Supramolecular Inorganic Chemistry Leading to Functional Materials

J. Chem. Sci. (2020) 132:46 Indian Academy of Sciences

https://doi.org/10.1007/s12039-020-1744-0 Sadhana(0123456789().,-volV)FT3](0123456789().,-volV) REVIEW ARTICLE

Supramolecular inorganic chemistry leading to functional materials

OLIVIA BASU and SAMAR K DAS* School of Chemistry, University of Hyderabad, P.O. Central University, Hyderabad 500 046, India E-mail: [email protected]; [email protected]

MS received 28 August 2019; revised 17 November 2019; accepted 26 November 2019

Abstract. Functional inorganic materials are very important today to meet the needs of our society. The most demanding needs are sustainable and clean energy (it would be nice if that can be achieved from water splitting), smart materials for sensing toxic volatile as well as water-soluble substances (health care) and efﬁcient catalysts that can cycle multiple times without deterioration for useful chemical reactions. Supramolecular chemistry, that plays a vital role to design and synthesize such functional molecules, controls over the intermolecular interactions, thereby the molecular recognition processes leading to molecular functions, e.g., sensing, catalysis, etc. This article deals with inorganic supramolecular chemistry of a number of mono-nuclear coordination complexes to selected di-nuclear systems through trinuclear metal basic carboxylates, mostly in their solid-state, leading to the functional inorganic materials. We have demonstrated that some of the very old inorganic systems can be explored in the light of supramolecular chemistry to describe them as functional materials, which have potential in serving our society to some extent.

Keywords. Functional inorganic materials; mononuclear dithiolato complexes; dinuclear iron compounds; trinuclear basic carboxylates; supramolecular chemistry.

1. Introduction known as basic carboxylates. Many of these synthesized compounds do not exhibit any functional Self-assembly through metal coordination has shown behavior. This work has been planned to furnish a remarkable potential for the formation of functional glimpse that some inorganic compounds, including inorganic materials. In fact, today, in modern inor- these metal basic carboxylates, can exhibit unique ganic chemistry, the designed supramolecular molecular functions, mostly in the solid-state. We approach to the construction of new molecular and have mainly focused on three types of inorganic sys- supramolecular architectures is a challenging task to tems: (i) mono-nuclear metal coordination complexes, obtain functional inorganic materials. An important (ii) di-nuclear systems and (iii) tri-nuclear metal car- function of a supramolecular material is ‘molecular boxylates. Some of these systems to be described here, recognition’, which refers to the specific interaction indeed, are very old. For example, the mononuclear II between two or more molecules through noncovalent copper complex [Cu {C6H4(NH2)2}2SO4]H2O(C6- associations. These noncovalent interactions include H4(NH2)2 = benzene-1,2-diamine), which has recently metal coordination, hydrogen bonding, p–p interac- been shown by us to be an active functional material, tions, van der Waals forces, electrostatic effects, etc. was prepared in the 1960s. The tri-nuclear iron car- In the course of our inorganic chemistry research boxylates, one of which has recently been demon- during the last 19 years, we have been looking for strated by us to be a methanol recognizer in its solid- inorganic systems that can be explored further in their state, had been synthesized almost 200 years ago. solid-state to exhibit supramolecular functions. We Metal dithiolato coordination complexes, an old but have synthesized and characterized diverse types of still a contemporary system, have recently been inorganic materials including mono-nuclear metal demonstrated by us in the angle of supramolecular coordination complexes to the di-nuclear cluster recognitions. We have explored, in our laboratory, through tri-nuclear metal-l3-O-cluster containing these old systems in a new dimension in the perspec- compounds. These trinuclear compounds are popularly tive of inorganic supramolecular chemistry leading to functional materials. Particular attention is paid to *For correspondence crystalline state (solid to solid) transformations in 46 Page 2 of 31 J. Chem. Sci. (2020) 132:46 solid-liquid and solid-gas interface reactions. This article would contribute an important message that many inorganic systems, that are synthesized and characterized previously but functionally are not explored, can be revisited further to reconnoiter their molecular functions by analyzing their supramolecular chemistry. We have presented here a systematic supramolecular approach, mainly by analyzing weak interactions in the pertinent crystal structures that can be linked to their molecular functions or the potential to show function behavior. We have arranged the article by beginning with mono-nuclear dithiolato and ortho-phenylene diamine metal coordination complexes followed by selected dinuclear iron and zinc complexes through trinuclear metal carboxylates. Scheme 1. A schematic representation of the threading and dethreading process of pseudorotoxane (left). A pseu- 2. Mononuclear coordination complexes dorotoxane created with 1,2-bis(4,40-bipyridinium)ethane as an ‘‘axle’’ and dibenzo crown-8 ether as a ‘‘wheel’’ (right). 2.1 Mononuclear metal(dithiolate) complexes in the perspective of supramolecular chemistry This pseudorotaxane has been known to self- assemble by p–p and hydrogen bonding interactions to Transition metal bis(dithiolate) complexes are of form supramolecular architectures.22,23 We have continuing interest because of their potential applica- exploited the dipositive charge (2 ?) of this pseu- tions as near-infrared dyes, conducting-, magnetic- 2- 1–12 dorotaxane by its ion-pairing with [M(dithiolate)2] and nonlinear optical-materials. Besides their to obtain a new class of inorganic supramolecular materials chemistry aspect, they have received atten- compounds (Scheme 2).15,24 tion in the area of biomimetic chemistry due to the The structural analysis of these compounds shows existence of metal-dithiolene moiety in several met- 13 the occurrence of supramolecular grid-type channels/ alloenzymes as an active site. In recent times, they void spaces of diverse shapes depending on a partic- have been used as potential building blocks in 2- 14–17 ular coordination complex anion ([M(mnt)2] , supramolecular constructions. Even though we 2- [Zn(dmit)2] ) used, that is accommodated in the have contributed to diverse areas of metal(dithiolate) relevant channel/void space. In this system, the metal chemistry, the present discussion will be limited only coordination complex can be described as a template to the supramolecular aspect of this class of inorganic in the sense that complex anion extends its hydrogen- complexes emphasizing their functional performances. bonding interactions with its surrounding pseudoro- 2.1a Transition metal(dithiolate) complex anion as a taxane cations, as if, it brings the adjacent pseudoro- template and a conformational modulator in the taxane cations together around it (Figure 1a), whereby supramolecular construction the pseudorotaxane cations undergo hydrogen bonding The first part of this section will be described by a and pi-pi interactions among themselves to construct story of how a classical coordination compound the three-dimensional supramolecular architecture. In influences a pseudorotaxane to self-assemble into non- the case of compound [pseudorotaxane][Cu(mnt)2], covalent architectures of diverse topologies that are each void space (in one layer) accommodating two dictated by the size and shape of the complex used. A complex anions is constructed by surrounding eight pseudorotaxane is formed by an axle threaded into the pseudorotaxane cations. This situation is shown in wheel (Scheme 1); the axle is generally stabilized by Figure 1a. In the relevant crystal structure, the com- 2- the non-covalent interactions, such as hydrogen plex anion [Cu(mnt)2] has non-planar geometry and bonding or p–p stacking interactions of the axle with thus the shape of the concerned void space is rhom- the ring. For example, Loeb and co-workers18–21 used bus-/diamond-like, as shown in Figure 1b. On the 1,2-bis(4,40-bipyridinium)ethane as an ‘‘axle’’ and other hand, for compounds [pseudorotax- dibenzo crown-8 ether as a ‘‘wheel’’ to create a ane][Ni(mnt)2] and [pseudorotaxane][Pd(mnt)2], the 2- 2? 2? pseudorotaxane (Scheme 1, right), which can be complex anions [M(mnt)2] (M = Ni and Pd , obtained as PF6 salt. respectively) exist, expectedly, in a square-planar J. Chem. Sci. (2020) 132:46 Page 3 of 31 46

N N

R R O O O O S S N - N M O + O 2PF R R CH CN O + O 6 S S 3 S S + [Bu N] M 4 2 - + 2Bu4NPF6 O + O N O R S S R N O R R O O O O R S NC S S S = and S S S R S NC S N M = Cu(II), Ni(II), Pd(II), Pt(II), and Zn(II) N

Scheme 2. Structural representation of non-covalent supramolecular interaction built out of a metal bis(dithiolene) 2- coordination complex [M(dithiolato)2] as a counteranion of a pseudorotaxane cation.

Figure 1. (a) View of the formation of void space, formed by the aggregation of pseudorotaxane cations, influenced by 2- [Cu(mnt)2] complex anions that are accommodated in the void space ([pseudorotaxane][Cu(mnt)2]). The red dotted lines indicate C–HS hydrogen bonding interactions, the green dotted lines indicate C–HN hydrogen bonding interactions 2- between pseudorotaxane cations and [Cu(mnt)2] complex anions. (b) & (c) Packing diagrams of compounds [pseudorotaxane][Cu(mnt)2] and [pseudorotaxane][Pd(mnt)2] showing void spaces accommodating complex anions 2- 2- [Cu(mnt)2] and [Pd(mnt)2] , respectively. geometry and in both these cases, the square-like void of the benzimidazolium cations can be modulated by spaces (Figure 1c) are formed. In other words, the the supramolecular influence of metal dithiolate anion 2- variation in shapes of the void spaces from rhombus- [M(mnt)2] through its interactions with concerned type ([pseudorotaxane][Cu(mnt)2]) to square-type benzimidazolium cation as shown in Figure 2.We ([pseudorotaxane][Ni(mnt)2] and [pseudorotax- have explained this conformational modulation by ane][Pd(mnt)2]) can be correlated with the coordination offering a supramolecular model of eclipsed and geometry of the metal ion in the relevant coordination staggered conformations of two flexible –CH2– groups 2- complex [M(mnt)2] anions, that are encapsulated in of benzimidazolium cation with respect to the posi- 2- 24 therespectivevoidspaces([Cu(mnt)2] is quite non- tions of the hydrogen atoms of the –CH2– groups. 2- 2- planar, whereas [Ni(mnt)2] and [Pd(mnt)2] are So far, we have seen, in the preceding section, that a almost planar). transition metal-dithiolate coordination complex 2- A metal bis(dithiolato) complex anion [M(mnt)2] anion can offer supramolecular influences to its sur- can also modulate the conformation of an associated rounding organic cation in constructing diverse organic benzimidazolium cation. We synthesized three supramolecular architectures of the concerned pesu- organic benzimidazolium cations, namely, [Bz,H- dorotaxane cation and in modulating the conformation ? ? BzBimy] , [Bz,NO2-BzBimy] , and [Bz,Br- of the associated benzimidazolium cation. It can also BzBimy]?, which can exist either its syn conformation happen the other way around. The surrounding or its anti-conformation (Figure 2). The conformation cationic organic part can influence the central metal 46 Page 4 of 31 J. Chem. Sci. (2020) 132:46

? ? Figure 2. (i) Conformation of the benzimidazolium salts (from left to right: [Bz, H-BzBimy] , [Bz, NO2-BzBimy] , and ? [Bz,Br-BzBimy] in their BF4 salt), anion (tetraﬂuoroborate) moiety is omitted for clarity; (ii) conformation of the 2- benzimdazolium salts in metal bis(dithiolene) ion-pair complexes, ([M(mnt)2] anion is omitted for clarity). coordination complex anion. The geometry around the the resulting ion-pair compound in the solid-state.32 metal ion in the coordination complex can be tuned We have reported six ion pair compounds [C8H12 through non-covalent interactions with adjacent (CH2)nN4][Cu(mnt)2] (n = 1 to 6), shown in Scheme 3, organic cations, as described in the following section. in which the counter cations are varied by increasing chain length of the cation, retaining the same complex 2? 2- 2.1b Tuning coordination geometry around Cu anion [Cu(mnt)2] in all six compounds. The devia- 2- center in the [Cu(mnt)2] anion by varying alkyl tion from the planarity in the coordination complex 2- chain length in the associated cation anion [Cu(mnt)2] can be understood if we take the The transition-metal bis(dithiolate) complexes, that following factors (Scheme 4) into account: (a) lack of generally show square-planar geometry, exhibit devi- centre of symmetry (Ci) along metal hydrogen bond, ations from this planarity.8,25–30 This deviation can be (b) unbalanced supramolecular interactions and caused by putting constraints on the concerned metal- (c) crowdedness around the complex anion 31 2- chelate rings. [Cu(mnt)2] . As shown in Scheme 4a, there is a We have shown that the deviation can also be symmetry along the interaction of metal hydrogen caused by supramolecular interactions of a particular bond in which the metal is Cu2? ion of the complex 2- complex anion with the surrounding organic cation in anion [Cu(mnt)2] , showing identical hydrogen bonds

Scheme 3. General synthetic procedure for ion pair compounds. J. Chem. Sci. (2020) 132:46 Page 5 of 31 46

Scheme 4. (a) Centre of symmetry along metal-hydrogen interaction, (b) equivalent and un-equivalent hydrogen bonding interactions (purple and green colors showing balanced and unbalanced interactions respectively), (c) bulkiness around the 2- 32 anion [Cu(mnt)2] . Reprinted with permission from Ref. Copyright (2012) American Chemical Society. from both sides of the anion with an identical distance. (Figure 4, right), can be obtained with its unusual cis- There may be symmetrical or balanced hydrogen form (Figure 4, left) when it is ion paired with a II 2- bonding interactions around a complex anion that are [Zn (dmit)2] coordination complex anion with the shown in purple color in Scheme 4b. The same Sche- isolation of an ion pair compound [C28H24N4] 33 me 4b also shows un-balanced supramolecular inter- [Zn(dmit)2]2DMF (Figure 5, left). The cis form of 2? 00 actions involving different groups with dissimilar the cationic part, {C28H24N4} [1,1 -1,4-phenylene- distances as indicated in green color. This leads to a bis(methylene)bis-4,40-bipyridinium] of this ion pair change in spatial orientation of the chelate rings of the compound can be described as a molecular tweezer complex anion. Scheme 4c presents the possibility that (the molecular tweezers are nothing but non-cyclic in the crystals of the above-mentioned compounds (a-f) host molecules having open cavities of ﬂexible sizes [Cu(mnt)2], the required cations may be present close and potential to exhibit conformational changes, cap- to the complex anion and interact from top/bottom of able of binding guest molecules via supramolecular the molecular plane of the complex anion with a metal interactions). This molecular tweezer (the cis form of hydrogen bond. Thus, the bulkiness of the cation, the cationic receptor) recognizes the dithiolato com- II 2- hydrogen-bonded to the central metal ion, may affect plex anion [Zn (dmit)2] through p–p intermolecular the geometry around the metal ion in the coordination interactions involving the contact of the {C3S2} ring of 2- II 2- complex anion [Cu(mnt)2] . the [Zn (dmit)2] coordination complex anion with Based on these factors, we have demonstrated that the aromatic ring of the 4,40-bipyridinium end of the increasing length of the alkyl chain of imidazolium receptor cation (see Figure 5, left) in the ion pair 2- cation (which is ion paired with [Cu(mnt)2] anion) compound [C28H24N4][Zn(dmit)2]2DMF. can tune the geometry of the square planar copper The cis form of the cationic receptor is energetically bis(dithiolato) complex anion in solid state as shown unfavorable (stabilized through p–p intermolecular II 2- in Figure 3. interactions with [Zn (dmit)2] complex anion), thus As a part of the supramolecular inﬂuence of metal it has natural tendency to go it’s usual trans/anti form. dithiolate complex anion, now in the following section, When deep brown crystals of [C28H24N4] it has been demonstrated that a molecular machine [Zn(dmit)2]2DMF, that are insoluble in MeCN sol- (reversible syn-anti isomerization of an organic vent, are suspended in a saturated MeCN solution of cationic receptor) can be operated by mononuclear [Bu4N]Br, the deep-brown crystals start losing color 2- zinc dithiolato complex anion [Zn(dmit)2] . without their dissolution resulting in colorless off- white crystals of [C H N ]Br , in which the cationic 2.1c A molecular machine: an ion pair compound 28 24 4 2 receptor takes its usual trans/anti-form (Figure 5, [C H N ][Zn(dmit) ]2DMF exhibits reversible cis- 28 24 4 2 right). During this solid-liquid interface reaction trans isomerization of its cationic receptor in a solid- liquid interface reaction triggered by anion exchange ÂÃ ? ½ðÞ ðÞ 2 C28H24N4 Zn dmit 2 2DMF solid phase An organic cationic receptor [C28H24N4] , which generally exists in its usual trans conformation þ ½Bu4N BrðÞ! soln: phase 46 Page 6 of 31 J. Chem. Sci. (2020) 132:46

2- Figure 3. Deviations from the planarity in the complex anion [Cu(mnt)2] in ion-pair compounds (a–f) [Cu(mnt)2] decrease with increasing alkyl chain length in between two imidazolium moieties in the associated cations. Reprinted with permission from Ref.32 Copyright (2012) American Chemical Society.

2? Figure 4. Left: cis (or cleft-like) conformation of [C28H24N4] in ion-pair compound [C28H24N4][Zn(dmit)2]2DMF. 2? 33 Right: anti (or staircase-like) conformation of [C28H24N4] in its bromide salt, [C28H24N4]Br2. Reproduced from Ref. with permission from the Royal Society of Chemistry.

[C28H24N4]Br2 (solid phase) ? [Bu4N]2[Zn(dmit)2] [C28H24N4][Zn(dmit)2]2DMF (solid phase) ? [Bu4N] II 2- (soln. phase), the [Zn (dmit)2] complex anion is Br (soln. phase) is so sluggish (the low energy form of removed from the insoluble solid crystals of [C28H24N4] the cationic receptor goes to its high energy form) that it [Zn(dmit)2]2DMF forming pink MeCN solution of takes almost a week to be completed. This system can - [Bu4N]2[Zn(dmit)2] and dissolved Br ions get into the be described as a molecular machine (a molecular II 2- same crystals (that have removed their [Zn (dmit)2] system, where the constituent components produce anionic content) forming off-white crystals of [C28H24N4] mechanical motion in response to specific stimuli) in Br2 (that are again insoluble in MeCN). This reaction is which the cationic receptor (a molecular component) quite a facile one (completed within 4-5 minutes) because produces quasi-mechanical movement (output in the in this reaction, a high energy (cis) form of the cationic present work: cis-trans isomerization in the solid-state) receptor goes to its low-energy (anti/trans) form. Con- in response to specific stimuli (input; in the present versely, the reverse reaction case, the input is ion exchange). Metal dithiolate complexes, as shown above, not ½ ðÞ C28H24N4 Br2 solid phase only influence the conformations of the surrounding þ½ ½ ðÞðÞ: Bu4N 2 Zn dmit 2 soln phase molecules in a crystal but also can be responsible for ! an important organic functionalization at an ambient J. Chem. Sci. (2020) 132:46 Page 7 of 31 46

Figure 5. Left: the thermal ellipsoidal plot (50% probability) of [C28H24N4][Zn(dmit)2] in compound [C28H24N4] 33 [Zn(dmit)2]2DMF. Right: the crystal structure of [C28H24N4]Br2. Reproduced from Ref. with permission from the Royal Society of Chemistry. condition by a supramolecular ion pairing with an an ion pair compound [ODP]2[M(mnt)2] (in which organic heterocyclic dication as described below. ODP1? is the oxidized product of DDP2?)asshownin Scheme 6.34 The mechanism for the oxygenation of 2.1d Functionalization of an organic compound at an DDP2? to ODP1? (Scheme 6) is not clear to us. How- ambient condition mediated by a mononuclear 2- ever, we speculate that the supramolecular ion-pairing in [M(mnt)2] complex ? [DDP][M(mnt) ] (a hypothetical compound) promotes An organic heterocyclic dication DDP2 , 6,12-dihy- 2 0 0 ? inter-molecular electron transfer between cation and drodipyrido [1,2-a;1 ,2 -d] pyrazidinium ([C H N ]2 , 12 12 2 anion with the involvement of water (source of oxo Scheme 5 left), can be obtained as a halide salt. ? group) resulting in the isolation of [ODP] [M(mnt) ]. This DDP2 dication can be oxidized to its cyclic 2 2 ? The supramolecular ion paring has physically been quaternary ammonium monocation ODP (12-oxo-9H- 0 0 ? observed in the crystal structures of [ODP] [M(mnt) ], in dipyrido[1,2-a;1 ,2 -d] pyrazin-5-ium, [C H N O]1 ) 2 2 12 9 2 which two ODP1? cations interact with one [M(mnt) ]2- with the addition of an oxo group (Scheme 5, right). 2 anion through p–p interactions as shown in Figure 6. However, this oxidation had been known in a two-step This oxygenation has a real impact in organic synthesis ‘‘not so easy’’ reaction process. In the ﬁrst step, the ? of industrial importance, because this does not use any DDP2 was dehydrogenated by Pd/C as a catalyst and in speciﬁc oxidation reagent other than aerial oxygen. the second step, the oxidation was performed using SeO2 ? Besides supramolecular templating, molecular as an oxidizing agent to obtain ODP1 as a bromide salt. machine operating, and organic functionalization as We have demonstrated this organic transformation in a discussed in the preceding sections, dithiolene-based one-step facile oxidation, when we treat [DDP]Cl2 to an 2- supramolecular interactions can also lead to the aqueous solution of [M(mnt)2] (formed in situ from 2- emergence of a new molecular system as depicted in MCl2xH2OandNa2mnt (mnt = 1,2-dicyano-ethylene ? ? the following section. dithiolate; M = Ni2 ,Cu2 ) resulting in the isolation of 2.1e Emergence of an inorganic tetrathiafulvalene: a supramolecular approach Tetrathiafulvalene (TTF) is a non-aromatic organosulfur 14-p-electron system (Scheme 7a) which can be oxidized to its mono-radical cationic and diradical cationic states reversibly at relatively low oxidation potential (Scheme 7b). TTF being an organic compound has been shown to be able to conduct electricity, similarly to metals. This led to the synthetic community to design and synthesize new ? Scheme 5. Structural representation of DDP2 and TTF derivatives by the substitution of the periphery of 2? 34 ODP . Reprinted with permission from Ref. Copyright the TTF molecule. While working with (2006) American Chemical Society. 46 Page 8 of 31 J. Chem. Sci. (2020) 132:46

Scheme 6. One-pot oxygenation of DDP2?. Reprinted with permission from Ref.34 Copyright (2006) American Chemical Society.

2- Figure 6. Supramolecular interactions between ODP cations (oxidized product) and the [Ni(mnt)2] coordination complex anion. Ball and stick view (left) and space-ﬁlling view (right). Color code: C, medium gray; N, blue; O, red; S, yellow; Ni, cyan. Reprinted with permission from Ref.34 Copyright (2006) American Chemical Society.

Scheme 7. (a) Structural representation of tetrathiafulvalene. (b) Oxidation of TTF to its radical mono-cation and radical- dication. asymmetrically substituted bis(dithiolene) nickel(III) tetrathiafulvalene ClPhTTF (Scheme 9, right) in terms III compounds, for example, [Bu4N][Ni (ClPhdt)2] of their supramolecular interactions (for example, S (ClPhdt2- = 2-(p-chlorophenyl)-1,2-dithiolate), as S contacts, packing diagrams, etc.). as shown in presented in Scheme 8 (left), we could isolate a neu- Figure 7. From Schemes 8-9 and Figure 7,itis IV IV tral nickel(IV) compound [Ni (ClPhdt)2] (Scheme 8, apparent that both molecular structures {[Ni right), which can be described as an inorganic (ClPhdt)2] and ClPhTTF} would be identical if the 4? IV tetrathiafulvalene (TTF) as far as structural analogy metal center (Ni ion) in compound [Ni (ClPhdt)2] between this neutral Ni(IV) compound (Scheme 8, is substituted by a C=C double bond or the central right) and an organic TTF derivative (ClPhTTF, C=C double bond in compound ClPhTTF is replaced Scheme 9, right) is concerned. We have synthe- by a Ni4? ion. Both operations generate indistin- sized and structurally characterized the TTF deriva- guishable molecular structures. Based on notable sim- 5 IV tive ClPhTTF (Scheme 9). We have observed ilarity between [Ni (ClPhdt)2] (an inorganic crystallographically an interesting structural relation- coordination complex) and ClPhTTF (a tetrathiaful- ship between the neutral Ni(IV) dithiolato com- valene derivative) in terms of bond lengths and angles, IV pound [Ni (ClPhdt)2] (Scheme 8, right) and intermolecular SS contacts and packing diagrams in J. Chem. Sci. (2020) 132:46 Page 9 of 31 46

Scheme 8. Synthesis of neutral [Ni(ClPhdt)2] [ClPhdt = 2-(p-chlorophenyl)-1,2-dithiolate]. Reprinted with permission from Ref.5 Copyright (2008) American Chemical Society.

Scheme 9. Synthesis of 4,40-bis(4-chlorophenyl)-tetrathiafulvalene (ClPhTTF). Reprinted with permission from Ref.5 Copyright (2008) American Chemical Society.

Figure 7. Molecular structures of compound ClPhTTF (a) and compound [Ni(ClPhdt)2](b) showing the intermolecular SS interactions. Reprinted with permission from Ref.5 Copyright (2008) American Chemical Society. their crystal structures, the coordination complex copper-azide coordination polymer brings about the IV [Ni (ClPhdt)2] can be called an ‘‘inorganic tetrathia- molecular recognition of azide anion (a toxic sub- fulvalene (TTF)’’. stance) from an aqueous solution in a solid-liquid We, thus, could demonstrate in the preceding sec- interface reaction using a copper-orthophenanthroline tions that a metal dithiolate complex (i) can act as a coordination complex. template to generate diverse supramolecular architectures, (ii) can inﬂuence conformation of the molecules that supramolecularly interact with the metal 2.2 Molecular recognition of azide anion dithiolate complex, (iii) can regulate the reversible by a mononuclear Cu(II) complex cis-trans isomerization of an organic receptor P P II (molecular machine) using its supramolecular – Compound [Cu {C6H4(NH2)2}2SO4]H2O was syn- (pi–pi) interactions, (iv) can bring about organic thesized during 1967–68.35–37 However, its crystal functionalization and (v) can lead to the emergence of structure was not reported probably due to difﬁculty in an ‘‘inorganic tetrathiafulvalene (TTF)’’. These facts obtaining relevant single crystals. We have achieved support that the diverse supramolecular interactions its crystal structure38 and we found it to be a simple in the crystals of diverse mono-nuclear metal dithio- mononuclear copper complex (Figure 8, left) with late complexes can be utilized in obtaining functional square pyramidal geometry around copper ion: the two materials. In the same line, we have shown below that benzene-1,2-diamine ligands occupy the basal plane a driving force of supramolecular formation of a and the apical site is coordinated to an oxygen atom of 46 Page 10 of 31 J. Chem. Sci. (2020) 132:46

II Figure 8. Left: thermal ellipsoidal plot of [Cu {C6H4(NH2)2}2SO4]•H2O with 50% probability. The disordered water molecule and hydrogen atoms are omitted for clarity. Right: Chiral coordination network stabilizing homochiral helices (left-handed) running along crystallographic a-axis. A single helix is highlighted in purple color. Cu, cyan; N, blue. sulfate anion. Thus, its crystal structure is nothing According to the product analysis, the concerned unusual, that crystallizes in orthorhombic Pbca space chemical reaction, occurring at the solid-liquid interface, II group. When these achiral crystals (purple needles) of can be described as: [Cu {C6H4(NH2)2}2SO4] II II [Cu {C6H4(NH2)2}2SO4]H2O are dipped into an H2O ? 2NaN3 ? [Cu {C6H4(NH2)2}(N3)2] ? Na2SO4 ? aqueous solution of sodium azide, they transform to C6H4(NH2)2 ? H2O. In this reaction, the released sulfate chiral crystals (green needles) of a coordination anion and C6H4(NH2)2 were quantitatively estimated II polymer [Cu {C6H4(NH2)2}(N3)2]n in a unique solid- from the liquid phase by gravimetric and spectrophoto- liquid interface reaction, in which the metal coordi- metric determinations respectively; the respective yields II nated sulfate anions from crystals of [Cu {C6H4 conﬁrm that it is a quantitative reaction. The chirality II (NH2)2}2SO4]H2O are ion-exchanged with azide of the crystals of the green product [Cu anions and trigger the homochiral helical winding in {C6H4(NH2)2}(N3)2]n is achieved through the formation II the crystals of [Cu {C6H4(NH2)2}(N3)2]n as shown in of homochiral helices during its formation by sponta- Figure 8, right.38 neous resolution as shown in Figure 8, right. In the crystal The visual observation of color change from purple structure of the coordination polymer (compound [CuII II (compound [Cu {C6H4(NH2)2}2SO4]H2O) to green {C6H4(NH2)2}(N3)2]n), each copper ion is shared by two II (compound [Cu {C6H4(NH2)2}(N3)2]n) in this crystal homochiral helices, in which each copper is coordinated to crystal transformation is shown in Figure 9. The to four copper ions through bridging (azide) anions II green product [Cu {C6H4(NH2)2}(N3)2]n, obtained by forming nodal point of net-like coordination network this dipping method is nothing but a coordination (Figure 8, right). As expected, only one-handed helices polymer, that crystallizes in orthorhombic P212121 are observed throughout the crystal-network. The pro- chiral space group. duct formation modiﬁes the overall geometry around the

II Figure 9. A view showing solid-to-solid transformation: purple colored crystals of compound [Cu {C6H4(NH2)2}2SO4]•H2O II (1) go to green colored crystals of [Cu {C6H4(NH2)2}(N3)2]n (2), on dipping into an aqueous azide solution. Reproduced from Ref.38 with permission from the Royal Society of Chemistry. J. Chem. Sci. (2020) 132:46 Page 11 of 31 46 copper ion from square planar to octahedral. This unique ion exchange process can be introduced in the undergraduate laboratory for the qualitative identification of azide anion from an aqueous solution by making ‘‘azide paper’’ (like a ‘‘pH paper’’) by spreading the solid powder II form of compound [Cu {C6H4(NH2)2}2SO4]H2Oona filter paper. This ‘‘azide paper’’ can be used to detect azide anion from an aqueous solution simply by dipping the ‘‘azide paper’’ that would sense azide anion immediately by changing its color from violet to green (Figure 9). Thus the mainspring for this azide recognition is the supramolecular formation of copper-azide coordination polymer from a mononuclear Cu(II) orthophenylene diamine coordination complex with a monodentate sulfate ligand. Since the resulting Scheme 10. (a) catalytic proton reduction and (b) active supramolecular coordination polymer (Figure 8, right) site of [FeFe]–hydrogenase. is thermodynamically more stable than the starting II precursor [Cu {C6H4(NH2)2}2SO4]H2O, a reversible reaction (from supramolecular polymer to the copper fact, that this type of ligand is present in the active orthophenylene monomer) has not become possible. sites of Mo/W containing metalloenzymes catalyzing a With the success of mononuclear metal-dithiolate range of oxidation/reduction reactions64,65 and the and metal-orthophenylenediammine coordination Nature’s most efficient catalysts for hydrogen pro- complexes exhibiting rich supramolecular chemistry duction, the [FeFe]-hydrogenases ([FeFe]H2ases), leading to molecular functions (vide supra), we turned have the active site consisting of two iron centers our attention to dinuclear coordination complexes to bridged by a dithiolate ligand (Scheme 10), inspired us emphasize their functional performances, in connec- to synthesize model compounds (synthetic analogues) tion with their supramolecular properties. of the active site of [FeFe]-hydrogenases so that we can use these synthesized iron dimers as functional analogues to mimic the enzymatic function of 3. Dinuclear systems [FeFe]H2ase (catalytic reduction of proton to hydrogen) as shown in Scheme 10. We synthesized a Dinuclear metal complexes have extensively been stud- number of model compounds in relevance to the active 66–68 ied because of their potential applications in diverse areas site of [FeFe]H2ase). As a representative iron of chemical sciences including modeling the metalloen- dimeric system, we have described here three model zymes,39–53 catalysis,54–59 molecular devices for sensing compounds as shown in Scheme 11. Complexes 1, 2 selective ions.60–63 Our contribution to the dinuclear and 3 (Scheme 11) exhibit one quasi–reversible two- system includes the design and synthesis of dinuclear electron reduction process at E1/2 = -1.23, -1.24 and functional molecules with iron and zinc. The iron dinu- -1.25 V (vs. Fc/Fc?) respectively,67 as shown in clear compounds mimic the functional analogue reaction Figure 10 (a). of iron only hydrogenase, [FeFe]H2ase and zinc dinu- We have demonstrated electrocatalytic proton clear compound can act as a molecular device exhibiting reductions to molecular H2 using compounds 2 and 3 selective sensing of ferric and periodate ions. as catalysts taking p-toluene sulfonic acid (p-HOTs) as 67 proton source. For compound [Fe2{l-diph-6,7- qdt}(CO)6](2), when we added 2 mmol of p-HOTs, a 3.1 Electrocatalytic proton reduction new peak appeared at Epc = -0.53 V and the current to molecular hydrogen catalyzed by a dithiolato- intensity of the reduction at Epc = -1.25 V increased bridged dinuclear iron system with a cathode shift by 40 mV, which continuously grew as the acid concentrations increase from As mentioned in section 2.1, we have been working on 2-12 mM with final cathodic shift by 260 mV the metal-dithiolate system in the perspective of (Figure 10b). inorganic supramolecular chemistry using ene-1,2- This is characteristic of an electrocatalytic reduction dithiolate type ligand, that has long been known to of p-HOTs to hydrogen, which occurs at about serve as an effective electron transfer pathway. The -1.61 V vs.Fe?/Fe0 V with an overpotential of 46 Page 12 of 31 J. Chem. Sci. (2020) 132:46

Scheme 11. A representative synthetic scheme of three model compounds. Reproduced from Ref.67 with permission.

0.96 V. But the peak, appeared at Epc = -0.53 V after be responsible for the fat metabolism and growth of addition of 2 mmol of p-HOTs, did not grow further tissue.72 up during the addition of 4-12 mM p-HOTs. This Thus, sensing iron cation and periodate anion is a observation and electronic spectral changes (Fig- challenging task for an inorganic chemist. For this ure 11a), observed on addition of p-HOTs to the ace- purpose, we synthesized a cationic ligand, 1,3-bis(2,6- tonitrile solution of complex 2 showing a clean diisopropyl-4-(pyridin-4-yl)phenyl)-1H-imidazol-3- isosbestic point at 395 nm, indicating a chemical- ium chloride/bromide) (L) having two N donors and its electrochemical-electrochemical-chemical (CEEC) Zn(II)-metallacycle (1) as shown in Scheme 12.73 As mechanism (Figure 11b) for the electrocatalytic pro- shown in Figures 12a–d, the receptor 1 displays an ton reduction by diph-6,7-qdt-bridged all-carbonyl emission at 399.7 nm (kex = 320 nm, excitation and {Fe1Fe1} compound 2. emission slits 3 nm) which is ascribed to the emission of central imidazolium moiety. Upon addition of 10 lL of 5 mM solution of Mn? ions (Mg2?,Ca2?,Cr3?,Mn2?,Fe3?,Fe2?,Co2?, ? ? ? ? ? ? ? ? 3.2 A dinuclear Zn system as a metallacycle Ni2 ,Cd2 ,Zn2 ,Ag,Pb2 ,Ru3 ,Sm3 ,Eu3 , 3? - 3? 3? 3? 3? for selective recognition of Fe and IO4 Ions Gd ,Tb ,Dy and Er ) to 3 mL of host 1 (except Fe3? ions), the ﬂuorescence emission intensity of 1 3? The ferric ion (Fe ) is responsible for the oxygen does not obviously change, whereas the addition of metabolism, oxygen uptake and electron transfer in same amount of Fe3? ions causes a great decrease in 69,70 many living organisms. While iron deﬁciency emission intensity of 1 as shown in Figures 12a,b. 71 leads to the disease, anemia, an excess of iron can Similarly, out of a number of examined An- ions ------harm the DNA, proteins and lipids. Similarly, the (SCN ,N3 ,IO4 ,I ,H2PO4 , HCO3 ,CH3COO , - - - - - 2- 2- 2- periodate (IO4 ) anion is believed to act as an oxidant Cl ,Br,BF4 , ClO4 ,Cr2O7 ,CO3 ,SO3 , 2- 2- 2- 3- for the oxidation of L-serine, a non-essential amino SO4 ,S2O5 ,S2O8 and PO4 ), the decreased - - acid having –OH functionality. IO4 is also known to emission intensity was only noticed with IO4 ions. In J. Chem. Sci. (2020) 132:46 Page 13 of 31 46

- addition of excess CIO4 ions. The detection limit for the recognition of the Fe3? ion was found to be -6 - -5 2.5 9 10 mol/L and that for IO4 ion is 6.3 9 10 mol/L. This work of selective recognition of Fe3? and - IO4 ions by a metallacycle 1, a fluorescent-based receptor (Scheme 12) would uncover the entry for the design and synthesis of new N-heterocyclic ligand- based coordination materials as sensors. Thus, the connection between iron dinuclear complexes (discussed above) exhibiting electrocatalytic proton reduction and supramolecular chemistry is the design and synthesis of low-valent iron dimers having un-coordinated ‘N’ donors (Scheme 11), capable of undergoing supramolecular proto- nation (Figure 11) for subsequent reduction of the coordinated proton to molecular hydrogen. Like- wise, a zinc dimer has been designed and synthesized by supramolecular metal-ligand coordination (Scheme 12) to obtain specific-sized cavities for supramolecular recognitions/sensing of ferric and periodate ions (Figure 12). The successful demonstration of these selected iron and zinc dimers as supramolecular functional molecules prompted us to analyze the supramolecular Figure 10. (a) Cyclic voltammograms of complexes 1, 2 chemistry of trinuclear basic carboxylates (Scheme 13, and 3 (1.0 mM) in 0.1 M n-Bu4NClO4/MeCN at a scan rate -1 ? discussed below), an old known popular class of of 100 mV s . Potential are vs. Fc/Fc . (b) Cyclic voltammogram of complex 2 with 1.0 mM complex and inorganic compounds. This 200-years-old system has 0–12 mM p-HOTs in CH3CN solution (0.1 M n-Bu4NClO4) become a contemporary research area: (i) after we at a scan rate of 100 mV s-1. used this system as methanol and pyridine recognizer in the supramolecular gas-solid reactions; (ii) after its use as the building block for the supramolecular - addition, the titration results of 6 9 10 3 mM solution construction of diverse metal-organic frameworks - -3 of IO4 ion with 3 9 10 mM solution of 1 also having diverse functional performances and (iii) after show a regular decrease in the fluorescence intensities its use as the macro-cation for the formation of (Figure 12d) and almost completely quenched after supramolecular ionic crystals for selective gas

Figure 11. (a) Changes in the absorption spectra of compound 2, (5.0 9 10-5 M) upon the addition of 10–60 lL aliquots of dilute p-HOTs (0.1 mM) in CH3CN. (b) A schematic representation of plausible CEEC mechanism for electrocatalytic proton reduction in the presence of p-HOTs for compound 2. 46 Page 14 of 31 J. Chem. Sci. (2020) 132:46

Scheme 12. Formation of dinuclear zinc metallacycle from N-heterocyclic carbene precursor. Reprinted with permission from Ref.73 Copyright (2017) American Chemical Society.

Figure 12. (a) Fluorescence spectra of metallacycle 1 (1.0 9 10-6 mol L-1) upon addition of different cations (Mn?) (10 lL of 5 mM solution of all guest was added to 3 mL of host 1) in DMF (k = 399.7 nm). (b) Fluorescence titration of metallacycle 1 (2 9 10-3 mM) in DMF with increasing Fe3? concentration (addition of 10 lLFe3? ions per time; 3? kex = 320 nm). (Inset of b) Stern–Volmer plot for Fe ions recognition. (c) Fluorescence spectra of metallacycle 1 (1.0 9 10-6 mol L-1) upon addition of different anions (An-) (10 lL of 5 mM solution of all guest was added to 3 mL of host 1) in DMF (k = 399.7 nm). (d) Fluorescence titration of metallacycle 1 (3 9 10-3 mM) in DMF with increasing - - - IO4 concentration (addition of 20 lLIO4 ions per time kex = 320 nm). (Inset of d) Stern–Volmer plot for IO4 ions recognition. Reprinted with permission from Ref.73 Copyright (2017) American Chemical Society. adsorptions and breathing of crystals. We have dis- in the perspective of supramolecular chemistry that cussed below these recent advances of this old system brings about functional materials. J. Chem. Sci. (2020) 132:46 Page 15 of 31 46

explored its solid-state properties. This cluster (Scheme 14) is unusual in the sense that unlike symmetric l3-oxo-triiron(III) clusters (Scheme 13) in basic iron carboxylates, one of the iron centers in [Fe3(l3-O) (l2-CH3COO)6(C5H5NO)2(H2O)] ClO43H2O (our work),78 has water coordination with other two iron atoms, coordinating 2-pyridone ligands through neutral oxo donor atoms (Scheme 14). As observed in the crystal structure, the iron coordinated water molecule is hydrogen-bonded to three lattice water molecules forming a supramolecular water tetramer. We have investigated solid-state properties of this unusual trinuclear iron cluster containing compound that would make our system [Fe3(l3-O) (l2-CH3COO)6(C5H5NO)2(H2O)] ClO43H2O a solid- state functional material. The molecular functions, it shows, are (i) reversible loss and gain of water tetramer, (ii) molecular recognition of methanol and (iii) molecular recognition of pyridine in reversible gas-solid reactions that are discussed below under respective headings.

4.1a A cyclic supramolecular (H2O)4 cluster disappears on heating and regenerates from water vapor Scheme 13. A general schematic representation of ‘‘iron Small water clusters, for example, (H2O)2,(H2O)4, 1? 85–89 carboxylate’’ [Fe3(l3-O)(l2-RCOO)6L3] . (H2O)6,(H2O)8 and (H2O)10 have extensively studied to understand the bulk properties of mysterious solvent water. It has already been mentioned that a water 4. Tri-nuclear Metal-(l3-O)-Compounds (Basic cluster (H2O)4 can be formed from three solvent water carboxylates) — a very old system molecules and one iron-coordinated water in the crystal structure (Scheme 14)of[Fe3(l3-O)(l2-CH3COO)6 l3-Oxo-centered tri-nuclear carboxylate-bridged M(III) 78 ? (C H NO) (H O)] ClO 3H O. When we heat this complexes, such as [Fe (l -O)(l -RCOO) L ]1 5 5 2 2 4 2 3 3 2 6 3 substance at 130 C, it loses the water tetramer (three (where L is a neutral monodentate ligand, e.g., H Oand 2 lattice water molecules are lost) as evidenced from IR R is an alkyl or aryl group),74,75 known as ‘‘basic spectral- and PXRD studies. Remarkably, the dehydrated carboxylates’’ (Scheme 13), have long been solid on the exposure of water vapor at an ambient known.76,77 Basic carboxylates (Scheme 13) have been condition regenerates the water tetramer. This reversible explored extensively in terms of magnetism (for III loss and gain of water tetramer mediated by an unusual example, three high spin Fe bridged by a common l3- iron carboxylate [Fe3(l3-O)(l2-CH3COO)6(C5H5NO)2(- oxo ion), catalysis and modeling iron core formation in ? H O)]1 can be cycled several times without loss of any ferritins78–83 andrecentlyintheareaofMOF(metal- 2 compound.86 Thus, this compound can be described as a organic framework) research.84 The discrete basic car- functional material which works in a supramolecular boxylate was never used as a functional material in its reversible gas-solid reaction (Scheme 15) with the dis- solid-state. We have explored a unique trinuclear ruption of O–HO bonds, while covalent bonds are not Fe (l -O) system (basic carboxylate) exhibiting rever- 3 3 affected. sible and switchable materials properties as described We have just seen in this supramolecular reversible below. gas-solid reaction that, the water tetramer misplaces three (lattice) water molecules on heating the relevant crystals retaining the water molecule, coordinated to 4.1 A unique trinuclear Fe3(l3-O) system one of the iron centers of the trinuclear cluster. Can Several years ago (2003), we synthesized an unusual this water molecule be substituted by another oxygen iron basic carboxylate containing compound [Fe3(l3-O) donor molecule in the solid-state! If this happens with (l2-CH3COO)6(C5H5NO)2 (H2O)] ClO43H2Oandwe a toxic volatile substance, then the predicted vapor- 46 Page 16 of 31 J. Chem. Sci. (2020) 132:46

Scheme 14. Schematic representation of [Fe3(l3-O)(l2-CH3COO)6(C5H5NO)2(H2O)] ClO43H2O exhibiting un-symmetrical coordination with respect to monodentate ligand. In the relevant synthesis, 2-hydroxypyridine was used; but its tautomeric form 2-pyridone was found was a monodentate ligand in the relevant crystal structure. Reproduced from Ref.78 with permission from the Centre National de la Recherche Scientiﬁque (CNRS) and the Royal Society of Chemistry.

Scheme 15. A supramolecular reversible gas-solid reaction. solid reaction would be of immense importance in the 4.1b Molecular recognition of methanol in a reversible context of sensing a volatile toxic substance. Methanol crystal to crystal transformation with methanol/water is one such substance, which is greatly toxic as well as vapor volatile at room temperature. We have demonstrated Methanol, a common organic solvent, is not only used below that the above described trinuclear iron com- in undergraduate- and postgraduate-laboratories but pound [Fe3(l3-O)(l2-CH3COO)6(C5H5NO)2(H2O)] also is used extensively in industry. However, one ClO4•3H2O undergoes reversible single-crystal to should be careful in handling this general solvent single-crystal transformation with methanol and water because its exposure in its vapor state (a low boiling in their vapor state. solvent!) causes headaches, dizziness, nausea and J. Chem. Sci. (2020) 132:46 Page 17 of 31 46 blurred vision and drinking methanol causes even This reversible crystalline state reactions—a unique death.90 Thus molecular recognition of methanol system—is not only established by single crystallog- mediated by an inorganic metal complex in a rever- raphy but also by IR spectral studies including isotope sible gas-solid reaction is important in developing effect studies.94 The novelty of this gas-solid reaction functional materials. We were dealing with our unique is that the solid, [Fe3(l3-O)(l2-CH3COO)6(C5H5NO)2 basic carboxylate [Fe3(l3-O)(l2-CH3COO)6(C5H5NO)2 (H2O)] ClO43H2O not only senses a toxic substance, (H2O)] ClO43H2O(Scheme14) and we have demon- methanol from its vapor state but also selectively strated that this compound exhibits molecular recogni- recognizes methanol vapor from the heated mixture of tion of methanol in its vapor state in a reversible single- few other alcohols (ethanol, 2-propanol, butanol, etc.) crystal-to-single-crystal conversion. This reaction, in and even low boiling acetone. However, the major principle, is not favoured because a non-porous crystal, drawbacks of this methanol recognition are: (i) it is a in which molecules are densely packed, is generally slow gas-solid reaction and (ii) there is no colour likely to be destroyed/collapsed during their single- change after and before methanol recognition. We crystal-to-single-crystal transformation interacting with were then looking for a toxic substrate which can be a gaseous molecule.91 Hence, the loss of crystallinity is recognized in a relatively faster gas-solid reaction a common event in such solid-gas reaction. Koten and with a visual color change. And we found that pyridine co-workers reported a unique gas-solid reaction is one such toxic molecule, which exhibits relatively involving bond-forming in a forward direction (sub- faster reaction with a color change as depicted below. strate binding) and bond breaking in the opposite direction in a crystalline state reaction but without 4.1c Molecular recognition of pyridine: relatively retaining the single crystallinity.92,93 We have shown faster gas-solid reaction with visual color change that the single crystals of [Fe3(l3-O)(l2-CH3COO)6 The present compound of interest ([Fe3(l3-O) (C5H5NO)2(H2O)] ClO43H2O, on exposure to metha- (l2-CH3COO)6(C5H5NO)2(H2O)] ClO4 3H2O), dis- nol (CH3OH) vapor at an ambient condition (Figure 13, cussed in the preceding section in the context of see cartoon of bottle) for 24 h, undergo crystalline state methanol recognition, consists of single iron-coordi- reaction to form the single crystals of [Fe3(l3-O) nated water and propensity of this site towards ligand 94 (l2-CH3COO)6(C5H5NO)2(CH3OH)] ClO43H2O. substitution reactions in solid-state prompted us to Notably in the reaction, only the iron-coordinated investigate further on ligand exchange properties using 95 water molecule is replaced by a methanol molecule pyridine as a monodentate ligand. The primary rea- (in its vapor state); the lattice water molecules get son for choosing ‘‘pyridine’’ is manifolds: (i) recog- retained. Interestingly, the methanol coordinated nizing/sensing pyridine from its vapor state (in a gas- compound undergoes again single crystal to single- solid reaction) has a practical importance, (ii) pyridine crystal transformation with water vapor (even with is well-known toxic base, and (iii) it remains as par- atmospheric water vapor) to regenerate the parent tially as a vapor state in an open atmospheric condi- crystals of compound [Fe3(l3-O)(l2-CH3COO)6 tions — thus detection of pyridine vapor by a (C5H5NO)2(H2O)] ClO43H2O(Figure13). crystalline solid is a great challenge from the

Figure 13. Reversible single crystal to single-crystal transformation at an ambient gas-solid reaction. 46 Page 18 of 31 J. Chem. Sci. (2020) 132:46 application point of view. Brown colored crystals of 4.1d Trinuclear basic carboxylate as a building unit to [Fe3(l3-O)(l2-CH3COO)6(C5H5NO)2(H2O)]ClO43H2O, obtain MOF compounds as functional materials on exposure to pyridine vapor at an ambient condition, In the construction of a class of metal-organic frame- result in the formation of green compound [Fe3(l3-O) work (MOF) materials/metal-organic materials (l2-CH3COO)6(C5H5NO)2(C5H5N)] ClO4•C5H5N, with (MOMs), the trinuclear basic carboxylates, [M3(l3-O) n? the iron coordinated water molecule replaced by a pyr- (O2CR)6] (Scheme 13) act as building units, that are idine molecule (Figure 14).95 popularly known as ‘‘trigonal prism’’ molecular This green-coloured pyridine coordinated compound building blocks (MBBs).96 The most versatile MBB is ? goes back to parent brown compound [Fe3(l3-O) [Cr3(l3-O)(O2CCH3)6(H2O)3] . The relevant basic (l2-CH3COO)6(C5H5NO)2(H2O)]ClO43H2Owhenthe carboxylate compound [Cr3(l3-O)(O2CCH3)6 pyridine compound is exposed to water vapor as shown (H2O)3]Cl6H2O was prepared in 1919. This chro- in Figure 14. This regeneration can also be performed mium basic carboxylate has become a legendary by exposing the pyridine compound under atmospheric MBB, when MIL-100 and MIL-101 metal-organic water vapor at an ambient condition. Thus, we have materials were discovered in 2004 and 2005 respec- shown that an iron basic carboxylate, as a discrete tively by Fe´rey and co-workers.97,98 Kim and group, in molecule, brings about reversible crystalline state 2000, for the first time, used a basic carboxylate reactions through molecular recognition of toxic sub- (trigonal prism) [Zn3(l3-O)(O2CR)6]N3 as a MBB for stances (pyridine, methanol, etc.) in their vapor state the construction of a 2-periodic honeycomb (hcb) net, via supramolecular bond breaking and bond making in when the ‘zinc trigonal prism’ MBBs were connected reversible vapor-solid interface reactions at an ambient by chiral and flexible pyridine carboxylates. The condition. Now we wish to describe that the metal basic resulting MOM exhibits enantioselective catalytic carboxylates can also be used as building blocks to activity towards transesterification reactions.99 construct supramolecular architectures of diverse Even though several research groups used trigonal topologies having well-defined void spaces, that belong prismatic tri-nuclear cluster unit (Scheme 13)asa to the distinct class of metal-organic framework (MOF) MBB for the construction diverse MOMs/MOF com- compounds exhibiting unique functional performances pounds, Fe´rey, Serre and group dominated this area of as depicted below. research by discovering important MILs (Materials of

Figure 14. Reversible conversion of compound ([Fe3(l3-O)(l2-CH3COO)6(C5H5NO)2(H2O)]ClO43H2O to compound [Fe3(l3-O)(l2-CH3COO)6(C5H5NO)2(C5H5N)] ClO4C5H5N, with the Fe(III) coordinated water replaced by a pyridine molecule and vice versa. Reproduced from Ref.95 with permission from the Royal Society of Chemistry. J. Chem. Sci. (2020) 132:46 Page 19 of 31 46

Institute Lavoisier) of diverse topologies. Among these, three systems MIL-88, MIL-100 and MIL 101 have received particular attention in terms of application point of view. MIL-88 shows a coordination network structure, constructed from trimers (Scheme 13) of iron(III) octahedral, linearly linked by fumarate dianions to create a 3-dimensional framework structure having one-dimensional pore channels as shown in Figure 15, left (viewed down to the crystallographic c axis).99 In the meantime, Yaghi and co-workers reported MIL-88 related MOFs [Fe3O(1,4- BDC)3(DMF)3][FeCl4] (DMF)3 (MOF-235, orange) and Fe3O(1,3-BDC)3(py)3(py)0.5(H2O)1.5 (MOF-236, green) that have {Fe3O(CO2)6} trigonal prismatic unit as SBU (secondary building unit) as shown in Figure 15, right.100 In these MOFs (MOF-235 and MOF-236), the concerned linkers are 1,4-benzenedi- Figure 15. Left: Structural representation of MIL-88: carboxylate (1,4-BDC) and 1,3-benzenedicarboxylate (a) viewed down to the crystallographic c axis and (1,3-BDC) respectively. The other two important (b) viewed down to the crystallographic b axis; Iron metal basic carboxylate {Cr3O(CO2)6} MOFs are octahedra, oxygen and carbon atoms are shown in white, MIL-100 and MIL101. Their construction can be white and black respectively. Reproduced from Ref.100 with 100 described by the formation of super tetrahedral permission Right: crystal structure view of (a) MOF-235 95 (Fe, blue; O, red; Cl, teal; C, gray) and (b) MOF-236 (Fe, supramolecular building blocks, that are linked in a light blue; O, red; C, gray; N, green) composed of oxo- corner-sharing manner to form 5- and 6-connected centred iron trimers and benzene dicarboxylate links (views rings resulting in the formation of a topology, similar down z-axis. Reprinted with permission from Ref.101 101 to that of mtn (Mobile Thirty Nine) zeolite as shown in Copyright (2005) American Chemical Society. Figure 16.

Figure 16. The underlying mtn topology network in MIL-100 and MIL-101, consisting of two types of cages. The nodes are different, corner-sharing supertetrahedral supramolecular building blocks.96 Reproduced from Ref.96 with permission from the Royal Society of Chemistry. 46 Page 20 of 31 J. Chem. Sci. (2020) 132:46

Figure 17. (a) Two different types of {Co3} units in MOF 2. (b) View of the pore space partition through the planar {Co3} units in MOF 2. (c) Two types of intersecting free spaces in MOF 2. (d) View of the solvent-accessible volume in MOF 2 composed of the two types of intersecting free spaces. Reprinted with permission from Ref.104 Copyright (2018) American Chemical Society.104

4.1e Functional behaviors of MOF compounds enhanced C2H2 uptake and C2H2/C2H4 separation per- 104 Zhang and co-workers reported NH2-MIL-88 (Fe) formance as shown in Figure 18. MOF compound that exhibits bifunctional behaviors When we treated CoCl26H2O with dpe and H3BTB of highly sensitive detection and efﬁcient removal (see Figure 19a and b) in 10 mL water/DMF at 120 C of arsenate in an aqueous medium.102 They have under solvothermal conditions, we could generate the shown the determination of As(V) in tap water and trigonal prism molecular building block (metal basic 1- Chaohu lake water based on NH2-MIL-88 (Fe) carboxylate) {Co3(l3-OH)(COO)6} with a negative MOF. charge (Figure 19c), that acts as a building unit in the Feng, Bu and their co-workers have introduced pore construction of a three-dimensional interpenetrated space partition (PSP) in MIL-88 type system (the acs net) framework having void space, occupied by mono-nu- 2? by using PSP agent (for example, C3-symmetric 2,4,6- clear {Co(H2O)4(DMF)2} coordination complex tri(4-pyridyl)-1,3,5-triazine) and demonstrated that, the (Figure 19d).105 resulting network (pacs net) exhibits dramatically This host-guest compound [{Co3(l3-OH)(BTB)2 103 enhanced CO2 uptake. Liu, Du and their group have, (dpe)2}{Co(H2O)4(DMF)2}0.5]nnH2O acts as an efﬁ- very recently, reported two MIL-88 type MOFs with pacs cient catalyst for electrochemical water oxidation as net (Figure 17), [Co3(l3-OH)(cpt)3Co3(l3-OH)(L)3 shown in Figure 20. We have established that the 2? (H2O)9](NO3)4(guests)n [L = 3-amino-1,2,4-triazole guest mono-nuclear compound {Co(H2O)4(DMF)2} , (MOF 1) and 3,5-diamino-1,2,4-triazole (MOF 2); locked in the void space of the interpenetrated Hcpt = 4-(4-carboxyphenyl)-1,2,4-triazole] and showed framework (a MOF of cobalt basic carboxylate), acts J. Chem. Sci. (2020) 132:46 Page 21 of 31 46

Figure 18. (a) Gas sorption isotherms for activated MOF 1 at 298 K. (b) Gas sorption isotherms for activated MOF 2 at 298 K. (c) Adsorption selectivity predicted by ideal adsorbed solution theory (IAST) for MOF 1. (d) Adsorption selectivity predicted by IAST for MOF 2. Reprinted with permission from Ref.104 Copyright (2018) American Chemical Society.104

1- Figure 19. (a) Linker dpe, (b) linker H3BTB, (c) basic trinuclear {Co3(l3-OH)(COO)6} unit and (d) interpenetrated framework (frameworks shown in blue and green) along the crystallographic b axis with sterically crowded 2? {Co(H2O)4(DMF)2} complex (purple). as the actual water oxidation catalyst, as proposed in KOH) exhibits linear behaviour for a wide potential Scheme 16. range with a Tafel slope of 128 mV/decade, estab- The A1/C1 couple (Figure 20a) can be assigned to lishing this host-guest compound an efﬁcient catalyst the oxidation of CoII to CoIII. A second but little for water oxidation with an overpotential (g)of broader response (A2, Figure 20a) that appears prior 390 mV at a current density of 1 mA cm-2.105 to oxygen evolution, can be ascribed to the formation Sere, Fe´rey and their co-workers have described of higher valent CoIV species (most probably adsorption and delivery of an analgesic and anti-in- CoIV = O) which is the active intermediate for water ﬂammatory drug, Ibuprofen, by using MIL-100 and oxidation. We have constructed the Tafel plots, relat- MIL-101 MOFs.106 They synthesized and character- ing overpotential (g) and catalytic current density, by ized two cubic (Fd3m) zeolite type chromium car- galvanostatic polarization technique at steady state. As boxylates, MIL-100 and MIL-101 from chromium shown in Figure 20b, the Tafel plot at pH 13 (0.1 m basic carboxylate trimers and di- or tricarboxylic acids 46 Page 22 of 31 J. Chem. Sci. (2020) 132:46

Figure 20. (a) Cyclic voltammograms of [{Co3(l3-OH)(BTB)2(dpe)2} {Co(H2O)4(DMF)2}0.5]nnH2O (solid line) coated on a glassy carbon electrode (3 mm diameter) in 0.1 m KOH (pH 13) at scan rates of 100 mV s-1 and the proﬁle for glassy carbon electrode (dotted line). Inset: Photograph of an FTO electrode coated with the compound during electrolysis 105 105 showing O2 bubbles. (b) The relevant Galvanostatic iR corrected Tafel plot at pH 13. Reproduced from Ref. with permission.

II 2+ ‒ III 3+ [Co (H2O)4L2] ‒ e → [Co (H2O)4L2] These exhibit ‘breathing of crystal properties’ as well III 3+ + III 2+ as unique sorption properties, as described below. [Co (H2O)4L2] ‒ H → [Co (OH)(H2O)3L2] [CoIII(OH)(H O) L ]2+ ‒ e‒ → [CoIV(OH)(H O) L ]3+ 2 3 2 2 3 2 4.1d Ionic crystals from trinuclear basic carboxylate IV 3+ + IV 2+ [Co (OH)(H2O)3L2] ‒ H → [Co =O(H2O)3L2] as a macrocation and polyoxometalate anion: sorption [CoIV=O(H O) L ]2+ + H O → [CoII(H O) L ]2+ + ½ O properties and breathing of crystals 2 3 2 2 2 4 2 2 Mizuno and co-workers reported that the association of Scheme 16. Proposed mechanism of the water oxidation metal basic carboxylates (Scheme 13) type macrocation 2? process performed by the catalyst {Co(H2O)4L2} with Keggin-type polyoxometalate (POM) anion (e.g., ÂÃ ÂÃ (L = DMF). II VI 6 IV VI 4 aCo W 12O40 , a Si W 12O40 , etc.) results in the formation of ionic crystals of compounds, ˚ (Scheme 17) with giant pores (25–34 A) and huge K3[Cr3O(OOCH)6(H2O)3][a-SiW12O40]16H2O, Cs5[- 2 -1 110–117 surface areas (3100–5900 m g ) without any loss of Cr3O(OOCH)6(H2O)3][a-CoW12O40]7.5H2Oetc. crystallinity even after water evacuation. Scheme 18 has described syntheses and crystal structures The MIL-100 and MIL-101 materials showed of 1a, 2a, 3a and 4a. remarkable Ibuprofen adsorption. Figure 21 describes ÂÃ ðÞðÞ½a ðÞ; the kinetics of Ibuprofen delivery to simulated body Na2ÂÃCr3O OOCH 6 H2O 3 PW12O40 16H2O 1a ðÞðÞ½a ðÞ; ﬂuid (SBF) at 37 C. The delivered Ibuprofen con- KÂÃ3 Cr3O OOCH 6 H2O 3 SiW12O40 16H2O 2a Rb Cr OðÞ OOCH ðÞH O ½aBW O 16H OðÞ3a ; and centration was determined by HPLC. 4 ÂÃ3 6 2 3 12 40 2 Cs Cr OðÞ OOCH ðÞH O ½a CoW O 7:5H OðÞ4a : Besides drug delivery applications, MIL-100 and 5 3 6 2 3 12 40 2 MIL-101 show diverse functional behaviors, such as adsorption of indole and quinoline from a fuel,107 catalytic activity for the cycloaddition of CO2 and As shown in Scheme 18, compounds 1b, 2b, 3b and epoxide under mild and co-catalyst free conditions,108 4b are dehydrated compounds of 1a, 2a, 3a and 4a selective removal of cesium and strontium from high- respectively. 1b–3b are completely dehydrated, level nuclear waste,109 etc. whereas 4b is partially dehydrated (50%). The water Metal basic carboxylates as building blocks, not of crystallization in 1a–3a can completely be desol- only construct extended structures of functional MOF vated by evacuation at room temperature to form compounds (as discussed above), but also serve as 1b–3b, while 50% of the water of crystallization in 4a potential macrocations for the supramolecular for- can be desolvated to form 4b. mation of ionic crystals, where the anions are mostly The desolvated molecules 1b–4b can recognize polyoxometalates (POMs). In this class of the ionic small polar molecules as shown in Figure 22. Com- crystals, the POM anions and metal basic carboxy- pound 1b shows reversible sorption with various polar lates are held together by the Columbic type of hydrophilic organic molecules (alcohols, nitriles, interactions forming the supramolecular adducts. esters etc.). Compound 2b sorbs water, methanol, J. Chem. Sci. (2020) 132:46 Page 23 of 31 46

Scheme 17. Top: Tetrahedra, built up from chromium trimer octahedra and 1,4-benzenedicarboxylate moieties or 1,3,5- benzene-tricarboxylate groups in MIL-101 and MIL-100, respectively. Bottom: a three-dimensional representation of the zeolite type architecture of MIL-100 and MIL-101.106 Reproduced from Ref.106 with permission.

Figure 21. Left: Ibuprofen delivery (% IBU vs t) from MIL-101 and MIL-100. Right: Ibuprofen delivery (mg IBU/g dehydrated material vs t) from MIL-101 and MIL-100 in comparison with MCM-41.106 Reproduced from Ref.106 with permission. 46 Page 24 of 31 J. Chem. Sci. (2020) 132:46

Scheme 18. A schematic representation of the syntheses and crystal structures of ionic crystals Red and green polyhedra showed the [WO6] units of the polyoxometalate and the [CrO6] units of the macrocation, respectively. Blue and light-blue spheres show the alkali-metal ions and the water of crystallization, respectively.113 Reprinted with permission from Ref.113 Copyright (2006) American Chemical Society. ethanol, acetonitrile and methyl formate. Compound In their (1a–4a) crystal structures, the POMs 3b is found to sorb water and methanol, whereas (anions) and metal basic carboxylates (macrocations) compound 4b sorbs only water. line up alternatively to form columns, that are arran- Mizuno and group successfully demonstrated the ged in a honeycomb structure in 1a and 2a, layered selective sorption properties of the dehydrated ionic structure in 3a and densely packed arrangement in 4a. crystals 1b, 2b, 3b and 4b, for example, ethanol was The crystal structure of the dehydrated compounds selectively sorbed by 1b from a mixture of 1:1:1 1b–4b show close packing of the columns with C2-C4 alcohols. Methanol is a low volatile toxic reduced cell volumes, as expected. More speciﬁcally, substance, which can selectively be sorbed/separated the crystal structure of compound 2a111 shows that in by 2b and 3b from a mixture of 1:1:1 C1-C3 alcohols. the crystal, the polyoxometalates (POMs) and the Water could selectively be sorbed by 4b from a mix- macro-cations (tri-nuclear chromium carboxylates) ture (1:1:1) of water, methanol, and ethanol. As far as line up along crystallographic c axis to form the col- nitriles separation is concerned, valeronitrile is selec- umns, that are hexagonally arranged to result in the tively sorbed by 1b from a mixture (1:1) of valeroni- straight channels. Mizuno group argued that the use of trile and capronitrile. Acetonitrile was found to be such macro-cation (metal basic carboxylates) with a selectively sorbed by 2b from a mixture (1:1) of ace- large size (around 70 A˚ ) and a small charge of ?1 tonitrile and propionitrile. In the case of separation of (that reduces the ion-ion interactions) is crucial for the esters, methyl propionate has selectively been shown formation of these channels. The lattice waters, that to be sorbed by 1b from a mixture (1:1) of methyl are located in these channels through hydrogen propionate and ethyl propionate. Methyl formate can bonding, can be easily desorbed by simply evacuation be selectively sorbed by 2b from a mixture (1:1) of at room temperature forming the desolvated crystals of methyl formate and methyl acetate. 2b. The crystal structure of 2b shows the close packing J. Chem. Sci. (2020) 132:46 Page 25 of 31 46

Figure 22. Sorption isotherms of (A) alcohols, (B) nitriles, and (C) esters for 1b at 298 K. Sorption isotherms for (D) 2b, (E) 3b, and (F) 4b at 298 K. (a) Water, (b) methanol, (c) ethanol, (d) 1-propanol, (e) 2-propanol, (f) 1-butanol, (g) 2-butanol, (h) isobutyl alcohol, (i) tert-butyl alcohol, (j) acetonitrile, (k) propionitrile, (l) butylonitrile, (m) isobutylonitrile, (n) valeronitrile, (o) benzonitrile (313 K), (p) methyl formate, (q) methyl acetate, (r) ethyl acetate, (s) methyl propionate, and (t) ethyl propionate.113 Reprinted with permission from Ref.113 Copyright (2006) American Chemical Society. of the columns explaining the broadness of the compound 1 experiences molecular motion in the solid- observed PXRD peaks of dehydrated compound 2b state; molecules come closer in the crystals of dehydrated (consistent with low BET surface area). The desorbed compound with the shrinkage of unit cell volume of the compound 2b absorbs water molecules upon exposure resulting dehydrated crystal as shown in Figure 23.These to saturated water vapor at room temperature with the dehydrated crystals (dehydrated 1–85 and dehydrated regeneration of the PXRD pattern of compound 2a. 1–135), remarkably, upon exposure of water vapor at an And this water loss/gain along with crystal volume ambient condition regenerate to the crystals of parent shrinkage/growth back can be cycled several times, compound 1 with the expansion of the unit cell volume. which has been described as breathing of ionic The solid-state properties of compound 1 and crystals.111,116 dehydrated 1–85 are described in the view of We have also contributed to the area of ionic crystals. We exclusion (shrinkage of unit cell volume) and inclu- synthesized the ionic crystals of compounds [M3(l3-O) sion (expansion of unit cell volume) of water mole- (ClCH2COO)6(H2O)3]4[SiW12O40]xH2O2ClCH2CO cules in the crystal lattice. This phenomenon can be OH [M = Fe3?,x=18(1); M = Cr3? x = 14(2)]. When described as breathing of the crystals, as originally the compound 1 crystals are heated at 85 C and 135 C described by Mizuno research group. in an open atmospheric condition for about 3.5 h, compound 1 undergoes single-crystal to single-crystal transformations during its dehydration with the formation of 5. Conclusions [Fe3(l3-O)(ClCH2COO)6(H2O)3]4[SiW12O40]10H2O 2ClCH2COOH (dehydrated 1–85), and [Fe3(l3-O) In this article, we have tried to convey an important (ClCH2COO)6(H2O)3]4[SiW12O40]8H2O2ClCH2CO message that a meaningful and logical analysis of OH (dehydrated 1–135), respectively with the loss of supramolecular interactions in the crystal structures— considerable amount of lattice water molecules.118 We thereby analyzing the supramolecular chemistry—of have observed that during this dehydration process, inorganic compounds can be linked to some level of 46 Page 26 of 31 J. Chem. Sci. (2020) 132:46

Figure 23. A perspective view of packing diagram along the crystallographic b axis: (a) [Fe3(l3-O)(ClCH2COO)6 (H2O)3]4[SiW12O40]18H2O2ClCH2COOH (1) and (b) [Fe3(l3-O)(ClCH2COO)6(H2O)3]4[SiW12O40]10H2O2ClCH2- 1? 4- COOH (dehydrated 1–85); Color code: [Fe3(l3-O)(ClCH2COO)6(H2O)3] cluster, purple polyhedra; [SiW12O40] cluster, golden yellow polyhedra; non-coordinated water molecules, blue; Cl, light green. Reproduced from Ref.118 with permission. understanding of their potential to act as functional prompted us to perform electrocatalytic proton reduction. materials. We have demonstrated this, by taking three Therefore, the take-home message is that there are types of inorganic systems: mononuclear dithiolato- diverse inorganic systems, even they are characterized and orthophenylenediamine-coordination complexes, and reported, can be re-looked in the perspective of dinuclear iron- and zinc-compounds and trinuclear supramolecular chemistry and can further be explored, basic carboxylates. For example, supramolecular by applying some basic inorganic chemistry knowledge, analysis of the trinuclear iron basic carboxylate as functional inorganic materials mostly in the solid- [Fe3(l3-O)(l2-CH3COO)6(C5H5NO)2(H2O)] ClO43H2O state. This message should not be limited to only having a water coordination to one of the three iron mononuclear, dinuclear and trinuclear systems, we have centers, indicates, by applying the knowledge of basic explored, rather it is open to any inorganic system (re- coordination chemistry, that this coordinated water can ported as well as newly synthesized systems). For be substituted by other oxygen and nitrogen donor example, we have now turned our attention to multi- molecules. This clue makes this compound a functional nuclear polyoxometalate clusters for the exploration of material for sensing methanol and pyridine in solid-va- their new solid-state properties. por interface reactions. It has also be shown that these basic carboxylates, which have been used as building Acknowledgement units (called as trigonal prisms), can be linked to extended structures of stable metal-organic frameworks We acknowledge SERB, Department of Science and Technology, Government of India, for the financial support (MOFs) having well-defined cages/cavities (e.g., MIL- (Project No. EMR/2017/002971) of this work. We would 100 and MIL-101); these supramolecular materials have also like to thank UGS-CAS, UPE-II, DST-PURSE and been shown to adsorb and deliver analgesic and anti- DST-FIST. OB thanks UGC for her fellowship. inflammatory drug, Ibuprofen. Likewise, careful inves- tigation on the molecular structure of the mono-nuclear II • copper compound [Cu {C6H4(NH2)2}2SO4] H2Ohav- References ing 5-coordinated Cu(II) with weakly coordinated sulfate anion hinted us that the unsaturated copper(II) can easily 1. Singh N and Gupta S 1999 Solid State Electrical be coordinated to other nitrogen donor ligands; this Conductance Properties of Some New Bimetallic Salts indication makes this old-known molecule azide recog- and Heterometallic Coordination Polymers Derived nizer in a solid-liquid interface reaction. In the case of di- from Bis (1-Ethoxycarbonyl-1-Cyanoethylene-2,2- Dithiolato) Cuprate(II) Ion Synth. Met. 107 167 nuclear iron system, the relevant crystal structures hav- 2. Singh N and Gupta S 2000 Magnetic and Electrical ing iron in ?1 oxidation state and ring nitrogen, capable Properties of New Inorganic Complex Based Materials of being protonated by supramolecular interactions, Derived from Bis(1-Ethoxycarbonyl-1-Cyanoethylene- J. Chem. Sci. (2020) 132:46 Page 27 of 31 46

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