Denise Zacher

Controlling interpenetration in metal|[ndash]|organic frameworks by liquid-phase epitaxy

Metal-organic frameworks (MOFs) are highly porous materials generally consisting of two building elements: inorganic coupling units and organic linkers. These frameworks offer an enormous porosity, which can be used to store large amounts of gases and, as demonstrated in more recent applications, makes these compounds suitable for drug release. The huge sizes of the pores inside MOFs, however, also give rise to a fundamental complication, namely the formation of sublattices occupying the same space. This interpenetration greatly reduces the pore size and thus the available space within the MOF structure. We demonstrate here that the formation of the second, interpenetrated framework can be suppressed by using liquid-phase epitaxy on an organic template. This success demonstrates the potential of the step-by-step method to synthesize new classes of MOFs not accessible by conventional solvothermal methods.

Surface chemistry of metal-organic frameworks at the liquid-solid interface.

Metal-organic frameworks (MOFs) are a fascinating class of novel inorganic-organic hybrid materials. They are essentially based on classic coordination chemistry and hold much promise for unique applications ranging from gas storage and separation to chemical sensing, catalysis, and drug release. The evolution of the full innovative potential of MOFs, in particular for nanotechnology and device integration, however requires a fundamental understanding of the formation process of MOFs. Also necessary is the ability to control the growth of thin MOF films and the positioning of size- and shape-selected crystals as well as MOF heterostructures on a given surface in a well-defined and oriented fashion. MOFs are solid-state materials typically formed by solvothermal reactions and their crystallization from the liquid phase involves the surface chemistry of their building blocks. This Review brings together various key aspects of the surface chemistry of MOFs.

Growth Mechanism of Metal–Organic Frameworks: Insights into the Nucleation by Employing a Step‐by‐Step Route

One step at a time: The in situ monitoring of the step-by-step formation of metal-organic frameworks (MOFs) by using surface plasmon resonance (SPR), allows the nucleation process and the formation of the secondary building units to be investigated. Growth rates on functionalized organic surfaces with different crystallographic orientations can also be studied.

Deposition of microcrystalline [Cu3(btc)2] and [Zn2(bdc)2(dabco)] at alumina and silica surfaces modified with patterned self assembled organic monolayers: evidence of surface selective and oriented growth

The deposition of microcrystalline, phase pure [Cu3(btc)2] (1) and [Zn2(bdc)2(dabco)] (2) under solvothermal conditions on alumina and on silica surfaces modified with self assembled organic monolayers has been studied. The growth of 1 is remarkably surface selective and does not take place on silica but does on alumina. In contrast the growth of 2 is not surface selective at all and densely packed coatings were obtained on silica and on alumina surfaces. Most interestingly, [Cu3(btc)2] shows an oriented growth mode on both c-plane sapphire and COOH-terminated SAMs at SiO2/Si(100) wafers. The single crystals of 1 are attached to the organic surface via the (111) face, while on c-plane sapphire the crystals interact via the (100) lattice plane leading to different shapes of the crystals: pyramids on sapphire and fully developed octahedra on the COOH-SAM. The gas adsorption properties of the obtained coatings were demonstrated by reversible loading with water and with pyridine in the case of 1 and loading with [(η5-C5H5)Pd(η3-C3H5)] as a typical example of a MOCVD precursor for both 1 and 2 which allows the fabrication of Pd nanoparticle loaded MOF thin films.

Liquid-Phase Epitaxy of Multicomponent Layer-Based Porous Coordination Polymer Thin Films of [M(L)(P)0.5] Type: Importance of Deposition Sequence on the Oriented Growth

The progressive liquid-phase layer-by-layer (LbL) growth of anisotropic multicomponent layer-based porous coordination polymers (PCPs) of the general formula [M(L)(P)(0.5)] (M: Cu(2+), Zn(2+); L: dicarboxylate linker; P: dinitrogen pillar ligand) was investigated by using either pyridyl- or carboxyl-terminated self-assembled monolayers (SAMs) on gold substrates as templates. It was found that the deposition of smooth, highly crystalline, and oriented multilayer films of these PCPs depends on the conditions at the early growth cycles. In the case of a two-step process with an equimolar mixture of L and P, growth along the [001] direction is strongly preferred. However, employing a three-step scheme with full separation of all components allows deposition along the [100] direction on carboxyl-terminated SAMs. Interestingly, the growth of additional layers on top of previously grown oriented seeding layers proved to be insensitive to the particular growth scheme and full retention of the initial orientation, either along the [001] or [100] direction, was observed. This homo- and heteroepitaxial LbL growth allows full control over the orientation and the layer sequence, including introduction of functionalized linkers and pillars.

Chemistry of SURMOFs: layer-selective installation of functional groups and post-synthetic covalent modification probed by fluorescence microscopy.

Layer-selective installation of functional groups at SURMOFs (surface-attached metal-organic framework multilayers) is reported. Multilayers of [Cu(ndc)(dabco)(0.5)] grown in [001] orientation on pyridine-terminated organic self-assembled monolayers on Au substrates were functionalized with amino groups by step-by-step liquid-phase epitaxy. The method allows the growth of samples exhibiting one monolayer of functional groups at the external thin-film surface. In situ quartz crystal microbalance monitoring confirmed the presence of amino groups by turning the multilayer film from a non-reactive to a reactive material for covalent binding of fluoresceinisothiocyanate, and fluorescence microscopy displays the expected luminous property.

Surface structure of metal-organic framework grown on self-assembled monolayers revealed by high-resolution atomic force microscopy.

The surface structure of an individual metal-organic framework (MOF) microcrystal grown on a functionalized surface has been successfully investigated for the first time in air and vacuum using high-resolution atomic force microscopy. Moreover, this detailed surface analysis has been utilized to optimize the MOF formation procedure to obtain a defect-free surface structure. Comparison of obtained data with recent microscopic studies performed on the same MOF crystal but grown by a conventional procedure clearly shows a much higher quality of crystals produced by surface oriented growth. Importantly, this method of preparing crystals suitable for microscopic analysis is also much faster (3 days compared to 2 years) and, in contrast to the conventional method, produces material suitable for in situ study. These results thus demonstrate for the first time the possibility of nanoscale investigation/modification of MOF surface structure.

Methylgallium as a Terminal Ligand in [(Cp*Ga)4Rh(GaCH3)]+

As “exotic” ligands at transition-metal centers M, carbenoid Group 13 metal compounds ER (E=Al, Ga, In; R=bulky substituent: e.g. alkyl, aryl, C5Me5 (Cp*), bisketoiminates, amidinates, guanidinates) are attracting a great deal of attention because their properties can be compared with those of the related borylenes and N-heterocyclic carbenes (NHCs). In particular, complexes with ECp* ligands show interesting reactivity that is related to the soft binding properties and facile haptotropic shifts of the Cp* ring, which allows a modulation of the electrophilicity at the Group 13 center E. Accordingly, even selective protolysis of coordinated GaCp* is possible: the treatment of the complex [Pt(GaCp*)4] with [H(OEt2)2]BAr F 4 (Ar F = 3,5-(CF3)2C6H3) yields the dimer [Pt2H(Ga)(GaCp*)7] 2+ by elimination of Cp*H via the intermediate [GaCp*)4PtH] . By using the Ga transfer reagent [Ga2Cp*] , the [(GaCp*)4PtGa] + complex has been generated in which the bonding of naked Ga as a strong s/p-acceptor ligand without s-donor properties has been demonstrated. As part of our continuing work in this area, we set out to generate otherwise elusive GaR moieties by protolytic cleavage of Cp*H from coordinated Ga(R)Cp* groups. The choice of the R substituent for isolable and thus synthetically useful monovalent ER compounds is limited because of the inherent instability of E and its disproportionation into E and E. Methylgallium, for example, has to date only been studied by matrix studies at low temperatures. Very few complexes bearing ER ligands with sterically nondemanding groups R without p-donor/acceptor properties, such as the anion [{Fe(CO)4}2GaCH3] and the dimer [{Cp*IrAlEt}2], are known. [6,7] However, all these complexes feature the ER ligand in a bridging (tricoordinate) binding mode, which rules out direct comparisons with other dicoordinate (terminal) ER ligands. Analogously, the first terminal alkyl borylene complex, [Cp(CO2)MnBtBu], has been reported recently. The Mn BR (R= tBu) bond was described as weakly polar but with significant p-backbonding. The reaction of [Rh(CH3)(cod)(py)] with excess of GaCp* in hexane at room temperature leads to the substitution of the labile ligands pyridine (py) and 1,5-cyclooctadiene (cod), as well as to the insertion of the carbenoid GaCp* into the Rh CH3 s bond to give the all-Ga-coordinated neutral complex [(Cp*Ga)4Rh(h -Cp*GaCH3)] (1) in 89% yield (Scheme 1).

Use of confocal fluorescence microscopy to compare different methods of modifying metal–organic framework (MOF) crystals with dyes

Dye modified MOF microcrystals were characterized by fluorescence microscopy (FM) and confocal laser scanning microscopy (CLSM) which visualized the position and distribution of fluorescent dyes encapsulated into MOF crystals and provided proof for selective, post-synthetic covalent modification of the external surface of MOF crystals.

Step-by-step growth of highly oriented and continuous seeding layers of [Cu2(ndc)2(dabco)] on bare oxide and nitride substrates

The step-by-step growth of highly oriented and continuous thin films of [Cu2(ndc)2(dabco)] (1) at 50 °C was studied and compared with growth directly from solvothermal mother solution at 120 °C. The substrates were bare unmodified SiO2, Al2O3 grown by atomic layer deposition (ALD), Ta2O5 and Si3N4. The deposited layers of 1 were characterized via in-plane and out-of-plane X-ray powder diffraction (PXRD) and Scanning Electron Microscopy (SEM). The stepwise film formation process was studied by the variation of the reaction conditions and washing procedures indicating an island growth mode and the importance of storage effects. The highly oriented layers obtained by the step-by-step method were used as seeds for the deposition of thicker films of 500–700 nm with the same orientation directly from solvothermal mother solution.