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Structural and reductive chemistry of low-valent lanthanide complexes featuring modified porphyrinogens

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Frey, ASP 2009 , 'Structural and reductive chemistry of low-valent lanthanide complexes featuring modified porphyrinogens', PhD thesis, University of Tasmania.

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Abstract

This thesis describes studies into the synthesis, characterisation and reactivity of samarium(II) and samarium(III), europium(II) and ytterbium(II) complexes derived from the modified porphyrinogens \(trans-N,N'\)-dimethyl-meso-octaethylporphyrinogen, Et\(_8\)N\(_4\)Me\(_2\)H\(_2\), and \(trans\)-calix[2]benzene[2]pyrrole, Me\(_8\)N\(_2\)Ph\(_2\)H\(_2\).
Chapter 2 is concerned with the synthesis of a new modified porphyrinogen \(N,N'\)-dimethyl-\(meso\)-octamethylporphyrinogen, Me\(_8\)N\(_4\)Me\(_2\)H\(_2\), via a convergent "3+1" procedure from the condensation of 1-methy1-2,5-bis(1,1'-dimethylhydroxymethyl)pyrrole with 1-methy1-2,5-bis {(2'-pyrroly)dimethylmethyl}pyrrole in acetonitrile in the presence of scandium trifluoromethanesulfonate. The stepwise nature of the synthesis potentially allows independent functionalisation of various parts of the molecule.
The unique electronic and steric properties of complexes derived from doubly deprotonated \(N,N'\)-dimethyl-\(meso\)-octaethylporphyrinogen were exploited to force unusual reactivity and/or structural features in a range of lanthanide(II) and lanthanide(III) complexes. Chapter 3 details the synthesis of samarium(II), europium(II) and ytterbium(II) complexes of this macrocycle. Subsequent reaction with a range of 1,4-diazabuta-1,3-dienes (R-N=C(H)-C(H)=N-R, R = \(t\)-Bu, \(i\)-Pr and \(n\)-Bu) gave complexes featuring 1,4-diazabuta-1,3-diene binding to the lanthanide centres as neutral Lewis base donors, chelating radical anions and bridging reduced dianions. Steric limitations were found to alter these structural outcomes, and the complexes were characterised by X-ray crystal structure determination and NMR spectroscopy. Steric factors were also implicated in the observation of an unusual solvent mediated Sm(II)/Sm(III) reversibility in which a Sm(III) centre reverted to Sm(II) upon addition of coordinating solvent (R = \(t\)-Bu).
Steric competition in the \(N,N'\)-dimethyl-\(meso\)-octaethylporphyrinogen system was further examined in Chapter 4 by synthesis of a highly strained cyclopentadienyl Sm(III) complex [(Et\(_8\)N\(_4\)Me\(_2\))Sm(C\(_5\)H\(_5\))] featuring a major conformational deformation in the macrocycle. Also synthesised was a centrosymmetric bimetallic cyclooctatetraenediyl bound Sm(III) complex [{(Et\(_8\)N\(_4\)Me\(_2\))Sm}\(_2\)(µ:ƞ\(^2\):ƞ\(^2\)-COT)] in which the cyclooctatetraenediyl dianion is forced to adopt an ƞ\(^2\) -binding mode to each Sm centre. Solid state molecular structures of these strained molecules were complemented by \(^1\)H, \(^{13}\)C, 2D and variable temperature NMR studies of the cyclopentadienyl complex to examine fluctional processes in solution.
Chapter 5 describes ligand substitution reactions in which Sm(III) complexes of 1,4-diazabuta-1,3-dienes were reacted with reducible substrates. Samarium(III) complexes of \(t\)-butyl- 1,4-diaza-1 ,3-diene and \(i\)-propyl-1,4-diaza-1,3-diene were found to reduce benzil to give the binuclear complex [{(Et\(_8\)N\(_4\)Me\(_2\))Sm}\(_2\){µ-OC(Ph)C(Ph)O}] with the concomitant formation of the free 1,4-diazabuta-1,3-diene. Also investigated was the highly strained [(Et\(_8\)N\(_4\)Me\(_2\))Sm(C\(_5\)H\(_5\))], which was found to react with 1,4-benzoquinone to give the binuclear complex [{(Et\(_8\)N\(_4\)Me\(_2\))Sm}\(_2\){µ-O(C\(_6\)H\(_4\))O}].
Chapter 6 describes the reductive chemistry of the Sm(II) complex [(Et\(_8\)N\(_4\)Me\(_2\))Sm(THF)\(_2\)]. It was found to reduce CO\(_2\) in a disproportionation reaction to give carbon monoxide and a bridging CO\(_3\)\(^{2-}\) moiety. The resulting binuclear samarium(III) complex was characterised by X-ray crystal structure determination and NMR spectroscopy. The Sm(II) complex was also used in redox transmetallation reactions with mercury, thallium and silver salts. The reaction with silver tetrafluoroborate gave a Sm(III) tetrafluoroborate intermediate which underwent subsequent salt metathesis reactions with sodium cyclopentadienide and lithium iodide to give the respective samarium(III) derivatives.
Chapter 7 details the synthesis of \(trans\)-calix[2]benzene[2]pyrrole by condensation of pyrrole with 1,3-bis(1',1'- dimethylhydroxymethyl)benzene in acetonitrile. The literature procedure for the synthesis of this macrocycle was improved by the use of a catalytic amount of scandium trifluoromethanesulfonate in place of stoichiometric boron trifluoride as Lewis acid. As a counterpoint to the conformationally restricted \(N,N'\)-dimethyl-\(meso\)-octaethylporphyrinogen, the less rigid doubly deprotonated \(trans\)-calix[2]benzene[2]pyrrole was investigated as a ligand for lanthanide metals. The potassium salt was synthesised by deprotonation of the neutral porphyrinogen with potassium metal. The lanthanide chemistry was investigated by reaction of the dipotassium salt with SmI\(_2\). The reaction was sensitive to conditions and resulted in mixtures from which a number of Sm(II) complexes featuring varying degrees of solvation and an unsolvated "\(N\)-confused" dimer were isolated. Molecular structures of the dipotassium salt, mono- and bis-THF Sm(II) adducts and Sm(II) \(N\)-confused binuclear dimer were obtained. As derivatives, a cationic Sm(III) cyclooctatetraenediyl complex and a potassium containing Sm(III) cyclooctatetraendiyl complex were obtained and characterised by X-ray crystal structure determination. Macrocyclic binding modes fell between the extremes of the samarium(II) mono-THF adduct (featuring a bis(ƞ\(^3\)-arene) structural motif with only a slight metallocene bend angle, ƞ\(^5\) -bound pyrrolide rings) and the cyclooctatetraenediyl Sm(III) complexes in which the macrocycle splays back to allow the large, planar COT full access to the Sm coordination sphere, resulting in an ƞ\(^8\) Sm-COT interaction and concomitant reduction in arene hapticity to a slipped ƞ\(^1\)-arrangement with pyrrolide rings ƞ\(^1\) -bound through the nitrogen. The Sm(II) complexes were also characterised by \(^1\)H NMR and/or variable temperature \(^1\)H NMR spectroscopy.

Item Type: Thesis - PhD
Authors/Creators:Frey, ASP
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Thesis (PhD)--University of Tasmania, 2009. Includes bibliographical references

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