Soft routes to inorganic frameworks via assembly of molecular building blocks
Abstract
The assembly of mononuclear and polynuclear molecular building blocks has been
investigated as a route to extended metal oxide structures. Various [MX[subscript(y)]]ⁿ⁻ (X = Cl,
OMe, OH) and [(RO)M’M₅O₁₈]³⁻ (R = MeO, M’ = Sn, M = W) building blocks have
been synthesised. Controlled hydrolysis has been explored for transition and main
group metal hexahalides [MCl₆]ⁿ⁻ M=Ti, Sn using ¹⁷O enriched water.
An attempted synthesis of [Me₃NCH₂Ph] [Sn(OMe)₆] gave the dinuclear product
[Me₃NCH₂Ph] [Sn₂(OMe)₉]. Variable temperature ¹H NMR studies revealed exchange
between terminal and bridging alkoxides and the limiting spectrum is consistent with
solid state structure. 2,6-lutidinium hydrochloride was synthesised and used to
chlorinate the metal alkoxides {M(OR)[subscript(n)]} (M = Nb). A route to monochloro niobium
alkoxide {NbCl(OMe)₄} was developed and variable temperature
¹H NMR
spectroscopy studies of {NbCl(OMe)₄} in different solvents revealed exchange
between bridging and terminal alkoxides and also suggested the presence of different
structural isomers in solution.
A novel heterometallic Lindqvist type of POM containing Sn has been successfully
synthesised by using controlled hydrolytic aggregation. A mixture of (TBA)₂WO₄,
WO(OMe)₄ and {Sn(O[superscript(t)] Bu)₄} was partially hydrolysed in a non-aqueous solvent to
give (TBA)₃[(MeO)SnW₅O₁₈] (1).
¹¹⁹Sn NMR INEPT and selective tin decoupled
proton NMR experiments have been carried out to determine the axial and equatorial
²J {¹¹⁹Sn*¹⁸³W} coupling constants and to estimate
³J{¹¹⁹Sn¹H} and ³J{¹¹⁷Sn¹H}
coupling constants. The electrochemistry of (1) was studied by cyclic voltammetry
(CV), and was shown to undergo a reversible one electron reduction close to the
solvent limit.
Hydrolysis of [(MeO)SnW₅O₁₈]³⁻
produced [(OH)SnW₅O₁₈]³⁻
(2) which is stable in the
solid state but in solution undergoes a condensation reaction to give [(µ-
O)(SnW₅O₁₈)₂]⁶⁻
(3).
The chloro stannotungstate [ClSnW₅O₁₈]³⁻ (4)
was also obtained during the synthesis of (1)
and (2). Compounds (2) and (4) are crystallographically isostructural and Sn hetero site was
disordered over all six metal positions in both anions. The redox properties of (4) were
studied by CV and showed an irreversible reduction peak at -1.67 V. Compound (4) is stable in air and did not react with H₂O or PhOH but did react with MeOH or NaOMe
to give (1). It also reacted with diisopropylamine (DIPA) to produce the H-bonded
aggregate [(Prⁱ₂NH₂)₂(µ-O)(SnW₅O₁₈)₂]⁴⁻. The adduct structure is related to a recently
characterised titanium analogue [(µ-O)(TiW₅O₁₈H)₂]⁴⁻
, which forms the H-bonded
THF adduct [(µ-O)(TiW₅O₁₈H)₂(THF)]⁴⁻
.
2D-¹H EXSY NMR studies of the mixtures of (1) and MeOH did not show any
exchange peaks between (1) and methanol which demonstrates that exchange is slow
but reactivity studies of (1) have been carried out with various alcohols and phenols to
give substituted products. Sterically smaller alkyl groups gave trans disordered
structures, but no disorder is present in structures of anion with bulkier aliphatic
alkoxide and aryloxide groups. Hydrogen bonding was observed between the POM
cage and pendant phenolic OH groups All the alkyl and aryloxido derivatives have
been characterised by single crystal X-Ray diffraction,
¹H and multinuclear NMR
spectroscopy, infrared spectroscopy and CHN analysis.
Preliminary studies to explore the immobilisation of metal alkoxides [Ti(OPrⁱ )₄] and (1)
on ~30% OH functionalised Si(111) surfaces have been carried out. The attempted
covalent immobilisation of (1) to Si(111) surfaces appeared to be successful from
Atomic Force Microscopy (AFM) measurements.
Type
Thesis, PhD Doctor of Philosophy
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