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The ambiphilic and small-bite-angle diphosphanylborane ligands Ph2PCH(PPh2)CH2B(C8H14) (2) and Ph2PCH2CH[B(C8H14)]PPh2 (3) containing the Lewis acidic 9-boranorbornyl group B(C8H14) have been prepared in one step by regioselective anti-Markovnikov hydroborations of 1,1-bis(diphenylphosphanyl)ethylene and 1,2-bis(diphenylphosphanyl)ethylene with 9-borabicyclo[3.3.1]nonane (9-BBN) under relatively drastic reaction condition. The coordination properties of Ph2PCH2CH2B(C8H14) (1) and the newly prepared 2 and 3 towards the tungsten nitrosyl fragment {WNO}6 have been studied structurally and spectroscopically. Herein, we report the cis and trans tungsten nitrosyl complexes cis-[W(CO)2(1)2(NO)(Cl)] (cis-4a), cis-[W(CO)2(1)2(NO)(H)] (cis-4b), trans-[W(CO)2(1)2(NO)(Cl)] (trans-4a), trans-[W(CO)2(1)2(NO)(H)] (trans-4b), cis-[W(CO)2(2)(NO)(Cl)] (cis-5a), cis-[W(CO)2(2)(NO)(H)] (cis-5b), cis-[W(CO)2(3)(NO)(Cl)] (cis-6a), cis-[W(CO)2(3)(NO)(H)] (cis-6b), which were readily obtained from straightforward ligand-substitution reactions of the tungsten carbonyl nitrosyl precursor [W(NO)(CO)4(ClAlCl3)] in tetrahydrofuran. The crystal structures of the tungsten chloride complexes cis-4a, trans-4a, and cis-5a revealed that the trialkyl boron centers are free pendants in the secondary coordination spheres, whereas in the hydride complexes trans-4b, cis-5b, and cis-6b the hydride ligand trans to the nitrosyl ligand is σ coordinated to the tethered B(C8H14) group and form three-center, two-electron (3c–2e) W–H–B bonds, which were observed in their crystal structures. The authenticity of the 3c–2e W–H–B bond in the tungsten hydride complex cis-5b was further checked by a DFT structural optimization and natural bond order (NBO) analysis.

Reaction of 1 with B(C6F5)3 under 1 bar of CO2 led to the instantaneous formation of the frustrated Lewis pair (FLP)-type species [ReHBr(NO)(PR3)2(η(2)-O═C═O-B(C6F5)3)] (2, R = iPr a, Cy b) possessing two cis-phosphines and O(CO2)-coordinated B(C6F5)3 groups as verified by NMR spectroscopy and supported by DFT calculations. The attachment of B(C6F5)3 in 2a,b establishes cooperative CO2 activation via the Re-H/B(C6F5)3 Lewis pair, with the Re-H bond playing the role of a Lewis base. The Re(I) η(1)-formato dimer [{Re(μ-Br)(NO)(η(1)-OCH═O-B(C6F5)3)(PiPr3)2}2] (3a) was generated from 2a and represents the first example of a stable rhenium complex bearing two cis-aligned, sterically bulky PiPr3 ligands. Reaction of 3a with H2 cleaved the μ-Br bridges, producing the stable and fully characterized formato dihydrogen complex [ReBrH2(NO)(η(1)-OCH═O-B(C6F5)3)(PiPr3)2] (4a) bearing trans-phosphines. Stoichiometric CO2 reduction of 4a with Et3SiH led to heterolytic splitting of H2 along with formation of bis(triethylsilyl)acetal ((Et3SiO)2CH2, 7). Catalytic reduction of CO2 with Et3SiH was also accomplished with the catalysts 1a,b/B(C6F5)3, 3a, and 4a, showing turnover frequencies (TOFs) between 4 and 9 h(-1). The stoichiometric reaction of 4a with the sterically hindered base 2,2,6,6-tetramethylpiperidine (TMP) furnished H2 ligand deprotonation. Hydrogenations of CO2 using 1a,b/B(C6F5)3, 3a, and 4a as catalysts gave in the presence of TMP TOFs of up to 7.5 h(-1), producing [TMPH][formate] (11). The influence of various bases (R2NH, R = iPr, Cy, SiMe3, 2,4,6-tri-tert-butylpyridine, NEt3, PtBu3) was studied in greater detail, pointing to two crucial factors of the CO2 hydrogenations: the steric bulk and the basicity of the base.

Several manganese(I) and rhenium(I) carbonyl complexes of the chelating diphosphanylborane ligands Ph2PCH(PPh2)CH2B(C8H14) (1) and Ph2PCH2CH[B(C8H14)]PPh2 (2) were prepared to study Lewis acid assisted stoichiometric CO reductions. The phosphanylborane complexes fac,cis-Mn(CO)3(1)(Br) (3a), fac,cis-Re(CO)3(1)(Br) (4a), fac,cis-Mn(CO)3(2)(Br) (5a), and fac,cis-Re(CO)3(2)(Br) (6a) were obtained from the reactions of Mn(CO)5Br or Re(CO)5Br with diphosphanylborane ligands 1 and 2. Further treatment of all four complexes with one equivalent of AgBF4 in CH2Cl2 in the dark followed by 1 atm of CO afforded the corresponding monocationic complexes [Mn(CO)4(1)][BF4] (3b), [Re(CO)4(1)][BF4] (4b), [Mn(CO)4(2)][BF4] (5b), and [Re(CO)4(2)](BF4) [6b]. An X-ray diffraction study confirmed the expected molecular structure of 3b. In addition to NMR and elemental analysis characterizations, the structures of these complexes could be unambiguously assigned based on two major ν(CO) IR bands, which established the cis-(CO)4 local ligand environment. The cationic complexes 3b, 4b, 5b, and 6b were tested for stoichiometric CO reductions by reaction with NaHBEt3 in chlorobenzene. However, the reductions of the complexes 3b and 4b produced mixtures of formyl and hydride species. The reaction of complexes 5b and 6b led to the formation of the stable formyl derivatives [Mn(CHO)(CO)3(2)] (5c) and [Re(CHO)(CO)3(2)] (6c), which were isolated in pure form and characterized by spectroscopic means. In addition, 5c could be characterized by a single-crystal X-ray study, which revealed the formyl structure and a stabilizing coordination of the pendant Lewis acidic B(C8H14) group to the formyl oxygen atom.

Without a catalyst ketones and aldehydes were reacted in THF with ammonia borane (AB) to proceed hydroboration forming alkyl borates. Mechanistic studies revealed that dissociation of ammonia from AB occurred before the hydroboration step. When methanol was used as the solvent, metal free methanolysis of AB would take place with the ketone/aldehyde being directly hydrogenated by the MeOH·BH3 complex.

[ReBr 2 (NO)(CH 3 CN)(PTA) 2 ] (PTA = 1, 3, 5-triaza-7-phosphaadamantane) catalyzes the alcoholysis of ammonia–borane and amine–boranes and the catalytic transfer hydrogenations of various terminal olefins. Excellent yields were achieved at 70 °C in isopropanol using t BuOK as a co-catalyst affording TOF values up to 396 h −1 .

Five-coordinated rhenium(I) hydride complexes of the type [Re(Br)(H)(NO)(PR3)2] (R = Cy 2a, iPr 2b) were prepared from [Re(Br)2(NO)(PR3)2(η2-H2)] (R = Cy 1a, iPr 1b) via deprotonation of the η2-H2 ligands with various bases. Filling the vacant site of 2a or 2b by various less bulky two-electron donors produced the 18-electron complexes [Re(Br)(H)(NO)(PR3)2(L)] (L = O2 3, CH2═CH2 4, acetylene 5, H2 6, CO 7, CH3CN 8). The influence of the trans-coordinated ligand on the Re−H bond was examined. The 1H NMR chemical shift of the hydride depends on L in the order O2 > acetylene > CH2═CH2 > H2 > CO > CH3CN. The reactions of 2a or 2b with the IMes or SIMes ligands afforded the five-coordinated complex [Re(Br)(H)(NO)(PR3)(NHC)] (NHC = IMes 9 (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), SIMes 10 (SIMes = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene)) via replacement of one phosphine. The reaction of 2a or 2b with n-BuLi leads to the formation of the n-butene-coordinated dihydride compl...

Choosy chemicals: Rhenium(I) complexes of type [ReBr(2)(L)(NO)(PR(3))(2)] (L=H(2) (1), CH(3)CN (2), ethylene (3); R=iPr (a), cyclohexyl (b)) proved to be suitable catalyst precursors for the highly selective dehydrogenative silylation of alkenes. Two types of rhenium(I) hydride species, [ReBrH(NO)(PR(3))(2)] (4) and [ReBr(eta(2)-CH(2)=CHR(1))H(NO)(PR(3))(2)] (5), were found in the [ReBr(2)(L)(NO)(PR(3))(2)]-catalyzed dehydrogenative silylation of alkenes.Rhenium(I) complexes of type [ReBr(2)(L)(NO)(PR(3))(2)] (L=H(2) (1), CH(3)CN (2), and ethylene (3); R=iPr (a) and cyclohexyl (Cy; b)) catalyze dehydrogenative silylation of alkenes in a highly selective manner to yield silyl alkenes and the corresponding alkanes. Hydrosilylation products appear only rarely depending on the type of olefinic substituent, and if they do appear then it is in very minor amounts. Mechanistic studies showed that two rhenium(I) hydride species of type [ReBrH(NO)(PR(3))(2)] (R=iPr (4 a) and Cy (4 b)) and [ReBr(eta(2)-CH(2)=CHR(1))H(NO)(PR(3))(2)] (R(1)=p-CH(3)C(6)H(4), R=iPr (5 a), Cy (5 b); R(1)=H, R=iPr (5 a'), Cy (5 b')) are involved in the initiation pathway of the catalysis. The rate-determining steps of the catalytic cycle are the phosphine dissociation from complexes of type 5 and the reductive eliminations to form the alkane components. The catalytic cycle implies that the given rhenium systems have the ability to activate C-H and Si-H bonds through the aid of a facile redox interplay of Re(I) and Re(III) species. The molecular structures of 4 b and 5 a were established by means of X-ray diffraction studies.

Treatment of the complexes [Re(NO)2(PR3)2][BAr(F)4] (R = Cy, 1 a; R = iPr, 1 b) with phenyldiazomethane gave the cationic benzylidene species [Re{CH(C6H5)}(NO)2(PR3)2][BAr(F)4] (2 a and 2 b) in good yields. Upon reaction of 2 a and 2 b with acetonitrile, the consecutive formation of [Re(N[triple bond]CCH3)(N[triple bond]CPh)(NO)(OC(CH3)=NH)(PR3)][BAr(F)4] (3 a and 3 b) and [Re(NCCH3)(OC{CH3}NH{C6H5})(NO)(PR3)2][BAr(F)4] (4 a and 4 b) was observed. The proposed reaction sequence involves the coupling of coordinated NO, carbene and acetonitrile molecules to yield the (1Z)-N-[imino(phenyl)methyl]ethanimidate ligand. The coupling of the nitrosyl and the benzylidene is anticipated to occur first, forming an oximate species. The subsequent acetonitrile addition can be envisaged as a heteroene reaction of the oximate and the acetonitrile ligand yielding 3 a and 3 b, which in turn can cyclise and undergo a prototropic shift initiated by an internal attack of the ethaneimidate ligand on the benzonitrile moiety to afford 4 a and 4 b.

trans-Fe(depe)2I2 (depe =1,2-bis(diethylphosphino)ethane) was employed to stepwise incorporate Fe(II) centers into a rigid-rod butadiyne based 5,10,15,20-tetraferratetracosa-1,3,6,8,11,13,16,18,21,23-decayne. The iterative synthesis first connects two Fe(II) centers via a central butadiynediyl ligand to provide I-Fe(depe)2-C4-Fe(depe)2-I (2), then extends the system by substituting the terminal halides of 2 to yield Me3SiC4-Fe(depe)2-C4-Fe(depe)2-C4SiMe3 (3). Further modification of the termini gives the deprotected and stannylated compounds RC4-Fe(depe)2-C4-Fe(depe)2-C4R (4 and 5; R = H, Sn(CH3)3, respectively). Transmetalation with two more mononuclear units furnishes the homometallic tetranuclear compound I-Fe(depe)2-C4-Fe(depe)2-C4-Fe(depe)2-C4-Fe(depe)2-I (6), to which two more butadiynyl units were attached to give Me3SiC4-Fe(depe)2-C4-Fe(depe)2-C4-Fe(depe)2-C4-Fe(depe)2-C4SiMe3 (7). All compounds were characterized by NMR, IR, and Raman spectroscopies and by elemental analyses. X-ray diffraction studies were carried out on the dinuclear complexes revealing highly symmetrical rigid-rod structures. Cyclic voltammetric studies showed that compounds 2-7 undergo reversible and well-defined oxidations with high Kc values indicating thermodynamically stable mixed valence species. While the number of the oxidation waves of compounds 2, 6, and 7 are equivalent to the number of metal centers, the dinuclear complexes 3, 4, and 5 exhibit three reversible oxidation waves, one at significantly more positive potential. Two redox waves were attributed to the oxidation of the metal centers, while the remaining one is due to the oxidation of the butadiynediyl ligand. The electronic properties of complexes 2, 3, and 7 were investigated by spectroelectrochemical measurements.

Diiodo Re(I) complexes [ReI2(NO)(PR3)2(L)] (3, L = H2O; 4 , L = H2; R = iPr a, Cy b) were prepared and found to exhibit in the presence of "hydrosilane/B(C6F5)3" co-catalytic systems excellent activities and longevities in the hydrogenation of terminal and internal alkenes. Comprehensive mechanistic studies showed an inverse kinetic isotope effect, fast H2/D2 scrambling and slow alkene isomerizations pointing to an Osborn type hydrogenation cycle with rate determining reductive elimination of the alkane. In the catalysts' activation stage phosphonium borates [R3PH][HB(C6F5)3] (6, R = iPr a, Cy b) are formed. VT (29)Si- and (15)N NMR experiments, and dispersion corrected DFT calculations verified the following facts: (1) Coordination of the silylium cation to the ONO atom facilitates nitrosyl bending; (2) The bent nitrosyl promotes the heterolytic cleavage of the H-H bond and protonation of a phosphine ligand; (3) H2 adds in a bifunctional manner across the Re-N bond. Nitrosyl bending and phosphine loss help to create two vacant sites, thus triggering the high hydrogenation activities of the formed "superelectrophilic" rhenium centers.

Treatment of the complex Mo(NO)Cl3(NCMe)2 with the large bite angle diphosphine, 2,2′-bis(diphenylphosphino)diphenylether (DPEphos) afforded the dinuclear species [Mo(NO)(P∩P)Cl2]2[μCl]2 (P∩P = DPEphos = (Ph2PC6H4)2O (1). 1 could be reduced in the presence of Zn and MeCN to the cationic complex [Mo(NO)(P∩P)(NCMe)3]+[Zn2Cl6]2–1/2 (2). In a metathetical reaction the [Zn2Cl6]2–1/2 counteranion was replaced with NaBArF4 (BArF4 = [B{3,5-(CF3)2C6H3}4]) to obtain the [BArF4]− salt [Mo(NO)(P∩P)(NCMe)3]+[BArF4]− (3). 3 was found to catalyze hydrosilylations of various para substituted benzaldehydes, cyclohexanecarboxaldehyde, 2-thiophenecarboxaldehyde, and 2-furfural at 120 °C. A screening of silanes revealed primary and secondary aromatic silanes to be most effective in the catalytic hydrosilylation with 3. Also ketones could be hydrosilylated at room temperature using 3 and PhMeSiH2. A maximum turnover frequency (TOF) of 3.2 × 104 h–1 at 120 °C and a TOF of 4400 h–1 was obtained at room temperature for the hydro...

Sterically congested Lewis pairs cannot form Lewis adducts; instead they establish encounter complexes of "frustrated" Lewis pairs (FLPs). These encounter complexes have recently been recognized to be capable of activating, i.e., splitting, homopolar and polar single and double bonds, which rendered a new reactivity principle. With the help of qualitative orbital considerations this chapter reviews and explains the reactivity of FLPs toward homopolar Z-Z or Z-Z' single bonded molecules, such as H-H and C-H single bonds, assuming in the encounter complexes the action of strongly polarizing Coulombic fields originating from the FLP constituents. This reactivity principle has been extended in its view to the activating potential for homopolar Z-Z or Z-Z' single bonds of strongly polarized [X-Y ↔ X(+)-Y|(-)] σ and [X=Y ↔ X(+)-Y|(-)] π bonded molecules (X,Y = atoms or molecular fragments; electronegativity of X < electronegativity of Y). A striking analogy in the reaction behavior of FLPs and strongly polarized σ and π bonded molecules could be revealed based on the analyses of selected examples of "metal-free" (main group element reactions) or metal-based (containing transition metals) σ bond metathesis reactions and σ bond additions of H2 and alkanes to polarized main group element and metal to ligand π bonds. Related to the described polar reaction types are Z,Z' double atom or group transfers between highly polarized double bonds of XY and X'Y' molecules combining a Z,Z' elimination with an addition process. Multiple consecutive Z,Z' double atom or group transfers are denoted as double H transfer cascade reactions. Analyzed by examples are concerted or stepwise double H transfers and double H transfer cascades with Z,Z'=H,H from the "metal-free" and metal-based realms: the Meerwein- Pondorf-Verley reduction, H,H exchanges between amine boranes and between amine boranes and unsaturated organic compounds, and the crucial H,H transfer steps of Noyori's bifunctional and Shvo type transfer hydrogenation catalyses.

The reactions of [M(NO)(CO)4(ClAlCl3)] (M=Mo, W) with (iPr2PCH2CH2)2NH, (PNHP) at 90 °C afforded [M(NO)(CO)(PNHP)Cl] complexes (M=Mo, 1 a; W, 1 b). The treatment of compound 1 a with KOtBu as a base at room temperature yielded the alkoxide complex [Mo(NO)(CO)(PNHP)(OtBu)] (2 a). In contrast, with the amide base Na[N(SiMe3)2], the PNHP ligand moieties in compounds 1 a and 1 b could be deprotonated at room temperature, thereby inducing dehydrochlorination into amido complexes [M(NO)(CO)(PNP)] (M=Mo, 3 a; W, 3 b; PNP=(iPr2PCH2CH2)2N)). Compounds 3 a and 3 b have pseudo-trigonal-bipyramidal geometries, in which the amido nitrogen atom is in the equatorial plane. At room temperature, compounds 3 a and 3 b were capable of adding dihydrogen, with heterolytic splitting, thereby forming pairs of isomeric amine-hydride complexes [Mo(NO)(CO)H(PNHP)] (4 a(cis) and 4 a(trans)) and [W(NO)(CO)H(PNHP)] (4 b(cis) and 4 b(trans); cis and trans correspond to the position of the H and NO groups). H2 approaches the Mo/W[DOUBLE BOND]N bond in compounds 3 a,b from either the CO-ligand side or from the NO-ligand side. Compounds 4 a(cis) and 4 a(trans) were only found to be stable under a H2 atmosphere and could not be isolated. At 140 °C and 60 bar H2, compounds 3 a and 3 b catalyzed the hydrogenation of imines, thereby showing maximum turnover frequencies (TOFs) of 2912 and 1120 h−1, respectively, for the hydrogenation of N-(4-methoxybenzylidene)aniline. A Hammett plot for various para-substituted imines revealed linear correlations with a negative slope of −3.69 for para substitution on the benzylidene side and a positive slope of 0.68 for para substitution on the aniline side. Kinetics analysis revealed the initial rate of the hydrogenation reactions to be first order in c(cat.) and zeroth order in c(imine). Deuterium kinetic isotope effect (DKIE) experiments furnished a low kH/kD value (1.28), which supported a Noyori-type metal–ligand bifunctional mechanism with H2 addition as the rate-limiting step.

Shelf-available alkali metal tert-butoxides, hydrides and bis(trimethylsilyl)amides were shown to be highly efficient homogeneous precatalysts for the disproportionation of aldehydes to the corresponding carboxylic esters. Potassium compounds in combination with 18-crown-6 ether could drastically increase the rate of reaction in a few cases. Alternatively, efficient aldol condensations were found for aldehydes possessing an enolizable methylene group at the α-position to the aldehyde functionality. The active species involved in this esterification using any of these alkali metal catalysts is expected to be the metal alkoxide. Potassium compounds were found to be much more efficient when compared to analogous sodium compounds and kinetic studies revealed the rate-determining step to be a second order concerted hydride transfer from a potassium hemiacetal species to another molecule of aldehyde.

Highly efficient alkene hydrogenations were developed using NO-functionalized hydrido dinitrosyl rhenium catalysts of the type [ReH(PR3)2(NO)(NO(LA))][Z] (2, LA = B(C6F5)3; 3, LA = [Et]+, Z = [B(C6F5)4]−; 4, LA = [SiEt3]+, Z = [HB(C6F5)3]−; R = iPr a, Cy b). Lewis acid attachment to the NO ligand was found to facilitate bending at the NOLA atom and concomitantly to open up a vacant site at the rhenium center. According to DFT calculations, the ability to bend follows the order 4 > 3 > 2, which did not match with the order of increasing hydrogenation activities: 3 > 4 > 2. The main factor spoiling catalytic performance was catalyst deactivation by detachment of the LA group occurring during the catalytic reaction course, which was found to go along with the decrease in order of DFT-calculated strengths of the ONO–LA bonds. LA detachment from the ONO atom could at least partly be prevented by LA addition as cocatalysts, which led to an extraordinary boost of the hydrogenation activities. For instance the “1...

A series of [ReBr2(MeCN)(NO)(P∩P)] complexes (P∩P = 1,1′-bis(diphenylphosphino)ferrocene (dppfc) (1a), 1,1′-bis(diisopropylphosphino)ferrocene (diprpfc) (1b), 2,2′-bis(diphenylphosphino)diphenyl et...

Transfer hydrogenation of polarized olefins bearing strongly electron-withdrawing groups on one side of the double bond was achieved with ammonia borane under mild conditions without using a catalyst. Mechanistic studies proved the character of the direct H transfers proceeding stepwise with a unique hydroboration intermediate and hydride before proton transfer.

Dihydrogen reactions are mainly reaction-center-mediated. In a systematic approach, these reactions are divided into the steps of splitting of H(2) and H-atom transfers with homolytic and heterolytic splitting at mono- and dinuclear reaction centers, and further homopolar H transfers in Wilkinson-type hydrogenations (insertion and reductive elimination) and heteropolar transfers in ionic hydrogenations (hydride before proton, proton before hydride, simultaneous hydride/proton transfers). Based on these categories, transfer hydrogenations are also surveyed. A separate class of reactions is one-electron reactions of dihydrogen.

The five-coordinated Re(I) hydride complexes [Re(Br)(H)(NO)(PR(3))(2)] (R=Cy 1 a, iPr 1 b) were reacted with benzylbromide, thereby affording the 17-electron mononuclear Re(II) hydride complexes [Re(Br)(2)(H)(NO)(PR(3))(2)] (R=Cy 3 a, iPr 3 b), which were characterized by EPR, cyclic voltammetry, and magnetic susceptibility measurements. In the case of dibromomethane or bromoform, the reaction of 1 afforded Re(II) hydrides 3 in addition to Re(I) carbene hydrides [Re(=CHR(1))(Br)(H)(NO)(PR(3))(2)] (R(1)=H 4, Br 5; R=Cy a, iPr b) in which the hydride ligand is positioned cis to the carbene ligand. For comparison, the dihydrogen Re(I) dibromide complexes [Re(Br)(2)(NO)(PR(3))(2)(eta(2)-H(2))] (R=Cy 2 a, iPr 2 b) were reacted with allyl- or benzylbromide, thereby affording the monophosphine Re(II) complex salts [R(3)PCH(2)R'][Re(Br)(4)(NO)(PR(3))] (R'=-CH=CH(2) 6, Ph 7). The reduction of Re(II) complexes has also been examined. Complex 3 a or 3 b can be reduced by zinc to afford 1 a or 1 b in high yield. Under catalytic conditions, this reaction enables homocoupling of benzylbromide (turnover frequency (TOF): 3 a 150, 3 b 134 h(-1)) or allylbromide (TOF: 3 a 575, 3 b 562 h(-1)). The reaction of 6 a and 6 b with zinc in acetonitrile affords in good yields the monophosphine Re(I) complexes [Re(Br)(2)(NO)(MeCN)(2)(PR(3))] (R=Cy 8 a, iPr 8 b), which showed high catalytic activity toward highly selective dehydrogenative silylation of styrenes (maximum TOF of 61 h(-1)). Single-electron transfer (SET) mechanisms were proposed for all these transformations. The molecular structures of 3 a, 6 a, 6 b, 7 a, 7 b, and 8 a were established by single-crystal X-ray diffraction studies.

The reactions of frustrated Lewis pairs (FLPs) derived from B(C6F5)3 and the bulky Lewis bases 2,2,6,6-tetramethylpiperidine (TMP), tri-tert-butylphosphine, and lutidine (Lut) with terminal alkynes (acetylene, phenylacetylene, 3-ethynylthiophene) were investigated. The FLPs TMP···B(C6F5)3, t-Bu3P···B(C6F5)3 and Lut···(C6F5)3 reacted with acetylene (HC≡CH) to yield the apparently thermodynamically more stable E isomers [TMPH][(C6F5)2B−C(C6F5)═C(H)B(C6F5)3] (1-E), t-Bu3PC(H)═C(H)B(C6F5)3 (2-E; 90%), and [t-Bu3PH][(C6F5)2B−C(C6F5)═C(H)B(C6F5)3] (3-E; 10%), and LutC(H)═C(H)B(C6F5)3 (4-E), respectively. A mechanistic pathway for the reaction of acetylene is suggested to start with the formation of a weak B(C6F5)3/acetylene adduct followed by a deprotonation of this species with any mentioned Lewis bases (LB), yielding the acetylide salts [LBH][(C6F5)3BC≡CH]. Alternatively, nucleophilic addition of the LB to this adduct occurs to yield LBC(H)═C(H)B(C6F5)3 compounds. Formation of 1 and 3-E is explained by the re...

Two transition-metal hydride complexes of the type [M(dippe) 2 (NO)(H)] [M = W (2a), Mo (2b); dippe = 1,2-bis(di-isopropylphosphanyl)ethane] have been prepared by the reaction of [M(dippe) 2 (NO)(Cl)] [M = W (1a), Mo (1b)] with LiBH 4 . The nitrosyl groups of the tungsten complexes 1a and 2a are capable to coordinate a LiBH 4 molecule to form the stable adducts W(dippe) 2 (Cl)(NO···LiBH 4 ) (1c) and W(dippe) 2 -(H)(NO...LiBH4) (2c). Addition of ethylenediamine to a toluene solution of 2c led to rupture of the 2c adduct and afforded 2a in good yield. After the interaction of 2a with [H(Et 2 O)][BF 4 ], the stable seven-coordinated cationic dihydride [W(dippe)(H) 2 (dippe)(NO)][BF 4] (6a) was isolated. The reaction between 2b and [H(Et 2 O)][BF 4 ] led to the formation of [Mo(dippe) 2 (NO)(FBF 3 )] (6b) in which BF 4 - was found to be coordinated to the metal centre. The [H(Et 2 O) 2 ][BAr F 4 ] [Ar F = 3,5-(CF 3 ) 2 C 6 H 3 ] acid interacted with 2a to yield the seven-coordinated complex [W(dippe)(Hb(dippe)(NO)]-[BAr F 4 ] (5a) similar to 6a. In the reaction between 2b and [H(Et 2 O) 2 ][BAr F 4 ], the 16e - five-coordinated complex [Mo-(dippe) 2 (NO)][BAr F 4 ] (4b) was formed. X-ray diffraction revealed that 4a has a weak agostic interaction trans to the NO ligand. Complex 4b was found to react rapidly with hydrogen gas under ambient conditions to form the dihydride complex [Mo(dippe)(H) 2 (dippe)(NO)][BAr F 4 ] (5b), which is unstable in the absence of a hydrogen atmosphere. The equilibrium constant for the reversible reaction of 4b with hydrogen was found to be K = 2.6 bar -1 at 25 °C. Complex 4b was tested as a catalyst for acetone hydrogenation; a maximum TON of 7 was found.

Dibromonitrosyl(dihydrogen)rhenium(I) complexes [ReBr(2)(NO)(PR(3))(2)(η(2)-H(2))] (1; R = iPr, a; Cy, b) and Me(2)NH·BH(3) (DMAB) catalyze at either 90 °C or ambient temperature under 10 bar of H(2) the hydrogenation of various terminal and cyclic alkenes (1-hexene, 1-octene, cyclooctene, styrene, 1,5-cyclooctadiene, 1,7-octadiene, α-methylstyrene). Maximum turnover frequency (TOF) values of 3.6 × 10(4) h(-1) at 90 °C and 1.7 × 10(4) h(-1) at 23 °C were achieved in the hydrogenation of 1-hexene. The extraordinary catalytic performance of the 1/DMAB system is attributed to the formation of five-coordinate rhenium(I) hydride complexes [Re(Br)(H)(NO)(PR(3))(2)] (2; R = iPr, a; Cy, b) and the action of the Lewis acid BH(3) originating from DMAB. The related 2/BH(3)·THF catalytic system also exhibits under the same conditions high activity in the hydrogenation of various alkenes with a maximum turnover number (TON) of 1.2 × 10(4) and a maximum TOF of 4.0 × 10(4) h(-1). For the hydrogenations of 1-hexene with 2a and 2b, the effect of the strength of the boron Lewis acid was studied, the acidity being in the following order: BCl(3)BH(3)BEt(3) ≈ BF(3)B(C(6)F(5))(3)BPh(3) ≫ B(OMe)(3). The order in catalytic activity was found to be B(C(6)F(5))(3)BEt(3) ≈ BH(3)·THFBPh(3) ≫ BF(3)·OEt(2)B(OMe)(3) ≫ BCl(3). The stability of the catalytic systems was checked via TON vs time plots, which revealed the boron Lewis acids to cause an approximate inverse order with the Lewis acid strength: BPh(3)BEt(3) ≈ BH(3)·THFB(C(6)F(5))(3). For the 2a/BPh(3) system a maximum TON of 3.1 × 10(4) and for the 2a/B(C(6)F(5))(3) system a maximum TOF of 5.6 × 10(4) h(-1) were obtained in the hydrogenation of 1-hexene. On the basis of kinetic isotope effect determinations, H(2)/D(2) scrambling, halide exchange experiments, Lewis acid variations, and isomerization of terminal alkenes, an Osborn-type catalytic cycle is proposed with olefin before H(2) addition. The active rhenium(I) monohydride species is assumed to be formed via reversible bromide abstraction with the "cocatalytic" Lewis acid. Homogeneity of the hydrogenations was tested with filtration and mercury poisoning experiments. These "rhenium(I) hydride/boron Lewis acid" systems demonstrate catalytic activities comparable to those of Wilkinson- or Schrock-Osborn-type hydrogenations accomplished with precious metal catalysts.

The hydridic reactivity of the complex W(CO)(H)(NO)(PMe3)3 (1) was investigated applying a variety of protic donors. Formation of organyloxide complexes W(CO)(NO)(PMe3)3(OR) (R = C6H5 (2), 3,4,5-Me3C6H2 (3), CF3CH2 (4), C6H5CH2 (5), Me (6) and iPr (7)) and H2 evolution was observed. The reactions of 1 accelerated with increasing acidity of the protic donor: Me2CHOH (pKa = 17) Regioselective hydrogen bonding of 1 was probed with two of the protic donors furnishing equilibrium formation of the dihydrogen bonded complexes ROH···HW(CO)(NO)(PMe3)3 (R = 3,4,5-Me3C6H2, 3a and iPr, 7a) and the ONO hydrogen bonded species ROH···ONW(CO)(H)(PMe3)3 (R = C6H2Me3, 3b and iPr, 7b) which were studied in hexane and d8-toluene solutions using variable temperature IR and NMR spectroscopy. Quantitative IR experiments at low temperatures using 3,4,5-trimethylphenol (TMP) confirmed the two types of competitive equilibria: dihydrogen bonding to give 3a (ΔH1 = −5.8 ± 0.4 kcal/mol and ΔS1 = −15.3 ± 1.4 e.u.) and hydrogen bonding to give 3b (ΔH2 = −2.8 ± 0.1 kcal/mol and ΔS2 = −5.8 ± 0.3 e.u.). Additional data for the hydrogen bonded complexes 3a,b and 7a,b were determined via NMR titrations in d8-toluene from the equilibrium constants K(Δδ) and K ( Δ R 1 ) measuring either changes in the chemical shifts of HW(Δδ) or the excess relaxation rates of HW (ΔR1) (3a,b: ΔH(Δδ) = −0.8 ± 0.1 kcal/mol; ΔS(Δδ) = −1.4 ± 0.3 e.u. and Δ H ( Δ R 1 ) = −5.8 ± 0.4 kcal/mol; Δ S ( Δ R 1 ) = −22.9 ± 1.9 e.u) (7a,b: ΔH(Δδ) = −2.3 ± 0.2 kcal/mol; ΔS(Δδ) = −11.7 ± 0.9 e.u. and Δ H ( Δ R 1 ) = −2.9 ± 0.2 kcal/mol; Δ S ( Δ R 1 ) = −14.6 ± 1.0 e.u). Dihydrogen bonding distances of 1.9 A and 2.1 A were derived for 3a and 7a from the NMR excess relaxation rate measurements of HW in d8-toluene. An X-ray diffraction study was carried out on compound 2.

Reduction of [NMe4]2[ReBr5(NO)] (1) with zinc in acetonitrile leads to the known trisacetonitrile compound [ReBr2(CH3CN)3(NO)] (2). Attempts to turn 2 into a dihydrogen or a hydride complex applying direct reaction with H2 or with H2 and a base were unsuccessful. Complex 2 could be transformed into [ReBr(BF4)mer(CH3CN)3(NO)] (2a) with AgBF4 in acetonitrile and was used as a starting material in a ligand exchange reaction with the water soluble phosphine 1,3,5-triaza-7-phosphadamantane (PTA) to obtain the complex [ReBr2(NO)(PTA)3 ]( 3). When the reduction of 1 with zinc was carried out in the presence of PTA in acetonitrile, the disubstituted complex [ReBr2(CH3CN)(NO)(PTA)2 ]( 4) was formed. The olefin-coordinated rhenium complexes [ReBr2(NO)(CH2@CH2)(PTA)2 ]( 5a) and [ReBr2(NO)(PhCH@CH2)(PTA)2 ]( 5b) were obtained from the reaction of 4 with the corresponding olefins. Complex 4 reacts further with NaHBEt3 in THF to give the dihydride [ReH2(THF)(NO)(PTA)2 ]( 6). In the presence of ethylene 6 is transformed into the ethyl hydride complex [ReH(CH2CH3)(g

The chemistry of nitrosyl tungsten hydrides bearing the (2-aminoethyl)dimethylphosphane ligand (edmp) with an acidic NH functionality was studied. Such complexes were targeted by applying the reaction of WCl 3 (NO)(CH3CN) 2 with edmp, which gave [WCl 2 (NO)(η 2 -edmp) 2 ][Cl] (1a) and [WCl 2 (NO)-( η 2 -edmpb) 2 ][BPh 4 ] (1b). Complex 1b was treated with Zn to produce [W(H)(Cl)(NO)(η 2 -edmp) 2 ][BPh4] (2b). Subsequent attempts with the use of WCl(NO)[P(OMe) 3 ] 4 and edmp as starting materials produced {W(NO)(η 2 -edmp) 2 [P(OMe) 3 ]}-[Cl] (3a) at 85 °C and [W(H)(Cl)(NO)(η 2 -edmp) 2 ][Cl] (2a) at 120 °C. Substitution of the P(OMe) 3 ligand in (W(η 2 -edmp)2-(NO)[P(OMe) 3 ]}[BPh 4 ] (3b) with CO afforded [W(CO)(η 2 -edmp) 2 (NO)][BPh 4 ] (4b). Complex 4b was treated with NaHBEt 3 , resulting in a dihydride product [W(CO)trans-(η 1 -edmp) 2 (H) 2 (NO)]Na (4c). The chlorido complexes containing one η 2 -edmp ligand WCl(NO)(η 2 -edmp)(PMe3) 2 (5a), WCl(NO)(η 2 -edmp)(CO)(PMe 3 ) (6a), WCl(NO)(η 2 -edmp)[P-(OMe) 3 ] 2 (5b), and WCl(NO)(CO)(η 2 -edmp)[P(OMe)3] (6b) were prepared and treated with NaHBEt 3 to obtain the corresponding hydride species WH(NO)(η 2 -edmp)[P(OMe)) 3 ] 2 (8b), WH(NO)(CO)(η 2 -edmp)(PMe 3 ) (7a), and WH(NO)(CO)-(η 2 -edmp)[P(OMeb) 3 ] (7b). The initially formed WH(NO)(η 2 -edmp)(PMe 3 ) 2 (8a) was spontaneously transformed into the amide complex W(NO)(CO)(NHCH 2 CH 2 PMe 2 )(PMe 3 ) (11a) with loss of H 2 . The anionic dihydride products [W(H) 2 (η 1 -edmp)(NO)L 2] Na [L = PMe 3 (9a) and L = PMe 3 (9b)] were produced from 7a and 7b by the reaction with NaHBEt 3 . Hydride 8b turned out to be the most stable hydride complex in the given series. NMR spectroscopic experiments and deuterium labeling showed that 8b coexists in equilibrium with corresponding dihydrogen amide complex 10b. The structures of compounds 1a, 2b, 3a, 4b, 5b, 6a, and 6b were studied by single-crystal X-ray diffraction.

Stable paramagnetic Cr(II) and Cr(III) bis(alkynyl) complexes of the type [trans(RC≡C)2Cr(dmpe)2]n+ (R = Ph, SiMe3, SiEt3, C≡C–SiMe3 n = 0, 1) were prepared and characterised by NMR, cyclic voltammetry, EPR, magnetic measurements, and X-ray single-crystal diffraction studies.

The tris(acetonitrile)dibromonitrosylrhenium(I) compound (1a) was obtained by reduction of the paramagnetic [NMe4]2[Re(NO)Br5] salt with Zn in MeCN. Subsequent reaction of 1a with THF produced the THF derivative [Re(NO)(THF)(MeCN)2Br2] (1b). Reaction of 1b with PiPr3, Pcy3, or P(p-tolyl)3 yielded bis(acetonitrile)-cis-dibromo(nitrosyl)-trans-bis(phosphine)rhenium complexes (R = iPr 2a, cy 2b, p-tolyl 2c). Treatment of 2a,b with excess NaBH4 produced the known borohydride complexes [Re(H)(η2-BH4)(NO)(PR3)2] (R = iPr 3a, cy 3b). Replacement of the BH3 moiety of 3a,b in THF by ethylene (1 bar) produced the dihydride complexes [Re(H)2(η2-C2H4)(NO)(PR3)2] (4) (R = iPr a, cy b). Protonation of 4a,b with HBF4·OEt2 afforded H2 and the monohydrido tetrafluoroborato species [Re(H)(NO)(PR3)2(η2-C2H4)(BF4)] (R = iPr 5a, cy 5b). X-ray diffraction studies were carried out on 1a, 2b,c, and 5b. Complexes 4a,b are catalytically active in olefin, imine, and ketone hydrogenations and in olefin and ketone hydrosilylations, a...