International Journal of Molecular Sciences

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Oligonuclear Metal Complexes with Schiff Base Ligands

by Luca Rigamonti

Int. J. Mol. Sci. 2023, 24(13), 11014; https://doi.org/10.3390/ijms241311014 - 03 Jul 2023

Cited by 3 | Viewed by 1088

Abstract

As stated by two of the seven papers [...] Full article

(This article belongs to the Special Issue Oligonuclear Metal Complexes with Schiff Base Ligands)

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19 pages, 2749 KiB

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Selective Formation, Reactivity, Redox and Magnetic Properties of Mn^III and Fe^III Dinuclear Complexes with Shortened Salen-Type Schiff Base Ligands

by Luca Rigamonti, Paolo Zardi, Stefano Carlino, Francesco Demartin, Carlo Castellano, Laura Pigani, Alessandro Ponti, Anna Maria Ferretti and Alessandro Pasini

Int. J. Mol. Sci. 2020, 21(21), 7882; https://doi.org/10.3390/ijms21217882 - 23 Oct 2020

Cited by 12 | Viewed by 2483

Abstract

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 [...] Read more.

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) is presented. The selective formation of the dinuclear complexes [M₂(μ-salmp)₂], M = Mn (1a), Fe (2a), [M₂(μ-salmen)₂(μ-OR)₂)], R = Et, Me, H and M = Mn (3a–c) or Fe (4a–c), and (M₂(μ-sal[p-X]ben)₂(μ-OMe)₂), X = tBu, Me, H, F, Cl, CF₃, NO₂ and M = Mn (5a–g) or Fe (6a–g), could be identified by reaction of the Schiff bases with metal salts and the base NEt₃, and their characterization through elemental analysis, infrared spectroscopy, mass spectrometry and single-crystal X-ray diffraction of 2a·2AcOEt, 2a·2CH₃CN and 3c·2DMF was performed. In the case of iron(III) and H₃salmp, when using NaOH as a base instead of NEt₃, the dinuclear complexes [Fe₂(μ-salmp)(μ-OR)(salim)₂], R = Me, H (2b,c) could be isolated and spectroscopically characterized, including the crystal structure of 2b·1.5H₂O, which showed that rupture of one salmp³⁻ to two coordinated salim⁻ ligands and release of one salH molecule occurred. The same hydrolytic tendency could be identified with sal(p-X)ben ligands in the case of iron(III) also by using NEt₃ or upon standing in solution, while manganese(III) did not promote such a C–N bond breakage. Cyclic voltammetry studies were performed for 3b, 4b, 5a and 6a, revealing that the iron(III) complexes can be irreversibly reduced to the mixed-valence Fe^IIFe^III and Fe^II₂ dinuclear species, while the manganese(III) derivatives can be reversibly oxidized to either the mixed-valence Mn^IIIMn^IV or to the Mn^IV₂ dinuclear species. The super-exchange interaction between the metal centers, mediated by the bridging ligands, resulted in being antiferromagnetic (AFM) for the selected dinuclear compounds 3b, 4b, 5a, 5e,5f, 6a and 6e. The coupling constants J (–2JŜ₁·Ŝ₂ formalism) had values around −13 cm⁻¹ for manganese(III) compounds, among the largest AFM coupling constants reported so far for dinuclear Mn^III₂ derivatives, while values between −3 and −10 cm⁻¹ were obtained for iron(III) compounds. Full article

(This article belongs to the Special Issue Oligonuclear Metal Complexes with Schiff Base Ligands)

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27 pages, 7173 KiB

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Dinuclear Copper(II) Complexes with Schiff Bases Derived from 2-Hydroxy-5-Methylisophthalaldehyde and Histamine or 2-(2-Aminoethyl)pyridine and Their Application as Magnetic and Fluorescent Materials in Thin Film Deposition

by Magdalena Barwiolek, Anna Kaczmarek-Kędziera, Tadeusz M. Muziol, Dominika Jankowska, Julia Jezierska and Alina Bieńko

Int. J. Mol. Sci. 2020, 21(13), 4587; https://doi.org/10.3390/ijms21134587 - 28 Jun 2020

Cited by 13 | Viewed by 3259

Abstract

Two Cu(II) complexes, 1 and 2, with tridentate Schiff bases derived from 2-hydroxy-5-methylisophthalaldehyde and histamine HL1 or 2-(2-aminoethyl)pyridine HL2, respectively, were obtained and characterized by X-ray crystallography, spectroscopic (UV-vis, fluorescence, IR, and EPR), magnetic, and thermal methods. Despite the fact that [...] Read more.

Two Cu(II) complexes, 1 and 2, with tridentate Schiff bases derived from 2-hydroxy-5-methylisophthalaldehyde and histamine HL1 or 2-(2-aminoethyl)pyridine HL2, respectively, were obtained and characterized by X-ray crystallography, spectroscopic (UV-vis, fluorescence, IR, and EPR), magnetic, and thermal methods. Despite the fact that the chelate formed by the NNO ligand donors (C26-C25H₂-C24H₂-N23=C23H-C22-C19Ph(O1)-C2(Ph)-C3H=N3-C4H₂-C5H₂-C6 fragment) are identical, as well as the synthesis of Cu(II) complexes (Cu:L = 2:1 molar ratio) was performed in the same manner, the structures of the complexes differ significantly. The complex 1, {[Cu₂(L1)Cl₂]₂[CuCl₄]}·2MeCN·2H₂O, consists of [Cu₂(L1)Cl₂]⁺ units in which Cu(II) ions are bridged by the HL1 ligand oxygen and each of these Cu(II) ions is connected with Cu(II) ions of the next dimeric unit via two bridging Cl⁻ ions to form a chain structure. In the dinuclear [Cu₂(L2)Cl₃]⋅0.5MeCN complex 2, each Cu(II) is asymmetrically bridged by the ligand oxygen and chloride anions, whereas the remaining chloride anions are apically bound to Cu(II) cations. In contrast to the complex 1, the square-pyramidal geometry of the both Cu(II) centers is strongly distorted. The magnetic study revealed that antiferromagnetic interactions in the complex 2 are much stronger than in the complex 1, which was corresponded with magneto-structural examination. Thin layers of the studied Cu(II) complexes were deposited on Si(111) by the spin coating method and studied by scanning electron microscopy (SEM/EDS), atomic force microscopy (AFM), and fluorescence spectroscopy. The Cu(II) complexes and their thin layers exhibited fluorescence between 489–509 nm and 460–464 nm for the compounds and the layers, respectively. Additionally, DFT calculations were performed to explain the structures and electronic spectral properties of the ligands. Full article

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14 pages, 2993 KiB

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Cathecol and Naphtol Groups in Salphen-Type Schiff Bases for the Preparation of Polynuclear Complexes

by Samira Gholizadeh Dogaheh, Sara Barbero, Joel Barrientos, Jan Janczak, Janet Soleimannejad and E. Carolina Sañudo

Int. J. Mol. Sci. 2020, 21(10), 3574; https://doi.org/10.3390/ijms21103574 - 18 May 2020

Cited by 4 | Viewed by 3159

Abstract

In this paper, we show a strategy to modify salphen-type Schiff base ligands with naphtol (SYML1) and pyrocathecol (2,3-dihydroxyphenyl) groups (SYML2), or a combination of both (ASYML). Each of these ligands can be used to obtain polynuclear metal complexes following two different strategies. [...] Read more.

In this paper, we show a strategy to modify salphen-type Schiff base ligands with naphtol (SYML1) and pyrocathecol (2,3-dihydroxyphenyl) groups (SYML2), or a combination of both (ASYML). Each of these ligands can be used to obtain polynuclear metal complexes following two different strategies. One relies on using metals that are either too large for the N₂O₂ cavity or not fond of coordination number 4 and the other one relies on forcing the polynuclear species by adding functional groups to the hydroxybenzaldehayde in order to have extra coordination sites in the ligand. We report and characterize the mononuclear complexes SYML1-Cu and SYML1-Ce, along with the dinuclear complex SYML1-Fe and the tetranuclear species SYML2-Mn. The asymmetric ligand ASYML routinely hydrolyzes into the symmetric ligands in the reaction mixtures. SYML1-Fe displays a nearly linear Fe-O-Fe bridge with very strong antiferromagnetic coupling between the Fe(III) ions. Full article

(This article belongs to the Special Issue Oligonuclear Metal Complexes with Schiff Base Ligands)

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15 pages, 2610 KiB

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Aroylhydrazone Schiff Base Derived Cu(II) and V(V) Complexes: Efficient Catalysts towards Neat Microwave-Assisted Oxidation of Alcohols

by Manas Sutradhar, Tannistha Roy Barman, Armando J. L. Pombeiro and Luísa M. D. R. S. Martins

Int. J. Mol. Sci. 2020, 21(8), 2832; https://doi.org/10.3390/ijms21082832 - 18 Apr 2020

Cited by 16 | Viewed by 2449

Abstract

A new hexa-nuclear Cu(II) complex [Cu₃(μ₂-1κNO²,2κNO²-L)(μ-Cl)₂(Cl)(MeOH)(DMF)₂]₂ (1), where H₄L = N′¹,N′²-bis(2-hydroxybenzylidene)oxalohydrazide, was synthesized and fully characterized by IR spectroscopy, ESI-MS, [...] Read more.

A new hexa-nuclear Cu(II) complex [Cu₃(μ₂-1κNO²,2κNO²-L)(μ-Cl)₂(Cl)(MeOH)(DMF)₂]₂ (1), where H₄L = N′¹,N′²-bis(2-hydroxybenzylidene)oxalohydrazide, was synthesized and fully characterized by IR spectroscopy, ESI-MS, elemental analysis, and single crystal X-ray diffraction. Complex 1 and the dinuclear oxidovanadium(V) one [{VO(OEt)(EtOH)}₂(1κNO²,2κNO²-L)]·2H₂O (2) were used as catalyst precursors for the neat oxidation of primary (cinnamyl alcohol) and secondary (1-phenyl ethanol, benzhydrol) benzyl alcohols and of the secondary aliphatic alcohol cyclohexanol, under microwave irradiation using tert-butyl hydroperoxide (TBHP) as oxidant. Oxidations proceed via radical mechanisms. The copper(II) compound 1 exhibited higher catalytic activity than the vanadium(V) complex 2 for all the tested alcohol substrates. The highest conversion was found for 1-phenylethanol, yielding 95.3% of acetophenone in the presence of 1 and in solvent and promoter-free conditions. This new Cu(II) complex was found to exhibit higher activity under milder reaction conditions than the reported aroylhydrazone Cu(II) analogues. Full article

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19 pages, 3706 KiB

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Schiff Base Ancillary Ligands in Bis(diimine) Copper(I) Dye-Sensitized Solar Cells

by Elias Lüthi, Paola Andrea Forero Cortés, Alessandro Prescimone, Edwin C. Constable and Catherine E. Housecroft

Int. J. Mol. Sci. 2020, 21(5), 1735; https://doi.org/10.3390/ijms21051735 - 03 Mar 2020

Cited by 12 | Viewed by 2962

Abstract

Five 6,6′-dimethyl-2,2′-bipyridine ligands bearing N-arylmethaniminyl substituents in the 4- and 4′-positions were prepared by Schiff base condensation in which the aryl group is Ph (1), 4-tolyl (2), 4-^tBuC₆H₄ (3), 4-MeOC₆ [...] Read more.

Five 6,6′-dimethyl-2,2′-bipyridine ligands bearing N-arylmethaniminyl substituents in the 4- and 4′-positions were prepared by Schiff base condensation in which the aryl group is Ph (1), 4-tolyl (2), 4-^tBuC₆H₄ (3), 4-MeOC₆H₄ (4), and 4-Me₂NC₆H₄ (5). The homoleptic copper(I) complexes [CuL₂][PF₆] (L = 1–5) were synthesized and characterized, and the single crystal structure of [Cu(1)₂][PF₆]·Et₂O was determined. By using the “surfaces-as-ligands, surfaces-as-complexes” (SALSAC) approach, the heteroleptic complexes [Cu(6)(L_ancillary)]⁺ in which 6 is the anchoring ligand ((6,6′-dimethyl-[2,2′-bipyridine]-4,4′-diyl)bis(4,1-phenylene))bis(phosphonic acid)) and L_ancillary = 1–5 were assembled on FTO-TiO₂ electrodes and incorporated as dyes into n-type dye-sensitized solar cells (DSCs). Data from triplicate, fully-masked DSCs for each dye revealed that the best-performing sensitizer is [Cu(6)(1)]⁺, which exhibits photoconversion efficiencies (η) of up to 1.51% compared to 5.74% for the standard reference dye N719. The introduction of the electron-donating MeO and Me₂N groups (L_ancillary = 4 and 5) is detrimental, leading to a decrease in the short-circuit current densities and external quantum efficiencies of the solar cells. In addition, a significant loss in open-circuit voltage is observed for DSCs sensitized with [Cu(6)(5)]⁺, which contributes to low values of η for this dye. Comparisons between performances of DSCs containing [Cu(6)(1)]⁺ and [Cu(6)(4)]⁺ with those sensitized by analogous dyes lacking the imine bond indicate that the latter prevents efficient electron transfer across the dye. Full article

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Review

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28 pages, 6960 KiB

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Homo- and Hetero-Oligonuclear Complexes of Platinum Group Metals (PGM) Coordinated by Imine Schiff Base Ligands

by Barbara Miroslaw

Int. J. Mol. Sci. 2020, 21(10), 3493; https://doi.org/10.3390/ijms21103493 - 15 May 2020

Cited by 26 | Viewed by 4763

Abstract

Chemistry of Schiff base (SB) ligands began in 1864 due to the discovery made by Hugo Schiff (Schiff, H., Justus Liebigs Ann. der Chemie 1864, 131 (1), 118–119). However, there is still a vivid interest in coordination compounds based on imine ligands. The [...] Read more.

Chemistry of Schiff base (SB) ligands began in 1864 due to the discovery made by Hugo Schiff (Schiff, H., Justus Liebigs Ann. der Chemie 1864, 131 (1), 118–119). However, there is still a vivid interest in coordination compounds based on imine ligands. The aim of this paper is to review the most recent concepts on construction of homo- and hetero-oligonuclear Schiff base coordination compounds narrowed down to the less frequently considered complexes of platinum group metals (PGM). The combination of SB and PGM in oligonuclear entities has several advantages over mononuclear or polynuclear species. Such complexes usually exhibit better electroluminescent, magnetic and/or catalytic properties than mononuclear ones due to intermetallic interactions and frequently have better solubility than polymers. Various construction strategies of oligodentate imine ligands for coordination of PGM are surveyed including simple imine ligands, non-innocent 1,2-diimines, chelating imine systems with additional N/O/S atoms, classic N₂O₂-compartmental Schiff bases and their modifications resulting in acyclic fused ligands, macrocycles such as calixsalens, metallohelical structures, nano-sized molecular wheels and hybrid materials incorporating mesoionic species. Co-crystallization and formation of metallophilic interactions to extend the mononuclear entities up to oligonuclear coordination species are also discussed. Full article

(This article belongs to the Special Issue Oligonuclear Metal Complexes with Schiff Base Ligands)

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49 pages, 18531 KiB

Open AccessReview

Oligonuclear Actinoid Complexes with Schiff Bases as Ligands—Older Achievements and Recent Progress

by Sokratis T. Tsantis, Demetrios I. Tzimopoulos, Malgorzata Holynska and Spyros P. Perlepes

Int. J. Mol. Sci. 2020, 21(2), 555; https://doi.org/10.3390/ijms21020555 - 15 Jan 2020

Cited by 35 | Viewed by 5422

Abstract

Even 155 years after their first synthesis, Schiff bases continue to surprise inorganic chemists. Schiff-base ligands have played a major role in the development of modern coordination chemistry because of their relevance to a number of interdisciplinary research fields. The chemistry, properties and [...] Read more.

Even 155 years after their first synthesis, Schiff bases continue to surprise inorganic chemists. Schiff-base ligands have played a major role in the development of modern coordination chemistry because of their relevance to a number of interdisciplinary research fields. The chemistry, properties and applications of transition metal and lanthanoid complexes with Schiff-base ligands are now quite mature. On the contrary, the coordination chemistry of Schiff bases with actinoid (5f-metal) ions is an emerging area, and impressive research discoveries have appeared in the last 10 years or so. The chemistry of actinoid ions continues to attract the intense interest of many inorganic groups around the world. Important scientific challenges are the understanding the basic chemistry associated with handling and recycling of nuclear materials; investigating the redox properties of these elements and the formation of complexes with unusual metal oxidation states; discovering materials for the recovery of trans-{U^VIO₂}²⁺ from the oceans; elucidating and manipulating actinoid-element multiple bonds; discovering methods to carry out multi-electron reactions; and improving the 5f-metal ions’ potential for activation of small molecules. The study of 5f-metal complexes with Schiff-base ligands is a currently “hot” topic for a variety of reasons, including issues of synthetic inorganic chemistry, metalosupramolecular chemistry, homogeneous catalysis, separation strategies for nuclear fuel processing and nuclear waste management, bioinorganic and environmental chemistry, materials chemistry and theoretical chemistry. This almost-comprehensive review, covers aspects of synthetic chemistry, reactivity and the properties of dinuclear and oligonuclear actinoid complexes based on Schiff-base ligands. Our work focuses on the significant advances that have occurred since 2000, with special attention on recent developments. The review is divided into eight sections (chapters). After an introductory section describing the organization of the scientific information, Sections 2 and 3 deal with general information about Schiff bases and their coordination chemistry, and the chemistry of actinoids, respectively. Section 4 highlights the relevance of Schiff bases to actinoid chemistry. Sections 5–7 are the “main menu” of the scientific meal of this review. The discussion is arranged according the actinoid (only for Np, Th and U are Schiff-base complexes known). Sections 5 and 7 are further arranged into parts according to the oxidation states of Np and U, respectively, because the coordination chemistry of these metals is very much dependent on their oxidation state. In Section 8, some concluding comments are presented and a brief prognosis for the future is attempted. Full article

(This article belongs to the Special Issue Oligonuclear Metal Complexes with Schiff Base Ligands)

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