MOF-71 as a degradation product in single crystal to single crystal transformation of new three-dimensional Co(II) 1,4-benzenedicarboxylate

Abstract

A new three-dimensional coordination polymer, {[Co(μ₂-H₂O)(bdc)(H₂O)(dmf)]·0.5H₂O}_n (1), was obtained as a by-product in the synthesis of the laminar solid {[Co₂(μ₂-H₂O)(bdc)₂(S-nia)₂(H₂O)(dmf)]·(dmf)₂·H₂O}_n (2) (where H₂bdc = 1,4-benzenedicarboxylic acid, S-nia = thionicotinamide, dmf = N,N′-dimethylformamide). The three-dimensional network in 1 is organized of Co(II) atoms in O₆ octahedral coordination environments separated by μ₂-water molecules in coordination chains. The bdc²⁻ anions coordinate in monodentate mode and also act as bridging ligands that extend the structure in two other directions. The coordination cores of two crystallographically different Co(II) atoms are completed by either two dmf or two water molecules. The rhombohedral open channels contain water molecules. The evacuation of solvents from 1 occurs as a single crystal to single crystal transformation and results in MOF-71, [Co(bdc)(dmf)]_n, whose structure was confirmed by single crystal X-ray diffraction.

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The past two decades have witnessed continued interest in coordination polymers from the viewpoint of their fascinating architectures and searching for possibilities to use them as storage devices for harmful gases, explosives, solvents and drug molecules due to such specific features as rigidity of their coordination scaffolds, and potential to form large, well-defined and easily modified internal cavities.^1–20 Efforts have been undertaken to comprehend the impact of exogenous stimuli where the coordination networks can either survive under external stress, or be subject to soft transformations with retention of the coordination scaffold, or be subject to significant structural transformations accompanied by reconstruction of the coordination scaffold. These so-called single crystal to single crystal (SC–SC) transformations have been covered by tens of original and review publications reporting the starting and recovered coordination networks, the external stimuli applied, and methods that allow one to follow these transitions (most often X-ray powder or single crystal diffraction) which result in substantial changes in measurable properties (magnetism, luminescence etc.).^21–30

Between the most intriguing and the very prospective from the viewpoint of future sensor materials are the SC–SC transformations accompanied by visible changes, for example solids' colors. Such changes are widely discussed for Co(II) coordination networks, and are associated with processes of solvent adhesion/removal and occur either with or without transformation of coordination frameworks and Co(II) coordination nodes.^31–44 Among the Co(II) coordination networks, MOF-71 [Co(bdc)(dmf)]_n being the isostructural analogue of MIL-47 (metal = VO)⁴⁵ and MIL-53 (metal = Cr)⁴⁶ is the most well-known one having been first reported by Yaghi et al. in their seminal work in 2005.⁴⁷ In the next decade the excellent selectivity of Co-MOF-74 (Co-dioxidoterephthalate) for adsorption of hydrocarbons was documented as well.⁴⁸ There is interest not only in the properties of metal–organic frameworks (MOFs) themselves, but also in their degradation and deconstruction products.^49,50

On the route towards deeper understanding of the transformation pathways for rigid MOF materials and soft coordination networks, rather delicate and gradual changes of the exogenous stimuli (temperature, pressure, mechanical indentation) are needed that allow one to follow the steady changes in these materials. From our own recent work, we investigated the softness of crystals of the 1D coordination polymer catena-{(μ₂-adipato-O,O′)-bis-(pyridine-4-aldoxime)–copper(II)} subjected to mechanical stress in conditions of different indentation loadings upon which the crystal survived most probably due to flexibility of supramolecular H-bonded network, and the non-rigid aliphatic backbone of the adipato-linker;⁵¹ and the pronounced anisotropy of hardness for the layered coordination polymer {[Co-(OAc)₂(bpe)(H₂O)]·0.5(dmf)}_n (OAc = acetate, bpe = 1,2-bis(4-pyridyl)ethane) with a very soft layer facet.⁵² Under thermal stress we have registered the SC–SC transformation in the crystal of 1D coordination polymer catena-{(μ₂-bpe-N,N′)-[bis-(formate)]-(1,2-cyclohexanedionedioxime)–cadmium(II)} dimethylformamide solvate that resulted in a desolvated apohost with the retention of the coordination network and the triclinic unit cell although with a 10.3% contraction of the unit cell volume,⁵³ and, most recently, we have recorded the SC–SC transformation of 2D Co(II) coordination polymer {[Co₂(μ₂-H₂O)(bdc)₂(S-nia)₂(H₂O)(dmf)]·(dmf)₂·H₂O}_n, accompanied by a visible change of the crystal color, transformation of Co(II) coordination polyhedron, and an essential transformation of the coordination network.⁵⁴

Herein we present our recent findings concerning the SC–SC transformation of previously unknown 3D polymeric Co(II) 1,4-benzenedicarboxylate to the well-known MOF-71. Compound {[Co(μ₂-H₂O)(bdc)(H₂O)(dmf)]·0.5H₂O}_n (1) precipitated as a by-product in the preparation of the 2D coordination polymer {[Co₂(μ₂-H₂O)(bdc)₂(S-nia)₂(H₂O)(dmf)]·(dmf)₂·H₂O}_n (2) obtained by solvent co-crystallization from a mixture of solvents, CH₃OH : dmf : H₂O (6 : 3 : 2 mL), of the following starting compounds: Co(CH₃COO)₂·4H₂O, thionicotinamide (S-nia), and 1,4-benzenedicarboxylic acid (H₂bdc). The details are given in ref. 54. After the precipitated crystals of the main product 2 were removed from the flask, the mother liquor remained at ambient conditions, and light brown crystals (Fig. 1a) were separated in a week. Single crystal X-ray structural analysis identified this compound as a previously unknown three-dimensional (3D) coordination polymer: 1. Our search of the Cambridge Structural Database (CSD version 5.36; ConQuest version 1.17) did not reveal any structural analogues of 1. Compound 1 crystallizes in the triclinic centrosymmetric P1̄ space group.⁵⁵ Two Co(II) atoms and two bdc²⁻ residues reside on inversion centers. The 3D network in 1 is organized from chains of octahedral Co(II) atoms in O₆ coordination environments separated by μ₂-water molecules and running along the [0 −2 10] direction in the crystal lattice (Fig. 1b and c). The Co(1)⋯Co(2) separation across the bridging water molecule is 3.9379(4) Å. The bdc²⁻ anions that coordinate in monodentate mode act as bridging ligands as well, and extend the structure in two other directions. The second oxygen atom of each carboxylic group that does not participate in coordination to the metal center is involved in the OH⋯O hydrogen bonds with bridging water molecule, O(1W)⋯O(4) (1 − x, −y, 1 − z) = 2.646(3) Å; O(1W)⋯O(2) (2 − x, −y, 1 − z) = 2.623(3) Å. The coordination cores of two crystallographically different Co(II) atoms are completed by either two terminal dmf or two terminal water molecules. The Co–O distances in two coordination polyhedra vary in the ranges 2.061(2)–2.153(2) Å (to bridging water molecule) for Co(1), and 2.046(2)–2.180(2) Å (to bridging water molecule) for Co(2). The rhombohedral open channels with the dimensions 11.343(1) × 11.490(1) Å host water molecules that are held in the cavities via OH⋯O hydrogen bonds.

Fig. 1 View of crystals of 1 (a). Coordination environment of two crystallographically different Co(II) atoms in 1 (b). Fragment of coordination chain (c). Fragment of coordination chain indicating Co(II) octahedra (d). Crystal packing in 1. View along the a axis. Accommodated water molecules are shown in space filling mode (e). In all cases, C-bound H atoms are omitted for clarity.

The evacuation of solvents from 1 in mild conditions by heating the crystalline solid at 105 °C for 4 h in vacuum was accompanied by a color change from light brown of 1 to pink of the desolvated product (Fig. 2a). The process occurred in the form of SC–SC transformation and resulted in MOF-71, [Co(bdc)(dmf)]_n, that crystallizes in the orthorhombic Imma (no. 74) space group and whose structure was confirmed by single crystal X-ray diffraction. The evacuated solvents are all water molecules that include the two coordinated to the Co(2) atom and one water of crystallization, and one of the dmf molecules coordinated to the Co(1) atom. The transformation results in significant reconstruction of the 3D coordination skeleton (Fig. 2b) where all Co atoms become crystallographically equivalent, and the bdc²⁻ and dmf ligands change their coordination functions from monodentate to bidentate bridging ones, accompanied by contraction of Co⋯Co separation across the bridging dmf molecule to 3.620 Å (vs. 3.9379(4) Å in 1).

Fig. 2 View of a crystal of MOF-71 obtained in conditions of thermal stress (a). Fragment of coordination chain in MOF-71 (b). Crystal packing viewed along the b axis; dmf molecules are disordered along the mirror plane (c). In all cases, C-bound H atoms are omitted for clarity.

The process of SC–SC transformation of 1 accompanied by the color change from light brown to pink occurred in the form of visible agglomeration of single crystals. Repeated attempts have been undertaken to cut the crystalline block accessible for single crystal X-ray experiments. Since the single crystal X-ray data were obtained from a crystal subjected to considerable thermal stress, we were unable to get a better refinement result due to the high mosaicity of the tested samples. Nevertheless, the exact match of our⁵⁵ and reported^47,56 crystallographic data does not leave any doubts that MOF-71 is the product of the SC–SC transformation in this system. In conclusion, we report herein a new 3D coordination polymer, {[Co(μ₂-H₂O)(bdc)(H₂O)(dmf)]·0.5H₂O}_n (1), whose SC–SC transformation under thermal stress resulted in the MOF-71 coordination network. To the best of our knowledge, this is the first example of a 3D predecessor for MOF material having been reported.

Acknowledgements

The authors are indebted to the State Program 14.518.02.04A for support.

Notes and references

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55. All reagents and solvents were obtained from commercial sources and were used without further purification. Elemental analysis for 1 was performed on an Elementar Analysensysteme GmbH Vario El III elemental analyzer. The desorption experiment for 1 was carried out in derivatograph Q-1500 thermal analyzer in an air flow at a heating rate of 5 °C min⁻¹ in the temperature range of 25–110 °C. Crystallographic Studies. Diffraction measurements for 1 and MOF-71 were carried out on an Xcalibur Oxford Diffraction 4-circle diffractometer equipped with CCD area detector and with Mo-Kα radiation at room temperature. Final unit cell dimensions were obtained and refined on an entire data set. All calculations to solve the structures and to refine the model proposed were carried out with the programs SHELXS-2014 and SHELXL-2014 [G. M. Sheldrick, Acta Cryst. (2015). C71, 3–8]. Hydrogen atoms attached to carbon atoms were positioned geometrically and treated as riding atoms using SHELXL default parameters with U_iso(H) = 1.2U_eq(C), the O-bounded H-atoms were found from differential Fourier maps at the intermediate stages of the refinement and their positions were restrained during the refinement. The outer sphere water molecule was refined as disordered with the oxygen atom occupying two positions with the occupancies 0.879(15) and 0.121(15). The crystal purity for 1 was confirmed by elemental analysis, Found for C₁₁H₁₇CoNO₈ C 37.45, H 4.67, N 3.81; calcd C 37.73, H 4.89, N 3.99. Crystal data for 1: M_r = 350.18, triclinic, space group P1̄ (no. 2), a = 7.8758(7), b = 9.1877(10), c = 11.3427(8) Å; α = 112.883(8), β = 91.057(7), γ = 105.877(9)°; V = 720.07(12) ų; Z = 2; D_c = 1.615 g cm³; F(000) = 362; μ = 1.230 mm⁻¹; R₁ = 0.0446, wR₂ = 0.0849 for 1994 independent reflections with I > 2σ(I); R₁ = 0.0672, wR₂ = 0.0954 for all 2664 independent reflections. MOF-71: single crystals were obtained by heating the bulk crystalline sample of 1 at 105 °C for 4 h in vacuum. The data collection was carried out from the crystal subjected to the thermal stress that explains the significant deterioration of the crystal quality. The X-ray data were collected from a rather weak survived crystal, which explained the insufficient quality of the crystallographic data. The dmf molecule is disordered along the mirror plane, H-atoms were not added to this molecule. Crystal data for MOF-71: C₁₁H₁₁CoNO₅, M_r = 296.14, orthorhombic, space group Imma (no. 74), a = 19.228(5), b = 7.240(4), c = 8.874(4) Å; V = 1235.4(9) ų; Z = 4; D_c = 1.592 g cm³; F(000) = 604; μ = 1.401 mm⁻¹; R₁ = 0.1264, wR₂ = 0.2807 for 273 independent reflections with I > 2σ(I); R₁ = 0.2033, wR₂ = 0.3300 for all 607 independent reflections. CCDC 1430630–1430631 contain the supplementary crystallographic data for 1 and MOF-71..
56. Y.-L. Fu, J.-L. Ren and S. W. Ng, Acta Crystallogr., Sect. E: Crystallogr. Commun., 2004, 60, m1507–1509.

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Footnote

† CCDC 1430630 and 1430631. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ce02094h

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