Experimental And Theoretical Studies Of Binding Interactions In Divalent Transition Metal Cation-N-Donor Ligand Complexes: Structures, Sequential Bond Dissociation Energies, Mechanisms And Energetics Of Collision-Induced Dissociation

摘要

The thesis research described here involves a series of experiments that have been designed to probe the influence of the electronic structure of the metal cation, the nature and number of ligands, as well as the effects of chelation and steric interactions on the geometry and binding strength of transition metal cation-ligand complexes. The experimental studies make use of energy-resolved collision-induced dissociation (CID) techniques that are carried out in a custom-built guided ion beam tandem mass spectrometer (GIBMS) to probe the structures, energetics, and fragmentation behavior of the complexes of interest. Electronic structure theory calculations including several density functional theory methods are employed to determine stable low-energy structures of the M2+(N L)x complexes and the relevant species associated with their CID behavior. The five late first-row transition metal cations in their 2+ oxidation states, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+, are included in this work. The N-donor ligands (N-L) investigated here include pyridine (Pyr), a monodentate ligand, and two pyridine based bidentate ligands, 2,2-bipyridine (Bpy), and 1,10-phenanthroline (Phen). The structures and energetics of these complexes are investigated theoretically, while the CID behavior is investigated experimentally.In Chapters 3 and 4, we found that the dominant dissociation pathway for all M2+(Phen)3 and M2+(Bpy)3 complexes is loss of an intact Phen and Bpy ligand, respectively. In both cases, the BDEs computed using the M06 theory are found to be the largest, BHandHYP values are intermediate, whereas B3LYP produced the smallest values. Very good agreement between the B3LYP theoretically calculated and TCID experimentally determined BDEs was found for both M2+(Phen)3 and M2+(Bpy)3 complexes, suggesting that the B3LYP functional is capable of accurately describing the binding in these complexes. The sequential BDEs of M2+(Phen)x and M2+(Bpy)x complexes are observed to decrease monotonically with increasing ligation for all five metal cations regardless of which theory is employed. The sd hybridization of the M2+ cation plays a major role in enhancing the binding energy of the first Phen and Bpy ligand. The decline in effective charge retained by M2+ cation upon binding of Phen and Bpy ligand (s), Pauli repulsion between the valence electrons of the metal cation and those donated by Phen and Bpy ligands, and ligand-ligand repulsive interactions with each successive ligand bound also contribute to the fall off in the strength of binding with increasing ligation. Periodic trends indicate that the binding in all M2+(Phen)x and M2+(Bpy)x complexes is dominated by the electronic structure of the metal cation and to a lesser extent by the nature of the ligand. For both Phen and Bpy complexes, the charge of the metal cation is found to be the dominant contributing factor to the differences in the strength of binding between M2+ and M+ complexes, however the differences in the strength of binding are much smaller for cations of the same charge. Comparisons between the Phen and Bpy complexes suggest that the flexibility of the Bpy ligand plays a significant role in enhancing its binding interactions with the M2+ cations.Chapter 5 examines the ground-state structures and sequential binding energies of the M2+(Pyr)x complexes, x = 1f{6 by density functional theory methods. Structures of the Ca2+(Pyr)x complexes are compared to those of the M2+(Pyr)x complexes to Fe2+, Co2+, Ni2+, Cu2+, and Zn2+ to further assess the effects of the d-orbital occupation of the preferred binding geometries. The B3LYP, BHandHLYP, and M06 levels of theory yield very similar geometries for the analogous M2+(Pyr)x complexes. The overall trends in the sequential BDEs for all five metal cations at all three levels of theory examined are highly parallel, and are determined by a balance of the effects of the valence electronic configuration and hybridization of the metal cation, but are also influenced by ligand-ligand repulsive interactions. Present results for the M2+(Pyr)x complexes are compared to the analogous complexes to the late first-row monovalent transition metal cations, Co+, Ni+, Cu+, and Zn+ previously investigated to assess the effect of the charge/oxidation state on the structures and sequential binding energies. Trends in the sequential binding energies of the M2+(Pyr)x complexes are also compared to the analogous M2+(water)x, M2+(imidazole)x, M2+(Bpy)x, and M2+(Phen)x complexes.Preliminary studies covered in Appendices D and E are inter-related and describe the results of mapping the mechanisms and energetics of fragmentation pathways of M2+(Phen)2 and M2+(Bpy)2 complexes, respectively. Four types of reaction pathways are observed in competition in all the M2+(Phen)2 and M2+(Bpy)2 complexes including ETCF, PTCF, simple CID, and dehydrogenation. For all the M2+(Phen)2 and M2+(Bpy)2 complexes, severe overlap of the products separated by 1 Da originating from the ETCF and PTCF pathways is observed because the experiments were performed under low-resolution conditions. Preliminary data analysis of the cross sections is performed for the ETCF and simple CID pathways, without consideration of the PTCF pathway for all the M2+(Phen)2 and M2+(Bpy)2 complexes. As a result, the activation energies and bond dissociation energies extracted are only approximate. To extract accurate thermochemistry for the ETCF, PTCF, and simple CID pathways of both the M2+(Phen)2 and M2+(Bpy)2 complexes, experimental studies under high-resolution conditions are needed. Only one mechanism is investigated for ETCF and PTCF activated dissociation of each M2+(Phen)2 and M2+(Bpy)2 complexes. Investigation into other plausible mechanisms involved in the PTCF activated dissociation is needed because the current PES is likely not be the lowest energy PTCF pathway. Because of the low-resolution and incomplete experimental data, the strengths and limitations of the theoretical methods employed cannot be evaluated. Therefore, further experimental and theoretical studies of M2+(Phen)2 and M2+(Bpy)2 systems are needed to enable appropriate interpretation of the experimental data and accurate thermochemistry to be extracted.