Ion pairing in clusters and ion transport in porous nanocapsules

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Ion pairing in clusters and ion transport in
porous nanocapsules
Controlling complex function in
polyoxometalate-based systems

• Craig L. Hill and Group
• M. Khaled Sarker
• Emory University
• A. Merca, H. Bögge, M. Schmidtmann, A. Müller

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Synthetic units synthons
Structural units tectons
Catalytic units ?
Sensing units ?
Other macro, micro or nanofunction units
Incorporate functionality into the building
blocks
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Structural units (building blocks) in nanospheres
and nanorings
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d1 delocalized gt very intense IVCT transitions
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Catalytic units for functional materials
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Sensing and catalytic units in POM-based materials
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POM-based network color change sensor and catalyst
Pores not large enough for effective sensing and
catalysis !
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Carboxylate-terminated triesterified V6O19
(TBA)3H3V10O28

DMA, 85 C, 22h under O2
Yield 29 (based on vanadium)
TBA (n-C4H9)4N
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Pure by 1H and 51V NMR
TBA (n-C4H9)4N

S
water
CH2OV

S DMSO-d6 D DMF ? (n-C4H9)4N


D
D
CH2N
o
m
D
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Formation of catalytic sensing MOF
1.1 Ln(NO3)3

DMA or DMF (10 mM) (Ln Gd, Tb, or Yb)
TBA salt in DMF (10 mM)
stir 1h
slow MeOH diffusion 5 days
Yield of MOF 60-70 based on (HOOC-tris)2V6
unit)
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nanocapsules things of beauty
Goal to understand all their physico- chemical
dynamic properties
Insights from and into Nature
Do they have uses? (function beauty)
Achim Müller ? W. H. Casey, J. R. Rustad MM code
developed by JRR
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ion pairing stoichiometry K values
protonation, geometric structure, electronic
structure, reduction potentials, other
thermodynamic properties
experimental computational
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Nanocapsule counterion chemistry

• Ih point group

12 C5, 12 C52,
12 S10, 12 S103
20 C3, 20 S6
15 C2, 15?, i
Internal (SO4K)5 rings lower symmetry
AM pict.
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Arrangement of cations in Mo72V30
Cations have D5d point group symmetry
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Arrangement of cations in keplerates
Cations have icosahedral symmetry in a regular
keplerate
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Al3-capped large ball keplerate (Mo2 linkers)

• (CH3)2NH254Al6(MoVI)MoVI5O21(H2O)612MoV2O4(SO
  4)30250H2O
• X-ray (Bögge) Ten Al3 bound to outsides of
  pores none located inside
• Myriad challenges
• Fractional occupany,
• position disorder,
• etc.

• Merca, H. Bögge,
• M. Schmidtmann, A. Müller

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Al3 binding to nanocapsule

• Al3 bound outside pores by H bonds (2.6 3.2 Å)

• Al3 not located but sulfates suggest location
  near C3

• Low occupancy positional disorder --
  refinement continuing
• (Bögge, Merca)

D. Rehder, A. Müller, co-workers Li NMR
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27Al NMR spectra before and after excess Al3
freely diffusing Al(H2O)63
after
before
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27Al NMR experimental

• 27Al NMR spectra collected at 156.2 MHz on a
  Varian UNITY 600 MHz spectrometer.
• External standard aluminum nitrate solution
  (1.1 mol/kg)
• Al(H2O)63 0 ppm
• NMR spectra were processed using NUTS (by Acorn
  NMR Inc.), MS Excel and Varian NMR software.
• Deconvoluted spectra were used to calculate peak
  areas.

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spectrum acquisition

• Keplerate (50 mg) was dissolved in 0.75 ml D2O
• Almost saturated solution
• AlW12 (20 mg) added as reference
• T1 0.1 s at 55?C, increases with temperature
• Magnetization recovery delay (d1)0.5 s
• 5x T1 to allow linear response
• Probe tuned to sample for further signal
  enhancement
• 1024 scans for high resolution spectra
• Acquisition time 30 min (for high res)
• Temperature 23?C - 55?C

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T1 relaxation

• T1, spin-lattice relaxation time,
• the rate at which magnetization recovers in the
  z axis
• ? 2?e2qzzQ/h
• ? (?yy-?xx)/?zz asymmetry parameter
• ?c correlation time

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Many other sources of error

• Acquisition time
• Spectra take 2- 5 minutes to acquire, thus
  producing a snapshot that is an average of a
  changing spectrum
• Spectrum noise
• Shortened acquisition time results in higher
  noise
• High resolution spectra take 15-30 minutes to
  acquire

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Effect of temperature on 27Al NMR spectrum
Linewidth increases with temperature (characterist
ic of Al(H2O)63)
23? C
55? C
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Effect of removal of Al3
Selective binding of Al(H2O)63 using
2-hydroxypyridine-N-oxide (pyrh) -- same chem
shift
Al(H2O)5(OH)2
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Evidence for peak assignments
T 55 ?C
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Temperature dependence of 27Al NMR peak integrals

• Constant reference
• Peak areas are slightly dependent of temperature

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Variation of NMR spectrum of keplerate with
temperature
23? C
56? C
23? C
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Sample integrity 27Al peak position with
temperature

• Temperature-dependent chemical shifts are
  reversible
• Thus, sample is stable during changes in
  temperature

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Al3-capped nanocapsules in solution
keplerate
Al3
Al3

Al3
pore-capping
internal
free
Full kinetic model very difficult impact of one
Al3 association on others, etc., etc.
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Equilibrium constants
Al3 P Al (P)3
Al3(P) I P Al (I)3
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Temperature dependence of equilibrium constants
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Temperature dependence of ?G
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?Ho and ?So (approx.) for Al3/nanocapsule

• Al3(free) Al3(pore)
• ?S 130 J mol-1 K-1
• ?H? -20 kJ mol-1
• Al3(pore) Al3(internal)
• ?S 19 J mol-1 K-1
• ?H? 4.7 kJ mol-1
• Favorable entropy and enthalpy associated with
  cation binding to outside of pore
• Energy is required to take cation into the
  keplerate (unfovaorable electrostatic repulsion?)

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Removal of pore-capping aluminum cations by
2-hydroxypyridine-N-oxide
Pore-capping Al3 is removed
Free Al3 is restored
Temp 55? C
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Determination of rate of removal of porecapping
aluminum cations
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Determination of rate of removal of porecapping
aluminum cations
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Removal of pore-capping aluminum cations by
2-hydroxypyridine-N-oxide
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Rate of removal of pore-capping Al3

• Relatively slow rate of exchange with freely
  diffusing Al3 in solution
• Rates increase with temperature as expected
• Rates calculated vary from
• 2 X 10-2 s-1 at 55 C
• 6 X 10-4 s-1 at 40 C

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Thank You

• Questions?