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Combined Experimental and Theoretical Investigation of Ligand and Anion Controlled Complex Formation with Unprecedented Structural Features and Photoluminescence Properties of Zinc(II) Complexes Prateeti Chakraborty,† Jaydeep Adhikary,† Sugata Samanta,§ Daniel Escudero,‡ Abril C. Castro,# Marcel Swart,#,∇ Sanjib Ghosh,§ Antonio Bauzá,∥ Antonio Frontera,*,∥ Ennio Zangrando,*,⊥ and Debasis Das*,† †

Department of Chemistry, University of Calcutta, 92 A. P. C. Road, Kolkata 700009, India Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany § Department of Chemistry, Presidency University, Kolkata 700073, India ∥ Departament de Química, Universitat de les Illes Balears, Crta. de Valldemossa km 7.5, 07122 Palma, Baleares, Spain ⊥ Department of Chemical and Pharmaceutical Sciences, University of Trieste, Via L. Giorgieri 1, 34127 Trieste, Italy # Institut de Química Computacional i Catàlisi and Departament de Química, Universitat de Girona, Campus de Montilivi, 17071 Girona, Catalonia, Spain ∇ Institució Catalana de Recerca i Estudis Avançats (ICREA), Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain ‡

S Supporting Information *

ABSTRACT: By using two potential tridentate ligands, HL1 [4chloro-2-[(2-morpholin-4-yl-ethylimino)-methyl]-phenol] and HL2 [4-chloro-2-[(3-morpholin-4-yl-propylimino)-methyl]-phenol], which differ by one methylene group in the alkyl chain, four new ZnII complexes, namely, [Zn(L2H)2](ClO4)2 (1), [Zn(L1)(H2O)2][Zn(L1)(SCN)2] (2), [Zn(L1)(dca)]n (3), and [Zn2(L1)2(N3)2(H2O)2] (4) [where dca = dicyanamide anion] were synthesized and structurally characterized. The results indicate that the slight structural difference between the ligands, HL1 and HL2, because of the one methylene group connecting the nitrogen atoms provokes a chemical behavior completely different from what was expected. Any attempt to isolate the Zn(L2) complexes with thiocyanato, dicyanamido, and azide was unsuccessful, and perchlorate complex 1 was always obtained. In contrast, with HL1 we obtained structural diversity on varying the anions, but we failed to isolate the analogous perchlorate complex of HL1. Single-crystal X-ray analyses revealed that the morpholine nitrogen of ligand L2 is protonated and thus does not take part in coordination with ZnII in complex 1. On the other hand, the morpholine nitrogen of L1 is coordinated to ZnII in 2−4. Of these, 2 and 4 are rare examples of a cocrystallized cationic/anionic complex and of a dinuclear complex bridged by a single azide, respectively. Some of these unexpected findings and some interesting noncovalent interactions leading to the formation of dimeric entities in solid-state compound 4 were rationalized by a DFT approach. Photoluminescence properties of the complexes as well as the ligands were investigated in solution at ambient temperature and at 77 K. The very fast photoinduced electron transfer (PET) from the nitrogen lone pair to the conjugated phenolic moiety is responsible for very low quantum yield (Φ) exhibited by the ligands, whereas complexation prevents PET, thus enhancing the Φ in the complexes. The origin of the electronic and photoluminescence properties of the ligands and complexes was assessed in light of theoretical calculations.

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sity.14−18 In the course of producing complexes with structural diversity, counteranions play a very vital role. It has been observed that acetate, thiocyanate, dicyanamide, and others, which have a versatile mode of coordination, may be useful in

INTRODUCTION Schiff bases have become one of the main pillars in the foundation of modern coordination chemistry.1−8 Tetradentate salen-type ligands are by far the most popular class9−13 of Schiff-base ligands for coordination chemists. However, recently, the higher flexibility and coordinating unsaturation property of tridentate Schiff-base ligands have made them interesting for generating complexes with structural diver© XXXX American Chemical Society

Received: May 16, 2014 Revised: June 17, 2014

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dx.doi.org/10.1021/cg500717n | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

(400−4000 cm−1) with a Perkin-Elmer RXI FTIR spectrophotometer. Ground-state absorption was measured with a JASCO V-530 UV−vis spectrophotometer. 1H and 13C NMR spectra (300 MHz) were recorded in CDCl3 solvent at 298 K on a Bruker AC300 NMR spectrometer using the solvent signal as the internal standard in a 5 mm BBO probe. UV−vis absorption spectra were recorded on a Hitachi U-4010 spectrophotometer at 298 K. The steady-state emission measurements were carried out using a Hitachi Model F7000 spectrofluorimeter equipped with a 150 W xenon lamp at 298 K using a stoppered cell with a 1 cm path length. Emission studies at 77 K were made using a Dewar system having a 5 mm o.d. quartz tube, and the freezing of all of the samples to 77 K was achieved at the same rate. Phosphorescence spectra were recorded in a Hitachi Model F7000 spectrofluorimeter equipped with phosphorescence accessories at 77 K. Fluorescence quantum yield (Φ) was determined in each case by comparing the corrected emission spectrum of the samples with that of anthracene in MeOH (Φ 0.20)24 using eq 125 by taking the total area under the emission curve into account.

making interesting molecular networks.15,19−21 On the other hand, noncoordinating anions, like perchlorate, tetraphenyl borate, triphenyl phosphate, and others, do not have much influence on controlling the molecular structure of the complexes.22 However, the above-mentioned three noncoordinating anions have the potential to favor the crystallization of their complexes and to act as templates for a variety of complexes. 23 For the present study, we selected two homologous tridentate Schiff-base ligands, HL1 and HL2 [HL1 = {4-chloro-2-[(2-morpholin-4-yl-ethylimino)-methyl]phenol} and HL2 = {4-chloro-2-[(3-morpholin-4-yl-propylimino)-methyl]-phenol}], which differ by only one −CH2− group in their ligating backbone, and zinc(II) as the metal ion. Initially, our focal interest was to investigate how this small difference in the ligating backbone of the two tridentate Schiffbase ligands may affect the structural features and properties of the resulting ZnII complexes by exploiting the versatile coordinating property of coligands thiocyanate, dicyanamide, and azide starting from the metal perchlorate complex as the precursor. To our surprise, we failed to isolate any thiocyanato, dicyanamido, and azido complexes of ZnII with HL2, and in each case the perchlorate complex was crystallized out (Scheme 1). On the contrary, with HL1 we failed to isolate the analogous

Q = QR

(1)

where Q is the quantum yield of the compounds, F is the integrated fluorescence intensity (area under the emission curve), OD is the optical density, and n is the refractive index of the medium. It is assumed that the reference and the unknown samples are excited at the same wavelength (360 nm). The subscript R refers to the reference fluorophore (anthracene in this case) of known quantum yield. The standard quantum yield value was then used for the calculation of radiative and nonradiative rate constants of the systems. Singlet state lifetimes were measured by a time master fluorimeter from Photon Technology International (PTI, USA). The system consists of a pulsed laser driver PDL-800-B (from Pico-Quant, Germany) with interchangeable subnanosecond pulsed LEDs and pico-diode lasers (PicoQuant, Germany) with a TCSPC setup (PTI, USA). The lifetimes of the free ligands and complexes were measured by using a diode laser, LDH-405 (pulse width, 200 ps) and PLS-370 (pulse width, 600 ps), from PicoQuant, Germany, at a repetition frequency of 10 MHz. Instrument response functions (IRF) were measured at the respective excitation wavelengths of 370 nm (in the case of a LED) and 405 nm (in the case of a diode laser) using slits with a band pass of ∼1−3 nm using Ludox silica as a scatterer. Intensity decay curves were fitted as the summation of exponential terms:

Scheme 1. Synthetic Route of the Complexes

perchlorate derivative; however, very interesting structural diversities were achieved on varying the pseudohalide anions (Scheme 1). Dicyanamide (dca) yields a polynuclear [Zn(L1)(dca)]n species, a usual consequence of dca chemistry. Strikingly, thiocyanate generates a cocrystal of mononuclear cationic and anionic complexes, namely, [Zn(L1)(H2O)2]+ and [Zn(L1)(SCN)2]−, where the water molecule helps to construct a 1D chain by participating in H-bonding, an unprecedented consequence, and azide produces a self-assembled azidobridged dinuclear complex, [Zn2(L1)2(N3)2(H2O)2], stabilized by anion−π interactions in solid state, a rare find in zinc coordination chemistry. Some aspects of these unforeseen findings were rationalized by DFT calculations. Photoluminescence properties of the complexes as well as the ligands were also investigated, and DFT and time-dependent DFT (TD-DFT) calculations were performed to rationalize the origin of the observed properties. The solid-state structure of the complexes linger in solution, as revealed from the combined experimental and theoretical NMR spectral analyses. All of these interesting findings are lucidly portrayed in this article.

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F ODR n2 FR OD nR2

F(t ) =

∑ αi exp(−t /τi)

The decay parameters were recovered using a nonlinear iterative fitting procedure based on the Marquardt algorithm.26 A deconvolution technique was used to determine the lifetime up to 150−200 ps with a LED, and the time resolution was 100 ps with diode laser. The quality of the fit was assessed over the entire decay, including the rising edge, and was tested with a plot of weighted residuals and other statistical parameters, e.g., the reduced χ2 ratio and the Durbin− Watson (DW) parameters.27 The decay times of the four complexes in the milliseconds or longer range were also acquired by phosphorescence time-based acquisition mode in which the emission intensity was measured as a function of time in the QM-30 fluorimeter (PTI, USA) using a gated detection system with start and end window times of 200 and 2000 μs, respectively. The decay parameters were recovered using a nonlinear iterative fitting procedure based on the Marquardt algorithm.27 High-purity N-(2-aminoethyl)morpholine, N-(3-aminopropyl)morpholine, 5-chloro salicylaldehyde, and sodium dicyanamide were purchased from Sigma-Aldrich and used as received. Zinc perchlorate was purchased from Merck Chemical Company, and sodium thiocyanate, from LOBA Chemicals Company. Solvents were dried according to a standard procedure and distilled prior to use. All other chemicals used were of AR grade. Water used in all physical measurement experiments was Milli-Q grade.

EXPERIMENTAL SECTION

Physical Methods and Materials. Elemental analyses (carbon, hydrogen, and nitrogen) were performed using a Perkin-Elmer 240C elemental analyzer. Infrared spectra were recorded on KBr disks B



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Table 1. Crystallographic Data and Details of Refinement for Complexes 1−4 complex

1

2

3

4

empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm−3) μ(Mo Kα) (mm−1) F(000) θ range (deg) no. of reflns collected no. of indep. reflns Rint no. of reflns (I > 2σ(I)) no. of refined params GOF (F2) R1, wR2 (I > 2σ(I)) residuals (e Å−3)

C28H38Cl4N4O12Zn 829.79 triclinic P1̅ 9.835(2) 12.723(3) 14.592(4) 84.277(3) 87.288(3) 76.841(3) 1768.5(7) 2 1.558 1.061 856 1.40−25.89 12 498 6671 0.0251 4875 448 1.065 0.0489, 0.1416 0.800, −0.417

C28H36Cl2N6O6S2Zn2 818.39 triclinic P1̅ 9.0372(12) 12.5384(16) 15.1211(19) 89.705(2) 86.941(2) 84.171(2) 1702.1(4) 2 1.597 1.739 840 1.35−24.56 11 278 5592 0.0266 4251 443 1.088 0.0432, 0.1198 0.694, −0.636

C15H16ClN5O2Zn 399.15 monoclinic P21/c 16.9993(6) 7.5278(3) 13.5447(5) 90.0 100.0370(10) 90.0 1706.75(11) 4 1.553 1.613 816 1.22−29.18 25 919 4582 0.0361 3541 217 1.024 0.0347, 0.0864 0.440, −0.412

C26H34Cl2N10O5Zn2 768.27 monoclinic P21/n 8.8137(7) 14.6565(11) 25.2937(19) 90.0 96.030(3) 90.0 3249.3(4) 4 1.570 1.693 1576 1.61−25.75 36 770 6142 0.0759 4194 412 1.092 0.0623, 0.1425 0.742, −0.552

Synthesis of Ligand 1 (HL1). A methanolic solution (5 mL) of N(2-aminoethyl)morpholine (0.260 g, 2 mmol) was added dropwise to 10 mL of methanol containing 5-chlorosalicylaldehyde (0.312 g, 2 mmol), and the resulting deep yellow solution was refluxed for 2 h. Then, the solvent was removed under reduced pressure, giving a yellow liquid from which pure HL1 was isolated through column chromatography. Yield: 90%. 1H NMR (300 MHz, CDCl3): 2.42−2.45 (4H, m, −CH2−N−CH2−), 2.58−2.63 (2H, m, −CH2−N), 3.61− 3.67 (4H, m, −CH2−O−CH2−; 2H, m, N−CH2−), 6.80−7.18 (3H, m, Ar−H), 8.20 (H, s, −CHN−), 13.33 (1H, s, Ar−OH). 13C NMR (300 MHz, CDCl3): 164.5 (−CHN−), 159.87 (Ar−C−OH), 118.64−130.0(Ar−C), 53.8−67.22 (morpholine−C and N−CH2− CH2−N). Synthesis of Ligand 2 (HL2). This ligand was prepared by adopting a similar procedure as that for HL1 by replacing N-(2aminopropyl)morpholine with N-(2-aminopropyl) morpholine (0.288 g, 2 mmol). Yield: 85%. 1H NMR (300 MHz, CDCl3): 1.75−1.84 (2H, m, −CH2−), 2.31−2.35 (4H, m, −CH2−N−CH2−; 2H, m, −CH2− N), 3.55−3.63 (4H, m, −CH2−O−CH2−; 2H, m, N−CH2−), 6.79−7.18 (3H, m, Ar−H), 8.19 (H, s, −CHN−), 13.42 (1H, s, Ar−OH). 13C NMR (300 MHz, CDCl3): 163.87 (−CHN−), 159.95 (Ar−C−OH), 118.61−131.99 (Ar−C), 53.68−67.02 (morpholine−C and N−CH2−CH2−N), 27.46 (−CH2−). Syntheses of the Complexes. All of the complexes were prepared by adopting two different routes. One is a template synthesis technique, and the other is a conventional sequential route; however, in both cases, the complexes obtained were the same. The preparation, composition, and other physicochemical properties of all four complexes using the template technique are given below. Caution! Metal perchlorate salts are potentially explosive and should be handled in small amounts and with proper precautions. Synthesis of Complex [Zn(L2H)2](ClO4)2 (1). A methanolic solution (10 mL) of N-(2-aminopropyl) morpholine (0.144 g, 1 mmol) was added dropwise to a methanolic solution (5 mL) of 5-chlorosalicylaldehyde (0.156 g, 1 mmol), and the resulting mixture was refluxed for 2 h. Then, a methanolic solution of zinc perchlorate (0.372 g, 1.5 mmol) was added to it, and the mixture was allowed to stir for 2.5 h at ambient temperature. At that point, the solution was kept in a CaCl2 desiccator. Single crystals suitable for X-ray data collection were

obtained from the solution after 2 days (yield: 90%). Anal. Calcd for C28H38Cl4N4O12Zn: C, 40.53; H, 4.62; N, 6.75%. Found: C, 40.50; H, 4.60; N, 6.72%. IR: ν (CN) 1635.96 cm−1; ν (skeletal vibration) 1528.33 cm−1. 1H NMR (300 MHz, CDCl3): 1.62−1.67 (H, m, −CH2−), 1.82−1.84 (H, m, −CH2−), 2.36−2.82 (4H, m, −CH2−N− CH2−; 2H, m, −CH2−N), 3.45−3.58 (6H, m, −CH2−O−CH2−,  N−CH2−), 6.65−7.31 (3H, m, Ar−H), 8.34 (H, s, −CHN−). Synthesis of Complex [Zn(L1)(H2O)2][Zn(L1)(SCN)2] (2). A methanolic solution (5 mL) of 5-chloro salicylaldehyde (0.156 g, 1 mmol) was added dropwise to a methanolic solution (10 mL) of N-(2aminoethyl) morpholine (0.130 g, 1 mmol) under stirring, and the resulting mixture was refluxed for 1 h. Then, a methanolic solution of zinc perchlorate (0.372 g, 1.5 mmol) was added to it, and the resulting mixture was refluxed for another 2 h. After cooling the mixture to ambient temperature, a water−methanolic solution (5 mL) of sodium thiocyanate (0.246 g, 3 mmol) was added to it with stirring, and stirring was continued for a further 2 h. The solution thus obtained was kept in a CaCl2 desiccator. Single crystals suitable for X-ray diffraction were separated out from the solution after a few days (yield: 88%). Anal. Calcd for C28H36Cl2N6O6S2Zn2: C, 41.09; H, 4.43; N, 10.27%. Found: C, 41.05; H, 4.40; N, 10.22%. IR: ν (CN) 1636.3 cm−1; ν (skeletal vibration) 1576.5 cm−1. 1H NMR (300 MHz, CDCl3): 2.35−2.51 (4H, m, −CH2−N−CH2−; 2H, m,CH2−N), 3.53−3.63 (4H, m, −CH2−O−CH2−; 2H, m, N−CH2−), 6.47− 7.11 (3H, m, Ar−H), 8.25 (1H, s, −CHN−). Synthesis of Complex [Zn(L1)(dca)]n (3). 3 was synthesized by following a similar procedure as that for 2 by replacing sodium thiocyanate with sodium dicyanamide (0.267 g, 3 mmol). Single crystals suitable for X-ray diffraction were obtained from the final solution after 1 week (yield: 90%). Anal. Calcd for C15H16ClN5O2Zn: C, 45.14; H, 4.04; N, 17.54%. Found: C, 45.09; H, 4.02; N, 17.52%. IR: ν (CN) 1636.3 cm−1; ν (skeletal vibration) 1580 cm−1. 1H NMR (300 MHz, CDCl3): 2.28−2.54 (4H, m, −CH2−N−CH2−; 2H, m, CH2−N), 3.51−3.64 (4H, m, −CH2−O−CH2−; 2H, m, N− CH2−), 6.51−7.13 (3H, m, Ar−H), 8.25 (1H, s, −CHN−). Synthesis of Complex [Zn2(L1)2(N3)2(H2O)2] (4). 4 was prepared by adopting a similar procedure as that for 2 by replacing sodium thiocyanate with sodium azide (0.195 g, 3 mmol). Single crystals suitable for X-ray diffraction were obtained from the filtrate after few C



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adapted delocalized coordinates.47 This computational setup was shown to work well for transition-metal complexes.48 All chemical shift values are in ppm and are reported with respect to tetramethylsilane (TMS).

days (yield: 92%). Anal. Calcd for C26H34Cl2N10O5Zn2: C, 40.65; H, 4.46; N, 18.22%. Found: C, 40.19; H, 4.26; N, 18.15%. IR: ν (CN) 1639.4 cm−1; ν (skeletal vibration) 1580.8 cm−1. 1H NMR (300 MHz, CDCl3): 2.27−2.54 (4H, m, −CH2−N−CH2−; 2H, m, CH2−N), 3.41−3.74 (4H, m, −CH2−O−CH2−; 2H, m, N−CH2−), 6.51− 7.13 (3H, m, Ar−H), 8.13 (H, s, −CHN−). X-ray Data Collection and Structure Determination. Diffraction data for compounds 1−4 were collected on a Bruker Smart Apex diffractometer equipped with a CCD. All experiments were performed at room temperature with Mo Kα radiation (λ = 0.71073 Å). Cell refinement, indexing, and scaling of the data set were carried out using Bruker Smart Apex and Bruker Saint packages.28 The structures were solved by direct methods and subsequent Fourier analyses and refined by the full-matrix least-squares method based on F2 with all observed reflections.29 The contribution of H atoms (at calculated positions generated by SHELXL29 or located from the Fourier map for water molecules) was introduced in the final cycles of refinement. Crystallographic data and details of the refinements are reported in Table 1. All calculations were performed using the WinGX System, version 1.80.05.30 Theoretical Methods. The geometries and energies for all the complexes included in the TD-DFT part of this study were computed at the RI-BP86/def2-TZVPD level of theory with the program TURBOMOLE version 6.4.31 Relativistic effects were included for the Zn atom by using the ECP-10-mdf Stuttgart/Dresden pseudopotential.32 The UV−vis absorption spectra of HL1, HL2, and complexes 1− 4 were obtained by calculating the lowest-lying vertical singlet electronic excitation energies using TD-DFT at the ground-state optimized geometries. The computational models used for complexes 1−4 were those obtained from the X-ray diffraction analysis shown in Figures 1, 2a,b, 4, and 6, respectively. In the case of polymeric complex 3 (Figure 4), the N5 and C15 positions were saturated with H atoms. In the case of complex 4 (Figure 6), the N8 position was saturated with a methyl ligand. For the TD-DFT calculations, the hybrid B3LYP functional and the long-range corrected CAM-B3LYP functional in combination with the 6-31G* basis set were selected. To simulate the fluorescence spectra of HL1 and HL2, excited-state structures and energies were obtained using TD-DFT with the B3LYP functional. Hence, the geometries of HL1 and HL2 on their singlet emissive states were fully optimized. At these excited-state optimized geometries, the electronic transitions corresponding to the fluorescence energies were calculated. The phosphorescence emission spectra of complexes 1−4 were simulated on the basis of ΔSCF-DFT calculations, which yield the energy difference between the lowest triplet excited state at its optimized geometry (T1) and the closed-shell ground state at the same geometry. Thereby, the geometries of the lowest-emitting (T1) triplet excited states of complexes 1−4 were fully optimized. The single-point TD-DFT and ΔSCF-DFT calculations were both performed in solution using methanol as solvent with the polarization continuum model (PCM).33,34 Single-point TD-DFT and ΔSCF-DFT calculations and the excited state TD-DFT optimizations were performed with the Gaussian 09 program package.35 For the DFT mechanistic study, all of the density functional calculations were carried out with the Amsterdam Density Functional (ADF) program36,37 in combination with a dielectric continuum solvation model (COSMO),38−42 with a non-empirical setup41 for the solvent parameters appropriate for methanol (used both in the geometry optimization and NMR calculations) and for the atomic radii. The geometries were calculated by using the PBE-D2 functional that include Grimme’s dispersion (D2) corrections,43,44 and the molecular orbitals were expanded in an uncontracted set of Slater-type orbitals (STOs) of triple-ζ quality containing diffuse functions and two sets of polarization functions (TZ2P) or an even-tempered basis of quadruple-ζ quality (ET-pVQZ45). All NMR chemical shifts reported here were obtained with the ET-pVQZ basis and the KT2 functional in single-point calculations at geometries that were obtained with the TZ2P basis (e.g., KT2/ET-pVQZ//PBE-D2/TZ2P). The geometry optimizations were carried out with the QUILD program,46 which uses superior optimization routines based on

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RESULTS AND DISCUSSION

Synthesis and Characterization. From the homologous Schiff-base ligands, HL1 and HL2, and four anionic coligands, namely, ClO4−, SCN−, N(CN)2−, and N3−, we expected to obtain at least eight complexes (excluding mixed coligand complex formation). Surprisingly, we were able to isolate complexes [Zn(HL2)2](ClO4)2 (1), [Zn2(L1)2(H2O)2(SCN)2] (2), [Zn(L1)(dca)] (3), and [Zn2(L1)2(N3)2(H2O)2] (4), as confirmed from elemental analyses, irrespective of the synthetic routes we adopted, i.e., the template synthetic technique or sequential route. In the FT-IR spectra, the complexes show bands due to the CN stretching in the range 1600−1645 cm−1 and skeletal vibration in the range 1580−1585 cm−1. The symmetric band centered in complexes 2 and 3 around 2097 and 2187 cm−1, respectively, indicates the presence of thiocyanate and dicyanamide anion in the respective species. The split band observed in complex 4 at 2036 cm−1 was due to presence of azide in the complex with two different modes of coordination. The band observed at 1110 cm−1 in complex 1 is due to the presence of the perchlorate anion. In order to follow the sequential method to prepare the complexes, ligands HL1 and HL2 were synthesized by the conventional condensation technique and were characterized by 1H and 13C NMR (see the Experimental Section and Supporting Information Figures S13−S16). To confirm the solution-state structure of the complexes, 1H spectra were recorded (see the Experimental Section and Supporting Information Figures S17−S20) and verified by DFT calculations (given in the Supporting Information). Crystal Structure Description of the Complexes. Complex [Zn(HL2)2](ClO4)2 (1). The structure of complex 1 is a cationic species with two HL2 ligands chelating the zinc(II) ion through the phenol oxygen and the imino N donor, with the −(CH2)3−morpholine chains that are far apart containing the nitrogen of these fragments that is protonated. The metal has a distorted tetrahedral coordination sphere with bond angles in the range of 96.21(11)−124.46(11)°, with the Schiffbase’s mean planes almost orthogonal, forming a dihedral angle of 86.70(6)°. The N(1)−Zn−O(1) and N(3)−Zn−O(3) chelating bond angles of 96.21(11) and 96.69(11)Å, respectively, are the largest among the structures reported. Both of the morpholine nitrogen atoms are protonated and form H-bonds with perchlorate oxygens, resulting in the anions being appended to the morpholine moieties. The NH···O distances of these interactions are 2.841(5) and 2.942(4) Å (Figure 1 and Table 2). Complex [Zn(L1)(H2O)2][Zn(L1)(SCN)2] (2). Compound 2 is a cocrystal of mononuclear cationic and anionic complexes, namely, [Zn(L1)(H2O)2]+ and [Zn(L1)(SCN)2]−. The ORTEP drawings of the two species are depicted in Figure 2a,b, a selection of coordination bond distances and angles are reported in Table 3, and the H-bond geometry is shown in Table 4. In both species, the deprotonated HL1 chelates the metal through the phenoxo oxygen atom, the imine-nitrogen, and amine donor of the morpholine ring. The metal completes the coordination sphere through two aqua ligands and two isothiocyanate anions in the cationic and anionic fragments, respectively. However, it is worth noting that the coordination D



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Figure 1. ORTEP drawing (ellipsoids at 30% probability) of complex 1.

geometry is different between these the two cases, trigonal bipyramidal and square pyramidal, respectively, as clearly indicated by the τ parameters49a of 0.717 and 0.011. In the two complexes, the N2O donors of the trichelating ligand L1 are coplanar with the metal ion, and the Zn−O(phenol), Zn− N(imino), and Zn−N(amino) bond distances show differences likely induced by the different coordination sphere. In particular, the Zn−N(amino) values (Zn(1)−N(2) of 2.490(4) Å and Zn(2)−N(12) of 2.263(3) Å) manifest the larger variation, with the long value measured in the former being forced by the presence of the N(3)−C(14)−S(1) ligand. The water molecules are involved in a hydrogen-bonding scheme connecting the cationic species to form a 1D chain (Figure 3), which is flanked by the anionic complexes that are connected through O(2w)−H···O(1) interactions (O···O distance of 2.621(4) Å). Cocrystals of cation/anion complexes are not very common, and we found only one example in the literature of this type of peculiar Zn complex containing carbaldehyde thiosemicarbazone as ligand.49b Complex [Zn(L1)(dca)]n (3). The X-ray structural analysis revealed that complex 3 is formed by 1D coordination polymers realized through dca anions connecting Zn(L1) units. The ORTEP drawing of complex 3 is shown in Figure 4. The metal is chelated by the phenol oxygen and the imino and amino N donors in a coordination sphere in between trigonal bipyramidal and square pyramidal, with a calculated τ trigonal index of 0.465. The coordination environment is completed by the cyano groups of the bridging dca anion. The coordination distances (Table 5) are similar to those measured in the anionic complex of 2, but it is worth noting the significantly longer Zn−N(amino) bond length (Zn−N(2) = 2.5732(17) Å), which avoids steric clashes with the bridging dca units and favors the formation of the polymeric chains

Figure 2. (a) ORTEP drawing of the anionic complex of 2. (b) ORTEP drawing of the cationic complex of 2.

shown in Figure 5. Moreover, the crystal packing evidences that the polymeric chains are interdigitated, giving rise to π−π interactions between the phenol rings about an inversion center (centroid-to-centroid distance of 3.950(2) Å). Along the chains, the metals are separated by 7.528 Å, corresponding to the crystallographic b axis. Complex [Zn2 (L 1 ) 2 (N 3) 2(H 2 O) 2] (4). Compound 4 is composed of two complexes with metals connected by a μ1,3 bridging azide separated by 5.833 Å. For the binuclear unit, the ORTEP drawing is shown in Figure 6, H-bond interactions are presented in Figure 7, and pertinent coordination bond distances and angles are reported in Table 6. The deprotonated HL1 ligand chelates the metal as in 2 and 3, and the Zn1 and Zn2 ions, bridged by the azide ion, complete the coordination sphere through a second monocoordinated azide and an aqua ligand, respectively. The trigonal τ values calculated from the metal bond angles of 0.631 and 0.529 indicate a coordination geometry in between a square planar and a trigonal bipyramid environment. Here, both the Zn(1)−N(2) and Zn(2)−N(10) bond distances involving the morpholine fragment are rather long, 2.436(5) and 2.556(5) Å, respectively, but they are slightly shorter than the value observed in 3. The end-on and the pendant N3 anions are practically linear (N−N−N of 176.8(8), 173.6(7)°) and are coordinated with Zn−N−N angles of 151.7(7) (monocoordinated azide), 155.5(6), and

Table 2. Coordination Bond Lengths (Å) and Angles (Deg) for Complex 1 Zn(1)−N(1) Zn(1)−N(3)

2.005(3) 1.991(3)

Zn(1)−O(1) Zn(1)−O(3)

1.927(3) 1.911(2)

N(1)−Zn(1)−O(1) N(1)−Zn(1)−N(3) N(1)−Zn(1)−O(3)

96.21(11) 124.46(11) 114.77(11)

N(3)−Zn(1)−O(3) O(1)−Zn(1)−O(3) N(3)−Zn(1)−O(1)

96.69(11) 114.04(12) 111.70(12)

E



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Table 3. Coordination Bond Lengths (Å) and Angles (Deg) for the Two Different Species of Complex 2 Zn(1)−N(1) Zn(1)−N(2) Zn(1)−O(1) Zn(1)−N(3) Zn(1)−N(4)

2.018(3) 2.490(4) 2.052(3) 1.957(4) 1.966(4)

Zn(2)−N(11) Zn(2)−N(12) Zn(2)−O(11) Zn(2)−O(1w) Zn(2)−O(2w)

2.063(3) 2.263(3) 2.008(3) 2.029(3) 2.091(3)

N(1)−Zn(1)−O(1) N(1)−Zn(1)−N(2) N(1)−Zn(1)−N(3) N(1)−Zn(1)−N(4) N(2)−Zn(1)−N(3) N(2)−Zn(1)−N(4) N(2)−Zn(1)−O(1) N(3)−Zn(1)−O(1) N(4)−Zn(1)−O(1) N(3)−Zn(1)−N(4) τ parameter

89.06(13) 77.67(13) 123.48(16) 121.81(15) 95.07(15) 89.69(14) 166.52(11) 94.26(15) 95.38(14) 114.00(17) 0.717

N(11)−Zn(2)−O(11) N(11)−Zn(2)−N(12) N(11)−Zn(2)−O(1w) N(11)−Zn(2)−O(2w) N(12)−Zn(2)−O(11) N(12)−Zn(2)−O(1w) N(12)−Zn(2)−O(2w) O(11)−Zn(2)−O(1w) O(11)−Zn(2)−O(2w) O(1w)-Zn(2)−O(2w)

89.00(13) 80.66(13) 168.96(14) 106.63(13) 169.65(12) 100.02(13) 89.83(13) 90.19(13) 92.88(13) 84.40(14) 0.011

Table 4. H-Bond Geometry (Å/Deg) for Complex 2 donor−H

acceptor

D−H

H···A

D···A

D−H···A

symmetry code for A

O(1w)−H(1A) O(1w)−H(1B) O(2w)−H(2A) O(2w)−H(2B)

O(12) O(11) O(1) O(11)

1.05(6) 0.78(5) 0.77(6) 0.75(6)

1.70(6) 1.91(5) 1.85(6) 2.19(6)

2.686(5) 2.684(5) 2.621(4) 2.872(4)

155(5) 174(4) 177(9) 151(6)

1 − x, −y, 1 − z 1 − x, −y, 1 − z −x, −y, 1 − z −x, −y, 1 − z

Table 5. Coordination Bond Lengths (Å) and Angles (Deg) for Complex 3

Figure 3. Crystal packing of 2 showing the H-bonds occurring between the cationic complexes, which also interact with the anionic species, of which only oxygen atoms O(1) are shown for clarity.

Zn−O(1) Zn−N(1) Zn−N(2)

1.9709(17) 2.0009(16) 2.5732(17)

Zn−N(3) Zn−N(4)

1.9962(18) 2.0109(18)

O(1)−Zn−N(3) O(1)−Zn−N(1) N(3)−Zn−N(1) O(1)−Zn−N(4) N(3)−Zn−N(4) C(14)−N(3)−Zn

93.60(8) 93.17(7) 139.45(7) 102.33(8) 106.28(8) 162.47(18)

N(1)−Zn−N(4) O(1)−Zn−N(2) N(3)−Zn−N(2) N(1)−Zn−N(2) N(4)−Zn−N(2) C(15)−N(4)−Zn

111.16(7) 167.33(6) 89.77(6) 76.48(6) 88.40(7) 164.20(17)

Figure 5. Crystal packing of complex 3: interdigitated 1D coordination polymers show π−π interactions.

Figure 4. ORTEP drawing of the independent unit of polymeric complex 3.

bridging azide have never been reported. The packing evidences dinuclear units arranged in pairs about a center of symmetry to form dimers and connected by short H-bonds involving the coordinated water molecules (Ow1···O1 and Ow1···O3 distances of 2.611(6) and 2.651(6) Å and Ow1−

143.2(5)° for the bridging anion. A single azide anion bridging two distinct metal entities to form a dinuclear species is not unusual,50 and in rare cases, the connected fragments are different.51 However, dinuclear Zn complexes with a single F



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Figure 6. ORTEP drawing of complex 4. C21 represents the interacting atom with terminal azide N5 of the symmetry-related molecule in the dimer.

Figure 8. Absorption spectra of ligands in methanol (1 × 10−4 M) at 298 K.

Figure 7. Centrosymmetrical dimer of complex 4 with indication of Hbond interactions (primed atoms at 1 −x, −y, 1 − z; only H atoms of the aqua ligand indicated).

Figure 9. Absorption spectra of complexes 1−4 in methanol (1 × 10−4 M) at 298 K.

The band maxima along with the ε values are provided in Table S1 of the Supporting Information. The lowest-energy band of the ligands observed at 415 nm is blue-shifted to ∼375 nm in the complexes. The distinct band around 325 nm observed in the ligands appears as a weak shoulder in the complexes.

H1···O1 and Ow1−H2···O3 distances of 177(9) and 168(9)°, respectively). Electronic Spectra. The electronic spectra of ligands HL1 and HL2 and of complexes 1−4 were studied in methanol at 298 K and are depicted in Figures 8 and 9, respectively.

Table 6. Coordination Bond Lengths (Å) and Angles (Deg) for Complex 4 Zn(1)−N(1) Zn(1)−N(2) Zn(1)−N(3) Zn(1)−N(6) Zn(1)−O(1)

2.020(5) 2.436(5) 1.937(7) 2.019(7) 2.049(4)

Zn(2)−N(9) Zn(2)−N(10) Zn(2)−N(8) Zn(2)−O(3) Zn(2)−O(1w)

1.993(5) 2.556(5) 2.017(6) 2.023(4) 1.966(4)

N(1)−Zn(1)−N(2) N(1)−Zn(1)−N(3) N(1)−Zn(1)−N(6) N(1)−Zn(1)−O(1) N(2)−Zn(1)−N(3) N(2)−Zn(1)−N(6) N(2)−Zn(1)−O(1) N(3)−Zn(1)−N(6) N(3)−Zn(1)−O(1) N(6)−Zn(1)−O(1)

78.3(2) 128.1(4) 118.2(3) 88.20(18) 99.1(3) 91.6(3) 165.98(17) 113.7(4) 92.0(3) 91.6(2)

N(9)−Zn(2)−N(10) N(9)−Zn(2)−N(8) N(9)−Zn(2)−O(3) N(9)−Zn(2)−O(1w) N(8)−Zn(2)−N(10) N(10)−Zn(2)−O(3) N(10)−Zn(2)−O(1w) N(8)−Zn(2)−O(3) N(8)−Zn(2)−O(1w) O(1w)−Zn(2)−O(3)

77.96(18) 121.5(2) 90.37(18) 135.83(18) 93.2(2) 167.58(17) 89.07(16) 96.6(2) 101.1(2) 96.49(17)

G



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In order to access the UV−vis absorption characteristics of ligands HL1 and HL2 and of complexes 1−4, TD-DFT (B3LYP/6-31G* and CAM-B3LYP/6-31G*) calculations were performed by accounting for solvent effects with the PCM model (see Theoretical Methods). The main TD-DFT singlet electronic excitations of HL1 and HL2 are presented in Table 7, whereas the spectra are shown in Supporting Table 7. Main PCM-TDDFTa Electronic Singlet−Singlet Transition Energies (ΔE) with Oscillator Strengths ( f) and Assignments for HL1 and HL2b state S1

ΔE (eV) [nm]

f

S7 S5

3.75 4.25 3.96 4.79 4.92 5.31 5.64 6.06

[331] [292] [313] [259] [252] [233] [220] [204]

0.117 0.197 0.078 0.02 0.080 0.099 0.476 0.842

S1 S2 S4 S6 S7

3.43 3.96 4.70 5.13 5.64

[361] [313] [264] [242] [220]

0.079 0.074 0.010 0.196 0.435

S2 S4

assignment HL1 H → L (0.66) π → π* H → L (0.68) π → π* H-1 → L (0.65) π + pN → π* (CT) H-1 → L (0.53) π + pN→ π* (CT) H-2 → L (0.61) π → π* H-2 → L (0.50) π → π* H → L+1 (0.56) π → π* H-1→ L+1 (0.48) π → π* HL2 H → L (0.70) π + pN → π* (CT) H-1 → L (0.68) π → π* H-2 → L (0.66) π → π* H-3 → L+1 (0.53) π → π* H-1 → L+1 (0.56) π → π*

Figure 10. Fluorescence spectra of HL1, HL2 (λex = 325 nm), and complexes 1−4 (λex = 375 nm) in methanol at room temperature; all solutions have an optical density of 0.2.

states were performed. Hence, for both ligands, TD-DFT optimization of the lowest-emissive singlet excited state (S1) was performed. The main geometrical parameters of the excited-state optimized geometries are shown in Supporting Information Figure S11 and are compared to the S0 optimized ones. In the S1 optimized geometries, some rearrangements are detected among the bond distances of the phenyl unit. These rearrangements root on the relaxation on the lowest π−π* excited state potential energy surface. The theoretical fluorescence electronic energies are presented in Table 8. These values are blue-shifted with respect to those of the experiment. This is due to the limited accuracy of the PCMTD-B3LYP protocol to describe the low-energy π−π* bands of the ligands. On complexation, the emission intensity is enhanced by 70 to 75 times compared to that of the ligands. The quantum yield (Φ) calculated in each case is given in Table 8. Although the λmax of the emission of complex 1 and of the corresponding ligand HL2 are the same, complexes 2−4 exhibit a slightly redshifted band compared to the ligand HL1. Excitation spectra of the ligands and the complexes monitoring the corresponding λ(F) max match well with the absorption spectra (Figure 8 and Table 8). The low quantum yield of the ligands is ascribed to a very fast photoinduced electron transfer (PET) from the nitrogen lone pair to the conjugated phenolic moiety. Complexation prevents PET and thus enhances the Φ in the complexes.54 The fluorescence decay of the ligands and complexes is shown in Figure 11a,b, with λexc = 405 nm and λexc = 370 nm, respectively. The decay process in each case is found to be fitted with two exponentials (Table 8). The average lifetimes observed (Table 8) clearly indicate the π−π* nature of the excited state, thus corroborating our theoretical predictions. A shorter component (∼1.4−1.7 ns) contributes significantly in the case of the ligands, whereas a longer component of 7 ns is predominant in the complexes (Table 8). The radiative (kr) and the nonradiative (knr) rate constants are calculated using Φ and ⟨τ⟩. The ratio (kr)complex/(kr)ligand for different complexes (∼25−30) gives support to the PET effect. Phosphorescence Spectra. The photoluminescence properties of complexes 1−4 were studied by carrying out the emission measurement at 77 K in CH3OH glass (Figure

a PCM-B3LYP/6-31G*. bValues in italic correspond to the PCMCAM-B3LYP/6-31G* singlet electronic excitations.

Information Figures S4 and S5. For HL1, all of the PCMCAM-B3LYP excitations are clearly energetically overestimated (i.e., blue-shifted) compared to the data obtained from PCMB3LYP and from experimental evidence (Table 7). This is a common trend observed for local π−π* excitations of organic compounds.52 Hence, a better agreement with the experiment is gained with the B3LYP functional rather than with the CAMB3LYP one. Therefore, the PCM-B3LYP protocol was selected for the rest of the compounds. The spectra of HL1 and HL2 are assigned as follows: The high-energy band peaking experimentally at ca. 221 nm is assigned mainly to high-energy π−π* excitations (i.e., S7 for HL1 and S6−7 for HL2; Table 7). These theoretical transitions are in perfect agreement with the experiment results. The low-energy bands, peaking experimentally at ca. 413 and 325 nm, are mainly due to less intense π−π* transitions, for example, see S1 and S2 for HL2 in Table 7. These bands are theoretically blue-shifted with respect to the experiment. The computed PCM-TDDFT spectra of all of the complexes are depicted in Supporting Information Figures S6−S10. The overall agreement between the experimental and theoretical spectra of complexes 1−4 is remarkable. Indeed, the protocol PCM-TD-B3LYP proved to be realistic for other transition-metal complexes.53 The nature and character of the main low-lying singlet excited states are maintained upon coordination of Zn(II). Photoluminescence Study (Steady-State and TimeResolved Emission Spectra). The photoluminescence spectra of the ligands (λexc = 325 nm) and complexes 1−4 (λexc= 375 nm) in CH3OH at 298 K are shown in Figures 10. To gain insight into the fluorescence characteristics of HL1 and HL2, excited-state optimizations of the lowest-lying emissive H



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Table 8. Fluorescence and Time-Resolved Emission Data for Ligands and Complexes 1−4 at 298 K and Low-Temperature Phosphorescence Data for 1−4 at 77 K in Methanol phosphorescence spectra (λex = 375 nm)

singlet state lifetime monitoring the λ(F) max

a

ligand/ complex

a (F) λ(F) max (theor. λmax) (nm)

Φ

HL1 HL2 1 2 3 4

445.0 (343.0) 444.0 (357.0) 449.0 463.0 462.0 462.0

0.0016 0.0018 0.1318 0.1230 0.1127 0.1096

τ1 (ns) 1.40 1.67 1.10 0.62 0.93 1.34

(88.7%) (85.0%) (24.6%) (27.3%) (31.8%) (35.3%)

τ2 (ns) 6.25 5.26 7.15 7.05 7.15 6.96

(11.3%) (15.0%) (75.4%) (72.7%) (68.2%) (64.6%)

⟨τ⟩ (ns)

χ2

kr × 10−7 (s−1)

knr × 10−7 (s−1)

(P) λ(P) max (theor. λmax) (nm)

τp (s)

1.95 2.21 5.66 5.30 5.17 4.97

1.0 1.0 1.1 1.1 1.0 1.0

0.082 0.081 2.328 2.320 2.179 2.205

51.200 45.167 15.339 16.547 17.162 17.915

488.0 (506) 500.0 (578/591)b 502.0 (594) 499.0

0.31 0.29 0.28 0.26

λex = 325 nm for HL1 and HL2 and λex = 375 nm for complexes 1−4. bTheor. λ(P) max values of cationic and anionic species of complex 2.

Figure 12. Low-temperature phosphorescence spectra of complexes 1−4 in methanol glass at 77 K (λex = 375 nm; b/p = 10:2.5); all solutions have an optical density of 0.2.

especially in the case of complex 1. The values of the phosphorescence lifetime (∼0.3 s) shown in Table 8 corroborate the π−π* nature of the lowest triplet in each case. Among complexes 1−4, complex 1 possesses slightly larger quantum yields of photoluminescence (Table 8 and Figures 10 and 12) compared to those of complexes 2−4. We have theoretically demonstrated that the emissive state for all of the complexes studied herein is of π−π* character. Accordingly, the values of the radiative rates of photoluminescence (kr) should be very similar, as corroborated experimentally (Table 8). Consequently, the nonradiative decay rates (knr) are responsible for the larger values of Φ in complex 1. This is in accordance with the energy gap,55 which predicts the knr values to increase exponentially with decreasing T1/S1−S0 gaps. Hence, complexes 2−4, bearing red-shifted peaks compared to that of complex 1, possess larger knr values. These are the ultimate reasons for the increased photoluminescence quantum yields of complex 1. Theoretical Rationalization Regarding Complex Formation. We are still left with the intriguing question of why HL1 and HL2 behave differently in complex when they are prepared under similar reaction conditions (Scheme 1). In order to understand this apparent peculiar behavior, we performed thorough theoretical calculations. We start by considering the energy of formation of the different complexes with either the HL1 or HL2 ligand (Scheme 1). Given the intrinsic problems related to the calculation of the solvation energies for ionic species (especially the proton or transition metals),56,57 which are further complicated when the solvent is

Figure 11. (a) Fluorescence decay of the ligands in methanol at 298 K; λexc = 405 nm. (b) Fluorescence decay of complexes 1−4 in methanol at 298 K; λexc = 370 nm.

12). The phosphorescence spectrum was isolated using a chopper (Figure 12 and Table 8). In order to unveil the nature of the emissive states, the lowest-emissive triplet excited states (T1) of complexes 1−4 were optimized with DFT. The emissive T1 state of all of the complexes studied is also of π−π* character, as unambiguously reflected by the spin density distributions at the T1 optimized geometries (see Supporting Information Figure S12). The phosphorescence maxima at 77 K are slightly blueshifted compared to those recorded at room temperature. The theoretical phosphorescence electronic energies computed are presented in Table 8, and on the basis of ΔSCF-DFT calculations, they are red-shifted with respect to the experiment. The overall agreement with the experimental values is good, I



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not water,58,59 we have considered here only neutral species. For simplicity, we consider only the following processes: 2HLx + Zn(ClO4 )2 → [Zn(LxH)2 ](ClO4 )2

Theoretical Study on Weak Interactions. Complex 4 is a unique example of single azido bridged dinuclear zinc(II) complex, and, interestingly, it exhibits a self-assembling property to yield a dimer where the weak interactions exerted by water and azide moieties are likely to play the key role. In order to unveil the contribution of these weak interactions in formulating the dimer, we have performed a detailed theoretical investigation. Figure 14 shows the centrosymmetric dimer and the molecular electrostatic potential surface of the monomer. It can be observed that in the self-assembled dimer two strong hydrogen bonds between each water molecule and the phenolic oxygen atoms of the symmetry-related complex are formed and characterized by very short O−H···O distances (1.65 and 1.85 Å). In addition, the ending nitrogen atom of the azide is interacting with the imine carbon atom (N9···C21′ distance of 3.05 Å), which belongs to the extended π system of the ligand, thus forming a pseudo anion−π interaction. The MEP surface shows that the highest positive potential (106 kcal mol−1) is located on the coordinated water molecule and the lowest negative potential is located in the phenoxide group (−75 kcal mol−1). Taking into account that the maximum and minimum energy values are very large (in absolute value), this molecule has a strong tendency to form self-assembled dimers, as observed in solid state. It is interesting to note that the shortest hydrogen bond (1.65 Å) is formed with the oxygen atom that is coordinated to the Zn(II) metal center that does not have the coordinated water molecule. This is the oxygen atom that presents the most negative electrostatic potential, as shown by the MEP surface. In order to evaluate the contribution of the hydrogen bonds and pseudo anion−π interactions that account for the formation of the dimer, we have considered different theoretical models, as depicted in Figure 15. In the first model (left), we used the dimer retrieved from the X-ray structure, and the dimerization energy is very large and negative (ΔE1 = −121.6 kcal mol−1), in agreement with the MEP energies described above. In the figure, we show the relative interaction energies for the three models using ΔE1 as reference. In the second model (middle), we removed the water molecules, and the binding energy is drastically reduced (ΔE2rel = 51.9 kcal mol−1), indicating that the contribution of each hydrogen bond is approximately 1/4 × ΔE2rel = 12.9 kcal mol−1. This strong hydrogen-bonding interaction energy is due to the combination of the partially anionic nature of the hydrogen-bond acceptor and the enhanced acidity of the hydrogen-bond donor due to the coordination of the water molecule to the metal center. Finally, in the third model (right), we have replaced the N3− anions by H− in order to evaluate the pseudo anion−π interactions. The relative interaction energy is reduced to ΔE3rel = +15.3 kcal mol−1, indicating that the contribution for a single anion−π interaction is 1/2 × ΔE3rel = 7.7 kcal mol−1.

(i)

2HLx + 2Zn(Cl04)2 + 2H 2O + 2HSCN → [Zn(Lx)2 (H 2O)2 ][Zn(Lx)2 (SCN)2 ] + 4HClO4

(ii)

Lx = {L1, L2}

The other two reactions (with NaN3 and Na(dca)) lead to polymeric species, which are more difficult to model, and when cut off, they may lead to ionic species again. If we look at the reaction energies for the two processes (Table 9), then we see that with ligand HL1 the formation of Table 9. Reaction Energies for the Processes i and iia

a

ligand

HL1

HL2

ΔER, process i ΔER, process ii

−84.52 −88.62

−82.38 −79.41

Given in kcal mol−1; at COSMO(methanol)-PBE-D2/TZ2P.

the dimeric [Zn(L1)2(H2O)2][Zn(L1)2(SCN)2] complex (process ii) is clearly favored by ca. 4 kcal mol−1. On the other hand, for ligand HL2, the formation of [Zn(L2H)2](ClO4)2 is clearly favored by ca. 3 kcal mol−1. Therefore, it does not really matter that the reaction energy for the formation of [Zn(LxH)2](ClO4)2 is ca. 2 kcal mol−1 lower for HL2 than for HL1, because the formation of the dimeric species, [Zn(L2)2(H2O)2][Zn(L2)2(SCN)2], is clearly unfavorable. The question that now arises is what are the factors that brings this about. The most important factor turns out to be the number of methylene groups (n). For HL1, n = 2, which enables a coordination around the zinc atom where both waters are able to coordinate to the zinc (see crystal structure and the computed structure, Supporting Information). Instead, for HL2 with n = 3, the morpholine moiety has to adapt its orientation in order to optimize its metal−ligand bonding with the zinc. This reorientation blocks the coordination possibility for the second water, which instead moves away from zinc and forms hydrogen bonds with the coordinating water and with the oxygen of the 5-chlorosalicylaldehyde (Figure 13). Therefore, it is the inability of this second water molecule to coordinate to water that makes the formation of the dimeric complex less favorable.

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CONCLUSIONS In this article, we report that homologues tridentate Schiff-base ligands, HL1 and HL2, differing by one −CH2− group, exhibit exciting different reactivities upon complexation with zinc(II) in the presence of ClO4−, SCN−, N(CN)2−, and N3− as coligands. No perchlorate complex of HL1 could be isolated, whereas HL2 yielded only the perchlorate derivative (1) under our reaction conditions. Dicyanamide (dca) forms a polynuclear [Zn(L1)(dca)]n (3) species, thiocyanate generates cocrystal of mononuclear cationic and anionic complexes, namely, [Zn(L1)(H2O)2]+ and [Zn(L1)(SCN)2]− (2), and

Figure 13. Hydrogen-bonding position of second water molecule. J



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Figure 14. (Left) Self-assembled dimer of 4 observed in solid state (distances in Å). (Right) Molecular electrostatic potential surface of 4.

Figure 15. Theoretical models used to evaluate the contribution to the dimerization energy in compound 4. The hydrogen atoms have been omitted for clarity except for the water hydrogen atoms and the hydride bonded to Zn in the model shown on the right.

azide produces self-assembled azido-bridged dinuclear complex [Zn2(L1)2(N3)2(H2O)2] (4) stabilized by anion−π interactions in solid state, which is a rare finding in zinc coordination chemistry. These unprecedented findings, specifically the formation of thiocyanato complex, were rationalized by DFT calculations. The theoretical calculations unambiguously demonstrate that because of the different ligating backbone of the two homologous Schiff-base ligands the formation of [Zn(L2)2(H2O)2][Zn(L2)2(SCN)2] is clearly unfavorable, a consequence that seldom leads to the formation of dimeric species with HL2. All of the complexes exhibit enhanced photoluminescence compared to that of the ligands. The theoretical prediction of the electronic and photoluminescence properties of the ligands as well as of the complexes corroborates the experimental results well, thus validating the origin of these properties.

■

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank CSIR, New Delhi [01(2464)/11/ EMR-II dt 16-05-11 to D.D.] for financial support and the University of Calcutta for providing the single-crystal X-ray diffractometer facility from the DST FIST program. A.B. and A.F. thank the DGICYT of Spain (project nos. CTQ201127512/BQU and CONSOLIDER INGENIO 2010 CSD201000065, FEDER funds) and the Direcció General de Recercai Innovació del Govern Balear (project no. 23/2011, FEDER funds) for financial support. S.G. gratefully acknowledges CSIR (no. 21(0871)/11/EMR-II), DST (no. SR/S1/PC-57/2008), and DST (no. SB/S1/PC-003/2013) for financially supporting this work. S.S. thanks CSIR for the JRF fellowship (no. 21(0871)/11/EMR-II).

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic data of complexes 1−4 in CIF format; FT-IR spectra; computed PCM-TDDFT UV−vis absorption spectrum of complexes 1−3 as well as ligands HL1 and HL2; bond distances of the lowest singlet excited-state (S1) and ground-state (S0) optimized geometries; spin density distributions (B3LYP/6-31G*) plots of complexes 1−3; 1H and 13C NMR spectra of ligands HL1 and HL2; 1H NMR spectra of complexes 1−4; Cartesian coordinates of all species; computed NMR data of ligands and complexes; and absorption data of two ligands and three complexes in tabular form. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*(A.F.) E-mail: [email protected]. *(E.Z.) E-mail: [email protected]. *(D.D.) E-mail: [email protected]. K



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