Redox-active MOFs

The high specific surface area and the presence of redox-active centers make MOFs interesting materials for electrochemical sensing and electrocatalysis. By the first token, size selectivity can be combined with the coordinating ability of redox-active centers. An example of the possibility of selective electrochemical sensing using MOFs is shown in Figure 5.15, which displays the square-wave voltammetries... 

Suh et al. studied the nanoparticle formation in MOFs using metal salts as precursors which are reduced to form metal clusters by a special redox-active framework... 

Also interesting is the Mn(II) D-saccharide MOF ([Mn2(sac)2(H20)2] with H2sac = D-saccharic acid) with chiral helicity and redox activity centered on manganese. In this structure hydrophilic channels have been identified and two distinct redox processes (see Fig. 9) have been observed in distilled water but so far not further investigated. 

In a careful investigation of redox activity of Co-MOF-71 ([Co(bdc)(DMF)] ) it was observed that this Co-benzenedicarboxylate... 

The potential for application of highly porous electrode materials is very broad. The release of redox active and anti-bacterial components such as silver from MOF pores has been reported. Gas accumulation and storage... 

A number of research groups have investigated the uptake of redox active metal complexes as a precursor to the formation of metal nanoparticles inside MOFs. This area has recently been reviewed by Suh and co-workers, hence only representative examples will be discussed. In most examples, pores within the MOF are loaded with a metal complex or salt through CVD or through immersion of the MOF in a solution containing the complex, and subsequent chemical reduction gives the metal nanoparticle embedded in the MOF. The MOFs with embedded nanoparticles may have enhanced H2 storage because of M-H2 interactions to the particles, and they are effective solid-state catalysts. " ... 

In this section, pure MOF systems as well as composites comprising MOF-encapsulated redox-active materials or interpenetrated networks are considered (see Interpenetration and Entanglement in Coordination Polymers and Patterning Techniques for Metal-Organic Frameworks). The reversibility and stability of their electrochemical response are discussed. Stability may be defined as a retained electrochemical response on repeated cycling, whereas electrochemical reversibility is linked to the concept of fast electron transfer at the... 

The oxygen-bound sulfate complex [Co(tren)(OH2)OS02] is formed by reaction of [Co(tren)(OH2)OH] with SO3 and undergoes internal electron transfer to give cobalt(II) and SO4 with a rate constant 1.0 X 10 s" at 25 C and pH less than 5.4. Activation parameters are MI 24 kcal moF and AS 8.2 cal moF. At higher pH a bis-sulfite complex which is not subject to internal redox is formed. Infrared evidence points to both S and O binding in this species. 

Once formed, MOF materials can maintain electrochemical activity or indeed exhibit electrolytic degradation. The reverse-synthesis where the building blocks (e.g. Cu or Fe ) are removed from the MOF structure via switching of the redox state in solution (even at very low concentration or aided by formation of chloro-complexes) is commonly observed, and may be described as a CE-type reduction (a chemical dissolution step followed by the electrochemical redox switch step) as shown for the com-mercially-available iron-btc MOF Basolite-F300. Electrochemical reactivity of MOFs without direct framework degradation or reactivity may be achieved by either (i) covalently bound or (ii) physisorbed redox systems within MOF nanopores. This topic is addressed next. 

In the future, similar potential driven release mechanisms could be useful in cases where active compounds or drugs are released depending on environmental factors. However, for the redox behaviour of the MOF, this process causes irreversibility and although several redox cycles can be... 

Babu et al. investigated the case of the commercially available iron-benzenetricarboxylate-based MOF Basolite F300 and its electrocatalytic activity towards the oxidation of hydroxide to O2 (anodic water splitting). In aqueous media well-defined Fe(III/II) redox processes of the solid microcrystalline powder attached to platinum electrodes were observed, but only in the presence of hydrochloric acid. The HCl concentration effect as well as the Fe(III/II) reversible potential for this process clearly indicated a CE-type reaction with framework dissolution prior to reduction of Fe (aq)... 

The importance of MOFs in heterogeneous catalyst derives from the consideration that most of the transition metal ions have catalytic activity as Lewis acids, in redox reactions and in... 

---