Search

Multivariate MOFs (MTV-MOFs) constructed from multiple components with atomistic precision hold the promise for many fascinating developments in both fundamental sciences and applications. Sequential linker installation can be an effective method to introduce different functional linkers into an MOF that contains coordinatively unsaturated metal sites. However, in many cases, these linkers must be installed according to a specific sequence and the complete synthetic flexibility and freedom is yet to be realized. Here, we rationally decreased the size of the primary ligand used in NPF-300, a Zr-MOF with scu topology (NPF = Nebraska Porous Framework), and synthesized its isostructure, NPF-320. NPF-320 exhibits optimized pocket sizes which allow for the post-synthetic installation of three secondary linkers in all six permuted sequences via both linker exchange and installation, forming a final quinary MTV-MOF via single-crystal-to-single-crystal transformation. With the functionalization of the linkers from the quinary MOF system, one will be able to construct MTV-MOFs not only with variable porosity but also with unprecedented complexity and encoded synthetic sequence information. The utility of sequential linker installation was further demonstrated by the construction of a donor-acceptor pair-based energy transfer system.

Nitrosobenzene (PhNO) and phenylhydroxylamine (PhNHOH) are of paramount importance because of their involvement as crucial intermediates in the biological metabolism and catalytic transformation of nitrobenzene (PhNO 2 ) to aniline (PhNH 2 ). However, a complete reductive transformation cycle of PhNO to PhNH 2 via the PhNHOH intermediate has not been reported yet. In this context, we design and construct a new thiolate-bridged dicobalt scaffold that can accomplish coordination activation and reductive transformation of PhNO. Notably, an unprecedented reversible ligand-based redox sequence PhNO 0 ↔ PhNO •- ↔ PhNO 2- can be achieved on this well-defined {Co III (μ-SPh) 2 Co III } functional platform. Further detailed reactivity investigations demonstrate that the PhNO 0 and PhNO •- complexes cannot react with the usual hydrogen and hydride donors to afford the corresponding phenylhydroxylamino (PhNHO - ) species. However, the double reduced PhNO 2- complex can readily undergo N-protonation with an uncommon weak proton donor PhSH to selectively yield a stable dicobalt PhNHO - bridged complex with a high p K a value of 13-16. Cyclic voltammetry shows that there are two successive reduction events at E 1/2 = -0.075 V and E p = -1.08 V for the PhNO 0 complex, which allows us to determine both bond dissociation energy (BDE N-H ) of 59-63 kcal·mol -1 and thermodynamic hydricity (Δ G H - ) of 71-75 kcal·mol -1 of the PhNHO - complex. Both values indicate that the PhNO •- and a dicobalt hydroxide-bridged complex. Overall, we present the first stepwise reductive sequence, PhNO 0 complex is not an efficient hydride acceptor. In the presence of BH 3 as a combination of protons and electrons, facile N-O bond cleavage of the coordinated PhNHO - group can be realized to generate PhNH 2 and a dicobalt hydroxide-bridged complex. Overall, we present the first stepwise reductive sequence, PhNO 0 ↔ PhNO •- ↔ PhNO 2- ↔ PhNHO - → PhNH 2 .

Biradicaloids in π-conjugated organic molecules have been extensively studied in recent years because of the fundamental insights into the chemical bonds and unique optical, electrical, and magnetic properties. Several studies have reported that the spin-spin interactions of biradicaloids with flexible molecular frameworks dynamically evolve correlating with molecular structural changes. Although these dynamic behaviors will provide important insights into the relationship between molecular structures and spin properties, studies on such behaviors have been limited to two-spin systems. Here, we investigated the stepwise photochromic properties of biphotochromic molecules involving multiple spin interactions by double-pulse laser flash photolysis. The one-photon photochromic reaction generates the o -biradical form as the open-closed form, which thermally isomerizes to the o -quinoidal form and reaches the thermal equilibrium state between them. The additional absorption of a photon by the open-closed form leads to the photochromic reaction of the other photochromic unit, resulting in the generation of unpaired spins at the p -position of the central aromatic bridge of the biradical or quinoidal form. Under the situation, while the interaction between the unpaired spins and the o -biradical preferentially produces the p -quinoidal form in which the antiferromagnetic interaction at the p -position is dominant, that between the spins and the o -quinoidal form kinetically produces the bis( o -quinoidal) form followed by the thermal isomerization to the thermodynamically stable p -quinoidal form. These dynamic spin-spin interactions along with the rearrangement of chemical bonds will give a deeper understanding of the singlet biradicaloids and that to bridge organic multiradicals in molecular systems to cooperative spin behaviors in bulk materials.

Iodide is an essential promoter in the industrial production of acetic acid via methanol carbonylation, but it also contributes to reactor corrosion and catalyst deactivation. Here we report that iridium pincer complexes mediate the individual steps of methanol carbonylation to methyl acetate in the absence of methyl iodide or iodide salts. Iodide-free methylation is achieved under mild conditions by an aminophenylphosphinite pincer iridium(I) dinitrogen complex through net C-O oxidative addition of methyl acetate to produce an isolable methyliridium(III) acetate complex. Experimental and computational studies provide evidence for methylation via initial C-H bond activation followed by acetate migration, facilitated by amine hemilability. Subsequent CO insertion and reductive elimination in methanol solution produced methyl acetate and acetic acid. The net reaction is methanol carbonylation to acetic acid using methyl acetate as a promoter alongside conversion of an iridium dinitrogen complex to an iridium carbonyl complex. Kinetic studies of migratory insertion and reductive elimination reveal essential roles of the solvent methanol and distinct features of acetate and iodide anions that are relevant to the design of future catalysts for iodide-free carbonylation.

An efficient stepwise synthesis method for discovering new heteromultinuclear metal clusters using a robotic workflow is developed where numerous reaction conditions for constructing heteromultinuclear metal oxo clusters in polyoxometalates (POMs) were explored using a custom-built automated platform. As a result, new nonanuclear tetrametallic oxo clusters {FeMn 4 }Lu 2 A 2 in TBA 5 [(A-α-SiW 9 O 34 ) 2 FeMn 4 O 2 {Lu(acac) 2 } 2 A 2 ] ( II A ; A = Ag, Na, K; TBA = tetra- n -butylammonium; acac = acetylacetonate) were discovered by the installation of diamagnetic metal cations A + into a paramagnetic {FeMn 4 }Lu 2 unit in TBA 7 [(A-α-SiW 9 O 34 ) 2 FeMn 4 O 2 {Lu(acac) 2 } 2 ] ( I ). POMs II A exhibited single-molecule magnet properties with the higher energy barriers for magnetization reversal ( II Ag , 40.0 K; II Na , 40.3 K; II K , 26.7 K) compared with that of the parent I (19.7 K). Importantly, these clusters with unique properties were constructed as designed by a step of the predictable sequential multistep reactions with the time-efficient platform.

Stepwise synthesis of nanosized Pd−Ru heteronuclear metal–organic cages from predesigned redox- and photo-active Ru(II)-metalloligand and naked Pd(II) ion is described. The resulting cage shows rhombododecahedral shape and contains a 5350 Å3 cavity and 12 open windows, facilitating effective trapping of both polar and nonpolar guest molecules. Protection of photosensitive guests against UV radiation is studied. [ABSTRACT FROM AUTHOR]

A mechanistic pathway for the formation of the structurally characterized manganese-amide-hydrazide pinned butterfly complex, Mn4(μ3-PhN-NPh-γ³N,N')2(μ-PhN-NPh-γ³-N,N')(μ-NHPh)2L4 (L = THF, py), is proposed and supported by the use of labeling studies, kinetic measurements, kinetic competition experiments, kinetic isotope effects, and hydrogen atom transfer reagent substitution, and via the isolation and characterization of intermediates using X-ray diffraction and electron paramagnetic resonance spectroscopy. The data support a formation mechanism whereby bis[bis-(trimethylsilyl)amido]manganese(II) (Mn(NR2)2, where R = SiMe3) reacts with N,N'-diphenylhydrazine (PhNHNHPh) via initial proton transfer, followed by reductive N--N bond cleavage to form a long-lived Mnlv imido multinuclear complex. Coordinating solvents activate this cluster for abstraction of hydrogen atoms from an additional equivalent of PhNHNHPh resulting in a Mn(II)phenylamido dimer, Mn2(μ-NHPh)2-(NR2)2L2. This dimeric complex further assembles in fast steps with two additional equivalents of PhNHNHPh replacing the terminal silylamido ligands with η¹-hydrazine ligands to give a dimeric Mn20-NHPh)2 (PhN-NHPh)2L4 intermediate, and finally, the addition of two additional equivalents of Mn(NR2)2 and PhNHNHPh gives the pinned butterfly cluster. [ABSTRACT FROM AUTHOR]

The characteristic distribution of transverse relaxation times (T2) within dendrimers (shorter values at the core than the periphery) can be exploited in T2-edited 1D and 2D NMR experiments for the stepwise filtering of internal nuclei according to their topology within the dendritic structure. The resulting filtered spectra, which can be conceived as corresponding to virtual hollow dendrimers, benefit from reduced signal overlap, thus facilitating signal assignment and characterization. The generality of the method as a powerful tool in structural and end-group analysis has been confirmed with various dendritic families and nuclei (1H, 13C, 31P). [ABSTRACT FROM AUTHOR]

The text-book mechanism of bimolecular nucleophilic aromatic substitutions (S N Ar) reactions is a stepwise process that proceeds via a so-called Meisenheimer intermediate. Only recently the alternative, concerted version of this mechanism has gained acceptance as more and more examples thereof have been reported. But so far only isolated examples of concerted S N Ar reactions have been described and a coherent picture of when a S N Ar reaction proceeds via a stepwise and when via a concerted mechanism has not yet been established. Here key factors are identified that influence the mechanistic choice of S N Ar reactions. Moreover, the electron affinity is used as a simple descriptor to make a prediction on whether a given aryl fluoride substrate favors a concerted or stepwise mechanism.

Hydrogen-bonded organic frameworks (HOFs) show great potential in many applications, but few structure-property correlations have been explored in this field. In this work, we report that self-assembly of a rigid and planar ligand gives rise to flat hexagonal honeycomb motifs which are extended into undulated two-dimensional (2D) layers and finally generate three polycatenated HOFs with record complexity. This kind of undulation is absent in the 2D layers built from a very similar but nonplanar ligand, indicating that a slight torsion of ligand produces overwhelming structural change. This change delivers materials with unique stepwise adsorption behaviors under a certain pressure originating from the movement between mutually interwoven hexagonal networks. Meanwhile, high chemical stability, phase transformation, and preferential adsorption of aromatic compounds were observed in these HOFs. The results presented in this work would help us to understand the self-assembly behaviors of HOFs and shed light on the rational design of HOF materials for practical applications.

Here we report a synthetic protocol toward a merocyanine (MC) pentamer 1 which represents the first merocyanine oligomer longer than a dimer. By continuously decreasing the solvent polarity, we demonstrate the stepwise folding from partially folded monomeric and dimeric MC subunits (in chloroform) up to the full pentamer π-stack (in 75% methylcyclohexane/25% chloroform) and a subsequent self-assembly of pentamer 1 into larger aggregates (in 80% methylcyclohexane/20% chloroform). This hierarchical structure formation process became possible due to the predominant dipole-dipole interactions among MC dyes that allowed for a precise modulation of the energy landscape by the solvent polarity. This unprecedented stepwise control of dye assembly via hierarchical dipole-dipole interactions opens the door for a more precise control of dye-dye interactions in artificial multichromophoric ensembles.