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1.
Chemistry ; 29(34): e202300771, 2023 Jun 19.
Artigo em Inglês | MEDLINE | ID: mdl-37042487

RESUMO

Mesoionic carbenes (MIC) are a popular class of compound that are heavily investigated at the moment. The access to cationic MICs, and the ability of MICs to stabilize radicals are two highly attractive fields that have hardly been explored until now. Here the synthesis and characterisation of three different cationic azide-substituted 1,2,3-triazolium salts, used as building blocks for studying their reactivity towards triphenylphosphine are reported, where the reactivity is dependent on the nature of the starting triazolium salt. Furthermore, the cationic triazolium salts were used to develop a series of unsymmetrical MIC-triazene-NHC/MIC' compounds, which can be readily converted to the radical form either by electrochemical or chemical methods. These radicals, which display NIR electrochromism, were investigated using a battery of techniques such as electrochemistry, UV/Vis/NIR and EPR spectroelectrochemistry, and theoretical calculations. Interestingly, the MIC plays an important role in the stabilization of the triazenyl radical, particularly in a competitive role vis-à-vis their NHC counterparts. These results shed new light on the ability of MICs to stabilize radicals, and possibly also on their π-accepting ability.

2.
Chemistry ; 29(31): e202300405, 2023 Jun 02.
Artigo em Inglês | MEDLINE | ID: mdl-36971603

RESUMO

In recent years terpyridines (tpy) and mesoionic carbenes (MIC) have been widely used in metal complexes. With the right combination with a metal center, both of these ligands are individually known to generate excellent catalysts for CO2 reduction. In this study, we combine the potentials of PFC (PFC=polyfluorocarbon) substituted tpy and MIC ligands within the same platform to obtain a new class of complexes, which we investigated with respect to their structural, electrochemical and UV/Vis/NIR spectroelectrochemical properties. We further show that the resulting metal complexes are potent electrocatalysts for CO2 reduction in which CO is exclusively formed with a faradaic efficiency of 92 %. A preliminary mechanistic study, including the isolation and characterization of a key intermediate is also reported.

3.
Chemistry ; 29(46): e202301246, 2023 Aug 15.
Artigo em Inglês | MEDLINE | ID: mdl-37191067

RESUMO

In coordination chemistry and materials science, terpyridine ligands are of great interest, due to their ability to form stable complexes with a broad range of transition metal ions. We report three terpyridine ligands containing different perfluorocarbon (PFC) tails on the backbone and the corresponding FeII and CoII complexes. The CoII complexes display spin crossover close to ambient temperature, and the nature of this spin transition is influenced by the length of the PFC tail on the ligand backbone. The electrochemical properties of the metal complexes were investigated with cyclic voltammetry revealing one oxidation and several reduction processes. The fluorine-specific interactions were investigated by EPR measurements. Analysis of the EPR spectra of the complexes as microcrystalline powders and in solution reveals exchange-narrowed spectra without resolved hyperfine splittings arising from the 59 Co nucleus; this suggests complex aggregation in solution mediated by interactions of the PFC tails. Interestingly, addition of perfluoro-octanol in different ratios to the acetonitrile solution of the sample resulted in the disruption of the F ⋯ ${\cdots }$ F interactions of the tails. To the best of our knowledge, this is the first investigation of fluorine-specific interactions in metal complexes through EPR spectroscopy, as exemplified by exchange narrowing.

4.
Angew Chem Int Ed Engl ; 60(1): 499-506, 2021 Jan 04.
Artigo em Inglês | MEDLINE | ID: mdl-33080102

RESUMO

We present herein anionic borate-based bi-mesoionic carbene compounds of the 1,2,3-triazol-4-ylidene type that undergo C-N isomerization reactions. The isomerized compounds are excellent ligands for CoII  centers. Strong agostic interactions with the "C-H"-groups of the cyclohexyl substituents result in an unusual low-spin square planar CoII  complex, which is unreactive towards external substrates. Such agostic interactions are absent in the complex with phenyl substituents on the borate backbone. This complex displays a high-spin tetrahedral CoII  center, which is reactive towards external substrates including dioxygen. To the best of our knowledge, this is also the first investigation of agostic interactions through single-crystal EPR spectroscopy. We conclusively show here that the structure and properties of these CoII complexes can be strongly influenced through interactions in the secondary coordination sphere. Additionally, we unravel a unique ligand rearrangement for these classes of anionic mesoionic carbene-based ligands.

5.
Chemistry ; 26(6): 1314-1327, 2020 Jan 27.
Artigo em Inglês | MEDLINE | ID: mdl-31778594

RESUMO

Asymmetric platinum donor-acceptor complexes [(pimp)Pt(Q2- )] are presented in this work, in which pimp=[(2,4,6-trimethylphenylimino)methyl]pyridine and Q2- =catecholate-type donor ligands. The properties of the complexes are evaluated as a function of the donor ligands, and correlations are drawn among electrochemical, optical, and theoretical data. Special focus has been put on the spectroelectrochemical investigation of the complexes featuring sulfonyl-substituted phenylendiamide ligands, which show redox-induced linkage isomerism upon oxidation. Time-dependent density functional theory (TD-DFT) as well as electron flux density analysis have been employed to rationalize the optical spectra of the complexes and their reactivity. Compound 1 ([(pimp)Pt(Q2- )] with Q2- =3,5-di-tert-butylcatecholate) was shown to be an efficient photosensitizer for molecular oxygen and was subsequently employed in photochemical cross-dehydrogenative coupling (CDC) reactions. The results thus display new avenues for donor-acceptor systems, including their role as photocatalysts for organic transformations, and the possibility to introduce redox-induced linkage isomerism in these compounds through the use of sulfonamide substituents on the donor ligands.

6.
Dalton Trans ; 51(27): 10507-10517, 2022 Jul 12.
Artigo em Inglês | MEDLINE | ID: mdl-35766191

RESUMO

Control of the spin state of metal complexes is important because it leads to a precise control over the physical properties and the chemical reactivity of the metal complexes. Currently, controlling the spin state in metal complexes is challenging because a precise control of the properties of the secondary coordination sphere is often difficult. It has been shown that non-covalent interactions in the secondary coordination sphere of transition metal complexes can enable spin state control. Here we exploit this strategy for fluorinated triazole ligands and present mononuclear CoII and FeII complexes with "click"-derived tripodal ligands that contain mono-fluorinated benzyl substituents on the backbone. Structural characterization of 1 and 2 at 100 K revealed Co-N bond lengths that are typical of high spin (HS) CoII complexes. In contrast, the Fe-N bond lengths for 3 are characteristic of a low spin (LS) FeII state. All complexes show an intramolecular face-to-face non-covalent interaction between two arms of the ligand. The influence of the substituents and of their geometric structure on the spin state of the metal center was investigated through SQUID magnetometry, which revealed spin crossover occurring in compounds 1 and 3. EPR spectroscopy sheds further light on the electronic structures of 1 and 2 in their low- and high-spin states. Quantum-chemical calculations of the fluorobenzene molecule were performed to obtain insight into the influence of fluorine-specific interactions. Interestingly, this work shows that the same fluorinated tripodal ligands induce SCO behavior in both FeII and CoII complexes.

7.
Chem Sci ; 12(9): 3170-3178, 2021 Feb 11.
Artigo em Inglês | MEDLINE | ID: mdl-34164084

RESUMO

Unusual and unexpected chemical transformations often provide access to completely new types of functional molecules. We report here the synthesis of a methylene-bridged bis-triazolium salt designed as a precursor for a new bis-mesoionic carbene (MIC) ligand. The direct metalation with silver oxide led to the isolation and crystallographic characterization of a cationic tetranuclear octacarbene-silver(i) complex. During metalation the formal bis-MIC precursor undergoes significant structural changes and chemical transformations. A combined synthetic, crystallographic and (spectro-)electrochemical approach is used to elucidate the mechanistic pathway: starting from the methylene-bridged bis-triazolium salt a single deprotonation leads to a NacNac analogue, which is followed by a redox-induced radical dimerization reaction, generating a new tetra-MIC ligand coordinated to silver(i) central atoms. Decomplexation led to the isolation of the corresponding tetratriazoliumethylene, a profoundly electron-poor alkene, which is an analogue of TCNE.

8.
Dalton Trans ; 50(48): 18097-18106, 2021 Dec 14.
Artigo em Inglês | MEDLINE | ID: mdl-34851330

RESUMO

The fine-tuning of intermolecular or intramolecular non-covalent interactions (NCIs) and thus the precise synthesis of metal complexes in which the spin states can be controlled by NCIs remains challenging, even though several such complexes have been intensively studied. In this regard, we present mononuclear cobalt(II) and iron(II) complexes with "click"-derived tripodal ligands that contain fluorinated benzyl substituents in the secondary coordination sphere. The complexes were co-crystallized with different solvent molecules to decipher the effect of the crystallized solvents on NCIs, and on the spin state of the metal ion. Additionally, the fluorine-specific interactions in the secondary coordination sphere were examined. We present a first structure-property correlation between the nature of interaction of the (per)fluorinated aromatic substituents on the ligand periphery, and the spin state of the metal complexes. In particular, the TF5TA containing ligand show interesting stacking motifs depending on the used solvent, and these interactions have an influence on the spin state of the cobalt(II) complexes. Furthermore, the iron(II) complex thereof, Fe(TF5TA)2(BF4)2·2EtOH displays spin crossover (SCO).

9.
Chem Sci ; 8(9): 6300-6306, 2017 Sep 01.
Artigo em Inglês | MEDLINE | ID: mdl-28989663

RESUMO

Switchable crown ether-ammonium [2]rotaxanes with a redox-active tetrathiafulvalene (TTF) unit implemented in their wheels were synthesised and fully characterised. Reversible operation in two modes is possible, in which the [2]rotaxane's axle is either charged or neutral. Cyclic voltammetry experiments reveal the effects of mechanical bonding on the electrochemical properties of TTF and show the [2]rotaxanes to perform a distinct function in both modes. In the charged mode, redox-switching is dominated by strong electrostatic repulsion in the [2]rotaxane which subsequently leads to a macrocycle translation along the axle. In the non-charged mode, a selective energetic stabilisation of TTF radical cations is observed, which can be attributed to an interplay of weak electrostatic interactions between wheel and axle.

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