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 μ-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.