Transformation of various multicenter bondings within bicapped-square antiprismatic motifs: -rearrangement.

Reported herein are mutual rearrangements in the whole series of seven bicapped-square antiprismatic -CBH by means of high-quality computations that disprove the earlier postulated (diamond-square-diamond) scheme for these isomerizations. The experimentally existing -1,2-CBH was able to be converted to 1,6-, and 1,10-isomers by pyrolysis, and the (diamond-square-diamond) mechanism was offered as an explanation of these processes. However, these computations disprove the postulated scheme for these isomerizations that take place in the ten-vertex series.

Electrophilic Methylation of Decaborane(14): Selective Synthesis of Tetramethylated and Heptamethylated Decaboranes and Their Conjugated Bases.

The paper reports specific syntheses of methylated decaborane(14), -BH (), derivatives. The reaction of with an excess of neat MeI and AlCl yields 1,2,3,4-Me--BH () essentially quantitatively when performed at room temperature. Heating the same mixture to 120 °C provides 1-I-2,3,4,5,7,8,10-Me--BH ().

Synthesis of closo-1,2-HCBMe and 1,2-HCBMeX (X = I and OTf) Dicarbaboranes and Their Rearrangement Reactions.

Methyl-camouflaged dicarbaboranes closo-1,2- and 1,10-HCBMe have been prepared in high yields either from nido-5,6-HCBH or closo-1,2-HCBH via electrophilic methylation reactions and cluster-rearrangement methods. Prepared were also monosubstituted derivatives of general formulation closo-HCBMe-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 CB carborane scaffolding.

Quantitative syntheses of permethylated -1,10-RCBMe (R = H, Me) carboranes. Egg-shaped hydrocarbons on the Frontier between inorganic and organic chemistry.

Electrophilic methylation of the -1,10-RCBH (1) (R = H or Me) dicarbaboranes at higher temperatures or thermal rearrangement of the 1,6-RCBMe (3) compounds at 400-500 °C generated the B-permethylated derivatives -1,10-RCBMe (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-CB carborane core.

Methyl camouflage in the ten-vertex closo-dicarbaborane(10) series. Isolation of closo-1,6-RCBMe (R = H and Me) and their monosubstituted analogues.

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.

Unusual Closure of the Ten-Vertex Nido Cage via Alkylation: Regiospecific Synthesis of 3-Alkyl Derivatives of closo-1,2-CBH.

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

Quantitative Assessment of Substitution NMR Effects in the Model Series of o-Carborane Derivatives: α-Shift Correlation Method.

The principles of a new α-shift correlation (ASC) NMR method are demonstrated on a model series of substituted derivatives of o-carborane for which reliable NMR data are available. This graphical method revealed an acceptable linear correlation between α(B) or α(C) shifts and those induced by substituents in unsubstituted (u) positions of the carborane cluster. The linearity holds for all nuclei involved in skeletal bonding: Δδ(N) = g × α (where N = B, C, and H).

Electrophilic Halogenation of closo-1,2-CBH.

Initial studies on electrophilic halogenation of the dicarbaborane closo-1,2-CBH (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-CBH-10-X (2) (where X = F, I) and 1,2-CBH-7-X (3) (where X = Cl, I).

Unusual Cage Rearrangements in 10-Vertex nido-5,6-Dicarbaborane Derivatives: An Interplay between Theory and Experiment.

The reaction between selected X-nido-5,6-CBH 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-CBH compounds with one equivalent of PS in hexane or CHCl at ambient temperature led to a 7 → 10 halogen rearrangement, forming a series of PSH[10-X-5,6-CBH] salts. Reprotonation using concentrated HSO in CHCl generates a series of neutral carbaboranes 10-X-5,6-CBH, with the overall 7 → 10 conversion being 75%, 95%, and 100% for X = Cl, Br, and I, respectively.

Prototropic μ-H and μ-H Tautomers Derived from the [nido-5,6-CBH] Anion.

Reported is an unusual tautomeric behavior within the [nido-5,6-CBH] (1a) cage that has no precedence in the whole area of carborane chemistry. Isolated were two skeletal tautomers, anions [6-Ph-nido-5,6-CBH-μ] (2d) and [5,6-Me-nido-5,6-CBH-μ] (3b), which differ in the positioning of the open-face hydrogen bridge. Their structures have been determined by X-ray diffraction analyses.

Click Dehydrogenation of Carbon-Substituted nido-5,6-C2B8H12 Carboranes: A General Route to closo-1,2-C2B8H10 Derivatives.

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).

Sequential Camouflage of the arachno-6,9-C2B8H14 Cage by Substituents.

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%).

Unique stereocontrol in carborane chemistry: skeletal alkylcarbonation (SAC) versus exoskeletal alkylmethylation (EAM) reactions.

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).

Carbon insertion into arachno-6,9-C2B8H14 via acyl chlorides. skeletal alkylcarbonation (SAC) reactions: a new route for tricarbollides.

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.

Half-pseudoferrocene cations from nucleophilic addition of o-carboranyl anions to the [(η6-mesitylene)2Fe](2+) dication.

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%.

Thermal isomerization of η6-arene ferradicarbolllides. Experimental proof for isolobal relation between (η6-arene)Fe and (η5-cyclopentadienyl)Co cluster units.

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.

Polymethylated [Fe(η6-arene)2]2+ dications: methyl-group rearrangements and application of the EINS mechanism.

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%.

Additive character of electron donation by methyl substituents within a complete series of polymethylated [1-(η6-Me9n)C6H(6-n))-closo-1,2,3-FeC2B9H11] complexes. Linear correlations of the NMR parameters and Fe(II/III) redox potentials with the number of arene methyls.

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.

Synthesis of C-substituted t-BuNH-8,9-R,R'-nido-7,8,9-C3B8H9 (R,R' = H,H; MeH; Me,Me; Ph,H and Ph,Ph) tricarbollide compounds and their tautomeric conversions. Effect of substituents on tautomeric equilibria between neutral and zwitterionic forms.

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).

Synthesis and electrochemistry of some ferrocenyl substituted dicarba- and tricarbaborane compounds.

Treatment of arachno-4-Me(2)S-B(9)H(13) (1) with ethynylferrocene, FcC[triple bond]CH (Fc = ferrocenyl), in boiling benzene afforded an inseparable 2 : 1 mixture of 6-Fc-nido-5,6-C(2)B(8)H(11) (2a) and 5-Fc-nido-5,6-C(2)B(8)H(11) (2b) in a 10% yield. However, compound 2b was converted quantitatively to the pure, violet 2a by the action of proton sponge (PS), followed by acidification. The structure of 2a was determined by X-ray diffraction analysis.

Toward the synthesis of high boron content polyanionic multicluster macromolecules.

Reported are further consequences of the dioxane ring opening in [3,3'-Co(8-(CH(2)CH(2)O)(2)-1,2-C(2)B(9)H(10))(1',2'-C(2)B(9)H(11))], [1], with 12-vertex carborane mono- and dianions. The removal of one BH vertex from the 1,2-closo-C(2)B(9)H(12) part of the double-cluster monoanions of type [1''-X-2''-R-closo-1'',2''-C(2)B(10)H(11)](-), [2](-) (where X = [3,3'-Co(8-(CH(2)CH(2)O)(2)-1,2-C(2)B(9)H(10))(1',2'-C(2)B(9)H(11))](-) and R = H, [2](-); CH(3), [8](-) and C(6)H(5), [9](-)), via heating with ethanolic KOH or CsF led to the isolation of a series of orange dianions having the general formula [7''-X-8''-R-7'',8''-nido -C(2)B(9)H(11)](2-) (R = H, [11](2-); CH(3), [12](2-); and C(6)H(5), [13](2-)). The same procedure applied to the dianionic triple-cluster compound [1'',2''-X(2)-1'',2''-closo-C(2)B(10)H(10)](2-), [5](2-), yielded the trianionic species [7'',8''-X(2)-7'',8''-nido -C(2)B(9)H(10)](3-), [14](3-).

Ten-vertex polyhedral azametallaborane chemistry: a unique nido-6,9 to nido-6,8-cluster isomerization.

Reaction of the [arachno-4-NB(8)H(12)](-) anion 2 with [RhCl(2)(eta(5)-C(5)Me(5))](2) in CH(2)Cl(2) at room temperature affords a mixture of red '6,9' isomer [9-(eta(5)-C(5)Me(5))-nido-6,9-NRhB(8)H(11)] (3) and its yellow '6,8' isomer, [8-(eta(5)-C(5)Me(5))-nido-6,8-NRhB(8)H(11)] (4). Under the same conditions, reactions of 2 with [IrCl(2)(eta(5)-C(5)Me(5))](2) and [RuCl(2)(eta(6)-MeC(6)H(4)-4-(iso)Pr)](2) give the '6,8' isomers, yellow [8-(eta(5)-C(5)Me(5))-nido-6,8-NIrB(8)H(11)] (5) and red [8-(eta(6)-MeC(6)H(4)-4-(iso)Pr)-nido-6,8-NRuB(8)H(11)] (6), respectively. In contrast, [IrCl(PPh(3))(3)] yields orange [9,9-(PPh(3))(2)-9-H-nido-6,9-NIrB(8)H(11)] (7), which exhibits the '6,9' configuration.

Reductive degradation of nido-1-CB8H12 into smaller-cage carborane systems via new monocarbaboranes [arachno-5-CB8H13](-) and closo-2-CB6H8.

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.

Azatricarbaborane 7-t-Bu-arachno-7,1,5,12-NC3B8H12 and Parent Tricarbaboranes nido-[5,6,9-C3B7H10]- and -5,6,9-C3B7H11.

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).