Marina V. Kirillova

Topologically Unique Heterometallic CuII/Li Coordination Polymers Self-Assembled from N,N-bis(2-Hydroxyethyl)-2-aminoethanesulfonic Acid Biobuffer: Versatile Catalyst Precursors for Mild Hydrocarboxylation of Alkanes to Carboxylic Acids

The facile aqueous medium reactions of copper(II) nitrate with BES biobuffer [(HOCH(2)CH(2))(2)N(CH(2)CH(2)SO(3)H), hereinafter referred as H(3)bes] in the presence of various benzenecarboxylic acids [benzoic (Hba), 3-hydroxybenzoic (Hhba), and 3,5-dihydroxybenzoic (Hdhba) acid] and lithium hydroxide gave rise to the self-assembly generation of three new heterometallic Cu(II)/Li materials, [Li(H(2)O)(4)][Cu(4)(μ(2)-Hbes)(4)(μ(2)-ba)]·H(2)O (1) and [Cu(4)(μ(3)-Hbes)(4)(L){Li(H(2)O)(2)}](n)·3nH(2)O {L = μ(2)-hba (2) and μ(2)-dhba (3)}. They were isolated as air-stable crystalline solids and fully characterized by infrared (IR) and UV-vis spectroscopy and electrospray ionization (ESI)-MS(±), elemental, thermal, and single-crystal X-ray diffraction analyses. The latter revealed that 1-3 have comparable packing patterns and unit cell parameters, being composed of similar [Cu(4)(μ-Hbes)(4)(μ-carboxylate)](-) cores and [Li(H(2)O)(4)](+) cations (in 1) or [μ-Li(H(2)O)(2)](+) groups (in 2 and 3), which are arranged into discrete 0D aggregates in 1 or infinite 3D noninterpenetrating metal-organic networks in 2 and 3. The topological analysis of the coordination polymers 2 and 3 disclosed the trinodal 3,3,4-connected underlying nets with an unprecedented topology defined by the point symbol of (4.6.8)(4)(4(2).6)(2)(6(2).16(2).18(2)), further simplification of which resulted in the binodal 4,4-connected nets with the pts (PtS) topology. Apart from representing very rare examples of coordination compounds derived from H(3)bes, 1-3 feature solubility in water and were applied as efficient and versatile catalyst precursors for the mild (60 °C) single-pot hydrocarboxylation, by CO and H(2)O, of various gaseous, linear, and cyclic C(n) (n = 2-9) alkanes into the corresponding C(n+1) carboxylic acids, in H(2)O/MeCN medium under homogeneous conditions and in the presence of potassium peroxodisulfate. Total yields (based on alkane) of carboxylic acids up to 78% were achieved, which are remarkable in the field of alkane functionalization under mild conditions, especially for a C-C bond formation reaction in aqueous acid-solvent-free medium.

Alkanes to carboxylic acids in aqueous medium: metal-free and metal-promoted highly efficient and mild conversions

A convenient and clean route to transform, in aqueous medium, various alkanes to carboxylic acids via single-pot carboxylation with CO and water, under mild conditions, has been achieved, proceeding efficiently and selectively even without any metal catalyst and any acid additive, at low temperatures; the relevant hydroxylating role of H(2)O and radical mechanisms are disclosed by radical-trap, H(2)(18)O and DFT studies.

Copper(II) complexes with a new carboxylic-functionalized arylhydrazone of β-diketone as effective catalysts for acid-free oxidations

The aquasoluble [Cu(H2O)((CH3)2NCHO)(HL)] (2) and [Cu2(CH3OH)2(μ-HL)2] (3) CuII complexes were prepared by reaction of CuII nitrate hydrate with the new 3-(2-hydroxy-4-carboxyphenylhydrazone)pentane-2,4-dione (H3L, 1), in the presence (for 2) or absence (for 3) of (n-C4H9)2SnO, and characterized by elemental analysis, IR spectroscopy and X-ray single crystal diffraction. Magnetic susceptibility measurements, in compound 3, reveal strong antiferromagnetic coupling between the CuII ions through the μ2-phenoxido-O atoms, J = −203(1) cm−1. Complexes 2 and 3 act as catalyst precursors for the acid-free peroxidative oxidation of cyclohexane to the mixture of cyclohexyl hydroperoxide (primary product), cyclohexanol and cyclohexanone (TONs and yields up to 163 and 14.4%, respectively), as well as for the selective aerobic oxidation of benzyl alcohols to benzaldehydes in aqueous solution, mediated by a TEMPO radical, under mild conditions (TONs and yields up to 390 and 94%, respectively). In the alkane oxidations, 2 and 3 appear to behave as “dual role catalysts” combining, in one molecule, an active metal centre and an acidic promoting group, to provide a high activity of the system even without any acid promoter.

Bringing an Old Biological Buffer to Coordination Chemistry: New 1D and 3D Coordination Polymers with [Cu4(HbeS)4] Cores for Mild Hydrocarboxylation of Alkanes

New water-soluble 1D and 3D Cu(II)/Na coordination polymers 1-3 bearing unprecedented [Cu(4)(Hbes)(4)] cores have been easily generated by aqueous-medium self-assembly and fully characterized, thus opening up the use of the common biological buffer H(3)bes, (HO(3)SCH(2)CH(2))N(CH(2)CH(2)OH)(2), in synthetic coordination chemistry. Apart from representing the first isolated and structurally characterized coordination compounds derived from H(3)bes, 1-3 show a remarkable promoting effect in the mild aqueous-medium hydrocarboxylation, by CO and H(2)O, of gaseous alkanes (C(3)H(8) and n-C(4)H(10)) to the corresponding carboxylic acids, which are obtained in up to 95% yields based on the alkane.

A Hexanuclear Mixed-Valence Oxovanadium(IV,V) Complex as a Highly Efficient Alkane Oxidation Catalyst

The new hexanuclear mixed-valence vanadium complex [V(3)O(3)(OEt)(ashz)(2)(μ-OEt)](2) (1) with an N,O-donor ligand is reported. It acts as a highly efficient catalyst toward alkane oxidations by aqueous H(2)O(2). Remarkably, high turnover numbers up to 25000 with product yields of up to 27% (based on alkane) stand for one of the most active systems for such reactions.

Mild, single-pot hydrocarboxylation of gaseous alkanes to carboxylic acids in metal-free and copper-promoted aqueous systems.

A direct, selective and highly efficient method has been developed for the hydrocarboxylation of C(n) gaseous alkanes into C(n+1) carboxylic acids, in aqueous acid-solvent-free medium at low temperatures. The approach is based on a metal-free or copper-promoted reaction of ethane, propane or n-butane with carbon monoxide and water, in a H(2)O/MeCN medium at 50-60 degrees C, in the presence of potassium peroxodisulfate. The effects of various reaction parameters, such as the absence or presence of a copper promoter, solvent composition, temperature, time, CO and alkane pressure, were studied. A free radical mechanism was confirmed by radical trap experiments involving acyl radical formation, oxidation and subsequent hydroxylation by water. Remarkable yields (based on alkane) of carboxylic acids in the 34-41 % range were achieved even in the metal-free systems, although in the presence of a tetracopper(II) triethanolaminate derived promoter they reach superior values of 58 and 87 % for the hydrocarboxylations of propane and n-butane, respectively; in these cases, branched isobutyric and 2-methylbutyric acids were the predominant products. From a green perspective, important features of the present alkane hydrocarboxylations include the exceptional metal-free, mild and acid-solvent-free reaction conditions, the operation in aqueous medium with a rare hydroxylating role of water, with high selectivities, and yields of carboxylic acids. Taken together, these conditions correspond to the mildest and the most efficient method so far reported for the oxidative functionalisation of gaseous alkanes.

Alkali Metal Directed Assembly of Heterometallic Vv/M (M = Na, K, Cs) Coordination Polymers: Structures, Topological Analysis, and Oxidation Catalytic Properties

The reactions of [VO(acac)2] with bis(salicylaldehyde)-oxaloyldihydrazone (H4L) and an alkali metal carbonate M2CO3 (M = K, Na, Cs), in EtOH/H2O medium upon reflux, resulted in the generation of three new heterometallic V(V)/M materials, namely the 1D [(VO2)2(μ4-L){Na2(μ-H2O)2(H2O)2}]n (1), 2D [{V(μ-O)2}2(μ4-L){K2(μ-H2O)2(H2O)2}]n (2), and 3D [{V(μ-O)(μ3-O)}2(μ8-L){Cs2(μ-H2O)2(H2O)2}]n (3) coordination polymers. They were isolated as air-stable solids and fully characterized by IR, UV-vis, (1)H, and (51)V NMR spectroscopy, ESI-MS(±), elemental, thermal, and single-crystal X-ray diffraction analyses, the latter showing that 1-3 are constructed from the resembling [(VO2)2(μ(4/8)-L)](2-) blocks assembled by the differently bound aqua-metal [M2(μ-H2O)2(H2O)2](2+) moieties (M = Na, K, Cs). The main distinctive features of 1-3 arise from the different coordination numbers of Na (5), K (7), and Cs (9) atoms, thus increasing the complexity of the resulting networks from the ladder-like 1D chains in 1 to double 2D layers in 2, and layer-pillared 3D framework in 3. The topological analysis of 2 disclosed a uninodal 4-connected underlying net with a rare kgm [Shubnikov plane net (3.6.3.6)/kagome pattern] topology, while 3 features a trinodal 4,7,8-connected underlying net with an unprecedented topology. Compounds 1-3 also show solubility in water (S(25 °C) ≈ 4-7 mg mL(-1)) and were applied as efficient precatalysts for the homogeneous oxidation of cyclohexane by aqueous H2O2, under mild conditions (50 °C) in MeCN/H2O medium and in the presence of an acid promoter. Total yields (based on substrate) of cyclohexanol and cyclohexanone up to 36% and turnover numbers (TONs) up to 5700 were achieved.

A new binuclear oxovanadium(V) complex as a catalyst in combination with pyrazinecarboxylic acid (PCA) for efficient alkane oxygenation by H2O2

A new binuclear oxovanadium(V) complex [{VO(OEt)(EtOH)}2L] (1) where H4L is bis(2-hydroxybenzylidene)terephthalohydrazide has been synthesized and fully characterized. The combination of 1 with pyrazine-2-carboxylic acid (PCA; a cocatalyst) affords a catalytic system for the efficient oxidation of saturated hydrocarbons, RH, with hydrogen peroxide and air in acetonitrile solution at 50 °C to produce alkyl hydroperoxides, ROOH, as the main primary products. Very high turnover numbers (TONs) have been attained in this reaction: for example, after 2220 min, TON = 44,000 and initial TOF (turnover frequency) = 3300 h(-1) per molecule of complex 1. The estimated activation energy of the cyclohexane oxygenation in the presence of 1/PCA is E(a) = 16 ± 2 kcal mol(-1). This value is identical to that obtained for the cyclohexane oxidation with H2O2 catalyzed by the (n-Bu4N)[VO3]/PCA combination (17 ± 2 kcal mol(-1)). The dependences of initial oxidation rates W0 on the initial concentrations of all components of the reaction mixture have been determined. Based on these kinetic data and on the regio- and bond-selectivity parameters measured in the oxidation of linear and branched alkanes a mechanism of the oxidation has been proposed which includes the generation of hydroxyl radicals in the crucial stage.

Copper(II) Coordination Polymers Self-Assembled from Aminoalcohols and Pyromellitic Acid: Highly Active Precatalysts for the Mild Water-Promoted Oxidation of Alkanes

Three novel water-soluble 2D copper(II) coordination polymers-[{Cu2(μ2-dmea)2(H2O)}2(μ4-pma)]n·4nH2O (1), [{Cu2(μ2-Hedea)2}2(μ4-pma)]n·4nH2O (2), and [{Cu(bea)(Hbea)}4(μ4-pma)]n·2nH2O (3)-were generated by an aqueous medium self-assembly method from copper(II) nitrate, pyromellitic acid (H4pma), and different aminoalcohols [N,N-dimethylethanolamine (Hdmea), N-ethyldiethanolamine (H2edea), and N-benzylethanolamine (Hbea)]. Compounds 2 and 3 represent the first coordination polymers derived from H2edea and Hbea. All the products were characterized by infrared (IR), electron paramagnetic resonance (EPR), and ultraviolet-visible light (UV-vis) spectroscopy, electrospray ionization-mass spectroscopy (ESI-MS(±)), thermogravimetric and elemental analysis, and single-crystal X-ray diffraction (XRD), which revealed that their two-dimensional (2D) metal-organic networks are composed of distinct dicopper(II) or monocopper(II) aminoalcoholate units and μ4-pyromellitate spacers. From the topological viewpoint, the underlying 2D nets of 1-3 can be classified as uninodal 4-connected layers with the sql topology. The structures of 1 and 2 are further extended by multiple intermolecular hydrogen bonds, resulting in three-dimensional (3D) hydrogen-bonded networks with rare or unique topologies. The obtained compounds also act as highly efficient precatalysts for the mild homogeneous oxidation, by aqueous H2O2 in acidic MeCN/H2O medium, of various cycloalkanes to the corresponding alcohols and ketones. Overall product yields up to 45% (based on cycloalkane) were attained and the effects of various reaction parameters were investigated, including the type of precatalyst and acid promoter, influence of water, and substrate scope. Although water usually strongly inhibits the alkane oxidations, a very pronounced promoting behavior of H2O was detected when using the precatalyst 1, resulting in a 15-fold growth of an initial reaction rate in the cyclohexane oxidation on increasing the amount of H2O from ∼4 M to 17 M in the reaction mixture, followed by a 2-fold product yield growth.

New tricopper(II) cores self-assembled from aminoalcohol biobuffers and homophthalic acid: synthesis, structural and topological features, magnetic properties and mild catalytic oxidation of cyclic and linear C5–C8 alkanes

Two new crystalline materials [Cu3(μ2-H3bis-tris)2(μ2-Hhpa)2]·H2O (1) and [Cu3(μ2-H2tea)2(μ2-hpa)(μ3-hpa)]n (2) bearing distinct tricopper(II) cores were easily generated by the aqueous medium self-assembly method from copper(II) nitrate, bis(2-hydroxyethyl)amino-tris(hydroxymethyl)methane (H5bis-tris) or triethanolamine (H3tea) aminoalcohol biobuffers and homophthalic acid (H2hpa). The obtained products were characterised by IR, UV-vis and EPR spectroscopy, ESI-MS(±), thermogravimetric, elemental and single crystal X-ray diffraction analysis. Apart from possessing different dimensionality, the crystal structures of the discrete 0D trimer 1 and the zigzag 1D coordination polymer 2 show distinct symmetric [Cu3(μ-O)4(μ-COO)2] and asymmetric [Cu3(μ-O)3(μ-COO)2] tricopper(II) cores, respectively. An intense pattern of intermolecular O–H⋯O hydrogen bonds provides a 0D → 3D (1) or 1D → 2D (2) extension of the structures into intricate topologically unique H-bonded nets. After additional simplification, these were classified as a uninodal 6-connected 3D framework with the snk topology in 1 and a binodal 3,5-connected 2D layer with the 3,5L50 topology in 2. Variable-temperature magnetic susceptibility studies indicate a predominant ferromagnetic coupling [J = 39.1(1) and 29.5(1) cm−1 for 1 and 2, respectively] within the mixed-bridged tricopper(II) cores. Both compounds 1 and 2 were also applied as rather efficient bio-inspired pre-catalysts for the mild homogeneous oxidation, by aqueous H2O2 at 50 °C in acidic MeCN–H2O medium, of cyclic (cyclopentane, cyclohexane, cycloheptane and cyclooctane) and linear (n-pentane, n-hexane, n-heptane and n-octane) alkanes to the corresponding alcohols and ketones with overall yields up to 26% based on the alkane. The effects of different reaction parameters (type of pre-catalyst and acid promoter, reaction time and substrate scope) and various selectivity features were investigated and discussed, supporting a free-radical mechanism in the present alkane oxidations.