Fabrication of Desired Metal-Organic Frameworks via Post-Synthetic

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Fabrication of Desired Metal-Organic Frameworks via PostSynthetic Exchange and Sequential Linker Installation Peipei Cui, Peng Wang, Yue Zhao, and Wei-Yin Sun Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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Crystal Growth & Design

Fabrication of Desired Metal-Organic Frameworks via Post-Synthetic Exchange and Sequential Linker Installation Peipei Cui,†,‡ Peng Wang,‡ Yue Zhao,‡ and Wei-Yin Sun*,‡ †

Shandong Provincial Key Laboratory of Functional Macromolecular Biophysics, Shandong

Universities Key Laboratory of Functional Biological Resources Utilization and Development, College of Life Science, Dezhou University, Dezhou 253023, China ‡

Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, School

of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, China

ABSTRACT: Metal-organic frameworks (MOFs) have attracted much attention not only because of their diverse framework structures but also due to their interesting properties as well as potential applications in varied fields. The rational design and fabrication of MOFs with desired structures and properties is important but still a challenge. Generally, the construction of new MOFs is mainly by mixing metal salts/clusters and organic ligands via conventional reaction process. However, the conventional reaction process is not always available for the targeted MOFs. In this review, we will highlight the post-synthetic methods based on stepwise reactions including metal-ion metathesis, ligand exchange and sequential linker installation, which has been proved to be powerful for fabrication of desired MOFs that are not available by direct synthesis.

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INTRODUCTION Metal-organic frameworks (MOFs), as a new subset of multifunctional materials, have drawn extensive attention from scientific researchers in the past years due to their unique structures, fascinating properties and varied potential applications.1-5 In order to achieve desired MOFs, the design of organic ligands, choice of metal centers and use of synthetic method are crucial. Although extensive works have been carried out, the design and synthesis of MOFs with desired structures and properties is still a challenge up to now, and there is a long way to realize the controlled syntheses for MOFs.6-8 This is because that varied factors have been reported to affect the formation MOFs. On the one hand, the structural uncertainty of MOFs is an inherent character for the assembly process. The nature of ligands, coordination tendency of metal centers, reaction conditions such as temperature, solvent, molar ratio of reactants can influence the structure of resulted MOFs.9-11 On the other hand, the current well-used technics like single crystal x-ray diffraction can not characterize too small crystalline products or unknown structure powder.12-14 Therefore, alternative synthetic route is required. In order to get desired MOFs, varied methods have been utilized such as one-pot reaction, liquid diffusion, layer-by-layer, hydro/solvothermal reactions, microwave assisted methods and so on.15-18 However, these conventional synthetic methods are not always available, alternative methods are essential, for example post-synthetic modification (PSM) has been developed to fabricate new MOFs.19-24 PSM not only provides new pathway for construction of MOFs, particularly for those MOFs not available by direct synthesis, but also can improve the property and functionality of MOFs. Namely, compared with parent MOFs, the daughter MOFs may have enhanced capabilities for gas adsorption/separation, sensing, catalysis and so on. In the post-synthetic method, most of well-examined examples are single-crystal-tosingle-crystal transformations (SCSCs), which generally emphasize unanimous singlecrystallinity without disintegration even though accompanying breaking and forming bonds.25-


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Some structural characteristics about SCSCs have been summarized in previous reviews: (i)

metal-ion exchange, coordination geometric change, metal redox reaction of metal centers with unstable or metastable electronic configuration and/or coordination geometry; (ii) exchange, modification, polymerization of organic ligands with active/functional groups; (iii) structural variation such as bending, rotation, swelling, or shrinking of flexible frameworks; (iv) removal or exchange of lattice guest molecules.28-32 In addition to these extensively studied post-synthetic approaches, conceptually different post-synthetic routes are now emerging and sequential linker installation (SLI) is one that cannot be ignored. Compared with other post-synthetic methods, SLI is new and related reviews are rare.33 Here, we will summarize and comment on post-synthetic exchange (PSE) and SLI as outlined in Scheme 1. PSE strategies involve metal-ion metathesis and organic ligand exchange. In addition, the examples presented in this work will exhibit drastic structure changes, normally with breakage and formation of bonds.

Scheme 1. Three main strategies via SCSCs described in this article.


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METAL-ION METATHESIS Post-synthetic metalation is an important subset of PSM approach to achieve functional MOFs. To enhance the performance of MOFs, post-synthetic metalation offers feasibility for the targeted functionality.34-38 Varied methods have been reported for the post-synthetic metalation, and four main strategies can be categorized: 1) metal-ion metathesis at the nodes of MOFs; 2) metal-ion exchange in charged frameworks; 3) coordination to the functional groups in the organic ligands; 4) metal-containing entities introduced into the pores of the framework via encapsulation. The latter three methods have been summarized by Doonan et al.39 In this review, we will focus on metal-ion metathesis at the nodes of MOFs as one major part of post-synthetic exchange, which is normally observed by SCSCs. Metal-ion metathesis, also known as metal-ion exchange, is a powerful way for fabricating new MOFs materials. Broadly speaking, metal-ion metathesis is partial or complete substitution of metal ions at nodes to another. This method not only offers an alternative route for generating new MOFs, but also provides opportunity for preparing MOFs when conventional synthesis fails. More importantly, it is also a useful way to enhance the properties of MOFs via metal-ion metathesis. In general, the metal-ion exchange often occurs at the metal nodes or secondary-building units (SBUs) of MOFs. Although SBUs are integral to the MOF structure, the metal ions can still be exchanged by others. In this review, we sum up the reported examples with different metal nodes or SBUs covered common features from mononuclear to multinuclear, which is believed to be helpful for the exploration of metal-ion metathesis (Figure 1).


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Figure 1. SBUs come from published MOFs that undergo metal-ion metathesis (C: grey; N: blue; O: red; Cl: green; metal ions: grey green).

Early studies on metal-ion metathesis have been reported by Hou et al. in 2007 for the transmetalation of mononuclear SBUs.40 They observed partial exchange via SCSCs within a


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threefold interpenetrated three-dimensional (3D) framework [Cd(bpy)2(O3SFcSO3)]n (Fc = ferrocene, bpy = 4,4’-bipyridine).40 In the original framework, Cd2+ adopts a slightly distorted octahedral coordination environment with four N atoms of pyridyl groups from four bpy and two O ones from two different -O3SFcSO3- ligands to form [CdN4O2] SBU (Figure 1a). The Cd2+ can be replaced by Cu2+ using Cu(NO3)2 solution and the resulted product is {[Cd0.5Cu0.5(bpy)2(O3SFcSO3)]·(CH3OH)4}n.

After

that,

as

the

isostructuralism

to

[Cd(bpy)2(O3SFcSO3)]n, when the crystals of [Zn(bpy)2(FcphSO3)2]n (FcphSO3Na = sodium m-ferrocenyl benzenesulfonate) were immersed into concentrated solutions of Cd(NO3)2, Cu(NO3)2 and Pb(NO3)2, respectively, different products induced by metal-ion exchange [Cd0.6Zn0.4(bpy)2(FcphSO3)2]n,

[Cu0.5Zn0.5(bpy)2(FcphSO3)2]n

and

[Zn0.75Pb0.25(bpy)2(FcphSO3)2]n were obtained.41 In the mononuclear [MN4O2] SBU, when the axial positions were occupied by water molecules instead of the -O3SFcSO3- ligands, metalion metathesis also occurred. Biradha group reported a series of isostructuralism {[M(L)2(H2O)2]·2(PF6)·pyrene·2(H2O)}n (M = Zn2+, Cd2+ and Cu2+, L = benzene-1,3,5triyltriisonicotinate) with [MN4O2] SBU. The metal-ion exchanges, such as Zn2+ to Cu2+, Cd2+ to Cu2+ and Cu2+ to Cd2+, have been realized by immersing the crystals in the MeOH solution with targeted metal salt, which was verified by single crystal diffraction, field emission scanning spectroscopy (FESEM), energy dispersive X-ray (EDX) and atomic absorption spectroscopy (AAS).42 It should be pointed out that the exchange of Cu2+ or Cd2+ to Zn2+ could not occur since the coordination ability of M2+ with L is in order of Cu2+ > Cd2+ > Zn2+. In addition, metal-ion metathesis has also been reported for other mononuclear SBUs including four-coordinated [MO4] (Figure 1b), six-coordinated [MO6] (Figure 1c and 1e) and [MN3O3] (Figure 1d).43-46 To evaluate the metal-ion metathesis process, Harris et al. recently examined the transportation of Co2+ and Zn2+ during metal-ion exchange within two-dimensional (2D) network (Et4N)2[Mn2L3] (H2L = 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone) (Figure 2) by


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X-ray crystallography, electron microscopy (EM), and EDX.46 In (Et4N)2[Mn2L3], there are [MnO6] SBUs (Figure 1e and Figure 2), and Mn2+ was exchanged by immersing the solvated crystals into a DMF solution of Co(ClO4)2·6H2O or Zn(NO3)2·6H2O. EDX mapping studies on crystals with hetero-metal ions Co0.62Mn1.38 revealed that Co2+ ions diffuse through the one-dimensional (1D) channel along the c axis (Figure 2) and metal-ion exchange occurred.

Figure 2. 2D network of [Mn2L3]2- as viewed along the c axis.

Compared with the mononuclear SBUs, binuclear SBUs, particularly the well-known [M2(COO)x] paddlewheel SBUs, have been investigated extensively. The metal-ion exchange of binuclear SBUs was succeeded for SNU-51 {[Zn2(bdcppi)(dmf)3]·6DMF·4H2O}n (H4bdcppi = N,N’-bis(3,5-dicarboxyphenyl)pyromellitic diimide, dmf refers to the coordinated DMF) with [Zn2(OCO)3(COO)(dmf)3] paddlewheel SBUs as illustrated in Figure 1f.47 When the SNU-51 crystals were immersed in MeOH with mixed salts Co(NO3)2·6H2O, Ni(NO3)2·6H2O, Cu(NO3)2·2.5H2O and Cd(NO3)2·6H2O, only Cu2+ replacing Zn2+ in SNU-51 was detected. In addition, if DMF or 1-pentanol was employed as solvent instead of MeOH, no metal-ion exchanges happened. The results imply that the metal-ion metathesis is sensitive to the metal ions and reaction solvent. It is noteworthy that the metal-ion exchanged {[Cu2(bdcppi)(dmf)3]·7DMF·5H2O}n (SNU-51-CuDMF) could not be obtained by direct synthesis. Similar metal-ion metathesis has also been reported for the MOFs with such


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[M2(OCO)3(COO)x(solvent)y] paddlewheel SBUs like NTU-101(Zn),48 MOF-4,49 and SDU1,50 as well as other similar binuclear paddlewheel SBUs illustrated in Figure 1g-1i.51-53 Interestingly, metal-ion exchange was further observed between two MOFs with different metal ions MIL-53(Al)-Br and MIL-53(Fe)-Br.53 MIL-53(Al)(Fe) containing both Al3+ and Fe3+ was detected by mixing MIL-53(Al)-Br and MIL-53(Fe)-Br in water. In addition, metal-ion exchange was also found in UiO-66(Zr) with hexanuclear [Zr6O4(OH)4(COO)12] SBU (Figure 1w). Zr4+ was found to be exchanged by Ti4+ to generate the Ti4+ analogue framework of UiO-66(Zr), even though UiO-66(Zr) is well known for its high structural stability under varied conditions.53 Metal-ion metathesis has also been observed in the typical [M2(COO)4] paddlewheel SBUs. The successful examples covered different MOFs including HKUST-1(Zn),54 PMOF2(Zn),54 PCN-921(Zn) (PCN = porous coordination network),55 M-FJI-1 (M = Zn, Co)56,57, SUMOF-1(Zn)58 and NU-505(Zn).59 From these studies, it was found that the metal-ion exchange is feasible and the reaction condition is simple if the axial positions occupied by solvent molecules in the [M2(COO)4] paddlewheel SBUs. In addition, the replacement of Zn2+ by Cu2+ is more common due to the comparable higher thermodynamic stability of the Cu2+ SBUs. For the metal-ion metathesis of [M2(COO)4] paddlewheel SBUs, we found that partial metal-ion exchange took place in the isostructural MOFs {[Zn3(L)2(DABCO)(H2O)]·9DMF} (named as Zn-1) and {[Cu3(L)2(DABCO)(H2O)]·15H2O·9DMF} (named as Cu-1) [H3L = [1,1’:3’,1’’-terphenyl]-4,4’’,5’-tricarboxylic

acid,

and

DABCO

=

1,4-

diazabicyclo[2.2.2]octane] to give Zn2+ and Cu2+ heterometallic frameworks.60,61 In the structure of Zn-1, there are two different kinds of [Zn2(OCO)4] SBUs (Figure 3), one is the axial sites coordinated by N atoms from DABCO ligands (Figure 1j) and the other one is the axial sites occupied by H2O molecules (Figure 1i). When the crystals of Zn-1 were suspended in Cu(NO3)2 solution at room temperature, only Zn2+ atoms in the [Zn2(OCO)4] SBUs with


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axial

H2O

molecules

were

exchanged

by

Cu2+

to

produce

{[Zn2Cu(L)2(DABCO)(H2O)]·11DMF·7H2O} (named as Zn-1’). Similar partial metal-ion exchange was also observed for Cu-1 to form Cu-1’. It should be point out that no metal-ion exchange was detected by using DMF solution of Mg(NO3)2, Mn(NO3)2, Ni(NO3)2, Co(NO3)2, and Cd(NO3)2, implying Zn2+ in Zn-1 can selectively be exchanged by Cu2+. Furthermore, heterometallic frameworks Zn-1’ and Cu-1’ not only could not be achieved by direct synthesis, but also show improved adsorption and luminescence properties.60 Partial metal-ion metathesis was also reported for MOF {[Zn7(L)3(H2O)7]n·[Zn5(L)3(H2O)5]n} (H4L = Nphenyl-N’-phenyl bicyclo[2,2,2]oct-7-ene-2,3,5,6-tetracarboxdiimide tetracarboxylic acid) with one mononuclear SBU (Figure 1b) and two different binuclear paddlewheel SBUs (Figure 1k and 1m).43

Figure 3 Framework structure of Zn-1 with two different kinds of paddlewheel SBUs and metal-ion metathesis occurred at the tagged SBU.

When chelating carboxylate groups locate at the axial positions of the paddlewheel [M2(OCO)4] SBU to form [M2(OCO)6] SBU (Figure 1l and Figure 4), the metal ions in the [M2(OCO)6] SBU can be exchanged by other metal ions, however, accompanying the variation of binuclear to tetranuclear SBU (Figure 4) as observed in porph@MOM-11 (P11)


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reported by Zaworotko group.62 When the crystalline porphyrin-encapsulated metal-organic material P11 was immersed in methanol solution of Cu2+, the [Cd2(COO)6] SBU of P11 not only undergoes metal-ion exchange, but also converts into a new [Cu4X2(COO)6(S)2] (X = OH-, CH3O-; S = H2O, CH3OH) tetranuclear SBU (Figure 4).

Figure 4. The SBU from [Cd2(COO)6] to [Cu4X2(COO)6(S)2].

In addition, porph@MOM-10 isolated by reaction of biphenyl-3,4’,5-tricarboxylate, CdCl2 and meso-tetra(N-methyl-4-pyridyl)porphine tetratosylate (TMPyP) contains trinuclear [Cd3(COO)4(Cl)2] SBUs, in which three Cd2+ are linked together by four carboxylate groups and two bridging Cl-, while the other sites occupied by water molecules and non-bridging carboxylate groups as exhibited in Figure 1n.63 The results of metal-ion exchange confirm that Cd2+ can be completely exchanged by Mn2+ to form Mnporph@MOM-10-Mn, but partially replaced by Cu2+ to give Cuporph@MOM-10-CdCu. Other examples of metal-ion exchange for MOFs with tri- and tetranuclear paddlewheel SBUs (Figure 1o and 1p) have been investigated.64,65 Metal-ion exchange together with structural transformation has been reported by Dalgarno and Thallapally.66 Anionic 3D framework {[Mn3(L)2]2−·2[NH2(CH3)2]+·9DMF} (H4L = tetrakis[4-(carboxyphenyl)oxamethyl]methane acid) was found to transform to a neutral one MnCo3L2(H2O)2·12DMF upon immersing in DMF solution of Co(NO3)2 via SCSCs. Obviously, homometallic trinuclear SBU in the anionic framework was changed to a heterometallic one with the breakage and formation of bond cooperatively (Figure 5).


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Figure 5. Transformation of SBUs with bond breakage and formation.

Metal-ion exchange of trinuclear SBUs was realized for both PCN and MIL series MOFs such as PCN-426, PCN-333 and MIL-101.67-69 PCN-426-Fe3+ and PCN-426-Cr3+ have been synthesized by using PCN-426-Mg through metal-ion metathesis followed by oxidation of the metal nodes.67 In PCN-426-Mg, each Mg2+ has octahedral coordination environment and three Mg2+ share a common O to form a [Mg3(μ3-O)] SBU (Figure 1q).67 Metal-ion exchange within the [M3(μ3-O)] SBU has also been used to prepare PCN-333-Cr3+ from PCN-333-Fe3+,68 as well as MIL-101-Al3+ and MIL-101-Fe3+ from MIL-101-Cr3+.69 Metal-ion exchange was also examined for MOFs with tetranclear SBUs as schematically shown in Figure 1r-1u.70-78 Dincӑ and Long reported a 3D cubic framework Mn3[(Mn4Cl)3(BTT)8(CH3OH)10]2 [1-Mn2+; H3BTT = 1,3,5-tris(tetrazol-5-yl)benzene] with chloride-centered [Mn4Cl]7+ tetranclear SBUs (Figure 1r and Figure 6).70,71 It was demonstrated that Mn2+ ions in 1-Mn2+ can be selectively exchanged by Li+, Cu+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+.70 The results of metal-ion exchange were determined by Inductively Coupled Plasma-Atomic Absorption (ICP-AA), and isostructural frameworks, e.g. Fe3[(Mn4Cl)3(BTT)8]2∙FeCl2

(1-Fe),

Co3[(Mn4Cl)3(BTT)8]2∙1.7CoCl2

(1-Co),

Ni2.75Mn0.25[(Mn4Cl)3(BTT)8]2 (1-Ni), Cu3[(Cu2.9Mn1.1Cl)3(BTT)8]2∙2CuCl2 (1-Cu) and Zn3[(Zn0.7Mn3.3Cl)3(BTT)8]2∙2ZnCl (1-Zn) were isolated. It implies that metal-ion exchange in MOFs is not only feasible for charge balancing extra-framework metal ions but also for metal nodes of intra-framework. The impact of metal-ion exchange on the hydrogen storage properties was further investigated.71,72


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Figure 6. Structure of Mn3[(Mn4Cl)3(BTT)8(CH3OH)10]2 with two different kinds of Mn2+ (intra-framework: gray green and extra-framework: yellow).

Another example of the successful metal-ion metathesis in single crystal of MOF with tetranuclear SBUs was reported by Kim et al. in 2009 (Figure 1s and Figure 7).73 A cubic framework {Cd1.5(H3O)3[(Cd4O)3(hett)8]·6H2O} was achieved by solvothermal reaction of Cd(NO3)2 ·4H2O and ligand ethyl substituted truxene tricarboxylic acid (H3hett, Figure 7b) in DMF. Four Cd2+ ions link a μ4-bridged O atom to form a square-planar [Cd4O]6+ SBU and the other sits of Cd2+ are coordinated by carboxylate groups and coordinated water molecules (Figure 7c). Six such [Cd4O]6+ tetranclear SBUs are joined together by eight hett3- ligands to form an octahedron as basic building unit in the framework (Figure 7a). Metal-ion exchange experiments confirm that Cd2+ ions can be replaced not only by Pb2+ completely and reversibly but also by lanthanide ions Dy3+ or Nd3+. In addition, the Kim group reported another example of metal-ion metathesis of POST-65 containing teranuclear SBUs (Figure 1t).74 It was proved that Mn2+ ions in POST-65 Mn(H3O)[(Mn4Cl)3(hmtt)8] (H3hmtt = methyl substituted truxene tricarboxylic acid) can be exchanged by Fe2+, Co2+, Ni2+ and Cu2+.74


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Figure 7. (a) Structure of {Cd1.5(H3O)3[(Cd4O)3(hett)8]·6H2O}. (b) Ligand H3hett. (c) Tetranuclear SBU.

Metal-ion metathesis of MOF-5 with typical [Zn4O(COO)6] tetranuclear SBUs (Figure 1u) was also studied. Dincӑ’s group investigated the substitution Zn2+ in MOF-5 by di- and tri-valent metal ions to fabricate analogues, some of which are not accessible by direct synthetic reactions.75-77 The crystals of MOF-5 were soaked into DMF solution of different metal salts, and M-MOF-5 (M = V2+, Cr2+, Mn2+, Fe2+, or Ni2+)75, 76 and Cl-M-MOF-5 (M = Ti3+, V3+, or Cr3+)47 with complete or partial exchanged products were achieved. It is noticeable that the daughter MOFs with inserted metal ions endowed M-MOF-5 has redoxactive property, for example Cr2+-MOF-5 can be oxidized to Cr3+-MOF-5. In addition, Fe2+ in Fe2+-MOF-5 can disproportionate NO into N2O and Fe3+-NO2 complex,77 and selective olefin epoxidation catalyzed by partially substituted Mn2+-MOF-5 [MnZn3(terephthalate)3] was reported recently.78 Denysenko et al. reported a Zn2+-Co2+ heterometallic framework with pentanuclear SBUs (Figure 1v) and investigated its reversible catalytic property for gas-phase oxidation.79 The Zn2+-Co2+ heterometallic framework {ZnCo4Cl4(btdd)3} was synthesized by postsynthetic metal-ion exchange from {Zn5Cl4(btdd)3} (MFU-4l, H2btdd = bis(1H-1,2,3-triazolo[4,5-b],[4’,5’-i])dibenzo-[1,4]-dioxin) (Figure 8a, b).79 In the pentanuclear SBU, the central Zn2+ ion coordinated by six N atoms could not be exchanged by Co2+, while the four Zn2+ ions


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in the corner of tetrahedron coordinated by three N atoms and one Cl- can be substituted by Co2+ (Figure 8c). Furthermore, it was recently reported that the pore hydrophilicity of MFU4l could be controlled by the radio of Co2+ replaced Zn2+.80 The results imply that the metalion exchange is a possible route for tuning the hydrophilicity of MOFs.

Figure 8. (a) Structure of MFU-4l. (b) Ligand H2btdd. (c) Reversible exchange of Zn2+ and Co2+ in the pentanuclear SBU.

Selectively metal-ion exchange for the MOFs containing metalloporphyrin SBUs (Figure 1x) was examined. For example, crystals MMPF-5 based on Cd2+ and tetrakis(3,5dicarboxyphenyl)porphine were suspended in DMSO solution of Co(NO3)2, and Cd2+ ions were partially exchanged by Co2+ to form MMPF-5(Co).81 The results remonstrated that in MMPF-5(Co) only the Cd2+ in the porphyrin macrocycles are substituted by Co2+ while the other sites in the framework are still Cd2+ (Figure 9). Moreover, MMPF-5(Co) exhibits porosity as MMPF-5 and interesting performance for the catalytic epoxidation of transstilbene.


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Figure 9. Metal-ion exchange in the metalloporphyrin within MMPF-5.

Ren and Jiang’s group reported a family of porphyrin-salen-based chiral MOFs through post-synthetic metalation in both porphyrin core and metal nodes (Figure 10).82,83 The parent framework [Cd2(NiL1)(CdL2)][Cd2(NiL1)(H2L2)]·6DMF·5MeOH was obtained by reaction of (R,R)-N,N’-bis(3-tert-butyl-5-(4-pyridyl)salicylidene)-1,2-diphenyldiamine Ni2+ (NiL1), tetra-(4-carboxyphenyl)porphyrin (H6L2) and Cd(NO3)2·6H2O.83 In the parent framework, there are twisted Cd2(COO)4 paddlewheel SBUs and porphyrin core occupied by Cd2+ (Figure 10). The crystals of the parent framework were soaked in DMF solution containing Cu2+, Zn2+, Ni2+, or Co2+. Four different kinds of bimetallic (Cd-Cu, Cd-Zn, Cd-Ni, and Cd-Co) paddlewheel SBUs were detected and the Cd2+ in the porphyrin core was also exchanged. The multimetallic MOFs resulted from the metal-ion exchange show enhanced surface area and adsorption sites as well as improved synergistic effect.


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Figure 10. Metal-ion exchange in both porphyrin core and metal nodes of porphyrin-salenbased MOFs.

The reported examples of metal-ion metathesis have been summed up. The examples with different metal nodes or SBUs cover from mononuclear to multinuclear and the results show that post-synthetic metal-ion metathesis represents a powerful synthetic method to fabricate MOFs that are not accessible through direct synthesis and it is also a prominent strategy to improve the properties of MOFs.

LIGAND EXCHANGE As an another essential component of MOFs, organic ligand exchanges also attracted attention of researchers and recently reported works will be described here. To get more and/or enhanced functionalities of MOFs, introducing ligands with definite functional groups is an effective way. As a pioneering work, Yaghi and his co-workers have proved that a series of MOFs were achieved by using mixed 1,4-benzenedicarboxylate (BDC2-) derivative ligands with up to eight functional groups and found that they show fascinating properties superior to their parent MOF-5.84 After that, they reported 24 isomorphous structures of multivariate MOF-177 with varied functional groups on 1,3,5-benzenetribenzoate (BTB3-) ligand.85 These works demonstrate that functionalized organic ligands are important for modifying and ACS Paragon Plus Environment

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improving properties of the resulted MOFs, however, it is a challenge to isolate and characterize the multivariate MOFs using mixed organic ligands due to the uncertainty of the assembly process and the tendency of disordering the functional groups in the crystal lattice. To avoid uncertainty and disorder, Telfer et al. reported series isoreticular MOFs by using combination of ligands with different structures.86,87 Even though such one-pot reaction is useful for fabrication of MOFs with mixed organic ligands, the synthetic reaction from the beginning is not always workable for the desired MOFs. Compared with the one-pot method, ligand exchange or solvent-assisted linker exchange (SALE) appears to be a more promising approach. As for the ligand metathesis, generally, powdered sample of parent MOFs was immersed in a solution of other organic ligands in a definite solvent to produce daughter MOFs containing new ligands with exchange ratio from 1% to 100%. Normally, the daughter MOFs retain the framework structures but with exchanged ligands. The reported works have demonstrated that ligand exchange approach can be used to fabricate new MOFs, introduce new functionalities, tune pore size or shape and so on.88,89 In 2011, Choe et al. reported that PPF-18 and PPF-20 obtained by reactions of Zn(NO3)2∙6H2O with N,N’-di-4pyridylnaphthalenetetracarboxydiimide

(DPNI)

and

tetrakis(4-carboxyphenyl)porphyrin

(TCPP) have 2D bilayer and 3D framework structure, respectively, in which DPNI ligands act as pillars.90 It was found that the DPNI ligands were replaced by bpy by immersing the crystals of PPF-18 into a DMA/EtOH solution of bpy to provide a new MOF PPF-27 (Figure 11), while in the case of PPF-20 the ligand replacement resulted in formation of isostructural PPF-4.


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Figure 11. Schematic representation of ligand replacement for PPF-18 to PPF-27 or PPF-20 to PPF-4.

The Farha and Hupp group has done outstanding works on pillared paddlewheel MOFs and their ligand exchanges as well as SALE.91-101 For instance, DO-MOF with noncatenated structure formed by reaction of tetracarboxylate ligand, Zn(NO3)2∙6H2O and meso-1,2-di(4-pyridyl)-1,2-ethanediol undergoes pillared ligand replacement of meso-1,2di(4-pyridyl)-1,2-ethanediol by bpy leading to the formation of SALEM-3.96 More interestingly, they successfully utilize post-synthetic ligand exchange method to incorporate a variety of ligands with different lengths into the pillared paddlewheel frameworks.97 As an analogue of DO-MOF, SALEM-5 based on meso-1,2-di(4-pyridyl)-1,2-ethanediol pillars with approximately 9 Å length was employed as parent material. Daughter MOFs SALEM-6, SALEM-7 and SALEM-8 were achieved by the ligand exchange of the pillared ligand meso1,2-di(4-pyridyl)-1,2-ethanediol using 4,4’-(2,3,5,6-tetramethyl-1,4-phenylene)dipyridine (11 Å length), 2,6-di(pyridin-4-yl)naphthalene (14 Å length) and 4,4’-[(2,3,5,6-tetramethyl-1,4phenylene)bis(ethyne-2,1-diyl)]dipyridine (17 Å length), respectively (Figure 12). Obviously, the daughter MOFs have larger voids than the parent one due to the longer pillared ligands.97 Ligand exchanges with similar strategy were also reported by other groups.102-105 For example, Park and Lah et al. obtained [Ni(HBTC)(DABCO)] and [Ni2(HBTC)2(bpy)0.6(DABCO)1.4]


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with complete and partial ligand exchanges of bpy in [Ni(HBTC)(bpy)] (H3BTC = 1,3,5benzenetricarboxylic acid) through tuning the concentration of DABCO.105

Figure 12. Insertion of progressively longer ligands into the pillared paddlewheel frameworks through the SALE strategy.

Recently, Hou and Ding group reported an efficient transformation for a 3D MOF {[Ni1.5(L)(4,4′-azobpy)(H2O)]·6.5H2O}n (1) (H3L = 1-aminobenzene-3,4,5-tricarboxylic acid, 4,4′-azobpy = 4,4′-azopyridine) via SCSC SALE.106 After immersing crystals of 1 in aqueous CH3CN solution of 4,4′-bpy or 4,4′-vinylenedipyridine (bpe) or NH2-bpy, daughter MOFs {[Ni1.5(L)(4,4′-bpy)(H2O)]·6H2O}n

(2),

{[Ni1.5(L)(bpe)(H2O)]·8.5H2O}n

(3),

and

{[Ni1.5(L)(NH2-bpy)(H2O)]·7.5H2O}n (4) were achieved (Figure 13). The complete SALE was evidence by single crystal and powder X-ray diffractions. Importantly, MOFs 2 and 3 exhibit different magnetic property from the parent MOF 1, furthermore, 4 shows high adsorption capacity of 93.693 mg/g for removal of Hg2+ due to the presence of functional NH2 group.


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Figure 13. Structural variation through SALE.

Ligand exchange strategy has also been used for zeolitic imidazolate frameworks (ZIFs), an unique family of MOFs with high chemical and thermal stability, crystallinity and porosity. In 2012, it was reported that solvothermal reactions of the crystals of ZIF with 2ethylimidazolate (eim), namely CdIF-4, with solution of 2-nitroimidazole (nim, excess) and 2-methylimidazole (mim, excess) resulted in formation of previously reported ZIF CdIF-9 and a new ZIF SALEM-1, respectively (Figure 14).107 Similarly, using imidazole (im) to replace the mim ligand with methyl group in ZIF-8 [Zn(mim)2] via SALE, new material SALEM-2 [Zn(im)2] with catalytically active sites by reaction with n-BuLi was obtained.108 For ZIFs with mixed imidazolate ligands, selective ligand exchange was found.109 When ZIF69 was used as parent framework, it was found that only 5-chlorobenzimidazolate (cbim) could be replaced by 5-(trifluoromethyl)benzimidazole (fbim), while the nim retains, to give the daughter framework SALEM-10. Similar phenomenon also occurred for ZIF-78 with mixed nim and 5-nitrobenzimidazolate (nbim) ligands as well as ZIF-76 with both im and cbim linkers. Only nbim and cbim could be exchanged by fbim in ZIF-78 and ZIF-76 to generate exchanged products SALEM-10b and SALEM-11, respectively.109 In addition, Cohen group reported post-synthetic both metal-ion and ligand exchanges in ZIF-71 and ZIF-8. They realized not only the replacement of Zn2+ by Mn2+ to give porous Mn-ZIF, but also the exchange of 4,5-dichloroimidazole in ZIF-71 and mim in ZIF-8 by 4-bromo-1Himidazole.110 There are additional studies on ligand exchange of ZIFs including introduction of 1,2,3-triazole and 3-amino-1,2,4-triazole into ZIF-67 and ZIF-8,111,112 the modification of ZIF-8 and ZIF-90,113-115 drawing benzotriazole into ZIF-7.116 The results illustrate that the daughter ZIFs resulted from post-synthetic ligand exchange not only maintain the superiority of parent ZIFs, but also provides improved and/or extra functionalities.


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Figure 14. Schematic representation of ligand replacement within CdIF-4, CdIF-9 and SALEM-1.

In addition to the above mentioned imidazolate bridging ligands, pyrazine and its derivatives are also well-used in the construction of MOFs as bridging linkers. As an analogue of aforementioned Cu-1, MOF [Cu3(L)2(pyz)(H2O)]·13DMF (1) was achieved by using pyrazine (pyz) instead of DABCO. Fortunately, we found that pyz in MOF 1 can be exchanged by its derivatives 2,5-dimethylpyrazine (2,5-Me2pyz), quinoxaline (qx), = pyrazine-2,5-diylbis(methylene) diacetate [2,5-(C3H5O2)2-pyz] and pyrazine-2-amine (2-NH2-pyz) via SCSCs to generate new MOFs [Cu3(L)2(2,5-Me2pyz)(H2O)]·12DMF (2), [Cu3(L)2(qx)(H2O)]·12DMF (3), [Cu3(L)2(H2O)2(H2O)]·12DMF (4), [Cu3(L)2(2,5-(C3H5O2)2pyz)(H2O)]·8DMF (5) and [Cu3(L)2(2-NH2-pyz)(H2O)]·12DMF (6) as schematically illustrated in Figure 15.117 Furthermore, the ligand exchange process was monitored by in situ X-ray diffraction, which provides direct evidence for the SCSC process. It is worthy to note that MOFs 5 and 6 could not be achieved by direct synthesis by using 2,5-(C3H5O2)2-pyz and 2-NH2-pyz ligands, respectively, implying that ligand exchange is a valid method for preparation of MOFs. More importantly, MOF 6 with functional amino groups not only shows the largest adsorption enthalpy for the CO2 adsorption, but also exhibits higher catalytic activity than the parent MOF 1 for the Knoevenagel condensation reactions.117 The


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results indicate the improvement of sorption and catalytic properties of MOFs by incorporating definite functional groups through ligand exchange. Therefore, it is further confirming that ligand exchange is a powerful strategy to fabricate desired MOFs.

Figure 15. Schematic illustration of MOFs with pyrazine and its derivatives together with DABCO and water molecules.

Multicarboxylate compounds are also an important family of bridging organic linkers and well employed in construction of MOFs, which are the same as the N-donor ligands. Metal-carboxylate frameworks with varied structures and properties have been extensively exploited. The reported research works revealed that carboxylate ligands in MOFs may also be able to be exchanged by others via PSM. UiO-66 series MOFs are particularly interested and well studied due to their high stability. For instance, functionalized ligands such as BrBDC2- and NH2-BDC2- were introduced into the MOF UiO-66 based on BDC2- through the ligand exchanges (Figure 16).118 In addition, catalytic active moiety FeFe(dcbdt)(CO)6 (dcbdt = 1,4-dicarboxylbenzene-2,3-dithiolate) was successfully incorporated into UiO-66 via ligand exchange to produce UiO-66-[FeFe](dcbdt)(CO)6 (Figure 16).119 [FeFe](dcbdt)(CO)6 can be regarded as model for the active center of [FeFe]-hydrogenase due to their structural similarity. It is interesting to find that UiO-66-[FeFe](dcbdt)(CO)6 shows higher catalytic


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activity towards photochemical hydrogen evolution than FeFe(dcbdt)(CO)6 in solution using [Ru(2,2’-bpy)3]2+ (2,2’-bpy = 2,2’-bipyridine) and ascorbate as photosensitizer and electron donor, respectively. The high catalytic activity of UiO-66-[FeFe](dcbdt)(CO)6 is considered to be due to its high stability by incorporation the catalytic active moiety FeFe(dcbdt)(CO)6 into the framework.119 Ligand exchange process was examined for UiO-66 (Figure 16), MOF-5, and UMCM-8 using H2BDC-d4 and the results show that diffusion determines the ligand exchange and arise the formation of core-shell structures.120 Furthermore, postsynthetic annealing (PSA) was reported to be a facile way to create defects through the selfexchange of labile terephthalate (-OOC-C6H4-COO-) and benzoate (C6H5-COO-) ligands on the surface of UiO-66.121

Figure 16. Ligands can be exchanged in UiO-66.

Ligand exchanges can occur even in highly robust MOFs like MIL-53 and MIL-68 with dicarboxylate linkers. Cohen et al. demonstrated that ligands NH2-BDC2- and Br-BDC2can be mutually exchanged in MIL-53(Al) and MIL-68(In) (Figure 17).53 Similarly, reversible ligand exchange between BDC2- and Br-BDC2- in MOF-5 was confirmed by powder X-ray diffraction (PXRD), 1H and 13C NMR, SEM, EDX and nitrogen adsorption.122


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Figure 17. Ligands can be exchanged in MIL-53 and MIL-68.

For biomedical applications of MOFs, it is essential to consider their biocompatibility in addition to the functionality. To reach this purpose, bio-MOFs are generally constructed by using biocompatible metal centers and small biomolecule linkers.123 However, there are difficulties in construction of bio-MOFs with reasonable porosity and stability coming from the low symmetry, large flexibility and weak coordination ability of biomolecule ligands.124126

Accordingly, ligand exchange strategy provides additional route for fabrication of desired

bio-MOFs. Rosi et al. reported a series of bio-MOFs with different length of dicarboxylate ligands. The results of ligand exchanges revealed that the short dicarboxylate ligand can be replaced by a longer one, for example, 4,4’-biphenyldicarboxylate (BPDC2-) ligands in bioMOF-100 can be exchanged by azobenzene-4,4'-dicarboxylate (ABDC2-) leading to the formation of bio-MOF-102, and similar conversion from bio-MOF-102 to bio-MOF-103 was also realized by using 2’-amino-1,1’:4,1’’-terphenyl-4,4’’-dicarboxylate (NH2-TPDC2-) (Figure 18).127 The important thing is that bio-MOF-100 and bio-MOF-101 can be prepared by direct synthetic reactions using H2BPDC and naphthalenedicarboxylic acid (H2NDC), respectively. In contrast, bio-MOF-102 and bio-MOF-103 with longer ABDC2- and NH2TPDC2- ligands are not available by direct synthetic reactions. Furthermore, ligand exchange with longer linker increases the porosity of the resulted MOFs. To further enhance the properties of bio-MOFs, they expanded the isoreticular series of bio-MOF-100 by installing


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functional groups such as –N3, -CHO and –NH2 into the frameworks via post-synthetic ligand exchange.128 Starting from bio-MOF-102 as parent MOF, bio-MOF-106 and bio-MOF-107 were achieved by sequential ligand exchange using 2’-nitro-1,1’:4’,1”-terphenyl-4,4”dicarboxylate (NO2-TPDC2-) to give bio-MOF-106, followed by using 4-(3’-nitro-4’-(4’’carboxylphenylethynyl)phenyl)benzoate (NO2-eTPDC2-) to generate bio-MOF-107.129

Figure 18. Schematic illustration of sequential ligand exchanges from bio-MOF-101 to bioMOF-103.

MOFs with multi-components provide opportunity to tune their structures and properties and have attracted remarkable attention from researchers. Recently, a series of multi-component MOFs PCN-900(RE) (RE = rare-earth metal ions) were established by using three-components of rare-earth hexanuclear clusters, tetracarboxylate porphyrinic ligands and dicarboxylate linear linkers.130 Interestingly, the linear ligand 4,4’-


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dicarboxydiphenyl sulfone (DCDPS) can be exchanged by other dicarboxylate ligands with functional groups including -CHO, -NO2 and so on (Figure 19). The results of heterogeneous catalytic cycloaddition reactions of CO2 with epoxides confirm that installation of functional moieties into the frameworks together with tunable metal sites provides desired platform for catalysis.

Figure 19. Schematic illustration of post synthetic ligand exchange in PCN-900.

Despite limited reports, the published ligand exchange examples demonstrate that both N-donor linkers and carboxylate-based ligands can be replaced by others, and such ligand exchange is efficient and useful technique for not only generating new MOFs particularly for those not available by direct synthesis, but also installing functional moieties into MOFs to improve the functionality of MOFs.

SEQUENTIAL LINKER INSTALLATION (SLI) The above-mentioned ligand exchange is one kind of ligand in the framework partially or completely replaced by another, and has been demonstrated to be an efficient and important strategy for fabrication of new MOFs as well as for tuning the structure and property of MOFs. In addition to the ligand exchange, additional approach has been developed by installation or


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insertion, rather than the exchange or replacement, of organic ligands into the existed frameworks, which is called sequential linker installation (SLI). Such approach can generate multivariate MOFs (MTV-MOFs) and further broaden the structure and functionality of MOFs. Obviously, SLI is a post-synthetic stepwise approach, and is different from the onepot reaction by using mixed organic ligands.

Figure 20. Schematic illustration of step-wise insertion of two sequential linkers from PCN700 to give PCN-703 and PCN-704.

MOF PCN-700 with Zr6O4(OH)8(H2O)4 clusters and 2,2’-dimethylbiphenyl-4,4’dicarboxylate (Me2-BPDC2-) linkers was achieved by one-pot reaction.131 It is noticeable that there are terminal OH- and H2O ligands in the Zr6O4(OH)8(H2O)4 clusters, which may be replaceable by other species. Accordingly, SLI strategy is applied to PCN-700, and varied linear dicarboxylate linkers with different lengths and functional moieties were tested. It is interesting to find that PCN-703 can be obtained by sequential insertion of BDC2- (to give PCN-701) and 2’,5’-dimethylterphenyl-4,4’’-dicarboxylate (Me2-TPDC2-) linkers into PCN-


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700 (Figure 20). However, reverse sequential insertion of Me2-TPDC2- giving PCN-702 and BDC2- could not produce PCN-703 (Figure 20), implying that the insertion sequence is important. Similarly, PCN-704 was achieved by using ligands with distinct functional groups via SLI (Figure 20).131 Then, further extended studies were performed using Me2-BPDC2analogues with different functional moieties. PCN-700 and UiO-67 series MOFs were achieved and their SLI was systematically examined.132 These studies demonstrate that SLI provides a powerful way to construct MTV-MOFs with predesigned functionalities and controlled pore size and environment. As a result, adsorption properties, particularly the H2 adsorption capacity, of MOFs after SLI were significantly improved. Furthermore, PCN-700BPYDC(Cu) (BPYDC2- = 2,2’-bipyridne-5,5’-dicarboxylate) with catalytic active moiety of BPYDC(Cu) shows high activity and selectivity for the alcohol oxidation reactions.132 To further increase the complexity of MOFs, not only BDC2- and TPDC2- were introduced in PCN-606, but also three linear linkers of BDC2-, BPDC2- and TPDC2- were installed into PCN-609 by SLI.133 By immersing the crystals of PCN-606 in DMF solution of BDC, PCN-606-BDC was produced. After that the crystals of PCN-606-BDC were treated with TPDC solution, PCN-606-BDC-TPDC was achieved. Alternatively, PCN-606-BDCTPDC is also available by installation of TPDC first to give PCN-606-TPDC followed by insertion of BDC (Figure 21). In the case of PCN-609 with low-symmetry trapezoidal linker, there are three pockets with different sizes, providing possibility to install three different linkers. As a result, a series of linkers with different lengths and functional groups can be employed to fabricate multivariate MOFs through SLI (Figure 22). The results of this work demonstrate that the symmetry of organic linkers is important not only in the creation of pore environments but also in the SLI.


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Figure 21. Transformations of PCN-606 by installation of BDC2- and TPDC2-.

Figure 22. Schematic representation of three pockets I, II and III with a variety of functionalized linkers in PCN-609.

Further sequential ligand elimination and installation was reported for PCN-160, a Zrbased MOF with azobenzene-4,4’-dicarboxylate linker (AZDC2- or ABDC2-).134,135 Detailed study revealed that the AZDC2- linker in PCN-160 could not be exchanged by 4pyridinecarboxylate (INA-) or M-INA2 but can be exchanged by imine-based linker like 4-


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carboxybenzylidene-4-aminobenzate (CBAB2-).134,136 More importantly, M-INA2 (M = Ni, Mn, Fe, Co, Cu and Pd) were successfully installed into PCN-160 by CBAB2- exchange first, followed by subsequent treatment in the solution of INA and metal salt (Figure 23). This occurred because CBAB2- can be eliminated by hydrolyzation into 4-aminobenzoic acid and 4-formylbenzoic acid.

Figure 23. Transformation of linker in PCN-160 by sequential ligand exchange, elimination and installation.

Another term of post-synthetic variable spacer installation (PVSI) was used to fabricate MTV-MOFs.137-139 As a prototypical MOF, LIFM-28 is a Zr4+-MOF with 2,2’bis(trifluoromethyl)-4,4’-biphenyldicarboxylate linkers, and there are two distinct types of replaceable binding sites A and B (Figure 24). It is interesting to find that varied dicarboxylate linkers with distinct substituents can be installed into the Site-A and removed by addition water, implying a reversible process. In addition, the installation and removal of linkers causes expansion and contraction of the framework leading to the breathing behavior.137 Then, to further functionalize proto-LIFM-28, two different kinds of dicarboxylate ligands with varied functional moieties were installed into both Site-A and SiteB, and MOFs LIFM-70-86 were obtained as schematically illustrated in Figure 24.138 Furthermore, the results show that the properties of MOFs such as gas adsorption/separation,


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catalytic activity can be fine tuned by installation of ligands with definite functional groups. More recently, LIFM-31 and LIFM-50-53 were generated by immersing the crystals of LIFM-28 into DMF solution of H2L15-HL19, respectively.139

Figure 24. Schematic illustration of two types of spacers at sites A and B with varied substituents installed into LIFM-28.

Research work on SLI has been further extended by installation of three different secondary ligands into the framework NPF-300.140 MOF NPF-300 with Zr6O4(OH)8(H2O)4 clusters and {5’,5’’’’-(buta-1,3-diyne-1,4-diyl)bis([1,1’:3’,1’’-terphenyl]-4,4’’-dicarboxylate)} primary ligands is able to install different secondary dicarboxylate linkers step-wisely to generate MOFs NPF-301-NPF-305 as shown in Figure 25.140 Importantly, the porosity as well as the adsorption property of the frameworks can be efficiently tuned by such step-wise linker installation.


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Figure 25. Step-wise insertion of sequential linkers from NPF-300 to NPF-301-NPF-305.

OUTLOOK It can be seen from the research works described above that post-synthetic method is effective and powerful strategy for fabricating desired frameworks especially for those not available by direct synthesis, creating or enhancing the property and functionality of MOFs. In this review, we summarized post-synthetic exchange (PSE) and sequential linker installation (SLI). For PSE, parent MOFs were usually immersed into a solution containing excess targeted metal ions or ligands for exchange or replacement. The driving force of PSE can be the difference of coordination ability and/or concentration before and after the exchange. In addition, the structure and topology of daughter MOFs are generally same to the parent, and the complexity of MOFs changes less before and after the exchange. Thus, the PSE method has limitation in


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broadening the utility of MOFs. In contrast, SLI may increase the complexity of MOFs via insertion of new ligands. However, to now the reported SLI is limited to the Zr4+-MOFs with Zr6O4(OH)8(H2O)4 clusters. Although PSE and SLI have advantages for increasing thermal stability, introducing functional groups and enabling catalytic sites, it is still a great challenge to realize PSE and SLI in designable, predictable and tunable manner. For exchange or installation, basically coordination ability between metal center and ligand, coordination number and geometry of metal ions, size and flexibility of ligands should be taken into account. In addition, thermodynamics and kinetics within the PSE and SLI should be considered. More importantly, further systematic research is needed to understanding the exchange and installation process. Fortunately, there are many researchers pay attention on this area and much progress can be expected in the near future.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; Fax: +86 25 89682309. ORCID Wei-Yin Sun: 0000-0001-8966-9728 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (grant no. 21701021).

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140, 7710-7715.


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For Table of Contents Use Only:

Fabrication of Desired Metal-Organic Frameworks via Post-Synthetic Exchange and Sequential Linker Installation Peipei Cui, Peng Wang, Yue Zhao, and Wei-Yin Sun*

This review mainly introduces the post-synthetic method including post-synthetic exchange and sequential linker installation to synthesize new and desired frameworks that could not be obtained by direct synthesis.


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Fabrication of Desired Metal-Organic Frameworks via Post-Synthetic

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