Role of non-redox innocent ligand units in the oxidation of alcohols with H2O2 catalyzed by μ-oxido-diiron(III) bis-phenolato polypyridyl complexes

Redox non-innocent ligands hold the potential to expand the redox chemistry and activity of transition metal catalysts. The impact of the additional redox chemistry of phenol ligands in oxidation catalysis is explored here in the complex μ-oxido-diiron(III) polypyridyl (1) [(L)Fe(III)(μ-O)Fe(III)(L)](ClO4)2 (where HL is 2-(((di(pyridin-2-yl)methyl) (pyridin-2-ylmethyl) amino)methyl)phenol) and its tert-butyl substituted analog 2, in which each of the Fe(III) centers is coordinated to a phenolato moiety of the ligand. Complex 1 was shown earlier to catalyse the oxidation of benzyl alcohols to aldehydes with H2O2. In particular acid was found to accelerate the reactions by removal of a lag period before catalysis initiated. Here, we use reaction monitoring with resonance Raman, UV/vis absorption and EPR spectroscopy to show that under catalytic conditions, i.e. with excess H2O2, rapid (< 5 s) loss of the phenolato moiety occurs, resulting in the formation of an N4 ligated Fe(III) complex. This N4 coordinated complex forms a Fe(III)-OOH species, which is responsible for alcohol oxidation and over time a relatively stable oxido-bridged dinuclear Fe(III) complex forms as a resting state in the catalytic system. The main role of acid in the catalysis is shown to be to facilitate the initial coordination of H2O2 by driving the formation of mononuclear complexes from 1 and 2. The data show that although the phenolato moiety imparts interesting redox properties on complex 1, it does not contribute directly to the oxidation catalysis observed with H2O2.

Iron(III) Complexes with Pyridine Group Coordination and Dissociation Reversible Equilibrium: Cooperative Activation of CO2 and Epoxides into Cyclic Carbonates

Herein, a series of [ONSN]-type iron(III) complexes were synthesized. A binary catalytic system in combination with iron complexes and tetrabutylammonium bromide (TBAB) exhibited high activity for the synthesis of cyclic carbonates from CO2 (1 atm) and terminal epoxides at room temperature. Additionally, single-component iron complexes without using additional TBAB as nucleophiles also showed high activity for the cycloaddition of CO2 and terminal epoxides under 80 °C and 0.5 MPa of CO2. This study demonstrates that single-component iron catalysts provide a competitive alternative to binary catalytic systems for the synthesis of cyclic carbonates from CO2 and epoxides. Mechanistic studies on a single-component iron catalytic system suggest that the temperature serves as a role of responsive switch for controlling the coordination and dissociation of pyridine bearing iron catalysts detected using in situ infrared spectroscopy, and uncoordinated pyridine activates CO2 to form carbamate. Studies of electrospray ionization high-resolution mass spectrometry reveal that an iron center was used as a Lewis acidic site, free halogen anions from the iron center were used as a nucleophilic site, and coordinated pyridine was released from iron complexes to activate CO2.

The Complex Reactivity of [(salen)Fe]2(μ-O) with HBpin and Its Implications in Catalysis

We report a detailed study into the method of precatalyst activation during alkyne cyclotrimerization. During these studies we have prepared a homologous series of Fe(III)-μ-oxo(salen) complexes and use a range of techniques including UV-vis, reaction monitoring studies, single crystal X-ray diffraction, NMR spectroscopy, and LIFDI mass spectrometry to provide experimental evidence for the nature of the on-cycle iron catalyst. These data infer the likelihood of ligand reduction, generating an iron(salan)-boryl complex as a key on-cycle intermediate. We use DFT studies to interrogate spin states, connecting this to experimentally identified diamagnetic and paramagnetic species. The extreme conformational flexibility of the salan system appears connected to challenges associated with crystallization of likely on-cycle species.

Nickel(II) Complexes of Tripodal Ligands as Catalysts for Fixation of Atmospheric CO2 as Organic Carbonates

The fixation of atmospheric CO₂ into value-added products is a promising methodology. A series of novel nickel(II) complexes of the type [Ni(L)(CH₃ CN)₂ ](BPh₄ )₂ 1-5, where L=N,N-bis(2-pyridylmethyl)-N', N'-dimethylpropane-1,3-diamine (L1), N,N-dimethyl-N'-(2-(pyridin-2-yl)ethyl)-N'-(pyridin-2-ylmethyl) propane-1,3-diamine (L2), N,N-bis((4-methoxy-3,5-dimethylpyridin-2-ylmethyl)-N',N'-dimethylpropane-1,3-diamine (L3), N-(2-(dimethylamino) benzyl)-N',N'-dimethyl-N-(pyridin-2-ylmethyl) propane-1,3-diamine (L4) and N,N-bis(2-(dimethylamino)benzyl)-N', N'-dimethylpropane-1,3-diamine (L5) have been synthesized and characterized as the catalysts for the conversion of atmospheric CO₂ into organic cyclic carbonates. The single-crystal X-ray structure of 2 was determined and exhibited distorted octahedral coordination geometry with cis-α configuration. The complexes have been used as a catalyst for converting CO₂ and epoxides into five-membered cyclic carbonates under 1 atmospheric (atm) pressure at room temperature in the presence of Bu₄ NBr. The catalyst containing electron-releasing -Me and -OMe groups afforded the maximum yield of cyclic carbonates, 34% (TON, 680) under 1 atm air. It was drastically enhanced to 89% (TON, 1780) under pure CO₂ gas at 1 atm. It is the highest catalytic efficiency known for CO₂ fixation using nickel-based catalysts at room temperature and 1 atm pressure. The electronic and steric factors of the ligands strongly influence the catalytic efficiency. Furthermore, all the catalysts can convert a wide range of epoxides (ten examples) into corresponding cyclic carbonate with excellent selectivity (>99%) under this mild condition.

Ionic Conjugated Polymers as Heterogeneous Catalysts for the Cycloaddition of Carbon Dioxide to Epoxides to Form Carbonates under Solvent- and Cocatalyst-Free Conditions

The generation of cyclic carbonates by the cycloaddition of CO₂ with epoxides is attractive in the industry, by which CO₂ is efficiently used as C1 source. Herein, a series of catalysts were developed to efficient mediate the cycloaddition of CO₂ with epoxides to generate carbonates. The catalysts were easily synthesized via the amine-formaldehyde condensation of ethidium bromide with a variety of linkers. The newly prepared heterogeneous catalysts have high thermal stability and degradation temperatures. The surface of the catalysts is smooth and spherical in shape. The effect of temperature, pressure, reaction time and catalyst dosage on the cycloaddition of CO₂ with epoxide were investigated. The results show that the catalyst with 1,3,5-tris(4-formylphenyl)benzene as the linker can achieve 97.4 % conversion efficiency at the conditions of 100 °C, reaction time of 12 h, and the reaction pressure of 1.2 MPa in a solvent-free environment. Notably, the polymers serve as homogeneous catalysts during the reaction (reaction temperature above T_g ) and can be separated and recovered easily as homogeneous catalysts at room temperature. In addition, the catalyst is not only suitable for a wide range of epoxide substrates, but also can be recycled many times. Furthermore, DFT calculations show that the coordination between the electrophilic center of the catalyst and the epoxide reduces the energy barrier, and the reaction mechanism is proposed based on the reaction kinetic studies and DFT calculations.

Rapid atmospheric carbon dioxide fixation by nickel(II) complexes: meridionally coordinated diazepane-based 3N ligands facilitate fixation

Octahedral complexes of the type [Ni(L)(H2O)3](ClO4)2 (1 and 2), where L is the tridentate 3N ligand 4-methyl-1-(pyrid-2-ylmethyl)-1,4-diazacycloheptane (L1, 1), or 4-methyl-1-(N-methylimidazolyl)-1,4-diazacycloheptane (L2, 2), have been isolated and characterized using elemental analysis, ESI-MS and electronic absorption spectroscopy. The DFT optimized structures of 1 and 2 reveal that the tridentate 3N ligands are coordinated meridionally constituting a distorted octahedral coordination geometry around nickel(ii). In methanol solution, the complexes, upon treatment with triethylamine, generate the reactive red colored low-spin square planar Ni-OH intermediate [Ni(L1/L2)(OH)]+ (1a and 2a), as characterized by ESI-MS and electronic absorption spectroscopy, and energy minimized structures. The latter when exposed to the atmosphere rapidly absorbs atmospheric CO2 to produce the carbonate bridged dinickel(ii) complexes [Ni2(L1/L2)2(μ-CO3)(H2O)2](ClO4)2 (3 and 4), as characterized by elemental analysis and the IR spectral feature (∼1608 cm-1) characteristic of bridging carbonate. The single crystal X-ray structure of 3 reveals the presence of a dinickel(ii) core bridged by a carbonate anion in a symmetric mode. Both the Ni(ii) centers are identical to each other with each Ni(ii) possessing a distorted octahedral coordination geometry constituted by a meridionally coordinated 3N ligand, a carbonate ion and a water molecule. The decay kinetics of the red intermediates generated by 1 (kobs, 7.7 ± 0.1 × 10-5 s-1) and 2 (kobs, 5.8 ± 0.3 × 10-4 s-1) in basic methanol solution with atmospheric CO2 has been determined by absorption spectroscopy. DFT studies illustrate that meridional coordination of the 3N ligand and the electron-releasing imidazole ring as in 2 facilitate fixation of CO2. The carbonate complex 3 efficiently catalyzes the conversion of styrene oxide into cyclic carbonate by absorbing atmospheric and pure CO2 with excellent selectivity.

Fixation of atmospheric CO2 as C1-feedstock by nickel(ii) complexes

The development of molecular catalysts for the activation and conversion of atmospheric carbon dioxide (CO₂) into a value-added product is a great challenge. A series of nickel(ii) complexes, [Ni(L)(CH₃CN)₃](BPh₄)₂, 1-4 of diazepane based ligands, 4-methyl-1-[(pyridin-2-yl-methyl)]-1,4-diazepane (L1), 4-methyl-1-[2-(pyridine-2-yl)ethyl]-1,4-diazepane (L2), 4-methyl-1-[(quinoline-2-yl)-methyl]-1,4-diazepane (L3) and 1-[(4-methoxy-3,5-dimethyl-pyridin-2-yl)methyl]-4-methyl-1,4-diazepane (L4), have been synthesized and characterized as catalysts for the activation of atmospheric CO₂. The single-crystal X-ray structure of 1 shows a distorted octahedral geometry with a cis-β configuration around the NiN₆ coordination sphere. All the complexes are used as catalysts for the conversion of atmospheric CO₂ and epoxides into cyclic carbonates at 1 atmosphere (atm) pressure and in the presence of Et₃N. Catalyst 4 was found to be the most efficient catalyst and showed a 31% formation of cyclic carbonates with a TON of 620 under 1 atm air as the CO₂ source. This yield was enhanced to 94% with a TON of 1880 under 1 atm pure CO₂ gas and it is the highest catalytic efficiency known for nickel(ii)-based catalysts. Catalyst 4 enabled the transformation of a wide range of epoxides (eight examples) into corresponding cyclic carbonates with excellent selectivity (>99%) and yields of 59-94% and 11-31% under pure CO₂ and atmospheric CO₂, respectively. The catalytic efficiency is strongly influenced by the electronic nature of the complexes. The CO₂ fixation reactions without an epoxide substrate led to the formation of the carbonate bridged dinuclear nickel(ii) complexes [(LNi^II)₂CO₃](BPh₄)₂1a-4a, which are speculated as catalytically active intermediates. The formation of these species was accompanied by the formation of new absorption bands around 592-681 nm and was further confirmed by the ESI-MS and IR spectral studies. The molecular structures of these carbonate-bridged key intermediates were determined by X-ray analysis. The structures contain two Ni²⁺-centers bridged via a carbonate ion that originated from CO₂. Distorted square pyramidal geometries are adopted around each Ni(ii) center. All these results support that CO₂ fixation reactions occur via CO₂-bound nickel key intermediates.

Efficient and Reusable Iron Catalyst to Convert CO2 into Valuable Cyclic Carbonates

The production of cyclic carbonates from CO2 cycloaddition to epoxides, using the C-scorpionate iron(II) complex [FeCl2{κ3-HC(pz)3}] (pz = 1H-pyrazol-1-yl) as a catalyst, is achieved in excellent yields (up to 98%) in a tailor-made ionic liquid (IL) medium under mild conditions (80 °C; 1-8 bar). A favorable synergistic catalytic effect was found in the [FeCl2{κ3-HC(pz)3}]/IL system. Notably, in addition to exhibiting remarkable activity, the catalyst is stable during ten consecutive cycles, the first decrease (11%) on the cyclic carbonate yield being observed during the 11th cycle. The use of C-scorpionate complexes in ionic liquids to afford cyclic carbonates is presented herein for the first time.

Synthesis of amino-phenolate manganese complexes and their catalytic activity in carbon dioxide activation and oxidation reactions

Two manganese(III) compounds were studied as catalysts for the reaction of carbon dioxide with propylene oxide, styrene oxide, and cyclohexene oxide, and formed cyclic carbonate products selectively under solvent free conditions in the presence of an ionic co-catalyst such as TBAB or PPNCl. Variable temperature kinetic studies allowed the activation energy for propylene carbonate formation to be determined (64 kJ mol−1). The catalysts showed good stability in these reactions and overall turnover numbers (TON) of up to 6000 were observed. The complexes showed low activity for the aerobic oxidation of 4-methyoxybenzylalcohol to the corresponding aldehyde, achieving up to 40% conversion in 72 h. However, near quantitative conversion of 1,2-diphenyl-2-methoxyethanol to provide up to 90% yield of benzaldehyde could be achieved over the course of 5 days. Both complexes showed similar reactivity in the two catalytic processes, and this is likely due to the weakly coordinating nature of the pendant donor within the tetradentate ligand.

FeIII and FeII Phosphasalen Complexes: Synthesis, Characterization, and Catalytic Application for 2-Naphthol Oxidative Coupling

We report on the synthesis and characterization of three iron(III) phosphasalen complexes, [Fe^III (Psalen)(X)] differing in the nature of the counter-anion/exogenous ligand (X^- =Cl^- , NO₃ ^- , OTf^- ), as well as the neutral iron(II) analogue, [Fe^II (Psalen)]. Phosphasalen (Psalen) differs from salen by the presence of iminophosphorane (P=N) functions in place of the imines. All the complexes were characterized by single-crystal X-ray diffraction, UV/Vis, EPR, and cyclic voltammetry. The [Fe^II (Psalen)] complex was shown to remain tetracoordinated even in coordinating solvent but surprisingly exhibits a magnetic moment in line with a Fe^II high-spin ground state. For the Fe^III complexes, the higher lability of triflate anion compared to nitrate was demonstrated. As they exhibit lower reduction potentials compared to their salen analogues, these complexes were tested for the coupling of 2-naphthol using O₂ from air as oxidant. In order to shed light on this reaction, the interaction between 2-naphthol and the Fe^III (Psalen) complexes was studied by cyclic voltammetry as well as UV/Vis spectroscopy.

Chromium Diamino-bis(phenolate) Complexes as Catalysts for the Ring-Opening Copolymerization of Cyclohexene Oxide and Carbon Dioxide

Chromium diamino-bis(phenolate) complexes, CrXL [where L = 6,6'-((1,4-diazepane-1,4-diyl)bis(methylene))bis(2,4-dimethylphenolato) and X = Cl^- (1), OH^- (2), and N₃^- (3)], were prepared and characterized by MALDI-TOF MS and single-crystal X-ray diffraction. Complex 1 crystallized as two linkage isomers, specifically a green chloride-bridged dimer (1) and a pink asymmetrically bridged isomer exhibiting one chloride bridging atom and one bridging phenolate oxygen (1'). Adventitious moisture during sample handling causes the formation of hydroxide-containing complex 2. The reaction of 1 with PPNN₃ (where PPN = bis(triphenylphosphine)iminium) permits the isolation of a crystalline chromium azide complex, 3, which was structurally authenticated. Complex 1 showed good activity toward the ring-opening copolymerization of cyclohexene oxide and carbon dioxide with an added chloride, azide, or 4-(dimethylamino)pyridine (DMAP) cocatalyst to give a completely alternating polycarbonate with a narrow molecular weight dispersity.

Mechanistic guidelines in nonreductive conversion of CO2: the case of cyclic carbonates

This perspective provides general mechanistic guidelines for the catalytic formation of cyclic organic carbonates from CO₂ and cyclic ethers.

Salalen vs. thiolen: in the ring(-opening of epoxide and cyclic carbonate formation)

A range of Fe(iii)-salalen and -thiolen–chloride complexes have been prepared and are shown to be active catalysts for the selective coupling of CO₂ and cyclohexene oxide (CHO).

Highly Selective Single-Component Formazanate Ferrate(II) Catalysts for the Conversion of CO2 into Cyclic Carbonates

The development of new families of active and selective single-component catalysts based on earth-abundant metal is of interest from a sustainable chemistry perspective. In this context, anionic mono(formazanate) iron(II) complexes bearing labile halide ligands, which possess both Lewis acidic and nucleophilic functionalities, have been developed as novel single-component homogeneous catalysts for the reaction of CO₂ with epoxides to produce cyclic carbonates. The influence of the halide ligand and the electronic properties of the formazanate ligand backbone on the catalytic activity are investigated by employing the iron(II) complexes with and without an additional nucleophile. Very high selectivity is achieved towards the formation of the cyclic carbonate products from various terminal and internal epoxides without the need of a cocatalyst.

The synthesis, characterisation and application of iron(iii)–acetate complexes for cyclic carbonate formation and the polymerisation of lactide

Cracking the whip: Simple iron(iii) acetate complexes have been applied to the catalytic cyclic carbonate formation and lactide ROP.

Highly active dinuclear cobalt complexes for solvent-free cycloaddition of CO2 to epoxides at ambient pressure

Dinuclear Co-based catalysts are used for the coupling reaction of epoxides and CO₂ in the presence of a cocatalyst.

Cobalt amino-bis(phenolate) complexes for coupling and copolymerization of epoxides with carbon dioxide

Electron-withdrawing groups on phenolate donors enhance polycarbonate production from CO₂ and cyclohexene oxide by Co(ii) amino-bis(phenolate) complexes.

Iron(III) N,N-Dialkylcarbamate-Catalyzed Formation of Cyclic Carbonates from CO2 and Epoxides under Ambient Conditions by Dynamic CO2 Trapping as Carbamato Ligands

Easily available and inexpensive Fe^III carbamates were employed in the solvent-free synthesis of a series of cyclic carbonates from epoxides and CO₂ at room temperature and atmospheric pressure, in the presence of a cocatalyst. Different experimental conditions (type and concentration of catalyst and cocatalyst, as well as reaction time) were investigated: Fe(O₂ CNEt₂ )₃ and NBu₄ Br acted as the best catalyst/cocatalyst combination, allowing the formation of propylene carbonate and 1,2-butylene carbonate with quantitative yield and selectivity in 24 h. According to NMR and DFT studies, the reaction proceeds with the dynamic trapping of carbon dioxide as a carbamato ligand.

Salen, salan and salalen iron(iii) complexes as catalysts for CO2/epoxide reactions and ROP of cyclic esters

Salan, salen and salalen iron complexes as catalysts in CO₂/epoxide reactions and in the ROP of ε-caprolactone and l-lactide.

Distinct properties of the triplet pair state from singlet fission

Singlet fission, the conversion of a singlet exciton (S1) to two triplets (2 × T1), may increase the solar energy conversion efficiency beyond the Shockley-Queisser limit. This process is believed to involve the correlated triplet pair state 1(TT). Despite extensive research, the nature of the 1(TT) state and its spectroscopic signature remain actively debated. We use an end-connected pentacene dimer (BP0) as a model system and show evidence for a tightly bound 1(TT) state. It is characterized in the near-infrared (IR) region (~1.0 eV) by a distinct excited-state absorption (ESA) spectral feature, which closely resembles that of the S1 state; both show vibronic progressions of the aromatic ring breathing mode. We assign these near-IR spectra to 1(TT)→Sn and S1→Sn' transitions; Sn and Sn' likely come from the antisymmetric and symmetric linear combinations, respectively, of the S2 state localized on each pentacene unit in the dimer molecule. The 1(TT)→Sn transition is an indicator of the intertriplet electronic coupling strength, because inserting a phenylene spacer or twisting the dihedral angle between the two pentacene chromophores decreases the intertriplet electronic coupling and diminishes this ESA peak. In addition to spectroscopic signature, the tightly bound 1(TT) state also shows chemical reactivity that is distinctively different from that of an individual T1 state. Using an electron-accepting iron oxide molecular cluster [Fe8O4] linked to the pentacene or pentacene dimer (BP0), we show that electron transfer to the cluster occurs efficiently from an individual T1 in pentacene but not from the tightly bound 1(TT) state. Thus, reducing intertriplet electronic coupling in 1(TT) via molecular design might be necessary for the efficient harvesting of triplets from intramolecular singlet fission.