Catalytic redistribution and polymerization of diborazanes: unexpected observation of metal-free hydrogen transfer between aminoboranes and amine-boranes

Ir-catalyzed (20 °C) or thermal (70 °C) dehydrocoupling of the linear diborazane MeNH(2)-BH(2)-NHMe-BH(3) led to the formation of poly- or oligoaminoboranes [MeNH-BH(2)](x) (x = 3 to >1000) via an initial redistribution process that forms MeNH(2)·BH(3) and also transient MeNH═BH(2), which exists in the predominantly metal-bound and free forms, respectively. Studies of analogous chemistry led to the discovery of metal-free hydrogenation of the B═N bond in the "model" aminoborane iPr(2)N═BH(2) to give iPr(2)NH·BH(3) upon treatment with the diborazane Me(3)N-BH(2)-NHMe-BH(3) or amine-boranes RR'NH·BH(3) (R, R' = H or Me).     

Transition metal-catalyzed formation of boron-nitrogen bonds: catalytic dehydrocoupling of amine-borane adducts to form aminoboranes and borazines

A mild, catalytic dehydrocoupling route to aminoboranes and borazine derivatives from either primary or secondary amine-borane adducts has been developed using late transition metal complexes as precatalysts. The adduct Me(2)NH.BH(3) thermally eliminates hydrogen at 130 degrees C in the condensed phase to afford [Me(2)N-BH(2)](2) (1). Evidence for an intermolecular process, rather than an intramolecular reaction to form Me(2)N=BH(2) as an intermediate, was forthcoming from "hot tube" experiments where no appreciable dehydrocoupling of gaseous Me(2)NH.BH(3) was detected in the range 150-450 degrees C. The dehydrocoupling of Me(2)NH.BH(3) was found to be catalyzed by 0.5 mol % [Rh(1,5-cod)(mu-Cl)](2) in solution at 25 degrees C to give 1 quantitatively after ca. 8 h. The rate of dehydrocoupling was significantly enhanced if the temperature was raised or if the catalyst loading was increased. The catalytic activity of various other transition metal complexes (Ir, Ru, Pd) for the dehydrocoupling of Me(2)NH.BH(3) was also demonstrated. This new catalytic method was extended to other secondary adducts RR'NH.BH(3) which afforded the dimeric species [(1,4-C(4)H(8))N-BH(2)](2) (2) and [PhCH(2)(Me)N-BH(2)](2) (3) or the monomeric aminoborane (i)Pr(2)N=BH(2) (4) under mild conditions. A new synthetic approach to the linear compounds R(2)NH-BH(2)-NR(2)-BH(3) (5: R = Me; 6: R = 1,4-C(4)H(8)) was developed and subsequent catalytic dehydrocoupling of these species yielded the cyclics 1 and 2. The species 5 and 6 are postulated to be intermediates in the formation of 1 and 2 directly from the catalytic dehydrocoupling of the adducts R(2)NH.BH(3). The catalytic dehydrocoupling of NH(3).BH(3), MeNH(2).BH(3), and PhNH(2).BH(3) at 45 degrees C to give the borazine derivatives [RN-BH](3) (10: R = H; 11: R = Me; 12: R = Ph) was demonstrated. TEM analysis of the contents of the reaction solution for the [Rh(1,5-cod)(mu-Cl)](2) catalyzed dehydrocoupling of Me(2)NH.BH(3) together with Hg poisoning experiments suggested a heterogeneous catalytic process involving Rh(0) colloids.     

Heterogeneous or homogeneous catalysis? Mechanistic studies of the rhodium-catalyzed dehydrocoupling of amine-borane and phosphine-borane adducts

In depth, comparative studies on the catalytic dehydrocoupling of the amine-borane adduct Me(2)NH.BH(3) (to form [Me(2)N-BH(2)](2)) and the phosphine-borane adduct Ph(2)PH.BH(3) (to form Ph(2)PH-BH(2)-PPh(2)-BH(3)) with a variety of Rh (pre)catalysts such as [[Rh(1,5-cod)(micro-Cl)](2)], Rh/Al(2)O(3), Rh(colloid)/[Oct(4)N]Cl, and [Rh(1,5-cod)(2)]OTf have been performed in order to determine whether the dehydrocoupling proceeds by a homogeneous or heterogeneous mechanism. The results obtained suggest that the catalytic dehydrocoupling of Me(2)NH.BH(3) is heterogeneous in nature involving Rh(0) colloids, while that of Ph(2)PH.BH(3) proceeds by a homogeneous mechanism even when starting with Rh(0) precursors such as Rh/Al(2)O(3). The catalytic dehydrocoupling reactions are thought to proceed by different mechanisms due to a combination of factors such as (i) the greater reducing strength of amine-borane adducts, (ii) the increased ease of dissociation of phosphine-borane adducts, and (iii) phosphine ligation and/or poisoning of active catalytic sites on metal colloids.     

Development of a generic mechanism for the dehydrocoupling of amine-boranes: a stoichiometric, catalytic, and kinetic study of H3B·NMe2H using the [Rh(PCy3)2]+ fragment

The multistage Rh-catalyzed dehydrocoupling of the secondary amine-borane H(3)B·NMe(2)H, to give the cyclic amino-borane [H(2)BNMe(2)](2), has been explored using catalysts based upon cationic [Rh(PCy(3))(2)](+) (Cy = cyclo-C(6)H(11)). These were systematically investigated (NMR/MS), under both stoichiometric and catalytic regimes, with the resulting mechanistic proposals for parallel catalysis and autocatalysis evaluated by kinetic simulation. These studies demonstrate a rich and complex mechanistic landscape that involves dehydrogenation of H(3)B·NMe(2)H to give the amino-borane H(2)B═NMe(2), dimerization of this to give the final product, formation of the linear diborazane H(3)B·NMe(2)BH(2)·NMe(2)H as an intermediate, and its consumption by both B-N bond cleavage and dehydrocyclization. Subtleties of the system include the following: the product [H(2)BNMe(2)](2) is a modifier in catalysis and acts in an autocatalytic role; there is a parallel, neutral catalyst present in low but constant concentration, suggested to be Rh(PCy(3))(2)H(2)Cl; the dimerization of H(2)B═NMe(2) can be accelerated by MeCN; and complementary nonclassical BH···HN interactions are likely to play a role in lowering barriers to many of the processes occurring at the metal center. These observations lead to a generic mechanistic scheme that can be readily tailored for application to many of the transition-metal and main-group systems that catalyze the dehydrocoupling of H(3)B·NMe(2)H.     

Synthesis and the thermal and catalytic dehydrogenation reactions of amine-thioboranes

A series of trimethylamine-thioborane adducts, Me(3)N·BH(2)SR (R = tBu [2a], nBu [2b], iPr [2c], Ph [2d], C(6)F(5) [2e]) have been prepared and characterized. Attempts to access secondary and primary amine adducts of thioboranes via amine-exchange reactions involving these species proved unsuccessful, with the thiolate moiety shown to be vulnerable to displacement by free amine. However, treatment of the arylthioboranes, [BH(2)-SPh](3) (9) and C(6)F(5)SBH(2)·SMe(2) (10) with Me(2)NH and iPr(2)NH successfully yielded the adducts Me(2)NH·BH(2)SR (R = Ph [11a], C(6)F(5) [12a]) and iPr(2)NH·BH(2)SR (R = Ph [11b], C(6)F(5) [12b]) in high yield. These adducts were also shown to be accessible via thermally induced hydrothiolation of the aminoboranes Me(2)N═BH(2), derived from the cyclic dimer [Me(2)N-BH(2)](2) (13), and iPr(2)N═BH(2) (14), respectively. Attempts to prepare the aliphatic thiolate substituted adducts R(2)NH·BH(2)SR' (R = Me, iPr; R' = tBu, nBu, iPr) via this method, however, proved unsuccessful, with the temperatures required to facilitate hydrothiolation also inducing thermal dehydrogenation of the amine-thioborane products to form aminothioboranes, R(2)N═BH(SR'). Thermal and catalytic dehydrogenation of the targeted amine-thioboranes, 11a/11b and 12a/12b were also investigated. Adducts 11b and 12b were cleanly dehydrogenated to yield iPr(2)N═BH(SPh) (22) and iPr(2)N═BH(SC(6)F(5)) (23), respectively, at 100 °C (18 h, toluene), with dehydrogenation also possible at 20 °C (42 h, toluene) with a 2 mol % loading of [Rh(μ-Cl)cod](2) in the case of the former species. Similar studies with adduct 11a evidenced a competitive elimination of H(2) and HSPh upon thermolysis, and other complex reactivity under catalytic conditions, whereas the fluorinated analogue 12a was found to be resistant to dehydrogenation.     

Linear hybrid aminoborane/phosphinoborane chains: synthesis, proton-hydride interactions, and thermolysis behavior

The reaction of the lithiated phosphine-borane adducts Li[PPhR.BH(3)] or Li[CH(2)-PR(2).BH(3)] with Me(2)NH.BH(2)Cl afforded the hybrid linear species Me(2)NH-BH(2)-PPhR-BH(3) (1, R = Ph; 2, R = H) or Me(2)NH-BH(2)-CH(2)-PR(2)-BH(3) (3, R = Ph; 4, R = Me). Single-crystal X-ray diffraction studies on 1 and 3, the first for linear hybrid aminoborane/phosphinoborane adducts, confirmed the expected four-coordinate N-B-P-B and N-B-C-P-B frameworks. In addition, interactions between the protic N-H and hydridic B-H hydrogen atoms resulted in short intermolecular H...H contacts for 1, whereas 3 was found to possess an exceptionally short intramolecular H...H distance of 1.95 A. Solution and solid state infrared studies on 3 and 4 also suggest that these dihydrogen interactions were maintained even in dilute solution. Hydrogen bond strengths in the range of 7.9 to 10.9 kJ mol(-1) indicate the presence of a relatively weak interaction. The thermal and catalytic dehydrocoupling reactivities of 1-4 were also investigated. Chain cleavage reactions were observed for 1 and 2 upon thermolysis at 130 degrees C to afford species such as Me(2)NH.BH(3), [Me(2)N-BH(2)](2), PhPRH.BH(3) (R = Ph, H), PhPRH (R = Ph, H), Ph(2)PH-BH(2)-PPh(2)-BH(3), and also the low molecular weight polyphosphinoborane [PhPH-BH(2)](n) (M(w) approximately 5000). Similar products were observed for the attempted catalytic dehydrocoupling reactions but under milder reaction conditions (50 degrees C). Thermolysis of 3 at 130 degrees C yielded the six-membered ring [BH(2)-CH(2)-PPh(2)](2) (5), which presumably results from the dissociation of Me(2)NH.BH(3) from 3. Thermolysis of 4 at 90 degrees C afforded Me(2)NH.BH(3) and Me(3)P.BH(3), in addition to a product tentatively assigned as [BH(2)-CH(2)-PMe(2)](2) (6).     

Syntheses and structural characterizations of anionic borane-capped ammonia borane oligomers: evidence for ammonia borane H2 release via a base-promoted anionic dehydropolymerization mechanism

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 H(2) 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 H(2) release reactions. The salt VBH(+)[H(3)BNH(2)BH(2)NH(2)BH(3)](-) (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(NH(2)BH(3))(3)](-) (2a) and VBH(+)[H(3)BNH(2)BH(2)NH(2)BH(2)NH(2)BH(3)](-) (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 H(2) release, involving the initial formation of the previously known [H(3)BNH(2)BH(3)](-) 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.     

Probing the catalytic potential of chloro nitrosyl rhenium(I) complexes

The reduction of the mononitrosyl Re(II) salt [NMe(4)](2)[ReCl(5)(NO)] (1) with zinc in acetonitrile afforded the Re(i) dichloride complex [ReCl(2)(NO)(CH(3)CN)(3)] (2). Subsequent ligand substitution reactions with PCy(3), PiPr(3) and P(p-tolyl)(3) afforded the bisphosphine Re(i) complexes [ReCl(2)(NO)(PR(3))(2)(CH(3)CN)] (3, R = Cy a, iPr b, p-tolyl c) in good yields. The acetonitrile ligand in 3 is labile, permitting its replacement with H(2) (1 bar) to afford the dihydrogen Re(I) complexes [ReCl(2)(NO)(PR(3))(2)(η(2)-H(2))] (4, R = Cy a, iPr b). The catalytic activity of 2, 3 and 4 in hydrogen-related catalyses including dehydrocoupling of Me(2)NH·BH(3), dehydrogenative silylation of styrenes, and hydrosilylation of ketones and aryl aldehydes were investigated, with the main focus on phosphine and halide effects. In the dehydrocoupling of Me(2)NH·BH(3), the phosphine-free complex 2 exhibits the same activity as the bisphosphine-substituted systems. In the dehydrogenative silylation of styrenes, 3a and 4a bearing PCy(3) ligands exhibit high catalytic activities. Monochloro Re(I) hydrides [Re(Cl)(H)(NO)(PR(3))(2)(CH(3)CN)] (5, R = Cy a, iPr b) were proven to be formed in the initiation pathway. The phosphine-free complex 2 showed in dehydrogenative silylations even higher activity than the bisphosphine derivatives, which further emphasizes the importance of a facile phosphine dissociation in the catalytic process. In the hydrosilylation of ketones and aryl aldehydes, at least one rhenium-bound phosphine is required to ensure high catalytic activity.     

Iron complexes for the catalytic transfer hydrogenation of acetophenone: steric and electronic effects imposed by alkyl substituents at phosphorus

A series of iron(II) complexes, trans-[Fe(NCMe)(2)(PR(2)CH(2)CH═NCH(2)CH(2)N═CHCH(2)PR(2))][BPh(4)](2) (5, R = Cy; 7, R = iPr; 9, R = Et) were prepared via the template synthesis in one-pot involving air-stable phosphonium dimers, [cyclo-(-PR(2)CH(2)CH(OH)-)(2)](Br)(2) (4, R = Cy; 6, R = iPr; 8, R = Et), KOtBu, [Fe(H(2)O)(6)][BF(4)](2) and ethylenediamine in acetonitrile. In the synthesis of 9, a methanol/acetonitrile solvent mixture was required; otherwise an intermediate iron bis(tridentate) complex, [Fe(PEt(2)CH(2)CH═NCH(2)CH(2)NH(2))(2)](2+), formed as determined by electrospray ionization mass spectrometry (ESI-MS). The crude iron(II) complexes from a template synthesis with ethylenediamine or (S,S)-1,2-diphenylethylenediamine are stirred in acetone under a CO atmosphere (～2 atm) overnight to displace a NCMe ligand; however, in addition to this, bromide displaces an NCMe ligand as well to form a new class of the iron complexes trans-[Fe(CO)(Br)(PR(2)CH(2)CH═NCHR'CHR'N═CHCH(2)PR(2))](+) (10 R = Cy, R' = H; (S,S)-11, R = Cy, R' = Ph; 12, R = iPr, R' = H; (S,S)-13, R = iPr, R' = Ph; 14, R = Et, R' = H; (S,S)-15, R = Et, R' = Ph). These complexes were isolated in moderate yields (55-84%) as tetraphenylborate salts. Complexes 10-15 were tested for the catalytic transfer hydrogenation of acetophenone in basic iso-propanol at 25 and 50 °C. The complexes 10-13 (where R = Cy or iPr) were inactive while the complexes 14 and (S,S)-15 (where R = Et) were active at 25 °C but had better activity at 50 °C. Complex (S,S)-15 was higher in activity than complex 14, achieving turnover frequencies as high as 4100 h(-1), conversions of acetophenone to (R)-1-phenylethanol as high as 80% and an enantiomeric excess (e.e.) of 50% in the product. As catalysis progressed, the e.e. diminished to as low as 26%.     

Transition metal-catalyzed dissociation of phosphine-gallane adducts: isolation of mechanistic model complexes and heterogeneous catalyst poisoning studies

Attempts to induce the catalytic dehydrocoupling of the phosphine-gallane adduct Cy2PH.GaH3 (Cy=cyclohexyl) (1) by treatment with ca. 5 mol% of either the Rh(I) complex [{Rh(mu-Cl)(1,5-cod)}2] (cod=cyclooctadiene) or the Rh(0) species Rh/Al2O3 and [Oct4N]Cl-stabilized colloidal Rh led to catalytic P-Ga bond cleavage to generate the phosphine, H2, and Ga metal. Interestingly, subsequent treatment of the reaction mixtures with Me2NH.BH3 failed to lead to the formation of [Me2N-BH2]2 via Rh-catalyzed dehydrocoupling, which suggested that catalyst deactivation was taking place. Poisoning studies involving the treatment of the active Rh(0) catalyst with Cy2PH, PMe3, or GaH3.OEt2 showed that deactivation indeed occurred as the dehydrocoupling of Me2NH.BH3 either dramatically decreased in rate or did not take place at all. The X-ray photoelectron spectroscopy analysis of colloidal Rh(0) that had been treated with Cy2PH and PMe3 confirmed the presence of phosphorus on the catalyst surface in each case, consistent with catalyst poisoning via phosphine ligation. A mechanism for the Rh-catalyzed P-Ga bond cleavage reaction of 1 and Me3P.GaH3 (2) is proposed and involves the initial reaction of Ga-H bonds with the Rh colloid surface, which weakens and ultimately breaks the P-Ga bond. The reasonable nature of this mechanism is supported by a model reaction between the zerovalent group 9 complex Co2(CO)8 and 2 which afforded Me3P.Ga[Co(CO)4]3 (3). Consistent with the elongated and thus weakened P-Ga bond in 3, solutions of this species in Et2O subsequently form the known complex [(Me3P)Co(CO)3]2 (4) and Ga metal after 4 h at 25 degrees C.     

Stoichiometric reactivity of dialkylamine boranes with alkaline earth silylamides

Reactions of β-diketiminato group 2 silylamides, [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)M(THF)(n){N(SiMe(3))(2)}] (M = Mg, n = 0; M = Ca, Sr, n = 1), and an equimolar quantity of pyrrolidine borane, (CH(2))(4)NH·BH(3), were found to produce amidoborane derivatives of the form [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)MN(CH(2))(4)·BH(3)]. In reactivity reminiscent of analogous reactions performed with dimethylamine borane, addition of a second equivalent of (CH(2))(4)NH·BH(3) to the Mg derivative induced the formation of a species, [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)Mg{N(CH(2))(4) BH(2)NMe(2)BH(3)}], containing an anion in which two molecules of the amine borane substrate have been coupled together through the elimination of one molecule of H(2). Both this species and a calcium amidoborane derivative have been characterised by X-ray diffraction techniques and the coupled species is proposed as a key intermediate in catalytic amine borane dehydrocoupling, in reactivity dictated by the charge density of the group 2 centre involved. On the basis of further stoichiometric reactions of the homoleptic group 2 silylamides, [M{N(SiMe(3))(2)}(2)] (M = Mg, Ca, Sr, Ba), with (CH(3))(2)NH·BH(3) and (i)Pr(2)NH·BH(3) reactivity consistent with successive amidoborane β-hydride elimination and [R(2)N[double bond, length as m-dash]BH(2)] insertion is described as a means to induce the B-N dehydrocoupling between amine borane substrates.     

Metal‐Free Dehydrogenation of Amine‐Boranes by Tunable N‐Heterocyclic Iminoboranes

We report the synthesis of structurally tunable boron complexes supported by N‐heterocyclic imine ligands IPr=N?BR₂ (IPr=[(HCNDipp)₂C], Dipp=2,6‐iPr₂C₆H₃, R=Cl and/or Ph) that have the ability to abstract dihydrogen from amine‐boranes, and instigate their dehydrocoupling. In one instance, mild heating of the hydrogen addition product IPr=NH?B(Ph)HCl releases H₂ to regenerate the starting N‐heterocyclic iminoborane; accordingly IPr=N?B(Ph)Cl can be used as a metal‐free catalyst to promote the dehydrocoupling of MeNH₂ ? BH₃ to yield N‐methylaminoborane oligomers [MeNH‐BH₂]_x.     

Chemistry of phosphine-borane adducts at platinum centers: dehydrocoupling reactivity of Pt(II) dihydrides with P-H bonds

The reaction of the Pt(II) dihydride complex cis-[PtH2(dcype)](dcype = 1,2-bis(dicyclohexylphosphino)ethane) with the primary or secondary phosphine-borane adducts PhRPH x BH3(R = H, Ph) was found to exclusively afford the mono-substituted complexes cis-[PtH(PPhR x BH3)(dcype)](1: R = H; 2: R = Ph)via a dehydrocoupling reaction between Pt-H and P-H bonds. Similar reactivity was observed between the uncoordinated phosphines PhRPH (R = H, Ph) and cis-[PtH2(dcype)], which gave cis-[PtH(PPhR)(dcype)](4: R = H; 5: R = Ph). The complexes were characterized by 1H, 11B, 13C and 31P NMR spectroscopy, IR, MS and, in the case of 2, X-ray crystallography that confirmed the cis geometries. The di-substituted complex cis-[Pt(PhPH x BH3)2(dcype)](3) was prepared from the reaction of cis-[PtCl2(dcype)] with two equivalents of Li[PPhH x BH3]. This suggested that steric reasons alone cannot be used to explain the lack of reactivity with respect to a second dehydrocoupling reaction involving the remaining Pt-H bond in complexes 1, 2, 4 and 5.     

Role of NH3 in the dehydrogenation of calcium amidoborane ammoniate and magnesium amidoborane ammoniate: a first-principles study

First-principles calculations show that [NH(3)] molecules play crucial roles as both activator for the break-up of B-H bond and supplier of protic H for the establishment of dihydrogen bonding, which could facilitate the dehydrogenation of Ca(NH(2)BH(3))(2)·2NH(3) or Mg(NH(2)BH(3))(2)·NH(3) occurring at lower temperatures compared to those of Ca(NH(2)BH(3))(2) and Mg(NH(2)BH(3))(2). Moreover, the calculations of Helmholtz Free energy and [NH(3)] molecule removal energy evidence that coordination between [NH(3)] and Mg cation is stronger than that between [NH(3)] and Ca cation; therefore, Mg(NH(2)BH(3))(2)·NH(3) will undergo directly dehydrogenation rather than deammoniation at lower temperatures.     

Synthetic and mechanistic studies of metal-free transfer hydrogenations applying polarized olefins as hydrogen acceptors and amine borane adducts as hydrogen donors

Metal-free transfer hydrogenation of polarized olefins (RR'C=CEE': R, R' = H or organyl, E, E' = CN or CO(2)Me) using amine borane adducts RR'NH-BH(3) (R = R' = H, AB; R = Me, R' = H, MAB; R = (t)Bu, R' = H, tBAB; R = R' = Me, DMAB) as hydrogen donors, were studied by means of in situ NMR spectroscopy. Deuterium kinetic isotope effects and the traced hydroboration intermediate revealed that the double H transfer process occurred regio-specifically in two steps with hydride before proton transfer characteristics. Studies on substituent effects and Hammett correlation indicated that the rate determining step of the H(N) transfer is in agreement with a concerted transition state. The very reactive intermediate [NH(2)=BH(2)] generated from AB was trapped by addition of cyclohexene into the reaction mixture forming Cy(2)BNH(2). The final product borazine (BHNH)(3) is assumed to be formed by dehydrocoupling of [NH(2)=BH(2)] or its solvent stabilized derivative [NH(2)=BH(2)]-(solvent), rather than by dehydrogenation of cyclotriborazane (BH(2)NH(2))(3) which is the trimerization product of [NH(2)=BH(2)].     

Synthesis and dehydrocoupling reactivity of iron and ruthenium phosphine-borane complexes

The Fe and Ru phosphine-borane complexes CpM(CO)2PPh2 x BH3 (1, M = Fe, 4, M = Ru) were synthesized utilizing the reaction of the phosphine-borane anion Li[PPh2 x BH3] with the iodo complexes CpM(CO)2I. The Fe complex 1 reacted with PMe3 to yield CpFe(CO)(PMe3)(PPh2 x BH3) (2) and CpFe(PMe3)2(PPh2 x BH3) (3) whereas the Ru species 4 gave only CpRu(CO)(PMe3)(PPh2 x BH3) (5). The complexes 1-5 were characterized by 1H, 11B, 13C and 31P NMR spectroscopy, MS, IR and X-ray crystallography for 1 to 4, and EA for 1, 2 and 4. The reactivity of 1 and 4 towards PPh2H x BH3 was explored. Although no stoichiometric reactions were detected under mild conditions, both 1 and 4 were found to function as dehydrocoupling catalysts to afford Ph2PH x BH2 x PPh2 x BH3 in the melt at elevated temperature (120 degrees C). The carbonyl Fe2(CO)9 also functioned as a dehydrocoupling catalyst under similar conditions. Complex 1 and Fe2(CO)9 represent the first reported active Fe complexes for the catalytic dehydrocoupling of phosphine-borane adducts.     

Coordination and organometallic chemistry of relevance to the rhodium-based catalyst for ethylene hydroamination

The RhCl(3)·3H(2)O/PPh(3)/nBu(4)PI catalytic system for the hydroamination of ethylene by aniline is shown to be thermally stable by a recycle experiment and by a kinetic profile study. The hypothesis of the reduction under catalytic conditions to a Rh(I) species is supported by the observation of a high catalytic activity for complex [RhI(PPh(3))(2)](2). New solution equilibrium studies on [RhX(PPh(3))(2)](2) (X = Cl, I) in the presence of ligands of relevance to the catalytic reaction (PPh(3), C(2)H(4), PhNH(2), X(-), and the model Et(2)NH amine) are reported. Complex [RhCl(PPh(3))(2)](2) shows broadening of the (31)P NMR signal upon addition of PhNH(2), indicating rapid equilibrium with a less thermodynamically stable adduct. The reaction with Et(2)NH gives extensive conversion into cis-RhCl(PPh(3))(2)(NHEt(2)), which is however in equilibrium with the starting material and free Et(2)NH. Excess NHEt(2) yields a H-bonded adduct cis-RhCl(PPh(3))(2)(Et(2)NH)···NHEt(2), in equilibrium with the precursors, as shown by IR spectroscopy. The iodide analogue [RhI(PPh(3))(2)](2) shows less pronounced reactions (no change with PhNH(2), less extensive addition of Et(2)NH with formation of cis-RhI(PPh(3))(2)(NHEt(2)), less extensive reaction of the latter with additional Et(2)NH to yield cis-RhI(PPh(3))(2)(Et(2)NH)···NHEt(2). The two [RhX(PPh(3))(2)](2) compounds do not show any evidence for addition of the corresponding X(-) to yield a putative [RhX(2)(PPh(3))(2)](-) adduct. The product of C(2)H(4) addition to [RhI(PPh(3))(2)](2), trans-RhI(PPh(3))(2)(C(2)H(4)), has been characterized in solution. Treatment of the RhCl(3)·3H(2)O/PPh(3)/nBu(4)PI/PhNH(2) mixture under catalytic conditions yields mostly [RhCl(PPh(3))(2)](2), and no significant halide exchange, demonstrating that the promoting effect of iodide must take place at the level of high energy catalytic intermediates. The equilibria have also been investigated at the computational level by DFT with treatment at the full QM level including solvation effects. The calculations confirm that the bridge splitting reaction is slightly less favorable for the iodido derivative. Overall, the study confirms the active role of rhodium(I) species in ethylene hydroamination catalyzed by RhCl(3)·3H(2)O/PPh(3)/nBu(4)PI and suggest that the catalyst resting state is [RhCl(PPh(3))(2)](2) or its C(2)H(4) adduct, RhCl(PPh(3))(2)(C(2)H(4)), under high ethylene pressure.     

Catalytic dehydrocoupling of Me2NHBH3 with Al(NMe2)3

The catalytic dehydrocoupling reaction of Me(2)NHBH(3) with Al(NMe(2))(3) gives the dimer [Me(2)NBH(2)](2) and the chain [(Me(2)N)(2)BH], involving the thermally-stable Al(III) hydride catalyst [{(Me(2)N)(2)BH(2)}(2)AlH].     

"Spontaneous" ambient temperature dehydrocoupling of aromatic amine-boranes

The dehydrocoupling/dehydrogenation behavior of primary arylamine-borane adducts ArNH(2)?BH(3) (3?a-c; Ar = a: Ph, b: p-MeOC(6)H(4), c: p-CF(3)C(6)H(4)) has been studied in detail both in solution at ambient temperature as well as in the solid state at ambient or elevated temperatures. The presence of a metal catalyst was found to be unnecessary for the release of H(2). From reactions of 3?a,b in concentrated solutions in THF at 22?°C over 24?h cyclotriborazanes (ArNH-BH(2))(3) (7?a,b) were isolated as THF adducts, 7?a,b?THF, or solvent-free 7?a, which could not be obtained via heating of 3?a-c in the melt. The μ-(anilino)diborane [H(2)B(μ-PhNH)(μ-H)BH(2)] (4?a) was observed in the reaction of 3?a with BH(3)?THF and was characterized in situ. The reaction of 3?a with PhNH(2) (2?a) was found to provide a new, convenient method for the preparation of dianilinoborane (PhNH)(2)BH (5?a), which has potential generality. This observation, together with further studies of reactions of 4?a, 5?a, and 7?a,b, provided insight into the mechanism of the catalyst-free ambient temperature dehydrocoupling of 3?a-c in solution. For example, the reaction of 4?a with 5?a yields 6?a and 7?a. It was found that borazines (ArN-BH)(3) (6?a-c) are not simply formed via dehydrogenation of cyclotriborazanes 7?a-c in solution. The transformation of 7?a to 6?a is slowly induced by 5?a and proceeds via regeneration of 3?a. The adducts 3?a-c also underwent rapid dehydrocoupling in the solid state at elevated temperatures and even very slowly at ambient temperature. From aniline-borane derivative 3?c, the linear iminoborane oligomer (p-CF(3)C(6)H(4))N[BH-NH(p-CF(3)C(6)H(4))](2) (11) was obtained. The single-crystal X-ray structures of 3?a-c, 5?a, 7?a, 7?b?THF, and 11 are discussed.     

In situ formed catalytically active ruthenium nanocatalyst in room temperature dehydrogenation/dehydrocoupling of ammonia-borane from Ru(cod)(cot) precatalyst

The development of simply prepared and effective catalytic materials for dehydrocoupling/dehydrogenation of ammonia-borane (AB; NH(3)BH(3)) under mild conditions remains a challenge in the field of hydrogen economy and material science. Reported herein is the discovery of in situ generated ruthenium nanocatalyst as a new catalytic system for this important reaction. They are formed in situ during the dehydrogenation of AB in THF at 25 °C in the absence of any stabilizing agent starting with homogeneous Ru(cod)(cot) precatalyst (cod = 1,5-η(2)-cyclooctadiene; cot = 1,3,5-η(3)-cyclooctatriene). The preliminary characterization of the reaction solutions and the products was done by using ICP-OES, ATR-IR, TEM, XPS, ZC-TEM, GC, EA, and (11)B, (15)N, and (1)H NMR, which reveal that ruthenium nanocatalyst is generated in situ during the dehydrogenation of AB from homogeneous Ru(cod)(cot) precatalyst and B-N polymers formed at the initial stage of the catalytic reaction take part in the stabilization of this ruthenium nanocatalyst. Moreover, following the recently updated approach (Bayram, E.; et al. J. Am. Chem. Soc.2011, 133, 18889) by performing Hg(0), CS(2) poisoning experiments, nanofiltration, time-dependent TEM analyses, and kinetic investigation of active catalyst formation to distinguish single metal or in the present case subnanometer Ru(n) cluster-based catalysis from polymetallic Ru(0)(n) nanoparticle catalysis reveals that in situ formed Ru(n) clusters (not Ru(0)(n) nanoparticles) are kinetically dominant catalytically active species in our catalytic system. The resulting ruthenium catalyst provides 120 total turnovers over 5 h with an initial turnover frequency (TOF) value of 35 h(-1) at room temperature with the generation of more than 1.0 equiv H(2) at the complete conversion of AB to polyaminoborane (PAB; [NH(2)BH(2)](n)) and polyborazylene (PB; [NHBH](n)) units.