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Title: Redox properties of metal complexes with internal hydrogen bonding
Author: Metteau, Laurent
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2008
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In this thesis we incorporate hydrogen bonding features NH2 (Am), NHCH2tBu (Np), and NHCOtBu (Piv) in the pyridine 6th position of the tripodal ligand TPA (tris(2-pyridylmethyl)amine, L). Thus, thirteen ligands LX-n (X = Am, Np, Piv; n = 0, 1, 2, 3) derived from and including TPA have been made. Also, one and two amino groups have been introduced in a similar position of the ligand N,N-Bis(2-pyridilmethyl)-N-(glycine)amine (BPG), (one amino group, APPG and two amino groups, BAPG) have been made. The structures of several [LMCl]0 or +1 complexes (M = Zn(II), Cu(I), Cu(II)) were determined by X-ray crystallography ([(LAm-3)ZnCl]BPh4, [(LPiv-3)CuCl], and [(BAPG)CuCl]Cl). All these complexes exhibit intramolecular NHCl hydrogen bonding. For the Zn(II)Cl complexes of the LX-n ligands, the main structural changes caused by hydrogen bonds are the lengthening of the Zn-Cl and the Zn-NPy bonds and shortening of the Zn-NAx bond. In general, these changes are more significant when the number of hydrogen bonds increases, and vary according to their strength. In related Cu(I) and Cu(II) complexes, an important factor affecting the strength of the N-HCl hydrogen bonds is the oxidation state of the metal centre. Thus, the N-HCl interaction is stronger in the Cu(I) oxidation state. The [(LX-n)CuCl]+ complexes were used to investigate the effect of the N-HCl hydrogen bonding on the redox properties of the metal centre. Cyclic voltammetry studies showed that the reduced state of the metal centre is stabilised when stronger and more numerous hydrogen bonds are introduced. The half wave potential of the Cu(II)/Cu(I) redox couple shifted to more positive values by 58 mV in [(LAm-1)CuCl]+ to 541 mV in [(LPiv-3)CuCl]+ relative to the TPA complex. We estimated that in these complexes, hydrogen bonds can contribute as much as 30% to the stabilisation of the Cu(I) over the Cu(II) oxidation state. The individual contribution of each hydrogen bond towards stabilising the Cu(I) state is greater the fewer interactions. The effect of hydrogen bonding on the redox properties of zinc-bound DTBC (3,5-di-tert-butylcatechol) in [LX-nZnDTBC) complexes was also investigated. The X-ray structure of [LAm-1ZnDTBC] showed internal N-HO hydrogen bonding. Cyclic voltammetry and spectroelectrochemical studies showed that the reduction of DTBC complexes occurs at more positive potentials in the presence of hydrogen bonds. For [LPiv-3ZnDTBC], the reduction of DTBC was facilitated by as much as 470 mV relative to the TPA complex, and is accompanied in this case by a binding enhancement of 3.9 x 106 fold. These changes are compared with those obtained by changing the metal or its coordination sphere and with hydrogen bonds alone.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available