American Chemical Society
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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

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journal contribution
posted on 2006-06-19, 00:00 authored by Marcos N. Eberlin, Daniela M. Tomazela, Koiti Araki, Anamaria D. P. Alexiou, André L. B. Formiga, Henrique E. Toma, Sofia Nikolaou
[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.