摘要:
Tellurium compounds of the main-group elements: progress andprospects?Tristram ChiversDepartment of Chemistry, The University of Calgary, Calgary, Alberta T2N IN4, CanadaThe discovery of tellurium (Latin tellus, earth) in ores mined inTransylvania in 1782 predated the isolation of selenium by 35years. ' Despite this historical precedence, investigations oftellurium compounds have lagged behind developments inselenium chemistry. There is, however, an increasing interest intellurium chemistry that is reflected in the number of recentreviews dealing with various aspects of the subject. Theseinclude tellurium-rich tellurides,2 the co-ordination chemistryof telluride ligands, the synthesis and structures of tellurium-nitrogen compound^,^ stereochemical aspects of telluriumcomplexes with sulfur ligands, intramolecular co-ordination intellurium chemistry,6 tellurium polycations, and electrochemi-cally generated tellurium-containing polyanions.The major consumers of tellurium (worldwide production ca.260 tonnes in 1991) l b have traditionally been the metallurgicaland rubber industries, but electronic applications of tellurides,particularly of the Zn/Cd/Hg triad, in solar cells, IR detectors,and optical fibre communication devices are attractingincreasing attention.The potential uses of transition-metaltellurides have been the source of much of the extensive recentwork on the co-ordination chemistry of telluride l i g a n d ~ . ~Many main-group element tellurides, e.g. SnTe, GaTe anionsand Bi,Te,, also have desirable properties for applications inoptical or magnetic devices and in the semiconductorindustries.8n The present technology for the preparation of thesetellurides includes aqueous precipitation, gas-phase deposition,or the reaction of the elements at elevated temperatures. Amajor focus of recent work8b has been the development ofalternative routes to these interesting materials.From the viewpoint of fundamental chemistry, the synthesisof multiply bonded tellurium derivatives represents aconsiderable, but worthwhile, challenge since such compoundsexhibit unusual reactivities. At the same time inorganictellurium compounds generate excitement for the syntheticchemist because unique structural features are often encoun-tered. The intention of this article is to reflect upon these threeaspects of the chemistry of main-group element-telluriumcompounds with emphasis on significant developments duringthe past 5 years.Structural Chemistry of Tellurium CompoundsInorganic chemistry textbooks, with one notable exception, laaccord scant attention to tellurium compounds.Consequently,most chemistry students are unaware of the remarkablepolymeric structures of the tellurium subhalides Te,CI,, Te,X(X = Br or I) and TeI. The fascinating structural chemistry ofelemental tellurium is highlighted by recent discoveries ofpolyanions and polycations with unique frameworks. Forexample, the Te,,,- which consists of a puckered chain[d(Te-Te) 2.71-2.78 A, cf: 2.75 8, for a Te-Te single, bond] l ointerconnected by weak Te-Te interactions [d(Te-Te) 3.16-3.50 8,] to give a two-dimensional network [Fig.l(a)] is thelongest structurally characterized polychalcogenide ion. TheS,,'- ion has been claimed as the product of the reaction ofsulfur and K,CO, in acetone, but the characterization relied onchemical analyses. l 4 Attempts to prepare Se,,' - from seleniumunder similar conditions yielded the novel [Se,C(Se)COMe] -anion in which the six-membered Se,C ring adopts a chair con-formation." The unbranched structure of Te, ,,- is particu-larly surprising in view of the disposition of the heavier chalco-gens to form 'non-classical' anions,2b e.g. in polyselenides withbicyclic (SelO2-)l6 and spirocyclic (Se, 12- and Se,,'-) " , I 8structures. Indeed the most common structural motifs forpolytellurides are the T-shaped [TeTe,]" - or square [TeTe,]" -arrangements as found, for example in the spirocyclic Te,,-anion [Fig.l(b)]." In contrast, the SB2- anion has anunbranched chain structure. 19,20The anions in tellurium-rich polytellurides form two-dimensional infinite networks, e.g. (Te,-),, in RbTe," and(Te,,-),, in CS,T~,,.'~ The polytelluride in the former consistsof Te, rings in a chair conformation connected by four Te-Tebonds in a layer structure. T-Shaped TeTe, units are apparentin this structure [Fig. l(c)]. The layer anions (Te,,-),, inCs,Te, , contain two crystallographically independent Teatoms with linear or T-shaped co-ordination geometriesconnected to give Te, and Te,, squares [Fig.I(d)]. Theremarkable structure of Cs,Te,, also incorporates Cs + ions co-ordinated to a cubic arrangement of eight tellurium atoms fromtwo crown-shaped Te, rings [Fig. 2(b)].I3 Cyclic allotropes oftellurium have not previously been characterized, althoughthere is NMR evidence for cyclo-TeS, in sulfur-telluriummelts.,' This discovery suggests the prospect of interestingdevelopments in the chemistry of homocyclic Te rings.Like the other chalcogens tellurium also forms polycationswith weakly nucleophilic anions in highly electrophilic media.The square planar Te,, + and trigonal prismatic Te6,+ cationshave been known for many years., More recently, the oxidationof tellurium by high oxidation state transition-metal halides hasemerged as an alternative to the use of more traditionaloxidizing agents, e.g.MF, (M = As or Sb) or S206F,, for thegeneration of novel tellurium polycations. As indicated in Fig.2 a plethora of structural types has been discovered, includingexamples of polycations for which several isomers exist. Forexample, the monomeric square-planar cation Te,2 + 27 alsoexists as a dimer in the Teg4+ cation [Fig. 2(a)] and in a one-dimensional polymeric form (Te,, +)" [Fig. 2(b)].22b In Te,4+the Te, squares [ld(Te-Te)( 2.76 8,] are linked by two Te Teinteractions of 3.01 8, and the elongated Te, cubes form strands.In addition to Te,,' [Fig. 2(c)],', with a boat conformation,and (Tel,2f),,22b three isomers of Te,,+ [Fig. 2(d), 2(e) and2 0 1 2 2 b 7 c 3 2 4 and two isomers ~ f T e , ~ + [Fig.2(g) and 2(h)] 2 5 , 2 6have been structurally characterized. A comparison of theframeworks of these new polycations reveals some interestingstructural patterns that may be indicative of additionalpossibilities. The tricyclic Teg2+ [Fig. 2(d)] is derived fromTe," by the formal insertion of a Te-Te bridge. The infinitechain structures of (Te,,'),, [Fig. 2(g)] and (Te,,+),, [Fig.2 ( f ) ] involve Te, rings bridged by one or two Te atoms,respectively. The polymeric structures of (Te72 +)n [Fig. 2(h)]and (TelO2+), [Fig. 2(i)] are closely related and, reminiscent of f Non-SI unit employed: Torr z 133.322 Pa.J. Chem. Soc., Dalton Trans., 1996, Pages 1185-1194 118TeITe - Te - Te- Te -TeTeTe- Te - TeI ITe - Te- Te -TeI ITe TeITeII I ITe - Te- Te- Te - TeI I IIII ITe- Te - Te - Te - TeITe -Te -TeI( d )Fig.1pentaoxacyclopentadecane)," (c) (Te,-),, in RbTe,12 and (d) (Te63-)n in Cs,Te,,I3Structures of tellurium polyanions: (a) Te,,'- in (NEt4),Tel,,9 (h) TeE2 ~ in [K( 1 5-crown-5),],Te8 (1 5-crown-5 = 1,4,7,10,13-1 *+ Te-Te/Te-Te,Te-TeTe-Te- Teo 'Tea Te- Te- -Tea )TeaTe -Te - Te/ \ \Te,,-, display spirocyclic Te centres. For example, the Te, unitin (Te,, +), consists of a central tellurium co-ordinated via fourlong Te-Te bonds (mean value 2.95 .$) to two Te, units in whichthe Te-Te distances are 2.76 A. The Te, units are connected bycovalent Te-Te bonds of 2.88 8, to give the one-dimensionalstrands.The bicyclic structure of Te,' + [Fig. 2(e)] resemblesthat of the well known s S 2 + and Se," cations, but thetransannular interaction in Te,, + [d(Te-Te) 2.99 A] is strongerthan those in SB2+ or Se,,+. Thus a wide variety of poly-cationic alternatives to Te,' + now exist as potential sourcesof polytellurium fragments or rings in metal complexes. 3bPolyanions that incorporate tellurium and other p-blockelements, especially those of Group 15, exhibit tremendousstructural variety. This field originated with the pioneeringwork of Zintl and co-workers in the 1930s.Ib It has experienceda rebirth in view of the possible applications of ternary main-group element tellurides, e.g. MM',Te, (M = Cd or Hg, M' =Ga or in the semiconductor industries resulting fromtheir optical, thermal and electrophysical properties.Thetraditional methods for the synthesis of Ga/In-Te anions, e.g.K,Ga,Te,,,' involve either the high temperature fusion ofthe elements or hydrothermal techniques. A milder approachto these materials involves the exploitation of Zintl's electro-chemical method. This approach is potentially very versatile.For example, the cathodic dissolution of a Ga,Te, electrode inan ethylenediamine (en) solution of [PPh,]Br yields [PPh,]-[GaTe,(en)] 31 and a similar electrochemical synthesis hasbeen used to prepare [NBU",][I~T~,],~~ which consists ofone-dimensional chains of InTe, tetrahedra. The structurallyrelated polymer RbInTe, has been prepared by high temper-ature fusion of Rb,Te,, In and Te at 400 OC.,, The iso-electronic polymeric anion (GeInTe, -) in [PPh,][GeInTe,]also forms one-dimensional chains [Fig.3 ( ~ ) ] . ~ , An altern-ative, low temperature route to telluridometalates involves thereduction of main-group element tellurides with potassium inliquid ammonia in the presence of an encapsulating ligand,e.g. 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane). 34,35This procedure has been used to generate the [GaTe,13- ionwhich, in K[K( 1 8-crown-6)],[GaTe3]-2MeCN, has a some-what distorted trigonal planar structure and the novel In,Te,'-ion.34 The latter consists of a cubic [In,Te,]+ framework(where one corner is vacant) with a terminal Te2- ion attachedto each In3+ centre [Fig. 3(b)].The best known examples of Group 14 element-telluriumanions are of the type [M,Te312 - (1, M = Sn or Pb) for the + 2oxidation state of the metal and [SnTe414- (2) for Sn in the +4oxidation state.The trigonal-bipyramidal structure of 1 (M =Sn3,0 or Pb35,36b) is well established. The anion in M,GeTe,(M = Rb or Cs) consists of GeTe, tetrahedra linked by Te-Tebonds.The known arsenic telluride ions include [As,Te,14- (3),38[As2Te5l2- (4),39 [AS,Te614- (5),,' [AS2Te6]2- (6) 38740 and[As,,T~,]~ - (7).,' These As-Te anions adopt structures withthree-co-ordinate As, two-co-ordinate Te and terminal(unidentate) Te atoms (Fig. 4), which can readily berationalized by a two-centre two-electron bonding description.The [As,Te,14- ion (3) adopts a staggered (C,,) structure,1186 J.Chem. SOC., Dalton Trans., 1996, Pages 1185-119Te IFig. 3 ( a ) One-dimensional chain in [(In2Te,2-},] or [(GeIn-Te4-]J 33 and ( h ) the [In,Te,]' ion in [NEt4],[In3Te,].0.5Et20 342 1 M = Sn or Pbreminiscent of the isoelectronic P214, with a normal As-Asbond length whereas the [As2Te5l2- ion (4) exists as infiniteone-dimensional chains. 39The electrochemical approach is a particularly fruitful sourceof the unique antimony-tellurium anions [SbTe413 - (8),",[Sb,Te,14- (9),", [Sb4Te414- (lo),,, [Sb6Te914- (ll)", and[Sb9Te613 - (12) 44 (Fig. 4). These anions are all prepared by thecathodic dissolution of an Sb,Te, electrode in ethylenediaminein the presence of tetraalkylammonium cations, NR4+ (R =Me, Et or Pr), which facilitate the isolation of crystalline salts.The pyramidal Sb atom in [SbTe413- is attached to a Te22-and two Te2 - ligands.,, Interestingly, the closely relatedpyramidal [SbTe,13- ion (two Te,'- and one Te2- ligands) isfound in the ternary polytelluride ion [Cu4SbTe,,13 - .45 The[Sb2Te512 - anion consists of two SbTe, trigonal pyramids thatshare a corner Te atom4, while [Sb,Te,14- adopts a foldedfour-membered Sb, ring with terminal Te atoms in equatorialpositions.44 The [Sb6Te,14- ion involves the weak co-ordination of a Te2- ion in a square-pyramidal fashion to thefour Sb atoms of the central Sb4Te, ring of the tricyclic[Sb6Te,12 - ion.43 The unusual structure of the [Sb9Te613- iondefies a simple description.One of the Sb atoms is four-co-ordinate with a weak interaction of 3.205 8, (dashed line) to aterminal Te atom and an unusually long terminal Sb-Te bond(2.994 A, c$ typical values of 2.65-2.75 A).In view of the exten-sive range of frameworks already established for As- or Sb-Teanions (Fig. 4), it is remarkable that, as yet, none of thesestructural types is known for both As and Sb.The ability of tellurium to accommodate high co-ordinationnumbers (i.e. seven or eight) in ions such as [TeF60] -, [TeF,] -and [TeF,]2-46347 is another important feature of thestructural chemistry of tellurium compounds. The I2,Te NMRspectrum of [TeF,]- at 25 "C consists of a binomial octetindicating a stereochemically non-rigid pentagonal-bipyramidalstructure (DSh) like the isoelectronic IF7 r n o l e c ~ l e .~ ~ , ~ ~ The[TeF,I2- ion has not been obtained in a pure state but, on thebasis of vibrational spectroscopy, it exists as a square antiprism(0,,).46" Although the octahedral structure of TeMe6,determined by a gas-phase electron diffraction study,,," is'unremarkable, the high thermal stability of this uniquehexaalkylated derivative of a main-group element isnoteworthy.48bMultiple Bonding in Main-group Element-Tellurium CompoundsMultiple bonding involving the heavier p-block elements hasbeen a topic of considerable interest in recent years andtellurium is no exception. The reluctance of tellurium to engagein p(n)-p(n) bonding with other main-group elements isevident, however, from the observations that Te=C=Te andRN=Te=NR (R = alkyl or aryl) are unknown while (TeO,), isa three-dimensional polymer. By contrast, the correspondingsulfur compounds are stable, multiply bonded monomers witha very extensive chemistry.Although tellurocarbonyls R,C=Te(e.g. R = H) can be stabilized by co-ordination to a transitionmetal, no example of a stable telluroaldehyde or telluroketonewas known until recently.49 The dimer 13a is obtained from thetreatment of adamantanone with the tellurium-transfer reagentMe,A1TeA1Me2. 5 0 The reaction of Hg[Te(CF,)], with AIIEt,in the absence of a solvent produces a deep violet, amorphousmaterial, presumably T d F , , at - 196 OC,'l but the darkred 1,3-ditelluretane 13b is formed upon warming. The firsttelluroketone that is sufficiently stable to be characterizedspectroscopically in solution, 1,1,3,3-tetramethylindantelIurone(14), exhibits a weak visible absorption band at 825 nm,attributed to the n --+ n* transition of the C=Te bond andremarkably deshielded 13C and I2'Te NMR resonances (6 I3C301,6 "'Te 2858).49Although stable compounds containing [Si=Te] multiplebonds are unknown, it is possible that the silatelluroneCp*,Si=Te (Cp* = C,Me,) is an intermediate in the formationof the cyclodisilatellurolane 15 from the reaction of Cp*2Si andTe=PBu,.5 2 The remarkable macrocyclic germanium complex16, in which a terminal telluride ligand is stabilized by thetetramethyldibenzotetraaza[ 14lannulene dianion, has beenreported recently. 53 The Ge-Te bond distance of 2.466( 1) A in 16suggests that the bonding involves contributions from both theGe+-Te- and Ge=Te resonance structures (cf: predicted valuesof 2.59 and 2.39 A for single and double bonds, respectively).53The molecular structure of 16 differs from that of thecorresponding selenide or sulfide complexes in that the benzogroups of the macrocyclic ligand are tilted away from the[GeTe] moiety (as opposed to towards the [GeE] group whenE = S or Se).Attempts to make the Sn analogue of 16 wereunsuccessful, although the corresponding Sn sulfide andselenide have been structurally characterized. 54 Recentsuccesses in the generation and characterization of otherkinetically stabilized 'heavy ketones' involving the [Ge=Se],[Sn=Se] 5 5 b or [Pb=S] 5 6 groups suggest that telluronesR2M=Te (M = Si, Ge or Sn) may be accessible for extremelybulky R groups, at least in situ.The thermal instability of multiply bonded tellurium-nitrogen compounds is manifested by the explosive natureof Te,N,' 7 9 5 8 and potassium triimidotellurite(Iv), K2Te-(NH),.58 Although the formula of Te3N4, the only knownbinary tellurium nitride, represents an obvious difference fromthat of the most common nitrides of sulfur and selenium,i.e.S4N, and Se4N,, it is the expected composition for atellurium(rv) nitride. Both S4N4 and Se,N, are known to adopta cradle-shaped (D2& structure with weak, transannularchalcogen-chalcogen interactions, but the structure of Te,N4 isunknown. However, the unusual framework of derivatives ofthe type N(XTeNSN), (17, X = F or C1) 5 9 consists of a twelve-membered Te,N,S, ring with a central nitrido ligand co-ordinated to three tellurium atoms.It is conceivable that Te,N,also incorporates the p3-nitrido structural motif and that itJ, Chem. Soc., Dalton Trans., 1996, Pages 1185-1194 1184 5,Te-Te, / TeI As/As Te- Te Te6e8 97'Sb1011 12Fig. 4 Structural frameworks of arsenic-- and antimony-tellurium polyanions (see text for negative charges on the anions 3-12)13a R = adamantyl13b R = F14cp+2si /Te\ Si c P , ~\ /Te-Te15 16has a highly associated structure (e.g. 18) as a result ofintermolecular Te - N interactions, cf telluradia~oles,~"*~~accounting for its insoluble nature.Diorganotellurimides R,Te=NR' formally contain atellurium-nitrogen double bond, but the Te-N bond distance of1.98 8, in (4-MeOC,H4),Te=NS0,C,H,Me-461 [ c j d(Te-N)2.05 and 1.83 8, for single and double bonds, respectively]indicates predominantly dipolar character for this bond.17 18Compounds formulated as tellurium diimides 'RN=Te=NR'(R = COMe or ArSO,) have been reported,62 but theirinsolubility and high melting points imply highly associatedstructures. The search for genuine Te-N double bonds hasintensified recently 63-65 and the first structural determinationof a tellurium diimide was reported in 1 994.66 The derivative 19is a dimer with exocyclic Te-N bond lengths of 1.90 A.Theexocyclic imido substituents are in a ?runs orientation withrespect to the Te,N, ring, but 'H, 31P and '"Te NMR studiesindicate that conversion to the cis isomer 20a occurs slowly insolution at 23 0C.60*66 The cis isomer 20b, with exocyclic Te-Ndistances of 1.88 A, and the cyclic tellurium(I1) imide (Bu'NTe),,which is a six-membered ring in the chair conformation, areobtained from the reaction of TeCl, with LiNHBu' in a 1 : 4molar ratio in toluene at -78 0C.67 When this reaction iscarried out in a 2:7 molar ratio, the major product is 21, aprotonated derivative of 20b, in which the exocyclic Te=NBu'bond length of 1.84 A is very close to the predicted double bondvalue.67 The dimer 20b sublimes at 85 "C, 10 Torr and meltsat 100-1 02 "C without decomposition.This high thermal1188 J. Chem. SOC., Dalton Trans., 1996, Pages 1185-119But RU'N NR .N.But But20a R = PPh2NSiMe320b R = But19 R = PPh2NSiMe3But21stability is in marked contrast to that of the correspondingselenium diimide Bu'N=Se=NBu', which decomposes at roomtemperature.68 On the basis of spectroscopic evidence seleniumdiimides are monomeric in solution,68 but the solid-statestructures are unknown.The facile synthesis of the tellurium diimide dimer 20bprovides a unique opportunity to investigate the reactivity ofTe=N double bonds.A characteristic reaction of sulfur diimidesis the quantitative formation of sulfinimidamates Li[RN-S(R')NR], whose dimeric structures have only recently beene ~ t a b l i s h e d , ~ ~ upon treatment with organolithium reagents,LiR'.Surprisingly, the product of the reaction of the telluriumdiimide dimer 20b with phenyllithium contains no Ph groups.7oIt was identified by X-ray crystallography as the dimer[{Li,Te(NBu'),},], 22, which can be obtained in much betteryields ( > 90%) from 20b and four mol equivalents of LiNHBu'.The Te,N,Li, cage in 22 consists of a distorted hexagonalprism containing two pyramidal tris( tert-buty1imido)telluriteions [Te(NBut),l2-. The essentially equal Te-N bond lengthsof ca. 1.98 8, are consistent with the delocalized structure 23.In contrast to the parent [Te(NH),I2- ion,58 the tert-butylderivative 22 can be handled without fear of explosion. Themonoanionic tellurite analogues Te(NBu'),(EBu') (24a, E =0; 24b, E = NH) have also been structurally characterizedrecently in the clusters [{ K(thf)[Te(NBu'),(0But)]},] (thf =tetrahydrofuran) (25) and [{ [LiTe(NBu'),(NHBu'),]LiCl),](26), respectively.The structural parameters for these isoelec-tronic anions are consistent with a Te-E single bond [d(Te-0)2.049(5) 8, in 24a and Id(Te-N)I 2.057(6) 8, in 24bl and a Te-Nbond order of ca. 1.5 [dJ(Te-N)J 1.915(5) 8, and 1.943(5) 8, in24a and 24b, respectively]. The cluster 25 is prepared by thereaction of the tellurium diimide dimer 20b with potassiumtert-butoxide, while the centrosymmetric dimer 26 is obtainedwhen the cage 22 is treated with 2 equivalents of dry HC1 gas.The pyramidal trithiotellurite ion, [TeS3]2-,72 is a versatileligand for transition metals. The bonding modes that have beenestablished to date [see Fig.5(a)] give some indication of thepotential ligand behaviour of the isoelectronic ions, 23, 24aand 24b. For example, the dimeric anion [CU,(T~S,),(S,),]~-consists of two CuS, rings bridged by two monodentate[TeS,12- anions.73 The ability of the [TeS,I2- ligand togenerate novel two- or three-dimensional materials is illustratedby compounds of the type AMTeS, (A = K, Rb or Cs; M =Cu or Ag)74" and A,Mn(TeS,), (A = Cs or Rb).74b Thecomplex CsCuTeS, has a three-dimensional structure in whichthe [TeS,I2- ions bridges three trigonal planar Cu+ atoms. Bycontrast, the other complexes of the type AMTeS, adoptstructures with alternating layers of (A'), cations and(MTeS,)," - anions comprised of [TeS,], - ligands bridgingfour tetrahedral Cu+ or Ag+ centres.A layer structure is alsoobserved for Cs,Mn(TeS,),, but the [TeS,I2 - ligands in thiscomplex bridge three octahedral Mn2 + cations. A differentbonding mode is observed for [TeS3I2- in the anion[Au,(TeS,),I2 - . 7 5 In this complex the linearly co-ordinated-I4/S-Te-s \Te p SS SFig. 5[Te(TeS,),I4- ligand '' Bonding modes of (a) the [TeS,]'- ligand73-75 and (b) the22 23 24a24bAu+ centres are bridged by two bidentate [TeS,I2- ligands togive an eight-membered ring. Interestingly, the reaction of[TeS,I2- or [TeSe,12- with AgNO, generates the new ligands[Te(TeE,),I4- (E = S, Se) in which a tellurium atom bridgestwo [TeE,]*- ions [Fig. 5(b)]. In the isostructural anions[Ag,Te(TeE,),I2- (E = S or Se) this ligand gives rise to anovel cage structure in which a triangular Ag,Te plane issandwiched by two pyramidal [TeE,], - ligands.Significantly,co-ordination of the lone pair on Te to a metal centre has notbeen observed in any of these complexes.The [Te(NBu'),12 - dianion (23) and the tellurium diimidedimer Te,(NBu'), (20b) are isoelectronic with the recentlydescribed Sb-N anions, [Sb(NR),I3- (R = PhCH,CH,),76and [Sb2(NR),l2 - (R = cy~lohexyl),~~ respectively. Parallelsbetween the chemistry of Te-N species and their isoelectronicSb-N counterparts can be anticipated. The dianion[Sb,(NR),I2- has been used to incorporate metals into anSb-N cage, e.g. [Cu,(Sb,(NC,H, and it seems likelythat [Te(NBu'),I2- will also prove to be a useful building blockfor incorporating other elements into Te-N rings and cages.Indeed the usefulness of the reagent 22 has already beenestablished by reactions with BC1,Ph and PC1,Ph whichproduce the four-membered ring 27 containing an exocyclicTe=N double bond and, uia a redox process, the spirocycle 28,respectively.70The recently reported cation [Te,N,C18]2+, 29,79 the dimerof [Cl,Te-N=Te-Cl] +, provides another example of thetendency of multiply bonded tellurium-nitrogen compounds todimerize.J. Chem. SOC., DuIton Trans., 1996, Pages 1185-1194 118Bu' But ButPhJ3\ /.N.But27~ NP h N N N28But But Bur0"'Te - Te' (a ) ( b )CITe@/ \Fig. 6 Bonding contributions in TePMe, ( a ) (3 donation and (6) 'N*Ls :NC1 N,'S=N Tc-back donation (negative hyperconjugation)29 30Me,P'-T? Me,€?!Tt?31a 31bR RTeH32a R = Me32bR=Pr'3 2 ~ R = ButIn contrast, the related selenium-nitrogen-chlorine cation[Cl,SeNSeCl,] + is monomeric in the solid state."Two other examples from Te-N chemistry illustrate theunique structural features or unusual reactivity of telluriumcompounds. The bicyclic structure of [Te,S,N,]' + [d(Te-Te)2.88 is of interest in comparison with the planar,delocalized 10 n-electron ring system [S,N,12 + .Presumably,the folded structure of 30 is a reflection of the weakness ofTe(SpkN(2p) n bonding and the pronounced tendency fortellurium to catenate (cf tellurium polycations). The dark bluetellurium(1rI) cation radical [Te{ N(SiMe,),),]'+ [AsF,] - ismonomeric in the solid state with Id(Te-N)) 1.97 8,82a (cf 2.058, for Te[N(SiMe,),]) 8 2 b consistent with some multiple bondcharacter.The ESR spectrum of a CDC1, solution of thisradical indicates that the unpaired electron is located primarilyon tellurium.Triorganophosphine tellurides TePR, were first preparedmore than 30 years ago by the combination of phosphines withTe powder in boiling toluene,', but the nature of the bonding inthese (and other) four-co-ordinate phosphorus(v) chalcogenidesis still a matter of debate. 84 X-Ray structural determinationsof TePR, derivatives reveal almost identical P-Te distances[d(P-Te) 2.368 8, for T~PBu',,~'" 2.365 A for TePPr',,85b and2.371 8, for Bu',P(T~)NH(C,H,,).~~ It was suggested thatthese bond lengths imply a bond order of 1.5.85 However,solution and solid-state 12'Te NMR and "'Te Mossbauerspectroscopic data have been interpreted to indicate the absenceof multiple bonding in TePR, corn pound^.^^ Recent densityfunctional theory (DFT) calculations on TePMe, revealcontributions from resonance structures 31a and 31 b,consistent with substantial multiple bonding.88 As indicatedin Fig.6, structure 31a involves CJ donation from PMe, toTe, while the n interactions in 31b are due to negativehyperconjugation from the filled p orbitals on Te to thedegenerate (T* orbitals of the PMe, fragment. The backdonation is enhanced by the synergic effect and, in the seriesEPMe, (E =0, S, Se or Te), becomes more important relativeto cr donation, as one descends the series.88Although the thermal instability of TePR, has been angeneration of transition-metal-tellurium clusters, which maysubsequently be pyrolysed to yield binary metal tellurides, bythe reaction of TePR, reagents with low-valent transition-metal compounds.Examples of clusters prepared in this way(PEt3)8],90' [Ni,Te,(PEt,),] and [Ni,,Te, 8(PEt3)l andthe ditelluride [ { Co,MnTe(PEt,),} ,] .90e Other importantapplications of TePR, as Te transfer reagents involve thepreparation of terminal transition-metal tellurides, e.g.[W(PMe,)4(Te),],9 chalcogen-exchange reactions to producetellurometalates from selen~metalates,~~ and insertions intometal-carbon 93*94 or metal-metal bond^.^'include [Fe4Te4(PEt3),],90U [C0,Te8(PEt,),],90b [Pd,Te,-Single Source Precursors of Metal TelluridesTellurides of the Group 12 metals (Zn, Cd or Hg), includingcadmium mercury telluride, are important semiconductors withapplications in infrared, solar cell and optoelectronic devices.The technique of MOCVD (metal-organic chemical vapourdeposition) applied to single source precursors is a potentialsource of thin films at temperatures significantly lower thanthose used in the traditional methods for producing suchmaterials.Tellurolates are important precursors for thegeneration of tellurides. For example, the thermolysis ofmercury tellurolates at 200 "C produces HgTe.96 The phosphinetelluride TePEt, serves as a source of tellurium incombination with HgEt, or, preferably, HgPh, to give pureHgTe in toluene at reflux, presumably via the insertion of Teatoms into Hg-C bonds [equationHgPh, + 2TePEt, Hg(TePh), - HgTe + TePh, (1)Generally tellurols RTeH (R = Ph or alkyl) are consideredto be undesirable reagents on account of their foul smell,toxicity and thermal instability.However, important advanceshave been made recently with the discovery of the remarkablystable tellurol [(Me,Si),Si]TeH (m.p. 128-1 30 "C) 98 andsterically hindered benzenetellurols 32a-32c which, unlikePhTeH, can be isolated as colourless, low melting solids.99"These new reagents have been used for the preparation oftellurolates via tellurolysis of M-C or M-N bonds [equations(2) and (3)].98399" The Cd tellurolate [{Cd[Te(mes)],),][or 2NH(SiMe,),]impediment to the development of the chemistry of phosphinetellurides, and very few metal complexes are known,89 thisproperty has been exploited imaginatively in the use of thesereagents as a more reactive source of tellurium than Te metal."Foremost in this development has been the low-temperature(mes = 2,4,6-Me3C,H,) is a one-dimensional polymer withbridging Te(mes) ligands and tetrahedrally co-ordinated Cd.99"The monomeric adduct [Zn{Te(mes)),(NC,H,),] has alsobeen structurally ~haracterized.~~'1190 J.Chem. SOC., Dalton Trans., 1996, Pages 1185-119NTe LiBu"But2P - Li[But2P(Te)NRl Bu'zPNHR Te(R = Pr' or C6Hll) toluene, reflux'NHR IRScheme 1The gas-phase pyrolysis of M[Te{E(SiMe,),}], (M = Zn,Cd or Hg; E = C or Si) produces crystalline thin films of thecorresponding tellurides; ' O0*' O' CdTe may also be formed bythe decomposition of the polymer [{ Cd[Te(mes)],},] in boilingmesitylene. 99A promising new strategy for the generation of binary metaltellurides films from single source precursors involves thesynthesis, and subsequent thermolysis, of chelate complexes ofphosphine tellurides of the type Bu',P(Te)NHR (R = Pr' orc6H11).86 The general approach is summarized in Scheme 1.This method provides an alternative to the tellurolate-basedroutes to metal tellurides, since it produces thermally stablemetal complexes sublimable in the temperature range 1 15-200 "C.The technique of MOCVD has been widely investigated forthe generation of Group 13-15 semiconductors, e.g.GaAs andInP,'02 from organometallic precursors. In the last 4-5 yearsan interest has developed in complexes containing Al, Ga or Inbonded to tellurium as potential precursors for generating filmsof the corresponding tellurides with useful optoelectronicproperties. A variety of mono-, di- and tetra-meric complexeshave been prepared and structurally characterized.The monomeric aluminium tellurolate Bu',AlTeBu", a usefulreagent in organic synthesis, has been generated fromBu'TeTeBu" and AlHBu',. '03,104 The metal-hydride cleavageof ditellurides has also been used to prepare the monomericadduct AI(NMe,)(TePh), [equation (4)]. lo'AlH,(NMe,) + ; Te,Ph, --+Al(NMe,)(TePh), + ;H, (4)The monomeric gallium tellurolate [(Me,Si),CH],GaTe-[Si(SiMe,),] has been prepared by two routes [equations (5)and (6)] '06 and the homoleptic complex Ga~e{Si(SiMe,),}],GaBr[CH(SiMe,),] , + LiTe[Si(SiMe,),][(Me , Si) , CHI ,Ga-Te [Si( SiMe,) ,] (5)~[(Me,Si),CH],GaGa[CH(SiMe,),] +~Me,Si),SiTeTeSi(SiMe,), - [(Me , Si,)CH] ,Ga-Te [ Si( SiMe,) ,] (6)has also been obtained by the metathetical approach.lo7 Ingeneral, the M-Te distances are significantly shorter (ca.0.15 8,for A1 and ca. 0.20 A for Ga) in monomeric compared to dimeric(bridged) Group 13 tellurolate complexes.Dimers of the type 33 have been obtained by the reaction of(a) GaCl(CH,CMe,), with LiTePh,"* (b) trimesitylindiumwith ditellurides TeR, (R = Pr" or Ph) log and (c) tri-tert-butyl-aluminium or -gallium with elemental tellurium.' "3' ' ' Thedimers 33a and 33b adopt a butterfly arrangement for the four-membered M,Te, ring '' ,*' ', whereas the Al,Te, ring in 33cis planar. ' ' The latter dimer was shown to be an intermediatein the formation of the tetramer (BU'AIT~),.~'~ Tetramers of33a M = Ga, R = CH2CMe333b M = In, R = PI-" or Ph33c M = Al, R = But33d M = Ga, R = But34a M = Cia, R = But34b M = Al, R = Cp*34c M = Ga. R = Cp'R35 R = Si(SiMe3)3the type 34, which have cubane structures,' ''3' 12,' l 3 may beobtained from (a) tri-t-butylgallium and elemental tellu-rium,' '' (b) (AICp*), and elemental tellurium ' l4 and ( c )(GaCl,Cp*), and Te(SiMe,),.' l 3 Interestingly, the Cp* ligandin the Ga complex 34c is CJ bonded", whereas thecorresponding A1 complex 34b exhibits q ' co-ordination of theCp* rings.' 'Although impure In,Te, is produced by heating[ {(mes),In(,u-TePh)},] at 600 OC,' O9 the conversion of theorganometallic complexes 33 or 34 to pure Group 13 telluridesby MOCVD techniques has not yet been achieved. Clearly thereis a need for further investigations of suitable low-temperatureprecursors of binary tellurides of Group 13 and other main-group elements.Several groups have investigated organometallic andinorganometallic Sn-Te complexes as potential single sourceprecursors for SnTe, which is a narrow band gap semiconductorof interest for use in infrared detectors. The homoleptic tin(r1)and lead(n) tellurolates M[Te{Si(SiMe,),)], (M = Sn orPb) are conveniently prepared by protonolysis using the bulkytellurol [(Me,Si),Si]TeH [equation (7)I.l l 4[(Me,Si),Si]TeH + M[N(SiMe,),], --+M[Te{Si(SiMe,),}], + 2NH(SiMe,), (7)(M = SnorPb)The tin(I1) compound 35 is dimeric in the solid state with abutterfly-shaped Sn,Te, ring and exocyclic substituents in a cisorientation.Pyrolysis of the complexes M[Te(Si(SiMe,),}],(M = Sn or Pb) at 250 "C proceeds cleanly to give cubic SnTeand PbTe [equation (S)] containing small amounts of carbonand hydrogen.' l4M[Te{Si(SiMe,),}], - MTe + Te[Si(SiMe,),], (8)(M = Snor Pb)Bis(triphenylstanny1) telluride is readily obtained from thereaction of SnClPh, with anhydrous Na,Te in thf. ' ' ' Pyrolysisof this organometallic telluride at 450 "C under N, producescubooctahedral SnTe (containing 1-5% carbon) according toequation (9).' ' ' This transformation may occur via the terminal3Te(SnPh,), 3 2SnTe + 4SnPh4 + TePh, (9)telluride Ph,Sn=Te, which trimerizes to the unstable six-membered ring (Ph,SnTe),. Surprisingly, in view of thereducing nature of tellurolates, the reaction of Sn"X, (X = C1or Br) with K[Te(mes)] yields the Sn'" derivative Sn[Te(mes)],,which generates SnTe rather than SnTe, upon pyrolysis. ''The tellurides M,Te, (M = As or Sb) and, especially,J. Chem. SOC., Dalton Trans., 1996, Pages 1185-1194 119bismuth-telluride-based alloys are the most efficient semicon-ductor materials for thermoelectric cooling devices. They arealso of interest for their optoelectronic properties. These alloysare tygically made by comelting the constituent elements, inappropriate amounts, above 600 "C.A remarkably facileprocess for the generation of Sb,Te, films by MOCVD has beenreported recently. l1 Polycrystalline Sb,Te, films can be grownat room temperature under reduced pressure ( - 0.25 Torr) bythe combination of Sb(NMe,), and Te(SiMe,), in a gas-phaseMOCVD reactor. When this reaction is carried out in hexanessolution, polycrystalline powders of Sb,Te, (or Bi,Te,) areprecipitated at temperatures as low as -30 "C [equation( 10)l.i 1 72M(NMe,), + 3Te(SiMe,),M,Te, + 6NMe,(SiMe,) (10)(M = Sb or Bi)The thermally unstable organometallic Bi-Te compoundsTe(BiR,), (R = Me or mes), which are prepared by theinsertion of Te into the Bi-Bi bond of Bi,R,, decompose to giveBi,Te, and BiR,.' '* The corresponding antimony compoundEt,SbTeSbEt,, prepared in a similar manner from Sb2Et4 andTe, is not a useful source of Sb,Te,."g Antimony tellurolatesare obtained in high yields by the disportionation of distibanesand ditellurides [equation (1 l)] or, in the case of Et,SbTeEt,R,SbSbR, + R'TeTeR' 2R,SbTeR' (1 1)(R = Me or Et) (R' = Me, Et orp-CH,C,H,)from SbBrEt, and LiTeEt.However, the thermal degradationof Et,SbTeEt produces an Sb-rich film.Polycrystalline Bi,Te, may also be prepared at room temper-ature by a two-step process involving the aqueous reaction ofBi,O, with [Te0,12 - (generated by dissolving Te in 6 mol dm-,nitric acid) followed by hydrogen reduction of the precipitate soformed. lZo Alternatively, Bi,Te, may be synthesized from thecomplex formed by adding tartaric acid and ethylene glycol toa mixture of Bi203 and [Te0,]2-.'21 The product is heatedto 350 "C to remove carbonaceous material and then reducedunder an H, atmosphere to give Bi,Te,.The mechanism of thistransformation merits further investigation.ConclusionThe chemistry of tellurium compounds of the main-groupelements is still relatively unexplored, but recent studiesreinforce the notion that tellurium is often unique among thechalcogens in its structural chemistry. Moreover, multiplybonded tellurium compounds, particularly those involvingnitrogen or phosphorus, exhibit unusual reactivity. Theprospect of discovering novel chemistry should provide a strongincentive for future studies of tellurium compounds which, incertain cases, will be aided by NMR studies ("'Te, I = i,7%)."' In addition, the unique solid-state properties of manybinary or ternary tellurides is likely to lead to an increasinginterest in organometallic or inorganometallic telluriumcompounds as single-source precursors for the generation ofthin films of these desirable materials. Finally, there is anexciting possibility of controlling the design of novel materialswith specific electronic properties based on layered polytelluridestructures." The blending of the pure and applied aspects of thesubject will undoubtedly result in the most significant advances.AcknowledgementsThe author thanks Drs.J. Beck, R. C. Haushalter, J.Ibers,J. Kolis and J. 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ISSN:1477-9226
DOI:10.1039/DT9960001185
出版商:RSC
年代:1996
数据来源: RSC