Synthesis, Crystal Structure, and Optical Properties of a Trinuclear Zinc(II) Complex with Rhodamine B

Abstract

A heteroleptic homotrinuclear complex, [Zn₃(valhydr)₂(RhB)₂(EtOH)₂](ClO₄)₂ (H₂valhydr = 1,2-bis(2-hydroxy-3-methoxybenzylidene)hydrazine, RhB = rhodamine B), was synthesized and structurally characterized by X-ray diffraction on single crystal. In the centrosymmetric complex, the zinc ions are positioned in a linear manner. The external zinc ions are pentacoordinated with a distorted square pyramidal stereochemistry, while the central zinc ion presents an elongated octahedral geometry. Luminescent properties of the complex were investigated in solid state and solution.

1. Introduction

Xanthene derivatives such as fluoresceins and rhodamines are fluorophores with excellent photophysical properties (high extinction coefficients, excellent quantum yields, great photostability, and relatively long emission wavelengths) [1]. These features, together with the good water solubility, visible excitation, and emission, make them valuable candidates for biomedical applications [2]. Spirocyclic derivatives of these two xanthenes are useful molecule-based sensors because the ring opening process leads in solution to a turn-on fluorescence change. Fluorescein and rhodamine derivatives are non-fluorescent when they exist in the lactone form, and the ring opening can induce colour changes and fluorescence enhancements (Scheme 1). In solid state, the reduced separation between chromophores and supramolecular interactions established in crystals can increase the non-radiative de-excitation pathways and quench the fluorescence.

The ring opening process offers to the carboxylate group the possibility of coordination to various metal ions. In fluorescein derivatives, the phenol and/or carbonyl groups can also be involved in coordination. For the fluorescein derivatives coordinating solely by the carboxylate group were reported discrete [3,4,5] and polymeric species [4,6,7]. The coordination through the carboxylate, and phenol or carbonyl groups generates coordination polymers [3,4,8]. The binding of the metal ions to the phenol oxygen atoms can be directed by functionalization of the fluorescein with additional potential coordinating groups in the ortho positions. Fluorescein derivatives modified by Mannich reactions were used as fluorescence sensors for quantifying biological Zn(II) ions [9,10].

The coordination abilities of the rhodamine are limited only to the carboxylate group. A survey on the CSD (Cambridge Structural Database) revealed only six examples of crystal structures of metal complexes containing rhodamine as a ligand: two mononuclear Zn(II) complexes [11,12], one mononuclear Cd(II) complex [13], respectively, two Zn(II) [14,15], and one Cd(II) coordination polymer [15]. Additionally, the Zn(II) and Cd(II) coordination polymers also contain polycarboxylates as bridging ligands.

The interest for the rhodamine derivatives is justified by the wide range of applications. Apart from the biological applications, the rhodamine derivatives have found applications in chemodosimetry [16], photon up-conversion via cooperative energy pooling [17], as luminescent sensors [15], and as chromophores in dye-sensitized solar cells [18].

Herein, we report on the synthesis and characterization of trinuclear Zn(II) complexes containing the rhodamine B, and the dianion of the Schiff base resulted from the condensation of o-vanillin with hydrazine as ligands. We used an anionic ligand containing aromatic systems in order to increase the chance of a heteroleptic complex formation with rhodamine B by the possibility of π–π interactions development between the two types of ligands.

2. Materials and Methods

The chemicals used as well as all the solvents were of reagent grade, and were purchased from commercial sources.

2.1. Synthesis of [Zn₃(valhydr)₂(RhB)₂(EtOH)₂](ClO₄)₂ (1)

A mixture of 0.0958 g (0.2 mmol) of rhodamine B, 0.0653 g (0.2 mmol) of H₂valhydr (the Schiff base resulted from the condensation of o-vanillin with hydrazine), and 0.1129 g (0.3 mmol) of Zn(ClO₄)₂·6H₂O in 30 mL ethanol was stirred for an hour at room temperature in the presence of triethylamine. The resulting dark violet solution was filtered. Suitable crystals for X-ray diffraction were obtained after approximately one week by slow evaporation of the solvent at room temperature. The dark violet crystals (0.1083 g) were separated by filtration prior to total evaporation of the solvent (yield 55%).

Selected IR data (KBr pellet, cm⁻¹): 3435(bw), 2975(m-w), 2929(m-w), 1648(m), 1589(vs), 1556(s-m), 1530(m), 1510(m), 1465(s), 1442(s), 1412(s), 1394(s), 1346(s), 1336(s), 1299(m), 1276(s-m), 1245(s), 1208(s), 1181(s), 1134(m), 1079(br, s), 1010(m-w), 925(w), 740(m-w), 682(m-w), and 622(w) (where vs = very strong, s = strong, m = medium, w = weak, br = broad). Anal. calcd. for C₉₂H₁₀₀Cl₂N₈O₂₄Zn₃ (%): C, 56.12; H, 5.12; N, 5.69. Found: C, 56.33; H, 4.95; N, 5.41.

Caution! Perchlorate salts are potentially explosive and they should be handled in small quantities. The synthesis was carried out at the mmol scale and the crystals were grown by slow evaporation at room temperature.

The purity of the crystalline material obtained was also checked by powder X-ray diffraction with a very good agreement between the experimental and single-crystal simulated XRPD patterns (Figure S1).

2.2. Physical Measurements

The IR spectra were recorded on KBr pellets on a Bruker Tensor 37 spectrophotometer in the 4000–400 cm⁻¹ range. Absorption spectra were made with a JASCO V-670 spectrophotometer. The photoluminescence measurements in solid state were carried out at room temperature using a JASCO FP 6500 spectrofluorometer. Emission steady-state and time resolved data were recorded on a TCSPC Edinburgh Instrument FLS920 spectrofluorimeter with an excitation wavelength of 510 nm from a Xe lamp and 374.6 nm from a laser diode. The fluorescence quantum yield was determined relative to fluorescein in 0.1 N NaOH with a quantum yield of 0.91 [19], using solutions with an absorbance lower than 0.1 to avoid the inner-filter effect. The quantum yield of the sample was determined according to the equation:

$$\mathsf{\Phi }={\mathsf{\Phi }}_s\frac{F·\left(1-{10}^{-A_s}\right)}{F_s·\left(1-{10}^{-A}\right)}·\frac{n^2}{n_s^2}$$

(1)

where the s index designates the standard, F is the integrated emission, A is the absorbance at the excitation wavelength, and n is the refractive index. The ratios between the integrated fluorescence and the absorptivity at the excitation wavelength were taken as the slope of the plot of emission band area versus absorptivity in triplicate measurements. The NaOH 0.1 N refractive index was taken as 1.3340 [20]. The emission decays were fitted as monoexponentials and the goodness of fit assessed on grounds of χ² ≤ 1.2.

Thermal measurements were performed on a Netzsch STA 449 F1 Jupiter simultaneous thermal analyzer in dynamic air (30 mL/min) with a heating rate of 5 °C min⁻¹, using alumina crucibles covered with pierced alumina lids. Elemental analysis of C, N, and H was performed on a EuroEA3000 elemental analyzer.

2.3. X-ray Structure Determination

X-ray diffraction measurements were performed on a STOE IPDS II diffractometer, operating with a Mo-Kα (λ = 0.71073 Å) X-ray tube with a graphite monochromator. The structure was solved by direct methods and refined by full-matrix least squares techniques based on F². The non-H atoms were refined with anisotropic displacement parameters. Calculations were performed using the SHELX-2014 crystallographic software package. A summary of the crystallographic data and the structure refinement for crystal 1 are given in

Table 1. Crystallographic data, details of data collection, and structure refinement parameters for compound 1.

Compound	1
Chemical formula	C92H100Cl2N8O24Zn3
M (g mol−1)	1968.80
Temperature, (K)	293 (2)
Wavelength, (Å)	0.71073
Crystal system	Triclinic
Space group	P-1
a (Å)	11.4535 (9)
b (Å)	12.3264 (9)
c (Å)	17.7024 (13)
α (°)	105.604 (6)
β (°)	97.473 (6)
γ (°)	101.851 (6)
V (Å3)	2309.8 (3)
Z	1
Dc (g cm−3)	1.415
μ (mm−1)	0.909
F(000)	1024
Goodness-of-fit on F2	0.874
Final R1, wR2 [I > 2σ(I)]	0.0538, 0.1261
R1, wR2 (all data)	0.1193, 0.1530

; CCDC reference number: 2078101.

The X-ray powder diffraction measurements (XRPD) were carried out on a Proto AXRD Benchtop using the Cu-Kα radiation with a wavelength of 1.54059 Å in the 2θ range of 5–35°.

2.4. Hirshfeld Surface Analysis

The Hirshfeld surface and 2D-fingerprint analyses were performed with CrystalExplorer17 [21]. The normalized distance, d_norm, and the shape index were mapped on it in order to evidence structural features of the crystal (see [22] for a thorough description). d_norm takes negative values (red) when the atoms are closer than the sum of their Van der Waals radii and positive (blue) when they are further apart. The shape index takes negative values (red) for a concave surface and positive for a convex one (blue). The d_i-d_e fingerprints represent the distribution of the closest distance from the surface to an atom outside the surface (d_e) vs. the closest one to an atom within it (d_i).

3. Results

The trinuclear complex [Zn₃(valhydr)₂(RhB)₂(EtOH)₂](ClO₄)₂ (1) was synthetized using the rhodamine B pigment and the Schiff base resulted from the condensation of o-vanillin with hydrazine (valhydr is the dianion of the Schiff base, RhB stands for rhodamine B, and EtOH denotes ethanol). The valhydr ligand was already used for synthesis of trinuclear Ni(II) [23], Co(II) [24], Cu(II) [25], or Zn(II) complexes [26,27]. We chose to use valhydr as co-ligand for the synthesis of heteroleptic Zn(II) complexes with rhodamine B due to the fact that in the [Zn₃(valhydr)₂]²⁺ trinuclear complexes, the axial positions of the metal ions are accessible for the coordination of the carboxylate group of the rhodamine. Moreover, the stability of such heteroleptic complexes can be reinforced by π–π interactions between the aromatic systems of the two types of ligands.

3.1. Description of the Structure

The compound 1 crystallises in the triclinic P-1 space group and the crystal structure consists in centrosymmetric trinuclear cations, [Zn₃(valhydr)₂(RhB)₂(EtOH)₂]²⁺, and disordered perchlorate anions. The oxygen atoms of the perchlorate anions are disordered on two crystallographic positions with site occupancy factors of 0.6 and 0.4. In the trinuclear cations, [Zn₃(valhydr)₂(RhB)₂(EtOH)₂]²⁺, the zinc ions are placed in a linear manner. The external metal ions present a distorted square pyramidal stereochemistry and the central zinc ion is hexacoordinated with an elongated octahedral geometry (Figure 1).

The basal planes of the external zinc ions and the equatorial plane of the central one are formed by the two Schiff base ligands that resulted from the condensation of o-vanillin with hydrazine. In the basal plane of the external zinc ions (Zn1) are coordinated one imino nitrogen atom (N1) and one phenoxo oxygen atom (O2) from one ligand; and two oxygen atoms, phenoxo (O3′), and methoxy (O4′) from the second ligand. The corresponding bond lengths are: Zn1-N1 = 2.078(4), Zn1-O2 = 1.949(3), Zn1-O3′ = 2.053(3), and Zn1-O4′ = 2.201(3) Å (symmetry code: ’ = 1-x, 2-y, -z). In the apical position is coordinated monodentate, the carboxylato group of the rhodamine B, Zn1-O5 = 1.958(3) Å. The rhodamine is in the zwitterionic open-ring form. The central metal ion is surrounded in the equatorial plane by two imino nitrogen atoms (Zn2-N2 = 2.040(3) Å) and two phenoxo oxygen atoms (Zn2-O3 = 2.066(3) Å). The phenoxo oxygen atoms act as bridging atoms between the metal ions. In the axial positions are coordinated two ethanol molecules, Zn2-O8 = 2.205(3) Å. In each Schiff base ligand, the dihedral angle between the mean planes of the two extended π systems formed by the benzene ring and the corresponding imino group is 68.4°. The rhodamine B ligands are involved in intramolecular π−π interactions with the valhydr ligands (3.56–3.74 Å).

The rhodamine ligands also establish intermolecular π–π interactions through the xanthene fragments with neighbouring complexes generating supramolecular chains running along the crystallographic c axis (Figure 2a). There is an important overlap between these fragments with a separation ranging from 3.53 to 3.72 Å. CH···π interactions (2.80–3.28 Å) were evidenced between the supramolecular chains involving the benzene rings of the Schiff base ligands (Figure 2b).

3.2. Spectral Properties

The absorption spectrum of compound 1 was acquired over a wavelength range from 200 to 800 nm on a solid sample (using the diffuse reflectance technique). The solid-state electronic spectrum of 1 displays broad absorption bands in the domain of 220–660 nm with peaks at 475, 545, and 590, respectively; and two “shoulders” at 225 and 260 nm (Figure 3a). The band from 545 nm belongs probably to the rhodamine ligand and it is blue shifted compared to the rhodamine B sole, 560nm [15]. From the UV–Vis spectrum, the corresponding band gap was determined to the value of 1.7 eV using the Tauc plot [28] (Figure S2).

The room temperature solid-state photoluminescence of compound 1 was investigated using different wavelengths for excitation in the 430–480 nm range. The resulting emission spectra with maxima around 630 nm are presented in Figure 3b. The shape of the spectrum varies with the excitation wavelength, indicating the presence of multiple excited states in the solid state.

The absorption and emission spectra of 1 were also measured in methanol, together with those of RhB. In diluted solution, there are essentially no differences between their spectra, with maxima at 545 nm in absorption and 570 nm in emission, which is specific to RhB [29]. The fluorescence quantum yield of the trinuclear Zn(II) complex solution is 0.61, whereas for RhB, it is 0.57. The emission decays were fitted as monoexponentials, and the lifetime value is 2.05 ns for the complex dissolved in methanol and 2.02 ns for the ligand solution. RhB is the only species that fluoresces in diluted solution due to the disassembling of the complex (Figure S3). In concentrated solutions (0.5–5 mM), some traces of the complex can be observed with emission maxima at around 630 nm.

3.3. Thermal Behaviour

TGA/DSC curves were recorded for complex 1 using a very small amount of compound as precaution for the perchlorate content (Figure 4). Thus, from its starting point (25 °C) up to around 85 °C, the TGA curve displays a smooth slope that indicates a decrease in the sample weight of about 1.15%, which, in the absence of guest solvent molecules from the crystal structure, could be assigned to the traces of ethanol used for washing and/or adsorbed moisture. A well-defined step in the TGA signal occurs within the 125–175 °C temperature interval; it corresponds to a mass loss of 4.54% and is accompanied by an endothermic DSC event (T_peak = 156.0 °C). Both the temperature range and the weight-loss percent are in agreement with the elimination of the two coordinated ethanol molecules (theoretically, 4.67%). Further, the TGA curve evolves as a plateau up to approximately 250 °C, when it changes into a rather steep slope towards 350 °C. Although the ending of this third step is not clearly defined as the sample mass slowly, but constantly, decreases towards 480 °C, it is associated with an asymmetric and broad exothermic DSC event (T_peak = 311.5 °C). The expected process for this temperature range (250–350 °C) is perchlorate decomposition (theoretically, 10.11%). The corresponding mass loss from TGA is significantly larger (experimentally, 16.19%); the full decarboxylation of the coordinative compound should also occur (theoretically, 4.47%). The further decomposition of the organic ligand is moderate up to 480 °C; the continuous drift of the TGA curve indicates a weight loss of around 10.55%. Within the 480–670 °C temperature domain, this decomposition is more dramatic (total weight loss = 57.80%) and ends up with the formation of ZnO (theoretically, 12.39% vs. a residual mass of 9.77%).

3.4. Hirshfeld Surface Analysis

In the aim to assess the non-covalent interactions in the trinuclear complex 1 in the solid state, Hirshfeld surface and fingerprints analyses were performed. The Hirshfeld surfaces mapped with d_norm and the shape index are depicted along the three axes in Figure 5. This density maps analysis backs up the one based on geometrical X-ray parameters. The analysis of d_norm projected on the Hirshfeld surface (Figure 5a–c) reveals some very weak H bonds of the type O…H-C, indicated by the red dots (negative d_norm values—interatomic distances lower than the sum of vdW radii). They establish between Schiff bases of two adjacent crystallographic units along the a axis, together with C-H∙∙∙π interactions, seen as the complementary concave (red)-convex (blue) patterns in Figure 5d. There is a π–π stacking interaction comprising RhB along the c axis, where two xanthene fragments from two adjacent structural units overlap (characteristic red–blue triangular patterns in Figure 5f).

Another useful analysis comes from the d_i-d_e fingerprints plots seen in Figure S4. The main interaction as a percent from the Hirshfeld surface it spans is the dispersion H···H one with 53.9% and the largest extension along the b axis, followed by O···H with 25.8%, even if from weak H bonds. These are the closest contact interactions; however, they are unusually high for a crystal at d_i/d_e pairs of 1 Å or more. Correlating the fingerprints to the shape index, one can see that C···C, C···O and part of the C···H interactions come from π-π stacking, while the principal part of the C···H ones are involved in the C-H···π type.

4. Conclusions

In this article, we described the synthesis and characterization of a Zn(II) trinuclear complex containing rhodamine B, and the Schiff base resulted from the condensation of o-vanillin with hydrazine as ligands. Despite the π–π interactions established by the rhodamine B and the Schiff base ligand in the solid state, in solution, the rhodamine B is easily removed from the coordination sphere of the metal ion.

A better understanding of the intra- and/or intermolecular interactions in the crystalline materials can offer the possibility of designing tailored fluorescent materials for very specific applications.