A novel monoanionic tetradentate N4 ligand (F5DPPy) based on a dipyrromethene skeleton as a molecular platform and decorated with pyridine rings at the 1‐ and 9‐positions of the dipyrrin motif has ...been prepared and characterized. Interestingly, although this ligand is weakly fluorescent, it presents a chelation‐enhanced fluorescence effect of around 150 times upon coordination to ZnII. Time‐dependent (TD) DFT calculations reproduce nicely the spectroscopic features of both the ligand and the complex, and analysis of the electron density redistribution in the excited state suggests that a better orbital overlap of the HOMO and LUMO in F5DPPyZnCl compared with F5DPPy is responsible for the more intense transitions observed with the former system. As such, this ligand opens interesting perspectives in the design of ratiometric sensors.
A new N4 ligand featuring a dipyrrin skeleton decorated with 2‐pyridyl groups was synthesized and characterized. Upon zinc complexation, a 150‐fold chelation‐enhanced fluorescence was observed, which was rationalized by DFT calculations. This result opens interesting perspectives toward the use of this type of ligands as ratiometric sensors, following the example of their famous BODIPY analogues.
Two new heterobimetallic LNiO
Cu(RPY2)
(RPY2 = N-substituted bis 2-pyridyl(ethylamine) ligands with R = indane, 3a or R = Me, 3b) complexes have been spectroscopically trapped at low temperatures. ...They were prepared by reacting the mononuclear side-on LNi
superoxo precursor bearing a β-diketiminate ligand (L = HC-(CMeNC
H
(iPr)
)
) with the Cu(i) complexes. In contrast to the oxo groups in known high-valent M
(μ-O)
(M = Fe, Co, Ni, Cu) cores that display electrophilic reactivities, 3a and 3b display rather nucleophilic oxo cores active in aldehyde deformylation reactions. However, the spectroscopic and reactivity properties of 3a/3b are found to be distinct relative to that of the previously reported LNiO
Cu(MeAN)
complex containing a more basic (nucleophilic) N,N,N',N',N'-pentamethyl-dipropylenetriamine (MeAN) ligand at the copper centre. The geometry and electronic properties of the copper ligands affect the electron density of the oxygen atoms of the heterodinuclear {Ni(μ-O)
} core and 3a/3b undergo slower nucleophilic and faster electrophilic reactions than the previously reported LNiO
Cu(MeAN)
intermediate. The present study therefore demonstrates the tuning of the electrophilicity/nucleophilicity of the oxygen atoms of the heterobimetallic Ni(μ-O)
Cu
cores by controlling the electron donation from the ancillary ligands, and underlines the significance of subtle electronic changes in the physical and chemical properties of the biologically relevant heterobimetallic metal-dioxygen intermediates.
A bisamine aliphatic dithiolate NiIIN2S2 complex that does yield a metal-based oxidation has been synthesized. A square pyramidal NiIIIN3S2+ complex is generated by electrochemical oxidation in the ...presence of imidazole, mimicking the redox structural changes of NiSOD. In addition, EPR measurements coupled to DFT calculations demonstrate that the metal character in the redox active orbital increases drastically upon imidazole binding, implicating that these geometrical modifications are crucial for the stabilization of the NiIII state.
Metalloenzymes use earth-abundant non-noble metals to perform high-fidelity transformations in the biological world. To ensure chemical efficiency, metalloenzymes have acquired evolutionary ...reactivity-enhancing tools. Among these, the entatic state model states that a strongly distorted geometry induced by ligands around a metal center gives rise to an energized structure called entatic state, strongly improving the reactivity. However, the original definition refers both to the transfer of electrons or chemical groups, whereas the chemical application of this concept in synthetic systems has mostly focused on electron transfer, therefore eluding chemical transformations. Here we report that a highly strained redox-active ligand enables a copper complex to perform catalytic nitrogen- and carbon-group transfer in as fast as 2 min, thus exhibiting a strong increase in reactivity compared with its unstrained analogue. This report combines two reactivity-enhancing features from metalloenzymes, entasis and redox cofactors, applied to group-transfer catalysis.
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•We design a catalyst interfacing two reactivity-enhancing tools from metalloenzymes•This work merges redox-active cofactors and entatic state reactivity•The modifications in the coordination sphere lead to enhanced catalytic behavior•These results open perspectives in bioinspired catalysis and group-transfer reactions
Inorganic Chemistry; Molecular Inorganic Chemistry; Catalysis
In the following work, we carried out a systematic study investigating the behavior of a thiosemicarbazone-nickel (II) complex (
) as a molecular catalyst for photo-induced hydrogen production. A ...comprehensive comparison regarding the combination of three different chromophores with this catalyst has been performed, using
,
and
as photosensitizers. Thorough evaluation of the parameters affecting the hydrogen evolution experiments (i.e., concentration, pH, solvent nature, and ratio), has been performed in order to probe the most efficient photocatalytic system, which was comprised by
and
as catalyst and chromophore, respectively. The electrochemical together with the photophysical investigation clarified the properties of this photocatalytic system and allowed us to propose a possible reaction mechanism for hydrogen production.
Sulfur-rich nickel metalloenzymes are capable of stabilizing NiI and NiIII oxidation states in catalytically relevant species. In an effort to better understand the structural and electronic features ...that allow the stabilization of such species, we have investigated the electrochemical properties of two mononuclear N2S2 NiII complexes that differ in their sulfur environment. Complex 1 features aliphatic dithiolate coordination (NiL, 1), and complex 2I is characterized by mixed thiolate/thioether coordination (NiLMeI, 2I). The latter results from the methylation of a single sulfur of 1. The X-ray structure of 2I reveals a distorted square planar geometry around the NiII ion, similar to what was previously reported by us for 1. The electrochemical investigation of 1 and 2 + shows that the addition of a methyl group shifts the potentials of both redox NiII/NiI and NiIII/NiII redox couples by about 0.7 and 0.6 V to more positive values. Through bulk electrolyses, only the mononuclear dithiolate NiIL− (1 - ) and the mixed thiolate/thioether NiIIILMe2+ (2 2+ ) complexes were generated, and their electronic properties were investigated by UV−vis and EPR spectroscopy. For 1 - (NiI, d9 configuration) the EPR data are consistent with a d x 2 - y 2 based singly occupied molecular orbitals (SOMOs). However, DFT calculations suggest that there is also pronounced radical character. This is consistent with the small g-anisotropy observed in the EPR experiments. The spin population (Mulliken analysis) analysis of 1 - reveals that the main contribution to the SOMO (64%) is due to the bipyridine unit. Time dependent density functional theory (TD-DFT) calculations attribute the most prominent features observed in the electronic absorption spectrum of 1 - to metal to ligand charge transfer (MLCT) transitions. Concerning 2 2+, the EPR spectrum displays a rhombic signal with g x = 2.236, g y = 2.180, and g z = 2.039 in CH3CN. The g iso value is larger than 2.0, which is consistent with metal based oxidation. The unpaired electron (NiIII, d7 configuration) occupies a Ni-d z 2 based molecular orbital, consistent with DFT calculations. Nitrogen hyperfine structure is observed as a triplet in the g z component of the EPR spectrum with A N = 51 MHz. This result indicates the coordination of a CH3CN molecule in the axial position. DFT calculations confirm that the presence of a fifth ligand in the coordination sphere of the Ni ion is required for the metal-based oxidation process. Finally, we have shown that 1 exhibits catalytic reductive dehalogenation activity below potentials of −2.00 V versus Fc/Fc+ in CH2Cl2.
The synthesis of a dinuclear copper(II) complex, supported by a 1,3‐diamino‐2‐propanol‐based tetraamide ligand, is reported. Structural properties in the solid state and in solution, by means of XRD ...analysis and NMR spectroscopy, respectively, provide evidence of a highly flexible complex that can display several conformations, leading to the image of the wings of a butterfly. The complex was fully characterized and the redox properties were investigated. Room‐temperature spectro‐electrochemistry was used to monitor the formation of a metastable mono‐oxidized product that displayed an absorption band centered at λ=463 nm. EPR investigation of the low‐temperature, chemically generated, mono‐oxidized product reveals the presence of an intermediate described as a mixed‐valent CuIICuIII species, which is a model of the possible highly oxidizing intermediate in particulate methane monooxygenase.
Fleeting species: A flexible dinuclear copper(II) complex based on a tetraamide ligand that resembles a butterfly flapping its wings is characterized, along with its one‐electron oxidized species, which consists of an unstable mixed‐valent copper(II)/copper(III) species (see figure).
Density functional theory Orio, Maylis; Pantazis, Dimitrios A.; Neese, Frank
Photosynthesis research,
12/2009, Letnik:
102, Številka:
2-3
Journal Article
Recenzirano
Odprti dostop
Density functional theory (DFT) finds increasing use in applications related to biological systems. Advancements in methodology and implementations have reached a point where predicted properties of ...reasonable to high quality can be obtained. Thus, DFT studies can complement experimental investigations, or even venture with some confidence into experimentally unexplored territory. In the present contribution, we provide an overview of the properties that can be calculated with DFT, such as geometries, energies, reaction mechanisms, and spectroscopic properties. A wide range of spectroscopic parameters is nowadays accessible with DFT, including quantities related to infrared and optical spectra, X-ray absorption and Mössbauer, as well as all of the magnetic properties connected with electron paramagnetic resonance spectroscopy except relaxation times. We highlight each of these fields of application with selected examples from the recent literature and comment on the capabilities and limitations of current methods.
Thiosemicarbazone-based complexes have been explored as a new class of redox-active catalysts H
2
production due to their flexibility for extensive optimization. To rationalize the process, we need ...to understand how these complexes function. In this work, we used DFT calculations to investigate the various mechanisms that could take place for three previously characterized Ni complexes. We found that two possible mechanisms are compatible with previously published experimental data, involving protonation of two adjacent N atoms close to the metal center. The first step likely involves a proton-coupled electron transfer process from a proton source to one of the distal N atoms in the ligand. From here, a second proton can be transferred either to the coordinating N atom situated in between the first protonated atom and the Ni atom, or to the second distal N atom. The former case then has the protons in close distance for H
2
production. However, the latter will require a third protonation event to occur, which would fall in one of the N atoms adjacent to the Ni center, resulting in a similar mechanism. Finally, we show that the H-H bond formation is the rate-limiting step, and suggest additional strategies that can be taken into account to further optimize these complexes.
The reaction mechanism of nickel thiosemicarbazone complexes, a new class of redox-active catalysts for H
2
evolution, is investigated by the means of DFT methods.
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