Structure, Spectra, and DFT Simulation of Nickel Benzazolate

Article pubs.acs.org/IC

Structure, Spectra, and DFT Simulation of Nickel Benzazolate Complexes with Tris(2-aminoethyl)amine Ligand Javier Cerezo, Alberto Requena, and José Zúñiga* Departamento de Química Física, Universidad de Murcia, E-30071 Murcia, Spain

María José Piernas and M. Dolores Santana* Departamento de Química Inorgánica, Universidad de Murcia, E-30071 Murcia, Spain

José Pérez and Luís García Departamento de Ingeniería Minera, Geológica y Cartográfica, Á rea de Química Inorgánica, Universidad Politécnica de Cartagena, E-30203 Cartagena, Spain S Supporting Information *

ABSTRACT: Benzazolate complexes of Ni(II), [Ni(pbz)(tren)]ClO4 (pbz = 2-(2′-hydroxyphenyl)-benzimidazole (pbm), 1, 2(2′-hydroxyphenyl)-benzoxazole (pbx), 2, 2-(2′-hydroxyphenyl)-benzothiazole (pbt), 3; tren = tris(2-aminoethyl)amine), are prepared by self-assembly reaction and structurally characterized. Theoretical DFT simulations are carried out to reproduce the features of their crystal structures and their spectroscopic and photophysic properties. The three complexes are moderately luminescent at room temperature both in acetonitrile solution and in the solid state. The simulations indicate that the absorption spectrum is dominated by two well-defined transitions, and the electronic density concentrates in three MOs around the benzazole ligands. The Stokes shifts of the emission spectra of complexes 1−3 are determined by optimizing the electronic excited state.

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INTRODUCTION Molecules with intramolecular hydrogen bonds often exhibit photoinduced proton transfer processes, a phenomenon that has broad implications for the action of many laser dyes,1,2 and as detectors of high-energy radiation,3−5 UV-light stabilizers,6,7 and fluorescent probes.8−10 The excited state intramolecular proton transfer mechanism, ESIPT, involves the migration of a proton, usually pertaining to a hydroxyl or amino group covalently linkedto a neighboring atom located less than 2 Å away, by a hydrogen bond mechanism.11 ESIPT molecules exhibit red-shifted tautomeric fluorescence emission whose spectrum depends on the nature of the ground state and excited state and which is often strongly influenced by the pH, by the polarity of the solvent, and by the ability of the complex to form hydrogen bonds.12,13 Because the photo© 2017 American Chemical Society

tautomer formed is more stable in the excited state than in the ground state, the Stokes shift observed in fluorescence emission is unusually large, with values ranging from 100 to 500 nm.14 These properties make ESIPT molecules very interesting targets for the development of radiometric fluorescent probes. Henary et al.11 studied the influence of the coordination of a metal cation in the proton transfer process of ESIPT ligands, finding that the emission is sensitive to the metal bond, since the chelating of cations often competes with the protonation of donor atoms of the ligands. If the coordination of the metal cation is accompanied by the displacement of the hydroxyl proton, inhibition of the ESIPT process occurs and red-shifted Received: January 14, 2017 Published: March 2, 2017 3663

DOI: 10.1021/acs.inorgchem.7b00059 Inorg. Chem. 2017, 56, 3663−3673

Article

Inorganic Chemistry Scheme 1

Figure 1. ORTEP drawing of cations of complexes 1 (A), 2 (B), and 3 (C) with atom-labeling scheme.

complexes 23 should be red shifted compared to the corresponding free ligands. However, Hpbm, Hpbx, and Hpbt emit at 444, 494, and 505 nm, while their corresponding beryllium complexes emit at 428, 440, and 474 nm, respectively, that is, there is a blue shift in the complexes which constitutes a violation of the expected red shift. We have previously contributed to this area by reporting the synthesis N,O-benzazolate derivatives of pentacoordinate nickel(II)-containing fragments “Ni(Tp*)” and “Ni(mcN3)” (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate; mcN3 = 2,4,4trimethyl-1,5,9-triazacyclododec-1-ene or 2,4,4,9-tetramethyl1,5,9-triazacyclododec-1-ene) and by theoretically studying their crystal structures and spectroscopic and photophysical properties by density functional theory (DFT) methods.24,25 In this paper, we describe the synthesis of benzazolate complexes of nickel(II) by self-assembly using the ligand tris(2aminoethyl)amine (tren) as ancillary ligand. The crystal structures and spectra of these octahedral complexes are recorded and interpreted by conducting theoretical DFT simulations. The influence of the environment coordination of nickel(II) on the spectroscopic and luminescence properties of complexes is also discussed.

emission of the tautomer disappears in favor of the normal emission of the metal complex to higher energy. In other words, a significant variation in wavelength takes place, and a blue shift in the emission peak, relative to the keto tautomer, is observed. 2-(2′-Hydroxyphenyl)-benzazole ligands have been studied widely in order to explain the mechanism of the ESIPT process. The photophysical information available for these ligands and the provision of an intramolecular hydrogen bond, essential for the process ESIPT to occur, make them good candidates to explore the influence of the bounded cation in the proton transfer process.12,15−22 Derivatives based on benzimidazole (2-(2′-hydroxyphenyl)benzimidazole, Hpbm) are a new class of luminescent N,Odonor ligands that can be used as possible electroluminescent materials whose luminescent properties can be tuned by binding metal ions. It is also expected that homologous ligands containing different heteroatoms like O (2-(2′-hydroxyphenyl)benzoxazole, Hpbx) and S (2-(2′-hydroxyphenyl)-benzothiazole, Hpbt) could exhibit luminescent properties. Tong et al. reported the synthesis of monomeric complexes of Be(II) with N,O-benzazolate ligands with slightly distorted tetrahedral geometry.23 They also studied the effect of both the Be(II) complexation and the substitution of the heteroatom on the electronic transitions accounting for the luminescent processes and spectral variations by carrying out theoretical calculations (TD-DFT) and concluded that the maximum absorption energy corresponds to the π → π * transition for the free ligands, Hpbm, Hpbx, and Hpbt, and also for their Be(II) complexes.23 As described for the formation of metal chelates with ligands ESIPT, the luminescence of the Be(II)

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RESULTS

Synthesis and Crystal Structures. The self-assembly reaction (Scheme 1) of a solution of compound Ni(ClO4)2· 6H2O with 1 equiv of capping amine ligand tris(2-aminoethyl)amine (tren) in ethanol with another solution of 1 equiv of the corresponding benzazole ligand and KOH in ethanol led to complexes 1−3. 3664


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Inorganic Chemistry Table 1. Selected Bond Lengths (Angstroms) and Bond Angles (degrees) for Complexes 1−3 complex 1

complex 3

complex 2

parameter

exp.

calcd

exp.

calcd

exp.

parameter

exp.

calcd

Ni(1)−N(1) Ni(1)−N(2) Ni(1)−N(3) Ni(1)−N(4) Ni(1)−N(5) Ni(1)−O(1) N(1)−Ni(1)−N(2) N(1)−Ni(1)−N(3) N(1)−Ni(1)−N(4) N(1)−Ni(1)−N(5) N(1)−Ni(1)−O(1) N(2)−Ni(1)−N(3) N(2)−Ni(1)−N(4) N(2)−Ni(1)−N(5) N(2)−Ni(1)−O(1) N(3)−Ni(1)−N(4) N(3)−Ni(1)−N(5) N(3)−Ni(1)−O(1) N(4)−Ni(1)−N(5) N(4)−Ni(1)−O(1) O(1)−Ni(1)−N(5)

2.063(2) 2.104(2) 2.122(2) 2.114(2) 2.145(2) 2.093(2) 178.48(8) 98.37(8) 96.12(9) 98.61(9) 85.06(8) 82.91(8) 82.91(9) 82.16(9) 93.69(7) 95.22(9) 91.73(9) 176.31(7) 162.61(9) 85.74(8) 86.39(8)

2.046 2.162 2.156 2.154 2.178 2.058 177.07 100.85 97.92 96.68 88.42 81.88 82.73 81.99 88.88 97.12 94.24 170.61 159.45 79.99 86.04

2.0991(15) 2.1054(16) 2.1623(16) 2.1147(17) 2.0952(16) 2.0705(13) 175.53(6) 99.45(6) 101.09(6) 94.61(6) 84.56(5) 81.79(6) 83.14(6) 83.50(6) 91.23(6) 91.57(6) 163.04(7) 86.99(6) 95.01(7) 174.33(6) 84.93(6)

2.074 2.165 2.182 2.158 2.152 2.046 175.57 95.65 102.20 98.82 87.24 81.75 81.58 82.87 89.03 93.15 159.89 86.68 97.34 170.52 80.17

2.0873(16) 2.1021(15) 2.1109(16) 2.1330(17) 2.1247(16) 2.0338(13) 175.53(6) 101.31(6) 97.45(6) 95.42(6) 88.30(6) 83.03(6) 83.47(6) 82.92(6) 87.41(6) 91.10(7) 96.64(7) 170.11(6) 163.39(7) 85.26(7) 84.66(6)

Ni(2)−N(6) Ni(2)−N(7) Ni(2)−N(8) Ni(2)−N(10) Ni(2)−N(9) Ni(2)−O(3) N(6)−Ni(2)−N(7) N(6)−Ni(2)−N(8) N(6)−Ni(2)−N(10) N(6)−Ni(2)−N(9) O(3)−Ni(2)−N(6) N(7)−Ni(2)−N(8) N(7)−Ni(2)−N(1) N(7)−Ni(2)−N(9) O(3)−Ni(2)−N(7) N(8)−Ni(2)−N(10) N(8)−Ni(2)−N(9) O(3)−Ni(2)−N(8) N(9)−Ni(2)−N(11) O(3)−Ni(2)−N(10) O(3)−Ni(2)−N(9)

2.0894(15) 2.1059(15) 2.1092(17) 2.1296(17) 2.1069(18) 2.0436(14) 176.15(6) 100.27(6) 94.83(6) 98.47(6) 87.71(6) 82.84(6) 82.70(6) 83.48(6) 89.19(6) 92.62(7) 94.56(7) 172.03(6) 163.51(7) 86.66(7) 84.17(7)

2.054 2.163 2.150 2.182 2.152 2.049 176.56 100.73 95.64 98.95 88.72 81.96 81.94 82.79 88.64 94.91 96.60 170.46 159.37 85.38 80.47

Geometry Optimization. The DFT values calculated for the bond distances and angles of the Ni coordination geometries are also included in Table 1. For complex 2, which crystallizes as a dimer, the optimized structures obtained starting from each monomer are basically identical, since optimization does not consider either the interactions between the monomers or the possible crystal packing effects. Nevertheless, the optimized geometries are reasonably accurate, especially for use in the simulations of the electronic spectra. The root-mean-squares deviation (RMSD) of the Ni bond lengths are on the order of 0.04 Å for the three complexes and decrease to 0.02 Å when considering all lengths, and the RMSD values for the bond angles are 3.01° (1), 2.18° and 1.80° (2), and 2.63° (3) for the Ni coordination structure and decrease to 1.72° (1), 1.47° and 1.28° (2), and 1.62° (3) for the entire complexes. All values lie within the limits established for the optimized geometries of the complexes to be accepted as good geometries.33,34 IR Spectra. The IR spectra of complexes 1−3 show bands at ∼3350 and 3300 cm−1 which are attributed to the ν(N−H) of the tren ligand. The NH2 bending mode of the tren ligand appears31 at ∼1600 cm−1. Bands corresponding to the benzazolate ligands are also observed in the IR spectra. Specifically, a band corresponding to the stretching vibration of the CN group appears in the region 1622−1600 cm−1, and a broad band associated with the vibrations ν(C−C) + ν(C−N) lies between 1531 and 1520 cm−1. It is noteworthy in addition that all infrared spectra present two bands at ∼1080 and 620 cm−1, which are assigned to the stretching vibrations ν3 and ν4 of the no-coordinated perchlorate anion. The IR spectra of complexes 1−3 are shown in Figures S3−S5. Electronic Absorption Spectra. The absorption spectra of the three complexes were recorded in acetonitrile solution at room temperature. These spectra are shown in Figure 2 (central panels), while Table 2 contains the spectral data for both the complexes and the benzazolate ligands for comparison.

The capping amine ligand (tren) can occupy four coordination sites on the metal center, thus leaving two sites available for coordination of the benzazolate. In Figure 1 we show the crystal structures of the cations of complexes 1−3 as elucidated by single-crystal X-ray analysis, and in Table 1 we include a number of selected bond lengths and bond angles. The three complexes present an octahedral geometry around the nickel ions. The four nitrogen atoms of the amine tren occupy three equatorial positions (N3, N4, and N5) and one apical position (N2), and the remaining positions are occupied by the benzazolate ligands. The benzazolate ligands are bonded as chelate ligands η2(N,O) to the nickel atoms. The nitrogen atom of the benzazolate (N1) is in the trans position to the nitrogen N2 of the amine tren. The oxygen atom of the benzazolate (O1) is in an equatorial position. The Ni−Ntren bond distances (2.102−2.162 Å) are within the range reported for complexes containing the [Ni(tren)] moiety.26−32 The Ni− N1 bond distances are 2.063, 2.087, 2.099 Å for complexes 1, 2, and 3, respectively. The bond distances Ni−O1 are 2.093, 2.034, and 2.071 Å for 1, 2, and 3, respectively. The bond distances Ni−N lie in the range reported for nickel compounds containing benzazolates,24,25 and the Ni−O1 distances are a bit longer than the reported ones. The O−Ni−N “bite” angles of the chelating benzazolates are 85.06°, 88.30°, and 84.56° for complexes 1, 2, and 3, respectively. The six-membered (Ni− O1−C7−C12−C13−N1) chelate rings are nonplanar. The dihedral angles between the phenolate ring (O1−C7···C12) and the plane defined by Ni−N1−C13 are 24.91° in 1, 17.74° in 2, and 31.83° in 3. In complex 2 there are two molecules in the asymmetric unit. The most relevant difference between them is the displacement of the Ni atom about the benzoxazole ring: in one Ni is coplanar with benzoxazol (the previous angle is for this molecule) and in the other displaced by 0.605 Å. There are some intermolecular hydrogen bondings in the complexes, which are shown in the Supporting Information (Tables S1−S3 and Figures S1 and S2) 3665


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Figure 2. UV−vis absorption spectra measured for ligands 1 (Hpbm), 2 (Hpbx), and 3 (Hpbt) (top panels) and complexes 1−3 (central panels), and spectra of the complexes calculated using the TD-CAM-B3LYP method (bottom panels) in CH3CN solution; vertical lines in calculated spectra indicate excited states and their corresponding oscillator strengths.

intraligand charge transfer (ILCT) band depends on the metal, its oxidation state, the ligands present in the molecule, and the polarity of the solvent. In Figure 2 we also show the calculated electronic absorption spectra of the complexes (bottom panels). As noticed, the calculated spectra are all blue shifted from the experiment by about 40 nm, an effect which has been reported to be characteristic of simulations carried out using the CAM-B3LYP density functional.25,36−38 The simulations indicate that the structure of the absorption spectra is dominated by the two well-defined transitions to the excited states T10 and T15 plus a number of increasing transitions that congest the spectra at lower wavelengths. In Table 3 we give the absorption spectral data for the most important transitions of the complexes to the excited states T10 and T15. The major MO contributions clearly indicate that the lowest energy transition to the T10 state is the HOMO → LUMO for the three complexes, presenting similar weights for the α- and β-spin electronic densities, whereas the main contribution for the transitions to the T15 state comes from the HOMO-1 → LUMO component. In Figure 3 we plot the electronic densities of the HOMO-1, HOMO, and LUMO orbitals of complex 1. These three MOs concentrate the electronic density around the benzazole ligand

Table 2. Absorption Spectra Data for Benzazolate Nickel Complexes in CH3CN Solutions at RT λmax/nm (ε/M−1 cm−1)

complex 1 2 3 Hpbm Hpbx Hpbt

292 292 289 290 290 286

(11 870), (12 590), (12 490), (25 000), (35 200), (19 400),

317 318 330 316 317 330

(6850), 331 (7550), 372 (9380) (3820), 331 (3950), 391 (10 510) (6430), 344 (6030), 413 (11 010) (39 930), 330 (38 400) (33 200), 330 (29 800) (23 800), 343 (21 700)

The absorption spectra show intense absorption bands in the region between 290 and 420 nm, with molar extinction coefficients between 13 000 and 4000 M−1 cm−1. These bands obscure the much less intense metal-centered (MC) spinallowed bands due to the octahedral environment of nickel(II).35 In general, the absorption spectra of benzazolate complexes are characterized by intraligand transitions (IL). The band at the shorter wavelength is assigned to the π−π* transition of the benzazolate aromatic system, and the bands at longer wavelength are attributed to IL transition associated with a charge transfer (CT) from lone pairs of phenolate (donor) to the heterocycle (acceptor). The energy corresponding to this 3666


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Table 3. Selected Spin-Allowed Triplet−Triplet Electronic Transitions of Complexes 1−3 Calculated Using the TD-CAMB3LYP Method in CH3CN Solution complex

transition

energy (eV)

λcalcd (nm)

oscillator strength

major contribution

λexp. (nm)

1

10

3.73

332

0.3704

372

15

4.69

264

0.1674

10

3.58

346

0.3607

15

4.60

269

0.3036

10

3.40

364

0.3437

15

4.42

280

0.1920

HOMO(α) → LUMO(α) (47%) HOMO(β) → LUMO(β) (46%) HOMO-1(α) → LUMO(α) (21%) HOMO-1(β) → LUMO(β) (31%) HOMO(α) → LUMO(α) (47%) HOMO(β) → LUMO(β) (48%) HOMO-1(α) → LUMO(α) (32%) HOMO-1(β) → LUMO(β) (40%) HOMO(α) → LUMO(α) (48%) HOMO(β) → LUMO(β) (46%) HOMO-1(α) → LUMO(α) (30%) HOMO-1(β) → LUMO(β) (41%)

2

3

292 391 292 413 289

from the lonely pairs of phenolate (donor) to the π* orbital of the heterocycle (acceptor), while the essentially HOMO-1 → LUMO transition to the T15 state is the intraligand (IL) π → π* transition occurring within the aromatic system of the benzazolate ligand. To confirm the charge transfer occurring in the HOMO → LUMO transition and to quantify its amount and spreading, we used the model developed by Le Bahers and co-workers,39,40 which gives a charge transfer of 0.59 electron units for complex 1 with a shift of the charge centers of the positive and negative electronic density regions of 1.17 Å (see Figure 5). For complexes 2 and 3, the amounts of charge transferred are similar, 0.58 and 0.60 electron units, respectively, and the corresponding shifts are 1.18 and 1.37 Å. 1 H NMR Spectra. 1H NMR spectroscopy has become a very useful tool in the characterization of paramagnetic metal complexes, including five- and six-coordinate complexes of nickel(II).35 Complexes 1−3 were therefore characterized by 1 H NMR spectroscopy (see the Experimental Section), and Figure 6 shows the proton NMR spectra of complex 1 as an example. The 1H NMR spectra of complexes 2 and 3 are shown in the Supporting Information Figures S7 and S9. The 1H NMR spectra of the three complexes present isotropically shifted resonances from 140 (downfield) to 1 ppm in acetonitrile solution at RT. The broad signals between 140 and 70 ppm are attributed to methylene protons of the tren ligand. The assignments of all isotropically shifted signals were achieved by two-dimensional 1H NMR techniques. The {1H−1H} COSY spectra of complexes 1−3 are given in the Supporting Information. The signals assigned to H1−H8 protons of the benzazolate rings show an alternating downfield−upfield shift pattern with regard to the diamagnetic positions. This shift pattern is consistent with a π-polarizationdominant pathway of spin density, although the ground state of Ni(II) contains unpaired electrons in the dx2−y2 and dz2 orbitals. However, those electrons could polarize the net spin density in the dπ orbitals, a behavior which has been observed before in other paramagnetic complexes.35,41,42 Emission Spectra. The photophysical properties of all benzazolate complexes have been studied in acetonitrile solutions and in the solid state. The luminescent data of the spectra are given in Table 4. The three complexes show an unstructured band in the blue region between 456 and 475 nm (Table 4), as shown in Figure 7 for complex 1 and for complexes 2 and 3 in the Supporting Information Figures S11and S12 (in acetonitrile solution) and Figures S13 and S14 (solid state).

Figure 3. Ground-state molecular orbital energy diagram for transitions 10 and 15 of complex 1 and MO plots, calculated using the TD-CAM-B3LYP method in CH3CN solution.

and exhibit, as expected, similar distributions for the α- and βspin electronic densities. The same happens for the MOs of complexes 2 and 3. To reinforce the interpretation of the main electronic transitions, we calculated the natural transition orbital NTO, which best describes the electronic excitation from one donor (hole) orbital to one acceptor (electron) orbital. In Figure 4, we show the NTO surfaces for the transition to states T10 and T15 of complex 1, which are very similar to the corresponding canonical MO contours. The NTO electronic density distributions are compatible with the interpretation that the lowest energy HOMO → LUMO transition to state T10 can be identified as an intraligand transition with a certain charge transfer (ILCT) 3667


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Figure 4. NTO plots for transitions 10 and 15 of complex 1 calculated using the TD-CAM-B3LYP method in CH3CN solution.

within the range ΔSt = 2500−5000 cm−1, with complex 3 containing the ligand 2-(2′-hydroxyphenyl)benzothiazole exhibiting the smallest shift. The emission lifetimes measured are included in Table 4. The excited states for the three complexes at room temperature in dilute solution show a singleexponential decay with lifetimes in the range 0.11−0.19 μs. The emission spectra of the complexes in the solid state at room temperature show similar characteristics to those in solution, with the band at 463 nm of complex 1 being unstructured and the bands at 489 and 496 nm of complexes 2 and 3, respectively, being structureda behavior which has been reported before.25 In general, the complexes emit in the solid state at lower energy, that is, the bands are red shifted with respect to those in solution. The quantum yields of the complexes were also measured using coumarin-314 as standard, and their values are included in Table 4. As noticed, the quantum yields are high in general, and they are difficult to put in perspective since data collected in the literature are scarce. Complex 1 containing the ligand 2(2′-hydroxyphenyl)benzimidazole has both the highest quantum yield and the largest Stokes shift. Neither octahedral geometry nor the presence of the tren ligand in the coordination sphere of nickel(II) inhibits, therefore, the luminescent properties. Accordingly, the coordination of benzazole ligands to Ni(II) results in slightly red-shifted emissions with respect to free ligands, with similar quantum yields. However, the Stokes shifts are smaller and the lifetimes of the emissions are longer, all of which can be attributed to the deprotonation of the benzazole ligands and their coordination to the Ni(II) ion. We determined the Stokes shifts of the emission spectra of the complexes by optimizing the structure of the bright excited state T10 and by calculating the vertical energy differences between the equilibrium geometries of the ground and the excited state T10. To describe properly the charge transfer

Figure 5. Representation of the charge transfer distance and direction for the lowest energy transition of complex 1.

According to the decrease in the energy differences between the HOMO and the LUMO orbitals produced by deprotonation of the organic ligands and their coordination to the metal ions,43 the emission of the complexes is expected to be red shifted with respect to the free ligands emission. The ligands used to prepare the complexes emit, in particular, at 452 (Hpbm), 475 (Hpbx), and 453 nm (Hpbt) in acetonitrile solutions at room temperature,24 whereas the complexes emit at 456 (1), 475 (2), and 461 (3) nm, with their excitation spectra being similar to the corresponding absorption spectra (see Table 4). The complexes emissions are therefore slightly shifted to longer wavelengths with respect to the free benzazoles. Moreover, the Stokes shifts of the complexes lie 3668


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Figure 6. 1H NMR spectrum of 1 in CD3CN solution at room temperature.

Table 4. Luminescent Spectra Data for Complexes 1−3 in Acetonitrile Solutions and in the Solid State excitation λem/nm

complex 1 2 3 a

solution solid solution solid solution solid

456 466 475 491 461 467

λmax/nm 283, 318 283, 278, 292, 263,

290, 313, 330 291, 317, 329 328 305, 388 334, 355

emission λexc/nm

λmax/nm

ΔStokes/cm−1

Φa

τ/μs

313 320 317 333 389 336

456 463 475 377, 402, 489 461 408, 465, 497

4952

0.37

0.15

4523

0.12

0.19

2521

0.07

0.11

Quantum yields were determinate relative to coumarin-314 ΦCum314 = 0.68 in ethanol.

Figure 7. Emission (solid line) and excitation (dashed line) spectra for complex 1 (blue) and ligand 1 (Hpbm) (red) in acetonitrile solution (left) and in the solid state (right).

nonadiabatic coupling between the emitting state and other nearby states. A proper treatment of this problem would require a detailed exploration of the involved electronic states starting from the Franck−Condon (FC) point, carried out using a multireference method able to offer a balanced description of the electronic effects operating on all relevant states. This is, however, a formidable task and lies outside the scope of this work, and we limit ourselves here to the simpler, yet informative, analysis of the energetics along with structural optimization of the bright state from the FC point, which is carried out by keeping the dihedral angle between the two rings in the benzazolate ligand fixed at the FC value. In Figure 8, we show the vertical energies and oscillator strengths calculated along the optimization steps over the bright state starting from the FC point. We observe first that the bright state T10 undergoes a large solvent reorganization, which does not occur in either the T9 or the T8 state,

character of the electronic state T10, it is necessary to use a density functional which includes long-range corrections, like CAM-B3LYP.44,45 Accordingly, we used this density functional to optimize the structure of the T10 state and to calculate the transition energies at the equilibrium geometries. To keep the consistency between the computational levels employed for the ground and the excited electronic states, DFT optimization of the ground state was carried out also using the CAM-B3LYP functional. The Stokes shifts obtained for complexes 1 and 2 are, respectively, 4779 and 4214 cm−1 and agree reasonably well with the experimental values of 4952 and 4523 cm−1. Complex 3 shows, however, a much higher disagreement, with a calculated Stokes shift of 4024 cm−1 versus the experimental value of 2521 cm−1. The emission observed for this complex is, therefore, likely caused by some photochemical mechanism not properly described in our model, such as a significant 3669


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Figure 8. Vertical energy (in eV) and oscillator strength (in atomic units) calculated along the optimization steps over the bright state from the FC point. Solvent is described with LR-PCM in the equilibrium regime along the optimization, while the nonequilibrium values at the initial geometry are displayed as empty circles.

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evidencing a significant polarity change for T10, in agreement with the CT character of this state. The relaxation of the solvent then reduces the energy gap between the T10 and the T9 states, eventually reversing their order in complex 3. Moreover, in the first optimization steps, state T10 clearly drops below T9 for all complexes, while the distance with T8 decreases only slightly. Along the optimization path, the couplings of T10 with T9 and T8 can be qualitatively tracked by the amount of oscillator strength that T8 and T9 borrow from T10. It is observed that the coupling between T10 and T9 peaks when both states cross each other, as expected, although in complex 3 the crossing occurs along the solvent relaxation and the effect is not properly observed. As for the coupling between T8 and T10, this shows up only in complex 3, even despite there being no crossing of the states. If such a coupling is larger in the real system, the excited electronic state of complex 3 can be perturbed in such a way so as to create a local minimum from the interaction of T10 with T8, and the emission from such a minimum would involve less structural reorganization with respect to the predicted TD-DFT minimum, thus explaining the distinct Stokes shift observed for complex 3.

EXPERIMENTAL SECTION

All chemicals were of reagent grade and used without further purification. Solvents were dried and distilled by general methods before use. Physical Techniques. C, H, N, and S analyses were carried out with a microanalyzer, Carlo Erba model EA 1108. IR spectra were recorded on a PerkinElmer 16F PC FT-IR spectrophotometer using Nujol mulls between polyethylene sheets. ESI-MS analyses were performed on an Agilent VL mass spectrometer. The ionization mechanism used was electrospray in positive- and negative-ion full scan mode using acetonitrile as solvent and nitrogen gas for desolvation. The 1H NMR spectra of CD3CN solutions were recorded on a Bruker spectrometer AC 600 MHz. {1H−1H} COSY spectra were recorded on the Bruker 600 MHz spectrometer at 20 °C in CD3CN solutions with 1024 data points in the F1 dimension and 1024 data points in the F2 dimension with a delay time of 50 ms. An unshifted sine-bell-squared weighting function was applied prior to Fourier transformation followed by baseline correction in both dimensions and symmetrization. The UV−vis spectra (in acetonitrile) were recorded on a PerkinElmer Lambda 750S spectrophotometer for the 230−800 nm range. Excitation and emission spectra were recorded on a Jobin Yvon Fluorolog 3-22 spectrofluorometer with a 450 W xenon lamp double-grating monochromators and a TBX-04 photomultiplier. The solution measurements were carried out in a right angle configuration using degassed acetonitrile solutions of the samples in 10 mm quartz fluorescence. The solid state emission spectra were recorded by placing a uniform layer or powder between two quartz plates in the front-face configuration of the spectrofluorometer. X-ray Data Collection and Structure Determination. Diffraction data were collected in a Bruker Smart Apex diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The diffraction frames were integrated using the SAINT package46 and corrected for absorption with SADABS.47 Crystallographic data are shown in Table S4. The structures were solved by direct methods and refined anisotropically on F2.48 Hydrogen atoms were introduced in calculated positions. In complex 1, the highest residual peak in the refinement is 2.801 e/Å3. It is 1.522 Å from Cl(2); its origin is another oxygen position caused by the ClO4− disorder. Unfortunately, the remaining positions could not be located, and that peak was not introduced in the model. Computational Details. The theoretical study of the three nickel complexes was carried out using the DFT and TD-DFT methods with the Gaussian 09 suite of programs.49 As in our previous work on this type of complexes,25 the geometries of the triplet electronic ground states (T0) were optimized using the BP86 density functional50,51 and the bases LANL2DTZ+52 for nickel and cc-pVDZ53,54 for the rest of atoms. The optimizations were made in vacuo starting from the X-ray

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CONCLUSIONS We report the synthesis of three new Ni(II) complexes containing benzazole ligands and tren as ancillary ligand. The complexes have been structurally characterized, and their photophysical behavior has been studied. We carried out theoretical DFT simulations to understand their features. The simulations indicate that the structure of the absorption spectra is dominated by two well-defined transitions. The lowest energy HOMO → LUMO transition can be identified as an intraligand transition with a certain charge transfer from the lonely pairs of the phenolate to the π* orbital of the heterocycle. The model of Le Bahers et al. is used to quantify this charge transfer. We determine that the incorporation of a sulfur atom in the backbone on the benzazole ligand influences the emissive properties of its Ni(II) complex. In particular, complex 3 containing the ligand 2-(2′-hydroxyphenyl)benzothiazole exhibits both the smallest Stokes shift and the lowest quantum yield. 3670


Inorganic Chemistry

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structures. The UV−vis absorption spectra were in turn simulated using the TD-DFT method and the CAM-B3LYP functional,55 which is expressly designed to account for charge transfer transitions properly. The simulations were carried out using the in vacuo optimized geometries, and the effect of the solvent was accounted for by the polarizable continuum model with the integral equation formalism (IEF-PCM).56,57 We calculated specifically the first 100 triplet excited states, which extend up to about 6.8 eV, and used Gaussian functions with a width of 0.25 eV to widen the spectral lines. Besides the canonical molecular orbital (MO), the natural transition orbitals (NTO),58 which provide the best representations of the electronic excitation in terms of single particles, were employed to analyze the electronic transitions. Caution! Perchlorates are very dangerous. They should be handled in small quantities with great care. Synthesis. In separate experiments, KOH (0.410 mmol) and the corresponding benzazole (0.410 mmol) were dissolved in EtOH, obtaining a colorless solution. Furthermore, to a solution of Ni(ClO 4)2·6H2O (0.410 mmol) in EtOH was added tris(2aminoethyl)amine (tren) in a stoichiometric amount (0.410 mmol) that led to a light purple suspension. The colorless solution and the above suspension were mixed and stirred at room temperature for 30 min. Then the reaction mixture was filtered off to remove the byproduct (KClO4). The solvent of the filtrate was reduced until a final volume of 5 mL. By adding ethyl ether, a precipitate was obtained which was filtered, washed several times with ether (3 × 5 mL), and dried under reduced pressure. [Ni(pbm)(tren)]ClO4 [1] greenish solid. The yield was 126 mg (59.8%). Anal. Calcd for C19H27ClN6NiO5 (513.60): C, 44.43; H, 5.30; N, 16.36. Found: C, 44.76; H, 5.07; N, 16.43. ESI-MS (m/z): 413.5 [M+]. IR (Nujol): 3348 (νN−H), 3297 (νN−H), 1622 (νCN + δN−H), 1544 (νC−N), 1529 (νC−C), 1068 (νCl−Ot), 623 (νCl−Ot) cm−1. 1 H NMR (600 MHz; CD3CN): 103.0 (−CH− tren, 2H), 90.7 (−CH− tren, 2H), 80.1 (−CH− tren, 6H),74.1 (−CH− tren, 2H), 27.1 (H7, 1H), 26.2 (H8, 1H), 12.7 (H1, 1H), 10.4 (H2, 1H), 7.6 (H5, 1H), 6.8 (H6, 1H), 1.5 (H3, 1H), 1.1 (H4, 1H). [Ni(pbx)(tren)]ClO4 [2] yellowish solid. The yield was 131 mg (61.9%). Anal. Calcd for C19H26ClN5NiO6 (514.59): C, 44.35; H, 5.09; N, 13.61. Found: C, 44.40; H, 4.94; N, 13.64. ESI-MS (m/z): 515.5 [M + H]. IR (Nujol): 3350 (νN−H), 3298 (νN−H), 1613 (νCN + δN−H), 1550 (νC−N), 1520 (νC−C), 1079 (νCl−Ot), 624 (νCl−Ot) cm−1. 1 H NMR (600 MHz; CD3CN): 103.3 (−CH− tren, 4H), 80.6 (−CH− tren, 6H), 75.5 (−CH− tren, 2H), 27.7 (H8, 1H), 27.3 (H7, 1H), 13.1(H1, 1H), 10.0 (H2, 1H), 7.5 (H3, 1H), 6.7 (H6, 1H), 4.7 (H5, 1H), 3.0 (H4, 1H). [Ni(pbt)(tren)]ClO4 [3] yellow solid. The yield was 157 mg (72.2%). Anal. Calcd for C19H26ClN5NiO5S (530.65): C, 43.00; H, 4.94; N, 13.20; S, 6.04. Found: C, 43.25; H, 4.63; N, 13.06; S, 6.19. ESI-MS (m/z): 533.4 [M + 2H]. IR (Nujol): 3347 (νN−H), 3297 (νN−H), 1600 (νCN + δN−H), 1531 (νC−N + νC−C), 1083 (νCl−Ot), 623 (νCl−Ot) cm−1. 1H NMR (600 MHz; CD3CN): 137.6 (−CH− tren, 2H), 74.6 (−CH− tren, 6H), 72.5 (−CH− tren, 4H), 24.8 (H8, 1H), 23.1 (H7, 1H), 13.4 (H1, 1H), 9.1 (H2, 1H), 7.0 (H3, 1H), 6.2 (H5, 1H), 3.0 (H6, 1H), 1.3(H4, 1H).

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

M. Dolores Santana: 0000-0002-1446-8232 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Spanish Ministerio de Ciencia e Innovación under Projects CONSOLIDER CSD2009-00038, CTQ2015-64319-R, CTQ2015-67927-R (MINECO/FEDER), and CTQ2016-79345-P and by the Fundación Séneca del Centro de Coordinación de la Investigación de la Región de Murcia under Project 19419/ PI/14-2.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00059. Crystallographic data in CIF format; figures and tables of hydrogen bonds for complexes 1−3; IR spectra; 1H NMR spectra; {1H−1H} COSY spectra and emission spectra; details of the X-ray crystal structure determinations (PDF) 3671


Article

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