Fragmentation of doubly charged metal–acetamide complexes: Second ionization energies and dissociation chemistries

The dissociation chemistries of [M(L)n]2+ (M = Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn; L = acetamide; n = 2–6) have been examined experimentally by tandem mass spectrometry and theoretically by density functional theory (DFT). At low collision energies, three primary reactions were observed: loss of acetamide to produce [M(L)n−m]2+; inter-ligand proton transfer followed by dissociation to form [M(L − H)(L)n−2]+ and protonated acetamide, [L + H]+; and amide-bond cleavage, producing [M(NH2)(L)n−1]+ and [CH3CO]+. Dissociative electron transfer from acetamide to the metal forming [M(L)n−1]+ and [L]+ was only observed for [Cu(L)2,3]2+ complexes. At higher collision energies, complexes containing deprotonated acetamide [M(L − H)(L)n−2]+ (except for [Cu(L − H)(L)n−2]+) further fragmented by eliminating small molecules (H2O, CH3CN, H2CCO, HNCO) or by acetamide loss to produce [M(L − H)]+; product ions [M(NH2)(L)n−1]+ either eliminated ammonia to produce [M(L − H)(L)n−2]+ by inter-ligand proton transfer from one NH2 group to another one, or lost acetamides to form [M(NH2)]+. Collision-induced dissociations of [M(L − H)]+ yielded three common product ions, [M(CH3)]+, [M(OH)]+, and M+ by elimination of neutral molecules, HNCO, CH3CN, and a neutral radical, (L − H), respectively. Elimination of methane from [M(L − H)]+ was only observed for M = Ca, and elimination of the methyl group occurred for M = Co, Ni, and Zn. Copper complexes exhibited different chemistries; [Cu(L − H)(L)]+ fragmented to either produce [Cu(L)]+ by elimination of (L − H), or [Cu(HNCO)(L)]+ by elimination of CH3; [Cu(HNCO)(L)]+ further fragmented to produce [Cu(L)]+ by elimination of HNCO. DFT calculations show that the gas-phase reactivities of [M(L)n]2+ complexes are closely related to the second ionization energies (IE2) of the metals. For the doubly charged [M(L)n]2+ species, as IE2 increases, fragmentations involving charge separation become more competitive: inter-ligand proton transfer becomes energetically more favorable than dissociation of a neutral ligand, and amide-bond cleavage occurs more readily. For singly charged ions [M(L − H)L]+, for metals with low IE2 values, loss of L has a considerably lower enthalpy than loss of (L − H); however, for metals with higher IE2 values, loss of (L − H), which effectively reduces the oxidation state of the metal, becomes energetically competitive with the loss of L. The enthalpies for eliminating methane and the methyl radical from [M(L − H)]+ (M = Ca, Mg, and Zn) have been calculated and correlate well with the experimental observations.