Ion pairing in clusters and ion transport in porous nanocapsules

1. 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

2. Functional nanosystems composed of fundamental units Synthetic units: synthons Structural units: tectons Catalytic units: ? Sensing units: ? Other macro, micro or nanofunction units • Incorporate functionality into the building blocks

3. Structural units (building blocks) in nanospheres and nanorings

4. POM-based detection and catalytic O2/air-based oxidation POMox substrate • pollutants (environment) • toxins (protection) • biological targets (medical applications) tunable extremely selective 1/2 O2 oxidized product POMred POM Net rxn: substrate + O2 oxidized product (catalyst) d1 delocalized => very intense IVCT transitions

5. Selective O2-based organic oxidations Okun, N. M.; Anderson, T. M.; Hill, C. L. J. Am. Chem. Soc. 2003, 125, 3194. Enantiopure POMs Fang, X.; Anderson, T. M.; Hill, C. L. Angew. Chem. Int. Ed.2005, 44, 3540 (feature article) Noble metal-oxo compounds & O2-based oxidations Science, 2004, 306, 2074. J. Am. Chem. Soc.2005, 127, 11948. Catalytic units for functional materials

6. 2- L L linker linker L L Oxygen Vanadium Sensing and catalytic units in POM-based materials

7. 2- M(NO3)2 + M2+ = Mn2+, Co2+, Ni2+, or Zn2+ [M(H2O)2(DMF)2{4-pyV6}] Oxygen Vanadium Vanadium Oxygen Nitrogen Carbon M 2+ J.-W. Han, K. I. Hardcastle, C. L. Hill, EJIC, 2006, 2598-2603 POM-based network color change sensor and catalyst Pores not large enough for effective sensing and catalysis !

8. Carboxylate-terminated triesterified {V6O19} 2 - (TBA)2 (TBA)3[H3V10O28] + DMA, 85 °C, 22h under O2 Yield: 29% (based on vanadium) TBA = (n-C4H9)4N

9. 2 - (TBA)2 o m 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

10. Formation of catalytic & sensing MOF 2 - 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)

11. X-ray structure of giant-pore detecting & catalytic material based on bis(triester)V6 units POM (V6) units detect and catalyze air-based oxidations 28.6 Å Ln2(OOCR)4 units catalyze hydrolysis; Detect by emission quenching These two-dimensional (2D) coordination layers are linked by hydrogen bonds to form three-dimensional porous structure.

12. Pores of detecting, catalytic nanomaterial

13. 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

14. Towards understanding POM reactivity clarify Dynamics: kinetics & mechanism meaningful creative design of functional POMs & POM materials • ion pairing stoichiometry K values • protonation, • geometric structure,• electronic structure, • reduction potentials, • other thermodynamic properties experimental & computational

15. 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.

16. Arrangement of cations in Mo72V30 Cations have D5d point group symmetry

17. Arrangement of cations in keplerates Cations have icosahedral symmetry in a regular keplerate

18. Al3+-capped “large ball” keplerate (Mo2 linkers) [(CH3)2NH2]54Al6{(MoVI)MoVI5O21(H2O)6}12{MoV2O4(SO4)}30]·250H2O 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

19. known location possible internal location Al3+ binding to nanocapsule • Al3+ not located but sulfates suggest location near C3 • Al3+ bound outside pores by H bonds (2.6 – 3.2 Å) • Low occupancy; positional disorder -- refinement continuing • (Bögge, Merca) D. Rehder, A. Müller, co-workers: Li NMR

20. 27Al NMR spectra before and after excess Al3+ freely diffusing [Al(H2O)6]3+ after before

21. 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)6]3+ = 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.

22. 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

23. 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

24. 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

25. Effect of temperature on 27Al NMR spectrum Linewidth increases with temperature (characteristic of [Al(H2O)6]3+) 23 C 55 C

26. Effect of removal of Al3+ Selective binding of [Al(H2O)6]3+ using 2-hydroxypyridine-N-oxide (pyrh) -- same chem shift [Al(H2O)5(OH)]2+

27. 1.5 ppm • freely diffusing • Removed first by chelator • Increases on addition of Al3+ • Chemical shift • Linewidth • ~0 ppm • Internal • Removed last by chelator • Chemical shift lowered by binding to sulfate • 16.5 ppm • capping outside of pore • Removed second by excess chelator • Asymmetry tends to increase • chemical shift Evidence for peak assignments T = 55 C

28. Temperature dependence of 27Al NMR peak integrals • Constant reference • Peak areas are slightly dependent of temperature

29. Variation of NMR spectrum of keplerate with temperature 23 C 56 C 23 C

30. Sample integrity: 27Al peak position with temperature • Temperature-dependent chemical shifts are reversible • Thus, sample is stable during changes in temperature

31. + Al3+ 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.

32. Equilibrium constants Al3+ + P [Al (P)]3+ Al3+(P) + I P + [Al (I)]3+

33. Temperature dependence of equilibrium constants

34. Temperature dependence of G

35. 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?)

36. Removal of pore-capping aluminum cations by 2-hydroxypyridine-N-oxide Pore-capping Al3+ is removed Free Al3+ is restored Temp = 55 C

37. Determination of rate of removal of porecapping aluminum cations

38. Determination of rate of removal of porecapping aluminum cations

39. Removal of pore-capping aluminum cations by 2-hydroxypyridine-N-oxide

40. 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

41. Thank You • Questions?