Nitrogen ligands

Nitrogen occupies a relevant position in the periodic table and this Dalton Transactions themed collection on nitrogen ligands, appearing in 2019, can be viewed as a contribution to the celebrations of the International Year of the Periodic Table of Chemical Elements as proclaimed by the United Nations and UNESCO.

This collection was inspired by the theme of the 7^th EuCheMS Conference on nitrogen ligands, which was held (2018) in Lisboa, Portugal. It followed the previous editions of this series of conferences held in Beaune (2015), Granada (2011), Garmisch Partenkirchen (2008), Camerino (2004), Como (1996) and Alghero (1992). Their success testifies to the relevance and current interest in nitrogen ligands, the development of which has greatly contributed to the progress of Coordination Chemistry and related fields.

In fact, nitrogen ligands have played a key role since the foundation of Coordination Chemistry by Alfred Werner in the late XIX^th century (his seminal publication on the constitution of ammine metal compounds appeared in 1893, i.e., 125 years before the abovementioned 7^th EuCheMS conference on nitrogen ligands).¹

Since then, the complexity of nitrogen ligands has greatly increased and nowadays they are key players (even ubiquitous in some particular areas) in a wide diversity of fields, namely coordination, metal–organic, inorganic and bioinorganic, pharmaceutical and medicinal chemistries, biologically active compounds, materials, catalysis (both homogeneous and heterogeneous), metal-mediated synthesis, non-covalent interactions and supramolecular assemblies, electrochemistry, C–H bond activation and functionalization, etc.

This themed collection highlights the roles of such ligands in modern chemistry, showing also their multi- and transdisciplinary characters. Nitrogen ligands can behave as mono- or multidentate ligands, eventually hemilabile ones, components of more complex assemblies such as MOFs (metal organic frameworks), dendrimers and aza-crowns, being also able to be involved in non-covalent interactions, etc.

Although a full coverage of the numerous types of nitrogen ligands is obviously not possible, in this collection various important perspectives, approaches and applications are addressed by internationally recognized authors from different laboratories in Europe, Asia, America and Australia, who were invited to contribute.

In this Dalton Transactions collection, the following types of nitrogen ligands are reviewed:

- Various classes of redox non-innocent chelating nitrogen ligands in chelates of different ring sizes, e.g., (i) amidinates and triazenides, (ii) α-diimines and (iii) formazanates, in 4-, 5- or 6-membered metallacycles, respectively, and comparison with related O or S ligands (W. Kaim); DOI: 10.1039/C9DT01411J

- α-Diimines in Ni catalysts for ethylene polymerization (K. Bryliakov, W.-H. Sun, E. P. Talsi et al.); DOI: 10.1039/C9DT01297D

- Guanidinates in complexes, e.g., with luminescent behaviour and/or used as homogeneous catalysts (olefin isomerization, transfer hydrogenation and hydroxylation of carbonyl compounds, etc.) (V. Cadierno et al.); DOI: 10.1039/C9DT01289C

- Tris(pyrazolyl)methanes in group 11 metal catalysts for reactions under homogeneous or heterogeneous conditions (P. J. Pérez et al.); DOI: 10.1039/C9DT01661A

- 2,2′-Dipyridylamines in metal complexes as homogeneous catalysts and luminescent materials (C. Bruneau, S. Gaillard, C. Fischmeister et al.); DOI: 10.1039/C9DT02165E

- Cyanoximes (mono-, bis- or even tris-cyanoximes) in complexes and biological properties (N. Gerasimchuk); DOI: 10.1039/C9DT01057B

- Phosphorus dendrimers functionalised with nitrogen ligands (N,N or P,N or N,N,N ligands) in catalysis (Pd, Cu and Sc complexes) or with anti-cancer activity (Cu and Au complexes) (A.-M. Caminade et al.); DOI: 10.1039/C9DT01305A

- Nitrogen moieties and functionalities not acting as conventional ligands but playing a key role in nitrogen-doped metal-free carbon catalysts for chemical or electrochemical CO₂ conversion and valorization (D. M. Fernandes, A. F. Peixoto et al.). DOI: 10.1039/C9DT01691K

Moreover, the following classes of nitrogen ligands (ordered preferentially according to the number of coordinated N atoms, N_x, indicated before the ligand name) are addressed in the following research articles:

- (N₁) Pyridine-based PNP-type pincer ligands in Mo catalysts for nitrogen fixation (dinitrogen reduction to ammonia) under ambient reaction conditions (Y. Nishibayashi et al.); DOI: 10.1039/C8DT04975K

- (N₁) Bis(dimethylsilyl)amides in heterobimetallic Ba/Li and Ca/Li storable compounds that are inert towards Et₂O and THF (Y. Sarazin et al.); DOI: 10.1039/C9DT00771G

- (N₁) Amido [bis(trimethylsilyl)amido] and imidazolin-2-iminato in half-sandwich pentamethylcyclopentadienyl Ru complexes, the latter with a rare one-legged piano stool geometry and a short Ru–N bond (M. Tamm et al.); DOI: 10.1039/C9DT00577C

- (N₁) Bis(N-heterocyclic carbenes) (bis-NHC) involving pyrazine or pyrimidine as the central N-heteroaromatic coordinated moiety in homoleptic Fe(II) complexes with an improved excited-state lifetime (P. C. Gros et al.); DOI: 10.1039/C9DT01731C

- (N₁) Bifunctional pyrazole-isophthalates as T-shaped ligands in Cu(II) and Zn(II) MOFs with rutile topology, the former showing flexibility and high saturation uptake to CO₂ (C. Janiak et al.); DOI: 10.1039/C9DT01499C

- (N₁) Phenolato-dimethyloxazolines (as N,O-ligands) in oxido-Re(V) complexes as catalysts for cyclooctene epoxidation and perchlorate reduction (J. A. Schachner, N. C. Mösch-Zanetti et al.); DOI: 10.1039/C9DT01352K

- (N₁) Imidazole-based monocarboxylates (as N,O-ligands) in Na(I), Cu(II), Ag(I) and Zn(II) homochiral coordination polymers (E. Álvarez, A. Galindo et al.); DOI: 10.1039/C9DT01237K

- (N₁) 3H-imidazo[4,5-f]quinolin-5-ol derivatives as artificial nucleobases used for Cu(II)-mediated base pairing (N. L. Doltsinis, J. Müller et al.); DOI: 10.1039/C9DT02043H

- (N₁) Monodentate banana shaped complementary ligands based on a fluorenone-, carbazole- or phenanthrene-backbone with hanging pyridyl arms, in Pd(II) heteroleptic cages which are differentiated by high-resolution trapped ion-mobility mass spectrometry (TIMS) (G. H. Clever et al.); DOI: 10.1039/C9DT01814J

- (N₂) Amidinates in mono- and bimetallic samarium complexes as catalysts for hydroamination/cyclization reactions (P. W. Roesky et al.); DOI: 10.1039/C9DT01418G

- (N₂) Ansa-bis(amidinate) ligand (sterically demanding) with a rare k¹-N(amido):η⁶-arene coordination (favorable to the classic κ²-N,N-chelation) in complexes of Yb(II) (A. A. Trifonov et al.); DOI: 10.1039/C9DT01162E

- (N₂ and N₃) Aza-ether-crown functionalised amidinates and iminoanilides in Ba(II) silylamide complexes which catalyse/mediate the hydrophosphination of styrene with primary and secondary phosphines (Y. Sarazin et al.); DOI: 10.1039/C9DT01512D

- (N₂) Pyrazolates and 1,10-phenanthrolines (e.g., neo- and bathocuproine) in multinuclear luminescent Ag(I) assemblies (E. S. Shubina et al.); DOI: 10.1039/C9DT01355E

- (N₂) Pyrroles (bidentate amino or imino derivatives) in K(I) complexes as catalysts for isocyanates cyclotrimerization (X. Wei, C. Xi et al.); DOI: 10.1039/C9DT01246J

- (N₂) Diiminoacenaphthenes (or bis(aryl)acenaphthenequinonediimines) in allyl-Mo(II) complexes with antitumor activity (M. J. Calhorda et al.); DOI: 10.1039/C9DT00469F

- (N₂) Diiminoacenaphthenes (unsymmetrical, with electron-withdrawing nitro and fluoride-substituted benzhydryl substituents) in Ni(II) catalysts for ethylene polymerization (G. A. Solan, W.-H. Sun et al.); DOI: 10.1039/C8DT04427A

- (N₂) 1,4-Diazabutadienes (sterically hindered) in dioxolene Ni complexes which undergo intermolecular coupling through the back-bonded methyl groups of diazabutadienes (M. P. Bubnov, G. A. Abakumov et al.); DOI: 10.1039/C9DT01309A

- (N₂ and N₁) Pyridylaminobisphenolates and hydroxyquinolinates in high spin Fe(III) complexes with anti-tumor activity (I. Correia, C. Acilan et al.); DOI: 10.1039/C9DT01193E

- (N₂) Amphiphilic Schiff bases (N,N,O-ligands with a long alkyl chain) in Fe(III) complexes with spin crossover and self-assembly with aggregation in solution (P. N. Martinho et al.); DOI: 10.1039/C9DT00032A

- (N₂) A Schiff base (N,N,O-ligand) in V(V) catecholate complexes wherein the hydrophobicity enhances the membrane affinity and the anti-cancer activity (D. C. Crans, P. A. Lay et al.); DOI: 10.1039/C9DT00601J

- (N₂) N,N,N′,N′-Tetramethylethylenediamine (TMEDA) in Zn-aryloxide initiators for ring-opening polymerization of L-lactide to afford antifungal biodegradable materials (R. Petrus, P. Sobota); DOI: 10.1039/C9DT00627C

- (N₃) Bis(imino)pyridines of the type substituted bis(imino)dihydroquinolines in Co catalysts for ethylene polymerization to linear polyethylene waxes (G. A. Solan, W. Zhang, X. Hu, W.-H. Sun et al.); DOI: 10.1039/C9DT01345H

- (N₃) (Triazol)amines in Ru(II) catalysts for hydrogenation of ketones and aldehydes (V. Beghetto, J. G. de Vries et al.); DOI: 10.1039/C9DT01822K

- (N₃) 1,4,7-Triazacyclononane-based ligands substituted by methylthiazolylcarboxylate and/or methylthiazolyl arms in Cu(II) or Zn(II) complexes, namely exocyclic and polynuclear ones (R. Delgado, C. Platas-Iglesias, V. Patinec, R. Tripier et al.); DOI: 10.1039/C9DT01366K

- (N₃) Latonduine and paullone modified derivatives with a tridentate N,N,N binding site in Cu(II) complexes with antitumor activity (F. Bacher, V. B. Arion et al.); DOI: 10.1039/C9DT01238A

- (N₃) Terpyridines and 1,3,5-triaza-7-phosphaadamantane (PTA) or 1,3,5-triaza-7-phosphaadamantane-7-sulfide (PTA=S) in light-stable and water-soluble Ag(I) complexes with cytotoxic and antitumor activities (P. Smolenski et al.); DOI: 10.1039/C9DT01646E

- (N₃) Terpyridines in Zn(II) complexes with anti-tumor activity (H. Chen, L. Pan, Z. Ma et al.); DOI: 10.1039/C8DT04924F

- (N₄) Bis(pyridyl-triazolyl)alkanes in Fe(II), Co(II) and Ni(II) mono- or binuclear complexes (magnetic and luminescent properties) (A. Gusev, W. Linert et al.); DOI: 10.1039/C9DT01391A

- (N₄) Corroles with various functionalities and their Ag(III) complexes (K. M. Smith, K. M. Kadish, R. Paolesse et al.); DOI: 10.1039/C9DT03166A

- (N₄) Corroles as free bases or their Co complexes as macrocyclic linkers in porous organic polymers (POPs) with CO adsorption properties and selectivity over other gases (S. Brandès et al.); DOI: 10.1039/C9DT01599J

- (N₄) Pyrene-labeled dendronized porphyrins in Zn(II), Cu(II), Mg(II) and Mn(III) dendritic complexes (optical and photophysical properties) (D. Morales-Morales, E. Rivera et al.); DOI: 10.1039/C9DT00855A

- (N₆) Hexaazamacrocycles based on a bis(imino)pyridine (and on chiral trans-1,2-diaminocyclohexane or on achiral ethylenediamine) in heterodinuclear rare earth complexes with chirality transfer from the former to the latter macrocycle (J. Lisowski et al.); DOI: 10.1039/C9DT01318K

- (N₆) N-Rich aroylhydrazones (with dipyridylpyrazine groups) in Cu(II) multinuclear complexes as catalysts for the microwave-assisted peroxidative oxidation of xylenes to the corresponding methyl benzyl alcohols, tolualdehydes and toluic acids (M. Sutradhar, E. C. B. A. Alegria, A. J. L. Pombeiro et al.); DOI: 10.1039/C9DT02196E

- (N₈) A phthalocyanine bearing four S-pyridyl hanging moieties in a Cu(II) coordination polymer as an heterogeneous catechol oxidase biomimetic catalyst (3,5-di-tert-butylcatechol oxidation) (K. A. D. F. Castro, F. Figueira, M. M. Q. Simões et al.); DOI: 10.1039/C9DT00378A

- (N₁₀) Glycinehydroximate as ligand in a water-soluble polynuclear metallamacrocyclic Sr(II)–Cu(II) complex of the 15-MC-5 metallacrown type which undergoes easy replacement of Sr(II) by Y(III) and shows low toxicity (S. Y. Ketkov et al.). DOI: 10.1039/C9DT01368G

Although without involving metal coordination bonds, studies on nitrogen compounds (which nevertheless are potential nitrogen ligands) to disclose supramolecular self-assembly processes driven by non-covalent interactions, of significance towards the selective formation of functional aggregates or fabrication of nanostructures at the interface, are also included in this themed collection:

- A molecular tweezer with two acridinium moieties linked by a 1,3-dipyridylbenzene spacer displaying self-complementary (as a result of π–π stacking and hydrophobic interactions) and narcissistic self-sorting in water (H.-P. J. Rouville, V. Heitz et al.); DOI: 10.1039/C9DT01465A

- Porphyrins in truxene-porphyrin assembled structures studied on a highly oriented pyrolytic graphite surface by STM, STS and DFT calculations (Y. Liu, C. P. Gros, K. Deng, Q. Zeng et al.). DOI: 10.1039/C9DT01078E

The above works collected herein illustrate the variety of compositions, structures, properties and applications of nitrogen ligands and their complexes. These ligands have contributed greatly to the development of chemistry and related sciences, and particular emphasis on their design and application towards further promotion of sustainability under different viewpoints is expected.

The coordination ability of nitrogen is immense, taking into account not only the properties of this element, but also of its pending arms and of backbones to which it can belong to, apart from the features of the binding sites. It provides seeds for our imagination to sow fields of unlimited scope…

I would like to thank the authors for their valuable contributions and the reviewers, as well as the Royal Society of Chemistry (RSC) and Dalton Transactions for the publication of this themed collection and for the invitation to act as Guest Editor. Particular thanks are due to Mike Andrews (Deputy Editor), Andrew Shore (Executive Editor), Paige Boxhall (Publishing Editor) and the RSC team involved, for their kind assistance and competence.

Nevertheless, the assessment of this initiative will concern mainly the readers’ judgement and I hope they will find this themed collection to be a fruitful and inspiring motivation for their research and a contribution towards fostering the relevance of nitrogen ligands in science and its applications.

Acknowledgements

Thanks are due to the Fundação para a Ciência e Tecnologia (FCT, project UID/QUI/00100/2019), Portugal, for financial support.

References

1. A. Werner, Beitrag zur konstitution anorganischer verbindungen , Z. Anorg. Allg. Chem., 1893, 3, 267–330,  DOI:10.1002/zaac.18930030136.

---