: As stated by two of the seven papers [...]

Rigamonti L.

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Citation: Rigamonti, L. OligonuclearMetal Complexes with Schiff BaseLigands. Int. J. Mol. Sci. 2023, 24,11014. https://doi.org/10.3390/ijms241311014Received: 8 June 2023Accepted: 26 June 2023Published: 3 July 2023Copyright: © 2023 by the author.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/). International Journal of Molecular SciencesEditorialOligonuclear Metal Complexes with Schiff Base LigandsLuca RigamontiDipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Modena e Reggio Emilia, Via G. Campi103, 41125 Modena, Italy; luca.rigamonti@unimore.it; Tel.: +39-0592058646As stated by two of the seven papers [1,2] published in the Special Issue entitled‘Oligonuclear Metal Complexes with Schiff Base Ligands’ that I guest edited, the chemistryof Schiff bases began in 1864 due to the discovery made by Hugo Schiff [3], but even morethan one and a half centuries after their first synthesis, Schiff bases continue to surpriseinorganic chemists.The condensation of a carbonyl moiety RR’C=O (R, R’ = alkyl or aryl groups forketones, R’ = H for aldehydes) with a primary amine R”NH2 (R” = alkyl or aryl group)forms an iminic RR’C=NR” bond, referred to as azomethine or secondary aldimmine whenR’ = H [4], and the lone pair of the iminic nitrogen atom, along with being basic, is able toact as a ligand toward metal centers [5]. C=N is usually incorporated into more articulatedarchitectures in addition to other donor atoms, the so-called polydentate ligands [6], sincemetal complexes with macrocyclic and acyclic polydentate Schiff bases [7,8] show enhancedthermodynamic stability due to the chelating effect [9]. It is of course mandatory to name themost famous acyclic tetradentate N2O2 Schiff base N,N′-bis(salicylidene)ethylenediamine,H2salen, obtained via the condensation of two moles of salicylaldehyde, salH, and onemole of ethylenediamine, en, and known for almost one century [10]. Depending onthe disposition of the donor atoms in the organic skeleton, Schiff base ligands can formmononuclear or oligonuclear metal complexes and, for example, the salen scaffold suitablyhosts metal ions by occupying four coordination positions in a square planar fashion, thuspreferably forming mononuclear species [11–13].The aliphatic diamine en in H2salen can be replaced by the aromatic1,2-phenylenediamine, phen, obtaining the more rigid tetradentate N2O2 H2salphen [14,15].Janet Soleimannejad, E. Carolina Sañudo and co-workers suitably engineered this ligandby introducing cathecol and naphtol groups on salH in their paper entitled ‘Cathecol andNaphtol Groups in Salphen-Type Schiff Bases for the Preparation of Polynuclear Com-plexes’ [16]. Mononuclear copper(II) and cerium(IV) complexes could be suitably isolated,as well as a dinuclear iron(III) species with a bridging oxido ion, displaying a nearly linearFe–O–Fe bridge with very strong antiferromagnetic coupling constant J = −255 cm−1 (–JŜ1·Ŝ2 formalism) between the two metal ions. A tetranuclear cluster with mixed-valencemanganese(II)/(III) ions was also assembled when the ligand containing the cathecolfragments were employed, thanks to the two phenolato oxygen atoms in ortho position.Longer spacers between the two salH moieties can be also employed, such as forexample 1,3-diaminopropane [17,18], or even more articulated molecules like oxalyldihy-drazide, as shown by Manas Sutradhar, Luísa M. D. R. S. Martins and co-workers intheir contribution entitled ‘Aroylhydrazone Schiff Base Derived Cu(II) and V(V) Com-plexes: Efficient Catalysts towards Neat Microwave-Assisted Oxidation of Alcohols’ [19].The presence of further nitrogen and oxygen donor atoms along with the Schiff baseN′1,N′2-bis(2-hydroxybenzylidene)oxalohydrazide induces the formation of a hexanuclearcopper(II) cluster and a dinuclear oxidovanadium(V) species. Both complexes were testedas homogeneous catalyst precursors for the microwave-assisted neat oxidation of primaryor secondary alcohols to the corresponding carbonyl compounds (aldehydes for primaryalcohols and ketones for secondary alcohols) under low-power (5–10 W) irradiation usingInt. J. Mol. Sci. 2023, 24, 11014. https://doi.org/10.3390/ijms241311014 https://www.mdpi.com/journal/ijmsInt. J. Mol. Sci. 2023, 24, 11014 2 of 4aqueous tert-butyl hydroperoxide (TBHP) as an oxidizing agent without any added solvent.The copper(II) derivative exhibited higher catalytic activity than the vanadium(V) one forall of the tested substrates, and its performances were even better under milder reactionconditions than the reported aroylhydrazone copper(II) analogues [20].The reactivity of the shortened salen-type ligands H3salmp, H2salmen and H2sal(p-X)ben with variable para-substituent on the central aromatic ring (X = tBu, Me, H, F,Cl, CF3, NO2) towards the trivalent metal ions manganese(III) and iron(III) was insteadpresented in the research paper I co-authored entitled ‘Selective Formation, Reactivity,Redox and Magnetic Properties of MnIII and FeIII Dinuclear Complexes with ShortenedSalen-Type Schiff Base Ligands’ [21]. The one-carbon bridge between the two iminicnitrogen atoms drives toward the selective formation of dinuclear complexes, togetherwith hydrolytic C–N bond breakage in the case of iron(III). The antiferromagnetic (AFM)coupling constants J (−2J Ŝ1·Ŝ2 formalism) between the metal centers had values around−13 cm−1 for manganese(III) compounds, among the largest AFM coupling constantsreported so far for dinuclear MnIII2 derivatives, while values between −3 and −10 cm−1were obtained for iron(III) compounds.Schiff bases derived from 2-hydroxy-5-methylisophthalaldehyde and histamine or2-(2-aminoethyl)pyridine were instead employed by Magdalena Barwiolek, AnnaKaczmarek-Kędziera and co-workers to assemble dinuclear copper(II) derivatives and pre-sented in their article entitled ‘Dinuclear Copper(II) Complexes with Schiff Bases Derivedfrom 2-Hydroxy-5-Methylisophthalaldehyde and Histamine or 2-(2-Aminoethyl)pyridineand Their Application as Magnetic and Fluorescent Materials in Thin Film Deposition’ [22].The complexes were studied for their magnetic properties, revealing AFM interactions be-tween the two metal centers, and for their optical behavior, both in the bulk and depositedon Si(111) by spin coating, showing fluorescence between 489 and 509 and 460 and 464 nmfor the compounds and the layers, respectively.The last research paper by Catherine E. Housecroft and co-workers entitled ‘SchiffBase Ancillary Ligands in Bis(diimine) Copper(I) Dye-Sensitized Solar Cells’ reported anelegant surfaces-as-ligands, surfaces-as-complexes (SALSAC) approach [23] for obtainingthe heteroleptic copper(I) complexes [Cu(Lanchor)(Lancillary)]+ assembled on FTO-TiO2 elec-trodes and incorporated as dyes into n-type dye-sensitized solar cells (DSCs), where Lanchoris a phosphonic acid-functionalized 2,2′-bipyridyl anchoring ligand and Lancillary is a 6,6′-dimethyl-2,2′-bipyridyl ligand bearing N-arylmethaniminyl substituents [24]. Comparisonsbetween performances of DSCs containing the Schiff base complexes with those sensitizedby analogous dyes lacking the imine bond indicate that the latter prevents efficient electrontransfer across the dye.The first review of the Special Issue by Demetrios I. Tzimopoulos, Malgorzata Holyn-ska, Spyros P. Perlepes and co-workers entitled ‘Oligonuclear Actinoid Complexes withSchiff Bases as Ligands—Older Achievements and Recent Progress’ deals with the coordi-nation chemistry of Schiff bases with actinoid (5f elements) ions [2]. This is an emergingarea in recent years, compared to the more mature chemistry of transition metals and lan-thanides. The handling and recycling of nuclear materials, the recovery of trans-{UVIO2}2+from the oceans and the catalytic activation of small molecules are among the possibleapplications of Schiff base ligands with actinides. This review thoroughly covers aspects ofsynthetic chemistry, reactivity, and the properties of dinuclear and oligonuclear actinoidcomplexes based on Schiff base ligands within the last 20 years in an extensive way andwith clear presentation.The second review in this Special Issue by Barbara Miroslaw entitled ‘Homo- andHetero-Oligonuclear Complexes of Platinum Group Metals (PGM) Coordinated by ImineSchiff Base Ligands’ presents the most recent concepts on the construction of homo- andhetero-oligonuclear Schiff base coordination compounds of PGMs (iridium, osmium, pal-ladium, platinum, rhodium, and ruthenium) [1]. Ranging from mono- to poly-dentateSchiff bases, this review elegantly shows the potency of these ligands to form variegateInt. J. Mol. Sci. 2023, 24, 11014 3 of 4coordination and metallorganic architectures with a very efficient visual approach, andalways focusing on the potential applications of the obtained compounds.All papers and reviews of the Special Issue have added fundamental knowledge inthe field of Schiff bases and related oligonuclear metal complexes. I personally thank theauthors for participating with their extraordinary articles, which are worthy of reading andbeing used as references in the ongoing research on Schiff base complexes.Acknowledgments: I strongly acknowledge Alessandro Pasini, Università degli Studi di Milano,my mentor during my PhD and in the early stages of my career, for his continuous support and forfostering in me a passion for Schiff base chemistry.Conflicts of Interest: The author declares no conflict of interest.References1. Miroslaw, B. Homo- and Hetero-Oligonuclear Complexes of Platinum Group Metals (PGM) Coordinated by Imine Schiff BaseLigands. Int. J. Mol. Sci. 2020, 21, 3493. [CrossRef]2. Tsantis, S.T.; Tzimopoulos, D.I.; Holynska, M.; Perlepes, S.P. Oligonuclear Actinoid Complexes with Schiff Bases as Ligands—Older Achievements and Recent Progress. Int. J. Mol. Sci. 2020, 21, 555. [CrossRef]3. Schiff, H. Mittheilungen aus dem Universitätslaboratorium in Pisa: Eine neue Reihe organischer Basen. Ann. Der Chem. UndPharm. 1864, 131, 118–119. [CrossRef]4. Carey, F.A.; Sundberg, R.J. Advanced Organic Chemistry, 5th ed.; Springer: New York, NY, USA, 2007; ISBN 978-0-387-44897-8.5. Qin, W.; Long, S.; Panunzio, M.; Biondi, S. Schiff Bases: A Short Survey on an Evergreen Chemistry Tool. Molecules 2013, 18,12264–12289. [CrossRef]6. Liu, X.; Hamon, J.-R. Recent Developments in Penta-, Hexa- and Heptadentate Schiff Base Ligands and Their Metal Complexes.Coord. Chem. Rev. 2019, 389, 94–118. [CrossRef]7. Vigato, P.A.; Peruzzo, V.; Tamburini, S. Acyclic and Cyclic Compartmental Ligands: Recent Results and Perspectives. Coord. Chem.Rev. 2012, 256, 953–1114. [CrossRef]8. Gavey, E.L.; Pilkington, M. Coordination Complexes of 15-Membered Pentadentate Aza, Oxoaza and Thiaaza Schiff BaseMacrocycles “Old Complexes Offer New Attractions”. Coord. Chem. Rev. 2015, 296, 125–152. [CrossRef]9. Cotton, F.A. Advanced Inorganic Chemistry; Wiley: Bengaluru, India, 2008; ISBN 978-81-265-1338-3.10. Pfeiffer, P.; Breith, E.; Lübbe, E.; Tsumaki, T. Tricyclische orthokondensierte Nebenvalenzringe. Justus Liebig’s Ann. Der Chem.1933, 503, 84–130. [CrossRef]11. Hobday, M.D.; Smith, T.D. N,N′-Ethylenebis(Salicylideneiminato) Transition Metal Ion Chelates. Coord. Chem. Rev. 1973, 9,311–337. [CrossRef]12. Rigamonti, L.; Demartin, F.; Forni, A.; Righetto, S.; Pasini, A. Copper(II) Complexes of Salen Analogues with Two DifferentlySubstituted (Push−Pull) Salicylaldehyde Moieties. A Study on the Modulation of Electronic Asymmetry and Nonlinear OpticalProperties. Inorg. Chem. 2006, 45, 10976–10989. [CrossRef]13. Mazzoni, R.; Roncaglia, F.; Rigamonti, L. When the Metal Makes the Difference: Template Syntheses of Tridentate and TetradentateSalen-Type Schiff Base Ligands and Related Complexes. Crystals 2021, 11, 483. [CrossRef]14. Leoni, L.; Dalla Cort, A. The Supramolecular Attitude of Metal–Salophen and Metal–Salen Complexes. Inorganics 2018, 6, 42.[CrossRef]15. Consiglio, G.; Oliveri, I.; Failla, S.; Di Bella, S. Supramolecular Aggregation of a New Substituted Bis(Salicylaldiminato)Zinc(II)Schiff-Base Complex Derived from Trans-1,2-Diaminocyclohexane. Inorganics 2018, 6, 8. [CrossRef]16. Gholizadeh Dogaheh, S.; Barbero, S.; Barrientos, J.; Janczak, J.; Soleimannejad, J.; Sañudo, E.C. Cathecol and Naphtol Groups inSalphen-Type Schiff Bases for the Preparation of Polynuclear Complexes. Int. J. Mol. Sci. 2020, 21, 3574. [CrossRef]17. Rigamonti, L.; Forni, A.; Righetto, S.; Pasini, A. Push-Pull Unsymmetrical Substitution in Nickel(II) Complexes with TetradentateN2O2 Schiff Base Ligands: Synthesis, Structures and Linear-Nonlinear Optical Studies. Dalton Trans. 2019, 48, 11217–11234.[CrossRef]18. Rigamonti, L.; Forni, A. Effect of Crystal Packing and Coordinated Solvent Molecules on Metal-Ligand Bond Distances in LinearTrinuclear Nickel Compounds with Bridging Acetato and Schiff Base Ligands. Inorg. Chim. Acta 2018, 473, 216–222. [CrossRef]19. Sutradhar, M.; Roy Barman, T.; Pombeiro, A.J.L.; Martins, L.M.D.R.S. Aroylhydrazone Schiff Base Derived Cu(II) and V(V)Complexes: Efficient Catalysts towards Neat Microwave-Assisted Oxidation of Alcohols. Int. J. Mol. Sci. 2020, 21, 2832. [CrossRef]20. Tojo, G.; Fernández, M.I. Oxidation of Alcohols to Aldehydes and Ketones: A Guide to Current Common Practice; Basic reactions inorganic synthesis; Springer: New York, NY, USA, 2006; ISBN 978-0-387-25725-9.21. Rigamonti, L.; Zardi, P.; Carlino, S.; Demartin, F.; Castellano, C.; Pigani, L.; Ponti, A.; Ferretti, A.M.; Pasini, A. Selective Formation,Reactivity, Redox and Magnetic Properties of MnIII and FeIII Dinuclear Complexes with Shortened Salen-Type Schiff Base Ligands.Int. J. Mol. Sci. 2020, 21, 7882. [CrossRef]Int. J. Mol. Sci. 2023, 24, 11014 4 of 422. Barwiolek, M.; Kaczmarek-Kędziera, A.; Muziol, T.M.; Jankowska, D.; Jezierska, J.; Bieńko, A. Dinuclear Copper(II) Complexeswith Schiff Bases Derived from 2-Hydroxy-5-Methylisophthalaldehyde and Histamine or 2-(2-Aminoethyl)Pyridine and TheirApplication as Magnetic and Fluorescent Materials in Thin Film Deposition. Int. J. Mol. Sci. 2020, 21, 4587. [CrossRef]23. Malzner, F.; Housecroft, C.; Constable, E. The Versatile SALSAC Approach to Heteroleptic Copper(I) Dye Assembly in Dye-Sensitized Solar Cells. Inorganics 2018, 6, 57. [CrossRef]24. Lüthi, E.; Forero Cortés, P.A.; Prescimone, A.; Constable, E.C.; Housecroft, C.E. Schiff Base Ancillary Ligands in Bis(Diimine)Copper(I) Dye-Sensitized Solar Cells. Int. J. Mol. Sci. 2020, 21, 1735. [CrossRef] [PubMed]Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individualauthor(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury topeople or property resulting from any ideas, methods, instructions or products referred to in the content.

Oligonuclear Metal Complexes with Schiff Base Ligands

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Oligonuclear Metal Complexes with Schiff Base Ligands

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