Chromium Complexes Research Articles

Synthesis of novel bis(cyano) meso-tetraphenylporphyrinato-chromium(III), [K(2 2 2)] [CrIII(TPP)(CN)2] 2.(C7H6O2) (III), and (Chloro) meso-tetraphenylporphyrinato-chromium(III), [CrIII(TPP)Cl] (II): Spectroscopic, physico-chemical, and decolorization properties

The design of metalloporphyrins has attracted the attention of scientists owing to their valuable properties. In this work, the synthesis of meso-tetraphenyl porphyrin H2TPP (I), meso-tetraphenylporphyrinato-chromium(III) complex [CrIII(TPP)Cl] (II), and bis(cyano) meso-tetraphenylporphyrinato-chromium(III) complex [K(222)] [CrIII(TPP)(CN)2] 2.(C7H6O2) (III) was described. The physicochemical properties of (ll)-(IIII) were investigated. X-ray molecular structure of (III) displayed that the crystal lattice of the six-coordinated chromium (III) coordination compound, was made by two-dimensional chain pairs parallel to the [010] direction sustained by C__H…Cg and Cg…Cg π inter and intra-molecular interactions. Hirshfeld surface analysis allowed us to understand the intermolecular contacts, where the fingerprint plot provided the information about the percentage contribution. UV–vis data showed a red-shift for the chromium complex and slightly blue-shift after coordination with cyano axial ligand. The Eg-op value suggested that (I)-(III) compounds are semi-conductor. The complex (II) efficiently and promptly degraded calmagite dye solution at a yield of 100% (calmagite = 40 mg/L, pH = 6, time = 10 min, catalyst dose = 3 mg, H2O2 dose = 4 mL/L, and T = 20 °C).

Bright Long-Lived Circularly Polarized Luminescence in Chiral Chromium(III) Complexes.

A series of highly emissive inert and chiral CrIII complexes displaying dual circularly polarized luminescence (CPL) within the NIR region have been prepared and characterized. The helical [Cr(dqpR)2 ]3+ (dqp=2,6-di(quinolin-8-yl)pyridine; R=OCH3 , Br or C≡CH) complexes were synthesized as racemic mixtures and resolved into their respective PP and MM enantiomers by chiral stationary phase HPLC. The corresponding enantiomers show large glum ≈0.2 and high quantum yield of up to 17 %, which afford important CPL brightness of up to 170 m-1 cm-1 , a key point for applications as chiral luminescent probes. Moreover, the long-lived CP-NIR emission provided by these chromophores (ms range) in aqueous solution opens the way toward the quantification of chiral targets in biological systems with time-gated detection. Thus, such chiral chromophores based on earth abundant and inert 3d metals open new perspectives in the field of CPL and represent an alternative to precious 4d, 5d and to labile 4f metal-based complexes.

Bright Long‐Lived Circularly Polarized Luminescence in Chiral Chromium(III) Complexes

AbstractA series of highly emissive inert and chiral CrIII complexes displaying dual circularly polarized luminescence (CPL) within the NIR region have been prepared and characterized. The helical [Cr(dqpR)2]3+ (dqp=2,6‐di(quinolin‐8‐yl)pyridine; R=OCH3, Br or C≡CH) complexes were synthesized as racemic mixtures and resolved into their respective PP and MM enantiomers by chiral stationary phase HPLC. The corresponding enantiomers show large glum≈0.2 and high quantum yield of up to 17 %, which afford important CPL brightness of up to 170 m−1 cm−1, a key point for applications as chiral luminescent probes. Moreover, the long‐lived CP‐NIR emission provided by these chromophores (ms range) in aqueous solution opens the way toward the quantification of chiral targets in biological systems with time‐gated detection. Thus, such chiral chromophores based on earth abundant and inert 3d metals open new perspectives in the field of CPL and represent an alternative to precious 4d, 5d and to labile 4f metal‐based complexes.

Back Cover: Bright Long‐Lived Circularly Polarized Luminescence in Chiral Chromium(III) Complexes (Angew. Chem. Int. Ed. 18/2021)

Back Cover: Bright Long‐Lived Circularly Polarized Luminescence in Chiral Chromium(III) Complexes (Angew. Chem. Int. Ed. 18/2021)

Highly Active Chromium Complexes Supported by Constrained Schiff-Base Ligands for Cycloaddition of Carbon Dioxide to Epoxides.

Novel constrained Schiff-base ligands (inden) were developed based on the well-known salen ligands. Chromium complexes supported by the constrained inden ligands were successfully synthesized and used as catalysts for the synthesis of cyclic carbonates from epoxides and carbon dioxide (CO2). The catalyst having tert-butyl (tBu) groups as substituents in combination with tetrabutylammonium bromide (TBAB) as a cocatalyst exhibited very high catalytic activity with a turnover frequency of up to 14800 h-1 for the conversion of CO2 and propylene oxide into propylene carbonate exclusively at 100 °C and 300 psi of CO2 under solvent-free conditions. The catalyst was found to be highly active for various epoxide substrates to produce terminal cyclic carbonates in 100% selectivity.

DFT Study on the Electrocatalytic Reduction of CO2 to CO by a Molecular Chromium Complex.

A variety of molecular transition metal-based electrocatalysts for the reduction of carbon dioxide (CO2) have been developed to explore the viability of utilization strategies for addressing its rising atmospheric concentrations and the corresponding effects of global warming. Concomitantly, this approach could also meet steadily increasing global energy demands for value-added carbon-based chemical feedstocks as nonrenewable petrochemical resources are consumed. Reports on the molecular electrocatalytic reduction of CO2 mediated by chromium (Cr) complexes are scarce relative to other earth-abundant transition metals. Recently, our group reported a Cr complex that can efficiently catalyze the reduction of CO2 to carbon monoxide (CO) at low overpotentials. Here, we present new mechanistic insight through a computational (density functional theory) study, exploring the origin of kinetic selectivity, relative energetic positioning of the intermediates, speciation with respect to solvent coordination and spin state, as well as the role of the redox-active bipyridine moiety. Importantly, these studies suggest that under certain reducing conditions, the formation of bicarbonate could become a competitive reaction pathway, informing new areas of interest for future experimental studies.

Nano-Synthesis, characterization and spectroscopic Studies of chromium (III) complex derived from new quinoline-2-one for solar cell fabrication

In this work, Schiff base ligand (L) has been synthesized by condensation reaction of N-amino quinoline-2-one with 4-chlorobenzaldehyede in ethanol, for 5 hours. The synthesized ligand was characterized using (13C,1H NMR), (U.V-Vis), X-ray diffraction (XRD), (FT-IR), (C.H.N) elemental analysis, atomic force microscope (AFM) and melting point. The chromium complex was obtained by (2:1) (L: M) molar ratio and then characterized by FT-IR, UV-Vis, molar conductivity, magnetic susceptibility measurements, AFM, XRD and flame atomic absorption technique (FAA). The results confirmed an octahedral geometry of chrome ion (III). Drop casting techniques was used to prepare nano-thin films of the synthesized compounds. The aim of this study was to fabricate solar cells using the prepared nano-thin films. To achieve, the morphological, structural and optical properties of the nano-thin films were studied, and then they were precipitated on the silicon slides. The fabricated solar cells showed a high efficiency promising to be used for improving silicon solar cells.

Structure Forming Role of Alkaline Metal Cations in the Formation of Chromium(III) Complexes with Anions of Cyclopropane-1,1-Dicarboxylic Acid

The reactions of Cr(NO3)3·9H2O with potassium, sodium, and lithium salts of cyclopropane-1,1-dicarboxylic acid (H2Cpdc) in a ratio of 1 : 3 are studied in order to elucidate the chemical assembling processes of the coordination polymers with Cr3+ and alkaline metal ions. The nature of the reacted alkaline metal cation affects the compositions and structures of the formed compounds. The reaction of Cr(NO3)3·9H2O with K2(Cpdc) affords the 3D polymeric complex [KCr(Cpdc)2(H2O)2]n (I) formed by the bis-chelate anionic fragments [Cr(Cpdc)2(H2O)2]–. Under similar conditions, the use of Na2(Cpdc) and Li2(Cpdc) results in the formation of the 3D polymers [Na4Cr(Cpdc)3(H2O)6(NO3)]n (II) and {[Li8Cr2(Cpdc)8(H2O)5]·H2O}n (III), respectively, containing the tris-chelate anionic fragments [Cr(Cpdc)3]3–. A distinctive feature of compound III is the presence in its structure of the {Li(Cpdc)}– fragments in which the Cpdc2– anion forms a six-membered chelate cycle with the Li+ ion. The crystal structures of compounds I–III are determined by X-ray diffraction (CIF files CCDC nos. 2031258 (I), 2031260 (II), and 2031261 (III)).

Improved Single‐Site Chromium Catalysts with Electron Rich Indenyl Ligands for the Formation of Ultrahigh Molecular Weight Polyethylene

AbstractQuinolyl functionalized indenyl chromium complexes with different substitution patterns were synthesized and evaluated as single site catalysts in ethylene polymerization. Adding substituents like a fused cyclohexyl ring or a Si(CH3)3 group to the indenyl moiety improve complex stability, solubility and catalytic activity compared to the unsubstituted analogues. Furthermore, the ability to incorporate 1‐hexene into the polymer chain increased. Ultrahigh molecular weight polyethylene (UHMW‐PE) or linear low density polyethylene (LLD‐PE) with very high molecular weight was obtained. Although other catalysts are able to incorporate more 1‐hexene only a few systems are known, that combine substantial 1‐hexene incorporation with very high activity and such high molecular weights. Consequently, these catalytic systems are attractive candidates in industrial processes for the production of improved polyethylene materials.

Two-Coordinate, Nonlinear Vanadium(II) and Chromium(II) Complexes of the Silylamide Ligand–N(SiMePh2)2: Characterization and Confirmation of Orbitally Quenched Magnetic Moments in Complexes with Sub-d5 Electron Configurations

The two-coordinate metal amide complexes V{N(SiMePh2)2}2 (1) and Cr{N(SiMe2Ph)2}2 (2) were synthesized by reaction of two equivalents of LiN(SiMePh2)2 with VI2(THF)4 or CrCl2(THF)2 in n-hexane. Their crystal structures showed that they have bent coordination, N-V-N = 137.0(4)°, N-Cr-N = 139.19(5)°, at the metals. The vanadium complex (1) displayed no tendency to isomerize as previously observed for some V(II) amido complexes. Curie fits of SQUID magnetic measurements afforded magnetic moments of 3.36 (1) and 4.68 (2) μB, consistent with high-spin configurations. These values are lower than the spin-only values of 3.88 and 4.90 μB expected for d3 and d4 complexes, suggesting a significant unquenched orbital angular momentum contribution to the overall moment, which is lower as a result of the positive spin-orbit coupling constants.

Heteroleptic trivalent chromium in coordination chemistry: Novel building blocks for addressing old challenges in multimetallic luminescent complexes

Tailored heteroleptic trivalent chromium complexes combine kinetic inertness and structural control with tuneable photophysical properties, all aspects which are crucial for the design of a next gereneration of cheap (supra)molecular light-converting assemblies and devices. • Photoactive Cr(III) complexes as an alternative to precious Ru(II) complexes. • Lability-induced Cr(III) complexes as precursors for heteroleptic complexes. • Inert Cr(III)N 6 complexes might be exploited in metallo-supramolecular structures. • Cr(III)N 6 sensitizers can be programmed for providing long NIR excited state lifetimes. Although less famous than low-spin trivalent cobalt found in [Co III (NH 3 ) 6− x Cl x ]Cl 3− x ( x ≤ 3), which was exploited by Alfred Werner during the early part of the 20th century for establishing the basic rules of coordination chemistry, related trivalent chromium complexes exhibit comparable kinetic inertness, a rare property along the 3d-transition series. The associated slow isomerisation processes are compatible with the isolation of well-defined heteroleptic complexes. Some subtle energetic differences between Cr III -X (X = halide, pseudo-halide, cyanide, solvent) and Cr-N bonds can be exploited for preparing stable and inert cis / trans -[Cr(N ∩ N) 2 X 2 ] + and fac / mer -[Cr(N ∩ N ∩ N)X 3 ] primary heteroleptic complexes incorporating multidentate nitrogen-containing chelate ligands. The use of these building blocks within the frame of the ‘complex-as-ligand’ strategy, or via the labilization of the remaining Cr-X bonds pave the way for the design of discrete polymetallic assemblies in which the photophysically appealing [Cr III N 6 ] chromophores could find applications as functional light-converting devices.

Effect of Synthesis Conditions on the Composition and Structure of Chromium(III) Complexes with Cyclobutane-1,1-Dicarboxylic Acid Anions

The reaction of Cr(NO3)3·9H2O with potassium cyclobutane-1,1-dicarboxylate (K2(Cbdc)) in 1 : 3 ratio was studied. It was shown that, depending on the reaction conditions and the solvents used for synthesis and crystallization, compounds of various compositions and structures can be isolated from the reaction system. Conducting the reaction in water followed by recrystallization from ethanol and methanol results in isolation of 2D polymeric complexes [KCr(Cbdc)2(H2O)2]n (I) and {[K4Cr2(OMe)2(Cbdc)4(MeOH)6]· MeOH}n (II), respectively. The addition of excess KOH to the same reaction mixture and recrystallization from alcohols gives rise to the crystals of 3D polymeric compound [K3Cr(Cbdc)3(H2O)2]n (III). Conducting the reaction in methanol followed by slow diffusion of diethyl ether affords complex [K3Cr(Cbdc)3(MeOH)2(H2O)2]n (IV) with a 2D polymeric structure. The crystal structures of compounds I–IV were determined by X-ray diffraction (CIF files CCDC nos. 2006090 (I), 2006109 (II), 2006116 (III), and 2 006 117 (IV)).

Plant-based functional food ingredients: zinc, chromium and bilberry polyphenols complexes

This work aimed to develop the technology for the production of functional food ingredients (FFI) with hypoglycemic and hypocholesterolemic action. These are organic forms of zinc and chromium and complex of bilberry polyphenols sorbed on brown buckwheat flour. Soy protein isolate hydrolysate was obtained using pancreatin as an enzymatic agent. The hydrolysis was conducted at 50-52°C for 3 hours. The product was then pasteurized at 75°C for 20 min. Organic forms of zinc and chromium were obtained by mixing the hydrolysate with zinc chloride and chromium chloride water solutions followed by micro- and nanofiltration. The bilberry fruits polyphenols were sorbed on the buckwheat flour by mixing them. The mixture was then centrifuged, the precipitate was lyophilized. Organic sources of zinc and chromium were included in the composition of specialized food product (SFP) for patients with metabolic syndrome (MS) as FFI. The introduction of the product into the hypocaloric diet of MS patients led to improvements in lipid metabolism. In essence, lowered total cholesterol decreased low-density lipoproteins and triglycerides levels, lowered atherogenic coefficient. During in vivo study hypoglycemic properties of FFI (bilberry polyphenols complex with buckwheat flour) were established. Experimental mice had significantly lower glucose, glycated haemoglobin and relative liver weight on the background of FFI consumption. Firstly, the obtained data proves the possibility to extract, concentrate and save beneficial properties of polyphenols through their sorption on the biopolymer plant matrices. Secondly, the obtained data proves the prospects of the use of essential microelements organic forms for preventive diet therapy.

NEt3-Triggered Synthesis of UHMWPE Using Chromium Complexes Bearing Non-innocent Iminopyridine Ligands

Chromium complexes bearing non-innocent iminopyridine ligands were investigated as precatalysts for the polymerization of ethylene using different aluminum cocatalysts and Lewis base NEt3 as additive. Beyond confirming the key role of the chromium to ligand synergy in accessing the active complex, other factors that play a crucial role are (i) the nature of the aluminum activator that influences the ion pair generated, (ii) the presence of the additive that boosts the synthesis of UHMWPE with narrow and unimodal molecular weight distribution even at 40 °C, and (iii) the polymerization temperature that affects the polymerization catalysis and the polymer molecular weight. UV–vis–NIR and Fourier transform infrared (FT-IR) spectroscopic techniques have been applied to inspect the activation process and to understand the mode of action by which NEt3 affects the catalytic conversion of ethylene. NEt3 interacts with the aluminum cocatalyst (rather than with the chromium complex) to form an Et3N*AlRx adduct, thus affecting the catalytic ion pair.

Chromium(III) complexes based on phenoxy-imine ligands with pendant N- and O-donor groups as precatalysts for ethylene oligomerization: synthesis, characterization, and DFT studies

Chromium complexes of general formula [Cr{ZNO}(THF)Cl2] [2a, ZNO = C9H6N-8-(N=CH)-2,4-tert-butyl-2-(OC6H2); 2b, ZNO = Ph(NH)-C2H4-(N=CH)-2,4-tert-butyl-2-(OC6H2); 2c, ZNO = 2-MeO-Ph-CH2-(N=CH)-2,4-tert-butyl-2-(OC6H2); 2d, ZNO = 2-PhO-Ph-(N=CH)-2,4-tert-butyl-2-(OC6H2); 2e, ZNO = PhO-C2H4-(N=CH)-2,4-tert-butyl-2-(OC6H2)] and the bis(ligand) complex [Cr{C9H6N-8-NH2}2Cl2]Cl (4) were synthetized and characterized by elemental analysis, IR spectroscopy, and by X-ray crystallography for 4. In the solid state, 4 is monomeric with two 8-amino-quinoline acting as bidentate ligands and two chloride ligands in cis position. The DFT calculations showed slightly higher HOMO energy for 2d. In addition, the energy levels of the LUMO are slightly influenced by pendant O- and N-donor group. Particularly, the LUMOs for complexes 2a and 2d show a small contribution from Cr and Cl atoms as compared to other chromium complexes (2b, 2c and 2e), and the orbitals are almost entirely delocalized over the phenoxy-imine unit. Upon activation with methylaluminoxane (MAO), chromium precatalysts 2a-2e showed good activity in ethylene oligomerization (TOF = 22.0 – 52.7 × 103 (mol ethylene)(mol Cr)−1⋅h−1 at 80°C) with Schultz-Flory distribution of oligomers (KC4-C10 ≈ 0.92), and production of polymer varying from 2.9 to 22.3 wt.%. The catalytic performance is mainly controlled by electronic effects at the phenoxy-imine ligands. The bis(ligand) chromium complex 4 showed good activity in ethylene oligomerization (TOF = 39,400 (mol ethylene)(mol Cr)−1 h−1), producing mostly oligomers (95.2 wt% of total products) with high selectivity for α-olefins. The highest activity among the six precatalysts screened was reached with 2c (TOF = 52,700 mol(ethylene)•mol(Cr)−1•h−1).

Magnetic Anisotropy: Structural Correlation of a Series of Chromium(II)-Amidinate Complexes.

Systematic substituent variations on amidinate ligands bring delicate changes of CrN4 coordination in a family of chromium(II) complexes with the common formula of Cr(RNC(CH3)NR)2, where R = iPr (1), Cy (2), Dipp (Dipp = 2, 6-diisopropylphenyl) (3), and tBu (4). With the largest substituent group, 4 shows the largest distortion of the N4 coordination geometry from square-planar to seesaw shape, which leads to its field-induced single-molecule magnet (SMM) behavior. This is an indication that 4 has the strongest axial magnetic anisotropy and/or optimized magnetic relaxation process. Combined with high-frequency/field electron paramagnetic resonance (HF-EPR) experiments and ab initio calculations, we deduce that the smallest energy gap between ground 4Ψ0 and the first excited 4Ψ1 orbitals in 4 contributes the most to its strongest magnetic anisotropy. Moreover, the lower E value of 4 ensures its being a field-induced SMM. Specifically, the D and E values were found to be correlated to the dihedral angle between the ΔN1CrN2 and ΔN3CrN4 triangles, indicating that distortion from ideal square-planar geometry to the seesaw help increase axial magnetic anisotropy and suppress the transversal part. Thus, the study on this system not only expands the family of Cr(II)-based SMMs but also contributes to a deeper understanding of magneto-structural correlation in four-coordinate Cr(II) SMMs.

Characterization of chromium (III)-glycine complexes in an acidic medium by UV-visible spectrophotometry and capillary electrophoresis

Since the 2017 REACH restrictions on hexavalent chromium, the aeronautics industries have been seeking substitute processes for chromium plating. So far, use of trivalent chromium-based precursors seems to be the best solution in terms of process adaptation. The development of such processes requires a better understanding of plating bath solution chemistry. More specifically, there is a need to characterize the complexation mechanisms occurring with the chromium (III), since they are responsible for higher electrodeposition efficiency. Chromium (III) complexation occurs under specific conditions, such as in an acidic solution, where chromium hydrolysis takes place, leading to a stable aqua chromium complex, which is suspected to be detrimental to an even deposit. To tackle this issue, a complexing agent is added to destabilize hexaaquachromium complex. It is reported that ligands such as glycine allow the formation of chromium-ligand complexes under specific pH and chromium-ligand ratios. In order to identify and understand the formation of the various complexes, two characterization methods have been tested: UV–visible spectrophotometry, which allows identification of chromium complexes through d-d electronic transition in the UV–Visible range, and capillary electrophoresis as a separation method, coupled with UV–visible detection in order to identify and quantify each species. UV–visible spectrophotometry and capillary electrophoresis analyses revealed that no chromium-glycine complex was observed for pH lower than 2, which is the regular condition for electrodeposition. Only a higher Gly:Cr ratio and/or pH>2.4 (i.e pKa1 of glycine) made it possible to observe glycine complexes in solution. As these conditions could be met during electrodeposition near the substrate/electrolyte interface, it is highly likely that chromium depletion, together with the pH increase that occurs, could favor the formation of glycine complexes at the interface, which have been reported as being required to achieve homogeneous chromium deposits.

Single Electron Transistor Based on Chromium Complex of Thiophene: First Principle Study

The ab initio approach based on density functional theory (DFT) and non-equilibrium green’s function (NEGF) for the modelling of single electron transistor (SET) based on thiophene and chromium complex of thiophene, operating in the coulomb blockade (CB) regime has been discussed. Both thiol-ended thiophene molecule and its chromium complex have been analysed for the charge stability and conductance dependence on the source-drain bias and gate voltage. The charging energies for the isolated and SET environment for both the islands have been calculated and discussed. The analysis shows remarkably improved conductance for the chromium complex of thiol-ended thiophene molecule in SET environment than the thiol-ended thiophene molecule in SET environment.

Chromium complexes supported by bidentate thioether-imine [N,S] ligands: synthesis and ethylene oligomerization studies

Chromium complexes bearing thioether-imine ligands were synthesized and their catalytic behavior in ethylene oligomerization has been investigated, evaluating the effect of the ligand and the experimental parameters on the activity, selectivity, and product distribution.

Transient FTIR spectroscopy after one- and two-colour excitation on a highly luminescent chromium(III) complex.

The development of photoactive transition metal complexes with Earth-abundant metals is a rapidly growing research field, where a deeper understanding of the underlying photophysical processes is of great importance. A multitude of potential applications in the fields of photosensitizing, optical sensing, photoluminescence and photoredox catalysis motivates demanding spectroscopic studies. We applied a series of high-level spectroscopic methods on the previously reported highly luminescent chromium(iii) complex [Cr(ddpd)2](BF4)3 (ddpd = N,N'-dimethyl-N,N'-dipyridine-2-ylpyridine-2,6-diamine) possessing two near-IR emissive doublet states with microsecond lifetimes. Luminescence measurements were performed at temperatures down to about 10 K, showing a remarkable rise of the integrated emission intensity by more than a factor of three. The emissive doublet states were structurally characterized by transient FTIR spectroscopy at 290 K and 20 K, supplemented by ground state FTIR and Raman spectroscopy in combination with density functional theory. According to emission and step-scan FT-IR spectroscopy, the stronger luminescence at lower temperature arises from decreased non-radiative decay via energy transfer to CH vibrational overtones and increased radiative decay based on lowered symmetry. Pump/pump/probe (FTIR) and pump/dump/probe (FTIR) schemes were developed to modulate the excited doublet state populations at 290 and 20 K as a function of specific near-IR pump vs. dump wavelengths. The effect of the second near-IR pulse can be explained by combinations of excited state absorption, ground state absorption and stimulated emission. The successful establishment of these two-colour step-scan FTIR experiments is an important step towards profound studies on further transition metal complexes with energetically close-lying excited states in the near future.