Coordination-Induced Self-Assembly of a Heteroleptic Paddlewheel-Type Dirhodium Complex

: A novel heteroleptic paddlewheel-type dirhodium (Rh 2 ) complex [Rh 2 (O 2 CCH 3 ) 3 (PABC)] ( 1 ; PABC = para -aminobenzenecarboxylate), which has an amino group as a potential donor site for coordination with the metal ion, was synthesized and characterized by 1 H NMR, ESI-TOF-MS, infrared spectra, and elemental analysis. The slow evaporation of N,N -dimethylformamide (DMF)-dissolved 1 produces the purple-colored crystalline polymeric species [Rh 2 (O 2 CCH 3 ) 3 (PABC)(DMF)] n ( 1P ). Single-crystal and powder X-ray di ﬀ raction analyses, as well as thermo-gravimetric analysis, clariﬁed that 1P formed a one-dimensional polymeric structure, in which the two axial sites of the Rh 2 ion in 1P are coordinated by a DMF molecule and an amino group of the PABC ligand of the neighboring molecule 1 , by coordination-induced self-assembly (polymerization) with an Rh-amino bond. The reversible structural change (self-assembly and disassembly transformations) between the discrete species [Rh 2 (O 2 CCH 3 ) 3 (PABC)(DMF) 2 ] ( 1D ; green solution) and the polymeric species 1P (purple solid) was accompanied by a color change, which easily occurred by the dissolution and evaporation procedures with DMF.

With this background in mind, this study integrates the two abovementioned synthetic strategies to establish a new synthetic strategy for the synthesis of Rh2-SBU-based CPs and MOFs. Specifically, we prepared a new heteroleptic Rh2 complex [Rh2(O2CCH3)3(PABC)] (1; PABC = para-aminobenzenecarboxylate), in which the amino group of the PABC ligand is coordinated with the axial sites of the Rh2 unit. As shown in Figure 1, although complex 1 exists as a discrete molecular structure [Rh2(O2CCH3)3(PABC)(DMF)2] (1D) in N,N-dimethylformamide (DMF), it was easily self-assembled to form a one-dimensional polymeric structure [Rh2(O2CCH3)3(PABC)(DMF)]n (1P) by the evaporation of the DMF solvent. Evidently, the polymeric species 1P can be re-dissolved in DMF, which indicates that the self-assembly and disassembly transformations between 1D and 1P easily occurred by the dissolution and evaporation procedures with DMF. We believe that this study affords a new strategy for the construction of self-assembled CPs with paddlewheel-type Rh2-SBUs.

General
All materials, solvents, and gases used in this study were purchased from commercial suppliers and were used without further purifications. [Rh2(O2CCH3)4(H2O)2] was prepared according to the literature [36]. Proton nuclear magnetic resonance ( 1 H NMR) spectra were performed with a JEOL-500SS (500 MHz) spectrometer (Tokyo, Japan) with DMF-d7 as the solvent. Chemical shifts were referenced to residual DMF ( = 2.75 ppm). Electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) was performed with a Bruker micrO-TOF II analyzer (Billerica, MS, USA). The instrument was calibrated with a solution of sodium formate. Infrared spectra were obtained with a JASCO FT/IR 6300 spectrometer equipped with a JASCO ATR PRO ONE module (Tokyo, Japan). Powder X-ray diffraction analysis was performed on a Rigaku SmartLab X-ray diffractometer with Cu K radiation (Tokyo, Japan). Thermo-gravimetric (TG) analysis was conducted with a MAC Science TG-DTA 2000S (Billerica, MS, USA) with a heating rate of 4 K/min under a nitrogen atmosphere. The UV-visible absorption spectrum in DMF was recorded with a JASCO V-670 spectrometer (Tokyo, Japan). The diffuse reflectance spectrum was measured with a JASCO V-670 spectrometer equipped with a JASCO ISN-923 integrating sphere (Tokyo, Japan). Elemental analysis was conducted on a Yanaco CHN CORDER MT-6 installed at Shimane University, Japan.

General
All materials, solvents, and gases used in this study were purchased from commercial suppliers and were used without further purifications. [Rh 2 (O 2 CCH 3 ) 4 (H 2 O) 2 ] was prepared according to the literature [36]. Proton nuclear magnetic resonance ( 1 H NMR) spectra were performed with a JEOL-500SS (500 MHz) spectrometer (Tokyo, Japan) with DMF-d 7 as the solvent. Chemical shifts were referenced to residual DMF (δ = 2.75 ppm). Electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) was performed with a Bruker micrO-TOF II analyzer (Billerica, MS, USA). The instrument was calibrated with a solution of sodium formate. Infrared spectra were obtained with a JASCO FT/IR 6300 spectrometer equipped with a JASCO ATR PRO ONE module (Tokyo, Japan). Powder X-ray diffraction analysis was performed on a Rigaku SmartLab X-ray diffractometer with Cu Kα radiation (Tokyo, Japan). Thermo-gravimetric (TG) analysis was conducted with a MAC Science TG-DTA 2000S (Billerica, MS, USA) with a heating rate of 4 K/min under a nitrogen atmosphere. The UV-visible absorption spectrum in DMF was recorded with a JASCO V-670 spectrometer (Tokyo, Japan). The diffuse reflectance spectrum was measured with a JASCO V-670 spectrometer equipped with a JASCO ISN-923 integrating sphere (Tokyo, Japan). Elemental analysis was conducted on a Yanaco CHN CORDER MT-6 installed at Shimane University, Japan.

Single-Crystal X-Ray Diffraction Analysis
Single crystals of 1P suitable for single-crystal X-ray diffraction analysis were obtained by the slow evaporation of the DMF solution of 1. The X-ray diffraction data of 1P were collected at 150 K using a RIGAKU Mercury system equipped with a rotating-anode X-ray generator with Mo Kα radiation (λ = 0.71075 Å) installed at the Institute of Molecular Science and were processed using the RIGAKU CrystalClear-SM Expert 2.0 program. The structure of 1P was solved by the direct method SIR-2011 [37] and was refined using the full-matrix least-squares technique F 2 with SHELXL2014 [38] equipped in the RIGAKU CrystalStructure 4.3 software. Non-hydrogen atoms were refined with anisotropic displacement, and hydrogen atoms were located at the calculated positions and were refined as riding models. The crystal data and details of the data collection and refinement of 1P are summarized in Table 1 and can be obtained as a CIF from the Cambridge Crystallographic Data Center (CCDC). The deposition number of 1P is CCDC-1973889.

Theoretical Calculation Method
All density functional theory (DFT) calculations applied in this study were performed with B3LYP functional with the LANL08(f) basis set for Rh atoms and cc-pVDZ for other atoms using the Gaussian 09 (C.02) program package [39]. The geometry optimizations of [Rh 2 [Rh 2 (O 2 CCH 3 ) 3 (PABC)(DMF)], and 1D were carried out without symmetry constraints (see Supplementary Materials Figure S1 and Tables S1-S3), and the resulting optimized structures were confirmed to be energy minima by the frequency analyses.

Synthesis and Characterizations
Complex 1 was synthesized by the solvothermal reaction of [Rh 2 (O 2 CCH 3 ) 4 (H 2 O) 2 ] and HPABC (in 1:1 molar ratio) in CH 3 CN at 413 K for 6 h with a Teflon-lined steel autoclave. The pure product of 1 can be purified by silica-gel column chromatography (Eluent: 5% MeOH in CHCl 3 ) with a 15.9% yield. The obtained 1 is stable in air (even in moist air) and can be dissolved in DMF. Complex 1 was characterized by 1 H NMR, ESI-TOF-MS, infrared spectroscopy, and elemental analysis. In the 1 H NMR spectrum measured in DMF-d 7 (see Figure S2), the two doublet signals, which were assigned as aromatic protons in the PABC ligand, were detected at 7.55 and 6.54 ppm. Moreover, a singlet signal that was derived from the amino protons appeared at 5.78 ppm. The signals for methyl protons in O 2 CCH 3 ligands were divided into two signals (1.71 and 1.79 ppm) with the integral intensities of 2:1, which correspond to the cisand trans-positions, respectively, relative to the PABC ligand. As depicted in Figure S3, the 1 H NMR spectral features of 1 measured in DMSO-d 6 were similar to those measured in DMF-d 7 . In the positive-ion mode ESI-TOF-MS, the signal was observed at 541.8783 m/z, which corresponds to the [M+Na] + value of complex 1 (541.8800 m/z). As depicted in Figure S4, the observed shape and pattern of the isotope pattern also fit well with the simulated isotope distribution of complex 1. Moreover, no signal assignable to unreacted [

Crystal Structure
Small purple-colored single crystals of 1P, which are suitable for single-crystal X-ray diffraction analysis, can be obtained by the slow evaporation of the DMF solution of 1. The X-ray diffraction pattern revealed that this complex crystallized in the monoclinic system with the space group P 2 1 /c. The final refined model of 1P is shown in Figure 2, and the crystallographic data and structural parameters of 1P are summarized in Tables 1 and 2, respectively.  The asymmetric unit of 1P contains two Rh ions, three O2CCH3 ligands, one PABC ligand, and one DMF molecule (Figure 2a) and adopts a typical paddlewheel-type structure in which the Rh2 core is coordinated by the bridging carboxylates of three O2CCH3 ligands and one PABC ligand at the equatorial positions. Figure 2b shows that the structural unit of 1P was self-assembled to form the one-dimensional chain (polymeric) structure using the rhodium-amine coordination bond. Therefore, the axial position of the  The asymmetric unit of 1P contains two Rh ions, three O 2 CCH 3 ligands, one PABC ligand, and one DMF molecule (Figure 2a) and adopts a typical paddlewheel-type structure in which the Rh 2 core is coordinated by the bridging carboxylates of three O 2 CCH 3 ligands and one PABC ligand at the equatorial positions. Figure 2b shows that the structural unit of 1P was self-assembled to form the one-dimensional chain (polymeric) structure using the rhodium-amine coordination bond. Therefore  [40]. However, the Rh-N(amine) and Rh-O(DMF) bond lengths in 1P are 2.293(5) and 2.308(5) Å, respectively, which are clearly longer than those of Rh-O(carboxylate). These results indicate that the coordination bonds in the axial sites of 1P were weaker than those in the equatorial sites. The phenyl rings of the PABC ligand in 1P are almost coplanar (2.87 • ) relative to the Rh 2 -carboxylate(PABC) plane. This result suggests the formation of the transition dipole moment between the PABC ligand and the Rh 2 moiety [25]. As shown in Figure  S5, the one-dimensional chains of 1P were closely assembled in the crystal without non-coordinated DMF molecules.

Self-Assembly and Disassembly Phenomena
As shown in Figure 3, a green solution was obtained when 1 was dissolved in DMF, and the evaporation of DMF from the green solution produced a purple-colored crystalline product 1P. On the basis of the color of the solution and the knowledge of previously reported paddlewheel-type Rh 2 complexes [7], it is speculated that [Rh 2 (O 2 CCH 3 ) 3 (PABC)(DMF) 2 ] (1D), in which two DMF molecules are coordinated with the axial sites of Rh 2 ions, is formed in the DMF solution because the solution and solids of the paddlewheel-type Rh 2 complexes with axial O-donor ligands typically have a green color while those with axial N-donor ligands have a purple (or red) color [7]. Remarkably, we confirmed that the polymeric species 1P can be easily re-dissolved in DMF and afford 1D, which results in the green color of the solution. This indicates that the self-assembly and disassembly transformations between 1D and 1P easily occur by the dissolution and evaporation procedures with DMF. PABC ligand in 1P are almost coplanar (2.87˚) relative to the Rh2-carboxylate(PABC) plane. This result suggests the formation of the transition dipole moment between the PABC ligand and the Rh2 moiety [25]. As shown in Figure S5, the one-dimensional chains of 1P were closely assembled in the crystal without non-coordinated DMF molecules.

Self-Assembly and Disassembly Phenomena
As shown in Figure 3, a green solution was obtained when 1 was dissolved in DMF, and the evaporation of DMF from the green solution produced a purple-colored crystalline product 1P. On the basis of the color of the solution and the knowledge of previously reported paddlewheel-type Rh2 complexes [42], it is speculated that [Rh2(O2CCH3)3(PABC)(DMF)2] (1D), in which two DMF molecules are coordinated with the axial sites of Rh2 ions, is formed in the DMF solution because the solution and solids of the paddlewheel-type Rh2 complexes with axial O-donor ligands typically have a green color while those with axial N-donor ligands have a purple (or red) color [42]. Remarkably, we confirmed that the polymeric species 1P can be easily re-dissolved in DMF and afford 1D, which results in the green color of the solution. This indicates that the self-assembly and disassembly transformat To investigate the absorption properties of 1P and 1D, the absorption spectrum of 1D in DMF and the diffuse reflectance spectrum of 1P were measured at room temperature (see Figure 4). In the visible light region, both 1D and 1P show two absorption maxima. The lower energy absorption band of 1P was observed at 572 nm, which was slightly blue-shifted compared to that of 1D (586 nm), while the higher energy absorption bands of 1D (444 nm) and 1P (441 nm) were located at approximately the same position. Hence, the origin of the color difference between 1D and 1P is primarily due to the difference in the location of their lower energy absorption bands. The other difference in the absorption bands between 1D and 1P was also confirmed in the UV region; 1P shows an intense absorption band at 341 nm, which is not detected in the absorption spectrum of 1D. Since this intense band of 1P arises from the visible light region (~441 nm), it is confirmed that this absorption band edge slightly influences the color of 1P. Although 1D shows an intense absorption band at 295 nm (= 29,472 M −1 cm −1 ) in the UV region, 1P shows three continuous absorption bands at 281, 260, and 214 nm. To investigate the absorption properties of 1P and 1D, the absorption spectrum of 1D in DMF and the diffuse reflectance spectrum of 1P were measured at room temperature (see Figure 4). In the visible light region, both 1D and 1P show two absorption maxima. The lower energy absorption band of 1P was observed at 572 nm, which was slightly blue-shifted compared to that of 1D (586 nm), while the higher energy absorption bands of 1D (444 nm) and 1P (441 nm) were located at approximately the same position. Hence, the origin of the color difference between 1D and 1P is primarily due to the difference in the location of their lower energy absorption bands. The other difference in the absorption bands between 1D and 1P was also confirmed in the UV region; 1P shows an intense absorption band at 341 nm, which is not detected in the absorption spectrum of 1D. Since this intense band of 1P arises from the visible light region (~441 nm), it is confirmed that this absorption band edge slightly influences the color of 1P. Although 1D shows an intense absorption band at 295 nm (ε = 29,472 M −1 cm −1 ) in the UV region, 1P shows three continuous absorption bands at 281, 260, and 214 nm.  The coordination nature of paddlewheel-type Rh2 complexes can be easily discerned from their absorption spectra. In detail, Cotton et al. confirmed that paddlewheel-type Rh2 complexes without axial donor ligands show an intense d-d(Rh2) absorption band at approximately 760 nm [43]. In contrast, the absorption spectrum of 1D shows no related band in the same region. This result indicates that two DMF molecules are coordinated at the two axial sites of Rh2 ions of 1D, which is similar to [Rh2(O2CCH3)4(DMF)2] [40].

Powder X-Ray Diffraction and TG Analyses of 1P
To investigate the phase purity and thermal stability of 1P, powder X-ray diffraction (XRD) and TG analyses were performed. As depicted in Figure 5a, the observed XRD pattern of 1P is in good agreement with the simulated pattern of the crystal structure of 1P. This result indicates that the obtained 1P has a crystalline single phase. The TG analysis of 1P shows a distinct three-step weight loss process (see Figure 5b). The initial step in the temperature range of 27-249 ˚C corresponds to the loss of the DMF molecule (Obs. -12.8%, Calcd. -12.3%), which is coordinated to the axial site of Rh2 ions in 1P, whereas the second (249-315 ˚C) and third (315-700 ˚C) steps are attributed to the decomposition of the O2CCH3 and PABC ligands of 1P, respectively. This TG result is also consistent with the chemical formula of the crystal structure of 1P. The coordination nature of paddlewheel-type Rh 2 complexes can be easily discerned from their absorption spectra. In detail, Cotton et al. confirmed that paddlewheel-type Rh 2 complexes without axial donor ligands show an intense d-d (Rh 2 ) absorption band at approximately 760 nm [42]. In contrast, the absorption spectrum of 1D shows no related band in the same region. This result indicates that two DMF molecules are coordinated at the two axial sites of Rh 2 ions of 1D, which is similar to [Rh 2 (O 2 CCH 3 ) 4 (DMF) 2 ] [40].

Powder X-Ray Diffraction and TG Analyses of 1P
To investigate the phase purity and thermal stability of 1P, powder X-ray diffraction (XRD) and TG analyses were performed. As depicted in Figure 5a, the observed XRD pattern of 1P is in good agreement with the simulated pattern of the crystal structure of 1P. This result indicates that the obtained 1P has a crystalline single phase. The TG analysis of 1P shows a distinct three-step weight loss process (see Figure 5b). The initial step in the temperature range of 27-249 • C corresponds to the loss of the DMF molecule (Obs. -12.8%, Calcd. -12.3%), which is coordinated to the axial site of Rh 2 ions in 1P, whereas the second (249-315 • C) and third (315-700 • C) steps are attributed to the decomposition of the O 2 CCH 3 and PABC ligands of 1P, respectively. This TG result is also consistent with the chemical formula of the crystal structure of 1P.

Conclusions
In this study, we successfully developed a new synthetic strategy for CPs with heteroleptic paddlewheel-type Rh2-SBUs using coordination-induced self-assembly with an amino group of the PABC ligand. Single-crystal X-ray diffraction analyses clarified that 1P, which is obtained by the slow evaporation of DMF-dissolved 1, forms a one-dimensional polymeric structure in which two axial sites of the Rh2 ion in 1P are coordinated by a DMF molecule and an amino group of the PABC ligand of the neighboring molecule 1. It is also confirmed that polymeric species 1P can be easily re-dissolved in DMF, which indicates that the self-assembly and disassembly transformations between 1D and 1P easily occurred by dissolution and evaporation procedures with DMF. As previously reported, the types of organic solvents in the synthetic processes of CPs and MOFs affect their molecular and self-assembly structures [44][45][46]. We speculate that weak O-donor solvents are preferable in the synthetic strategy with Rh2-SBUs. It is expected that this synthetic strategy can be applied to the synthesis of various novel CPs with heteroleptic Rh2-SBUs using similar ligands with potential coordination donor sites.

Conclusions
In this study, we successfully developed a new synthetic strategy for CPs with heteroleptic paddlewheel-type Rh 2 -SBUs using coordination-induced self-assembly with an amino group of the PABC ligand. Single-crystal X-ray diffraction analyses clarified that 1P, which is obtained by the slow evaporation of DMF-dissolved 1, forms a one-dimensional polymeric structure in which two axial sites of the Rh 2 ion in 1P are coordinated by a DMF molecule and an amino group of the PABC ligand of the neighboring molecule 1. It is also confirmed that polymeric species 1P can be easily re-dissolved in DMF, which indicates that the self-assembly and disassembly transformations between 1D and 1P easily occurred by dissolution and evaporation procedures with DMF. As previously reported, the types of organic solvents in the synthetic processes of CPs and MOFs affect their molecular and self-assembly structures [43][44][45]. We speculate that weak O-donor solvents are preferable in the synthetic strategy with Rh 2 -SBUs. It is expected that this synthetic strategy can be applied to the synthesis of various novel CPs with heteroleptic Rh 2 -SBUs using similar ligands with potential coordination donor sites.