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Developed was a systematic method for the incorporation of the ((η6-arene)Fe) fragment into a carborane cage. Reactions between [(η6-arene)2Fe](PF6)2, salts and Tl2[nido-7,8-C2B9H11] in refluxing (CH2)2C12 generated a series of [3-(η6-arene)-closo-3,1,2-FeC2B9H11] neutral complexes with variable types of polymethylated arene ligands in the structure. The structures of durene and hexamethylbenzene complexes were established by X-ray diffraction analyses. The work also shows that the original hi%htemperature method for the preparation of the [(η6-arene)2Fe]2+ dications fails for mesitylene and durene compounds, which was overcome by a reliable room-temperature modification. [ABSTRACT FROM AUTHOR]

A series of nido -tricarbollides based on 10,11-X 2 -7-Me 3 N- nido -7,8,9-C 3 B 8 H 8 (X = H, Cl, Br, I) and their protonated, i.e. cationic, counterparts, which have an extra H-bridge over the B10-B11 vector in the nido open pentagonal belt, were prepared. Their structures have been verified by the joint ab initio (DFT)/GIAO(ZORA)/NMR approach, MS and HR-MS spectroscopy, and X-ray diffraction techniques. The iodination process is quite slow and could be monitored by 11 B NMR spectroscopy in conjunction with a computational search for the corresponding reaction pathway. These efforts have revealed the existence of monoiodo nido tricarbollide cationic motifs with an iodine atom in the positions that have features of an asymmetrical bridge over B10(H)-B11(H) in 10,11-H 2 -7-Me 3 N- nido -7,8,9-C 3 B 8 H 8 or B10(I)-B11(H) in 10,11-I,H-7-Me 3 N- nido -7,8,9-C 3 B 8 H 8 . Interestingly, 1,2-(COOH) 2 - closo -1,2-C 2 B 10 H 10 is one of the active acids, which confirms the earlier observation that a dicarboxylic acid based on o -carborane acts as a superacid.

We report the difunctional modification of an anionic cobalta bis(dicarbollide)(1-) cluster with a B(8,8')-oxygen bridging unit that provides structural rigidity and an organic alkylazide substituent(s) on the carbon atoms of the metallacarborane cage. These ions present a good binding motif for incorporation into organic molecules using Huisgen-Sharpless (2+3) cycloaddition reactions. In addition, the compounds are chiral, as verified by separation of enantiomers using HPLC on chiral stationary phases (CSPs) and provide a high electrochemical peak in the window located outside of typical signals of biomolecules.

Reported herein are mutual rearrangements in the whole series of seven bicapped-square antiprismatic closo -C 2 B 8 H 10 by means of high-quality computations that disprove the earlier postulated dsd (diamond-square-diamond) scheme for these isomerizations. The experimentally existing closo -1,2-C 2 B 8 H 10 was able to be converted to 1,6-, and 1,10-isomers by pyrolysis, and the dsd (diamond-square-diamond) mechanism was offered as an explanation of these processes. However, these computations disprove the postulated dsd scheme for these isomerizations that take place in the ten-vertex closo series. Experimentally observed thermal rearrangements, both in the parent and substituted closo -1,2-C 2 B 8 H 10 , closo -1-CB 9 H 10 - , and closo -B 10 H 10 2- , indirectly support these refined computations. All these processes are based on the new concept of the so-called Z -mechanism, being consistent with a transition state of a boat shape with an open hexagonal belt that results from the initial breakage of three bonds. Such bond breakings and the consequent bond formations bring to mind the shape of the letter Z . In effect, the pattern of multicenter bonding shifts from reactant through a transition state to product. The molecular rearrangements that are available experimentally favour either the axial or equatorial isomers, and this ratio depends on temperature and the type of cluster and its substitution.

The electrophilic substitution of icosahedral closo- 1-SB 11 H 11 with methyl iodide has resulted in two B-functionalized thiaboranes, 7,12-I 2 -2,3,4,5,6,8,9,10,11-(CH 3 ) 9 -1- closo -SB 11 and 7,8,12-I 3 -2,3,4,5,6,9,10,11-(CH 3 ) 8 - closo -1-SB 11 , with the former being significantly predominant. These two icosahedral thiaboranes are the first cases of polysubstituted polyhedral boron clusters with another vertex that differs from B and C. Such polyfunctionalizations have increased the earlier observed thiaborane icosahedral barrier, not exhibiting any reactivity toward bases, unlike the parent thiaborane. The search for methylation pathways has revealed that the complete B 11 -methylation is impossible, like in the case of decaborane(14), where this seems to be a result of the positively charged upper parts of these two molecules.

Ring cleavage of cyclic ether substituents attached to a boron cage via an oxonium oxygen atom are amongst the most versatile methods for conjoining boron closo -cages with organic functional groups. Here we focus on much less tackled chemistry of the 11-vertex zwitterionic compound [10-( O -(CH 2 -CH 2 ) 2 O)- nido -7,8-C 2 B 9 H 11 ] ( 1 ), which is the only known representative of cyclic ether substitution at nido -cages, and explore the scope for the use of this zwitterion 1 in reactions with various types of nucleophiles including bifunctional ones. Most of the nitrogen, oxygen, halogen, and sulphur nucleophiles studied react via nucleophilic substitution at the C1 atom of the dioxane ring, followed by its cleavage that produces six atom chain between the cage and the respective organic moiety. We also report the differences in reactivity of this nido -cage system with the simplest oxygen nucleophile, i.e., OH - . With compound 1 , reaction proceeds in two possible directions, either via typical ring cleavage, or by replacement of the whole dioxane ring with -OH at higher temperatures. Furthermore, an easy deprotonation of the hydrogen bridge in 1 was observed that proceeds even in diluted aqueous KOH. We believe this knowledge can be further applied in the design of functional molecules, materials, and drugs.

Methyl-camouflaged dicarbaboranes closo-1,2- and 1,10-H 2 C 2 B 8 Me 8 have been prepared in high yields either from nido-5,6-H 2 C 2 B 8 H 10 or closo-1,2-H 2 C 2 B 8 H 8 via electrophilic methylation reactions and cluster-rearrangement methods. Prepared were also monosubstituted derivatives of general formulation closo-H 2 C 2 B 8 Me 7 -X (X = I or OTf). The permethylated compounds exhibit extreme air stability in comparison to unprotected counterparts as a consequence of rigid, egg-shaped hydrocarbon structures incorporating inner C 2 B 8 carborane scaffolding. The structures of all compounds isolated were confirmed unambiguously by multinuclear ( 11 B, 1 H, 13 C, and 19 F) NMR measurements, supported by X-ray diffraction analyses and geometry optimization methods on several compounds.

Electrophilic methylation of the closo -1,10-R 2 C 2 B 8 H 8 (1) (R = H or Me) dicarbaboranes at higher temperatures or thermal rearrangement of the 1,6-R 2 C 2 B 8 Me 8 (3) compounds at 400-500 °C generated the B-permethylated derivatives closo -1,10-R 2 C 2 B 8 Me 8 (2) in quantitative (>95%) yields. The compounds exhibit extreme air stability as a consequence of a rigid, egg shaped hydrocarbon structures incorporating inner 1,10-C 2 B 8 carborane core., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Reported are procedures leading to the first types of methyl camouflaged dicarbadecaboranes with fewer than eleven vertices. The compounds contain the closo-1,6-C2B8 scaffolding inside the egg-shaped hepta - decamethyl sheath, which imparts unusually high air and solvolytic stability to all of these compounds.

Alkylation of the [nido-5,6-R 1 2 C 2 B 8 H 9 ] - anions (where R 1 = H and Me) with alkyl halides (RX, where R = primary and secondary alkyls) in boiling tetrahydrofuran (THF) proceeds via unusual H 2 elimination, followed by cage closure to give a series of the neutral closo-1,2-R 1 2 C 2 B 8 H 7 -3-R derivatives in ∼70-80% yields. In contrast, treatment of the unsubstituted [nido-5,6-C 2 B 8 H 11 ] - anion with tert-butyl bromide (t-BuBr) led to the formation of the parent closo-1,2-C 2 B 8 H 10 in >85% yield. The constitution of all compounds isolated has been confirmed unambiguously by multinuclear ( 11 B, 1 H, and 13 C) nuclear magnetic resonance measurements and α-shift correlation assessments.

Initial studies on electrophilic halogenation of the dicarbaborane closo-1,2-C 2 B 8 H 10 (1) have been carried out to reveal that the substitution takes place at B7 and B10 vertexes, which are the most removed from the CH positions. The course of the halogenation is strongly dependent on the nature of the halogenation agent and reaction conditions. Individual reactions led to the isolation of the monosubstituted compounds 1,2-C 2 B 8 H 9 -10-X (2) (where X = F, I) and 1,2-C 2 B 8 H 9 -7-X (3) (where X = Cl, I). Disubstituted carboranes 1,2-C 2 B 8 H 8 -7,10-X 2 (4) (where X = Cl, Br, I) were obtained under more forcing conditions. Individual halo derivatives were characterized by mass spectrometry and high-field NMR ( 11 B, 1 H, 13 C) spectroscopy combined with two-dimensional [ 11 B- 11 B]-COSY, 1 H{ 11 B(selective)}, and [ 11 B- 1 H]-correlation NMR techniques. All of the derivatives bearing a halogen substituent in the B10 position exhibit a remarkable antipodal 13 C and 1 H NMR shielding at the CH1 vertex, increasing in the order H < I < Br < Cl < F. The structures of 1,2-C 2 B 8 H 8 -7,10-X 2 derivatives (where X = Cl, I, 4b,d) were established by X-ray diffraction analyses.

The reaction between selected X-nido-5,6-C 2 B 8 H 11 compounds (where X = Cl, Br, I) and "Proton Sponge" [PS; 1,8-bis(dimethylamino)naphthalene], followed by acidification, results in extensive rearrangement of all cage vertices. Specifically, deprotonation of 7-X-5,6-C 2 B 8 H 11 compounds with one equivalent of PS in hexane or CH 2 Cl 2 at ambient temperature led to a 7 → 10 halogen rearrangement, forming a series of PSH + [10-X-5,6-C 2 B 8 H 10 ] - salts. Reprotonation using concentrated H 2 SO 4 in CH 2 Cl 2 generates a series of neutral carbaboranes 10-X-5,6-C 2 B 8 H 11 , with the overall 7 → 10 conversion being 75%, 95%, and 100% for X = Cl, Br, and I, respectively. Under similar conditions, 4-Cl-5,6-C 2 B 8 H 11 gave ∼66% conversion to 3-Cl-5,6-C 2 B 8 H 11 . Since these rearrangements could not be rationalized using the B-vertex swing mechanism, new cage rearrangement mechanisms, which are substantiated using DFT calculations, have been proposed. Experimental 11 B NMR chemical shifts are well reproduced by the computations; as expected δ( 11 B) for B(10) atoms in derivatives with X = Br and I are heavily affected by spin-orbit coupling.

Triethylamine-catalyzed dehydrogenation of carbon-disubstituted dicarbaboranes 5,6-R2-nido-5,6-C2B8H10 [1, where R = H (1a), Me (1b), and Ph (1c)] in refluxing acetonitrile leads to a high-yield (up to 85-95%) formation of a series of dicarbaboranes 1,2-R2-closo-1,2-C2B8H8 (2). The monosubstituted 6-R-nido-5,6-C2B8H11 (3) analogues [where R = Ph (3a), naph (1-naphthyl; 3b), Bu (3c)] afforded 1-R-1,2-closo C2B8H9 (4) isomers [where R = Ph (4a), naph (4b), n-Bu (4c)] as the main products; compounds 4a and 4c were accompanied by 2-R-1,2-C2B8H9 (5) isomers (total yields up to 90%), with the 4/5 molar ratio being strongly dependent on the nature of R (4:1 and 1:1, respectively). All of these cage-closure reactions are supposed to proceed via the stage of the corresponding Et3NH(+) salts of nido anions [5,6-R2-5,6-C2B8H9](-) (1(-)) and [6-R-5,6-C2B8H10](-) (3(-)), which lose H2 and Et3N upon heating (dehydrodeamination). The cage-closure mechanisms leading to closo isomers 2, 4, and 5 have been substantiated by B3LYP/6-31+G* calculations of the reaction profile for a simple 1a(-) → 2a + H(-) conversion. All of the compounds isolated have been characterized by multinuclear ((11)B, (1)H, and (13)C) NMR spectroscopy, mass spectrometry, and elemental analyses, and the structure of 1-Ph-closo-1,2-C2B8H9 (4a) was established by an X-ray diffraction study.

Sequential methylation of arachno-6,9-C2B8H14 (1) led to a series of methyl derivatives and finally to the camouflaging of all boron positions by mixed persubstitution. Thus, deprotonation of 1 produced the [arachno-6,9-C2B8H13] anion (1(-)), the methylation of which with MeI in tetrahydrofuran proceeded on the open-face boron vertexes with the formation of 5-Me-arachno-6,9-C2B8H13 (2; yield 28%) and 5,8-Me2-arachno-6,9-C2B8H12 (3; yield 36%). Observed in this reaction was also a side formation of 2-Me-closo-1,6-C2B8H9 (4; yield 6%).The electrophilic AlCl3-catalyzed CH3(+) attack of the neutral 1 in neat MeI at ambient temperature afforded 1,3-Me2-arachno-6,9-C2B8H12 (5), while a 76-h heating at 120 °C generated a mixture of the di- and triiodo derivatives 1,2,3,4,8,10-Me6-5,7-I2-arachno-6,9-C2B8H6 (6) and 1,2,3,4,7-Me5-5,7,10-I3-arachno-6,9-C2B8H6 (7). On the other hand, a HOTf-catalyzed reaction between 1 and MeOTf at reflux resulted in the isolation of 2-TfO-1,3.4,5,7,8,10-Me7-arachno-6,9-C2B8H6 (8; Tf = CF3SO2; yield 65%). The compounds were characterized by multinuclear ((11)B, (1)H, (13)C, and (19)F) NMR spectroscopy, mass spectrometry, and elemental analysis, and the structures of compounds 1, 1(-), 5, and 6 were established by X-ray diffraction analysis.

Reactions between the arachno-6,9-C2B8H14 (1) dicarbaborane and acyl chlorides, RCOCl (2), are subject to stereocontrol that completely changes the nature of the reaction products. While most chlorides produce the 8-R-nido-7,8,9-C3B8H11 (3) tricarbollides (by skeletal alkylcarbonation=SAC), bulky RCOCls (2; where R=1-adamantyl, 2 a; 1-mesityl, 2 b; 9-anthranyl, 2 c; 1-naphthyl, 2 d) in 1,2-dichloroethane (DCE) in the presence of triethylamine at 40-60 °C gave a series of entirely different 1-R-2-CH3-closo-1,6-C2B8H8 (4) dicarbaboranes upon acidification with conc. H2SO4 (by exosleletal alkylmehylation=EAM). Both types of reactions seem to proceed via a common [8-R-nido-7,8,9-C3B8H10](-) (3(-)) anion which in the EAM case is unstable because of steric crowd and undergoes rearrangement via the isomeric [R-nido-7,8,10-C3B8H10](-) tricarbollide structures which, on protonation, undergo reductive extraction of one CH vertex to generate the 2-CH3 substituent in structure 4., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Reactions between arachno-6,9-C2B8H14 (1) and selected acyl chlorides, RCOCl, in the presence of PS (PS = "proton sponge", 1,8-dimethylamino naphthalene) in CH2Cl2 for 24 h at reflux, followed by in situ acidification with concentrated H2SO4 at 0 °C, generate a series of neutral alkyl and aryl tricarbollides 8-R-nido-7,8,9-C3B8H11 (2) (where R = CH3, 2a; C2H5, 2b; n-C4H9, 2c; C6H5, 2d; 4-Cl-C6H4, 2e; 4-Br-C6H4, 2f; 4-I-C6H4, 2g; 1-C10H7, 2h; and 2-C10H7, 2i). The best yields were achieved for aryl derivatives (80-95%) while the yields of the corresponding alkyl substituted compounds are lower (60-70%). These skeletal alkylcarbonation (SAC) reactions are consistent with an aldol-type condensation between the RCO group and open-face hydrogen atoms on the dicarbaborane 1, which is associated with the insertion of the carbonyl carbon atom into the structure of arachno-6,9-C2B8H14 (1) under elimination of three extra hydrogen atoms as H2O and HCl. The reactions thus result in an effective R-tricarbaborane cross-coupling. Individual compounds of structure 2 have been purified by chromatography on a silica gel support, using hexane as the mobile phase (R(F) = ∼0.3). Deprotonation agents, such as NEt3, NaOH, NaH, etc., convert tricarbaboranes 2 into the corresponding conjugated anions [8-R-nido-7,8,9-C3B8H10](-) (2(-)) which were isolated as salts with suitable countercations (for example, Et3NH(+), Tl(+), NEt4(+), etc.). The compounds have been characterized by multinuclear ((11)B, (1)H, and (13)C) NMR spectroscopy, mass spectrometry, and elemental analyses. The structures of anions [8-R-nido-7,8,9-C3B8H10]¯ (where R = C6H5, 4-I-C6H4 and 1-C10H7; 2a(-), 2g(-), and 2h(-)) and that of the neutral 8-(1-C10H7)-nido-7,8,9-C3B8H11 (2h) have been established by X-ray diffraction analyses.

Reactions between the mesitylene (mes) dication [(η(6)-mes)(2)Fe](2+) (1a) [(PF(6)(-))(2) salt] and lithium o-carboranes Li[1-R-1,2-C(2)B(10)H(11)] (2) (R = H, 2a; Me, 2b; Ph, 2c) at low temperature (-60 °C, 1 h, followed by stirring for 2 h at r.t.) in THF resulted in a clean addition of the corresponding carborane anions to one of the unsubstituted arene sites in 1a, forming a series of orange monocations of general structure [(η(5)-mes-exo-6-{2-R-1,2-C(2)B(10)H(11)})Fe(η(6)-mes)](+) (3) (R = H, 3a; Me, 3b; Ph, 3c) which were isolated as PF(6)(-) salts (3PF(6)) in yields ranging 50-75%. Individual complexes were obtained on purification by LC or preparative TLC on a silica gel substrate, using MeCN-CH(2)Cl(2) mixtures as the mobile phase. Interestingly, the room-temperature reaction between 2a (threefold excess) and 1a(PF(6))(2) with a reverse order of addition of the reaction components yielded an orange salt [(η(5)-mes-exo-6-{1,2-C(2)B(10)H(11)})Fe(η(6)-mes)](+)[closo-nido-H(11)B(10)C(2)-C(2)B(10)H(12)](-) (3acCA) (cCA = conjucto-carborane anion = [closo-nido-H(11)B(10)C(2)-C(2)B(10)H(12)](-)) as a sole product in 71% yield. The formation of this conjucto anion can be taken as a strong support for the participation of a radical-chain mechanism in the ostensible nucleophilic addition which we suppose to be initiated by the formation of the [(mes)(2)Fe(+)]˙ radical cation. The structures of both 3PF(6) and 3acCA have been established by X-ray diffraction and the constitution of all compounds isolated is in agreement with elemental analyses, multinuclear NMR data, and MS spectra.

The heating of selected [1-(η(6)-arene)-closo-1,2,3-FeC(2)B(9)H(11)] complexes resulted in the thermal rearrangement and isolation of the corresponding 1,2,4-, 1,2,7-, and 1,2,8-cage isomers. Demonstrated here is a similar rearrangement and the NMR behaviour for isostructural [1-(η(5)-cyclopentadienyl)-closo-1,2,3-CoC(2)B(9)H(11)] compounds., (This journal is © The Royal Society of Chemistry 2011)

Reactions between the methylated arenes ArMe(n) [where ArMe(n) = C(6)Me(n)H((6-n)), and n = 1-6] and FeCl(2) in heptane at 90 °C in the presence of anhydrous AlCl(3) give, for the arenes with n = 1-5, extensive isomerisations and disproportionations involving the methyl groups on the arene rings, and the formation of mixtures of [Fe(ArMe(n))(2)](2+) dications that defy separation into pure species. GC-MS studies of AlCl(3)/mesitylene and AlCl(3)/durene reactions in the absence of FeCl(2) (90 °C, 2 h) allow quantitative assessments of the rearrangements, and the EINS mechanism (electrophile-induced nucleophilic substitution) is applied to rationalise the phenomena. By contrast, ArMe(n) / FeCl(2) /AlCl(3) reactions in heptane for 24-36 h at room-temperature proceed with no rearrangements, allowing the synthesis of the complete series of pure [Fe(ArMen)](2+) cations in yields of 48-71%. The pure compounds are characterised by (1)H NMR spectroscopy and electrospray-ionization mass-spectrometry (ESI-MS), and the structures of [Fe(m-xylene)(2)][PF(6)](2) and [Fe(durene)(2)][PF(6)](2) are established by single-crystal X-ray diffraction analyses.

A systematic method for the incorporation of the {(η(6)-Me(n)C(6)H(6-n))Fe} fragment into the dicarbollide cage was developed based on reactions between [(η(6)-Me(n)C(6)H(6-n))(2)Fe][PF(6)](2) salts (1) and Tl(2)[nido-7,8-C(2)B(9)H(11)]. These reactions proceed with elimination of one arene ligand to generate a complete series of the neutral [1-(η(6)-Me(n)C(6)H(6-n))-closo-1,2,3-FeC(2)B(9)H(11)] (2) complexes with n = 1-6 in yields ranging 15-70% depending on the arene. The structures of mesitylene and pentamethylbenzene complexes were established by X-ray diffraction analyses. All compounds were characterized by (11)B and (1)H NMR measurements, mass spectra, melting points and elemental analyses. Correlations between selected (1)H and (11)B NMR parameters and the Fe(II/III) redox potentials and the number of arene methyls for complexes 2 are linear. These facts establish direct evidence for a strictly additive character of electron donation by the methyl substituents to the arene ring and further to the Fe center and the second (dicarbollide) ligand.Correlations between the number of arene methyls (n) and selected (1)H and (11)B NMR parameters or the Fe(II/III) redox potentials for complexes [1-(η(6)-MenC(6)H(6-n))-closo-1,2,3-FeC(2)B(9)H(11)] are of strictly linear character.

Treatment of C-substituted nido dicarbadecaboranes 5,6-R',R-5,6-C(2)B(8)H(10) (1) (where R',R = H,H (1a); H,Me, (1b); Me,Me, (1c); H,Ph, (1d) and Ph,Ph, (1e) with 1,8-bis-(dimethylamino)naphthalene (proton sponge = PS) and t-BuNC in CH(2)Cl(2), followed by acidification, generated a series of pure neutral compounds 7-t-BuNH-8,9-R,R'-nido-7,8,9-C(3)B(8)H(9) (N2) (where R,R' = H,H (N2a); H,Me (N2b); Me,Me (N2c); H,Ph (N2d), and Ph,Ph (N2e)), each of which exhibits tautomerism. Dissolution of the substituted compounds (N2b-N2e) in protic solvents (PRS), such as MeCN and Me(2)CO, leads to tautomeric equilibrium with the zwitterionic tautomers 7-t-BuNH(2)-8,9-R,R'-nido-7,8,9-C(3)B(8)H(8) (Z2) (where R,R'= H,H (Z2a); H,Me (Z2b); Me,Me (Z2c); H,Ph (Z2d) and Ph,Ph (Z2e)), while the unsubstituted compound N2a exhibits absolute tautomerism--a complete conversion into the zwitterionic tautomer Z2a. The tautomeric behaviour of individual compounds is therefore strongly affected by the nature of the substituent, as assessed via NMR spectroscopy in terms of tautomerisation constants K(T) = C(Z2)/C(N2) (where C(Z2) and C(N2) are equilibrium concentrations of Z2 and N2 forms in a given solvent). Individual tautomers were characterised by (11)B and (1)H NMR spectroscopy and the structure of the monomethylated N2b tautomer was determined by an X-ray diffraction study.

The macropolyhedral [B(19)H(22)](-) monoanion 1 and the dianion [B(19)H(21)](2-) 2 are synthesized in consistent 86-92% yields by the reaction of [PSH](+)[syn-B(18)H(21)](-) with BH(3)(SMe(2)) in 1,2-Cl(2)C(2)H(4) at 72 degrees C. ['PS' is an abbreviation for 'Proton Sponge', 1,8-bis-(dimethylamino)naphthalene. 'PSH' is its protonated derivative.] The molecular structures of 1 and 2 were elucidated as their [PS{BH(2)}](+) and [PS{BH(2)}](2)(+) salts 1a and 2a by single-crystal X-ray diffraction studies, in which all atoms were located, and supported by mass spectrometric analyses together with calculations of the cluster molecular geometries (ab ignitio and/or DFT) and of (11)B chemical shifts based on GIAO-DFT shielding tensors. Acidification of dianion 2 with CF(3)COOH in acetonitrile, H(2)SO(4) in dichloromethane, or aqueous HCl results in the clean formation of the monoanion [B(19)H(22)](-) 1. Conversely, shaking a concentrated acetonitrile solution of 1 in 0.5 M aqueous NaOH cleanly yields the [B(19)H(21)](2-) dianion 2. Reaction of a dichloromethane solution of 1 with a 36% aqueous solution of HCHO in the presence of H(2)SO(4) quantitatively converts 1 at room temperature to a 1:1 mixture of the syn- and anti-isomers of B(18)H(22). This cluster dismantling process is the first example of a syn- to anti-B(18)H(22) isomer conversion.

The neutral azatricarbaborane 7-t-Bu-arachno-7,1,5,12-NC(3)B(8)H(12), isolated as a side product (yield 2%) from the new synthesis of 7-t-BuNH2-nido-7,8,9-C(3)B(8)H(10) (yield 70%), can be easily converted to the first parent representatives of the 10-vertex nido family of tricarbaboranes, [5,6,9-C(3)B(7)H(10)]- and 5,6,9-C(3)B(7)H(11).

Treatment of the nido-1-CB8H12 (1) carborane with NaBH4 in THF at ambient temperature led to the isolation of the stable [arachno-5-CB8H13]- (2(-)), which was isolated as Na+[5-CB8H13]-.1.5 THF and PPh4 +[5-CB8H13]- in almost quantitative yield. Compound 2(-) underwent a boron-degradation reaction with concentrated hydrochloric acid to afford the arachno-4-CB7H13 (3) carborane in 70 % yield, whereas reaction between 2(-) and excess phenyl acetylene in refluxing THF gave the [closo-2-CB6H7]- (4-) in 66 % yield. Protonation of the Cs+4(-) salt with concentrated H2SO4 or CF3COOH in CH2Cl2 afforded a new, highly volatile 2-CB6H8 (4) carborane in 95 % yield, the deprotonation of which with Et3N in CH2Cl2 leads quantitatively to Et3NH+[2-CB6H7](-) (Et3NH+4(-)). Both compounds 4- and 4 can be deboronated through treatment with concentrated hydrochloric acid in CH2Cl2 to yield the carbahexaborane nido-2-CB5H9 (5) in 60 % yield. New compounds 2-, 3, and 4 were structurally characterised by the ab initio/GIAO/MP2/NMR method. The method gave superior results to those carried out using GIAO-HF when relating the calculated 11B NMR chemical shifts to experimental data.

Alkynes R(1)R(2)C(2) react with the neutral monocarbaborane arachno-4-CB(8)H(14) (1) at elevated temperatures (115-120 degrees C) under the formation of the derivatives of the ten-vertex dicarbaborane nido-5,6-C(2)B(8)H(12) (2) of general formula 9-Me-5,6-R1,R2-nido-5,6-C(2)B(8)H(9) (where R1,R2 = H,H 2a; Me,Me 2b; Et,Et 2c, H,Ph 2d, and Ph,Ph 2e) in moderate yields (26-52%). Side reaction with PhC(2)H also yields 1-Ph-6-Me-closo-1,2-C(2)B(8)H(8) (3d). In contrast, the reaction between [arachno-4-CB(8)H(13)](-) anion ((-)) and PhC(2)H produces a mixture of the closo anions [1-CB7H8]- (4-) and [1-CB6H7]- (5-) (yields 32 and 24%, respectively). Individual compounds were isolated and purified by liquid chromatography and characterized by NMR spectroscopy ((11)B, (1)H and (13)C) combined with two-dimensional [(11)B-(11)B]-COSY and (1)H-{(11)B(selective)}NMR techniques.

Treatment of a solution of excess PCl(3) and PS (PS = "proton sponge" = 1,8-dimethylamino naphthalene) with arachno-4-CB(8)H(14) (1) in CH(2)Cl(2), followed by hydrolysis of the reaction mixture, resulted in the isolation of the eleven-vertex diphosphacarbaborane nido-7,8,9-P(2)CB(8)H(10) (2) (yield 34%) as the main product. Other products isolated from this reaction were the phosphacarboranes nido-7,8,9,10-P(3)CB(7)H(8) (3) (yield 5%) and closo-2,1-PCB(8)H(9) (4) (yield 15%). Compound 2 can be deprotonated by PS in CH(2)Cl(2) or NaH in diethyl ether to give the [nido-7,8,9-P(2)CB(8)H(9)](-) (2(-)()) anion, which gives back the original compound, 2, upon re-protonation. Thermal rearrangement of anion 2(-) (Na(+) salt) at 350 degrees C for 2 h produced the isomeric [nido-7,8,10-P(2)CB(8)H(9)](-) (5(-)()) anion, which was isolated as a PPh(4)(+) salt (yield 86%). Multinuclear ((1)H, (11)B, (31)P, and (13)C), two-dimensional [(11)B-(11)B] COSY, (1)H{(11)B(selective)}, (1)H{(31)P(selective)}, and gradient-enhanced ([(1)H-(13)C] HSQC) magnetic resonance measurements led to complete assignments of all resonances which are in excellent agreement with the structures proposed. Coupling constants, (1)J((31)P,(13)C), (2)J((31)P,C,(1)H), and (1)J((31)P,(31)P), were calculated using the DFT method B3LYP/6-311+G(d,p). The molecular geometries of all compounds were optimized ab initio at a correlated level of theory (RMP2(fc)) using the 6-31G basis set, and their correctness was assessed by comparison of the experimental (11)B and (13)C chemical shifts with those calculated by the GIAO-SCF/II//RMP2(fc)/6-31G method. The computations also include the structures and chemical shieldings of the still unknown isomers [nido-7,10,8-P(2)CB(8)H(9)](-) (6(-)) and [nido-7,9,8-P(2)CB(8)H(9)](-) (7(-)).

The reaction of the Tl+ salt of the [nido-7,8,9-P2CB8H9]- anion (1-) with [CpFe(CO)2I](Cp =eta(5)-C5H5) in refluxing mesitylene for 12 h gives mixed-sandwich [1-Cp-closo-1,2,3,4-FeP2CB8H9] (2) (yield 63%). Reaction of the PPh4+ salt of the isomeric [nido-7,8,10-P2CB8H9]- anion 3- with [CpFe(CO)2I] in refluxing mesitylene gives [1-Cp-closo-1,2,3,5-FeP2CB8H9]4 (yield 56%), isomeric with 2. Compound 4 also results (yield 92%) from the sublimation of 2 under argon at ca. 350 degrees C. The constitution of all compounds is established by mass spectrometry, IR spectroscopy and multinuclear NMR spectroscopy (1H, 11B, 31P, and 13C; two-dimensional [11B-11B]-COSY, and 1H- 11B(selective)), further confirmed in the case of 4 by a single-crystal X-ray diffraction analysis.

The reaction of nido-[7,8,9-PC(2)B(8)H(11)] (1) with [[CpFe(CO)(2)](2)] (Cp=eta(5)-C(5)H(5) (-)) in benzene (reflux, 3 days) gave an eta(1)-bonded complex [7-Fp-(eta(1)-nido-7,8,9,-PC(2)B(8)H(10))] (2; Fp=CpFe(CO)(2); yield 38 %). A similar reaction at elevated temperatures (xylene, reflux 24 h) gave the isomeric complex [7-Fp-(eta(1)-nido-7,9,10-PC(2)B(8)H(10))] (3; yield 28 %) together with the fully sandwiched complexes [1-Cp-closo-1,2,4,5-FePC(2)B(8)H(10)] 4 a (yield 30%) and [1-Cp-closo-1,2,4,8-FePC(2)B(8)H(10)] 4 b (yield 5%). Compounds 2 and 3 are isolable intermediates along the full eta(5)-complexation pathway of the phosphadicarbaborane cage; their heating (xylene, reflux, 24 h) leads finally to the isolation of compounds 4 a (yields 46 and 52%, respectively) and 4 b (yields 4 and 5%, respectively). Moreover, compound 3 is isolated as a side product from the heating of 2 (yield 10%). The structure of compound 4 a was determined by an X-ray structural analysis and the constitution of all compounds is consistent with the results of mass spectrometry and IR spectroscopy. Multinuclear ((1)H, (11)B, (31)P, and (13)C), two-dimensional [(11)B-(11)B]-COSY, and (1)H[(11)B(selective)] magnetic resonance measurements led to complete assignments of all resonances and are in excellent agreement with the structures proposed.

The reaction between arachno-4-CB(8)H(14) and PCl(3) in the presence of PS (PS = proton sponge = 1,8-dimethylamino naphthalene) (dichloromethane, rt, 24 h) produced the neutral phosphacarborane closo-2,1-PCB(8)H(9) (35% yield), while a similar reaction of nido-1-CB(8)H(12) gave the isomeric compound closo-6,1-PCB(8)H(9) (27% yield). The structures of both compounds were derived on the basis of the combined ab initio/GIAO/NMR ((1)H, (11)B, (13)C) approach. The optimized structures at a correlated level of theory (MP2) with 6-31G* basis set were used as a basis for calculations of the (11)B and (13)C chemical shifts at GIAO-SCF/II and GIAO-MP2/II, the latter showing excellent agreement with experimental data.

Thermolysis in the solid state of Cs+[arachno-CB9H14]-, or of Cs+[nido-CB9H12]-, or the oxidation of nido-1-CB8H12 with I2 in THF at -78 degrees C in the presence of NEt3, gives the first nine-vertex closo monocarbaborane, the stable [closo-4-CB8H9]- anion, in yields of 56, 61 and 75%, respectively.

The reaction between the carborane arachno-4,6-C2B7H13 (1) and PCl3 in dichloromethane in the presence of a "proton sponge" (PS = 1,8-dimethylaminonaphthalene) resulted in the isolation of the eleven-vertex nido-diphosphadicarbaboranes 7,8,9,11-P2C2B7H, (2) and 3-Cl-7,8,11-P2C2B7H, (3-Cl-2) in yields of 54 and 7%, respectively. Replacement of the PS by NEt3 in the same reaction gave diphosphadicarbaboranes 2 and 3-CI-2 together with the isomeric species nido-7,9,8,10-P2C2B7H, (3) in yields of 28, 15 and 3%, respectively. The reaction between the isomeric carborane arachno-4,5-C2B7H13 (4) and PCl3 in dichloromethane in the presence of PS gave the asymmetrical isomer, nido-7,8,9,10-P2C2B7H, (5). along with the chloro derivatives 4-Cl-7,8,9,10-P2C2B7H8 (4-Cl-5) and 11-Cl-7,8,9,10-PC2B7,H8 (11-Cl-5) (yields of 21, 1 and 13%, respectively). The structures of the chlorinated derivatives 3-Cl2 and 11 -Cl-5 were determined by X-ray diffraction analysis. In addition, the structures of all compounds isolated were geometry-optimised and confirmed by comparison of experimental 11B chemical shifts with those calculated by the GIAO-SCF/II//RMP2(fc)/6-31G* method. The calculations also include the structure and 11B NMR shifts of the isomer nido-7,10,8,9-P2C2B7H9 (6) which has not yet been isolated.