Unique vapochromism of a paddlewheel-type dirhodium complex accompanied by dynamic structural and phase transitions

Yusuke Kataoka *a, Yoshihiro Kohara a, Natsumi Yano b and Tatsuya Kawamoto c
aDepartment of Chemistry, Graduate School of Natural Science and Technology, Shimane University, 1060, Nishikawatsu, Matsue, Shimane 690-8504, Japan. E-mail: kataoka@riko.shimane-u.ac.jp
bSpecial Course of Science and Engineering, Graduate School of Natural Science and Technology, Shimane University, 1060, Nishikawatsu, Matsue, Shimane 690-8504, Japan
cDepartment of Chemistry, Faculty of Science, Kanagawa University, 2946, Tsuchiya, Hiratsuka, Kanagawa 259-1293, Japan

Received 30th July 2020 , Accepted 18th August 2020

First published on 19th August 2020


The one-dimensional coordination polymer [Rh2(HA)4]n (1G; HA = hexanoate) exhibits a drastic vapochromic color change from green to red upon exposure to pyridine (py) vapor. Heating the red discrete complex [Rh2(HA)4(py)2] (1R) at 338 K affords the purple discrete tetrarhodium complex [Rh2(HA)4(py)]2 (1P), which is an intermediate species in the vapochromic transformation of 1G to 1R. The obtained complexes 1G, 1R, and 1P differ not only in their color in the solid state, but also in their temperature-dependent phase transition properties.


The development of vapochromic metal complexes that can detect volatile organic compounds (VOCs) via a reversible color change has recently received increased attention.1 The majority of these complexes contain a Pt(II),2 Au(I),3 or Cu(I) ion4 as the chromophore, and their apparent vapochromic behavior arises from the ON/OFF switching of inter- or intra-molecular metal–metal or π–π interactions via the sideslip of their self-assembly structures upon the adsorption/desorption of the organic vapor into their crystal packing spaces. Although the molecular structures and photophysical properties of these metal complexes are well understood, the ab initio prediction and design of metal complexes that exhibit specific phase transition and vapochromic properties are still difficult. Moreover, examples of metal complexes with stepwise vapochromic properties or vapor-induced changes in functionality remain scarce.5 Additionally, while numerous complexes have been reported whose vapochromism is triggered by inter- or intramolecular interactions, examples of coordination-induced vapochromic metal complexes, in which vapochromism is triggered by the coordination and dissociation of vaporous small organic molecules, remain limited.6,7 The vapochromic behavior of such complexes is often accompanied by changes in their functionality, such as their magnetic properties.6 Metal complexes with multiple unsaturated sites or coordination sites that can engage in ligand exchange reactions could likely also show multistep or stepwise vapochromic behavior with multiple functional changes. Therefore, the proposal and realization of new prospective coordination-induced vapochromic metal complex with multiple unsaturated sites or coordination sites that can engage in ligand exchange are highly desirable. In this context, the present study focuses on a paddlewheel-type dirhodium (Rh2) tetracarboxylate complex8 as a potential new coordination-induced vapochromic metal complex. This type of Rh2 complex contains two coordination sites for ligand exchange at the axial positions of the Rh–Rh bond, and often shows apparent solvatochromism in organic solvents.9 Herein, we describe the first example of vapochromic behavior of a paddlewheel-type Rh2 complex. Interestingly, the developed Rh2 complex shows highly selective vapochromic behavior in response to pyridine (py) vapor and undergoes dynamic structural and phase transitions. Moreover, multiple color changes were observed, and an intermediate species in the vapochromic process was successfully isolated.

Small green single crystals of [Rh2(HA)4]n (1G; HA = hexanoate) were obtained via a ligand–exchange reaction between [Rh2(O2CCH3)4(H2O)2] and hexanoic acid under an N2 atmosphere followed by recrystallization from THF (yield: 98.2%). 1G smoothly reacted with py in CHCl3 under ambient conditions to afford the red Rh2 complex [Rh2(HA)4(py)2] (1R) in excellent yield (97.7%). The obtained complexes 1G and 1R were fully characterized using 1H NMR and infrared (IR) spectroscopy, ESI mass spectrometry, and elemental analysis (Fig. S1–S3, ESI). A single crystal X-ray diffraction (SCXRD) analysis clarified that 1G forms a one-dimensional coordination polymer (Fig. 1(a)), in which [Rh2(HA)4] units self-assemble via intermolecular Rh–O coordination bonds (2.310(2) Å) between neighboring [Rh2(HA)4] units at the axial positions of the Rh2 bonds. An SCXRD analysis of 1R revealed discrete Rh2 units, in which two py ligands are coordinated to the two axial positions of the Rh2 core (Fig. 1(b)). The Rh–Rh bond lengths in 1G (2.3700(4) Å) and 1R (2.3951(5) Å) are similar to those of the similar polymeric complex [Rh2(O2CCH3)4]n (2.381(1) Å)10 and the discrete complex [Rh2(O2CC6H5)4(py)2] (2.402(1) Å),11 respectively. Two of the HA ligands in 1G and 1R are fully extended in all-trans conformations, while the other two adopt gauche conformations with respect to the C(8)–C(9) bond (dihedral torsion angle: 69.2(3)° for 1G and 54.5(6)° for 1R). The two py rings in 1R are aligned in parallel to the two fully extended HA ligands due to the formation of hydrogen bonds between an H atom in the py ligand and an O atom in the HA ligand. The powder X-ray diffraction (PXRD) patterns of 1G and 1R agree with the simulated patterns generated based on the their SCXRD structures (Fig. S4 and S5, ESI), indicating that the obtained 1G and 1R both exhibit a single-phase structure (1G: triclinic P[1 with combining macron]; 1R: triclinic P[1 with combining macron]).


image file: d0dt02672g-f1.tif
Fig. 1 Crystal structures of (a) 1G, (b) 1R, and (c) 1P with thermal ellipsoids at 30% probability; color code: green = Rh; red = O; gray = C; blue = N; white = H.

As depicted in Fig. 2, 1G shows vapochromic behavior; the green color of 1G gradually became red upon exposure to py vapor. The total py-uptake of 1G, i.e., approximately 2.0 molecules of py per Rh2 unit, was estimated from the difference in the weight of 1G before and after exposure to py vapor. The PXRD pattern and 1H NMR spectrum of the py-exposed powder agreed well with those of 1R (Fig. S5 and S6, ESI) synthesized in solution (CHCl3 and py), indicating that a dynamic structural transformation from the one-dimensional structure of 1G to the discrete structure 1R occurred upon exposing 1G to py vapor, whereby the crystallinity of 1G was maintained. Remarkably, no clear vapochromic response or structural transformation of 1G was observed in response to the vapor of other polar donor solvents such as water, methanol, ethanol, and acetone, indicating that 1G exhibits selective vapor-sensing properties. Moreover, 1R showed unique and dramatic temperature-dependent phase and structural transformation behavior; heating 1R to 338 K resulted in a gradual transition to the purple complex 1P (99.2% yield); 1R also instantly exhibited a phase transition to a dark-red liquid crystal phase at approximately 348 K when subjected to rapid heating. Additionally, 1H NMR spectroscopy and elemental analyses clearly indicated that the composition ratio between [Rh2(HA)4] and py in 1P is 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (Fig. S7, ESI). Heating 1P at 363 K for 3 h afforded 1G as a crystalline solid, and 1P could be reverted to 1R by exposing 1P to py vapor. These results suggest that 1P is an intermediate in the vapochromic transition of 1G to 1R. Surprisingly, the purple color of 1P was maintained in ethanol, indicating that the coordination bonds between the Rh2 units and the py ligands are maintained in ethanol. Therefore, 1P was recrystallized by the slow diffusion of water into an ethanol solution of 1P, and the obtained single crystals of 1P were analyzed by SCXRD. As depicted in Fig. 1(c), 1P forms a discrete “dimer-of-dimers”-type tetrarhodium (Rh4) unit of the type [Rh2(HA)4(py)]2, in which two Rh2(HA)4(py) units are connected by intermolecular Rh–O coordination bonds (Rh(3)–O(4): 2.372(3) Å; Rh(2)–O(15): 2.392(3) Å) through the axial positions of the Rh2 bonds in two Rh2(HA)4(py) units. The Rh–Rh bond lengths of 1P were estimated to be 2.3884(5) Å (Rh(1)–Rh(2)) and 2.3877(5) Å (Rh(3)–Rh(4)); these distances are longer and shorter than those of 1G and 1R, respectively, indicating that the coordination of the py ligands at the axial positions of the Rh2 bonds elongates the Rh–Rh bonds and affects both their absorption features and solid-state color. No solvent molecules, e.g., water or ethanol, were found in the crystal structure (packing space) of 1P. Moreover, the PXRD pattern of as-synthesized 1P agreed well with the pattern simulated from its SCXRD structure (Fig. S8, ESI), indicating that as-synthesized 1P also exhibits a single-phase structure (monoclinic P21/c).


image file: d0dt02672g-f2.tif
Fig. 2 Photographs of the crystalline powders of 1G, 1R, and 1P.

To clarify the structural and phase-transition characteristics of 1G, 1R, and 1P, the behavior of the samples was observed in real-time using thermo-gravimetric (TG) and differential scanning calorimetry (DSC) analyses. As shown in Fig. 3(A), the TG profile of 1G shows a one-step weight decrease starting at approximately 529 K, with a weight decrease of 67.1 wt% at 553 K. This weight loss is due to the thermal decomposition of the HA ligands (calcd 69.1 wt% of 1G). On the other hand, 1R and 1P exhibit multiple gradual weight decrease steps starting at relatively low temperatures compared to that of 1G due to the desorption of the py ligands from 1R and 1P. Remarkably, the onset of the weight loss for 1R occurs at approximately 335 K, while 1P is thermally stable up to 360 K. These results indicate that the two py ligands in 1R desorb in a stepwise manner with increasing temperature. Moreover, we confirmed that 1R and 1P show phase transitions from single crystals (SCs) to liquid crystals (LCs) during heating. Therefore, the details of these phase transitions were investigated by DSC using a scan rate of 10.0 K min−1. As depicted in Fig. 3(B), the DSC curve of 1G exhibits a single endothermic peak (−38.7 kJ mol−1) at 403 K during the heating process; this peak was assigned to a change in the conformation of the alkyl chains12 of the HA ligands similar to those observed for paddlewheel-type dicopper alkylcarboxylate complexes.13 In other words, 1G did not show an SC-to-LC phase transition. In contrast, the curve of 1P shows a single endothermic peak (−54.7 kJ mol−1) at 398 K, which was assigned to an SC-to-LC phase transition; however, no peak corresponding to a change in the alkyl-chain conformations of the HA ligands is present. Interestingly, for 1R, two endothermic peaks were observed at 338 and 346 K; the former (−22.8 kJ mol−1) was attributed to changes in the alkyl-chain conformation of the HA ligands, while the latter (−23.8 kJ mol−1) was ascribed to the SC-to-LC phase transition. These results indicate that exposure to py vapor controls not only the color of the Rh2 complexes, but also their phase-transition properties.


image file: d0dt02672g-f3.tif
Fig. 3 (A) TG and (B) DSC diagrams of (i) 1G, (ii) 1R, and (iii) 1P. Enlarged photographs of 1G, 1R, and 1P are provided in the ESI (Fig. S9–S11, ESI).

To understand the origin of the color differences among 1G, 1R, and 1P, their diffuse reflectance (DR) spectra were recorded. As depicted in Fig. 4, 1G, 1R, and 1P show two absorption bands in the visible region. The low-energy absorption bands (A-bands) of 1G, 1R, and 1P were observed at 627, 520, and 555 nm, respectively, indicating that the absorption wavelengths of the Rh2 complexes are blue-shifted with increasing number of py ligands coordinated to the Rh2 moieties. On the other hand, higher energy absorption bands (B-bands) of 1G (451 nm), 1R (448 nm), and 1P (449 nm) were located at almost the same position. These results clearly demonstrate that the color differences among 1G, 1R, and 1P originate from the changes in the absorption wavelengths of their A-bands. Noteworthy, reversible vapochromic behaviour and same absorption features were observed even after five py-exposure and heating cycles (see Fig. S12). Similar absorption features in the visible region were also observed in ethanol solutions of 1G, 1R, and 1P, and their molar absorption coefficients (ε) were lower than 420 M−1 cm−1, indicating that the A- and B-bands of the Rh2 complexes are dominated by d–d excitation at the Rh2 centers and not by charge-transfer excitation (see Fig. S13, ESI).


image file: d0dt02672g-f4.tif
Fig. 4 Solid-state diffuse reflectance (DR) spectra of 1G (green), 1R (red), and 1P (purple) measured at room temperature.

Finally, to further understand the electronic structures and absorption features of 1G, 1R, and 1P, density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were carried out. For these calculations, the polymeric structure of 1G was reduced to the discrete model structure [Rh2(HA)4(H2O)2], in which two H2O ligands are coordinated to the axial sites of the Rh2 bond. As depicted in Fig. S14, the occupied molecular orbitals (MOs) of 1G, 1R, and 1P between the Rh2 ions are described as π4δ2σ2δ*2π*4, which is typical for paddlewheel-type Rh24+ complexes.14 Our analyses indicate that the energies of the MOs with σ(Rh2) and σ*(Rh2) characters are strongly destabilized upon coordination of additional py ligands to the Rh2 moieties, whereas other orbitals with d(Rh2) character are only slightly destabilized. Thereby, the d-orbital gaps, i.e., the energy differences between the occupied MOs with π*(Rh2) character and unoccupied MOs with σ*(Rh2) character, also increase with increasing number of py ligands coordinated to the Rh2 moieties. As summarized in Tables S1–S3, our TDDFT results revealed that the major excitation character of the A-bands of 1G, 1R, and 1P is the π*(Rh2) → σ*(Rh2) excitation and that their excitation energies are dramatically red-shifted with increasing number of py ligands coordinated to the Rh2 moieties. Based on these results, we concluded that the coordination and dissociation of py ligands at the axial sites of Rh2 moieties affect the energies of MOs with σ*(Rh2) character, dramatically shift the absorption wavelength of the A-band, and, as a result, change the color of the Rh2 complexes.

In conclusion, we have designed and synthesized a paddlewheel-type Rh2 complex that exhibits vapochromic behavior that is highly sensitive and selective toward pyridine (py). Single-crystal X-ray diffraction analyses clearly revealed that the polymeric structure of [Rh2(HA)4]n (1G; HA = hexanoate) is transformed into the discrete structure of [Rh2(HA)4(py)2] (1R) upon exposure to py vapor, and that 1R can be easily reverted to 1G in a stepwise manner through the intermediate Rh4 complex [Rh2(HA)4(py)]2 (1P) by moderate heating. Diffuse reflectance (DR) spectra and DFT/TDDFT calculations revealed the origins of the color differences among these three Rh2 complexes. Remarkably, the phase-transition properties of these Rh2 complexes are strikingly different, i.e., 1R and 1P show in contrast to 1G temperature-dependent SC-to-LC phase transitions. This dynamic structural and phase-transition behavior could potentially be controlled by judicious choice of the alkylcarboxylate ligands in such Rh2 complexes. Moreover, similar alkylcarboxylate-bridged paddlewheel-type complexes of other metal ions using various VOCs may show similar and/or otherwise interesting behavior.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by JSPS KAKENHI Grant Numbers 20H05102, 19K15588, and 18H05166.

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Footnote

Electronic supplementary information (ESI) available: Instruments and methods, synthetic procedure, 13 figures, 3 tables, and CIFs. CCDC 2018470–2018472. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0dt02672g

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