Electronic and Photophysical Properties of ReI(CO)3Br Complexes

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Article Cite This: ACS Omega 2019, 4, 4679−4690

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Electronic and Photophysical Properties of ReI(CO)3Br Complexes Modulated by Pyrazolyl−Pyridazine Ligands Marianela Saldías,†,∥ Nicolaś Guzmań ,† Franco Palominos,† Catalina Sandoval-Altamirano,† Germań Günther,‡ Nancy Pizarro,† and Andreś Vega*,†,§ †

ACS Omega 2019.4:4679-4690. Downloaded from pubs.acs.org by 178.159.100.209 on 03/07/19. For personal use only.

Departamento de Ciencias Químicas, Facultad de Ciencias Exactas, Universidad Andrés Bello, Av. Quillota 980, Viña del Mar 2531015, Chile ‡ Departamento de Química Orgánica y Fisicoquímica, Facultad de Ciencias Químicas y Farmacéuticas, Universidad de Chile, Sergio Livingstone 1007, Santiago 8380492, Chile § Centro para el Desarrollo de la Nanociencia y la Nanotecnología, CEDENNA, Santiago, Chile S Supporting Information *

ABSTRACT: The direct reaction of a series of substituted (1H-pyrazol-1yl)pyridazine (LI: 6-(1H-pyrazolyl)pyridazine; LII: 3-chloro-6-(1H-pyrazole-1-yl)-pyridazine; LIII: 6-(1H-3,5-dimethylpyrazolyl)pyridazine-3-carboxylic acid; LIV: 3,6-bis-N-pyrazolyl-pyridazine; and LV: 3,6-bis-N-3methylpyrazolyl-pyridazine) with the bromotricarbonyl(tetrahydrofuran)rhenium(I) dimer leads to the monometallic complexes [(LX)Re(CO)3Br] (I−V), which displays a nonregular octahedral geometry around the ReI center and a fac-isomerism for the carbonyl groups, whereas pyridazine and pyrazolyl rings remain highly coplanar after coordination to rhenium. Cyclic voltammetry shows one irreversible oxidation and one irreversible reduction for each compound as measured in N,N-dimethylformamide. Oxidation ranges from 0.94 V for III to 1.04 V for I and have been attributed to the ReI/ReII couple. In contrast, the reductions are ligand centered, ranging from −1.64 V for II to −1.90 V for III and V. Density functional theory calculations on the vertical one electron oxidized and one electron reduced species, using the gas-phase optimized geometry for the neutral complex confirm this assignment. Compounds I−V show two absorption bands, one around 410 nm (metal-to-ligand charge transfer (MLCT), Redπ → π*) and the other at ∼300 nm (intraligand, π → π*). Excitation at 400 nm at 77 K leads to unstructured and monoexponential emission with large Stokes shift, whose maxima vary between 570 (III) and 636 (II) nm. The quantum yields for these emissions in solution are intensified strongly going from air to argon equilibrated solution. Singlet oxygen quantum yields change from 0.03 (III) to 0.21 (IV). These data are consistent with emission from 3MLCT. The emission undergoes a bathochromic shift when R1 is a π-donating group (Cl or N-pyrazolyl) and a hypsochromic shift for a π-acceptor (COOH). The bimolecular emission quenching rate constant by triethylamine (TEA) for II, IV, and V is 1.09, 0.745, and 0.583 × 108 M−1 s−1, respectively. Photolysis in dichloromethane−CO2 saturated solution with TEA as a sacrificial electron donor leads in all cases to formic acid generation.

1. INTRODUCTION

(from water) or products stemming from the reduction of carbon dioxide. The proper light absorber must be a dye serving as an antenna,27 where the energy absorbed creates an electron−hole pair to be used for a given purpose.28 For any of these purposes, simple imines like pyrazolyl or triazolyl-pyridazine derivatives are appealing candidates to explore. Their planarity and limited conformational flexibility disfavor the nonradiative deactivation paths.29,30 From the synthetic point of view, they are also relatively easy to be prepared and modified. Examples of complexes with pyrazolyl−pyridazine ligands coordinated to metals belonging

Rhenium(I) diimine tricarbonyl complexes have attracted the attention of researchers during the past few decades because of their useful photophysical properties, well-behaved and predictable synthesis, stability, and potential applications.1−3 Potential applications ranges from photosensitization,4−8 anion sensing,9,10 biolabeling, and therapy11−15 to carbon dioxide photoreduction.5,16−23 Rhenium complexes with chelating diimines characteristically exhibit light absorption with moderate values of molar extinction coefficient in the visible to near UV region, resulting from metal-to-ligand charge transfer (MLCT) transitions.24−26 The excess of energy present in the excited states may be used for a wide range of objectives. Of particular interest is the use of excited states to drive reactions that result in storable fuels such as hydrogen © 2019 American Chemical Society

Received: November 27, 2018 Accepted: February 19, 2019 Published: March 4, 2019 4679

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and the pyridazine rings that coordinated to rhenium(I) remain highly coplanar, which is reflected by the low N1 N2C4N3 torsion angle: −5.2(9), −9.3(12), −4.3(12), 0.7(16), and −3.2(8)° for I, II, III, IV, and V respectively. For compounds IV and V, even the uncoordinated pyrazolyl ring remains coplanar to pyrazine, as reflected by the N4C7 N5N6 torsion angle, 1(2) and −172.8(6)°, respectively. These uncoordinated pyrazolyl rings show unambiguously an S-conformation. Co-planarity to the pyridazine is observed even for the carboxylic acid group in III, (N4C7C16 O1 torsion angle, −3.6(15)°). The structures clearly show the availability of a vacant coordination site in compounds IV and V, to connect to a second metal, opening an interesting challenge for synthetic chemists. 2.3. Electronic Structure and Electrochemical Properties. A thorough understanding of the chemical behavior requires a detailed knowledge of the molecular structure of the compounds, described above, and their electronic properties. Ground state electronic structures were explored by means of density functional theory (DFT) calculations, based on the optimized structures of the compounds. Table S2 shows a comparison of distances and angles for I−V between experimental X-ray determinations and DFT computed values. A close agreement is obtained. As normally found for diimine ReI carbonyls,60 the highest occupied molecular orbital (HOMO) is dominated by orbital density on Re(5d) orbitals, whereas the lowest unoccupied molecular orbital (LUMO) corresponds to π*diimine. Figure 2a,b show that this is exactly the case for I. Figure S1a−e show frontier orbitals in detail for the complete series I to V, respectively. To explore the redox behavior for the five ReI compounds, cyclic voltammograms were obtained using a glassy carbon electrode in nitrogensaturated N,N-dimethylformamide (DMF) solutions. Table 2 shows a summary of the redox parameters, whereas Figure S2a−e show voltammograms for each complex. A positive potential scan between +0.1 and +1.2 V at 100 mV s−1 displays a single and fully irreversible oxidation process for complexes I−V (Table 2, Ep,a; Figure S2a−e, right). The oxidation potential across the series of compounds varies from +0.935 V for III to +1.037 V for I. This oxidative wave around +1.0 V is assigned to the metal-centered one-electron oxidation couple ReI/ReII. This has been previously observed and reported for mononuclear ReI tricarbonyl complexes with multidentate nitrogen donor ligands.27,61−63 Spin-density diagrams computed by DFT by the vertical oxidation (Figure 2c) of I supports this interpretation. Cathodic voltammograms of complexes I−V show one reduction; quasi reversible for I (ΔV = 130 mV, one electron) and fully irreversible for II−V; waves between −1.64 V for II and −1.90 V for V (Figure S3a−e, left) (Table 2, Ep,c). The reduction process shows a ligand-centered nature and the significative shift of the peak of reduction modulated by different substituents in pyrazolyl−pyridazine rings are confirmed by the DFT computed vertical reduction of I (Figure 2d). Comparison with the literature supports that the reduction waves could be in all cases associated with the reduction of the pyrazolyl−pyridazine moiety.27 Additionally, complex III shows a second reduction wave at −1.90 V, consistent with a ReI/Re0 couple. This has been previously described for tricarbonyl ReI complexes with different diimine derivates.27,64 The values for the reduction potential show to be increasingly more negative going from II to V. This means

to any of the transition series have been previously described.31−53 Compounds having pyrazolyl or the related triazolyl-pyridazine ligands chelate to ReI(CO)3X (X = Cl or Br),38,54−56 and have shown the influence of the presence of substituents in the ligand on the compound properties, i.e., they can modulate their ability for being used as the photocatalyst.57,58 This effect can also be used for the design of efficient light-emitting materials.59 In the present work, we describe the synthesis, electronic and photophysical properties of a series of monometallic ReI complexes (see Scheme 1) Scheme 1. Pyrazolyl−Pyridazine ReI Series of Complexes

based on the pyrazolyl−pyridazine moiety with different substituents to explore the tuning of the photophysical properties and their potential use as ligands with a second metal site.

2. RESULTS AND DISCUSSION 2.1. IR and NMR Spectroscopies. The IR spectra of the pyrazolyl−pyridazine ligands, LI and LII, exhibit νC−H aromatic stretching bands at 3087−3011 and 3133−3150 cm−1, respectively, whereas for symmetric heterocyclic ligands, LIV and LV, these stretching bands appear at 3100 cm−1. νCN stretching bands for all ligands are observed around 1460− 1580 cm−1. The five complexes [(LX)Re(CO)3Br] exhibit carbonyl stretching frequencies around 2030−1900 cm−1, whereas C−H and CN stretching bands shift by approximately 20 cm−1 at a longer wavenumber with respect to free ligands. For all complexes, the 1H NMR spectra showed the presence of a single compound. In the cases of III and V, the signal due to the methyl protons of R2 and R3 groups of III and R1, R2 of V (Scheme 1) were also observed at approximately δ 3.0 ppm. It is important to note that the presence of the electronwithdrawing [BrRe(CO)3] moiety has a clear effect on the chemical shifts of pyrazolyl−pyridazine protons. The 13C NMR data showed, for all complexes, the carbon nuclei of the organometallic fragments, CN groups, and the carbonyl Re(CO)3 moiety to be low field around 160 and 200 ppm chemical shifts, respectively. 2.2. Structural Description. Figure 1I−V shows the molecular structure of I to V, respectively. Table 1 shows the most relevant bond distances and angles. The structure of the complexes can be described as a central ReI ion octahedrally surrounded by three carbonyl groups (in a fac geometry), one of the bidentate sites of each ligand, and a bromide anion. Along the series of compounds, the pyrazolyl 4680

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Figure 1. Molecular Structure diagram for I, II, III, IV, and V. Full numbering scheme included. Hydrogen atoms omitted for clarity except that from the carboxylic acid group in III. Displacement ellipsoids at 50%.

coefficients in the order of 103 M−1 cm−1, whereas the second band lies in the UV region (around 300 nm), showing extinction coefficients at least 1 order of magnitude higher (104 M−1 cm−1). This higher energy transition, which is also observed for the uncoordinated ligands and similar systems,66 would be assigned to an intraligand (IL) process with π → π* character (Figure S5). On the other hand, the band at lower energy (410 nm) would be assigned to a metal-to-ligand charge transfer (MLCT) transition.2,67 This assignment is consistent with computational results from time-dependent density functional theory (TD-DFT) shown in Figure 5 and Table S3 (detailed frontier orbital calculations for I−V are shown in Figure S1). For compound I, TD-DFT results show that the absorption at

R1: = H (I) to N-methylpyrazolyl (V) (Scheme 1). To define the influence of these substituents, we have compared the difference between Ep,c and Ep,a with the HOMO−LUMO gap as determined from DFT modeling (Figure 3). It is clear that the larger the computed gap the larger the difference between Ep,c and Ep,a. This kind of dependence has been reported for [Re2(μ-X)2(CO)6(1,2-diazine)] complexes.65 In practice, the net effect of each substituent reflects a subtle balance among inductive and π-effects, which influences the redox potential. 2.4. Photophysical Properties. Figure 4a shows the UV− vis absorption spectra for all studied ReI complexes in dichloromethane (DCM) solution at room temperature. The complete series of compounds presents two absorption bands. The first one centered at 410 nm, displaying extinction 4681

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Table 1. Selected Bond Distances (Å), Angles (°), and Torsion Angles (°) for Compounds I−V Re1C13 Re1C14 Re1C15 Re1N1 Re1N3 Re1Br1 C13Re1C14 C13Re1C15 C14Re1C15 C13Re1N1 C14Re1N1 C15Re1N1 C13Re1N3 C14Re1N3 C15Re1N3 N1Re1N3 C13Re1Br1 C14Re1Br1 C15Re1Br1 N1Re1Br1 N3Re1Br1 N1N2C4N3 N4C7C16O1 N4C7N5N6

I

II

III

IV

V

1.908(8) 1.918(8) 1.929(9) 2.164(6) 2.170(5) 2.6240(9) 88.4(3) 90.6(3) 91.2(3) 171.7(3) 99.1(3) 92.8(3) 98.1(3) 172.6(3) 92.3(3) 74.2(2) 91.6(2) 92.8(2) 175.5(2) 84.6(2) 83.5(2) −5.2(9)

1.933(5) 1.912(5) 1.922(5) 2.154(4) 2.162(4) 2.6402(8) 91.2(2) 90.7(2) 88.4(2) 169.4(2) 96.8(2) 96.4(2) 98.2(2) 170.5(2) 92.4(2) 73.7(1) 87.7(2) 93.1(1) 177.9(1) 84.9(1) 86.3(1) −8.6(6)

1.918(10) 1.919(10) 1.898(11) 2.153(7) 2.138(7) 2.6493(12) 88.9(4) 89.9(4) 91.2(5) 169.4(3) 101.7(3) 90.9(4) 96.1(3) 172.6(4) 94.4(4) 73.3(3) 92.9(3) 90.2(3) 176.8(3) 86.1(2) 84.0(2) −4.3(12) −3.6(15)

1.96(3) 1.99(2) 1.95(2) 2.146(13) 2.148(12) 2.633(2) 86.6(8) 90.1(8) 91.7(7) 170.6(7) 101.6(7) 92.9(6) 97.8(7) 173.7(6) 94.4(6) 73.6(5) 92.7(5) 90.2(5) 176.7(6) 84.8(3) 82.8(3) 1.9(2)

1.927 (4) 1.916 (4) 1.915 (4) 2.171 (3) 2.175 (3) 2.6219 (4) 89.72(16) 89.31(15) 89.03(16) 169.55(12) 99.91(14) 94.87(12) 96.31(13) 172.96(13) 94.67(12) 73.83(10) 91.84(11) 91.69(12) 178.65(10) 83.88(7) 84.50(7) −3.6(4)

0.3(2)

−172.7(3)

Figure 3. (Ep,a − Ep,c) values vs the computed HOMO−LUMO gap energies. Figure 2. Frontier Kohn−Sham orbitals ((a) HOMO and (b) LUMO) for I and spin density for the corresponding oxidized (I+•, (c)) and reduced (I−•, (d)) species.

complete series. As mentioned, TD-DFT also confirms a major π → π* character for the absorption band at higher energy. Figure S4 displays the TD-DFT computed spectra for I−V, showing a close correlation with the experimentally measured ones. The effect of the substituent bearing the pyrazolyl− pyridazine rings can be noticed in the energy level diagram shown in Figure 5, where a smaller HOMO−LUMO gap can be observed for II and III (R1 = Cl and COOH, respectively). Moreover, the frontier orbital calculations of ligands included in Figure S1a−e, demonstrate that R1-substitution modifies both the energy and the nature of the frontier orbitals. When complexes are excited at their lower energy band (λexc = 400 nm), the complete series exhibits a broad and structureless emission band centered at around 600 nm (Figure 4b), with large Stokes shifts compatible with a MLCT transition. As shown in Figure S5b, emission bands

Table 2. Cathodic and Anodic Peak Potentials for ReI Complexes I−V in DMF(N2-saturated) solutiona compound

Ep,c (V)

Ep,a (V)

I II III IV V

−1.65 −1.64 −1.75; −1.90 −1.83 −1.90

1.04 0.95 0.94 0.97 1.01

a

Values are reported in volts referred to the Fc+/Fc couple.

376 nm (f = 0.078) corresponds to an excitation from HOMO-4, a highly metal content orbital, to the LUMO, a ligand π*-orbital. Similar transitions are observed for the 4682

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absorption and emission maxima are dependent on the substituents bearing pyrazolyl−pyridazine rings. Replacing the hydrogen in R1 with different substituents promotes a bathochromic shift of the lower energy absorption band, behavior that could be mainly attributed to the electronic stabilization of the LUMO by the electron-inductive withdrawing effect. In addition, this band is also sensitive to solvent polarity, showing hypsochromic shifts in more polar solvents for all studied complexes (see Figure S6 and Table S4 in the Supporting Information), behavior usually found for MLCT transitions.69,70 On the other hand, the nature of R1 influences the position of emission maxima through the resonance electronic effect. The replacement of the H atom by chlorine or N-pyrazolyl substituents, π-donating groups, produces a bathochromic shift of the emission band for compounds II and IV, whereas the presence of a carboxylic moiety, electron acceptor, induces a considerable hypsochromic shift for III. Moreover, the emission at a longer wavelength and the lowest emission quantum yield determined for II can be related with the Energy gap Law.71 The solvent effect on the emission band is only observed for complexes I and III (see Figure S7). The slight increase in the emission quantum yields of deaerated solutions for transitions with MLCT character indicates some triplet multiplicity of the states involved.72 Indeed, complexes I−V were able to generate singlet oxygen, O2(1Δg), with moderate to good quantum yields (with values in the order of 0.03−0.21, see Table 3). The lowest values were observed for II and III, whose radiative deactivation paths do not show sensitivity to the presence of oxygen. Then, the excited states of these complexes have less triplet character or have shorter lifetimes. The highest capacity of singlet oxygen generation found for I, IV, and V, allows us to propose them for potential applications like photodynamic therapy, as it has been reported for other rhenium complexes.73,74 To clarify the nature of the excited states of these complexes, their quenching by triethylamine (TEA) was explored. Typical Stern−Volmer plots were obtained just for II, IV, and V, with KSV values from the slope of the linear fit of 8.89, 14.1, and 16.6 M−1, respectively (see Figure S8). The bimolecular rate constant for the dynamic emission quenching by TEA were 1.09 × 108 M−1 s−1 for II, 7.45 × 107 M−1 s−1 for IV, and 5.83 × 107 M−1 s−1 for V. These results support an electron-transfer mechanism for the quenching process, the excited state of II being the most susceptible to be deactivated by the electron donor TEA. Single exponential behavior of the time-resolved emission was found for all the complexes at room temperature in aerated or deaerated DCM solutions, with lifetimes ranging from 30 to 300 ns (see Table 3). Low temperature emission spectra are shown in Figure 6. The emission bands remain structureless, displaying an hypsochromic shift relative to room temperature measure-

Figure 4. a) Absorption and (b) emission spectra (λexc = 400 nm) for I−V in DCM aerated solutions at room temperature.

Figure 5. DFT computed energy level diagram for I−V.

of free ligands present their maxima between 375 and 460 nm upon excitation at 280 nm, with short Stokes shifts, as reported for π → π* IL transitions.68 The photophysical properties determined in DCM aerated or argon-saturated-solutions for the five complexes are summarized in Table 3. It can be observed that both the

Table 3. Summary of Photophysical Properties for Compounds I−V in DCM Solutions at Room Temperature and 77 K compound I II III IV V

λabs/nm (ε/103 M−1 cm−1) 390 412 412 400 390

(2.6) (4.2) (1.6) (1.5) (6.2)

λem

Φem,air

Φem,Ar

τair/ns

τAr/ns

τ77K/μs

ΦΔ

602 636 570 612 602

0.021 0.006 0.022 0.023 0.025

0.050 0.007 0.029 0.035 0.049

130.4 69.3 25.4 112.0 137.5

293.3 81.5 29.9 188.6 284.9

22.8 20.7 13.4 23.1 29.2

0.20 0.08 0.03 0.21 0.17

4683

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III are also in agreement with the lower singlet oxygen generation quantum yields (ΦΔ) determined for these complexes. Similar transient absorption shape has been reported for a parent complex with the pyrazolyl-pyrazine ligand, which exhibited a mixture of 3IL and 3MLCT excited state, displaying biexponential transient decays.64 In contrast, we discard the participation of transitions centered onto the ligands,66 based on the single exponential decays, and the transient lifetimes and the transient absorption maxima wavelengths were determined. A preliminary exploration of the potential capability of these complexes to reduce CO2, photolysis experiments of I−V in N2- and CO2-saturated DCM solutions were performed with TEA as a sacrificial electron donor at room temperature (Figure S10). Irradiation of the N2- and CO2-saturated DCM solutions with a blue-LED leads to different behaviors between the studied complexes. According our preliminary results, the more promising candidates as photocatalysts are complexes II and III, which after 1 h of irradiation under a CO2 atmosphere yielded significant amounts of formic acid/formate anions, species with typical signals at 1724 and 1606 cm−1 in IR experiments and that have been related to the photoreduction mechanism of carbon dioxide.20,21

Figure 6. Emission spectra (λexc = 400 nm) for I−V in MeOH/EtOH (1:4) at 77 K. Inset: Emission decays at 77 K.

ments, as expected for MLCT transitions affected by the rigidochromic effect.75,76 At this temperature (77 K), longer lifetimes ranging from 13 to 29 μs were found for the complete series (see inset in Figure 6 and Table 3). Transient absorption experiments in argon-saturated DCM solutions were performed for all studied compounds. Figure 7

3. CONCLUSIONS Monometallic ReI complexes derived from the 3,6-bis-Npyrazolyl-pyridazine ligands are available from direct reaction with the bromotricarbonyl(tetrahydrofuran)-rhenium(I) dimer. The complexes present a chelating bidentate site allowing coordination to a second metal center, making them interesting systems for the preparation of multimetallic complexes. Their electrochemistry and photophysical properties together with DFT modeling clearly show that the properties of the compounds can be modulated by systematically changing the substituents on the pyrazolyl−pyridazine skeleton. The experimental results show that the replacement of the hydrogen atom by acceptor groups decreases the HOMO−LUMO gap, whereas donors increase it across the series of compounds. In this way, despite the low emission quantum yields, at least three of the studied compounds show the capability of generating singlet oxygen in moderate yields, besides with the potential capability to act as photocatalysts. Then, if the ease of synthesis of this kind of ligand is considered, the family of complexes is very appealing for several potential applications, i.e., for photodynamic therapy due to the significant values of singlet oxygen generation quantum yields or for carbon dioxide photoreduction because of the photoreduction observed activity.

Figure 7. Transient absorption spectra (λexc = 355 nm) for I and II in argon-saturated DCM solution at 298 K.

shows transient absorption spectra of complexes I and II upon excitation at 355 nm. (The absorption spectra for all the series and their kinetic traces at the maximum absorption wavelength can be found in Figure S9). Broad transient absorption bands with maxima around 510 nm were observed for all complexes. Transient lifetimes, τT, summarized in Table 4, were coincident with the luminescent ones, reported in Table 3 for deaerated solutions (τAr). These results allow us to ascribe them to the triplet MLCT excited state (3MLCT). The shorter lifetime values found for II and

4. EXPERIMENTAL SECTION All reagents were used as received from the supplier (Aldrich), with no purification before use. Solvents: dichloromethane (DCM, UvaSol or SeccoSolv grade, Merck), chloroform (CHCl3, Analysis grade, EMSURE Merck), acetonitrile (MeCN, Analysis grade, EMSURE Merck), ethanol (EtOH, Analysis grade, EMSURE Merck), N,N-dimethylformamide (DMF, Uvasol grade, Merck), benzene (C6H6, Analysis grade, EMSURE Merck), and toluene (Analysis grade, EMSURE Merck), were employed as received. Standard Schlenk techniques were used for all manipulations. Ligand LIII (6(1H-3,5-dimethylpyrazolyl)pyridazine-3-carboxylic acid) and 6-(1H-pyrazolyl)pyridazine-3-carboxylic acid were purchased

Table 4. Transient Absorption Maxima and Transient Lifetimes for I−V in Argon-Saturated DCM Solutions compound

λT (nm)

τT (ns)

I II III IV V

510 540 500 510 505

298.0 79.8 32.0 188.0 290.0 4684

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whereas those at 127.79 and 120.22 ppm to pyridazine ring positions 3 and 6, respectively. The signal at 109.51 ppm correspond to pyrazolyl position 4. IR: νC−H(arom)3105, 3132, 3150, νCN 1583, 1521, 1456, and 1386 cm−1. 4.1.1.3. 3,6-Bis-N-pyrazolyl-pyridazine (LIV). Ligand LIV was prepared following a similar procedure described above for LII. Pyrazole (4.89 g, 0.072 mmol) was mixed with lithium (0.50 g, 0.072 mol) and then 5.36 g (0.036 mol) of 3,6dichloropyridazine. The product recrystallized as colorless microcrystals from a dichloromethane/chloroform mixture. Yield: 5.96 g, 78%. Calculated (experimental) for C10H8N6: C, 56.60 (57.4); H, 3.80 (4.07); N, 39.60 (40.0)). 1H NMR (400 MHz, CDCl3, δ(ppm)) δ 8.75 (d, J = 2.6 Hz, 2H) corresponding to two pyridazine protons, whereas to the lower chemical shifts of 8.37 (s, 2H), 7.83 (d, J = 0.8 Hz, 2H), 6.58−6.56 (m, 2H) were attributed to both pyrazolyl proton groups. 13C NMR (101 MHz, CDCl3, δ(ppm)) δ 154.11 was assigned to positions 3 and 6 within the pyridazine ring. Signal at 121.16 corresponds to pyridazine positions 4 and 5. Pyrazolyl carbons appear at 143.68, 127.90, and 109.71 ppm for positions 3, 4, and 5 respectively. IR: νC−H(arom) 3101, νCN 1457, and 1375 cm−1. 4.1.1.4. 3,6-Bis-N-3-methylpyrazolyl-pyridazine (LV). For LV, a procedure similar to that described for LII was used. A solution of 3-methylpyrazole (5.00 g, 0.061 mol) in 100 mL of tetrahydrofuran was mixed with lithium (0.42 g, 0. 060 mol) and stirred at 70 °C until the metal dissolved after 3 h. To this solution was added a solution of 4.53 g (0.0304 mol) of 3,6dichloropyridazine in 100 mL of tetrahydrofuran. The resulting mixture was stirred at 70 °C for 72 h and finally the solvent was then removed at reduced pressure. Colorless rectangular crystals were obtained from the dichloromethane/hexane mixture (5.90 g, 82%). Calculated (experimental) for C12H12N6: C, 59.99 (59.44); H, 5.03 (4.86); N, 34.98 (35.57)%. 1H NMR (400 MHz, CDCl3, δ(ppm)) δ 8.29 (s, 2H) was assigned to two pyridazine protons, whereas 7.98 (t, J = 6.9 Hz, 2H) and 6.04 (s, 2H) peaks were associated with pyrazolyl moiety protons, and finally at 2.18 ppm (s, 6H) one singlet corresponding to methyl protons was observed. 13C NMR spectrum (101 MHz, CDCl3, δ(ppm)) shows two peaks at δ 153.10 and 152.77 ppm, which correspond to pyridazine positions 3/6 and 4/5, respectively. Signals at 127.92, 120.23, and 109.39 correspond to pyrazolyl positions 3, 4, and 5 respectively. Finally, methyl carbon appears at 13.91 ppm. IR: νC−H(arom) 3100, νCN 1537, and 1443 cm−1. 4.1.2. Complexes. The ReI complexes I−V were prepared following a previously described procedure,26 by the direct reaction of each ligand with the bromotricarbonyl(tetrahydrofuran)-rhenium(I) dimer, as shown in Scheme 3. 4.1.2.1. [(LI)Re(CO)3Br] (I). The monometallic compound was obtained by the direct reaction of the LI ligand (0.352 g, 1.85 mmol) with the bromotricarbonyl(tetrahydrofuran)rhenium(I) dimer (0.781 g, 9.25 mmol), at room temperature in toluene solution. A yellow microcrystalline product was instantly obtained. The yellow solid was dissolved in acetone and slow evaporation lead to yellow crystals corresponding to the mononuclear complex, as shown in Scheme 2. Yield: 94%. Calculated (experimental) for C10H6N4O3BrRe: C, 24.2 (24.6); H, 1.22 (1.62); N, 11.3 (10.4)%. For I, the 1H NMR spectra (400 MHz, DMSO, δ(ppm)) shows at δ 9.12 (d, J = 9.2 Hz, 1H), 8.77 (d, J = 9.2 Hz, 1H), and 6.91 (d, J = 19.9 Hz, 1H) chemical shifts assigned to pyridazine protons, moreover the peaks from pyrazolyl protons were assigned to 8.59 (d, J =

from Life Chemical Corp. The ligands LII (3-chloro-6-(1Hpyrazole-1-yl)-pyridazine), LIV (3,6-bis-N-pyrazolyl-pyridazine), and LV (3,6-bis-N-3-methylpyrazolyl-pyridazine) were prepared from the corresponding pyrazole and 3,6-dichloropyridazine by following slightly modified literature methods.77 4.1. Syntheses. 4.1.1. Ligands. 4.1.1.1. 6-(1H-Pyrazolyl)pyridazine (LI). The soft warming of 6-(1H-pyrazolyl)pyridazine-3-carboxylic acid in toluene solution lead to its decarboxylation, as depicted in Scheme 2. A solution of 6-(1HScheme 2. Synthetic Route to Ligand LI

pyrazolyl)pyridazine-3-carboxylic acid in toluene was stirred at 60 °C for 3 h to produce the decarboxylation reaction. After a volume reduction of 75% at reduced pressure, the solution was allowed to stand for one day at 5 °C. A colorless solid was obtained, then filtered, and finally dried at reduced pressure. Yield: 31%. Calculated (experimental) for C7H6N4: C, 57.53 (58.2); H, 4.14 (4.27); N, 38.34 (36.5). 1H NMR (400 MHz, dimethyl sulfoxide (DMSO), δ(ppm)) δ 9.20 (dd, J = 4.7, 1.1 Hz, 1H), 8.83 (d, J = 2.6 Hz, 1H) (d, 1H), and 7.95 (m, 1H), peaks corresponding to pyridazine moiety protons. Chemical shifts at 8.22 (d, J = 8.9 Hz, 1H), 7.92 (m, 1H), and 6.68 (m, 1H) ppm were assigned to pyrazolyl protons. 13C NMR (101 MHz, DMSO) with δ 153.96 and 150.60 ppm were assigned to 3,6-pyridazine CN carbon nuclei and 127.31 and 117.20 peaks to pyridazine positions 4 and 5, respectively. Signals at 143.03 and 129.87 ppm corresponds to pyrazolyl carbons 3 and 5, respectively. Finally, at 108.99 ppm, the remaining carbon (4-pyrazolyl) nucleus from this ring. IR: νC−H(arom) 3147, 3087 νCN 1580, 1521, and 1461 cm−1. 4.1.1.2. 3-Chloro-6-(1H-pyrazole-1-yl)-pyridazine (LII). A solution of pyrazole (4.94 g, 0.072 mol) in 100 mL of tetrahydrofuran was mixed with lithium (0.50 g, 0.072 mol) and stirred at 70 °C until the metal dissolved after 3 h. To this solution was added a solution of 10.7 g (0.072 mol) of 3,6dichloropyridazine in 100 mL of tetrahydrofuran. The resulting mixture was stirred at 70 °C for 24 h. The solvent was then removed at reduced pressure. The crude was dissolved in 150 mL of dichloromethane and washed with 250 mL of water. The dichloromethane solution was dried with magnesium sulfate overnight, and the solvent was then removed under reduced pressure. The product was recrystallized from an acetone/dichloromethane mixture to form colorless microcrystals. Yield: 4.68 g, 37%. Calculated (experimental) for C7N4H5Cl C, 48.56 (49.4) H, 2.79 (3.01); N, 31.02 (32.0)%. LII shows a 1H and 13C NMR similar spectra than LI, however, the peaks shift slightly due to R1: chloride effect (400 MHz, DMSO, δ(ppm)) then 8.73 (d, J = 2.7 Hz, 1H), and 8.21 (d, J = 9.2 Hz, 1H) were assigned to pyridazine protons and chemical shifts 7.81 (s, 1H), 7.62 (d, J = 9.2 Hz, 1H), and 6.58−6.52 (m, 1H) come from the pyrazolyl fragment. 13C NMR (101 MHz, DMSO) spectra shows at δ 154.59, 152.0 ppm (CN, pyridazine) and (NCH−Cl, pyridazine) carbon nuclei, respectively. Signals at 143.65 and 130.70 ppm were assigned to CN carbon from the pyrazolyl ring, 4685

DOI: 10.1021/acsomega.8b03306 ACS Omega 2019, 4, 4679−4690

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4.1.2.4. [(LIV)Re(CO)3Br] (IV). The direct reaction of the ligand for IV (0.238 g, 1.125 mmol) with the bromotricarbonyl(tetrahydrofuran)-rhenium(I) dimer (0.951 g, 1.125 mmol) at room temperature in toluene solution. A red microcrystalline product was instantly obtained. Warming the red solid in acetonitrile easily generates a yellow solution corresponding to IV. Slow evaporation of the solvent at room temperature leads to yellow crystals of IV [+ solvent]. Yield: 82.7%. Calculated for nonsolvated formula (experimental) for C13H8N6O3BrRe (IV): C, 27.77 (27.75); H, 1.43 (1.34); N, 14.94 (14.46). 1H NMR (400 MHz, DMSO, δ(ppm)) is consistent with chemical shifts expected for LIV coordinated, thus δ 9.28 (d, J = 2.6 Hz, 1H), 8.97 (d, J = 9.6 Hz, 1H) signals come from the pyridazine ring, whereas pyrazolyl groups present at 8.76 (d, J = 9.6 Hz, 1H), 8.59 (d, J = 2.0 Hz, 1H), 8.48 (s, 1H), 7.99 (s, 1H), 6.98 (m, 1H), and 6.75 (m, 1H) ppm. 13C NMR (101 MHz, DMSO, δ(ppm)) signal at 196.31, 196.11, and 188.90 from three carbonyl groups. 152.66, 152.13 ppm (CN, 3,6-pyridazine carbons) and 146.92, 144.29, 132.86, 127.37 ppm for CN carbon nuclei from two different pyrazolyl groups at 123.40 and 122.78 ppm corresponding to the 4,5-pyridazine position, whereas at 112.01 and 110.45 ppm were attributed to −CC− carbon from two different pyrazolyl rings. IR: νC−H(arom) 3132, 3091, νCN 1652 and 1577 cm−1, νCO 2033, 1938, 1906 cm−1. 4.1.2.5. [(LV)Re(CO)3Br] (V). The direct reaction of the ligand for V (0.271 g, 1.125 mmol) with the bromotricarbonyl(tetrahydrofuran)-rhenium(I) dimer (0.951 g, 1.125 mmol) at room temperature in toluene solution. A red microcrystalline product was obtained, whose warming in acetonitrile leads to a yellow solution. Slow evaporation of the solvent at room temperature leads to yellow crystals of V [+ solvent]. Yield: 81.5%. Calculated for nonsolvated formula (experimental) for C15H12N6O3BrRe (V): C, 30.51 (29.98); H, 2.06 (1.89); N, 14.23 (13.72)%. 1H NMR (400 MHz, DMSO) signals at δ 9.15 (d, J = 2.9 Hz, 1H) and 8.91 (d, J = 9.6 Hz, 1H) come from pyridazine protons and the pyrazolyl and methyl protons were assigned at 8.64 (d, J = 9.6 Hz, 1H), 8.49 (d, J = 2.5 Hz, 1H), 6.83 (d, J = 2.9 Hz, 1H), 6.54 (d, J = 2.5 Hz, 1H), 2.61 (s, 3H), 2.37 (s, 3H) peaks. 13C NMR (101 MHz, DMSO) presents carbonyl groups at 196.02, 195.52, and 187.59 ppm. Pyridazine ring signals at 155.42 ppm (position 3), 122.20 ppm (position 4), 121.16 ppm (position 5), and 152.63 ppm (position 6). Pyrazolyl signals at 151.54 and 150.28 ppm (position 3 and 3′), 110.80 and 109.93 ppm (position 4 and 4′), 132.19 and 127.08 ppm (position 5 and 5′). Finally, methyl group signals at 14.74 and 13.00 ppm. IR: νC−H(arom) 3109, 3093, νCN 1652 and 1577 cm−1, νCO 2030, 1934, 1903 cm−1. 4.2. Cyclic Voltammetry. Cyclic voltammograms at room temperature for I−V complexes and LI−LV ligands were recorded in DMF solutions (1.0 mM) using tetrabutylammonium perchlorate (0.10 M) as the supporting electrolyte. Cyclic voltammograms were recorded at 100 mV s−1 between +1.4 and −2.3 V. Before runs, the sample solutions were deoxygenated by bubbling nitrogen for 15 min. A vitreous carbon electrode was used as the working electrode, a platinum electrode as the auxiliary electrode, and an Ag/AgCl electrode the reference electrode. Ferrocene was used as an internal standard (E1/2 = 0.195 V; ΔE = 0.170 V). 4.3. Structural Determination. The crystal structures of I−V were determined by X-ray diffraction at 273 K, using a SMART-APEX II CCD diffractometer system. Data reduction

Scheme 3. Synthetic Route to Compounds I−V

9.2 Hz, 1H), 8.43 (s, 1H), and 8.16 (dd, J = 9.0, 4.8 Hz, 1H), all of them being slightly shifted in 0.8 ppm toward a higher field with respect to the uncoordinated ligand. 13C NMR spectra (101 MHz, DMSO, δ(ppm)) shows three small signals at low field at 197.22, 197.16, and 196.98 ppm corresponding to the carbonyl groups. Signals at 163.74 and 154.34 ppm correspond to pyridazine ring positions 3 and 6 respectively, whereas positions 4 and 5 appear at 133.84 and 120.71 ppm, respectively. Signals at 152.67, 113.09, and 148.02 for positions 3, 4, and 5 of pyrazolyl, respectively. IR: νC−H(arom) 3164, 3046 νCN 1725, 1589 and νCO 2030, 1934, 1905 cm−1. 4.1.2.2. [(LII)Re(CO)3Br] (II). A similar procedure to that described for I yielded yellow microcrystals (5.90 g, 53%). Calculated (experimental) for C10H5N4O3BrReCl: C, 22.63 (23.08); H, 0.95 (0.97); N, 10.56 (10.38)%. 1H NMR spectra (400 MHz, DMSO, δ(ppm)) presents the predicted signals from two pyridazine protons at δ 9.27 (t, J = 7.8 Hz, 1H) and 8.90 (d, J = 9.5 Hz, 1H). At 8.58 (dd, J = 11.5, 5.7 Hz, 2H), we found two nearly doublets and one triplet corresponding to pyrazolyl protons (t, 7.09−7.02, 1H). As well as in I, the 13C NMR (101 MHz, DMSO, δ(ppm))) shows 3 peaks assigned to carbonyl groups at 199.39, 196.58, and 196.28 ppm. Signals at 154.16 ppm (CN, pyridazine), 152.57 ppm (NC−Cl, pyridazine), 147.88 ppm (CN, pyrazolyl), 147.01 ppm (C N, pyrazolyl), 133.79, and 122.91 ppm (CC, pyridazine), and 112.80 (CC, pyrazolyl) were attributed to ligands. In general, the chloride electron-withdrawing effect over all of peaks was observed. Their chemical shifts appear at lower field compared to I (when R1 = H). IR: νC−H(arom) 3122, 3066 νCN 2025, 1475, 1398 and νCO 2033, 1940, 1909 cm−1. 4.1.2.3. [(LIII)Re(CO)3Br] (III). The direct reaction of 6-(1H3,5-dimethylpyrazolyl)pyridazine-3-carboxylic acid (LIII) (352 mg, 1.850 mmol) with the bromotricarbonyl(tetrahydrofuran)rhenium(I) dimer (780 mg, 0.925 mmol) at room temperature in N,N-dimethylformamide solution. Yield: 86%. Calculated (experimental) for C13H10N4O5BrRe: 27.47 (26.4); H, 1.77 (1.97); N, 9.86 (9.47)%. 1H NMR (400 MHz, DMSO) shows the predicted signals such as δ 8.58 (s, 1H) and 8.55 (dd, J = 28.2, 9.5 Hz, 1H) from the pyridazine group, 6.73 (s, 1H), 2.84 (s, 3H), and 2.54 (s, 3H) from pyrazolyl hydrogen nucleus with two methyl groups. 13C NMR (101 MHz, DMSO) presents around δ 206.64 a single weak peak from carbonyl groups. Pyridazine peaks at 162.85 (CO2H), 155.88 ppm (position 3), 132.36 ppm (position 4), 120.79 ppm (position 5), and 150.29 ppm (position 6). Pyrazolyl peaks at 154.27 ppm (position 3), 113.87 ppm (position 4), and 146.95 ppm (position 5). Methyl groups appear at 16.08 and 14.44 ppm. IR: νC−H(arom) 3099, 3066 νCN 1739, 1589 and νCO 2025, 1928, 1900 cm−1. 4686

DOI: 10.1021/acsomega.8b03306 ACS Omega 2019, 4, 4679−4690

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τo = 1 + KSV[TEA] τ

with SAINT.78 Multi-scan absorption corrections were applied using SADABS.79 Structure solution by direct methods, completion by Difference Fourier Synthesis and refinement by least-squares using SHELXL.8,80,81 The hydrogen atoms positions were calculated after each cycle of refinement with SHELXL using a riding model for each structure, with CH distance of 0.96 Å. Uiso(H) values were set equal to 1.2 Ueq of the parent carbon atom. Despite our efforts, no completely satisfactory model for solvent was found for compounds IV and V. At this point, we choose to use SQUEEZE,82,83 a welldocumented method for the modeling of unresolved electron density, to consider the effect of the disordered solvent. The number of electrons suggested by SQUEEZE in addition to the experimental data, suggest 12 acetonitrile molecules per unit cell for IV and one of acetone for V. It is important to note that the use of SQUEEZE almost does not affect the structural parameters of both molecules. Table S1 shows a summary of structural and refinement details for I−V. 4.4. Spectroscopic and Photophysical Measurements. The infrared spectra (in the range 4000−400 cm−1) were measured using a Jasco FTIR-4600 spectrophotometer equipped with an ATR PRO ONE. 1H NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer at 298 K, using CDCl3 as the solvent. UV−vis spectra were recorded on an Agilent 8453 Diode-Array spectrophotometer in the range of 250−700 nm in aerated solvent solutions at room temperature. Emission spectra were measured in a Horiba Jobin-Yvon FluoroMax-4 spectrofluorometer in different solvents at room temperature or in ethanol−methanol glass (4:1, v/v) at 77 K. Luminescence lifetime measurements were carried out with the time correlated single photon counting technique using a PicoQuant FluoTime300 fluorescence lifetime spectrometer. A sub-nanosecond Pulsed Laser 405 nm was employed as pulsed light sources (full width at halfmaximum (FWHM) ∼500 ps; average power 2 mW). Timeresolved experiments were made in solution either airequilibrated or argon-saturated. Emission quantum yields (Φem) were measured using the procedures described in the literature with [Ru(bpy)3](PF6)2 in acetonitrile solution as the actinometer.84,85 Singlet oxygen, O2(1Δg) emission measurements were carried out with a FluoTime 200 equipped with a NanoHarp 200 multichannel scaler. Excitation at 355 nm was achieved with a laser FTSS355-Q3 (Crystal Laser, Berlin, Germany) working at 1 kHz repetition rate. For the detection at 1270 nm a NIR PMT H10330A (Hamamatsu) was employed. The O 2 ( 1 Δ g ) quantum yields (Φ Δ ) were determined by comparing the intensity at zero time of the 1270 nm signals to those of optically-matched solutions of phenalenone as a ref 86. Nanosecond laser flash photolysis experiments were performed on nitrogen-saturated dichloromethane solutions by exciting at 420 nm and obtaining absorption spectra by collecting decays at 10 nm intervals between 350 and 750 nm. An Applied Photophysics LKS 60 optical system was used with 420 nm excitation light (10 mJ/ pulse;