Alexandre Burgun

Capturing snapshots of post-synthetic metallation chemistry in metal-organic frameworks.

Post-synthetic metallation is employed strategically to imbue metal–organic frameworks (MOFs) with enhanced performance characteristics. However, obtaining precise structural information for metal-centred reactions that take place within the pores of these materials has remained an elusive goal, because of issues with high symmetry in certain MOFs, lower initial crystallinity for some chemically robust MOFs, and the reduction in crystallinity that can result from carrying out post-synthetic reactions on parent crystals. Here, we report a new three-dimensional MOF possessing pore cavities that are lined with vacant di-pyrazole groups poised for post-synthetic metallation. These metallations occur quantitatively without appreciable loss of crystallinity, thereby enabling examination of the products by single-crystal X-ray diffraction. To illustrate the potential of this platform to garner fundamental insight into metal-catalysed reactions in porous solids we use single-crystal X-ray diffraction studies to structurally elucidate the reaction products of consecutive oxidative addition and methyl migration steps that occur within the pores of the Rh-metallated MOF, 1·[Rh(CO)2][Rh(CO)2Cl2]. Obtaining precise structural information for metal-centred reactions that take place within the pores of metal–organic frameworks continues to be an elusive goal. Now, a flexible framework has been synthesized that enables the direct elucidation of the products of post-synthetic metallation reactions and subsequent chemical transformations by single-crystal X-ray crystallography. Camera image:

Kinetically controlled porosity in a robust organic cage material

Microporous materials are of significant interest owing to their central role in gas storage, separation processes, and catalysis. Recently, microporous molecular solids composed of discrete, shape-persistent organic cages have received growing attention because they possess unique properties that set them apart from conventional, extended network materials, such as zeolites, metal–organic frameworks, and covalent organic frameworks. For example, molecular solids are readily solution-processable, provide facile access to multicomponent materials by mix-and-match synthesis, and, by virtue of their noncovalent intermolecular packing, can exhibit advanced properties, such as adsorbatetriggered on/off porosity switching. Unlike extended networks, where solvent-accessible voids are linked through rigid covalent framework solids composed of discrete organic cages predominantly aggregate by relatively weak dispersion forces. Predicting the crystal structures of such weakly aggregating materials is a long-standing challenge in solid-state chemistry, and is, in this field, inherently coupled to estimating the ultimate porosity of a molecular solid from its building units, as different polymorphs can afford solids with dramatically different surface areas. Accordingly, relatively few examples of porous organic solids have been reported. Nevertheless, recent work from the laboratories of Cooper and Mastalerz have demonstrated that the porosity of such materials can be modified through crystal engineering strategies and synthetic processing. 10] Herein we describe the synthesis and characterization of a novel, permanently porous, shape-persistent cage molecule (C1) that is constructed entirely from thermodynamically robust carbon–carbon bonds and has the molecular formula C112H62O2 (Scheme 1). Furthermore, we demonstrate kinetically controlled access to two crystalline polymorphs C1a and C1b that possess dramatically different N2 porosities: polymorph C1a, which is nonporous to N2, and polymorph C1b, which affords a BET surface area of 1153 m g .

Mapping-Out Catalytic Processes in a Metal–Organic Framework with Single-Crystal X-ray Crystallography

Single-crystal X-ray crystallography is employed to characterize the reaction species of a full catalytic carbonylation cycle within a MnII -based metal-organic framework (MOF) material. The structural insights explain why the Rh metalated MOF is catalytically competent toward the carbonylation of MeBr but only affords stoichiometric turn-over in the case of MeI. This work highlights the capability of MOFs to act as platform materials for studying single-site catalysis in heterogeneous systems.

Reactions of 7,7,8,8-Tetracyanoquinodimethane (TCNQ) with Alkynyl-Iron- and -Ruthenium Complexes: Synthesis of Ru{C≡CC(CN)=C6H4=C(CN)2}(PPh3)2Cp, a New Donor-Acceptor Molecular Array

Reactions of 7,7,8,8-tetracyanoquinodimethane (TCNQ) with the alkynyl-iron and ruthenium complexes [M](C≡CR) {[M] = Fe(dppe)Cp*, Ru(PPh3)2Cp; R = H, Ph} are described. The iron complex Fe(C≡CPh)(dppe)Cp* (2a) is oxidized by TCNQ to give the kinetically stable salt [2a•+][TCNQ]•– . Displacement of [TCNQ]•– is achieved by ionic metathesis upon addition of KPF6 to produce [2a•+]PF6. In contrast, Fe(C≡CH)(dppe)Cp* (2b) reacted with TCNQ to give a mixture of compounds containing Fe(=C=CH2)(dppe)Cp* (3a), {Fe(dppe)Cp*}2(μ-C=CHCH=C) (3b), and the zwitterionic complex Fe+{=C=CHC(CN)2C6H4C–(CN)2}(dppe)Cp* (3c). In contrast, the reaction of TCNQ with Ru(C≡CR)(PPh3)2Cp (4a, R = Ph; 4b, R = H) gave selectively the zwitterionic vinylidenes Ru+{=C=CRC(CN)2C6H4C–(CN)2}(PPh3)2Cp (5a, R = Ph; 5b, R = H), in which the Ru centres are positively charged and the counter-anion is located on the further C(CN)2 group. On heating 5b, elimination of HCN affords Ru{C≡CC(CN)=C6H4=C(CN)2}(PPh3)2Cp (1), while similar treatment of 5a gives Ru{η3-C(CN)2CPh=C6H4=C(CN)2}(PPh3)Cp (6) with loss of PPh3. X-ray structures of 1, 5a, and 6, cyclic voltammetry, and UV-vis spectroscopy of 1 provided evidence for the electronic structures of the new complexes.

Hydrogen adsorption in azolium and metalated N-heterocyclic carbene containing MOFs

Azolium and metalated N-heterocyclic carbene (NHC) functionalised metal–organic frameworks (MOFs) have been investigated as adsorbents and heterogenous catalysts. Here we describe the structures of two sets of 1D polymeric structures, {[M2(μ2-HCOO)(HL)2](NO3)·xDMF}n (M = Zn, x = 1, 1; Cu, x = 1.75, 2) and {[M3(HL)4(H2O)4](NO3)2·xDMF}n (M = Co, x = 2.75, 3; Mg, x = 0, 4; Mn, x = 1.5, 5), prepared from reactions of 1,3-bis(3-carboxyphenyl)-1H-imidazol-3-ium bromide (H3LBr) with M(NO3)2·xH2O (M = Zn, Cu, Co and Mg) and MnCl2·4H2O. These pack as porous and non-porous 3D materials, respectively. In the case of the known Zn(II) material 1, which we reveal to be porous, we also report metalation of the NHC precursor concomitant with framework synthesis to give {[Zn2(μ2-HCOO)(HL)1.6(L–Cu–Br)0.4](NO3)0.6·0.75DMF}n (1-Cu). Compounds 1, 2, and 1-Cu show H2 adsorption enthalpies of −9.9, −9.1, and −9.7 kJ mol−1 respectively, and allow the effect of micropores decorated with charged imidazolium moieties, or NHC–CuBr entities, on H2 adsorption to be probed.

Triazolium-Containing Metal–Organic Frameworks: Control of Catenation in 2D Copper(II) Paddlewheel Structures

One approach to exploit metal–organic frameworks (MOFs) as heterogeneous catalyst platforms requires the development of materials containing groups that can be utilised to anchor a catalytic moiety into the links within the structure. Here we report the synthesis of the first integrated triazolium-containing MOF linker and the first MOFs containing linkers of this type. 1,4-Bis(4-benzoic acid)-1-methyl-1H-1,2,3-triazolium chloride, H2L1Me, was synthesised in three steps by a ‘click’ reaction of methyl 4-ethynylbenzoate with methyl 4-azidobenzoate, methylation using methyl triflate, followed by ester hydrolysis in overall 74 % yield. The equivalent neutral triazole precursor, 1,4-bis(4-benzoic acid)-1H-1,2,3-triazole hydrochloride, H2L1(HCl), was also prepared and a comparison of the chemistry with Zn(NO3)2·6H2O and Cu(NO3)2·3H2O is presented. The results support the use of reaction conditions to control interpenetration and provide additional evidence that the charge on structurally similar ligands can drastically alter the types of structures that are accessible due to the requirements for charge balance in the final product.

X-ray crystallographic insights into post-synthetic metalation products in a metal–organic framework

Post-synthetic modification of metal–organic frameworks (MOFs) facilitates a strategic transformation of potentially inert frameworks into functionalized materials, tailoring them for specific applications. In particular, the post-synthetic incorporation of transition-metal complexes within MOFs, a process known as ‘metalation’, is a particularly promising avenue towards functionalizing MOFs. Herein, we describe the post-synthetic metalation of a microporous MOF with various transition-metal nitrates. The parent framework, 1, contains free-nitrogen donor chelation sites, which readily coordinate metal complexes in a single-crystal to single-crystal transformation which, remarkably, can be readily monitored by X-ray crystallography. The presence of an open void surrounding the chelation site in 1 prompted us to investigate the effect of the MOF pore environment on included metal complexes, particularly examining whether void space would induce changes in the coordination sphere of chelated complexes reminiscent of those found in the solution state. To test this hypothesis, we systematically metalated 1 with first-row transition-metal nitrates and elucidated the coordination environment of the respective transition-metal complexes using X-ray crystallography. Comparison of the coordination sphere parameters of coordinated transition-metal complexes in 1 against equivalent solid- and solution-state species suggests that the void space in 1 does not markedly influence the coordination sphere of chelated species but we show notably different post-synthetic metalation outcomes when different solvents are used. This article is part of the themed issue ‘Coordination polymers and metal–organic frameworks: materials by design’.

Engineering Isoreticular 2D Metal–Organic Frameworks with Inherent Structural Flexibility

The chemical mutability of metal–organic frameworks (MOFs) is an advantageous feature that allows fine-tuning of their physical and chemical properties. Herein, we report the successful isoreticulation of a MOF with an outstanding gas selectivity for CO2 versus N2: [Cu(L1)(H2O)]·xS (CuL1), where H2L1 = bis(4-(4-carboxyphenyl)-1H-pyrazolyl)methane) and S = solvate. By modifying the steric bulk and length of the original ligand, we synthesised three new MOFs with 2D networks isoreticular to CuL1, namely [Cu(L1Me)(H2O)]·xS (CuL1Me), [Cu(L2)(H2O)]·xS (CuL2), and [Cu(L2Me)(H2O)]·xS (CuL2Me) (where H2L1Me = bis(4-(4-carboxyphenyl)-3,5-dimethyl-1H-pyrazolyl)methane, H2L2 = bis(4-(4-carboxyphenyl)-(ethyne-2,1-yl)-1H-pyrazolyl)methane, and H2L2Me = bis(4-(4-carboxyphenyl)-(ethyne-2,1-yl)-3,5-dimethyl-1H-pyrazolyl)methane). Depending on the steric hindrance and structure metrics of the organic links, staggered and eclipsed arrangements of 2D 44 net layers were obtained. The anisotropy of the pore dimensions is proportional to the linker length (L2 and L2Me), which when increased, renders these materials non-porous. However, the more sterically demanding ligand L1Me gives a material that shows gate-opening behaviour in response to a CO2 absorbate. The synthesis and structure of an unexpected mixed-valence CuII/CuI 3D MOF, Cu3[Cu(L2Me)2]2(H2O)4]·xS (Cu5(L2Me)4), containing an unusual trimeric CuII node are also reported.

A Facile Synthesis Procedure for Sulfonated Aniline Oligomers with Distinct Microstructures

Well-defined sulfonated aniline oligomer (SAO) microstructures with rod and flake morphologies were successfully synthesized using an aniline and oxidant with a molar ratio of 10:1 in ethanol and acidic conditions (pH 4.8). The synthesized oligomers showed excellent dispersibility and assembled as well-defined structures in contrast to the shapeless aggregated material produced in a water medium. The synergistic effects among the monomer concentration, oxidant concentration, pH, and reaction medium are shown to be controlling parameters to generate SAO microstructures with distinct morphologies, whether micro sheets or micro rods.