Crystal Structures, Photoluminescence, and Magnetism of Two Novel Transition-Metal Complex Cocrystals with Three-Dimensional H-Bonding Organic Framework or Alternating Noncovalent Anionic and Cationic Layers.

Cocrystallization may alter material physicochemical properties; thus, the strategy of forming a cocrystal is generally used to improve the material performance for practical applications. In this study, two transition-metal complex cocrystals [Zn(bpy)3]H0.5BDC·H1.5BDC·0.5bpy·3H2O (1) and [Cu2(BDC)(bpy)4]BDC·bpy·2H2O (2) have been achieved using a hydrothermal reaction, where bpy and H2BDC represent 2,2'-bipyridine and benzene-1,3-dicarboxylic acid, respectively. Cocrystals were characterized by microanalysis, infrared spectroscopy, and UV-visible spectroscopy. Cocrystal 1 contains five components and crystallizes in a monoclinic space group P21/n. The H0.5BDC1.5-, H1.5BDC0.5-, and H2O molecules construct three-dimensional H-bonding organic framework; the [Zn(bpy)3]2+ coordination cations and uncoordinated bpy molecules reside in channels, where two coordinated bpy ligands in [Zn(bpy)3]2+ and one uncoordinated bpy adopt sandwich-type alignment via π···π stacking interactions. Cocrystal 2 with four components crystallizes in a triclinic space group P-1 to form alternating layers; the binuclear [Cu2(bpy)4(BDC)]2+ cations and uncoordinated bpy molecules build the cationic layers, and the BDC2- species with disordered lattice water molecules form the anionic layers. Cocrystal 1 shows intense photoluminescence at an ambient condition with a quantum yield of 14.96% and decay time of 0.48 ns, attributed to the π* → π electron transition within phenyl/pyridyl rings, and 2 exhibits magnetic behavior of an almost isolated spin system with rather weak antiferromagnetic coupling in the [Cu2(bpy)4(BDC)]2+ cation.

Introduction

Cocrystals, multicomponent molecular complexes defined by Bond,[1] are at the forefront of the quest for novel crystal forms.[2] Design and synthesis of cocrystals have gained significant interest in recent years, and this is because that the cocrystals may alter the material physicochemical properties and further improve the material performance for practical applications. In pharmaceutical materials, cocrystals can enhance solubility and dissolution by forming a crystal of drug and other benign molecule or coformer with specific stoichiometric compositions. Meanwhile, the cocrystallization process does not affect the chemical integrity of molecule compounds.[3−12] Pharmaceutical cocrystals, formed between an active pharmaceutical ingredient (API) and a cocrystal precursor that is a solid under ambient conditions, represent a new paradigm in API formulation that might address important intellectual and physical property issues in the context of drug development and delivery.[13] In other material science, the cocrystal of two different molecules is a possible way of intentionally influencing the position of molecules in a crystal lattice and allows for the investigation of newly generated macroscopic properties.[14−18] It has been known that the material properties, including magnetism,[19] photoluminescence,[15,20−23] mechanical strength,[14] etc., are modifiable using a cocrystallization strategy, and the desired physical and chemical properties are delivered by means of selecting a suitable cocrystal precursor to form a cocrystal with the active compound.[2]

The organic cocrystals have been widely reported hitherto,[14,15,19,24−31] and in this context, the multicomponents generally have similar molecular structures and connect together via noncovalent interactions, such as π···π stacking and C–H···π or H-bonding interactions. For example, a survey of the Cambridge Structural Database[32] reveals that many cocrystal compounds of carboxylate salts contain the neutral carboxylic acid molecules and numerous neutral pyridyl derivative residues in the cocrystals of corresponding pyridinium derivatives. In contrast, the cocrystals containing metal coordination compounds are infrequently reported, and this situation is due to the fact that, commonly, the metal complexes have different coordination modes, and the compounds with different coordination modes rarely possess similar lattice packing forces and exhibit similar crystallization kinetics.[33−38]

In this study, we have prepared two metal complex cocrystals of [Zn(bpy)3]H0.5BDC·H1.5BDC·0.5bpy·3H2O (1) and [Cu2(BDC)(bpy)4]BDC·bpy·2H2O (2) utilizing the self-assembly strategy of auxiliary ligands with transition-metal zinc(II) and copper(II) ions, respectively. We found that the uncoordinated ligands form a cocrystal with the transition-metal complex cations, and the H-bonding and π···π stacking interactions play a critical role in the formation of two metal complex cocrystals. Herein, we present the crystal structure analyses for both 1 and 2, the solid state photoluminescence at an ambient condition for 1 and the magnetic property in 2–300 K for 2.

Results and Discussion

Crystal Structures

Cocrystal 1 crystallizes in a monoclinic space group P21/n. An asymmetry unit of 1, as shown in Figure , includes one coordination cation [Zn(bpy)3]2+, one H0.5BDC1.5–, one H1.5BDC0.5–, one half uncoordinated bpy molecule, and three lattice H2O molecules.

Figure 1

Asymmetry unit of 1 (the hydrogen atoms except those of the carboxylic groups are omitted for clarity, and the thermal ellipsoids are drawn at the 50% probability level).

The Zn2+ ion in [Zn(bpy)3]2+ shows the distortedly octahedral coordination sphere, which is comprised of six nitrogen atoms (labeled as N1–N6) from three bpy ligands (Figure a). The bond lengths of Zn–N range from 2.149(1) to 2.181(1) Å, and three bite angles of N–Zn–N show the similar values of 75.25(7)°, 76.32(7)°, and 76.49(7)°; the bond angles of N–Zn–N in which two N atoms adopt in a trans manner arrangement are equal to 167.99(7), 164.54(7), and 165.59(7), respectively. These geometric parameters in [Zn(bpy)3]2+ are comparable to those in other analogues.[39−43] The uncoordinated bpy molecule shows a centrosymmetric conformation, as shown in Figure b, the inversion center locates at the midpoint of C43-C43#7 (symmetry code: #7 = 2 – x, −y, −z), and the inversion symmetry constrains two pyridyl rings being coplanar. The partially deprotonated H0.5BDC1.5– and H1.5BDC0.5– anions form two types of centrosymmetrical supramolecular dimers through H-bonds. In a dimer, two monomers share one proton, which occupies at an inversion center, and two phenyl rings are coplanar (Figure c,d).

Figure 2

(a) Distorted octahedral coordination geometry of Zn1 center, (b) centrosymmetric bpy molecule, (c) dimer of [H(BDC)2]3–, and (d) dimer of [H3(BDC)2]− (the hydrogen atoms in both pyridyl and phenyl rings are omitted for clarity).

As shown in Figure a, the neighboring [H(BDC)2]3– and [H3(BDC)2]− acidic dimers are connected together via strong H-bonds to form a zigzag H-bond chain along the c-axis direction, where the phenyl rings in the neighboring [H(BDC)2]3– and [H3(BDC)2]− dimers make a dihedral angle of 69.6°. Besides the strong H-bonds between different acidic dimers as well as within each type of acidic dimer, as displayed in Figure b, the strong O–H···O H-bond interactions also appear between different water molecules and between water molecules and each of two types of acidic dimers in which the typical distances between the atoms of the H-bond donor and acceptor (O···O), as listed in Table , fall in the ranges of 2.801(3)–2.856(3) Å. As shown in Table , the H-bond parameters in 1 are comparable to those in other O–H···O bond systems.[44−46]

Figure 3

(a) 1D zigzag H-bond chain formed between [H(BDC)2]3– (magenta color) and [H3(BDC)2]− (blue color) dimers running along c-axis direction, (b) H-bonds between water molecules, between acids, and between water molecules and acids, (c) 3D HOF with channels along b axis, (d) crystal packing diagram showing HOF and components in channels, and (e) π···π stacking between coordinated and lattice bpy molecules in channels of HOF in the crystal structure of 1.

Table 1

Geometric Parameters of H-Bonds in the Crystal Structure of 1

DHA	symmetry	d(D-H) (Å)	d(H···A) (Å)	d(D···A) (Å)	∠D-H···A (°)
O12–H12A···O11		0.795(18)	2.09(2)	2.853(4)	160(4)
O9–H9A···O1#3	x, y + 1, z	0.86(5)	1.97(5)	2.826(3)	172(4)
O11–H11A···O8#3	x, y + 1, z	0.86(4)	1.95(4)	2.801(3)	169(3)
O5–H5A···O2#2	x – 1/2, −y + 1/2, z + 1/2	0.86(4)	1.73(4)	2.565(2)	162(4)
O9–H9B···O2#1	–x + 3/2, y + 1/2, −z + 1/2	0.92(5)	1.94(5)	2.856(3)	177(4)

The zigzag H-bond chains formed by two types of acidic dimers are further developed into a 3D H-bonding organic framework (HOF) through the H-bonding interactions between lattice water molecules and two types of acidic dimers, and the HOF shows channels along the b-axis direction, which is illustrated in Figure c. Both [Zn(bpy)3]2+ coordination cations and lattice bpy molecules are accommodated in the channels (Figure d). As shown in Figure e, each uncoordinated bpy molecule is sandwiched between two bpy molecules from two neighboring [Zn(bpy)3]2+ coordination cations, with a mean plane distance of 2.981 Å between the neighboring bpy rings and a closest interatomic distance of 2.868 Å. Obviously, the five different components cocrystallized in 1 are due to multiple strong H-bonding[47,48] and π···π stacking interactions.[49,50]

Cocrystal 2 belongs to a triclinic space group P-1, and its asymmetric unit consists of one binuclear Cu2+ coordination cation, one disordered BDC2–, one lattice bpy molecule, and two heavily disordered lattice water molecules, forming a cocrystal with a 1:1:1:2 stoichiometry, which is shown in Figure a. Two crystallographically inequivalent Cu2+ ions in the binuclear unit show a distorted square pyramid coordination sphere, as displayed in Figure b, and each coordination square pyramid is comprised of four N atoms from two different bpy ligands and one O atom from one BDC2– ligand, where each carboxylate adopts the monodentate binding manner in the BDC2– ligand. The Cu–N distances range from 1.975(2) to 2.177(2) Å, and the Cu–O bonds show the distances of Cu1–O1 = 1.991(2) Å and Cu2–O3 = 2.029(2) Å. These bond lengths are similar to those reported in the literature.[51−54] It is worth noting that two different types of metal coordination cations [Zn(bpy)3]2+ and [Cu2(bpy)4(BDC)]2+ appear in 1 and 2, respectively, although almost the same reaction conditions were used for preparation of them. The formation of distinct metal coordination cations in 1 and 2 may be relevant to two reasons: (1) the bpy is a chelating ligand, and the entropy effect promotes the Zn2+/Cu2+ ion to easily form the species of the metal ion with bpy compared to carboxylate using a monodentate binding manner and (2) the Cu2+ ion favors to form quadrilateral or square pyramidal coordination geometry owing to the Jahn–Teller effect arising from its electron configuration of d9.

Figure 4

(a) Asymmetry unit (all hydrogen atoms are omitted for clarity, and the thermal ellipsoids are drawn at the 50% probability level) and (b) distorted coordination square pyramids of two crystallographically different Cu2+ ions in the crystal structure of 2.

In the crystal of 2, the uncoordinated bpy molecule and deprotonated BDC2– coexist in the lattice, and the bpy molecule shows a transoid conformation, which is similar to that in 1. One carboxylate shows disordered, and the oxygen atoms have two possible positions (Figure a).

As shown in Figure a, 2 shows lamellar packing fashion, and the layers are parallel to the crystallographic (0 1 –1) plane. The anionic layer contains the deprotonated BDC2– ions and heavily disordered lattice water molecules (removed in the refined crystal structure, see Figure S3), and the cationic layer is comprised of binuclear [Cu2(bpy)4(BDC)]2+ units and uncoordinated bpy molecules. As displayed in Figure b,c, in a cationic layer, the uncoordinated and coordinated bpy molecules form a ladder-like motif in a manner of ···(coordinated bpy)2(uncoordinated bpy)··· along the b + c direction, where the bpy molecules act as the rungs of a ladder and interact with each other via π···π interactions. The mean plane of molecules between the neighboring uncoordinated bpy and coordinated bpy molecules shows a distance of 3.493/3.657/3.405/3.523 Å, and the closest interatomic distance is 3.268/3.288/3.323/3.329 Å. The mean plane of molecules between the neighboring coordinated bpy molecules shows a distance of 3.429/3.433 Å, and the shortest interatomic distance is 3.382/3.380 Å. These distances fall within the typical separation range of π···π interactions. In addition, two neighboring coordinated bpy molecules show the π···π interaction along the a + b + c direction (Figure b,d) with a mean plane distance of 3.354/3.496 Å and a shortest interatomic separation of 3.292/3.476 Å.

Figure 5

(a) Crystal packing diagram showing a lamellar structure, and the layers are parallel to the (0 1 –1) plane, (b) a cationic layer in 2 (all H atoms are omitted for clarity) and π···π stacking between bpy molecules in 2, (c) ladder-like motif in a manner of ···(coordinated bpy)2(uncoordinated bpy)··· along the b + c direction, and (d) two neighboring coordinated bpy molecules along the a + b + c direction.

PXRD, UV–Vis Absorption Spectra of 1 and 2, and Photoluminescence Spectrum of 1

The synthesized and simulated PXRD patterns are shown in Figure S4 for 1 and 2, respectively. The well-matched synthesized and simulated PXRD patterns indicate the high phase purity of the crystalline samples.

UV–vis spectra of cocrystals 1 and 2 in the solid state are shown in Figure a. Two compounds show similar ultraviolet spectra in a region of 250–400 nm, with two intense absorption bands located at ∼287 and 285 nm and three shoulders; these absorption bands correspond to the π → π* electron transitions within the aromatic rings, including the pyridyl rings in bpy and phenyl rings in BDC species.[52,55] Cocrystals 1 and 2 display different spectra in the visible spectral regime (400–800 nm). Cocrystal 2 exhibits a broad absorption band spanning from 550 to 800 nm, attributed to the d–d electron transition in the Cu2+ ion. In the spectrum of 1, there is no sizable absorption in a spectral regime of 400–800 nm, this is because that the Zn2+ ion has d10 electron configuration, and no d–d transition is possible. The solid-state excitation and emission spectra of 1 were investigated at an ambient condition. Cocrystal 1 was irradiated by ultraviolet light with λex = 335 nm, giving rise to an emission band with the maximum λem = 368 nm (Figure b), which is attributed to the π* → π transitions within the aromatic rings. A similar emission band was also observed in other compounds containing [Zn(bpy)3]2+ coordination cations.[56,57] The quantum yield of 1 was determined to be 14.96% under an ambient condition, and the curves of fluorescence decay are shown for 1 in Figure S5. The process of fluorescence decay follows a single exponential decay law. The best fit of the time-dependent fluorescence intensity to eq led to a fluorescence lifetime τ of 0.48 ns, which falls within the nanosecond range and shows fluorescence character.

Figure 6

(a) UV–vis spectra of 1 and 2 and (b) excitation and emission spectra of 1 in solid state.

Magnetic Property of 2

Temperature-dependent magnetic susceptibility is shown in Figure a, where χm denotes the molar magnetic susceptibility of 2 with two Cu2+ ions per formula unit, and the diamagnetism was not subtracted from χm. The plot of χm–T shows the typical Curie–Weiss magnetic behavior; thus, first, the Curie–Weiss law was used for the fit of temperature-dependent magnetic susceptibility data,In eq , C and θ represent the Curie and Weiss constants, respectively, and χ0 denotes the temperature-independent magnetic susceptibility term, including the diamagnetism and possible van Vleck paramagnetism. The best fit gave the parameters of C = 0.773 emu·K·mol–1, θ = −0.35 K, and χ0 = −1.8 × 10–6 emu·mol–1. The fitted Curie constant is slightly larger than a spin-only value of 0.75 emu·K·mol–1 for two uncoupled S = 1/2 spins with g = 2.0. Too small θ value indicates that the magnetic coupling is quite weak between the neighboring magnetic centers, and this result is in agreement with the crystal structure analysis (refer to the Crystal Structures description section). Next, the Bleaney–Bowers equation was further used for the magnetic susceptibility fit to estimate the magnetic coupling interaction within a dimer. The function of molar magnetic susceptibility against temperature is expressed in eq (Bleaney–Bowers equation) for an S = 1/2 spin dimer, which is deduced from the spin Hamiltonian H = −2JS1S2.In eq , the symbols of N, g, μB, and J have normal meanings, and then, the total experimental magnetic susceptibility follows the eq ,In eq , the symbol χ0 represents the temperature-independent magnetic susceptibility and is fixed using the value obtained from the fit by eq . The best fit using eq and eq produced a g of 2.02 and J of −0.52 K. The g factor value is quite reasonable for the Cu2+ ion, and the small negative J value also indicates the existence of weak AFM coupling interaction within a dimer. The corresponding χm(dimer)T versus T is plotted in Figure b, which also shows the presence of weak antiferromagnetic interaction within the dimer of Cu2(bpy)4(BDC)2+.

Figure 7

Plots of (a) χm vs T (the black squares denote the experimental magnetic susceptibility; the red line is theoretically reproduced using fitted parameters) and (b) χm(dimer)T vs T for 2.

Conclusions

In summary, we have achieved two novel cocrystal compounds, which contain correspondingly Zn2+/Cu2+ coordination cations with the uncoordinated ligands, using a hydrothermal reaction. In cocrystal 1, the partially deprotonated H2BDC dimers of [H(BDC)2]3– and [H3(BDC)2]− form 3D HOF with channels, and the [Zn(bpy)3]2+ cations together the uncoordinated bpy reside in channels; there are strong π···π stacking interactions between the uncoordinated and coordinated bpy molecules and between the coordinated bpy molecules. Cocrystal 2 shows the layered structure with alternating anionic and cationic layers, the dimers of Cu2(bpy)4(BDC)2+ are comprised of the cationic layers together with the uncoordinated bpy molecules, and the BDC2– ions are consistent of anionic layers together with heavily disordered water molecules. The formation of cocrystals 1 and 2 is attributed to the strong H-bonding and π···π stacking interactions between multicomponents. Cocrystal 1 emits intense photoluminescence in the solid state at an ambient condition with a quantum yield of 14.96% and decay time of 0.48 ns, assigned to the π–π* electron transition within the aromatic rings, and 2 shows the magnetic characterization of an almost isolated magnetic system with rather weak antiferromagnetic coupling in the Cu2(bpy)4(BDC)2+ dimer.

Experimental Section

Materials and Physical Measurements

Chemicals, including Zn(NO3)2·6H2O, Cu(NO3)2·3H2O, H2BDC, and bpy, and all solvents were commercially purchased and directly used without further purification.

Elemental analyses (EA) for carbon, hydrogen, and nitrogen were performed on a PerkinElmer CHN-2400 elemental analyzer. Infrared spectra (IR) were recorded in the wavenumber ranges of 400–4000 cm–1 on an AVATAR-360 spectrophotometer using KBr pellets. The UV–vis spectra were recorded using a PerkinElmer Lambda 950 UV–vis spectrometer. The 1H NMR spectra were recorded in DMSO-d6 on a Bruker 400 MHz. The photoluminescence (PL) spectra were measured with an Edinburgh Instruments FLS920P fluorescence spectrometer at room temperature. The emission decay lifetime and quantum yield were recorded at room temperature in an Edinburgh FLS980 fluorescence spectrometer. Magnetic susceptibility was measured for the polycrystalline sample in the temperature ranges of 2–300 K using a Quantum Design MPMS-5S superconducting quantum interference device (SQUID) magnetometer, and the diamagnetism arising from the core of atoms was not removed from the experimental magnetic susceptibility.

Preparation and Characterization of 1 and 2

Preparation of [Zn(bpy)3]H0.5BDC·H1.5BDC·0.5bpy·3H2O (1)

A mixture of Zn(NO3)2·6H2O (0.15 g, 0.50 mmol), H2BDC (0.17 g, 1.0 mmol), bpy (0.080 g, 0.50 mmol), and NaOH (0.060 g, 1.5 mmol) was dissolved in 15 mL of distilled water. The mixture was stirred for 0.5 h, sealed in a 23 mL Teflon-lined stainless steel autoclave, heated at 170 °C for 3 days, and then slowly cooled to ambient temperature. The solutions were filtered, and the filtration was allowed to slowly evaporate solvent. Pink crystals suitable for single-crystal X-ray structure analysis were obtained after 3 days and dried in air yielding 0.025 g (∼50% yield calculated using the reactant Zn(NO3)2·6H2O). Anal. calcd for C51H44N7O11Zn (Mr = 996.32): C, 61.43, H, 4.45, and N, 9.84%. Found: C, 60.97, H, 4.54, and N, 9.63%. IR spectrum (KBr disc, cm–1): 3419(m, br), 3076(m), 1698(s), 1605(m), 1562(m), 1474(m), 1442(m), 775(s), and 736(s). 1H NMR data (400 MHz, DMSO, 25 °C, TMS, δ): =7.47–8.70(m, 38H, Hpheyl-ring and Hpyrid-ring).

Preparation of [Cu2(BDC)(bpy)4]BDC·bpy·2H2O (2)

This cocrystal compound was prepared using the similar procedure to that of 1, instead of Zn(NO3)2·6H2O by Cu(NO3)2·3H2O (0.18 g, 0.75 mmol). Dark green crystals suitable for single-crystal X-ray structure analysis were obtained after 3 days and dried in air yielding 0.20 g (65% yield calculated using the reactant Cu(NO3)2·3H2O) of 2. Anal. calcd for C66H52Cu2N10O10 (Mr = 1272.26): C, 62.30, H, 4.12, and N, 11.01%. Found: C, 61.99, H, 4.52, and N, 11.63%. IR spectrum (KBr disc, cm–1): 3431(m, br), 3074(m), 1606(s), 1560(m), 1473(m), 1444(m),765(s), and 718(s).

X-ray Crystallography

Single-crystal X-ray diffraction data of 1 and 2 were collected on a Siemens SMART-CCD diffractometer with graphite monochromatic Mo Kα radiation (λ = 0.71073 Å). The structures were solved by direct methods and refined on F2 using full-matrix least-squares methods with SHELXTL package.[58] All nonhydrogen atoms were refined anisotropically, and hydrogen atoms were theoretically added, riding on their parent atoms.

It is worth mentioning that two heavily disordered and diffused outer-sphere water molecules were found in the structure of 2, which were removed by the program SQUEEZE in Platon during the structural refinements.[59]

Crystallographic data and structural refinement details of 1 and 2 are summarized in Table . Selected bond lengths and angles are tabulated in the Supporting Information (Table S1). The Diamond 3.1 program was used for the creation of figures and analysis of hydrogen bonding interactions in the crystal lattice.[60]

Table 2

Crystallographic Data and Structure Refinement Details of 1 and 2

compound	1	2
empirical formula	C51H44N7O11Zn	C66H48N10O8Cu2
CCDC deposit no.	1525050	1525309
Fw (g·mol–1)	996.32	1236.26
crystal system	monoclinic	triclinic
space group	P21/n	P-1
crystal color	pink	green
crystal size	0.30 × 0.20 × 0.10 mm	0.30 × 0.20 × 0.10 mm
temperature (K)	296(2)	293(2)
a (Å)	12.5732(17)	14.5367(16)
b (Å)	13.1278(18)	16.3208(18)
c (Å)	28.584(4)	17.3434(19)
α (°)	90	94.183(3)
β (°)	98.849(2)	104.754(3)
γ (°)	90	114.463(2)
V (Å3)	4661.9(11)	3546.7(7)
Z	4	2
Dc (g·cm–3)	1.419	1.158
F(000)	2068	1272
reflections collected/unique (Rint)	41116/10647 Rint = 0.0551	43305/16138 Rint = 0.0375
observed reflections	8140	11529
refinement method	full-matrix least squares on F2	full-matrix least squares on F2
data/restraints/parameters	10647/6/656	16138/125/794
goodness-of-fit on F2	1.054	1.030
final R factor	0.0444	0.0583
wR2	0.1158	0.1466