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Coordination chemistry of bicyclic guanidines and guanidinates

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posted on 2023-06-07, 16:10 authored by Majid Shannon Khalaf
A [5:5] ligand based on the bicyclic guanidine 1,4,6-triazabicyclo[3.3.0]oct-4-ene [Htbo] has been synthesised. Two of these tbo units have been linked via a carbon bridgehead to form a novel bidentate ligand. Based on the knowledge that the electronic properties and specific steric interactions at a metal can lead to new insights into the relationship between crystal structure and ligand activity, the synthesised compounds have been investigated as polydentate ligands on the assumption that that they could be used to precisely control coordination environment. The corresponding [6:6] system which is based on the bicyclic guanidine 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) [hppH] has also been synthesised and its coordination to silyl groups investigated. 1,4,6-triazabicyclo[3.3.0]oct-4-ene (Htbo) has been deprotonated to form the [tbo]- anion, which has been assessed as a ligand at Li, Al and Zn centres, allowing direct structural comparison to be made with the [hpp]- analogues. The coordination chemistry of Htbo and its derivatives has been explored through formation of a variety of transition metal complexes. Additionally, crystal structures showed that Htbo coordinates to the metal through the N-imine atom as a monodentate ligand to form trigonal planar, tetrahedral, square planar and octahedral complexes. The bis (tbo) methane H2C{tbo}2 ligand was found to support a range of coordination geometries and upon coordination, ionic structures such as [Pd(H2C{tbo}2)2][Cl]2 were formed. The synthesis of the monosubstituted compound hppSiCl3 afforded a series of (hpp) silanes which were isolated and reacted with LiOAr (Ar= 2,6-tBu2C6H3), providing a proof of N-Si bond stabilisation using a metal. Five examples of pentacoordinated silicon species have been synthesised via the reaction of [hpp]- and R2SiCl2 (R= Ph, Me).


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