Review All-boron aromatic clusters as potential new inorganic

56 Pages·2006·3.55 MB·English

CoordinationChemistryReviews250(2006)2811–2866
Review
All-boron aromatic clusters as potential new inorganic
ligands and building blocks in chemistry
Anastassia N. Alexandrovaa, Alexander I. Boldyreva,∗, Hua-Jin Zhaib,c, Lai-Sheng Wangb,c,∗
aDepartmentofChemistryandBiochemistry,UtahStateUniversity,Logan,UT84322-0300,UnitedStates
bDepartmentofPhysics,WashingtonStateUniversity,2710UniversityDrive,Richland,WA99354,UnitedStates
cW.R.WileyEnvironmentalMolecularSciencesLaboratoryandChemicalSciencesDivision,PacificNorthwestNationalLaboratory,
MSK8-88,P.O.Box999,Richland,WA99352,UnitedStates
Received5December2005;accepted11March2006
Availableonline9May2006
Contents
1. Introduction........................................................................................................... 2813
1.1. Boranesandcarboranes.......................................................................................... 2813
1.2. Bareboronclusters.............................................................................................. 2815
2. Methodology.......................................................................................................... 2815
2.1. Theoreticalmethods............................................................................................. 2815
2.2. Photoelectronspectroscopy....................................................................................... 2816
3. Pure-boronclusters..................................................................................................... 2817
3.1. ThediatomicB molecule........................................................................................ 2817
2
3.2. Triatomicclusters:B ,B +,andB −............................................................................... 2819
3 3 3
3.2.1. B ...................................................................................................... 2819
3
3.2.2. B +..................................................................................................... 2820
3
3.2.3. B −anditsphotoelectronspectra.......................................................................... 2820
3
3.3. Tetraatomicclusters:B ,B +,B −,B 2−........................................................................... 2822
4 4 4 4
3.3.1. B ...................................................................................................... 2822
4
3.3.2. B +..................................................................................................... 2824
4
3.3.3. B −anditsphotoelectronspectra.......................................................................... 2825
4
3.3.4. B 2−.................................................................................................... 2827
4
3.4. Pentaatomicclusters:B ,B +,B − ................................................................................ 2828
5 5 5
3.4.1. B ...................................................................................................... 2828
5
3.4.2. B +..................................................................................................... 2828
5
3.4.3. B −anditsphotoelectronspectra.......................................................................... 2829
5
3.5. Hexaatomicclusters:B ,B +,B −,B 2−........................................................................... 2832
6 6 6 6
3.5.1. B ...................................................................................................... 2832
6
3.5.2. B +..................................................................................................... 2833
6
Abbreviations: ADE,adiabaticdetachmentenergy;AO,atomicorbital;B3LYP,ahybridmethod,amixtureofHartree–Fockexchangewithdensityfunctional
exchange-correlation;BE,bindingenergy;2c-2e,two-centertwo-electron(bond);3c-2e,three-centertwo-electron(bond);CASSCF,completeactive-spaceself-
consistent-field;CCSD(T),coupled-clustermethodattheallsinglesanddoubleslevelwiththenoniterativeinclusionoftripleexcitations;2D,two-dimensional;3D,
three-dimensional;DFT,densityfunctionaltheory;GEGA,gradientembeddedgeneticalgorithm;HF,Hartree–Fock;HOMO,highestoccupiedmolecularorbital;IR,
infrared;LUMO,lowestunoccupiedmolecularorbital;MO,molecularorbital;MP2,second-orderMøller–Plessetperturbationtheory;MPn,nth-orderMøller–Plesset
perturbationtheory;MRCISD,multireferencesinglesplusdoublesconfigurationinteraction;NICS,nucleus-independentchemicalshift;NMR,nuclearmagnetic
resonance;NPA,naturalpopulationanalysis;OVGF,outervalenceGreenfunction;PES,photoelectronspectroscopy;RE,resonanceenergy;ROVGF,restricted
OVGF;TD-B3LYP,time-dependentB3LYP;UCCSD(T),unrestrictedCCSD(T);UOVGF,unrestrictedOVGF;UV,ultraviolet;VB,valencebond;VDEvertical
detachmentenergy;ZPE,zeropointenergy;NBO,naturalbondinganalysis;KE,kineticenergy;ON,occupationnumber;LSD,localspindensity
∗ Correspondingauthors.
E-mailaddresses:[email protected](A.I.Boldyrev),[email protected](L.-S.Wang).
0010-8545/$–seefrontmatter©2006ElsevierB.V.Allrightsreserved.
doi:10.1016/j.ccr.2006.03.032
2812 A.N.Alexandrovaetal./CoordinationChemistryReviews250(2006)2811–2866
3.5.3. B −anditsphotoelectronspectra.......................................................................... 2833
6
3.5.4. B 2−.................................................................................................... 2834
6
3.5.5. LiB −,Li B ,andMB (M=Be,Mg,Ca,andSr)........................................................... 2837
6 2 6 6
3.6. Heptaatomicclusters:B ,B +,B −................................................................................ 2839
7 7 7
3.6.1. B +..................................................................................................... 2839
7
3.6.2. B ...................................................................................................... 2840
7
3.6.3. B −anditsphotoelectronspectra.......................................................................... 2840
7
3.6.4. H B −andAu B −...................................................................................... 2844
2 7 2 7
3.7. Octaatomicclusters:B ,B +,B −,B 2−............................................................................ 2847
8 8 8 8
3.7.1. B +..................................................................................................... 2847
8
3.7.2. B ...................................................................................................... 2848
8
3.7.3. B −anditsphotoelectronspectra,B 2−.................................................................... 2849
8 8
3.7.4. LiB −halfsandwichandFe(B ) 2−sandwichstructures..................................................... 2850
8 8 2
3.8. Nonaatomicclusters:B ,B +,B −................................................................................. 2852
9 9 9
3.8.1. B +..................................................................................................... 2852
9
3.8.2. B ...................................................................................................... 2852
9
3.8.3. B − .................................................................................................... 2852
9
3.9. The10-atomicclustersB +,B ,B −............................................................................. 2853
10 10 10
3.9.1. B +.................................................................................................... 2853
10
3.9.2. B ..................................................................................................... 2853
10
3.9.3. B −anditsphotoelectronspectra......................................................................... 2854
10
3.10. The11-atomicclustersB +,B ,B −............................................................................ 2855
11 11 11
3.10.1. B +................................................................................................... 2855
11
3.10.2. B .................................................................................................... 2856
11
3.10.3. B −anditsphotoelectronspectra........................................................................ 2856
11
3.11. The12-atomicclustersB +,B ,B −............................................................................ 2856
12 12 12
3.11.1. B +................................................................................................... 2856
12
3.11.2. B .................................................................................................... 2856
12
3.11.3. B −anditsphotoelectronspectra........................................................................ 2857
12
3.12. The13-atomicclustersB +,B ,B −............................................................................ 2857
13 13 13
3.12.1. B +................................................................................................... 2857
13
3.12.2. B .................................................................................................... 2858
13
3.12.3. B −anditsphotoelectronspectra........................................................................ 2858
13
3.13. The14-atomicclustersB +,B ,B −............................................................................ 2859
14 14 14
3.13.1. B +................................................................................................... 2859
14
3.13.2. B .................................................................................................... 2859
14
3.13.3. B −anditsphotoelectronspectra........................................................................ 2859
14
3.14. The15-atomclustersB ,B − .................................................................................. 2859
15 15
3.14.1. B .................................................................................................... 2859
15
3.14.2. B −anditsphotoelectronspectra........................................................................ 2859
15
3.15. Planar-to-tubularstructuraltransitionatB ....................................................................... 2860
20
3.16. Areallplanarboronclustershighlyaromatic?..................................................................... 2862
4. Conclusions........................................................................................................... 2863
Acknowledgements.................................................................................................... 2864
References............................................................................................................ 2864
Abstract
Smallboronclustersasindividualspeciesinthegasphasearereviewed.Whilethefamilyofknownboroncompoundsisrichanddiverse,alarge
bodyofhithertounknownchemistryofboronhasbeenrecentlyidentified.Freeboronclustershavebeenrecentlycharacterizedusingphotoelectron
spectroscopyandabinitiocalculations,whichhaveestablishedtheplanarorquasi-planarshapesofsmallboronclustersforthefirsttime.This
hassurprisedthescientificcommunity,asthechemistryofboronhasbeendiverselyfeaturedbythree-dimensionalstructures.Theplanarityofthe
specieshasbeenfurtherelucidatedonthebasisofmultiplearomaticity,multipleantiaromaticity,andconflictingaromaticity.
Althoughmostlyobservedinthegasphase,pureboronclustersarepromisingmoleculesforcoordinationchemistryaspotentialnewligandsand
formaterialsscienceasnewbuildingblocks.Theuseofpureboronspeciesasnovelligandshascommenced,suggestingmanynewchemistries
areaheadofus.
©2006ElsevierB.V.Allrightsreserved.
Keywords: Boronclusters;Multiplearomaticity;Multipleantiaromaticity;Conflictingaromaticity;Photoelectronspectroscopy
A.N.Alexandrovaetal./CoordinationChemistryReviews250(2006)2811–2866 2813
1. Introduction earlyworksrepresentedthefirstexperimentaldemonstrationof
closed boron polyhedra in a chemical structure. A subsequent
Boron compounds have been known to humankind since relatedworkbyLonguet-HigginsandRoberts[14]usedmolec-
ancient times, when it was used to prepare hard glasses and ularorbitaltheorytoshowthatthe[B ]2−unithasaclosed-shell
6
glazes[1].Nowadaystheuseofboroncompoundsrangesfrom electronic arrangement of high stability. Longuet-Higgins and
hardmaterialsandsemiconductorstoantitumormedicines,and Roberts[15]alsousedasimilarapproachtostudytheB icosa-
12
itsimportancecannotbeoverestimated.Neighboringcarbonin hedron,adominantstructuralunitofvariousallotropesofboron
the periodic table, boron has one electron less than valence [16].TheirworkindicatedthattheB icosahedronhas13skele-
12
orbitals, and that makes a huge difference in determining the tal bonding orbitals and 12 outward pointing external orbitals.
chemistryofboron.Althoughboronhasarichanddiversechem- They concluded that a borane, B H , would be stable only
12 12
istry,itdifferssubstantiallyfromthatofcarbon[1,2]. as a dianion, B H 2−, as well as a series of stable cage-like
12 12
boranedianionsofthegeneralformula,B H 2−.Theconclusion
n n
1.1. Boranesandcarboranes wassupportedbyexperimentaldataprovidedbyHawthorneand
Pitochelli,whosynthesizedsaltsoftheboraneanion,B H 2−
12 12
Boroncompoundsplayedessentialrolesinadvancingchem- [17,18]. From X-ray diffraction experiments, the structure of
icalbondingmodels.Afewmilestonesinthehistoryofcontem- the B H 2− anion was indeed shown to be icosahedral [19]
12 12
porary boron chemistry should be pointed out. In 1912 Stock andanotherB H 2−anionwasshowntobeabicappedsquare
10 10
reported his pioneering work on boranes [3], which led to the antiprism[20].Furtherexperiments[21–23]indicatedtheexis-
identificationoftheneutralboronhydrideswithformulasB H , tenceofotherdeltahedralboranes,B H 2−,B H 2−,B H 2−,
2 6 11 11 9 9 8 8
B H ,B H ,B H ,andB H .Thesecompoundswerechar- B H 2−,andB H 2−.
4 10 5 9 5 11 6 10 7 7 6 6
acterized as toxic, air-, and water-sensitive gases, or volatile Different rules were developed for the number of atoms,
liquids.Alargercompound,B H ,wasisolatedasavolatile bonds,electrons,andorbitalsinstableboranes[24–28].Dixon
10 14
solid. Although the structures of the boranes were established et al. made the first attempt to describe boranes in terms of
then, the chemical bonding within them remained unclear, as resonanceofKekule-typestructureswithalternating2c-2eand
the stoichiometry of the species contradicted the postulates of 3c-2ebonds[29].In1971Williams[30]recognizedthecloso-,
valence theory. Even the reason for the rapid dimerization of nido-,andarachno-structuralmotifsinthechemistryofdelta-
BH intoB H wasapuzzle.ThestructureofB H withbridg- hedral boranes. The most spherical deltahedra have a formula
3 2 6 2 6
ing H-atoms was proposed in 1921 by Dilthey [4]. However, B H (orB H 2−).Thelossofavertexfromthiscloso-form
n n+2 n n
it was not considered seriously until the 1940s, when infrared resultsintheB H (B H 2−)nido-structure.Thearachno-
n n+4 n n+2
spectroscopydata[5–7]supportedthestructure.Later,electron structure, B H (B H 2−), can be formed by the removal
n n+6 n n+4
diffraction [8] and low-temperature X-ray diffraction [9] also of yet another vertex from the deltahedron. Wade [31] recog-
confirmedthebridgedstructureforthediborane.Thechemical nized that this structural relationship could be associated with
bondinginboraneswasfirstconsideredbyPitzer,whoproposed thenumberofskeletalelectronsinaborane.Namely,heunder-
theconceptofa“protonateddoublebond”[10]. stood that closo-, and the nido- and arachno-shapes, have the
Further,Lipscombandco-workers[11]putforwardthecon- same number of skeletal electrons: 2n+2. These species have
cept of three-center two-electron (3c-2e) bonding, which, in thesamenumberofmolecularorbitalsbelongingtotheboron
thecaseoftheB H diborane,consistedoftwo3c-2eB–H–B skeleton: closo-boranes have to have n+1 MOs, nido-boranes
2 6
bonds involving the bridging hydrogen atoms. Lipscomb also havetohaven+2MOs,andarachno-boraneshavetohaven+3
explained the structure of all known boron hydrides, in which MOs. This set of rules is known as Wade’s rules for boranes
thebridgingB–H–Bbondappearedtobethekeystructuralunit andcarboranes.Gellespieetal.alsoproposedanotherscheme,
[9].Inthe3c-2ebondingthreeatomssupplythreeorbitals,one in which skeletal electron pairs (2n electrons) remained local-
oneachatom.Theseatomicorbitalsinteracttoformonebond- ized on each vertex, whereas two electrons participated in the
ing and two antibonding orbitals. The two available electrons delocalizedbondingoverthesphericallysymmetricpolyhedron
may thus fill the bonding orbital to form a 3c-2e bond. In the [32].Furthermore,therelativestabilitiesofdeltahedralboranes
n-atomicspecies,therearenatomicorbitals,andonlyn/3bond- formaseries[33]:
ingmolecularorbitals,whichcanbeoccupiedby2n/3electrons.
Thus,thereasonforcertainboranestoexhibitspecialstability B H 2−> B H 2−> B H 2−
12 12 10 10 11 11
was elucidated. In principle, Lipscomb’s concept of the 3c-2e
bond, along with aromaticity, is one of the ways of describ- > B9H92−∼ B8H82−∼ B6H62−> B7H72−
ingelectrondeficientbonding,eventhougharomaticityismore
common in chemistry and, in a way, more clear. The work of In 1959 Lipscomb et al. [34] proposed the term “superaro-
Lipscomb on the chemical bonding of the boranes eventually maticity”toexplainthe3DaromaticityofB H 2−.Chenand
12 12
ledtohiswinningoftheNobelPrizeandopenedthegatewayto Kingrecentlyreviewed[35]theintroductionof3Daromaticity
understandingthechemistryofboron. in chemistry. Explicitly, the idea of aromaticity of deltahedral
Allardin1932[12],andPaulingandWeinbaumin1934[13] boranes was put forward by Aihara [36], and by King and
showed the existence of regular octahedra of boron atoms in Rouvray [37] in 1978. Carboranes of the formula C2Bn−2Hn
several metal hexaborides of the general formula MB . These also exhibit deltahedral topology, and the chemical bonding
6
2814 A.N.Alexandrovaetal./CoordinationChemistryReviews250(2006)2811–2866
Fig.1. (A–E)Lowest-energyisomersoftheLi6B6H6saltmoleculeattheB3LYP/6-311++G**leveloftheory.(ReprintedwithpermissionfromRef.[41].Copyright
2004AmericanChemicalSociety.)
within them can be described in the same manner [38]. The planarandnonplanarisomersinvolvingB H 6− hasbeenper-
6 6
onlydifferenceintheborane-carborane-electronicrelationship formed[41b].SaltslikeLi B H withtheB H 6−benzene-like
6 6 6 6 6
is that C2B3H5, being an electronic analog of B5H52−, is a analog are still a theoretical prediction, but Fehlner and co-
knowncompound,whileB5H52−itselfhasnotbeensynthesized. workers[43]recentlyreportedsynthesisandcrystalstructuresof
Thethree-dimensionalaromaticityinboranes,BnHn2−,closo- remarkabletriple-decker(Cp*ReH2)B5Cl5and(Cp*)2B6H4Cl2
−
monocarborane anions, CBn−1Hn , and closo-dicarboranes, compounds containing planar B5Cl5 and B6H4Cl2 structural
C2Bn−2Hn was also computationally studied by Schleyer and fragments,respectively(Fig.2).
Najafian [39], who used the NICS index values as criteria of It is believed that the planar B Cl and B H Cl structural
5 5 6 4 2
aromaticity.Theauthorsshowedthegeneraltrendinsuchsys- fragments in Felhner’s compounds acquire six electrons from
tems:thestabilityincreaseswithincreasingvertecesfrom5to theReatomsformallyandthusbecomesix(cid:1)-electronaromatic
12. The comparison of the chemistry of boron and carbon on compoundssimilartothepredictedB H 6−andB H 6−build-
5 5 6 6
the example of boron and carbon hydrides was also discussed ingblocksintheLi B H andLi B H saltmolecules.
6 5 5 6 6 6
byJemmisandJayasree[40]. Beyond boranes and carboranes a very rich family of
In2003,Boldyrevandco-workerstheoreticallypredictedthe metallocarboranes has been synthesized and characterized
newfamilyofplanararomatichighlychargedboranes,suchas [1,2,38,43–47]. One of the exciting new applications of these
B6H66− stabilized by six Li+ cations surrounding the species species was a demonstration of rotary motion of a carborane
(structureA,Fig.1)[41]. cage ligand (7,8-dicarbollide) around a nickel axle controlled
Importantly, the planar B6-fragment has been previously byelectricalorlightenergy,thusmovingusclosertoinorganic
known to be the building block of the MgB2 solid, which is a nanomachines[45].Boronatomscanalsobeincorporatedinto
recentlydiscoveredhigh-temperaturesuperconductor[42].An transitionmetalclusters[44–47]furtherextendingtherichboron
extendedanalysisofthechemicalbondinginthelowestenergy chemistry.
Fig.2. Molecularstructuresof:(Cp*ReH)2B5Cl5and(Cp*Re)2B6H4Cl2.(ReprintedwithpermissionfromRef.[43a].Copyright2004AmericanChemicalSociety.)
A.N.Alexandrovaetal./CoordinationChemistryReviews250(2006)2811–2866 2815
1.2. Bareboronclusters all-boronclusterswiththehopethatthelatestexperimentaland
theoretical understanding may stimulate further investigations
Experimental studies of bare boron clusters have been sur- leadingtonovelcoordinationcompoundscontainingtheplanar
prisinglylimiteddespiteitsproximitytocarbonintheperiodic all-boronclustersasligandsornewbuildingblocks.
tableandtheextensiveresearcheffortoncarbonclustersandthe
fullerenes([48–52]andreferencestherein).Untilveryrecently 2. Methodology
the only experimental studies on boron clusters were carried
out by Anderson and co-workers in the late 1980s [53–55]. 2.1. Theoreticalmethods
Theseauthorsproducedboronclustercationsusinglaservapor-
izationandstudiedtheirchemicalreactivityandfragmentation Oneofthemajorchallengesinanyclustertheoreticalstudy
properties [53–59]. Their observation of the prominent B + istofindreliablytheglobalminimumstructure.Differentthe-
13
peak in mass spectra stimulated further computational efforts oreticalstudiessometimespredictdifferent“globalminimum”
[60–64]. In 1992, La Placa et al. reported a mass spectrum of structures because of human bias. A number of methods have
bareboronclustersconsistingof2–52atoms,generatedbylaser beendevelopedtosearchforglobalminimumstructures,which
vaporization of a boron nitride target in an effort to produce aredesignedtoexcludehumanbias,suchastheCar–Parrinello
BNclusters[65].Littleinformationregardingthestructuraland method, molecular dynamics methods with density functional
electronic properties of boron clusters can be drawn from the theory for energy calculations, Monte Carlo annealing meth-
mass-spectrometry-basedstudies.Andtherehavebeennospec- ods, and genetic algorithms for global minimum searches. We
troscopicstudiesuntilveryrecently. recently developed a new genetic algorithm method for the
Startingfrom2001,wehaveconductedextensivephotoelec- search for global minima, called ab initio gradient embedded
tron spectroscopy (PES) studies on a series of boron cluster genetic algorithm (GEGA). The GEGA program was written
−
anions,B (n=3–20),whichwerecombinedwithstate-of-the- by Alexandrova and described in details in Refs. [78,79]. The
n
art computational studies to elucidate the structural and elec- hybridmethod,knownasB3LYP[80–82],withrelativelysmall
tronicpropertiesandchemicalbondinginthespecies[66–72]. basisset,3-21G,isusuallyemployedthroughouttheexecution
These works established for the first time experimentally pla- oftheGEGA.Forlargeclusters,wealsousethesemiempirical
narity or quasi-planarity in small boron clusters for as large PM3method.
as 20 atoms. Among our findings are the observation and Briefly, within the GEGA procedure, the initial geometries
characterizationofthehepta-andocta-coordinatedpureboron ofindividuals(structures)inapopulation(setofstructures)are
molecular wheels in B and B [70], the observation of an randomly generated and further optimized to the nearest local
8 9
unusually large HOMO–LUMO gap in B [71], and the dis- minimaonthepotentialenergysurface,usingtheGaussian03
12
covery that the planarity or quasi-planarity of boron clusters package[83].Ifasaddlepointisencounteredthenormalmodeof
canbeexplainedonthebasisofmultiple(cid:2)-and(cid:1)-aromaticity, thefirstimaginaryfrequencyisfolloweduntilalocalminimumis
multiple (cid:2)- and (cid:1)-antiaromaticity, and conflicting aromaticity found.Thepopulation,composedofthethusselectedgoodindi-
(simultaneouspresenceof(cid:2)-aromaticityand(cid:1)-antiaromaticity viduals(structureswiththelowestenergies),undergoesbreeding
or(cid:2)-antiaromaticityand(cid:1)-aromaticity)[66–72].Wealsofound andmutations.Duringthe“matingprocess”someofthegeomet-
thatthe(cid:1)-aromaticityinsmallboronclustersupton=15seems ricfeaturesofgoodindividualsinthepopulation(“parents”)are
to follow the Hu¨ckel’s rules, analogous to hydrocarbons [71]. combined and passed to new individuals (“children”). Parents
We showed that the B neutral cluster appears to possess a arelocalminimumstructuresobtainedeitherduringtheinitial
20
ring-like 3D ground state geometry although both planar and orsubsequentiterations.Childrenarenewstructuresmadeout
−
3D structures are nearly isoenergetic for the B anion [72]. of two parent structures. Probabilities to be bred (to produce
20
TheB neutralthereforerepresentstheplanar-to-tubulartran- child structures) are assigned to parents according to the best-
20
sitioninsmallboronclustersandmaybeviewedastheembryo fitcriterion(i.e.,thelowestenergy).Basedontheprobabilities,
of the thinnest single-walled boron nanotubes with a diameter couplesofparentsarerandomlyselected.Thegeometriesofthe
assmallas5.2A˚ [72].Veryrecently,severalotherexperimental parentsarecutbyarandomcuttingplane(XY,XZ,orYZ),and
effortshavealsoappearedonboronclusters[73–77]. the thus obtained halves (“genes”) are then recombined either
In spite of the enormous variety of boron chemical com- inasimpleorinahead-to-tailmannertoforma“child”(anew
pounds and their great influence on developing modern chem- structure).Thenumberofatomsinthenewlygeneratedgeome-
ical bonding theory, bare boron clusters as ligands in chemi- tryischecked,andthenewstructureisoptimizedtothenearest
cal compounds are still absent. Given the fact that our recent localminimum.
−
spectroscopic and theoretical studies have shown that isolated Fig. 3A shows a typical breeding procedure using the B
9
boronclustershaveplanargeometriesandexhibitaromaticand clusterasanexample.Inthiscase,theYZplaneischosenasthe
antiaromatic electronic properties analogous to hydrocarbons, cuttingplaneandrecombinationofthehalvesoccursinasim-
itisbelievedthatbareboronclusterscouldpotentiallybenew plemanner,i.e.,thepartofgeometryofparentstructure1taken
ligandsorbuildingblocksofnewsolids.Itisworthmentioning fromtheleftofthecuttingplaneisrecombinedwiththepartof
−
thataplanararomaticcyclopentadienanionC H isoneofthe geometryofparentstructure2takenfromtherightofthecutting
5 5
mostcommonligandsincoordinationchemistry.Inthisreview plane.Thenumberofatomsinthenewlygeneratedgeometryis
wesummarizetheoreticalandspectroscopicstudiesofisolated checked,andthechildstructureisoptimizedtothenearestlocal
2816 A.N.Alexandrovaetal./CoordinationChemistryReviews250(2006)2811–2866
atoms)weusethreelevelsoftheoryforcalculatingtheVDEs:
U(R)CCSD(T)/6-311+G(2df), the outer valence Green Func-
tionmethod(OVGF/6-311+G(2df))[92–95]attheCCSD(T)/6-
311+G*geometries,aswellasthetime-dependentDFTmethod
[96,97] (TD-B3LYP/6-311+G(2df)) at the B3LYP/6-311+G*
geometries. Core electrons were frozen in treating the elec-
tron correlation at the CCSD(T) and OVGF levels of theory.
Forthelargeboronclusters(>9atoms),weusedprimarilythe
TD-B3LYP/6-311+G*leveloftheorytocalculatetheVDEs.
Natural Bond Order (NBO) analysis [98] was employed
to examine the detailed chemical bonding. Molecular orbitals
(MOs) were calculated at the HF/6-311+G* level of theory.
B3LYP, HF, and CCSD(T) calculations were performed using
Gaussian2003andNWChem[99]programs.Molecularorbitals
Fig.3. IllustrationoftheGEGAprocedure.(A)Breeding,whenXYplaneis picturesweremadeusingtheMOLDEN3.4program[100].
randomlychosen,geometriesoftwoselectedparentsarecutbyXY,andpartsof
parentsarerecombinedinasimplemanner;theobtainedchildisthenoptimized
2.2. Photoelectronspectroscopy
tothenearestminimum.(B)Mutations,whentherandomnumberofkicksis
introducedtodistortthestructurestronglyenoughtocrossthebarrieronthe
potentialenergysurface,andtheobtainedmutantisthenoptimizedtothelocal The photoelectron spectroscopic studies were carried out
minimum. usingamagnetic-bottletime-of-flightPESapparatusequipped
with a laser vaporization supersonic cluster source [101,102].
minimum.Ifthenumberofatomsisincorrect,thecuttingplane Detailsofthisapparatushavebeenreportedelsewhere,andFig.4
is shifted so that the child structure has the correct number of showsaschematicviewofit.
−
atoms.Afterthenumberofindividualsinthepopulationisdou- TheB clusteranionswereproducedbylaservaporization
n
bled within the breeding process, the best-fit group is selected ofanisotopicallyenriched10Bdisktargetinthepresenceofa
andconvergenceofthealgorithmischecked.TheGEGAiscon- heliumcarriergas.Theclusteranionsweremassanalyzedusing
sidered converged if the current lowest energy species (global atime-of-flightmassspectrometerandtheclusterofinterestwas
minimumoratleastverystablelocalminimum)remainsfor20 mass selected and decelerated before being photodetached. A
iterations.Iftheconvergenceisnotyetmet,thehighestenergy varietyofdetachmentlaserphotonenergieshavebeenusedin
speciesinthepopulationundergomutations.Themutationrateis thecurrentexperiments(532nm,355nm,266nm,and193nm).
setto33.33%.Mutationsareshiftsofrandomatomsofaspecies Photoemitted electrons are collected by the magnetic-bottle at
in random directions, with the purpose of changing the initial nearly 100% efficiency and analyzed in a 3.5m long electron
geometrysoastopushthestructureoutofthecurrentlocalmin- time-of-flighttube.Thephotoelectrontime-of-flightspectraare
− −
imumtoanotherwellonthepotentialenergysurface(Fig.3B). calibrated using the known spectra of Rh or Au and con-
Mutants(structuresobtainedfrommutations)areoptimizedto verted to kinetic energy (KE) spectra. The reported binding
thenearestlocalminima.Afterthatthealgorithmproceedswith energy (BE) spectra are obtained by subtracting the KE spec-
the new cycle of breeding. All low-lying isomers are detected tra from the photon energy (hν) using Einstein’s photoelectric
and stored throughout the execution and they are reported to equation:BE=hν−KE.Highphotonenergyspectraarepartic-
the user at the end of the run. A few runs of GEGA are done
on the system in order to confirm the found global minimum
structure.
Thegeometryandvibrationalfrequenciesofthethusidenti-
fiedglobalminimum,aswellaslow-lyingisomers,arefurther
refinedathigherlevelsoftheory.TheB3LYPandthecoupled-
cluster CCSD(T) [84–86] methods with the polarized split-
valencebasissets(6-311+G*)[87–89]areusuallyusedforthis
purpose.Finalrelativeenergiesforthefewloweststructuresare
calculatedattheCCSD(T)/6-311+G(2df)leveloftheoryusing
theCCSD(T)/6-311+G*geometry.Wealsoruncalculationsfor
the global minimum structure at the CASSCF(X,Y)/6-311+G*
[90,91](XisanumberofactiveelectronsandYisanumberof
activeorbitals)leveloftheory,inordertotesttheapplicabilityof
theoreticalmethodssuchasMPn,CCSD(T),andotherswhich
arebasedontheone-electronapproximation.
The next step is to calculate the ab initio vertical detach-
ment energies (VDEs), which will facilitate comparison with
Fig.4. Schematicviewofthelaservaporizationmagnetic-bottledtime-of-flight
the experimental PES data. For small systems (three to nine photoelectronspectroscopyapparatus.
A.N.Alexandrovaetal./CoordinationChemistryReviews250(2006)2811–2866 2817
ularlyimportantbecausetheyrevealmoreelectronictransitions, bondingofsmallboronclusterstobedeterminedandunderstood
whichareessentialtofacilitatecomparisonswiththeoreticalpre- [66–72].
dictions. Low photon energies in general yield better-resolved
spectraforthegroundstatetransitions,allowingmoreaccurate 3. Pure-boronclusters
determination of adiabatic detachment energies (ADEs) of the
neutralspeciesandvibrationalresolutioninfavorablecases.The 3.1. ThediatomicB molecule
2
resolutionofourapparatusis(cid:2)KE/KE∼2.5%,i.e.,∼25meV
for1eVelectrons. The diatomic B molecule provides the first example of
2
Itshouldbepointedoutthatalthoughitwasnotdifficultto unusual chemical bonding in boron species. It is known
observemassspectrawithawidesizerangeofBn−clustersby [106,107] that B2 has a 3(cid:3)g− ground electronic state with the
laservaporization,itturnedouttoberatherchallengingtoobtain 1(cid:2)21(cid:2)21(cid:1)22(cid:2)0electronicconfiguration.Thiselectronicconfig-
g u u g
highqualityPESdata,primarilyduetothelowphotodetachment urationcontradictstheconventionalchemicalbondingpicture,
cross-sectionsoftheselightclustersandthedifficultyobtaining because the 1(cid:1) bonding orbital is occupied before the 2(cid:2)
u g
coldclusteranions.Thekeytothecurrentprogresswastheuse bondingorbitalisoccupied.Thefirsttwo(cid:2)-MOs,1(cid:2)2and1(cid:2)2,
g u
of a large waiting-room nozzle, which could more efficiently are pairs of bonding and antibonding orbitals formed primar-
cool cluster anions [103–105]. The temperature effects were ily by the 2s-AOs (with rather small s–p hybridization) and
furthercontrolledbytuningthefiringtimingofthevaporization do not contribute to bonding significantly. Thus, most of the
laserrelativetothecarriergasandselectingclustersthathavea ∼70kcal/molbondingenergyinB comesprimarilyfromchem-
2
longerresidencetimeinthenozzleforphotodetachment.These icalbondingofthe1(cid:1)2 orbital.Earlieroccupationofthe1(cid:1) -
u u
−
effortshaveallowedustoobtainwell-resolvedPESdataforBn MOisinconsistentwiththegeneralconceptionofhowchemical
clustersatdifferentphotodetachmentenergies. bonding should occur in molecules. According to the conven-
Asanexample,Fig.5showsthedramatictemperatureeffects tionalchemicalbondmodel,acovalentbondisformedwhena
−
observedintheB20 speciesatthreedifferentdetachmentpho- portionofanatomicorbitalofoneatomoverlapsaportionofan
tonenergies.Clearly,onlythe“cold”spectra(toppanel)yielded atomicorbitalofanotheratom.Ina(cid:2)-bond,anorbitaloverlap
the rich and well-resolved PES features, which represent the occursalongtheaxisjoiningtwonuclei.Ina(cid:1)-bondanorbital
electronicfingerprintoftheunderlyingclusters.Asmanyasnine overlap is formed by parallel p-orbitals. Because of the lesser
definitive spectral features (X–H) were readily resolved in the degreeoftheorbitaloverlapsforming(cid:1)-bond,comparedwith
coldspectrumofB20−at193nm,whichwereotherwisesmeared those forming (cid:2)-orbitals, (cid:1)-bonds are generally weaker than
out in the “hot” spectra even under the same instrumental res- (cid:2)-bonds,and,therefore,(cid:2)-orbitalsshouldbeoccupiedbefore(cid:1)-
olution. The well-defined and well-resolved PES spectra have orbitals.Theconventionalchemicalbondingmodelwasinitially
been used to compare with theoretical calculations, allowing developed for organic molecules, where this prediction holds
the structural and electronic properties, isomers, and chemical ratherwell.However,forelectropositiveandelectrondeficient
Fig.5. PhotoelectronspectraofB20−atdifferentsourceconditionstodemonstratethesignificanttemperatureeffectsonthephotoelectronspectra.Toppanel,cold
clusters;bottompanel,hotclusters.Notethathotclustersresultinbroadspectra,smearingoutthespectralfeaturesevenunderhighinstrumentalresolution.(a,c,
ande)Coldand(b,d,andf)hot.
2818 A.N.Alexandrovaetal./CoordinationChemistryReviews250(2006)2811–2866
atoms,suchasB,itdoesnotworkwell,notonlyforthediatomic 2(cid:3) + (1(cid:2)21(cid:2)22(cid:2)11(cid:1)0). However, one can see that the 21(cid:3) +
g g u g u g
B2 molecule, but also for larger clusters, as we will show stateofB withthe1(cid:2)21(cid:2)22(cid:2)21(cid:1)0 electronicconfigurationis
2 g u g u
belLowow. -lying excited states for B , B +, B 2+, and B − from about 1.46eV higher than the 3(cid:3)g− ground state. We believe
2 2 2 2 thatthereasonwhyB andotherdiatomicmoleculescomposed
BrunaandWright[108]aresummarizedinFig.6.B +isshown 2
2 ofelectropositiveatomshaveunconventionalgroundelectronic
to have conventional bonding with the ground electronic state
Fig.6. Low-lyingstatesof:(a)B2,(b)B2+,(c)B22+,and(d)B2−.(ReprintedwithpermissionfromRef.[108].Copyright1990Elsevier.)
A.N.Alexandrovaetal./CoordinationChemistryReviews250(2006)2811–2866 2819
Table1
MolecularpropertiesoftheB3(D3h,2A(cid:3)1)cluster
B3LYP/6-311+G*a UCCSD(T)/6-311+G*a,b CASSCF/DZPc CASSCF-MRCISD/aug-cc-pvTZd Experiment
Etotal(a.u.) −74.298272 −74.065078 −73.906715 e
Re(B–B)(A˚) 1.548 1.586 1.587 1.5706 1.60377(106)f
ω1(a(cid:3)1)(cm−1) 1223 1165 1214 e 1020(50)a
ω2(e(cid:3))(cm−1) 934 871 869 e
a Ref.[66].
b Etotal=−74.091367auatRCCSD(T)/6-311+G(2df)//RCCSD(T)/6-311+G*.
c Ref.[110].
d Aug-cc-pvTZbasissetwasaugmentedbytwosandonepfunctionsatthebondcenters(Refs.[75,76]).
e Propertywasnotcalculatedatthisleveloftheory.
f R0(B–B),B=1.19064(157)cm−1andC=0.59532(79)cm−1(Ref.[76]).
states with (cid:1)-orbitals being occupied before (cid:2)-orbitals is due electron excitation from HOMO-1 to HOMO. The D 2A(cid:3)(cid:3)
3h 2
tothelowvalenceatomiccharges.B2 simplycannotfavorably species was theoretically predicted to be 0.66eV higher in
support six (cid:2) valence electrons. As we will show below this energythantheglobalminimum(CCSD(T)/6-311+G(2df)level
factor is crucial for understanding the unusual planar shape of oftheory)[66]andthatnumberreasonablyagreeswiththeexper-
all-boronclusters. imental value of 0.74eV [66]. An extensive ab initio study of
alternativestructuresofB indifferentspectroscopicstateshas
3
3.2. Triatomicclusters:B ,B +,andB − beencarriedoutbyHernandezandSimons[110].Theoverview
3 3 3
of the theoretically identified spectroscopic states of the trian-
3.2.1. B gular,distorted,andlinearspeciesisshowninFig.8.
3
ThefirstabinitiostudyonB isdated1981,whenWhiteside Nospecieswithstablegeometrieswerefoundtohavesym-
3
performedunrestrictedHartree–FockandUMP4calculationson metrylowerthanC2v.TheZPE-correctedbarriersbetweenthe
clustersofberylliumandboron[109].Sincethen,thetriatomic minimawerealsocalculated[110].
boronclusterhasreceivedasignificantattentionintheliterature A spectroscopic study of B3 has been performed by Maier
[53,54,60–62,66,75–77,109–130]. andco-workers[75,76].Theyassignedtwobandswithorigins
There is a consensus in the literature that the neutral B is at736nmand458nmtotheX2A(cid:3) →12E(cid:3) andX2A(cid:3) →22E(cid:3)
3 1 1
a D 2A(cid:3) (1a(cid:3)21e(cid:3)41a(cid:3)(cid:3)22a(cid:3)1) triangular radical species in its transitions,respectively,onthebasisoftheirabinitiocalcula-
3h 1 1 2 1
groundelectronicstate.Molecularpropertiesoftheglobalmin-
imum B structure computed at various levels of theory and
3
experimentallydeterminedaregiveninTable1[66].
TheB valenceMOsareshowninFig.7.
3
TheHOMO(2a(cid:3))ofB isa(cid:2)-MO,formedbytheradialover-
1 3
lapofthe2p-atomicorbitalsonboronatoms,asschematically
illustrated in Fig. 7. The HOMO is a completely delocalized
orbital with single occupation in neutral B , giving rise to a
3
(cid:3)(cid:3)
three-center one-electron (3c-1e) bond. The HOMO-1 (1a ) is
2
a(cid:1)-MO,formedbytheout-of-planeoverlapof2p -AOsfrom
z
the three B atoms. We localized the remaining set of valence
(cid:3) (cid:3)
MOs (1e-HOMO-2 and 1a -HOMO-3) into three 2-center 2-
1
electron(2c-2e)B–Bbondswiththeoccupationnumbers(ON)
1.89eusingNBOanalysisattheRHF/6-311+G*leveloftheory
in the putative B 3+ cation (1a(cid:3)21e(cid:3)4 electronic configuration)
3 1
at the geometry of the neutral B cluster [130]. The strong
3
s–p hybridization in the B 3+ cation (occupation numbers are
3
2s0.962p1.03)isresponsibleforbondingcharacterofthelowest
threevalenceMOs(correctiontoourstatementintheRef.[66]).
Thetwoelectronsinthefullydelocalized(cid:1)HOMO-1makeB
3
(cid:1)-aromatic,obeyingthe4n+2Hu¨ckel’sruleforn=0.Although
nottechnicallyobeyingthe4n+2Hu¨ckel’sruleforaromaticity,
B isalsopartially(cid:2)-aromaticderivedfromthesingleoccupa-
3
tionofthefully(cid:2)-delocalizedHOMO.
1a(cid:3)2T1hee(cid:3)41sae(cid:3)(cid:3)c1o2nad(cid:3)2 elolewctersotn-eicnecrgoynfigsutaratetionDa3hrise2sAf(cid:3)2r(cid:3)omwitthhe otnhee Foribgi.t7al.sMgiovleencuinlaarcocrobridtaalncpeicwtuirtehoRf(Uth)eOBV3G(FD/36h-321A1(cid:3)1+)Gc(l2udstfe)rc.aTlchuelaotridoenrs.ofthe
1 2 1
2820 A.N.Alexandrovaetal./CoordinationChemistryReviews250(2006)2811–2866
LinearB excitedisomerswerefirstidentifiedbyPellegatti
3
etal.[119]intheirCI-calculationsandhavebeenlaterfurther
studiedbyHernandesandSimons[113]andCaoetal.[120].All
linearisomersaresignificantlyhigherinenergythantheglobal
minimumstructure.
The hydrogenation of B has been studied by Hernandez
3
and Simons [113]. Two stable B H, and three stable and one
3
metastable B H species have been identified. The hydro-
3 2
gen atom abstraction (B +H →B H+H) was found to be
3 2 3
endothermicandlessfavorablethantheB +H →B H reac-
3 2 3 2
tion.B H (x=0–3)havebeenobservedbymassspectrometry
3 x
andalternativestructuresandtheirstabilitieshavebeenstudied
viaMOcalculations[114].
3.2.2. B +
3
Theclosed-shellcationicB + clusterwaspredictedcompu-
3
tationallytohaveaD symmetrywiththe1A(cid:3) (1a(cid:3)21e(cid:3)41a(cid:3)(cid:3)2)
3h 1 1 2
ground electronic state. It originates from B by removing an
3
electronfromthesinglyoccupiedHOMO[116,120,122–128].
TheoreticalmolecularparametersofthegroundstateofB +are
3
presentedinTable2[131].
Otherspectroscopicstatesofthetriangularstructureandthe
Fig.8. Low-lyingstatesinB3.Curvesrepresentstatesthathavebeengeomet- linear isomers of B + were found to be higher in energy. The
ricallyoptimized.Thehorizontallinesrepresentstatesforwhichenergiesare 3
knownbutwhosegeometrieshavenotbeenoptimized.(Reprintedwithpermis- atomization energy for B3+ (B3+(1A(cid:3)1)→2B(2P)+B+(1S))
sionfromRef.[110].Copyright1991AmericanInstituteofPhysics.) attheCCSD(T)/6-311+G(2df)//CCSD(T)/6-311+G*+ZPElevel
is 151kcal/mol [131]. Positively charged boron clusters B +
n
tions. The two low-lying 12E(cid:3) and 22E(cid:3) states were assigned (n=2–6)werestudiedbyGarcia-Molinaetal.[126].Theauthors
analyzed theoretically the effect of stopping power due to the
as originating from the ground electronic state through the
2a(cid:3)-HOMO→2e(cid:3)-LUMO and 1e(cid:3)-HOMO-1→2a(cid:3)-HOMO geometricstructuresofpure-boroncationicclustersincidenton
1 1 anamorphouscarbontarget.Thestoppingpowerwasfoundto
excitations, respectively, though with the substantial mixing
betweenthetwo.Thecomplexvibrationalstructureinthe12E(cid:3) increasewithincreasingclustersize.
ThemolecularorbitalpictureoftheglobalminimumofB +
statewasinterpretedonthebasisofthestrongJahn–Tellerdistor- 3
tion.The22E(cid:3)stateundergoesonlyarelativelyweakJahn–Teller icsomveprlyetseilmyiblaorndtointghaftulolyf tdheelonceaulitzraeldB(cid:1)3-MclOust(e1ra(cid:3)((cid:3)F)iigs.n7o)w. Tthhee
distortionandshowsashortprogressionwithanobservedfre- 2
quency of 981(10)cm−1 which compares favorably with the HOMO,whichisdoublyoccupied.Thecationthusremains(cid:1)-
theoreticalfrequencyof973cm−1.Thecalculatedpositionsfor aromatic according to the 4n+2 Hu¨ckel’s rule, but it is not a
the12E(cid:3) and22E(cid:3) bandoriginsattheCASSCF-MRCISD/aug- (cid:2)-aromatic species anymore. The cation still has three 2c-2e
B–Bbonds.
cc-pvTZ (augmented by 2s1p bond functions) level of theory
were found to be T =12,829cm−1 and 22,272cm−1 which
00
−
are in reasonable agreement with the experimentally observed 3.2.3. B3 anditsphotoelectronspectra
13,587cm−1and21,828cm−1,respectively[75,76]. The anionic Bn− clusters have received considerably less
TheatomizationenergyfortheB clusterinthegroundstate attentionthantheneutralandcationicspecies.However,anionic
3
wasfirstcalculatedbyMartinetal.[115]tobe193kcal/moland clustersareespeciallyimportantforthecontemporaryinorganic
189kcal/molattheG1andmodifiedG1levelsoftheory,respec- chemistry,sincepotentiallytheycanbeusedasnewinorganic
tively. Our value at the CCSD(T)/6-311+G(2df)/CCSD(T)/6- ligands, for example, by coordination to cationic metal atoms.
311+G*+ZPEleveloftheoryis185kcal/mol[131]. Onlyalimitednumberofexperimentalandtheoreticalstudies
−
An experimental electron resonance spectrum of B3 was havebeendoneontheanionicB3 cluster[66,75,120,130].The
recorded by Weltner and co-workers [111] and a follow-up anionic B − species has D symmetry with 1a(cid:3)21e(cid:3)41a(cid:3)(cid:3)21a(cid:3)2
3 3h 1 2 1
theoretical study (MC-SCF) on the isotropic and anisotropic closed-shell electronic configuration. Molecular properties of
−
components of the hyperfine coupling tensor of B was car- theB clusteraregiveninTable2[66].
3 3
ried out [117]. The authors of the theoretical work concluded Kuznetsov and Boldyrev [130] predicted the vertical one-
−
thatinnoblegasmatricesB rotatesrapidlyarounditsC -axis, electrondetachmentenergiesforB .Zhaietal.obtainedthe
3 3 3
−
andthehyperfinecouplingpredictedtheoreticallyiscompletely firstPESspectraofB atvariousphotonenergies,asshownin
3
averaged out by such free rotation. Reis et al. performed DFT Fig.9[66].
calculationsondipolepolarizabilitiesandhyperpolarizabilities Allfeatures(exceptpeakB)ofthespectracorrespondtothe
ofboronclusters[118]. previously predicted VDEs [130]. However, the peak B with