Carbene-Like Reactivity in an Iron Azametallacyclobutene Complex: Insights from Electronic Structure.

Herein we describe our investigation into the electronic structure of the first isolated monometallic iron azametallacyclobutene complex. Computational analysis through density functional theory calculations reveals electron delocalization throughout the four atoms of the ring system, in line with experimental observations and supporting the classification of this complex as a conjugated metallacycle. The results of this study also point to significant contribution from an imine-substituted iron carbene resonance structure to the overall bonding picture for the azametallacyclobutene. Accordingly, this complex participates in carbene-like reactivity in the presence of an isocyanide substrate to generate a ketenimine product. The related reaction with carbon monoxide leads to the isolation of a five-membered metallacycle that is analogous to the proposed intermediate in ketenimine formation, and confirms the α-carbon as the site of reactivity.

Iron(II) complexes supported by pyrazolyl-substituted cyclopentadienyl ligands: synthesis and reactivity.

The preparation of bis(pyrazolyl)cyclopentadienyl iron complexes is described. Isopropyl substitution promotes solubility of the iron chloride complex that serves as a precursor to several derivatives through ligand exchange. Modification of the cyclopentadienyl substituent to replace a pyrazolyl unit with a phenyl group favors formation of a substituted ferrocene complex.

Acid-assisted hydrogenation of CO2 to methanol using Ru(II) and Rh(III) RAPTA-type catalysts under mild conditions.

A highly efficient homogeneous catalyst system for production of CH3OH from CO2 using single molecular defined ruthenium and rhodium RAPTA-type catalysts [Ru(η6-p-cymene)X2(PTA)] (X = I(1), Cl(2); PTA = 1,3,5-triaza-7-phosphaadamantane) and rhodium catalysts [Rh(η5-C5Me5)X2(PTA/PTA-BH3)] (X = Cl(3), H(4) and PTA-BH3, H(5)) developed in acidic media under mild conditions. A TON of 4752 is achieved using a [Ru(η6-p-cymene)I2(PTA)] catalyst which represents the first example of CO2 hydrogenation to CH3OH using single molecular defined Ru and Rh RAPTA-type catalysts.

Photoreductive chlorine elimination from a Ni(iii)Cl2 complex supported by a tetradentate pyridinophane ligand.

Herein we report the isolation, characterization, and photoreactivity of a stable NiIII dichloride complex supported by a tetradentate pyridinophane N-donor ligand. Upon irradiation, this complex undergoes an efficient photoreductive chlorine elimination reaction, both in solution and the solid-state. Subsequently, the NiIIICl2 species can be regenerated via a reaction with PhICl2.

Transition-metal complexes of group 12 with 1,1'-bis(phosphanyl)ferrocene ligands.

The syntheses of four new cadmium and zinc complexes with 1,1'-bis(phosphanyl)ferrocene ligands and their phosphine chalcogenide derivatives are reported. The complexes were characterized by elemental analyses and IR, 1H NMR, 31P NMR and electronic absorption spectroscopy. The crystal structures of dichlorido[1-diphenylphosphinoyl-1'-(di-tert-butylphosphanyl)ferrocene-κ2O,P]cadmium(II), [CdCl2{(C17H14OP)(C13H22P)Fe}] or CdCl2(κ2P,O-dppOdtbpf) (1), bis[µ-(tert-butyl)(1'-diphenylphosphinoylferrocen-1-yl)phosphinato-κ3O,O':O'']bis[chloridozinc(II)], [Zn2{(C9H13O2P)(C17H14OP)Fe}2Cl2] or [ZnOCl{κ2O,O'-Ph2POFcPO2(t-Bu)}]2 (2), 1,1'-bis(di-tert-butylthiophosphinoyl)ferrocene, [Fe(C13H22PS)2] or dtbpfS2 (3), and [1,1'-bis(dicyclohexylphosphanyl)ferrocene-κ2P,P'][chlorido/cyanido(0.25/1.75)]zinc(II), [Zn(CN)1.75Cl0.25{(C17H26P)2Fe}] or Zn(CN)2(κ2-dcpf) (4), were determined crystallographically. Compound 1 has tetrahedral geometry in which the CdII centre is coordinated by one dppOdtbpf ligand in a κ2-manner and by two Cl atoms, while compound 2 displays a centrosymmetric dimeric unit in which two oxide atoms bridge the two Zn atoms to generate an eight-membered ring. Compound 3 revealed a sandwich structure with both phosphane groups sulfurized. In compound 4, the ZnII atom adopts a tetrahedral geometry by coordinating to the 1,1'-bis(dicyclohexylphosphanyl)ferrocene ligand in a κ2-manner and to two cyanide ligands.

Copper(I) Complexes Containing PCP Ligand Catalyzed Hydrogenation of Carbon Dioxide to Formate under Ambient Conditions.

The five new copper(I) complexes [Cu2(µ-Cl)2(κ1-PCPt-Bu)] (1), [Cu2(µ-Br)2(κ1-PCPt-Bu)] (2), [Cu2(µ-I)2(κ1-PCPt-Bu)] (3), [Cu2(µ-CN)2(κ1-PCPt-Bu)] (4), and [Cu4(µ3-SCN)4(κ1-PCPt-Bu)2]·CH2Cl2 (5) bearing a 1,3-bis[(di-tert-butylphosphino)methyl]benzene ligand were synthesized and characterized spectroscopically, and the molecular structures of 1, 3, and 5 were determined by single-crystal X-ray diffraction techniques. Structural studies for 1 and 3 revealed their binuclear structures with Cu···Cu separations of 2.609(3) and 2.6359(19) Å, respectively. However, 5 has a tetranuclear cubane structure with an 18-electron configuration at each copper without any metal-metal bonds. The two copper centers in 1 and 3 are bonded to one bridging PCPt-Bu ligand in a κ1-manner and two bridging (pseudo)halido ligands in a µ2-bonding mode to generate a nonplanar Cu2(µ-X)2 framework. The four copper centers in 5 are at the vertices of a tetrahedron. Each copper center has pseudo-tetrahedral coordination provided by two bridging PCPt-Bu ligands in a κ1-manner and the four bridging thiocyanate groups in a µ3-manner. These complexes were used as catalysts for the hydrogenation of CO2 to formate in the presence of DBU as a base to produce valuable energy-rich chemicals, and therefore it is a promising, safe, and simple strategy to conduct reactions under ambient pressure at room temperature. Among all of the five copper(I) complex based catalysts, 3 displayed the best catalytic performance with turnover number (TON) values of 38-8700 in 12-48 h of reaction at 25-80 °C. The outstanding catalytic performance of [Cu2(µ-I)2(κ1-PCPt-Bu)] (3) makes it a potential candidate for realizing the large-scale production of formate by CO2 hydrogenation.

Mechanistic insight into initiation and regioselectivity in the copolymerization of epoxides and anhydrides by Al complexes.

Pentacoordinate Al catalysts comprising bipyridine (bpy) and phenanthroline (phen) backbones were synthesized and their catalytic activity in epoxide/anhydride copolymerization was investigated and compared to (t-Busalph)AlCl. Stoichiometric reactions of tricyclic anhydrides with Al alkoxide complexes produced ring-opened products that were characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography, revealing key regio- and stereochemical aspects.

Detection and Characterization of Mononuclear Pd(I) Complexes Supported by N2S2 and N4 Tetradentate Ligands.

Palladium is a versatile transition metal used to catalyze a large number of chemical transformations, largely due to its ability to access various oxidation states (0, I, II, III, and IV). Among these oxidation states, Pd(I) is arguably the least studied, and while dinuclear Pd(I) complexes are more common, mononuclear Pd(I) species are very rare. Reported herein are spectroscopic studies of a series of Pd(I) intermediates generated by the chemical reduction at low temperatures of Pd(II) precursors supported by the tetradentate ligands 2,11-dithia[3.3](2,6)pyridinophane (N2S2) and N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane (tBuN4): [(N2S2)PdII(MeCN)]2(OTf)4 (1), [(N2S2)PdIIMe]2(OTf)2 (2), [(N2S2)PdIICl](OTf) (3), [(N2S2)PdIIX](OTf)2 (X = tBuNC 4, PPh3 5), [(N2S2)PdIIMe(PPh3)](OTf) (6), and [(tBuN4)PdIIX2](OTf)2 (X = MeCN 8, tBuNC 9). In addition, a stable Pd(I) dinuclear species, [(N2S2)PdI(µ-tBuNC)]2(ClO4)2 (7), was isolated upon the electrochemical reduction of 4 and structurally characterized. Moreover, the (tBuN4)PdI intermediates, formed from the chemical reduction of [(tBuN4)PdIIX2](OTf)2 (X = MeCN 8, tBuNC 9) complexes, were investigated by EPR spectroscopy, X-ray absorption spectroscopy (XAS), and DFT calculations and compared with the analogous (N2S2)PdI systems. Upon probing the stability of Pd(I) species under different ligand environments, it is apparent that the presence of soft ligands such as tBuNC and PPh3 significantly improves the stability of Pd(I) species, which should make the isolation of mononuclear Pd(I) species possible.

Improved Oxidative C-C Bond Formation Reactivity of High-Valent Pd Complexes Supported by a Pseudo-Tridentate Ligand.

There is a large interest in developing oxidative transformations catalyzed by palladium complexes that employ environmentally friendly and economical oxidizing reagents such as dioxygen. Recently, we have reported the isolation and characterization of various mononuclear PdIII and PdIV complexes supported by the tetradentate ligands N,N'-dialkyl-2,11-diaza[3.3](2,6)pyridinophane (RN4, R = tBu, iPr, Me), and the aerobically induced C-C and C-heteroatom bond formation reactivity was investigated in detail. Given that the steric and electronic properties of the multidentate ligands were shown to tune the stability and reactivity of the corresponding high-valent Pd complexes, herein we report the use of an asymmetric N4 ligand, N-mehtyl-N'-tosyl-2,11-diaza[3.3](2,6)pyridinophane (TsMeN4), in which one amine N atom contains a tosyl group. The N-Ts donor atom exhibits a markedly reduced donating ability, which led to the formation of transiently stable PdIII and PdIV complexes, and consequently the corresponding O2 oxidation reactivity and the subsequent C-C bond formation were improved significantly.

Indolylthio Glycosides As Effective Building Blocks for Chemical Glycosylation.

The S-indolyl (SIn) anomeric moiety was investigated as a new leaving group that can be activated for chemical glycosylation under a variety of conditions including thiophilic and metal-assisted pathways. Understanding of the reaction pathways for the SIn moiety activation was achieved via the extended mechanistic study. Also reported is how the new SIn donors fit into selective activation strategies for oligosaccharide synthesis.

Synthesis of Phosphonomethyl Tetrahydrofurans via the Mori-Tamaru Reaction of Phosphonodienes.

Nickel-catalyzed reductive addition of phosphonodienes to aldehydes (the Mori-Tamaru reaction) gives hydroxy vinyl phosphonates in good yields with excellent control of the relative stereochemistry. Base-induced cyclization of the vinyl phosphonates yields phosphonomethyl-substituted tetrahydrofurans. Inversion of the hydroxyl stereochemistry by Mitsunobu reaction and then cyclization yields a different set of phosphonomethyl-substituted tetrahydrofuran diastereoisomers.

A highly active copper catalyst for the hydrogenation of carbon dioxide to formate under ambient conditions.

Carbon dioxide (CO2) is an important reactant and can be used for the syntheses of various types of industrially important chemicals. Hence, investigation concerning the conversion of CO2 into valuable energy-rich chemicals is an important and current topic in molecular catalysis. Recent research on molecular catalysts has led to improved rates for conversion of CO2 to energy-rich products such as formate, but the catalysts based on first-row transition metals are underdeveloped. Copper(i) complexes containing the 1,1'-bis(di-tert-butylphosphino) ferrocene ligand were found to promote the catalytic hydrogenation of CO2 to formate in the presence of DBU as the base, where the catalytic conversion of CO2via hydrogenation is achieved using in situ gaseous H2 (granulated tin metal and concentrated HCl) to produce valuable energy-rich chemicals, and therefore it is a promising, safe and simple strategy to conduct reactions under ambient pressure at room temperature. Towards this goal, we report an efficient copper(i) complex based catalyst [CuI(dtbpf)] to achieve ambient-pressure CO2 hydrogenation catalysis for generating the formate salt (HCO2-) with turnover number (TON) values of 326 to 1.065 × 105 in 12 to 48 h of reaction at 25 °C to 80 °C. The outstanding catalytic performance of [CuI(dtbpf)] makes it a potential candidate for realizing the large-scale production of formate by CO2 hydrogenation.

Synthetic, spectral, structural and catalytic activity of infinite 3-D and 2-D copper(ii) coordination polymers for substrate size-dependent catalysis for CO2 conversion.

Two copper(ii) coordination polymers, viz. [Cu2(OAc)4(µ4-hmt)0.5]n (1) and [Cu{C6H4(COO-)2}2]n·2C9H14N3 (2), have been synthesized solvothermally and characterized. The solid-state structure reveals that 1 is an infinite three-dimensional (3D) motif with fused hexagonal rings consisting of Cu(ii) and hmt in a µ4-bridging mode, while 2 is an infinite two dimensional (2D) motif containing Pht-2 in a µ1-bridging mode. CP 1 has a two-fold interpenetrated diamondoid network composed of 4-connected sqc6 topology with the point symbol of {66}, while 2 has a Shubnikov tetragonal plane network possessing a 4-connected node with an sql topology with a point symbol of {44·.62}-VS [4·4·4·4·*·*]. Both CPs 1 and 2 serve as efficient catalysts for CO2-based chemical fixation. Moreover, 1 demonstrates one of the highest reported catalytic activity values (%yield) among Cu-based MOFs for the chemical fixation of CO2 with epoxides. 1 shows high efficiency for CO2 cycloaddition with small epoxides but its catalytic activity decreases sharply with the increase in the size of epoxide substrates. The catalytic results suggested that the copper(ii) motif-catalyzed CO2 cycloaddition of small substrates had been carried out within the framework, while large substrates could not enter into the framework for catalytic reactions. The high efficiency and size-dependent selectivity toward small epoxides on catalytic CO2 cycloaddition make 1 a promising heterogeneous catalyst for carbon fixation and it can be used as a recoverable stable heterogeneous catalyst without any loss of performance. The solvent-free synthesis of the cyclic carbonate from CO2 and an epoxide was monitored by in situ FT-IR spectroscopy and an exposed Lewis-acid metal site catalysis mechanism was proposed.

Synthesis of Phostones via the Palladium-Catalyzed Ring Opening of Epoxy Vinyl Phosphonates.

The reaction of epoxy aldehydes with tetraethyl methylenediphosphonate gave γ,δ-epoxy vinyl phosphonates. The palladium-catalyzed addition of primary alcohols gave the monoprotected diols as single diastereoisomers. The trans- and cis-epoxides lead to opposite ( syn and anti) diastereoisomers of the addition products. The alkene of the vinyl phosphonates was subjected to hydrogenation, and the resulting saturated phosphonates underwent base-catalyzed cyclization to give phostones with a very high diastereoselectivity in the formation of the new chiral center at the phosphorus atom.

Regioselective photoreactions within a series of mixed co-crystals containing isosteric dihalogenated resorcinols with 4-stilbazole.

The formation and photoreactivity of a series of mixed co-crystals containing 4-stilbazole along with both 4,6-dichlororesorcinol and 4,6-dibromoresorcinol at various ratios is reported. In all cases, the quantitative [2 + 2] cycloaddition reaction yields an identical head-to-tail photoproduct, namely rctt-1,3-bis(4-pyridyl)-2,4-bis(phenyl)cyclobutane. The selectivity in the cycloaddition reaction occurred due to a shorter distance observed between two carbon-carbon double bonds that were found between and not within the hydrogen-bonded assemblies.

Isolation of Amine Derivatives of (ZnSe)34 and (CdTe)34. Spectroscopic Comparisons of the (II-VI)13 and (II-VI)34 Magic-Size Nanoclusters.

The spectroscopically observed magic-size nanoclusters (ZnSe)34 and (CdTe)34 are isolated as amine derivatives. The nanoclusters [(ZnSe)34( n-octylamine)29±6(di- n-octylamine)5±4] and [(CdTe)34( n-octylamine)4±3(di- n-pentylamine)13±3] are fully characterized by combustion-based elemental analysis, UV-visible spectroscopy, IR spectroscopy, and mass spectrometry. Amine derivatives of both (ZnSe)34 and (CdTe)34 are observed to convert to the corresponding (ZnSe)13 and (CdTe)13 derivatives, indicating that the former are kinetic products and the latter thermodynamic products, under the conditions employed. This conversion process is significantly inhibited in the presence of secondary amines. The isolation of the two new nanocluster derivatives adds to a total of nine of 12 possible isolated derivatives in the (II-VI)13 and (II-VI)34 families (II = Zn, Cd; VI = S, Se, Te), allowing comparisons of their properties. The members of these two families exhibit extensive spectroscopic homologies. In both the (II-VI)13 and (II-VI)34 families, linear relationships are established between the lowest-energy nanocluster electronic transition and the band gap of the corresponding bulk semiconductor phase.

Aerobic C-C and C-O bond formation reactions mediated by high-valent nickel species.

Nickel complexes have been widely employed as catalysts in C-C and C-heteroatom bond formation reactions. While Ni(0), Ni(i), and Ni(ii) intermediates are most relevant in these transformations, recently Ni(iii) and Ni(iv) species have also been proposed to play a role in catalysis. Reported herein is the synthesis, detailed characterization, and reactivity of a series of Ni(ii) and Ni(iii) metallacycle complexes stabilized by tetradentate pyridinophane ligands with various N-substituents. Interestingly, while the oxidation of the Ni(ii) complexes with various other oxidants led to exclusive C-C bond formation in very good yields, the use of O2 or H2O2 as oxidants led to formation of appreciable amounts of C-O bond formation products, especially for the Ni(ii) complex supported by an asymmetric pyridinophane ligand containing one tosyl N-substituent. Moreover, cryo-ESI-MS studies support the formation of several high-valent Ni species as key intermediates in this uncommon Ni-mediated oxygenase-type chemistry.

Crystal structures of 2-bromo-1,1,1,3,3,3-hexa-methyl-2-(tri-methyl-sil-yl)tris-ilane and 2-bromo-1,1,1,3,3,3-hexa-isopropyl-2-(triiso-propyl-sil-yl)tri-silane.

The synthesis and crystal structures of two tris-(tri-alkyl-sil-yl)silyl bromide compounds, C9H27BrSi4 (I, HypSiBr) and C27H63BrSi4 (II, TipSiBr), are described. Compound I was prepared in 85% yield by free-radical bromination of 1,1,1,3,3,3-hexa-methyl-2-(tri-methyl-sil-yl)tris-ilane using bromo-butane and 2,2'-azobis(2-methyl-propio-nitrile) as a radical initiator at 333 K. The mol-ecule possesses threefold rotational symmetry, with the central Si atom and the Br atom being located on the threefold rotation axis. The Si-Br bond distance is 2.2990 (12) Šand the Si-Si bond lengths are 2.3477 (8) Å. The Br-Si-Si bond angles are 104.83 (3)° and the Si-Si-Si bond angles are 113.69 (2)°, reflecting the steric hindrance inherent in the three tri-methyl-silyl groups attached to the central Si atom. Compound II was prepared in 55% yield by free-radical bromination of 1,1,1,3,3,3-hexa-isopropyl-2-(triiso-propyl-sil-yl)tris-ilane using N-bromo-succinimide and 2,2'-azobis(2-methyl-propio-nitrile) as a radical initiator at 353 K. Here the Si-Br bond length is 2.3185 (7) Šand the Si-Si bond lengths range from 2.443 (1) to 2.4628 (9) Å. The Br-Si-Si bond angles range from 98.44 (3) to 103.77 (3)°, indicating steric hindrance between the three triiso-propyl-silyl groups.

Glycosyl nitrates in synthesis: streamlined access to glucopyranose building blocks differentiated at C-2.

In an attempt to refine a CAN-mediated synthesis of 1,3,4,6-tetra-O-acetyl-α-d-glucopyranose (2-OH glucose) we unexpectedly discovered that this reaction proceeds via the intermediacy of glycosyl nitrates. Improved mechanistic understanding of this reaction led to the development of a more versatile synthesis of 2-OH glucose from a variety of precursors. Also demonstrated is the conversion of 2-OH glucose into a variety of building blocks differentially protected at C-2, a position that is generally hard to protect regioselectively in the glucopyranose series.

Evaluation of novel platinum(ii) based AIE compound-encapsulated mesoporous silica nanoparticles for cancer theranostic application.

Advanced biomedical research has established that cancer is a multifactorial disorder which is highly heterogeneous in nature and responds differently to different treatment modalities, due to which constant monitoring of therapy response is becoming extremely important. To accomplish this, different theranostic formulations have been evaluated. However, most of them are found to suffer from several limitations extending from poor resolution, radiation damage, to high costs. In order to develop a better theranostic modality, we have designed and synthesized a novel platinum(ii)-based 'aggregation induced emission' (AIE) molecule (named BMPP-Pt) which showed strong intra-cellular fluorescence and also simultaneously exhibited potent cytotoxic activity. Due to this dual functionality, we wanted to explore the possibility of using this compound as a single molecule based theranostic modality. This compound was characterized using elemental analysis, NMR and IR spectroscopy, mass spectrometry and single crystal X-ray structure determination. BMPP-Pt was found to exhibit a high AIE property with emission maxima at 497 nm. For more efficient cancer cell targeting, BMPP-Pt was encapsulated into mesoporous silica nanoparticles (Pt-MSNPs) and the MSNPs were further surface modified with an anti-EpCAM aptamer (Pt-MSNP-E). Pt-MSNPs exhibited higher intracellular fluorescence compared to free BMPP-Pt, though both of them induced a similar degree of cell death via the apoptosis pathway, possibly via cell cycle arrest in the G1 phase. Anti-EpCAM aptamer modification was found to increase both cytotoxicity and intracellular fluorescence compared to unmodified MSNPs. Our study showed that EpCAM functionalized BMPP-Pt loaded MSNPs can efficiently internalize and induce apoptosis of cancer cells as well as show strong intracellular fluorescence. This study provides clues towards the development of a potential single compound based theranostic modality in future.