C O M M U N I C A T I O N S
sufficiently robust that the pore solvent molecules can be removed
(and commensurately higher porosity and catalytic rates can be
seen).
Acknowledgment. We thank the ARO (Grant W911NF-05-1-
0200) and TDA Research Corp. (Grant W911NF-04-C-0136) for
support and K. I. Hardcastle and X. Fang for X-ray crystallography.
Supporting Information Available: Experimental details, CIF files
for squeezed and nonsqueezed Tb1, table of crystallographic informa-
tion of H21 and Tb1, space-filling model of channel structure, catalytic
data for PrSH and THT oxidation, TGA and DSC for Tb1, powder
X-ray patterns, and IR data. This material is available free of charge
Figure 2. Aerobic oxidation of PrSH catalyzed by Tb1. PrSH (0.662 mmol,
0.220 M) decane (internal standard), and the catalyst (0.0074 mmol or
milliequivalents of V6 units in Tb1), were stirred in chlorobenzene in a
Schlenk tube fitted with a PTFE plug under air at 45 °C. The control reaction
shown was run under identical reaction conditions except without the
catalyst, Tb1. See text for results of other control reactions.
References
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273 K).24 Although this surface area is still much smaller than the
PLATON calculated void volume because of the residual DMF
molecules in the pores, the CO2 adsorption data show that Tb1
can uptake some guest molecules including potential targets for
catalytic oxidative decontamination.
Tb1 catalyzes sulfoxidation by peroxides (rates are increased
several-fold over the Tb1-free control reactions; see SI) but more
importantly, Tb1 catalyzes O2-based oxidations and does so under
very mild conditions. The aerobic oxidation of PrSH, a model for
odorants and mild toxics ubiquitous in human environments, was
examined (Figure 2). Tb1 catalyzes eq 1 producing 18.5 turnovers
based on the molar equivalents of V6 groups in the Tb1 (41% yield
of the desired nonodorous disulfide at 45 °C after 30 days), using
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2PrSH + 0.5O2 f PrSSPr + H2O
(1)
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only ambient air as the oxidant. Four control reactions were run:
(1) no catalyst, (2) TbCl3 only, (3) a strong acid (p-TsOH) only
(all three gave no disulfide) and (4) an equivalent molar quantity
of soluble monomer, 1 (half as active as insoluble Tb1). Tb1 can
be isolated and reused without loss of catalytic activity while the
supernatant shows no catalytic activity. The IR spectra (Figure S11)
and X-ray powder diffraction patterns (Figure S3) of Tb1 collected
before and after the catalytic reactions indicate the framework
structure is maintained under turnover conditions. Kinetic studies
of eq 1 catalyzed by Tb1 indicate that the rate is first order in both
PrSH and Tb1 (Figures S9 and S10) but independent of the partial
pressure of O2. The same reaction (PrSH + Tb1) under N2 results
in reduction of the hexavanadate units (indicated by the change in
color from orange to green; see SI) and subsequent loss of the
catalytic activity. Adding O2 or air reoxidizes the V6 units. All these
findings are consistent with a mechanism involving rate-limiting
bimolecular reaction between of PrSH and the V6 units in Tb1
primarily on the outside of the particles and fast reoxidation of the
reduced V6 units in Tb1 by O2/air.
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(20) As is the usual case for highly porous materials, the pores of Tb1 contain
many disordered and dynamic solvent molecules which significantly
impact the R value. To improve the refinement, the SQUEEZE routine of
PLATON was applied to treat the diffuse electron density. Crystal data
for Tb1: monoclinic P21/c, a ) 13.102(4) Å, b ) 26.279(9) Å, c )
26.104(6) Å, â ) 93.923(5)°, V ) 8967(5) Å, Z ) 4, Mo KR (0.71073
Å) radiation (T ) 100 K), R1 ) 0.1378 [0.0879], wR2 ) 0.3474 [0.2295]
with I > 2σ(I). Statistics after using SQUEEZE are bracketed.
(21) Elemental analysis for Tb1: [Tb(bpdo)2(H1)]‚1.5DMF‚3.0EG. Anal. Calcd
for C54.5H71.5N7.5O34.5V6Tb: C, 35.42; H, 3.90; N, 5.68; V, 16.5; Tb, 8.6.
Found: C, 35.51; H, 4.02; N, 5.60; V, 16.0; Tb, 8.5.
-
(22) IR shows no typical vibrational band from NO3 ((D3h) 1734-1790,
1300-1550, 801-834 cm-1; KNO3 has 1770, 1360, and 827 cm-1).
Nyquist, R. A.; Putzig, C. L.; Leugers, M. A. The Handbook of Infrared
and Raman Spectra of Inorganic Compounds and Organic Salts; Academic
Press: San Diego, CA, 1997; Vol. 4.
In summary, we have prepared a rare example of a heterogeneous
aerobic oxidation catalyst, Tb1, a three-dimensional coordination
polymer from a predesigned bis(triester)hexavanadate derivative
(1), bis(pyridine-N-oxide) (bpdo), and Tb(III) ions. Catalytic
turnover does not disrupt the open-framework structure. However,
the channels in Tb1 are largely blocked by solvent molecules. Thus
future work will seek similar open-framework redox-active materials
(23) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7-13.
(24) Adsorption is an exothermic process so as the temperature is raised the
amount of sorption generally decreases. However, constrictions in very
small micropores constitute an energy barrier to the adsorbate’s passage
through the pores, resulting in an activated uptake. Gregg, S. J.; Sing, K.
S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press:
London, 1982.
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