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Article

6-Bromo-2-hydroxypyridinate-bridged Paddlewheel-Type Dirhodium Complex Isomers: Synthesis, Crystal Structures, Electrochemical Properties, and Structure-Dependent Absorption Properties

by
Kozo Sato
,
Natsumi Yano
and
Yusuke Kataoka
*
Department of Chemistry, Graduate School of Natural Science and Technology, Shimane University, 1060, Nishikawatsu, Matsue 690-8504, Shimane, Japan
*
Author to whom correspondence should be addressed.
Inorganics 2024, 12(3), 70; https://doi.org/10.3390/inorganics12030070
Submission received: 27 January 2024 / Revised: 19 February 2024 / Accepted: 22 February 2024 / Published: 26 February 2024
(This article belongs to the Section Coordination Chemistry)
Browse Figures
Versions Notes

Abstract

:
Two new paddlewheel-type dirhodium (Rh2) complex isomers, formulated as trans-2,2- and 3,1-forms of [Rh2(bhp)4] (bhp = 6-bromo-2-hydroxypyridinate), were obtained by the reaction of 6-bromo-2-hydroxypyridine with [Rh2(O2CCH3)4(H2O)2] and characterized by NMR, ESI-MS, and elemental analyses. Single crystal X-ray diffraction analyses clarified that the crystal structure of trans-2,2-form takes a conventional paddlewheel-type dimer structure with no axial coordination ligands, i.e., trans-2,2-[Rh2(bhp)4], whereas that of the 3,1-form changed significantly depending on the kinds of solvent used for crystallization processes; dimer-of-dimers-type tetrarhodium complex, i.e., 3,1-[Rh2(bhp)4]2, and a conventional paddlewheel-type dimer complex with an axial DMF ligand, i.e., 3,1-[Rh2(bhp)4(DMF)], were observed. The 3,1-form showed unique absorption changes that were not observed in the trans-2,2-form; the trans-2,2-form showed an absorption band at approximately 780 nm both in the solid state and in solution (CH2Cl2 and DMF), whereas the 3,1-form showed a similar absorption band at 783 nm in CH2Cl2 solution, but their corresponding bands were blue-shifted in solid state (655 nm) and in DMF solution (608 nm). The molecular structures and the origin of their unique absorption properties of these Rh2 complexes were investigated using density functional theory (DFT) and time-dependent DFT (TDDFT).

1. Introduction

Paddlewheel-type dinuclear complexes with multiple bonds or orbital interactions between two metal atoms have been developed extensively not only because of the interest in their molecular structures, electronic features, and functionalities but also because they are useful building blocks for supramolecular complexes, coordination polymers, and metal-organic frameworks (MOFs) [1,2,3,4,5,6]. As is well known, this structural motif can be formed using most transition metal ions. Among them, dirhodium(II) (Rh24+) complexes [1,7,8,9,10], which have a single Rh-Rh bond originating from the σ2π4δ2δ*2π*4 orbital interactions, are particularly interesting because of their excellent catalytic [11,12,13,14,15,16,17], sensing [18,19], and medical functionalities [20]. These functionalities are strongly influenced by the type of bridging ligand coordinated at the equatorial positions of the Rh2 core. The majority of bridging ligands in Rh2 complexes are carboxylates, but other bidentate ligands with heterodonor sites, such as amidates [1,10] and amidinates [8], can also be used as bridging ligands for Rh2 complexes.
2-Hydroxypyridine (hp) derivatives [21,22,23,24,25,26] are also available as bridging ligands, and homoleptic paddlewheel-type dinuclear complexes with four hp derivative ligands, [M2(hp)4], are expected to form four coordination isomers: 4,0-, 3,1-, trans-2,2-, and cis-2,2-form structures (Scheme 1). In this regard, it is known that the introduction of a substituent at the 6-position of the hp ligand can somewhat control its coordination isomerization. For example, 6-fluoro-2-hydroxypyridinate (fhp)-coordinated Rh2 complex, [Rh2(fhp)4], adopts the 4,0-form structure as the main product [21,22], whereas 6-chloro-2-hydroxypyridine (chp) or 6-methyl-2-hydroxypyridine (mhp) coordinated Rh2 complexes, i.e., [Rh2(chp)4] or [Rh2(mhp)4], afford trans-2,2- and 3,1-form structures as the main and minor products, respectively [23]. Single crystal X-ray diffraction (SCXRD) studies of 6-substituted hp-bridged Rh2 complexes clarified that the 4,0- and 3,1-form structures can coordinate an axial donor ligand at the Rh atom, which is surrounded by four and three O atoms of hp derivatives, respectively, whereas trans-2,2-form structure cannot possess donor ligands at the axial positions of the Rh-Rh bond because of the steric hindrance of the substituents [21,22,23,24].
Although reports on Rh2 complexes coordinated with four hp derivatives or related N^O-bridging ligands are still relatively limited, they exhibit excellent catalytic activity in electrochemical and photochemical hydrogen evolution reactions [22], enantioselective S-H and C-H insertion reactions [15,27], and cyclopropanation reactions [28]. Therefore, further development and detailed fundamental studies of Rh2 complexes with hp derivatives are required. Herein, we report the synthesis, characterization, crystal structures, electrochemical properties, and absorption spectral features of paddlewheel-type Rh2 complexes with 6-bromo-2-hydroxypyridinate (bhp). From NMR, ESI-MS, elemental analyses, and SCXRD analyses, it was revealed that two coordination isomers, trans-2,2- and 3,1-forms, can be isolated. Interestingly, the crystal structures of the 3,1-form change significantly depending on the type of solvent used for the crystallization processes; a dimer-of-dimers-type complex, 3,1-[Rh2(bhp)4]2, and a dimer complex with an axial DMF ligand, 3,1-[Rh2(bhp)4(DMF)], were obtained. Moreover, the 3,1-form exhibited unique absorption spectral changes that were not observed for the trans-2,2-form. The molecular structures and the origin of unique absorption properties were also closely investigated by the density-functional theory (DFT) and time-dependent DFT (TDDFT) calculations and were discussed in this study.

2. Results

2.1. Synthesis and Characterizations

The coordination isomers, trans-2,2- and 3,1-forms of [Rh2(bhp)4], were obtained by the reaction of [Rh2(O2CCH3)4(H2O)2] with 10 equivalents of 6-bromo-2-hydroxypyridine in chlorobenzene under N2 atmosphere, followed by column chromatography (silica gel) and drying at 393 K under vacuum. An orange powder of trans-2,2-form and a yellowish green powder of 3,1-form were isolated in 63.6% and 2.5% yields, respectively, indicating that the coordination structure of trans-2,2-form is energetically more stable than that of 3,1-form. Although some precipitates, including rhodium black, were found in the reaction solution after refluxing, they are insoluble in common solvents and could not be structurally characterized.
The isolated trans-2,2- and 3,1-forms were characterized using ESI-MS, NMR spectroscopy, and elemental analyses. In the ESI-MS spectra of the trans-2,2- and 3,1-forms, intense peaks appeared at 920.5531 and 920.5535 m/z, respectively, which correspond to the calculated [M + Na]+ value (920.5555 m/z) for the complex of two Rh atoms and four bhp ligands. As shown in Figures S1 and S2, the isotope distributions of the trans-2,2- and 3,1-forms matched well with each other and with the simulation results for [Rh2(bhp)4]. No other intense peaks were observed in the spectra. Elemental analyses also revealed that the observed CHN ratios of trans-2,2- and 3,1-forms correspond to the calculated ratio of dehydrated [Rh2(bhp)4]. The 1H NMR of trans-2,2-form showed three doublet of doublet signals at 7.24, 6.78, and 6.35 ppm, with integral ratio of 1:1:1 (Figure S3). This spectral feature is similar to that of trans-2,2-[Rh2(chp)4] with D2h molecular symmetry [29]. In the 13C{1H} NMR spectrum of trans-2,2-form, five resonances were observed as well resolved signals in the aromatic region. On the other hand, the proton signals of 3,1-form were observed in the region of 6.21–7.20 ppm, with integral ratio of 1:1:1:1:1:2:2:3[overlapped signal] (Figure S4). In addition, fifteen resonances were overserved in the 13C{1H} NMR spectrum of 3,1-form. These NMR results are consistent with the 3,1-arrangement structure with C2v molecular symmetry.

2.2. Single Crystal X-ray Diffraction Analyses

Single crystals of trans-2,2-[Rh2(bhp)4], 3,1-[Rh2(bhp)4]2, and 3,1-[Rh2(bhp)4(DMF)] suitable for SCXRD analysis were grown using the methods described in the experimental section. Diffraction analyses revealed that trans-2,2-[Rh2(bhp)4] crystallized in the P 21/n space groups (monoclinic system), whereas 3,1-[Rh2(bhp)4]2 and 3,1-[Rh2(bhp)4(DMF)] crystallized in the P-1 (triclinic system) and P 212121 space groups (orthorhombic system), respectively. Figure 1 shows the crystal structures with selected numbering schemes of trans-2,2-[Rh2(bhp)4], 3,1-[Rh2(bhp)4]2, and 3,1-[Rh2(bhp)4(DMF)]. Selected bond lengths and angles of their crystal structures are summarized in Tables S1–S3.
As expected, the obtained structures consist of the paddlewheel core and are isomeric. In trans-2,2-[Rh2(bhp)4], the Rh2 core is bridged by four bhp ligands with trans-2,2-arrangemnet and is not coordinated by solvent molecules as the axial ligands because of the bulky bromide groups, similar to the structures of trans-2,2-[Rh2(chp)4] and trans-2,2-[Rh2(mhp)4] [23]. By contrast, the Rh2 core in 3,1-[Rh2(bhp)4(DMF)] is coordinated by four bhp ligands with 3,1-arrangement, and the Rh atom surrounded by three oxygen atoms and one nitrogen atom of the bhp ligands is further coordinated by a DMF molecule, which was used as the recrystallization solvent. In the 3,1-[Rh2(bhp)4]2 structure, two 3,1-[Rh2(bhp)4] units self-aggregate with bonds between the oxygen atom of one bhp ligand in 3,1-[Rh2(bhp)4] and the axial position of another Rh2 unit. Similar “dimer-of-dimers-type” structure was found in the crystal structure of 3,1-[Ru2(chp)4]2 [30]. The averaged Rh2-Oaxial bond lengths in 3,1-[Rh2(bhp)4]2 and 3,1-[Rh2(bhp)4(DMF)] are 2.293 and 2.173 Å, respectively, which are sizably longer than the averaged Rh2-Oequatrial bond lengths in 3,1-[Rh2(bhp)4]2 (2.022 Å) and 3,1-[Rh2(bhp)4(DMF)] (2.031 Å). These results indicate that the coordination energies of the Rh-Oaxial bonds in 3,1-[Rh2(bhp)4]2 and 3,1-[Rh2(bhp)4(DMF)] are considerably weaker than those of the Rh2-Oequatrial bonds. The bhp ligands in 3,1-[Rh2(bhp)4]2 and 3,1-[Rh2(bhp)4(DMF)] are slightly twisted, whereas those in trans-2,2-[Rh2(bhp)4] are almost planar: averaged dihedral angles of N-Rh-Rh-O in 3,1-[Rh2(bhp)4]2, 3,1-[Rh2(bhp)4(DMF)], and trans-2,2-[Rh2(bhp)4] are 22.59°, 23.39°, and 1.66°, respectively. Ligand twisting is presumed to be a structural uniqueness of the 3,1-form, because similar twisted (3,1-form) structures were found when mhp or chp were used as the bridging ligands for Rh2 complexes [23]. The Rh-Rh bond length of trans-2,2-[Rh2(bhp)4] was determined to be 2.3902(4) Å, which is slightly longer than that of trans-2,2-[Rh2(chp)4] (2.379(1) Å) and trans-2,2-[Rh2(mhp)4] (2.359(1) Å) [23]. This slight increase in the Rh-Rh bond length is presumably due to the electron-withdrawing effects of the bromide groups of the bhp ligands. The Rh-Rh bond length of 3,1-[Rh2(bhp)4]2 and 3,1-[Rh2(bhp)4(DMF)] are 2.3704(4) and 2.3726(11) Å, which are slightly (0.02 Å) shorter than those of trans-2,2-[Rh2(bhp)4] and 4,0-[Rh2(fhp)4(DMF)] (2.3970 Å) [22]. The shorter Rh-Rh bond lengths in 3,1-[Rh2(bhp)4]2 and 3,1-[Rh2(bhp)4(DMF)] than in trans-2,2-[Rh2(bhp)4] may be owing to the axial coordination effect rather than the influence of the structural arrangement.

2.3. Structural Stabilities and Favorable Spin States

To investigate the structural characteristics of 3,1-[Rh2(bhp)4], restricted DFT (B3LYP functional) calculations of 3,1-[Rh2(bhp)4(DMF)] in DMF and 3,1-[Rh2(bhp)4]2 in the gas phase were computed. The zero-point energy (ZPE) of optimized geometry of 3,1-[Rh2(bhp)4]2 is 5.67 kcal/mol more stable than the double value of ZPE of optimized geometry of 3,1-[Rh2(bhp)4]. Moreover, the basis set superposition error (BSSE)-corrected aggregation energy between two 3,1-[Rh2(bhp)4] fragments in 3,1-[Rh2(bhp)4]2 was calculated to be −9.69 kcal/mol. These results suggest that the desolvated 3,1-[Rh2(bhp)4] is more stable in forming a dimer-of-dimers structure than the discrete dimer structure in the solid state. In 3,1-[Rh2(bhp)4(DMF)], the BSSE-corrected binding energy between 3,1-[Rh2(bhp)4] and DMF fragments was estimated to be −11.23 kcal/mol, indicating that 3,1-[Rh2(bhp)4] in DMF solution is more favorable to form the DMF-adducted structure 3,1-[Rh2(bhp)4(DMF)] than the dimer-of-dimers-type structure 3,1-[Rh2(bhp)4]2. These calculated results are consistent with the experimentally obtained crystal structures.
The frontier molecular orbitals (MOs) of the optimized geometries of trans-2,2-[Rh2(bhp)4] and 3,1-[Rh2(bhp)4] in CH2Cl2, 3,1-[Rh2(bhp)4(DMF)] in DMF, and 3,1-[Rh2(bhp)4]2 in the gas phase are shown in Figure 2. In all the complexes, the highest occupied MO (HOMO) and lowest unoccupied MO (LUMO) are mainly localized on the δ*(Rh2)/π(bhp) and σ*(Rh2) orbitals, respectively. The orbital energies of HOMO and LUMO of trans-2,2-[Rh2(bhp)4] and 3,1-[Rh2(bhp)4] are almost identical, between the complexes. The orbital energies of LUMO of 3,1-[Rh2(bhp)4(DMF)] and 3,1-[Rh2(bhp)4]2 are more destabilized than that of 3,1-[Rh2(bhp)4] because of the anti-bonding orbital interaction between the σ*(Rh2) and p(Oaxial) orbitals.

2.4. Electrochemical Properties

Cyclic voltammetry (CV) measurements were performed to investigate the electrochemical properties of trans-2,2- and 3,1-forms of [Rh2(bhp)4] in CH2Cl2 and DMF (see Figure 3). In CH2Cl2, the CV diagram of trans-2,2-[Rh2(bhp)4] exhibited two reversible redox waves at E1/2 = 1.15 and −1.28 V vs. SCE, whereas that of 3,1-[Rh2(bhp)4] showed one reversible wave and one irreversible wave at E1/2 = 1.07 and −1.17 V vs. SCE, respectively. From the results of the DFT calculations of trans-2,2- and 3,1-forms of [Rh2(bhp)4] in CH2Cl2, the one-electron oxidation and reduction processes could be assigned to occur at the δ*(Rh2)/π(bhp) and σ*(Rh2) orbitals, respectively, which are similar to those of other paddlewheel-type Rh2 complexes [16,31]. Observed E1/2 values of trans-2,2- and 3,1-forms of [Rh2(bhp)4] are more positive than those of trans-2,2-[Rh2(mhp)4] (E1/2 = 0.91 V and −1.36 V vs. Ag/AgCl in CH2Cl2) [29] because of the electron-withdrawing effect of bromide groups in bhp ligands. Similar CV features were also found when DMF was used as the solvent instead of CH2Cl2, whereas observed potentials in DMF were relatively shifted to positive direction than those in CH2Cl2; the E1/2 values of trans-2,2-[Rh2(bhp)4] in DMF are 1.25 and −1.14 V vs. SCE, whereas those of 3,1-[Rh2(bhp)4] are 1.13 and −1.12 V vs. SCE. These results indicate that (i) 3,1-[Rh2(bhp)4] is more susceptible to one-electron oxidation than trans-2,2-[Rh2(bhp)4], and (ii) the one-electron reduction process of 3,1-[Rh2(bhp)4] is accompanied by structural changes, whereas that of trans-2,2-[Rh2(bhp)4] is not.

2.5. Absorption Properties

The visible absorption spectra in solution (CH2Cl2 and DMF) and diffuse reflectance (DR) spectra in the solid state were measured for trans-2,2- and 3,1-forms of [Rh2(bhp)4]. As shown in Figure 4a, the spectral shape of trans-2,2-[Rh2(bhp)4] in CH2Cl2 is nearly identical to that in DMF; an intense “A-band” and a shoulder “B-band” were observed at approximately 780 nm [CH2Cl2: 783 nm (ε = 815). DMF: 780 nm (ε = 816)] and 450–460 nm, respectively. Similar spectral characteristics were also observed in the DR spectrum of trans-2,2-[Rh2(bhp)4] (see Figure 4b), although the band maxima of the DR spectrum are slightly blue-shifted compared to those of the absorption spectra. On the other hand, the spectral feature of 3,1-[Rh2(bhp)4] drastically changed depending on the solvent used; the A-band in CH2Cl2 was observed at 783 nm (ε = 475), similar to trans-2,2-[Rh2(bhp)4], whereas that in DMF was observed at 608 nm (ε = 260). The absorption coefficient of the A-band of 3,1-[Rh2(bhp)4] is relatively lower than that of trans-2,2-[Rh2(bhp)4]. The B-band of 3,1-[Rh2(bhp)4] was observed at approximately 450–460 nm, which is nearly the same as that of trans-2,2-[Rh2(bhp)4]. The shape of the DR spectrum of 3,1-[Rh2(bhp)4], which possesses an A-band at 655 nm and a B-band at 450–460 nm, is apparently different from the shapes of the absorption spectrum of 3,1-[Rh2(bhp)4] in CH2Cl2 and the DR spectrum of trans-2,2-[Rh2(bhp)4], but similar to that of 3,1-[Rh2(bhp)4] in DMF. The previous literature reports that the A-band of a paddlewheel-type Rh2 complex is blue-shifted when the ligands are coordinated to the axial positions of the Rh2 core [32,33]. Therefore, it is considered that nearly all the as-synthesized 3,1-[Rh2(bhp)4] in the solid state forms a dimer-of-dimers structure by self-aggregation, as found in the crystal structure of 3,1-[Rh2(bhp)4]2 but does not form a discrete dimer structure.
To clarify the absorption spectral features of trans-2,2 and 3,1-forms of [Rh2(bhp)4], TDDFT calculations of trans-2,2-[Rh2(bhp)4] and 3,1-[Rh2(bhp)4] in CH2Cl2, 3,1-[Rh2(bhp)4-(DMF)] in DMF, and 3,1-[Rh2(bhp)4]2 in the gas phase were performed. The computed results, such as the excitation wavelengths, oscillator strengths, and dominant excitation characters are summarized in Tables S5–S7. The simulated excitation wavelengths of the complexes generally reproduced the observed absorption wavelengths. That is, the simulated excitation wavelengths of the A-bands of 3,1-[Rh2(bhp)4(DMF)] in DMF and 3,1-[Rh2(bhp)4]2 were blue-shifted with respect to that of 3,1-[Rh2(bhp)4] in CH2Cl2, similarly to their observed spectra. The excitation characters of A-band of trans-2,2-[Rh2(bhp)4] and 3,1-[Rh2(bhp)4] in CH2Cl2 possess the π*(Rh2)→σ*(Rh2) excitations as the dominant characters, whereas those of 3,1-[Rh2(bhp)4(DMF)] and 3,1-[Rh2(bhp)4]2 comprise π*(Rh2)→σ*(Rh2) excitations as the dominant characters with δ*(Rh2)→σ*(Rh2) excitations as the minor contribution. These results indicated that the main characteristics of A-band of 3,1-[Rh2(bhp)4], 3,1-[Rh2(bhp)4(DMF)], and 3,1-[Rh2(bhp)4]2 are commonly d(Rh2)→σ*(Rh2) excitation characters. That is, it was clarified that the reason why the absorption wavelengths of the A-band of 3,1-[Rh2(bhp)4(DMF)] and 3,1-[Rh2(bhp)4]2 were found at shorter wavelengths than that of 3,1-[Rh2(bhp)4] is owing to the stabilization and destabilization of the orbital energies of σ*(Rh2), which are obviously changed by the axial coordination of O-donor molecules.

3. Materials and Methods

3.1. Chemicals and Instruments

6-bromo-2hydroxypyridine was purchased from Tokyo Chemical Industries (Tokyo, Japan), and organic solvents were obtained from Wako Pure Chemical Industries (Osaka, Japan) and used as received without further purification. [Rh2(O2CCH3)4(H2O)2] was synthesized according to methods described in the literatures [34]. Electrospray ionization mass spectroscopy (ESI-MS) were performed with a Bruker micrOTOF-II instrument (Bruker, Billerica, MA, USA) in the positive-ion mode. Nuclear magnetic resonance spectra (NMR) were recorded on a JEOL JNM-ECX500 spectrometer (JEOL, Tokyo, Japan) operating at 500 MHz for 1H and 126 MHz for 13C in a DMSO-d6. Chemical shifts are referenced to the residual DMSO signal. Elemental analyses were performed with a YANACO CHN corder MT-6 instrument (Yanaco, Tokyo, Japan). Absorption spectra were measured with a JASCO V-670 spectrophotometer (JASCO, Tokyo, Japan). Diffuse reflectance (DR) spectra were acquired using a JASCO V-670 spectrophotometer equipped with an ISN-923 integrating sphere (JASCO, Tokyo, Japan). Cyclic voltammetry (CV) measurements of the [Rh2(bhp)4] (0.50 mM) in dried DMF and CH2Cl2 solutions containing 0.10 M TBAPF6 were performed using a HOKUTO DENKO HZ-7000 HAG1232m system (Meiden Hokuto Coorporation, Tokyo, Japan) at a scan rate of 100 mV/s. The glassy carbon, platinum wire, and saturated calomel electrode were employed as the working, counter, and reference electrodes, respectively.

3.2. Synthesis of trans-2,2-form and 3,1-Form of [Rh2(bhp)4]

A mixture of [Rh2(O2CCH3)4(H2O)2] (95.6 mg, 0.20 mmol) and 6-bromo-2-hydroxypyridine (348.0 mg, 2.0 mmol) in 15.0 mL chlorobenzene was refluxed under nitrogen atmosphere for 24 h. After cooled to room temperature, the reaction solution was filtered, and the filtrate was then evaporated under reduced pressure. Obtained residue was separated by a silica-gel column chromatography (eluent: CH2Cl2/MeCN/EtOH = 17:2:1 v/v/v). The first and second fractions were evaporated to dryness, dried at 393 K under reduced pressure, and obtained as an orange powder (trans-2,2-form: 114.2 mg, 63.6%) and a yellowish green powder (3,1-form: 4.4 mg, 2.5%), respectively.
Analysis data for trans-2,2-form. 1H NMR (500 MHz, DMSO-d6, δ): 7.24 (dd, J = 8.6, 7.5 Hz, 4H), 6.78 (dd, J = 7.3, 0.8 Hz, 4H), 6.35 (dd, J = 8.6, 1.0 Hz, 4H) ppm. 13C{1H} NMR (126 MHz, DMSO-d6) δ = 177.87, 139.68, 138.88, 114.87, 114.25 ppm. ESI-MS calcd for C20H12Br4N4O4Rh2Na [M + Na]+: 920.5555 m/z; found 920.5531 m/z. EA calcd (%) for C20H12Br4N4O4Rh2: C 26.76, H 1.35, N 6.24; found C 26.76, H 1.51, N 6.34.
Analysis data for 3,1-form. 1H NMR (500 MHz, DMSO-d6, δ): 7.20 (m, 3H), 7.08 (dd, J = 8.6, 7.1 Hz, 1H), 6.78 (dd, J = 7.3, 0.8 Hz, 2H), 6.75 (dd, J = 7.3, 1.2 Hz, 1H), 6.58 (dd, J = 6.9, 1.2 Hz, 1H), 6.38 (dd, J = 8.6, 1.0 Hz, 1H), 6.30 (dd, J = 8.4, 0.8 Hz, 2H), 6.21 (dd, J = 8.8, 1.1 Hz, 1H) ppm. 13C{1H} NMR (126 MHz, DMSO-d6, δ): 180.08, 179.39, 179.35, 140.95, 140.10, 140.03, 139.20, 139.11, 139.02, 117.00, 116.36, 115.63, 114.55, 114.20, 113.99 ppm. ESI-MS calcd for C20H12Br4N4O4Rh2Na [M + Na]+: 920.5555 m/z; found 920.5535 m/z. EA calcd (%) for C20H12Br4N4O4Rh2: C 26.76, H 1.35, N 6.24; found C 26.69, H 1.46, N 6.11.

3.3. Crystallography

Single crystals of trans-2,2-[Rh2(bhp)4] suitable for X-ray diffraction were obtained by slow diffusion of hexane into CH2Cl2 solution containing trans-2,2-form, whereas those of 3,1-[Rh2(bhp)4]2 and 3,1-[Rh2(bhp)4(DMF)] were grown by slow diffusion of diethyl ether into CH2Cl2 solution containing 3,1-form and slow evaporation of DMF solution containing 3,1-form, respectively. Obtained single crystals were carefully mounted on a MiteGen micromount using a paratone-N oil and then were quickly transferred to the cold nitrogen-steam for data collection. X-ray diffraction data of trans-2,2-[Rh2(bhp)4] and 3,1-[Rh2(bhp)4(DMF)] were collected on a Rigaku HyPix-6000 detector system (Tokyo, Japan) with a graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 150 K, whereas that of 3,1-[Rh2(bhp)4]2 was collected on a Rigaku Mercury CCD detector system (Tokyo, Japan) with a graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 150 K. Data processing were performed with CrysAlisPro software (version 1.171.42.49) [35]. The structures were initially solved with SHELXT-2018 program [36] and were then refined with full-matrix least square on F2 using SHELXL program [37] in the Olex 2 software (version 1.5) [38]. All non-hydrogen atoms were refined using anisotropic displacement parameters, whereas hydrogen atoms were placed in calculated positions and refined as riding model. In the refinement of trans-2,2-[Rh2(bhp)4], the residual electron density of disorder solvents was removed by using the solvent mask routine of the Olex 2. Crystallographic data of final refined structures are summarized in Table 1. These crystallographic data can be obtained free of charge from Cambridge Crystallographic Data Centre (CCDC); deposition numbers of trans-2,2-[Rh2(bhp)4], 3,1-[Rh2(bhp)4]2, and 3,1-[Rh2(bhp)4(DMF)] are CCDC-2328797, 2328798, and 2328799, respectively.

3.4. Calculation Details

DFT calculations were performed using the hybrid B3LYP functional [39] with SDD for Rh atoms, aug-cc-pVDZ for N and O atoms, and cc-pVDZ for other atoms by using the Gaussian 16 version C.02 program package [40]. Molecular structures were fully optimized without any symmetry constraints and were then checked by frequency analyses. The polarizable continuum models (PCM) were employed to take account of the solvent effects for the calculations of geometry optimizations, frequency analyses, and MO analyses of 3,1-[Rh2(bhp)4(DMF)] (ε = 37.219 for DMF) and trans-2,2- and 3,1-forms of [Rh2(bhp)4] (ε = 8.93 for CH2Cl2). The aggregation energy and binding energy were estimated with counterpoise method for removing the basis set superposition error (BSSE). Vertical singlet excitation energies, oscillator strengths (f), and assignments of excitation characters were computed with a time-dependent DFT (TDDFT).

4. Conclusions

In this study, two paddlewheel-type Rh2 complex isomers coordinated with bhp ligands, trans-2,2- and 3,1-forms of [Rh2(bhp)4], were successfully prepared and characterized. SCXRD analyses revealed that crystal structure of 3,1-form changed significantly depending on the kinds of solvent used for crystallization processes; dimer-of-dimers-type tetrarhodium complex, i.e., 3,1-[Rh2(bhp)4]2, and a conventional paddlewheel-type dimer complex with an axial DMF ligand, i.e., 3,1-[Rh2(bhp)4(DMF)], are observed, whereas that of trans-2,2-form takes a conventional paddlewheel-type dimer structure with no axial coordination ligands. DFT calculations revealed that 3,1-[Rh2(bhp)4] prefers to be coordinated by DMF solvent or to form the self-aggregated dimer-of-dimers structure rather than the formation of discrete structure with no axial coordination ligand in the solid state. We confirmed that there were no significant differences in the absorption features of trans-2,2-[Rh2(bhp)4] in solution (CH2Cl2 and DMF) and solid states. By contrast, spectral feature of 3,1-[Rh2(bhp)4] drastically changes; the A-band in CH2Cl2 was observed at 783 nm, whereas those in DMF and in solid state were observed at 608 nm and 655 nm, respectively. TDDFT calculations clarified that this spectral difference is caused by the changes in the coordination environments as illustrated in Scheme 2.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics12030070/s1, Figure S1: Observed and simulated ESI-MS spectra of trans-2,2-[Rh2(bhp)4]; Figure S2: Observed and simulated ESI-MS spectra of 3,1-[Rh2(bhp)4]; Figure S3: 1H NMR spectrum of trans-2,2-[Rh2(bhp)4] in DMSO-d6; Figure S4: 1H NMR spectrum of 3,1-[Rh2(bhp)4] in DMSO-d6; Table S1: Selected bond lengths (Å) and angles (°) of trans-2,2-[Rh2(bhp)4]; Table S2: Selected bond lengths (Å) and angles (°) of 3,1-[Rh2(bhp)4(DMF)]; Table S3: Selected bond lengths (Å) and angles (°) of 3,1-[Rh2(bhp)4]2; Table S4: Averaged bond lengths (Å) of optimized geometries of [Rh2(bhp)4] isomers; Table S5: TDDFT results of trans-2,2-[Rh2(bhp)4] in CH2Cl2; Table S6: TDDFT results of 3,1-[Rh2(bhp)4] in CH2Cl2; Table S7: TDDFT results of 3,1-[Rh2(bhp)4] in DMF; Table S8: TDDFT results of 3,1-[Rh2(bhp)4] in gas phase.

Author Contributions

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

Funding

This research was funded by JSPS KAKENHI Grant Numbers 22K14765 and 19K15588.

Data Availability Statement

Crystallographic data of trans-2,2-[Rh2(bhp)4], 3,1-[Rh2(bhp)4(DMF)], and 3,1-[Rh2(bhp)4]2 can be obtained free of charge from Cambridge Crystallographic Data Centre (CCDC); deposition numbers of trans-2,2-[Rh2(bhp)4], 3,1-[Rh2(bhp)4(DMF)], and 3,1-[Rh2(bhp)4]2 are CCDC-2328797, 2328798, and 2328799, respectively.

Acknowledgments

A part of this work was conducted at the Institute for Molecular Science (IMS), supported by Nanotechnology Platform Program <Molecule and Material Synthesis> of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

Conflicts of Interest

The authors declare no conflicts of interest.

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Inorganics 12 00070 sch001
Scheme 1. Molecular structures of (a) bhp- ligand and (b) coordination isomers of [Rh2(bhp)4].
Scheme 1. Molecular structures of (a) bhp- ligand and (b) coordination isomers of [Rh2(bhp)4].
Inorganics 12 00070 sch001
Inorganics 12 00070 g001
Figure 1. Crystal structures of (a) trans-2,2-[Rh2(bhp)4], (b) 3,1-[Rh2(bhp)4(DMF)], and (c) 3,1-[Rh2(bhp)4]2. Thermal ellipsoids were drawn at 30% probability. Hydrogen atoms and solvents were omitted for clarity.
Figure 1. Crystal structures of (a) trans-2,2-[Rh2(bhp)4], (b) 3,1-[Rh2(bhp)4(DMF)], and (c) 3,1-[Rh2(bhp)4]2. Thermal ellipsoids were drawn at 30% probability. Hydrogen atoms and solvents were omitted for clarity.
Inorganics 12 00070 g001
Inorganics 12 00070 g002
Figure 2. Frontier MOs of trans-2,2-[Rh2(bhp)4] and 3,1-[Rh2(bhp)4] in CH2Cl2, 3,1-[Rh2(bhp)4(DMF)] in DMF, and 3,1-[Rh2(bhp)4]2 in the gas phase.
Figure 2. Frontier MOs of trans-2,2-[Rh2(bhp)4] and 3,1-[Rh2(bhp)4] in CH2Cl2, 3,1-[Rh2(bhp)4(DMF)] in DMF, and 3,1-[Rh2(bhp)4]2 in the gas phase.
Inorganics 12 00070 g002
Inorganics 12 00070 g003
Figure 3. CV diagrams of 0.50 mM trans-2,2-[Rh2(bhp)4] (red) and 3,1-[Rh2(bhp)4] (blue) in CH2Cl2 (solid line) and DMF (dashed line) solutions containing 0.10 M TBAPF6.
Figure 3. CV diagrams of 0.50 mM trans-2,2-[Rh2(bhp)4] (red) and 3,1-[Rh2(bhp)4] (blue) in CH2Cl2 (solid line) and DMF (dashed line) solutions containing 0.10 M TBAPF6.
Inorganics 12 00070 g003
Inorganics 12 00070 g004
Figure 4. (a) Absorption spectra of trans-2,2-[Rh2(bhp)4] (red) and 3,1-[Rh2(bhp)4] (blue) in CH2Cl2 (solid line) and DMF (dashed line), (b) DR spectra of trans-2,2-[Rh2(bhp)4] (red) and 3,1-[Rh2(bhp)4] (blue).
Figure 4. (a) Absorption spectra of trans-2,2-[Rh2(bhp)4] (red) and 3,1-[Rh2(bhp)4] (blue) in CH2Cl2 (solid line) and DMF (dashed line), (b) DR spectra of trans-2,2-[Rh2(bhp)4] (red) and 3,1-[Rh2(bhp)4] (blue).
Inorganics 12 00070 g004
Inorganics 12 00070 sch002
Scheme 2. Structural changes of 3,1-[Rh2(bhp)4]2 in DMF and CH2Cl2.
Scheme 2. Structural changes of 3,1-[Rh2(bhp)4]2 in DMF and CH2Cl2.
Inorganics 12 00070 sch002
Table 1. Crystallographic data of trans-2,2-[Rh2(bhp)4], 3,1-[Rh2(bhp)4]2, and 3,1-[Rh2(bhp)4(DMF)].
Table 1. Crystallographic data of trans-2,2-[Rh2(bhp)4], 3,1-[Rh2(bhp)4]2, and 3,1-[Rh2(bhp)4(DMF)].
trans-2,2-[Rh2(bhp)4]3,1-[Rh2(bhp)4]23,1-[Rh2(bhp)4(DMF)]
Chemical formulaC20H12Br4N4O4Rh2C42H28Br8Cl4N8O8Rh4C23H19Br4N5O5Rh2
Formula weight897.801965.44970.89
Crystal systemmonoclinictriclinicorthorhombic
Space groupP 21/nP-1P 212121
a (Å)10.7274(3)11.7389(3)9.7976(4)
b (Å)15.1958(4)14.1558(4)12.1892(6)
c (Å)15.1861(4)18.6200(4)23.3582(10)
α (deg)90109.831(2)90
β (deg)103.275(3)94.751(2)90
γ (deg)90109.617(2)90
V (Å3)2409.36(12)2673.30(13)2789.6(2)
Z424
Dcalc (g cm−3)2.4752.4422.312
μ (mm−1)8.0407.4716.957
F(000)1688.018561848.0
R1 (I > 2σ(I))0.03740.03190.0465
wR2 (I > 2σ(I))0.08790.06670.1007
R1 (all data)0.04430.04650.0608
wR2 (all data)0.09040.07160.1055
GOF on F21.0691.0131.064
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Sato, K.; Yano, N.; Kataoka, Y. 6-Bromo-2-hydroxypyridinate-bridged Paddlewheel-Type Dirhodium Complex Isomers: Synthesis, Crystal Structures, Electrochemical Properties, and Structure-Dependent Absorption Properties. Inorganics 2024, 12, 70. https://doi.org/10.3390/inorganics12030070

AMA Style

Sato K, Yano N, Kataoka Y. 6-Bromo-2-hydroxypyridinate-bridged Paddlewheel-Type Dirhodium Complex Isomers: Synthesis, Crystal Structures, Electrochemical Properties, and Structure-Dependent Absorption Properties. Inorganics. 2024; 12(3):70. https://doi.org/10.3390/inorganics12030070

Chicago/Turabian Style

Sato, Kozo, Natsumi Yano, and Yusuke Kataoka. 2024. "6-Bromo-2-hydroxypyridinate-bridged Paddlewheel-Type Dirhodium Complex Isomers: Synthesis, Crystal Structures, Electrochemical Properties, and Structure-Dependent Absorption Properties" Inorganics 12, no. 3: 70. https://doi.org/10.3390/inorganics12030070

APA Style

Sato, K., Yano, N., & Kataoka, Y. (2024). 6-Bromo-2-hydroxypyridinate-bridged Paddlewheel-Type Dirhodium Complex Isomers: Synthesis, Crystal Structures, Electrochemical Properties, and Structure-Dependent Absorption Properties. Inorganics, 12(3), 70. https://doi.org/10.3390/inorganics12030070

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