Abstract
This thesis details the development of the organometallic chemistry of a tetra-arylated pentalenide (1,3,4,6-Ph4Pn2-) with a particular focus on the synthesis, structure and reactivity of bimetallic complexes of the s- and late d-block.As an introduction to the work presented, Chapter One reviews bimetallic complexes of selected polycyclic π-ligands (indenide, naphthalene, azulene and pentalenide), with a particular focus on how the ligand provides a framework to form bimetallic complexes. Different classifications of bimetallic systems are introduced and the factors that appear to influence syn or anti metalation are discussed. Previous work on the Ph4Pn2- ligand is also reviewed, leading to the aims & objectives of this thesis.
Results Chapter Two extends the group 1 chemistry of Ph4Pn2- to the heavier alkali metals, in collaboration with Dr. Stuart Robertson at the University of Strathclyde, with the synthesis of [M(THF)]2[Ph4Pn] (M = Rb, Cs) which were characterised by X-ray diffraction and mark the first examples of rubidium and caesium pentalenides. The structural trends in the solid state as the alkali metal cation increases in size from lithium through to caesium are examined and compared with an earlier reported computational study on M2Pn (M = Li, Na, K, Rb, Cs). The influence of the donor ligand has been explored with the synthesis of the DME adducts, [Li(DME)]2[Ph4Pn] and [Na(DME)2]2[Ph4Pn], and Me6TREN adducts [M(Me6TREN)]2[Ph4Pn] (M = K, Rb, Cs).
In results Chapter Three the synthesis of the first alkaline earth pentalenide Mg[Ph4Pn] is described, including the solvent-dependent outcome on the structure of the magnesium pentalenide that forms. The interconversion between these two structures was studied by NMR and UV-vis and is akin to a Schlenk-type equilibrium. The addition of electrophiles (H2O, D2O, MeI and TMSCl) to Ph4Pn2- was studied to examine the influence of the 1,3,4,6-Ph4 substitution pattern on charge localisation within the pentalenide.
Results Chapter Four expands on the chemistry of the Ph4Pn2- dianion by examining its oxidation to the neutral 8π antiaromatic Ph4Pn which was characterised by NMR, XRD and UV-vis. Ph4Pn marks a rare example of a room temperature stable antiaromatic hydrocarbon, which could be fully reduced back to Ph4Pn2- by addition of potassium. The antiaromaticity of Ph4Pn, including the role of the aromatic substituents, was explored by nucleus-independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations by Prof. Holger Helten at the Julius-Maximilians-Universität of Würzburg.
Results Chapter Five explores the organometallic chemistry of Ph4Pn2-by way of transmetalation with Rh(I) and Ir(I) precursors. The importance of the identity of the cation(s) is explored by comparing Mg[Ph4Pn] and Li·K[Ph4Pn]. Other factors deemed to influence syn and anti-selectivity, including the steric and electronic nature of the auxiliary ligands on the d-block precursor, are explored with the synthesis of anti-[RhI(L)n]2[Ph4Pn] (L = 1,5-cyclooctadiene, norbornadiene, ethylene; n = 1, 2) and syn-[RhI(CO)2]2[Ph4Pn]. Additional DFT calculations, performed by Dr. Claire McMullin, shed light on the intrinsic preference for syn metalation of Rh(I) and Ir(I). The tolerance of Ph4Pn2- towards a range of sterically and electronically diverse ligands is surveyed by way of ligand substitution of syn-[RhI(CO)2]2[Ph4Pn] with sequential substitution of CO, demetallation and disproportionation encountered, including the synthesis of syn-[Rh(CO)(P{OPh}3)]2[Ph4Pn].
Results Chapter Six examines the reactivity of syn-[RhI(CO)2]2[Ph4Pn] and syn-[Rh(CO)(P{OPh}3)]2[Ph4Pn] towards chemical oxidants such as AgPF6 and iodine. Oxidation of the di-Rh(I) systems resulted in demetallation to form the monometallic complexes [RhIII(CO)(η5-Ph4Pn)(µ-I)]2 and [RhIII(P{OPh}3)2(η5-Ph4Pn)][PF6], which are the first examples of Rh(III) pentalenide complexes.
Results Chapter Seven chronicles attempts to synthesise bimetallic Ph4Pn2- complexes of first-row transition metals, with transmetalation of Mg[Ph4Pn] to Mn(I) and Fe(II) precursors resulting in oxidation to give Ph4Pn, whilst Cu(I) precursors underwent clean transmetalation to yield the first copper-pentalenide [Cu(PtBu3)]2[Ph4Pn]. Unsuccessful attempts to form a Zn(II) pentalenide through transmetalation or deprotonative metalation are discussed. The direct redox metalation of Ph4PnH2 with Fe2(CO)9 resulted in formation of [Fe2(CO)4(µ-CO)][Ph4Pn] which was fully characterised by NMR and XRD, and the crystal structure of the first asymmetric pentalenide [Fe2(CO)4(µ-CO)][1,3-Me2-4,6-Ph2Pn] is reported.
The main findings and conclusions presented in this thesis are summarised in Chapter Eight, alongside insight as to direction future work on the organometallic chemistry of Ph4Pn2- could take.
The Appendix contains supplementary information of results chapters 2-7.
| Date of Award | 24 Jul 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Ulrich Hintermair (Supervisor) & Michael Hill (Supervisor) |
Keywords
- Pentalenides
- Organometallics