X = various neutral, mono-dentate ligands) represent a class of compounds that meets the synthetic criteria for the preparation of potential carbon monoxide (CO) release molecules (CORMs) for medicinal application. The aim of our investigation was to achieve qualitative information whether the nature of the ancillary X ligand might influence the release of CO.


METHODS
The release of CO has been investigated by means of product ion spectrometry of electrospray ionization (ESI)-generated [M + H]+ species, produced by multiple collisional experiments, using an ion trap mass spectrometer.


RESULTS
Tandem mass spectrometry applied to the protonated species [Re(CO)3 (PO)(X) + H]+ of seven complexes (those including X = OH2 (1), isonitrile (2, 3), imidazole (4), pyridine (5) and phosphine (6, 7)) shows initial loss of coordinated water (1) or pyridine (5), whereas the majority of investigated entries display initial, sequential release of CO groups. The energetics of CO release have been investigated by breakdown curves for selected collisionally activated decomposition processes involving CO, and compared with those involving X groups.


CONCLUSIONS
The nature of the co-ligand X drives the primary loss in the MSn processes of [Re(CO)3 (PO)(X) + H]+ compounds. When X = solvent, the energetics of these decompositions follow the trend H2 O < MeOH < CO. In each case, loss of CO is a favored fragmentation route with associated energies following the trend: N-py ≤ P-phosphine < C-isonitrile. Overall, MSn pathways indicate that [Re(PO)] (Re with chelated PO phosphine) constitutes the residual moiety. This behavior indicates that the presence of a functionalized phosphine is essential for a sequential, controlled release of CO.