Review Review Combining Coordination and Hydrogen Bonds to DevelopBonds to Discrete Develop Supramolecular Discrete Metalla- AssembliesSupramolecular Metalla-Assemblies Bruno Therrien Institute of Chemistry, 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 proteins. 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; supramolecular chemistry; chemistry; metalla-assemblies; metalla- assemblies;coordination-driven coordination-drive self-assembly;n self-assembly; orthogonality; orthogonality; ligands; 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], molecules 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 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 structures,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,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 interactions 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 nucleobases were were