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

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Frey, Alistair Sydney Paul (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,
Et8N4Me2H2, and trans-calix[2]benzene[2]pyrrole, Me8N2Ph2F12.
Chapter 2 is concerned with the synthesis of a new modified porphyrinogen N,N'-
dimethyl-meso-octamethylporphyrinogen, Me8N4Me2H2, 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 [(Et8N4Me2)Sm(C5H5)] featuring a major conformational deformation
in the macrocycle. Also synthesised was a centrosymmetric bimetallic cyclooctatetraenediyl bound Sm(III) complex [{(Et8N4Me2)Sm}2(µ:ƞ2:ƞ2-COT)] in
which the cyclooctatetraenediyl dianion is forced to adopt an ƞ2 -binding mode to each
Sm centre. Solid state molecular structures of these strained molecules were
complemented by 1H, 13C, 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 [{(Et8N4Me2)Sm}2{µ-0C(Ph)C(Ph)0}] with
the concomitant formation of the free 1,4-diazabuta-1,3-diene. Also investigated was the
highly strained [(Et8N4Me2)Sm(C5H5)], which was found to react with 1,4-benzoquinone
to give the binuclear complex [{(Et8N4Me2)Sm}2{µ-0(C6H4)0}].

Chapter 6 describes the reductive chemistry of the Sm(II) complex
[(Et8N4Me2)Sm(THF)2]. It was found to reduce CO2 in a disproportionation reaction to
give carbon monoxide and a bridging CO32- 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 SmI2. 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, ƞ5 -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 ƞ8 Sm-COT interaction and concomitant reduction in arene hapticity to a slipped
ƞ1-arrangement with pyrrolide rings ƞ1 -bound through the nitrogen. The Sm(II)
complexes were also characterised by IH NMR and/or variable temperature NMR
spectroscopy.

Item Type: Thesis (PhD)
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Copyright 2009 the Author - The University is continuing to endeavour to trace the copyright
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No access or viewing until March 2011. Available for library use only and copying in accordance with the Copyright Act 1968, as amended. Thesis (PhD)--University of Tasmania, 2009. Includes bibliographical references

Date Deposited: 09 Dec 2014 00:13
Last Modified: 06 May 2016 01:23
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