Electrospray ionization tandem mass spectrometry of polymetallic μ-oxo- and carboxylate-bridged [Ru3O(CH3COO)6(Py)2(L)]+ complexes:: Intrinsic ligand (L) affinities with direct access to steric effects

[Ru3O(CH3COO)(6)(py)(2)(L)](+) (1(+)) are polymetallic singly charged cationic complexes with a unique structural arrangement in which three neutral ligands are bound via ruthenium atoms to a highly delocalized symmetrical triangular tridentate [Ru3O(CH3COO)(6)]+ core bonded by a mu-oxo and six carboxylate bridges. Several 1(PF6) complexes with various terminal ligands (L) and two pyridines (py) used as reference ligands were synthesized. These complexes were directly transferred from a methanol solution to the gas phase and characterized by electrospray ionization mass spectrometry (ESI-MS) and subsequently dissociated by gentle collisions with argon molecules via ESI-MS/MS. The applicability of Cooks' kinetic method (CKM) to rank the binding affinities of a set of L to the [Ru3O(CH3COO)(6)](+) core was demonstrated by the good linear correlation (R=0.98) observed in a CKM plot for which the relative peak intensities of the two fragment ions arising from the competitive loss of py or L as well as L affinities predicted by PM3(tm) calculations were used. Steric effects greatly affect L affinities, as evidenced by the values measured for ortho-substituted pyridines. The gaseous 1(+) were found to display a relatively high effective temperature of 1258 K. We have therefore extended CKM to investigate metal-ligand interactions, showing its usefulness to order and measure intrinsic (no solvent or counterion effects) ligand affinities to large and structurally intricate polymetallic complexes.