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Title: Redox non-innocence in transition metal macrocyclic complexes
Author: Stephen, Emma Louise
Awarding Body: University of Nottingham
Current Institution: University of Nottingham
Date of Award: 2011
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Abstract Chapter one introduces this thesis with a brief overview of macrocyclic chemistry and provides examples of macrocyclic complexes as well as briefly discussing the applications of macrocyclic complexes, the macrocyclic effect and the co-ordination chemistry of sulfur-based macro cyclic transition metal complexes. The concepts of paramagnetism and Jahn- Teller distortion are approached with respect to the target metal oxidation states Ni(III), Pt(III) and Pd(III). The overall aim of the work presented in this thesis is also stated. Chapter two continues from the concept of paramagnetism briefly discussed in chapter one and describes the technique of EPR spectroscopy and the energy terms associated with the EPR spin Hamiltonian such as the electron Zeeman interaction (g) and the hyperfine and superhyperfine coupling parameters (A, a). The limitation of this technique is discussed in terms of observed hyperfine splitting and the technique of ENDOR spectroscopy is introduced. Chapter three describes the chemistry of Ni(III) macrocyclic complexes and a brief history of 61Ni EPR spectroscopic studies. The Ni(II) complexes [Ni([9]aneNS2-CH3)2]2+ ([9]aneNS2-CH3 = N-methyl-l-aza-4, 7 -dithiacyclononane), 1 2-bis-(1-aza-4 7- , , dithiacyclononylethane), [Ni([9]aneS3)2]2+ ([9]aneS3 = 1,4,7 -trithiacyclononane) and I-phenyl-l-phospha-4,7- dithiacyclononane) have been prepared and can be chemically oxidized to gave the formal Ni(III) products [Ni([9]aneS3)2]3+, [Ni(bis[9]aneNS2-C2H4)]3+ and [Ni([9]aneNS2-CH3)2]3+ and [Ni([9]aneS2P-C6Hs)2]3+ which have been characterized by multi-frequency EPR spectroscopy. The multi-frequency X-, L-, S-, K- and Q- [Ni([9]aneS3)2]3+ and their 86.2% 61Ni-enriched analogues as both fluid and frozen 11 Abstract solutions were simulated. Treatment of the spm Hamiltonian parameters by perturbation theory reveals that the SOMO has 50.6%, 42.8% and 37.2% Ni 3dz' character m and [Ni([9]aneS3)2]3+ respectively, consistent with DFT calculations and reflecting delocalization of charge onto the S-thioether centres. EPR spectra for C8Ni([9]aneS3)2]3+ and [61,S8Ni([9]aneS3)2]3+ are consistent with a dynamic Jahn- Teller distortion in this compound. In contrast to [Ni([9]aneNS2-CH3)2]3+, [Ni(bis[9]aneNS2-C2H4)]3+ and [Ni([9]aneS3)2]3+, DFT calculations on [Ni([9]aneS2P-C6Hs)2]3+ reveal that the SOMO for this complex has 30.9% Ni 3dxy character. Chapter four details the chemistry of Pt(III) and that of Pt(II) macrocyclic complexes. The Pt(II) complexes [Pt([9]aneS3)2]2+ ([9]aneS3 1,4,7- trithiacyclononane), [Pt([18]aneS6)]2+ ([18]aneS6 1,4,7,1 0, 13, 16- hexathiacyclooctadecane) and [Pt([ 1 0]aneS3)2f+ ([ 1 0]aneS3 1,4,7- trithiacyclodecane) have been synthesised. The electrochemical two electron oxidation of [Pt([9]aneS3)2]2+ to [Pt([9]aneS3)2t+ in MeCN [0.2 M NBli4PF6, vs. Fe + IFc, N2, chemically reversible at 243 K] proceeds via an unusually stable [Pt([9]aneS3)2]3+ intermediate [E« = +0.37 V vs. Fc+IFc, ~E = 142 mY, A = 396 nm (5700)]. Single crystals of [Pt([9]aneS3)2](PF6kMeCN were grown from a solution following a limited one electron electrochemical oxidation of the parent Pt(II) complex [Pt([9]aneS3)2]2+. [Pt([9]aneS3)2](PF6)3.MeCN possesses a distorted octahedral geometry consistent with a J ahn- Teller distorted low-spin 5d7 Pt(III) centre. Multi-frequency EPR spectroscopy of chemically-generated [Pt([9]aneS3)2]3+ in HCI04 at Q-, X-, S- and L-band show that [Pt([9]aneS3)2]3+ undergoes a dynamic Jahn-Teller distortion in solution which was minimised at 80 K to reveal rhombic parameters. Treatment of the Spin Hamiltonian reveals that the Pt Sd» orbital iii Abstract contribution to the SOMO of this Pt(III) species, supported by DFT calculations is :S 30.4% and reflecting the non-innocence of the thioether crown macrocyc1e. In addition to this, DFT calculations identify significant spin density on four protons pendant to the macrocyc1ic backbone. The couplings associated with these protons were confirmed by S- and X-band EPR spectroscopy and studied further by double resonance methods to reveal that these protons interact with the Pt 5dz2 orbital through an orbital through-bond interaction. A comparative multi-frequency EPR spectroscopic study was undertaken on the chemically-generated complexes [Pt([18]aneS6)]3+ and [Pt([10]aneS3)2]3+ in 70% HCl04. Interpretation of the spin Hamiltonian parameters reveals that the Pt 5dz> orbital contributions to the SOMOs of these Pt(III) species are :S23.8%. Pt(III) solid state structures were unobtainable for [Pt([18]aneS6)]3+ and [Pt([10]aneS3)2]3+, but attempts to isolate [Pt([l 0]aneS3)2]3+ led to the isolation of the octahedral Pt(IV) complex [Pt([l 0]aneS3)2] (Cl04k(H30)2 [Pt- S = 2.3610(11), 2.3552(10), 2.3792(11) A]. Chapter five begins with a discussion of Pd(III) chemistry and the chemistry of Pd(II) with macrocyc1ic ligands. The Pd(II) complexes [Pd([9]aneS3)2]2+ ([9]aneS3 = 1,4,7-trithiacyc1ononane) and [Pd([18]aneS6)f+ ([18]aneS6 = 1,4,7,10,13,16- hexathiacyc1ooctadecane) have been prepared and can be chemically oxidized to gave the formal Pd(III) products [Pd([9]aneS3)2]3+ and [Pd([18]aneS6)]3+, which have been characterized by X-ray crystallography and multifrequency EPR spectroscopy. The single of crystal X-ray structures distorted octahedral stereochemistries for the Pd centres with Pd-S distances of [2.3692, 2.3695(8), 2.5356(9) A] and [2.3454(5), 2.3490(6), 2.5474(6) A] respectively, consistent with the Jahn- Teller distorted geometry expected for d7 Pd(III) complexes. Further to the chemical oxidation, [Pd([9]aneS3)2](PF6)2 shows a iv Abstract one-electron oxidation process in MeCN [0.2 M NBll4PF6, 293 K] at Ey, = +0.57 V vs. F c + IF c assigned to a formal Pd(III)/Pd(II) couple. Multi-frequency EPR spectra of [Pd([9]aneS3)2]3+ and [Pd([18]aneS6)]3+ as both fluid (X-band) and frozen (Q-, X-, S- and L-band) solutions were simulated. Treatment of the spin Hamiltonian parameters by perturbation theory reveals that the SOMO has 21.8% and 25% Pd character in [Pd([9]aneS3)2]3+ and [Pd([18]aneS6)]3+ respectively, consistent with DFT calculations and reflecting the significant electronic charge distributed over the S-thioether centres. Both [Pd([9]aneS3)2]3+ and [Pd([18]aneS6)]3+ exhibit a clear five line superhyperfine splitting in the g== region in the EPR spectra. Double resonance spectroscopic measurements supported by DFT calculations have shown that four protons pendant to the macrocyclic backbone give rise to this feature through a through-bond interaction with the Pd 4dz2orbital. Chapter six concludes this thesis with a discussion on the overall conclusions to the work on the transition metal macrocyclic complexes presented in this thesis. v
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available