Cationic Bis(cyclometalated) Ir(III) Complexes with PyridineâCarbene

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Cationic Bis(cyclometalated) Ir(III) Complexes with Pyridine−Carbene Ligands. Photophysical Properties and Photocatalytic Hydrogen Production from Water Javier Torres,⊥ M. Carmen Carrión,*,⊥,† Jorge Leal,⊥ Félix A. Jalón,⊥ José V. Cuevas,§ Ana M. Rodríguez,¶ Gregorio Castañeda,∥ and Blanca R. Manzano*,⊥ ⊥

Departamento de Química Inorgánica, Orgánica y Bioquímica, Facultad de Ciencias y Tecnologías Químicas-IRICA and Departamento de Química Analítica y Tecnología de los Alimentos, Universidad de Castilla-La Mancha, Avda. Camilo J. Cela 10, 13071 Ciudad Real, Spain † Fundación Parque Científico y Tecnológico de Castilla-La Mancha, Bulevar Rio Alberche s/n, 45007 Toledo, Spain § Departamento de Química, Facultad de Ciencias, Universidad de Burgos, Pza. Misael Bañuelos s/n, 09001 Burgos, Spain ¶ Escuela Superior de Ingenieros Industriales, Avda, C. J. Cela, 3, 13071 Ciudad Real, Spain ∥

S Supporting Information *

ABSTRACT: Precursors of chelate pyridine−N-heterocyclic carbene (N^C:) ligands with methyl- or benzyl-substituted imidazolylidene fragments were synthesized. They were used to obtain 12 iridium bis-cyclometalated complexes of the type [Ir(C^N)2(N^C:)]+ (C^N = 2-(phenyl)pyridinato-C2,N, ppy, 2(4,6-difluorophenyl)pyridinato-C2,N, dfppy). The ancillary N^C: ligands contain different structural modifications. The aim of the work was to analyze the effect that changes in the two types of ligands have on the photophysical and electrochemical properties and also on the behavior of these materials as photosensitizers. The X-ray crystal structures of five complexes were determined. The complexes emitted in the blue-green region. It was expected that the frontier orbitals and thus the photophysical and electrochemical properties would be controlled by the main C^N ligands, and it was demonstrated that the effect of the modifications in the N^C: ligand, especially the presence of a nitro group in the pyridine ring or the interruption of conjugation between the two rings, also affected these properties. The quenching with O2 and photostability studies were also performed. Density functional theory calculations were used to explain the behavior of some derivatives. The complexes and other previously reported compounds were employed as photosensitizers (PS) in preliminary studies on the production of H2 from water using [Co(bpy)3]Cl2 (bpy = 2,2′-bipyridine) as catalyst and triethanolamine (TEOA) as the sacrificial reductant. The absence of quenching of the PS with TEOA allowed us to propose an oxidative quenching mechanism.

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(LFSE) and less thermally accessible 3MC states than ruthenium derivatives, usually exhibit relatively long-lived (microsecond time scale) and often highly luminescent triplet excited states, and they have allowed broad color tuning by means of ligand modification.7,14−17 In the widely used complexes of the type [Ir(C^N)2(N^N)]+ the emission originates from a metal-to-ligand charge transfer (MLCT) triplet state that mainly involves the metal center and the ancillary N^N ligand.1,16,18−23 Recently, Ortı ́ and coworkers24,25 reported iridium complexes in which the N^N ligand is substituted with the strong σ-donor N-heterocyclic carbenes (NHCs)26,27 of the type N^C: with the main aim of

INTRODUCTION

Luminescent transition-metal complexes have attracted significant interest in the scientific community because of their great potential in a range of photonic applications.1,2 For example, a range of complexes can be used as emissive dopants in organic light-emitting devices (OLEDs),3−7 light-emitting electrochemical cells (LECs) based on ionic transition-metal complexes (iTMCs),8 oxygen detection,9 photovoltaic cells,10,11 and as biological labeling reagents.1,12 Early work was mainly focused on complexes of the type [Ru(bpy)3]2+ (bpy = 2,2′bipyridine), but the thermal population of a nonemissive metalcentered state (3MC) rendered limited color-tuning ability.13 Among the different complexes, special attention has been paid to cyclometalated Ir(III) species.1 These iridium(III) complexes, which have a high ligand-field stabilization energy © XXXX American Chemical Society

Received: September 5, 2017

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DOI: 10.1021/acs.inorgchem.7b02289 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Chart 1. Ligands Used in This Work

Scheme 1. Iridium Complexes Synthesized in This Work

an electron relay (ER) is required to allow the electron transfer from the PS to the catalyst. Catalyst based on Pd,42,43 Pt,44−46 Rh,47−50 Co,51−57 Re,57 and bioinspired Fe−Fe derivatives58,59 has been described as WRC. In terms of the PS, early60−63 and also recent reports49,50,53,54,57,58 concerned the successful use of [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine). Other chromophores such as Zn(II) porphyrins64−66 or cyclometalated Pt(II) complexes46,67,68 have also been described to have moderate performance. Bernhard and co-workers reported pioneering work with excellent results concerning the use of [Ir(C^N)2(N^N)]+15,52,69,70 or [IrCl(C^N)(N^N^N)]+ complexes71 as the PS.52,72,73 [Cp*Ir] derivatives have also been used.48 The photosensitizer and its efficient cooperation with the catalysts is perhaps the most important part of the system. A clear correlation between the photophysical properties and the behavior as photosensitizers has not been found.47,69,70 However, in principle the favorable characteristics of a PS, besides photostability, are a strong visible absorption and efficient electron transfer to the catalyst, a fact that should be favored by sufficiently long excited lifetimes. We report here the synthesis of two families of iridium complexes of the type [Ir(C^N)2(N^C:)]+ (C^N = phenylpyridinato ligands, N^C: = pyridine-NHC ligands) with the aim of evaluating if it was possible to tune the photophysical and electrochemical properties not only by modifying the C^N ligand as described in the literature but also after changing the N^C: ligand. In addition to the incorporation of F atoms in the C^N ligands, we introduced different types of substituent in the N^C: ligand, including methylene bridges to interrupt the πconjugation of the chelate 74,75 or a nitro substituent. Considering that this type of carbene complex, which potentially has high stability, has not been tested in the

obtaining deep blue emitters that are necessary for LECs. Other authors have reported relatively similar complexes.28−31 In these complexes, the very high energy of the lowest unoccupied molecular orbital (LUMO) centered on the NHC ligand led to a predominantly ligand-centered (LC) emission from the cyclometalated ligands, and tuning of the emission was achieved by modification of these C^N ligands. Changes on the N^C: ligand did not affect the photophysical properties, but modifications as the interruption of the π-conjugation of the chelate or the introduction of a nitro substituent were not reported. The nitro group is widely recognized as a strong quencher of fluorescent dyes,32 and its electron-withdrawing character leads to stabilization of the orbitals in which it participates. A very weak luminescence was obtained in iridium complexes with cyclometalating ligands such as 2-(phenyl)pyridinato-C2,N (ppy)33 or 1-(phenyl)pyrazolate with nitro substituents in the phenyl ring34 or with the ancillary ligand 5-nitro-8-hydroxyquinolate.35 However, relatively high quantum yields were found in substituted ppy36 or difluoropyridinato derivatives37 when the nitro group was present in the pyridine ring. Aoki and co-workers38 studied (phenyl)isoquinoline and ppy iridium derivatives and deduced by computational studies that quenching of the emission occurred when the LUMO orbital was localized on the nitro groups of the molecule. A very interesting and challenging application of luminescent transition-metal complexes is the catalytic photoreduction of water.39−41 The generation of hydrogen using sunlight is a way of harnessing solar energy without using nonrenewable resources. Typical systems for photocatalytic hydrogen production consist of a photosensitizer (PS), a water-reduction catalyst (WRC), and a sacrificial reductant (SR). In some cases B


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Inorganic Chemistry

with a benzyl (bz) group in the N^C: ligand a π−π interaction80 is established between the phenyl ring of this group and a ring of one cyclometalating ligand (9, 11 with one difluorophenyl ring and 12 with one pyridine fragment; see Figure 1), while this is not the case for complex 4. We decided

photoproduction of hydrogen from water, we also decided to perform preliminary studies of their behavior in this catalytic reaction and to analyze their photostability and their interaction with the SR and the catalyst. It was of interest to verify whether the nitro group would lead to quenching of the emission and to assess the possible influence of this change on the behavior of the material as a photosensitizer.

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RESULTS AND DISCUSSION Several cyclometalated Ir(III) complexes of formula [Ir(C^N)2(N^C:)]X (C^N = 2-(phenyl)pyridinato-C2,N, ppy, 2(4,6-difluorophenyl)pyridinato-C2,N, dfppy) with different NHC ligands were synthesized (N^C: = pyridine carbene ligands). The precursors of the N^C: ligands are shown in Chart 1. All of these compounds contain a pyridine ring bonded to an imidazolium fragment that bears a methyl or benzyl substituent. Methyl or nitro substituents were introduced into the pyridine ring, and in two cases a methylene group is present between the two heterocycles. The compounds L1H-5NO2, L2H-5Me, L2H-4Me, and L2H5NO2 are described in this work for the first time. All ligand precursors, except those containing nitro groups, were prepared by the previous formation of a 2(imidazolyl)pyridine derivative and subsequent functionalization with an alkyl halide.76,77 L1H-5NO2 and L2H-5NO2 were obtained by the reaction of the corresponding 1-substituted imidazolium salt and 2-bromo-pyridine, a process that requires high temperatures78,79 (see Supporting Information). The iridium complexes were synthesized by a splitting reaction of the corresponding chloro-bridged dinuclear derivatives with the silver carbene species formed by treatment of the precursors of the N^C: ligands with Ag2O according to the method reported for similar complexes.25 The subsequent addition of KPF6 led to anion interchange (see Scheme 1). Products I−IV have been reported previously.25,28,29 The new products were characterized by elemental analysis, mass spectrometry, and IR and NMR (1H, 13C{1H}, and 19F where applicable) spectroscopies. In some cases, the solid-state structure was determined by X-ray diffraction (complexes 4, 8, 9, 11, and 12). As one would expect, all of the rings appear different in the 1 H and 13C{1H} NMR spectra. Although a large number of resonances appear in the aromatic region, and assignment of the peaks is not reported in the literature for similar complexes, we were able to assign the majority of the signals, a fact that may be useful in the characterization of similar new complexes that may be synthesized. The asymmetry of the molecules makes the methylene protons of L3 and L4 ligands and those of the benzyl groups diastereotopic in all derivatives. The chemical shift of the methylene protons of the benzyl groups warrants further comment. In all complexes with “L2” type of ligands, one of these protons appears in the narrow range of 4.71−4.73 ppm, while the chemical shift of the other proton depends on the presence of phenyl (4.83−4.86 ppm) or difluorophenyl (4.99−5.03 ppm) rings on the cyclometalated ligands. Clearly, this second proton is oriented toward one of these main ligands. In the 19F NMR spectra of complexes 8−12 two quadruplets (assigned to F5 atoms) and two triplets (assigned to F3 atoms) are observed. These signals show the presence of two different difluorophenyl rings, and 3JH−F and 4JF−F have similar values. As detailed in the following section on solid structure determination by X-ray diffraction, in the case of complexes

Figure 1. Molecular structure of 11. The red and purple dotted lines represent the π−π and anion−π interactions, respectively. Hydrogen atoms were omitted for clarity.

to determine whether this interaction is, at least partially, maintained in solution.81 Comparison of the resonances of the phenyl group of the benzyl fragment of these four complexes shows a difference in the ortho protons of complex 4 (6.35 ppm) with respect those of the complexes with the π−π interaction (6.21−6.26 ppm). The shielding of the protons in the latter case must be due to effect of the ring current anisotropy of the aromatic ring that participates in the aforementioned interaction. The fact that the two ortho or meta protons give rise to a single signal means that the ring is freely rotating, but the effect on the chemical shift indicates that the conformation that allows the π−π interaction is more populated. The proton resonances of the difluorophenyl rings of complexes 9 and 11 were also analyzed and compared with those of complexes without the π−π interaction (containing “L1” type of ligands). A shielding (0.11−0.21 ppm) is observed for the protons of one of the difluorophenyl rings (the moreshielded H4 and the less-shielded H6 resonances, which belong to the same ring) again indicating maintenance to some extent of the aforementioned interaction with one of the cyclometalating ligands. X-ray Crystal Structures. The X-ray crystal structures of complexes 4, 8, 9, 11, and 12 were determined from the appropriate single crystals. The crystallographic data, the ORTEP82 representations (Figures S1−S5), and detailed information are provided in the Supporting Information. In all of the complexes the iridium center has a distorted octahedral geometry. The cyclometalated ligands, in a similar way to those found in other phenyl-pyridinato complexes,25,28,29 are oriented with the two nitrogen atoms trans with respect to one another, and the two orthometalated phenyl rings are in cis positions (see Figure 1). As far as the Ir− N or Ir−C distances are concerned, a clear effect of the higher trans influence of C-donor groups is observed (see Table S1). C


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Inorganic Chemistry

nm) is observed for complexes with dfppy as the cyclometalating ligand when compared with similar ppy complexes. This is a common finding in neutral20 and cationic iridium(III) complexes.25,69,87 To evaluate whether aggregation of the molecules takes place in solution, the linearity of the Lambert−Beer law was tested in the 1 × 10−4 to 1 × 10−5 M range. Perfect agreement with the Lambert−Beer law was found. As an example, the result for complex 1 can be seen in Figure S10. A graphical representation of the absorbance of the MLCT band against concentration gives a straight line with R2 > 0.999 99. Thus, aggregation does not occur in this concentration range. Besides, by means of DLS we did not observe the formation of nanoparticules in acetonitrile or water/acetonitrile (1:1) solutions. The room-temperature luminiscence spectra of 1−7 and 8− 11 with ppy and dfppy ligands are gathered in Figures 3 and

As stated previously, apart from complex 4 all of the derivatives with a benzyl group on the N^C: ligand exhibit a π−π interaction between this phenyl ring and a ring of ligand LB (see Figure 1 and Table S2 for the parameters of this interaction). In the cases of 9 and 11, this ring is difluorophenyl (see Figures S6 and 1), while for 12, which contains ligand L4, the interaction is established with the pyridine ring (Figure S7). This difference could be related to the presence of the methylene bridge in ligand L4. This interaction is not present in the case of derivative 4 with ppy as the cyclometalating ligand (Figure S8). The phenyl group is rotated, and instead a double CH−π interaction83,84 is established with an ortho proton and a proton of the methylene bridge, with the phenyl group of ligand LB as H-acceptor (see Figure S8). In the crystal structure the PF6− anion gives rise not only to hydrogen bonds but also to anion−π interactions85 in some complexes (8, 9, 11, and 12; see Figure 1 and Table S3). Photophysical Properties. The ultraviolet−visible (UV− vis) absorption spectra of complexes 1−11 were recorded in acetonitrile solution at room temperature, and these are shown in Figure 2 for the ppy complexes 1−7 (see Figure S9 for the

Figure 3. Normalized emission spectra of 1 (red), 2 (brown), 3 (black), 4 (green), 5 (cyan), 6 (blue), and 7 (pink) in CH3CN solution at 298 K (1 × 10−4 M; see Table 1).

S11, respectively, and the positions of the bands are provided in Table 1. The complexes can be divided into two groups: the Figure 2. Absorption spectra of 1 (red), 2 (brown), 3 (black), 4 (green), 5 (cyan), 6 (blue), and 7 (pink) in CH3CN solution at 298 K (1 × 10−5 M).

Table 1. Photophysical Data for Complexes 1−11 at Room Temperature in CH3CN Solution (1 × 10−4 M)

absorption spectra of complexes with dfppy ligands and Table S4 for the absorption data). The strong absorption bands at high energy (350 nm) can be assigned, according to the bibliography, to spin-allowed metal-to-ligand charge-transfer (1MLCT) transitions and spin-forbidden 3 MLCT transitions (promoted by the strong spin−orbit coupling of the iridium center) with a strong π−π* character.19,86 In the complexes with ppy ligands, the maximum of this band appears at 375−378 nm for complexes with a N^C: ligand bearing methyl substituents on the pyridine ring; a very small red shift is observed for those derivatives with a methylene bridge in the N^C: ligand (380 nm), while the complexes with the electron-withdrawing nitro substituent exhibit a blue shift of 10 nm. A larger hypsochromic shift (20

complex

λexc (nm)

1 2 3 4 5 6 7 8 9 10 11

380 399 378 372 399 362 355 351 352 352 354

a

λem (nm)

stra

ΦPL (%)

Stokes shift (cm−1)

473, 510 473, 471, 511 512 514 450, 451, 452, 451,

yes no yes yes no no no yes yes yes yes

2.52 0.33 2.20 6.49 0.58 2.57 9.42 5.89 4.16 5.09 8.49

6316 5455 6535 6882 5493 8094 8714 7526 7445 7489 7284

500 502 500

477 477 478 477

Structured emission band.

ones with a structured band (1, 3, 4, 8−11) and those with broad and structureless emission spectra (2, 5, 6, 7). The previously reported complexes I−V also exhibited25,28,29 structured bands. The presence of a structured band is indicative of a pronounced LC π−π* character and a weaker MLCT contribution, while the absence of this structure points to a substantial MLCT character of the emitting state.88 This is D


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Inorganic Chemistry because the increasing role of the π−π* LC transitions in the emission process is usually associated with a more pronounced vibronic substructure in the emission spectra, even at room temperature. Interestingly, the complexes with unstructured bands are those with the nitro group on the N^C: ligand or with a methylene group between the two heterocycles of this ligand, which interrupts the conjugation. The positions of the emission bands for complexes that contain the same cyclometalating ligand and pyridine-methyl substituted L1 or L2 type of ligands are very similar and are consistent with previous results indicating that the ancillary ligand does not affect the nature of the excited state. This is because the frontier electronic levels primarily involve the Ir(III) dπ- and π-orbitals of the C^N ligands,24,25,28,29 and it is proposed that the substituents on the N^C: ligands are too far from the iridium center to alter significantly the ligand field strength of the metal.25 However, a red shift was observed (Figure 3) when a nitro group in the pyridine ring or a methylene group between the two heterocycles is present in the N^C: ligands, and this highlights the possibility of modifying the photophysical properties with chemical changes in the N^C: ancillary ligand. As expected, a blue shift (21−24 nm) is observed in the emission spectra of dfppy complexes with respect those containing ppy ligands due mainly to the lower energy of the highest occupied molecular orbital (HOMO) orbital25,28 (see below for Theoretical Calculations). Three-dimensional (3D) excitation−emission spectra were obtained for all products by registering all the emission wavelengths for each excitation wavelength (5 nm between consecutive points). The real image of the band was obtained in this way. This process allowed us to verify that there is, for all compounds, a sole, although broad, excitation−emission band (see Figure S12 for the example of complex 8). The ΦPL values (see Table 1) are rather low. They are lower than those observed for [Ir(ppy)2(C^C:)] complexes (42− 68%), 89 similar to or higher than those of similar complexes,24,25,28 and higher, in general, than those of [Ru(bpy)2(N^C:)]2+ complexes.90 Interestingly, some influence of the substituents or their positions in the N^C: ligand was observed. The effect of the presence of methyl or benzyl groups on the nitrogen of the imidazolyl ring is not uniform. However, other effects are clearer. Higher values are obtained when a methyl group is present in the 4-position, and very low values are observed in the case of a nitro substituent in the pyridine ring reflecting a quenching process. The highest value is obtained for complex 7 with ligand L4. Comparison of complexes with the same N^C: ligand shows that higher quantum yields are obtained on going from ppy to dfppy main ligands. The excited-state lifetime values for selected complexes are shown in Table 2. The highest values are obtained for complex 2 with the L1−5NO2 ligand (0.5 μs) and mainly in the case of complex 7 with ligand L4 (1.4 μs). The radiative and the overall nonradiative rate constants (kr and knr, respectively) were calculated from the ΦPL and the τ values,91 assuming an intersystem crossing efficiency of 1 (kr = Φ/τ and knr = (1 − Φ)/τ). The data are gathered in Table 2. The lowest kr value was obtained for complex 2, which contains the nitro group, and in this case the knr is 3 orders of magnitude higher, thus showing a rapid deactivation. Complex 7, with the methylene bridge in the ancillary ligand, showed the smallest difference between the rate constants (knr is 9.6 times higher than kr). For

Table 2. Photophysical Data for Complexes 1−3, 7, 8, and 10 at Room Temperature in CH3CN Solution complex

τ (ns)

kr (s−1)

1 2 3 7 8 10

41.64 ± 0.027 499.9 ± 9.016 43.13 ± 0.127 1405 ± 12.00 119.5 ± 1.200 132.4 ± 1.100

× × × × × ×

6.05 6.60 5.10 6.70 4.93 3.84

knr (s−1) 5

10 103 105 104 105 105

2.34 1.99 2.27 6.45 7.87 7.17

× × × × × ×

107 106 107 105 106 106

complexes that contain Me substituents in the N^C: ligand with both ppy or dfppy cyclometalating ligands, comparable kr values were obtained (3.8−6.1 × 105 s−1). This shows the existence of T1 excited states that are very similar in nature. In these cases, the knr values are one (dfppy complexes) or two (ppy complexes) orders of magnitude higher, and this is consistent with a more rapid deactivation for the ppy complexes. Although differences were found in all these complexes, the results indicate the existence of nonradiative deactivation pathways that should involve thermally accessible upper-lying states from the T1 excited states.89,86 Quenching with O2 and Stability Studies. It was verified that the complexes were quenched with O2, but the process was found to be reversible (see Figures S13 and S14). This opens the possibility of being used as oxygen sensors. The stability of the complexes under irradiation conditions was also analyzed. The behavior of two samples of complex 4 (2 h) both prepared under inert conditions and irradiated at the excitation maximum was compared. In one sample irradiation was performed only during the data acquisition time, and in the other sample the irradiation was maintained during the whole experiment. A similar behavior was observed in the two samples reflecting photostability (see Figure S15). Two-Photon Luminescence. In the classical luminescence processes, the molecule is excited by a photon whose energy is the difference between the ground and the excited levels. However, excitation is also possible if two photons of lower energy are absorbed simultaneously, if the sum of energies correspond to this gap.92 This double photon luminescence (DPF) is interesting, as it makes possible the study of substances where the excitation and emission bands are overlapped, for example, due to a low Stokes shift. This process is also interesting for biological purposes, mainly because red or infrared light penetrates further into tissue and has reduced phototoxicity. Thus, two-photon luminescence can find applications in imaging diagnosis and photodynamic therapy.93 Complex 4 was selected for this study due to its emission intensity. The maximum excitation wavelength for this complex is 372 nm, and the maximum emission is at 473 nm (Figure S16a), with the emission spectrum having a vibronic structure with four peaks. If the emission spectra are registered when the sample is excited at half energy (744 nm, Figure S16b), the spectrum is similar to the initial one, although the intensity is very low due to the low possibility of the process occurring compared with the classical process. The peak observed at 744 nm corresponds to the Rayleigh scattering of the irradiated light. It was verified that the excitation at 744 nm is a true twophoton process and not an excitation of the lower energy part of the band due to excitation of one electron (see Figure S17). It was concluded that the complex exhibits two-photon E


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Inorganic Chemistry

molecular structures and electronic configurations of the cations of selected complexes (1, 2, 7, and 8) in the presence of solvent molecules (acetonitrile). Details of these calculations are provided in the Experimental Section. Ground State. The calculations predicted a near-octahedral structure for the cations in their ground electronic state (S0), and this is in agreement with the results obtained for previously reported complexes25,28,29 and those obtained in this work. The relative arrangement of the donor atoms was also correctly predicted. In the case of complex 8, the structure of which was solved by X-ray diffraction, the values of the Ir−N, Ir−C, and the bite angles predicted are in good agreement with those determined, and the only exception was the Ir−N bond of the ancillary ligand (see Table S5). The composition (in %) calculated for different orbitals for complexes 1, 2, 7, and 8 is gathered in Table S6, and the energy levels of their frontier orbitals are shown in Figure 4. In all cases

luminescence, and this makes it suitable for use in biological experiments. Electrochemical Properties. Cyclic voltammetry (CV) experiments were performed on complexes 1−11 to obtain information about the influence that the changes introduced both in the cyclometalating and the NHC ligands have on the redox behavior. The complexes exhibited quasi-reversible behavior in acetonitrile solutions at a scan rate of 100 mV/s (see Figures S18−S21). In the case of complex 10 different scan rates were tested (from 25 to 200 mV/s), and minor changes were observed in the corresponding values (see Figure S22). The results (scan rate of 100 mV/s) are gathered in Table 3. As expected, the attachment of electron-withdrawing fluoro Table 3. Redox Properties of Iridium Complexes 1−11 complex

Eoxa (V)

ΔEp(ox) (mV)

Ereda (V)

ΔEp(red) (mV)

ΔE (Eox − Ered) (V)

1 2b 3 4 5b 6 7 8 9 10 11

0.79 0.73 0.82 0.80 0.84 0.82 0.80 1.09 1.04 1.07 1.06

80 210 90 100 100 100 100 90 100 80 80

−2.34 −2.50 −2.31 −2.38 −2.48 −2.37 −2.34 −2.32 −2.30 −2.34 −2.32

65 50 70 70 50 75 120 70 70 70 80

3.13 3.23 3.13 3.18 3.32 3.19 3.14 3.41 3.34 3.41 3.38

a

From CV measurements, E = 1/2(Epa + Epc); 0.7 mM in acetonitrile/ tetrabutylamomiun hexafluorophosphate versus Fc+/Fc. bReduction peaks are also observed for 2 at −2.35, −1.17, and −0.96 eV and for 5 at −2.36, −1.18, and −0.95.

substituents on the phenyl rings of the cyclometalating ligands led to an anodic shift, a finding in accordance with the expected lower energy of the HOMO orbital.25,28 As explained in the section on Theoretical Calculations, the HOMO orbital has a contribution from the dπ orbitals of the iridium center and π orbitals of the phenyl group of the cyclometalating ligands. Concerning the reduction potentials, a cathodic shift, albeit small, is observed on passing from ppy to dfppy derivatives. Although the LUMO orbital is located in the cyclometalating ligands, the higher contribution is from the pyridine ringsa fact that could explain the small effect observed on the reduction potentials. The characteristics of the HOMO and LUMO orbitals explain the small effect observed on the potentials of the substituents on the ancillary ligand, especially in the case of the dfppy derivatives, which only differ in the presence of methyl groups on the pyridine ring or in the Nsubstituent on the imidazolyl fragment. The presence of the nitro group led to a negative shift in the reduction potentials, and, furthermore, there were several extra small peaks at lower reduction potentials (see footnote in the table). The higher ΔE values found for the derivatives with dfppy ligands (Table 3) correlates with the blue shift observed in the absorption spectra for these fluorinated complexes with respect to derivatives with ppy cyclometalating ligands (see Figures 2 and S9 and Table S4). Theoretical Calculations. In an effort to gain an insight into the electrochemical and photophysical properties of these compounds, density functional theory (DFT) calculations were performed at the B3LYP/(6-31G**/LANL2DZ) level on the

Figure 4. Energy diagram for the frontier orbitals of complexes [1]+, [2]+, [7]+, and [8]+.

the HOMO, with energy values ranging between −5.45 and −5.73 eV, is composed of a mixture of Ir(III) dπ orbitals (t2g) and phenyl π orbitals of the cyclometalating ligands, with a higher participation of the phenyl group of LB (see Figure 1 for the numbering of LA and LB). The isovalue contours of the frontier orbitals calculated for compounds 1 and 2, as examples, are shown in Figure 5. The energy values for 1, 2, and 7, all of which contain the ppy ligand, are very similar. The higher stabilization of the HOMO in compound 8 is related to the electron-withdrawing effect of the F atoms.91 The oxidation therefore takes place in the cyclometalating ligand-Ir fragment and, as stated, these results are in good agreement with the experimental Eox values, which indicated the effect of the cyclometalating ligand and small influence of the ancillary ligand (Table 3). In contrast to results found for [Ir(C^N)2(N^N)]+ species, where the LUMO is located mainly on the N^N ligand,1,16,18−20,69 and similar to that found in analogous complexes with N^C: ligands,25,28,29 the LUMO in compounds 1, 7, and 8 is located on the phenyl-pyridine or difluorophenyl-pyridine ligands but with a higher participation of the pyridine fragment (especially that of ligand LA, see Figure 1). This latter finding could explain the lower difference in the energy level between the LUMO of the dfppy derivative (8) and the LUMO of the ppy complexes 1 and 7 when compared F


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Inorganic Chemistry

give rise to a similar HOMO−LUMO gap (∼3.90 eV) for 1 and 7 and a higher value for 8 (4.13 eV) according to the blue shift in the absorption data of the dfppy derivatives. The case of complex 2, with a nitro group in the ancillary ligand, is different, and in this case the LUMO is located over the NO2 substituent and the pyridine ring of the N^C: ligand. This different topology of the LUMO in compound 2 is the reason for its lower energy value (∼1.3 eV when compared with the energy of the LUMO of the other complexes) and the smaller HOMO−LUMO gap found in this complex. Interestingly, the LUMO+2 in compound 2 has a similar topology to the LUMO in compound 1, and in both cases these orbitals correspond to π* orbitals of the cyclometalating ligand. It is not surprising that the LUMO of complex 1 and LUMO+2 of complex 2 have very similar energy values. The extremely small orbital overlap between the HOMO and LUMO and LUMO+1 orbitals may explain the absence of the corresponding absorptions. The extra reduction peaks observed for complexes with a nitro group may be related to the reduction of this functional group. Excited States. With the aim of investigating the nature of the emitting excited state, the low-lying triplet states of complexes 1, 2, 7, and 8 were calculated starting from the optimized geometries of the ground state (S0) using the timedependent (TD) DFT approach. The vertical excitation energies calculated for the first three triplets are listed in Table 4 together with their molecular orbital description and electronic nature. In the case of complexes 1, 7, and 8, the two lowest-lying triplet states (T1 and T2) have very similar energy. T1 is mainly described by the HOMO → LUMO excitation, while more

Figure 5. Isovalue contours (0.03 au) for some orbitals calculated for [1]+ and [2]+.

with the difference in energy of the HOMO of the same compounds. In any case, this higher stabilization of the LUMO orbital of 8 correlates with the relative Ered values. These data

Table 4. Lowest Triplet Excited States Calculated at the TD-DFT B3LYP/(6-31G*+LANL2DZ) Level for Complexes 1, 2, 7, and 8 in Acetonitrile Solutiona cmp 1

2

7

8

state

E (eV)

λ (nm)

T1 T2

2.819 2.850

439.86 434.97

T3

3.227

384.17

T1 T2 T3 T1 T2

2.107 2.560 2.707 2.801 2.830

588.55 484.32 458.11 442.63 438.10

T3

3.196

387.90

T1

2.933

422.74

T2

2.960

418.93

T3

3.358

369.25

natureb

monoexcitations H→L (59) H-2→L (22) H→L+1 (21) H→L+2 (41) H-1→L+2 (16) H→L (21) H→L+1 (51) H→L (100) H-2→L (88) H-1→L (85) H→L (57) H-2→L (22) H→L (56) H-3→L (17) H→L (57) H-2→L (28) H-2→L+1 (16) H→L (39) H→L+1 (25) H→L+2 (22) H-1→L+2 (23)

dπ(Ir) dπ(Ir) dπ(Ir) dπ(Ir) dπ(Ir) dπ(Ir) dπ(Ir) dπ(Ir) dπ(Ir) dπ(Ir) dπ(Ir) dπ(Ir) dπ(Ir) dπ(Ir) dπ(Ir) dπ(Ir) dπ(Ir) dπ(Ir) dπ(Ir) dπ(Ir) dπ(Ir)

+ + + + + + + + + + + + + + + + + + + + +

πppy → π*ppy πppy → π*ppy πppy → π*py(N^C:) πppy → π*py(N^C:) + π*ppy πpy(N^C:) → π*py(N^C:) + π*ppy πppy → π*ppy πppy → π*py(N^C:) πppy → π*py(N^C:)d πim(N^C:) → π*py(N^C:)d πppy → π*py(N^C:)d πppy → π*ppy πppy → π*ppy πppy → π*ppy πppy → π*ppy πppy → π*ppy πdfppy → π*dfppy + πdfppy → π*dfppy + π*py(N^C:) πdfppy → π*dfppy πdfppy → π*dfppy + π*py(N^C:) πdfppy → π*dfppy + π*py(N^C:) πim(N^C:) → π*dfppy + π*py(N^C:)

descriptionc LC/3MLCT LC/3MLCT 3 MLCT/3LLCT 3 LC/3MLCT/3LLCT 3 LC/3MLCT/3LLCT 3 LC/3MLCT 3 LLCT/3MLCT 3 MLCT/3LLCT 3 MLCT/3LLCT 3 MLCT/3LLCT 3 LC/3MLCT 3 LC/3MLCT 3 LC/3MLCT 3 LC/3MLCT 3 LC/3MLCT 3 LC/3MLCT 3 LLCT/3MLCT 3 LC/3MLCT 3 LLCT/3LC/3MLCT 3 LLCT/3LC/3MLCT 3 LLCT/3LC/3MLCT 3 3

a

Vertical excitation energies (E), dominant monoexcitations with contributions (within parentheses) of greater than 15%, the nature of the electronic transition, and the description of the excited state are summarized. bpy(N^C:) = pyridine ring in the N^C: ligand. im(N^C:) = imidazolyl ring in the N^C: ligand. cLC, MLCT, and LLCT denote ligand-centered, metal-to-ligand charge transfer, and ligand-to-ligand charge transfer, respectively. dpy(N^C:) high participation of the NO2 group. H and L denote HOMO and LUMO, respectively. G


Article

Inorganic Chemistry

0.001 eV, respectively), but for complex 8 the difference between T1 and T2 is bigger (0.204 eV). For this reason, a situation with two emitting states cannot be ruled out for complexes 1 and 7. The DFT values predicted for Eem are slightly underestimated when compared with the experimental values. They correctly reproduce the main experimental trends featuring similar emission energies for complexes 1 and 7 and higher energy emission energy for complex 8. H2 Generation. An initial analysis of the comparative behavior of the ppy and dfppy iridium complexes as photosensitizers in the photogeneration of H2 from water was performed. The complex [Co(bpy)3]Cl2, which has previously been used in these kinds of reactions,52 was chosen as the catalyst. This complex has the advantage that it does not require the use of an electron relay. A 1:1 mixture of H2O/ CH3CN was used as the solvent. Acetonitrile was required to ensure the solubility of the complexes. It was verified that, under the experimental conditions (temperature, species dissolved), the two solvents were miscible. Triethanolamine (TEOA) was used as a sacrificial reductant. This was preferred over triethylamine due to its higher solubility in the reaction medium and the lower toxicity.95 It was verified that the PS (complex 7 as an example) was stable in the presence of TEOA in a range including the ratio of the catalytic experiments (ratio from 100 to 10 000; see Figures S27−S29). LiCl was also added to reduce the H2 solubility in the reaction medium, and HCl was added to counterbalance the basicity of the TEOA (pH = 8).45 It was estimated that a time of 2 h was sufficient to compare the behavior of the different photosensitizers. The reactions were performed in a multireactor (see Figures S30 and S31) with voltage sensors. The voltage values were converted to pressures using the appropriate calibration lines. Smoothing of the data was performed using natural splines, and the best node number for the spline curve was calculated by cross validation. Finally, to avoid any effect from any other component of the solution except the photosensitizer, the curve from a simultaneously performed experiment carried out without photosensitizer was subtracted from the main experiment. The kinetic traces were obtained in this way. The stability of the PS and the catalyst was verified before and after catalysis by means of high-performance liquid chromatography (HPLC) analysis (Figure S32). No noticeable H2 production was observed in the absence of PS or WRC. The quantities of H2 obtained (μmol) for each photosensitizer under the conditions used (see Experimental Section) after 2 h are shown in Table 6 (see Table S8 for the amount of H2 at different times). The results, grouped according to the ligands on the complexes, are provided in Figure S33. It is observed that, although the average value is higher for the dfppy derivatives, there are some complexes with ppy that give rise to higher values (complexes 2 and II). The ppy derivatives show a higher variation within the series, a fact that may indicate greater sensitivity to modifications on the ligands, and this opens the way to new candidates with improved behavior. The comparison between complexes with ligands of type L1 or L2 (with Me or benzyl groups, respectively) can be made by considering Figure S33b. In both families (with metalating ligands ppy or dfppy) better results were obtained on using complexes with L1 ligands. The same detrimental effect of the benzyl group is observed when comparing complexes 6 and 7 with L3 and L4 ligands. It is noteworthy that the very weak luminescence of complex 2 is not detrimental to its behavior as

variability is observed for T2 that has a multiconfigurational character. These triplet states are described as LC states, involving the cyclometalating ligands, and MLCT states due to the participation of the metal in the HOMO orbital. The high participation of the cyclometalating ligands in the orbitals involved in the case of 1 and 8 points to a large LC character for these derivatives in accordance with the structured emission band, while the higher participation of the metal in the HOMO orbital of 7 may explain the nonstructured emission band (higher MLCT character). The participation of the ancillary ligand in the LUMO+1 in 1 and 8 (and LUMO+2 for 1) leads to a small contribution of an LLCT state for these complexes. In the case of compound 2, the low value of the quantum yield can be explained, according to the literature, by the high participation of the nitro group in the LUMO orbital.38 For this complex T1, T2, and T3 correspond to excitation to the LUMO orbital, and this may involve nonradiative deactivation. The lowest triplet states for these compounds were further examined by optimizing their geometries using the spinunrestricted DFT approach. The adiabatic energy differences between S0 and T1 (ΔE) and the emission energies (Eem) estimated as the vertical energy difference between T1 and S0 at the optimized minimum energy of T1 are listed in Table 5.94 Table 5. Adiabatic Energy Difference (ΔE) between S0 and T1 States and the Emission Energy (Eem) from T1 Calculated for Complexes 1, 7, and 8 compound

ΔE (eV; nm)

Eem (eV; nm)

1 7 8

2.67; 464 2.67; 465 2.81; 442

2.36; 525 2.39; 519 2.47; 502

The unpaired-electron spin-density distributions computed for the fully relaxed T1 state of complex 1 is shown in Figure 6 as a

Figure 6. Unpaired-electron spin-density contours (0.006 au) calculated for the fully relaxed T1 state of complex 1.

representative example (see Figures S23−S26 for other examples). The unpaired electrons in the T1 or T2 states for the complexes 1 and 7 are mostly concentrated in the LA or LB ligand (see Table S7; the data for 1 as an example are T1: Ir, 0.31e; LA, 1.59e, LB, 0.08e; N^C:, 0.02e; T2: Ir, 0.25e; LA, 0.05e, LB, 1.68e; N^C:, 0.02e). In complex 8, the influence of the fluoro substituents of the cyclometalating ligand causes a decrease of the electron density over the iridium atom (Ir, 0.16e, LA, 1.8e, LB, 0.005e, N^C:,0.005e) in accordance with results of other compounds.25 The calculated difference of energy for T1 and T2 in complexes 1 and 7 is small (0.011 and H


Article

Inorganic Chemistry Table 6. Amount of H2 Produced (μmol) Using Different Photosensitizersa C^N ligand: ppy

Table 7. Excited-State Redox Properties of the Photosensitizers 1−11 complex

Eemis (eV)

Eox (V vs SCE)

Ered (V vs SCE)

E (PS+/PS*) (V vs SCE)

E (PS*/PS−) (V vs SCE)

1 2 3 4 5 6 7 8 9 10 11

2.55 2.43 2.55 2.56 2.43 2.42 2.41 2.68 2.68 2.67 2.68

1.17 1.11 1.20 1.18 1.22 1.20 1.18 1.47 1.42 1.45 1.44

−1.96 −2.12 −1.93 −2.00 −2.10 −1.99 −1.96 −1.94 −1.92 −1.96 −1.94

−1.38 −1.32 −1.35 −1.38 −1.21 −1.22 −1.23 −1.21 −1.26 −1.22 −1.24

0.59 0.22 0.62 0.56 0.33 0.43 0.45 0.74 0.76 0.71 0.74

C^N ligand: dfppy

N^C: ligand

complex

μmol H2

complex

μmol H2

L1 L1−5Me L1−4Me L1−5NO2 L2 L2−5Me L2−4Me L2−5NO2 L3 L4

I 1 II 2 III 3 4 5 6 7

44 51 82 80 13 13 40 36 39 23

IV 8 V

54 52 52

9 10 11

55 32 40

6 μmol of PS, 0.06 mmol of [Co(bpy)3]Cl2, 11.30 mmol of TEOA, 5.424 mmol of LiCl, 4.83 mmol of HCl, 10 mL H2O + 10 mL of CH3CN, 2 h, 27 °C.

a

(N^N)]+.52,69 On considering the E(PS+/PS*) data it can be concluded that complexes 1−11 have a strong and relatively similar reducing power, which is higher than that of complexes with similar C^N ligands but with N^N ancillary ligands (values in the range from −0.82 to −0.96, except for one case with a value of −1.21 V).52,69 However, the strength as oxidants is lower than the N^N counterparts with identical main ligands. In this respect, the dfppy derivatives are more powerful oxidants than those containing ppy cyclometalating ligands. The derivatives with NO2 groups are weaker oxidants but are stronger reductors than the rest of the complexes. We also performed experiments to analyze the possible quenching of the excited photosensitizer by the SR, in our case TEOA. It was previously reported that [Ru(bpy)3]2+ and similar derivatives cannot be quenched by TEOA,96 while iridium derivatives of the type [Ir(C^N)2(N^N)] are quenched by this SR.52 The intensity of the emission of the PS was evaluated after adding from 100 to 10 000 mol per mol of PS (this includes the ratio of 1880 used in the catalytic experiments) for a selection of compounds, both with ppy or dfppy ligands ([Ir(ppy)2(L4)](PF6), [Ir(ppy)2(L1−4Me)](PF6), and [Ir(dfppy)2(L1)](PF6); see Figures S34−S36). A change in the emission intensity of the PS after adding the different amounts of TEOA was not observed, thus showing that TEOA is not able to quench our iridium derivatives. It was also verified that the photosensitizer 7 (as an example) was quenched by the catalyst. The intensity of the emission of 7 dropped to 0.7% of the initial intensity (at catalytic concentrations of PS and catalyst). Thus, the process must take place exclusively by oxidative quenching. The potentials E(PS+/PS*) of the different PSs acting as redutors can overcome the potential of the WRC (measured in acetonitrile, Ered = −1.09 V, vs SCE). In any case, the amount of H2 produced did not correlate with the precise value of the oxidation potential or with the photophysical or other electrochemical properties, thus indicating that another factor (or factors) influences the activity in the overall photocatalytic process.

photosensitizer in the production of H2 and that the rupture of conjugation on the N^C: ligand does not prevent the formation of H2. It is considered that the process for the photogeneration of H2 from water with a photosensitizer can take place through reductive or oxidative quenching (see Scheme 2).47,70,73 In the Scheme 2. (a) Mechanism of Oxidative Quenching. (b) Mechanism of Reductive Quenching

oxidative quenching the excited photosensitizer (PS*) reduces the catalyst (or the electron relay where required), and the reduced catalyst is able to reduce protons to hydrogen. The oxidized photosensitizer is then reduced thanks to the sacrificial reductant (SR), and the cycle can start again. Conversely, in the reductive quenching the excited photosensitizer interacts with the SR that is oxidized by the PS*. The reduced PS− transfers one electron to the catalyst. The process is complex, and several factors may influence the production of H2. In any case, bearing in mind that it is the excited state of the PS that acts as the reducing or oxidizing agent (or both), we decided to calculate the “excited-state redox” properties of the new iridium derivatives, taking into account the emission energy13 (see Table 7). The values included in the table are relative to the saturated calomel electrode (SCE) to facilitate comparison with those of previously reported complexes of the type [Ir(C^N)-

■

CONCLUSIONS The 12 cationic iridium bis(cyclometalated) complexes synthesized [Ir(C^N)2(N^C:)]+ (C^N = ppy or dfppy) contained substituted pyridine-carbene ligands (N^C:) instead of the N^N most common ligands. In previous complexes of the same type it was concluded that the photophysical properties were only modified by changes in the cycloI


Article

Inorganic Chemistry

16 transients of 2048 data points were collected for each of the 256 increments, with a pulse time of 1 s and mixing time of 1 s. In the NMR analysis s, d, t, q, sept, m, and br s denote singlet, doublet, triplet, quartet, septet, multiplet, and broad signal, respectively; o, m, and p stand for ortho, meta, and para; Im for imidazole ring, py for a pyridine ring, Ph for a phenyl ring, and bz for the benzyl substituent. Unless otherwise stated, the 13C{1H} NMR signals are singlets. Coupling constants are in hertz. NMR signals for the hexafluorophosphate anion were omitted in the characterization of the complexes for simplification, being for all of them the following: 19 1 F{ H} NMR: −70.2 (d, 6F, J = 710 Hz) ppm. 31P{1H} NMR: −144.2 (sept) ppm. The ligands L1H, L1H-5Me, L1H-4Me, L2H, L3H, and L4H were previously reported. References where the same methodology as that used by us (Ullmann coupling, see Supporting Information) are indicated: L1H,25,29,90,97 L1H-5Me,97 L1H-4Me,25 L2H,98 and L3H (with Br as counteranion). 29,90 L4H was prepared in the bibliography97 by the method indicated in Scheme 2. Iridium complexes I,25,28 II,25 III,29 IV,25,28 and V25 have been already reported. The assignment of the NMR resonances of all of them is included in the Supporting Information. The starting materials [Ir(ppy)2Cl]299 and [Ir(dfppy)2Cl]2100 and the catalyst [Co(bpy)3]Cl252 were prepared according to literature procedures. The synthesis of the new ligand precursors is indicated in the Supporting Information. Measurement of UV/Vis Absorption and Luminescence Spectra. UV/Vis absorption spectra were recorded on a UVIKON XS UV/vis spectrophotometer, while fluorescence excitation and emission spectra were recorded on a PTI Quanta Master TM spectrofluorimeter from Photon Technology International (PTI) equipped with a 75 W xenon short arc lamp and a model 814PTM detection system. Felix32 software was used to collect and process fluorescence data. For all optical measurements, 1 × 10−4 M solutions in CH3CN were prepared in a glovebox under a nitrogen atmosphere, and the solutions were kept under an inert atmosphere in 1 cm closed quartz cuvettes equipped with Teflon septum screw caps. The emission and excitation slit widths were 1 or 3 nm, depending on the emission intensity of the compounds, to avoid saturation of the detector. All optical measurements were made at room temperature. The emission quantum yield (Φ) was calculated for each complex according to the equation:101

metallating ligands, and no influence of the N^C: ligand was observed. However, we have demonstrated that, besides the changes in the C^N ligands, modifications such as introducing a nitro group or a methylene fragment to induce a rupture of the conjugation between the two heterocycles of the N^C: ligand make possible the modifications of these properties. The complexes, that emitted in the blue-green region, displayed relatively low photoluminescence quantum yields, and the excited-state lifetimes were in the range of 41−1405 ns. The presence of a nitro group led to very low quantum yields, and the highest values for the ΦPL and τ parameters were obtained for the complex that contains a methylene bridge in the N^C: ligand. The electrochemical properties were influenced by the presence of fluorine atoms in the C^N ligands but also by the nitro fragment in the carbene ligand. DFT studies have allowed us to explain the photophysical and electrochemical properties, including the low ΦPL of the nitro derivatives. TDDFT studies allowed us to conclude that the emission triplets are LC states, involving the cyclometalating ligands, and MLCT states. Preliminary studies indicate that the iridium complexes behave as photosensitizers (PS) in the photogeneration of H2 from water with [Co(bpy)3]Cl2 as the catalyst and TEOA as a sacrificial reductant. Interestingly, complex 2, which contains a nitro group and exhibits a very low ΦPL value, gave rise to one of the best results, which shows that a high value for this parameter is not a prerequisite for H2 production. The complexes exhibit a strong and relatively similar reducing power of the excited state, higher than that of similar complexes with N^N ancillary ligands. The absence of quenching of the PS with TEOA leads to the conclusion that oxidative quenching is operating. These studies demonstrate that, in certain cases, modification of the ancillary ligand in these kinds of complexes affects their properties and provides helpful insights into the factors that could be taken into account when designing new NHC-based luminescent and photosensitizer complexes.

■

Φs = Φr·(Is/Ir) ·(Ar/As) where Φs is the quantum yield of the sample, As and Ar are the absorbance of the sample and the reference at the excitation wavelength, and Is and Ir represent the points of maximum intensity in the corrected emission spectra. Φr is the quantum yield for the reference complex [Ir(ppy)2(bpy)](PF6), which is assumed to be 7.07 under the experimental conditions.72 For the determination of the luminescence lifetime of the compounds, the fluorescence decay was measured on a PTI Time Master fluorimeter equipped with a picosecond nitrogen laser and a wavelength selector when necessary. This system was used with the dyes 2-[1,1′-biphenyl]-4-yl-6-phenyl-benzoxazole (PBBO) or 2-[1,1′biphenyl]-4-yl-5-[4-(1,1′-dimethylethyl)phenyl]-1,3,4-oxadiazole (BPBD). The astroboscopic detector was coupled to a Czerny− Turner monochromator on the emission port. The system was connected to a personal computer (PC) via an ethernet interface and governed by Felix32. The instrumental parameters used were as follows: maximum λexc and λem for each compound, 300 channels, integration time = 50 μs, 15 averages per decay, 5 shots per channel, laser pulse frequency = 5 Hz, 1 or 3 nm excitation and emission slit widths, and a logarithmic collection step. Two-Photon Luminiscence. The samples were prepared in high purity CH3CN in a glovebox under a nitrogen atmosphere, and the solutions (1 × 10−4 M) were kept under an inert atmosphere in 1 cm closed quartz cuvettes equipped with Teflon/silicone septum screw caps and further protected with Teflon film. The closed cuvettes were transported to the equipment in a closed box under an inert atmosphere. The parameters were as follows: maximum of excitation,

EXPERIMENTAL SECTION

General Comments. All manipulations were performed under an atmosphere of dry oxygen-free nitrogen using standard Schlenk techniques and an MBraun glovebox; MB-20-G TP170b was used for some preparations (O2 < 1 ppm). Solvents were distilled under inert atmosphere from the appropriate drying agents and degassed before use or purified in an MBraun SPS MB-SPS-800 from commercial HPLC solvents. Elemental analyses were performed with a Thermo Quest FlashEA 1112 microanalyser, and IR spectra were obtained on a Shimadzu IR Prestige-21 infrared spectrometer equipped with a Pike Technologies ATR. The fast atom bombardment (FAB+) mass spectrometry (MS) measurements were obtained with a Thermo MAT95XP mass spectrophotometer with a magnetic sector. 1H and 13 C{1H} NMR spectra were recorded on Varian Innova 500 and Varian Unity 400, and the latter was also used for the recording of the 19 F and 31P{1H} NMR. Shifts (in ppm) are related to tetramethylsilane, Si(CH3)4, (for 1H and 13C), CFCl3 (19F), and 85% H3PO4 (31P). 1 H−1H COSY spectra: standard pulse sequence with an acquisition time of 0.214 s, pulse width of 10 ms, and relaxation delay of 1 s was used, optimizing the number of scans, width of the window, and number of increments depending on the sample conditions. For 1 H−13C g-HMBC and g-HMQC spectra the standard Varian pulse sequences were used (VNMR 6.1 C software). The spectra were acquired using 7996 Hz (1H) and 25 133.5 Hz (13C) widths; 16 transients of 2048 data points were collected for each of the 256 increments. NOESY spectra were acquired using 8000 Hz width, and J


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Inorganic Chemistry 372 nm; maximum of excitation DPF, 744 nm; emission width, 375− 775 nm; scanning rate, 1 nm/s; excitation and emission slit widths, 3 nm (BP, BandPass)/0.75 mm (width), maximum of emission, 473 nm; excitation window, 300−800 nm. Electrochemical Measurements. CV was performed on a Metrohm potentiostat/galvanostat model 757 VA Computrace at a potential sweep rate of 100 mV/s. A glassy carbon (GC) disk electrode (3 mm diameter) served as the working electrode, a coiled platinum wire was used as the counter electrode, and a Ag/AgCl (3 M KCl) electrode was used as reference. Electrochemical measurements were performed in a 10 mL cell at an approximate complex concentration of 0.7 mM using 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile as supporting electrolyte. The samples were degassed by bubbling acetonitrile-saturated N2 into the solution for 10 min before the voltammograms were recorded to ensure that they were oxygen-free. The GC electrode was polished with 0.05 μm γ-alumina slurry, rinsed clean with deionized water and ethanol, and subsequently dried. Each compound was obtained against its corresponding background, and all voltammograms were background-corrected. Theoretical Calculations. DFT calculations were performed with the D.01 revision of the Gaussian 09 package (see Supporting Information for the reference) using the Becke’s three-parameter B3LYP exchange-correlation functional,102,103 together with the 631G(d) basis set for H, C, N, O, and F,104,105 and the “double-ζ” quality LANL2DZ basis set for the Ir element.106 The geometries of the singlet ground states (S0) and the lowest-energy triplet states (Topt) were fully optimized without imposing any symmetry restriction. The geometries of the triplet states were calculated at the spin-unrestricted UB3LYP level with a spin multiplicity of 3. All the calculations were performed in the presence of the solvent (acetonitrile). Solvent effects were considered within the selfconsistent reaction field (SCRF) theory using the SMD keyword that performs a polarized continuum model (PCM)107calculation using the solvatation model of Thrular and co-workers.108 TD-DFT calculations of the lowest-lying 15 singlets and triplets were performed in the presence of the solvent at the minimum-energy geometry optimized for the ground state (S0). The rest of the general information on this section is given in the Supporting Information. Syntheses of the New Complexes. The synthesis and characterization of complexes 1 and 8 is described here. The rest is reflected in the Supporitng Information. In Chart 2 the atom numbering for the new complexes is indicated. [Ir(ppy)2(L1−5Me)](PF6), 1. L1H-5Me (120.5 mg, 0.4 mmol), 214.4 mg (0.2 mmol) of [Ir(ppy)2Cl]2, and 92.7 mg (0.4 mmol) of Ag2O were suspended in 10 mL of 1,2-dichloroethane under inert atmosphere in a light-protected Schlenk flask, and the mixture was stirred at reflux overnight. The deep yellow solution was filtered to remove rests of silver compounds, and the organic solvent was washed with 10 mL of deionized water containing 368.1 mg (2 mmol, 5 equiv)

of KPF6 with intense agitation for 5 min. After the aqueous phase was carefully removed and discarded, the organic layer was washed with 2 × 10 mL of deionized water, dried with MgSO4 for at least 30 min, and finally evaporated under vacuum to yield a brown oil. The oil was triturated with 15 mL of pentane with ultrasound until a powder was obtained. Once filtered, the solid was washed with 5 mL of water and dried under vacuum. The solid was further purified with chromatography using aluminum oxide as the stationary phase using dichloromethane to extract all impurities. The desired product is obtained using methanol as eluent. Evaporation of the methanol solution to dryness yields the product as a yellow powder. Yield: 49% (159.9 mg). Anal. Calcd for C32H27F6IrN5P·1.5H2O C: 45.44, H: 3.58, N: 8.28. Found: C: 45.51, H: 3.31, N: 7.72%. 1H NMR (DMSO-d6, 298 K), δ: 8.47 (d, 1H, J = 1.9 Hz, Im4), 8.28−8.20 (m, 3H, py3′ + 2py6), 8.09 (d, 1H, J = 8.2 Hz, py4′), 7.98 (d, 1H, J = 5.9 Hz, py3), 7.96−7.86 (m, 4H, 2py5 + 2Ph), 7.63 (d, 1H, J = 6.4 Hz, py3), 7.52 (d, 1H, J = 1.9 Hz, Im5), 7.35 (s, 1H, py6′), 7.15 (br s, 2H, 2py4), 7.04 (t, 1H, J = 6.8 Hz, Ph), 6.96 (t, 1H, J = 7.6 Hz, Ph), 6.93 (t, 1H, J = 7.6 Hz, Ph), 6.82 (d, 1H, J = 6.6 Hz, Ph), 6.27 (d, 1H, J = 8.2 Hz, Ph), 6.15 (d, 1H, J = 8.2 Hz, Ph), 3.09 (s, 3H, NMe), 2.12 (s, 3H, pyMe) ppm. 13C{1H} NMR (DMSO-d6, 298 K) δ: 177.0, 167.9 (py3), 166.6 (Ph), 164.0 (Ph), 153.5 (py6′), 151.2 (Ph), 149.1, 149.0, 148.3 (py4′), 144.5 (Ph), 142.8 (Ph), 142.3 (Ph), 138.4, 137.6, 133.8, 130.5, 130.5, 130.3, 129.6, 125.3 (Im), 124.9, 124.9, 124.5, 123.5, 122.7, 121.0, 120.2, 119.8, 117.7 (Im), 112.3, 36.2 (NMe), 17.6 (pyMe) ppm. FTIR (ATR): 3167, 3136, 3041 ν(C−H aromatic); 2963 ν(CH3); 1499, 1476 δ(C−H aromatic); 837 ν(PF6) cm−1. MS-FAB+ (%): 674 (100%), [M]+; 519 (32%), [M − ppy − H]+; 501 (15%), [M − L1−5Me]+; 174 (49%), [L1−5Me + H]+; 154 (48%), ppy+ m/z. [Ir(dfppy)2(L1−5Me)](PF6), 8. L1H-5Me (78.9 mg, 0.26 mmol), 134.0 mg (0.13 mmol) of [Ir(dfppy)2Cl]2, and 60.7 mg (0.26 mmol) of Ag2O were added to a light-protected Schlenk flask. 1,2Dichloroethane (10 mL) was added, and the suspension was stirred under inert atmosphere at reflux temperature for 14 h. The deep yellow solution was filtered to remove rests of silver compounds, and the organic solvent was washed with 10 mL of deionized water containing 241.1 mg (1.31 mmol, 5 equiv) of KPF6 with intense agitation for 5 min. After the aqueous phase was carefully removed and discarded, the organic layer was washed with 2 × 10 mL deionized water, dried with MgSO4 for at least 30 min, and dried under vacuum to yield a yellow powder. Once perfectly dried, the highly hydrophobic solid was washed with 5 mL of methanol, again with 5 mL methanol/ water (1:1), and dried under vacuum to get a yellow powder. Yield: 53% (123.7 mg). Anal. Calcd for C32H23F10IrN5P·1.2H2O C: 42.13, H: 2.81, N: 7.68. Found: C: 42.25, H: 3.15, N: 7.20%. 1H NMR (DMSOd6, 298 K), δ: 8.50 (d, 1H, J = 2.4 Hz, Im4), 8.32−8.25 (m, 3H, 2py6 + py3′), 8.15 (d, 1H, J = 9.0 Hz, py4′), 8.06−8.00 (m, 3H, 2py5 + py3), 7.70 (d, 1H, J = 6.4 Hz, py3), 7.60 (d, 1H, J = 2.4 Hz, Im5), 7.39 (s, 1H, py6′), 7.25 (t, 1H, J = 6.0 Hz, py4), 7.23 (t, 1H, J = 6.1 Hz, py4), 6.94 (dd, 1H, J = 12.0 Hz, J = 9.1 Hz, Ph4), 6.88 (dd, 1H, J = 12.0 Hz, J = 9.1 Hz, Ph4), 5.68 (dd, 1H, J = 9.0 Hz, J = 2.6 Hz, Ph6), 5.56 (dd, 1H, J = 8.4 Hz, J = 2.6 Hz, Ph6), 3.17 (s, 3H, NMe), 2.19 (s, 3H, pyMe) ppm. 13C{1H} NMR (DMSO-d6, 298 K)) δ: 154.6 (py5 or py3), 153.7, 151.3, 150.1 (py3), 148.8 (py6′), 143.3 (py4′), 140.1, 139.3, 135.0, 125.9 (Im), 125.6 (py4), 124.5 (py4), 123.6, 118.3 (Im), 113.3 (Ph6), 100.0 (Ph4), 98.2 (Ph4), 36.8 (NMe), 18.1 (pyMe) ppm. 19 F NMR (DMSO-d6, 298 K), δ: −106.6 (q, 1F, F5, J = 8.2 Hz), −107.3 (q, 1F, F5, J = 10.0 Hz), −108.4 (t, 1F, F3, J = 12.4 Hz), −109.2 (t, 1F, F3, J = 11.5 Hz) ppm. FTIR (ATR): 3178, 3130, 3088, 3074, 3045 ν(C−H aromatic); 2952 ν(C−H alkylic); 1500, 1475 δ(C−H aromatic); 839 ν(PF6) cm−1. MS-FAB+ (%): 746 (100%), [M]+; 573 (11%) [M − L1−5Me]+; 556 (16%) [M − dfppy]+ m/z. Single crystals suitable for X-ray diffraction studies were obtained by diffusion of hexane into a 1,2-dichloroethane solution of the complex.

Chart 2. Atom Numbering Scheme for the New Complexes

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Inorganic Chemistry

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ionic transition metal complexes. J. Mater. Chem. 2007, 17, 2976− 2988. (8) Costa, R. D.; Ortí, E.; Bolink, H. J.; Monti, F.; Accorsi, G.; Armaroli, N. Luminescent Ionic Transition-Metal Complexes for Light-Emitting Electrochemical Cells. Angew. Chem., Int. Ed. 2012, 51, 8178−8211 and references therein. (9) See, for example, Borisov, S. M.; Klimant, I. Ultrabright Oxygen Optodes Based on Cyclometalated Iridium(III) Coumarin Complexes. Anal. Chem. 2007, 79, 7501−7509. (10) Wong, H. L.; Lam, L. S. M.; Cheng, K. W.; Man, K. Y. K.; Chan, W. K.; Kwong, C. Y.; Djurisic, A. B. Low-band-gap, sublimable rhenium(I) diimine complex for efficient bulk heterojunction photovoltaic devices. Appl. Phys. Lett. 2004, 84, 2557−2559. (11) Atkins, P. J.; Gold, V.; Marsh, R. The decarboxylation of trichloroacetic acid and the reactions of the trichloromethyl anion with 1,3,5-trinitrobenzene and with hydrogen ions: kinetic measurements in dimethyl sulfoxide solution. J. Chem. Soc., Perkin Trans. 2 1984, 1239− 1245. (12) Lo, K. K.-W.; Chung, C.-K.; Lee, T. K.-M.; Lui, L.-K.; Tsang, K. H.-K.; Zhu, N. New Luminescent Cyclometalated Iridium(III) Diimine Complexes as Biological Labeling Reagents. Inorg. Chem. 2003, 42, 6886−6897. (13) Kalyanasundaram Photochemsitry of Polypyridine and Porphyrin Complexes; Academic Press: San Diego, CA, 1992. (14) Lowry, M. S.; Bernhard, S. Synthetically Tailored Excited States: Phosphorescent, Cyclometalated Iridium(III) Complexes and Their Applications. Chem. - Eur. J. 2006, 12, 7970−7977. (15) Mills, I. N.; Kagalwala, H. N.; Bernhard, S. Cyano-decorated ligands: a powerful alternative to fluorination for tuning the photochemical properties of cyclometalated Ir(III) complexes. Dalton Trans. 2016, 45, 10411−10419. (16) Tamayo, A. B.; Garon, S.; Sajoto, T.; Djurovich, P. I.; Tsyba, I. M.; Bau, R.; Thompson, M. E. Cationic Bis-cyclometalated Iridium(III) Diimine Complexes and Their Use in Efficient Blue, Green, and Red Electroluminescent Devices. Inorg. Chem. 2005, 44, 8723−8732. (17) Hwang, F.-M.; Chen, H.-Y.; Chen, P.-S.; Liu, C.-S.; Chi, Y.; Shu, C.-F; Wu, F.-I.; Chou, P.-T.; Peng, S.-M; Lee, G.-H. Iridium(III) Complexes with Orthometalated Quinoxaline Ligands: Subtle Tuning of Emission to the Saturated Red Color. Inorg. Chem. 2005, 44, 1344− 1353. (18) Stagni, S.; Colella, S.; Palazzi, A.; Valenti, G.; Zacchini, S.; Paolucci, F.; Marcaccio, M.; Albuquerque, R. Q.; De Cola, L. Essential Role of the Ancillary Ligand in the Color Tuning of Iridium Tetrazolate Complexes. Inorg. Chem. 2008, 47, 10509−10521. (19) Mydlak, M.; Bizzarri, C.; Hartmann, D.; Sarfert, W.; Schmid, G.; De Cola, L. Positively Charged Iridium(III) Triazole Derivatives as Blue Emitters for Light-Emitting Electrochemical Cells. Adv. Funct. Mater. 2010, 20, 1812−1820. (20) Li, J.; Djurovich, P. I.; Alleyne, B. D.; Yousufuddin, M.; Ho, N. N.; Thomas, J. C.; Peters, J. C.; Bau, R.; Thompson, M. E. Synthetic Control of Excited-State Properties in Cyclometalated Ir(III) Complexes Using Ancillary Ligands. Inorg. Chem. 2005, 44, 1713− 1727. (21) Zhao, Q.; Yu, M.; Shi, L.; Liu, S.; Li, C.; Shi, M.; Zhou, Z.; Huang, C.; Li, F. Cationic Iridium(III) Complexes with Tunable Emission Color as Phosphorescent Dyes for Live Cell Imaging. Organometallics 2010, 29, 1085−1091. (22) Ladouceur, S.; Fortin, D.; Zysman-Colman, E. Enhanced Luminescent Iridium(III) Complexes Bearing Aryltriazole Cyclometallated Ligands. Inorg. Chem. 2011, 50, 11514−11526. (23) Sun, L.; Galan, A.; Ladouceur, S.; Slinker, J. D.; ZysmanColman, E. High stability light-emitting electrochemical cells from cationic iridium complexes with bulky 5,5′ substituents. J. Mater. Chem. 2011, 21, 18083−18088. (24) Kessler, F.; Costa, R. D.; Di Censo, D.; Scopelliti, R.; Ortí, E.; Bolink, H. J.; Meier, S.; Sarfert, W.; Grätzel, M.; Nazeeruddin, Md. K.; Baranoff, E. Near-UV to red-emitting charged bis-cyclometalated iridium(III) complexes for light-emitting electrochemical cells. Dalton Trans. 2012, 41, 180−191.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02289. Experimental details of the synthesis of the new precursors of ligands and the majority of the complexes. X-ray crystallographic files in CIF format and ORTEP representations of the structures. Tables with data of noncovalent interactions. Complementary information concerning the photophysical and electrochemical properties and data of the hydrogen production catalytic tests. Data from the computational studies (PDF) Accession Codes

CCDC 1562951−1562955 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (M.C.C.) *E-mail: [email protected]. (B.R.M.) ORCID

Félix A. Jalón: 0000-0002-6622-044X José V. Cuevas: 0000-0002-2421-1529 Blanca R. Manzano: 0000-0002-4908-4503 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Junta de Comunidades de Castilla-La Mancha (Project No. PEII-0214-9492), Spanish Ministerio de Economı ́a y Competitividad-FEDER (Project Nos. CTQ2014-58812-C2-1-R and CTQ2015-71353-R), the INCRECYT program (contract to M.C.C.), and Junta de Castilla y León, Consejerı ́a de Educación y Cultura y Fondo Social Europeo (BU051U16).

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REFERENCES

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Inorganic Chemistry

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Cationic Bis(cyclometalated) Ir(III) Complexes with PyridineâCarbene

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