Third themed collection on Nitrogen Ligands

This collection was initiated during the 8^th EuCheMS Conference on Nitrogen Ligands which was held in June 2024 in Cassis, France. It followed the previous editions of this themed conference held in Lisboa (2018), Beaune (2015), Granada (2011), Garmisch Partenkirchen (2008), Camerino (2004), Como (1996) and Alghero (1992). More than 130 enthusiastic participants coming from eighteen different countries have joined this event. This number illustrates the success of this themed conference and testifies the ongoing interest in nitrogen ligands and their major importance in coordination chemistry and related fields.

Two previous and impressive themed collections of peer-reviewed manuscripts were initiated during the events located in Camerino and Lisboa.^1,2 These successes prompted us to propose this second themed collection in Dalton Transactions that will highlight the role of nitrogen ligands in a wide diversity of areas, namely in coordination chemistry, metal–organic chemistry, inorganic and bioinorganic chemistry, biologically active compounds, materials, catalysis (both homogeneous and heterogeneous), metal-mediated synthesis, non-covalent interactions and supramolecular assemblies, C–H bond activation and functionalization, electrochemistry (Fig. 1).

Fig. 1 Logo of the 8^th EuCheMS Conference on Nitrogen ligands.

In this second Dalton Transactions collection some of these aspects are highlighted or reviewed through twenty-nine contributions including two perspectives, one frontier, one communication and twenty-five research articles that will illustrate how:

- Rigid bispidine ligands are giving rise to fascinating complexes with applications in catalysis, the activation of small molecules as well as in the development of biological probes (K. Bleher et al.; https://doi.org/10.1039/D5DT00050E).

- Natural cyclic polypeptides are able to chelate two copper ions giving rise to complexes with biological activities (P. Baur and P. Comba; https://doi.org/10.1039/D4DT03002H).

- The strength of the metal–nitrogen bond can be finely tuned to get a spin crossover of an iron(II) center at room temperature (T. Lathion et al.; https://doi.org/10.1039/D4DT01868K).

- Ruthenium(II) naphthalocyanines are efficient homogeneous catalysts for the conversion of amines to glycine derivatives through carbene insertion into N–H bonds (A. P. Kroitor et al.; https://doi.org/10.1039/D4DT03263B).

- NH–CO fragments born by terpyridine or bipyridine ligands, are efficient anchoring sites towards enzyme pockets for platinum(II) or ruthenium(II) complexes that can thereby inhibit enzyme activities (M. Scarpi-Luttenauer et al.; https://doi.org/10.1039/D4DT02984D).

- Copper(I) ions chelated by dipyridylamine ligands produce phosphorescent complexes that are active Earth-abundant metal photocatalysts in the chlorosulfonylation or bromonitromethylation of styrene (A. Villegas-Menares et al.; https://doi.org/10.1039/D4DT03337J).

- Simple nickel(II) salen complexes are efficient precatalysts for the hydrosilylation of olefins (B. Szarłan et al.; https://doi.org/10.1039/D4DT03257H).

- Pyrazole bearing two appended phenol groups and one N-phenyl substituent can act as a tridendate ligand for silicon(IV), giving rise to highly luminescent and chiral complexes (V. Giuso et al.; https://doi.org/10.1039/D5DT00392J).

- The physico-chemical properties of subphthalocyanines will differ significantly when changing the chelated central metal ion in the macrocyclic cavity (J. Labella et al.; https://doi.org/10.1039/D4DT03173C).

- Tridentate ligands based on triazolyl-pyridine moieties can be used to obtain water soluble platinum(II) complexes for the selective binding of guanine quadruplexes (S. Kroos et al.; https://doi.org/10.1039/D4DT03067B).

- Replacing carboxylate groups by imidazole moieties in polydentate ligands chelating lanthanides can be detrimental to the kinetic inertness of the complexes even if their overall geometry is only slightly altered (M. Sanadar et al.; https://doi.org/10.1039/D5DT00236B).

- Introducing two strong electron-withdrawing β-cyano substituents on one pyrrole unit of metalloporphyrins significantly modulates their physico-chemical properties and can be used in naked-eye detection of cyanide ions (V. Bhardwaj and M. Sankar; https://doi.org/10.1039/D4DT03528C).

- Triazamacrocycles with pendant methylpyridine arms produce zinc complexes that can be effective for the apoptosis of B-cells (A. Marlin et al.; https://doi.org/10.1039/D4DT02962C).

- Cadmium complexes built on a tris-2-aminoethylamine substituted by three phenol groups can be used in the preparation of Pd–Cd core–shell nanoparticles that are active as photocatalysts in the reductive amination of furfural in water (J. Rohilla et al.; https://doi.org/10.1039/D4DT03058C).

- The selectivity of a cytochrome c oxidase model towards the reduction of oxygen into water can be significantly improved by replacing the terpyridine ligand chelating the copper(II) ion with a bipyridine one (M. Berthe et al.; https://doi.org/10.1039/D4DT03188A).

- Trifused porphyrins are showing red-shifted absorption spectra and reduced HOMO–LUMO gaps together with promising three-photon absorption coefficients and cross-section values (A. S. Bulbul et al.; https://doi.org/10.1039/D4DT02930E).

- Heptacoordinated molybdenum(II) complexes can be obtained when using a non-innocent tridentate and pincer-type bis-pyrazolyl-pyridine ligand (A. Estival et al.; https://doi.org/10.1039/D4DT03264K).

- Introducing two ethyl phosphonic acid groups on the meso-positions of Pd(II) or Pt(II) porphyrin prevents their aggregation and gives access to water soluble and phosphorescent complexes active as photosensitizers or as photocatalysts (M. V. Volostnykh et al.; https://doi.org/10.1039/D4DT03068K).

- A simple copper(II) complex bearing two bidentate pyridine-imidazolyl ligands and self-assembled as a monolayer on gold gives access to an epinephrine selective voltammetric sensor in the presence of uric acid and in aqueous media (D. Nowicka et al.; https://doi.org/10.1039/D4DT02702G).

- Model systems for electron transfer proteins built on copper(I) complexes bearing a tripodal tetradentate ligand may show different electron transfer mechanisms depending on the distortion of their coordination sphere (T. Seitz et al.; https://doi.org/10.1039/D4DT02917H).

- Phosphine or oxazoline bioconjugated with an optically pure amino acid moiety can self-assemble into dimers giving rise to supramolecular bidentate ligands that render rhodium(I) or iridium(I) complexes active as enantioselective catalysts in asymmetric hydrogenation or hydroformylation reactions (M. Bakija et al.; https://doi.org/10.1039/D4DT02519A).

- The conjugation of two BODIPY complexes by a thienothiophene-based bridge gives access to dyes with high molar extinction coefficients, large Stoke shifts and high two-photon absorption cross section values (Z. Chai et al.; https://doi.org/10.1039/D4DT02655A).

- A flexible tripodal pyrrole-imine ligand can be used to prepare uranium(VI) complexes and facilitates their reduction into uranium(V) or even to uranium(IV) when considering polymetallic derivatives gathering uranium and rare-earth metals (T. J. N. Obey et al.; https://doi.org/10.1039/D4DT02367F).

- Sterically hindered imino-pyridine bidentate ligands when chelating nickel(II) produces efficient precatalysts of ethylene polymerization towards low-molecular weight and branched polyethylenes (Z. Ning et al.; https://doi.org/10.1039/D4DT02159B).

- Replacing two of the three biimidazole bidentate ligands in homoleptic and poorly soluble chromium(III) octahedral complexes, with two phenanthrolines, gives rise to heteroleptic and soluble complexes that have fascinating pH-dependent photophysical properties and could be further used to build polymetallic assemblies (J. Chong et al.; https://doi.org/10.1039/D4DT01608D).

- A deprotonation–alkylation sequence applied to tris-(2-pyridylmethyl)amines gives access to new functionalized tripodal and tetradentate ligands, that when chelated to cobalt(II) centers, produce HER photocatalysts (P. Zardi et al.; https://doi.org/10.1039/D4DT02022G).

- β–β BODIPY dimers featuring conjugated triphenylamine or carbazole units are efficient and low-molecular weight electron donors when they are incorporated into bulk heterojunction organic solar cells (M. Zhu et al.; https://doi.org/10.1039/D4DT01163E).

- Heteroleptic ruthenium(II) or osmium(II) complexes having two phenanthrolines or two tetraazaphenanthrene ligands and one diamino-tetraazaphenanthrene moiety, are promising and original photosensitizers (S. De Kreijger et al.; https://doi.org/10.1039/D4DT01077A).

- 1,2-Diamidobenzene can be a good ligand to bind cobalt(II), nickel(II) or zinc(II) and thereby produce metallic or polymetallic synthons with magnetic and/or redox properties suitable for constructing discrete metal–organic architectures (D. Hunger et al.; https://doi.org/10.1039/D4DT01115E).

Overall, we believe that this second themed collection of manuscripts gathered in Dalton Transactions will illustrate the unlimited and fascinating variety of nitrogen ligands together with a selection of their applications.

Acknowledgements

We are grateful to all the authors who have contributed to this collection. The success of the conference held in Cassis was due to the help of the International Scientific Committee: Claude Gros (France), Jens Mueller (Germany), Jorge Navarro (Spain), Claudio Pettinari (Italy), Andrew Phillips (Ireland), Armando Pombeiro (Portugal) and Anna Trzeciak (Poland). We would like to thank our colleagues who joined us in the Local Organizing Committee: Aura Tintaru, Arnault Heynderickx, Anthony Kermagoret and Elena Zaborova; as well as our young coworkers Gaelle Nassar, Valentin Jubault, Tatiana Munteanu and Vlad Tarpa, for their assistance during the conference.

References

1. C. Pettinari and N. Masciocchi, J. Organomet. Chem., 2005, 690, 1871–1877.
2. A. J. L. Pombeiro, Dalton Trans., 2019, 48, 13904–13906.

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