Exploration of Ruthenium(II/III/VI)–Salen Complexes: From Synthesis to Functional Applications

Abstract

This review provides a comprehensive overview of recent advances in the synthesis, structural characterization, and applications of Ru(II), Ru(III), and Ru(VI) complexes, which bear tetradentate Schiff bases of salen type. Ruthenium complexes exhibit catalytic, electrochemical, and biological properties, serving as multifunctional platforms that integrate fundamental aspects of coordination chemistry with potential practical applications.

1. Introduction

In recent years, ruthenium–salen complexes have been reported as a highly versatile class of compounds, attracting considerable attention due to their diverse potential applications across multiple scientific disciplines. In the context of homogeneous catalysis, these compounds have been the subject of extensive research in the field of oxidation reactions [1,2,3,4,5,6,7,8,9,10,11,12]. The tunable electronic and steric properties of these compounds are conducive to high levels of selectivity, efficiency, and robustness under a range of reaction conditions. Beyond catalysis, their remarkable photophysical features, such as strong absorption, tunable emission, and nonlinear optical responses, have positioned these complexes as promising candidates for the development of advanced optical materials [13,14,15,16,17,18]. Furthermore, within the domain of medicinal chemistry, ruthenium–salen complexes have exhibited remarkable biological activities, including anticancer and antimicrobial properties, along with potential applications as diagnostic and therapeutic agents [18,19,20]. Concurrent with these developments, their rich and reversible redox chemistry has engendered opportunities in electrochemical applications [21], including molecular sensing, electrocatalysis, and the design of novel energy storage systems. The salen type Schiff base ligands used for preparation of the Ru–salen compounds are synthesized via the condensation of two equivalents of an ortho-hydroxy substituted aromatic aldehyde, most commonly a salicylaldehyde derivative, with a bidentate diamine. The resulting diimine ligand framework is characterized by its ability to chelate metal centers through both nitrogen and oxygen donor atoms, making it highly versatile in coordination chemistry. In the context of an octahedral ruthenium–salen complex, accompanied by two ancillary ligands, three distinct configurations are conceivable. As illustrated in Figure 1, the structures in question are as follows: trans, with two apical ancillary ligands, cis-α, with two equatorial ligands, and cis-β, with one apical and one equatorial ligand [22,23]. The N₂O₂ donor atoms of the salen ligand and the ruthenium ion in the trans complexes are approximately coplanar, while at least one of the two oxygen donor atoms occupies the axial coordination site in the cis-α or cis-β complexes. In the case of monodentate ancillary ligands, salen–metal complexes typically adopt a trans configuration.

This review highlights recent developments in the synthesis and functional applications of ruthenium(II), ruthenium(III), and ruthenium(VI) salen complexes (salen ligands, herein designated as Lⁿsalen, are presented in Figure 2), emphasizing their versatile coordination chemistry, tunable redox properties, and structural diversity. Ru–salen complexes represent promising multifunctional platforms for both fundamental research and practical applications.

2. Ruthenium(II/III/VI)–Salen Complexes—Synthesis and Crystal Structures

Mononuclear ruthenium(II/III/VI)–salen complexes 1a, 2a, 3a, 3b, 4a, 5a, 5b, 6a, 6b, 6c, 7a, 8a, 9a, 9b, 9c, 10a, 11a, 12a, 12b, 13a, 13b, 13c, 13d, 14a, 14b, 15a, and 15b (

Table 1. The formula and selected bond distances of Ru(II)–, Ru(III)–, and Ru(VI)–salen complexes [24].

The Complex Formula	CSD Refcodes	Ru–Nimine (Å)	Ru–Ophen (Å)	Ref.
1a	IQIBUF	2.030(3) 2.018(4)	1.977(3) 1.971(3)	[25]
2a	EVEHAQ	1.994(3) 1.988(2)	2.015(2) 2.021(2)	[26]
3a	LEJLIY	1.984(4) 1.990(4)	2.030(3) 2.023(2)	[27]
3b	LEJLOE	1.981 1.981	2.032 2.032	[27]
4a	NIGDEP	1.989(3) 1.988(3)	2.015(2) 2.028(2)	[28]
5a	QAKJIZ	1.987(2) 1.989(3)	2.024(2) 2.015(2)	[29]
5b	QAKJOF	1.990(3) 2.067(3)	2.023(2) 2.027(2)	[29]
6a	ERUQIU	2.123(4) 2.054(4)	2.060(4) 2.089(3)	[30]
6b	ERUQOA	2.163(3) 2.036(3)	2.078(2) 2.101(2)	[30]
6c	ERUQUG	2.055(2) 2.178(2)	2.070(2) 2.082(2)	[30]
7a	GOJKEY	1.980(7) 1.975(7)	2.025(6) 2.004(4)	[31]
8a	HUKLEH	1.958(8) 1.985(9)	2.008(6) 2.009(7)	[32]
9a	KUSVAY	2.026(3) 2.052(4)	2.081(4) 2.071(3)	[33]
9b	KUSVEC	2.031(3) 2.202(3)	2.064(2) 2.060(2)	[33]
9c	KUSXII	2.073(2) 2.052(2)	2.060(1) 2.101(1)	[33]
10a	LEQBAN	2.020(4) 2.022(4)	2.025(3) 2.023(4)	[34]
11a	MEKGIV	2.003(6) 1.983(6)	2.008(5) 2.007(6)	[35]
12a	YOBXAR	1.983(3) 2.008(3)	2.016(3) 2.008(3)	[36]
12b	YOBXEV	1.981(6) 1.966(7)	2.102(5) 2.105(5)	[36]
13a	ZIBNOQ	1.987(2) 1.992(2)	2.011(2) 2.022(2)	[37]
13b	ZIBNUW	2.009(5) 1.986(5)	2.016(4) 2.014(4)	[37]
13c	ZIBPAE	2.035(4) 2.051(5)	2.007(4) 2.014(3)	[37]
13d	ZIBPEI	2.017(4) 2.005(4)	2.004(3) 2.012(3)	[37]
14a	CENXAY	2.031(5) 2.028(5)	2.037(4) 2.035(4)	[38]
14b	CENXIG	2.005(3) 2.007(2)	2.027(2) 2.033(3)	[38]
15a	ECEZIZ	2.065(2) 2.038(2)	2.092(1) 2.041(1)	[39]
15b	ECEZOF	2.066(3) 2.075(4)	2.078(3) 2.076(3) 2.092(3) 2.218(4) 2.066(3)	[39]
16a	GOJHAR	2.004(6) 1.992(4) 1.996(4) 2.008(4)	2.026(4) 2.016(3) 2.019(4) 2.024(4)	[31]
16b	GOJGUK	1.999(4) 2.021(6) 1.993(5) 1.999(5)	2.026(3) 2.029(5) 2.028(4) 2.030(4)	[31]
16c	GOJKAU	1.965(7) 1.993(9) 1.993(8) 1.990(8)	2.025(6) 2.000(5) 2.004(5) 2.022(7)	[31]
17a	VIVCOV	1.99(3) 1.94(2) 2.03(2) 2.01(2)	2.03(1) 1.95(2) 2.06(1) 2.01(1)	[40]
18a	YUFTAW	1.982(9) 2.000(9)	2.018(6) 2.025(7)	[41]
18b	YUFTEA	1.971(7) 1.967(9) 1.992(6) 1.994(8)	2.019(7) 2.008(6) 2.016(6) 2.009(5)	[41]
19a	KILRAB	1.94(1) 1.983(7) 2.02(1) 1.980(8)	1.988(8) 1.981(6) 1.987(6) 2.042(7)	[42]
19b	KILREF	2.00(1) 1.997(9) 2.01(1) 2.002(7)	2.009(6) 1.994(9) 2.017(6) 2.010(9)	[42]
19c	KILRIJ	2.01(1) 1.97(1) 1.99(1) 1.97(1)	1.976(9) 2.002(7) 2.016(8) 1.980(9)	[42]
19d	KILROP	2.008(4) 1.996(4) 1.984(5) 1.987(5)	1.990(4) 2.006(4) 1.995(4) 1.989(4)	[42]

) were prepared using different synthetic strategies. For instance, complexes 1a, 9a, 9c, 10a, 11a, 12a, 12b, 13a, 13b, 13c, 13d, 15a, and 15b were synthesized through an indirect route, involving the reaction of ruthenium precursors with the respective tetradentate Schiff base ligands. Complexes 14a and 14b, on the other hand, were obtained via a procedure in which the Schiff base ligand was formed in situ during complexation. The heteronuclear ruthenium(III)–salen complexes 16a, 16b, 16c, 17a, 18a, 18b, 19c, 19b, 19c, and 19d (Table 1) were synthesized using a variety of coordination strategies involving different metal precursors and reaction conditions. For instance, 16a, 16b, and 16c were obtained through the direct reaction of a preformed ruthenium(III)–salen metalloligand with divalent metal perchlorates, such as Mn²⁺, Co²⁺, and Ni²⁺. In the case of 18b, terbium(III) nitrate was used. The choice of solvent for these reactions varies, with methanol, dichloromethane, ethanol, tetrahydrofuran (THF), acetonitrile, dimethylformamide (DMF), or mixtures commonly employed, depending on the desired reactivity, solubility, and stability of intermediates and final products [24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42]. In the synthesis of Ru(II/III/VI)–salen complexes, solvent choice is a critical factor that governs the coordination environment and stability of the metal precursor, ligands, and intermediates and directly impacts the catalytic performance of the resulting compounds. It also plays a key role in controlling the oxidation state of ruthenium during complexation. Polar aprotic solvents, such as DMF, DMSO, and ACN, are widely used during the complexation of Ru ions with Schiff bases due to their ability to solvate both the metal precursors and the tetradentate salen ligands. This facilitates efficient complexation and prevents ligand decomposition [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42]. Solvents with low coordinating ability, such as CH₂Cl₂ or THF, are often preferred for purification and crystallization processes after synthesis since they allow Ru(II/III) centers to retain high reactivity toward salen coordination, preserve cis/trans stereochemistry, and prevent the hydrolysis of undesired Ru–O_phen or Ru–N_imine bonds [24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42]. In summary, the solvents and synthetic strategies chosen for Ru–salen complexes are not arbitrary, but instead reflect a balance between solubility, stabilization of Ru oxidation states, prevention of unwanted ligand competition, and control of complex nuclearity.

All complexes 1a–19d were comprehensively characterized using a wide range of analytical and spectroscopic techniques. Elemental analysis (C, H, N) was performed for all compounds, and their molecular structures were unambiguously determined by single-crystal X-ray crystallography. Additional spectroscopic studies included IR spectroscopy for all complexes, UV–Vis spectrophotometry for 2a, 5a, 5b, 6a, 6b, 6c, 7a, 8a, 9a–9c, 11a, 14a, 14b, and 16a–16c, NMR spectroscopy for 1a, 4a, 9a–9c, 10a, 12a, 12b, 14a, 14b, 15a, and 15b, and EPR spectroscopy for 14a and 14b. Electrospray ionization mass spectrometry (ESI-MS) was employed for 3a, 3b, 4a, 5a, 5b, and 9a–9c, while cyclic voltammetry was used to investigate the electrochemical properties of 3a, 3b, 5a, 5b, 12a, and 12b. The purity of selected compounds (1a, 2a, 10a, and 13a–13d) was confirmed by gas chromatography–mass spectrometry (GC–MS). Magnetic susceptibility measurements were carried out for 3a, 3b, 5a, 5b, 7a, 8a, 13a–13d, 16a–16c, 17a, 18a, 18b, and 19a–19d. UV–visible absorption spectra of 6a, 6b, and 6c in CH₂Cl₂ at room temperature exhibited bands at ~380 nm and intense absorption below 300 nm, both primarily attributed to intra-ligand charge transfer (ILCT) transitions of the L²salen ligand. An additional band near 460 nm was assigned to a metal-to-ligand charge transfer (MLCT) transition. No substantial shifts in these absorption features were observed for complexes 2–4, with complex 2 displaying the longest MLCT wavelength (λ_max = 463 nm). In the UV-vis spectra of 9a and 9c, broad absorption bands with λ_max between 419 and 442 nm were assigned to MLCT transitions. The UV-vis spectrum of 9b also shows a broad band with λ_max at 410 nm, together with a weak low-energy shoulder absorption band at ca. 630−700 nm (log ε~2.69 dm³ mol⁻¹cm⁻¹), which were tentatively assigned to d-d transition. The complexes 6a–6c, 11a, 14a, and 14b were analyzed by density functional theory (DFT) calculations. Single crystals suitable for X-ray crystallographic analysis of the complexes listed in Table 1 were obtained using several approaches. Most frequently, they were grown by recrystallization from the solvent or solvent mixture previously used for synthesis, or from a different solvent. Another employed method involved slow diffusion of a solvent (most often diethyl ether) into a solution of the complexes in an appropriate solvent (commonly methanol or a methanol/dichloromethane mixture). In a few cases, crystals were obtained directly during the synthesis (1a) or through slow evaporation of the solvent from the reaction mixture (4a, 19a–d).

Structurally, the Ru–salen complexes exhibit a pseudo-octahedral geometry and adopt mainly a trans configuration (1a, 2a, 3a, 3b, 4a, 5a, 5b, 7a, 8a, 10a, 11a, 12a, 12b, 13a, 13b, 13c, 13d, 14a, 14b), wherein the ruthenium center is coordinated equatorially by two nitrogen and two oxygen donor atoms of the Schiff base ligand, forming a stable N₂O₂ coordination environment. The axial positions of the octahedron are typically occupied by either neutral ligands, such as carbon monoxide (CO), nitric oxide (NO), ammonia (NH₃), water (H₂O), methanol (CH₃OH), acetonitrile (CH₃CN), pyridine (Py), or triphenylphosphine (PPh₃), or anionic ligands, such as chloride (Cl⁻) or cyanide (CN⁻). The cis configuration occurs less frequently (6a, 6b, 6c, 9a, 9b, 9c, 15a). The nature of these axial ligands can significantly influence the electronic properties, redox behavior, and catalytic activity of the complexes.

In the case of cationic complexes, counterions such as ClO₄⁻ (1a, 4a, 16a, 16b, 16c) or PF₆⁻ (2a, 3a, 3b, 5a, 5b, 11a) are incorporated into the crystal structure to maintain overall charge neutrality. These anions are typically found in the lattice as non-coordinating species and are often located in the voids or channels of the crystal packing. Although they do not directly participate in coordination to the metal center, their presence can influence the overall crystal packing, intermolecular interactions, and, in some cases, the solubility and stability of the complex. In certain instances, weak hydrogen bonding or electrostatic interactions between the counterions and the coordinated ligands may also contribute to the supramolecular organization of the crystal structure [24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42].

A rare high-valent ruthenium–salen complex, [Ru^VI(N)(L¹salen)(MeOH)]ClO₄ 1a (where L¹salen = dianion of N,N′-bis(salicylidene)-o-cyclohexyldiamine), was synthesized by reacting [NBuⁿ₄][Ru^VI(N)Cl₄] with N,N′-bis(salicylidene)-o-cyclohexyldiamine in methanol followed by the addition of ClO₄⁻ [25]. Compound 1a (Figure 3, Table 1) exhibits a distorted octahedral geometry, with the L¹salen ligand coordinated to the Ru(VI) center in the equatorial plane and an axial methanol ligand. The other axial site is occupied by a terminal nitride ion. The distances between Ru(VI) and N_imine are 2.030(3) and 2.018(4) Å, while those between Ru(VI) and O_phen are 1.977(3) and 1.971(3) Å (Table 1) [24,25].

The mononuclear cationic complex [Ru^III(L¹salen)(NH₃)(py)]PF₆·2a was obtained as an oxidation product of the hydroquinone by an electrophilic nitrido species [Ru^IVN(L¹salen salen)(MeOH)](PF₆) (L¹salen = dianion of N,N′-bis(salicylidene)-o-cyclohexyldiamine) in the presence of pyridine in dichlorometane solution [26]. In the crystal structure of 2a (Figure 4, Table 1), the Ru(III) ion occupies the N₂O₂ site of the Schiff base and exhibits a disordered octahedral environment, where the equatorial plane is composed of two imine nitrogen and two phenolato oxygen atoms of the salen ligand, while two axial positions are occupied by one nitrogen atom of the pyridine and one ammonia nitrogen atom. Bond lengths between Ru(III) and N_imine are found to be 1.988(2) and 1.994(3) Å, while those between Ru(III) and O_phen measure 2.015(2) and 2.021(2) Å (Table 1) [24,26].

The cationic complexes trans-{Ru^III(L¹salen)[NH=C(NH₂)₂]₂}PF₆·Et₂O 3a and trans-{Ru^III(L¹salen)[NH=C(NH₂)(NHC₂H₄OH)]₂}PF₆ 3b (where L¹salen = dianion of N,N′-bis(salicylidene)-o-cyclohexyldiamine) were synthesized by treating the solution of trans-[Ru^III(L¹salen)(N≡CNH₂)₂]PF₆ in THF at a high temperature with gaseous NH₃, or by adding ethanolamine to the solution of trans-[Ru^III(L¹salen)(N≡CCH₃)₂]PF₆ in THF [27]. It is evident (Figure 5 and Figure 6, Table 1) that the ruthenium(III) ion in both mononuclear complexes exhibits a distorted octahedral geometry, with the tetradentate Schiff base situated in the equatorial plane while the axial positions are occupied by two nitrogen atoms from two guanidine 3a or two amidine 3b ligands. The distances between Ru(III) and N_imine range from 1.981 to 1.990 Å, while those between Ru(III) and O_phen lie between 2.023 and 2.032 Å (Table 1) [24,27].

Another mononuclear cationic complex [Ru^III(L¹salen)(NH₃)(MeOH)]ClO₄·MeOH 4a was isolated as the final product resulting from the simple reduction of a ruthenium(VI) nitride complex [Ru^VI(L¹salen)(N)(OH₂)]⁺ to [Ru^III(L¹salen)(NH₃)(OH₂)]⁺ [28]. This transformation proceeds via ruthenium(IV) sulfilamido, ruthenium(III) sulfilamine, and ruthenium(IV) amido intermediates upon reaction with L-cysteine in aqueous acidic solution. Kinetic and mechanistic studies in this case suggest that the reaction involves proton-coupled electron transfer processes. In the molecular structure of 4a (Figure 7, Table 1), the ruthenium(III) is octahedrally coordinated by a tetradentate salen Schiff base ligand in the plane as well as by one ammonia and one methanol molecule in the apical positions. As shown in Table 1, Ru–N_imine bond lengths were determined to be 1.988(3) and 1.989(3) Å, and the Ru–O_phen distances were found to be 2.015(2) and 2.028(2) Å [24,28].

The mononuclear paramagnetic complexes [Ru^III(L¹salen)(NHC(NHCH₂Py)Py)₂]PF₆ 5a and [Ru^III(L¹salen)(NHC(NHCH₂Ph)Ph)₂]PF₆ 5b were synthesized during reaction of [Ru^III(L¹salen)(H₂O)₂](PF₆) with 2-(aminomethyl)pyridine (Py) or benzylamine (Ph) [29]. Compounds 5a and 5b (Figure 8 and Figure 9, Table 1) exhibit a distorted octahedral geometry, with the ruthenium(III) centers coordinated to two oxygen atoms and two nitrogen atoms from the N₂O₂ salen ligand in the equatorial plane. The axial positions are occupied by two nitrogen atoms from the amidine ligands. In the crystal structures of 5a and 5b, the axial arylamidine ligands were generated in situ through the oxidative coupling of two molecular amines, marking the first reported instance of a direct metal-mediated coupling between amines. This transformation is air-sensitive, as no analogous products were formed under an inert atmosphere, and it is strongly dependent on the nature of the amine substrate. The distances between Ru(III) and N_imine range from 1.987 to 2.067 Å, while those between Ru(III) and O_phen lie between 2.015 and 2.027 Å (Table 1) [24,29].

The mononuclear neutral complexes cis-β-[Ru^II(L²salen)(CO)L^2a]·2CH₂Cl₂ 6a, cis-β-[Ru^II(L²salen)(CO)L^2b]·0.5C₆H₁₄ 6b, and cis-β-[Ru^II(L²salen)(CO)L^2c]·CH₂Cl₂ 6c were synthesized by the reaction of cis-β-[Ru^II(L²salen)(CO)₂] (L²salen = dianion of N,N′-bis(3-R¹-5-R²-salicylidene)-(S)-(−)-1,1′-binaphthalene-2,2′-diamine; R¹ = R² = Cl) with heteroatom-stabilized carbene ligands, such as 3-dimethyl-1H-benzimidazolium iodide (L^2a) in tetrahydrofuran, p-chloro-aniline and p-methyl-phenylacetylene (L^2b) 6b in 1,2-dichlorometane, and 1,1-diphenyl-2-propyn-1-ol (L^2c) in ethanol [30]. The synthesis of the compounds did not require photolytic decarbonylation, i.e., irradiation with a 300 W incandescent lamp. In all complexes (Figure 10, Figure 11 and Figure 12, Table 1), the ruthenium(II) ion adopts an octahedral configuration, with the coordination sphere of the metal center consisting of two nitrogen and two oxygen atoms from the doubly deprotonated tetradenate Schiff base (in a cis-salen configuration), as well as two carbon atoms from the carbonyl group and the carbene ligand. The distances between Ru(II) and N_imine range from 2.036 to 2.178 Å, while those between Ru(II) and O_phen lie between 2.060 and 2.101 Å (Table 1) [24,30].

The donor atoms of the L²salen ligand (N or O) adjacent to the carbene C atom contribute to the high stability of 6a, 6b, and 6c both in solution and in solid state [30].

The mononuclear anionic complex AsPh₄[Ru^III(L³salen))(CN)₂]·8.5H₂O 7a (where L³salen = dianion of N,N′-bis(3-methoxysalicylidene)-1,2-ethylenediamine) was obtained during reaction of [Ru^III(L³salen)(PPh₃)Cl] with KCN and AsPh₄Cl, respectively [31]. The crystal structure of 7a (Figure 13, Table 1) is composed of individual trans-[Ru^III(L³salen)(CN)₂]⁻ anions, tetraphenylarsonium counterions, and water molecules. The ruthenium(III) center is located within the N₂O₂ donor set of the Schiff base, adopting an octahedral coordination geometry with two imino nitrogen and two phenoxido oxygen atoms in the equatorial plane, and two cyanide ligands occupying the axial positions. The Ru–N_imine bond lengths are measured at 1.980(7) and 1.975(7) Å, whereas the Ru–O_phen distances are 2.004(4) and 2.025(6) Å (see Table 1) [24,31].

The mononuclear neutral complex [Ru^III(L³salen)(PPh₃)Cl]·3H₂O 8a (where PPh₃ = triphenylphosphine) was synthesized in air by the reaction of [Ru^II(PPh₃)₃Cl₂] with N,N′-bis(3-methoxysalicylidene)-1,2-ethylenediamine in methanol, in the presence of triethylamine [32]. As illustrated in Figure 14, the ruthenium(III) ion, which displays an octahedral geometry, is positioned within the N₂O₂ compartment of the Schiff base ligand. The equatorial plane is defined by two imino nitrogen atoms and two phenoxido oxygen atoms, while the axial positions are occupied by a chloride ion and the phosphorus atom of the PPh₃ ligand. Bond lengths between Ru(III) and N_imine are found to be 1.958(8) and 1.985(9) Å, while those between Ru(III) and O_phen measure 2.008(6) and 2.009(7) Å, as presented in Table 1 [24,32].

The unique neutral complexes cis-β-[Ru^II(L⁴salen)(H₂O)(CO)]·2CH₂Cl₂ 9a, cis-β-[Ru^II(L⁵salen)(CO)(CPh₂)]·0.5CH₂Cl₂ 9b, cis-β-[Ru^II(L⁶salen)(CO)₂]·0.25MeOH 9c (where L⁴salen = dianion of N,N′-bis(3-R¹-5-R²-salicylidene)-(S)-(−)-1,1′-binaphthalene-2,2′-diamine; R¹ = Bu^t R² = CPh₃, L⁵salen = dianion of N,N′-bis(3-R¹-5-R²-salicylidene)-(S)-(−)-1,1′-binaphthalene-2,2′-diamine; R¹ = R² = Bu^t, L⁶salen = dianion of N,N′-bis(3-R¹-5-R²-salicylidene)-1,2-cyclohexenediamine dianion; R¹ = R² = Bu^t) bearing nonplanar N₂O₂ salen type ligands were obtained in different ways, i.e., 9a and 9c by reaction of Ru₃(CO)₁₂ and the corresponding H₂L⁴salen/H₂L⁶salen ligand in 1,2,4-trichlorobenzene under an argon atmosphere in the absence of light; the synthesis of 9b was carried out by irradiation of a refluxing solution of cis-β-[Ru^II(L⁵salen)(CO)₂] in acetonitrile with an incandescent lamp (300 W) with the objective of removing one of the two coordinated CO groups, and subsequent treatment with a solution of the diazo compound N₂CPh₂ dissolved in dichloromethane at room temperature [22,33]. In all crystal structures (Figure 15, Figure 16 and Figure 17, Table 1), the central metal ion is six-coordinated by two nitrogen and two oxygen atoms of the Schiff base in a cis-salen configuration and, additionally, the coordination sphere is completed by one carbon atom of the carbonyl group and one aqua oxygen atom (9a); a carbon atom of the carbonyl group and a carbon atom of the carbene ligand (9b); and two carbon atoms of the carbonyl groups (9c). Ru–N_imine and Ru–O_phen bond lengths are observed within the ranges of 2.026–2.202 Å and 2.060–2.101 Å, respectively.

A neutral ruthenium(II) nitrosyl complex [Ru^II(L⁷salen)(NO)Cl]·CH₂Cl₂ 10a (where L⁷salen = dianion of N,N′-bis(salicylidene)-1,2-phenyldiamine) was obtained at a higher temperature during the reaction of Ru(NO)Cl₃·xH₂O, which was dissolved in dimethylformamide (DMF), with N,N′-bis(salicylidene)-1,2-phenylenediamine in tetrahydrofuran (THF), in the presence of triethylamine [34]. In the crystal structure of 10a (Figure 18, Table 1), the ruthenium(II) ion is in an octahedral coordination environment containing one tetradentate Schiff base ligand, one chloride ion, and one nitrosyl group. The Ru–N_imine bond lengths are reported as 2.020(4) and 2.022(4) Å, whereas the Ru–O_phen bonds are 2.023(4) and 2.025(3) Å (Table 1) [24,34].

The cationic complex [Ru^III(L⁸salen)(H₂O)(PPh₃)]PF₆·0.5CH₂Cl₂ 11a (where L⁸salen = dianion of N,N′-bis(salicylidene)-1,2-ethylenediamine, PPh₃ = triphenylphosphine) was obtained in the reaction of [RuCl₂(PPh₃)₃] with a methanolic solution of N,N′-bis(salicylidene)-1,2-ethylenediamine [35]. Single crystals of the compound were obtained from dichloromethane. The central ruthenium(III) ion in the [Ru^III(L⁸salen)(H₂O)(PPh₃)]⁺ has a distorted octahedral geometry, with two imino nitrogen atoms and two phenoxy oxygen atoms of the Schiff base forming the equatorial plane in the coordination polyhedron and the axial positions occupied by an aqua oxygen and phosphorus atom of a PPh₃ moiety (Figure 19, Table 1). The bond distances between the salen donor atoms N1, N2, O1, O2, and Ru(III) are 2.003(6), 1.983(6), 2.008(5), and 2.007(6) Å, respectively (see Table 1) [24,35].

The neutral complexes [Ru^IIICl(L⁹salen)(PPh₃)] 12a and [Ru^IIICl(L¹⁰salen)(PPh₃)] 12b (where L⁹salen = dianion of N,N′-bis(salicylidene)-1,2-(1-methyl)ethylenediamine, L¹⁰salen = dianion of N,N′-bis(3,5-dibromosalicylidene)-1,2-(1-methyl)ethylenediamine, PPh₃ = triphenylphosphine) were prepared during reaction of [Ru^IICl₂(PPh₃)₃] with the respective unsymmetrical tetradentate Schiff bases in tetrahydrofuran solution [36]. During the synthesis of 12a and 12b, oxidation of Ru(II) to Ru(III) by air is reported. In the crystals of 12a and 12b (Figure 20 and Figure 21, Table 1), N₂O₂-donor ligands coordinate to ruthenium(III) in a tetradentate mode. The compounds are octahedral in nature and adopt trans configuration. The basal plane of the central atom is formed by the two phenolic oxygen atoms and the two imine nitrogen atoms of the respective Schiff base, while the axial positions are occupied by the chlorine and phosphine atoms. Ru–N_imine and Ru–O_phen bond lengths are reported within the ranges of 1.966–2.008 Å and 2.008–2.105 Å, respectively (Table 1) [24,36].

The series of mononuclear neutral complexes [Ru^IIICl(L⁸salen)(PPh₃)] 13a, [Ru^IIICl(L⁹salen)PPh₃)]·2CH₂Cl₂ 13b, [Ru^IIICl(L¹¹salen)(PPh₃)]·CH₂Cl₂ 13c (where L¹¹salen = dianion of N,N′-bis(salicylidene)-1,2-propanediamine), [Ru^IIICl(L¹²salen)(PPh₃)]·CH₂Cl₂ 13d (where L¹²salen = dianion of N,N′-bis(salicylidene)-1,2-tolyldiamine) were obtained during reactions in nitrogen atmosphere [Ru^IICl₂(PPh₃)₃] and equal equivalents of the respective Schiff bases in THF/CH₂Cl₂ with the presence of a small excess of Et₃N [37]. In the crystals of all complexes (Figure 22, Figure 23, Figure 24 and Figure 25, Table 1), the central ruthenium(III) ion is in an octahedral coordination environment, containing N₂O₂-donor atoms of one Schiff base in the equatorial position and triphenylphosine and chloride ions in the apical positions. The distances between Ru(III) and N_imine range from 1.986 to 2.051 Å, while those between Ru(III) and O_phen are between 2.004 and 2.022 Å (Table 1) [24,37].

The other neutral complexes [Ru^IIICl(L¹³salen)(NO)]·CH₃CN 14a and [Ru^IIICl(L¹⁴salen)(NO)]·CH₃CN 14b (where L¹³salen = dianion of N,N′-(1,2-phenylene)-bis(salicylideneimine) and L¹⁴salen = dianion of N,N′-1,2-phenylene-bis(2-hydroxy-1-naphthylmethyleneimine) were synthesized in a one-pot reaction in ethanol, using salicylaldehyde or 2-hydroxy-1-naphthaldehyde and 1,2-phenylenediamine, along with [Ru(NO)Cl₃]·xH₂O and a small amount of Et₃N [38]. Compounds 14a and 14b (Figure 26 and Figure 27, Table 1) exhibit a slightly distorted octahedral coordination environment around the Ru(III) center, where the equatorial plane is defined by the nitrogen and oxygen atoms of the L¹³salen/L¹⁴salen ligand, and the axial positions are occupied by a NO molecule and a chloride ion. As shown in Table 1, Ru–N_imine bond lengths were determined to be in the range of 2.005–2.031Å, and the Ru–O_phen distances were found to be 2.027–2.037 Å [24,38].

The inert mononuclear complex cis-β-[Ru^II(L¹⁴salen)(CO)₂]·MeOH 15a was synthesized in a one-pot reaction at high temperature between Ru₃(CO)₁₂ and N,N′-1,2-phenylene-bis(2-hydroxy-1-naphthylmethyleneimine) in the presence of 1,2,4-trichlorobenzene [39]. The axial sites of the octahedrally coordinated Ru(II) center are occupied by a phenolate oxygen from the Schiff base ligand and an oxygen atom from a carbonyl group (Figure 28, Table 1). Bond lengths between Ru(II) and N_imine are found to be 2.038(2) and 2.065(2) Å, while those between Ru(II) and O_phen measure 2.041(1) and 2.092(1) Å, as presented in Table 1. An alkoxo-bridged dinuclear Ru(II) complex 15b was obtained as a minor product during 15a preparation. In its crystal structure (Table 1), each ruthenium(II) center adopts an octahedral coordination geometry, bonded to two CO ligands and a tridentate dibasic ONO-type Schiff base. Notably, the alkoxo oxygen atoms also bridge to the ruthenium(II) ion of an adjacent monomer unit. The Ru–N_imine bond lengths are measured at 2.066(3) and 2.075(4) Å, whereas the Ru–O_phen distances are in the range of 2.066–2.218 Å (Table 1) [24,39].

The ^∞₂[{Ru^III(L³salen)(CN)₂}₄{M^II(DMF)₃}₂{Mn^II(DMF)₄}](ClO₄)₂·4DMF 16a, 16b, and 16c (where M^II = Mn 16a, Co 16b, Ni 16c) 2D isostructural coordination polymers with ratios of Ru(III):M(II) equal to 4:3 were synthesized through the reaction of AsPh₄[Ru^III(L³salen))(CN)₂]·8.5H₂O with the perchlorate salt of the corresponding divalent metal in a DMF solution [31]. In the crystal structure of 16a (Figure 29, Table 1), the recurring structural motif features 48-membered metallacycles incorporating two crystallographically distinct ruthenium(III) and manganese(II) centers. One of the hexacoordinated M(II) ions is coordinated by three nitrogen atoms originating from three [Ru^III(L³salen)(CN)₂]⁻ units and by three oxygen atoms from DMF molecules. In contrast, the second Mn(II) ion is bonded to two nitrogen atoms from two [Ru(L³salen)(CN)₂]⁻ units and four oxygen atoms contributed by four DMF molecules. Additionally, a different spatial arrangement of the cyanido bridges was observed for the hexacoordinated Mn(II) ions. The octahedral geometry of the Ru(III) center is defined by two imine nitrogen atoms and two phenoxide oxygen atoms from the Schiff base ion, as well as two cyanide ions. Ru–N_imine and Ru–O_phen bond lengths are observed within the ranges of 1.990–2.021 Å and 2.000–2.030 Å (Table 1) [24,31].

The neutral dodecanuclear cluster containing a 36-membered macrocycle {[Ru^III(L⁸salen)(CN)₂][R,R-Mn^III(L¹salen)]}₆·PPh₃·6CH₃CN·6MeOH·12H₂O 17a was synthesized in air by reacting a methanol solution of [PPh₄][Ru(L⁸salen)(CN)₂] with R,R-[Mn(L⁸salen)(H₂O)₂]ClO₄ dissolved in a mixture of CH₃OH/CH₃CN [40]. In the crystal structure of 17a (Figure 30, Table 1), both Ru(III) and Mn(III) ions are six-coordinate, each adopting a distorted octahedral geometry. The equatorial plane is defined by N₂O₂ donor atoms from the Schiff base ligand, while the axial positions are occupied by carbon or nitrogen atoms from the bridging cyanide ligands. In the case of the Mn(III) ion, the axial Mn–N_cyanide bond lengths are noticeably longer than the Mn–O_phen and Mn–N_imine distances, reflecting an elongated octahedral geometry characteristic of the Jahn–Teller effect (Table 1). Complex 17a is very rare example of a cyanide-bridged 4d–3d metallic-based nano-sized chiral magnetic molecular wheel compound [40].

The heteronuclear complex [K(H₂O)₂Ru^III(L⁸salen)(CN)₂]·H₂O 18a was synthesized via the reaction of [Ru^III(L⁸salen)(PPh₃)Cl] with KCN in methanol solution [41]. The Ru(III) ion (Figure 31, Table 1) is situated within the N₂O₂ donor set of the Schiff base ligand, with the two nitrogen and two oxygen atoms occupying the equatorial positions of its octahedral coordination environment, while the cyanide ligands are located at the axial sites. The Ru–N_imine bond lengths are measured at 1.982(9) and 2.000(9) Å, whereas the Ru–O_phen distances are 2.018(6) and 2.025(7) Å (Table 1). The K ion is situated in the open coordination pocket of the Schiff base ligand, where it is coordinated by two bridging phenoxo oxygen atoms, two methoxy oxygen atoms, and two water molecules. In the structure of 18a, the presence of supramolecular dimeric units formulated as {[K^I(H₂O)₂Ru^III(L⁸salen)(CN)₂]}₂ is observed. These dimers are stabilized through weak non-covalent interactions, specifically anagostic contacts, formed between a hydrogen atom of a methyl group on one [K^IRu^III] moiety and the potassium ion of a neighboring dimer. This interaction contributes to the overall stability and organization of the crystal structure, illustrating the role of subtle intermolecular forces in the formation of extended supramolecular architectures.

The isomorphous compound with a one-dimensional structure, ∞[{Ru^III(L⁸salen)(CN)₂KRu^III(L⁸salen)(CN)₂}{Ln(NO₃)₂(MeOH)₃}]·2MeOH 18b (Ln = Gd, Tb, Dy), was formed through the reaction between 18a and respective lanthanide(III) nitrates [41]. In the crystal structure of 18b, trinuclear cyanido-bridged units {Ru^III−CN−Tb^III−NC−Ru^III} are linked via K ions, which are coordinated by two O₂O₂ donor sets from two distinct L⁸salen ligands, resulting in the formation of an infinite one-dimensional coordination polymer (Figure 32). Each [Ru^III(L⁸salen)(CN)₂]⁻ metalloligand coordinates to a Ln(III) ion via one bridging cyanide group, while the second cyanide ligand remains terminal. The Ln(III) center adopts a nine-coordinate geometry, bonded to two cyanide bridges, four oxygen atoms from two bidentate nitrate ligands, and three methanol molecules.

Two pairs of chiral cyano-bridged heterobimetallic compounds, [Mn(L⁸salen)Ru^III((R,R)-L¹⁵salen))(CN)₂]_n 19a, [Mn(L⁸salen)Ru^III((S,S)-(L¹⁵salen))(CN)₂]_n 19b, [Ni(tren)][Ru^III((R,R)-(L¹⁶salen)(CN)₂]₂ 19c, and [Ni(tren)][Ru^III((S,S)-(L¹⁶salen))(CN)₂]₂ 19d (where L¹⁵salen = dianion of N,N′-bis(5-bromosalicylidene)-o-cyclohexyldiamine, L¹⁵salen = dianion of N,N′-bis(5-chlorosalicylidene)-o-cyclohexyldiamine tren = tri(2-aminoethyl)amine) were obtained in MeOH/MeCN solution by the reaction of ((R,R) or (S,S))-[Ru^III(L¹⁵salen/L¹⁶salen(CN)₂]⁻ with [Mn(L⁸salen)(H₂O)₂]ClO₄ or Ni(tren)(NO₃)₂, respectively [42]. The structures of an enantiomeric pair, 19a and 19b (Figure 33, Table 1), consist of a neutral cyano-bridged zigzag chain built from repeating (–Ru^III–CN–Mn^III–NC–)_n units, with adjacent Mn(III) ions adopting a trans configuration. Each Ru(III) center adopts a slightly distorted octahedral geometry, coordinated by two nitrogen and two oxygen atoms from the Schiff base ligand, along with two carbon atoms from cyanide groups. The Mn(III) center adopts a highly distorted octahedral geometry, with the equatorial plane defined by two nitrogen and two oxygen atoms from the Schiff base ligand, while the axial sites are occupied by two nitrogen atoms from bridging cyanide ligands. The distances between Ru and N_imine range from 1.94 to 2.02 Å, while those between Ru and O_phen are 1.981–2.042 Å (Table 1). The axial bonds are noticeably longer than the equatorial ones, a consequence of the well-known Jahn–Teller distortion typically observed in high-spin Mn(III) ions with octahedral coordination. Compounds 19c and 19d (Figure 34, Table 1) are trinuclear complexes where the central Ni(II) ion is linked to two terminal (R,R/S,S)-trans-[Ru(L¹⁶salen)(CN)₂]⁻ units via cyanide bridges. Each Ni(II) center exhibits a distorted octahedral coordination geometry, being six-coordinated, with four nitrogen atoms from the tren ligand and two nitrogen atoms from the cyanide groups of the two separate (R,R)-[Ru^III(L¹⁶salen)(CN)₂]⁻ units. Bond lengths between Ru(III) and N_imine are reported to be 1.97–2.01 Å, while those between Ru(III) and O_phen measure 1.976–2.016Å, as presented in Table 1. Intermolecular interactions connect each trinuclear molecule, leading to the formation of a two-dimensional (2D) supramolecular structure [42].

Schiff base ligands coordinate to ruthenium through imine nitrogen (Ru–N_imine) and phenolic oxygen (Ru–O_phen), forming chelate rings whose geometries are influenced by the oxidation state of Ru and the cis/trans configuration (

Table 2. Average Ru–O_phen and Ru–N_imine bond lengths in salen–ruthenium complexes depending on oxidation state and cis/trans isomerism [25,26,27,28,29,30,31,32,33,34,35,36,37,38,39].

Ru Oxidation State	Geometry	Bond Length (Å)		Ref.
		Ru–Ophen	Ru–Nimine
II	cis	2.06–2.10	2.03–2.20	[30,33,39]
II	trans	2.02–2.05	2.02	[34]
III	trans	2.00–2.11	1.96–2.07	[26,27,28,29,31,32,35,36,37,38]
VI	trans	1.97–1.98	2.02–2.03	[25]

) [25,26,27,28,29,30,31,32,33,34,35,36,37,38].

The Ru–O_phen bond lengths in Schiff base complexes are particularly sensitive to the oxidation state of ruthenium and the geometric arrangement of ligands. In general, higher oxidation states of Ru, with some exceptions, result in shorter Ru–O_phen bonds due to increased electrostatic attraction between the positively charged metal center and donor atoms of the Schiff base ligands. The Ru–N_imine bond lengths are less affected by changes in the oxidation state of ruthenium. The isomeric form of the complex also influences the lengths of both the Ru–O_phen and Ru–N_imine bonds. The trans geometry generally leads to shorter Ru–O_phen bond lengths than the cis form, likely due to reduced steric repulsion and differences in electronic delocalization. In cis isomers, the metal–ligand interactions are more sterically hindered in the coordination environment, resulting in longer Ru–O_phen bond lengths. Isomerism has a more subtle effect on Ru–N_imine bond lengths than on Ru–O_phen bonds. The cis isomers typically exhibit slightly longer and more variable Ru–N_imine distances, likely due to steric interactions between ligands and uneven electron distribution. In contrast, trans isomers maintain shorter and more consistent Ru–N_imine bond lengths, reflecting a more symmetrical coordination environment and reduced steric hindrance.

3. The Potential Applications of Ru(II/III/VI)–Salen Complexes

Ru(II/III/VI)–salen complexes represent a fascinating class of coordination compounds that have attracted increasing interest due to their structural tunability, redox versatility, and stability under diverse conditions. The potential applications of these materials have been explored across a range of domains, including catalysis, medicine, and advanced materials science (Figure 35).

In catalysis, for example, they can act as efficient and selective mediators in oxidation and hydrogenation reactions (Figure 36 and Figure 37). In materials science, their redox versatility and structural tunability facilitate the design of functional materials, including molecular switches and conductive polymers. In medicinal chemistry, their capacity to interact with biomolecules and regulate redox processes is driving ongoing efforts toward the development of anticancer, antimicrobial, and diagnostic agents.

Ruthenium–salen complexes with trans configurations are investigated as catalysts in a wide variety of organic transformations. Their stability, predictable geometry, and well-defined electronic structures have made them attractive model systems for mechanistic studies as well as practical applications, e.g., the nitrido Ru(VI) complex 1a exhibits rich chemical reactivity, as demonstrated by its ability to oxidize phenols to p-benzoquinone imines. The proton-coupled electron-transfer reactions of phenols have attracted a great deal of attention due to their fundamental importance and relevance to numerous biological processes. In the case of 1a, the reaction proceeds through a two-step sequential mechanism. Initially, an electrophilic attack by 1a on the aromatic ring generates a Ru(IV) p-hydroxyanilido intermediate. In the subsequent step, a pyridyne-assisted intramolecular redox process takes place, resulting in the formation of the Ru(II) p-benzoquinone imine product. This transformation highlights the strong oxidative potential of the metal center and the important role of the Schiff base ligand in stabilizing high-valent ruthenium species, thereby enabling selective activation of phenolic O–H bonds [7,25]. An investigation into the catalytic properties of 10a revealed that it is an effective catalytic precursor for the hydrogenation transfer of acetophenone [34]. Complexes 13a, 13b, 13c, and 13d have been shown to effectively catalyze the oxidation of primary alcohols into their corresponding aldehydes and secondary alcohols into ketones, using dichloromethane as the solvent and N-methylmorpholine-N-oxide (NMO) as a co-oxidant [37].

The cis-Ru–salen complexes, though less studied than their trans counterparts, exhibit unique reactivity and, in some cases, superior catalytic performance. The nonequivalent coordination sites in the cis configuration create distinct steric and electronic environments, offering a powerful platform for the design of asymmetric catalysts. These features position cis-Ru–salen complexes as promising candidates for enantioselective synthesis and novel catalytic applications, e.g., complexes 6a, 6b, and 6c are reported as catalysts for organic transformations, including intramolecular carbene transfer reactions [30], the mononuclear 9c and dinuclear ruthenium 15b complexes exhibit high catalytic efficiency in the intramolecular cyclopropanation of trans-allylic diazoacetates under irradiation with a 300 W incandescent lamp in the absence of light exposure (the irradiation can enhance the dissociation rate of CO ligands from carbene M(CO)₂-type complexes to give more reactive mono(carbonyl) species). The corresponding cyclopropanation products were obtained in yields ranging from 84% to 96% (9c) and 85% to 94% (15b), respectively [22,33,39]. In the case of 15b, the influence of solvent on this transformation was examined, revealing that coordinating solvents such as MeOH or THF suppressed the reaction, whereas the use of the non-coordinating solvent CH₂Cl₂ containing 0.05% (v/v) MeOH afforded superior results [39].

The cytotoxicity profiles of ruthenium(III) complexes 3a and 3b were evaluated using several human cancer cell lines, including HeLa (cervical), A549 (lung), MCF-7 (breast), and HepG2 (liver), which are commonly employed in the biological assessment of metal-based drugs. It is important to highlight that the anticancer mechanisms of these complexes are strongly influenced by the nature of the axial ligand (guanidine or amidine). Remarkably, their modes of action differ substantially from those of the clinically established drug cisplatin [27].

Converting metal nitrides [Ru^VI(L¹salen)(N)(OH₂)]⁺ into ammonia 4a represents a crucial step in both biological and chemical nitrogen fixation. The straightforward reduction of a Ru(VI)–nitrido complex containing a Schiff base ligand to a Ru(III)-ammonia species by L-cysteine in aqueous solution constitutes a rare example of metal nitrido reduction. This transformation provides valuable insight into the redox behavior of nitrido complexes under aqueous conditions [28].

The synthetic strategy employed to prepare complexes 5a and 5b provides an innovative and effective approach to the production of arylamidine-containing metal complexes. Amidines are a fundamental class of functional group in both medicinal and coordination chemistry. Their well-documented roles range from acting as pharmacophores in drug discovery to serving as highly adaptable ligands that can fine-tune the steric and electronic environment of metal centers [29]. Therefore, developing new methodologies to access arylamidine derivatives is of significant interest in order to expand the toolbox of both bioinorganic and organometallic chemistry.

Compound 7a is a key precursor in the synthesis of novel, cyanide-bridged, heterometallic coordination polymers of the {Ru(III)–Mn(II)/Co(II)/Ni(II)} type [31]. These coordination polymers are of great interest due to their adjustable architectures and multifunctional properties, which result from the interaction between the ruthenium(III) centers and the incorporated 3d transition metals. These materials have potential applications in areas such as molecular magnetism, electronic and photonic devices, and catalytic processes. This highlights the importance of developing efficient synthetic methods for preparing them.

The novel hybrid material 8a, when immobilized on SBA-15, has been shown to exhibit significant antibacterial and anti-biofilm activity against the bacteria Enterococcus faecalis, Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli. Furthermore, in vitro assays assessing its cytotoxic effects on HeLa cells demonstrated a dose-dependent reduction in cell viability, highlighting the compound’s considerable potential for biomedical applications [32].

Compound 11a was investigated as a potential chemosensor for the selective recognition of acetate anions, which play a crucial role in various metabolic processes, such as enzyme activity and antibody function, and can form strong hydrogen bonds with different donor groups, including urea and boronic acid derivatives. The results showed that the fluorescence intensity of 11a increased with rising acetate concentrations, with no interference from other anions [35].

Complexes 14a and 14b, incorporating planar N₂O₂ ligands with π-extended aromatic systems, efficiently release nitric oxide (NO) under visible-light irradiation, highlighting their potential as photoresponsive NO donors [38].

Magnetic studies of the cyanido-bridged heterometallic coordination polymers 16a, 16b and 16c revealed antiferromagnetic interactions between Ru(III) and either Mn(II) or Co(II) ions. In contrast, ferromagnetic coupling was observed between Ru(III) and Ni(II) centers. These findings emphasize the pivotal role of the cyanide ion, whose strong bridging ability and effectiveness in facilitating exchange interactions establish it as a vital linker in the formation of heterometallic structures. Cyanide-bridged compounds can generate systems with tunable spin arrangements by enabling diverse magnetic coupling pathways, thereby offering significant potential for the development of molecule-based magnetic materials and multifunctional coordination polymers [31]. The subsequent compound, 17a, a cyanide-bridged 4d–3d heterobimetallic chiral macrocyclic enantiomeric complex, emerges as a particularly promising candidate for advanced magnetic material applications. In-depth magnetic investigations have demonstrated that 17a exhibits pronounced single-molecule magnet (SMM) behavior, characterized by slow magnetization relaxation and magnetic bistability at the molecular level [40].

The chiral cyano-bridged bimetallic systems 19a, 19b, 19c, and 19d are also promising candidates for the design of magnetic materials, such as molecular multiferroics and chiral magnets, owing to their delocalized magnetic orbitals, pronounced magnetic anisotropy driven by strong spin–orbit coupling, and capacity to adopt multiple oxidation states with varying coordination environments. Magnetic investigations revealed antiferromagnetic interactions between the Ru(III) and Mn(III) centers bridged by cyanide ligands (19a, 19b), while ferromagnetic coupling occurs between Ru(III) and Ni(II) ions (J = 2.73 cm⁻¹) (19c) [42].

4. Conclusions

Ruthenium(II/III/VI)–salen complexes can be obtained using various synthetic methods. These include the direct coordination of pre-formed salen ligands to ruthenium precursors; stepwise assembly from salicylaldehyde and amine components in the presence of a ruthenium salt; and post-synthetic modification of pre-assembled ruthenium–ligand frameworks. The oxidation state of the metal center and the geometry of the complex can be controlled by tuning reaction conditions such as solvent, temperature, counterion, and oxidizing or reducing agents. The modular nature of the salen ligand framework not only stabilizes the metal center but also allows for systematic modification of the steric and electronic environment around the metal center. The complexes usually adopt an octahedral trans configuration, with the ruthenium(II/III/VI) center being tightly bound to two nitrogen and two oxygen atoms of the tetradentate Schiff base ligands. In addition, neutral (e.g., CO, NH₃, CH₃OH, CH₃CN) or anionic (e.g., Cl⁻) ligands occupy the two vacant sites. In the case of cationic complexes, counterions such as ClO₄⁻ or PF₆⁻ are present in their crystal structure. Ru(II/III/VI)–salen complexes have potential applications in many fields, including catalysis, materials science, and medicinal chemistry. Their stability and redox activity enable their use in the design of advanced functional materials, sensors, and molecular devices. Furthermore, due to their ability to interact with biological molecules and generate reactive oxygen species, Ru–salen complexes are being investigated for applications in anticancer therapy and photodynamic treatments.

Future Perspectives: The multifunctionality of ruthenium–salen complexes highlights their potential to act as molecular platforms for interdisciplinary applications. Advances in ligand design, computational modeling, and experimental techniques are expected to enable these complexes to continue providing innovative solutions in catalysis, materials science, and biomedicine. Future research could focus on designing Ru(II/III/VI) complexes that are multifunctional and have enhanced selectivity in C–H, C–O, C–N, and C–C bond activation. These complexes would also have tunable redox properties for sustainable oxidation reactions. Additionally, light-responsive ruthenium–salen complexes containing a Ru–NO or Ru–CO bond offer potential for use in photochemical and theranostic applications, combining catalysis with the controlled release of reactive species or drug molecules. In summary, their ability to combine catalytic, optical, biological, and electrochemical properties within a single molecular structure establishes them as one of the most promising and versatile classes of coordination compounds.