The Chemistry of Three-coordinate Bis(trimethylsilyl)amido Complexes of the First-Row Transition Metals And Designing a Solution-Stable Distannene: The Decisive Role of London Dispersion Effects
Skip to main content
Open Access Publications from the University of California

UC Davis

UC Davis Electronic Theses and Dissertations bannerUC Davis

The Chemistry of Three-coordinate Bis(trimethylsilyl)amido Complexes of the First-Row Transition Metals And Designing a Solution-Stable Distannene: The Decisive Role of London Dispersion Effects


Abstract. A series of studies of the chemistry of first-row transition metal M{N(SiMe3)2}3 complexes is described herein. Both divalent M{N(SiMe3)2}2 and trivalent M{N(SiMe3)2}3 transition metal complexes have been described in the literature since the early 1960s. However, while the divalent complexes have been the subject of a great deal of study, comparatively little attention has been paid to the trivalent complexes since their initial discovery. The reason for this disparity appears to be the notion that the steric crowding in these molecules results in their low reactivity. Although work in this laboratory has periodically examined these complexes, our most recent interest stemmed from the observation that the detailed molecular structure of the vanadium complex V{N(SiMe3)2}3 had been absent from the literature as recently as 2019, despite the complex having been reported in 1971. By isolating this complex as violet-colored crystals, Wagner and coworkers determined that the original report of ‘brown’ V{N(SiMe3)2}3 had probably mischaracterized this material. This observation encouraged us to reexamine the chemistry of these classic metal amido complexes, which revealed a vibrant chemistry that had not been described until now.

Chapter 2 describes the synthesis and characterization of a series of M{N(SiMe3)2}3L2 complexes (L = nitrile or isocyanide ligand) of titanium and vanadium. Although it had been initially thought that the considerable steric crowding in M{N(SiMe3)2}3 complexes precluded the formation of Lewis base complexes, it is shown that the formation of such donor complexes is possible by judicious selection of the donor molecule (isocyanide or nitrile bases). A spectroscopic study of these complexes revealed that the donors are only weakly bound in solution, with stronger binding in the case of the vanadium complexes. Furthermore, we observed no complex formation for M = Cr, Mn, Fe, or Co, a property that we attribute to the absence of empty, low-energy orbitals available in the later metals to accept electrons from donor molecules.

In Chapter 3, we exploit the observations made in Chapter 2 to show that V{N(SiMe3)2}3 can be oxidized by reagents similar in shape to the Lewis bases used to form M{N(SiMe3)2}3L2 complexes. Thus, the reaction of V{N(SiMe3)2}3 with iodosylbenzene (PhIO) or trimethylsilylazide (Me3SiN3) afforded the vanadium(V) complexes V(=O){N(SiMe3)2}3 or V(=NSiMe¬3){N(SiMe¬3)2}3, respectively, which were studied structurally and spectroscopically. Despite several reports of its attempted synthesis, V(=O){N(SiMe3)2}3 had not been isolated until now. It had been suggested that the V=O moiety reacts with a ligand trimethylsilyl group to give the V(OSiMe3)(=NSiMe3){N(SiMe3)2}2 ¬isomer. Indeed, we show this to be the case, and provide a kinetic study of this transformation in solution.

Chapter 4 describes the reduction of M{N(SiMe3)2}3 (M = V, Cr, Fe) complexes with the metal hydride reagents LiAlH4 or AlH3(NMe3). Thus, the reaction of V{N(SiMe3)2}3 with LiAlH4 afforded the highly unstable polyhydride [V(µ2-H)6[Al{N(SiMe3)2}2]3][Li(OEt2)3]. In contrast, the reaction of V{N(SiMe3)2}3 with LiAlH4 in the presence of 12-crown-4 gave the rare terminal hydride [VH{N(SiMe3)2}3][Li(12-crown-4)2]. The presence of this hydride was verified by the preparation and spectroscopic study of the corresponding deuteride complex [VD{N(SiMe3)2}3][Li(12-crown-4)2] by the reaction of V{N(SiMe3)2}3 with LiAlD4 in the presence of 12-crown-4. The reaction of LiAlH4 with M{N(SiMe3)2}3 (M = Cr, Fe) in the presence of 12-crown-4 afforded the ionic metal(II) complexes [M{N(SiMe3)2}3][Li(12-crown-4)2], rather than any hydride complexes. In the absence of 12-crown-4, the reaction of Fe{N(SiMe3)2}3 with LiAlH4 gave the so-called “hydrido inverse crown” complex [Fe(µ2-H){N(SiMe3)2}2(µ2-Li)]2, while treatment of the same trisamide with AlH3·NMe3 afforded the mixed-metal polyhydride Fe(µ2-H)6[Al{N(SiMe3)2}2]2[Al{N(SiMe3)2}(NMe3)].

Chapter 5 describes attempts to prepare divalent M{N(SiMe3)2}2 complexes of titanium and vanadium by reduction of the corresponding M(Cl){N(SiMe3)2}2 complexes. Additionally, efforts to determine the fate and role of the bromine atom in the synthesis of M{N(SiMe3)2}3 (M = Mn, Co) complexes by oxidation of the divalent M{N(SiMe3)2}2 with BrN(SiMe3)2 are detailed. Reduction of Ti(Cl){N(SiMe3)2}2 with 5 %(wt) sodium on sodium chloride afforded the unusual mixed-metal, mixed-valent Ti(III/IV) hydride complex Ti2(μ-H)2{N(SiMe3)2}3{N(SiMe3)(SiMe2CH)}(Na) rather than “Ti{N(SiMe3)2}2,” while the analogous reaction using V(Cl){N(SiMe3)2}2 gave only a mixture of intractable products. After isolation of Co{N(SiMe3)2}3 formed by the reaction of BrN(SiMe¬3)2 with Co{N(SiMe3)2}2, storage of the mother liquor afforded a small amount of crystalline, polymeric [(μ-Br)Co{μ-N(SiMe3)(SiMe2CH2CH2Me2Si)(Me3Si)μ-N}Co(μ-Br)]ꝏ, indicating that the three products of this reaction are Co{N(SiMe3)2}3, HN(SiMe3)2, and the aforementioned polymer. In order to better understand the role of bromine in these oxidations, we also treated M{N(SiMe3)2}2 (M = Mn or Co) with elemental bromine. Surprisingly, treatment of Mn{N(SiMe3)2}2 with bromine gave the trisamide Mn{N(SiMe¬3)2}3 as the only isolated product. In contrast, the reaction of Co{N(SiMe¬3¬)2}2 with elemental bromine gave the heteroleptic [Co(Br){μ-N(SiMe3)2}]2.

Chapters 6 and 7 differ from the preceding chapters in that the focus is no longer the chemistry of transition metal amide complexes. In Chapter 6, the synthesis and characterization of several homoleptic, dimeric aryloxide and thiolate complexes of iron are described. Thus, treatment of [Fe{N(SiMe¬3)2]2¬ with the appropriate phenol or thiol affords the dimers {Fe(OC6H2-2,6-But2-4-Me)2}2 and {Fe(OC6H3-2,6-But2)2}2, or the monomeric Fe{SC6H3-2,6-(C6H3-2,6-Pri2)2}2. Recrystallization of {Fe(OC6H2-2,6-But2-4-Me)2}2 or {Fe(OC6H3-2,6-But2)2}2 from diethyl ether gives the corresponding three-coordinate ether complexes Fe(OC6H3-2,6-But2-4-Me)2(OEt2) and Fe(OC6H3-2,6-But2)2(OEt2). However, recrystallization of the thiolate Fe{SC6H3-2,6-(C6H3-2,6-Pri2)2}2 from diethyl ether afforded no new products. Fe(OC6H3-2,6-But2-4-Me)2(OEt2) and Fe(OC6H3-2,6-But2)2(OEt2) were found to weakly coordinate ether in solution, a property which allowed the complete assignment of the 1H NMR spectra of the parent dimers and the monomeric diethyl ether complexes. In contrast to the weak coordination of diethyl ether, the iron aryloxides were shown to strongly complex ammonia as exemplified by the four-coordinate Fe(OC6H2-2,6-But2-4-Me)2(NH3)2, which does not lose diethyl ether in solution or under heating and low pressure. Alternatively, Fe{SC6H3-2,6-(C6H3-2,6-Pri2)2}2 was shown to reversibly bind either one or two ammonia molecules, giving the products Fe{SC6H3-2,6-(2,6-Pri2-C6H3)2}2(NH3) and Fe{SC6H3-2,6-(2,6-Pri2-C6H3)2}2(NH3)2.

In Chapter 7, work on the design of a ‘dispersion effect donor’ ligand which is capable of stabilizing a dimeric R2SnSnR2 species in solution is described. The vast majority of distannene (R2SnSnR2) molecules possess only weak Sn-Sn interactions and thus dissociate into monomeric stannylene (SnR2) fragments in solution. However, we show that the reaction of 2 eq. of LiC6H2-2,4,6-Cy¬3¬·OEt2 (Cy = cyclohexyl) with SnCl2 afforded the distannene {Sn(C6H2-2,4,6-Cy3)2}2, which remains a dimer in solution even at elevated temperatures. A computational study revealed that the origin of its stability is the London dispersion attraction between multiple close contacts of ligand C-H moieties across the Sn-Sn bond. Whereas the only previous example of a distannene to remain a dimer in solution at ambient temperature was shown to be stabilized by strong covalent interactions between tin atoms, we show here that Sn-Sn interactions in {Sn(C6H2-2,4,6-Cy3)2}2 are comparatively weak, and that its stability is mainly due to London dispersion effects.

Main Content
For improved accessibility of PDF content, download the file to your device.
Current View