Effect of solvent inclusion on the structures and solid-state fluorescence of coordination compounds of naphthalimide derivatives and metal halides

Single crystal X-ray diffraction

Single crystal X-ray diffraction data for structures 3-pnCoCl·CH ₃ OH, 3-pnCoBr·CH ₃ OH, 3-pnZnCl·CH ₃ OH, 3-pnCoCl·CH ₃ CN and 3-pnZnBr·CHCl ₃ were collected on a Bruker D8 Venture diffractometer, with a Photon 100 CMOS detector, at 150(2) K, employing a combination of ϕ and ω scans. Monochromatic MoK-α radiation of wavelength 0.71073 Å, from an Iμs source, was used as the irradiation source. Cooling was achieved using an Oxford Cryogenics Cryostat. Data reduction was performed using the software SAINT+ [26] and absorption corrections were performed using SADABS [27] as part of the APEX II suite [28].

Single crystal X-ray diffraction data for structure 3-pnZnBr·CH ₃ OH were collected on a Rigaku XtaLAB Synergy R diffractometer, with a HyPix CCD detector at 150(2) K using ω scans and a rotating-anode X-ray source. Monochromatic Mo radiation of wavelength 0.71073 Å, was employed as the irradiation source for this structure. Cooling was achieved using an Oxford Cryogenics Cryostat. Data reduction and absorption was carried out using the CrysAlisPro (Version 1.171.40.23a) software package [29].

All the crystal structures were solved either by direct methods or intrinsic phasing using SHELX-2013 [30], as part of the WinGX suite [31]. Structure refinements were done using SHELXL [32] in WinGX [31] as GUI. Graphics and publication material were generated using Mercury 3.5 [33] and PLATON [34]. In all the structures the hydrogen atoms were placed in calculated positions, with U _iso(H) = 1.5U _eq (parent atom).

Powder X-ray diffraction

Powder X-ray diffraction patterns were measured at room temperature on a Bruker D2 Phaser powder diffractometer, employing a Si low-background sample holder, and experimental powder patterns of bulk samples were compared with powder patterns calculated from single crystal structures using the software DiffractWD [35]. Calculated and experimental power patterns are included in Section 1 of the Supplementary Information Section.

Solid-state fluorescence spectra

The solid-state fluorescence spectra were collected at room temperature under ambient conditions employing a Horiba Fluoromax-4 spectrofluorometer, fitted with a xenon lamp light source and a photomultiplier detector. The optical system was a plane-grating Czery–Turner monochromator, with a wavelength range of 200–950 nm. A front entrance and exit slit of 5.00 nm band pass was used for both excitation and emission spectra. Ground samples of the coordination compounds and the organic ligand were sandwiched between two glass microscope slides, and measured at a 45° geometry. In order to minimise scattering caused by re-absorption of the emitted light, which may affect the solid-state spectra, the samples were prepared to be as thin as possible, while still being uniform [36].

Isostructural series: 3-pnCoCl·CH₃OH, 3-pnCoBr·CH₃OH, 3-pnZnCl·CH₃OH and 3-pnZnBr·CH₃OH

The structures 3-pnCoCl·CH ₃ OH, 3-pnCoBr·CH ₃ OH, 3-pnZnCl·CH ₃ OH and 3-pnZnBr·CH ₃ OH are isostructural, crystallising in the triclinic space group P 1 ‾ , with the asymmetric unit comprising two crystallographically independent 3-pn ligands coordinating via the nitrogen atom of the pyridyl group, and two halido ligands coordinated to a metal cation. An isolated methanol solvent molecule is also present in the asymmetric unit, as shown in Fig. 1a. The site occupancy of the methanol solvent molecule in structures 3-pnCoCl·CH ₃ OH, 3-pnCoBr·CH ₃ OH, 3-pnZnCl·CH ₃ OH and 3-pnZnBr·CH ₃ OH was refined independently, and was found to be 0.284(4), 0.404(10), 0.268(4) and 0.33(2) respectively. These values indicate a relatively low occupancy of solvent molecules in the structures. Compounds containing the 1,8-naphthalimide moiety are often isostructural [38, 39].

Structure 3-pnCoCl·CH ₃ OH will be discussed in detail, and is representative of the rest of the structures in the isostructural series. The crystallographically independent organic 3-pn ligands will be distinguished according to their pyridyl nitrogen atoms, with ligand 1 containing atom N(1) and ligand 2, atom N(3). The complex adopts a twisted conformation, with one of the 3-pn ligands, ligand 2, folding over, with the oxygen atom of the folded ligand accepting a hydrogen bond from a methanol solvent molecule, as shown in Fig. 2a. The second organic ligand, ligand 1, exhibits an extended conformation. The angle between the pyridyl plane and naphthalimide plane is indicative of the conformation of the 3-pn ligand. In the methanol-containing structures, this angle ranges from 71.89° to 72.92° for ligand 1, and from 62.55° to 63.72° for ligand 2, indicating the difference in conformation between ligand 1 and ligand 2.

Fig. 2:

(a) Complex 3-pnCoCl·CH ₃ OH, illustrating the hydrogen bond between the methanol solvent molecule and the complex molecule. (b) Dimeric unit consisting of two complex molecules connected by aromatic interactions, and two methanol molecules. (c) Packing of methanol solvent molecules in cavities in the structure. (d) Layered structure of 3-pnCoCl·CH ₃ OH viewed down the b-axis. (e) Aromatic stack in structure 3-pnCoCl·CH ₃ OH showing aromatic interactions between the 13-membered rings. Note that structure 3-pnCoCl·CH ₃ OH is representative of the structures in the isostructural series.

The M(II) ions display a distorted tetrahedral geometry with the N-M-N angle increasing when the halido ligand changes from chlorido to bromido while the X-M-X angle is mainly unaffected (see Table S2.1 for values).

A dimeric unit is formed between two complex molecules and two solvent molecules, as illustrated in Fig. 2b. Cavities containing the methanol solvent molecules are formed by complex molecules, as shown in Fig. 2c.

The packing diagram of structure 3-pnCoCl·CH ₃ OH, viewed down the b-axis, is illustrated in Fig. 2d. A layered structure, consisting of alternating layers of stacked, interdigitated 1,8-naphthalimide groups and layers containing the pyridyl groups and the metal halido portion, is formed. The naphthtalimide moieties pack approximately parallel, forming a slipped stack, in which ligand 1 and ligand 2 alternate, with alternating centroid-to-centroid distances ranging from 71.89° to 72.94° for ligand 1 and from 62.55° to 63.72° for ligand 2 (see Table S2.1 in Supplementary Information Section 2), indicating weak interactions between aromatic groups, as illustrated in Fig. 2e.

Structure 3-pnCoCl·CH₃CN

Structure 3-pnCoCl·CH ₃ CN crystallises in space group C2/c and its asymmetric unit comprises one 3-pn ligand and one chlorido ligand coordinated to the Co(II) ion, as well as a very disordered acetonitrile molecule, as shown in Fig. 1. The chlorido ligand is disordered over two positions, with occupancies of 0.54 and 0.46, and the Co(II) metal ion is located on a two-fold rotation axis, which generates the rest of the molecule, as shown in Fig. 3a.

Fig. 3:

(a) Complex molecule in structure 3-pnCoCl·CH ₃ CN. (b) Packing of solvent and complex molecules in structure 3-pnCoCl·CH ₃ CN. (c) Packing of structure 3-pnCoCl·CH ₃ CN viewed down the a-axis. (d) Aromatic interactions between 3-pn groups in structure 3-pnCoCl·CH ₃ CN. (e) Packing of structure UCASEZ [13] viewed down the a-axis.

The metal ion exhibits a distorted tetrahedral geometry with the Cl–Co–Cl angle having the largest value, while the N-M-N angle exhibits the smallest value. The CH₃CN solvent molecules pack between the complex molecules in the structure. A layered structure is formed, as illustrated in Fig. 3b, c, with the CH₃CN solvent molecules and metal halido portion of the complex comprising one type of layer, while the 3-pn portion of the complex molecules form a second type of layer. When viewed down the a-axis, the complexes in neighbouring layers alternate in orientation, as shown in Fig. 3c. The naphthalimide moieties pack approximately parallel, in a slipped stack, with the complexes zippered together by aromatic interactions, as illustrated in Fig. 3d. Centroid-to-centroid distances between the naphthalimide moieties alternate between 3.759 and 3.933 Å, indicating weak aromatic interactions between the naphthalimide groups.

The crystal structure of the ZnCl₂ analogue of 3-pnCoCl·CH ₃ CN has been reported in the literature, with CSD [12] reference code UCASEZ [13], and is isostructural to structure 3-pnCoCl·CH ₃ CN, as illustrated in Fig. 3c, e.

Structure 3-pnZnBr·CHCl₃

Structure 3-pnZnBr·CHCl ₃ crystallises in space group Pnma and its asymmetric unit comprises half a Zn(II) metal centre, to which two half bromido ligands and a 3-pn ligand are coordinated, with the 3-pn ligand coordinating via the pyridyl nitrogen atom. The Zn(II) ion and bromido ligands lie on a mirror plane, and reflection across this mirror plane generates the rest of the complex molecule. Half a disordered chloroform solvent molecule is also present in the asymmetric unit, as shown in Fig. 1, with the carbon atom, one of the chlorine atoms and the hydrogen atom of the chloroform molecule located on a mirror plane. Reflection across the mirror plane generates the full chloroform molecule. The occupancy of the chloroform molecule was refined separately from the complex molecule, yielding an occupancy of 0.549(6).

Two 3-pn organic ligands and two bromido ligands are coordinated to the distorted tetrahedral metal ion, as shown in Fig. 4a. Due to the symmetry of the complex, the angle between the pyridyl plane and naphthalimide plane is the same for both 3-pn ligands, with a value of 56.95°.

Fig. 4:

(a) Complex molecule in structure 3-pnZnBr·CHCl ₃ . (b) Dimeric unit consisting of two complex molecules and two solvent molecules. (c) Channel containing solvent molecules. (d) Alternating orientation of chloroform molecules in channel. (e) Packing diagram of structure 3-pnZnBr·CHCl ₃ , viewed down the a-axis.

Two complex molecules and two chloroform solvent molecules pack to form a dimeric unit in which the two solvent molecules are encapsulated by the complex molecules, as shown in Fig. 4b. Packing of this dimeric unit along the a-direction results in channels formed by the complex molecules, with the solvent molecules contained in the channels, as illustrated in Fig. 4c. In a channel, neighbouring solvent molecules alternate in orientation, as illustrated in Fig. 4d. One of the bromido ligands coordinated to the Zn(II) ion is located on the inside of the channel, while the second bromido ligand points to the outside of the channel. Weak C–H … Br–Zn hydrogen bonding interactions are formed between the chloroform molecules and the halido ligands of the complex molecules on the channel surface.

Figure 4e shows the packing diagram of structure 3-pnZnBr·CHCl ₃ as viewed down the a-axis, and illustrates the solvent-filled channels. The naphthalimide groups pack in layers, with the metal halide portion, pyridyl groups and solvent molecules packing in a second layer, resulting in a layered structure.

Discussion of 3-pnMX.solvent structures

In all six new complexes, solvent molecules were included in the crystal lattice. It was found that the same complex could crystallise with different solvent molecules in the crystal lattice, depending on the solvent of crystallisation employed, and that different conformations are adopted by these complexes, to accommodate the solvent molecules in the crystal structure.

A distorted tetrahedral geometry is displayed by all the metal centres, and the 1,8-naphthalimide portion of the coordinated ligands is fairly rigid, but rotational flexibility around the M−N bond and the methylene group allows the complexes a degree of conformational flexibility.

In general, all the complexes containing the same solvent form isostructural structures, and thus show a similar geometry, packing of the complex and packing of the solvent molecules. The same conformation is adopted by complex molecules in all the structures containing methanol solvent, namely structures 3-pnCoX·CH ₃ OH and 3-pnZnX·CH ₃ OH, with X = Cl⁻ and Br⁻, as is evident from the overlay of the complex molecules as shown in Fig. 5a, The complex molecule in structure 3-pnCoCl·CH ₃ CN, as well as the complex molecule in its ZnCl₂ analogue, UCASEZ [13], adopt the same conformation, as illustrated by the overlay in Fig. 5b.

Fig. 5:

(a) Overlay of 3-pnMX·CH ₃ OH complex molecules. Red: M = Co, X = Cl, blue: M = Co, X = Br, green: M = Zn, X = Cl, black: M = Zn, X = Br. (b) Overlay of complex molecule in structure 3-pnCoCl·CH ₃ CN and UCASEZ [13]. Red: 3-pnCoCl·CH ₃ CN, black: complex in UCASEZ [13]. (c) overlay of complex molecule in UCASEZ [13] (green), 3-pnZnBr·CHCl ₃ (red) and 3-pnZnBr·CH ₃ OH (black).

Comparison of the conformations of these six complexes show that overall three different conformations are exhibited, as illustrated in Fig. 5c. The complexes in the methanol-containing structures exhibit a specific conformation, while the complexes in the acetonitrile-containing structures exhibit a second conformation, and the complex in the chloroform containing structure exhibits a third conformation. In the conformation of the complex adopted in the structure 3-pnZnBr.CHCl ₃ , shown in red in Fig. 5c, the orientation of one of the 3-pn ligands is similar to the orientation of the ligand in the acetonitrile structures, however, overall geometry of the complex is different.

It can be concluded that the conformation of the complex molecule, and the packing in the solid state, can be controlled through the choice of solvent from which the complex is crystallised. This effect is independent from the identity of the metal halide component, since the same conformation and packing is displayed regardless of the MX₂ portion of the complex, provided that the same solvent molecule is present in the structure, as can be seen in the family of structures 3-pnCoX·CH ₃ OH and 3-pnZnX·CH ₃ OH, with X = Cl⁻ and Br⁻.

In addition, the same complex molecule can display different conformations to accommodate different solvent molecules into the structure, as evidenced when comparing the structures 3-pnZnBr·CH ₃ OH and 3-pnZnBr·CHCl ₃ .

Comparison with related literature structures

It is informative to compare the structures determined in this study with related structures reported in the literature, including the parent ligand 3-pn (WEZDUB [14], WEZDUB01 [13]) and the ZnCl₂ analogue of 3-pnCoCl·CH ₃ CN (UCASEZ [13]), which will be abbreviated 3-pnZnCl·CH ₃ CN. Structures containing the protonated version of 3-pn, thus a 3-picolinium 1,8-naphthalimide cation (3-pn ⁺ ), will also be included in the comparison. These structures include the combination of 3-pn ⁺ with a tetrachlorocuprate(II) anion, with dichloromethane solvent (HATRIE [15]), abbreviated 3-pn ⁺ CuCl·CH ₂ Cl ₂ , and the perchlorate salt of 3-pn ⁺ (WICBEQ [14]), abbreviated 3-pn ⁺ ·ClO ₄ .

Firstly, the comparison will focus on the 3-pn parent compound, as well as the 3-pn and 3-pn ⁺ portions of the structures reported here and in the literature. Table S2.2 lists the angle between the naphthalimide plane and the pyridyl plane, as well as the N _{naphthalimide}–C–C–C_pyridyl torsion angles for the 3-pn moieties in the different structures. Where two 3-pn moieties are present in a structure, two values are listed. The angle between the naphthalimide plane and pyridyl plane ranges from 54.87° to 74.18° across the different structures. In the structures of the pure 3-pn ligand (WEZDUB [14], WEZDUB01 [13]), this angle has the largest value of 74°. The smallest angle between the planes (54.87°) is observed for one of the cations in structure 3-pn ⁺ ·ClO ₄ (WICBEQ [14]). Thus, some variation in the conformation of the 3-pn moiety is observed across the different structures, with the 3-pn molecule showing the largest angle between the planes, which decreases on coordination or the formation of a 3-pn ⁺ cation.

Sarma et al. [14] used theoretical calculations to determine the N _{naphthalimide}–C–C–C_pyridyl torsion angles of the most stable conformer of the isolated 3-pn molecule, and found them to be 47.0 and 131.3°. The experimental values of this angle listed in Table S2.2 for the different crystal structures show that the experimental torsion angles differ significantly from this theoretical value. As concluded by Sarma et al. [14], this indicates that non-covalent interactions play a significant role in determining the conformation of the 3-pn moiety.

Due to the presence of the naphthalimide portion and the pyridyl moiety, aromatic interactions are important non-covalent interactions in these structures, and are worth considering when comparing the structures. In the structure of 3-pn (WEZDUB [14]) pairs of 3-pn molecules interact through π–π interactions with a centroid-to-centroid distance of 3.686 Å, and edge-to-face C–H ⋯ π interactions (H ⋯ centroid distance 2.745 Å) to form a parallel fourfold aryl embrace (P4AE) [40] dimer, as illustrated in Fig. 6a. Neighbouring dimers pack to form a stack, with a centroid-to-centroid distance of 5.568 Å. A P4AE dimer is also formed in structure 3-pn ⁺ CuCl·CH ₂ Cl ₂ [15], as illustrated in Fig. 6b with a centroid-to-centroid distance of 4.015 Å and a H ⋯ centroid distance of 2.806 Å. Two single 3-pn ⁺ cations stack on both sides of the dimer, at centroid-to-centroid distance of 3.878 Å, with a disordered dichloromethane molecule completing the stack. This pattern repeats throughout the structure. In structure 3-pn ⁺ ·ClO ₄ two crystallographically independent 3-pn ⁺ cations are present. One type of cation forms a centrosymmetric, head-to-tail dimer, with a centroid to centroid distance of 4.253 Å, with neighbouring dimers of the same cation stacking with a centroid-to-centroid distance of 5.851 Å, as illustrated in Fig. 6c. The second cation forms a head-to-tail centrosymmetric dimer, with a centroid-to-centroid distance of 3.600 Å, shown in Fig. 6d. In structures 3-pnZnCl·CH ₃ CN (UCASEZ [13]) and 3-pnCoCl·CH ₃ CN, the complex molecules pack in a head-to-tail fashion, forming a stack of naphthalimide moieties, shown in Fig. 6e with centroid-to-centroid distances alternating between 3.740 Å and 3.927 Å in structure 3-pnZnCl·CH ₃ CN and values listed in Table S2.1 for structure 3-pnCoCl·CH ₃ CN. In structure 3-pnCoCl·CH ₃ OH, which is representative of the structures in the isostructural series, the twisted conformation of the molecule results in stacking of the naphthalimide groups, as illustrated in Fig. 6f, with alternating centroid-to-centroid distances listed in Table S2.1. In the stack, the planes of the naphthalimide groups are rotated by approximately 40°. The naphthalimide rings in structure 3-pnZnBr·CHCl ₃ form a slipped stack, illustrated in Fig. 6g, with large centroid-to-centroid distances of 5.571 Å and 7.528 Å, where pairs of neighbouring naphthalimide moieties point in the same direction.

Fig. 6:

Aromatic interactions in structures (a) 3-pn, (b) 3-pn ⁺ CuCl·CH ₂ Cl ₂ , (c) 3-pn ⁺ ·ClO ₄ (cation 1), (d) 3-pn ⁺ ·ClO ₄ (cation 2), (e) 3-pnZnCl·CH ₃ CN (UCASEZ [13]), which is also representative of structure 3-pnCoCl·CH ₃ CN, (f) 3-pnCoCl·CH ₃ OH (which is also representative of the rest of the structures in the isostructural series) and (g) 3-pnZnBr·CHCl ₃ .