**5. Solvent effects on supramolecular architecture**

In the previous sections, the role of solvents in supramolecular functions and processes has been discussed. As described, in those cases solvents are capable of influencing the thermodynamics of supramolecular binding processes as well as the energetics of supramolecular systems so as to induce solvatochromism or some nano-mechanical functions like molecular shuttle movements in rotaxanes and catenanes or other conformational changes.

In this section focus is placed on the effect of solvents in supramolecular architectures. Their regulating role in metallosupramolecular solids has been thoroughly investigated in recent years [57] as well as their aptitude to affect crystal growth and assembly and dynamic transformations. Many solvents especially those bearing N, O, or S atoms exhibit aptitude to coordinate (i.e., to specifically interact) with metals in various coordination complexes, and thus they are capable of forming new complexes. In these complexes solvent molecules can demonstrate a stabilizing role as building blocks and variation of the solvent can lead to alternative molecular architectures. A very nice example illustrating this ability of solvents is that of the assembly of the ligand 1,4-benzene dicarboxylic acid (*bda*) with MgII in the solvents dimethyl acetamide (DMA), EtOH, and dimethylformamide (DMF). In the case of DMA, polymeric 2D layers (**Figure 16a**) of the type [Mg3(*bda*)3(DMA)4]*<sup>n</sup>* are obtained. The situation is very different when EtOH or DMF are used as solvents.

#### **Figure 16.**

*Different supramolecular architectures involving MgII, the ligand bda and one of the solvents DMA (a), EtOH (b), and DMF (c). Reprinted with permission from: Li et al. [57].*

**221**

**Figure 17.**

*Solvent Effects in Supramolecular Systems DOI: http://dx.doi.org/10.5772/intechopen.86981*

sp2

with sp1


acetone (sp2


In the latter two cases, 3D frameworks are obtained instead of the following: [Mg3(*bda*)3(EtOH)2]*m* and [Mg(*bda*)(DMF)]*k*, respectively (**Figure 16b,c**). Note that *n*, *m*, and *k* correspond to the number of repeated units in each case [58–60]. Another important feature of the solvents which readily affects the supramolecular architecture of coordination complexes is the steric effect they might introduce. Noro et al. reported the steric effect of different solvents with sp1

ligand 1,2-bis(4-pyridyl)ethane (*bpe*) [61]. As depicted in **Figure 17A**, solvents

other hand, alcohols, THF, and dioxane (which all have sp3

variation of the steric effect introduced by different solvents.

*meso*-*α*,*β*-bi(4-pyridyl)glygol (*bpg*) and azide (N3


form H-bonds with the H-atoms of coordinated water molecules already coordinated at the axial positions. This stimulating divisibility is achieved through the

the solvents in these two cases happens axially with a Jahn-Teller distortion. On the

Synergistic solvent effects can also drastically influence the structure of supramolecular coordination polymers illustrated in 3D pillar-layered coordination polymers prepared by Wang et al. [62]. These complexes comprise CoII metal centers and

{[Co(bpg)(azide)2]S*x*}*n* (S represents a solvent and *x* the corresponding stoichiometric number), and they are prepared through a reaction of CoII, azide, and *bpg* in the following solvent mixtures: MeOH–H2O, DMSO–H2O, and DMF–H2O. The use of different solvent systems was found to drastically influence the 2D [Co(azide)2]*m* layers involved in the 3D networks. In the case of the protic mixture MeOH-water, a square net was observed without incorporation of solvent. On the other hand from DMSO–H2O and DMF–H2O, the aforementioned 2D layers obtained were of honeycomb and *kagomé* geometry, respectively, and solvent molecules were incorporated

in the 3D polymers (DMSO and DMF, respectively) (see **Figure 17B**) [62].

*A) Illustration depicting the steric effect introduced by various types of solvents in the polymeric* 

*supramolecular architecture involving CuII and bpe. B) Three different 2D polymeric [Co(azide)2]m layers involved in the complexes obtained by Wang et al. [62] Reprinted with permission from: Li et al. [57].*

Moreover, the protic or aprotic nature of the solvent can have a significant impact on the crystal structures of coordination compounds. A characteristic example is that of Mn(OAc)2 and its complexation with the ligand 3-(2-pyridyl)-5-(4-pyridyl)-1,2,4 triazole (*ppt*). The use of binary solvent mixtures involving a protic and an aprotic solvent (viz., DMF-EtOH and toluene-MeOH) or neat MeCN led to three different 2D systems according to Lin et al. [63]. What is noteworthy is that in the case of MeCN (aprotic) as a solvent, the obtained supramolecular system did not incorporate any solvent molecule: [Mn(*ppt*)2]*n*. Strikingly, in the case of the aforementioned binary solvent mixtures, two supramolecular 2D polymers involving the corresponding


O carbonyl atom) are able to directly ligate CuII. The coordination of


N) and DMF or

O atoms) preferably

<sup>−</sup>) ligands. Their general formula is

### *Solvent Effects in Supramolecular Systems DOI: http://dx.doi.org/10.5772/intechopen.86981*

*Solvents, Ionic Liquids and Solvent Effects*

catenanes or other conformational changes.

**5. Solvent effects on supramolecular architecture**

In the previous sections, the role of solvents in supramolecular functions and processes has been discussed. As described, in those cases solvents are capable of influencing the thermodynamics of supramolecular binding processes as well as the energetics of supramolecular systems so as to induce solvatochromism or some nano-mechanical functions like molecular shuttle movements in rotaxanes and

In this section focus is placed on the effect of solvents in supramolecular architectures. Their regulating role in metallosupramolecular solids has been thoroughly investigated in recent years [57] as well as their aptitude to affect crystal growth and assembly and dynamic transformations. Many solvents especially those bearing N, O, or S atoms exhibit aptitude to coordinate (i.e., to specifically interact) with metals in various coordination complexes, and thus they are capable of forming new complexes. In these complexes solvent molecules can demonstrate a stabilizing role as building blocks and variation of the solvent can lead to alternative molecular architectures. A very nice example illustrating this ability of solvents is that of the assembly of the ligand 1,4-benzene dicarboxylic acid (*bda*) with MgII in the solvents dimethyl acetamide (DMA), EtOH, and dimethylformamide (DMF). In the case of DMA, polymeric 2D layers (**Figure 16a**) of the type [Mg3(*bda*)3(DMA)4]*<sup>n</sup>* are obtained. The situation is very different when EtOH or DMF are used as solvents.

*Different supramolecular architectures involving MgII, the ligand bda and one of the solvents DMA (a), EtOH* 

*(b), and DMF (c). Reprinted with permission from: Li et al. [57].*

**220**

**Figure 16.**

In the latter two cases, 3D frameworks are obtained instead of the following: [Mg3(*bda*)3(EtOH)2]*m* and [Mg(*bda*)(DMF)]*k*, respectively (**Figure 16b,c**). Note that *n*, *m*, and *k* correspond to the number of repeated units in each case [58–60].

Another important feature of the solvents which readily affects the supramolecular architecture of coordination complexes is the steric effect they might introduce. Noro et al. reported the steric effect of different solvents with sp1 -, sp2 -, or sp3 -hybridized coordinating atoms on the assembly of CuII(PF6)2 and the ligand 1,2-bis(4-pyridyl)ethane (*bpe*) [61]. As depicted in **Figure 17A**, solvents with sp1 - or sp2 -hybridized coordinating atoms such as MeCN (sp1 N) and DMF or acetone (sp2 O carbonyl atom) are able to directly ligate CuII. The coordination of the solvents in these two cases happens axially with a Jahn-Teller distortion. On the other hand, alcohols, THF, and dioxane (which all have sp3 O atoms) preferably form H-bonds with the H-atoms of coordinated water molecules already coordinated at the axial positions. This stimulating divisibility is achieved through the variation of the steric effect introduced by different solvents.

Synergistic solvent effects can also drastically influence the structure of supramolecular coordination polymers illustrated in 3D pillar-layered coordination polymers prepared by Wang et al. [62]. These complexes comprise CoII metal centers and *meso*-*α*,*β*-bi(4-pyridyl)glygol (*bpg*) and azide (N3 <sup>−</sup>) ligands. Their general formula is {[Co(bpg)(azide)2]S*x*}*n* (S represents a solvent and *x* the corresponding stoichiometric number), and they are prepared through a reaction of CoII, azide, and *bpg* in the following solvent mixtures: MeOH–H2O, DMSO–H2O, and DMF–H2O. The use of different solvent systems was found to drastically influence the 2D [Co(azide)2]*m* layers involved in the 3D networks. In the case of the protic mixture MeOH-water, a square net was observed without incorporation of solvent. On the other hand from DMSO–H2O and DMF–H2O, the aforementioned 2D layers obtained were of honeycomb and *kagomé* geometry, respectively, and solvent molecules were incorporated in the 3D polymers (DMSO and DMF, respectively) (see **Figure 17B**) [62].

Moreover, the protic or aprotic nature of the solvent can have a significant impact on the crystal structures of coordination compounds. A characteristic example is that of Mn(OAc)2 and its complexation with the ligand 3-(2-pyridyl)-5-(4-pyridyl)-1,2,4 triazole (*ppt*). The use of binary solvent mixtures involving a protic and an aprotic solvent (viz., DMF-EtOH and toluene-MeOH) or neat MeCN led to three different 2D systems according to Lin et al. [63]. What is noteworthy is that in the case of MeCN (aprotic) as a solvent, the obtained supramolecular system did not incorporate any solvent molecule: [Mn(*ppt*)2]*n*. Strikingly, in the case of the aforementioned binary solvent mixtures, two supramolecular 2D polymers involving the corresponding

#### **Figure 17.**

*A) Illustration depicting the steric effect introduced by various types of solvents in the polymeric supramolecular architecture involving CuII and bpe. B) Three different 2D polymeric [Co(azide)2]m layers involved in the complexes obtained by Wang et al. [62] Reprinted with permission from: Li et al. [57].*

**Figure 18.**

*(A) The zigzag (…HOH…HCF…)*<sup>n</sup> *polymers involved in a viologen/HCF CTC reported by Papadakis et al. (hydrogen atoms have been omitted). A cluster consisting of four viologen molecules around a HCF anion and the corresponding Hirshfeld surface representations shown along the b-(B), a-(C) and c-(D) axis. Reprinted with permission from: Papadakis et al. [65].*

solvents were obtained: [Mn(*ppt*)2(DMF)1/2(H2O)3]*m* and [Mn(*ppt*)2(toluene)1/2(M eOH)3/2]*k*, respectively. This interesting phenomenon was attributed to the fact that when only an aprotic solvent is utilized, the neighboring 2D polymer grid-sheets in [Mn(*ppt*)2]*n* are stacked in a staggered mode, and this leads to a very compact 3D structure leaving out the solvent molecules. This effect is avoided when a protic solvent is employed as cosolvent [63].

In all above examples, the coordination of solvent molecules to the metal centers was found to affect the structure of the supramolecular coordination systems. Secondary interactions, which were already mentioned, mainly account to the H-bonding of a coordinating solvents and non-coordinating cosolvent. There are several cases however where secondary interactions alone can lead to stabilized 3D supramolecular coordination structures. The supramolecular charge transfer complexes (CTCs) of viologens with various electron donors have been reported long ago [64]. Such CTCs involving [FeII(CN)6] <sup>4</sup><sup>−</sup> (HCF) as a strong electron donor have been given some attention; however, their supramolecular structure has been scarcely investigated so far. An example pertaining to this category of supramolecular CTCs was recently reported by Papadakis et al. [65]. As depicted in **Figure 18**, the nonsymmetric dicationic viologen molecules tend to aggregate around the anionic HCF donor. However, the stability of a crystalline structure of such a CTC is achieved only if water is introduced in the reaction mixture. Water is found to readily form H-bonds with CN groups of HCF, and this results in the formation of a zigzag 2D polymer of the type: (…HOH…HCF…)*n*. The 3D supramolecular structure comprises the described 2D polymers and cationic channels of viologens perpendicular to these 2D polymers (**Figure 18**). Attempts to remove (by drying) or replace water (employing even another protic solvent) in these structures failed. Apparently the stabilizing interaction in these supramolecular systems is H-bonding which is stronger when H2O is utilized. The importance of H2O and the formation of a CTC in a similar fashion have been also reported earlier by Abouelwafa et al. [66]. In the aforementioned example, a symmetric viologen was utilized instead [66].
