Paddlewheel-type diruthenium(III,III) tetrakis(2-aminopyridinate) complexes with NIR absorption features: combined experimental and theoretical study

Yusuke Kataoka *a, Nanako Imasaki a, Kazuki Arakawa a, Natsumi Yano a, Hiroshi Sakiyama b, Tamotsu Sugimori c, Minoru Mitsumi d and Makoto Handa a
aDepartment of Chemistry, Interdisciplinary Graduate School of Science and Engineering, Shimane University, 1060, Nishikawatsu, Matsue, 690-8504, Japan. E-mail: kataoka@riko.shimane-u.ac.jp
bDepartment of Science, Faculty of Science, Yamagata University, 1-4-12, Kojirakawa, Yamagata, 990-8560, Japan
cInstitute of Liberal Arts and Sciences, University of Toyama, 2630, Sugitani, Toyama, 930-0194, Japan
dDepartment of Chemistry, Faculty of Science, Okayama University of Science, 1-1, Ridaicho, Kita-ku, Okayama, 700-005, Japan

Received 29th May 2019 , Accepted 22nd June 2019

First published on 24th June 2019


The reactions of [Ru2(O2CCH3)4Cl] with 2-aminopyridine (Hamp) and 2-amino-4-methylpyridine (Hammp) afforded two novel Ru2 complexes, [Ru2(amp)4Cl2] (1) and [Ru2(ammp)4Cl2] (2), respectively. Single crystal X-ray diffraction analyses revealed that 1 and 2 adopted typical paddlewheel-type structures, where the Ru2 units are coordinated with four aminopyridinate ligands with a cis-2:2 arrangement at the equatorial positions and two chloride ligands at the axial positions. The stabilities of 1 and 2 were supported by unrestricted density functional theory (uDFT) calculations. The zero-point energies of the three structural isomers (trans-2:2, 3:1, and 4:0 arrangements) of 1 and 2 were less stable than those of the respective cis-2:2 arrangements. Temperature-dependences of the magnetic susceptibility measurements and uDFT calculations showed that the oxidation and spin states of the Ru2 units in 1 and 2 were commonly Ru26+ and triplet states, respectively. Cyclic voltammetry showed that 1 and 2 underwent one-electron reduction processes, i.e., 1/1 and 2/2, at redox potentials (E1/2) of −0.08 and −0.18 V vs. SCE, respectively. These results agreed well with the DFT-calculated E1/2 values of 1 (−0.08 V vs. SCE) and 2 (−0.18 V vs. SCE) and were theoretically attributed to Ru2-centred (δ*(Ru2)) redoxes. Moreover, 1 and 2 showed unique near-infrared absorption bands at approximately 1200–1500 nm, which were theoretically attributed to the ligand-to-metal charge transfer (LMCT) with π(amp or ammp) → δ*(Ru2) transition characteristics.


Introduction

Paddlewheel-type dinuclear complexes with multiple metal–metal bonds or interactions have attracted significant attention due to fundamental interest in their molecular geometries and electronic structures1–3 as well as their intriguing functional properties for catalysis,4–7 magnetism,8–10 sensors,11,12 and so on.13,14 Among them, diruthenium (Ru2) complexes, [Ru2(L)4Xn] (L = equatorial ligands, X = axial ligands; n = 0, 1, and 2), are especially interesting because the paramagnetic Ru2 centres in their complexes are redox-active and can adopt three oxidation states (Ru26+, Ru25+, and Ru24+) depending on the types of equatorial and axial ligands.15 Most of the oxidation states of paddlewheel-type Ru2 complexes reported in the literature are Ru25+ and Ru24+ states, and their electronic structures and magnetic properties have been thoroughly investigated.16–22 On the other hand, the corresponding properties of Ru26+ complexes have not been sufficiently investigated to date due to the limited number of Ru26+ complexes. Prior studies revealed that several symmetric bidentate N^N bridging ligands, such as amidinate, can afford Ru26+ complexes. For example, Ru2 complexes with benzamidinate (bam) or dimethylbenzamidinate (DMBA), i.e., [Ru2(bam)4Cl2]23 or [Ru2(DMBA)4Cl2],24 respectively, afforded a Ru26+ centre with a spin state of S = 1, while similar Ru2 complexes with diphenylformamidinate (dpf), [Ru2(dpf)4Cl], and its derivatives yielded Ru25+ complexes with a spin state of S = 3/2.25,26 In contrast, in the asymmetric bidentate N^N bridging ligands, it is well-known that the Ru2 complexes with 2-anilinopyridinate (ap) or its derivatives rarely adopt an oxidation state of Ru26+ depending on the type of axially coordinated ligand.27–29 However, only a few examples of these compounds have been reported so far and their structures and properties have not been fully established. To more deeply understand the electronic states (oxidation state, spin state, and electronic structure) of Ru26+ complexes, it is considered that the quests of suitable bridging ligands, which afford the Ru26+ complexes, are indispensible, and the combination of experimental and theoretical studies are quite essential for the systematic study of the Ru26+ complexes.

Herein, two novel paddlewheel-type Ru2 complexes with 2-aminopyridinate (amp) and 2-amino-4-methylpyridinate (ammp) ligands, [Ru2(amp)4Cl2] (1) and [Ru2(ammp)4Cl2] (2), were synthesized and characterized by experimental and theoretical techniques. The results of single crystal X-ray diffraction, temperature dependences of the magnetic susceptibilities, and unrestricted density functional theory (uDFT) calculations showed that the oxidation and spin states of 1 and 2 are Ru26+ and triplet (S = 1) states, respectively. To the best of our knowledge, this is the first example of a 2-aminopyridinate ligand coordinated Ru26+ complex. Moreover, as shown in Fig. 1, although the formation of four isomers, cis-2:2, trans-2:2, 3:1, and 4:0 arrangements,27–29 was expected for 1 and 2, both complexes commonly adopted a cis-2:2 arrangement, representing the first such examples in the Ru2 complexes. Specific and unique intense near-infrared (NIR) absorptions of 1 and 2 were also observed and discussed in the context of time-dependent DFT (TDDFT) calculations. We believe that this combination of experimental and theoretical techniques will extend and deepen the understanding of the chemistry of the paddlewheel-type Ru26+ complex.


image file: c9dt02271f-f1.tif
Fig. 1 (a) Molecular structures of aminopyridinate ligands used in this study and (b) possible arrangements of regioisomers of Ru2 complexes with aminopyridinate ligands.

Results and discussion

Synthesis and characterization

Complexes 1 and 2 were synthesized by reactions of [Ru2(O2CCH3)4Cl] and 10.0 molar equivalents of 2-aminopyridine (Hamp) and 2-amino-4-methylpyridine (Hammp), respectively, in a mixture of tetrahydrofuran (THF) and triethylamine (TEA) at 348 K in air, followed by purification via column chromatography (eluent; 8% MeOH in CHCl3). Here, TEA is necessary for the reactions to proceed. The yield of 1 was only 18.1% because of the low reactivity of Hamp and poor solubility of the intermediate heteroleptic Ru2 species i.e., [Ru2(amp)x(O2CCH3)4−xCl] (x = 1–3), whereas that of 2 reached 86.1%. Although other reaction solvents (such as 2-propanol) or excess Hamp were used for the synthesis of 1, the yield was not improved. In addition, solvothermal synthesis using an autoclave vessel was also not effective for improving the yield of 1. Thus, we concluded that the present synthetic procedures were appropriate and effective for the synthesis of 1 and 2. The obtained 1 and 2 were commonly paramagnetic black powders that were stable in air. Although 2 is easily soluble in common organic solvents such as CH2Cl2, CHCl3, CH3CN, THF, MeOH, DMSO, and DMF, 1 is less soluble in their solutions.

The obtained 1 and 2 were characterized via electrospray ionization time-of-flight mass spectroscopy (ESI-TOF-MS), infrared (IR) and Raman spectra, and elemental analyses. The ESI-TOF-MS spectra of 1 and 2 showed intense signals at 610.9587 and 667.0197 m/z, respectively, which are close to the theoretical [M − Cl]+ values of 1 (610.9581 m/z) and 2 (667.0207 m/z). Moreover, intense signals corresponding to the intermediate heteroleptic Ru2 species were not observed. The results of MALDI-TOF-MS spectra also supported the formations and results of the ESI-TOF-MS spectra of 1 and 2. In their IR spectra, the N–H vibration modes of 1 and 2 were observed at 3281 and 3247 cm−1, respectively. The Raman spectra of 1 and 2 showed intense Ru–Ru vibration modes at 334 and 315 cm−1, respectively, which are close to the typical Raman shift of Ru–Ru vibration modes of paddlewheel-type Ru2 complexes such as [Ru2(O2CCH3)4Cl] (326 cm−1), indicating that the Ru–Ru bonds exist between the two Ru ions within 1 and 2.1 The purities of 1 and 2 were confirmed by CHN elemental analyses, as the observed CHN ratios of 1 and 2 were very close to the theoretical anhydrous compositions.

Crystal structures

Small single crystals of 1 and 2 suitable for X-ray crystal diffraction analyses were obtained by slow evaporation of mixed acetone/DMSO solutions of 1 and 2. Diffraction analyses at 150 K indicated that 1 and 2 crystallize in the monoclinic space group P21/n and the triclinic space group P[1 with combining macron], respectively. The crystallographic data and selected structural parameters of 1 and 2 are summarized in Table 1 and Tables S1, S2 in the ESI, respectively. Fig. 2 and 3 show the Ortep view of 1 and 2 (thermal ellipsoids set at 40% probabilities), respectively.
image file: c9dt02271f-f2.tif
Fig. 2 Crystal structure of complex 1 (Ru: brown, Cl: green, N: blue, C: gray, H: white. Thermal ellipsoids are shown with 40% probability). Here, the DMSO molecule is omitted for clarity.

image file: c9dt02271f-f3.tif
Fig. 3 Crystal structure of complex 2 (Ru: brown, Cl: green, N: blue, C: gray, H: white. Thermal ellipsoids are shown with 40% probability). Here, the acetone molecule is omitted for clarity.
Table 1 Crystallographic data of 1 and 2
Complexes 1 2
Formula C24H32Cl2N8O2Ru2S2 C30H40Cl2N8O2Ru2
M r (g mol−1) 801.73 817.74
Crystal system Monoclinic Triclinic
Space group P21/n P[1 with combining macron]
a (Å) 9.052(2) 8.8746(3)
b (Å) 8.533(2) 9.9417(3)
c (Å) 19.979(5) 10.5840(4)
α (deg) 90 85.558(3)
β (deg) 102.023(4) 85.197(3)
γ (deg) 90 65.274(3)
V3) 1509.3(6) 844.29(5)
Z 2 1
D Calc (g cm−3) 1.764 1.577
μ (mm−1) 1.354 1.077
F(000) 804.0 1572.0
R 1 (I > 2σ(I)) 0.0288 0.0546
wR2 (I > 2σ(I)) 0.0640 0.1446
R 1 (all data) 0.0361 0.0614
wR2 (all data) 0.0670 0.1500
GOF on F2 1.028 1.086


The crystal structure analyses revealed that the asymmetric units of 1 and 2 both contained a half of a molecule, consisting of one Ru ion, two amp or ammp ligands, one Cl ligand, and one crystallization solvent. Thus, the inversion centres of 1 and 2 were located at the midpoints of the Ru–Ru bonds. Here, the crystals of 1 and 2 selectively included DMSO and acetone, respectively, which were used as the crystallization solvents. As shown in Fig. 2 and 3, the overall structures of 1 and 2 formed the typical paddlewheel-type structures, where four aminopyridinate and two Cl ligands are coordinated with the equatorial and axial positions of the Ru2 units, respectively. It should be noted that four amp and ammp ligands in 1 and 2, respectively, selectively adopted the cis-(2,2) arrangement (regioisomer). It was deduced that the donations from NH moieties to Ru2 units in 1 and 2 were relatively larger than those from py moieties because the Ru–N(H) bond lengths of 1 (Ru(1)–N(2′): 2.001(2) Å, and Ru(1)–N(4′): 1.997(2) Å) and 2 (Ru(1)–N(2′): 2.004(3) Å, and Ru(1)–N(4′): 1.999(3) Å) are apparently shorter than their Ru–N(py) bond lengths (1; Ru(1)–N(1): 2.082(2) Å, and Ru(1)–N(3): 2.068(2) Å. 2; Ru(1)–N(1): 2.079(3) Å, and Ru(1)–N(3): 2.076(4)). The Ru–Ru bond lengths of 1 and 2 were 2.3364(6) and 2.3336(6) Å, respectively, indicating the presence of multiple metal–metal bonds between the two Ru ions.1 These Ru–Ru bond lengths of 1 and 2 are largely consistent with those of previously reported Ru26+ complexes coordinated with two axial Cl ligands such as [Ru2(bam)4Cl2] (2.342(1) Å)23 and are longer than those of Ru25+ complexes coordinated with an axial Cl ligand, such as [Ru2(O2CCH3)4Cl] (2.281(3) Å)30 and [Ru2(ap)4 Cl] (2.275(3) Å).31 The Ru–Cl bond lengths of 1 and 2 were 2.5434(8) and 2.5262(11) Å, respectively, which are nearly equal to that of [Ru2(bam)4Cl2] (2.5269(10) Å).23 The Ru(1′)–Ru(1)–Cl(1) angles of 1 and 2 were 170.33(2) and 171.35(3)°, respectively. That is, the two Cl ions of 1 and 2 were slightly deviated from the Ru2 axial directions, which was also observed for [Ru2(bam)4Cl2],23 indicating that Ru2 units of 1 and 2 have a similar electronic configuration to that of [Ru2(bam)4Cl2] because d orbital interactions between two Ru atoms typically affect the primary coordination sphere and bond lengths of Ru2 complexes. From the total charge of the coordinated ligands and Ru–Ru bond lengths, it is presumed that 1 and 2 exhibited an oxidation state of Ru26+, i.e., the Ru(III)–Ru(III) state.

Magnetic susceptibilities

To investigate the magnetic properties and the oxidation and spin states of 1 and 2, their magnetic susceptibilities were measured from 300 to 2 K at 5000 Oe. Fig. 4 shows the temperature dependence of the molar magnetic susceptibilities (χM) and effective magnetic moments (μeff) of 1 and 2. At 300 K, the μeff values of 1 and 2 were 2.80 and 2.81μB, respectively, similar to the expected spin-only value for the S = 1 state with two unpaired electrons (2.83μB). With decreasing temperature, the μeff values of 1 and 2 drastically decreased due to the large zero-field splitting (ZFS) at the Ru2 centres. These magnetic behaviours are quite similar to those of the other Ru26+ complexes, indicating that 1 and 2 have an oxidation state of Ru26+ with S = 1 spin states.23,24,32,33
image file: c9dt02271f-f4.tif
Fig. 4 Temperature dependence of magnetic susceptibilities (blue dots) and effective magnetic moments (red dots) of (a) 1 and (b) 2. The solid lines represent the best fit of the data.

The ZFS parameters (D) and average g values of 1 and 2 were estimated by fitting the obtained magnetic data to eqn (1) using the Magsaki(B) W0.6.3 software.34

 
image file: c9dt02271f-t1.tif(1)

Here, k is the Boltzmann constant, N is the Avogadro number, and β is the Bohr magneton. Eqn (1) includes the correction terms for a small amount of paramagnetic impurity (ρ) with S = 3/2 [Ru25+ species: ρ = 0.00215 for 1 and 0.01055 for 2] and gimp values were assumed to be equal to the average g values. The resulting average g values of 1 and 2 were each 2.05 and the D values of 1 and 2 were estimated to be 225 and 214 cm−1, respectively. These values are also similar to the reported D values of Ru26+ complexes, including [Ru2(bam)4Cl2] (213.0 cm−1),23 [Ru2(DMBA)4Cl2] (203 cm−1),24 [Ru2(hpp)4Cl2] (227 cm−1; hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine),32 and [Ru2(tbn)4Cl2] (261 cm−1; tbn = 1,5,7-triazabicylo[4.3.0]non-6-ene),33 but much larger than the reported D values of Ru25+ complexes, including [Ru2(ap)4Cl] (57 cm−1).35

Favourable spin state, stabilities of isomeric structures, and electronic structures

To deeply understand the favourable spin states, stabilities of the isomeric structures, and electronic structures of 1 and 2, uDFT calculations were performed.36

Initially, in order to investigate the favourable spin states and structural stabilities of the observed structures of 1 and 2, geometric optimizations and zero-point energy (ZPE) calculations of the observed structures and assumed structural isomers of 1 and 2 in the singlet, triplet, and quintet states were performed. For these calculations, three structural isomers, trans-2:2, 3:1, and 4:0 arrangements, of 1 and 2 were assumed as depicted in Fig. 1. That is, a total of 12 geometric optimizations were performed and their frequencies were calculated for each Ru2 complex. The optimized geometries are summarized in Tables S3–S26. As their results, we clearly found that the ZPEs of the observed structures, i.e., a cis-2:2 arrangement, of 1 and 2 in the triplet state were the most stable in terms of their structures and spin states (see Fig. 5). These results provide clear evidence that the observed structures and spin states of 1 and 2 are energetically favourable.


image file: c9dt02271f-f5.tif
Fig. 5 Relative zero-point energies of regioisomers of 1 and 2 at singlet, triplet, and quintet states. Here, the zero point energies of the cis-2:2 arrangement of 1 and 2 at the triplet state were set at 0 kcal mol−1.

The electronic structures of the optimized geometries of 1 and 2 in the triplet state in CH2Cl2 were then investigated. Fig. 6 shows the electronic structures with the frontier molecular orbitals (MOs) of 1 and 2; the enlarged figures on the MOs of 1 and 2 are given in Fig. S1 and S2, respectively. Here, the SOMO, HOMO, and LUMO are the singly occupied MO, highest-occupied MO, and lowest-unoccupied MO, respectively, and the MOs were obtained as α and β orbitals due to the spin-unrestricted formalism. Although orbital energies differed slightly between 1 and 2 due to the effects of the methyl groups on the ammp ligands in 2, their configurations and orders were similar.


image file: c9dt02271f-f6.tif
Fig. 6 Electronic structures and selected MOs of (a) 1 and (b) 2.

In the occupied MO spaces in 1 and 2, higher SOMOs, i.e. the HOMO(α), and HOMO−2(α) were localized on the amp or ammp moieties with the π orbital characteristic, while lower SOMOs, i.e. the HOMO−1(α), were delocalized over the amp or ammp and Ru2(d) moieties. The HOMO(β) showed some π orbital characteristics of amp or ammp ligands with minor d(Ru2) contributions, and the HOMO−1(β) and HOMO−2(β) were localized on the amp or ammp ligands with the π orbital characteristic. That is, unstable occupied MOs mainly composed of the π orbital characteristic from the amp or ammp ligands. The MOs with Ru–Ru orbital interactions were observed at the HOMO−3(α) to HOMO−8(α) and the HOMO−3(β) to HOMO−5(β) orbitals, and the orders of their orbital configurations were both described as π4δ2σ2π*2.

On the other hand, in the unoccupied MO spaces of 1 and 2, the LUMO(α) and LUMO+1(α) mainly localized on the δ*(Ru2) and σ*(Ru2) orbitals, respectively, and the π* orbital characteristic of amp and ammp was observed from the LUMO+2(α) to LUMO+5(α). In addition, the unoccupied MOs with Ru–Ru orbital interaction characteristics were observed at the LUMO(β), LUMO+1(β), LUMO+2(β), and LUMO+7(β), which are attributed to π*(Ru2), δ*(Ru2), π*(Ru2), and σ*(Ru2) orbitals, respectively. The LUMO+3(β) to LUMO+6(β) of 1 and 2 were attributed to π*(amp) and π*(ammp) orbitals, respectively.

The instability of the singlet and quintet states of 1 and 2 compared with those in the triplet state was further investigated by MO analyses. The orbital configurations of both complexes in the singlet and quintet states can be described as π4δ2σ2π*2 and π4σ2δ1π*2δ*1, respectively. That is, (i) although the orbital configurations of both complexes in the singlet and triplet states have two electrons in the two π* orbitals, these electrons preferentially behave as lone pairs and (ii) the electron occupation of the δ* orbitals destabilize 1 and 2.

Finally, the orbital occupancies and stabilities of the Ru2 moieties in 1 and 2 were elucidated by their natural orbitals (NOs), which were obtained by diagonalizing the first-order density matrices. The occupation numbers (ni) of two π*(Ru2) orbitals in 1 and 2 are both 1.00, indicating that unpaired electrons are fully localized on their orbitals. It is obvious that the σ(Ru2) and two π(Ru2) orbitals of 1 and 2 are formed by stable orbital interactions because their ni values are almost 2.00. Moreover, it is also confirmed that the δ(Ru2) orbital interaction in 1 and 2 are slightly unstable than their σ(Ru2) and two π(Ru2) orbitals because the ni values of δ(Ru2) orbitals in 1 and 2 are 1.88 and 1.87, respectively.

Electrochemical properties

The electrochemical properties of 1 and 2 were investigated via cyclic voltammetry (CV). Fig. 7 shows the CVs of 1 and 2 in CHCl3 containing 0.1 M tetrabutylammonium chloride (TBACl) as an electrolyte, where 1 and 2 showed one reversible wave, i.e., 1/1 and 2/2, at −0.08 and −0.18 V vs. SCE, respectively. The slight negative shift of the redox potentials of 2 compared to that of 1 is due to the effects of the electron-donating methyl groups on the aminopyridinate ligands. The observed redox potentials of 1 and 2 were positively shifted compared to that of [Ru2(bam)4Cl2] (−0.231 V vs. SCE).23 This indicated that the electronic donation from the equatorial ligands to Ru2 units in 1 and 2 are lower than that in [Ru2(bam)4Cl2].
image file: c9dt02271f-f7.tif
Fig. 7 CVs of 1 and 2 in CHCl3 containing TBACl as an electrolyte.

To more deeply understand the electrochemical properties of 1 and 2, DFT calculations were performed.37 Here, we theoretically estimated the redox potentials of 1 and 2 to support the accuracy of the observed potentials and their assignments. The calculated one-electron redox potentials of 1 and 2 were −0.08 and −0.18 V vs. SCE, respectively, in good agreement with their observed potentials. The spin states of the reduced species were both quartet states (S = 3/2), which are well-known spin states in the Ru25+ complexes. The electronic structure analyses of 1 and 2 indicated that the reductions occur at the Ru2 moieties with δ*(Ru2) orbitals.

Absorption properties

Finally, the UV-visible-NIR absorption and diffuse reflectance (DR) spectra of 1 and 2 were measured at 300 K. Fig. 8 shows the UV-visible-NIR absorption spectra of 1 and 2 in CHCl3. The shapes of the absorption spectra of 1 and 2 were similar, but the wavelengths of the absorption band maxima of 2 were slightly red-shifted compared to those of 1. Surprisingly, 1 and 2 showed continuous multiple absorption bands in the visible and NIR region. Specifically, in the NIR region, 1 and 2 showed one low-lying absorption band at approximately 1200–1500 nm and double absorption bands at approximately 880 and 940 nm. Although the NIR absorption features of Ru26+ complexes in a closed-shell singlet state (S = 0) have been reported in the literature,38 the absorption wavelengths of their bands were located at approximately 800–1000 nm, and their molar absorption coefficients (ε) were relatively lower than those of the double NIR absorption bands of 1 and 2. Thus, it can be speculated that the NIR absorption bands of 1 and 2 have some specific absorption characteristics originating from their corresponding electronic structures. Therefore, we then performed time-dependent DFT (TDDFT) calculations for them. Based on the results, the low-lying absorption bands of 1 and 2 at approximately 1200–1500 nm were attributed to the transitions from π(amp or ammp) to δ*(Ru2) orbitals [mixed characteristic of major HOMO(α) → LUMO(α) and minor HOMO−1(β) → LUMO+1(β) transitions], which can be classified as ligand-to-metal charge transfers (LMCTs). In the double NIR absorption bands of 1 and 2, the low-energy bands observed at 930 and 938 nm for 1 and 2, respectively, were attributed to the mixed characteristics of major π(amp or ammp) → δ*(Ru2) transitions with minor π(amp or ammp) → π*(Ru2) and σ(Ru2) → δ*(Ru2) transitions. In contrast, high-energy (shoulder) bands observed at 875 and 880 nm for 1 and 2, respectively, were attributed to the mixed characteristics of π(Ru2) → π*(Ru2) and π(amp or ammp) → δ*(Ru2) transitions. Thus, the double NIR absorption bands of 1 and 2 were determined to be composed of LMCT and d–d transition characteristics.
image file: c9dt02271f-f8.tif
Fig. 8 UV-visible-NIR absorption spectra of 1 (red line) and 2 (blue line) in CHCl3.

In the visible light region, a single intense absorption (1: 685 nm, 2: 683 nm) and two low-intensity absorption bands (1: 545 and 413 nm, 2: 552 and 416 nm) were observed. The former intense bands were theoretically attributed to the mixed characteristics of major π(amp or ammp) → δ*(Ru2) and minor π(Ru2) → δ*(Ru2) transitions. On the other hand, although the two low-intensity absorption bands were composed of various transition characteristics, their transitions mainly occurred from π(amp or ammp) or d(Ru2) orbitals to d(Ru2) orbitals. Therefore, these two bands were determined to be composed of LMCT and d–d transition characteristics. From these results, we confirmed that the dominant transition characteristics of the absorption bands of 1 and 2 in the NIR and visible regions are LMCT and d–d transitions. That is, the π → π* transitions at the amp or ammp moieties and metal-to-ligand charge transfer (MLCT) were not dominant in these regions.

The DR spectra of the solid state of 1 and 2 are provided in Fig. S3 and S4, respectively. Although the relative intensities of the bands of the DR spectra of 1 and 2 slightly differ from those of the absorption spectra of 1 and 2, their band positions were similar. These indicated that 1 and 2 in solution were isostructural with their solid-state forms.

Experimental

Materials and equipment

All chemical reagents, solvents, and gases used in this study were purchased from commercial suppliers and were used without further purification. Infrared spectra (IR) were measured using a Jasco FT-IR 6300 spectrophotometer in KBr pellets at room temperature. Raman spectra were recorded using a Renishaw Raman system 2000 spectrometer equipped with a He–Ne laser (633 nm) as the excitation source. ESI-TOF-MS were recorded using a Bruker micrO-TOF II instrument in a positive ion mode. Here, sodium formate was used as the calibrant. MALDI-TOF-MS (matrix: α-Cyano-4-hydroxycinnamic acid) were recorded using a Bruker-Daltonics-Autoflex mass spectrometer operating in the positive ion mode. Elemental analyses were performed using a Yanaco CHN CORDER MT-6 elemental analyzer installed at Shimane University, Japan. The magnetic susceptibilities were measured in the temperature range of 2.0–300 K using a Quantum Design MPMS SQUID magnetometer in an applied field of 5000 G. Cyclic voltammetry (CV) measurements were performed in dried CHCl3 containing 0.1 M tetra-n-butylammonium chloride (TBACl) as an electrolyte using a BAS ALS-DY 2325 CV system. Here, a glassy carbon disk, platinum wire, and saturated calomel electrode (SCE) were used as the working, counter, and reference electrodes, respectively. UV-visible-NIR absorption spectra were recorded in CHCl3 using a Jasco V-670 spectrophotometer. The solid-state diffuse reflectance (DR) spectra were measured using a Jasco V-670 spectrophotometer equipped with an ISN-923 integrating sphere.

Synthesis of complex 1

A mixture of [Ru2(O2CCH3)4Cl] (236.9 mg, 0.50 mmol), Hamp (470.6 mg, 5.00 mmol), LiCl (423.9 mg, 10.00 mmol), THF (60.0 mL), and triethylamine (1.0 mL) were heated at 348 K for 24 h in air. The resulting solution was filtered through a Celite pad to remove unreacted LiCl, and its filtrate was then evaporated to dryness. The crude product was purified by silica-gel column chromatography (eluent: 8% MeOH in CHCl3), and the obtained solution was evaporated to dryness. The resulting residue was collected using a membrane filter using hexane and then by drying under vacuum at 353 K for 2 hours. Yield: 58.4 mg (18.1%). Anal. Calc. for H20C20N8Cl2Ru2: C, 37.22%; H, 3.12%; N, 17.36%. Found: C, 37.23%; H, 3.13%; N, 17.18%. ESI-TOF-MS: calc. for [M − Cl]+: 610.9581 m/z; found 610.9587 m/z. IR (KBr pellet, cm−1): 3437(w), 3359(w), 3281(w), 3097(w), 3062(w), 3028(w), 2956(w), 2888(w), 1607(m), 1539(w), 1476(s), 1443(s), 1365(w), 1338(w), 1157(w), 1112(w), 1043(w), 1013(w), 857(m), 764(m), 736(w), 666(w), 612(w), 590(w), 558(w), 525(w), 439(w), 410(w). Raman (ν(Ru–Ru)): 334 cm−1.

Synthesis of complex 2

A similar synthetic procedure to that of 1 was used for the synthesis of 2, but Hammp (540.7 mg, 5.00 mmol) was used instead of Hamp. Yield: 302.1 mg (86.1%). Anal. Calc. for H28C24N8Cl2Ru2: C, 41.09%; H, 4.02%; N, 15.97%. Found: C, 40.72%; H, 3.63%; N, 15.94%. ESI-TOF-MS: calc. for [M − Cl]+: 667.0207 m/z; found 667.0197 m/z. IR (KBr pellet, cm−1): 3450(w), 3367(w), 3247(w), 3052(w), 2919(w), 2854(w), 1617(m), 1458(s), 1317(w), 1177(w), 1024(w), 944(w), 853(w), 795(w), 757(w), 656(w), 607(w), 548(w), 463(w). Raman (ν(Ru–Ru)): 315 cm−1.

Single crystal X-ray diffraction analyses

Single crystals of 1 and 2 suitable for X-ray diffraction analyses were obtained by the slow evaporation from DMSO/acetone solutions of 1 and 2. Diffraction data of 1 were collected at 150 K using a Rigaku Saturn 724 CCD system equipped with a Mo rotating-anode X-ray generator with monochromated Mo-Kα radiation (λ = 0.71075 Å) installed in the Okayama University of Science and were processed with the Rigaku CrystalClear program, while those of 2 were collected at 150 K using a Rigaku XtaLAB AFC11 system equipped with a Mo rotating-anode X-ray generator with monochromated Mo-Kα radiation (λ = 0.71075 Å) installed in the Institute of Molecular Science (IMS) and were processed using the CrysAlisPro program. The structures of 1 and 2 were solved by direct methods SIR-2011 and SIR-2004,39,40 respectively, and refined using the full-matrix least-squares technique F2 with SHELXL2014 equipped in the Rigaku CrystalStructure software.41 Non-hydrogen atoms were refined with anisotropic displacement and almost all of the hydrogen atoms were located at the calculated positions and refined as riding models. The crystal data and the details of the data collection and refinement of 1 and 2 are summarized in Table 1 and can be obtained as CIFs from Cambridge Crystallographic Data Center (CCDC). Deposition numbers of 1 and 2 are CCDC 1916683 and 1916684, respectively.

Details of uDFT calculations

All unrestricted density functional theory (uDFT) calculations were performed using the Gaussian 09 C.01 program package.36 From the benchmark calculations of the redox potentials of the Ru26+ complexes in our group, the hybrid DFT functional method, uB3P86, was selected and used in all calculations of this study. The SDD, 6-311G*, and 6-31G* basis sets were employed for Ru, Cl, and other atoms, respectively. Initial molecular geometries of 1 and 2 were obtained from CIF files, and then, their geometries are optimized as needed. The solvent effect of CHCl3 was considered by the polarizable continuum model (PCM). Zero-point energies (ZPEs) were obtained from the vibrational frequency analyses of the optimized geometries of 1 and 2. The redox potentials were estimated by the method (with the Gibbs free energy change) developed by Noodleman.37 The natural orbitals (NOs) were obtained by diagonalizing the first order density matrix.42,43 The spin-allowed excitation was calculated by the time-dependent DFT (TDDFT). The results of TDDFT (excitation wavelength, oscillator strength, and excitation characteristics) of 1 and 2 are summarized in Tables S27 and S28, respectively.

Conclusions

In this study, two Ru2 complexes with aminopyridinate ligands were prepared and characterized. Single crystal X-ray diffraction analyses revealed that 1 and 2 are Ru26+ complexes, where the aminopyridinate (and N^N-donor-type) ligands adopt the cis-2:2 arrangement about the Ru26+ core, the first of its kind being reported. The oxidation and spin states of the Ru2 units in 1 and 2 were determined to be Ru26+ and triplet states, respectively, using the combined technique of magnetic susceptibility measurements and uDFT calculations. Moreover, relative ZPE analyses of the regioisomers of 1 and 2 clearly showed that the cis-2:2 arrangement and triplet spin state were most energetically favourable (stable). To the best of our knowledge, this study is the first theoretical investigation of the regioisomers and spin states of the Ru2 complexes with N^N-donor-type bridging ligands. The interesting absorption features of 1 and 2 were also confirmed; low-lying NIR and intense NIR-visible absorption bands, which were theoretically attributed to LMCT and LMCT/d–d transitions, respectively, were observed in both the solution and solid states. These results indicate that aminopyridinate ligands are promising and suitable bridging ligands for the synthesis of Ru26+ complexes.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work has been supported by the Grant-in-Aid for Scientific Research (No. 19K15588, 18H05166, and 17J11019) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. N. Y. acknowledges the Grant-in-Aid for JSPS research fellow. The authors are grateful to Dr Michiko Egawa (Shimane University) for her measurements of elemental analyses.

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Footnote

Electronic supplementary information (ESI) available. CCDC 1916683 and 1916684. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9dt02271f

This journal is © The Royal Society of Chemistry 2019