Mechanistic Study on the Gas-Phase Generation of Rollover-Cyclometalated [M(bipy - H)]+ (M = Ni, Pd, Pt)

Rollover-cyclometalated [Pt(bipy - H)](+) (bipy = 2,2'-bipyridine) can be easily generated in the gas phase via HX elimination brought about by collision-induced dissociation (CID) of the cationic complexes [Pt(X)(bipy)](+) with X = CH3, Cl; the latter as well as other [M(X)(bipy)](+) complexes (M = Ni, Pd; X = CH3, F, Cl, Br, I, OAc) are accessible by electrospray ionization mass spectrometry. For the nickel and palladium methyl complexes [M(CH3)(bipy)](+), upon CID, no cyclometalation occurs; rather, homolytic cleavage of the M-CH3 bond takes place. The related chloro complexes [M(Cl)(bipy)](+) (M = Ni, Pd) undergo competitive eliminations of HCl and Cl upon CID, and the branching ratios depend strongly on the collision energy. On the basis of DFT calculations, this metal- and ligand-controlled behavior is a consequence of the rather different energetic requirements for the direct loss of X versus elimination of HX (X = CH3, Cl). Deuterium-labeling experiments reveal that formation of CH4 and MCl is only for the platinum complexes due to a genuine rollover cyclometalation process, i.e., selective abstraction of a hydrogen atom from the C(3)-position of bipy. In the series of halo-substituted complexes [Ni(X)(bipy)]+ (X = F, Cl, Br, I), the Ni-X bond strength decreases in the sequence F > Cl > Br > I. For X = F, one observes the elimination of HF, which benefits from the particular stability of this molecule; the hydrogen atom in HF is mostly (>90%) abstracted from position C(6) with <10% originating from C(3) of the bipy ligand; thus, for this system rollover cyclometalation occurs only to a small amount. [Ni(Cl)(bipy)](+) undergoes competitive losses of HCl and Cl upon collision with Xe, and for X = Br and I only homolytic Ni-X bond cleavage takes place. In the elimination of HCl from [Ni(Cl)(bipy)](+), >60% of the hydrogen atoms originate from C(3), and the remaining from C(4,5,6), as inferred from deuterium-labeling experiments. The acetate complexes [Ni(OAc)(bipy)](+) and[Pd(OAc)(bipy)](+) exhibit eliminations of neutral AcO center dot, and at elevated collision energies, decarboxylation occurs. C-H bond activation resulting in the formation of HOAc is absent at the detection limit.