Large Effect of a Small Substitution: Competition of Dehydration with Charge Retention and Coulomb Explosion in Gaseous [(bipy~R)Au(μ-O)_2Au(bipy~R)]~(2+) Dications

摘要

Dinuclear gold(III) clusters with a rhombic Au_2O_2 core and 2,2'-bipyridyl ligands substituted in the 6-position (bipy~R) are examined by tandem mass spectrometry. Electrospray ionization of the hexafluorophosphate salts affords the complexes [(bipy~R)Au(μ-O)_2Au(bipy~R)]~(2+) as free dications in the gas phase. The fragmentation behavior of the mass-selected dications is probed by means of collision-induced dissociation experiments which reveal an exceptionally pronounced effect of substitution. Thus, for the parent compound with R = H, i.e., [(bipy)Au(μ-O)_2Au(bipy)]~(2+), fragmentation at the dicationic stage prevails to result in a loss of neutral H_2O concomitant with an assumed rollover cyclometalation of the bipyridine ligands. In marked contrast, all complexes with alkyl substituents in the 6-position of the ligands (bipy~R with R = CH_3, CH(CH_3)_2, CH_2C(CH_3)_3, and 2,6-C_6H_3(CH_3)_2) as well as the corresponding complex with 6,6'-dimethyl-2,2'-dipyridyl as a ligand exclusively undergo Coulomb explosion to produce two monocationic fragments. It is proposed that the additional steric strain introduced to the central Au_2O_2 core by the substituents on the bipyridine ligand, in conjunction with the presence of oxidizable C-H bonds in the substituents, crucially affects the subtle balance between dication dissociation under maintenance of the 2-fold charge and Coulomb explosion into two singly charged fragments.