The syntheses and structural characterizations of the first extensive series of Group 9 (Co, Rh, and Ir) tricarbadecaboranyl half-sandwich complexes are reported. The carbonyl complexes 1,1-(CO)2-2-Ph-closo-1,2,3,4-MC3B7H9, M = Co (1), Rh (2), and 8,8,8-(CO)3-9-Ph-nido-8,7,9,10-IrC3B7H9 (3) were obtained by the reactions of Li+(6-Ph-nido-5,6,9-C3B7H9)− with Co(CO)4I, [Rh(CO)2Cl]2, and Ir(CO)3Cl, respectively. Further reactions of 1, 2, and 3 with 1,2-bis(diphenylphosphino)ethane (dppe) yielded the 1,1-dppe-2-Ph-closo-1,2,3,4-MC3B7H9, M = Co (4), Rh (5), and 8-CO-8,8-dppe-9-Ph-nido-8,7,9,10-IrC3B7H9 (6) derivatives with their crystallographic determinations showing that 1 and 2 contain the η6-2-Ph-2,3,4-C3B7H91– ligand with the metallatricarbadecaboranyl cluster fragments having closo-octadecahedral geometries, while 3 has a slipped-cage η4-9-Ph-7,9,10-C3B7H91– coordination and a nido-cluster framework. The reaction of Li+(6-Ph-nido-5,6,9-C3B7H9)− with [Rh(COD)Cl]2 and [Ir(COD)Cl]2 produced the COD coordinated complexes 1,1-COD-2-Ph-closo-1,2,3,4-MC3B7H9, M = Rh (7), Ir (8), with η6-2-Ph-2,3,4-C3B7H91– ligands and closo-cluster structures. On the other hand, slipped-cage structures with η4-9-Ph-7,9,10-C3B7H91– coordination were achieved by the reactions of 1, 3, or 8 with an excess of the stronger donor tert-butyl isocyanide to give 8,8,8-(CNtBu)3-9-Ph-nido-8,7,9,10-MC3B7H9, M = Co (9), Ir (10), respectively, or by the reaction of 8 with 1 equiv of tert-butyl isocyanide to give 8,8-COD-8-CNtBu-9-Ph-nido-8,7,9,10-IrC3B7H9 (11). Upon the reaction of 1 with diphenylacetylene, both carbonyls were displaced with subsequent alkyne cyclization to form the tetraphenylcyclobutadienyl complex 1,1-(η4-C4Ph4)-2-Ph-closo-1,2,3,4-CoC3B7H9 (12). The crystalline tetramethylcyclobutadienyl derivative 1,1-(η4-C4Me4)-2-Ph-closo-1,2,3,4-CoC3B7H9 (13) was synthesized by the reaction of Li+(6-Ph-nido-5,6,9-C3B7H9)− with (η4-C4Me4)Co(CO)2I, and its crystallographic determination confirmed the formation of a complex where a formal Co3+ ion is sandwiched between η4-C4Me42– and η6-2-Ph-2,3,4-C3B7H91– ligands. In contrast to the reactions with diphenylacetylene, the reaction of 8 with 3-hexyne resulted in cage deboronation to produce 2,2-COD-10-Ph-closo-2,1,6,10-C3B6H8 (14). Neither 7 nor 8 would undergo oxidative-addition when treated with I2. Although 11 reacted with I2 and perfluoro-1-iodohexane, oxidative-addition products were also not obtained, but instead, iodation of a cage boron occurred to produce 8,8-COD-1-CNtBu-9-Ph-11-I-nido-8,7,9,10-IrC3B7H8 (15).

In contrast to previous reactions carried out in cyclopentane solvent at room temperature that produced 6-TfO-B10H13 (TfO = CF3SO3), the reaction of closo-B10H102– with a large excess of trifluoromethanesulfonic acid in the ionic liquid 1-butyl-3-methylimidazolium trifluoromethanesulfonate (bmimOTf) gave exclusively the previously unknown 5-TfO-B10H13 isomer. Experimental and computational studies demonstrated that the difference in the products of the two reactions is a result of 6-TfO-B10H13 isomerizing to 5-TfO-B10H13 above room temperature in bmimOTf solutions. Reactivity studies showed that 5-TfO-B10H13: (1) is deprotonated by reaction with 1,8-bis(dimethylamino)naphthalene to form the 5-TfO-B10H121– anion; (2) reacts with alcohols to produce 6-RO-B10H13 boryl ethers (R = Me and 4-CH3O-C6H4); (3) undergoes olefin-hydroboration reactions to form 5-TfO-6,9-R2-B10H11 derivatives; and (4) forms a 5-TfO-6,9-(Me2S)2-B10H11 adduct at its Lewis acidic 6,9-borons upon reaction with dimethylsulfide. The 5-TfO-6,9-(Me2S)2-B10H11 adduct was also found to undergo alkyne-insertion reactions to form a range of previously unreported triflate-substituted 4-TfO-ortho-carboranes (1-R-4-TfO-1,2-C2B10H10) and reactions with triethylamine or ammonia to form the first TfO-substituted decaborate [R3NH+]2[2-TfO-B10H92–], and [R3NH+]2[1-TfO-B10H92–] (R = H, Et) salts.

New linked cyclopentadienyl-tricarbadecaboranyl and bis-tricarbadecaboranyl dianions have been used to form the first examples of ansa-metallatricarbadecaboranyl complexes. The hybrid cyclopentadienyl-tricarbadecaboranyl dianion, Li2+[6-C5H4-(CH2)2-nido-5,6,9-C3B7H9]2– (1), was produced by an initial carbon-insertion reaction of a nitrile-substituted cyclopentadiene with the arachno-4,6-C2B7H12– anion, followed by deprotonation to the dianion with LiH. The linked-cage bis-tricarbadecaboranyl dianion, Li2+[6,6′-(CH2)2-nido-(5,6,9-C3B7H9)2]2– (2), was produced by a similar carbon-insertion route involving the reaction of two equivalents of arachno-4,6-C2B7H12– with succinonitrile. The reaction of 1 with an equivalent of FeCl2 produced the hybrid complex, ansa-(2-(CH2)2)-(1-η5-C5H4-closo-1,2,3,4-C3B7H9)Fe (3), with a crystallographic determination confirming the formation of a sandwich structure where the ring and cage are linked by the ansa −CH2CH2– group with attachment to the cage at the C2 carbon. The reaction of 2 with FeCl2 produced three isomeric ansa-(CH2)2-ferrabistricarbadecaboranyl sandwich complexes, ansa-(CH2)2-(closo-C3B7H9)2Fe (4, 5 and 6). Crystallographic determinations showed that in 4, the two tricarbadecaboranyl ligands are linked by the ansa-CH2CH2- group at the C2 and C2′ cage carbons, whereas in 5 and 6 they are linked at their C2 and C4′ carbons, with the structures of 5 and 6 differing in the relative positions of the C4′ carbons in the two cages of each complex. The structural determinations also showed that, depending upon the linking position of the ansa-tether, constraints in cage-orientation, such as observed in 4, produce unfavorable intercage steric interactions. However, the cage fragments in these complexes can readily undergo a cage-carbon migration that moves one -carbon and its tether linkage to the more favorable 4-position. This isomerization reduces the cage steric interactions and produces configurations, such as those found for 5 and 6, where the iron cage bonding is enhanced as a result of the binding effect of the tether.

Reaction of the amine boranes NH2(R)BH3, where R = H, Me, and Bz, with 1/3 equiv of sodium hexamethyldisilazane produced the five-membered, linear aminoborane anions Na+[BH3N(R)HBH2N(R)HBH3–], where R = H (1), Me (1Me), and benzyl (1Bz). Reactions of 1 and 1Me with ammonium chloride and methylammonium chloride, respectively, resulted in elimination of NaCl and H2 to produce the linear triborazanes BH3(RNHBH2)2N(R)H2, where R = H (2) and Me (2Me), with the structure of 2 crystallographically confirmed. The reactions of 1 and 1Me with pyridine–HCl produced the pyridine-capped aminoboranes H3B(RNHBH2)2(NC5H5), where R = H (3) and Me (3Me). 2 and 2Me proved to be stable up to 90 °C but produced a mixture of products when heated above 90 °C. 2 was selectively monochlorinated at the terminal boron when treated with 1 equiv of HCl and dichlorinated when reacted with a second 1 equiv of HCl.

The selective syntheses of new classes of 6,9-dialkenyl- and 6-alkenyl-decaboranes and 6-alkyl-9-alkenyl-decaboranes have been achieved via iridium and ruthenium catalyzed decaborane and 6-alkyl-decaborane alkyne-hydroborations. Reactions employing [Cp*IrCl2]2 and [RuCl2(p-cymene)]2 precatalysts gave β-E-alkenyl-decaboranes, while the corresponding reactions with [RuI2(p-cymene)]2 gave the α-alkenyl-decaborane isomers, with the differences in product selectivity suggesting quite different mechanistic steps for the catalysts. The alkenyl-decaboranes were easily converted to other useful derivatives, including coupled-cage and functionally substituted compounds, via iridium-catalyzed hydroborations and ruthenium-catalyzed homo and cross olefin-metathesis reactions.

Multi- and double-walled boron nitride nanotubes (BNNTs) have been synthesized with the aid of a floating nickel catalyst via the catalytic chemical vapor deposition (CCVD) of either the amine-borane borazine (B3N3H6) or the polyhedral-borane decaborane (B10H14) molecular precursors in ammonia atmospheres. Both sets of BNNTs were crystalline with highly ordered structures. The BNNTs grown at 1200 °C from borazine were mainly double-walled, with lengths up to 0.2 μm and ∼2 nm diameters. The BNNTs grown at 1200–1300 °C from decaborane were double- and multiwalled, with the double-walled nanotubes having ∼2 nm inner diameters and the multiwalled nanotubes (∼10 walls) having ∼4–5 nm inner diameters and ∼12–14 nm outer diameters. BNNTs grown from decaborane at 1300 °C were longer, averaging ∼0.6 μm, whereas those grown at 1200 °C had average lengths of ∼0.2 μm. The BNNTs were characterized using scanning and transmission electron microscopies (SEM and TEM), and electron energy loss spectroscopy (EELS). The floating catalyst method provides a catalytic and potentially scalable route to BNNTs with low defect density from safe and commercially available precursor compounds.Keywords: borazine; boron nitride nanotubes; chemical vapor deposition; decaborane; floating catalyst; nickel;

A general method for the synthesis of cage-carbon-functionalized cyclopentadienyl iron and cyclopentadienyl ruthenium tricarbadecaboranyl complexes has been developed that employs palladium-catalyzed Sonogashira, Heck, and Stille cross-coupling reactions directed at a cage-carbon haloaryl substituent. The key Li+[6-(p-XC6H4)-nido-5,6,9-C3B7H9–] (X = I (1), Br (2), Cl (3)) haloaryl–tricarbadecaboranyl anionic ligands were synthesized in high yields via the reaction of the arachno-4,6-C2B7H12– anion with the corresponding p-halobenzonitriles (p-XC6H4-CN). The reactions of the salts 1–3 with (η5-C5H5)Fe(CO)2I and (η5-C5H5)Ru(CH3CN)3PF6 were then used to produce the haloaryl complexes 1-(η5-C5H5)-2-(p-XC6H4)-closo-1,2,3,4-MC3B7H9 (M = Fe, X = I (4), Br (5), Cl (6) and M = Ru, X = I (7), Br (8), Cl (9)). The sonication-promoted Sonogashira coupling reactions of 4 with terminal alkynes catalyzed by Pd(dppf)2Cl2/CuI yielded the alkynyl-linked derivatives 1-(η5-C5H5)-2-p-RC6H4-closo-1,2,3,4-FeC3B7H9 (R = (PhC≡C)- (10), (CH3CH2C(O)OCH2C≡C)- (11), ((η5-C5H5)Fe(η5-C5H4C≡C))- (12)). Heck reactions of 4 with terminal alkenes catalyzed by Pd(OAc)2 yielded the alkene-functionalized products 1-(η5-C5H5)-2-p-RC6H4-closo-1,2,3,4-FeC3B7H9 (R = (PhCH2CH═CH)- (13), (CH3(CH2)2CH═CH)- (14)), while the Stille cross-coupling reactions of 4 with organotin compounds catalyzed by Pd(PPh3)2Cl2 afforded the complexes 1-(η5-C5H5)-2-p-RC6H4-closo-1,2,3,4-FeC3B7H9 (R = Ph- (15), (CH2═CH)- (16), (CH2═CHCH2)- (17)). These reactions thus provide facile and systematic access to a wide variety of new types of functionalized metallatricarbadecaboranyl complexes with substituents needed for potential metallocene-like biomedical and/or optoelectronic applications.

A general method for the systematic syntheses of amino acid functionalized metallatricarbadecaboranyl complexes has been developed that employs the copper-catalyzed click addition reactions of N-azidoacetyl amino acid methyl esters to a p-ethynylphenyl substituent at a cage carbon of the metallatricarbadecaboranyl cage. The trimethylsilyl-protected p-ethynylphenyl Li+[6-(p-((CH3)3SiC≡C)C6H4)-nido-5,6,9-C3B7H9–] (1) tricarbadecaboranyl anion was synthesized via the reaction of arachno-4,6-C2B7H12– with p-((CH3)3SiC≡C)C6H5CN. Reaction of 1 with (η5-C5H5)Fe(CO)2I and (η5-C5H5)Ru(CH3CN)3PF6 produced 1-(η5-C5H5)-2-(p-((CH3)3SiC≡C)C6H4)-closo-1,2,3,4-MC3B7H9 (M = Fe (2), Ru (3)). Deprotection of 2 and 3 with K2CO3 in MeOH/THF afforded the ethynyl derivatives 1-(η5-C5H5)-2-(p-(HC≡C)C6H4)-closo-1,2,3,4-MC3B7H9 (M = Fe (4), Ru (5)). Click addition of the model compound benzyl azide to 4 and 5 to form the triazole complexes, 1-(η5-C5H5)-2-(p-(1-(C6H5CH2)-1H-1,2,3-N3C2-4-)C6H4)-closo-1,2,3,4-MC3B7H9 (M = Fe (6), Ru (7)) confirmed the reactivity of the p-ethynyl group toward copper-catalyzed click azide addition, with the triazole-linked structure of 7 crystallographically established. Subsequent click addition reactions of the N-azidoacetyl amino acid methyl esters to 4 yielded a wide range of amino acid functionalized ferratricarbadecaboranyl complexes: 1-(η5-C5H5)-2-(p-(1-(Xaa-C(O)CH2)-1H-1,2,3-N3C2-4-)C6H4)-closo-1,2,3,4-FeC3B7H9 (Xaa = PheOMe (8), LeuOMe (9), ValOMe (10), MetOMe (11), AlaOMe (12), TryOMe (13)).

Significant advantages result from combining the disparate hydrogen release pathways for ammonia-borane (AB) dehydrogenation using ionic liquids (ILs) and transition metal catalysts. With the RuCl2(PMe3)4 catalyst precursor, AB dehydrogenation selectivity and extent are maximized in an IL with a moderately coordinating ethylsulfate anion.

The selective syntheses of new classes of decaboranyl ethers containing a range of functional groups substituted at the B5 or B6 positions were achieved through the reaction of alcohols with halodecaboranes. The surprising regioselectivity of the reaction, where the reaction of the 6-halodecaboranes (6-X-B10H13) with alcohols yielded the 5-substituted decaboranyl ethers (5-RO-B10H13) and the reaction with 5-halodecaboranes (5-X-B10H13) gave the 6-substituted decaboranyl ethers (6-RO-B10H13), was confirmed by NMR and X-ray crystallographic analyses. The crystallographic determinations also showed that the decaboranyl ethers had shortened B−O bonds and apparent sp2 hybridization at oxygen indicating significant π-backbonding from oxygen to the cage boron. A possible substitution mechanism was computationally identified involving: (1) initial nucleophilic attack by the alcohol−oxygen at a site adjacent to the 5- or 6-halo-substituted boron, (2) movement of the terminal hydrogen at the point of attack to a bridging position, (3) formation of a 5-membered (B−O−H−Cl−B) cyclic transition state allowing the acidic methanolic−hydrogen to bond to the halogen, (4) release of HX, and finally (5) movement of a bridging hydrogen into the vacated terminal position. Deuterium labeling studies confirmed the movement of hydrogen from a bridging position of the halodecaborane into the halogen-vacated terminal position on the decaboranyl ether product. The relative reaction rates of the 6-X-B10H13 compounds (X = F, Cl, Br, I) with alcohols were likewise found to be consistent with this mechanism.

The fabrication of 3D diamond-like silicon-oxycarbide and silicon-carbide high-temperature ceramic photonic crystals has been achieved by a strategy involving (1) the use of four-beam interference lithography (IL) to construct a patterned silsesquioxane (POSS) template and (2) infiltration of the polymeric allylhydridopolycarbosilane (AHPCS) silicon-carbide precursor into the patterned POSS template followed by high temperature ceramic conversion and HF etching. Energy-dispersive X-ray mapping analysis and Fourier transform infrared (FT-IR) studies suggested that the 3D ceramic photonic crystals formed at 1100 °C were SiC-like silicon oxycarbide. Additional thermal treatment at 1300 °C in vacuo resulted in the carbothermic reduction of the 3D silicon-oxycarbide to form 3D β-SiC with less than 10% shrinkage in the (111) plane and [111] direction, respectively. The reflectivities of the inverse 3D ceramic photonic crystals obtained at different stages were characterized by FT-IR in the [111] direction. Both the inverse 3D silicon-oxycarbide and silicon-carbide crystals showed bandgaps at 1.84 μm. These experimental values matched well with the calculated bandgaps, further supporting the robustness of such fabricated 3D ceramic photonic crystals.

Transition-metal-catalyzed decaborane−alkyne hydroboration reactions have been developed that provide high-yield routes to the previously unknown di- and monoalkenyldecaboranes. These alkenyl derivatives should be easily modified starting materials for many biomedical and/or materials applications. Unusual catalyst product selectivity was observed that suggests quite different mechanistic steps, with the reactions catalyzed by the [RuCl2(p-cymene)]2 and [Cp*IrCl2]2 complexes giving the β−E alkenyldecaboranes and the corresponding reactions with the [RuI2(p-cymene)]2 complex giving the α-alkenyldecaborane isomers.

High yield syntheses of the 5-X-B10H13 (5X) halodecaboranes have been achieved through the photochemical (X = I) or base-catalyzed (X = Cl, Br, I) isomerization reactions of their 6-X-B10H13 (6X) isomers. 5I was obtained in 80% isolated yield upon the UV photolysis of 6I. Treatment of 6X (X = Cl, Br, I) with catalytic amounts of triethylamine at 60 °C led to the formation of 78:22 (Cl), 82:18 (Br), and 86:14 (I) ratio 5X/6X equilibrium mixtures. The 5X isomers were then separated from these mixtures by selective crystallization (Br and I) or column chromatography (Cl), with the supernatant mixtures in each case then subjected to another round of isomerization/separation to harvest a second crop of 5X. The combined isolated yields of pure products after two cycles were 71% 5-Cl-B10H13, 83% 5-Br-B10H13, and 68% 5-I-B10H13. The previously proposed structures of 5-Br-B10H13 and 5-I-B10H13 were crystallographically confirmed. Deprotonation of 6X and 5X with 1,8-bis(dimethylamino)naphthalene (PS) resulted in the formation of [PSH+][6X−] and [PSH+][5X−]. Density functional theory-gauge-independent atomic orbital (DFT/GIAO) calculations and crystallographic determinations of [PSH+][6Cl−] and [PSH+][6Cl−] confirmed bridge-deprotonation at a site adjacent to the halogen-substituted borons. NMR studies of the 6-Br-B10H13 isomerization induced by stoichiometric amounts of PS showed that following initial deprotonation to form 6-Br-B10H12−, isomerization occurred at 60 °C to form an equilibrium mixture of 6-Br-B10H12− and 5-Br-B10H12−. DFT calculations also showed that the observed 5-X-B10H13/6-X-B10H13 equilibrium ratios in the triethylamine-catalyzed reactions were consistent with the energetic differences of the 5-X-B10H12− and 6-X-B10H12− anions. These results strongly support a mechanistic pathway for the base-catalyzed 6X to 5X conversions involving the formation and subsequent isomerizations of the 6X− anions. While triethylamine did not catalyze the isomerization reactions of either 6-(C6H13)-B10H13 or 6,9-(C6H13)2-B10H12, it catalyzed the isomerization of 6-X-9-(C6H13)-B10H12 to 5-X-9-(C6H13)-B10H12 resulting from halo, but not alkyl rearrangement. Comparisons of the chemical shift values found in the temperature-dependent 11B NMR spectra of 6Cl− and 6F− with DFT/GIAO chemical shift calculations indicate the fluxional behavior observed for these anions results from a process involving hydrogen migration around the open face that leads to the averaging of some boron resonances at higher temperatures.

The ruthenium-catalyzed metathesis reactions of dialkenyl-substituted ortho- and meta-carboranes provide excellent routes to both cyclic-substituted o-carboranes and new types of main-chain m-carborane polymers. The adjacent positions of the two olefins in the 1,2-(alkenyl)2-o-carboranes strongly favor the formation of ring-closed (RCM) products with the reactions of 1,2-(CH2═CHCH2)2-1,2-C2B10H10 (1), 1,2-(CH2═CH(CH2)3CH2)2-1,2-C2B10H10 (2), 1,2-(CH2═CHSiMe2)2-1,2-C2B10H10 (3), 1,2-(CH2═CHCH2SiMe2)2-1,2-C2B10H10 (4), and 1,2-[CH2═CH(CH2)4SiMe2]2-1,2-C2B10H10 (5) affording 1,2-(−CH2CH═CHCH2−)-C2B10H10 (10), 1,2-[−CH2(CH2)3CH═CH(CH2)3CH2−]-1,2-C2B10H10 (11), 1,2-[−SiMe2CH═CHSiMe2−]-1,2-C2B10H10 (12), 1,2-[−SiMe2CH2CH═CHCH2SMe2−]-C2B10H10 (13), and 1,2-[−SiMe2(CH2)4CH═CH(CH2)4SiMe2−]-C2B10H10 (14), respectively, in 72−97% yields. On the other hand, the reaction of 1,2-(CH2═CHCH2OC(═O))2-1,2-C2B10H10 (6) gave cyclo-[1,2-(1′,8′-C(═O)OCH2CH═CHCH2OC(═O))-1,2-C2B10H10]2 (15a) and polymer 15b resulting from intermolecular metathesis reactions. The nonadjacent positions of the alkenyl groups in the 1,7-(alkenyl)2-m-carboranes, 1,7-(CH2═CHCH2)2-1,7-C2B10H10 (7), 1,7-(CH2═CH(CH2)3CH2)2-1,7-C2B10H10 (8), and 1,7-(CH2═CHCH2SiMe2)2-1,7-C2B10H10 (9), disfavor the formation of RCM products, and in these cases, acyclic diene metathesis polymerizations (ADMET) produced new types of main chain m-carborane polymers. The structures of 3, 9, 11, 12, 13, and 15a were crystallographically confirmed.

The potentials of a series of one-electron oxidation and reduction reactions have been determined for manganese group half-sandwich complexes of the tricarbadecaboranyl ligand PhC3B7H9 and the penta-organo fullerene ligand C60Bn2PhH2 (Bn = benzyl). The anodic processes were studied in CH2Cl2 and the cathodic processes were studied in both CH2Cl2 and THF, the supporting electrolyte being [NBu4][B(C6F5)4]. The manganese complex Mn(CO)2(PMe3)(PhC3B7H9) (1) is a member of a three-electron transfer series which includes oxidation to 1+ (0.51 V versus ferrocene) and successive reductions to 1− (−1.66 V) and 12− (−1.77 V). Both the oxidation and reduction of the closely-related complex Mn(CO)2(PPh3)(PhC3B7H9) (2) are chemically irreversible under slow-scan cyclic voltammetry conditions. The rhenium complex Re(CO)2(PPh3)(PhC3B7H9) (3) oxidizes (E1/2 = 0.82 V versus ferrocene) to a radical cation which, unlike its cyclopentadienyl analogue, shows no evidence of dimerization. Oxidation of the fullerene-based complex Re(CO)3(C60Bn2PhH2) is more facile than that of its cyclopentadienyl analogue, in contrast to previous findings in this class of metal-fullerene derivatives. An electrochemical ligand factor, EL, of 0.63 is calculated for the PhC3B7H9 ligand in manganese group half-sandwich complexes.The E1/2 potentials of both the oxidation and reduction of manganese and rhenium half-sandwich complexes have been measured in CH2Cl2 and THF containing [NBu4][B(C6F5)4]. Three tricarbadecaboranyl compounds and one penta-organo fullerene compound were studied. The one-electron oxidation of the Re compounds give radical cations that do not dimerize on the voltammetry time scale. A ligand electronic parameter of 0.63 has been determined for the PhC3B7H9 ligand in these complexes, 0.30 V positive of that of the cyclopentadienyl ligand.

The strong non-nucleophilic base bis(dimethylamino)naphthalene (Proton Sponge, PS) has been found to promote the rate and extent of H2-release from ammonia borane (AB) either in the solid state or in ionic-liquid and tetraglyme solutions. For example, AB reactions in 1-butyl-3-methylimidazolium chloride (bmimCl) containing 5.3 mol % PS released 2 equiv of H2 in 171 min at 85 °C and only 9 min at 110 °C, whereas comparable reactions without PS required 316 min at 85 °C and 20 min at 110 °C. Ionic-liquid solvents proved more favorable than tetraglyme since they reduced the formation of undesirable products such as borazine. Solid-state and solution 11B NMR studies of PS-promoted reactions in progress support a reaction pathway involving initial AB deprotonation to form the H3BNH2− anion. This anion can then initiate AB dehydropolymerization to form branched-chain polyaminoborane polymers. Subsequent chain-branching and dehydrogenation reactions lead ultimately to a cross-linked polyborazylene-type product. AB dehydrogenation by lithium and potassium triethylborohydride was found to produce the stabilized Et3BNH2BH3− anion, with the crystallographically determined structure of the [Et3BNH2BH3]−K+·18-crown-6 complex showing that, following AB nitrogen-deprotonation by the triethylborohydride, the Lewis-acidic triethylborane group coordinated at the nitrogen. Model studies of the reactions of [Et3BNH2BH3]−Li+ with AB show evidence of chain-growth, providing additional support for a PS-promoted AB anionic dehydropolymerization H2-release process.

Simple blends of the poly(norbornenyldecaborane) (PND) boron-carbide preceramic polymer with either of the commercial poly(methylcarbosilane) (PMCS) or allylhydridopolycarbosilane (AHPCS) silicon-carbide preceramic polymers have been found to provide excellent processable precursors to boron-carbide/silicon-carbide ceramic composite materials. The blends exhibited good char yields with tunable ceramic compositions. Certain compositions of the PND/AHPCS derived ceramics also exhibited significant oxidation resistance. Nonwoven mats of PND/PMCS polymer composite fibers were readily obtained by the electrostatic spinning of polymer blend solutions. The pyrolytic ceramic conversion reactions of the PND/PMCS polymer fibers then produced mats of micro- and nanodiameter boron-carbide/silicon-carbide ceramic composite fibers.

The rate and extent of H2-release from ammonia borane (AB), a promising, high-capacity hydrogen storage material, was found to be enhanced in ionic-liquid solutions. For example, AB reactions in 1-butyl-3-methylimidazolium chloride (bmimCl) (50:50-wt %) exhibited no induction period and released 1.0 H2-equiv in 67 min and 2.2 H2-equiv in 330 min at 85 °C, whereas comparable solid-state AB reactions at 85 °C had a 180 min induction period and required 360 min to release ∼0.8 H2-equiv, with the release of only another ∼0.1 H2-equiv at longer times. Significant rate enhancements for the ionic-liquid mixtures were obtained with only moderate increases in temperature, with, for example, a 50:50-wt % AB/bmimCl mixture releasing 1.0 H2-equiv in 5 min and 2.2 H2-equiv in only 20 min at 110 °C. Increasing the AB/bmimCl ratio to 80:20 still gave enhanced H2-release rates compared to the solid-state, and produced a system that achieved 11.4 materials-weight percent H2-release. Solid-state and solution 11B NMR studies of AB H2-release reactions in progress support a mechanistic pathway involving: (1) ionic-liquid promoted conversion of AB into its more reactive ionic diammoniate of diborane (DADB) form, (2) further intermolecular dehydrocoupling reactions between hydridic B−H hydrogens and protonic N−H hydrogens on DADB and/or AB to form neutral polyaminoborane polymers, and (3) polyaminoborane dehydrogenation to unsaturated cross-linked polyborazylene materials.

One-dimensional nanostructures exhibit quantum confinement which leads to unique electronic properties, making them attractive as the active elements for nanoscale electronic devices. Boron nitride nanotubes are of particular interest since, unlike carbon nanotubes, all chiralities are semiconducting. Here, we report a synthesis based on the use of low pressures of the molecular precursor borazine in conjunction with a floating nickelocene catalyst that resulted in the formation of double-walled boron nitride nanotubes. As has been shown for carbon nanotube production, the floating catalyst chemical vapor deposition method has the potential for creating high quality boron nitride nanostructures with high production volumes.

Unlike in conventional organic solvents, where Lewis base catalysts are required, decaborane dehydrogenative alkyne-insertion reactions proceed rapidly in biphasic ionic-liquid/toluene mixtures with a wide variety of terminal and internal alkynes, thus providing efficient, one-step routes to functional o-carborane 1-R-1,2-C2B10H11 and 1-R-2-R′-1,2-C2B10H10 derivatives, including R = C6H5− (1), C6H13− (2), HC≡C−(CH2)5− (3), (1-C2B10H11)−(CH2)5− (4), CH3CH2C(O)OCH2− (5), (C2H5)2NCH2− (6), NC−(CH2)3− (7), 3-HC≡C−C6H4− (8), (1-C2B10H11)−1,3-C6H4− (9), HC≡C−CH2−O−CH2− (10); R,R′ = C2H5− (11); R = HOCH2−, R′ = CH3− (12); R = BrCH2−; R′ = CH3− (13); R = H2C═C(CH3)−, R′ = C2H5− (14). The best results were obtained from reactions with only catalytic amounts of bmimCl (1-butyl-3-methylimidazolium chloride), where in many cases reaction times of less than 20 min were required. The experimental data for these reactions, the results observed for the reactions of B10H13— salts with alkynes, and the computational studies reported in the third paper in this series all support a reaction sequence involving (1) the initial ionic liquid promoted formation of the B10H13— anion, (2) addition of B10H13— to the alkyne to form an arachno-R,R′-C2B10H13— anion, and (3) protonation of arachno-R,R′-C2B10H13— to form the final neutral 1-R-2-R′-1,2-C2B10H10 product with loss of hydrogen.

The high-yield syntheses of 6-X-B10H13 [X = Cl (88%), Br (96%), I (84%)] resulted from the cage-opening reactions of the (NH4+)2B10H102− salt with ionic-liquid-based superacidic hydrogen halides, while both the previously unknown 6-F-B10H13 (77%) derivative and 6-Cl-B10H13 (90%) were synthesized in high yields via the reactions of (NH4+)2B10H102− with triflic acid in the presence of 1-fluoropentane and dichloromethane, respectively. Structural characterizations of 1−4 confirm the predicted structures and indicate strong halogen back-bonding interactions with the B6 boron. The reaction of 6-Br-B10H13 with Bu3SnH produced the parent B10H14 in 70% yield, and thus, this reaction, in conjunction with the haloacid-induced closo-B10H102− cage-opening reactions, has the potential to provide an alternative to the traditional diborane pyrolysis route to decaborane.

Quantum mechanical computational studies of possible mechanistic pathways for B10H13− dehydrogenative alkyne-insertion and olefin-hydroboration reactions demonstrate that, depending on the reactant and reaction conditions, B10H13− can function as either an electrophile or nucleophile. For reactions with nucleophilic alkynes, such as propyne, the calculations indicate that at the temperatures (∼110−120 °C) required for these reactions, the ground-state B10H13− (1) structure can rearrange to an electrophilic-type cage structure 3 having a LUMO orbital strongly localized on the B6 cage-boron. Alkyne binding at this site followed by subsequent steps involving the formation of additional boron−carbon bonds, hydrogen elimination, protonation, and further hydrogen elimination then lead in a straightforward manner to the experimentally observed ortho-carborane products resulting from alkyne insertion into the decaborane framework. A similar mechanistic sequence was identified for the reaction of propyne with 6-R-B10H12− leading to the formation of 1-Me-3-R-1,2-C2B10H11 carboranes. On the other hand, both B10H13− and 4,6-C2B7H12− have previously been shown to react at much lower temperatures with strongly polarized alkynes, and the DFT and IRC calculations support an alternative mechanism involving initial nucleophilic attack by these polyborane anions at the positive terminal acetylenic carbon to produce terminally substituted olefinic anions. In the case of the B10H13− reaction, subsequent cyclization steps were identified that provide a pathway to the experimentally observed arachno-8-(NC)-7,8-C2B10H14− carborane. The computational study of B10H13− propylene hydroboration also supports a mechanistic pathway involving a cage rearrangement to the electrophilic 3 structure. Olefin-binding at the LUMO orbital localized on the B6 cage-boron, followed by addition of the B6−H group across the olefinic double bond and protonation, then leads to the experimentally observed 6-R-B10H13 products.

Unlike in conventional organic solvents where transition metal catalysts are required, decaborane olefin-hydroboration reactions have been found to proceed in biphasic ionic-liquid/toluene mixtures with a wide variety of olefins, including alkyl, alkenyl, halo, phenyl, ether, ester, pinacolborane, ketone, and alcohol-substituted olefins, and these reactions now provide simple high-yield routes to 6-R-B10H13 derivatives. Best results were observed for reactions with bmimX (1-butyl-3-methylimidazolium, X = Cl− or BF4−) and bmpyX (1-butyl-4-methylpyridinium, X = Cl− or BF4−). Both the experimental data for these reactions and separate studies of the reactions of B10H13− salts with olefins indicate a reaction sequence involving (1) the ionic-liquid-promoted formation of the B10H13− anion as the essential initial step, (2) the addition of the B10H13− anion to the olefin to form a 6-R-B10H12− anion, and finally, (3) protonation of 6-R-B10H12− to form the final neutral 6-R-B10H13 product. The 6-R-B10H13 derivatives also undergo ionic-liquid-mediated dehydrogenative alkyne-insertion reactions in biphasic bmimCl/toluene mixtures, and these reactions provide high yield routes to 3-R-1,2-R′2-1,2-C2B10H9 ortho-carborane derivatives.

A general method for the selective functionalization of cyclopentadienyliron tricarbadecaboranyl complexes has been developed that employs selective halogenation of the tricarbadecaboranyl ligand followed by Sonogashira coupling reactions. The reaction of N-chloro (NCS) or N-bromosuccinimide (NBS) with 1-(η5-C5H5)-2-Ph-closo-1,2,3,4-FeC3B7H9 (2) resulted in selective halogenation at the B6-boron of the tricarbadecaboranyl ligand to form 1-(η5-C5H5)-2-Ph-6-X-closo-1,2,3,4-FeC3B7H8 (X = Cl (3), Br (4)), respectively. 4 was also formed selectively by the reaction of 2 with Br2. The AlCl3-catalyzed reaction of 2 with ICl gave an easily separated mixture of 1-(η5-C5H5)-2-Ph-6-I-closo-1,2,3,4-FeC3B7H8 (5), 1-(η5-C5H5)-2-Ph-11-I-closo-1,2,3,4-FeC3B7H8 (6), and 1-(η5-C5H5)-2-Ph-6-I-11-I-closo-1,2,3,4-FeC3B7H7 (7). Reaction of 5 with terminal acetylenes in the presence of (PPh3)2PdCl2/CuI in Et2NH solvent yielded a series of acetylene-functionalized metallatricarbadecaboranyl complexes, 1-(η5-C5H5)-2-Ph-6-(RC≡C)-closo-1,2,3,4-FeC3B7H8 (R = Ph (8), (CH3)3Si (9), CH2OC(O)CH2CH3 (11), (η5-C5H4)Fe(η5-C5H5) (12)). 9 reacted with fluoride ion to give the deprotected acetylene complex 1-(η5-C5H5)-2-Ph-6-(HC≡C)-closo-1,2,3,4-FeC3B7H8 (10). The structures of 3−12 have been crystallographically determined.

The first use of diatom frustules as shape-dictating 3-D templates for the syntheses of nanostructured, nanocrystalline micro-particles of a non-oxide ceramic, boron nitride, is demonstrated.

Studies of the activating effect of Verkade’s base, 2,8,9-triisobutyl-2,5,8,9-tetraaza-1-phosphabicyclo[3.3.3]undecane (VB), on the rate and extent of H2 release from ammonia borane (AB) have led to the syntheses and structural characterizations of three anionic aminoborane chain-growth products that provide direct support for anionic dehydropolymerization mechanistic steps in the initial stages of base-promoted AB H2 release reactions. The salt VBH+[H3BNH2BH2NH2BH3]− (1) containing a linear five-membered anionic aminoborane chain was produced in 74% yield via the room-temperature reaction of a 3:1 AB/VB mixture in fluorobenzene solvent, while the branched and linear-chain seven-membered anionic aminoborane oligomers VBH+[HB(NH2BH3)3]− (2a) and VBH+[H3BNH2BH2NH2BH2NH2BH3]− (2b) were obtained from VB/AB reactions carried out at 50 °C for 5 days when the AB/VB ratio was increased to 4:1. X-ray crystal structure determinations confirmed that these compounds are the isoelectronic and isostructural analogues of the hydrocarbons n-pentane, 3-ethylpentane, and n-heptane, respectively. The structural determinations also revealed significant interionic B–H···H–N dihydrogen-bonding interactions in these anions that could enhance dehydrocoupling chain-growth reactions. Such mechanistic pathways for AB H2 release, involving the initial formation of the previously known [H3BNH2BH3]− anion followed by sequential dehydrocoupling of B–H and H–N groups of growing borane-capped aminoborane anions with AB, are supported by the fact that 1 was observed to react with an additional AB equivalent to form 2a and 2b.

The first use of diatom frustules as shape-dictating 3-D templates for the syntheses of nanostructured, nanocrystalline micro-particles of a non-oxide ceramic, boron nitride, is demonstrated.

Significant advantages result from combining the disparate hydrogen release pathways for ammonia-borane (AB) dehydrogenation using ionic liquids (ILs) and transition metal catalysts. With the RuCl2(PMe3)4 catalyst precursor, AB dehydrogenation selectivity and extent are maximized in an IL with a moderately coordinating ethylsulfate anion.