Programmed Molecular Construction: Driving the Self-Assembly by Coordination and Hydrogen Bonds Using 6-(Pyridin-2-yl)-1,3,5-triazine-2,4-diamine with M(NO3)2 Salts.

A new series of hydrogen-bonded metallotecton networks 6-9 of the general formula [M(2)2(NO3)2] were obtained from the reaction of 6-pyridin-2-yl-[1,3,5]-triazine-2,4-diamine 2 with transition-metal ions [M: Co(II), Ni(II), Cu(II), and Zn(II)]. Their supramolecular networks and associated properties were characterized by single-crystal and powder X-ray diffraction, IR, solid-state UV-vis spectroscopy, and thermogravimetric analysis associated with differential scanning calorimetry. On the basis of standard patterns of coordination involving 2,2'-bipyridine and simple derivatives, compound 2 binds transition-metal ions with predictable constitution and the diaminotriazinyl (DAT) groups serve orthogonally to ensure the intermetallotecton interactions by hydrogen bonding according to well-established motifs I-III. As expected, compound 2 formed octahedral 2:1 metallotectons with M(NO3)2, and further self-assembled by hydrogen bonding of the DAT groups to produce pure, crystalline, homogeneous, and thermally stable materials. In these structures, nitrate counterions also play an important role in the cohesion of intermetallotectons to form two-dimensional and three-dimensional networks. These results illustrated the effectiveness of the synthetic approach to create a wide range of novel ordered materials with controllable architectures and tunable properties achieved by varying the central metal ion. Crystal morphologies of 6-9 were also investigated by scanning electron microscopy and calculation using Bravais-Friedel-Donnay-Harker method from their single-crystal structure.

Introduction

Enhancements and innovations in the fields of chemistry, physics, and engineering have provided a better understanding of the structure–property relationships in several classes of materials. As a result, n class="Chemical">considerable effort has been made to develop strategies to precisely control the chemical composition and the structure to tailor properties of novel materials. Among others, ordered materials have attracted much attention due to their importance in numerous technological fields such as catalysis, photovoltaics, batteries, nanotechnology, and so forth.[1−3] A common approach to prepare well-defined structures is based on the concept of crystal engineering in which molecular components with suitable topologies and ability to engage in multiple predictable interactions with neighbors produce reliable patterns.[4,5] Even though reliable methods have been used to create ordered materials, it should be noted that the synthesis of the molecular components is usually not trivial. Thus, a hybrid approach, combining both inorganic and organic chemistry, is exploited to produce ordered materials driven by the self-assembly of organic units linked by coordination to metal ions and other directional forces.[6−10] This approach has numerous advantages: (i) it allows to prepare molecular crystals with high yields; (ii) the self-assemblies are predictable; (iii) ordered materials with various properties owing to the presence of transition metal ions are developed; and (iv) the supramolecular networks and properties can be easily tuned by varying the metal center.

In coordination chemistry, ligand like 2,2′-n class="Chemical">bipyridine (1 = 2,2′-bipy) has been widely used due to its chelating ability (in self-assembly with metal ions) to form predictable coordination complexes.[11−17] Many functional materials with 2,2′-bipy and related ligands have been developed and applied to material separation/purification, catalysis, and ions exchange.[18−23] However, these materials are comparatively difficult to prepare because their syntheses are laborious. In addition, crystal structures with 2,2′-bipy complexes are not predictable due to lack of functional groups to direct the self-assembly.

Of special interest are diaminotriazinyl-substituted pyridine, n class="Chemical">pyrazine, and pyrimidine 2–4, a family of compounds that can engage in predictable intermolecular interactions such as hydrogen bonds and can simultaneously bind metal ions according to reliable patterns (Scheme ).[24−31] Because of this dual ability, they were called tectoligands.[31] In crystal engineering, the self-assembly of tectoligands with metal ions produces metallotectons.[32,33]

Scheme 1

Representation of the Molecular Structures of (a) 2,2′-Bipyridine 1, (b) Tectoligands 2–4, and (c) Cyclic Hydrogen Bonding Motifs I–III of Diamino-1,3,5-triazinyl Group (DAT)

Here, we report the synthesis and the solid-state characterizations of a new series of crystalline materials 6–9 obtained under mild reaction n class="Chemical">conditions by the assembly of 2 with Co(II), Ni(II), Cu(II), and Zn(II) ions (Scheme b). By focusing on a single tectoligand with a predictable behavior, bound with various transition metal ions, we designed our study to produce wide range of ordered materials with closely related structures to reveal the principles of the molecular construction based on the metallotectonic approach.

Scheme 2

Representation of the Molecular Structures of (a) Complexes Without Hydrogen-Bonding Sticky Site and (b) Metallotectons 6–9

Results and Discussion

Syntheses and Characterization

To evaluate the potential of the hybrid approach to form various molecular crystals, we have chosen to study the behavior of 2 to bind transition-n class="Chemical">metal ions (Co(II), Ni(II), Cu(II), and Zn(II)) that have d7 to d10 electronic configurations. Compound 2 is basically an organic compound consisting of a diamino-1,3,5-triazinyl group (DAT) and a pyridine, which are known as the building blocks of many inorganic materials and biological molecules.[34−43] In this work, we focused on M(NO3)2 salts because nitrate ions can engage in multiple hydrogen bonding, which can strengthen the supramolecular networks. 6-(Pyridin-2-yl)-1,3,5-triazine-2,4-diamine 2 was prepared by reported methods.[44] The reaction of 2 and metal(II) nitrate in methanol produces metallotectons 6–9, which were crystallized by slow diffusion in MeOH/Et2O in good yields. As a result, structures of 6–9 reflect intra- and intermetallotecton interactions and multiple strong hydrogen bonds characteristic of DAT groups.

Three-Dimensional Molecular and Supramolecular Structure

Crystal Structures of 6 and 7

Crystals of 6 grown from MeOH/n class="Chemical">Et2O are pink. They belong to the orthorhombic space group Fdd2. Views of the structure are shown in Figure . Other crystallographic data are summarized in Table . The structure of 6, with 2:1 pyDAT-to-Co ratio, is Co(pyDAT)2(NO3)2. The cobalt atom coordinated with two pyDAT and two nitrate ions form a cis-conformation (Figure a). The metallotecton 6 has a strongly distorted octahedral geometry. This coordination geometry is reinforced by intramolecular hydrogen bonds N–H···O involving oxygen atoms of the nitrate and adjacent NH2 group (2.917(4) Å). The average distance of Co–N and Co–ONO2 in the metallotecton is normal (2.131(4) and 2.146(3) Å, respectively).[45] The Co atom and one pair of coordinated N atoms of the triazine rings of different pyDAT ligands form almost a straight line, with a N2–Co–N2i angle of 167.4(2)°. In contrast, the other coordinated pair of N atoms of the pyridine rings is not linear, with a N1–Co–N1i angle of 109.0(2)°. The pyridine and DAT rings in both ligands are tilted around the C–C bond by 12.6°. The average plane of bound ligands forms an angle of 57.9°. As expected, the DAT groups in metallotecton 6 are placed in trans-orientation. In the structure, each DAT group is linked by two N–H···N hydrogen bonds of type IV (Scheme , distance N–H···N = 3.104(5) Å), giving rise to a three-dimensional network (Figure b). Additional N–H···O hydrogen bonds involving nitrates and free NH2 groups reinforced the supramolecular network. Details of the hydrogen bonds and their angles are provided in Table S1.

Figure 1

Views of the crystal structure of 6 grown from MeOH/Et2O. Hydrogen bonds are represented by dashed lines. Unless stated otherwise, carbon atoms are shown in gray, hydrogen atoms in white, oxygen atoms in red, nitrogen atoms in blue, and cobalt atoms in pink. (a) Structure of the metallotecton 6 and (b) alternating of enantiomeric metallotectons of 6 joined by N–H···N hydrogen bonds of DAT groups according to motif IV to produce the three-dimensional network. For clarity, few metallotectons are marked in green and pink.

Table 1

Crystallographic Data for 6–9

	6	7	8	9
formula	Co(C8H8N6)2(NO3)2	Ni(C8H8N6)2(NO3)2	Cu(C8H8N6)2(NO3)2	Zn(C8H8N6)2(NO3)2
Mr	559.36	559.14	563.97	565.80
crystal system	orthorhombic	orthorhombic	monoclinic	monoclinic
radiation	Ga Kα	Ga Kα	Ga Kα	Cu Kα
λ (Å)	1.34139	1.34139	1.34139	1.54178
F(000)	2280	2288	1148	1152
space group	Fdd2	Fdd2	P21/c	C2/c
a (Å)	39.7939(16)	39.702(2)	15.2840(11)	9.0912(1)
b (Å)	8.9352(4)	8.8392(4)	9.0360(7)	11.4484(2)
c (Å)	11.7989(5)	11.9162(6)	16.0867(11)	19.9334(3)
α (deg)	90	90	90	90
β (deg)	90	90	105.075(3)	94.019(1)
γ (deg)	90	90	90	90
V (Å3)	4195.3(3)	4181.9(4)	2145.2(3)	2069.56(5)
Z	8	8	4	4
ρcalcd (g/cm3)	1.771	1.776	1.746	1.816
T (K)	120	120	110	100
μ (mm–1)	4.911	5.483	5.940	2.316
measured reflns	21 847	20 085	36 457	20 812
independent reflns	2325	2253	4913	2028
Rint	0.0652	0.0798	N/A	0.0229
observed reflns I > 2σ(I)	2180	2070	4504	1966
R1, I > 2σ(I)	0.0426	0.0560	0.0672	0.0251
R1, all data	0.0467	0.0615	0.0758	0.0258
ωR2, I > 2σ(I)	0.0976	0.1430	0.1716	0.0690
ωR2, all data	0.0999	0.1470	0.1834	0.0695
GoF	1.071	1.078	1.086	1.067

Scheme 3

Representation of the Molecular Structures of a Polymeric Hydrogen Bonding Motifs IV of Diamino-1,3,5-triazinyl Group (DAT)

Views of the crystal structure of 6 grown from n class="Chemical">MeOH/Et2O. Hydrogen bonds are represented by dashed lines. Unless stated otherwise, carbon atoms are shown in gray, hydrogen atoms in white, oxygen atoms in red, nitrogen atoms in blue, and cobalt atoms in pink. (a) Structure of the metallotecton 6 and (b) alternating of enantiomeric metallotectons of 6 joined by N–H···N hydrogen bonds of DAT groups according to motif IV to produce the three-dimensional network. For clarity, few metallotectons are marked in green and pink.

The reaction of 2 with n class="Chemical">nickel(II) nitrate 2:1 ratio in methanol subsequently crystallized from MeOH/Et2O produced cyan crystals of 7. The crystal structure determined by single-crystal X-ray diffraction is isostructural to 6. Views of the structure of 7 are shown in Figure S3. Other crystallographic data are given in Table . The structural formula of metallotecton 7 consists of Ni(pyDAT)2(NO3)2. The nickel atom is coordinated with pyDAT and nitrates similarly to 6 (Ni–N and Ni–ONO2 average distances are 2.092(5) and 2.125(5) Å, respectively). In the structure of 7, the N2–Ni–N2i angle is 170.5(3)°, which is slightly different from that of 6. Selected hydrogen bonds and their angles are provided in Table S2.

Crystal Structure of 8

The blue crystals of 8 grown from MeOH/n class="Chemical">Et2O proved to belong to the monoclinic space group 21/ and have the composition Cu(pyDAT)2(NO3)2. Figure shows the views of the structure. Other crystallographic data are summarized in Table . The metallotecton 8 with a 2:1 pyDAT-to-Cu ratio consists of [(pyDAT)2Cu]2+ cation and two nitrate anions in trans-fashion (Figure a). The cationic [(pyDAT)2Cu]2+ is flattened, and the average plane of bound ligands forms an angle of 39.8°. The copper(II) atom is coordinated to two pyDAT and two nitrate ions. The copper coordination polyhedron can be described as a strongly distorted octahedron. This coordination geometry is reinforced by intramolecular N–H···O hydrogen bonds involving oxygen atoms and NH2 groups. In the metallotecton 8, the average Cu–N bonds length is 2.005(1) Å. The two nitrates are coordinated in an apical position with Cu–O nonequal bond lengths of 2.457(3) and 2.845(4) Å. These values are within the 2.4–2.9 Å range and consistent with an axial elongation caused by a Jahn–Teller distortion of the octahedral geometry of Cu(II).[46,47] The observed structure consists of enantiomeric metallotectons of 8, which are linked alternatively into chains joined via characteristic N–H···N hydrogen bonds of DAT groups according to the motif I (Figure b). The average N–H···N distances in hydrogen-bonded pairs of DAT groups (3.073(6) Å) have normal values.[48] The chains are held together by π-stacking of DAT groups and pyridyl rings to form sheets (average distance 3.988 Å). The sheets are reinforced by additional N–H···O hydrogen bonds involving oxygen atoms of nitrate and NH2 groups that are simultaneously engaged in hydrogen bonding according to motif I. Packing of the sheets directly by multiple N–H···O hydrogen bonds involving nitrate and free NH2 groups produce the three-dimensional structure (Figure c). Selected hydrogen bonds and their angles are given in Table S3.

Figure 2

Views of crystal structure of 8 grown from MeOH/Et2O. Hydrogen bonds are represented by dashed lines. Unless stated otherwise, carbon atoms are shown in gray, hydrogen atoms in white, oxygen atoms in red, nitrogen atoms in blue, and copper atoms in green. (a) Structure of the metallotecton 8. (b) Alternating zigzag chains of Cu(pyDAT)2(NO3)2 and its enantiomer joined together by hydrogen bonding of DAT groups according to the motif I, strengthened by hydrogen bonding involving bridging nitrates. (b) View showing sheets packed together to form the three-dimensional structure. For clarity, layers are marked in green and blue.

Views of crystal structure of 8 grown from n class="Chemical">MeOH/Et2O. Hydrogen bonds are represented by dashed lines. Unless stated otherwise, carbon atoms are shown in gray, hydrogen atoms in white, oxygen atoms in red, nitrogen atoms in blue, and copper atoms in green. (a) Structure of the metallotecton 8. (b) Alternating zigzag chains of Cu(pyDAT)2(NO3)2 and its enantiomer joined together by hydrogen bonding of DAT groups according to the motif I, strengthened by hydrogen bonding involving bridging nitrates. (b) View showing sheets packed together to form the three-dimensional structure. For clarity, layers are marked in green and blue.

Crystal Structure of 9

The reaction of n class="Chemical">zinc nitrate with 2 in a ratio of 1:2 produced 9 in high yield. Colorless crystals of 9 grown from MeOH/Et2O proved to belong to the monoclinic space group 2/c and have the composition Zn(pyDAT)2(NO3)2. Views of the structure are shown in Figure , and additional crystallographic data are given in Table . The observed structure incorporates enantiomer of metallotectons. The coordination around the zinc atom in 9 is identical to that in metallotectons 6 and 7 (Figure a). The average plane of bound ligands in cationic [(pyDAT)2Zn]2+ forms an angle of 57.3° and the two DAT groups are in trans orientation. The zinc atom can be considered as having a distorted octahedral geometry. Within each ligand, the triazine and pyridine rings are almost coplanar, and the average planes form angles of 12.9 and 17.6°. The nitrogen atoms of two different DATs are axially coordinated with an almost linear N2–Zn–N2i angle of 168.3°. The pyridine rings are in an equatorial position with the N1–Zn–N1i angle of 114.3°. The distances Zn–N in the metallotecton 9 are normal (average distance 2.160 Å).[49] The average distance of the Zn–O bond is 2.181(1) Å. This suggests that nitrates are strongly coordinated to the metal ion. Details of the hydrogen bonds and their angles are summarized in Table S4. In the structure of 9, the trans orientation of the DAT groups allows pairing of metallotectons by the formation of four hydrogen bonds according to motif I (average N–H···N distance = 3.019(2) Å) to form chains. With the assistance of hydrogen bonds involving nitrate and free NH2 groups (average distance N–H···O = 2.945(5) Å), the chains are held together to form sheets (Figure b). These sheets pack via π-stacking to produce the observed three-dimensional structure (Figure c).

Figure 3

Views of the crystal structure of 9 grown from MeOH/Et2O. Hydrogen bonds are represented by dashed lines. Unless stated otherwise, carbon atoms are shown in gray, hydrogen atoms in white, oxygen atoms in red, nitrogen atoms in blue, and zinc atoms in orange. (a) Structure of the metallotecton 9. (b) Racemic pairs of metallotecton held together by four N–H···N hydrogen bonds of type I and multiple hydrogen bonds involving bridging nitrates to form the two-dimensional (2D) sheet. (c) View showing sheets packed together to form the three-dimensional structure. For clarity, layers are marked in red and blue.

Views of the crystal structure of 9 grown from n class="Chemical">MeOH/Et2O. Hydrogen bonds are represented by dashed lines. Unless stated otherwise, carbon atoms are shown in gray, hydrogen atoms in white, oxygen atoms in red, nitrogen atoms in blue, and zinc atoms in orange. (a) Structure of the metallotecton 9. (b) Racemic pairs of metallotecton held together by four N–H···N hydrogen bonds of type I and multiple hydrogen bonds involving bridging nitrates to form the two-dimensional (2D) sheet. (c) View showing sheets packed together to form the three-dimensional structure. For clarity, layers are marked in red and blue.

It should be noted that although metallotectons in (6 and 7) and 9 have identical n class="Chemical">coordination geometry and topology, the supramolecular networks resulting from the same crystallization condition (MeOH/Et2O) provide different association motifs of DAT groups. Comparison of the structures of 9 and [Zn(2)2(N3)2] reported from the literature[50] shows similar association motifs of DAT groups. However, the replacement of azide ions by nitrate ions generates different cohesion of adjacent sheets, which play an important role in the self-assembly.

Together, crystal structures of 6–9 and [n class="Chemical">Zn(2)2(N3)2] indicate that the molecular organization is dependent not only on the topology of metallotectons but also on the integration of subtle details, which collectively direct the supramolecular association.

Crystallinity, Purity, and Homogeneity of Bulk Materials of 6–9

The evaluation of homogeneity and phase purity is an important aspect in material science. Indeed, in the crystallization process, the crystal growth of a sample might result in a blend of several crystalline phases, thereafter producing a sample with heterogeneous properties. Therefore, we first verified the homogeneity and purity of 6–9 by elemental analysis (n class="Chemical">EA) (see the Experimental Section). The compositions found by EA for each sample have the general chemical formula M(C8H8N6)2(NO3)2 (M = CoII, NiII, CuII, or ZnII), which are consistent with the single-crystal X-ray diffraction data. Since we cannot confirm the phase purity by EA, the bulk products of the as-grown crystals of 6–9 were evaluated by powder X-ray diffraction (PXRD). Generally, due to the fragility of crystals outside of the solvents of crystallization, the analysis by PXRD is difficult. In our case, all crystals were stable outside the mother liquors. The PXRD measurements in transmission mode for 6–9 confirmed the phase purity of products. Indeed, all peaks of measured PXRD (in black) match well with the simulated (in red) patterns (Figure ). This result demonstrate the absence of secondary phases for all samples. The phase purity of 6–9 was reinforced by scanning electron microscopy (SEM), which confirms the absence of contaminations by an amorphous phase that cannot be observed by PXRD (see Figure S8).

Figure 4

PXRD of 6–9. Comparison of the measured powder X-ray diffraction (in black) with simulated patterns (in red) calculated from single-crystal structures. (a–d) 6–9, respectively.

Together, the XRD, PXRD, EA, and SEM demonstrate that each bulk sample of 6–9 can be prepared in a single-phase, pure, and crystalline forms.

Thermal and Photophysical Properties

Thermal behaviors of 6–9 were determined using a combination of thermogravimetric analysis and differential scanning calorimetry (TGA–DSC) mn class="Chemical">easured simultaneously. TGA and DSC curves of these compounds are shown in Figure . All samples were studied from 35 to 800 °C, with a heating rate of 10 °C/min under air atmosphere.

Figure 5

Thermogravimetric analysis (TGA, black) and differential scanning calorimetry (DSC, red) curves of 6–9. (a–d) 6–9, respectively.

DSC curves of 6 and 7 showed a sharp exothermic peak at about 337 and 350 °C, respectively (Figure a,b, red curves). These are followed by exothermic peaks between 410 and 485 °C. DSC curve measured for 8 displayed three small recognizable exothermic peaks between 300 and 400 °C, followed by an intense broad exothermic peak (Figure c, red curve). The DSC curve of 9 showed a sharp exothermic peak at 345 °C, followed by broad exothermic peaks (Figure d).

Compounds 6 and 7 have virtually the same TG thermal den class="Chemical">composition curves (Figure a,b, black curves). Their DSC curves show sharp exothermic peaks, which are associated to a pronounced loss of mass of ∼44%. In the second decomposition step, a loss of mass of the same magnitude is again observed to give the final residues. TG curve of 8 displayed three distinguishable decomposition steps in the 300–517 °C temperature range, with mass losses of 13% for the first two and 49.4% for the last one (Figure c, black curve). Compound 9 decomposed in three steps between 345 and 592 °C (Figure d, black curve). The first two decomposition steps occurred at 345–532 °C, with a net loss of mass of ∼34% each. The third decomposition step between 532 and 592 °C showed ∼25% loss of mass. A summary of temperature ranges, mass losses (%) found and calculated, and proposed assignment of decomposition of 6–9 determined by TGA are listed in Table S5.

Solid-state UV–vis spectra of 2 and 6–9 were measured at room temperature (Figure ). The primary objective of the investigation in solid state is to n class="Chemical">correlate the absorption bands with the crystallographic data. Indeed, at liquid state, the geometry of the transition-metal complex and the oxidation state of the metal ion could be different from the single-crystal structure, which could vary the electronic transitions.

Figure 6

Solid state UV–vis spectra of 2 and 6–9.

In solid state, the UV–vis spectrum of the free ligand 2 showed a strong absorption band at 320 nm, which is attributed to the n–π* and π–π* electron transitions typical of aromatic n class="Chemical">compounds. As excepted for 9, d–d transitions centered on the metal ion are observed for 6–8. Because the electronic configuration of Zn(II) is d10, all electrons in the d orbitals are paired; therefore, d–d transitions do not occur for 9. However, in the UV region of the spectrum of 9, two absorption bands are observed at 321 and 331 nm. These bands can be attributed to the electronic transitions centered to the ligand. Compounds 6–8 have electronic configurations of d7, d8, and d9 for Co(II), Ni(II), and Cu(II) ions, respectively. Their d orbitals are not fully occupied, which allows d–d transitions to occur.[51] In solid state, the absorption bands in the visible region of Co(pyDAT)2(NO3)2 in 6 are situated at 560 and 478 nm, which may be assigned to ν1[4T1g(F) → 4T2g(F)] and ν2[4T1g(F) → 4T1g(P)] transitions, respectively, in an idealized O symmetry. Those of Ni(pyDAT)2(NO3)2 in 7 are at 788 and 615 nm, which could be attributed to ν1[3A2g(F) → 3T2g(F)] and ν2[3A2g(F) → 3T1g(F)] transitions, respectively, again in an O symmetry. That of Cu(pyDAT)2(NO3)2 in 8 is at 734 nm, which may correspond to ν1[2Eg(D) → 2T2g(D)] transition, also in an O symmetry. All these absorption bands are consistent with the spin-allowed d–d transitions with an octahedral environment around the metal ions. In the UV region, the absorption bands at (327 and 360 nm), (328, 347, and 380 nm), and (325 and 372 nm) for 6–8, respectively, are indicative of charge-transfer transitions occurring due to the coordination of the ligands to metal ions. Comparing the solid-state UV–vis spectrum of 2 with those of 6–9, we notice that when the same ligand is coordinated to different metal ions with oxidation state +II, the electronic absorption spectra of the metallotectons varied significantly due to the coordination geometry and the different extranuclear electron distribution of the metal ions.

Liquid UV–vis spectra of 2 and 6–9 in dimethyl sulfoxide solutions were mn class="Chemical">easured at room temperature at concentration 8.8 × 10–6 M (Figure S9). All spectra display similar electronic transition in the UV region. However, the positions and the intensities of the two absorption bands varies depending on the metal ions. In the visible region, none of the typical d–d electron transitions are observed for 6–9.

IR spectra were measured for samples 2 and 6–9 (Figure S10). The pn class="Chemical">eak positions for each spectrum are listed in Table S6 along with their proposed assignments.

As the pyDAT molen class="Chemical">cule is coordinated to metal ions, the vibration frequencies, intensities, and shapes of the N–H, C–N, C=C, C–C, and C–H bonds could change in the infrared absorption spectra of 6–9. Three factors, such as steric effect, field effect, and ring strain, can contribute to the spatial effect.

In the 2000–600 cm–1 region, several metal-sensitive bands are observed. The absorption bands between 680 and 700 cm–1 of 6–9 are sensitive to different n class="Chemical">metal ions. The main difference between compounds 2 and 6–9 is a strong wide band appearing near 1270–1295 cm–1. This is assigned to the NO3– stretching vibration. In the 3500–2800 cm–1 region, the infrared spectra of 2 and 6–9 show typical broad bands characteristics of symmetric and asymmetric N–H stretching of the NH2 groups. Positions and intensities of these bands in 6–9 are different from that of pyDAT, indicating different hydrogen bonding motifs of the DAT groups. The infrared spectra of 6 and 7 are almost identical. This is due to their similar crystal structures. Within the deformation region between 2000 and 600 cm–1, the infrared spectrum of 9 is almost the same as those of 6 and 7. However, in the stretch region, the absorption bands of NH2 groups are different from those of 6 and 7. These observations can be explained by the similar spatial configuration but the different intermolecular hydrogen bonding motifs of the DAT groups of 6 and 9 (7 and 9, respectively). These results concur with their crystal structures. The IR spectrum of 8 shows the characteristic vibrational bands of pyDAT and nitrate counterions. The absorption bands are different from those of 6, 7, and 9. This result is in good agreement with the crystallographic data.

Crystal morphology is an importance aspect for both research and industries applications as physical properties of many crystals are implicitly dependent on their shapes.[52,53] Thus, an attempt was made to understand the crystal growth of n class="Chemical">compounds 6–9. Microscopy observation of the crystal morphologies of 6–9 was carried out by scanning electronic microscopy (SEM) (Figure S11a–d). The calculated crystal morphology of 6–9 was obtained from the Bravais–Friedel–Donnay–Harker (BFDH) calculation method[54] using their crystal structures (Figure S11e–h). All the theoretical calculations were performed using Materials Studio 4.0.[55] Morphologies based on BFDH calculations were compared with the experimental data obtained from solution growth, showing a good correlation. Combining the calculated morphology and the crystal structure, assignment of the surface chemistry were proposed for the largest facets of crystals of 6–9 (Figure S11i–l). In the case of (6, 7), and 9, the facets (400) and (002), respectively, expose the −NH2 of DAT groups on the surface at the molecular level. For crystal of 8, the surface of facet (100) is composed of nitrate groups pointing outside. These results indicate that hydrogen bond interactions play a critical role in the crystal morphology of 6–9.

Conclusions

This work demonstrates how a large range of ordered materials can be prepared using the metallotectonic approach that exploits the use of n class="Chemical">coordination bonds to metal ions as a primary directing force assisted by other strong intermolecular interactions such as hydrogen bonds. An obvious advantage of building crystalline materials from metallotectons is that they allow (i) to decrease the number of synthetic steps and (ii) to quickly obtain molecular topologies and properties not otherwise available in pure organic chemistry.

We have successfully prepared a series of crystalline materials by coordination of 2 with n class="Chemical">Co(II), Ni(II), Cu(II), and Zn(II). All compounds 6–9 were obtained in high yields as single-phase materials, as determined by XRD, PXRD, EA, and SEM. Single-crystal X-ray diffraction investigations of the synthesized compounds revealed that the supramolecular networks are mainly directed by coordination and hydrogen bonds as well as by van der Waals forces. Their physical properties were characterized by UV–vis, IR, TGA, and DSC. The solid-state UV–vis absorption spectroscopy spectra of 6–9 concord with their d–d transitions. All samples exhibit high stability under thermal conditions, as determined by TGA. The IR spectra of 6 and 7 are nearly identical, which is in agreement with the crystallographic data.

The external morphologies and crystal facets of 6–9 were measured by SEM and predicted by theoretical caln class="Chemical">culations using the BFDH method. Comparisons of experimental and calculated morphologies were in agreement, supporting the effectiveness of the BFDH method. Furthermore, the surface chemistry of crystals of 6–9 was proposed using a combination data including SEM measurement, morphology prediction, and XRD.

Our study demonstrates that properties of ordered materials can be tuned using the metallotectonic approach. The results show that the investigated approach allowed crn class="Chemical">eating materials with different properties while preserving identical supramolecular architectures as exemplified with compounds 6 and 7.

Experimental Section

General Notes and Procedures for the Synthesis of 6–9

6-(Pyridin-2-yl)-1,3,5-triazine-2,4-diamine 2 was synthesized by known reported methods.[44] Their n class="Chemical">complexes with Co, Ni, Cu, and Zn, respectively, were prepared by the experimental procedure described below. Other chemicals were commercially available and purchased and used without any additional purification. Solid of 2 (2.0 equiv) was added in small portions at 25 °C to the stirred solutions of M(NO3)2·xH2O (1 equiv) in MeOH (25 mL). The mixtures were refluxed for 12 h and the resulting homogeneous solutions were cooled to room temperature and subject to crystallization by slow diffusion with diethyl ether.

Compound 6

The reaction of 2 (0.05 g, 0.2656 mmol) with n class="Chemical">Co(NO3)2·6H2O (0.039 g, 0.1328 mmol) according to the general procedure yielded 90% of pink crystals of 6 with the composition of Co(2)2·(NO3)2. FTIR (ATR): 3436.52, 3306.53, 3214.29, 3149.30, 1645.50, 1610.88, 1588.36, 1563.40, 1538.40, 1506.64, 1487.93, 1464.66, 1449.55, 1435.91, 1395.07, 1291.03, 1259.03, 1194.19, 1146.45, 1054.25, 1020.08, 989.15, 911.93, 823.05, 787.32, 748.70, 686.35 cm–1. HRMS (ESI) calcd for [C16H16N12CoNO3]+m/z 497.0825, found 497.0831. Anal. calcd for C16H16CoN14O6: C, 34.36; H, 2.88; N, 35.06. Found: C, 34.38; H, 2.75; N, 34.78.

Compound 7

The reaction of 2 (0.05 g, 0.2656 mmol) with Ni(NO3)2·6H2O (0.039 g, 0.1328 mmol) according to the general procedure yielded 93% of cyan crystals of 7 with the composition of Ni(2)2·(NO3)2. FTIR (ATR): 3437.74, 3303.23, 3215.47, 3149.30, 1646.40, 1611.10, 1589.57, 1565.29, 1538.93, 1510.46, 1489.22, 1466.07, 1436.80, 1396.94, 1291.06, 1261.01, 1197.18, 1148.01, 1054.44, 1021.71, 991.07, 822.95, 786.86, 750.28, 687.91 cm–1. HRMS (ESI) calcd for [C16H16N12NiNO3]+m/z 496.0847, found 496.0856. Anal. calcd for C16H16N14NiO6: C, 34.37; H, 2.88; N, 35.07. Found: C, 34.63; H, 2.84; N, 34.62.

Compound 8

The reaction of 2 (0.05g, 0.2656 mmol) with n class="Chemical">Cu(NO3)2·H2O (0.031 g, 0.1328 mmol) according to the general procedure yielded 90% blue crystals 8 with the composition of Cu(2)2·(NO3)2. FTIR (ATR): 3461.75, 3321.61, 3205.23, 3164.44, 3061.00, 1612.35, 1589.04, 1567.94, 1519.67, 1488.64, 1447.38, 1393.23, 1315.35, 1271.13, 1057.87, 1040.90, 1016.51, 820.00, 789.50, 755.58, 719.14 cm–1. HRMS (ESI) calcd for [C16H16N12CuNO3]+m/z 501.08951, found 501.0800. Anal. calcd for C16H16CuN14O6: C, 34.08; H, 2.86; N, 34.77. Found: C, 34.34; H, 2.85; N 34.77.

Compound 9

The reaction of 2 (0.05 g, 0.2656 mmol) with n class="Chemical">Zn(NO3)2·6H2O (0.039 g, 0.1328 mmol) according to the general procedure yielded 99% colorless crystals 9 with the composition of Zn(2)2·(NO3)2. FTIR (ATR): 3443.23, 3371.81, 3312.99, 3211.12, 3171.54, 3113.98, 1672.16, 1644.16, 1610.99, 1587.80, 1558.28, 1516.19, 1489.62, 1470.80, 1446.24, 1428.34, 1395.70, 1292.37, 1265.96, 1194.79, 1144.84, 1095.17, 1064.21, 1021.02, 993.41, 911.39, 824.24, 787.76, 748.94, 681.16 cm–1. HRMS (ESI) calcd for [C16H16N12ZnNO3]+m/z 502.0785, found 502.0796. Anal. calcd for C16H16N14O6Zn: C, 33.97; H, 2.85; N, 34.66. Found: C, 33.93; H, 2.87; N, 34.39.

Instrumentation

Crystallographic data were collected using a Bruker Venture Metaljet diffractometer with Ga Kα radiation and a Bruker APEX2 diffractometer equipped with a Cu Kα radiation from a microfocus source. The structures were solved by direct methods using SHELXT,[56] and nonhydrogen atoms were refined anisotropically with least-squares minimization.[57] Hydrogen atoms were treated by first locating them from different Fourier maps, recalculating their positions using standard values for distances and angles, and then refining them as riding atoms. Microcrystalline powders were analyzed in transmission-mode geometry using a Bruker D8-Discover instrument (θ–θ geometry) equipped with a XYZ platform and a HI-STAR gas detector. X-rays were generated using a conventional sealed-tube source with a copper anode producing Cu Kα radiation (λ = 1.54178 Å). The samples were gently ground and then mounted on a flat Kapton sample holder. The data collection involved acquisition of two different sections with increasing angular position, giving two different 2D frames. These frames were integrated and combined to produce the final one-dimensional powder X-ray diffraction pattern. Calculated powder X-ray diffraction patterns were generated from the structural data in the corresponding CIF resulting from single-crystal analyses. The calculation was performed using MERCURY[58] software of the Cambridge Crystallographic Data Center. A unique value of the full width at half maximum for the diffraction peaks was adjusted to get a better match between the resolution of the experimental and the calculated patterns. The determination of the total carbon, hydrogen, nitrogen, and sulfur (C, H, N, and S) contents in the compounds was performed using a EA 1108 Fisons CHNS Element analyzer by quantitative “dynamic flash combustion” method. The solid-state UV–vis spectra were recorded on a Cary 5000 spectrometer. The crystals are gently ground and placed on quartz holders. The ATR–FTIR spectra were collected with a Nicolet iS 10 Smart FT-IR Spectrometer within 600–4000 cm–1. The thermogravimetric analysis and differential scanning calorimetry were performed simultaneously using a simultaneous thermal analysis (STA) System Setaram, model Labsys Evo STA. The samples were loaded in Al2O3 pans and isochronically heated from 35 to 800 °C with a heating rate of 10 °C/min. The scanning electron microscopy was performed using a Hitachi SU1510 microscope.