Boron: Its Role in Energy‐Related Processes and Applications

Abstract Boron's unique position in the Periodic Table, that is, at the apex of the line separating metals and nonmetals, makes it highly versatile in chemical reactions and applications. Contemporary demand for renewable and clean energy as well as energy‐efficient products has seen boron playing key roles in energy‐related research, such as 1) activating and synthesizing energy‐rich small molecules, 2) storing chemical and electrical energy, and 3) converting electrical energy into light. These applications are fundamentally associated with boron's unique characteristics, such as its electron‐deficiency and the availability of an unoccupied p orbital, which allow the formation of a myriad of compounds with a wide range of chemical and physical properties. For example, boron's ability to achieve a full octet of electrons with four covalent bonds and a negative charge has led to the synthesis of a wide variety of borate anions of high chemical and electrochemical stability—in particular, weakly coordinating anions. This Review summarizes recent advances in the study of boron compounds for energy‐related processes and applications.


Introduction
In terms of energy-related research, the element boron is currently enjoying significant attention from scientists working in various fields.M ost notably,r ecent advances in the fundamental understanding of boron chemistry and breakthroughs in the synthesis of its compounds have seen boron playing an increasingly important role in applications such as small-molecule activation for fuel synthesis,o rganic lightemitting diodes (OLEDs), hydrogen production and storage, and electrolyte materials.T he rich applications of boroncontaining compounds are closely associated with the unique properties of the element itself.F or example,f rom an electronic structure point of view,boronselectron deficiency means that many of its compounds can act as electrophiles and/or Lewis acids; [1] while under certain conditions the atom can be negatively charged or polarized and, therefore,behave like an ucleophile or Lewis base. [2] This flexibility allows boron to form ag reat variety of compounds with tunable properties for specific applications.F or example,b oron and hydrogen form al arge number of boranes and anions,a nd their high hydrogen capacity make them potential candidates for hydrogen storage materials. [3] Awide range of bulky and unsymmetrical borate anions and anionic boron clusters have also attracted interest because of their inherently weak affinity to cations,w hich makes them highly important building blocks for electrochemical devices. [4] This Review highlights several aspects of boron-containing compounds for energy-related research, including smallmolecule activation, hydrogen storage,e lectrolytes,a nd OLEDs,w ith the aim of emphasizing the diverse roles and high potential of this element. Each topic starts with as hort introduction followed by details of selected examples and discussion, and then closes with ab rief perspective.C ompared with carbon-based materials,which have been explored extensively for energy-related research, boron has attracted much less attention. This is likely because,in comparison with boron, it is much easier to control the morphology and structure and consequently the properties of carbon materials.T his Review aims to highlight borons ability to facilitate the development of efficient and economical methods and materials for future energy needs,a nd Boronsunique position in the Periodic Table,t hat is,a tthe apex of the line separating metals and nonmetals,m akes it highly versatile in chemical reactions and applications.Contemporary demand for renewable and clean energy as well as energy-efficient products has seen boron playing key roles in energy-related research, such as 1) activating and synthesizing energy-rich small molecules,2 )storing chemical and electrical energy,and 3) converting electrical energy into light. These applications are fundamentally associated with borons unique characteristics,s uch as its electron-deficiency and the availability of an unoccupied porbital, whichallowt he formation of amyriad of compounds with awide range of chemical and physical properties.F or example,boronsa bility to achieve afull octet of electrons with four covalent bonds and anegative charge has led to the synthesis of awide variety of borate anions of high chemical and electrochemical stability-in particular,weakly coordinating anions. This Review summarizes recent advances in the study of boron compounds for energy-related processes and applications. illustrates the great potential of this element to play an important, if not decisive,future role in this challenge.

Small-Molecule Activation with Molecular Boron: Scratching the Surface
Thereaction of transition-metal complexes with dihydrogen is an extremely well-established process,which forms the basis of both the practice and pedagogy of catalysis.I n contrast, for centuries,m ain-group compounds were thought to be unable to undergo reaction with H 2 under mild conditions.T his paradigm was abruptly shattered in the mid-2000s,w ith the first uncatalyzed reactions of H 2 with main-group molecules under ambient conditions,namely with ad igermyne in 2005, [5] an ambiphilic phosphine/borane compound in 2006, [6] and ac yclic (alkyl)(amino)carbene in 2007. [7] Thec ombination of filled and empty orbitals that are close in both space and energy is considered the key property of transition metals that allows them to interact with, and activate,r elatively inert small molecules.I n2 010, Power crystallized this concept in as hort review article in Nature entitled "Main-group elements as transition metals", wherein ap arallel was drawn between the filled/occupied orbital arrangements of transition metals and low-oxidation-state or multiply bound main-group species. [8] Ther esults of the interceding years,particularly in the field of low-valent boron chemistry,h ave made this review appear particularly prescient. In arecent article [9] in Chemical Reviews we attempted to comprehensively encapsulate the various ways that hypo-valent boron species are able to mimic the reactivity of transition-metal species,p articularly with regard to the activation of small molecules.
Herein we provide ashort overview of the abilities of lowvalent boron species to activate (or cooperate to activate)  small molecules of interest to catalysis and the sustainable use of energy and resources.T hese processes are in general promoted by three distinct systems based on boron ( Figure 1): 1) heterodinuclear activating species,namely frustrated Lewis pairs (FLPs), which function by the combination of aL ewisacidic boron component with aLewis base;2)homodinuclear activating species with two connected boron atoms,s uch as diboranes,d iborenes,a nd diborynes;a nd 3) monoboron species,namely monovalent, dicoordinate borylenes.

Cooperative Heterodinuclear Activation:B oron Plus Lewis Base
Thec oncept of frustrated Lewis pairs has truly captured the attention of chemists and beyond, and has led to an astounding number of reviews and books on the topic. [10] The discovery by Stephan and co-workers in 2006 of reversible dihydrogen splitting across ap hosphine/borane system marked the initiation of the field of FLP chemistry. [6] Since that time FLP-mediated activations of ar ange of small molecules have been reported, including CO 2 ,CO, N 2 O, NO, SO 2 ,s ilanes,h ydroboranes,o lefins,d ienes,c ycloalkanes,a nd alkynes.Recently,reactions of strongly Lewis-acidic boranes with N 2 -containing molecules,n amely transition-metal dinitrogen complexes [11] and diazoalkanes, [12] have also been reported, spurring speculation into the possibility of FLPbased dinitrogen activation. [13] This may indeed be possible if the right combination of strong Lewis base and acid can be found;h owever, such reactivity remains,a tp resent, speculative.
Thematurity of the field and the obvious applicability of the systems for small-molecule activation makes FLPs by far the most catalytically relevant boron species for this purpose. Thehydrogenation of simple multiple-bond-containing (C = C, C=N, C=O) species was the first and most obvious catalytic use of FLPs,c onsidering their facile dihydrogen splitting abilities.H owever, ar ange of other catalytic reactions have now been reported, such as SÀHb ond borylation, carbocyclizations,COreduction with syngas,and CO 2 hydrogenation. Recent advances have seen these catalytic procedures become even more attractive through increased efficiencies, [14] water-tolerance (A,F igure 2), [15] and even the devel-  opment of highly enantioselective conversions. [16] Perhaps the most exciting recent development in FLP catalysis is the intermolecular C(sp 2 ) À Hb ond activation and borylation of heteroarenes reported by Fontaine and co-workers. [17] By using an intramolecular FLP based on a1-amino-2-borylbenzene backbone as the catalyst, the authors were able to borylate ar ange of electron-rich heteroarenes in excellent yields-a process that has since been extended to include bench-stable and solid-immobilized precatalysts (A, Figure 2). As catalysis based on intermolecular C À Ha ctivation is highly desirable but also extremely challenging to achieve,t hese advances are particularly promising.

Cooperative Homodinuclear Activation:Systems with B À B Bonds
At first glance,c ompounds with BÀBs ingle,d ouble,o r triple bonds [18] do not seem particularly well-suited to smallmolecule activation. Given that acceptance of electron density from ab ond is often required to break bonds in small molecules,a ne mpty orbital is usually required in the activating system. Although base-stabilized diborenes and diborynes do not possess unoccupied porbitals like the FLPbased systems (see Section 2.1) and borylenes (see Section 2.3) described herein, anumber of other vacant orbitals exist that can potentially accept electron density from an incoming small molecule.T hese include the antibonding p*(BB) orbital(s) and partially vacant po rbitals from attached paccepting ligands (e.g. cyclic (alkyl)(amino)carbenes; CAACs).
Carbon monoxide serves as an excellent test case for small-molecule activations,a nd this molecule has shown diverse reactivity with diborenes and diborynes.T hese reactive species not only bind CO,b ut also lead to its reduction, coupling,a nd even scission. [19] Diborynes have been shown to undergo hydrogenation, [20] albeit reluctantly in many cases,w hile they readilly activate CÀHb onds of acetone, [21] and cyclize and activate CÀCb onds of alkynes (B,F igure 2). [22] Diborenes,i ns ome respects more reactive alternatives to diborynes,h ave been shown to undergo inverse-electron-demand Diels-Alder reactions with dienes, [23] and bind and cleave one C = Ob ond of CO 2 (C, Figure 2). [24] An interesting recent advance is the growth of boronbased systems for small-molecule activation with only BÀB single bonds or even two boron atoms not directly bonded to one another. Tr aditional diboranes(4) present the opposite case from diborenes and diborynes:they possess the requisite empty porbitals,but lack the filled p orbitals or lone pairs of the other species described herein. In this regard, some surprising activations of dihydrogen (and CO) have been reported with ar ange of singly bonded species,s uch as at etraaryldiborane(4) (D,F igure 2), ad iaryl, dithienobridged diborane(4), and ab orylborenium cation. [25] A number of electron-rich diboron systems without B À B bonds have not only shown reactivity with dihydrogen but also with other small molecules such as ethylene,alkynes,O 2 , and even NH 3 (E,F igure 2). [26] 2.3. Activation at aS ingle Boron Atom:Borylenes Borylenes of the form [DBR] are highly reactive species with both al one pair of electrons and an empty po rbital on boron, and as such have never been isolated in their free form. Early efforts towards generating and trapping borylene species hinted at their impressive ability to activate small molecules.The respective studies of Timms as well as Pachaly and West led to the reliable generation of transient borylenes and their trapping with alkanes,alkynes,and cyclic ethers by C À Hand C À Oinsertion reactions. [27] Recent studies from the group of Bettinger have shown that transient borylene species can be generated photolytically and react under matrix conditions with ar ange of small molecules such as CO, methane,ethene,e thyne,and even N 2 . [28] Foralong time,a ttempts to harness borylene reactivity under condensed-phase conditions were limited to transitionmetal borylene complexes, [29] as stabilized and synthetically convenient equivalents of "free" borylenes.T ransition-metal borylene complexes show diverse binding and reactivity patterns with CO and other donors,s uch as isonitriles and azides,l eading in some cases to metal-free,d oubly basestabilized borylene species. [30] Thed iscovery by Bertrand and co-workers in 2010 of asynthetic route to doubly base-stabilized borylenes opened anew chapter in borylene chemistry,which provided access to isolable borylenes stabilized not through boron-metal interactions but through Lewis-base stabilization. [31] Since that time,arange of metal-free borylene species have been prepared. However,e xamples of true metal-free smallmolecule activation by borylene species are still limited. Themost well-defined and "truest" isolable borylene species is likely adicoordinate,singly base-stabilized aminoborylene isolated by Stephan, Bertrand, and co-workers. [32] Although linear in the solid state,t his borylene activates both H 2 and CO at room temperature,t hus illustrating some of the most clear-cut borylene-based examples of small-molecule activation by isolated species.
Our work in this area led to the synthesis of two COstabilized borylene species that showed distinct metal-like photodecarbonylation reactivity. [30b,d] Depending on the reaction conditions,e ither intramolecular C À Ca ctivation or Lewis-base addition was observed. This work led to the development of chemistry centered around at ransient, reduction-generated borylene species.R eduction of monohaloboron radical species of the form [(CAAC)BBrAr]C (Ar = Mes or Dur) led to the putative generation of the dicoordinate borylenes [DBAr(CAAC)],w hich under an atmosphere of dinitrogen and further reduction led to either dipotassium di-or tetranitrogen complexes,depending on the nature of the aryl group at boron (F,F igure 2). [33] Further treatment of these metalated compounds with water provided the respective diprotonated derivatives,t hus providing the first binding and reduction of dinitrogen at ap-block element.

Summary and Perspectives
Frustrated Lewis pairs,b eing the most developed of the species described herein, are clearly advanced in terms of applications in small-molecule activation, with catalytic reactions now abundant and becoming increasingly efficient and practical. Thes toichiometric reactions of low-valent mono-and diboron species presented herein, while far from having any practical or large-scale utility,d os how an impressive ability to bind and activate small molecules.T his hints at av ery interesting future for metal-free reactions of boron and, by extension, other main-group elements that may have been historically overlooked for various reasons.

Boron Compounds for Hydrogen Storage
Boron is able to form aw ide range of hydrogen-rich molecules,s uch as boranes and borohydrides.A saconsequence of their high hydrogen capacity by weight, these compounds have been considered as hydrogen carriers. Hydrogen has as pecific energy that is much higher than common carbon-based fuels,b ut also av ery low energy density by volume. [34] Hydrogen storage has become the bottleneck for the wider deployment of hydrogen fuel cells in cars,w hich require am ethod featuring both the high volumetric and gravimetric densities of hydrogen. Conventional methods of compression and liquefaction offer quite low density,a nd they are further penalized by the energetic cost of converting H 2 gas into these physical states.Materialbased hydrogen storage has thus attracted attention, with boranes and borohydrides being intensively studied. [3]

Amine boranes
Interest in amine boranes for hydrogen storage is primarily driven by two factors:1 )their high hydrogen capacity and 2) their low hydrogen release temperature.T he high capacity is associated with their molecular compositions, wherein the light nitrogen and boron atoms typically bond with multiple hydrogen atoms.Amine boranes bear protic (N-H) and hydridic (B-H) hydrogen atoms in proximity,thereby leading to dihydrogen interactions that are conducive to H 2 formation. [3a] One of the best-known compounds for hydrogen storage is ammonia borane (Figure 3, NH 3 BH 3 ,c ommonly known as AB), which has ahigh capacity of 19.6 wt %. [35] Therelease of the chemically bound H 2 ,h owever,i so ften accompanied by the formation of impurities such as borazine and ammonia. Thea ctual thermal decomposition of pure AB involves several steps,n amely,i nduction, nucleation, and growth, which are directly related to the disruption of the dihydrogen bonds,t he formation of the diammoniate of diborane, [NH 3 BH 2 NH 3 ][BH 4 ]( DADB), and the bimolecular reaction between DADB and AB,r espectively. [35c] Together with volatile gaseous products,l inear and cyclic molecules are formed, and these are transformed into polymeric iminoborane as the temperature increases.S imilarly,D ADB has also been observed as akey intermediate when AB decomposes in ionic liquids or organic solvents. [35d] Notably,t he presence of ionic liquids seems to effectively reduce the induction and accelerate the reaction kinetics.T he derivatives MNH 2 BH 3 (M = Li, Na) have been synthesized and display improved kinetics,reduced decomposition temperature,less exothermic reactions,and negligible impurities.All these can be ascribed to the change in the electronic state of the nitrogen atom caused by the substitution of Hb ye lectron-donating metals. [35a] Dehydrogenation has been performed in organic solvents, but typically fewer than three equivalents of H 2 can be derived because of the formation of amino-or iminoboranes, BNH x .M etal-based catalysts and Lewis or Brønsted acids have been studied to improve the dehydrogenation rate and capacity. [36] Fore xample,acatalyst of aN i-NHC complex, obtained by reacting bis(cyclooctadiene)nickel with N-heterocyclic carbenes (NHCs), facilitates the release of 2.5 equiv H 2 ,a nd this is likely associated with the strong donating capacity of the NHC,which activates the B À Hbonds. [36c] Note that, compared with catalysts using expensive metals,F ebased catalysts are particularly interesting,s ince they will be cost-effective for large-scale application. [36d] Thet ypical spent fuel of AB consists of am ixture of polymers featuring strong B À Nb onds,i ncluding polyaminoborane (NH 2 BH 2 ) n and polyiminoborane (NHBH) n .F or practical application of AB as ahydrogen carrier,itiscritical, but challenging at this stage,t or egenerate AB in ac osteffective way.T he typical regeneration ( Figure 3) involves three steps:1 )the digestion of spent fuel to form BX n (X = anionic counterion), 2) the formation of BH 3 using areducing agent, and 3) the addition of NH 3 . [37] Thewhole process would require significant energy,a nd coupling with renewable energy sources could be an option. One-pot regeneration has been demonstrated, whereby hydrazine was used as both ad igesting agent and ar educing agent. [38] Thep roduction of hydrazine is,t herefore,at opic of discussion to make this process viable for large-scale application.
Hydrolysis has also been performed to drive hydrogen evolution. Ac atalyst is typically needed to promote the reaction, since AB is relatively stable in water. Heterogeneous catalysts such as metal nanoparticles are commonly used to facilitate combination between hydridic Ha toms in AB and protic Ha toms in water. [39] Ther esulting hydrolytic borate products,h owever,p ossess unfavorable thermodynamics for the regeneration of borane.

Light-Metal Borohydrides
Borohydrides with light cations such as Li + ,N a + ,a nd Mg 2+ have been considered for hydrogen storage. [40] Their thermolysis,h owever, requires high temperatures,t ypically above 250 8 8C, for dehydrogenation. Ther esulting stable borides and elemental boron also make it highly challenging to reform borohydrides under mild conditions.Jensen and coworkers achieved the direct hydrogenation of MgB 2 to Mg(BH 4 ) 2 under 950 bar H 2 at 400 8 8C. [41] Recently,t hey found that after MgB 2 has been ball-milled with THF and Mg (or MgH 2 ), it can be hydrogenated to Mg(BH 4 ) 2 at 300 8 8C under 700 bar of H 2 (Figure 4). [42] However,t his process still requires av ery high energy input, and other decomposition products of borohydrides such as amorphous boron have not been considered. NaBH 4 has been extensively studied as ahydrogen source for catalyzed hydrolysis,w hich leads to high-purity H 2 at room temperature.T he low solubility of both NaBH 4 and its hydrolytic products in water, however, necessitate the use of large amounts of water for effective hydrogen evolution, which eventually reduces the hydrogen capacity and, therefore,makes this system unsuitable for large-scale applications. Regeneration of NaBH 4 from the spent fuel involves breaking strong B À Obonds (dissociation energies:193 kcal mol À1 ), which indicates it is ahigh-energy process. [37] In ar egeneration method reported recently,m agnesium was ball-milled with NaBO 2 ·2 H 2 Ot op roduce NaBH 4 ( Figure 4). This process avoids the dehydration process to make NaBO 2 and requires no expensive MgH 2 ;therefore,itis comparably low-cost. [43]

Octahydrotriborates
This family of compounds,sometimes known as triborates, containing the B 3 H 8 À anion, are less well-studied than borohydrides,l ikely because they are not readily available. Recent progress in synthesis allows the preparation of these compounds to be carried out in at ypical chemistry laboratory. [44] Generally speaking,asthe number of Band Hatoms increases,the less reactive the hydridic Hatoms in the B m H n xÀ anions become as aresult of the increasing charge distribution across the large cluster.This is evident by NaB 3 H 8 being much more stable and soluble in water than NaBH 4 , [44b] which means that less water is needed to prepare al iquid phase. Finally,its boron-rich nature leads to the formation of various polyborates during hydrolysis that are relatively soluble in water, thus reducing the formation of precipitate.

BCN Heterocyclic Compounds
From the hydrogen storage perspective,a ni nteresting new class of compounds,n amely carbon-, boron-, and nitrogen-containing cyclic compounds,h ave been successfully synthesized and studied. These types of compounds were designed to take the form of some well-known hydrocarbons such as cyclopentanea nd cyclohexane,b ut feature much improved hydrogen evolution. Under the mild operating conditions required for practical applications,t he hydrogen capacity is determined by the reaction between the protic (N-H) and hydridic (B-H) hydrogen atoms.L iu and co-workers have made significant contributions to the discovery of several CBN compounds for hydrogen storage ( Figure 5). [45] Forexample,bis-BN cyclohexane,anisostere of cyclohexane with two BN units,has good thermal stability up to 150 8 8C, but it rapidly gives off pure H 2 at room temperature in the presence of ac atalyst. [45a] BN-methylcyclopentane is al iquid that can release 2equiv H 2 and the exclusive trimer spent fuel can be converted back into the starting material in good yields.
[45b] 1,2-BN-cyclohexane,a lso cleanly forms at rimer upon releasing H 2 .Asingle compound as the spent fuel that maintains the heterocyclics tructure of the starting materials could be highly advantageous for regeneration, compared to the various polymeric compounds formed during the thermal decomposition of ammonia borane.The hydrogen capacity of these BCN systems is still low and their regeneration requires two steps,adigestion reaction to form -NH 2 followed by areduction reaction to form -BH 2 . [45b]   While no single compound is so far able to reversibly store and release the desired amount of hydrogen with minimum energy input and high efficiency, much has been learned through these studies.T he attendant breakthroughs in the synthesis of new molecules and new synthetic methods have enriched the repertoire of methods for the exploration of hydrogen-storage materials.T he large variety of hydrogenrich boron-containing units/compounds,either by themselves or coupled with other units,offer fertile ground for research.

Borate Anions and Anionic Boron Clusters as Building Blocks for Energy-Related Materials
Liquid electrolytes are key components of many electrochemical devices used for the production of renewable energy for energy conversion, storage,a sw ell as transportation. Liion batteries are the current basis for mobile electrical energy and they are an important example of electrochemical devices based on liquid electrolytes. [46] Thed evelopment of Li-ion batteries in the 1980s and their commercial availability since 1991 was crucial for the spread of portable devices such as mobile phones and notebooks.T he capacity of commercial Li-ion batteries is often not sufficient, and they are not ideal with respect to the requirements of asecure power supply,for example,for electrical vehicles.New materials with improved properties are thus needed for mobile energy storage devices. Va rious battery systems based on Li-, Na-, Mg-and other metal-oxygen, -sulfur,a nd -air batteries are under development for mobile applications and flow batteries for stationary use. [46a-c,e,47] Supercapacitors (supercaps) are af urther important class of electrochemical devices for energy storage, which, similar to batteries,r ely on electrolytes as key components. [48] At hird example of electrochemical devices that often contain liquid electrolytes are dye-sensitized solar cells (DSSCs,Grätzel cells). [49] Borate anions and negatively charged boron clusters are widely applied building blocks for liquid electrolytes used in the aforementioned electrochemical devices.B oth types of boron-based ions belong to the class of weak-to mediumcoordinating anions that have been used for the stabilization of highly reactive cations. [50] In particular, anions with halogen substituents bonded to boron and anions with perfluorinated substituents are among the weakest coordinating anions (WCAs) that have been intensively studied and applied. Figure 6s hows examples of polyhalogenated WCAst hat are either tetracoordinate borate anions (e.g. [B(C 6 F 5 ) 4 ] À , [51] [B(CF 3 ) 4 ] À [52] )o ra re some of the most stable known monoand dianionic 12-vertex boron clusters (e.g. [1-CH 3 -closo-1-CB 11 F 11 ] À , [53] [closo-B 12 F 12 ] 2À , [54] and [(CH 3 ) 3 N-closo-B 12 Cl 11 ] À [55] ).
Aw eak interaction between the cations and anions underpins ah igh ion mobility-one of the requirements for salts used as an electrolyte or as acomponent of an electrolyte composition used in electrochemical devices.Afurther prerequisite is as ufficient chemical, electrochemical, and thermal stability of these salts.
In this section we provide ab rief survey of salts with borate anions and negatively charged boron clusters as building blocks of electrolytes for electrochemical devices. Thefirst section contains an overview of salts with tetracoordinate borate anions that are relevant to battery applications. Thes econd part focuses on cyanoborate anions as building blocks for low-viscosity ionic liquids and their use in electrochemical devices.T he third paragraph summarizes energyrelated applications of anionic boron clusters.

Borate Anions:B attery Applications
Lithium tetrafluoroborate Li[BF 4 ]w as studied as ac onducting salt for Li-ion batteries,a sa na lternative to Li-[PF 6 ], [46d] but most of its properties were found to be inferior. Fore xample,i tp rovides al ower ion conductivity than Li[PF 6 ]. Furthermore,i ti sh ydrolytically unstable,s imilar to Li[PF 6 ]. [46c,56] In addition to the tetrafluoroborate anion, an umber of other tetracoordinate borate anions have been studied for applications in battery electrolytes (Figure 7). Thereplacement of the fluorine substituents of [BF 4 ] À by perfluoroalkyl groups led to improved properties,e specially higher conductivities of the Li + salts in solution. Li[C 2 F 5 BF 3 ] (LiFAB, Figure 7) was studied in detail, [ 6 ]. [46d, 56b] Among these salts, lithium bis(oxalato)borate (LiBOB,F igure 7) is the most prominent fluorine-free example.T he disadvantages of LiBOB are its limited anodic stability and its rather low

Angewandte Chemie
Reviews solubility in carbonate solvents.N evertheless,L i BOB has significant potential for application in Li-battery technology. [4b,56b] Lithium difluoro(oxalato)borate (LiDFOB, Figure 7) offers ac ombination of the advantages of both parent salts, Li[BF 4 ]a nd LiBOB,t hat is,t hermal stability and high ionic conductivity in conventional carbonate solvents as aresult of its good solubility. [56b] Several related lithium spiroborates have been synthesized and tested in Li-ion batteries,f or example,l ithium bis(salicylato)borate (LiBSB)a nd lithium bis(1,2-benzenediolato)borate (LiBBB;F igure 7). [56b] Most of the aforementioned spiroborate anions,r elated borate anions,a nd some neutral boranes such as tris(pentafluorophenyl)borane (B(C 6 F 5 ) 3 ), have been employed as additives to electrolytes for Li-ion batteries. [56b, 58] In addition, ionomers with borate moieties have been studied as components for gel and polymer electrolytes,f or example,l ithium poly[oligo(ethyleneglycol)oxalato]borate. [56b] Beyond Li-ion batteries,b atteries based on other metals such as sodium, magnesium, and aluminum are of growing interest. [47,59] Arange of borate anions have been employed as counterions for these types of batteries.Since the demands of the battery are determined by the metal and the type of battery (e.g.metal-oxygen, metal-sulfur,ormetal-air battery), the borate anion has to be chosen carefully.T hus,b orate anions successfully employed in Li-ion batteries are not necessarily components of choice for other batteries.V ery recently,the Mg salt of the perfluorinated bis(pinacolato)borate anion ([B{O 2 C 2 (CF 3 ) 4 } 2 ] À , FPB;F igure 7) was found to be ap romising,c hloride-free component of electrolytes for Mg batteries. [60] 4.2. Low-Viscosity Room-Temperature Cyanoborate Ionic Liquids: Electrolyte Components for Electrochemical Devices Ionic liquids (ILs) based on the tetrafluoroborate anion [BF 4 ] À have been employed as electrolytes and as components of electrolytes in aw ealth of different electrochemical applications, [61] for example of e,62] and Na-ion batteries. [46e] Reasons for the widespread use of tetrafluoroborate-ILs are 1) the availability of salts of the [BF 4 ] À ion and 2) their beneficial properties.M any of them are roomtemperature ionic liquids (RTILs) that possess low viscosities and melting points and provide large electrochemical windows,for example,[EMIm][BF 4 ](EMIm = 1-ethyl-3-methylimidazolium;F igure 8).

Summary and Perspectives
Te tracoordinate borate anions and anionic boron clusters are highly important building blocks for diverse fields of materials chemistry,especially for electrochemical and optoelectronic devices,s uch as batteries,s upercapacitors,a nd solar cells.T heir versatility allows for facile tuning of the properties,a nd thus borate anions will be of increasing importance for the design of advanced conducting salts and ionomers.T his will stimulate research into both metal and organic salts of boron-based anions.

Boron Molecules in OLEDs
Organic light-emitting diodes (OLEDs) are devices that convert electrical energy into light. Since the demonstration Figure 9. The most relevant parent boron clusters with respect to metal-ion batteries (top) and acomplex Mg 2+ salt that contains1 ,7carboranyll igands (bottom, thf = tetrahydrofuran).

Angewandte Chemie
Reviews of low-driving-voltage OLEDs based on tris(8-hydroxyquinolinato)aluminum (Alq 3 )b yT ang and Va nSlyke in 1987, [87] OLED technologies have undergone tremendous progress and have found applications in av ariety of consumer products.T he use of boron-based molecules for applications in OLEDs have been an active research field for more than two decades.T he seminal work of Tang and Va nSlyke certainly inspired much of the early research into boron compounds as emitters in OLEDs,d riven by the need for stable and brighter emitters,especially blue emitters.

Boron-Based Molecules in Fluorescent and Phosphorescent OLEDs
Many examples of highly efficient and stable tetracoordinate boron fluorescent emitters with emission colors ranging from blue to red have been achieved and successfully used in OLEDs.R epresentative structures of tetracoordinate boron compounds as fluorescent emitters for OLEDs are shown in Figure 10 a. TheXand Yatoms are usually heteroatoms such as oxygen or nitrogen atoms.Insome cases,the Xatom can be acarbon atom of an aryl ring (e.g.phenyl). TheRgroups are typically aryl substituents or fluoride.T he p 1 and p 2 rings are typically five-or six-membered aryl or heteroaryl groups. Te tracoordinate boron compounds for applications in OLEDs have been covered in several reviews, [88][89][90] and thus will not be described further here.
Theu se of three-coordinate boron compounds,n amely triarylboranes,i nO LEDs was pioneered by Shirota and coworkers. [91] They first demonstrated the use of as eries of dithienyl-or trithienyl-linked bis(triarylborane) molecules, with the general structure shown in Figure 10 b, as highly efficient electron-transport materials in OLEDs. [91a] This stems from the empty p p orbital of the boron center, which undertakes p p -p conjugation with the neighboring aryl units, hence promoting electron transport. Many other triarylboranes or boron-embedded p-conjugated systems have since been demonstrated to be effective electron-transporting materials in OLEDs. [92] Subsequent studies by Shirota and co-workers revealed that the p-conjugated thiophene-bis-(triarylborane) compounds are also bright blue emitters and can be used as blue emitters in OLEDs. [91b] TheShirota group was also the first to demonstrate the use of triarylborane compounds as hole-blocking materials in OLEDs. [91c] When combining an electron-accepting triarylboron unit with acon-jugated electron donor unit such as an amino group,t he resulting "bipolar" molecules shown in Figure 10 ch ave an intense intramolecular charge-transfer transition from the donor to the boron center that leads to bright luminescence. TheS hirota group was the first to recognize the potential of such bipolar donor-acceptor boron compounds as emitters in OLEDs.T hey successfully demonstrated the use of several bipolar boron compounds in efficient OLEDs with tunable emission colors from blue-green to yellow,b ased on the extent of conjugation of the p-skeleton. [93] This early work inspired intensive subsequent research efforts into donoracceptor-based boron compounds for OLEDs. [94] Another important application of triarylboranes is their use as functional groups in phosphorescent transition-metal emitters for OLEDs.T he triarylborane unit has been shown by several research teams to be highly effective in enhancing the phosphorescence quantum yields of metal complexes, thereby leading to high-performance OLEDs.D etailed accounts are provided in several reviews/book chapters. [95]

Boron-Based TADF Emitters
Them ost significant recent development in boron-based molecules for OLEDs is their use as thermally activated delayed-fluorescence (TADF) emitters,w hich is described here in detail. Thes eminal discovery of TADF emitters and their use in OLEDs by Adachi and co-workers [96] ignited intense international interest and efforts in TADF research. [97] Like phosphorescent emitters,T ADF emitters have the advantages of capturing both singlet and triplet excitons as photons in OLEDs,t hus greatly increasing the internal quantum efficiency of OLEDs.T his is especially important for achieving high-efficiencyb lue OLEDs. [98] Blue emitters, especially triplet blue emitters,are well-known to suffer from low stability in the excited state and to have arelatively short operating lifetime in OLEDs because of the high excitation energy applied to these molecules. [99] Thed evelopment of blue phosphorescent emitters for OLEDs that possess high efficiency and high stability has been alongstanding challenge in OLED research and development. As illustrated in Figure 11, for phosphorescent emitters,b oth singlet and triplet excitons are harvested through intersystem crossing (ISC) through the phosphorescence decay channels.T herefore,toachieve blue emission (l phos % 450 nm), it is necessary to excite the molecule to the singlet excited state (S1 usually) in the near-UV region (l ex < 400 nm). ForT ADF,b ecause both the singlet and triplet exciton excitons are harnessed through the fluorescence decay channels (l FL % 450 nm) by ar everse intersystem crossing (RISC) process promoted by thermal energy,only the excitation of the molecule to the first singlet excited state is necessary.A saresult, the excitation energy,inprinciple,could be considerably lower than that for phosphorescent emitters,which may be beneficial for achieving efficient and stable blue emitters.I na ddition to the possibility of addressing the stability issue of blue emitters, TADF emitters do not require the presence of precious metals in the molecules,which could greatly reduce the manufacturing cost of blue OLEDs. [98] Figure 10. Representative structures of tri-and tetracoordinateboron compounds used as either fluorescentemitters or charge-transport/ blocking materials in OLEDs.

Reviews
To achieve efficient TADF emission, the RISC process must be efficient at ambient temperature,which requires the singlet-triplet energy separation (DE ST )tobeless than 0.2 eV. One commonly used strategy is to employ donor-acceptor systems in which the donor and the acceptor units are spatially separated with an approximate orthogonal arrangement to minimize exchange interactions and spatial overlap of the HOMO and the LUMO so that the DE ST can be minimized. [96][97][98][99][100] As triarylborane units are well-known to be very effective and stable electron acceptors with excellent structural tunability,i ti sn ot surprising that many highly effective TADF emitters use at riarylboron moiety as the electron-acceptor unit, and many such examples have been demonstrated recently. [101][102][103][104][105] Representative examples of this type of boron-containing TADF emitter (type (a)) are shown in Figure 11 a.
Boron-based TADF emitters in this category are in fact similar to the donor-acceptor fluorescent emitters (Figure 10 c) reported by Shirota and others,b ut with ag reater steric constraint on the donor and acceptor geometry to minimize the DE ST .B oron-based TADF emitters in this system typically contain one p-linker (most commonly used are phenyl or mesityl) between ad onor unit and the boron acceptor unit. Thetwo aryl substituents on boron are typically bulky groups such as mesityl or 2,4,6-triisopropylphenyl groups.T he donor units are usually bulky and highly constrained nitrogen donors such as carbazole or acridine (see examples 1-4 in Figure 11 a). [102,[104][105][106] Highly efficient blue-green/green OLEDs with external quantum efficiencies (EQEs) greater than 20 %h ave been achieved with this class of TADF emitters.I ti se specially noteworthy that OLEDs employing molecules 2 (R = H, 2a; t-Bu, 2b)a s emitters achieved record-breaking EQEs for TADF emitters, with EQE max values of 37.8 and 32.4 %, respectively. [104] This was attributed to the flat geometry of molecules 2,w hich causes alignment of their molecular axes with the OLED substrate surface upon vacuum deposition, thereby yielding thin films of the emitters with horizontally oriented dipoles. This results in an enhanced light output coupling factor, [107] which greatly increases the EQE max of the device while simultaneously decreasing its efficiencyr oll-off.R ecently, several examples of boron-based TADF emitters that rely on aD BNAu nit (5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene,F igure 11 b, top,with X = O) as the acceptor (e.g. 3 and 4)h ave also been demonstrated to produce highly efficient blue or sky-blue OLEDs. [105,106] In particular, compound 3, which contains three carbazole donor groups,h as been demonstrated to produce exceptionally bright and efficient blue OLEDs with an EQE max of 38.15 % AE 0.42 %a nd al ow efficiency roll-off. [106] In addition to the intrinsically high emission quantum efficiencyo f3,i ts flat and highly rigid structure,w hich leads to its horizontal orientation on the substrate,i sb elieved to greatly enhance the light output coupling and the overall EQE of the OLEDs. [106] Thes econd type of boron-based TADF emitters (Figure 11 b) was discovered by Hatakeyama and co-workers. [108][109][110] In contrast to type (a) molecules and the commonly used donor-acceptor TADF emitters,i nt ype (b) boron TADF emitters,t he donor and acceptor units are not physically located at different parts of the molecule.I nstead, the donor and the acceptor units in type (b) molecules are on the same domain of the molecule,w hich has ah ighly rigid, fully conjugated/aromatic structure.H owever,a si llustrated in Figure 11 b, the HOMO and the LUMO in this type of molecules involve contributions from two different sets of atoms,a lbeit on the same domain, which leads to as patial separation of the HOMO and LUMO and as mall DE ST .A s the atoms in the HOMO and the LUMO of this type of molecule exhibit opposite resonance features,H atakeyama and co-workers defined this type of TADF emitter as multiple resonance effect (or multiresonance effect) emitters. [108] Multiresonance TADF emitters have several key advantages over the classic donor-acceptor emitters.F irstly,b ecause the emission in multiresonance emitters originates from ahighly rigid p-conjugated/aromatic unit, it has am uch higher oscillator strength, thus they are often much more efficient than type (a) emitters,w hich are well-known to have av ery small oscillator strength and abroad charge-transfer emission maximum, owing to the orthogonal geometry of the donor and the acceptor units.S econdly,t ype (b) emitters have very small Stokes shifts,amuch narrower emission band with the full-width at half-maxima (FWHM) typically much less than 50 nm, which is highly beneficial for achieving OLEDs with ahigher color purity,compared to the type (a) emitters. [109] A recent computational study by Olivier and co-workers using highly correlated quantum-chemical calculations indicated that the unique TADF features displayed by type (b) boron Figure 11. Representative examples of donor-acceptor boron-based TADF emitters (a) and multiresonance boron-based TADF emitters (b), as well as ad iagrami llustrating the differenceb etween TADF and phosphorescence (c).

Angewandte Chemie
Reviews emitters can be ascribed to the short-range reorganization of the electron density that takes place upon electronic excitation of the multiresonant structures. [111] Them ultiresonance effect was first demonstrated by Hatakeyama and co-workers for the DBNAmolecule shown in Figure 11 b. [108] Thesubsequent study of related aza-DBNA molecules by Hatakeyama and co-workers demonstrated that substitution by adonor group at aposition para to boron can greatly enhance the multiresonance effect of the molecule and its performance in OLEDs. [109] Forexample,replacing R 2 in 6 by ad iphenylamino group produced ap ure blue TADF emitter that has an EQE max of 20.2 %, much higher than that based on the analogue without the amino substituent (EQE max = 13.5 %). Introducing ac arbazole donor at the alternative para position leads to molecule 7.T he blue OLEDs based on 7 exhibit an EQE max of 32.1 %and ahighly suppressed efficiency roll-off. [112] Thet hird type of boron-based TADF emitters (type (c)) involve tetracoordinate boron molecules with structural features similar to those depicted in Figure 10 a, except with the inclusion of an amino/N-heterocyclic donor group in the aryl substituents (R). Several examples of type (c) TADF emitters have been reported recently,w hich achieve TADF properties by taking advantage of the charge-transfer transition from the donor-appended aryl groups to the pconjugated chelate backbone. [113][114][115][116] Thep erformance of OLEDs based on tetracoordinate boron TADF emitters is generally not as impressive as the triarylboron-based emitters shown in Figure 11. Anotable example was an efficient NIR TADF emitter based on ab oron difluoride curcuminoid complex (5)r eported recently by Adachi and co-workers. [116] TheOLEDs based on this NIR emitter display EQE max values near 10 %and tunable l em from 700 to 780 nm, which is very impressive for NIR OLEDs.

Summary and Perspectives
In summary,the recent discoveries of boron-based TADF emitters demonstrate that the incorporation of ab oron unit can greatly improve the performance of OLEDs.B oronbased molecules will likely play an important role in practical and high-performance OLEDs,especially blue OLEDs.

Conclusion
Ther ich chemistry of boron is evidenced by its ability to form am yriad of molecules that are proving to be highly interesting in research related to energy conversion and storage.B oron forms unique interactions with itself through single,double,ortriple bonds,orwith other elements such as in FLPs,a ll of which allow effective activation of small molecules.T he combination of boron with hydrogen leads to aw ide range of hydrogen-rich molecules that hold potential as hydrogen-storage materials.A sac onsequence of its electron-deficient nature,b oron forms as eries of highly chemically,e lectrochemically,a nd thermally stable anions and negatively charged clusters.T he anionic boron clusters feature unique multicentered bonding,a nd are currently finding applications as building blocks for various materials such as very low viscosity room-temperature ionic liquids (RTILs) used in electrochemical devices.B oron-bearing molecules also enable rich donor-acceptor tunability,w hich is critical to obtain OLEDs with high efficiencies and light of desired wavelengths.
Thei mpressive recent advances in the synthesis of novel boron-containing molecules and the diverse materials derived therefrom, combined with as ignificantly improved understanding of the properties of these boron-based molecules and materials,p rovides ap owerful opportunity for the further exploration of boron as ak ey element in the field of energy research.