Metal open sites

Another SBU with open metal sites is the tri-p-oxo carboxylate cluster (see Section 4.2.2 and Figure 4.2). The tri-p-oxo Fe " clusters in MIL-100 are able to catalyze Friedel-Crafts benzylation reactions [44]. The tri-p-oxo Cr " clusters of MIL-101 are active for the cyanosilylation of benzaldehyde. This reaction is a popular test reaction in the MOF Hterature as a probe for catalytic activity an example has already been given above for [Cu3(BTC)2] [15]. In fact, the very first demonstration of the catalytic potential of MOFs had aheady been given in 1994 for a two-dimensional Cd bipyridine lattice that catalyzes the cyanosilylation of aldehydes [56]. A continuation of this work in 2004 for reactions with imines showed that the hydrophobic surroundings of the framework enhance the reaction in comparison with homogeneous Cd(pyridine) complexes [57]. The activity of MIL-lOl(Cr) is much higher than that of the Cd lattices, but in subsequent reaction rans the activity decreases [58]. A MOF with two different types of open Mn sites with pores of 7 and 10 A catalyzes the cyanosilylation of aromatic aldehydes and ketones with a remarkable reactant shape selectivity. This MOF also catalyzes the more demanding Mukaiyama-aldol reaction [59]. 

Additional adsorption sites are provided on open metal sites, when available. [Cu3(BTC)2] is performant in the selective adsorption and separation of olefinic compounds. The highly relevant separations of propene from propane and of isobutene from isobutane have been accomplished with separation factors of 2.0 and 2.1, respectively [101, 102]. [Cu3(BTC)2] also selectively takes up pentene isomers from aliphatic solvent in liquid phase, and even discriminates between a series of cis- and trans-olefin isomer mixtures with varying chain length, always preferring a double bond in cis-position. This behavior is ascribed to tt -complexation with the open Cu sites [100]. 

The confinement effects of the narrow pore on the ILs and the ionic interactions between [BMIM] favor the open pore while the anion, [PF6], was attached to the open metal sites, was observed in a simulation study. It was ascertained that C02 was favorably attached to the [PF6] anions sites. The study demonstrated that IL/MOF composites are a potential candidate for C02 adsorption and have displayed significantly high CO2/N2 selectivity. To the best of our... 

Chen B, Ockwig NW, Millward AR, Conteras DS, Yaghi OM, (2005). High H2 adsorption in a microporous metal-organic framework with open metal sites. Angewandte Chemie International Edition. 44,4745 1749... 

The importance of the pore size in Prussian blue analogues is supported by differential pair distribution function analysis of X-ray and neutron scattering data of hydrogen- and deuterium-loaded Mn3[Co(CN)6]2 [119]. This shows that no evidence for adsorption interactions with unsaturated metal sites exists and that the hydrogen molecules are disordered about the center of the pores defined by the cubic framework. In conclusion, experimental results indicate that optimum pore dimensions in Prussian blue analogues are predominantly responsible for the heat of adsorption at low loadings rather than the polarizing effect of open metal sites. 

Chen, B.L., Yang, Y., Zapata, F., etcd. (2007) Luminescent open metal sites within a metal-organic framework for sensing small molecules. Advanced Materials, 19, 1693-1696. 

Tetracarboxylate ligands such as 1,2,3,4-cyclobutanetetracarboxylate (190),287 1,3,5,7-adaman-tanetetracarboxylate (191),288 a bis-isophthalate derivative (192),289 (27 ,4i ,6i ,87 )-l,9-dihy-droxy-3,5,7-trioxanonane-2,4,6,8-tetracarboxylate (193),290 and 1,2,4,5-benzenetetracarboxylate (194)291 have been synthesized and applied as building blocks in open-metal-site porous materials. 

Q. Yang and C. Zhong, Understanding Hydrogen Adsorption in Metal-organic Frameworks with Open Metal Sites a Computational Study. J. Phys. Chem. B, 2006, 110, 655-658. 

If one can prepare PCP nanochannels with surface functional Lewis acidic or basic moiety [51], they can be applied to catalytic pores for a variety of polymerization reactions. In this regard, most PCPs have pore surfaces with coordinatively saturated functional groups. The introduction of Lewis-acidic sites into the pore however, requires coordinatively unsaturated metal cation(s) (open metal site(s)) in the pore surface [35, 36, 52-54], Taking into account that the coordinative saturation of metal ions in PCPs tends to occur in usual frameworks, a clue to create open... 

Yazaydin AO, Benin Al, Faheem SA et al (2009) Enhanced C02 adsorption in metal-organic frameworks via occupation of open-metal sites by coordinated water molecules. Chem Mater 21 1425-1430... 

Britt D, Furukawa H, Wang B et al (2009) Highly efficient separation of carbon dioxide by a metal-orgemic framework replete with open metal sites. Proc Natl Acad Sci USA 106 20637-20640... 

Recently, Phan et al. (2013) prepared an open metal site MOF Cu(BDC) and successfully applied it to the modified Friedlander reaction. In the presence of 3-5 mol% Cu(BDC), several 2-aryl quinolines were efficiently synthesized from 2-aminobenzyl alcohol and methyl ketones (Scheme 4.42). Different from the previous reports, the reaction of p-methoxyacetophenone or p-nitroacetophenone did not occur in this catalytic system. 

Phan, N. T. S., Nguyen, T. T., Nguyen, K. D., and Vo, A. X. T. 2013. An open metal site metal-organic framework Cu(BDC) as a promising heterogeneous catalyst for the modified Friedlander reaction. AppZ. Catal. A. 464-465 128-135. 

MOFs Formula BET surface area (mVg) Pore/channel diameter (A) Window diameter (A) Open metal sites. Thermostability (°C) Moisture ... 

The comparison of hydrogen uptake in the same MOFs with and without unsaturated metal sites will directly demonstrate how the open metal sites affect the hydrogen adsorption, e.g., SNU-5, Cu2(abtc), with open metal sites can store 2.87wt% H2 at 77K and Ibar, compared to 1.83 wt% on SNU-5, Cu2(abtc)(dmf)2, without open metal site [131]. The capacity of Mn-BTT with available Mn + sites (1 m ) was 6.9 wt% v.s. 3.9 wt% of Mn-BTT with Mn " sites passivated by DMF (1 ) [126] (Figure 6.12). Pretreatment processes and alignment of the open... 

FIGURE 6.13 The synthesis, open metal site alignment, and hydrogen uptake of the two MOF polymorphs PCN-12 and PCN-12 (Ref. [127]). 

Zhou W, Wu H, Yildirim T (2008) Enhanced H2 adsorption in isostructural metal-organic frameworks with open metal sites strong dependence of the binding strength on metal ions. J Am Chem Soc 130 15268-15269... 

Zhang Z, Xiang S, Rao X et al (2010) A rod packing microporous metal-organic framework with open metal sites for selective guest sorption and sensing of nitrobenzene. Chem Commun 46 7205... 

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