Search

The ethylene complex (PNP)Ti(η(2)-H2C═CH2)(CH2(t)Bu) or (PNP)Ti═CH(t)Bu(CH2(t)Bu) (PNP(-) = N[2-P(CHMe2)2-4-methylphenyl]2) reacts with H2CPPh3 to form the κ(2)-phosphinoalkylidene (PNP)Ti═CHPPh2(Ph) (1). Compound 1 activates benzene via the transient intermediate [(PNP)Ti≡CPPh2] (C). By treatment of (PNP)Ti═CH(t)Bu(OTf) with LiCH2PPh2, 1 or its isotopologue (PNP)Ti═CDPPh2(C6D5) (1-d6) can be produced by an independent route involving intermediate C, which activates benzene or benzene-d6 and dehydrogenates cyclohexane-d12. Addition of MeOTf to 1 results in elimination of benzene concomitant with the formation of the phosphonioalkylidyne complex, [(PNP)Ti≡CPPh2Me(OTf) (2). Theoretical studies of 2 suggest a resonance structure having dominant Ti-C triple-bond character with some contribution also from a C-P multiple bond.

The titanium alkylidene-triflate complex (PNP)Ti CH t Bu(OTf) was found to promote N–N bond cleavage of 9-diazofluorene and ditolyldiazomethane to give the imido complexes, (PNP)Ti N[C 13 H 9 ](OTf) ( 1 ) and (PNP)Ti N[CHtolyl 2 ](OTf) ( 2 ), respectively. The molecular structure of 2 was determined by single-crystal X-ray diffraction studies. Along with imido formation leading to 1 and 2 , the alkylidene Ti CH t Bu ligand in (PNP)Ti CH t Bu(OTf) was found to eliminate with the α-N atom of the diazoalkane to form the nitrile NC t Bu, which was confirmed by a combination of 1 H NMR spectroscopy and GC–MS. The reaction mechanism of the N–N bond cleavage promoted by the reactive Ti CH t Bu ligand is also discussed.

The alkyne complexes [(PNP)Ti(η2-HC≡CH)(CH2tBu)] (2) and [(PNP)Ti(η2-HC≡CMe)(CH2tBu)] (3) have been prepared by treatment of [(PNP)Ti═CHtBu(OTf)] (1) with the Grignard reagents H2C═CHMgCl and MeHC═CHMgBr, respectively. Complex 3 can be also prepared using the Grignard H2C═C(Me)MgBr and 1. The 2-butyne complex [(PNP)Ti(η2-MeC≡CMe)(CH2tBu)] (4) can be similarly prepared from 1 and MeHC═C(Me)MgBr. Complexes 2 and 3 have been characterized with a battery of multidimensional and multinuclear (1H, 13C, and 31P) NMR spectroscopic experiments, including selectively 31P decoupled 1H{31P}, 1H–31P HMBC, 1H–31P HOESY, and 31P EXSY. Variable-temperature 1H and 31P{1H} NMR spectroscopy reveals that the acetylene ligand in 2 exhibits a rotational barrier of 11 kcal mol–1, and such a process has been corroborated by theoretical studies. Formation of the titanium alkyne ligand in complexes 2 and 3 proceeds via the vinyl intermediate [(PNP)Ti═CHtBu(CH═CHR)] followed by a concerted, metal-mediated β-hydrogen abstraction ste...

Reacting (PNP)Ti[double bond, length as m-dash]CH(t)Bu(CH2(t)Bu) with 2,2'-bipyridine (bipy) in cyclohexane or heptane results in dehydrogenation, cleanly producing cyclohexene and 1-heptene, respectively, and a Ti(II) intermediate that is trapped by bipy to produce [(PNP)Ti(III)(CH2(t)Bu)(bipy˙(-))] (1). This titanium(ii) intermediate reduces the bipy ligand upon coordination to form a Ti(III) center, where the unpaired electron is antiferromagnetically coupled to the electron of the reduced [bipy˙(-)] π-radical moiety, giving an overall diamagnetic species. Complex 1 has been characterized by NMR and UV-vis spectroscopies as well as single crystal X-ray diffraction studies.

Many transition-metal complexes and some metal-free compounds are able to bind carbon monoxide, a molecule which has the strongest chemical bond in nature. However, very few of them have been shown to induce the cleavage of its C-O bond and even fewer are those that are able to transform CO into organic reagents with potential in organic synthesis. This work shows that bis(pinacolato)diboron, B2pin2, reacts with ruthenium carbonyl to give metallic complexes containing borylmethylidyne (CBpin) and diborylethyne (pinBC≡CBpin) ligands and also metal-free perborylated C1 and C2 products, such as C(Bpin)4 and C2 (Bpin)6, respectively, which have great potential as building blocks for Suzuki-Miyaura cross-coupling and other reactions. The use of (13)CO-enriched ruthenium carbonyl has demonstrated that the boron-bound carbon atoms of all of these reaction products arise from CO ligands.

The transient titanium alkylidyne, (PNP)Ti≡C(t)Bu (PNP = N[2-P(i)Pr(2)-4-methylphenyl](2)(-)), activates a C-H bond of ethane at room temperature, and a β-hydrogen of the resulting ethyl ligand is subsequently transferred to the adjacent alkylidene ligand to form an ethylene adduct of titanium. Treatment of the ethylene complex with two-electron oxidants such as organic azides results in extrusion of ethene concomitant with formation of a mononuclear titanium imido complex.

The homogeneous catalytic hydration of 2-, 3- and 4-cyanopyridines using 0.5 mol% of [(dippe)Ni(μ-H)]2 as catalyst precursor was achieved under heating. In the case of 4-cyanopyridine, production of isonicotinamide was observed at temperatures in the range of 80–120 °C. Heating to 180 °C resulted in formation of isonicotinic acid. In the case of 2- and 3-cyanopyridines the quantitative formation of their corresponding amides was achieved at 100 °C. The catalytic hydration of 2,6-dicyanopyridine was also undertaken in this work, in its case resulting in the synthesis of the mixed cyano/amide product, 2-cyanopyridine-6-carboxamide, at short reaction times.

The nickel complexes [(dippe)Ni(η2-O,C-benzophenone)] (2), [(dippe)Ni(η2-O,C-4-methylbenzophenone)] (3), [(dippe)Ni(η2-O,C-acetophenone)] (4), [(dippe)Ni(η2-O,C-acetone)] (5), [(dippe)Ni(η2-O,C-fluorenone)] (6), [(dippe)Ni(η2-O,C-di(2-pyridyl) ketone)] (7a) [(dippe)Ni(κ2-N,N-di(2-pyridyl) ketone)] (7b), [(dippe)Ni(κ2-O,O-2,2′-pyridil)] (8), [(dippe)Ni(κ2-O,O-benzil)] (9a), and [((dippe)Ni)2(η2-O,C-benzil)] (9b) were prepared by the reaction of [(dippe)Ni(μ-H)]2 (1) with the corresponding ketone or 1,2-diketone at room temperature. The structures of compounds 2, 6, 9a and 9b were confirmed by X-ray crystallography. The selective hydrogenation of the two types of substrates was undertaken using H2, giving high conversions to the corresponding reduction products, either alcohols or alkanes. Tunable reaction conditions to promote the partial or total hydrogenation (hydrogenolysis) of the substrates are described.

Nickel(0) catalysts of the type, [(dippe)Ni(η 2 – NC –R)] (R = –Ph, –PhCN) prepared in situ from the nickel(I) dimmer, [(dippe)Ni(μ-H)] 2 ( 1 ) in the presence of benzonitrile or the benzodinitriles (1,2-, 1,3- or 1,4-dicyanobenzenes) were used to hydrogenate these substrates. In the case of benzonitrile, 100% conversion was achieved after 72 h at 140 °C while pressurizing a reactor vessel with 60 psi of H 2 . N -Benzyl-benzylimine was obtained in 97% yield, accompanied by a small amount of dibenzylamine (2%). Hydrogenation of dicyanobenzenes was found to require more forceful conditions. In the case of 1,4-dicyanobenzene a 62% conversion of the substrate was achieved at 180 °C and 60 psi of H 2 after 72 h. Hydrogenation of 1,3-dicyanobenzene yielded only a 38% conversion, which could only be achieved at 180 °C while pressurizing at 120 psi. The major products from these reactions, 4-{[(4-cyanobenzyl)imino]methyl}benzonitrile and 3-{[(3-cyanobenzyl)imino]methyl}benzonitrile, were obtained in 97 and 98% yield respectively. The 1,2-dicyanobenzene did not show any reactivity under the conditions used for the other two substrates.

The reactions of [Ir(PMe3)4]Cl (1) with a variety of substrates (acetonitrile, p-aminobenzonitrile, p-cyanophenol) containing a nitrile group were examined. Cleavage of the X−H bonds occurred selectively over the coordination of the CN moiety in those substrates, and compounds derived from the corresponding X−H (X = C, N, O) oxidative addition reaction, namely, cis-[IrH(CH2CN)(PMe3)4]Cl (2), cis-[IrD(CD2CN)(PMe3)4]Cl (3), cis-[IrH(p-NHC6H4CN)(PMe3)4]Cl (4), and cis-[IrH(p-OC6H4CN)(PMe3)4]Cl (6), were obtained. X-ray diffraction studies have confirmed the structures of 3 and 4. In the case of 6, the compound trans-[IrClH(PMe3)4][p-OC6H4CN] (5b), resulting from exchange of Cl and p-OC6H4CN anions between inner and outer sphere, was also formed, and the solid-state structure (5b·HOC6H4CN), obtained by X-ray diffraction, contained the hydrogen-bonded NCC6H4O···H···OC6H4CN anion. The salt [Ir(PMe3)4][BPh4] (7) was also prepared, characterized by X-ray diffraction and reacted with p-HOC6H4CN. Reaction of 1 with...

The catalytic hydration of mono and dinitriles (dicyanoalkanes, DCAs) using nickel(0) catalysts of the type, [(dippe)Ni(η 2 - N , C –R)] (R = –Me,–Ph,–(CH 2 ) 2 –CN,–(CH 2 ) 4 –CN) and p -toluenesulfonic acid monohydrate (PTSA) as a co-catalyst, is reported. In the cases where DCAs (adipo and succinonitrile) were used, both the activity and the selectivity for the process were affected by length of the internal chains of the substrates. A competitive decomposition of the amide products was observed to be favoured to a greater extent in the case of succinonitrile, resulting in extensive carbonization and formation of ammonia. The carbonaceous by-products were analysed by scanning electron microscopy (SEM). The use of nickel(II) sources was addressed in the presence and in the absence of PTSA, with the aim of comparing their reactivity with that displayed by nickel(0) catalysts. The results obtained from these studies suggest that activity of the catalyst is affected by the oxidation state of the metal, the use of nickel(0) compounds resulting in better catalyst performance overall.

The homogeneous catalytic hydration of 1,2-, 1,3- and 1,4-dicyanobenzenes using organometallic nickel(0) catalysts of general formula [(dippe)Ni(η2-N,C-1,n-(CN)2-benzene)] (n = 2–4; complexes 2–4, respectively) was achieved under heating, the products of hydration, at least in the case of 1,3- and 1,4-dicyanobenzene being strongly dependent on the temperature used for the process; the production of the respective 1,3- and 1,4-cyanobenzamides been observed at 120 °C, while catalysis at 180 °C resulted in the quantitative formation of the 1,3- and 1,4-dicarboxylic acids. In the case of 1,2-dicyanobenzene, catalysis at either temperature yielded the hemihydration product 1,2-phthalamide, implying an important difference in overall reactivity for this system. All the hydration products were obtained in excellent selectivity and yield, using 0.5 mol% loadings of the corresponding nickel(0) catalyst and thus, the current work provides important evidences that could be of use for both synthetic organic chemistry and in the eventual production of polyamides.

The homogeneous catalytic hydration of benzo- and acetonitrile under thermal conditions was achieved using nickel(0) compounds of the type [(dippe)Ni(η 2 -NCR)] with R=phenyl or methyl (compounds 1 and 2, respectively), as the specific starting intermediates. Alternatively, the complexes may be prepared in situ by direct reaction of the precursor [(dippe)NiH] 2 (3) with the respective nitrile. Hydration appears to occur homogeneously, as tested by mercury drop experiments, producing benzamide and acetamide, respectively. Addition of Bu 4 NI did not lead to catalysis inhibition, suggesting the prevalence of Ni(0) intermediates during catalysis. Hydration using analogous complexes of 3, such as [(dtbpe)NiH] 2 (4) and [(dcype)NiH] 2 (5) was also addressed.

The reactions of the complexes [(dcype)NiH]2, 1, [(dippe)NiH]2, 2, and [(dtbpe)NiH]2, 3, with a mixture of BEt3 and Super-Hydride (LiHBEt3) afforded sigma-borane nickel(0) compounds of the type [(dcype)Ni(sigma-HBEt2)], 4, [(dippe)Ni(sigma-HBEt2)], 5, [(dtbpe)Ni(sigma-HBEt2)], 6, respectively, with the concomitant formation in each case of [(dcype)2Ni2)(H)3][BEt4], 7, [(dippe)2Ni2(H)3][BEt4], 8 and [(dtbpe)2Ni2(H)3][BEt4], 9, respectively. X-ray crystal structures are reported for 4 and 8. The reaction of BEt3 and LiHBEt3 was also reviewed in detail.

The transient titanium neopentylidyne, [(PNP)Ti≡C(t)Bu] (A; PNP(-)≡N[2-P(i)Pr2-4-methylphenyl]2(-)), dehydrogenates ethane to ethylene at room temperature over 24 h, by sequential 1,2-CH bond addition and β-hydrogen abstraction to afford [(PNP)Ti(η(2)-H2C═CH2)(CH2(t)Bu)] (1). Intermediate A can also dehydrogenate propane to propene, albeit not cleanly, as well as linear and volatile alkanes C4-C6 to form isolable α-olefin complexes of the type, [(PNP)Ti(η(2)-H2C═CHR)(CH2(t)Bu)] (R = CH3 (2), CH2CH3 (3), (n)Pr (4), and (n)Bu (5)). Complexes 1-5 can be independently prepared from [(PNP)Ti═CH(t)Bu(OTf)] and the corresponding alkylating reagents, LiCH2CHR (R = H, CH3(unstable), CH2CH3, (n)Pr, and (n)Bu). Olefin complexes 1 and 3-5 have all been characterized by a diverse array of multinuclear NMR spectroscopic experiments including (1)H-(31)P HOESY, and in the case of the α-olefin adducts 2-5, formation of mixtures of two diastereomers (each with their corresponding pair of enantiomers) has been unequivocally established. The latter has been spectroscopically elucidated by NMR via C-H coupled and decoupled (1)H-(13)C multiplicity edited gHSQC, (1)H-(31)P HMBC, and dqfCOSY experiments. Heavier linear alkanes (C7 and C8) are also dehydrogenated by A to form [(PNP)Ti(η(2)-H2C═CH(n)Pentyl)(CH2(t)Bu)] (6) and [(PNP)Ti(η(2)-H2C═CH(n)Hexyl)(CH2(t)Bu)] (7), respectively, but these species are unstable but can exchange with ethylene (1 atm) to form 1 and the free α-olefin. Complex 1 exchanges with D2C═CD2 with concomitant release of H2C═CH2. In addition, deuterium incorporation is observed in the neopentyl ligand as a result of this process. Cyclohexane and methylcyclohexane can be also dehydrogenated by transient A, and in the case of cyclohexane, ethylene (1 atm) can trap the [(PNP)Ti(CH2(t)Bu)] fragment to form 1. Dehydrogenation of the alkane is not rate-determining since pentane and pentane-d12 can be dehydrogenated to 4 and 4-d12 with comparable rates (KIE = 1.1(0) at ~29 °C). Computational studies have been applied to understand the formation and bonding pattern of the olefin complexes. Steric repulsion was shown to play an important role in determining the relative stability of several olefin adducts and their conformers. The olefin in 1 can be liberated by use of N2O, organic azides (N3R; R = 1-adamantyl or SiMe3), ketones (O═CPh2; 2 equiv) and the diazoalkane, N2CHtolyl2. For complexes 3-7, oxidation with N2O also liberates the α-olefin.

The anilide-methyl complex (PNP)Sc(NH[DIPP])(CH(3)) (1) [PNP(-) = bis(2-diisopropylphosphino-4-tolyl)amide, DIPP = 2,6-diisopropylphenyl] eliminates methane (k(avg) = 5.13 × 10(-4) M(-1) s(-1) at 50 °C) in the presence of pyridine to generate the transient scandium imido (PNP)Sc═N[DIPP](NC(5)H(5)) (A-py), which rapidly activates the C-H bond of pyridine in 1,2-addition fashion to form the stable pyridyl complex (PNP)Sc(NH[DIPP])(η(2)-NC(5)H(4)) (2). Mechanistic studies suggest the C-H activation process to be second order overall: first order in scandium and first order in substrate (pyridine). Pyridine binding precedes elimination of methane, and α-hydrogen abstraction is overall-rate-determining [the kinetic isotope effect (KIE) for 1-d(1) conversion to 2 was 5.37(6) at 35 °C and 4.9(14) at 50 °C] with activation parameters ΔH(‡) = 17.9(9) kcal/mol and ΔS(‡) = -18(3) cal/(mol K), consistent with an associative-type mechanism. No KIE or exchange with the anilide proton was observed when 1-d(3) was treated with pyridine or thermolyzed at 35 or 50 °C. The post-rate-determining step, C-H bond activation of pyridine, revealed a primary KIE of 1.1(2) at 35 °C for the intermolecular C-H activation reaction in pyridine versus pyridine-d(5). Complex 2 equilibrated back to the imide A-py slowly, as the isotopomer (PNP)Sc(ND[DIPP])(η(2)-NC(5)H(4)) (2-d(1)) converted to (PNP)Sc(NH[DIPP])(η(2)-NC(5)H(3)D) over 9 days at 60 °C. Molecular orbital analysis of A-py suggested that this species possesses a fairly linear scandium imido motif (169.7°) with a very short Sc-N distance of 1.84 Å. Substituted pyridines can also be activated, with the rates of C-H activation depending on both the steric and electronic properties of the substrate.

Addition of organic azides, N(3)R (R = 2,4,6-trimethylphenyl (Mes), phenyl (Ph), 1-adamantyl (Ad)), to a solution of the uranium(III) alkyl complex, Tp*(2)U(CH(2)Ph) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) (1), results in the formation of a family of uranium(iv) imido derivatives, Tp*(2)U(NR) (2-R). Notably, these complexes were synthesized in high yields by coupling of the benzyl groups to form bibenzyl. The uranium(IV) imido derivatives, 2-Mes, 2-Ph, and 2-Ad, were all characterized by both (1)H NMR and IR spectroscopy, and 2-Mes and 2-Ad were also characterized by X-ray crystallography. In the molecular structure of 2-Mes, typical κ(3)-coordination of the Tp* ligands was observed; however in the case of 2-Ad, one pyrazole ring of a Tp* ligand has rotated away from the metal centre, forcing a κ(2)-coordination of the pyrazoles. This results in a uranium-hydrogen interaction with the Tp* B-H. Treating these imido complexes with para-tolualdehyde results in multiple bond metathesis, forming the terminal uranium(IV) oxo complex, Tp*(2)U(O), and the corresponding imine.

Nickel(0) catalysts were used to produce substituted imidazoles in good to high yields using benzonitrile, p-substituted benzonitriles and 4-cyanopyridine as starting materials.

Nickel(0) catalysts were used to produce substituted imidazoles in good to high yields using benzonitrile, p-substituted benzonitriles and 4-cyanopyridine as starting materials.

Heteroleptic copper(I) complexes of the types [Cu(N,N)(P,P)] and [Cu(N,O)(P,P)], where (P,P) = phosphine (PPh(3)) or diphosphine (dppb, DPEPHOS, XANTPHOS), (N,N) = pyrrole-2-phenylcarbaldimine, 2PyN: [Cu(2PyN)(PPh(3))(2)] (1), [Cu(2PyN) (dppb)] (2), [Cu(2PyN)(DPEPHOS)] (3), and [Cu(2PyN)(XANTPHOS)] (4), (N,N) = indole-2-phenylcarbaldimine, 2IndN: [Cu(2IndN)(DPEPHOS)] (8), and (N,O) = pyrrole-2-carboxaldehyde, 2PyO: [Cu(2PyO)(DPEPHOS)] (5), [Cu(2PyO)(XANTPHOS)] (6), or (N,O) = indole-2-carboxaldehyde, 2IndO: [Cu(2IndO)(DPEPHOS)] (7), were synthesized and characterized by multinuclear NMR spectroscopy, electronic absorption spectroscopy, fluorescence spectroscopy, and X-ray crystallography (1-3, 5-8). The complexes with aldimine ligands are thermally stable, and sublimation of 2-4 was possible at T = 230-250 °C under vacuum. All complexes exhibit long-lived emission in solution, in the solid state, and in frozen glasses. The excited states have been assigned as mixed intraligand and metal-to-ligand charge transfer (3)(MLCT + π-π*) transitions through analysis of the photophysical properties and DFT calculations on representative examples.

The divergent reactivity of a transient titanium neopentylidyne, (PNP)TiCtBu (A) (PNP = N[2-PiPr2-4-methylphenyl]2−), that exhibits competing dehydrogenation and dehydroalkoxylation reaction pathways in the presence of acyclic ethers (Et2O, nPr2O, nBu2O, tBuOMe, tBuOEt, iPr2O) is presented. Although dehydrogenation takes place also in long-chain linear ethers, dehydroalkoxylation is disfavoured and takes place preferentially or even exclusively in the case of branched ethers. In all cases, dehydrogenation occurs at the terminal position of the aliphatic chain. Kinetics analyses performed using the alkylidene-alkyl precursor, (PNP)TiCHtBu(CH2tBu), show pseudo first-order decay rates on titanium (kavg = 6.2 ± 0.3 × 10−5 s−1, at 29.5 ± 0.1 °C, overall), regardless of the substrate or reaction pathway that ensues. Also, no significant kinetic isotope effect (kH/kD ∼ 1.1) was found between the activations of Et2O and Et2O-d10, in accord with dehydrogenation (C–H activation and abstraction) not being the slowest steps, but also consistent with formation of the transient alkylidyne A being rate-determining. An overall decay rate of (PNP)TiCHtBu(CH2tBu) with a t1/2 = 3.2 ± 0.4 h, across all ethers, confirms formation of A being a common intermediate. Isolated alkylidene-alkoxides, (PNP)TiCHtBu(OR) (R = Me, Et, nPr, nBu, iPr, tBu) formed from dehydroalkoxylation reactions were also independently prepared by salt metatheses, and extensive NMR characterization of these products is provided. Finally, combining theory and experiment we discuss how each reaction pathway can be altered and how the binding event of ethers plays a critical role in the outcome of the reaction.