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.

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-ß 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 [(LNiII)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.

Copper(II) complexes of tripodal ligand scaffold (N3O) as functional models for phenoxazinone synthase.

The copper(II) complexes [Cu(L)NO3] (1-9) of newer N3O ligands (L1-L9) have been synthesized and characterized. The molecular structure of 1, 4, and 7 exhibited nearly a perfect square pyramidal geometry (τ, 0.04-0.11). The Cu-OPhenolate bonds (~ 1.91 Å) are shorter than the Cu-N bonds (~ 2.06 Å) due to the stronger coordination of anionic phenolate oxygen. The Cu(II)/Cu(I) redox potentials of 1-9 appeared around -0.102 to -0.428 V versus Ag/Ag+ in water. The electronic spectra of the complexes showed the d-d transitions around 643-735 nm and axial EPR parameter (g||, 2.243-2.270; A||, 164-179 × 10-4 cm-1) that corresponds to square pyramidal geometry. The bonding parameters α2, 0.760-0.825; ß2, 0.761-0.994; γ2, 0.504-0.856 and K||, 0.698-0.954 and K⊥, 0.383-0.820 calculated from EPR spectra and energies of d-d transitions. The complexes catalyzed the conversion of substrate 2-aminophenol into 2-aminophenoxazine-3-one using molecular oxygen in the water and exhibited the yields of 41-61%. The formation of the product is accomplished by the appearance of a new absorption band at 430 nm and the rates of formation were calculated as 6.98-15.65 × 10-3 s-1 in water. The reaction follows Michaelis-Menten enzymatic reaction kinetics with turnover numbers (kcat) 9.11 × 105 h-1 for 1 and 4.66 × 105 h-1 for 9 in water. The spectral, redox and kinetic studies were performed in water to mimic the enzymatic oxidation reaction conditions.

Cu(I) complexes obtained via spontaneous reduction of Cu(II) complexes supported by designed bidentate ligands: bioinspired Cu(I) based catalysts for aromatic hydroxylation.

Copper(i) complexes [Cu(L1-7)2](ClO4) (1-7) of bidentate ligands (L1-L7) have been synthesized via spontaneous reduction and characterized as catalysts for aromatic C-H activation using H2O2 as the oxidant. The single crystal X-ray structure of 1 exhibited a distorted tetrahedral geometry. All the copper(i) complexes catalyzed direct hydroxylation of benzene to form phenol with good selectivity up to 98%. The determined kinetic isotope effect (KIE) values, 1.69-1.71, support the involvement of a radical type mechanism. The isotope-labeling experiments using H218O2 showed 92% incorporation of 18O into phenol and confirm that H2O2 is the key oxygen supplier. Overall, the catalytic efficiencies of the complexes are strongly influenced by the electronic and steric factor of the ligand, which is fine-tuned by the ligand architecture. The benzene hydroxylation reaction possibly proceeded via a radical mechanism, which was confirmed by the addition of radical scavengers (TEMPO) to the catalytic reaction that showed a reduction in phenol formation.

Benzene Hydroxylation by Bioinspired Copper(II) Complexes: Coordination Geometry versus Reactivity.

A series of bioinspired copper(II) complexes of N4-tripodal and sterically crowded diazepane-based ligands have been investigated as catalysts for functionalization of the aromatic C-H bond. The tripodal-ligand-based complexes exhibited distorted trigonal-bipyramidal (TBP) geometry (τ, 0.70) around the copper(II) center; however, diazepane-ligand-based complexes adopted square-pyramidal (SP) geometry (τ, 0.037). The Cu-NPy bonds (2.003-2.096 Å) are almost identical and shorter than Cu-Namine bonds (2.01-2.148 Å). Also, their Cu-O (Cu-Owater, 1.988 Å; Cu-Otriflate, 2.33 Å) bond distances are slightly varied. All of the complexes exhibited Cu2+ → Cu+ redox couples in acetonitrile, where the redox potentials of TBP-based complexes (-0.251 to -0.383 V) are higher than those of SP-based complexes (-0.450 to -0.527 V). The d-d bands around 582-757 nm and axial patterns of electron paramagnetic resonance spectra [g∥, 2.200-2.251; A∥, (146-166) × 10-4 cm-1] of the complexes suggest the existence of five-coordination geometry. The bonding parameters showed K∥ > K⊥ for all complexes, corresponding to out-of-plane π bonding. The complexes catalyzed direct hydroxylation of benzene using 30% H2O2 and afforded phenol exclusively. The complexes with TBP geometry exhibited the highest amount of phenol formation (37%) with selectivity (98%) superior to that of diazepane-based complexes (29%), which preferred to adopt SP-based geometry. Hydroxylation of benzene likely proceeded via a CuII-OOH key intermediate, and its formation has been established by electrospray ionization mass spectrometry, vibrational, and electronic spectra. Their formation constants have been calculated as (2.54-11.85) × 10-2 s-1 from the appearance of an O (π*σ) → Cu ligand-to-metal charge-transfer transition around 370-390 nm. The kinetic isotope effect (KIE) experiments showed values of 0.97-1.12 for all complexes, which further supports the crucial role of Cu-OOH in catalysis. The 18O-labeling studies using H218O2 showed a 92% incorporation of 18O into phenol, which confirms H2O2 as the key oxygen supplier. Overall, the coordination geometry of the complexes strongly influenced the catalytic efficiencies. The geometry of one of the CuII-OOH intermediates has been optimized by the density functional theory method, and its calculated electronic and vibrational spectra are almost similar to the experimentally observed values.

Catalytic Conversion of Atmospheric CO2 into Organic Carbonates by Nickel(II) Complexes of Diazepane-Based N4 Ligands.

Activation of CO2 and conversion into value-added products is an effective option to mitigate CO2 emission. The nickel(II) complexes [Ni(L1)](ClO4)2 1, [Ni(L2)](ClO4)2 2, and [Ni(L3)(CH3CN)2](Ph4B)2 3 of diazepane-based ligands [1,4-bis[(pyridin-2-yl-methyl)]-1,4-diazepane (L1), 1,4-bis[2-(pyridin-2-yl)ethyl]-1,4-diazepane (L2), and 4-bis[2-(quinoline-2-yl)-methyl]-1,4-diazepane (L3)] have been synthesized and structurally characterized. The complexes were employed as the catalysts for the conversion of atmospheric CO2 into organic carbonates in the absence of cocatalyst at 1 atm pressure. The single-crystal X-ray structures of 1 and 2 exhibit distorted square-planar geometry with almost identical Ni-N bond distances (1.891-1.946 Å). The geometry of the complexes rearranged into octahedral in acetonitrile, which was studied by paramagnetic 1H NMR and electronic spectra. The complexes selectively captured CO2 from the atmospheric air and readily converted epoxides into cyclic carbonates without any cocatalyst. They showed a maximum yield of 25% (TON, 500) using 1 atm air, which is drastically enhanced up to 89% (TON, 1780) using 1 atm pure CO2 gas. This is the highest catalytic efficiency reported for CO2 fixation using nickel-based catalysts to date. The CO2 fixation reaction without organic substrate showed the formation of carbonate-bridged dinuclear nickel(II) complexes. They showed characteristic absorption bands around 571-612 nm and were further confirmed by electrospray ionization mass spectrometry, IR, and single-crystal X-ray structures. The molecular structure of carbonate-bridged intermediates exhibited two Ni2+-centers with distorted square pyramidal geometries for 2a and 3a but distorted octahedral and square pyramidal geometries for 1a. The CO2 fixation reactions possibly proceeded via the formation of CO2-bound nickel species.

Catalytic fixation of atmospheric carbon dioxide by copper(ii) complexes of bidentate ligands.

New copper(ii) complexes, [Cu(L1)2(H2O)](ClO4)2, 1 [L1 = 2-pyridin-2-yl-quinoline], [Cu(L2)2(H2O)](ClO4)2, 2 [L2 = 2-pyridin-2-yl-quinoxaline], [Cu(L3)2(H2O)](ClO4)2, 3 [L3 = 6,7-dimethyl-2-pyridin-2-yl-quinoxaline], [Cu(L4)2(H2O)](ClO4)2, 4 [L4 = 4-phenyl-2-pyridin-2-yl-quinoline] and [Cu(L5)2(H2O)](ClO4)2, 5 [L5 = 4-phenyl-2-pyridin-2-yl-quinazoline], were synthesized and characterized as catalysts for selective fixation of atmospheric CO2. The molecular structure of 2 was determined by single-crystal X-ray studies and shown to have an unusual trigonal bipyramid geometry (τ, 0.936) around the copper(ii) center, with the coordination of two ligand units and a water molecule. The Cu-Nquin (2.040, 2.048 Å) bonds are slightly longer than the Cu-Npyr (1.987 Å) bonds but shorter than the Cu-Owater bond (2.117 Å). Well-defined Cu(ii)/Cu(i) redox potentials of around 0.352 to 0.401 V were observed for 1-5 in acetonitrile. The electronic absorption spectra of 1-5 showed ligand-based transitions at around 208-286 nm with a visible shoulder at around 342-370 nm. The d-d transitions appeared at around 750-800 and 930-955 nm in acetonitrile. The rhombic EPR spectra of 1-5 exhibited three different g values gx, 2.27-2.34; gy, 2.06-2.09; and gz, 1.95-1.98 at 70 K. Atmospheric CO2 was successfully fixed by 1-5 using Et3N as a sacrificial reducing agent, resulting in CO32--bound complexes of type [Cu(L)CO3(H2O)] that display an absorption band at around 614-673 nm and a νst at 1647 cm-1. This CO32--bound complex of 1 was crystallized from the reaction mixture and it displayed a distorted square pyramidal geometry (τ, 0.369) around the copper(ii) center via the coordination of only one ligand unit, a carbonate group, and water molecules. Furthermore, treatment of the carbonate-bound Cu(ii) complexes with one equivalent of H+ under N2 atmosphere resulted in the liberation of bicarbonate (HCO3-) and regenerated the parent complexes. These regenerated catalysts were active enough to fix CO2 in eight repeating cycles without any change in efficiency. The fixation of CO2 possibly occurs via the formation of Cu(i)-species, which is accompanied by the formation of an MLCT band at around 450-500 nm. The rates of Cu(i)-species formation, kobs, were determined and found to be 5.41-10.31 × 10-3 s-1 in the presence of Et3N in acetonitrile at 25 °C. Interestingly, the copper(i)-species of 3 has been successfully crystallized and displayed a distorted tetrahedral geometry through the coordination of two units of ligand L3.