Saeed Amirjalayer

Loading of porous metal–organic open frameworks with organometallic CVD precursors: inclusion compounds of the type [LnM]a@MOF-5

The highly porous coordination polymer [Zn4O(bdc)3] (bdc = benzene-1,4-dicarboxylate; MOF-5 or IRMOF-1) was loaded with typical MOCVD precursor molecules 1–10 for metals such as Fe, Pt, Pd, Au, Cu, Zn, Sn. Exposure of [Zn4O(bdc)3] to the vapour of the volatile organometallic compounds, e.g. ferrocene (3), resulted in the formation of inclusion compounds of the type [LnM]a@MOF-5, where [LnM] indicates the MOCVD precursor and a denotes the effective number of molecules per cavity of the MOF-5 lattice. The obtained inclusion compounds were characterised by C/H combustion analysis, determination of the metal content by atomic absorption spectroscopy, FT-IR and solid state NMR spectroscopy and by powder X-ray diffraction. The data prove that the host lattice and the guest molecules interact only by weak van der Waals forces without any change of the framework or the chemical nature of the included molecules. Rapid desorption is observed for small and comparably volatile compounds such as pentacarbonyliron or diethyl zinc. Less labile inclusion compounds were obtained for cyclopentadienyl complexes as guest molecules, e.g. a rather high loading of six molecules of ferrocene per cavity was observed. Careful hydrolysis/calcination of [Zn(C2H5)2]2@MOF-5 resulted in the composite (ZnO)2@MOF-5 pointing to the possibility to develop a subsequent chemistry of the embedded precursor molecules to yield novel nanocomposite materials based on MOFs as host matrices and MOCVD precursors in general.

Ab initio parametrized MM3 force field for the metal-organic framework MOF-5.

A new valence force field has been developed and validated for a particular class of coordination polymers known as nanoporous metal‐organic frameworks (MOFs), introduced recently by the group of Yaghi. The experimental, structural, and spectroscopic data in combination with density functional theory calculations on several model systems were used to parametrize the bonded terms of the force field, which explicitly treats the metal–oxygen interactions as partially covalent as well as distinguishes different types of oxygens in the framework. Both the experimental crystal structure of MOF‐5 and vibrational infrared spectrum are reproduced reasonably well. The proposed force field is believed to be useful in atomistic simulations of adsorption/diffusion of guest molecules inside the flexible pores of this important class of MOF materials.

Ballbot-type motion of N-heterocyclic carbenes on gold surfaces

Recently, N-heterocyclic carbenes (NHCs) were introduced as alternative anchors for surface modifications and so offered many attractive features, which might render them superior to thiol-based systems. However, little effort has been made to investigate the self-organization process of NHCs on surfaces, an important aspect for the formation of self-assembled monolayers (SAMs), which requires molecular mobility. Based on investigations with scanning tunnelling microscopy and first-principles calculations, we provide an understanding of the microscopic mechanism behind the high mobility observed for NHCs. These NHCs extract a gold atom from the surface, which leads to the formation of an NHC-gold adatom complex that displays a high surface mobility by a ballbot-type motion. Together with their high desorption barrier this enables the formation of ordered and strongly bound SAMs. In addition, this mechanism allows a complementary surface-assisted synthesis of dimeric and hitherto unknown trimeric NHC gold complexes on the surface.

Fast photodynamics of azobenzene probed by scanning excited-state potential energy surfaces using slow spectroscopy

Azobenzene, a versatile and polymorphic molecule, has been extensively and successfully used for photoswitching applications. The debate over its photoisomerization mechanism leveraged on the computational scrutiny with ever-increasing levels of theory. However, the most resolved absorption spectrum for the transition to S1(nπ*) has not followed the computational advances and is more than half a century old. Here, using jet-cooled molecular beam and multiphoton ionization techniques we report the first high-resolution spectra of S1(nπ*) and S2(ππ*). The photophysical characterization reveals directly the structural changes upon excitation and the timescales of dynamical processes. For S1(nπ*), we find that changes in the hybridization of the nitrogen atoms are the driving force that triggers isomerization. In combination with quantum chemical calculations we conclude that photoisomerization occurs along an inversion-assisted torsional pathway with a barrier of ~2 kcal mol−1. This methodology can be extended to photoresponsive molecular systems so far deemed non-accessible to high-resolution spectroscopy.

Amplified Vibrational Circular Dichroism as a Probe of Local Biomolecular Structure

We show that the VCD signal intensities of amino acids and oligopeptides can be enhanced by up to 2 orders of magnitude by coupling them to a paramagnetic metal ion. If the redox state of the metal ion is changed from paramagnetic to diamagnetic the VCD amplification vanishes completely. From this observation and from complementary quantum-chemical calculations we conclude that the observed VCD amplification finds its origin in vibronic coupling with low-lying electronic states. We find that the enhancement factor is strongly mode dependent and that it is determined by the distance between the oscillator and the paramagnetic metal ion. This localized character of the VCD amplification provides a unique tool to specifically probe the local structure surrounding a paramagnetic ion and to zoom in on such local structure within larger biomolecular systems.

Direct Probing of Photoinduced Electron Transfer in a Self-Assembled Biomimetic [2Fe2S]-Hydrogenase Complex Using Ultrafast Vibrational Spectroscopy

A pyridyl-functionalized diiron dithiolate complex, [μ-(4-pyCH2-NMI-S2)Fe2(CO)6] (3, py = pyridine (ligand), NMI = naphthalene monoimide) was synthesized and fully characterized. In the presence of zinc tetraphenylporphyrin (ZnTPP), a self-assembled 3·ZnTPP complex was readily formed in CH2Cl2 by the coordination of the pyridyl nitrogen to the porphyrin zinc center. Ultrafast photoinduced electron transfer from excited ZnTPP to complex 3 in the supramolecular assembly was observed in real time by monitoring the ν(C≡O) and ν(C═O)NMI spectral changes with femtosecond time-resolved infrared (TRIR) spectroscopy. We have confirmed that photoinduced charge separation produced the monoreduced species by comparing the time-resolved IR spectra with the conventional IR spectra of 3(•-) generated by reversible electrochemical reduction. The lifetimes for the charge separation and charge recombination processes were found to be τCS = 40 ± 3 ps and τCR = 205 ± 14 ps, respectively. The charge recombination is much slower than that in an analogous covalent complex, demonstrating the potential of a supramolecular approach to extend the lifetime of the charge-separated state in photocatalytic complexes. The observed vibrational frequency shifts provide a very sensitive probe of the delocalization of the electron-spin density over the different parts of the Fe2S2 complex. The TR and spectro-electrochemical IR spectra, electron paramagnetic resonance spectra, and density functional theory calculations all show that the spin density in 3(•-) is delocalized over the diiron core and the NMI bridge. This delocalization explains why the complex exhibits low catalytic dihydrogen production even though it features a very efficient photoinduced electron transfer. The ultrafast porphyrin-to-NMI-S2-Fe2(CO)6 photoinduced electron transfer is the first reported example of a supramolecular Fe2S2-hydrogenase model studied by femtosecond TRIR spectroscopy. Our results show that TRIR spectroscopy is a powerful tool to investigate photoinduced electron transfer in potential dihydrogen-producing catalytic complexes, and that way to optimize their performance by rational approaches.

Low‐Temperature CO Oxidation over Cu‐Based Metal–Organic Frameworks Monitored by using FTIR Spectroscopy

Low-temperature CO oxidation is among the most interesting reactions in heterogeneous catalysis. It is well known that this reaction can be catalyzed by gold deposited as nanoparticles on metal oxide surfaces (e.g. TiO2, MgO, and ZnO). [1] Developing highly efficient and low-cost catalysts for low-temperature CO oxidation represents a major challenge. Most recently, Cubased metal–organic frameworks (MOFs) have been identified for efficient CO oxidation at ambient pressure and elevated temperatures, namely [Cu3btc2] (btc = benzene-1,3,5-tricarboxylate ) and [Cu5(OH)2(nip)4(H2O)6] (H2O)4.25 (nip = 5-nitroisophthalate) with 100 % conversion at 240 8C and 200 8C, respectively. 3] However, the origin of the catalytic activity of such CuMOFs is not well understood. Coordinatively unsaturated metal ion sites (CUS) at the backbone of MOFs are known to play an important role for catalysis, gas storage, chemical sensing, and other applications. For catalysis, Schlichte et al. reported on the cyanosilylation of benzaldehyde and acetone with [Cu3btc2] as catalyst (HKUST-1, Figure S1) for which its activity was assigned to the Lewis acid properties of intrinsic Cu CUS at the copper-carboxylate paddle-wheel building unit (CuPW) of this prototypical MOF in its dehydrated form. The characterization of reactive CUS of MOFs with infrared spectroscopic techniques and suitable probe molecules, quite similar to classic heterogeneous catalysts, is expected to provide most valuable information for mechanistic understanding and fine tuning of the MOF materials. Herein, we present our studies on the in situ monitoring of the co-adsorption of CO and O2 at [Cu3btc2] (HKUST-1) [9] and its congener [Cu3btb2] (MOF-14, [11] btb = benzene-1,3,5-tribenzoate) by using an ultrahigh vacuum infrared spectroscopy tool (UHV-FTIRS). The high-quality IR data provide unambiguous spectroscopic evidence for the surprisingly high catalytic activity of both Cu-MOFs samples for CO oxidation at temperatures as low as 105 K, according to Scheme 1. Presented in Figure 1 are the UHV-FTIR spectra obtained after exposing activated [Cu3btc2] to CO (1 10 4 mbar) in the temperature range of 105 K to 135 K. Prior to CO adsorption, the thermal stability of the sample was confirmed up to 650 K (Figure S2). The H2O-related bands at ñ= 3647 (O H stretching mode) and 1617 cm 1 (d(H2O) scissoring mode) disappeared completely by heating the sample above 400 K, indicating the formation of Cu + CUS. The adsorption of CO at 105 K leads to the appearance of four IR bands at ñ= 2198, 2175, 2152, and 2128 cm . The dominant CO band at ñ= 2175 cm 1 exhibits a blue shift of 32 cm 1 with respect to the gas-phase value and is assigned to CO adsorbed at the intrinsic Cu + CUS of the CuPW in the axial position. After evacuation to 10 10 mbar, this band shifts to ñ= 2179 cm 1 and decreases in intensity (Figure 1 c). It is removed completely upon heating to 130 K, revealing that the majority CO species is weakly bound to intrinsic Cu + CUS sites. Density functional theory (DFT) calculations were performed in order to support the assignment of the IR bands. Based on our previous studies on copper paddle-wheel (CuPW) MOFs, we found it to be sufficient to use a non-periodic formate model of the inorganic unit to represent the carboxylate-linked Scheme 1. Low-temperature oxidation of CO to CO2 over activated Cu-based MOFs.

All-nitrogen coordinated amidinato/imido complexes of molybdenum and tungsten: syntheses and characterization.

The first all-nitrogen coordinated bis(alkylamidinato)/bis(alkylimido) complexes of molybdenum and tungsten, [Mo(NtBu)(2){(iPrN)(2)CMe}(2)]and [W(NtBu)(2){(iPrN)(2)CMe}(2)], have been synthesized and fully characterized by (1)H and (13)C NMR spectroscopy, elemental analyses, high-resolution electron impact mass spectrometry, and Fourier transform infrared spectroscopy. Density functional theory calculations of the tungsten complex allow for geometry optimization and structural characterization by assignment of the NMR data, in particular a comparison of the experimental (13)C NMR signals with the calculated ones. Both compounds sublime without decomposition at 130 °C and 1 mTorr and show rapid decomposition above 250 °C, hence representing promising vapor-phase deposition routes for metal nitride based thin-film materials.

An iron-iron hydrogenase mimic with appended electron reservoir for efficient proton reduction in aqueous media

A synthetic catalyst mimics its natural enzyme with improved stability. The transition from a fossil-based economy to a hydrogen-based economy requires cheap and abundant, yet stable and efficient, hydrogen production catalysts. Nature shows the potential of iron-based catalysts such as the iron-iron hydrogenase (H2ase) enzyme, which catalyzes hydrogen evolution at rates similar to platinum with low overpotential. However, existing synthetic H2ase mimics generally suffer from low efficiency and oxygen sensitivity and generally operate in organic solvents. We report on a synthetic H2ase mimic that contains a redox-active phosphole ligand as an electron reservoir, a feature that is also crucial for the working of the natural enzyme. Using a combination of (spectro)electrochemistry and time-resolved infrared spectroscopy, we elucidate the unique redox behavior of the catalyst. We find that the electron reservoir actively partakes in the reduction of protons and that its electron-rich redox states are stabilized through ligand protonation. In dilute sulfuric acid, the catalyst has a turnover frequency of 7.0 × 104 s−1 at an overpotential of 0.66 V. This catalyst is tolerant to the presence of oxygen, thereby paving the way for a new generation of synthetic H2ase mimics that combine the benefits of the enzyme with synthetic versatility and improved stability.

Isoreticular isomerism in 4,4-connected paddle-wheel metal–organic frameworks: structural prediction by the reverse topological approach

The theoretical structure prediction for a series of 4,4-connected copper paddle-wheel metal–organic frameworks has been performed by using the reverse topological approach, starting from the nbo-b topology. Since the rectangular-shaped tetracarboxylate linkers have a lower symmetry than the square vertices in nbo-b, two alternative insertion modes are possible for each linker. This leads, in principle, to the formation of multiple isoreticular isomers, which have been screened by a genetic global minimum search algorithm, using the first principles parameterized force field MOF-FF for structure optimization and ranking. It is found that isoreticular isomerism does, in this case, not lead to disorder but to a number of well-defined but structurally distinct phases, which all share the same network topology but have substantially different pore shapes and properties. In all cases, the experimentally observed structure is correctly predicted, but in addition a number of other slightly less stable phases are observed. Only one of these phases has been synthesized yet. The theoretical analysis of the molecular model systems of the pore cages revealed the reasons for the trends in conformational energy. This proof-of-concept study demonstrates that screening of isoreticular isomerism using an efficient but accurate force field allows prediction of the atomistic structure of even complex and flexible frameworks.