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

Abstract

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(NH 2)(L) n−1 ] + and [CH 3CO] +. 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 (H 2O, CH 3CN, H 2C C O, HNCO) or by acetamide loss to produce [M(L − H)] +; product ions [M(NH 2)(L) n−1 ] + either eliminated ammonia to produce [M(L − H)(L) n−2 ] + by inter-ligand proton transfer from one NH 2 group to another one, or lost acetamides to form [M(NH 2)] +. Collision-induced dissociations of [M(L − H)] + yielded three common product ions, [M(CH 3)] +, [M(OH)] +, and M + by elimination of neutral molecules, HNCO, CH 3CN, 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 CH 3 ; [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.