Electrospray ionization tandem mass spectrometric studies competitive pyridine loss from platinum(II) ethylenediamine complexes by the kinetic method

The competition between pyridine ligand loss in square planar Pt(II) comple xes has been examined using the doubly and singly charged ions of complexes consisting of platinum(ethylenediamine) coordinated to two different subst ituted pyridines, Collision induced dissociation (CID) of [Pt(en)Py1Py2](2) (where Py-1 = one of ten different substituted pyridines and Py-2 = pyrid ine) results in loss of the protonated pyridines to yield the singly charge d platinum ions [Pt(en)Py-1-H](+) and [Pt(en)Py-2-H](+). In contrast, fragm entation of [Pt(en)Py1Py2-H](+) results in neutral pyridine loss to yield t he ions [Pt(en)Py-1-H](+) and [Pt(en)Py-2-H](+). In the latter case, the co rrelation between relative losses of each pyridine compared to their gas-ph ase proton affinities is poor, A novel chloride ion abstraction reaction oc curs for the fragmentation of [Pt(en)Py1Py2](2+) when Py-1 = o-C5H4CIN and Py-2 = C5H5N, to yield the [Pt(en)(Cl)Py-2](+) and [o-C5H4N](+) pair of ion s, In order to model this process the competition between nitrogen and chlo rine binding in [Pt(NH3)(3)(o-NC5H4Cl)](2+) has been examined using density functional theory (DFT) calculations at the B3LYP/ LANL2DZ level of theory , Both adducts are minima with the N adduct being more stable than the Cl a dduct by 22.17 kcal mol(-1). Furthermore, the Cl adduct exhibits a signific ant stretching of the C-Cl bond (to 1.935 Angstrom), consistent with the ob served chloride ion abstraction reaction, which is endothermic by 9.0 kcal mol(-1) (relative to the N adduct). Copyright (C) 2001 John Wiley & Sons, L td.