Title: Structural and computational investigations into phosphine and scorpionate ligand complexes
Authors: Hamilton, Alexander J. #
Issue Date: 2010
Abstract: Structural techniques are used for probing the chemistry of transition metal complexes. In this thesis the techniques of choice are single crystal X-ray analysis, database mining of crystallographic data leading to structure-correlation relationships, and computational chemistry. These techniques are combined to study single molecular structures (using X-ray analysis) and then observe how they relate to other similar structures (with database mining). Computational techniques are used quantify the observed trends as well as explore the chemistry of the complex. The two main ligand classes focused on in this thesis are phosphine and scorpionate ligands.
Tertiary phosphine and phosphite ligand complexes are used extensively in catalysis. The steric profile of a ligand is an important property with direct impact on catalyst activity. However, conformational flexibility makes quantifying the steric profile problematic. In Chapter 2, using data mined from the Cambridge Structural Database (CSD) it has been shown that P(CH2Ph)3 and P(OPh)3 ligands can adopt 7 different conformers. For P(CH2Ph)3 only g+g+g+ (B), g+g+a (C), g+g_a (D) and g+aa (F) are significantly populated, whilst for P(OPh)3 only C, D and F are significantly populated. Conformer C is the most populated for both ligand types (36% for P(CH2Ph)3 and 54% for P(OPh)3). By mapping the conformational hypersurface, pathways connecting the conformers were observed. These pathways were more populated for the P(OPh)3 ligands, suggesting a more flexible structure allowing for interconversion between conformers. There is good agreement between the relative stabilities of the conformers (calculated with DFT) and their population in the CSD.
Nickel-catalysed hydrocyanation is an industrial homogeneous catalysis process involving phosphite ligands derived from that discussed in Chapter 2. The mechanism for hydrocyanation of ethene using [Ni(C2H4){P(O-o-tolyl)3}2] as the pre-catalyst, has been suggested from experimental data. In Chapter 3 this mechanism has been explored computationally, highlighting discrepancies in the experimental data. Possible new routes have been proposed from the computational evidence. The new mechanisms show different intermediates, cis/trans- [NiL2(CN)Et] (L = P(O-o-tolyl)3) and a different ordering of reaction steps. Ligand effects within the catalytic cycle have been explored; highlighting mono-dentate phosphonite and phosphinite ligands as possible new catalysts.
In Chapter 4, Phobanes, a class of phosphacyclic ligands, have been structurally characterised with interest in their coordination and catalytic properties. How the internal C-P-C angle affects the electronic properties (σ-donor and π-acceptor capabilities) of the phobane ligand has been explored. This has obvious implications for the complexes, which have been highlighted through the use of X-ray analysis and structure-correlation relationships.
Two donor-types of flexible scorpionate ligands have been studied, with specific interest in their B-H activation properties and metallaboratrane formation. In Chapter 5, the S-donor ligand (Tm) has been shown to adopt 3 different conformers, boat, chair and κ3-SSS. The conformational hypersurface has been explored using principal component analysis (PCA). The reaction step for forming the metallaboratrane complex has been studied qualitatively using structure-correlation relationships and quantitatively with computation. The effect of the auxiliary ligands on the metal for the B-H activation step has also been studied.
The mechanism for metal-mediated B-H activation and alkene insertion with a flexible N-donor (Tai) scorpionate complex has been explored in Chapter 6. Crystal structures of the reactant and product, and complexes analogous to reaction intermediates were used as starting points for the computational results. The mechanism for the reaction was elucidated, highlighting the key transition states and the rate determining step. Experimentally the reaction shows ligand and substrate sensitivity, and different activity for Ir and Rh systems, the computational results are in good agreement with these experimental observations. The importance of including a dispersion correction in the computational methodology is highlighted in this chapter.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Non-KU Leuven Association publications
# (joint) last author

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