INVESTIGATION OF HALOGEN BONDING USING XRAY CRYSTALLOGRAPHY

Abstract

This thesis describes work conducted on a series of halogen-bonded adducts derived from a series of fluoro-alkyl and -aryl bromides and iodides with 1,4-diazabicyclo[2.2.2]octane (DABCO). Vapour-phase diffusion of DABCO with 1,2-dibromotetrafluoroethane results in the formation of crystals of a 1:1 adduct of formula C2Br2F4.C6H12N2. This forms an infinite one-dimensional polymeric structure linked by intermolecular N…Br halogen bonds. These are characterised by d = 2.829 (3) Å, which is 0.57 Å shorter than the sum of the van der Waals radii and an N…Br-C angle of 175.6(1)°. Extending this work to the longer chained dibromo-perfluoroalkyl compounds BrCF2(CF2)nCF2Br (n = 2, 4, 6) gives rise to colourless crystals of Br2C4F8•C6H12N2, Br2C6F12•C6H12N2 and Br2C8F16•C6H12N2, each of which form one-dimensional halogen-bonded networks. All three adducts exhibit N···Br halogen bonds with N-Br. The shortest N…Br distances were observed in Br2C4F8•C6H12N2, 2.809 (3) and 2.818 (3) Å, which are 0.58 and 0.59 Å shorter than the sum of the van der Waals radii and the shortest N···Br halogen bond distance reported to date between a bromoperfluorocarbon and a nitrogen base. The X-ray structure of the second ever adduct based on an aromatic bromofluoroalkane and DABCO, is reported from the vapour phase diffusion of DABCO and 1bromoperfluorobenzene. The near-linear N…Br halogen bond (C-Br···N = 167.8 (2) to 169.3 (3) ˚) exhibits a N…Br bond distance of 2.814 (7) Å, which is 0.58 Å shorter than the sum of van der Waals radii. An investigation of the halogen bonded adducts formed from the vapour phase between four iodoperfluoroalkanes with DABCO resulted in the formation of one-dimensional structures of the formula CF3(CF2)nCF2I•C6H12N2 (n = 2, 4 and 6) with N…I distances ranging from 2.685 (6) to 2.799 (3) Å. Disorder was observed in structures of the longer chained adducts. When crystals were regrown from a dichloromethane solution of I(CF2)4I and DABCO single crystals containing (ClCH2+DABCO)I(CF2)4ICl-I(CF2)4IClI(CF2)4I(DABCO+CH2Cl) resulted, arising from mono-quaternisation of DABCO and the formation of a structure based on a mixture of N...I and I…Cl halogen bond interactions. The interaction between volatile perfluoroalkyl iodides, such as C3F7I, C4F9I, C6F13I and PPh3 in the vapour phase in glass containers results in the formation of crystals identified as F2PPh3 on the basis of single-crystal diffraction studies and NMR data. The mechanism of this reaction is not known, but in the absence of light the rate of reaction was shown to slow down, while the presence of glass was found to give rise to a number of silicon-containing decomposition products including (C6H5)3PO…HOP(C6H5)3.SiF5 and Ph3POSiF4OPPh3 consistent with the attack of the glass by Ph3PF2, or its hydrolysis products.

Date of Award	1 Aug 2018
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Alan Brisdon (Supervisor) & Pritchard (Supervisor)