Versatile Control of Directed Supramolecular Assembly via Subtle

Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Versatile Control of Directed Supramolecular Assembly via Subtle Changes of the Rhodium(I) Pincer Building Blocks Alan Kwun-Wa Chan, Maggie Ng, Kam-Hung Low, and Vivian Wing-Wah Yam* Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee, Hong Kong) and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P.R. China

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S Supporting Information *

ABSTRACT: Various rhodium(I) pincer complexes with different structural features have been prepared and found to display interesting self-assembly properties due to the extensive Rh(I)···Rh(I) interactions. The incorporation of electronwithdrawing −CF3 substituent has been found to improve the stability of the complexes and also facilitate the directed assembly of complex molecules, providing an opportunity for the systematic investigation of the various noncovalent interactions in their versatile self-assembly behaviors and insights into the structure−property relationship in governing the intermolecular interactions. An isodesmic growth mechanism is identified for the solvent-induced aggregation process. The complex molecules exhibit intense low-energy absorption bands corresponding to the absorptions of the dimers, trimers, and higher order oligomers upon aggregation, with energies related to the electronic properties of the tridentate N-donor ligand. Chiral auxiliaries have also been introduced into the rhodium(I) complexes to build up helical supramolecular assemblies and soft materials.

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INTRODUCTION It is well-known that square-planar d8 transition metal complexes can display noncovalent metal···metal interactions when the metal centers are brought into close proximity, which leads to their unique optical properties for various applications.1 The manipulation and utilizations of metal··· metal interactions with rational design are believed to provide a strategic approach for the versatile assembly of molecules into a well-aligned order, particularly in an anisotropic fashion,2 to result in one-dimensional (1D) nanomaterials through the ready coassembly of the desirable building blocks, which involves an interplay of the various natures of noncovalent interactions. Such strategies may lead to the potential development of multifunctional coassembled materials3 for a wide range of applications, for example, as charge transport materials,3f supramolecular hydrogels,3g and soft nanomaterials.3h Apart from the classical Magnus’ green salt4a and the mixedvalence Krogmann’s salt,4b it is rare to observe the employment of other square-planar platinum(II) complexes, which possess an extended linear unidirectional chain of close Pt(II)···Pt(II) contacts,1f,4f to construct self-assembled 1D materials.1e,f,2c,3c Besides, there has been a revival of interest only since the past decade on utilizing the isoelectronic square-planar rhodium(I) systems to construct such kinds of molecular functional materials5 despite their well-documented oligomerization and rich solid-state polymorphism originated from the extensive Rh(I)···Rh(I) interactions.6,7 In addition, detailed investiga© XXXX American Chemical Society

tions on their responsive properties by the manipulation of the fine balance of various types of intermolecular forces and interactions via molecular design have been relatively underexplored.7d,e Apart from the examples of [Rh(tpy)Cl]6g and [Rh(tpy)(NCMe)]+,7c most of the Rh(I) pincer complexes have been mainly confined to phosphine complexes and investigations directed toward catalysis and mechanistic studies of reactions.6g−i The self-assembly studies of a new class of rhodium(I) system with π-conjugated tridentate ligands have been demonstrated by us to exhibit infinite 1D molecular chain with close Rh···Rh contacts in the solid-state X-ray structure.8 Gray and co-workers have also revisited the rhodium(I) system and established the Rh···Rh bond energy for a model rhodium(I) isocyanide complex, [Rh2(TMB)4], to be 12 ± 6 kcalmol−1 (50 ± 25 kcal mol−1), and correlated the bond energy with the Rh···Rh distance, which causes a significant impact on the dimeric [dσ* → pσ] absorption energies and excited-state properties.12d,e It is also found that the utilization of an unconstrained ligand and oxidative addition processes (halides, NCS−, NO2−) can give rise to higher metal−metal stretching frequencies with conformational changes, indicating a strengthening of the Rh···Rh bonding interaction.12f By taking advantages of the extensive Rh(I)···Rh(I) interaction and the combined approaches of rational design, a detailed Received: May 3, 2018

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DOI: 10.1021/jacs.8b04687 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

by condensation reaction.13 The rhodium(I) complexes have been prepared by the reaction of [Rh(CO)2Cl]2 with silver triflate and 2 equiv of terpyridine ligands in tetrahydrofuran.8 The identities of the newly synthesized rhodium(I) pincer complexes have been confirmed by 1H NMR, positive FAB mass spectrometry, and satisfactory elemental analysis for their identities. The molecular structure of one of the rhodium(I) complexes has also been characterized by X-ray crystallography that indicates the head-to-head linearly stacked structure arranged in an eclipsed conformation. The chemical structures of cationic rhodium(I) complexes 1−6 with triflate as the counteranion have been depicted in Figure 1. X-ray Crystal Structures and Powder X-ray Diffraction Studies. Single crystals of complex 1 have been obtained by diffusion of diethyl ether into a concentrated complex solution in acetonitrile for structure determination by X-ray crystallography. Table S1a summarizes the crystal structure determination data, while Table S1b provides the information on the selected bond lengths and bond angles in the Supporting Information. Complex cations of 1 adopt a slightly distorted square-planar geometry,2,3,8 with two neighboring rhodium(I) centers connected by extensive Rh(I)···Rh(I) interactions (Figure 2). Between pairs of complex molecules, a head-tohead eclipsed stacking conformation has been observed in the crystal packing with the torsional angles between the neighboring complex cations determined to be about 4−5° when viewed along the Rh−Rh−Rh axis. The N−Rh−N angles (N1−Rh1−N2 79.03°, N1−Rh1−N3 158.22°, and N2−Rh1− N3 79.35°) have been determined to deviate from the ideal value of 90° and 180°, respectively (Table S1). This has been attributed to the strain exerted by the bite angle of the terpyridine ligand, commonly found in the range of 75−80° in other related compounds.9h The Rh···Rh distances within the dimer of neighboring rhodium(I) centers are found to be 3.464 Å, indicating the presence of extensive Rh(I)···Rh(I) interactions within those dimers, while the discrete dimeric units of the complex molecules are found to align in a zigzag fashion with Rh···Rh distances of 6.049 Å (Figure 2), indicating the absence of Rh(I)···Rh(I) interactions between those discrete dimers. The interplanar separation between the pyridine moieties on the terpyridine ligands between two adjacent molecules is determined to be 3.534 Å, indicating the presence of π−π interactions within the eclipsed rhodium dimers. Interestingly, the fluorine···fluorine distance has been found to be at 3.054 Å between the −CF3 substituents of the rhodium(I) complex molecules within the dimer, indicating the presence of F···F interaction. This has also been previously observed in the solid-state packing of molecules reported in the literature, with around 4 kcal mol−1 (17 kJ mol−1) stabilization energy contributed toward the crystal packing.14 Such F···F interaction has assisted the head-to-head alignment of the molecules. With reference to the powder X-ray diffraction (XRD) studies on 1, 2, and [Rh(tpy)(CO)]+, it is important to note that the peak at 2θ = 25−28° (d spacing = 0.31−0.34 nm) can be attributed to intermolecular Rh(I)···Rh(I) and π−π stacking interactions between the adjacent Rh(I) complexes in the aggregate species (Figure S2). The result indicates that complex 2 with more electron-drawing −CF3 units will end up with a shorter d spacing of 0.31 nm. As compared to the unsubstituted [Rh(tpy)(CO)]+, the introduction of the electron-withdrawing −CF3 substituent into the system has been found to alter the molecular stacking of the complex molecules, converting the conformation from the

investigatory study into new classes of rhodium(I) complexes with functionalized pincer ligands has been performed. This may provide important strategies to generate desirable building motifs for the construction of supramolecular assemblies.9,10 In the present work, we have attempted to further stabilize the electron-rich rhodium(I) centers by the introduction of πaccepting pincer ligands with −CF3 and −CN substituents for more diverse assembly studies. In addition, the increase in the electron-deficient character of the system is believed to foster the associations of complex molecules as a result of the reduction of mutual electronic repulsion between the molecules.11 It is envisaged that the introduction of the electron-withdrawing group may further induce donor− acceptor interactions to facilitate the assembly of molecules by the induced-dipole, as previously demonstrated in other organic systems in the literature.11,12 The strategy of directed supramolecular assembly has also been introduced to the current system for more sophisticated and enhanced association properties, especially given the high propensity of Rh(I) centers in the formation of Rh(I)···Rh(I) interactions, where higher oligomers [Rh]n in addition to dimers are found to be present in the solution state.6,7 Therefore, the exploitation of rhodium(I) complexes that are capable of forming extensive metal···metal interactions, in conjunction with other types of directional noncovalent interactions to control the orientation of the assembly mode, would be worth exploring. In the current work, the design and preparation of rhodium(I) pincer carbonyl complexes 1−6 consisting of subtle changes in the structural features and electronic properties (Figure 1) have been reported to realize the controlled directed 1D supramolecular assembly of squareplanar rhodium(I) systems.

Figure 1. Molecular structures of complexes 1−6.

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RESULTS AND DISCUSSION Synthesis and Characterization. Different terpyridine ligands have been synthesized by following the modified literature procedures that involve the condensation reaction of different substituted aldehydes with 2-acetlypyridine or 4(trifluoromethyl)picolinaldehyde.13 The chiral terpyridine ligand has been synthesized by the reaction of 4-hydroxybenzaldehyde with (S)-(+)-1-iodo-2-methylbutane followed B


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Table 1. Electronic Absorption Data for Rhodium(I) Complexes 1−6 complex 1 2 3 4 5 6 a

medium (T/K) acetone (298)a acetone (298)a acetone (298)a acetone (298)a acetone (298)a acetone (298)a

electronic absorption λmax/nm (ε/dm3 mol−1 cm−1) 358 (9290), 380 sh (5780), 490 (1060), 522 (980), 602 sh (320) 362 (10030), 390 (5730), 504 (1940), 536 (1960), 622 sh (610) 361 (3760), 386 (1870), 494 (400), 529 (430), 612 sh (120) 361 (7210), 385 (3860), 494 (920), 528 (930), 610 sh (370) 384 (12070), 515 (1110), 592 sh (340) 362 (9190), 420 (1080), 458 (810), 524 (520)

Measured at concentrations from 1 × 10−5 to 1 × 10−4 M.

Except for complex 6 with the bulky mesityl group, the complexes exhibit drastic color changes from pale yellow to reddish brown or purple on raising the concentration from 1 × 10−5 to 2 × 10−3 M. At around 600 nm, there is growth of an absorption shoulder. In addition, a gradual increase of broad absorption band at 740−930 nm has been observed. Both of them are found to deviate from Beer’s law for complexes 1−5 (Figure 3 and Figure S4). Application of dimerization plots according to a monomer−dimer equilibrium6d,15a has led to linear relationships of [Rh]/(A600)1/2 versus (A580)1/2. This suggests the presence of dimer formation and the assignment of the absorption shoulders as the dimeric [dσ*(Rh2) → π*(N^N^N)] transition of {[Rh(N^N^N)(CO)]}22+. The dimerization constants have also been determined (Table 2).

Figure 2. Crystal packing diagrams showing (a) the head-to-head eclipsed configuration, and (b) the zigzag alignment of complex cations of 1 with the dotted line showing the short Rh···Rh contacts in dimeric form.

head-to-tail stacked infinite rhodium(I) chain with similar Rh(I)···Rh(I) contacts to dimeric head-to-head eclipsed conformation to significantly enhance the π−π stacking interactions. In addition to F···F interactions, the reduction of electronic repulsion and the induced-dipole effect11 may also contribute to the enhancement of stacking of molecules upon the attachment of electron-withdrawing −CF3 substituent. Aggregation Affinities and Rh(I)···Rh(I) Interactions. To study the extensive Rh(I)···Rh(I) interactions of the rhodium(I) pincer complexes in solution state, their concentration-dependent UV−vis absorption studies in acetone solution have been carried out to investigate their self-assembly properties. Solutions of complexes 1−6 in acetone at a concentration of 5 × 10−5 M have been prepared and found to show intraligand absorptions at 350−390 nm, together with a moderately intense metal-to-ligand charge transfer (MLCT) [dπ(Rh) → π*(N^N^N)] transition at 420−540 nm (Figure S3 and Table 1), which originated from the monomeric species.

Figure 3. UV−vis spectra of (a) 2 (electron-withdrawing −CF3) and (b) 5 (electron-donating −CH2CH(CH3)CH2CH3) from 1 × 10−5 to 2 × 10−3 M in acetone at 298 K. The apparent absorbance values have been obtained by correcting to 1-cm path length equivalence. Insets: The trimerization plot for dimer−trimer equilibrium monitored at the absorption wavelength of the trimeric species. C


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the most electron-rich complex 5 (−chiral butyl, 13 480 cm−1) > [Rh(tpy)(CO)]+ (−H, 12 950 cm−1) > 1 (−CF3, 12 530 cm−1) > 4 (−NO2, 12 200 cm−1) > 2 (−3CF3, 11 600 cm−1) > 3 (−CN, 10 750 cm−1) as illustrated in Figure 4a. In addition,

Table 2. Association Constants (log K) of Complexes 1−6 complex

log Kda,b

log Kta,c

log Kad

trimer absorption energy/cm−1

1 2 3 4 5 6

2.40 4.78 3.43 4.72 3.44 −f

2.42 4.80 2.52 3.91 3.55 −f

1.64 2.02 −e −e 1.95 −f

12530 11600 10750 12200 13480 −f

a

Association constants are determined in the concentration range from 5 × 10−5 to 2 × 10−3 M in acetone solution from UV−vis absorption studies. bDimerization constant determined from dimerization plot of the monomer−dimer equilibrium. cTrimerization constant determined from trimerization plot of the dimer−trimer equilibrium. dAssociation constant determined from 1H NMR titration studies in the concentration range from 2 × 10−4 to 2 × 10−2 M in CD3CN solution. eThe poor solubility of the complexes has led to the unsatisfactory 1H NMR data fitting. fNo aggregation observed with the absorption data

As compared to the related square-planar rhodium(I)6d,e and platinum(II) complexes,15a their magnitudes have been found to be relatively higher. It is also noted that the incorporation of stronger or more electron-deficient substituents would lead to the increase in the dimerization constant with the order: 2 (three −CF3 units) > 1 (one −CF3 unit), and 4 (−NO2) > 3 (−CN) > 1 (−CF3).16 These observations suggest that the electron-withdrawing groups may enhance the self-aggregation of complex molecules by the induced-dipole and reduction of electronic repulsion, causing a cumulative effect on the Rh(I)··· Rh(I) and π−π stacking interactions between the π-conjugated pincer ligand as previously demonstrated.7e,8 The observation is also in line with the X-ray crystal structure of 1 in the solid state, which illustrates that the complex molecules with electron-withdrawing −CF3 substituent would preferentially adopt a head-to-head eclipsed conformation to facilitate their assembly as compared to the unsubstituted counterparts.16 Interestingly, a relatively larger dimerization constant of complex 5 (log kdimer = 3.44) with chiral alkyl chains as compared to 1 (log kdimer = 2.40) has been determined, which can be ascribed to the additional interactions by the hydrophobic chains that contribute to the intermolecular assembly.7e On the basis of the dimer−trimer equilibrium, the trimerization plot has also been constructed for the lowenergy absorption at 740−930 nm, which is red-shifted by around 3000−4000 cm−1 when compared to their corresponding dimer absorption, and such energy differences resemble the magnitude of the corresponding dimer-to-trimer energy shift of the related (tetrakisisocyano)rhodium(I) system.6a,d,e A linear correlation of A850 versus (A600)3/2 has been observed, which further supports its assignment as the absorption of the trimeric species, {[Rh(N^N^N)CO]}33+ with [dσ*(Rh3) → π*(N^N^N)] transition. The trimerization constants of complexes 1−5 have been determined by the trimerization plot. A trend similar to that for the dimerization constants has been found, further suggesting the enhancement of intermolecular interactions by the incorporation of the electrondeficient substituents. More interestingly, the absorption energies of the trimer absorption bands are found to exhibit a strong dependence on the electronic features of the complexes and their extent of aggregation through Rh(I)···Rh(I) interactions in the solution state. They have been found to be red-shifted gradually from

Figure 4. (a) UV−vis spectra of [Rh(tpy-C6H5-R)(CO)]+ with R = CH2CH(CH3)CH2CH3, H, CF3, NO2, 2,3,4-(CF3)3, CN in 2 × 10−3 M to indicate the growth and the energy difference of trimer absorption. The apparent absorbance values have been obtained by correcting to 1-cm path length equivalence. (b) Plot of the energy of the lowest absorption band (trimer) versus the Hammett σ value for [Rh(tpy-C6H5-R)(CO)]+ and its linear least-squares fit (−).

a plot of the trimer absorption energies versus the Hammett σ value16 results in a straight line with a slope of ca. −970 cm−1/ unit change in σ (Figure 4b). The result further illustrates that the trimer absorption energy is strongly dependent on the substituents of the pincer ligand. This can be explained by the stabilization of the π* orbital to reduce the energy gap of the trimeric [dσ*(Rh3) → π*(N^N^N)] transition. The phenomenon signifies the substantial perturbation by the electronic properties toward the transitions associated with extensive Rh(I)···Rh(I) interactions. The reduction in electron density in the current system may also enhance the Rh(I)···Rh(I) interactions to facilitate the stacking of molecules,12d−f resulting in a gradual red shift in dimeric and trimeric absorption energies. It is also important to realize that the electron-withdrawing effect of the −CF3 substituent (σ = 0.54) is additive for 1 and 2 of different number of −CF3 substituents and thus gives rise to the linear plot. Complex 3 with −CN substituent (σ = 0.66) has been found to deviate from the resulting plot with a relatively red-shifted trimer absorption energy (10 750 cm−1). The phenomenon can be attributed to the mesomeric effect of the −CN group, which is not accounted for in the σ value.17 The slope provided by the Hammett’s plot has been found to be similar to that of [Re(phen)(CO)3(C5H4N-R-4)] (790 cm−1/σ+) obtained from a relatively pure metal-to-ligand D


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Figure 5. 1H NMR spectra of 1 upon increasing the concentration in CD3CN at 298 K (left). The corresponding plot of chemical shift against the concentration fitted with the self-association model of monomer−dimer equilibrium (right).

postulated according to the NOE cross peak signals (Figure S7). It is noted that complex molecules of 2 with three electronwithdrawing −CF 3 substituents adopt a head-to-head staggered conformation to facilitate the Rh(I)···Rh(I) interactions with better molecular alignment, similar to the observation in its solid-state X-ray crystal structure, while complex molecules of 5 with the electron-donating chiral butyl chain are aligned in a head-to-tail arrangement to minimize the steric repulsion as is found in a similar system reported previously.11,12 The diffusion coefficients of complexes 2 and 5 (Figure S8) are also determined from diffusion ordered NMR spectroscopy (DOSY). According to the Stokes−Einstein equation,15b the hydrodynamic radii, Rh, are found to be very similar for the two complexes even though they have different electronic properties, with both 2 and 5 determined to be about 4.0 Å. This may indicate that the size of the aggregates is not significantly affected by their electronic properties and alignment. Temperature-Dependent Aggregation Studies. Temperature-dependent UV−vis absorption studies have been carried out for complexes 1 and 2 with different number of electron-withdrawing −CF3 substituents (Figures S9 and S10), and the data have been summarized in Table 3. By decreasing the temperature from 318 to 283 K, there is a gradual growth in the absorption of the trimers (1 at 807 nm and 2 at 840 nm). A well-defined sigmoidal curve can be obtained by plotting the degree of aggregation at the absorption maximum versus temperature. The data have also been fitted and analyzed according to the temperature-dependent aggregation

charge transfer (MLCT) transition, but relatively smaller than those of [Pt(tpy)(CC−C6H4-R-4)]+ (2200 cm−1/σ+)18a and [Pt(4,4′-R2-bpy)(tdt)] (bpy = 2,2′-bipyridine, tdt = toluene3,4-dithiolate at ca. 4900 cm−1/σ)18b,c obtained from their mixed MLCT/ligand-to-ligand charge-transfer (LLCT) transition bands, indicating a relatively less direct influence of the substituent effect on the trimeric [dσ*(Rh3) → π*(N^N^N)] transition that is more of a pure MLCT as well as intermolecular in nature. A careful comparison of the monomer−dimer and dimer− trimer energy shifts shows an extra energy shift (500−800 cm−1) in the trimer absorption (Figure S5 and Table S2) in complexes 1−4 relative to 5. While their monomer-to-dimer energy shifts are found to be relatively similar (2520 cm−1), the extra stabilization energy observed for the trimer may not be merely originated from the lowering of π*(N^N^N) orbital due to the substituent effect. The dimer−trimer energy shifts have also been correlated with the Hammett’s constant (σ) in the Hammett’s plot with a slope of ca. 542 cm−1/unit change in σ (Figure S5f). This may also suggest that the electronwithdrawing substituents can enhance the π−π stacking and anisotropic alignment of molecules by induced-dipole interactions together with the reduction in electronic repulsion, which may also synergistically assist the Rh(I)···Rh(I) interactions to facilitate the stacking of the complex molecules. Concentration-dependent 1H NMR studies have been performed for complexes 1, 2, and 5 of different electronic properties to further investigate their effects toward aggregation (Figure 5 and Figure S6). Upon increasing the concentration of 1 in CD3CN, upfield shifts of the proton signals have been noticed. This may suggest the presence of π−π interactions upon the aggregation process. Even though it is difficult to make a direct comparison, the trend of the dimerization constants of 1, 2, and 3 (Table 2) has been determined to be in accordance with those from the UV−vis absorption studies. The observed trend is in line with the extent of the electron deficiency and the hydrophobicity of the complex system, further indicating that the attachment of electron-deficient groups and hydrophobic chains would assist the aggregation. To obtain a more in-depth understanding of the association mode of the rhodium(I) complexes with respect to their electronic properties, 2D 1H−1H NOESY NMR experiments have been performed. Plausible association modes of 2 and 5 in CD3CN solution at 10−3 M have been

Table 3. Thermodynamic Parameters for the TemperatureInduced Aggregation Process of Complexes 1 and 2 complex 1 2

ΔH/ kJ mol−1

ΔS/ J mol−1 K−1

TΔS/ kJ mol−1

ΔG298/ kJ mol−1

melting temp/K

−140.52a −143.42b −148.33a −164.47b

−424.17a

−126.40a

−14.12a

299b

−392.64a

−117.01a

−31.32a

291b

a

Determined by van’t Hoff plot from UV−vis absorption studies at different temperatures. bDetermined from curve fitting by applying the temperature-dependent nucleation-elongation model with monitoring of the absorption band of trimers at different temperatures. E


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Journal of the American Chemical Society model,19 confirming the isodesmic nature of the cooling process. With increasing number of −CF3 substituents at the terpyridine ligand, the ΔH value (Table 3) for the aggregation process becomes much more negative from 1 (−143.42 kJ mol−1) to 2 (−164.47 kJ mol−1). The results indicate an enhancement of the molecular assembly, which may be attributed to the contribution from the additional F···F interactions (ca. 4 kcal mol−1 (17 kJ mol−1)), π−π stacking, as well as other noncovalent interactions in conjunction with the extensive Rh(I)···Rh(I) interactions with head-to-head eclipsed alignment,11,12 and are in line with the observations from concentration-dependent aggregation studies and the solid-state X-ray structure. By the van’t Hoff analysis (Figures S9 and S10), a linear correlation has been obtained by plotting the logarithm of the equilibrium constant (Ke) of aggregation against the reciprocal of temperature (1/T). The changes in the enthalpy (ΔH), entropy (ΔS), and Gibbs free energy (ΔG) of 1 and 2 have then been determined to provide further insights into the aggregation affinity (Table 3). The result is in good agreement with the data obtained from the temperaturedependent aggregation model, further suggesting the involvement of intermolecular F···F interactions in the aggregation. In addition, it is also believed that the electron-withdrawing −CF3 units can also contribute to the reduction of electronic repulsion and the formation of induced-dipole to further facilitate the stacking of molecules.11,12 Solvent-Induced Self-Assembly Studies. Because of the ease of rhodium(I) centers to undergo oxidative addition reactions, there is only very little investigation on induced selfassembly of rhodium(I) complexes in mixed solvent systems7e despite their ability to form dimers, trimers, and higher order oligomers associated with their proneness in the formation Rh(I)···Rh(I) interactions. The incorporation of three −CF3 substituents into the terpyridine ligands of the rhodium(I) complex has provided extra stability and solubility to the system, which allows the studies of their solution-state aggregation under different solvent mixtures. By the fine interplay of the extensive Rh(I)···Rh(I) interactions, F···F, and certain extent of dipole−dipole interactions, it is interesting to investigate how these noncovalent forces alter the aggregation affinities and association modes of the square-planar rhodium(I) complexes. By lowering the diethyl ether content in an acetone solution of complex 2, drastic color changes from pale yellow (100% acetone) to orange (50% acetone) to red (70% acetone) have been observed. This has been attributed to the reduced solvation that leads to the formation of aggregates. There is also a growth of absorption of the trimers at 713 nm with a concomitant drop in the absorptions of monomer and dimer at 534 and 600 nm, respectively, by increasing the diethyl ether content (Figure 6). A clear isosbestic point has been observed, and this indicates the conversion from the monomers and dimers to trimers in high diethyl ether content. The observations suggest that complex 2 would undergo solvent-induced aggregation by the addition of a poor solvent, stabilized by the extensive Rh(I)···Rh(I) interactions, as well as π−π stacking and dipole−dipole interactions. The electronic absorption spectral data of 2 have also been analyzed by the solvent-dependent equilibrium model previously developed by Meijer and co-workers.19b The plot of the degree of aggregation at 713 nm (trimer absorption) versus solvent composition results in a sigmoidal curve. The data are well-fitted to the model (Figure 6), suggesting that the assembly process of 2 adopts an isodesmic mechanism.

Figure 6. (a) Color changes of complex 2 upon changing the solvent composition from 0 (left) to 70% (right) diethyl ether content in acetone solution. (b) UV−vis spectra of complex 2 upon increasing diethyl ether content in acetone solution. The inset shows the corresponding plot of degree of aggregation at 713 nm against the volume fraction of acetone with the fitting by the nucleationelongation aggregation model.

Together with the enhanced solubility and stability brought about by the electron-deficient −CF3 substituents, it is anticipated that the rhodium(I) metal centers in the aggregates of 2 would be in a closer proximity and, more importantly, have a higher tendency to assemble together, giving rise to higher-ordered trimers from the monomers and dimers. The Gibbs free energy change (ΔG) of the solvent-induced assembly processes of the rhodium(I) complexes has been determined to be −36.90 kJ mol−1 with an m value of 30.00 J mol−1 K−1, attributed to the enhancement of Rh(I)···Rh(I), π−π stacking, and dipole−dipole interactions upon increasing the nonsolvent content. The versatility of the current system can possibly be employed for the desirable manipulation of assembly of molecules and their stimuli-responsiveness9,10 through simple structure−property control, rendering it the potential to serve as building blocks for functional molecular materials. Morphological Studies. Encouraged by the structure− property correlation observed in their photophysical properties upon aggregation, an attempt has been made to study the morphology of the assembly architectures. The supramolecular structures have been probed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) to rationalize the structure−morphology relationship and the interplay of the various noncovalent interactions as well as the influence of chirality in directing the modes of the assembly. Upon dissolving in acetone solution at 5 × 10−4 M, complex 5 is observed to form interesting self-assembled twisted ribbon-like nanostructures. Their diameters are found to be 50−100 nm with the lengths of 1−2 μm (Figure 7). As compared to the linear rod-like nanostructures observed in the related [Rh(tpy)(CO)]+ system,8 the incorporation of the chiral alkyl chain has been found to alter the assembly mode of the complex system, giving rise to the hierarchical ribbons supported by Rh(I)···Rh(I) and π−π stacking interactions. F


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Figure 7. (a) TEM and (b) SEM images showing the helical ribbon nanostructures formation upon dissolving 5 in acetone solution.

However, the chiral supramolecular assembly process is random, producing both the right-handed and the left-handed ribbons in statistical distribution such that no obvious CD signal is observed upon mixing. These observations further suggest that the structural manipulation may serve a crucial role in regulating the supramolecular assembly process, which is in accordance with the results obtained in other related rhodium(I) systems.7e,8 Computational Studies. Geometry optimizations have been performed on the dimers of 1 and 2 to study the effect of −CF3 substituent on the assembly of the complex molecules. The optimized structures of the dimers of 1 and 2, with some important structural parameters, are shown in Figure 8. The optimized structure of the dimer of 1 is in a head-to-head eclipsed stacking conformation, with a Rh···Rh distance of 3.44 Å and a F···F distance of 2.80 Å, indicating the presence of Rh(I)···Rh(I) and F···F interactions within the dimer, and this is in good agreement with the experimental X-ray crystal structure (Figure 2). The interplanar distance between the pyridine rings on the terpyridine ligands within the dimer is around 3.41 Å, implying that π−π interaction exists within the dimer. Regarding the dimer of 2, calculations have shown the presence of two conformations of comparable energies, one in a head-to-head eclipsed conformation (Figure 8b) and the other in a head-to-head slightly staggered conformation (Figure 8c). The one in an eclipsed conformation is 1.42 kJ mol−1 higher in energy than that in a slightly staggered conformation and has a Rh···Rh distance of 3.48 Å and a F···F distance of 2.80 Å, indicating the presence of Rh(I)···Rh(I) and F···F interactions within the dimer. The Rh···Rh distance is a bit longer than that in the dimer of 1, probably due to the increase of electron−electron repulsion between the complex molecules with increasing numbers of CF3 groups. The

Figure 8. Optimized ground-state structures of the front view (left) and side view (right) of the dimers of (a) 1, (b) 2 in an eclipsed conformation, and (c) 2 in a slightly staggered conformation at the PBE0-D3 level of theory. The bond lengths and bond angles are in angstroms and degrees, respectively. All hydrogen atoms are omitted for clarity.

interplanar separation between the two pyridine moieties in the terpyridine ligand within the dimer is around 3.41 Å, which implies the existence of π−π interaction within the eclipsed dimer. On the other hand, regarding the dimer of 2 in a slightly staggered conformation, although it has a shorter Rh···Rh distance and interplanar π−π distance of the adjacent terpyridine ligands within the dimer as compared to those in the eclipsed conformation, 3.17 and 3.30 Å respectively, the F···F distance between the −CF3 groups in the phenyl ring increases to 3.56 Å, and the planes of the two phenyl rings are nearly perpendicular to each other. Therefore, the current G


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Journal of the American Chemical Society system demonstrates an interplay of Rh(I)···Rh(I), π−π stacking, and F···F interactions in forming the molecular assembly, and more importantly in the control of their alignment and stacking mode that can effectively alter their spectroscopic properties. With the significant contribution from the Rh(I)···Rh(I) interactions as in complex 2, the extra F···F interactions may direct the assembly mode of the dimer into head-to-head conformation that can facilitate the π−π stacking interactions. However, this also leads to destabilization due to the steric hindrance of the −CF3 substituents and thus may preferably end up with a more stable head-to-head staggered mode of assembly. As a result, the current study provides an important message in that a balanced control of intermolecular forces is needed in determining the factors for arriving at the desirable aggregation mode of the molecular building blocks.

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Vivian Wing-Wah Yam: 0000-0001-8349-4429 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.W.-W.Y. acknowledges support from The University of Hong Kong under the URC Strategically Oriented Research Theme (SORT) on Functional Materials for Molecular Electronics. This work has been supported by the University Grants Committee Areas of Excellence Scheme (AoE/P-03/ 08) and a General Research Fund (GRF) grant from the Research Grants Council of Hong Kong Special Adminstrative Region, People’s Republic of China (HKU 17334216). The University of Hong Kong’s University Development Fund and Dr. Hui Wai Haan Fund are also acknowledged for funding the Bruker D8 VENTURE MetalJet Photon II CMOS X-ray Diffractometer. A.K.-W.C. acknowledges the receipt of a University Postdoctoral Fellowship and a Li Ka Shing Prize from The University of Hong Kong.

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CONCLUSIONS The induced self-assembly properties of a unique class of rhodium(I) complexes with different electron-deficient groups have been demonstrated through the modulation of the concentration and the temperature, resulting from the extensive Rh(I)···Rh(I), π−π stacking, and F···F interactions governed by the −CF3 substituents. More interestingly, the stacking mode and aggregation of complex molecules have also been found to be enhanced and well-controlled by the increase of their electron deficiency due to the reduction of electronic repulsion between the complex molecules, together with the introduction of F···F interactions that facilitate the π−π stacking interactions. The trimer absorption energies of the rhodium(I) complexes have also been observed to give rise to a linear relationship with the σ-parameters of different substituents in the Hammett’s plot with a slope of 970 cm−1 per σ unit, which further correlates with the Rh(I)···Rh(I) interactions in directing the self-assembly. The chiral complex 6 has also been found to aggregate to form helical nanoribbons as revealed by the microscopic studies. The present work has provided the fundamental understanding of cooperativity and the combination of various noncovalent interactions and their self-assembly. These qualitative and quantitative analyses have suggested the importance of a balanced control of various intermolecular forces, such as π−π stacking and F···F interactions, and their role in assisting the Rh(I)···Rh(I) interactions to arrive at the desirable assembly mode of the molecular building blocks. This work is also believed to provide a general strategic bottom-up approach for the control of assembly and coassembly of molecules into a well-aligned order appropriate for the design of different functional materials, such as supramolecular hydrogels and soft nanomaterials that may require certain specific orientation of interactions to achieve their functions.

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dependent supramolecular aggregation model and van’t Hoff analysis; temperature-dependent 1H NMR data; and diffusion order NMR data (PDF) X-ray crystallographic data for Complex 1 (CIF)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b04687. Experimental section; crystal and structure determination data (CCDC 1587238); photophysical data; concentration-dependent UV−vis absorption data and the corresponding dimerization plot for monomer− dimer equilibrium; temperature-dependent UV−vis absorption data and curve-fitting by the temperatureH


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