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Article

Electronic Influence of Trifluoromethyl Substituents on Benzoate Ligands in Paddlewheel-Type Diruthenium(II,II) Naphthyridine Complexes

by
Nozomi Tada
,
Natsumi Yano
,
Makoto Handa
* and
Yusuke Kataoka
*
Department of Chemistry, Graduate School of Natural Science and Technology, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, Shimane, Japan
*
Authors to whom correspondence should be addressed.
Magnetochemistry 2025, 11(12), 104; https://doi.org/10.3390/magnetochemistry11120104
Submission received: 3 November 2025 / Revised: 25 November 2025 / Accepted: 25 November 2025 / Published: 27 November 2025
(This article belongs to the Special Issue 10th Anniversary of Magnetochemistry: Past, Present and Future)
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Abstract

Two diruthenium(II,II) naphthyridine complexes coordinated with 4-trifluoromethylbenzoate (O2CPh-4-CF3) and 3,5-bis(trifluoromethyl)benzoate (O2CPh-3,5-diCF3) ligands, formulated as [Ru2(npc)2(O2CPh-4-CF3)2] (4; npc = 1,8-naphthyridine-2-carboxylate) and [Ru2(npc)2(O2CPh-3,5-diCF3)2] (5), respectively, were synthesized and structurally characterized. Single-crystal X-ray diffraction analysis revealed that both 4 and 5 form a direct metal–metal bond between the two Ru ions (2.2893(8) and 2.2896(7) Å, respectively) and adopt a paddlewheel-type structure in which two npc and two trifluoromethyl-substituted benzoate ligands are coordinated to a Ru24+ core with a cis-2:2 arrangement. The temperature dependence of the magnetic susceptibility measurements of 4 and 5 exhibited very large zero-field splitting (D = 242 and 246 cm−1, respectively) of the triplet ground state of the Ru24+ core, similar to that of [Ru2(npc)2(O2CPh)2] (3; D = 238 cm−1). Owing to the effects of the trifluoromethyl substituents, compared with 3, 4 and 5 showed (i) a significant blue shift of the absorption bands in the visible region and (ii) a positive shift of the redox potentials, with both shifts becoming more pronounced as the number of trifluoromethyl substituents increased. These experimental results are in good agreement with the electronic structure results obtained from density functional theory calculations.

1. Introduction

Paddlewheel-type dinuclear complexes, formulated as [M2(BL)4(AL)n] (M: metal ion, BL: bridging ligand, AL: axial ligand, n = 0–2), are intriguing coordination compounds, not only because of their fundamental interest in the study of metal–metal bonds and electronic structures [1,2,3,4,5,6,7,8,9], but also because of their rich functionalities arising from the unique d-orbital interactions within the M2 core [10,11,12,13,14,15,16,17,18]. The oxidation states and electronic structures of the M2 core in these types of complexes are influenced by the nature of the BL and AL [1]. In this type of structural motif, diruthenium (Ru2) complexes are especially interesting because (i) they are capable of forming three oxidation states (Ru26+/Ru25+/Ru24+) with an open-shell spin state arising from d-orbital interactions within the Ru2 core through proper selection and adjustment of the BL [1,4,5,6], and (ii) they exhibit intrinsic functional properties, such as magnetism [19,20,21,22], catalysis [23,24,25,26], metallodrug activity [27,28], and electrochromism [29]. The Ru25+ complexes are known to be stable when exposed to oxygen and can be readily handled in air, whereas Ru24+ complexes are generally unstable when exposed to oxygen and are readily oxidized in air [30,31]. Against this background, we recently developed a 1,8-naphthyridine-2-carboxylate (npc)-bridged Ru24+ complex, [Ru2(npc)2(O2CCH3)2] (1; Scheme 1a), which is stable in air in both solid and solution states [32]. Single crystal X-ray diffraction (SCXRD) analysis revealed that 1 adopts a paddlewheel-type structure in which the Ru24+ core is coordinated by two npc and two acetate ligands in a cis-2:2 coordination arrangement. Interestingly, 1 is a redox-active compound (redox potential: E1/2(Ru25+/4+) = 0.43 V vs. SCE) and possesses strong visible absorption bands (781–552 nm), arising from charge transfer from the Ru2 core to the npc ligands. This absorption feature of 1 is unique and not observed in conventional paddlewheel-type Ru2 complexes bridged by four carboxylate ligands (O2CR), [Ru2(O2CR)4(AL)2]. The two acetate ligands in 1 can be exchanged with other carboxylate ligands, thereby shifting their electrochemical and absorption properties [32,33,34]. For example, owing to the electron-withdrawing effect of the trifluoromethyl substituents, the E1/2(Ru25+/4+) value of [Ru2(npc)2(O2CCF3)2] (2; 0.72 V vs. SCE) is shifted in the positive direction, and the absorption bands are also blue-shifted by approximately 70–100 nm compared with those of 1 [33]. In addition, we confirmed that the absorption and redox potentials of [Ru2(npc)2(O2CPh)2] (3) are slightly shifted relative to those of 1 and that these properties show further pronounced shifts upon extension of the aromatic π-ring in the carboxylate ligands [34].
It has been reported that the introduction of substituents into the benzoate ligands of homoleptic paddlewheel-type Ru2 complexes enables control of their redox potentials [35,36,37]. Therefore, we became interested in how the introduction of substituents onto the benzoate ligands of 3 would influence the molecular and electronic structures of Ru2 naphthyridine complexes and how such modifications lead to differences in their characteristic absorption bands and redox potentials. In this study, two Ru2 naphthyridine complexes with trifluoromethyl-substituted benzoate ligands (Scheme 1b), [Ru2(npc)2(O2CPh-4-CF3)2] (4; O2CPh-4-CF3 = 4-trifluoromethylbenzoate) and [Ru2(npc)2(O2CPh-3,5-diCF3)2] (5; O2CPh-3,5-diCF3 = 3,5-bis(trifluoromethyl)benzoate), were synthesized and characterized. The detailed molecular structures of 4 and 5 were determined by SCXRD. Furthermore, the absorption and electrochemical properties as well as the electronic structures of 4 and 5 were closely investigated through a combination of experimental measurements and theoretical calculations and subsequently compared with those of 1–3.

2. Materials and Methods

2.1. Chemicals and Instruments

The chemicals used in this study were of reagent or analytical grade and were generally used as received from the suppliers; only tetra-n-butylammonium hexafluorophosphate (TBAPF6) was recrystallized from ethanol before use. 4-Trifluoromethylbenzoic acid (HO2CPh-4-CF3) was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan), and 3,5-bis(trifluoromethyl)benzoic acid (HO2CPh-3,5-diCF3) was obtained from Matrix Scientific (Elgin, SC, USA), and other chemicals were purchased from Fujifilm Wako Pure Chemical Industry (Osaka, Japan). 1–3 were prepared as described previously [32,33].
Proton nuclear magnetic resonance spectroscopy (1H NMR) was measured using a JEOL (Tokyo, Japan) JNM-ECZR500R spectrometer (500 MHz) in DMSO-d6. The chemical shifts were calibrated using the residual DMSO signal at δ = 2.49 ppm. Electrospray ionization mass spectrometry (ESI-MS) was performed with a Bruker (Billerica, MA, USA) micrOTOF-II instrument in the positive-ion mode using sodium formate as the calibrant. Elemental analyses of hydrogen, carbon, and nitrogen were performed using a Yanaco CHN Corder MT-6 instrument (Tokyo, Japan). The temperature dependence of the magnetic susceptibility was measured using a Quantum Design (San Diego, CA, USA) MPMS3 SQUID magnetometer under a magnetic field of 5000 Oe over the temperature range of 2–300 K. Cyclic voltammetry (CV) measurements of 1.0 mM 4 and 5 in dried N,N-dimethylformamide (DMF) solution containing 0.10 M TBAPF6 were performed using a HOKUTO DENKO HZ-7000 HAG1232m system (Tokyo, Japan) equipped with a glassy carbon working electrode, platinum wire counter electrode, and saturated calomel reference electrode (SCE) at a scan rate of 100 mV/s under an Ar atmosphere. Absorption spectrum was recorded in DMF using a JASCO (Tokyo, Japan) UV-670 spectrophotometer.

2.2. Synthesis of [Ru2(npc)2(O2CPh-4-CF3)2] (4)

1(H2O)0.5 (67.6 mg, 0.100 mmol) and HO2CPh-4-CF3 (380.2 mg, 2.00 mmol) were dissolved in 15.0 mL of CH3OH, and the mixture was heated at 433 K under Ar for 48 h using a Teflon-lined stainless-steel autoclave reactor. The resulting reaction solution was evaporated, and the crude product was purified by silica gel column chromatography using CHCl3/CH3OH (4:1) as the eluent. The dark blue fraction was collected and evaporated to dryness, and resulting dark blue residue was dried under vacuum at 433 K for 3 h. Yield: 82.3 mg (0.0871 mmol, 87.1%). EA calcd (%) for C34H18N4O8F6Ru2·(H2O): C 43.23, H 2.13, N 5.93; found C 43.50, H 2.14, N 5.99. 1H NMR (500 MHz, DMSO-d6, δ): 30.40 (d, 2H), 30.29 (d, 2H), 15.98 (s, 4H), 14.96 (d, 2H), 9.55 (d, 4H), −2.39 (d, 2H), –9.96 (bs, 2H). ESI-MS calcd for C34H18N4O8F6Ru2Na [M + Na]+: 950.9027 m/z, found 950.9021 m/z.

2.3. Synthesis of [Ru2(npc)2(O2CPh-3,5-diCF3)2] (5)

A synthetic method similar to 4 was applied for the preparation of 5, but HO2CPh-3,5-diCF3 (516.4 mg, 2.00 mmol) was used instead of HO2CPh-4-CF3. Yield: 88.5 mg (0.0833 mmol, 83.3%). EA calcd (%) for C36H16N4O8F12Ru2: C 40.69, H 1.52, N 5.27; found C 41.00, H 1.82, N 5.78. 1H NMR (500 MHz, DMSO-d6, δ): 29.90 (dd, 4H), 16.08 (s, 4H), 15.33 (d, 2H), 11.46 (s, 2H), −2.33(d, 2H), −11.56 (bs, 2H). ESI-MS calcd for C36H16N4O8F12Ru2Na [M + Na]+: 1086.8776 m/z, found 1086.8756 m/z.

2.4. Single Crystal X-Ray Diffraction Analysis

The SCXRD experiment of 4 was performed using a RIGAKU (Tokyo, Japan) XtaLAB Synergy-S system equipped with a Hypix-6000 detector and graphite-monochromated Cu Kα radiation (λ = 1.54184 Å) from a microfocus PhotonJet-S source, whereas that of 5 was conducted using a RIGAKU XtaLAB Synergy-R/DW system equipped with a Hypix-6000 detector and graphite-monochromated Cu Kα radiation (λ = 1.54184 Å) from a rotating anode PhotonJet-R source.
The crystals were coated with Paratone-N oil and mounted on a goniometer head using MiTeGen (Ithaca, NY, USA) microloops. Diffraction data were collected at 100 K under the N2 steam, and data processing and reduction were executed using CrysAlisPro software version 1.171.43.108a [38]. The initial structures were solved by intrinsic phasing with SHELXT program [39] and subsequently refined by full-matrix least-squares on F2 using SHELXL program [40] in the Olex2 software version 1.5 [41]. Hydrogen atoms were placed in calculated positions and refined as riding models, while all non-hydrogen atoms were refined anisotropically. The crystallographic data for the final refined structures of 4 and 5 are summarized in Table 1 and can be obtained free of charge from Cambridge Crystallographic Data Centre (CCDC). The deposition numbers of 4 and 5 are CCDC-2499389 and 2499390, respectively.

2.5. Quantum Chemical Calculation Methods

All quantum chemical calculations in this study were performed using the spin-unrestricted density functional theory (DFT) functional B3LYP [42], in combination with the SDD basis set for Ru atoms [43], the aug-cc-pVDZ basis set for N and O atoms, and the cc-pVDZ basis set for other atoms [44], as implemented in the Gaussian 16 (Rev. C.01) program suite [45]. Molecular geometries were fully optimized without symmetry constraints, and the obtained geometries were verified as minima using frequency analysis. The solvent effect of DMF (ε = 37.219) was considered by employing the polarizable continuum model [46]. Time-dependent DFT (TDDFT) calculations were performed to investigate the characteristics of vertical excitations from the ground state [47].

3. Results

3.1. Synthesis and Characterization

Complexes 4 and 5 were synthesized via ligand-exchange reactions at 433 K between complex 1 and 20 equivalents of HO2CPh-4-CF3 and HO2CPh-3,5-diCF3, respectively, under solvothermal conditions in an Ar atmosphere. Dark blue powders of 4 and 5 were obtained in 87.1 and 83.3% yields, respectively. Although the reaction was attempted under reflux conditions, the reaction did not procced sufficiently. The obtained 4 and 5 were paramagnetic compounds and stable both in the solid solution states in air.
Complexes 4 and 5 were characterized using paramagnetic 1H NMR, ESI-MS, and CHN elemental analyses. In the 1H NMR spectra (Figures S1 and S2), the signals assignable to the two npc ligands of 4 appear at 30.40, 30.29, 14.96, −2.39, and −9.96 ppm with an integral ratio of 2:2:2:2:2, whereas those of 5 appear at 29.90, 15.33, −2.33, and −11.56 ppm with an integral ratio of 4:2:2:2; the most downfield signal of 5 is a doublet of doublets. In addition, proton signals with total integral ratios of 8 and 6 derived from the two O2CPh-4-CF3 ligands in 4 (15.98, 9.55 ppm) and two HO2CPh-3,5-diCF3 ligands in 5 (16.08, 11.46 ppm) were also observed. The ESI-MS spectra of 4 and 5 show intense peaks at 950.9021 and 1086.8756 m/z, respectively, which are in good agreement with the [M + Na]+ values (950.9027 and 1086.8776 m/z, respectively) corresponding to the molecular formulas composed of two Ru ions, two npc ligands, and two O2CPh-4-CF3 or O2CPh-3,5-diCF3 ligands, respectively (Figures S3 and S4). The 1H NMR and ESI-MS results, as well as the elemental analysis, strongly indicate that 4 and 5 adopt structures with the chemical formula [Ru2(npc)2(O2CR)2], similar to those of 1–3.

3.2. Crystal Structures

Single crystals of 4 and 5 suitable for SCXRD measurements were obtained by the slow evaporation of mixed CH3CN/DMSO solutions containing 4 and 5, respectively. Diffraction analyses revealed that 4 crystallizes in a monoclinic system with the P 21/c space group, whereas 5 crystallizes in an orthorhombic system with the P na21 space group. The final refined crystal structures of 4 and 5 with the selected numbering schemes are shown in Figure 1, and their selected structural parameters (bond lengths and angles) are summarized in Tables S1 and S2, respectively.
Structural analysis revealed that the asymmetric units of both 4 and 5 comprised one crystallographically independent molecule of [Ru2(npc)2(O2CR)2], together with co-crystallized solvent molecules (for 4: three CH3CN and one H2O; for 5: one H2O). The final refined crystal structures of 4 and 5 adopted a paddlewheel-type structure, in which the Ru24+ core was coordinated by two npc and two trifluoromethyl-substituted benzoate ligands in a cis-2:2 coordination arrangement, similar to those of 1–3 [32,33]. The Ru–Ru bond lengths of 4 and 5 were estimated to be 2.2893(8) and 2.2896(7) Å, respectively, which show no significant differences from those of 1 (2.2893(4) Å), 2 (2.2978(4) Å), 3 (2.2834(6) Å), and other Ru24+ complexes such as [Ru2(O2CCH3)4(H2O)2] (2.262(3) Å) [30] and [Ru2(pynp)2(O2CCH3)2](PF6)2 (2.298(1) Å; pynp = 2-(2-pyridyl)-1,8-naphthyridine) [48], indicating the formation of a double bond between two Ru ions in 4 and 5. The two npc ligands of 4 and 5 were coordinated to the Ru2 core in a tridentate fashion; specifically, the naphthyridine and carboxylate moieties of each npc ligand were coordinated to the equatorial and axial positions of the Ru2 core, respectively. The average Ru–N(npc)/Ru–O(npc) bond lengths of 4 and 5 were 2.060/2.238 Å and 2.058/2.213 Å, respectively, with no significant difference between them. Two trifluoromethyl-substituted benzoate ligands bound to the Ru2 core in a syn–syn μ-bridging coordination mode [49]; the averaged Ru–O(μ-carboxylate) bond lengths of 4 and 5 were both 2.078 Å. The bond lengths in these primary coordination spheres differed by less than 0.01 Å from those of 3 [32]. Therefore, the influence of introducing trifluoromethyl substituents into the benzoate ligands of the [Ru2(npc)2(O2CR)2] complex on the Ru–Ru bond length and the bond lengths in the primary coordination sphere was negligibly small.
The two npc ligands coordinated in a coplanar manner to the Ru2 core, whereas the two trifluoromethyl-substituted benzoate ligands coordinated in a tilted manner, and their phenyl and carboxylate planes were slightly twisted (average rotation angles: 7.27° for 4 and 7.87° for 5). These distortions were not caused by the trifluoromethyl-substituents because the optimized structures of 4 and 5 obtained from DFT calculations (described later) did not show such tilting or twisting. In the packing structures of 4 and 5, the crystallization water molecules formed hydrogen bonds with the carboxylate moieties of the npc ligands, giving rise to a one-dimensional hydrogen-bonding network (see Figures S5 and S6).

3.3. Magnetic Susceptibilities

To investigate the magnetic properties as well as the electronic spins and oxidation states of 4 and 5, the temperature dependence of the magnetic susceptibilities were measured using a SQUID magnetometer under an applied field of 5000 Oe over a temperature range of 2–300 K. The effective magnetic moments of 4 and 5 at 300 K were estimated to be 2.90 and 2.87 μB, respectively; these values are close to the spin-only value of 2.83 μB for the triplet spin state of the Ru24+ complex but deviate significantly from the 3.87 μB expected for the quartet spin (S = 3/2) state of the Ru25+ complex. Figure 2 shows the temperature dependence of the molar magnetic susceptibility (χM) and effective moments (μeff) of 4 and 5. Similar to 1–3 [32,33], the μeff values decreased with decreasing temperature. At 2 K, the μeff values of 4 and 5 reached 0.38 and 0.35 μB, respectively. This temperature-dependent behavior can be ascribed to the zero-field splitting (ZFS) characteristics of the Ru2 complexes [50,51,52].
To elucidate the ZFS characteristics of 4 and 5 in detail, their D tensors and g values were determined by fitting the measured data to Equation (1) [32,33,34].
χ M = 1 − ρ 2 N β 2 g 2 3 k T · e − D k T + 2 k T D 1 − e − D k T 1 + 2 e − D k T + ρ ( 5 N g i m p 2 β 2 4 k T )
where ρ is the correction term for a small amount of paramagnetic impurities with S = 3/2 (0.003 for 4, 0.001 for 5), k is the Boltzmann constant, N is Avogadro’s number, β is the Bohr magneton, and the gimp value is assumed to be equal to the g value. The fitting results were as follows: (i) the calculated g values of both 4 and 5 were 2.12, similar to those of 1–3 (g = 2.03–2.13) and (ii) the D tensors of 4 and 5 were estimated to be 242 and 246 cm−1, respectively. These large D tensor magnitudes are characteristic of Ru24+ complexes [53,54], including 1 (227 cm−1), 2 (243 cm−1), and 3 (238 cm−1) [32,33,34], and significantly larger than those of Ru25+ complexes such as [Ru2(O2CCH3)4(H2O)2]BPh4 (71.8 cm−1) [55]. These results provide strong evidence that 4 and 5 possess a Ru24+ core with S = 1 and that the effects of the trifluoromethyl substituents on their magnetic properties are negligible.

3.4. Optimized Geometries, Spin Density Distributions, and Electronic Structures

DFT calculations were performed to investigate the molecular geometries, spin density distributions, and electronic structures of 4 and 5 in detail, and the results were compared with those of 3 [32]. The optimized geometries of 4 and 5 adopted a paddlewheel-type structure, in which two npc and two trifluoromethyl-substituted benzoate ligands were coordinated to the Ru2 core in a cis-2:2 arrangement, similar to their crystal structures. The Ru–Ru bond lengths of optimized geometries of 4 and 5 were calculated to be 2.321 and 2.324 Å, respectively. These bond lengths are almost identical to that of 3 (2.318 Å), indicating that the effect of introducing trifluoromethyl substituents into the benzoate ligands on the Ru–Ru bond length is negligibly small. The Ru–Ru bond lengths and the bond lengths between the Ru2 core and the first coordination sphere in 3–5 show excellent agreement with those obtained from SCXRD data, with the differences being within 0.035 Å. The trifluoromethyl-substituted benzoate ligands in the optimized geometries of 4 and 5 were coordinated to the Ru2 core without twisting or bending, as observed in 3 [32].
To clarify the origins of the magnetic spins of 4 and 5, their spin density distributions were estimated. As depicted in Figure 3, the spin density of 4 and 5 are predominantly localized on the Ru2 core (4: +2.043, 5: +2.037) rather than on the npc (4: −0.014, 5: −0.005) and trifluoromethyl-substituted benzoate ligands (4: −0.029, 5: −0.032), which is consistent with the magnetic susceptibility results.
Selected molecular orbital (MO) diagrams and frontier MO pictures, i.e., the highest occupied MO (HOMO) and lowest unoccupied MO (LUMO), of 3–5 are shown in Figure 4, where the MOs are separated into α and β spins due to the use of the unrestricted DFT approach. The calculated trends and energy ordering of the MOs for 4 and 5 closely resembled those of 3. Two singly occupied MOs, i.e., HOMO(α) and HOMO–1(α), of 4 and 5 were both predominantly localized on the π*(Ru2) orbitals, as observed in 3. The δ*(Ru2) orbital pairs of 4 and 5 are found at HOMO–2(α) and HOMO(β), whose energies were lower and higher, respectively, than those of the π*(Ru2) orbitals (HOMO(α)/HOMO–1(α)). The introduction of trifluoromethyl substituents into the benzoate ligands led to a significant stabilization of the occupied MOs. Specifically, the energies of HOMO(α)/HOMO(β) in 4 and 5 were stabilized by −0.06/−0.08 and −0.10/−0.14 eV, respectively, relative to those in 3, indicating that the MO stabilization increased with the number of trifluoromethyl substituents. The LUMO(α, β) and LUMO+1(α) of 4 and 5 were both mainly localized on the π*(npc) orbitals, as observed for 3. On the other hand, the LUMO+1(β) was primarily composed of the π*(Ru2) orbital with a minor contribution from π*(npc) orbital, whereas the LUMO+2(β) mainly consists of π*(npc) orbital with a small d*(Ru2) orbital contribution. In addition, the LUMO+3(β) is predominantly localized on the π*(Ru2) orbital. The energies of the unoccupied MOs of 4 and 5 were slightly stabilized by the trifluoromethyl substituents relative to those of 3, although the stabilization was less pronounced than for the occupied MOs. The energies of LUMO(α)/LUMO(β) in 4 and 5 were stabilized by −0.03/−0.06 and −0.06/−0.05 eV, respectively, relative to those in 3. The electronic configurations of the Ru2 core in 4 and 5 are π4δ2σ2δ*2π*2, which are similar to those of 1–3, indicating the formation of a double bond between the two Ru ions in 4 and 5.

3.5. Electrochemical Properties

To investigate the electrochemical properties of 4 and 5, cyclic voltammetry (CV) measurements were performed in DMF containing 0.10 M TBAPF6 over the potential range from 1.0 V to −1.8 V vs. SCE. The observed redox potentials (E1/2) and the peak separations of the reduction and oxidation waves (ΔE) of 4 and 5 as well as of those of 1–3 are summarized in Table 2. As shown in Figure 5, the CV diagrams of 4 and 5 both exhibit one and two reversible waves in the positive and negative regions, respectively, similar to those of 1–3 [32,33,34]. The E1/2 values of the waves in the positive region for 4 and 5, which can be assigned to the Ru25+/4+ redox couple, were observed at 0.56 and 0.58 V, respectively, and these values were slightly shifted in the positive direction compared with that of 3 (0.52 V). In addition, the E1/2 values of the two waves in the negative region for 4 and 5, attributed to the reduction of npc ligands, were −0.80/−1.15 V and −0.76/−1.12 V, respectively, showing a slight positive shift compared with those of 3 (−0.83/−1.19 V). All E1/2 values for 5 appeared at more positive potentials than those for 4. That is, the slight shifts in the E1/2 values can be attributed to the electron-withdrawing effect of the trifluoromethyl substituents. These CV results were consistent with the DFT-calculated electronic structures, in which both the HOMO and LUMO energy levels stabilized as the number of trifluoromethyl substituents increased. The E1/2 value of 2 shifted significantly to more positive potentials (~0.29 V) compared with those of 1. This indicates that a bridging ligand in which an electron-withdrawing group is directly connected to the carboxylate moiety, such as trifluoroacetate, has a greater influence on the E1/2 value of the Ru2 naphthyridine complex than bridging ligands in which similar substituents are introduced onto the benzoate ring, even at multiple positions.

3.6. Absorption Properties

Figure 6 shows the absorption spectra of 3–5 in DMF, and the wavelengths of the absorption maxima and molar absorption coefficients (ε) of 1–5 are summarized in Table 3. The lowest-energy absorption bands of 4 and 5, which were assigned to metal-to-ligand charge transfer (MLCT) transitions from the Ru2 core to the npc ligands, appeared at 736 (ε = 4135 M−1 cm−1) and 717 nm (ε = 4103 M−1 cm−1), respectively, and were apparently blue-shifted relative to the corresponding band of 3 (771 nm) [32]. The most intense absorption bands of 4 and 5 in the visible region were observed at 598 (ε = 6352 M−1 cm−1) and 586 nm (ε = 6424 M−1 cm−1), respectively, and intense shoulder bands were discernible at slightly higher energies, centered at 540 and 531 nm, respectively. These bands were also assigned to the MLCT transitions from the Ru2 core to the npc ligands according to TDDFT calculations (Tables S3 and S4). Based on these results, the absorption bands of 4 and 5 were clearly blue-shifted as the number of electron-withdrawing trifluoromethyl substituents on the benzoate increased. However, the energy differences in the absorption bands between 3 and 4 (or 5) were clearly smaller than those observed between 1 and 2 [32,33].

4. Conclusions

In summary, we successfully synthesized two paddlewheel-type Ru2 naphthyridine complexes, [Ru2(npc)2(O2CPh-4-CF3)2] (4; npc = 1,8-naphthyridine-2-carboxylate) and [Ru2(npc)2 (O2CPh-3,5-diCF3)2] (5), in which two npc and two trifluoromethyl-substituted benzoate ligands are coordinated to a Ru24+ core with a cis-2:2 arrangement. Direct Ru–Ru bonds were confirmed from the SCXRD results, and a very large zero-field splitting (D = 242 cm−1 for 4, 246 cm−1 for 5) associated with the triplet ground state Ru24+ core was also observed from magnetic susceptibility measurements, with all these findings further supported by DFT calculations. The obvious effects of the trifluoromethyl substituents on the benzoate ligands of the Ru2 complexes were confirmed from the absorption and electrochemical properties as well as from the MO energies obtained by DFT calculations. In the electrochemical analysis, compared with that of 3, the observed redox potentials of 4 and 5 were slightly shifted to more positive values with an increasing number of trifluoromethyl substituents on the benzoate ligands; the E1/2(Ru25+/4+) values of 3–5 were 0.52, 0.56, and 0.58 V vs. SCE, respectively. The low-energy absorption bands of 4 and 5 also showed a clear blue shift compared with those of 3. These shifts are consistent with the DFT calculation results, indicating that the introduction of trifluoromethyl substituents into the benzoate ligands stabilizes the MOs, particularly the occupied orbitals. The differences in the absorption and redox potentials among 3–5 were smaller than those observed between 1 and 2, suggesting that the direct introduction of trifluoromethyl substituents onto the carboxylate, as in trifluoroacetate, exerts a stronger electron-withdrawing effect on the absorption, redox potentials, and electronic structures of the Ru2 naphthyridine complexes than multiple trifluoromethyl substituents on the benzoate ligands.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/magnetochemistry11120104/s1, Figure S1: 1H NMR spectrum of 4 in DMSO-d6; Figure S2: 1H NMR spectrum of 5 in DMSO-d6; Figure S3: Observed and simulated ESI-MS spectra of 4; Figure S4: Observed and simulated ESI-MS spectra of 5; Figure S5: One-dimensional hydrogen-bonding network structure of 4 along the a-axis; Figure S6: One-dimensional hydrogen-bonding network structure of 5 along the b-axis; Table S1: Selected bond lengths (Å) and angles (°) of crystal structure of 4; Table S2: Selected bond lengths (Å) and angles (°) of crystal structure of 5; Table S3: TDDFT results (excitation wavelengths [nm], oscillator strengths [f], and major contributions) of 4. Here, transitions with oscillator strengths greater than 0.002 and excitation contributions exceeding 10% were selected; Table S4: TDDFT results (excitation wavelengths [nm], oscillator strengths [f], and major contributions) of 5. Here, transitions with oscillator strengths greater than 0.002 and excitation contributions exceeding 10% were selected.

Author Contributions

Conceptualization, Y.K.; methodology, Y.K.; validation, N.Y., M.H. and Y.K.; formal analysis, N.T., N.Y., M.H. and Y.K.; investigation, N.T. and Y.K.; resources, Y.K.; data curation, Y.K.; writing—original draft preparation, Y.K.; writing—review and editing, N.T., N.Y., M.H. and Y.K.; visualization, Y.K.; supervision, Y.K.; project administration, Y.K.; funding acquisition, N.Y., M.H. and Y.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by JSPS KAKENHI (grant numbers JP24K08494, JP24K08363, and JP25K18113).

Data Availability Statement

The original contributions presented in this study are included in the article/supplementary material. Further inquiries can be directed to the corresponding authors.

Acknowledgments

A part of this work was conducted at the Institute for Molecular Science, which was supported by “Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM)” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) (Proposal Number JPMXP1225MS1036).

Conflicts of Interest

The authors declare no conflicts of interest.

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Magnetochemistry 11 00104 sch001
Scheme 1. Molecular structures of (a) [Ru2(npc)2(O2CCH3)2] and (b) trifluoromethyl-substituted benzoate ligands used in this study.
Scheme 1. Molecular structures of (a) [Ru2(npc)2(O2CCH3)2] and (b) trifluoromethyl-substituted benzoate ligands used in this study.
Magnetochemistry 11 00104 sch001
Magnetochemistry 11 00104 g001
Figure 1. Crystal structures of (a) 4 and (b) 5. Thermal ellipsoids were represented at a 30% probability level. Hydrogen atoms and solvents were omitted for clarity.
Figure 1. Crystal structures of (a) 4 and (b) 5. Thermal ellipsoids were represented at a 30% probability level. Hydrogen atoms and solvents were omitted for clarity.
Magnetochemistry 11 00104 g001
Magnetochemistry 11 00104 g002
Figure 2. Temperature dependence of the magnetic susceptibility (red circle) and effective magnetic moment (blue rhombus) of (a) 4 and (b) 5. The solid lines represent the best fit of the data using Equation (1).
Figure 2. Temperature dependence of the magnetic susceptibility (red circle) and effective magnetic moment (blue rhombus) of (a) 4 and (b) 5. The solid lines represent the best fit of the data using Equation (1).
Magnetochemistry 11 00104 g002
Magnetochemistry 11 00104 g003
Figure 3. Spin density distributions of (a) 4 and (b) 5 (isovalue = 0.002).
Figure 3. Spin density distributions of (a) 4 and (b) 5 (isovalue = 0.002).
Magnetochemistry 11 00104 g003
Magnetochemistry 11 00104 g004
Figure 4. MO diagrams and frontier MO pictures of 3–5.
Figure 4. MO diagrams and frontier MO pictures of 3–5.
Magnetochemistry 11 00104 g004
Magnetochemistry 11 00104 g005
Figure 5. CV diagrams of 3 (green), 4 (blue), and 5 (red) in degassed DMF/TBAPF6 (scan rate: 100 mV/s).
Figure 5. CV diagrams of 3 (green), 4 (blue), and 5 (red) in degassed DMF/TBAPF6 (scan rate: 100 mV/s).
Magnetochemistry 11 00104 g005
Magnetochemistry 11 00104 g006
Figure 6. Absorption spectra of 3 (green), 4 (blue), and 5 (red) in DMF.
Figure 6. Absorption spectra of 3 (green), 4 (blue), and 5 (red) in DMF.
Magnetochemistry 11 00104 g006
Table 1. Crystallographic data of 4 and 5.
Table 1. Crystallographic data of 4 and 5.
45
Chemical formulaC40H29F6N7O9Ru2C36H18F12N4O9Ru2
Formula weight1067.841080.68
Crystal systemmonoclinicorthorhombic
Space groupP 21/cP na21
a [Å]14.3266(2)22.9594(3)
b [Å]10.25320(10)16.2295(2)
c [Å]27.8368(3)10.00890(10)
α [°]9090
β [°]99.0220(10)90
γ [°]9090
V [Å3]4038.46(8)3729.51(8)
Z44
Dcalc(g cm−3)1.7561.925
μ [mm−1]6.8897.685
F(000)2128.02120.0
R1 (I > 2σ(I))0.06920.0368
wR2 (I > 2σ(I))0.17230.0970
R1 (all data)0.07030.0369
wR2 (all data)0.17270.0971
GOF on F21.2401.077
Table 2. Redox potentials and their separation (ΔE: mV) of 1–5.
Table 2. Redox potentials and their separation (ΔE: mV) of 1–5.
ComplexE1/2: V vs. SCE (ΔE: mV)
10.43 (62), −0.92 (64), −1.19 (64)
20.72 (68), −0.67 (63), −1.10 (56)
30.52 (66), −0.83 (56), −1.19 (57)
40.56 (72), −0.80 (63), −1.15 (64)
50.58 (67), −0.76 (64), −1.12 (64)
Table 3. Absorption wavelengths (nm) and their molar absorption coefficients (ε) of 1–5 in DMF.
Table 3. Absorption wavelengths (nm) and their molar absorption coefficients (ε) of 1–5 in DMF.
ComplexWavelengths (nm) [ε (M−1 cm−1)]
1781 (4313), 621 (6421), 552 (5245)
2686 (3780) *, 555 (6599), 514 (6090) *
3771 (4528), 620 (7164), 548 (6374)
4736 (4135), 598 (6352), 540 (5683)
5717 (4103), 586 (6424), 531 (5760)
* Shoulder band.
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Tada, N.; Yano, N.; Handa, M.; Kataoka, Y. Electronic Influence of Trifluoromethyl Substituents on Benzoate Ligands in Paddlewheel-Type Diruthenium(II,II) Naphthyridine Complexes. Magnetochemistry 2025, 11, 104. https://doi.org/10.3390/magnetochemistry11120104

AMA Style

Tada N, Yano N, Handa M, Kataoka Y. Electronic Influence of Trifluoromethyl Substituents on Benzoate Ligands in Paddlewheel-Type Diruthenium(II,II) Naphthyridine Complexes. Magnetochemistry. 2025; 11(12):104. https://doi.org/10.3390/magnetochemistry11120104

Chicago/Turabian Style

Tada, Nozomi, Natsumi Yano, Makoto Handa, and Yusuke Kataoka. 2025. "Electronic Influence of Trifluoromethyl Substituents on Benzoate Ligands in Paddlewheel-Type Diruthenium(II,II) Naphthyridine Complexes" Magnetochemistry 11, no. 12: 104. https://doi.org/10.3390/magnetochemistry11120104

APA Style

Tada, N., Yano, N., Handa, M., & Kataoka, Y. (2025). Electronic Influence of Trifluoromethyl Substituents on Benzoate Ligands in Paddlewheel-Type Diruthenium(II,II) Naphthyridine Complexes. Magnetochemistry, 11(12), 104. https://doi.org/10.3390/magnetochemistry11120104

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