Desulfination versus decarboxylation as a means of generating three- and five-coordinate organopalladium complexes [(phen)nPd(C6H5)]+ (n = 1 and 2) to study their fundamental bimolecular reactivity

Routes to the formation of the 1,10-phenanthroline (phen) ligated organopalladium complexes [(phen)Pd(C₆H₅)]⁺ and [(phen)₂Pd(C₆H₅)]⁺ via thermal extrusion of CO₂ or SO₂ from mono-nuclear, mono-carboxylate or sulfinate complexes [(phen)_nPd(O₂XC₆H₅)]⁺ (X = C or S; n = 1 and 2) are examined using a combination of low energy collision induced dissociation experiments in an ion trap mass spectrometer and DFT calculations. [(phen)Pd(C₆H₅)]⁺ is formed from both [(phen)Pd(O₂CC₆H₅)]⁺ and [(phen)Pd(O₂SC₆H₅)]⁺, but only [(phen)₂Pd(O₂SC₆H₅)]⁺ fragments to form [(phen)₂Pd(C₆H₅)]⁺. In contrast, [(phen)₂Pd(O₂CC₆H₅)]⁺ fragments via loss of a phen ligand to form [(phen)Pd(O₂CC₆H₅)]⁺. The experimental results are consistent with DFT calculations, which show that the barriers associated with the desulfination reactions are lower than those for the decarboxylation reactions. Of the organopalladium cations [(phen)Pd(C₆H₅)]⁺ and [(phen)₂Pd(C₆H₅)]⁺, only the three-coordinate complex reacts with pyridine via a ligand coordination reaction to yield [(phen)Pd(C₆H₅) (NC₅H₅)]⁺ and with formic acid via an acid-base reaction to form [(phen)Pd(O₂CH)]⁺. DFT calculations highlight that the former reaction energy is -48 kcal/mol while the later reaction proceeds via a favourable six-centered transition structure.