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Metal-Organic Frameworks (MOFs)

 

Porous metal-organic frameworks (MOFs) are a rapidly emerging class of multifunctional hybrid materials, that might be useful for diverse technical applications such as gas or liquid adsorption and separation, molecular recognition, or catalysis.[i] Combining polycarboxylate ligands and (transition) metal ions, moderately robust MOFs can be prepared, among which 1,4-benzenedicarboxylate (BDC, terephthalic acid) and 4,4’-biphenyldicarboxylate (BPDC) represent archetypical linkers leading to highly porous non-interpenetrated frameworks such as the well-known MOF-5 ([Zn4O(BDC)3])[ii] or IRMOF-9 ([Zn4O(BPDC)3]).[iii]

MOF-5

MOF-5

 

These microporous MOFs in general show good thermal stabilities (decomposition occurs at T > 350°C). A fundamental disadvantage, however, is their low hydrolytic stability: Decomposition of the framework occurs rapidly if the gas or liquid phase contain a few percents of H2O,[iv] which imposes severe limitations on their usage in catalytic oxygenation reactions, where water constitutes a major reaction product.

 

Our current investigations focus on hydrolytically more stable redox active MOFs.

We achieve these advanced properties by using pyrazolate and triazolate linker instead of carboxylate groups and assembling of transition metal ions instead of zinc.

 

MFU-1

(a) Replacing acetate (ac) from basic zinc acetate  (= [Zn4O(ac)6]) with 1,4-benzenedicarboxylate (BDC) leads to the well-known metal-organic framework “MOF-5”  (= [Zn4O(BDC)3]).

(b) Similarly the novel redox-active MOF (“MFU-1”) can be derived from the pyrazolate (pz)-containing complex [CoII4O(3,5-dmpz)6] by replacing pyrazolate ligands with 1,4-bis-4’-(3’,5’-dimethyl)-pyrazolylbenzene (H2-BDPB).

 

Examples

MFU is a acronym for Metal-Organic Framework Ulm University. Here we want to present two examples:

1.) The redox-active cobalt(II)-containing MFU-1 is structually analogue to MOF-5. To the best of our knowledge it is the first MOF where catalytic test reactions clearly point to a heterogeneous catalytic reaction which takes place inside the pores of the MOF lattice.

2.) MFU-4, a zinc(II)-containing MOF with a bistriazolate ligands has a remarkable selective adsorption of hydrogen over nitrogen at -196°C.

MFU-1a

MFU-1

 

MFU-4

MFU-4

 


[i] Recent reviews highlighting different aspects of MOF applications: a) M. Eddaoudi, D. B. Moler, H. Li, B. Chen, T. M. Reineke, M. O'Keeffe, O. M. Yaghi, Acc. Chem. Res. 2001, 34, 319–330; b) G. Férey, C. Mellot-Draznieks, C. Serre, F. Millange, Acc. Chem. Res. 2005, 38, 217–225; c) U. Müller, M. Schubert, F. Teich, H. Puetter, K. Schierle-Arndt, J. Pastré, J. Mat. Chem. 2006, 16, 626–636.

[ii] H. Li, M. Eddaoudi, M. O’Keeffe, O. M. Yaghi, Nature 1999, 402, 276–279.

[iii] M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M. O’Keefe, O. M. Yaghi, Science 2002, 295, 469–472.

[iv] J. A. Greathouse, M. D. Allendorf, J. Am. Chem. Soc. 2006, 128, 10678–10679; Erratum: J. Am. Chem. Soc. 2006, 128, 13312.