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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)(CH3CN)2](BPh4)21–5, 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 Bu4NBr. 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. [ABSTRACT FROM AUTHOR]

Three new iron(III) complexes of sterically hindered tripodal monophenolate ligands are obtained and studied. The relatively high Fe-O-C bond angle of 136.1 degree is observed in one of the complexes, which is due to the steric demand imposed by the coordinated -NMe2 group nearby.

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 [(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. [ABSTRACT FROM AUTHOR]

A rhodamine appended Fe(III)-catecholate complex Fe(RhoCat)3 is reported as a smart dual-modal T1 MRI-optical imaging probe. The high spin Fe(III) coordination sphere and rhodamine unit act as MRI and optical reporters, respectively. The probe showed a r1-relaxivity of 4.37 mM−1 s−1 at 1.41 T via the interaction of second sphere water molecules to coordinated oxygen atoms. It produced an enhanced signal intensity of phantom images on the 7.0 T animal research MRI/MRS scanner at 25 °C and pH 7.3. The interaction of the probe with bovine serum albumin (BSA) significantly improved r1 relaxivity (7.09 mM−1 s−1). Moreover, the optical imaging reporter rhodamine moiety exhibited sensitivity towards biomolecule nitric oxide (NO) and acidic pH via the formation of a ring-opened tautomer of rhodamine, wherein the r1 relaxivity of the probe was enhanced to 5.19 mM−1 s−1 for NO and slightly decreased for acidic pH. Further, the probe visualized NO in adenocarcinoma gastric (AGS) cells via a turn-on fluorescence mechanism with 80% cell viability. Thus, Fe(RhoCat)3 is demonstrated as a potential dual "MRI-ON and Fluorescence-ON" molecular imaging probe to visualize the NO molecule and acidic pH in the tumour microenvironment. [ABSTRACT FROM AUTHOR]

The synthesis and characterization of the iron(III) complexes of three linear diazapane-based 4N ligands containing two (6-methyl)pyridylmethyl or pyridyl-methyl or quinolylmethyl arms as functional models for the extradiol cleaving catechol dioxygenase enzymes is described. The cleavage produces obtained for iron(III) complexes depend strongly upon the ligand architecture as well as the mode of coordination of catecholate substrate to iron(III).

The isolation of a series of iron(III) complexes of the type [Fe(L)[Cl.sub.3]] and its evaluation as structural and functional models for catechol dioxygenase enzymes is discussed. Results suggest that Lewis acidity of the iron(III) center and the steric demand of ligands are the main determinants of the cleavage yields and the rate of dioxygenation in dichloromethane (DCM) and dimethylformide (DMF) solvents.

The new iron(III) complex [Fe{L3)[Cl.sub.2]] (L3 = tripodal monophenolate ligand N,N-dimethyl-N'-(pyrid-2-ylmethyl)-N'-(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine) is isolated and analyzed as a structural and functional model for catechol dioxygenase enzymes. The intradiol and extradiol cleavage activities of the model complex have depended on the solvent, the base used to deprotonate the substrate and the mode of coordination of the catecholate substrate to the iron(III) center.

Copper(ii) complexes [Cu(L1)(H2O)2](SO3CF3)21 and [Cu(L2)(H2O)2](SO3CF3)22 based on 2,6-bis(benzimidazolyl)pyridine were synthesized and are reported herein as highly selective “turn-on” optical probes for l-cysteine. The Cu(ii)/Cu(i) redox potential of probe 1 (0.14 V vs. NHE) was lower than that of 2 (0.233 V vs. NHE) in water. The molecular structure of 2 adopted a square pyramidal geometry (τ = 0.2545), with the Cu–Npy bond (1.958 Å) of its middle pyridine unit being shorter than the other two Cu–Nbenzim bonds (Cu–N, 1.995, 2.000 Å). The axial Cu–O2 bond distance (2.247 Å) was slightly longer than the equatorial Cu–O1 bond distance (1.953 Å). The square-based geometry was further supported by the A value of 156 × 10−4 cm−1 in EPR at 70 K. The d–d and ligand-based transitions appeared at 662 and 314–356 nm for 1 and 651 and 313–360 nm for 2, respectively, in HEPES buffer at pH 7.34. These probes showed selective and efficient “turn-on” fluorescence behaviour towards Cys over other natural amino acids with a binding constant for 1 of 5.4 × 104 and 1.30 × 104 M−1 for 2 and a limit of detection of 2.9 × 10−8 M and 3.32 × 10−8 M, respectively, for 1 and 2 at pH 7.34. The quantum yield for the detection of Cys by 1 (14.7%) was much lower than by 2 (23%). The fluorescence intensity of 1 and 2 were also slightly enhanced by histidine, but at a relatively lower level than that exhibited by Cys. [ABSTRACT FROM AUTHOR]