Thesis (Ph. D.)--University of Hong Kong, 1993.

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

This dissertation concerns the discovery of the ability of the guanidinate ligand hpp (hpp = anion of 1,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidine) to stabilize high oxidation states of dimetal units (particularly dimolybdenum species) and attempts to control solubility and redox potential through modification of the ligand. Two general strategies were used for the ligand modifications: alteration of the ring size, and addition of alkyl substituents. All of the dimetal complexes using these ligands show a significant shift in redox potential compared to other commonly used classes of ligands (i.e. carboxylates and formamidinates) allowing access to the Mo2 4+, Mo2 5+ and Mo2 6+ oxidation states. The solubility of the complexes increases with increasing ring size, or with increasing length of the alkyl substituent. The physical and chemical properties of the ligands and their dimolybdenum complexes are described in detail.

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Principles involving the electronic structure of group VI metal-metal multiple bonded complexes are examined in order to provide insights into the intramolecular and intermolecular interactions of these systems. Examination of chromium, molybdenum and tungsten tetracarboxylate thin films by ultraviolet photoelectron spectroscopy has provided the first experimental evidence for the location of the σ ionization in dimolybdenum tetracarboxylate quadruple bonded complexes. These compounds have significant intermolecular interactions as thin films which destabilizes ionization of the valence σ state, allowing it to be experimentally observed. This is supported by the observed destabilization of the σ ionization feature of the ditungsten analogue in going to the thin film. The Cr₂(O₂CCH₃)₄ comparison shows a destabilization of the leading predominantly metal ionizations consistent with the broad range of M-M bond lengths observed for Cr₂ complexes. The Group VI M₂(mhp)₄ and M₂(chp)₄ [mhp=6-methyl-2-oxo-pyridine and chp=6-chloro-2-oxo-pyridine] complexes are also examined. The geometric constraints imposed on the compounds by the ligand effectively block intermolecular interaction axial to the metal-metal bond in the solid state. Comparison of the two ligand spectra, Hmhp and Hchp, has provided a unique opportunity to assign ionizations previously attributed to the keto form of the Hmhp tautomers. The spectra of the complexes exhibit minimal relative shifting of ionization features in going to the thin films because of this constraint. A band previously believed to be due to spin orbit coupling is assigned to the σ ionization in the ditungsten complex. The overall ionization band profiles of the two series correlate well, metal by metal, with the expected shifting due to substitution of the more electronegative chlorine atoms for a methyl group. The synthesis and characterization of Mo₂(N-t-butyl-acetamide)₄, the first tetraamidodimetal compound without large rings with delocalized pi structure, is described. The single crystal X-ray structure is presented, revealing the novel (one of three examples) cis configuration of the MoN₂O₂ ligand set. The Mo-Mo bond length of 2.063 Å is one of the shortest seen to date. A preliminary gas phase He I valence spectrum is reported.

This dissertation describes the experimental study of the electronic-structural relationships of selected mononuclear transition-metal sulfur dioxide, cyclopentadienyl and carbonyl complexes and the application of the information gained from these to the study of quadruply-bonded dimetallic complexes. These pertinent observations result from the application of photoelectron spectroscopy (p.e.s.) as a probe into the bonding, charge-distribution and excited state effects which contribute to the specifics of the ground and excited state molecular structures. The first part of this discussion centers around a specific study of the exemplary bonding probe, SO₂, with the well characterized ArM(CO)₂ metal fragment, where Ar = Bz and Cp and M = Cr, Mn and Re. A comparison of the ionization information with the structural details and molecular orbital calculations reveals not only the surprising coordinating similarity of SO₂ and CO in these complexes, but also the electronic origin for the counter-intuitive SO₂ bonding configuration. This work then moves to a more dramatic example of electronic control of ground state molecular structure; the crystallographically determined distortion of the coordinated Cp ring in Cp*Rh(CO)₂. The electronic origin of this distortion is graphically shown with the aid of two and three dimensional experimental and theoretical electron density maps. The structural effects of removing bonding electrons from quadruply-bonded dimetallic complexes is then investigated. This study incorporates the use of high-resolution p.e.s. for the novel observations of metal-metal vibrational structure in the predominantly metal ionizations providing direct information of the bonding influence of specific metal electrons. Particular attention is focused on the delta-ionization process of MO₂(O₂CCH₃)₄. The final chapter presents a comprehensive study of the valence and core ionizations of the series of quadruply-bonded M₂(X₂CR)₄ complexes, where M₂ = Cr₂, Mo₂, MoW, and W₂, X = O and S, and R = H, CH₃, CD₃, CF₃, CH₂CH₃, CH₂CH₂CH₃ and C(CH₃)₃. The changes in the electronic structure in both the ground and excited states of these molecules is presented and, where appropriate, compared to structural changes. The study of this series not only demonstrates how p.e.s. can be used to monitor the electronic effects of specific chemical modifications, but also reveals surprising excited state features related to facile charge-reorganization processes.

The molecular structures of [κ³-S₂H-Tmᴮᵘᵗ]Na(THF)₃ and [κ³-S₂H-Tmᴬᵈ]Na(THF)₃ have been obtained, which is significant as these are the first two examples of monomeric κ³-S₂H coordinate sodium compounds to be reported. Based on an extensive structural analysis of all of the [Tmᴿ]M compounds listed in the Cambridge Structural Database, a set of criteria has been generated that can be used to classify [Tmᴿ] ligands according to their coordination modes. Compounds exhibiting κ³-S₃ coordination are found to be the most prevalent, as are compounds exhibiting 0:3 conformation modes. A series of [Tmᴮᵘᵗ]CdO₂CR complexes (R = C₆H₄-4-Me; C₆H₄-4-F; C₆H₃-3,5-F₂; C₆H₃-2,6-F₂; C₃H₆Ph; 9-An; and tridecyl) has been prepared via the reaction of [Tmᴮᵘᵗ]CdMe with the corresponding carboxylic acids. [Tmᴮᵘᵗ]ZnO₂CR (R = C₆H₄-4-Me; C₆H₄-4-F; C₆H₃-3,5-F₂; C₆H₃-2,6-F₂; 9-An) have been prepared by an analogous method. In addition, two thiobenzoate complexes, [Tmᴮᵘᵗ]MSC(O)Ph (M = Zn, Cd), have been obtained via the treatment of [Tmᴮᵘᵗ]MR (R = Me) with thiobenzoic acid. An extensive structural analysis of the [Tmᴮᵘᵗ]MO₂CR and [Tmᴮᵘᵗ]MSC(O)Ph complexes has been provided, based on single crystal X-ray diffraction and NMR spectroscopy. In addition, degenerate benzoate exchange between [Tmᴮᵘᵗ]MO₂C(4-C₆H₄-F) and 4-fluorobenzoic acid has been investigated by ¹⁹F-NMR lineshape analysis over a large temperature range (195-262 K). The acid concentration dependence of the rate for the exchange process supports an associative exchange mechanism. [Tmᴮᵘᵗ]MO₂C(4-C₆H₄-F) benzoate exchange is extremely rapid on the 19F NMR timescale at 25˚, and has been observed to be faster for [Tmᴮᵘᵗ]CdO₂C(4-C₆H₄-F) than for [Tmᴮᵘᵗ]ZnO₂C(4-C₆H₄-F). The reactivity of [Tmᴮᵘᵗ]CdS(C₆H₄-4-F) towards different thiols, ArSH (Ar = C₆H₄-4-F, C₆H₄-4-Buᵗ, C₆H₄-4-OMe, C₆H₄-3-OMe), has been investigated using various NMR techniques. In contrast to the results of our degenerate benzoate exchange studies, thiolate exchange between [Tmᴮᵘᵗ]CdS(C₆H₄-4-F) and ArSH is slow on the ¹H NMR timescale. Even at elevated temperatures, the NMR signals for the reaction species remain resolved with minimal linebroadening. The equilibrium constants for the reactions of [Tmᴮᵘᵗ]CdS(C₆H₄-4-F) with ArSH (Ar = C₆H₄-4-But, C₆H₆-4-OMe, C₆H₄-3-OMe) have been calculated and determined to be indistinguishable, with equilibrium favoring the [Tmᴮᵘᵗ]CdS(C₆H₄-4-F) and ArSH species. Additionally, the reactivity of [Tmᴮᵘᵗ]CdS(C₆H₄-4-F) toward phenols, ArOH (Ar = Ph, 2,6-diphenylphenol), has been investigated. While [Tmᴮᵘᵗ]CdS(C₆H₄-4-F) has been found to be unreactive toward ArOH, [Tmᴮᵘᵗ]CdSOAr (Ar = 2,6-diphenylphenol) reacts immediately (C₆H₄-4-F)SH, resulting in complete conversion to [Tmᴮᵘᵗ]CdS(C₆H₄-4-F). Two monomeric [Tmᴮᵘᵗ]CdE(2-C₅H₄N) complexes (E = S, Se) have also been prepared, and structurally characterized. A monomeric, terminal zinc hydride complex, [Tmᴮᵘᵗ]ZnH, has been prepared via the reaction of [Tmᴮᵘᵗ]ZnOPh with phenylsilane. The molecular structure of [Tmᴮᵘᵗ]ZnH has been obtained by single crystal X-ray diffraction techniques, and the reactivity of [Tmᴮᵘᵗ]ZnH towards various reagents has been investigated. [Tmᴮᵘᵗ]ZnH reacts rapidly with ArEH (EAr = OPh, S(C₆H₄-4-F), SePh) to form [Tmᴮᵘᵗ]ZnEA via H₂ elimination. [Tmᴮᵘᵗ]ZnH reacts with CO₂ to form [Tmᴮᵘᵗ]ZnO₂CH via CO₂ insertion into the Zn-H bond. [Tmv]ZnO₂CH can also be prepared by the reaction of [Tmᴮᵘᵗ]ZnH with formic acid. [Tmᴮᵘᵗ]ZnH reacts rapidly with ZnEt₂ to form [Tmᴮᵘᵗ]ZnEt. The reaction of [Tmᴮᵘᵗ]ZnH with CpMo(CO)₃H resulted in the formation of a metal-metal bonded complex, namely [Tmᴮᵘᵗ]Zn-MoCp(CO)₃. A series of [Tmᴮᵘᵗ]M-M'Cp(CO)₃ heterobimetallic complexes (M = Zn, Cd; M' = Cr, Mo, W) has been prepared via the reaction of [Tmᴮᵘᵗ]MR (R = Me) with CpM'(CO)₃H. An extensive structural analysis of these complexes is provided, based on X-ray diffraction and NMR spectroscopy. Each of these complexes features a direct M-M' bond, which is supported by two partially bridging carbonyl ligands. Only a few complexes featuring an M-M' bond have been structurally characterized, and the molecular structures of [Tmᴮᵘᵗ]Zn-CrCp(CO)₃ and [Tmᴮᵘᵗ]Cd-WCp(CO)₃ represent the first two structures reported for compounds featuring either a Zn-Cr or Cd-W bond. The coordination chemistry of [tpyᴬʳ] (Ar = p-tolyl, mesityl) and [bppᴮᵘᵗ] with various main group and transition metals has been investigated. [tpyᴬʳ]MX₂ complexes (M = Co, Zn; X = Cl, I) are prepared by the reaction of [tpyᴬʳ] with the dihalide MX2 complex. [bppᴮᵘᵗ]MX2 (M = Fe, Co, Zn, Cd; X = Cl, I) complexes are prepared by an analogous method. Each of these [tpyᴬʳ]MX₂ and [bppᴮᵘᵗ]MX₂ complexes have been characterized by single crystal X-ray diffraction. The [bppᴮᵘᵗ]LiI compound was unexpectedly obtained from the reaction of [bppᴮᵘᵗ]FeCl₂ with MeLi, which is significant as it is the first example of an alkali metal complex featuring a [bppᴮᵘᵗ] ligand that has been structurally characterized.

Lin-Ping Zhang. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2003. / Includes bibliographical references (p. 125-138). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.

This dissertation concerns the use of bicyclic guanidinate ligands to prepare new dimetal paddlewheel compounds. Specifically, Ru_2^6+, Re_2^6+, Re_2^7+, and Os_2^7+ compounds will be examined to observe any changes brought about by using bicyclic guanidinate ligands with varying ring sizes. In the Ru_2^6+ compounds, different ligand ring sizes cause a change in the electronic configuration and magnetic properties. Bicyclic guanidinate ligands allow the preparation of Re_2^7+ compounds from Re_2^6+ compounds, both of which are examined structurally and electrochemically. [Os2(hpp)4]^+ is examined to improve upon earlier studies, yielding a model of the g-tensor components with respect to the compound structure. An additional project included in the dissertation involves the study of an asymmetric trinickel extended metal atom chain. The structural effects of the asymmetry are examined to help elucidate the magnetic behavior that differs significantly from symmetric trinickel extended metal atom chains.

The electron density function of molecular systems supplies a package of information. Quantum mechanical methods of producing and analyzing this function have been significantly improved during the past few years. The advent of accurate pseudopotentials and corresponding basis sets for Kohn-Sham density functional and for post-Hartree-Fock electron-correlated approaches have enabled the inclusion of scalar relativistic and spin-orbit coupling effects as well as electron correlation effects into the electron density function. The unpacking of the information embedded in such a function via the quantum theory of atoms in molecules (QTAIM) became possible by utilizing the very new subshell fitting method of reconstructing the density distribution of core electrons that had been replaced by the pseudopotentials. These theoretical advances were applied in this thesis to characterize and explore the topological features of metal-metal bonding as one of the fundamental types of bonds formed between two elements. Group 11 and 12 transition metals which include gold and mercury as the most relativistic elements were the main focus of this work. Mono and poly-nuclear compounds (with up to 4 metal atoms) in both pure metal clusters and chloro-complexes were studied by ab initio MØller-Plesset perturbation calculations followed by QTAIM analysis on the relaxed density. Some of these chloro-complexes of copper, gold, zinc and cadmium metals were identified in the gas phase by mass spectrometric experiments. The general formulas of the set of molecules studied in group 11 were : M2, MCl, MCl+, MCl2, MCl2+, M2Cl+, M2Cl2^(s+), M2Cl3+, M3Cl2+, M3Cl3+, M3Cl5+, M4Cl5+ and M4Cl7+ and in group 12 were : M2, MCl, MCl+, MCl2, M2Cl3+, M3Cl5+, M4Cl7+ and M2^(s+). The topological features of metal-metal bonding were calculated along with atomic properties for each individual local minimum isomer found. The comparison of the metal-metal bonding within the complexes and with the dimers revealed new features of metal-metal bonding in 3d, 4d and 5d transition metal elements of groups 11 and 12. With the aid of strong correlation between bond dissociation energy and electron density at the location of the bond critical points found in the case of dimers, the strength of the metal-metal bonding in the complexes was estimated. The electron density’s basin properties calculated accurately for all the clusters and their isomers in this thesis provided more insight also into the nature of M-Cl bondings in the group 11 and 12 chloride clusters. Ultimately the bonding information was used to predict the viability of these clusters in the gas phase. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy