BH3 Unit Research Articles

Synthesis of Os Hydride Complexes Supported by the Diarylamido/Bis(phosphine) PNP Ligand and Attempts at Using (PNP)Ru and (PNP)Os Complexes in C-H Borylation Catalysis.

This manuscript describes the synthesis of Os complexes supported by the diarylamido/bis(phosphine) PNP pincer ligand. Compound (PNP)OsH(CO) (3-Os) was prepared by analogy with the previously reported 3-Ru. However, attempts to make (PNP)OsH3 (4-Os) analogously to 4-Ru resulted in the formation of an unexpected compound (5-Os) that is a product of addition of a BH3 unit across the Os-N bond in 4-Os. Nonetheless, 4-Os was prepared via an alternative route. Unlike 4-Ru, 4-Os appears to be a classical trihydride. Compounds 3-Ru, 3-Os, 4-Os, 4-Ru, and 5-Os were tested as potential catalysts for (a) dehydrogenative borylation of terminal alkynes (DHBTA) and (b) dehydrogenative borylation of benzene. No catalytic C-H borylation was observed for any of them, but all of them catalyzed unselective hydroboration of 4-MeC6H4CCH.

Reversible Dissociation for Effective Storage of Diborane Gas within the UiO-66-NH2 Metal-Organic Framework.

There is an acute need for materials that can store the toxic and highly reactive diborane gas at room temperature. In this work, the interfacial chemistry leading to safe and reversible storage of diborane (B2H6) in the UiO-66-NH2 metal-organic framework (MOF) was investigated via in situ transmission infrared (IR) spectroscopy, temperature-programmed desorption (TPD), and electronic structure calculations. The infrared spectrum of B2H6 adsorbed within UiO-66-NH2 indicates hydrogen bonding with the μ3-OH groups of the MOF nodes and chemisorption at the -NH2 groups of the MOF linkers. The conversion of physisorbed to chemisorbed diborane, as observed through a spectroscopically unique intermediate species, occurred over a broad temperature regime from 80 to 410 K. During B2H6-TPD studies, both the weakly and strongly bound species were found to desorb exclusively as molecular B2H6. Infrared spectroscopic studies, performed during diborane adsorption and reaction, combined with electronic structure calculations, revealed that chemisorption occurred via a reversible dissociation reaction involving a "half-open" B2H6 intermediate and resulted in the formation of two NH2-bound BH3 units, which leave the MOF as B2H6 via recombinative desorption. The close spacing of -NH2 groups in the UiO-66-NH2 MOF is key to enabling high-temperature chemisorptive storage of B2H6, and the spatial arrangement of the amine groups has a significant effect on the dissociation energy profile. This work demonstrates that reversible dissociation of B2H6 on precisely engineered, nucleophile-rich materials represents a promising pathway to diborane stabilization and long-term storage.

Cooperative B-H and Si-H Bond Activations by κ2-N,S-Chelated Ruthenium Borate Complexes.

Cooperative E-H (E = B, Si) bond activations employing κ2-N,S-chelated ruthenium borate species, [PPh3{κ2-N,S-(NS2C7H4)}Ru{κ3-H,S,S'-H2B(NC7H4S2)2}], (1) are established. Treatment of 1 with BH3·SMe2 yielded the six-membered ruthenaheterocycle [PPh3{κ2-S,H-(BH3NS2C7H4)}Ru{κ3-H,S,S'-H2B(C7H4NS2)2}] (2) formed by a hemilabile ring opening of a Ru-N bond and capturing of a BH3 unit coordinated in an "end-on" fashion. On the other hand, the bulky borane H2BMes shows different reactivity with 1 that led to the formation of the two dihydroborate complexes [{κ3-S,H,H-(NBH2Mes)(S2C7H4)}Ru{κ3-H,S,S'-H2B(C7H4NS2)2}] (3) and [PPh3{κ3-S,H,H-(NBH2Mes)(S2C7H4)}Ru(κ2-N,S-C7H4NS2)] (4), in which H2BMes has been inserted into the Ru-N bond of the initial κ2-N,S-chelated ligand. In an attempt to directly activate hydrosilanes by 1, reactions were carried out with H2SiPh2 that yielded two isomeric five-membered ruthenium silyl complexes, namely [PPh3{κ2-S,Si-(NSiPh2)(S2C7H4)}Ru{κ3-H,S,S'-H2B(C7H4NS2)2}] (5a,b), and the hydridotrisilyl complex [Ru(H){κ2-S,Si-(SiPh2NC7H4S2}3] (6). These complexes were generated by Si-H bond activation with the release of H2 and the formation of N-Si and Ru-Si bonds. When the reaction of 1 was carried out in the presence of PhSiH3, the reaction only produced the analogous complexes [PPh3{κ2-S,Si-(NSiPhH)(S2C7H4)}Ru{κ3-H,S,S'-H2B(C7H4NS2)2}] (5a',b'). Density functional theory (DFT) calculations have been used to probe the bonding modes of boranes/silane with the ruthenium center.

Lewis acid-base adducts of zwitterionic alkali metal methanides and silanides with BH3

The synthesis, structures and spectroscopic properties of M(MeOCH2CH2OMe2Si)3CBH3 (M-1-BH3) and M(MeOCH2CH2OMe2Si)3SiBH3 (M-2-BH3) (M = Li, Na, K) derived from reactions of BH3 with the alkali metal zwitterions [M(MeOCH2CH2OMe2Si)3C] (M-1) and [M(MeOCH2CH2OMe2Si)3Si] (M-2) (M = Li, Na, K), resp., are reported. X-ray analysis and DFT calculations reveal discrete zwitterionic structures with the octahedral alkali metal cations rigidly locked and charge separated from the BH3 units via pendant donors groups. Solution experiments with the hydride acceptors B(C6F5)3 and [Ph3C]2[B12C12] indicate that Na-1-BH3 can donate hydrides to form cations of formula [Na(MeOCH2CH2OMe2Si)3CBH2]+.

On the Viability of Catalytic Turnover via Al-O/B-H Metathesis: The Reactivity of β-Diketiminate Aluminium Hydrides towards CO2 and Boranes.

A series of β-diketiminate stabilized aluminium hydrides of the type (Nacnac)Al(R)H has been synthesized offering variation in the auxiliary R substituent and in the Nacnac backbone. These show significant variation in the nature of the Al-H bond: electron-donating R groups give rise to weaker (and presumably more hydridic) Al-H bonds, leading to enhanced rates of reactivity towards CO2 . The resulting κ1 -formate complexes (Nacnac)Al(R){OC(O)H} have been isolated and their reactivity towards B-H-containing reductants probed. In the case of HBpin no reaction is observed (even under forcing conditions), while the more reactive boranes HBcat and {H(9-BBN)}2 ultimately yield boryloxy complexes of the type (Nacnac)Al(R)(OBX2 ) (X2 =cat, 9-BBN). However, no hint of Al-O/B-H metathesis is observed even under forcing conditions. With BH3 ⋅SMe2 the major product is a related boryloxy system, although (uniquely) in this case a minor product is observed which contains an Al-H bond. We hypothesize that (Nacnac)Al(R)(κ2 -BH4 ) is formed (despite the unfavourable thermodynamics of Al-O/B-H metathesis) due to the additional driving force provided by coordination of the strongly Lewis acidic BH3 unit to the Al-H bond. That said, we find that (unlike the analogous gallium systems) no catalytic turnover can be achieved in the reduction of CO2 by boranes mediated by these aluminium hydrides.

Dehydrogenation, Methyl Elimination and Insertion Reactions of the Agostic Methyl-Bridged Complex [Mo2 Cp2 (μ-κ1 :η2 -CH3 )(μ-PtBu2 )(μ-CO)

The high unsaturation of the title complex enabled it to react with a wide variety of molecules under mild conditions, whereby the agostic methyl ligand underwent unusual or unprecedented processes. Methane elimination occurred in the reactions with PPh2 H and SiPh2 H2 , this being followed in the latter case by Si-H bond oxidative addition to give the hydride silylene derivative [Mo2 Cp2 H(μ-PtBu2 )(μ-SiPh2 )(CO)]. Dehydrogenation, however, was the dominant process in the room temperature reaction with [Fe2 (CO)9 ], to give the unsaturated methylidyne cluster [Mo2 FeCp2 (μ3 -CH)(μ-PtBu2 )(CO)5 ] (Mo-Mo=2.6770(8) Å). In contrast, PMe elimination took place in the reaction with P4 , to give the unsaturated triphosphorus complex [Mo2 Cp2 (μ-η3 :η3 -P3 )(μ-PtBu2 )] (Mo-Mo=2.6221(3) Å). Yet a most remarkable reaction occurred with BH3 ⋅THF, involving insertion of two BH3 units and dehydrogenation to yield [Mo2 Cp2 (μ-B2 H4 Me)(μ-PtBu2 )(CO)], with the novel methyldiboranyl ligand acting as a 5-electron donor due to the presence of two 3-centre, 2-electron B-H-Mo interactions, according to spectroscopic data and DFT calculations (Mo-Mo ca. 2.65 Å).

Sequential Migrations between Boron and Rhodium Centers: A Cooperative Process between Rhodium and a Monosubstituted Borohydride Unit.

The sodium salt of a monosubstituted borohydride anion containing a 2-mercaptopyridyl unit (mp) is reported herein. This compound was coordinated to a rhodium(I) center providing the complex [Rh{κ3-H,H,S-H3B(mp)}(NBD)] (1) (where NBD = 2,5-norbornadiene) in which the boron-based ligand is coordinated to the rhodium center via the thione donor and two of the B-H bonds of the BH3 unit. Reaction of complex 1 with carbon monoxide results in the activation of the complex leading to the product of a formal intramolecular hydroboration reaction, where the NBD unit has, in effect, inserted into one of the B-H bonds. Three complexes were prepared in which the newly formed norbornenyl unit (nbe) is located at the boron center, namely, [Rh{κ3-H,H,S-H2B(nbe)(mp)}(CO)2] (2), [Rh{κ3-H,H,S-H2B(nbe)(mp)}(CO)(PCy3)] (3), and [Rh{κ3-H,H,S-H2B(nbe)(mp)}(CO)(PPh3)] (4). The identities of the three complexes were confirmed by spectroscopic and analytical techniques. Further confirmation was obtained via structural characterization of 3. Studies confirmed that the reactivity occurs at the metal center. A metal-ligand cooperative mechanism, involving initial migration of hydride from boron to metal center, was postulated for the formation of the new complexes based on previous investigations. The newly formed norbornenyl unit then migrates from metal center to boron.

Ammonia Borane Clusters: Energetics of Dihydrogen Bonding, Cooperativity, and the Role of Electrostatics.

Cluster formation of ammonia borane (NH3BH3) driven by noncovalent H···H dihydrogen interaction is investigated at the M06L/6-311+G(d,p) level of density functional theory. For clusters containing up to six monomers, ladder, cyclic, stacked, cross-stacked, end-on, mixed and hexagonal configurations have been screened for their energetic stability. In the dimer, 7.94 kcal/mol stabilization energy per monomer (Em) is observed. Compared to ladder and cyclic configurations, a tetramer consisting of stacked dimer units is more stable by 3.0 kcal/mol whereas a hexamer composed of hexagonally arranged monomers promoting side-on H···H interaction is more stable than a stacked configuration by 2.5 kcal/mol. The hexagonal packing of cluster is repeated to obtain (NH3BH3)12, (NH3BH3)18, (NH3BH3)36, (NH3BH3)48, and (NH3BH3)54 clusters. The Em 17.81 kcal/mol observed for (NH3BH3)54 is 2.24 fold higher than the dimer, suggesting strong cooperativity in cluster growth mechanism. The zwitterionic features of NH3BH3 is characterized in terms of molecular electrostatic potential (MESP) features. During cluster formation, donation of electron density from negatively charged BH3 unit of a monomer to the positively charged NH3 unit of other interacting monomers occurs through H···H dihydrogen bonding. The extent of electron donation is revealed through the value of MESP minium (Vmin) in every monomer. A strong linear correlation between the total value of Vmin for a cluster (ΣVmin) and the total stabilization energy of the cluster (Estb) is established. Further, MESP at the nuclei of N (VN) and B (VB) are found to be very sensitive to the strength of H···H bonding. With respect to free NH3BH3, the total change in VN (ΣΔVN) as well as the total change in VB (ΣΔVB) in a cluster shows near-perfect linear correlation with Estb. Further, the magnitude of the three quantities, viz. ΣΔVN, ΣΔVB, and Estb is nearly same and indicates that the cluster formation of NH3BH3 is almost effectively controlled by electrostatics of dihydrogen interactions. The extended network of dihydrogen interactions observed in large clusters and the significant positive cooperativity effect of such interactions support the use of ammonia borane as a potential hydrogen storage material.

Reducing Capsule Based on Electron Programming: Versatile Synthesizer for Size-Controlled Ultra-Small Metal Clusters.

Controlled reducing capsules with a specific number of reducing electrons were achieved by appropriately placed BH3 units in the dendritic polyphenylazomethines (DPAs). Using the 1:1 coordination fashion on their basic branches with radius affinity gradient, the 4th generation DPA (DPAG4) possessing four BH3 units in the central positions was prepared as a template synthesizer for size-controlled ultra-small metal clusters. This was well-demonstrated by reduction of Ag, Pt, and other metal ions resulting in monodispersed ultra-small clusters.

Stereodivergent SN2@P Reactions of Borane Oxazaphospholidines: Experimental and Theoretical Studies

The stereodivergent ring-opening of 2-phenyl oxazaphospholidines with alkyl lithium reagents is reported. N-H oxazaphospholidines derived from both (+)-cis-1-amino-2-indanol and (-)-norephedrine provide inversion products in a highly stereoselective process. In contrast, N-Me oxazaphospholidines yield ring-opening products with retention of configuration at the P center, as previously reported by Jugé and co-workers. As a result, from a single amino alcohol auxiliary, both enantiomers of key P-stereogenic intermediates could be synthesized. Theoretical studies of ring-opening with model oxazaphospholidines at the DFT level have elucidated the streochemical course of this process. N-H substrates react in a single step via preferential backside S(N)2@P substitution with inversion at phosphorus. N-methylated substrates react preferentially via a two-step frontside S(N)2@P, yielding a ring-opened product in which the nucleophilic methyl binds to P with retention of configuration. DFT calculations have shown that the BH3 unit is a potent directing group to which the methyl lithium reagent coordinates via Li in all the reactions studied.

Synthesis and structural characterization of group 5 dimetallaheteroboranes

Treatment of group 5 metal polychlorides such as [CpnMCl4−x] (M = Nb: n = 1, x = 0; M = V: n, x = 2), (Cp = η5−C5H5) with [LiBH4·THF] followed by thermolysis in presence of diphenyl dichalcogenide ligands, E2Ph2 (E = Se or Te) yielded dimetallaheteroborane clusters [(CpNb)2BH(Se)4], 1 and [(CpV)2B3H9(μ3-Te)], 2 in modest yields. Compound 1 can be considered as an edge fused cluster in which a trigonal bipyramidal unit [Nb2Se2B] has been fused with a tetrahedral core [Nb2Se2] by means of a common [Nb2] edge. Compound 2 can be described as a dimetallaheteroborane built from two edge-fused V2B2 tetrahedra, in which one of the BH3 units is replaced by an isoelectronic Te ligand. All the compounds have been characterized by mass spectrometry, 1H, 11B, 13C, 51V and 77Se NMR spectroscopy. In addition the geometry of compound 1 has been established by crystallographic analysis.

Chemistry of Vanadaboranes: Synthesis, Structures, and Characterization of Organovanadium Sulfide Clusters with Disulfido Linkage

Vanadaborane, [(CpV)(2)(B(2)H(6))(2)] (Cp = eta(5)-C(5)H(5)), 1 reacts with elemental sulfur to afford the hexasulfide cluster [(CpV)(2)S(4)(mu-eta(1)-S(2))], 2 in high yield. Compound 2 is a notable example of an organovanadium sulfide cluster in which the [V(2)S(4)] atoms define a bicapped tetrahedron framework, with one mu-eta(1)-S(2) ligand bridged the two (CpV) moieties. The sulfur atom in [V(2)S(4)] core in 2 is a four-skeletal-electron donor isoelectronic with the BH(3) unit; therefore, the replacement of boron hydride in 1 by four sulfur atoms necessitates the formation of a bicapped tetrahedron [V(2)S(4)] framework. Furthermore, this is the only reported example of a bimetallic hexasulfide cluster containing vanadium. Pyrolysis of 1 with bis-chalcogenide ligands such as Ph(2)S(2) and Bz(2)Se(2) (Bz = PhCH(2)), results in the formation of substituted vanadahexaboranes [(CpV)(2)B(4)H(12-x)L(x)], 3-5 (3: L = SPh: x = 3; 4: L = SPh, x = 2; 5: L = SeBz: x = 1) in modest yield. All these new compounds have been characterized by mass, (1)H, (11)B, (13)C NMR spectroscopy, and elemental analysis, and the structural types were unequivocally established by crystallographic analysis of compounds 2-5.

Unusual reactivity of lanthanide borohydride complexes leading to a borane complex

Depending on the ionic radius of the central metal atom the BH(4)(-) group, which usually reacts in lanthanide chemistry like a pseudo halide, can be involved in redox chemistry and the resulting product contains an N-BH(3) unit, which binds in an unusual eta(2)-fashion onto the metal atom.

Synthesis and Structure of a Dichromatetraborane Derivative [{(OC)(4)Cr}(2)(eta(4)-H,H',H",H"'-BH(2)BH(2).PMe(2)CH(2)PMe(2))

Photolysis of [Cr(CO)6] in the presence of BH3·PMe2CH2PMe2·BH3 (2) afforded a dichromatetraborane derivative [{(OC)4Cr}2(η4-H,H‘,H‘‘,H‘‘‘-BH2BH2·PMe2CH2PMe2)] (3) via an intramolecular dehydrogenative coupling of BH3 units on the metal. The complex 3 can be depicted as a dichromatetraborane derivative, with the wing-tip BH2+ groups in tetraborane(10) replaced by Cr(CO)4 fragments, which are isolobal with BH2+, and the terminal hydrides attaching to the basal boron atoms replaced by isoelectronic phosphine ligands.

Cyclic Silylhydrazines and Their Borane Adducts

A series of five-, six-, and seven-membered cyclic silylhydrazines have been prepared from the reactions of 1,2-bis(bromosilyl)ethane and 1,3-bis(bromosilyl)propane with 1,1-dimethylhydrazine [leading to 1-(dimethylamino)-1-aza-2,5-disilacyclopentane (3) and 1-(dimethylamino)-1-aza-2,6-disilacyclohexane (4)] and 1,2-dimethylhydrazine [leading to 1,2-dimethyl-1,2-diaza-3,6-disilacyclohexane (7) and 1,2-dimethyl-1,2-diaza-2,7-disilacycloheptane (8)] in the presence of triethylamine, respectively. The compounds with endocyclic Si-N-Si units (3, 4) are found to be stable for long periods of time, while those with Si-N-N-Si units (7, 8) decompose within a few days at ambient temperature. Compounds 3 and 4 have been reacted with the Lewis acid BH3 to give the dimethylamine-borane adducts 5 and 6. All compounds have been fully characterized by spectroscopic data [IR, MS, NMR (H-1, C-13, N-15, Si-29)]. Single crystals of 5 [6] grown from the melt and studied by low-temperature X-ray diffraction analyses are orthorhombic, space group Pbca (No. 61), with a = 11.385(1) [13.300 (1)] Angstrom, b = 9.938(1) [9.837(1)] Angstrom, c = 17.156(1) [16.364(1)] Angstrom, d(calc) = 1.096 (1.081) g cm(-3), and Z = 8 [8]. In both compounds, the BH3 unit is bound to the nonsilylated nitrogen atom, indicating the reduction of the basicity of nitrogen by Si substitution. The silylated nitrogen atoms show planar coordination, while the borylated amine unit is tetrahedrally coordinated. From a comparison of the ring geometries of 5 and 6 with known open-chain structures, it appears that the C2Si2N ring system of 5 is clearly more strained than that of 6 (C3Si2N) This argument also offers an explanation for the preferred formation of the compounds 1,6-diaza-2,5,7,10-tetrasila-[4.4.0]bicyclodecane (1) and bi(1-aza-2,6-disilacyclohexyl) (2) as compared to their isomers with different ring sizes. The relative stabilities of these isomers in question have been quantified by ab initio (MP2(fc)/6-31G*) calculations of geometries and energies of the systems [(CH2)(n)(SiH2)]N-N[(SiH2)(CH2)(n)] versus the annelated molecules (CH2)(n)(SiH2)N-2(CH2)(n)(SiH2) with n = 1-3. These results show the annelated isomers (ring enlarged) to be lower in energy for n = 1 and 2, while for n = 3 the N-N bridged nonannelated isomer is preferred.

Two-fragment addition of borohydride to iron complexes: synthesis of dithioformate iron complexes with B–H–Fe bond interaction. X-Ray structural characterization of Fe[η3-HC(SMe)S → B(H)H2](CO)(PMe3)2

Reaction of Fe(X)(µ2CS2R)(CO)(PR3)2(X = Cl, I) complexes with NaBH4 leads to both hydride and BH3 addition with the formation of dithiofromate–iron(O) Fe[µ3-HC(SR)S → BH3](CO)(PR3)2 complexes (4) containing an agostic B–H–Fe interaction; the addition of pyridine to (4) liberates a 16 electron-iron(O) moiety by trapping of the BH3 unit.

Reductive Cleavage of Acetals and Ketals by Borane. Part II. The Kinetics of the Reaction

The reductive cleavage of acetals with borane in tetrahydrofuran was found to be first order in acetal and close to third order in borane. The high order in borane may be due to the participation of three BH3 units in the transition state with subsequent formation of a B2H7− ion. All the hydrogen atoms of borane are available for the reduction of excess acetal at 30 °C.