Mononuclear and Tetranuclear Copper(II) Complexes Bearing Amino Acid Schiff Base Ligands: Structural Characterization and Catalytic Applications.

Two new glycine-Schiff base copper(II) complexes were synthesized. Single crystal X-ray diffraction (SCXRD) allowed us to establish the structure of both complexes in the solid state. The glycine-Schiff base copper(II) complex derived from 2'-hydroxy-5'-nitroacetophenone showed a mononuclear hydrated structure, in which the Schiff base acted as a tridentate ligand, and the glycine-Schiff base copper(II) complex derived from 2'-hydroxy-5'-methylacetophenone showed a less common tetranuclear anhydrous metallocyclic structure, in which the Schiff base acted as a tetradentate ligand. In both compounds, copper(II) had a tetracoordinated square planar geometry. The results of vibrational, electronic, and paramagnetic spectroscopies, as well as thermal analysis, were consistent with the crystal structures. Both complexes were evaluated as catalysts in the olefin cyclopropanation by carbene transference, and both led to very high diastereoselectivity (greater than 98%).

On the catalytic hydrodefluorination of fluoroaromatics using nickel complexes: the true role of the phosphine.

Homogeneous catalytic hydrodefluorination (HDF) of fluoroaromatics under thermal conditions was achieved using nickel(0) compounds of the type [(dippe)Ni(η(2)-C6F6-nHn)] where n = 0-2, as the catalytic precursors. These complexes were prepared in situ by reacting the compound [(dippe)Ni(µ-H)]2 with the respective fluoroaromatic substrate. HDF seems to occur homogeneously, as tested by mercury drop experiments, producing the hydrodefluorinated products. However, despite previous findings by other groups, we found that these HDF reactions were actually the result of direct reaction of the alkylphosphine with the fluoroaromatic substrate. This metal- and silane-free system is the first reported example of a phosphine being able to hydrodefluorinate on its own.

[1,2-Bis(diisopropyl-phosphan-yl)ethane-κ(2) P,P'](carbonato-κ(2) O,O')nickel(II).

In the crystal of the title compound, [Ni(CO3)(C14H32P2)], the metal center in each of three independent mol-ecules shows slight tetra-hedral distortion from ideal square-planar coordination geometry, with angles between the normals to the planes defined by the cis-P-Ni-P and cis-O-Ni-O fragments of 3.92 (17), 0.70 (16) and 2.17 (14)° in the three mol-ecules. In the crystal, there are inter-molecular C-H⋯O hydrogen bonds that show a laminar growth in the ab plane.

A second monoclinic polymorph of (E)-phen-yl(pyridin-2-yl)methanone oxime.

The title compound, C(12)H(10)N(2)O, a second monoclinic poly-morph of (E)-phen-yl(pyridin-2-yl)methanone oxime crystallizes in the space group P2(1)/n (Z = 4). The previously reported polymorph [Taga et al. (1990 ▶). Acta Cryst. C46, 2241-2243] occurs in the space group C2/c (Z = 8). In the crystal, pairs of bifurcated O-H⋯(N,O) hydrogen bonds link the mol-ecules into inversion dimers. The dimers are linked by C-H⋯π inter-actions, forming a linear arrangement. The dihedral angle between the pyridine and phenyl rings is 67.70 (8)°.

l-Isoleucine-Schiff Base Copper(II) Coordination Polymers: Crystal Structure, Spectroscopic, Hirshfeld Surface, and DFT Analyses.

A new copper(II) coordination polymer was synthesized from the l-isoleucine-Schiff base and characterized by elemental analysis, Fourier transform infrared (FT-IR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy, single-crystal X-ray diffraction (XRD) analysis, electronic paramagnetic resonance, and thermogravimetric analysis. XRD analysis confirmed the square planar coordination geometry of metallic centers and a zipper-like polymer structure. Vibrational, electronic, and paramagnetic spectroscopies and thermal analysis were consistent with the crystal structure. A Hirshfeld surface (HS) and density functional theory (DFT) analyses were employed to gain additional insight into interactions responsible for complex packing. The quantitative examination of two-dimensional (2D) fingerprint plots revealed, among other van der Waals forces, the dominating participation of H···H and H···Cl interactions in the molecular packing. The use of computational methods provided great help in detailing the supramolecular interactions occurring in the crystal, which were mainly van der Waals attractions. The electronic transition analysis helped corroborate the electronic transitions observed experimentally in the absorption spectrum. The frequency and vibrational mode analysis gave a deeper insight into the characterization of the CuLCL complex.

Homogeneous Manganese-Catalyzed Hydrofunctionalizations of Alkenes and Alkynes: Catalytic and Mechanistic Tendencies.

In recent years, many manganese-based homogeneous catalytic precursors have been developed as powerful alternatives in organic synthesis. Among these, the hydrofunctionalizations of unsaturated C-C bonds correspond to outstanding ways to afford compounds with more versatile functional groups, which are commonly used as building blocks in the production of fine chemicals and feedstock for the industrial field. Herein, we present an account of the Mn-catalyzed homogeneous hydrofunctionalizations of alkenes and alkynes with the main objective of finding catalytic and mechanistic tendencies that could serve as a platform for the works to come.

[1,2-Bis-(diisopropyl-phosphan-yl)ethane-κP,P']dichloridonickel(II).

In the crystal structure of title compound, [NiCl(2)(C(14)H(32)P(2))], the Ni(II) atom lies on a twofold rotation axis and shows a slightly distorted square-planar coordination geometry, with a dihedral angle of 10.01 (8)° between the cis-Cl-Ni-Cl and cis-P-Ni-P planes. There is no significant inter-molecular inter-action except very weak C-H⋯Cl inter-actions. The crystal studied was a racemic twin.

[1,2-Bis-(diisopropyl-phosphan-yl)ethane-κP,P']dichloridonickel(II)-9H-carbazole (1/2).

In the title compound, [NiCl(2)(C(14)H(32)P(2))]·2C(12)H(9)N, the neutral [Ni(dppe)Cl(2)] complex [dppe is 1,2-bis-(diisopropyl-phosphan-yl)ethane] consists of a tetracoordinated Ni(2+) cation and has a crystallographic twofold axis passing through the metal atom and the mid-point of the CH(2)-CH(2) bond of the dppe ligand. The metal atom shows slight tetra-hedral distortion from an ideal square-planar coordination geometry, as reflected in the dihedral angle between NiCl(2) and NiP(2) planes of 15.32 (2)°. The 9H-carbazole ring system is essentially planar (r.m.s. deviation = 0.022 Å). In the crystal packing, there are two symmetry-related 9H-carbazole mol-ecules between two adjacent Ni(II) complexes, with an angle between the carbazole mean planes of ca 77°.

[1,2-Bis(diiso-propyl-phosphan-yl)ethane-κ2 P,P'](2-fluoro-N-{[(2-fluoro-phen-yl)aza-nid-yl]carbon-yl}anilinido-κ2 N,N')nickel(II).

The mol-ecular structure of the title complex, [Ni(C13H8F2N2O)(C14H32P2)] or Ni(oFPU)(dippe), where oFPU is the dianion of bis-(2-fluoro-phen-yl)urea and dippe is 1,2-bis-(di-iso-propyl-phosphino)ethane, comprises an NiII atom with a distorted square-planar coordination environment (geometry index τ4 = 0.195). One of the fluoro-phenyl rings of the oFPU ligand is disordered over two sets of sites in an 0.832 (7):0.168 (7) ratio. The crystal structure displays C-H⋯O and C-H⋯F hydrogen-bonding inter-actions, leading to chains with R 2 2(12) motifs extending parallel to [100]. The title compound might be of inter-est with respect to the production of urea and carbamate derivatives of nickel(II).

Mechanistic insights on the hydrodesulfurization of biphenyl-2-thiol with nickel compounds.

The reactivity of the nickel(I) dimer [(dippe)Ni(mu-H)](2) (1) with biphenyl-2-thiol was explored with the aim of clarifying the key step of sulfur extrusion during the hydrodesulfurization process using dibenzothiophene (DBT). These reactions were monitored by variable temperature NMR experiments which allowed the complete characterization and isolation of [(dippe)(2)Ni(2)(mu-H)(mu-S-2-biphenyl)] (3). The latter compound evolves to the terminal nickel-hydride [(dippe)Ni (eta(1)-C-2-biphenyl)(H)] (4) and transient [(dippe)NiS] (5), to ultimately yield [(dippe)(2)Ni(2)(mu-S)] (2) and biphenyl as the resulting HDS products. The reactivity of 1 and biphenyl-2-thiol was examined using different ratios of reactants, which allowed preparation of [(dippe)Ni(eta(1)-S-biphenyl-2-thiolate)(2)] (6) when using an excess of this substrate. The reactivity of 6 with 1 was addressed, yielding compound 2 and an equivalent amount of biphenyl.

Stereoselective hydrogenation of aromatic alkynes using water, triethylsilane, or methanol, mediated and catalyzed by Ni(0) complexes.

The use of complexes of the type [(P-P)Ni(eta(2)-C,C-alkyne)] (P-P = 1,2-bis(di-isopropyl-phosphinoethane or 1,2-bis(diterbutylphosphino-ethane) in the presence of water, triethylsilane/water, or methanol as hydrogen sources yields the selective production of E- or Z- aromatic alkenes from the corresponding alkynes. For instance, in the case of diphenylacetylene (dpa) and water, a metal-mediated process was found to yield trans-stilbene stoichiometrically, whereas in the case of triethylsilane/water and methanol, a catalytic system (1% mol) was found. The catalytic systems gave >95% conversion to cis- or trans-stilbene, respectively. The use of a variety of substituents on the aromatic ring was also assessed. Deuterium-labeling studies using D(2)O allowed the confirmation of water as the hydrogen source for the alkyne reduction.

Hydrogenation and N-alkylation of anilines and imines via transfer hydrogenation with homogeneous nickel compounds.

The nickel-catalyzed N-alkylation of a variety of arylamines via transfer hydrogenation in the absence of pressurized hydrogen and basic or acidic additives was achieved in a tandem reaction. This process was further extended to the C[double bond, length as m-dash]N bond reduction and N-alkylation of a variety of imines with ethanol, the latter acting as a hydrogen and acetaldehyde source, which allowed for the reduction and subsequent condensation to yield the corresponding N-alkylated products.

Solvent effects and activation parameters in the competitive cleavage of C-CN and C-H bonds in 2-methyl-3-butenenitrile using [(dippe)NiH]2.

The reaction of [(dippe)NiH]2 with 2-methyl-3-butenenitrile (2M3BN) in solvents spanning a wide range of polarities shows significant differences in the ratio of C-H and C-CN activated products. C-H cleavage is favored in polar solvents, whereas C-C cleavage is favored in nonpolar solvents. This variation is attributed to the differential solvation of the transition states, which was further supported through the use of sterically bulky solvents and weakly coordinating solvents. Variation of the temperature of reaction of [(dippe)NiH]2 with 2M3BN in decane and N,N-dimethylformamide (DMF) allowed for the calculation of Eyring activation parameters for the C-CN activation and C-H activation mechanisms. The activation parameters for the C-H activation pathway were DeltaH(double dagger) = 11.4 +/- 5.3 kcal/mol and DeltaS(double dagger) = -45 +/- 15 e.u., compared with DeltaH(double dagger) = 17.3 +/- 2.6 kcal/mol and DeltaS(double dagger) = -29 +/- 7 e.u. for the C-CN activation pathway. These parameters indicate that C-H activation is favored enthalpically, but not entropically, over C-C activation, implying a more ordered transition state for the former.

Catalytic desulfurization of dibenzothiophene with palladium nanoparticles.

The thermal reduction of [(PEt3)2PdMe2] (1 mol%), which was produced in situ from [(PEt3)2PdCl2] (1) and 2 equiv of MeMgBr in toluene solvent, yielded palladium nanoparticles that in conjunction with MeMgBr effected the desulfurization of dibenzothiophene (DBT). The reaction resulted in the generation of the sulfur-free compound 2,2'-dimethylbiphenyl, in high yields (60%). The use of several stabilizing agents such as sodium 2-ethylhexanoate and hexadecylamine was also addressed herein, their use resulting in a significant improvement of the desulfurization reaction that reached up to 90% conversion of DBT into the mentioned biphenyl. The palladium nanoparticles formed during the reaction were characterized by transmission electron microscopy and exhibited a smaller size and a lesser extent of agglomeration whenever stabilizers were used.

Nickel-Catalyzed Transfer Hydrogenation of Benzonitriles with 2-Propanol and 1,4-Butanediol as the Hydrogen Source.

The homogeneous transfer hydrogenation of benzonitrile with 2-propanol or 1,4-butanediol produced N-benzylidene benzylamine (BBA, 85% yield) using 5 mol % [Ni(COD)2] as a catalytic precursor and a mixture of Cy2P(CH2)2PCy2 and Cy2P(CH2)2P(O)Cy2 as ancillary ligands, under mild reaction conditions (120 °C, 96 h, tetrahydrofuran). 1,4-Butanediol performed better than 2-propanol as a hydrogen donor and yielded γ-butyrolactone as the product of transfer dehydrogenation. Selectivity toward dibenzylamine (DBA, 62% yield) was achieved by varying the amount of 1,4-butanediol in the catalytic system. A reaction mechanism was proposed, involving a ligand-assisted O-H bond activation, end-on substrate coordination, and a key dihydrido-Ni(II) intermediate, leading to the in situ formation of primary imines and amines to ultimately yield the secondary imines observed.

Catalytic transfer hydrogenation of azobenzene by low-valent nickel complexes: a route to 1,2-disubstituted benzimidazoles and 2,4,5-trisubstituted imidazolines.

The one-pot synthesis of 1,2-disubstituted benzimidazoles by the transfer hydrogenation of azobenzene, using benzylamine as a hydrogen donor, sequential rearrangement of hydrazobenzene to semidine and further condensation with N-benzylideneamine is reported, catalyzed by 2 mol% of [Ni(COD)2] : dippe. The N2 substitution on benzimidazole can be controlled by the selection of different azobenzenes and C2 substitution will only depend on the chosen benzylamine. The current methodology avoids the addition of external oxidants, which are needed in the classical benzimidazole synthesis. In addition, the byproduct, N-benzylideneamine, obtained from dehydrogenation of benzylamine produced 2,4,5-trisubstituted imidazolines by cyclization and C-H functionalization, and this route was optimized with the use of 2 mol% of [Ni(COD)2] : 2PPh3.

Nickel-catalyzed transfer hydrogenation of ketones using ethanol as a solvent and a hydrogen donor.

We report a nickel(0)-catalyzed direct transfer hydrogenation (TH) of a variety of alkyl-aryl, diaryl, and aliphatic ketones with ethanol. This protocol implies a reaction in which a primary alcohol serves as a hydrogen atom source and solvent in a one-pot reaction without any added base. The catalytic activity of the nickel complex [(dcype)Ni(COD)] (e) (dcype: 1,2-bis(dicyclohexyl-phosphine)ethane, COD: 1,5-cyclooctadiene), towards transfer hydrogenation (TH) of carbonyl compounds using ethanol as the hydrogen donor was assessed using a broad scope of ketones, giving excellent results (up to 99% yield) compared to other homogeneous phosphine-nickel catalysts. Control experiments and a mercury poisoning experiment support a homogeneous catalytic system; the yield of the secondary alcohols formed in the TH reaction was monitored by gas chromatography (GC) and NMR spectroscopy.

Bond and small-molecule activation with low-valent nickel complexes.

The use of nickel compounds in low oxidation states allowed a variety of useful transformations of interest for academia, industry and in the solution of environmental issues.

Tandem hydrogenation and condensation of fluorinated α,ß-unsaturated ketones with primary amines, catalyzed by nickel.

A simple homogeneous catalytic system based on nickel phosphine complexes has been developed for the transfer hydrogenation and condensation of α,ß-unsaturated ketones to yield saturated ones and saturated imines using primary amines as hydrogen donors. Thus, a wide range of fluorinated 1,5-diaryl-1,4-pentadiene-3-ones were allowed to react with substituted benzylamines in the presence of [(dippe)Ni(µ-H)]2 (dippe = 1,2-bis-(diisopropylphosphino)-ethane) using ethanol as a solvent at 180 °C to give the corresponding saturated carbonyl compounds; here hydrogenation of the C[double bond, length as m-dash]C bond was preferred over the C[double bond, length as m-dash]O bond. Under the same reaction conditions but using an excess of benzylamine, a tandem process is then favoured, starting also with the reduction of the C[double bond, length as m-dash]C bond followed by a nucleophilic addition of the primary amine to yield valuable saturated imines with good to excellent yields (62%-91%).

Crystal structure of 1-mesityl-3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium iodide.

In the cation of the title salt, C18H20N3 (+)·I(-), the mesityl and phenyl rings are inclined to the central triazolium ring by 61.39 (16) and 30.99 (16)°, respectively, and to one another by 37.75 (15)°. In the crystal, mol-ecules are linked via C-H⋯I hydrogen bonds, forming slabs parallel to the ab plane. Within the slabs there are weak π-π inter-actions present involving the mesityl and phenyl rings [inter-centroid distances are 3.8663 (18) and 3.8141 (18) Å].