ABSTRACT
The reactions of Zn(NO3)2·6H2O with the polycarboxylic acids 1,3-benzenedicarboxylic acid (H2mbdc), 1,4-benzenedicarboxylic acid (H2bdc), 1,3,5-benzenetricarboxylic acid (H3btc) and 4,4'-biphenyldicarboxylic acid (H2bpdc) in the presence of methyl viologen iodide ([MV]I2) in DMF gave anionic frameworks with methyl viologen species incorporated as counter-ions. When the reactions were carried out at 120 °C, the blue products [MV][Zn3(mbdc)4] (1-ht), [MV]0.44[H2MV]0.36[NMe2H2]0.4[Zn3(bdc)4]·0.6DMF (2-ht), [MV]0.5[Zn(btc)]·DMF (4-ht) and [MV][Zn4(bpdc)5]·8DMF·10H2O (5-ht) were formed, and these were shown to contain the radical cation [MV]Ë+. In contrast, the same reactions carried out at 85 °C gave orange isostructural compounds containing the dication [MV]2+. Similar observations were made for reactions with ethyl viologen bromide. The compounds 1-ht, 2-ht and 4-ht contain similar framework topologies to analogues in which NMe2H2+ is the included cation. In contrast, 5-ht is based on a previously unreported interpenetrated network. Compound 2-ht contains the protonated species [H2MV]2+ in addition to [MV]Ë+ and the crystal structure shows that the two rings in the former are staggered with respect to each other. This species is believed to form under the reaction conditions employed in the synthesis and the formation of [H2MV]2+ is suppressed by using an alternative approach in which methyl viologen is formed in situ from viologen diacetic acid. In the bdc-containing products, the radical cation is rapidly oxidised to the dication on exposure to air, as witnessed by the colour change from blue to orange. This change is reversed either by heating to 120 °C or exposure to UV radiation, both under nitrogen. This is in contrast to observations with the mbdc and btc analogues 1-ht and 4-ht, as in these compounds the blue colour persists for weeks. The difference can be related to the structures, with the channels present in 2-ht allowing oxygen to reach the radical cations.
ABSTRACT
A series of lanthanide metal-organic frameworks (MOFs) of the general formula [Ln(Hodip)(H2O)]·nH2O (Sm, 1; Eu, 2; Gd, 3; Tb, 4; Dy, 5; Er, 6; H4odip = 5,5'-oxydiisophthalic acid) have been prepared and shown crystallographically to have isostructural three-dimensional frameworks. The fluorescence emission spectra of the europium compound 2, which is red, and the terbium compound 4, which is green, show characteristic peaks for transitions involving the metal centres, whereas that for the gadolinium compound 3 is dominated by transitions involving Hodip. Using a 1 : 1 : 1 mixture of europium, gadolinium and terbium nitrates in the synthesis resulted in the mixed-metal MOF [Gd0.17Tb0.19Eu0.64(Hodip)(H2O)]·nH2O 7, for which the ratio of the metal ions was determined using EDX spectroscopy. The fluorescence emission spectrum of 7 is dominated by europium emission bands reflecting the higher proportion of Eu3+ centres and quenching of the terbium fluorescence by metal-to-metal energy transfer. A series of core-shell MOF materials based on the Ln(Hodip)(H2O) framework have been prepared in order to isolate the lanthanides in different domains within the crystals. The emission spectra for materials with Gd@Tb@Eu (8) and Tb@Eu@Gd (9) are dominated by terbium emissions, suggesting that physical separation from europium suppresses quenching. In contrast, the material with Eu@Gd@Tb (10) shows only broad ligand bands and europium emissions. This confirms that core-shell MOFs have different fluorescence properties to simple mixed-metal MOFs, demonstrating that the spatial distribution of the metals within a mixed-lanthanide MOF affects the fluorescence behaviour.
ABSTRACT
Postsynthetic modification (PSM) of amino-functionalized metal-organic frameworks (MOFs) to those bearing pendant ß-amidoketone arms using diketene is herein reported. Three unique MOF families demonstrate the scope of this transformation, which both is atom-economical and yields high conversions. In each case, the crystallinity was retained, and instances of exceptional solid-state ordering were observed in the PSM products, which has allowed detailed crystallographic characterization in multiple instances.
ABSTRACT
A new approach is reported for tailoring the pore geometry in five series of multivariate metalorganic frameworks (MOFs) based on the structure [Zn2(bdc)2(dabco)] (bdc = 1,4-benzenedicarboxylate, dabco = 1,8-diazabicyclooctane), DMOF-1. A doping procedure has been adopted to form series of MOFs containing varying linker ratios. The series under investigation are [Zn2(bdc)(2-x)(bdc-Br)x(dabco)]·nDMF 1 (bdc-Br = 2-bromo-1,4-benzenedicarboxylate), [Zn2(bdc)(2-x)(bdc-I)x(dabco)]·nDMF 2 (bdc-I = 2-iodo-1,4-benzenedicarboxylate), [Zn2(bdc)(2-x)(bdc-NO2)x(dabco)]·nDMF 3 (bdc-NO2 = 2-nitro-1,4-benzenedicarboxylate), [Zn2(bdc)(2-x)(bdc-NH2)x(dabco)]·nDMF 4 (bdc-NH2 = 2-amino-1,4-benzenedicarboxylate) and [Zn2(bdc-Br)(2-x)(bdc-I)x(dabco)]·nDMF 5. Series 1-3 demonstrate a functionality-dependent pore geometry transition from the square, open pores of DMOF-1 to rhomboidal, narrow pores with increasing proportion of the 2-substituted bdc linker, with the rhomboidal-pore MOFs also showing a temperature-dependent phase change. In contrast, all members of series 4 and 5 have uniform pore geometries. In series 4 this is a square pore topology, whilst series 5 exhibits the rhomboidal pore form. Computational analyses reveal that the pore size and shape in systems 1 and 2 is altered through non-covalent interactions between the organic linkers within the framework, and that this can be controlled by the ligand functionality and ratio. This approach affords the potential to tailor pore geometry and shape within MOFs through judicious choice of ligand ratios.
