Supramolecular Assemblies of Dipyrrolyldiketone CuII Complexes

Dipyrrolyldiketones are essential building units of anion-responsive π-electronic molecules and ion-pairing assemblies. Here, we demonstrated that they form complexes with CuII characterized by planar geometries. The solid-state stacking assembled structures, as revealed by single-crystal X-ray analysis, were modulated by the substitution of pyrrole units. The rectangular shapes of the CuII complexes resulted in the formation of mesophases upon introduction of aliphatic chains.


Introduction
The fabrication of molecular assemblies is crucial for the development of functional materials [1]. In particular, π-electronic molecules with planar geometries may afford stacking assemblies, which are suitable for the generation of electrically conductive materials. Ion-pairing assemblies based on π-electronic ions form a variety of ordered states that act as dimension-controlled assemblies, such as crystals, liquid crystals, and supramolecular gels [2,3]. The preparation of π-electronic anions, which can serve as components of ion-pairing assemblies, is more difficult than that of π-electronic cations due to the lower stability of electron-rich species. Therefore, anion-responsive π-electronic molecules have been investigated for the preparation of anion complexes which are pseudo π-electronic anions. For example, boron complexes of dipyrrolyldiketones ( Figure 1a) provided planar anion complexes through hydrogen-bonding interactions with pyrrole-NH and bridging-CH units, which were used as components of various ion-pairing assemblies [4][5][6][7][8]. Anion binding requires the inversion of two pyrrole rings from the state where the NH units are oriented towards the carbonyl units, as the most stable conformations generally result from the opposite dipole arrangements of the pyrrole rings and carbonyl units.
Notably, the 1,3-diketone unit can act as a monovalent metal ligand for various metal complexes (Figure 1b) [9][10][11][12][13][14][15][16]. The complexation of 1,3-diketones with divalent metals, such as Cu II , Ni II , and Pt II , formed planar geometries, which are suitable for the formation of stacking assemblies. For example, various liquid-crystal materials based on 1,3-diketone metal complexes were fabricated using aryl units that served as scaffolds for aliphatic chains [11][12][13][14][15][16]. On the other hand, the introduction of divalent metals to dipyrrolyldiketones were not achieved due to the coordination by the pyrrole-N sites. Therefore, appropriate metal complexation conditions are required for the development of dipyrrolyldiketone metal complexes. Recently, Ti IV complexes of dipyrrolyldiketones were investigated by focusing on their crystal and liquid-crystal mesophase structures [17], prompting further investigations on other metal complexes [18]. This study describes the synthesis of Cu II complexes of dipyrrolyldiketones, as well as the investigation of their crystal structures and mesophase assemblies.

Results and Discussion
Dipyrrolyldiketone Cu II complexes 1a,b and 2a-f ( Figure 2) were synthesized according to the literature procedures, by treating dipyrrolyldiketones 1a′,b′ and 2a′-f′ [19][20][21][22] with Cu(OAc)2 in MeOH/CHCl3 [11]; moderate yields were obtained depending on the purification process. Due to the low stability of the 1,3-diketone-Cu coordination bonds under silica gel purification conditions, 1a,b and 2a-f were isolated by reprecipitation. The 1 H NMR of 1a,b and 2a-f displayed broad signals due to the paramagnetic properties of Cu II as d 9 electron configuration. Therefore, 1a,b and 2a-f were characterized by MALDI-TOF-and ESI-TOF-MS analysis. The UV/vis absorption maximum (λmax) of 1a as a representative example in CH2Cl2 was 399 nm, which can be attributed to the transition at the π-electronic systems, as suggested by time-dependent (TD)-DFT calculations at the B3LYP/6-31G(d,p) level with LanL2DZ for Cu [23].

Results and Discussion
Dipyrrolyldiketone Cu II complexes 1a,b and 2a-f ( Figure 2) were synthesized according to the literature procedures, by treating dipyrrolyldiketones 1a ,b and 2a -f [19][20][21][22] with Cu(OAc) 2 in MeOH/CHCl 3 [11]; moderate yields were obtained depending on the purification process. Due to the low stability of the 1,3-diketone-Cu coordination bonds under silica gel purification conditions, 1a,b and 2a-f were isolated by reprecipitation. The 1 H NMR of 1a,b and 2a-f displayed broad signals due to the paramagnetic properties of Cu II as d 9 electron configuration. Therefore, 1a,b and 2a-f were characterized by MALDI-TOF-and ESI-TOF-MS analysis. The UV/vis absorption maximum (λ max ) of 1a as a representative example in CH 2 Cl 2 was 399 nm, which can be attributed to the transition at the π-electronic systems, as suggested by time-dependent (TD)-DFT calculations at the B3LYP/6-31G(d,p) level with LanL2DZ for Cu [23].

Results and Discussion
Dipyrrolyldiketone Cu II complexes 1a,b and 2a-f ( Figure 2) were synthesized according to the literature procedures, by treating dipyrrolyldiketones 1a′,b′ and 2a′-f′ [19][20][21][22] with Cu(OAc)2 in MeOH/CHCl3 [11]; moderate yields were obtained depending on the purification process. Due to the low stability of the 1,3-diketone-Cu coordination bonds under silica gel purification conditions, 1a,b and 2a-f were isolated by reprecipitation. The 1 H NMR of 1a,b and 2a-f displayed broad signals due to the paramagnetic properties of Cu II as d 9 electron configuration. Therefore, 1a,b and 2a-f were characterized by MALDI-TOF-and ESI-TOF-MS analysis. The UV/vis absorption maximum (λmax) of 1a as a representative example in CH2Cl2 was 399 nm, which can be attributed to the transition at the π-electronic systems, as suggested by time-dependent (TD)-DFT calculations at the B3LYP/6-31G(d,p) level with LanL2DZ for Cu [23].  In the solid state, 1a and 2a-c,f exhibited planar geometries for the dipyrrolyldiketone Cu II unit with mean-plane deviations of 0.065(2) Å for 1a (core dipyrrolyldiketone Cu II unit as 31 atoms) and 0.041(4)/0.056(7), 0.127(4), 0.029 (2), and 0.091(2) Å for 2a-c,f (core dipyrrolyldiketone Cu II and aryl units as 55 atoms), respectively (Figure 3a; see Figures S14, S16 and S17 for 1b and 2b,c). On the other hand, 1b showed a distorted structure with a dihedral angle of 29.86(5) • for two oppositely arranged diketone mean planes (15 atoms of the two pyrrole and core diketone units). In each case, two pyrrole-NH units were oriented to the Cu II (carbonyl) side as the most stable conformation, which was also revealed by DFT calculations [23]. For example, 1a with uninverted pyrrole rings was more stabilized by 17.62 kcal/mol than the structure with completely inverted pyrrole rings, as suggested by DFT calculations at the B3LYP/6-31G(d,p) level with LanL2DZ for Cu. The antiparallel direction of the dipoles for the pyrrole and carbonyl moieties was a key factor for determining the planar structure. The C carbonyl -C bridging -C carbonyl angles for 1a,b and 2a-c,f were found to be 123.5(8)-126.4(2) • , which are similar to those of other 1,3-diketone Cu II complexes [11,12]. Owing to the core planar geometries of dipyrrolyldiketone Cu II complexes, slippedstacking structures were observed in the crystal states. For example, 1a showed a slippedstacking columnar structure with a stacking distance of 3.14 Å between the two mean planes of the core dipyrrolyldiketone Cu II unit (31 atoms of two dipyrrolyldiketone moieties and Cu) (Figure 4a). On the other hand, the distorted 1b showed a packing structure with no stacking, probably due to the steric repulsion between the β-ethyl units. Interestingly, 1b formed a hydrogen-bonding 1D-columnar structure based on the interaction of the pyrrole NH and the neighboring oxygen of the diketone unit ( Figure S14). Similar to 1a, aryl-substituted 2a-c formed slipped-stacking columnar structures with stacking distances of 3.24-3.22 Å between the two mean planes of the aryl-substituted dipyrrolyldiketone Cu II complex unit (55 atoms) (Figure 4b). The Cu II ···Cu II distances in the stacking structures of 1a and 2a were 5.901(3) and 9.08(2) Å, respectively. Furthermore, angles of 32.1 and 19.8 • were observed along the lines passing through two Cu II in the columnar structures of 1a and 2a to the mean planes of the core dipyrrolyldiketone Cu II complex units (31 atoms), respectively. The introduction of aryl units at the α-positions extended the planar structures to the long axis of the molecules, and thus, the aryl rings stacked on top of the diketone Cu II planes. In order to visualize the intermolecular interactions in the crystal structures, the Hirshfeld surface analysis was carried out [25][26][27]. In fact, the Hirshfeld surface analysis of 1a and 2a revealed an effective stacking of the core 1,3-diketone Cu II complex unit with the pyrrole and phenyl rings, respectively ( Figure 5). In particular, the surfaces of the phenyl rings of 2a showed the red and blue triangles arranged in bow-tie shapes on the shape-index surface and flat region on the curvedness surface, indicating the characteristic mapping pattern for the stacking of planar units including aryl rings [28]. The stacking structures of the dipyrrolyldiketone Cu II complexes as shown here were also stabilized by the chelate ring-π interaction [29]. The rectangular geometries formed by the introduction of aryl units are crucial for modulating the assembled structures. In contrast, 3,4,5-trimethoxysubstituted 2f exhibited a zigzag-arranged packing structure. The p-methoxy oxygen was coordinated to Cu II of the neighboring complex with an O···Cu II distance of 2.547(8) Å (Figure 4c). Owing to the core planar geometries of dipyrrolyldiketone Cu II complexes, slippedstacking structures were observed in the crystal states. For example, 1a showed a slippedstacking columnar structure with a stacking distance of 3.14 Å between the two mean planes of the core dipyrrolyldiketone Cu II unit (31 atoms of two dipyrrolyldiketone moieties and Cu) (Figure 4a). On the other hand, the distorted 1b showed a packing structure surface, indicating the characteristic mapping pattern for the stacking of planar units including aryl rings [28]. The stacking structures of the dipyrrolyldiketone Cu II complexes as shown here were also stabilized by the chelate ring-π interaction [29]. The rectangular geometries formed by the introduction of aryl units are crucial for modulating the assembled structures. In contrast, 3,4,5-trimethoxy-substituted 2f exhibited a zigzag-arranged packing structure. The p-methoxy oxygen was coordinated to Cu II of the neighboring complex with an O···Cu II distance of 2.547(8) Å (Figure 4c).  shown as packing diagrams with, in (c), green arrows indicating the coordination between the p-methoxy oxygen and Cu II . Solvent molecules are omitted for clarity. Color code: Brown, pink, cyan, red, and blue refer to carbon, hydrogen, nitrogen, oxygen, and copper, respectively.
On the basis of the rectangular discotic geometries of the dipyrrolyldiketone Cu II complexes, the formation of dimension-controlled assemblies as mesophases was investigated. To induce mesophases, 3,4-dihexadecyloxy and 3,4,5-trihexadecyloxy chains were introduced at the aryl rings of 3a,b (Figure 6a). Differential scanning calorimetry (DSC) analysis of dihexadecyloxy 3a revealed the formation of mesophases with transition temperatures of 104/86/36 and 28/96/107 • C upon cooling and heating, respectively, and a mosaic polarized optical microscopy (POM) texture (Figure 6b(i)). On the other hand, 3b exhibited broad transitions at the temperatures at 49/37/24 and 27/43/50 • C upon cooling and heating, respectively. X-ray diffraction (XRD) analysis of 3a at 90 • C upon cooling revealed peaks at 4.61, 3.99, 2.51, and 2.01 nm, which were indexed to (200), (110), (310), and (220), respectively, suggesting the formation of a Col r (P2/a) structure with a = 9.22 nm, b = 4.43 nm, c = 0.39 nm, and Z = 4 (ρ = 1.1) (Figure 6b(ii)) [30]. It should be noted that dipyrrolyldiketone Cu II complexes possessed rectangular geometries as the most stable structures based on the pyrrole NH directed toward the carbonyl moieties. In contrast to the columnar structure of 3a, trihexadecyloxy 3b exhibited XRD peaks at 6.77, 3.35, and 2.30 nm, which were indexed to (001), (002), and (003), respectively, suggesting the formation of a discotic lamellar structure with an interlayer distance of 6.77 nm. The length of 3b was estimated to be ca. 6 nm, which is in good agreement with the speculated packing structure formed by interdigitating the aliphatic chains. indicating the coordination between the p-methoxy oxygen and Cu II . Solvent molecules are omitted for clarity. Color code: Brown, pink, cyan, red, and blue refer to carbon, hydrogen, nitrogen, oxygen, and copper, respectively. On the basis of the rectangular discotic geometries of the dipyrrolyldiketone Cu II complexes, the formation of dimension-controlled assemblies as mesophases was investigated. To induce mesophases, 3,4-dihexadecyloxy and 3,4,5-trihexadecyloxy chains were introduced at the aryl rings of 3a,b (Figure 6a). Differential scanning calorimetry (DSC) analysis of dihexadecyloxy 3a revealed the formation of mesophases with transition temperatures of 104/86/36 and 28/96/107 °C upon cooling and heating, respectively, and a mosaic polarized optical microscopy (POM) texture (Figure 6b(i)). On the other hand, 3b exhibited broad transitions at the temperatures at 49/37/24 and 27/43/50 °C upon cooling and heating, respectively. X-ray diffraction (XRD) analysis of 3a at 90 °C upon cooling revealed peaks at 4.61, 3.99, 2.51, and 2.01 nm, which were indexed to (200), (110), (310), and (220), respectively, suggesting the formation of a Colr (P2/a) structure with a = 9.22 nm, b = 4.43 nm, c = 0.39 nm, and Z = 4 (ρ = 1.1) (Figure 6b(ii)) [30]. It should be noted that dipyrrolyldiketone Cu II complexes possessed rectangular geometries as the most stable structures based on the pyrrole NH directed toward the carbonyl moieties. In contrast to the columnar structure of 3a, trihexadecyloxy 3b exhibited XRD peaks at 6.77, 3.35, and 2.30 nm, which were indexed to (001), (002), and (003), respectively, suggesting the formation of a discotic lamellar structure with an interlayer distance of 6.77 nm. The length of 3b was estimated to be ca. 6 nm, which is in good agreement with the speculated packing structure formed by interdigitating the aliphatic chains.

General Procedures
Starting materials were purchased from FUJIFILM Wako Pure Chemical Corp.

General Procedures
Starting materials were purchased from FUJIFILM Wako Pure Chemical Corp. (Osaka, Japan), Nacalai Tesque Inc. (Kyoto, Japan), Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan), and Sigma-Aldrich Co. (Tokyo, Japan) and were used without further purification unless otherwise stated. NMR spectra used in the characterization of precursors of Cu II complexes were recorded on a JEOL ECA-600 600 MHz spectrometer (JEOL Ltd., Tokyo, Japan). UV-visible absorption spectra were recorded on a Hitachi U-3500 spectrometer (Hitachi High-Tech Science Corp., Tokyo, Japan). Matrix-assisted laser desorption ionization time-of-flight mass spectrometries (MALDI-TOF-MS) were recorded on a Shimadzu Axima-CFRplus (Shimadzu Corp., Kyoto, Japan). High-resolution (HR) electrospray ionization mass spectrometries (ESI-MS) were recorded on a BRUKER microTOF using the ESI-TOF method (Bruker, MA, USA). TLC analyses were carried out on aluminum sheets coated with silica gel 60 (Merck 5554). Column chromatography was performed on Wakogel C-300 and Merck silica gel 60H.

Method for Single-Crystal X-ray Analysis
Crystallographic data are summarized in the Supplementary Materials. A single crystal of 1a was obtained by vapor diffusion of n-hexane into a MeOH solution of 1a. The data crystal was a yellow prism of approximate dimensions 0.30 × 0.10 × 0.05 mm. A single crystal of 1b was obtained by vapor diffusion of n-hexane into a CHCl 3 solution of 1b. The data crystal was a violet prism of approximate dimensions 0.50 × 0.30 × 0.20 mm. A single crystal of 2a was obtained by vapor diffusion of water into an N-methyl-2-pyrrolidone and MeOH mixed solution of 2a. The data crystal was a yellow prism of approximate dimensions 0.10 × 0.10 × 0.10 mm. A single crystal of 2b was obtained by vapor diffusion of n-hexane into a MeOH solution of 2b. The data crystal was a yellow prism of approximate dimensions 0.20 × 0.15 × 0.10 mm. A single crystal of 2c was obtained by vapor diffusion of n-hexane into a MeOH solution of 2c. The data crystal was an orange prism of approximate dimensions 0.60 × 0.40 × 0.20 mm. A single crystal of 2f was obtained by vapor diffusion of n-hexane into a mixture of MeOH and CH 2 Cl 2 solution of 2f. The data crystal was an orange prism of approximate dimensions 0.40 × 0.20 × 0.10 mm. The data of 1a,b and 2a-c,f were collected at 123 K on a Rigaku RAXIS-RAPID diffractometer (Rigaku Corp., Tokyo, Japan) with graphite monochromated Mo-Kα (λ = 0.71075 Å). All the structures were solved by the dual-space method. The structures were refined by a full-matrix least-squares method using a SHELXL 2014 [34] (Yadokari-XG) [35,36]. In each structure, the non-hydrogen atoms were refined anisotropically. All the hydrogen atom positions were placed at calculated positions and rode on the atom of attachment except for the hydrogen atoms of water in 1a. For 2c,f, the disordered solvents, presumably N-methylpyrrolidone and CH 2 Cl 2 , respectively, were removed using the SQUEEZE protocol included in PLATON [37]. CIF files (CCDC-2051817-2051822) can be obtained free of charge from the Cambridge Crystallographic Data Centre.

DFT Caluculations
DFT calculations for the geometrical optimizations were carried out using the Gaussian 09 program [23].

Polarizing Optical Microscopy (POM)
The POM observation was carried out with a Nikon ECLIPSE E600POL polarizing optical microscope (Nikon Corp., Tokyo, Japan) equipped with a Mettler Toledo FP-82 HT hot stage system (Mettler Toledo, Columbus, OH, USA).

Synchrotron X-ray Diffraction Analysis (XRD)
High-resolution XRD analyses were carried out using a synchrotron radiation X-ray beam with a wavelength of 1.00 Å on BL40B2 at SPring-8 (Hyogo, Japan). The diffractions were detected by a large Debye-Scherrer camera with an imaging plate. The camera lengths were set at 550.5 mm. The diffraction patterns were obtained with a 0.01 • step in 2θ. An exposure time of the X-ray beam was 10 s.

Conclusions
Cu II complexes of dipyrrolyldiketones exhibited planar geometries based on their square planar coordination. Rectangular structures were formed upon extending the long axis of these molecules with the introduction of aryl units at the pyrrole α-positions. Planar dipyrrolyldiketone Cu II complexes were suitable for the generation of stackingbased molecular assemblies, as observed in single-crystal structures. Furthermore, the introduction of the aliphatic alkoxy chains induced mesophases as dimension-controlled assemblies based on the characteristic geometries of the formation of dipyrrolyldiketone Cu II complexes. The molecular design aimed at tuning the conformation of 1,3-diketone metal complexes developed in this study might enable further modifications of π-electronic systems for providing electronically and optically attractive materials.