DOI:10.1002/chem.201800517 Minireview

& Transmutation ElectronicTransmutation (ET): Chemically Turning One Element into Another XinxingZhang,*[a] Katie A. Lundell,[b] JaredK.Olson,[b] Kit H. Bowen,*[c] and Alexander I. Boldyrev*[b]

Chem. Eur.J.2018, 24,9200 –9210 9200  2018 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Minireview

Abstract: The concept of electronic transmutation (ET) de- the theoretical andexperimental fronts. Examples on the ET picts the processesthat by acquiring an extra electron, anel- of Group 13 elements into Group 14 elements, Group 14 ele- ement with the atomicnumber Zbegins to have properties ments into Group 15 elements, and Group 15 elements into that were known to only belongtoits neighboringelement Group 16 elements are discussed. Compoundsand chemical with the atomic number Z+1. Based on ET,signature com- bondingcomposed of carbon,silicon, germanium, phospho- pounds and chemical bonds that are composed of certain rous, oxygen and sulfur now have analoguesusing transmu- elements can now be designed and formed by other elec- tated boron, aluminum, , silicon, nitrogen, and phos- tronically transmutated elements. This Minireview summariz- phorous. es the recentdevelopments and applicationsofETonboth

Introduction ples on both the theoretical and experimental fronts, and out- look for wider applicationsand future research directions of Despite the successofthe valence-isoelectronic concept in this new concept. many examples of predicting reactivity,structures and exis- tence of compounds, such asimple electroncounting rule can 1. Electronic Transmutation of Group 13 nevertheless easily fail. For instance, being valence-isoelectron- Elements into Group 14 Elements ic to benzene (C6H6), the planar D6h silabenzene Si6H6 is not even aminimum on its potentialenergy surface.[1] The Group 13 elements, such as boron,[19] aluminum,[20] and galli- deformation from this planar structure to its real globalmini- um,[21] are well known to form clustered compounds through mum is attributable to the pseudo-Jahn–Teller effect. In view multicenter bonding, which is largely due to their electron-de- of this, astricter and narrower electronic transmutation (ET) ficient(s2p1 electron configuration) nature. The simplestexam- [2] [22] [23] concept was proposed in 2012, stating that by acquiring an ples are the (B2H6), dialane (Al2H6), and digallane [24] electron, acertain element with the atomic number Zbegins (Ga2H6) , where each of the two bridge hydrogen to behavesimilarly as its neighboringelement Z+ 1. For exam- atomsparticipates in forminga3-center 2-electron (3c–2e) M- ple, the transmutated boron,BÀ ,may well be functioning simi- Hbridge-M (M=B, Al, Ga) bond. Rather distinct from the larly to carbon.The similarities between the transmutated ele- Group 13 elements, Group 14 elements such as carbon, silicon, ment Zand the targeted elementZ+1could range from the and germanium usually form chain or ring compounds as a chemicalbonding they possess to the geometries of the com- result of spn (n= 1, 2, 3) hybridizations. With the difference of poundsthey form, so that many key features that were only one electron, Group 13 and 14 elements behavevery dif- thought only belong to element Z+ 1can now belong to ele- ferently in chemicalbondingand the compounds they can ment Z. Alchemists once spent great efforts in transmutating form. In this section, we discuss the theoretical and experimen- commonelements into precious others, which now we know tal advances of the electronic transmutation of Group 13 ele- is not possible merely with chemistry,but based on ET,the ele- ments into Group 14 elements, where the former yield similar ment Zcan now be chemically “turned into” elementZ+ 1. chemicalbondingand compounds as the latter after transmu- After the proposal of the ET concept, aplethora of successful tation. examples, including the transmutation of Groups 13, 14, and 15 elements into Group 14, 15, and 16 elements, have been re- 1.1. ET of boron into carbon ported.[2–18] In this Minireview,wewill summarize these exam- It is well-known that carbon forms alarge varietyofhydrocar- bons, including aromatic arene, , , and - [a] Prof. Dr.X.Zhang based compoundsthat feature chain (homocatenation)orring Collaborative Innovation CenterofChemical Science and Engineering College of Chemistry, Nankai University structures. Boron hydrides, or , on the other hand, Tianjin 300071 (P.R.China) prefer clustered structures.Inorder to satisfy the octet rule, E-mail:[email protected] the insufficient electrons in boronlead to the formation of 3c– [b] K. A. Lundell, Dr.J.K.Olson, Prof. Dr.A.I.Boldyrev 2e bonds,based on whichanextensiontomolecular orbital Department of Chemistry and Biochemistry (MO) theory was developed, known as the polyhedralskeletal Utah State University [19] 0300 Old Main Hill, Logan, UT,84322-0300 (USA) electronpairtheory (PSEPT) or simply Wade–Mingos rules. In E-mail:[email protected] this section, we present the successful examples of homo- [c] Prof. Dr.K.H.Bowen catenated and aromatic boron compounds when boron is elec- Departments of Chemistryand Material Science tronically transmutated into carbon. Johns Hopkins University We first discuss the theoretical predictions of homocatenat- Baltimore, MD, 21218 (USA) E-mail:[email protected] ed boron hydrides where the ET concept was firstly pro- [2] The ORCID identification number(s) for the author(s) of this articlecan be posed. follow the molecular formula CnH2n+2,sug- found under https://doi.org/10.1002/chem.201800517. gesting that their boron analogues should have the formula of

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n (BnH2n+2) À,inwhich each boron atom obtains one negative case of Li2B2H6 unless noted. The searchfor the global mini- charge to resemble carbon. These negative charges can be mum structure of the Li2B2H6 molecule was performed using provided by certain electron donors, preferably by alkali metals the Coalescence Kickprogram written by Averkiev.[26] Initially such as Li, so the first obvious example is the BH4À kernel in these calculations wereperformed at arelative low/cheap level [27] the LiBH4 salt, which is isoelectronic and isostructural to CH4, of theory (B3LYP/3–21G )tosearch for alarge quantity of iso- 1 and already commerciallyavailable. Here we focus on the ho- mers, and those lowest energy isomers(DE<60 kcalmolÀ ) mocatenation of boron, and Li2B2H6 seems to be asimple can- were then reoptimized and frequencies were calculated at didate to start with. However,the B Li, H Li, and B Hbond B3LYP/6–311++G**[28] and CCSD(T)/6–311++G**[29] and À À À dissociation energies are not too far away from each other,[25] single point calculations were performed using the RCCSD(T)/ one would expect arelatively flat potential energy surface and aug-cc-pVXZlevels of theory (X=Dand T).[30] The final relative many possible isomers that are close in energy for this mole- energies were obtained through extrapolationoftotal energies cule, which makes athorough, unbiased geometrical searchin- at the CCSD(T) level of theory to the complete basis set limit dispensable but very expensiveinorder to find the real global (CBS) using the Truhlar formula[31] (CCSD(T)/CBS//CCSD(T)/6– minimum. Here we present the detailedcalculation methods 311++G**) and corrected for zero-pointenergies calculated used in reference [2] in order to set an example for the search at CCSD(T)/6–311++G**. Chemical bonding analysis (B3LYP/ of the globalminimum of electronicallytransmutated mole- 6–311++G**) wasperformed using the AdNDPmethod.[32] All cules. The computational methods for other ET molecules in calculations were done using 03 and GAUSSI- the rest of this minireview are more or less the same as the AN 09[33] packages. Molekel 5.4.0.8 was used for MO ,[34] and MOLDENt3.4[35] was used for molecular structure visualization.

Figure 1A presents the globalminimum of Li2B2H6,which containsone 2c–2eB B s-bond and six 2c–2c B H s-bonds. Xinxing Zhang obtained his B.S. in chemistry À À at Fudan University in Shanghai, China These s-bonds are furtherconfirmed by AdNDP analysis (Fig- (2009), and his PhD at the Johns Hopkins ure 1D). From the structure, the B2H6 kernel is indeed very sim- University in the Kit Bowen research group ilar to the analog, ethane. However,the interac- (2015).Since 2016, he has been workingasa tion between theLiatoms and the B H kernel appearstobe postdoctoral scholaratCaltech in the J. L. 2 6 Beauchamp research group. His research in- critical to determine the existence of electronic transmutation. terests cover gas phase cluster reactivity and Calculated effective charges are + 0.94 e on each Li atom and j j spectroscopy, as well as the physical chemis- 1.88 e on the B H kernel. Thus, the interaction between À j j 2 6 try and biochemistry at the air–water interfa- the Li and the B H kernel is ionic, and the B H moiety is ces. He startedhis independent research 2 6 2 6 career as aprofessor at Nankai University, Tianjin, China, in 2018.

Katie Lundell received her B.S. in chemistry Kit H. Bowen, Jr.received his B.S. in chemistry and biochemistry at Idaho State University in at the University of Mississippi (1970), and his Pocatello, Idaho (2015). Since 2016 she has M.S. (1973) and Ph.D.(1977) in chemistry at been agraduate student in the Alex Boldyrev HarvardUniversity.Hewas also an NSF post- research group. Her research interests are the doc at HarvardUniversity.Dr. Bowen is now study of electronic transmutations and devel- the E. Emmet Reid Professor of Chemistry at opment of newchemical bonding models in the Johns Hopkins University.His research uti- clusters. lizes negativeion photoelectronspectroscopy and surface deposition techniques both of whichare applied to size-selected cluster anions.

Jared K. Olson received hisB.S. (2002) and Alexander I. Boldyrev received his B.S./M.S. in M.S. (2004)inphysics from the University of chemistry from Novosibirsk University(1974), Utah, MBA (2006) from Westminster College, his Ph.D. in physical chemistry from Moscow and Ph.D. (2010) in Physical Chemistry from State University (1978), and his Dr.Sci. in Utah State University. He is currently aSr. chemical physics from Moscow Physico-Chem- ProgramManager in the PropulsionSystems ical Institute (1987). He is currently aProfessor DivisionatOrbitalATK. His scientific interest at the Department of Chemistryand Biochem- is in chemical bonding models, the rational istry at Utah State University.His current sci- design of new molecules, and propulsion sys- entific interest is the development of new tems technology. chemical bondingmodels for clusters, mole- cules, solid state materials, novel two-dimen- sional materials and other chemical species, where conventional chemical bondingmodels are not applicable.

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Figure 1. Calculated global minimum structures of Li2B2H6 (A), Li3B3H8 (B),

Li4B4H10 (C), and the chemical bondingofLi2B2H6 recovered by the AdNDP analysis (D).

2 indeed in the form of B2H6 À.Inother words, electronic trans- mutationhas occurred.

From Li2B2H6 to Li3B3H8 and Li4B4H10,the expensiveness of the globalminimum search increases exponentially with the number of atoms involved, hence,the authors did not attempt to search for the global minimum, instead, they examined whether or not the - and n-butane-shaped molecules were local minima on their potential surfaces. The structuresin Figure 1B and 1C indeed display similar structures as propane and n-butane, manifesting the success of the electronic trans- mutationinthe homocatenation of boron hydrides. We next discussthe electronically transmutated aromatic boron hydrides, analogues of arenes. Benzene (C6H6), cyclopen- + [36] tadienide (C5H5À), tropylium (C7H7 ), and naphthalene (C10H8) are planar aromatic , following the 4n+2aroma- [19] 2 4 ticity rule. Closo-boranes, BnHn À or BnHn 2 À,feature poly- À hedral structures (Figure 2C,F,I,L). By donating negative chargestothese 3-dimensional closo-boranes, is it possible to “flatten” them into aromatic 2-dimensional arene analogues? Alexandrova and Boldyrev[3] examined the calculated global minimaofB6H6Li6 (Figure2A, B), B5H5Li6 (Figure 2D,E), B7H7Li6

(Figure 2G,H), and B10H8Li10 (Figure 2J,K), which are isoelec- + tronic to C6H6,C5H5À ,C7H7 and C10H8,respectively.All of these

BnHn kernels are planar, and the formal negative charges on [37] BnHn revealed by natural bond orbital(NBO) analysis are 1 close to -n,indicating that boron atoms in these molecules are Figure 2. Optimized structures of B6H6Li6 (D2h, Ag), side view (A);B6H6Li6 n 1 1 1 transmutated into carbon. More importantly,are these B H À (D2h, Ag), front view (B);B6H6Li2 (D3d, A1g)(C);B5H5Li6 (Cs, A’), side view (D); n n 1 2 1 1 B5H5Li6 (Cs, A’), front view (E);B5H5 À (D3h, A1’)(F);B7H7Li6 (C1, A), side view kernels aromatic?Nucleus-independent chemical shifts (NICS) 1 2 1 (G);BH Li (C , A), front view (H);BH À (D ,1A )(I);B H Li (D , A ), side [38] 7 7 6 1 7 7 5h 1’ 10 8 10 2h g indices were introduced by Schleyer as asimple probe for 1 4 1 view (J);B10H8Li10 (D2h, Ag), frontview (K);and B10H8 À (D2h, Ag)(L). p-molecu- 6 1 aromaticity.The NICS index at the center of the B6H6 À kernel is lar orbitalsofB6H6Li6 (D2h, Ag)(M). 7.2 ppm, very close to that of benzene ( 8.0ppm) calculated À À at the same level of theory.NICS indicesfor other speciesare 2.0 ppm for B H Li , 93.2 ppm for B H Li ,and 54.2 ppm Wade–Mingosrule and (4n+2) p Hückel rule using the elec- À 5 5 6 À 7 7 6 À for B10H8Li10,providing evidence of their aromaticity.Figure 2M tronic confined space analogy(ECSA)method, in whichthe presentsthe three aromatic p-molecular orbitals of B6H6Li6,all electronic transmutationconcept turned out to be akey of which are similar to that of benzene. factor. Similarly,Tiznado and co-workers[4] theoretically investigated We next present two extreme examples of transmutated + the Li6(BH)5 and Li7(BH)5 clusters, and discovered that the boron,the aromatic 2D boron films, which can be viewed as 6 B5H5 À kernels in the global minimaare the transmutated ana- the analogue of graphene. Graphene is one of the allotropes logues of cyclopentadienide (C5H5À). Solµ,Teixidor and co- of carbon consistingofasingle planar layer of carbon atoms workers[5] examined the intrinsic relationship between the arranged in ahexagonal lattice.[39] Graphene’sboron “cousin”

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Figure 3. (A) Crystal structure of MgB2 ;(B) Scanning tunnellingmicroscopy image of agraphene-like borophene sheet on an Al substrate.

n needs to have the stoichiometry of Bn À.Using powder X-ray diffraction, Akimitsu and co-workersconfirm that there are ex- traordinary 2D layers of honeycombstructures composed of boron atoms with Mg atoms located above and below the boron hexagon (Figure 3A)inthe well-known high-tempera- [6] ture superconductor MgB2. The 2D-lattice of boron appears to be structurally the same as graphene. Even though not clearly stated by the authors, acomplete charge transfer from 2+ 2 Mg to Binthe form of Mg B2 À due to the large electronega- tivity differencebetween Mg and Bcan be anticipated, and the electronic transmutation principle apparently plays akey n Figure 4. Globalminimum structures of Li2Al2H6 and Li3Al3H8 (A), and the role in forming this 2D structure. The Bn À sheet might be the chemical bonds of Li2Al2H6 (B) and Li3Al3H8 (C) recovered by the AdNDP anal- reason for this material’shigh-temperature superconducting yses. behavior.During the review process of this minireview, the successful preparation of ahoneycomb, graphene-like boro- phene (Figure 3B)byusing an Al surface as the substrate and full electron transferfrom Li to Al, therefore ET indeedoccurs [40] electron donorwas reported. The authors point out that in Li2Al2H6.Figure 4C exhibits the chemical bonds of Li3Al3H8, nearly one electron charge is transferredtoBfrom Al, which including two Al Al s single bonds and eight Al H s single À À makes agreat example of ET in the application of solid state bonds,and all the occupation numbersare more than 1.9 e , j j chemistry.Without electron transfer,the 2D boron film on Ag too. The natural population analysis(NPA) charges of Li and surfaceotherwise displays very different structure.[41] the Al H kernel are + 0.86 and 2.60, manifesting that ET is 3 8 À also present in this case. For the first time it has been shown that ET enablesaluminum atoms to homocatenate with the 1.2. ET of aluminum into silicon formation of /alkane-like species. As aresult of the development of modern gas-phase spectros- To experimentally investigate the existence of these exotic copy techniques, many aluminum hydrides (alanes) have been homocatenated aluminum hydrides in the gas phase, it is discovered using the pulsed arc cluster ionization source bettertostudy them in the form of ions. For the LinAlnH2n+ 2

(PACIS) and characterized using the anion photoelectron spec- molecules, one could study the Lin 1AlnH2n+2À anion by losing [20] + À troscopymethod. Without ET,aluminum hydrides prefer one Li counter ion from LinAlnH2n+2.When n=1, AlH4À ,the polyhedralstructures,following the Wade–Mingosrule just like simplest monosilane/methane analogue, was first examined by boranes.[20a] Similar to the ET of boron into carbon, the ET of anion photoelectronspectroscopy[43] in the gas phase. The ver- aluminuminto silicon also involves electron donation to alumi- tical detachment energy (VDE) of AlH4À is as high as 4.4 eV,in- num to make AlÀ . dicating that it is very stable. Anion photoelectron spectrosco- The first ET example of aluminum is the homocatenation of py is conducted by crossing amass-selected beam of negative aluminumhydrides. The stoichiometry LinAlnH2n+2 wasattempt- ions with afixed-frequency photon beam and energy-analy- ed to theoretically test the viability of ET for aluminum.Fig- sing the resultant photodetached electrons. It is governed by ure 4A presents the globalminimum structures of Li2Al2H6 and the energy-conserving relationship,hn= EBE+EKE, in whichhn [7] Li3Al3H8 and they do have similar structures as corresponding is the photonenergy,EBE is the electron binding (transition) [42]/ethane and trisilane[42]/propane. Figure 4B shows energy, and EKE is the electron kinetic energy.The anionpho- the chemical bonds of Li2Al2H6 revealed by AdNDP,including toelectron spectrometer,which has been described previous- one Al Al s-bond and six Al H s bonds, and all of the occupa- ly,[44] consists of one of many kinds of ion sources, alinear À À tion numbersare more than 1.9 e ,indicating effective single time-of-flight mass spectrometer,amass gate, amomentum j j bonds. The natural population analysis (NPA) charges of Li and decelerator,apulsed Nd:YAG photodetachmentlaser,and a the Al H kernel are +0.89 and 1.77, suggesting an almost magnetic bottle electron energy analyzer.Photoelectron spec- 2 6 À Chem.Eur.J.2018, 24,9200 –9210 www.chemeurj.org 9204  2018 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Minireview

tra were taken with 193 nm (6.42 eV) photon energy and cali- should be the electron affinity (EA) of Li2Al3H8.The first experi- brated against the well-known photoelectron spectrum of mental VDE, the energy difference between an anion andthe [45] CuÀ . The AlH4À cluster anions were generated in apulsed corresponding neutralspecies at the geometry of the anion, arc cluster ionization source (PACIS). During operation, a corresponds to the peak position of the band X, 2.70 eV.The pulsed valve backed by 200 psi of UHP hydrogen is openedfor width of the band Xsuggestsanappreciable geometry change about 200 microseconds and fills aregion between acopper between the ground state of Li2Al3H8À and that of its neutral. anode andgrounded aluminum cathode. A30microseconds In the 266 nm spectrum, asecond band (A) at the higher EBE long, 180 Vpulse is applied to the copper anode that discharg- end peaks at 4.32 eV,corresponding to the transition from the es through the hydrogen gas and subsequently vaporizes the ground state of the anion to the first excited state of the neu- aluminumcathode. The combination of free atomichydrogen tral molecule. More importantly,can the Li2Al3H8À cluster main- and vaporized aluminumisentrained with the remainingmo- tain the trisilane/propane-like structure after losing the Li+ lecular hydrogen and carriedalonga20 cm flow tube where it counter ion compared to the neutralLi3Al3H8?Athorough un- reacts, cools, and forms AlH4À ,which is then extracted and biasedtheoretical search finds that the global minimum struc- mass-selected before photodetachment. ture of Li2Al3H8À still possess the chain structure (Figure 5B), Li Al H À ,the aluminumanalogue of trisilane/propane, was and chemical bond analysis does show that the two Al Al 2 3 8 À also interrogated by anion photoelectron spectroscopybut bonds and the eight Al Hbonds are s-bonds with occupation À generatedinadifferent laser vaporizationsource.[8] Briefly,an numbersmore than 1.7 e .The vertical electronic transition j j aluminum rod was coatedbyavery thin layer of LiAlH4 calculations from the anion to the corresponding neutral powder, and then ablated by apulsed Nd:YAG laser beam op- match the Xand Apeaks, indicating that the experimentally erating at awavelength of 532 nm. The resulting plasma was observed clusterisindeed the calculated global minimum. cooled by supersonicallyexpanding aplume of helium gas NPAcharges also show asignificant electron transfer from the from apulsed gas valve (backing pressure of 100 psi). Nega- Li atoms to the Al H kernel. The discoveryofLiAl H À in the  3 8 2 3 8 tively charged anions were then extracted into the spectrome- gas phase makes the first successful experimentalexample of ter prior to mass selection and photodetachment. Figure 5A the homocatenationofaluminum. presentsthe photoelectron spectra of Li2Al3H8À taken with The above discussions are the ET of aluminumhydrides into 355 nm (3.49 eV) and 266 nm (4.66 eV) photon energies. Both saturated silane/alkane analogues, and an obvious question is spectra have an EBE band (X) starting from 2.20 eV and peak- that can one generate unsaturated aluminumhydrides with  2 ing at 2.70 eV.Incase of asufficientFranck–Condon overlap the ET concept, such as an Al=Al doublebond in the Al2H4 À between the ground state of the anion and the ground state kernel? Silicon hydrides are known to have homodinuclear of the neutral, the threshold of the first EBE band ( 2.20 eV) double bonds, such as that in the Si H molecule. The Al=Al  2 4 double bond has been otherwise notoriously difficult to syn- thesize. Astable neutralcompound with an Al=Al double bond was synthesized by Inoue and co-workers using bulky li- gands very recently.[46] For the gas phase study,the designed [9] ion is LiAl2H4À , which was generated and characterizedwith

the same methods as Li2Al3H8À .The measured anion photo-

electron spectrum of LiAl2H4À is presented in Figure 6A,and three EBE bands were observed (X, X’ and X“), among which X

Figure 6. Experimental photoelectron spectrum of LiAl2H4À using355 nm laser (black line), Gaussian fitting of isomers Iand II (red and blue dotted lines),and calculated stick spectra of isomers Iand II (red and blue vertical

Figure 5. Anion photoelectron spectraofthe Li2Al3H8À anion taken with 355 lines) (A);the structures of the two lowest-energyisomersIand II (B), and and 266 nm photons (A) and its global minimumstructure and the chemical the chemicalbonds of the global minimum structure recovered by the bonds recovered by the AdNDP analysis (B). AdNDP analysis (C).

Chem. Eur.J.2018, 24,9200 –9210 www.chemeurj.org 9205  2018 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Minireview belongs to the globalminimum Isomer I, X’ and X” belong to the second lowest lying IsomerII(Figure 6B). The next ques- tion is whether the Al=Al double bond exists in the global minimum structure IsomerI?AdNDP analysis showninFig- ure 6C presentstwo 2c–2e s-Al Hbonds (ON=2.00 e ), two À j j 3c–2e s-Li-H-Al bonds (ON=1.97 e )(these four bonds are j j analogoustothe s-Si Hbonds in Si H ), one s-3c–2e Al-Li-Al À 2 4 bond (ON=1.99 e )(an analogue of the s-Si Si bond in j j À Si H ), and one p-Al Al bond (ON=2.00 e )(an analogue of 2 4 À j j the p-Si Si bond in Si2H4). In order to claim the presence of À Figure 7. Structuralevolution and charge on the Si moiety with increasing the Al=Al double bond, one needs to evaluate how much the 4 numberofLiatoms in the LinSi4À series (n=0–5). lithium atom contributes to the s and p-3c–2e Al-Li-Albonds. AdNDP reveals that the s-3c–2e Al-Li-Al bond (ON= 1.99 e ) j j can be seen as one s-2c–2e Al Al bond (ON= 1.87 e )since has severaltypes of allotropes,[42] one of whichisthe white À j j the contribution of the lithium atom to this bond is as small as phosphorus,orsimply tetraphosphorus (P4), existing as mole- 0.13 e .The p-3c–2e Al-Li-Al bond (ON= 2.00 e )can be seen cules composed of four atoms in atetrahedral structure. In j j j j as one p-2c–2e Al Al bond (ON= 1.65 e ). That gives the order to transmutate Si into P, theoretical investigations using À j j (1.87+ 1.65)/2 =1.76 bond order for Al=Al double bond in the Li as the electron donorhave been attempted.[13,14] Figure 7[14] cluster.The optimal bond length between the two Al atoms in presentsthe structural evolution of the globalminimaand

LiAl2H4À structure is 2.46 Š (PBE0/6–311++G**), which is charges on the Si4 moiety with increasing number of Li atoms shorter than the single Al Al s-bond (2.59 Š,PBE0/6–311++ in the LinSi4À series (n=0–5). According to the ET designing À 4 G**) in the H2AlAlH2 molecule and the single Al Al s-bond principle, the first Si4 À kernel mimicking P4 should occur in the 2 [47] À (2.55 Š)inthe H3AlAlH3 À structure. The appreciably Li3Si4À cluster.Its global minimumindeed shows atetrahedral shorter Al Al bond length and the bond order indicate that Si4 moiety,and the NPAcharge on Si4 is 3.43 e ,making it À 4 À j j there is indeed adouble bond between the two aluminum effectively aSi4 À cluster. Surprisingly,byadding more Li atoms atoms. Additionally,the LiAl2H4À clusterisslightly distorted to Li3Si4À ,the NPAcharges on the Si4 moieties in the Li4Si4À from the planar structure, which is also the case in the Si2H4 and Li5Si4À clusters remainaround 3.5 e (Figure 7), suggest- [48] À j j molecule due to the pseudoJahn–Teller effect. ing that Si4 in theseclusters have astrong tendency to main- 4 tain the ET structure, tetrahedral Si4 À.The experimental obser- 4 vations of the Si À kernel have been achievedinheavieralkali 1.3. ET of gallium into germanium 4 monosilicides, MSi (M =Na, K, Rb, Cs) in the solid state.[51] Germanium hydrides (germanes), even though not in alarge scale, have been synthesized and shown hydrocarbon-like structures.[49] The ET of gallium into germanium enjoys fruitful 3. Electronic Transmutation of Group 15 experimentaldiscoveries.[10–12] Powders of gallium hydride (gal- Elements into Group 16 Elements 3 lane)-containing compounds, such as Cs10H[(Ga3H8) À]3, - 5 3.1. ET of nitrogen into oxygen (KxRb1 x)n[(GaH2) ]n,Rb8[Ga(GaH3)4 À]and Rbn[(GaH2)À]n have À been synthesized and characterized by X-ray diffraction, where The ET of nitrogen into oxygen enjoys both the experimental K, Rb and Cs are used as the electrondonors. In and theoretical developments.In2001, Kniep et al. synthesized 3 [10] [50] [15a, b] [15c] Cs10H[(Ga3H8) À]3, both ET and the Zintl–Klemm concept binary diazenides SrN2 and BaN2. Schnick et al. experi- 3 are utilized to design the compound, and the (Ga3H8) À kernel mentally confirmedthe stabilityofthe first alkali diazenide [11] is isostructural to propaneand Ge3H8.In(KxRb1 x)n[(GaH2)À]n Li2N2 under highpressure and high temperature conditions, [12] À and Rbn[(GaH2)À]n, the [(GaH2)À]n polyanions feature polyeth- where Sr,Baand Li functionasthe electron donors. In these 5 [12] 5 ylene structures, and in Rb8[Ga(GaH3)4 À], the Ga(GaH3)4 À three examples, the existence of the homonuclear dinitrogen 2 kernel has similarstructure as neopentane. In all these exam- anion N2 À,ananalogueofO2,are provenbyX-ray diffraction, ples, each of the Ga atoms obtain one negative charge for the neutron diffraction and infrared spectroscopy.The crystal struc- electronic transmutation. tures of these examples are displayed in Figure 8. In SrN2 and 2 BaN2,each N2 À kernel is surrounded by an octahedron formed 2+ 2+ 2 by six Sr or Ba ions (Figure 8A,8B), in Li2N2,each N2 À 2. Electronic Transmutation of Group 14 kernelissurrounded by acube formed by eightLi+ ions (Fig- Elements into Group 15 Elements ure 8C). The attemptoffinding the ozone analogue, Li N ,was per- 2.1. ET of silicon into phosphorous 3 3 formed theoretically.[15d] The bent ozone-like structure of the 3 In this section, we only discuss one example, the transmuta- N3 À kernel nevertheless is not the global minimum, but it is tion of silicon into phosphorous. Pure silicon forms the dia- only slightly higherinenergy than the globalminimum. Chem- [42] mond cubic crystal structure. Due to the structure and the ical bond analysis of Li3N3 and ozone (Figure 8D,8E) confirms high bond energy,silicon is hard. Its neighbour,phosphorus, the similarity in chemical bonding, including two N=Ndouble

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Figure 8. Crystal structures of SrN2 (A, Sr in red, Ningreen), BaN2 (B, Ba in grey,Ninblack),and Li2N2 (C, Li in yellow,Ninblue), as well as the chemical bond- ing of Li3N3 (D) and O3 (E) revealed by the AdNDP analysis.

bonds with occupation numbers (ON) of 1.98 e for s-bonds that resembles the Watson–Crick DNA structure[52] is reported j j and 1.93 e for p-bonds;one p-lone pair on the central nitro- and presented in Figure 9B.NBO analysis of the Li P circular j j 90 90 gen (ON=1.72 e )and two lone pairs of s and p type on each double-helix structure showsthat the bonding between lithi- j j side nitrogen atom, with ONs rangingfrom 1.71 e to 1.86 e . um and phosphorus atoms is quite ionic with effective atomic j j j j NBO chargeonthe N moiety is 2.04 e ,suggesting signifi- chargesranging from 0.4 to 0.8 e on P. Additionally,NBO 3 À j j À À j j cant chargetransfer from Li to N. analysis revealed the presence of 90 P P s-bonds with occupa- À tion numbers(ON) equalto1.92–1.95 e ,and two lone pairs j j of s- and p-type on each phosphorusatomwithONs ranging 3.2. ET of phosphorous into sulfur from 1.72 to 1.84 e .The results of the AdNDPanalysisare in j j In search of the transmutatedphosphorous, the global mini- excellent agreement with the NBO results(Figure 9C). [16] mum search was first performed for LixPx (x= 5–9). Remarka- We next provide evidencethat the ET of phosphorous into bly,inorganic double helix structures were found for all of sulfur indeed occurs in these structures. In LixPx (x= 7–9, 90), these clusters. Periodicrepetition of the LiP infinite double- the interaction between Li and Patoms are all ionic, and the P helix chain geometry is shown in Figure 9A for illustration. atoms obtain enoughnegative chargetotransmutate into S. 2 4 Startingfrom Li7P7 and up to Li9P9,the double heliceshave a The electronic configurations of PÀ and Sare [Ne]3s 3p ,indi- similar chemical bondingpattern:effective charges on Li range cating that there shouldbeone s-lone pair,one p-lone pair, from + 0.8 to + 0.9 e ;there are no Li P s-bonds. Six (Li P ), and two p-unpaired electrons. Therefore compounds with two j j À 7 7 seven (Li P ), and eight (Li P )PP s-bonds with ON=1.95– s-bonds formed by these two p-unpaired electrons can be ex- 8 8 9 9 À 1.98 e are observed. From this data, it can be concluded that pected. It is indeed the case in Li P (x=7–9, 90), where each P j j x x when the bonding between the Li and Patoms is ionic begin- atom forms two s-bonds with adjacent two other Patoms and ning from Li7P7 to Li9P9,and the double-helix structures are Pn chain structures are observed. However,there are many much more favourablerelative to other isomers. NBO analysis kinds of sulfur allotropes[42] including various chain and ring does not show any significant direct Li Li covalentbonding. In structures. The Satoms in these allotropes also possess two s À the graphical representation of double-helix structures, adja- bondsformed with adjacent two other Satoms. In 2013, Fuji- cent Li atoms are connected to make the double-helix struc- mori et al.[53] synthesized single chains of sulfur encapsulated ture look more apparent. The helix structure formed by lithium in carbon nanotubes and characterizedthem with transmission cations is due to the favourable electrostaticinteractions with electron microscopy (Figure 9D). Due to the confinementof neighbouring phosphorus anions. Further,atheoretical study the nanotube, the sulfur atoms can grow in asingle long chain [17] of Li90P90 whichpossesses acircular double-helix structure and do not form ring structures.This discovery justifies the ET

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Figure 9. Periodic repetitions of the LiP infinitedouble-helix chain geometry (A, Pingreen,Liinred), optimizedLi90P90 double-helicaltoroid structurewith in- ternaldiameter of 25.6 Š (B, Pingreen,Liinred), chemicalbondingpatternofLi90P90 shown by the AdNDPanalysis (C), the highresolution transmission elec- tron microscopy imagesand graphical representation of single-walled or double-walled carbon nanotubes encapsulated sulfur chains (D), and crystal struc- ture sections projectedalong the a and b axis of SnIP,aswell as the scanning electron microscopy (SEM) image of exfoliated SnIP (E).

Chem.Eur.J.2018, 24,9200 –9210 www.chemeurj.org 9208  2018 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Minireview of phosphorous presented in Figure9A. In view of this con- [2] J. K. Olson,A.I.Boldyrev, Chem. Phys. Lett. 2012, 523,83. [3] A. N. Alexandrova, K. A. Birch, A. I. Boldyrev, J. Am. Chem. Soc. 2003, 125, fined S-chain,the LinPn chain inside acarbon nanotube channel [18] 10786. was also calculated. More recently,first-principles investiga- [4] J. J. Torres-Vega, A. Vµsquez-Espinal, M. J. Beltran, L. Ruiz, R. Islas,W.Tiz- tions of aseries of inorganic double helical XY (X= Li, Na, K, nado, Phys. Chem. Chem. Phys. 2015, 17,19602. Rb, Cs;Y= P, As, Sb) structures were conducted.[54] Remarkably, [5] J. Poater,M.Solµ,C.ViÇas, F. Teixidor, Angew.Chem.Int. Ed. 2014, 53, the inorganic double helical SnIP semiconductors containing 12191; Angew.Chem. 2014, 126,12387. [6] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Nature one [SnI] helix and one [P] helix were synthesized by Nilges 2001, 410,63. [55] and co-workers in the solid state (Figure 9E). The Sn atoms [7] J. T. Gish, I. A. Popov,A.I.Boldyrev, Chem. Eur.J.2015, 21,5307. in this materialfunctionasthe electron donor,and both Iand [8] I. A. Popov,X.Zhang, B. W. Eichhorn,A.Boldyrev,K.H.Bowen, Phys. Pacquire significant negative charges from Sn. Consistent with Chem. Chem. Phys. 2015, 17,26079. [9] K. A. Lundell, X. Zhang,K.H.Bowen, A. I. Boldyrev, Angew. Chem. Int. Ed. the ET principle, it is the negative chargethat makes the [PÀ] 1 2017, 56,16593 –16596; Angew. Chem. 2017, 129,46820 –16823. chain resemble the sulfur chain. [10] H. Fahlquist, D. NorØus, Inorg. Chem. 2013, 52,7125. [11] H. Fahlquist, D. NorØus, M. H. Sorby, Inorg. Chem. 2013, 52,4771. [12] H. Fahlquist, D. NorØus, S. Callear,W.I.F.David, B. C. Hauback, J. Am. Outlook Chem. Soc. 2011, 133,14574. [13] B. H. Boo, S.-J. Kim, M. H. Lee, N. Nishi, Chem. Phys. Lett. 2008, 453,150. The major developmentofthe ET concept hasbeen in silico, [14] N. Perez-Peralta,A.I.Boldyrev, J. Phys. Chem. A 2011, 115,11551. where the unbiased global minimum searchand the chemical [15] a) G. Auffermann, Y. Prots, R. Kniep, Angew.Chem. Int. Ed. 2001, 40,547; bondinganalysisplay major roles. The advent of modern gas- Angew.Chem. 2001, 113,565;b)G.V.Vajenine, G. Auffermann, Y. Prots, W. Schnelle, R. K. Kremer,A.Simon,R.Kniep, Inorg. Chem. 2001, 40, phase and solid-statesynthesis and characterization tech- 4866;c)S.B.Schneider,R.Frankovsky, W. Schnick, Angew.Chem. Int. Ed. niques greatly helps to identify the ET compounds and justify 2012, 51,1873; Angew.Chem. 2012, 124,1909;d)J.K.Olson, A. S. the ET concept. The main spirit of ET is to discover new exotic Ivanov, A. I. Boldyrev, Chem. Eur.J.2014, 20,6636. compounds and chemical bonding. In the near future,wean- [16] A. S. Ivanov,A.J.Morris, K. V. Bozhenko, C. J. Pickard,A.I.Boldyrev, Angew.Chem. Int. Ed. 2012, 51,8330; Angew.Chem. 2012, 124,8455. ticipate that more experimental discoveries of electronically [17] A. S. Ivanov,A.I.Boldyrev, G. Frenking, Chem. Eur.J.2014, 20,2431. transmutated compounds can be achieved. For example, the [18] A. S. Ivanov,T.Kar,A.I.Boldyrev, Nanoscale 2016, 8,3454.

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