**4.1 Metals atoms tend to exist in clusters**

In order to bring down the structures to simpler diagrams, the patterns, or segments of atoms, that occur frequently throughout the framework must be well noticed. After taken the knowledge of the building blocks, we will look into a representative building to see how the blocks are assembled to a building. It is obvious that the organic linker will stay as it is used before the reaction in most structures, as it is very difficult for the benzene ring to disassemble in our BTC example. One thing, however, may fluctuate greatly from structure to structure: the coordination mode. Often there are many atoms, or sites, that are capable of coordinating to metal atoms, but almost always, not all of them do. It is very difficult to predict which coordination mode the ligand will take, since even under the same topology, the ligands are found to take structures with many different coordination modes [1, 30, 31]. Attempts have been made to collectively study coordination modes [34], but for successful discovery of any laws governing them, acquisition of more data is necessary.

In collaboration with the coordination modes, though it is difficult to distinguish causation from correlation at this level, the reaction environment determines the shape in which the metal atoms exist in the framework. From **Tables 4**, **5** and **7**, it has been shown the nuclear types the metal atoms take in the framework, but the concept has never been visited yet. This 'nucleus' is a small collection of metal atoms and atoms from the organic ligand coordinating to them and is more commonly called 'metal clusters' because many metal atoms are found together in most structures. These metal clusters are one of the most important character to determine the topology of MOFs, and the frameworks are named as binuclear, trinuclear, etc. according to the number metal atoms present in the metal cluster. If small variations within the same topology are ignored, the framework can be viewed as a collection of simple connections between the unvarying organic ligand and the metal clusters, just like vertices and edges of a mathematical 3D figure.

The simplification illustrated in **Figure 5** exemplifies the power of reduction in brining different structures together. Although it could have been inferred from the same molecular formulas, a great number of structures introduced in **Tables 4**, **5** and **7** actually have the exact same framework.

#### **4.2 Structure explains the popularity of [RMI][metal(BTC)] topology**

Some of the most commonly occurring structures need attention, not only because they will be frequently met in trials of novel conditions, but also they will provide a valuable starting point in relating to other structures occurring in the same system to understand the correlations like the ones we have visited.

The topology [RMI][Metal(BTC)] occurs in most metal systems that have been reported and in the highest frequency. With this topology as an example, we will show how a complex structure may be simplified. This way, details unnecessary for understanding of the topology can be ignored and attention may be more easily focused on the topology itself. The characteristics that may vary within the topology without changing it include coordination modes, bond angle, and bond in certain ranges.


**97**

*framework.*

*a similar manner to Table 4.*

**Table 7.**

*Ionothermal Synthesis of Metal-Organic Framework DOI: http://dx.doi.org/10.5772/intechopen.79156*

La [HMI][La2Cl(1,4-ndc)3] Poly-

Ce [HMI][Ce2Cl(1,4-ndc)3] Poly-

Pr [HMI][Pr2Cl(1,4-ndc)3] Poly-

Nd [HMI][Nd2Cl(1,4-ndc)3] Poly-

Gd [HMI][Gd2Cl(1,4-ndc)3] Poly-

Tb [HMI][Tb2Cl(1,4-ndc)3] Poly-

Dy [HMI][Dy2Cl(1,4-ndc)3] Poly-

Ho [HMI][Ho2Cl(1,4-ndc)3] Poly-

Er [HMI][Er2Cl(1,4-ndc)3] Poly-

Y [HMI][Y2Cl(1,4-ndc)3] Poly-

Na, Cu

**M Formula Nuclear # CCDC** 

nuclear

nuclear

nuclear

nuclear

nuclear

nuclear

nuclear

nuclear

nuclear

nuclear

nuclear

[EMI][NaCu(1,4-ndc)2] Poly-

**code**

WIKZAT [44]

MEGNIX [39]

MEGNOD [39]

MEGNUJ [39]

MEGPAR [39]

MEGPOF [39]

MEGPUL [39]

MEGQAS [39]

MEGQEW [39]

MEGQIA [39]

MEGQOG [39]

**Void with cation\***

0.6% 16.45 Å3 /2729.18 Å3

0% 0 Å3 /4328.43 Å3

0% 0 Å3 /4296.06 Å3

0% 0 Å3 /4262.42 Å3

0% 0 Å3 /4241.32 Å3

0% 0 Å3 /4164.69 Å3

0% 0 Å3 /4152.78 Å3

0% 0 Å3 /4124.91 Å3

0% 0 Å3 /4105.46 Å3

0% 0 Å3 /4103.22 Å3

0% 0 Å3 /4127.12 Å3 **Void without cation\***

25.7% 700.27 Å3 /2729.18 Å3

26.6% 1150.84 Å3 /4328.43 Å3

26.6% 1142.24 Å3 /4296.06 Å3

26.3% 1122.11 Å3 /4262.42 Å3

26.4% 1121.26 Å3 /4241.32 Å3

25.7% 1069.98 Å3 /4164.69 Å3

25.7% 1066.29 Å3 /4152.78 Å3

25.5% 1052.86 Å3 /4124.91 Å3

25.4% 1044.15 Å3 /4105.46 Å3

25.4% 1040.46 Å3 /4103.22 Å3

25.5% 1052.17 Å3 /4127.12 Å3

The simplification above is itself beautiful but is meaningless if description of the topology is not accompanied. Description gives meaning to the structure and

*Structures arising from imidazolium-based MOFs with 1,4-naphthalene dicarboxylic acid(NDC) presented in* 

*Calculation was done with the same setting. In the entry with reference code NUKBOM, TOLNAL, and TOLNEP, the calculation for the void volume with the cation removed was not conducted since the cations were bound to the* 

Based on face-centred cubic lattice (FCC), the unit cell of [RMI][Metal(BTC)] is very compact. Its binuclear metal cluster occupies all the FCC sites, and BTC occupies the interstitial sites. There are eight BTC ligands, and the rest of the interstitial sites appear empty in **Figure 6**. These sites, however, are not actually empty. There are eight metal clusters and eight BTC ligands in the unit cell, but each metal cluster has double positive charge while BTC ligand has triple negative. The framework is negatively charged, as nearly every ionothermally synthesised framework is, and the charge balance is maintained by the guest cations occupying the rest of the

explanations for many of the observed phenomena.


#### *Ionothermal Synthesis of Metal-Organic Framework DOI: http://dx.doi.org/10.5772/intechopen.79156*

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

Heptanuclear

Hexanuclear

Tetranuclear

Polynuclear

nuclear

nuclear

Polynuclear

Hexanuclear

Heptanuclear

Tri-nuclear EXUYEC

Di-nuclear TOLMOY

Di-nuclear TOLMUE

Tri-nuclear TOLNAL

Tri-nuclear TOLNEP

nuclear

**code**

[37]

AHIYOH [37]

AHIYUN [37]

AHIZAU [37]

EMUTUD [38]

MEGPIZ [39]

MEGPEV [39]

EMUVAL [38]

[40]

NUKBOM [41]\*\*

QATLEE [42]

[43]

[43]

[43]\*\*

[43]\*\*

[44]

VUXGOM [45]

**Void with cation\***

0% 0 Å3 /1739.88 Å3

0% 0 Å3 /8376.27 Å3

0% 0 Å3 /7998.16 Å3

0% 0 Å3 /6065.78 Å3

0% 0 Å3 /2033.18 Å3

0% 0 Å3 /4188.58 Å3

0% 0 Å3 /4203.36 Å3

0% 0 Å3 /2032.62 Å3

0% 0 Å3 /3424.65 Å3

0% 0 Å3 /4700.01 Å3

0% 0 Å3 /8044.7 Å3

0% 0 Å3 /1704.13 Å3

0% 0 Å3 /1721.79 Å3

0% 0 Å3 /3276.26 Å3

0% 0 Å3 /3387.26 Å3

0% 0 Å3 /1814.56 Å3

0.2% 17.02 Å3 /6950.81 Å3 **Void without cation\***

37.8% 658.13 Å3 /1739.88 Å3

23.4% 1958.42 Å3 /8376.27 Å3

21.5% 1717.36 Å3 /7998.16 Å3

31.3% 1897.74 Å3 /6065.78 Å3

34.8% 708.05 Å3 /2033.18 Å3

26.0% 1086.94 Å3 /4188.58 Å3

25.8% 1086.30 Å3 /4203.36 Å3

34.9% 709.04 Å3 /2032.62 Å3

24.7% 845.14 Å3 /3424.65 Å3

0% 0 Å3 /4700.01 Å3

23.2% 1864.05 Å3 /8044.7 Å3

36.5% 622.27 Å3 /1704.13 Å3

37.3% 642.55 Å3 /1721.79 Å3

0% 0 Å3 /3276.26 Å3

0% 0 Å3 /3387.26 Å3

38.4% 697.55 Å3 /1814.56 Å3

18.4% 1279.20 Å3 /6950.81 Å3

**M Formula Nuclear # CCDC** 

Co [EMI][Co(1,4-ndc)Br] Di-nuclear AHIYIB

[PMI]2[Co7(1,4 ndc)6(OH)4]

[BMI]2[Co6(1,4 ndc)6(OH)2]

[AMI]4[Co4Na5(1,4 ndc)8Br]

(ox)0.5Br]

Sm [HMI][Sm2Cl(1,4-ndc)3] Poly-

[EMI][Sm(1,4-ndc) (ox)0.5Br]

ndc)4(MeIm)2(H2O)2]·H2O

[BMI]2[Mg6(1,4 ndc)5(H2NDC)2(HCOO)2]

ndc)6]

C3Im)2(H2O)2]

[Ni3(1,4-ndc)4(Mim-C4Im)2(H2O)2]

[Ni3(1,4-ndc)4(Mim-C5Im)2(H2O)2]

[Ni3(1,4-ndc)4(Mim-C6Im)2(H2O)2]

Sr [EMI][Sr10(1,4-ndc)10Br4] Poly-

Cd [EMI][CdBr(1,4-ndc)] Di-nuclear UYUPUA

Mg [AMI]2[Mg3(1,4-

Zn [EMI]2[Zn7(μ4-O)2(1,4-

Ni [Ni3(1,4-ndc)4(Mim-

[HMI][Eu2Cl(1,4-ndc)3] Poly-

Eu [EMI][Eu(1,4-ndc)

**96**

*Calculation was done with the same setting. In the entry with reference code NUKBOM, TOLNAL, and TOLNEP, the calculation for the void volume with the cation removed was not conducted since the cations were bound to the framework.*

#### **Table 7.**

*Structures arising from imidazolium-based MOFs with 1,4-naphthalene dicarboxylic acid(NDC) presented in a similar manner to Table 4.*

The simplification above is itself beautiful but is meaningless if description of the topology is not accompanied. Description gives meaning to the structure and explanations for many of the observed phenomena.

Based on face-centred cubic lattice (FCC), the unit cell of [RMI][Metal(BTC)] is very compact. Its binuclear metal cluster occupies all the FCC sites, and BTC occupies the interstitial sites. There are eight BTC ligands, and the rest of the interstitial sites appear empty in **Figure 6**. These sites, however, are not actually empty. There are eight metal clusters and eight BTC ligands in the unit cell, but each metal cluster has double positive charge while BTC ligand has triple negative. The framework is negatively charged, as nearly every ionothermally synthesised framework is, and the charge balance is maintained by the guest cations occupying the rest of the

#### **Figure 5.**

*The structure represents the (2,6)-connected 2D network. The list of entries that exhibit this particular structure is: [EMI]2[Co3(BDC)3X2] (X = Cl, Br, I) (TACHUD for Cl; JOQXOE for Br; TACJOZ for I), [PMI]2[Co3(BDC)3X2] (X = Cl, Br, I) (TACJAL for Cl; JOQXUK for Br; TACJUF for I), [BMI]2[Co3(BDC)3X2] (X = Cl, Br, I) (TACJEP for Cl; JOQPUC for Br; TACKAM for I), [PEMI]2[Co3(BDC)3X2] (X = Cl, I) (TACJIT for Cl; TACKEQ for I), [EMI]2[Zn3(BDC)3X2] (X = Cl, Br, I) (SIVQEV for Cl; QUGVIZ for Br; QUGWEW for I), [PMI]2[Zn3(BDC)3X2] (X = Cl, Br, I) (QUGVAR for Cl; QUGVOF for Br; QUGWIA for I), [BMI]2[Zn3(BDC)3X2] (X = Cl, Br, I) (SIVCAD for Cl; QUGVUL for Br; QUGWOG for I), and [PEMI]2[Zn3(BDC)3X2] (X = I) (QUGWUM for I).*

**Figure 6.**

*The asymmetrical unit and the unit cell of [RMI][Metal(BTC)] topology with its binuclear cluster and organic ligand represented, respectively, by singular units.*

interstitial sites. This allows no void for the structure and is thus stable. Nevertheless, the structure may not house longer cations regardless of how preferred it is over other possible options; it is just impossible. This complies with the observation from **Table 5** that the structure is very much preferred with [EMI], but only with [EMI] and the preference drops greatly as we move on to longer cations.

### **4.3 Seemingly different structures may belong to the same topology class**

A large number of syntheses have been reported to the literature, but the number of novel topologies is much smaller. It will be very interesting to see so many structures that once appeared all different converging into one topology. In this example, a group of structures with a different formula and a different nuclear type will be merged with the [RMI][Metal(BTC)] class that has been described above.

**Figure 7** depicts the [RMI][Metal(BTC)] structure. This same structure, however, is shared by [EMI]2[In2Co(OH)2(BTC)2Br2] structure that has a remarkably different molecular formula. The formula is the simplest tool to represent frameworks, but it can sometimes be misleading. **Figure 8** shows an even more striking

**99**

clusters can have.

*represent acetate groups.*

**Figure 7.**

**Figure 8.**

*Ionothermal Synthesis of Metal-Organic Framework DOI: http://dx.doi.org/10.5772/intechopen.79156*

example with [RMI][Metal(BTC)] structure and [RMI]2[M3(HBTC)4(H2O)] structure. Although it is very difficult to catch any similarities from the formula nor the structure if at first glance they actually fall under the same topology umbrella. This remarkable similarity is possible because some coordination sites of the trinuclear metal cluster are occupied by another molecular moiety, OAc in this case as shown in **Figure 7**. These places the trinuclear clusters in the octahedral coordination mode, which is the maximum coordination that binuclear metal

*The left is the [RMI][Metal(BTC)] class with binuclear clusters and the right [RMI]2[Metal3(HBTC)4 (H2O)] with trinuclear clusters. Light blue stick represents the octahedral connections between metal clusters and BTC to better illustrate the topological identity of the two groups. The molecular moieties in the red circle* 

*The structure represents (3,6)-connected network. This corresponds to the following formulas: [BMI]*

*[Zn(BTC)] (FUTZEB), and [EMI]2[In2Co(OH)2(BTC)2Br2] (VUVZAP).*

**5. Outstanding properties of ionothermally prepared MOFs**

In previous sections, we have explored through the diverse structures prepared by ionothermal synthesis and several perspectives through which the groups of structures may be analysed to gain deeper insights. The last step is to find a practical use for those insights. The versatility of ionothermal synthesis, that its reaction environment may be easily altered and related to the change in products, directly

*Ionothermal Synthesis of Metal-Organic Framework DOI: http://dx.doi.org/10.5772/intechopen.79156*

#### **Figure 7.**

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

*The structure represents the (2,6)-connected 2D network. The list of entries that exhibit this particular structure is: [EMI]2[Co3(BDC)3X2] (X = Cl, Br, I) (TACHUD for Cl; JOQXOE for Br; TACJOZ for I), [PMI]2[Co3(BDC)3X2] (X = Cl, Br, I) (TACJAL for Cl; JOQXUK for Br; TACJUF for I), [BMI]2[Co3(BDC)3X2] (X = Cl, Br, I) (TACJEP for Cl; JOQPUC for Br; TACKAM for I),* 

*Br; QUGWOG for I), and [PEMI]2[Zn3(BDC)3X2] (X = I) (QUGWUM for I).*

*[PEMI]2[Co3(BDC)3X2] (X = Cl, I) (TACJIT for Cl; TACKEQ for I), [EMI]2[Zn3(BDC)3X2] (X = Cl, Br, I) (SIVQEV for Cl; QUGVIZ for Br; QUGWEW for I), [PMI]2[Zn3(BDC)3X2] (X = Cl, Br, I) (QUGVAR for Cl; QUGVOF for Br; QUGWIA for I), [BMI]2[Zn3(BDC)3X2] (X = Cl, Br, I) (SIVCAD for Cl; QUGVUL for* 

interstitial sites. This allows no void for the structure and is thus stable. Nevertheless, the structure may not house longer cations regardless of how preferred it is over other possible options; it is just impossible. This complies with the observation from **Table 5** that the structure is very much preferred with [EMI], but only with [EMI]

*The asymmetrical unit and the unit cell of [RMI][Metal(BTC)] topology with its binuclear cluster and* 

A large number of syntheses have been reported to the literature, but the number of novel topologies is much smaller. It will be very interesting to see so many structures that once appeared all different converging into one topology. In this example, a group of structures with a different formula and a different nuclear type will be merged with the [RMI][Metal(BTC)] class that has been described above. **Figure 7** depicts the [RMI][Metal(BTC)] structure. This same structure, however, is shared by [EMI]2[In2Co(OH)2(BTC)2Br2] structure that has a remarkably different molecular formula. The formula is the simplest tool to represent frameworks, but it can sometimes be misleading. **Figure 8** shows an even more striking

and the preference drops greatly as we move on to longer cations.

*organic ligand represented, respectively, by singular units.*

**4.3 Seemingly different structures may belong to the same topology class**

**98**

**Figure 5.**

**Figure 6.**

*The structure represents (3,6)-connected network. This corresponds to the following formulas: [BMI] [Zn(BTC)] (FUTZEB), and [EMI]2[In2Co(OH)2(BTC)2Br2] (VUVZAP).*

#### **Figure 8.**

*The left is the [RMI][Metal(BTC)] class with binuclear clusters and the right [RMI]2[Metal3(HBTC)4 (H2O)] with trinuclear clusters. Light blue stick represents the octahedral connections between metal clusters and BTC to better illustrate the topological identity of the two groups. The molecular moieties in the red circle represent acetate groups.*

example with [RMI][Metal(BTC)] structure and [RMI]2[M3(HBTC)4(H2O)] structure. Although it is very difficult to catch any similarities from the formula nor the structure if at first glance they actually fall under the same topology umbrella. This remarkable similarity is possible because some coordination sites of the trinuclear metal cluster are occupied by another molecular moiety, OAc in this case as shown in **Figure 7**. These places the trinuclear clusters in the octahedral coordination mode, which is the maximum coordination that binuclear metal clusters can have.
