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

Abstract

[Ru 3 O(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 [Ru 3 O(CH 3 COO) 6 ] + core bonded by a μ-oxo and six carboxylate bridges. Several 1(PF 6 ) 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 [Ru 3 O(CH 3 COO) 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.