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Metal-Assisted Salphen Organic Frameworks (MaSOFs) with Trinuclear Metal Units for Synergic Gas Sorption Sven M. Elbert, Wen-Shan Zhang, Yana Vaynzof, Nils Oberhof, Moritz Bernhardt, Markus Pernpointner, Frank Rominger, Rasmus R. Schroeder, and Michael Mastalerz Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b02165 • Publication Date (Web): 22 Jul 2019 Downloaded from pubs.acs.org on July 26, 2019
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Chemistry of Materials
Metal-Assisted Salphen Organic Frameworks (MaSOFs) with Trinuclear Metal Units for Synergic Gas Sorption. Sven M. Elbert,a,b Wen-Shan Zhang,b Yana Vaynzof,b,c Nils Oberhof,d Moritz Bernhardt,d Markus Pernpointner,d Frank Rominger,a Rasmus R. Schröderb and Michael Mastalerz*,a,b a) Dr. S. M. Elbert, Dr. F. Rominger and Prof. Dr. M. Mastalerz Organisch-Chemisches Institut Ruprecht-Karls-Universität Heidelberg Im Neuenheimer Feld 270, 69120 Heidelberg, Germany E-mail:
[email protected] b) Dr. S. M. Elbert, Dr. W.-S. Zhang, Prof. Dr. Y. Vaynzof, Prof. Dr. R. R. Schröder and Prof. Dr. M. Mastalerz Centre for Advanced Materials Ruprecht-Karls-Universität Heidelberg Im Neuenheimer Feld 225, 69120 Heidelberg, Germany c) Prof. Dr. Y. Vaynzof Kirchhoff-Institut für Physik Ruprecht-Karls-Universität Heidelberg Im Neuenheimer Feld 227, 69120 Heidelberg, Germany d) M. Bernhardt, N. Oberhof and Dr. M. Pernpointner Physikalisch-Chemisches Institut, Theoretische Chemie Ruprecht-Karls-Universität Heidelberg Im Neuenheimer Feld 229, 69120 Heidelberg, Germany
ABSTRACT: Metal-assisted salphen organic frameworks (MaSOFs) are known to possess high affinities to CO2 due to Lewis acidic metal sites and are therefore able to selectively adsorb CO2 over CH4 or N2. By aligning two metal centers in a carefully designed geometry, a “single molecular trap” (SMT) effect is generated, resulting in an interaction of two metal centers with one molecule CO2 by synergic effects. A condensation of a rigid triptycene based trissalicylaldehyde with tetrammino benzene is used to realize these metal alignments into MaSOFs. Characterization of the discrete trinuclear complexes proves that the chosen geometry is nearly optimal for synergic CO2 adsorption. The MaSOFs show high selectivities of CO2 against CH4 with a selectivity SIAST (according to the Ideal Adsorbed Solution Theorie) of up to 13 and a selectivity of SIAST up to 70 against N2, which are also reflected by isosteric heat of adsorptions (Qst) of up to 35 kJ/mol. Density functional theory (DFT) calculations support the hypothesis by geometry optimized models and furthermore show a positive cooperative effect by an energy gain of ~14 kJ/mol during the adsorption of CO2 in the second binding pocket of the trinuclear metal-salphen compared to a monomolecular adsorption.
MOFs with amine functions15-16 or free coordination sites on the metal centers17-20 have proven their potential as highly selective CO2 adsorbents.21 For the latter the M-MOF-74 (also known as M2(dobdc) or CPO-27-M)22-23 and HKUST-1 (also known as M3(BTC)2)24-26 series were intensively studied and clear evidence was obtained that the high selective adsorption of CO2 is due the interaction of the gas molecules with unsaturated coordination sites on the metal centers.27 Within the HKUST-1 series, specific surface areas (Brunauer-Emett-
Introduction Porous materials like zeolites,1-2 metal-organic frameworks (MOFs)3-7 or covalent organic frameworks (COFs)8-9 are generally used in a wide range of applications such as ion exchange,10 drug delivery,11 the selective binding of guest molecules,12 or separation of gas mixtures.13 In gas separation, the selective adsorption of CO2 over methane or nitrogen is of major interest to purify natural or flue gases.14 In particular
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Teller model) of up to SABET = 2031 m2/g (for Cr3(BTC)2)28 as well as high selectivities of up to SIAST = 101 for CO2 over nitrogen (for Cu3(BTC)2) were obtained.21,29 This already high value is even exceeded by Mg-MOF-74 (SIAST = 148), which in addition showed high uptakes of 37.8 wt.-% of CO2 at 1 bar and 298 K.23, 30
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O,N,N,O-binding pocket for the complexation of various metals73-75 was later exploited to generate an isostructural series of Zn2+, Ni2+, Cu2+, Pd2+ and Pt2+ MaSOFs based on a triptycene centered hexakissalicylaldehyde.76 With almost identical pore-size distributions, a reasonable comparison of the influence of the metal centers on the gas sorption behavior was available. Within this series, the Ni-MaSOF100 (the index is reffering to the reaction temperature of 100 °C) showed the highest uptake of CO2 at 273 K and 1 bar with 4.83 mmol/mmol (15.1 wt.-%) while the Pd-MaSOF100 gave a higher selectivity of CO2 over N2 (SIAST = 56; for a 20:80 mixture).
In 2012 Zhou and co-workers designed MOFs with local alignments of two metal centers possessing open coordination sites in a distance between 6.7 and 7.4 Å to each other, to generate a defined environment of the two binding sites to synergically bind to one molecule of CO2 during the adsorption process.31 According to the authors, these so-called single molecule traps (SMTs) bear the possibility to even activate CO2 in a confined space. The SMT approach was later taken up by Zhang and co-workers by synthesizing UNLPF-2, a MOF in which two porphyrinic cobalt centers are placed in a distance of dM-M = 6.1 Å by using a porphyrin octacarboxylate as the building block and zinc ions as connecting nodes. The authors postulated that “…an M–M distance of approximately 6.3 Å is likely to provide a perfect binding environment to trap CO2…”.32 It is worth mentioning, that comparable metal-tometal distances (dM-M = 6.2 Å) were also found by singlecrystal X-ray structure analyses of CO2 linked trinuclear magnesium complexes once more emphasizing the advantage of such alignments forsynergic CO2 binding.33
Besides the above mentioned three-dimensional networks,77 two dimensional porous (SABET up to 1258 m2/g) MaSOFs based on different C3-symmetrical precursors have been studied focusing more on heterogeneous catalysis than on gas-sorption.78-80 It is worth mentioning that metal-salphen moieties81-82 were previously used as linkers in MOFs,83-85 or used to construct organic molecules with intrinsic microporosity (OMIMs)86-87 as well as other POPs.88 To the best of our knowledge, no such approach that exploits the synergic effects of two precisely arranged metal centers with an ideal metal-to-metal distance of approx. dM-M = 6.5 Å was described for metal-containing POPs or COFs. Based on a triptycene trissalicylaldehyde,89 we present the synthesis and gas sorption properties of the first two MaSOFs containing trinuclear metal centers,90 having metal-to-metal distances of 6.65Å and 6.74Å, in an ideal range to exploit such synergic effects.
MOFs, especially those constructed via Zn2+-Ions or Zn4OClusters are unfortunately known to be chemically less stable against degradation such as hydrolysis under slightly acidic conditions,34-37 making the above mentioned structures less attractive for applications. In contrast to MOFs, porous organic polymers (POPs) are based on covalent bonds, which are formed under kinetic control and are therefore chemically more stable, especially those compounds with a carbonaceous backbone.38-42 The aforementioned POPs have gained a lot of interest in recent years and represent an alternative to more prominent porous materials like zeolites, MOFs and COFs.42 POPs can be well-designed in terms of their pore-size,43 as well as chemical44-46 and physical47-50 properties can be fine-tuned by modification of their molecular precursors.42, 51 Among these precursors, some bear the possibility of designing POPs that bear defined coordination sites within the pores of the network.44, 46, 52-70 For example, this was mainly realized by using phthalocyanine or porphyrin derivatives as building blocks.46, 52-60, 62
Syntheses and characterization of discrete trinuclear metal compounds and corresponding MaSOFs As all network POPs,42, 45, 91-98 MaSOF materials are usually insoluble and in most cases amorphous. Therefore, molecular model compounds were synthesized first to gain an insight in the detailed three-dimensional arrangement of the three metal ions, which should be beneficial for CO2 sorption. By reacting trissalicylaldehyde 189 o-phenylene diamine (2) and the corresponding metal acetates, the trinuclear [2+3]-nickel and copper complexes were isolated in 61% (3) and 57% yield (4) (Scheme 1, left) and fully characterized (see also Supporting Information).99-101 For instance, the 1H NMR spectrum of the trinuclear nickel complex 3 shows a characteristic peak at # = 8.86 ppm of the imine protons and a peak at # = 7.77 ppm, which can be assigned to the inner bridgehead proton as well as one at # = 5.60 ppm for the outer bridgehead proton (see Figure S1 in the Supporting Information). By MALDI-TOF mass spectrometry, defined
In 2012, the first metal-assisted salphen organic frameworks (MaSOFs) were introduced, where the metal-salphen unit was formed during the strut-formation and thus the generation of the porous polymer71 by the condensation of a tetraphenylmethane tetrasalicylaldehyde72 with orthophenylene diamine in the presence of nickel(II) or zinc(II) acetate.73 The resulting isostructural networks had specific surface areas of SABET = 647 m2/g (Ni-containing MaSOF-1) and SABET = 630 m2/g (Zn-containing MaSOF-2) and showed an influence of the metal centers on the corresponding CO2 uptakes at 273 K and 1 bar with MaSOF-1 adsorbing 1.83 mmol/g (8.1 wt.-%) while MaSOF-2 took up 2.23 mmol/g (9.8 wt.-%). The higher uptake of MaSOF-2 was attributed to its more Lewis acidic Zn(II) centers. The ability of the salphens
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Chemistry of Materials
Scheme 1. Syntheses of model compounds 3 and 4 and the network materials Ni3-MaSOF and Cu3-MaSOF from trissalicylaldehyde 1. a) Ni(OAc)2+#,2O or Cu(OAc)2+,2O, DMF, 100 °C, 16 h. b) Ni(OAc)2+#,2O or Cu(OAc)2+,2O, KOAc, DMF, 100 °C, 3 d.
N N MO O
H 2N
NH2
N N M O O
O
O O O N M N
OM
H 2N
NH2
H 2N NH2 4 HCl 5
HO
2 N OM N
N M O
O
O O M N N
HO
CHO
1 N MO
M = Ni (61%) M = Cu (57%)
OM N
O O
N NM O
OHC HO
N N M O O
N
N
N
3: 4:
N
O
OHC N MN O O
N OM N
N
O MN N
O O
N N MO O
O O M N N N N M O O
O
N OM N
N
NM O
O
O O M N N
O
Ni3-MaSOF: M = Ni (quant.) Cu3-MaSOF:M = Cu (quant.)
peaks for the corresponding sodium adducts [M+Na]+ with m/z = 1179.051 (3) and m/z = 1194.031 (4) were found for both compounds (see Supporting Information). Single-crystals of 3 and 4 of suitable quality were obtained by vapor phase diffusion of methanol into saturated DMSO solutions to study the compounds by X-ray diffraction (Figure 1). Nickel complex 3 crystallized in the trigonal space group R3 with six molecules in the asymmetric unit having a unit cell volume of Vcell = 9667 Å3. The crystalline packing is mainly driven by X X # motifs (dX X = 3.58 Å)102-103 between two 180° flipped salphen planes of two adjacent molecules (see Supporting Information). The repetition of this motif in the crystallographic ab-plane leads to a honeycomb-like 2D arrangements creating hexagonal pores with a maximal diameter of d = 1.83 Å (see Supporting Information). Along the crystallographic c-axis the layers are stacked in a staggered fashion impeding the formation of connected porous channels in the crystals (see Supporting Information). Enclathrated solvate molecules are disordered and could not be refined, therefore, the data set needed to be treated with the SQUEEZE routine function104-105 to solve the structure. The single-crystal structure of the trinuclear copper salphen 4 is isomorphic to the nickel analogue 3 with comparable crystal parameters (Vcell = 9963 Å3; dX X = 3.54 Å (4), see also Supporting Information). In contrast to the nickel salphen 3, here the solvate molecules (DMSO) could be refined, revealing a weak coordination of two DMSO molecules per copper atom in the axial positions with dCu-O = 2.84 Å and dCu-S = 3.17 Å having a
O MN N
Jahn-Teller distorted octahedral geometry around the metal centers. This finding indicates that the introduction of trinuclear salphen metal complexes into porous structures should enable free ligation sites on the copper centers upon suitable activation methods. Most importantly, the metal to metal distances were found to be dNi-Ni = 6.74 Å (3) and dCu-Cu = 6.65 Å (4), which is in the ideal range for synergic interactions of two metal centers with one molecule of CO2 as has been previously suggested.32 a)
b)
dNi-Ni = 6.74 Å
dCu-Cu = 6.65 Å
Figure 1. Single-crystal X-ray structures of trinuclear metal complexes 3 (a) and 4 (b) as stick models. The metal centers are depicted as balls with a size of 50% of their van-der-Waals radii and metal to metal distances are illustrated as green dotted lines and (values given under the structures). Coordinating DMSO molecules are omitted for clarity.
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Chemistry of Materials acetate (Scheme 1). In contrast to Ni3-MaSOFas, Cu3-MaSOFas shows a broad barely structured band at ( = 1584 cm-1 in the IR-spectrum and the imine band appears as a structurally less defined shoulder with ( = 1607 cm-1 whereas the discrete trinuclear copper complex 4 shows three sharp bands in this area with ( = 1608, 1590 and 1571 cm-1 (Figure 2b). Furthermore, the IR-spectrum of Cu3-MaSOFas shows distinct bands at ( = 1526 and 1425 cm-1 which are also found for copper complex 4 (( = 1514 and 1427 cm-1). In contrast to Ni3-MaSOFas, here a clear shoulder at approx. ( = 1650 cm-1 could be found, arising either from residual DMF, coordinated solvent molecules (see discussion of the crystal structures) or uncondensed carbonyl functions. However, further information of this observation is not given by this analytical technique. By nitrogen sorption at 77 K Cu3-MaSOFas shows a higher specific surface area of SABET = 625 m2/g compared to Ni3-MaSOFas. The isotherm is even more of an ideal type-I form, suggesting a larger fraction of micropores, which is estimated to be 85% of the overall pore volume according to the t-plot method (Figure 2d and Table 1). The pore volume of Cu3-MaSOFas is with Vpore = 0.276 cm3/g in a comparable range to Ni3-MaSOFas (Vpore = 0.243 cm3/g, Table 1). QS-DFT analysis (kernel: N2 on carbon at 77 K, cylindrical and spherical pores, adsorption branch, fitting error: 1.897%) again shows a sharp maximum with a comparable pore diameter in the microporous regime with dpore = 1.18 nm considering the networks to be nearly isostructural (Figure 2f and Table 1; for an idealised 3D model of the networks see Supporting Information Figure S43).
MaSOFas as a dark red insoluble solid in quantitative yields (Scheme 1). By comparison of the IR-spectrum with that of the discrete trinuclear complex 3 the formation of the trinuclear salphene units can be confirmed (Figure 2): The Ni3-MaSOFas shows an imine stretching band at ( = 1615 cm-1 (highlighted as dotted line in Figure 2a) and other defined bands at ( = 1584, 1525 and 1427 cm-1 which are comparable yet slightly shifted to higher frequencies to the corresponding bands of complex 3 with ( = 1608, 1574, 1515 and 1428 cm-1 (see Figure 2a). The C=O stretching band of the aldehyde moiety of the molecular precursor 1 (( = 1646 cm-1) is not detected anymore in the IRspectrum of Ni3-MaSOFas, suggesting a nearly quantitative condensation. Investigation of the material by nitrogen sorption at 77 K gave a type-I isotherm, revealing the microporous nature of the material with a specific surface area (BET-model)110 of SABET = 373 m2/g (Figure 2c and Table 1). The microporous surface area represent 60% of the overall surface area as calculated by the t-plot method111-113 with a pore volume of Vpore = 0.243 cm3/g (Table 1 and Supporting Information). By QS-DFT calculations (kernel: N2 on carbon at 77 K, cylindrical and spherical pores, adsorption branch, fitting error: 0.608%)114-115 a sharp maximum at dpore = 1.14 nm with negligible mesoporous proportions was indicated (Figure 2e). Under similar reaction conditions as for Ni3-MaSOFas, the corresponding copper network Cu3-MaSOFas was obtained in quantitative yield by using copper acetate instead of nickel
Table 1. Nitrogen sorption data of the M3-MaSOFs at 77 K. SABET [m2/g]
SALangmuir [m2/g]
dpore, [a] max
Vpore[a] [cm3/g]
Vmicro[b] [cm3/g]
Smicro[c] [%]
[nm] Compound Ni3-MaSOFas 373 n.d. [d] 1.14 0.243 0.095 60 Ni3-MaSOFw 441 497
