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Review Review Combining Coordination and Bonds to DevelopBonds to Discrete Develop Supramolecular Discrete Metalla- AssembliesSupramolecular Metalla-Assemblies

Bruno Therrien

Institute of , UniversityUniversity ofof Neuchatel,Neuchatel, AvenueAvenue dede BellevauxBellevaux 51,51, CH-2000CH-2000 Neuchatel,Neuchatel, Switzerland;Switzerland; [email protected]@unine.ch;h; Tel.: ++41-32-718-249941-32-718-2499  Received: 6 April 2020; Accepted: 18 18 May May 2020; 2020; Published: Published: date 8 June 2020 

Abstract: InIn Nature, Nature, metal metal ions ions play play critical critical roles roles at at different different levels, and they are often found in . Therefore, metal ions are naturally incorp incorporatedorated in hydrogen-bonde hydrogen-bondedd systems. In In addition, addition, the combination of metal coordination and hydrogen bonds have been used extensively to develop supramolecular materials. However, However, despite this win-win combination between coordination and hydrogen bonds in many supramolecular systems, the same combination remains scarce in the field field of coordination-driven self-assemblies. Indeed, as illustrated in this this mini-review, mini-review, only a few discrete supramolecular metalla-assembliesmetalla-assemblies combining combining coordination coordination and and hydrogen hydrogen bonds bonds can can be foundbe found in the in theliterature, literature, but but that that figure figure might might change change rapidly. rapidly.

Keywords: coordinationcoordination chemistry; chemistry; hydrogen hydrogen bonds; bond supramoleculars; ; chemistry; metalla-assemblies; metalla- assemblies;coordination-driven coordination-drive self-assembly;n self-assembly; orthogonality; orthogonality; ; metal ligands; ions metal ions

1. Introduction Introduction The preparation and and characterization characterization of of discrete discrete metal-based metal-based assemblies assemblies have have been been the the focus focus of severalof several research research groups. groups. Such Such metalla-assemblies metalla-assemblies are are obtained obtained by by combining combining metal metal ions ions and multidentate ligands ligands in in a pre-designed a pre-designed and andcontrolled controlled manner manner [1–5]. [These1–5]. supramolecular These supramolecular metalla- assembliesmetalla-assemblies can be used can as be sensors used [6–8], as sensors anticancer [6–8 agents], anticancer [9,10], hosts agents for [guest9,10], hosts for [11,12], guest drugmolecules carriers [11, 12[13],], drug mesogens carriers [14,15], [13], mesogens or molecular [14,15], flasks or molecular [16,17]. flasks About [16 ,4017 ].years About ago, 40 yearsthe first ago, coordination-driventhe first coordination-driven metal-based metal-based squares squares (Figure (Figure 1), composed1), composed of linear of linear diphosphine diphosphine ligands ligands and tetracarbonyland tetracarbonyl metal metal ions ions(Cr, (Cr,Mo, Mo,W), W),were were synthesized synthesized [18]. [ 18 ].

O O (CO)4 M P P M (CO)4 P O P O O O O O O P O P

(CO)4 M P O P M (CO)4 O M=Cr,Mo,W Figure 1. MolecularMolecular structure of the first first coordination-linked metalla-squares [[18].18].

A fewfew yearsyears later,later, the the field field really really took took off offwith with the the introduction introduction of 90 of◦ square-planar90° square-planar palladium palladium ions, whichions, which are versatile are versatile building building blocks inblocks supramolecular in supramolecular chemistry chemistry [19]. Nowadays, [19]. Nowadays, all kind of all transition kind of

Chemistry 2020, 2, Firstpage-Lastpage; doi: FOR PEER REVIEW www.mdpi.com/journal/chemistry Chemistry 2020, 2, 565–576; doi:10.3390/chemistry2020034 www.mdpi.com/journal/chemistry Chemistry 2020, 2 566 Chemistry 2020, 2, x 2 metaltransition ions withmetal di ffionserent with coordination different geometriescoordination have geometries been introduced have inbeen metalla-assemblies, introduced in metalla- and the fieldassemblies, is flourishing. and the field is flourishing. LikeLike thethe fieldfield ofof coordination-drivencoordination-driven self-assemblies,self-assemblies, hydrogen-bondedhydrogen-bonded self-assembledself-assembled systemssystems have receivedreceived for for many many years years a great a dealgreat of deal attention of attention [20–25]. The[20–25]. directionality, The directionality, stability, reversibility, stability, andreversibility, biological and importance biological of hydrogenimportance bonds of hydrogen have encouraged bonds have research encouraged groups to research use hydrogen-bonded groups to use motifshydrogen-bonded to construct supramolecularmotifs to construct assemblies. supramolecular An appropriate assemblies. selection An appropriate of donor and selection acceptor of groups, donor inand a pre-organizedacceptor groups, fashion, in a canpre-organized control the fashion, strength can and control geometry the of strength the designed and geometry supramolecular of the ,designed supramolecular and accordingly, structures, allow the formationand accordingly, of two-dimensional allow the formation and three-dimensional of two-dimensional assemblies. and Nowadays,three-dimensional the degree assemblies. of sophistication Nowadays, has the reacheddegree of an sophistication incredible level, has far reached beyond anthe incredible simple DNAlevel, helices,far beyond and the beautiful simple examplesDNA helices, are publishedand beautiful ata examples regular pace are published with different at a regular applications pace inwith mind different [20–25 applicatio]. ns in mind [20–25]. InIn thethe fieldfield ofof materialsmaterials science,science, coordinationcoordination and hydrogenhydrogen bonds have beenbeen jointjoint toto generategenerate ,polymers, dendrimers,dendrimers, and otherother supramolecularsupramolecular assembliesassemblies [[26–32].26–32]. An early example of suchsuch materials comes from the groupgroup ofof Reinhoudt,Reinhoudt, in which a barbituricbarbituric acid entityentity waswas coupledcoupled to palladium-basedpalladium-based metallo-dendrons metallo-dendrons to generateto generate metallo-dendrimers metallo-dendrimers [33]. In[33]. these In systems, these thesystems, barbituric the acidbarbituric residue acid forms residue a rosette forms type a rosette structure type viastruct hydrogenure via hydrogen bonds, while bonds, the while supramolecular the supramolecular network isnetwork further is extended further byextended the dendritic by the arms: dendritic The twoarms: functions The two are functions linked together are linked by coordinationtogether by chemistry.coordination Following chemistry. this pioneerFollowing report, this similarpioneer combinations report, similar have combinations been used to develop have been coordination used to anddevelop hydrogen-bonded coordination and materials hydrogen-bonded [26–35]. materials [26–35]. Surprisingly, despite this relative popularity,popularity, thethe combinationcombination ofof coordinationcoordination andand hydrogenhydrogen bonds to form discrete supramolecular metalla-assembliesmetalla-assemblies remainsremains scarce.scarce. Most examples are limited toto cycliccyclic andand planar planar entities entities (one (one and and two two dimensions), dimensions), and and only only recently, recently, systems systems showing showing cavities cavities and cage-likeand cage-like structures structures (three (three dimensions) dimensions) have appeared have appeared in the literature. in the literature. These hybrid These self-assembled hybrid self- systemsassembled involving systems coordination involving coordination chemistry and chemis hydrogen-bondedtry and hydrogen-bonded to forminteractions discrete to entities form arediscrete presented entities and are discussedpresented inand this discussed short review, in this thusshort showingreview, thus the showin great potentialg the great of potential combining of coordinationcombining coordination and hydrogen and bonds hydrogen to develop bonds newto develop supramolecular new supramolecular metalla-assemblies. metalla-assemblies.

2. Planar and Macrocyclic Assemblies Exploiting Coordination and Hydrogen BondsBonds InIn crystalcrystal engineering,engineering, the combinationcombination of metal ions andand hydrogenhydrogen bondsbonds hashas beenbeen extensivelyextensively explored [[36],36], andand thethe firstfirst examplesexamples ofof discretediscrete coordinationcoordination andand hydrogen-bondedhydrogen-bonded systemssystems werewere probablyprobably inspired by by solid-state solid-state ch chemistry.emistry. Joining Joining several several metal-base metal-basedd chromophores chromophores is needed is needed for the for thepreparation preparation of light-harvesting of light-harvesting systems, systems, however, however, to to better better understand understand the the electronic electronic pathway pathway andand metal-metal communications involved involved in in such syst systems,ems, having a dinuclear compound can be moremore appropriate. With that in mind, the groups of Ward andand Barigelletti have studied the electronic energy transfertransfer processprocess betweenbetween metal-polypyridylmetal-polypyridyl complexescomplexes linkedlinked byby complementarycomplementary hydrogen-bondedhydrogen-bonded groups [[37,38].37,38]. Bispyridyl ligands functionalized with with were were synthesized and used toto connect two metal ions, see see Figure Figure 22..

Figure 2. Cationic hydrogen-bonded dinuclear systems showing metal-metal energy transfertransfer [[37].37].

Chemistry 2020, 2 567 Chemistry 2020, 2, x 3 Chemistry 2020, 2, x 3 TheThe relativelyrelatively high high binding binding constant constant of the of nucleotide the nucleotide base pairs base and pairs the andnature the of naturethe dinuclear of the The relatively high binding constant of the nucleotide base pairs and the nature of the dinuclear dinuclearsystems have systems suggested have suggested that the energy that the transfer energy occurs, transfer even occurs, in even in(CH solution2Cl2), via (CH theCl hydrogen-), via the systems have suggested that the energy transfer occurs, even in solution (CH2Cl2), via the hydrogen-2 2 bonded interface. hydrogen-bondedbonded interface. interface. A similar bis-rhodium complex has been synthesized [39], and a single crystal X-ray structure A similarA similar bis-rhodium bis-rhodium complex complex has has beenbeen synthesizedsynthesized [39], [39], and and a asingle single crystal crystal X-ray X-ray structure structure analysisanalysisanalysis has has has confirmed confirmed confirmed the the the dimeric dimeric dimeric nature naturenature ofof thethe systemsysystemstem (Figure (Figure 3). 33).). Interestin Interestingly,Interestingly,gly, upon upon coordination coordination coordination toto the tothe the rhodiumrhodium rhodium pentamethylcyclopentadienylpentamethylcyclopentadienyl pentamethylcyclopentadienyl unit,unit, unit, thethe the hydrogenhydrogen hydrogen bond bond bond pairing pairing pairing between between between two two 7- two7- 7-diphenylphosphino-1diphenylphosphino-1diphenylphosphino-1HH-quinolin-2-one-quinolin-2-one ligandsligands is notisis disturbed.notnot didisturbed.sturbed. Diffusion-ordered Diffusion-orderedDiffusion-ordered NMR spectroscopy NMR NMR spectroscopy in CD2Cl2 shows that the dimeric structure is stable in aprotic . in CDspectroscopy2Cl2 shows in that CD the2Cl2 dimericshows that structure the dimeric is stable structure in aprotic is stable solvents. in aprotic solvents.

Figure 3. Dimeric structure of a hydrogen-bonded bis-rhodium complex [39]. FigureFigure 3. 3.Dimeric Dimeric structurestructure ofof aa hydrogen-bondedhydrogen-bonded bis-rhodium complex complex [39]. [39]. Other dinuclear systems have been prepared, in which a combination of coordination and Other dinuclear systems have been prepared, in which a combination of coordination and hydrogen-bondedOther dinuclear interactions systems have were beenused. prepared,For instance, in awhich platinum-based a combination hasof coordination been prepared and hydrogen-bonded interactions were used. For instance, a platinum-based dimer has been prepared in hydrogen-bondedin view to synthesize interactions higher were nuclearity used. For syst instemsance, [40]. a platinum-basedUnfortunately, the dimer self-complementary has been prepared viewin quinoloneview to synthesize to hydrogensynthesize higher bonds higher nuclearity were nuclearity too systems weak comparedsyst [40].ems Unfortunately, [40].to the Unfortunately, π-stacking the self-complementary interactions the self-complementary of the ligands, quinolone hydrogenquinolonethus forming bonds hydrogen werein solution toobonds weak awere coordination compared too weak to macroc thecomparedπ-stackingycle insteadto the interactions πof-stacking a hydrogen-bonded of interactions the ligands, of thustetranuclear the forming ligands, in solutionthussystem forming a coordination(Scheme in solution1). macrocycle a coordination instead ofmacroc a hydrogen-bondedycle instead of tetranucleara hydrogen-bonded system (Schemetetranuclear1). system (Scheme 1).

SchemeScheme 1. Self-assembly1. Self-assembly of of a a coordination coordination macrocyclemacrocycle ( (toptop) )over over a atetranuclear tetranuclear hydrogen-bonded hydrogen-bonded systemsystem (bottom (bottom)[40) [40].]. Scheme 1. Self-assembly of a coordination macrocycle (top) over a tetranuclear hydrogen-bonded Quinolone-basedsystem (bottom) [40]. ligands were also used with octahedral metal center. Indeed, a dinuclear rhodium-based complex was obtained by reacting [Cp*RhCl2]2 (Cp* = pentamethylcyclopentadienyl) with 7-diphenylphosphino-1H-quinolin-2-one in a 1:2 stoichiometry [39]. The cationic dinuclear

Chemistry 2020, 2, x 4 Chemistry 2020, 2, x 4 Quinolone-based ligands were also used with octahedral metal center. Indeed, a dinuclear Quinolone-based ligands were also used with octahedral metal center. Indeed, a dinuclear Chemistryrhodium-based2020, 2 complex was obtained by reacting [Cp*RhCl2]2 (Cp* = pentamethylcyclopentadienyl)568 withrhodium-based 7-diphenylphosphino-1 complex wasH obtained-quinolin-2-one by reacting in a[Cp*RhCl 1:2 stoichiometry2]2 (Cp* = pentamethylcyclopentadienyl) [39]. The cationic dinuclear with 7-diphenylphosphino-1H-quinolin-2-one in a 1:2 stoichiometry [39]. The cationic dinuclear complex (Figure 4) is stable in solution (CD2Cl2), and NMR studies suggest that no dynamic behavior complex (Figure 4) is stable in solution (CD2Cl2), and NMR studies suggest that no dynamic behavior (assembly-disassembly)complex (Figure4) is stable is occurring in solution at (CD room2Cl temperat2), and NMRure in studiesaprotic suggestsolvent. that As emphasized no dynamic in behavior Figure (assembly-disassembly)4,(assembly-disassembly) strong π-π stacking interactionsis is occurring occurring attake at room room place, temperat temperature whichure increases in aprotic in aprotic thesolvent. .stability As emphasized of As the emphasized macrocyclic in Figure in 4, strong π-π stacking interactions take place, which increases the stability of the macrocyclic structure.Figure4, strong Analogousπ-π stacking dinuclear interactions systems were take obtained place, which by reacting increases [(para the-cymene)RuCl stability of the2 macrocyclic]2 with 1-(4- structure. Analogous dinuclear systems were obtained by reacting [(para-cymene)RuCl2]2 with 1-(4- oxo-6-undecyl-1,4-dihydropyrimidin-2-yl)-3-(pyridine-4-ylethyl)ureastructure. Analogous dinuclear systems were obtained by reacting (UPy-L) [(para-cymene)RuCl in a 1:2 fashion2]2 with[41]. 1-(4-oxo-6-undecyl-1,4-dihydropyrimidin-2-yl)-3-(pyridine-4-ylethyl)ureaTheoxo-6-undecyl-1,4-dihydropyrimidin-2-yl)-3-(pyridine-4-ylethyl) neutral complex (Figure 5) is stable under ESI-MS (electro-spray ionization (UPy-L) (UPy-L) –in mass in a a1:2 1:2 spectrometry) fashion fashion [[41].41]. Theconditions.The neutralneutral The complexcomplex dinuclear (Figure(Figure complexes5 5)) isis stablestable were underunder also ESI-MSESI-MS incorporated (electro-spray(electro-spray in tetranuclear ionization–massionization systems, – mass spectrometry) spectrometry)in which the UPy-Lconditions.conditions. units Thewere dinucleardinuclear parallel to complexescomplexes each other werewere to generate alsoalso incorporatedinco metalla-rectanglesrporated inin tetranucleartetranuclear [42]. systems,systems, inin whichwhich thethe UPy-LUPy-L unitsunits werewere parallelparallel toto eacheach otherother toto generategenerate metalla-rectangles metalla-rectangles [ 42[42].].

Figure 4. Dinuclear rhodium-based metallacycle [39]. [39]. Figure 4. Dinuclear rhodium-based metallacycle [39].

Figure 5. A dinuclear complex incorporating piano-stool complexes and derived-ligands allowing hydrogen-bondedFigure 5. A dinuclear assemblies complex [[41].41 ].incorporating piano-stool complexes and derived-ligands allowing hydrogen-bonded assemblies [41]. Two-dimensional assemblies with more than two metal ions have also been synthesized, using for exampleTwo-dimensional square-planar assemblies complexes. with Tetranuclear more than two and metal hexanuclear ions have platinum-based also been synthesized, metalla-cycles using for example square-planar complexes. Tetranuclear and hexanuclear platinum-based metalla-cycles were prepared by RendinaRendina and hishis coworkerscoworkers [[43].43]. The nicotinic acid pair acts as a 120120°◦ bridging ,were prepared and upon by coordination Rendina and to his transtrans coworkers-bis(diphenylphosphine)platinum [43]. The nicotinic acid pair units, units, acts it asforms a 120° a dinuclear bridging ligand, and upon coordination to trans-bis(diphenylphosphine)platinum units, it forms a dinuclear sub-unit that that can can be be coup coupledled to toother other bidentate bidentate ligands. ligands. In comb In combinationination with a with 180° abidentate 180◦ bidentate spacer sub-unit that can be coupled to other bidentate ligands. In combination with a 180° bidentate spacer (4,4spacer′-biphenyl), (4,40-biphenyl), a hexanuclear a hexanuclear metalla-cycle metalla-cycle is obtained is (Figure obtained 6A), (Figure while6 A),in combination while in combination with a 120° with(4,4′-biphenyl), a 120 bidentate a hexanuclear ligand (4,4 metalla-cycle-benzophenone), is obtained a tetranuclear (Figure 6A), metalla-cycle while in combination is isolated (Figure with a 6120°B). ◦ 0

Chemistry 2020, 2, x 5 bidentate ligand (4,4′-benzophenone), a tetranuclear metalla-cycle is isolated (Figure 6B). When iso- ChemistrynicotinicChemistry2020 acid 2020, 2 is, 2 , xused instead, oligomeric and polymeric species are formed, demonstrating5 the569 importance of ligands and metal ions geometry for the preparation of discrete metalla-assemblies. bidentate ligand (4,4′-benzophenone), a tetranuclear metalla-cycle is isolated (Figure 6B). When iso- Whennicotiniciso-nicotinic acid is acid used is instead, used instead, oligomeric oligomeric and polymeric and polymeric species speciesare formed, are formed, demonstrating demonstrating the the importanceimportance of of ligands ligands and and metal metal ions ions geometry geometry for forthe thepreparation preparation of discrete of discrete metalla-assemblies. metalla-assemblies.

Figure 6. Hexanuclear (A) and tetranuclear (B) metalla-cycles built from nicotinic acid and square- planarFigureFigure 6.platinumHexanuclear 6. Hexanuclear ions [43]. (A) ( andA) and tetranuclear tetranuclear (B) ( metalla-cyclesB) metalla-cycles built built from from nicotinic nicotinic acid acid and and square-planar square- platinumplanar ions platinum [43]. ions [43]. Hydrogen-bonded dimers of para-pyridyl-substituted 2-ureido-4-1H-pyrimidinone and cis- coordinatedHydrogen-bondedHydrogen-bonded palladiumdimers complexes dimers of para ofhave-pyridyl-substituted para- beenpyridyl-substituted combined 2-ureido-4-1 to afford 2-ureido-4-1 aH tetranuclear-pyrimidinoneH-pyrimidinone metalla-cycle and cis -coordinatedand [44].cis- In coordinated palladium complexes have been combined to afford a tetranuclear metalla-cycle [44]. In palladiumsolution (CDCl complexes3), a mixture have been of combineda metalla-square to afford (Figure a tetranuclear 7) and metalla-cyclea metalla-triangle [44]. In was solution observed. (CDCl At3), lowa mixture solutionconcentrations of a(CDCl metalla-square3), (1 a mixturemM), (Figure the of triangulara 7metalla-square) and a metalla-triangle assembly (Figure is favored,7) was and observed. a metalla-trianglewhile at At higher low concentrations was concentrations, observed. (1 At mM), the low concentrations (1 mM), the triangular assembly is favored, while at higher concentrations, the amountthe triangular of the assembly square-like is favored, structure while is increasing at higher concentrations,significantly. This the amountstudy confirms of the square-like that the nature structure of amount of the square-like structure is increasing significantly. This study confirms that the nature of metalla-cyclesis increasing significantly. can be controlled This study by confirms steric factors, that the by nature the of metalla-cycles of the canfinal be entity, controlled and by by steric the metalla-cycles can be controlled by steric factors, by the solubility of the final entity, and by the factors, by the solubility of the final entity, and by the geometry of the different building blocks. geometrygeometry of the of the different different building building blocks. blocks.

Figure 7. Molecular structure of a palladium-based tetranuclear assembly [44]. Figure 7. MolecularMolecular structure of a palladium-based tetranuclear assembly [[44].44].

The same para-pyridyl-substituted 2-ureido-4-1H-pyrimidinone hydrogen-bonded dimer was used to construct tetranuclear arene ruthenium metalla-rectangles [42]. NMR spectroscopy and Chemistry 2020, 2, x 6

Chemistry 2020, 2 570 The same para-pyridyl-substituted 2-ureido-4-1H-pyrimidinone hydrogen-bonded dimer was used to construct tetranuclear arene ruthenium metalla-rectangles [42]. NMR spectroscopy and DFT DFTcalculations calculations showed showed that the that formation the formation of the hydr ofogen-bonded the hydrogen-bonded assembly results assembly in an energy results gain in an energyof ΔE gain = −146.8 of ∆ kJE mol= -1146.8, thus kJconfirming mol 1, thus the stability confirming in solution the stability of these in multiple solution hydrogen-bonded of these multiple − − hydrogen-bondedassemblies. assemblies. ̶ TheThe melamine–cyanuric melamine cyanuric acid acid (barbituric (barbituric acid) acid) pairing pairing is among is among the mostthe most studied studied hydrogen-bonded hydrogen- bonded system [45–48]. Rosette-type and tapelike structures can be achieved by the controlled system [45–48]. Rosette-type and tapelike structures can be achieved by the controlled functionalization functionalization of the sub-units [49,50]. Steric groups will favored the formation of discrete rosette- of the sub-units [49,50]. Steric groups will favored the formation of discrete rosette-type structures, type structures, while small and highly soluble groups will increase tapelike structures. Therefore, in while small and highly soluble groups will increase tapelike structures. Therefore, in view to obtain view to obtain discrete metal-coordinated rosette-type systems, a series of pyridyl-functionalized discretecyanuric metal-coordinated acid [51] and melamine rosette-type [52] derivatives systems, were a series synthesized. of pyridyl-functionalized Coordination of arene cyanuric ruthenium acid [ 51] andcomplexes melamine to [ 52the] derivativespyridyl groups were has synthesized. generated tr Coordinationinuclear (Figure of 8A) arene and ruthenium hexanuclear complexes (Figure 8B) to the pyridylrosette-type groups assemblies. has generated trinuclear (Figure8A) and hexanuclear (Figure8B) rosette-type assemblies.

FigureFigure 8. 8.Examples Examples of of trinuclear trinuclear ((AA)) andand hexanuclear ( (BB) )rosette-type rosette-type metalla-assemblies metalla-assemblies [51,52]. [51,52 ].

TheThe use use of of metalated metalated nucleobases nucleobases hashas beenbeen anothe anotherr approach approach in in supramolecular supramolecular chemistry chemistry that that combinescombines coordination coordination chemistry chemistry and and hydrogen hydrogen bonds bonds [ 53[53–59].–59]. InIn suchsuch systems,systems, the natural pairing pairing of nucleicof nucleic acids acids is replaced is replaced by by metal-mediated metal-mediated base base pairs, pairs, whichwhich allows the the generation generation of ofhybrid hybrid DNA DNA structures.structures. Various Various applications applications havehave beenbeen foreseenforeseen for for these these derivatives derivatives (sensing, (sensing, expending expending the the geneticgenetic code, code, forming forming nanoclusters nanoclusters or or nanowires, nanowires, DNA DNA technology) technology) andand theythey have been the the subject subject of manyof many reviews reviews [53–59 [53–59].]. Therefore, Therefore, this this abundant abundant literature literature will will not not be be covered covered here, here, and and the the readers readers who who are interested in that particular area are encouraged to refer to these reviews to complete the are interested in that particular area are encouraged to refer to these reviews to complete the discussion. discussion. 3. Cage-Like Assemblies Exploiting Coordination and Hydrogen Bonds 3. Cage-Like Assemblies Exploiting Coordination and Hydrogen Bonds Several supramolecular capsules built by a combination of two functionalized C2 symmetrical Several supramolecular capsules built by a combination of two functionalized C2 symmetrical calix[4]arene cavitands have been synthesized by Yamanaka and his coworkers [60–62]. In these calix[4]arene cavitands have been synthesized by Yamanaka and his coworkers [60–62]. In these systems,systems, the the two two capsules capsules are are linked linked byby twotwo metal ions ions and and two two pairs pairs of of hydrogen hydrogen bonds bonds (Figure (Figure 9). 9). TheThe size size of theof the calix[4]arene calix[4]arene and and the the length length of of the th functionale functional groups groups (hydrogen-bonded (hydrogen-bonded derivatives derivatives and pyridyland pyridyl groups) groups) dictate dictate the size the ofsize the of cavity.the cavity. In some In some cases, cases, the the cavity cavity is is filled filled by by anan anion,anion, while while in otherin other systems, systems, a guest a guest molecule is trapped. is trapped. The The guest guest exchange exchange dynamics dynamics areare linkedlinked to the nature nature of theof anions the anions used, andused, their and ability their to ability generate to conformationalgenerate conformational changes by changes disrupting by thedisrupting intramolecular the hydrogen-bondedintramolecular hydrogen-bonded system. system.

Chemistry 2020, 2 571 Chemistry 2020, 2, x 7

C7H15 C7H15 4+ C7H15 C7H15

O O OO O O O O

N N N O H HN O P P N M H O H O HN M C8H17 N P C8H17 P C H NH N 8 17 HN HN N C8H17

O O O O O O O O

C7H15 C7H15 C7H15 C7H15

Ph2 Ph2 Ph2 Ph2 P P P P P M = Pt Pd Pd Pd P P P P P Ph2 Ph2 Ph2 Ph2 FigureFigure 9. HybridHybrid capsules capsules built built from from two two functi functionalizedonalized calix[4]arene calix[4]arene cavitands [62]. [62].

The conical-shape conical-shape of of calix[4]arene calix[4]arene was was also also used used to togenerate generate giant giant uranyl-based uranyl-based cages cages [63]. [63 In]. theseIn these systems, systems, hydrogen hydrogen bond bond interactions interactions are are ex exploitedploited to to ensure ensure that that the the calixarene carboxylate ligandsligands adopt adopt a a stable stable and and symmetrical symmetrical conformation conformation,, which which ultimately ultimately forces forces the the carboxylate carboxylate anion anion 2+ toto coordinate to thethe uranyluranyl cationcation (UO(UO222+)) in in a a controlled controlled manner. This This strategy strategy has has allowed allowed the formationformation of of several several discrete discrete icosahedral icosahedral cage-like cage-like structure structure (Figure (Figure 10),10), in in which which the the metals metals are are not not locatedlocated at at the the corners corners or or edges edges of of the the assemblies, assemblies, an andd for for which which the the cavity cavity of of the the large large anionic anionic capsule capsule isis relatively relatively well shielded. The cooperative action of coordination bonds and quadruple hydrogen-bonded interactions has allowed the synthesis of tetrahedron cage-like structures [64]. The symmetry, size, and nature of the assembly are linked to the flexibility of the ligands, the choice of the metal ions (Hg2+, Fe2+, Zn2+) and the conditions used (solvent polarity, concentration, anion, temperature). In the (II) derivatives, the quadruple hydrogen-bonded units are linked to 2,20-bipyridyl group, to produce a tetrahedron cage-like structure (Figure 11). Stability studies have showed that protic solvents (DMSO, H2O) initiate the disassembly of the cage-like structure. However, some derivatives show remarkable stability in

Chemistry 2020, 2, x 8

Chemistry 2020, 2 572 polar solvents, and even in the presence of coordinating competing agents, thus suggesting that these Chemistrycapsules 2020 can, 2 be, x used as reactors for catalytic reactions. 8

Figure 10. Molecular structure of a hexameric cage-like structure built from six calix[4]arene carboxylates and eight uranyl cations [63].

The cooperative action of coordination bonds and quadruple hydrogen-bonded interactions has allowed the synthesis of tetrahedron cage-like structures [64]. The symmetry, size, and nature of the assembly are linked to the flexibility of the ligands, the choice of the metal ions (Hg2+, Fe2+, Zn2+) and the conditions used (solvent polarity, concentration, anion, temperature). In the iron(II) derivatives, the quadruple hydrogen-bonded units are linked to 2,2′-bipyridyl group, to produce a tetrahedron cage-like structure (Figure 11). Stability studies have showed that protic solvents (DMSO, H2O) initiate the disassembly of the cage-like structure. However, some derivatives show remarkable stability in polar solvents, and even in the presence of coordinating competing agents, thus Figure 10.10.Molecular Molecular structure structure of aof hexameric a hexameric cage-like cage-like structure structure built from built six calix[4]arene from six calix[4]arene carboxylates suggestingcarboxylatesand eight that uranyl these and cationseight capsules uranyl [63]. can cations be us [63].ed as reactors for catalytic reactions.

The cooperative action of coordination bonds and quadruple hydrogen-bonded interactions has allowed the synthesis of tetrahedron cage-like structures [64]. The symmetry, size, and nature of the assembly are linked to the flexibility of the ligands, the choice of the metal ions (Hg2+, Fe2+, Zn2+) and the conditions used (solvent polarity, concentration, anion, temperature). In the iron(II) derivatives, the quadruple hydrogen-bonded units are linked to 2,2′-bipyridyl group, to produce a tetrahedron cage-like structure (Figure 11). Stability studies have showed that protic solvents (DMSO, H2O) initiate the disassembly of the cage-like structure. However, some derivatives show remarkable stability in polar solvents, and even in the presence of coordinating competing agents, thus suggesting that these capsules can be used as reactors for catalytic reactions.

Figure 11. MolecularMolecular structure of a hydrogen-bonded tetrahedron cage-like system [[64].64].

The nature of the metal ion was also a critical point when dealing with these quadruple 2+ 2+ 2+ hydrogen-bonded units linked to 2,20-bipyridyl ligand [64]. Replacing Zn or Fe with Hg not only modified the stability, but also the overall geometry. The large ionic radius of Hg2+ provides a wider separation of the coordinated 2,20-bipyridyl ligands, thus allowing the quadruple hydrogen-bonded units to stack on top of each other and to form a triple decker system (Figure 12). The helicate structure 2+ is less stable than the tetrahedron systems, as the coordination energy of 2,20-bipyridyl to Hg remains relatively weak compared to Fe2+.

Figure 11. Molecular structure of a hydrogen-bonded tetrahedron cage-like system [64].

Chemistry 2020, 2, x 9

The nature of the metal ion was also a critical point when dealing with these quadruple hydrogen-bonded units linked to 2,2′-bipyridyl ligand [64]. Replacing Zn2+ or Fe2+ with Hg2+ not only modified the stability, but also the overall geometry. The large ionic radius of Hg2+ provides a wider separation of the coordinated 2,2′-bipyridyl ligands, thus allowing the quadruple hydrogen-bonded units to stack on top of each other and to form a triple decker system (Figure 12). The helicate structure is less stable than the tetrahedron systems, as the coordination energy of 2,2′-bipyridyl to Chemistry 2020, 2 573 Hg2+ remains relatively weak compared to Fe2+.

Figure 12. Dinuclear mercury-based hydrogen-bonded helicate [64]. Figure 12. Dinuclear mercury-based hydrogen-bonded helicate [64]. 4. Conclusions 4. Conclusions As pointed out here, as well as in several reviews and publications [26–35], allowing two or moreAs non-covalent pointed out interactionshere, as well toas takein several place reviews simultaneously and publications can be quite [26–35], challenging allowing (orthogonality two or more non-covalentconcept) [30]. interactions To be successful, to take a place compatibility simultaneously between can the be hydrogen-bonded quite challenging and (orthogonality coordination concept)interactions [30]. is essential,To be successful, as individually a compatibilit they showy between different solubility,the hydrogen-bonded different flexibility and coordination and different interactionsstability. Moreover, is essential, to form as a individually discrete supramolecular they show metalla-assembly,different solubility, the ligandsdifferent and flexibility the hydrogen and differentbonded unitsstability. should Moreover, not compete to form for a thediscrete metal supramolecular ions, and they metalla-assembly, should cooperate. the Therefore, ligands and it is thenot hydrogen so surprising bonded that sounits far theshould number not ofcompete discrete fo metalla-assembliesr the metal ions, and combining they should coordination cooperate. and Therefore,hydrogen bondsit is not remains so surprising limited. Nevertheless,that so far the we number can assume of discrete that considering metalla-assemblies recent progress combining in the coordinationunderstanding and on hydrogen how such orthogonalbonds remains concepts limited. can Nevertheless, be applied to supramolecularwe can assume systems,that considering and how recentinnovative progress strategies in the have understanding recently emerged on inhow the field,such thatorthogonal soon, we concepts will see more can ofbe these applied discrete to supramolecularsupramolecular systems, metalla-assemblies. and how innovative strategies have recently emerged in the field, that soon, we will see more of these discrete supramolecular metalla-assemblies. Funding: This research received no external funding. Funding: This research received no external funding. Acknowledgments: The author would like to thank past and present members of his group. Acknowledgments: The author would like to thank past and present members of his group. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References References 1. Chen, L.-J.; Yang, H.-B. Construction of stimuli-responsive functional materials via hierarchical self-assembly 1. Chen,involving L.-J.; coordination Yang, H.-B. interactions. ConstructionAcc. of Chem. stimuli-resp Res. 2018onsive, 51, 2699–2710.functional [materialCrossRefs][ viaPubMed hierarchical] self- 2. assemblyCook, T.R.; involving Stang, P.J.Recent coordination developments interactions. in the Acc. preparation Chem. Res. and 2018 chemistry, 51, 2699–2710. of metallacycles and metallacages 2. Cook,via coordination. T.R., Stang,Chem. P.J. Recent Rev. 2015 developments, 115, 7001–7045. in the [CrossRef preparation][PubMed and] chemistry of metallacycles and 3. metallacagesDatta, S.; Saha, via coordination. M.L.; Stang, Chem. P.J. Hierarchical Rev. 2015, 115 assemblies, 7001–7045. of supramolecular coordination complexes. 3. Datta,Acc. Chem. S.; Saha, Res. M.L.;2018, 51Stang,, 2047–2063. P.J. Hierarchical [CrossRef asse][PubMedmblies ]of supramolecular coordination complexes. Acc. 4. Chem.Stíbal, Res. D.; 2018 Therrien,, 51, 2047–2063. B. Organometallic chemistry in directed assembly. In Comprehensive Supramolecular 4. Stíbal,Chemistry D.; IITherrien,; Atwood, B. J.L., Organometallic Ed.; Elsevier: chemistry Oxford, UK, in 2017;directed Volume assembly. 6, pp. 305–329.In Comprehensive Supramolecular 5. ChemistryHosseini, II M.W.; Atwood, Molecular J.L., Ed.; tectonics: Elsevier: From Oxfo simplerd, UK, tectons 2017; toVolume complex 6, pp. molecular 305–329. networks. Acc. Chem. Res. 2005, 38, 313–323. [CrossRef][PubMed] 6. Li, M.; Chen, L.-J.; Zhang, Z.; Luo, Q.; Yang, H.-B.; Tian, H.; Zhu, W.-H. Conformer-dependent self-assembled metallacycles with photo-reversible response. Chem. Sci. 2019, 10, 4896–4904. [CrossRef][PubMed] 7. Percástegui, E.G.; Jancik, V. Coordination-driven assemblies based on meso-substituted porphyrins: Metal-organic cages and a new type of meso-metallaporphyrin macrocycles. Coord. Chem. Rev. 2020, 407, 213165. [CrossRef] Chemistry 2020, 2 574

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