Strategies for the anchoring of metal complexes, clusters, and colloids inside nanoporous alumina membranes

Transition metal oxide complexes as molecular catalysts for selective methane to methanol transformation: any prospects or time to retire?

Transition metal oxides have been extensively used in the literature for the conversion of methane to methanol. Despite the progress made over the past decades, no method with satisfactory performance or economic viability has been detected. The main bottleneck is that the produced methanol oxidizes further due to its weaker C-H bond than that of methane. Every improvement in the efficiency of a catalyst to activate methane leads to reduction of the selectivity towards methanol. Is it therefore prudent to keep studying (both theoretically and experimentally) metal oxides as catalysts for the quantitative conversion of methane to methanol? This perspective focuses on molecular metal oxide complexes and suggests strategies to bypass the current bottlenecks with higher weight on the computational chemistry side. We first discuss the electronic structure of metal oxides, followed by assessing the role of the ligands in the reactivity of the catalysts. For better selectivity, we propose that metal oxide anionic complexes should be explored further, while hydrophylic cavities in the vicinity of the metal oxide can perturb the transition-state structure for methanol increasing appreciably the activation barrier for methanol. We also emphasize that computational studies should target the activation reaction of methanol (and not only methane), the study of complete catalytic cycles (including the recombination and oxidation steps), and the use of molecular oxygen as an oxidant. The titled chemical conversion is an excellent challenge for theory and we believe that computational studies should lead the field in the future. It is finally shown that bottom-up approaches offer a systematic way for exploration of the chemical space and should still be applied in parallel with the recently popular machine learning techniques. To answer the question of the title, we believe that metal oxides should still be considered provided that we change our focus and perform more systematic investigations on the activation of methanol.

Mono- and polynuclear Ag(i) complexes of N-functionalized bis(diphenylphosphino)amine DPPA-type ligands: synthesis, solid-state structures and reactivity

DPPA-type ligands (Ph₂P)₂N(p-Z)C₆H₄ (Z = H, SMe, OMe) led to Ag(i) complexes of various nuclearities and an unexpected influence of the para-substituent Z is observed, including for CH₂Cl₂ activation.

Functional Short-Bite Ligands: Synthesis, Coordination Chemistry, and Applications of N-Functionalized Bis(diaryl/dialkylphosphino)amine-type Ligands

The aim of this review is to highlight how the diversity generated by N-substitution in the well-known short-bite ligand bis(diphenylphosphino)amine (DPPA) allows a fine-tuning of the ligand properties and offers a considerable scope for tailoring the properties and applications of their corresponding metal complexes. The various N-substituents include nitrogen-, oxygen-, phosphorus-, sulfur-, halogen-, and silicon-based functionalities and directly N-bound metals. Multiple DPPA-type ligands linked through an organic spacer and N-functionalized DRPA-type ligands, in which the PPh2 substituents are replaced by PR2 (R = alkyl, benzyl) groups, are also discussed. Owing to the considerable diversity of N-functionalized DPPA-type ligands available, the applications of their mono- and polynuclear metal complexes are very diverse and range from homogeneous catalysis with well-defined or in situ generated (pre)catalysts to heterogeneous catalysis and materials science. In particular, sustained interest for DPPA-type ligands has been motivated, at least in part, by their ability to promote selective ethylene tri- or tetramerization in combination with chromium. Ligands and metal complexes where the N-substituent is a pure hydrocarbon group (as opposed to N-functionalization) are outside the scope of this review. However, when possible, a comparison between the catalytic performances of N-functionalized systems with those of their N-substituted analogs will be provided.

Zwitterionic Cobalt Complexes with Bis(diphenylphosphino)(N-thioether)amine Assembling Ligands: Structural, EPR, Magnetic, and Computational Studies

The coordination of two heterofunctional P,P,S ligands of the N-functionalized DPPA-type bearing an alkylthioether or arylthioether N-substituent, (Ph2P)2N(CH2)3SMe (1) and (Ph2P)2N(p-C6H4)SMe (2), respectively, toward cobalt dichloride was investigated to examine the influence of the linker between the PNP nitrogen and the S atoms. The complexes [CoCl2(1)]2 (3) and [CoCl2(2)]2 (4) have been isolated, and 3 was shown by X-ray diffraction to be a unique dinuclear, zwitterion containing one CoCl moiety bis-chelated by two ligands 1 and one CoCl3 fragment coordinated by the S atom of a thioether function. The FT-IR, UV-vis, and EPR spectroscopic features of 3 were analyzed as the superposition of those of constitutive fragments identified by a retrosynthetic-type analysis. A similar approach provided insight into the nature of 4 for which no X-ray diffraction data could be obtained. A comparison between the spectroscopic features of 4 and of its constitutive fragments, [CoCl(2)2]PF6 (7) and [H2']2[CoCl4] (8) (2' = NH2(p-C6H4)SMe), and between those of 4 and 3 suggested that 4 could either have a zwitterionic structure, similar to that of 3, or contain a tetrahedral dicationic bis-chelated Co center associated with a CoCl4 dianion. Magnetic and EPR studies and theoretical calculations were performed. Doublet spin states were found for the pentacoordinated complexes [CoCl(1)2]PF6 (5) and 7 and anisotropic quadruplet spin states for the tetrahedral complexes [CoCl3(H1')] (6) (1' = NH2(CH2)3SMe) and 8. A very similar behavior was observed for 3 and 4, consisting in the juxtaposition of noninteracting doublet and quadruplet spin states. Antiferromagnetic interactions explain the formation of dimers for 6 and of layers for 8. The EPR signatures of 3 and 4 correspond to the superposition of low-spin nuclei in 5 and 7 and high-spin nuclei in 6 and 8, respectively. From DFT calculations, the solid-state structure of 4 appears best described as zwitterionic, with a low-spin state for the Co1 atom.

The novel [Ni{(Ph2P)2N(CH2)3Si(OCH3)3-P,P´}I2] complex: Structural features and catalytic reactivity in the oligomerization of ethylene

The novel [Ni{(Ph2P)2N(CH2)3Si(OCH3)3-P,P´}I2] complex (1) was synthesized and investigated by a variety of spectroscopic methods (IR, 1H and 31P NMR). The molecular structure of 1, determined by single crystal X-ray diffraction, was compared with those of the analogue [Ni{(Ph2P)2N(CH2)3Si(OCH3)3-P,P´}Cl2] complex (2) and the recently reported [Ni{iCPrP)2NH-P,P´}I2] complex. Complexes 1 and 2 were tested as homogeneous catalysts for the oligomerization of ethylene, leading to the formation of C4 and, to a smaller extent, C6 products, in moderate yields.

Synthesis, structure, and optical properties of Pt(II) and Pd(II) complexes with oxazolyl- and pyridyl-functionalized DPPM-type ligands: a combined experimental and theoretical study

New square-planar complexes [Pt(1(-H))2] (2a) [1(-H) = (oxazolin-2-yl)bis(diphenylphosphino)methanide] and [Pd(1(-H))2] (2b), of general formula [M{(Ph2P)2C---C---NCH2CH2O}2] (M = Pt, 2a; M = Pd, 2b), result from deprotonation of 2-{bis(diphenylphosphino)methyl}oxazoline (1) at the PCHP site. The new, functionalized dppm-type ligand 4-{bis(diphenylphosphino)methyl}pyridine, (Ph2P)2CH(4-C5H4N) (4), was prepared by double lithiation and phosphorylation of 4-picoline. In the presence of NEt3, the reactions of 2 equiv of 4 with [PtCl2(NCPh)2] and [Pd(acac)2] (acac = acetylacetonate) afforded [Pt(4(-H))2] (5a) [4(-H) = bis(diphenylphosphino)(pyridin-4-yl)methanide] and [Pd(4(-H))2] (5b), of general formula [M{(Ph2P)2C(4-C5H4N)}2] (M = Pt, 5a; M = Pd, 5b), respectively. In the absence of base, the reactions of 2 equiv of 4 with [PtCl2(NCPh)2] and [PdCl2(NCPh)2] afforded (5a·2HCl) (6a) and (5b·2HCl) (6b), respectively, in which the PCHP proton of 4 has migrated from carbon to nitrogen to give a pyridinium derivative of general formula [M{(Ph2P)2C(4-C5H4NH)}2]Cl2 (M = Pt, 6a; M = Pd, 6b). The complexes 3a, 5a·2MeOH, and 6b·4CH2Cl2 have been structurally characterized by X-ray diffraction. The absorption/emission properties of the Pt(II) complexes 2a and 5a and the Pd(II) complexes 2b and 5b have been investigated by UV-vis spectroscopy and theoretical analysis based on density functional theory. The UV-vis absorption spectra of the neutral complexes recorded in dilute N,N'-dimethylformamide solutions are dominated by intense spin-allowed intraligand transitions in the region below 350 nm. The complexes exhibit charge-transfer bands between 350 and 500 nm. The experimental and theoretical absorption spectra agree qualitatively and point to two low-lying ligand-to-metal charge transfer states that contribute to the bands observed between 350 and 500 nm. The complexes are emissive in frozen solutions at 77 K, in the pure solid state, and when doped into films of poly(methyl methacrylate) but are nonemissive in solution. A red shift is observed when Pt(II) is replaced by Pd(II).

Strings of metal half-shells fabricated using colloidal particle monolayer as a template

A two-dimensional (2-D) polystyrene (PS) particle monolayer is demonstrated as a template for the fabrication of one-dimensional (1-D) strings of metal shells. Metal is deposited by vacuum evaporation onto the PS particle monolayer, which is tilted at an angle (theta) between the normal of the monolayer plane and the direction of metal deposition. The effects of the metal deposition thickness, the tilt angle (theta) of the particle monolayer, and the type of metal on the formation of 1-D strings of metal shells are investigated. Strings of fan-shaped Au shells are obtained at a given tilt angle and deposition thickness. On the other hand, 1-D strings of Cr and Ag cannot be obtained, and they tend to form 2-D films and half-shell dots of the metal, respectively.

In situ synthesis of platinum nanocatalysts on a microstructured paperlike matrix for the catalytic purification of exhaust gases

The successful in situ synthesis of platinum nanoparticles (PtNPs) on a microstructured paperlike matrix, comprising ceramic fibers as main framework and zinc oxide whiskers as selective support for the PtNPs, is reported. The as-prepared hybrid material (PtNPs@ZnO "paper") resembles ordinary paper products because it is flexible, lightweight, and easy to handle. In the catalytic reduction of nitrogen oxide (NO(x)) with propene for exhaust gas purification, the PtNPs@ZnO paper demonstrates a high catalytic performance at a low reaction temperature, with one-third the dosage of precious platinum compared to conventional platinum-loaded honeycomb catalysts. These results imply that the combination of easily synthesized PtNPs and a unique fiber-network microstructure can provide excellent performances, promoting the effective transport of heat and reactants to the active sites of the platinum nanocatalysts. Thus, PtNPs@ZnO materials with paperlike practical aspects are promising catalytic materials for efficient NO(x) gas purification.

Nanoparticle-containing membranes for the catalytic reduction of nitroaromatic compounds

Layer-by-layer deposition of polyelectrolyte/metal nanoparticle films in porous alumina, track-etched polycarbonate, and nylon substrates yields catalytic membranes. With all three substrates, scanning electron microcopy images demonstrate a high density of well-separated nanoparticles in the membrane pores. These nanoparticles catalyze the reduction of nitroaromatic compounds by sodium borohydride with rate constants that are the same as those for nanoparticles immobilized on alumina powder. Moreover, the membranes selectively catalyze the reduction of nitro groups in compounds containing other reducible functionalities such as cyano, chloro, and styrenyl moieties. With nitrophenols and nitroanilines, the only reduction product is the corresponding amine. In contrast, nitrobenzene, nitrotoluenes, nitrobenzonitriles, chloronitrobenzenes, and m-nitrostyrene also form a nitroso product. Membrane catalysts are particularly attractive for controlling product distributions through variation of solution fluxes, as demonstrated by the formation of increased levels of nitroso compounds at high flux.

High Structural Control in Metal-Mediated Synthesis of New Functionalized Diphosphines Using Diphosphinoketenimines as Precursors

The diphosphinoketenimine ligand in the neutral complexes fac-[MnI(CO)(3){(PPh(2))(2)C=C=NR}] (1 a: R = Ph; 1 b: R = p-tolyl) undergoes nucleophilic attack by MeLi and nBuMgCl yielding, after hydrolysis, the diphosphinoenamine-containing complexes fac-[MnI(CO)(3){(PPh(2))(2)C=C(R')NHR}] (3 a,b: R' = Me; 4 a,b: R' = nBu). Complex 1 a reacts under the same conditions with H(2)C=C=CHMgBr to afford fac-[MnI(CO)(3){(PPh(2))(2)C=C(CH(2)CC[triple chemical bond]CH)NHR}] (5 a), which contains a terminal alkyne group on the alpha-carbon atom of the diphosphinoenamine ligand. The cationic complexes fac-[Mn(CO)(4){(PPh(2))(2)C=C=NR}](+) (6) react with H(2)C=C=CHMgBr to afford diphosphinomethanide derivatives bearing three different types of functional groups, depending upon the substituent on the nitrogen atom of the ketenimine: cumulene in fac-[Mn(CO)(4){(PPh(2))(2)C--C(CH=C=CH(2))=N-xylyl}] (7 d), internal alkyne in fac-[Mn(CO)(4){(PPh(2))(2)C--C(C[triple chemical bond]CCH(3))=NtBu}] (8), and quinoline in 9 (R = Ph), whose formation implies an unusual cyclization process. Protonation of 7 d, 8, and 9 with HBF(4) occurs at the nitrogen atom to give the cationic derivatives 10 d, 11, and 12, respectively, which contain the corresponding functionalized diphosphine ligands. Irradiation of 3 a,b and 4 a,b with Vis/UV light makes it possible to isolate the free ligands (PPh(2))(2)C=C(R')NHR (13 a,b and 14 a,b), completing the metal-assisted synthesis of these novel functionalized diphosphines. Irradiation of 12 with Vis/UV light generates free phosphinoquinoline ligand 15, which readily affords a complex 16 containing 15 as a P,N-chelating ligand when treated with [PdCl(2)(NCMe)(2)], thus demonstrating its coordination capability.

Characterization and Catalytic-Hydrogenation Behavior of SiO2-Embedded Nanoscopic Pd, Au, and Pd–Au Alloy Colloids

Colloids embedded in a silica sol-gel matrix were prepared by using fully alloyed Pd-Au colloids, and pure Pd and Au colloids stabilized with tetraalkylammonium bromide following a modified sol-gel procedure with tetrahydrofuran (THF) as the solvent. Tetraethoxysilicate (TEOS) was used as the precursor for the silica support. The molar composition of the sol was TEOS/THF/H2O/HCl = 1:3.5:4:0.05 for the bimetallic Pd-Au and TEOS/THF/H2O/HCl = 1:4.5:4:0.02 for Pd and Au monometallic systems. After refluxing, the colloid was added as a 4.5 wt % solution in THF for Pd-Au, 10.2 wt % solution in THF for Pd and 8.4 wt % solution in THF for Au at room temperature. The gelation was carried out with vigorous stirring (4 days) under an Ar atmosphere. Following these procedures, bimetallic Pd-Au-SiO2 catalysts with 0.6 and 1 wt % metal, and monometallic Pd- and Au-SiO2 catalysts with 1 wt % metal were prepared. These materials were further treated following four different routes: 1) by simple drying, 2) in which the dried catalysts were calcined in air at 723 K and then reduced at the same temperature, 3) in which they were directly reduced in hydrogen at 723 K, and 4) in which the surfactant was extracted using an ethanol-heptane azeotropic mixture. The catalysts were characterized by nitrogen adsorption-desorption isotherms at 77 K, H2 chemisorption measurements, solid-state 1H, 13C, 29Si-CP/MAS-NMR spectroscopy, powder X-ray diffraction (XRD), small angle X-ray scattering (SAXS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and 197Au Mössbauer spectroscopy. The physical characterization by a combination of these techniques has shown that the size and the structural characteristics of the Pd-Au colloid precursor are preserved when embedded in an SiO2 matrix. Catalytic tests were carried out in selective hydrogenation of 3-hexyn-1-ol, cinnamaldehyde, and styrene. These data showed evidence that alloying Pd with Au in bimetallic colloids leads to enhanced activity and most importantly to improved selectivity. Also, the combination of the two metals resulted in catalysts that were very stable against poisoning, as was evidenced for the hydrogenation of styrene in the presence of thiophene.

Chelating versus bridging bonding modes of N-substituted bis(diphenylphosphanyl)amine ligands in Pt complexes and Co2Pt clusters

N-substituted dppa ligands Ph2P-NR-PPh2 [R = -CH2CH2SCH2C6H5 (1), -CH2CH2S(CH2)5CH3 (2), -(CH2)9CH3 (3), -C6H5 (4)] were used for the synthesis of cis-[PtCl2{Ph2PN(R)PPh2}] complexes [R = -CH2CH2SCH2C6H5 (5), -CH2CH2S(CH2)5CH3 (6), -(CH2)9CH3 (7), -C6H5 (8)] and heterotrinuclear clusters of formula [PtCo2(CO)7{Ph2PN(R)PPh2}] [R = -CH2CH2SCH2C6H5 (9), -CH2CH2S(CH2)5CH3 (10), -(CH2)9CH3 (11), -C6H5 (12)]. The presence of relatively bulky substituents on N resulted in a higher chelating power of the ligands. The thermodynamic study of the equilibrium between the chelate and the bridged forms of clusters 9-11 showed that the bridged form is favoured by enthalpic factors whereas entropic factors favour chelation. The structures of 5 and 9 were determined by single crystal X-ray diffraction.

On the nature of metallic nanoparticles obtained from molecular Co3Ru–carbonyl clusters in mesoporous silica matrices

We report on the impregnation of THF solutions of the low-valent heterometallic cluster NEt(4)[Co(3)Ru(CO)(12)] into two mesoporous silica matrices, amorphous xerogels and ordered MCM-41, and a study of its thermal decomposition into metallic nanoparticles by X-ray diffraction, transmission electron microscopy and in situ magnetic measurements under controlled atmospheres. The decomposition of the cluster was monitored as a function of temperature by examining the chemical composition of the particles, their size distributions and their structures as well as their magnetic properties. Treatment under inert atmosphere (i.e. argon) at temperatures below 200 degrees C resulted in the formation of segregated spherical particles of hcp-ruthenium (2.3 +/- 1.0 nm) and hcp-cobalt (3.1 +/- 0.9 nm). The latter is transformed to fcc-cobalt (3.2 +/- 1.0 nm) above 270 degrees C. At higher temperatures, Co-Ru alloying takes place and the Ru content of the particles increases with increasing temperature to reach the nominal composition of the molecular precursor, Co(3)Ru. The particles are more evenly distributed in the MCM-41 framework compared to the disordered xerogel and also show a narrower size distribution. Owing to the different magnetic anisotropy of hcp- and fcc-cobalt, which results in different blocking temperatures, we were able to clearly identify the products formed at the early stages of the thermal decomposition procedure.

Easy one-pot synthesis of new dppm-type linkers for immobilizations

New synthetic routes to symmetric and unsymmetric dppm-type chelate linkers containing the ethoxysilane group are described and the compounds were characterized also by solid-state NMR spectroscopy and X-ray structures.