Coordination Chemistry of high valent early transition metals with neutral donor ligands: towards precursors for the electrodeposition and LPCVD of molybdenum and tungsten dichalcogenides

Abstract

The work in this thesis concerns the development of several new series of high valent Group VI (Mo, W) complexes and explores their application for the deposition of molybdenum and tungsten dichalcogenide thin films via electrodeposition and low-pressure chemical vapour deposition.

A review of the relevant background literature is presented in Chapter 1, while Chapter 2 presents an investigation to explore the chemistry of tungsten(VI) and develop the first systematic series of tungsten oxytetrachloride and thiotetrachloride coordination complexes, six-coordinate [WECl4(L)] or [(WECl4)2(µ-L-L)] (E = O or S) or seven-coordinate [WECl4(L-L)] and to explore their properties. The ligands used include phosphine oxides (dppeO2, dppmO2, OPPh3 and OPMe3) and N-heterocycles (2,2’-bipy, 1,10-phen and py).

Unusual seven-coordinate pentagonal bipyramidal tungsten(VI) oxo- and thio-tetrachloride complexes with a series of phosphine and arsine ligands to give complexes of the form [WECl4(LL)] (E = O or S), are reported in Chapter 3, together with investigation of monodentate ligands. The ligands used include o-C6H4(E’Me2)2 (E’ = P or As), Me2P(CH2)2PMe2, PMe3, AsEt3 and oC6H4(PPh2)2.

The coordination chemistry of a series of WSCl4 complexes with thioethers and comparative WOCl4 complexes are described in Chapter 4. Softer selenium donor ligands are also investigated and the properties of hard metal-soft ligand interactions explored. Potential precursors for low pressure chemical vapour deposition are identified on the basis of certain criteria i.e. butyl substituent ligands and M:E ratio.

Reported in Chapter 5, [(WSCl4)2( i PrS(CH2)2Si Pr)], [WSCl3( i PrS(CH2)2Si Pr)] and [WSCl4(Sn Bu2)] were tested as single source LPCVD precursors. [(WSCl4)2( i PrS(CH2)2Si Pr)] and [WSCl4(Sn Bu2)] deposit 4H-WS2 thin films, the two W(VI) precursors deposited continuous films with a W:S ratio of 1:2.2 consistent with WS2. These are the first examples of single source LPCVD precursors to deposit WS2 thin films and the films have been characterised by Raman spectroscopy, grazing incidence XRD and in-plane XRD, EDX, WDX, XPS and SEM.

Presented in Chapter 6, a family of MoOCl3 complexes with a range of neutral donor ligands, with a particular emphasis on thio-, seleno- and telluro-ether ligands, allowing comparisons to be drawn to the coordination complexes of WECl4 (E = O or S). The reaction of MoOCl4 with various ligands is investigated and compared to the MoOCl3 coordination complexes.

The development and synthesis of a series of potential precursors suitable for the electrodeposition of ME2 (M = Mo or W; E = S or Se) thin films using non-aqueous solvents is discussed in Chapter 7, particularly including the synthesis of precursors for the electrodeposition of MoS2 and WS2 and their respective electrochemical studies. Deposited films have been characterised by Raman spectroscopy, grazing incidence XRD, EDX, WDX and SEM analysis.

Where possible, new complexes have been characterised by IR, Raman, multinuclear NMR ( 1 H, 31P{1 H}, 77Se{1 H} and 95Mo where appropriate), UV-V is spectroscopy, magnetic measurements, elemental analysis and single crystal X-ray structures.

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Identifiers

ORCID for Danielle E. Smith: orcid.org/0000-0002-2446-1763

ORCID for Gillian Reid: orcid.org/0000-0001-5349-3468

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