Phenylbenzothiazole-Based Platinum(II) and Diplatinum(II) and (III) Complexes with Pyrazolate Groups: Optical Properties and Photocatalysis

Abstract

Based on 2-phenylbenzothiazole (pbt) and 2-(4-dimethylaminophenyl)benzothiazole (Me₂N-pbt), mononuclear [Pt(pbt)(R′₂-pzH)₂]PF₆ (R′₂-pzH = pzH 1a, 3,5-Me₂pzH 1b, 3,5-ⁱPr₂pzH 1c) and diplatinum (Pt^II–Pt^II) [Pt(pbt)(μ-R′₂pz)]₂ (R′₂-pz = pz 2a, 3,5-Me₂pz 2b, 3,5-ⁱPr₂pz 2c) and [Pt(Me₂N-pbt)(μ-pz)]₂ (3a) complexes have been prepared. In the presence of sunlight, 2a and 3a evolve, in CHCl₃ solution, to form the Pt^III–Pt^III complexes [Pt(R-pbt)(μ-pz)Cl]₂ (R = H 4a, NMe₂5a). Experimental and computational studies reveal the negligible influence of the pyrazole or pyrazolate ligands on the optical properties of 1a–c and 2a,b, which exhibit a typical ³IL/³MLCT emission, whereas in 2c the emission has some ³MMLCT contribution. 3a displays unusual dual, fluorescence (¹ILCT or ¹MLCT/¹LC), and phosphorescence (³ILCT) emissions depending on the excitation wavelength. The phosphorescence is lost in aerated solutions due to sensitization of ³O₂ and formation of ¹O₂, whose determined quantum yield is also wavelength dependent. The phosphorescence can be reversibly photoinduced (365 nm, ∼ 15 min) in oxygenated THF and DMSO solutions. In 4a and 5a, the lowest electronic transitions (S₁–S₃) have mixed characters (LMMCT/LXCT/L’XCT 4a and LMMCT/LXCT/ILCT 5a) and they are weakly emissive in rigid media. The ¹O₂ generation property of complex 3a is successfully used for the photooxidation of p-bromothioanisol showing its potential application toward photocatalysis.

Short abstract

We present a novel series of luminescent mononuclear (1a–c) and bimetallic (Pt^II–Pt^II) (2a−c, 3a) and (Pt^III–Pt^III) (4a, 5a) complexes based on phenylbenzothiazole (pbt, Me₂N-pbt) cyclometalating chromophores and pyrazole (1) or pyrazolate-bridging ligands (2–5). Interestingly, the Pt^III–Pt^III complexes (4a, 5a) are unusually emissive in rigid media and 3a, which displays a dual emission fluorescence (¹ILCT or ¹MLCT/¹LC) and an extremely oxygen-sensitive ³ILCT phosphorescence, depending on the excitation wavelength, has been successfully used for the photooxidation of p-bromothioanisol.

Introduction

Luminescent transition-metal compounds have attracted considerable attention owing to their wide range of applicability, such as emitters for light-emitting devices, optoelectronic materials, bioimagen probes, sensors, and photocatalysis.¹ Among these complexes, cyclometalated platinum complexes are considered one of the most promising materials due to their rich and tunable excited-state properties, which can be fine-tuned by molecular design.² Thus, mononuclear Pt^II complexes exhibit highly efficient triplet-state phosphorescence attributed to ³LC(³ππ), ³MLCT, or ³LC/³MLCT and ³LLCT/³MLCT mixtures, depending on the cyclometalating and ancillary ligands. A remarkable feature of these complexes is their propensity to develop, in the ground and/or upon excitation, noncovalent intermolecular platinum–platinum contacts through the interaction of the filled 5d_z² orbitals and ππ stacking interactions, particularly favored by the coordination of planar chelating aromatic ligands. These interactions are accompanied by simultaneous changes in the spectroscopic and optical properties, such as assembly-induced metal–metal–to ligand charge-transfer (MMLCT) transitions, which appear at lower energies than the corresponding monomers, and may be utilized to achieve single-doped white OLEDs³ or stimuli-responsive functional materials.⁴ Many of these systems have demonstrated to be solid-state low-red or near-infrared (NIR) emitters, with attractive applications in OLEDs⁵ or biological imaging.⁶

Another successful approach to modulate the optical properties is through the design of new bimetallic platinum(II) complexes in which the intramolecular Pt···Pt distance can be synthetically manipulated, primarily by the nature and bulkiness of the bridging ligands and to a minor extent by the cyclometalating groups. In addition, in bimetallic complexes, the efficiency is usually notably increased, which has been attributed to the enhanced coupling between the T₁ state and higher lying singlet states due to the incorporation of a second Pt center.⁷ Considerable interest has focused on two main categories, one featuring a butterfly shape and pyrazolate bridging ligands with relatively longer metal–metal separations and the other having a half-lantern shape and bridging thiolates with shorter intermetallic distances (Chart 1). Indeed, very efficient NIR phosphorescent OLED emitters have been reported using single-emissive binuclear half-lantern cycloplatinated complexes with various thiolates (pyridyl-thiolate, oxadiazole-thiolate, or benzo[d]thiazole-2-thiolate) as bridging ligands associated with a very strong ³MMLCT emission due to relatively short Pt···Pt distances.^5b,8 In the butterfly shape, Pt^II complexes bearing pyrazolate bridging groups, the electronic structure mainly depends on the steric bulkiness of the substituent groups of the pyrazolate ligands, which modulates the Pt···Pt separation not only in the ground state but also in the excited state. When the Pt···Pt separation is relatively large, the low-lying transitions are due to mixed LC/MLCT excitations located on separated platinum units because, upon excitation, there is a local T₁ minimum with a similar geometry to the S₀ ground state. However, if the Pt···Pt separation are shorter, upon excitation, easy intersystem crossing (on a subpicosecond time scale) to the ^1,3MMCT state takes place, leading to a T₁-global minimum in which further contraction of Pt–Pt distance (∼0.2–0.3 Å) occurs, due to depopulation of the dσ*, characteristic of the ³MMLCT excited state.⁹ In these systems, the photoinduced molecular structural change that takes place upon excitation, strongly depends on the surrounding environment of the molecule and, as a consequence, access to both excited states occurs giving rise to dual emissions depending on the media and the concentration.^8a,10 The dynamics and properties of the excited states of these binuclear complexes have been explored by ultrafast spectroscopic techniques (femtosecond absorption spectroscopy) and density functional theory (DFT) calculations in recent years.^9a,11 The electronic characteristics of the chromophoric cyclometalating group also play a key role on the final electronic nature of these diplatinum complexes, but its effect has been comparatively less explored.¹² In this field, despite the tremendous role that Pt···Pt nonbonded interactions play on the optical properties of cycloplatinated complexes, much less attention has been devoted to complexes featuring formal covalent Pt–Pt bonds (Chart 1).¹³ Indeed, in diplatinum cycloplatinated compounds, reports on emissive Pt^III–Pt^III (d⁷–d⁷) derivatives are quite rare.¹⁴ In these complexes, the lowest lying excited state has usually a remarkable metal center character (dσ_M–dσ*_M), being therefore nonemissive.

Chart 1. Schematic Drawings of Diplatinum(II) and (III) Complexes Bearing Pyrazolate and N^S Bridging Ligands.

Heterocycles featuring benzothiazole frameworks have been widely recognized as biologically active compounds¹⁵ and are also important fluorophores used to construct donor/acceptor systems displaying intramolecular charge-transfer properties.¹⁶ In this context, over the past few years, we have focused on the design and study of the optical and biological properties of cyclometalated Pt^II, Pt^IV, and Ir^III mononuclear complexes based on 2-phenylbenzothiazole (pbt) and 2-(4-dimethylaminophenyl)benzothiazole (Me₂N-pbt) frameworks.¹⁷ Following our interest in these systems, here we report new families of mononuclear [Pt(pbt)(R′₂-pzH)₂]PF₆ (R′₂-pzH = pzH 1a, 3,5-Me₂pzH 1b, 3,5-ⁱPr₂pzH 1c) and binuclear (Pt^II–Pt^II) [Pt(pbt)(μ-R′₂pz)]₂ (2a–c), [Pt(Me₂N-pbt)(μ-pz)]₂ (3a) and Pt^III–Pt^III [Pt(R-pbt)(μ-pz)Cl]₂ (R = H 4a, NMe₂5a) complexes based on phenylbenzothiazole (pbt) and 2-(4-dimethylaminophenyl)benzothiazole (Me₂N-pbt) as cyclometalated groups and different pyrazole (1) or pyrazolate (2–5) as bridging ligands. Their photophysical properties, supported by theoretical calculations, are presented.

On the other hand, sulfoxides, in particular asymmetric sulfoxides, are nowadays widely used in organic synthesis, fine chemicals, medicine, pesticides,¹⁸ and, recently, have also emerged as efficient ligands in transition-metal catalysis.¹⁹ Common oxidants for sulfide oxidation to sulfoxides (H₂O₂, K₂S₂O₈, or m-chloroperbenzoic acid) are usually not eco-friendly or cause serious environmental pollution. In recent years, photocatalytic oxidations have been successfully reported employing O₂ as an environmentally friendly oxidant and different organic photosensitizers, such as Rose Bengal,²⁰ Rivoflavin,²¹ or Bodipy.²² In this area, the employment of metal phosphorescent complexes as photosensitizers is rather less developed; although recently, the efficiency of several Ir^III, Ru^II and Au^I systems have been demonstrated.²³ To increase the knowledge on these systems, we sought to explore the utility of complex 3a, which displayed strong oxygen sensitivity on its phosphorescent band, for the photocatalytic oxidation of sulfides using O₂ as a green oxidant.

Results and Discussion

Synthesis and Characterization

Synthesis and Characterization of Cationic Bis-Pyrazole Pt^II Complexes

The reaction of the DMSO solvate [Pt(pbt)Cl(DMSO)]^17e with 1 equiv of TlPF₆ and 2 equiv of the corresponding pyrazole ligand in acetone at room temperature gives rise to new bis-pyrazole complexes of the type [Pt(pbt)(R′₂-pzH)₂]PF₆ (1 R′₂-pzH = pzH (a), 3,5-Me₂pzH (b), and 3,5-ⁱPr₂pzH (c)) (see Scheme 1 and Experimental Section). All attempts to generate related mononuclear complexes with the 2-(4-dimethylaminophenyl)benzothiazole as a cyclometalating ligand [Pt(Me₂N-pbt)(R′₂-pzH)₂]PF₆ starting from [Pt(Me₂N-pbt)Cl(DMSO)] have been unsuccessful. In this case, the reactions evolve with formation of complex mixtures in which the chelating dimethylaminophenylbenzothiazole (Me₂N-pbt) deprotonates the coordinated pyrazoles, giving rise to mixtures with the corresponding bridging pyrazolate bimetallic complexes.

Scheme 1. (a) Synthesis of Bis-pyrazole Complexes, (i) 3,5-R′₂-pzH (1 equiv), TlPF₆ (1 equiv), Acetone, 298 K, 6 h and (b) Schematic View of Atropisomers for 1b.

After a workup, complexes 1a–c were obtained as yellow solids in good yields (85–90%) and were fully characterized (see Figures S1–S3). They show strong IR absorptions due to the PF₆ anion (ν 557, 841–845 cm^–1) and broad bands at 3363–3384 cm^–1, assigned to the tension frequency N–H of the pyrazole groups. The corresponding molecular peaks [Pt(pbt)(R′₂pzH)₂]⁺, found in the ESI(+) mass spectra of 1b (m/z 597) and 1c (m/z 709), and their molar conductivity (1:1 electrolytes) agree with the proposed stoichiometry. The presence of peaks related to the formation of [Pt₂(pbt)₂(pz)₂] in 1a (m/z 967 [Pt₂(pbt)₂(pz)₂+Na]⁺, 945 [Pt₂(pbt)₂(pz)₂ + H]⁺, and 877 [Pt₂(pbt)₂(pz)]⁺) suggests that this complex has a clear tendency to form the binuclear complex. The ¹H and ¹³C{¹H} NMR spectra in CD₃COCD₃ show one set of signals corresponding to the cyclometalated group and nonequivalent R′₂pzH ligands. The assignment of the signals was made by ¹H–¹H (COSY and TOCSY) and ¹H–¹³C (HSQC and HMBC) correlations. The more characteristic protons of the pbt ligand, H⁷ and H¹¹, this last showing Pt coupling (³J_Pt–H ∼ 40 Hz), appear as doublets suffering an expected strong upfield shift [δ 6.18 (H⁷) and 6.26 (H¹¹) 1a, 6.26–6.19 (H⁷ and H¹¹) 1b, and 6.01 (H⁷) and 6.15 (H¹¹) 1c] compared to the precursor [Pt(pbt)Cl(DMSO)] [δ 9.62 (H⁷) and 8.42 (H¹¹)], due to the anisotropic shield of the aromatic pyrazole ligand. Bis-pyrazole compound 1a displays a broad signal with Pt satellites assigned to the H^3’ (8.10 ppm, ³J_Pt–H 20.3 Hz), whereas the derivatives 1b–1c show the corresponding signals of the alkyl substituents on the pyrazole co-ligands in the aliphatic region. Complexes 1a and 1c show two broad singlets at low frequencies due to the N–H protons of the pyrazole groups, whereas complex 1b displays four signals indicating the presence of two conformational isomers in a ca 35:65 molar ratio. We note that in these complexes there are two possible relative orientations of the NH units of the pyrazole ligands in relation to the platinum coordination plane leading to a syn/anti isomerism (Scheme 1b). In complex 1b, the interconversion between both conformers (atropisomers) seems to be slower than the NMR time scale. The presence of both conformers is also apparent in the H^4′,4″ protons (δ 6.40–6.28 range, 2H) and methyl resonances (2.47–2.35 ppm, 2H) of the nonequivalent 3,5-dimethylpyrazole ligands and in the corresponding ¹³C{¹H} NMR signals (See Figure S2). Finally, 1a–1c complexes display the expected doublet (∼−72.6 ppm, ¹J_F–P 706 Hz) and septuplet (∼−145 ppm) in the ³¹P{¹H} and ¹⁹F{¹H} NMR spectra, respectively, corresponding to the PF₆ counteranion.

Suitable crystals for complexes 1a and 1b have been obtained by slow diffusion of n-hexane through a solution of the corresponding compound in acetone at low temperatures. In the case of 1a, the X-ray diffraction study (Tables 1, S1 and S2) revealed that this complex crystallized with the PO₂F₂ anion generated by the partial hydrolysis of the PF₆^– group (1a·PO₂F₂). Hydrolysis of the PF₆^– anion has been previously observed.^17f A view of the molecular structures with the selected bond lengths and angles and a vision of the interaction of the cation and the anions is presented in Figure 1. Both cations showed the expected chelate C^N coordination of the pbt ligand and the cis arrangement of the two R′₂-pzH ligands. The bite angle of the pbt ligand [80.4(9)° 1a·PO₂F₂, 80.8(4)° 1b] and the Pt–C(1) [2.01(2) 1a·PO₂F₂, 2.007(10) Å 1b] distance (Table 1) are within the expected values for this type of complexes.²⁴ The Pt–N(2) distance [2.08(2) 1a·PO₂F₂, 2.117(9) Å 1b] trans to the C-metalated is longer than the Pt–N(4) distance [1.960(18) 1a·PO₂F₂, 2.002(9) Å 1b] (Table 1), reflecting the high trans influence of the metalated carbon. The phenyl benzothiazole ligand is almost coplanar with respect to the Pt coordination plane (deviations 2.60° 1a·PO₂F₂, 7.56° 1b). In the crystal, the pyrazole ligands display a syn orientation, which is stabilized by the occurrence of short hydrogen bonding interactions between the NH protons and the fluorine atoms of the counteranion. The F···H distances are shorter in 1a·PO₂F₂ than in 1b (1.868, 1.834 Å 1a·PO₂F₂ vs 2.090, 2.060 Å 1b) and within the range reported for F···H bonding interactions.²⁵ Both complexes form dimers through moderate intermolecular π···π (pbt···pbt, 3.462 1a·PO₂F₂ and 3.484 Å 1b) interactions (Figure 1).

Table 1. Selected Bond Distances (Å) and Angles (deg) of 1a·PO₂F₂, 1b, 2a, 3a·THF, 4a, and 5a·0.5CH₂Cl₂.

parameter	1a·PO2F2	1b	2a	3a·THF	4a	5a·0.5CH2Cl2
Pt-CC^N	2.01(2)	2.007(10)	2.001(3)	2.001(6)	2.013(2)	2.012(6)
			1.998(3)	1.993(6)		2.012(7)
Pt-NC^N	2.042(16)	2.038(8)	2.036(2)	2.043(5)	2.052(2)	2.055(5)
			2.033(2)	2.039(5)		2.049(6)
Pt-Npyr(trans-C)	2.08(2)	2.117(9)	2.099(2)	2.103(5)	2.118(2)	2.146(5)
			2.090(2)	2.105(5)		2.137(6)
Pt-Npyr(trans-N)	1.960(18)	2.002(9)	1.989(2)	2.004(5)	2.0038(19)	2.001(6)
			2.000(2)	1.998(5)		2.003(6)
Pt-Pt			3.344	3.1740(4)	2.58972(19)
Pt-Cl					2.4177(6)	2.4304(15)
						2.4112(19)
CC^N-Pt- Npyr	94.9(9)	92.6(4)	95.80(10)	95.0(2)	93.73(9)	91.4(2)
			95.80(11)	95.0(2)		92.2(3)
CC^N-Pt- NC^N	80.4(9)	80.8(4)	80.76(10)	81.6(2)	80.83(9)	80.9(2)
			80.92(11)	81.1(2)		80.8(3)
NC^N-Pt- Npyr	97.1(8)	99.2(3)	99.50(9)	98.7(2)	99.73(8)	102.3(2)
			98.42(9)	99.0(2)		100.8(2)
Npyr-Pt- Npyr	87.7(8)	87.45(3)	84.16(9)	84.8(2)	84.32(8)	84.0(2)
			84.72(9)	85.0(2)		84.3(2)
Cl-Pt-Pt					162.894(14)	165.71(6)
						166.71(6)

Figure 1.

Molecular structure and crystal stacking of complexes 1a·PO₂F₂ (a) and 1b (b).

Synthesis and Characterization of Bis(pyrazolate) Diplatinum Complexes

The binuclear pbt–platinum complexes with pyrazolate bridging ligands [Pt(pbt)(μ-R′₂pz)]₂ (R′₂-pz = pz 2a, 3,5-Me₂pz 2b, 3,5-ⁱPr₂pz 2c) were prepared by the reaction between the corresponding bis-pyrazole [Pt(pbt)(R′₂-pzH)₂]PF₆ (1a–1c) with excess of NEt₃ at room temperature (Scheme 2i). The related 2-(4-dimethylaminophenyl)benzothiazole derivative [Pt(Me₂N-pbt)(μ-pz)]₂3a was also prepared by reaction of [Pt(Me₂N-pbt)Cl(DMSO)] with 1 equiv of Hpz, TlPF₆, and NEt₃ (see Scheme 2ii) or by using Hpz (1 equiv) and excess of KOH (see the Supporting Information for details). Following these pathways, the binuclear complexes were selectively obtained as the anti-isomers. Complex 2a was also prepared starting from the mononuclear complex [Pt(pbt)Cl(DMSO)], by using Hpz (1 equiv) and excess of KOH (Scheme 2iii). However, in this case, the reaction evolves with formation of a mixture of the isomers 2a-anti and 2a-syn in relation ∼5:1, as assessed by ¹H NMR spectroscopy.

Scheme 2. Synthesis and Conditions for the Preparation of Pt₂^II Derivatives; (i) NEt₃ (exc.), Acetone, 8 h; (ii) Hpz (1 equiv), TlPF₆ (1 equiv), NEt₃ (exc.), Acetone, 6 h; and (iii) Hpz (1 equiv), KOH (exc.), EtOH/Acetone, 24 h.

The isomer 3a-anti was also obtained using via (iii).

The bimetallic nature of the complexes was supported by ESI(+) or MALDI(+)-MS analysis, as they show as the parent peak, the corresponding one due to [M + H]⁺ (m/z 945 2a, 1000 2b, 1112 2c, and 1030 3a) (Section S3 and Experimental Section in the Supporting Information). Unfortunately, only complexes 2a and 3a are soluble enough in THF-d⁸ to characterize them by ¹H NMR spectra. The anti-isomers (anti-2a and 3a) exhibit the presence of only one set of pbt ligands together with one set of pyrazolate groups. In both complexes, the most deshielded signal corresponds to the H⁷ of the pbt, which appear as a doublet (δ 7.89 2a-anti, 7.74 3a), whereas the most shielded is attributed to the H^4’ of bridging pyrazolate (δ 6.34 2a-anti, 6.3 3a) (Supporting Information, Section S2). In the anti/syn-2a mixture, the most distinct signal of the type of isomer corresponds to the H^4’ of the pyrazolate, which are equivalent for the anti-2a isomer (δ 6.34) and inequivalent in the syn-2a (δ 6.48; 6.32) (Figure S4b). Suitable yellow crystals for X-ray studies of anti-2a and 3a·THF were obtained from slow evaporation of a THF solution. Their molecular structures are depicted in Figures 2 and S11, and the corresponding structural bonding details are provided in Tables 1 and S2. The structures confirm the anti-arrangement of the Pt(R-pbt) units bridged by the two pyrazolate ligands, in coherence with the NMR spectra. Both Pt^II complexes display the typical butterfly-like structure with a C₂ symmetry. The intermetallic distances [3.344 2a and 3.1740(4) Å 3a, Table 1] are shorter than the sum of the van der Waals radii of the two Pt (3.5 Å) and comparable to those found in related μ-pyrazolate diplatinum complexes (2.834–3.486 Å).^12,26 It has been previously shown that in this type of complexes, both the Pt···Pt distance and the tilt angle decrease due to the increasing demanding of the substituents of the bridging pyrazolate ligand (3- and 5-positions).^26d Also the bulkiness of the cyclometalating group affects to the proximity of the platinum fragments in these type of binuclear complexes.²⁷ Surprisingly, despite the presence of bulky NMe₂ groups in 3a, the Pt···Pt is shorter and the angle between platinum planes is smaller than in 2a (84.23° for 2a and 73.50° for 3a), indicating stronger interactions between the platinum fragments, a fact that might be attributed to the coplanarity between the NMe₂ and the bt unit. On the basis of previous results, it is suggested that the Pt···Pt separation could likely be shorter in complex 2b (R’ = Me) and, particularly, in 2c featuring R’ = Prⁱ groups, what agrees with their intense orange colors. The Pt–C and Pt–N bond distances around the Pt centers are similar to those found in related complexes. In particular, the Pt–N_pz distance trans to the C_R-pbt [2.099(2) 2a and 2.103(5) Å 3a] is longer than that of trans to the N of the R-pbt group [2.000(2) 2a and 2.004(5) Å 3a] (Table 1), in coherence with the high trans influence of the metalated carbon. The extended structure of both compounds shows weak π···π interactions between cyclometalated groups of different molecules giving rise to dimers (3.585–3.641 Å) supported by secondary intermolecular sulfur···carbon (S_R-pbt···C_R-pbt ∼ 3.43 Å) and C–H···π (2.704–2.971 Å) interactions. These dimers additionally stack giving rise to a 1D infinite arrangement supported by C–H···π (2.778–2.836 Å) and (S_R-pbt···C_R-pbt ∼ 3.20 Å) (Figures 2 and S11).

Figure 2.

Molecular structure and crystal packing of complex 2a.

We observed that complexes [Pt(R-pbt)(μ-pz)]₂ (R = H 2a, Me₂N 3a) are stable in CHCl₃ solution in the dark, but in the presence of sunlight they evolve slowly (∼24 h) to form the metal–metal-bonded Pt^III–Pt^III complexes [Pt(R-pbt)(μ-pz)Cl]₂ (4a, 5a), which precipitate in the mixture in a ca yield of 60%. This type of two center-two electron oxidation reactions have been previously studied and depend on many factors involving either a radical-like mechanism (with the O₂ acting as a radical R* trap) and/or a thermally or photochemically activated processes.^26h,28 Recent studies on pyrazolate- and thiolate-bridged diplatinum complexes with very short Pt···Pt distances indicate that the ¹MMLCT excited states easily triggers the photooxidation of these complexes with CHCl₃.^10b As expected, the complexes were also obtained by reacting 2a and 3a with iodobenzenedichloride (PhICl₂) in CH₂Cl₂ at 0 °C, being precipitated and separated of the mixture in a 59–73% yield (Scheme 3). Complexes 4a and 5a have been characterized by mass spectrometry and NMR spectroscopy (Experimental Section and Sections S2 and S3 in the Supporting Information) and their structures confirmed by X-ray. The formation of the oxidized species is evident by the presence of peaks due to [M-Cl]⁺ (m/z 979 4a, 1065 5a) as parent peaks in their mass spectra.

Scheme 3. Synthesis and Conditions of Binuclear Bis-pyrazolate Pt₂^III Complexes; (i) CHCl₃, Sunlight, 298 K, 24 h or PhICl₂, CH₂Cl₂, 0 °C, 8 h.

The ¹H NMR spectra of 4a and 5a display only one set of signals for the R-pbt groups and for the pyrazolate ligands, indicating an anti-arrangement of the cyclometalated group. The ortho protons to the cyclometalated ligand, H¹¹, are notably shielded and the three-bond platinum-coupling constant (³J_Pt–H 31.1 4a, 35.2 Hz 5a) is smaller than in complex 1a (43.3 Hz), in agreement with the increased oxidation state. The molecular structures of complexes 4a and 5a·0.5CH₂Cl₂ were determined by single-crystal X-ray diffraction (Table 1 and Figures 3 and S12). The Pt^III–Pt^III complexes retain the boat-like structure of the Pt₂N₄ core and the anti-arrangement of the platinum(C^N) fragments. The interplanar angles between the platinum coordination planes are notably reduced with respect to those observed in Pt^II–Pt^II precursors (35.68° 4a and 34.30° 5a vs 84.23° 2a and 73.50° 3a) due to the formation of the Pt–Pt bond. Each Pt^III center shows a distorted octahedral coordination environment with axial positions occupied by chloride atoms and the other Pt^III center, with angles Cl–Pt–Pt of 162.894(14)° for 4a and 165.71(6)° and 166.71(6)° for 5a (Table 1), similar to those reported for related complexes.^12,26h As expected, the formation of a Pt–Pt formal bond produces a shortening in the Pt–Pt distance with respect to complexes 2a and 3a [2.5897(2) 4a 2.5776(3) Å 5a vs 3.344 2a, 3.1740(4) Å 3a, Table 1]. The molecular packing of these complexes shows extended π···π interactions through the R-pbt groups with distances of 3.458–3.544 Å in 4a and 3.416–3.538 Å for 5a, forming 1D chains (Figures 3 and S12).

Figure 3.

Molecular structure and crystal packing of 5a·0.5CH₂Cl₂.

Optical Properties

Absorption Properties and TD-DFT Calculations

The UV–vis spectra of bis(pyrazole) complexes 1a–1c in THF (5 × 10^–5 M) solution and in the solid state (Table S3, Figures 4 and S13a) are rather similar, indicating a negligible influence of the pyrazole groups. In solution, they show one intense absorption at ca. 260 nm, attributed to spin-allowed π–π* intraligand (¹IL, pbt) transitions. They exhibit an additional band in the 300–370 nm range and a less intense feature in the low-energy region at 385–420 nm (ε ∼ 10³ L·mol^–1·cm^–1). On the basis of theoretical calculations carried out in complex 1a (Figure 4 and Section S5, Supporting Information), which indicate that the calculated low energy transition (S₁ 382 nm) is mainly contributed by the HOMO (80% pbt, 20% Pt) to LUMO (93% pbt, 6% Pt) excitation, the low-energy feature is mainly attributed to an intraligand ¹IL (pbt) with some ¹MLCT. The band in the range of 300–370 nm has also a mixed ¹MLCT/¹IL character. The Hpz orbitals contribute from the LUMO+1 (63%) and they are involved in the S₆/S₈ transitions with very low oscillator strength (Table S4 and Figure S15).

Figure 4.

UV–vis absorption spectra of mononuclear complexes 1 and schematic representation of selected orbitals frontiers and transitions for 1a.

As noted, the binuclear Pt^II–Pt^II complexes 2b and 2c are insoluble in common solvents. Therefore, only the absorption spectra of the pyrazolate bridging complexes 2a and 3a could be recorded in THF solution (Figure 5). In both compounds, the band at λ < 350 nm are ascribed to π–π* intraligand (¹IL, pbt) and ligand-to-ligand (R₂pz to pbt) transitions. In 2a, the bands between 350 and 410 nm are ascribed to mixed ¹IL/¹MLCT and the low energy feature at ca. 440 nm to ¹MMLCT transitions, as supported by calculations (see Table S4). In the Me₂N-pbt 3a, the bands are more intense indicating a stronger intraligand charge-transfer contribution (NMe₂ to bt) and are slightly red-shifted (3a 430, 450 nm ε ∼ 20 × 10³ vs 2a 384, 440 nm ε 4.78 × 10³). As seen in Figure 5, which shows the frontier orbitals for both complexes, in both, the target LUMO and L+1 spread on the cyclometalated ligands. However, whereas in 2a, the highest orbital HOMO has a σ* (5d_z²–5d_z²) character and is located on the two platinum atoms; in 3a, a similar orbital is the H-2. In 3a, the HOMO and HOMO–1 are destabilized in relation to 2a and are located on the Me₂N-pbt. For 2a, the calculated S₁ (452 mn) and S₂ (439 nm) have ¹MMLCT characters, whereas for 3a the most intense calculated absorptions are S₂ (423 mn) and S₃ (415 nm) and have mixed ¹ILCT/¹MMLCT. It is interesting to note that in both complexes the calculated transitions with remarkable metal platinum contribution and mixed ¹MLCT/¹ILCT character appear at higher energy (S₄ 397 nm 2a; S₇ 351 nm 3a). The solid-state absorption spectra of 2a–2c have been also recorded. The main lowest-energy band ranges from 438 (2a), 455 (2b) to 503 nm (2c), with extending tails, in agreement with their color (Table S3 and Figures S13, S14), what are ascribed to mixed ¹IL/¹MMLCT, with higher contribution of this latter on going from 2a to 2c, likely due to a lower Pt–Pt distance by increasing the steric bulk of the substituents. Low intense features in the tails (>480 nm 2a, 2b or 540 nm, 2c) are observed, which are probably of spin-forbidden nature.

Figure 5.

UV–vis absorption spectra of mononuclear complexes 2a and 3a in THF (5 × 10^–5 M) and an schematic representation of their frontier orbitals and selected excitations.

The absorption spectra of Pt^III–Pt^III4a and 5a are included in Figure 6. The most significant feature in relation to complexes 2a and 3a is the notable hypsochromic shift in the low energy region. Thus, 4a displays a moderately intense absorption low energy band at 348 nm with a shoulder at 379 nm and 5a a band at 402 with a shoulder at 444 nm, clearly blue-shifted in relation to the corresponding Pt^II–Pt^II (440 2a, 430 nm 3a), reflecting to the oxidation of the platinum centers and the notable change in the frontier orbitals. DFT and time-dependent DFT (TD-DFT) calculations were performed with the Gaussian 16 program package to explore the orbital frontiers and the nature of the excited states and transitions (Tables S4–S7 and Figure S16). Figure 6 includes a selection of the orbitals and excitations. In both complexes, the LUMO is essentially identical located on the ClPtPtCl axis and formed by the antisymmetrical σ* combination of the 5d_z² of the two platinum centers and the p_z orbitals of the chloride atoms (58% Pt–Pt and 26% Cl–Cl). Therefore, population of this orbital should cause strong distorted excited states with elongation of the Pt–Pt separation and of the Pt–Cl bonds. The symmetrical combination σ of the 5d_z² is mainly located on the HOMO–2, but this orbital has also a notable contribution of the phenylbenzothiazolate groups. For complex 5a, the HOMO and H-1 are primarily located on the Me₂N-pbt groups, while in the pbt complex, 4a have also small platinum and Cl contributions (see Table S7). The calculated low lying S₁–S₃ (431 to 489 nm 4a and 748–489 nm 5a) transitions have very low oscillator strength and complex configuration (mixed LMMCT/LXCT/L’XCT 4a and LMMCT/LXCT/ILCT 5a) (L = R-pbt, X = Cl, L’ = pz). The most intense low energy-calculated band is S₄ ascribed to H-4, H-2 to LUMO in 4a and to a more complex configuration in 5a (H-2, H-5, H-6, and H-10 to LUMO) having a mainly L’MMCT/L’XCT nature in 4a and LMMCT/L’MCT/XC/MC in 5a. The solid-state spectra also reflect the oxidation of the Pt centers exhibiting blue-shifted low-energy features in relation to the Pt^II–Pt^II precursors (see Table S3, Figure S14).

Figure 6.

UV–vis absorption spectra in THF solution (5 × 10^–5 M) of 4a and 5a and an schematic representation of their frontier orbitals and selected excitations.

Emission Properties and DFT Calculations

The emission spectra of complexes 1 and 2a–5a were registered in THF fluid solution (5 × 10^–4 M for 1 and 2a–3a and 5 × 10^–5 M for 4a), THF glasses at 77 K, polystyrene (PS) films at 10% wt, and in the solid state for all complexes at room and low temperatures (Tables 2 and S8, Figures 7–10 and S17–S19). Mononuclear complexes 1a–1c show in the solid state, THF solution and doped films at 298 K a similar long-lived vibronically structured emission band (λ_max ∼ 540 nm), slight blue-shifted at low temperatures (λ_max ∼ 530 nm), attributed to a predominant intraligand transition (³IL) with some metal-to-ligand charge-transfer (³MLCT) characters. This emission is comparable to those reported for related neutral heteroleptic monomers [Pt(pbt)(L^X)] (L^X = O^O, N^O, P^O),^4d,17e,29 and the assignment is further supported by optimization of the triplet excited (T₁) state for complex 1a⁺. As illustrated in Figure 7, the spin density surface at the optimized T₁ state is centered on the pbt ligand with some contribution of the platinum (Pt ∼ 0.13). The adiabatic calculated emission wavelength (638 nm) exhibits the expected overestimated value in relation to the experimental data (∼540 nm, THF, 298 K). The calculated quantum yields are moderate in solution (10–21%, deoxygenated conditions, Table 2) but relatively low in rigid media (solid and PS, 1–8% Table S8), indicating aggregation-caused quenching characteristics (ACQs).³⁰ The relatively strong π···π and S··π interactions between the pbt ligands in these rigid media, as found in their X-ray structures, could provide easy deactivation pathways and may account for the reduced efficiencies. Notwithstanding, it is worth noting that reports on luminescent mononuclear bis-pyrazole Pt^II complexes are rare.³¹ A comparison of the radiative and nonradiative rate constants (k_r and k_nr) reveals that complex 1a presents the highest k_nr value in relation to 1b and 1c (k_nr 2 × 10⁵1a vs 1.4 × 10⁵1b, and 1.5 × 10⁵1c) and the lowest k_r (2.2 × 10⁴1a vs 3.5 × 10⁴1b and 4 × 10⁴1c), resulting in a less efficient phosphor (φ 10% 1a vs 20% 1b and 21% 1c). This could be associated with a less steric hindrance of the pyrazole ligand in 1a, which provides less rigidity.

Table 2. Photophysical Data for Complexes 1a–1c and 2a and 3a in THF Solution (5 × 10^–4 M)^a.

	298 K						77 K
compound	λem/nm	φb	φc	τd/μs	kre/s–1	knrf/s–1	λem/nm
1ag	543, 577max, 620	0.10	0.01	4.5	2.2 × 104	2.0 × 105	529, 569max, 618
1bg	540, 576max, 620	0.20	0.02	5.7	3.5 × 104	1.4 × 105	526, 567max, 616
1cg	534, 573max, 617	0.21	0.03	5.2	4.0 × 104	1.5 × 105	527, 568max, 615
2ag	550, 589max, 634	0.01	0.01	0.9	1.1 × 104	1.1 × 106	544, 585max, 634
3ag	405max, 570, 614b,g			0.0016 [405]			565max, 609
	500, 570max, 614b,h			0.0015 [500]
	500, 570max, 614c,h			8.7 [570]

^aλ_ex 365–450 nm.

^bDeoxygenated.

^cOxygenated.

^dλ_ex 390 nm(LED).

^ek_r = ϕ/τ_average.

^fk_nr = (1 – ϕ)/τ_average in deoxygenated conditions.

^gλ_ex 365 nm.

^hλ_ex 400–450 nm.

Figure 7.

(a) Emission spectra (λ_ex 420 nm) of complexes 1 in THF solution (5 × 10^–4 M) at 298 K and (b) spin density surface on T₁ state of complex 1a.

Figure 10.

(a) Photographs showing the switch-on and enhancement of the phosphorescent emission of 3a in an aerated THF solution upon excitation at 365 nm. (b) Emission spectra of 3a in THF 5 × 10^–4 M solution in the presence of O₂ with different irradiation times. (c) Relative energy and character of the more intense vertical singlet and triplet excitations at the S₀ geometry of 3a.

The bimetallic complex 2a displays in all media a well-resolved vibronic emission profile with λ_max (∼540–550 nm) (Figure 8), close to that observed for complexes 1, being therefore ascribed to a local mixed ³IL/³MLCT excited states. This behavior could be related to the relatively long Pt···Pt of 3.344 Å distance found for this complex, which makes it difficult to reach the σ²-σ^*1 configuration characteristic of the ³MMLCT excited state.^10a,11b,26k The optimized S₀ structure for complex 2a shows a Pt–Pt separation of 3.2069 Å, comparable to that found in the crystal structure, and similar separation is found in the optimized T₁ excited state (see Table S5), indicating the absence of short intramolecular platinum–platinum bonding interaction typical of the ³MMLCT excited state in T₁. To confirm the nature of the emission, the lowest T₁–T₃ vertical excitations at the S₀ geometry and the corresponding optimized T₁–T₃ were calculated (Tables S4, S9, S10 and Figure 8b). The two lowest vertical triplet excitations T_1,2, which are close in energy (2.4 and 2.5 eV), and their corresponding optimized T_1,2 states (676 nm) are located on one of the Pt(pbt) fragments having mixed ³IL/³MLCT nature. As illustrated in Figure 8b, the following excitation, T_3, has a ³MMLCT character but is located at 0.3 eV above T₁. In 2a, the metal contribution in T₁ increases in relation to the mononuclear complex 1a (0.2125 2a vs 0.1303 1a, Table S9) pointing to a higher metal-to-ligand charge-transfer (³MLCT) contribution in 2a, in correlation with the lower lifetime recorded in THF solution for this bimetallic complex in comparison to the 1a one (4.5, 1a vs 0.9 μs, 2a). The recorded quantum yield for 2a in deoxygenated THF solution (1%) is lower than for 1a (10%), but relatively similar in solid and PS (Table S8). This decrease in the quantum efficiency is attributable to an appreciable increase in k_nr (1.1 × 10⁶2a vs 2.0 × 10⁵1a), while there is a decrease in k_r (1.1 × 10⁴2a vs 2.2 × 10⁴1a).

Figure 8.

(a) Emission spectra (λ_ex 400–450 nm) of complex 2a in THF solution at 298 and 77 K. (b) Relative energies and character of the vertical triplet excitations (T₁–T₃) for complex 2a. Emission spectra of 2a–c in solid state (c) at 298 (d) at 77 K.

For complexes 2b and 2c, only emission properties in the solid state were recorded (Figure 8c,d). Dimethylpyrazolate-bridge derivative (2b) shows, at room temperature and at 77 K, a similar band to that obtained for complex 2a (∼550 nm) indicating emission from an ³IL/³MLCT excited state. For complex 2c, with a bulkier substituent on the bridge group (R = ⁱPr), the emission is broader at 298 K peaking at 560 nm and well stylized and red-shifted to 600 nm at 77 K (Figure 8d). This fact suggests the contribution of the low lying ³MMLCT excited state, which becomes predominant by decreasing the temperature.

Complex 3a, comprising the donor–acceptor Me₂N-pbt groups, displayed a significantly different behavior from that of the related complex 2a (Table 2). It exhibits, upon excitation on the low energy band at 400–450 nm, and in a carefully deoxygenated THF solution at 298 K, in addition to a strong long-lived low energy band (LE) in the yellow-orange region (570 nm), a minor high energy feature (HE) in the blue-green region (500 nm) (see Figure 9a, orange line). The high energy feature is short-lived (1.5 ns) and displays characteristic mirror band-shaped with the longest wavelength absorption band, being therefore ascribed to S₁ → S₀ fluorescence having metal-perturbed intraligand charge-transfer ¹ILCT (Me₂Nph-to-bt) characters, whereas the LE-structured phosphorescent band is ascribed, according to calculations, to ³ILCT. In this complex 3a, the two first triplet excitations T_1,2 at the S₀ geometry are essentially isoenergetic (556 nm) and exhibit an ILCT character (Figure 10c). The T₃, with a MMLCT character, lies more separated from T₁ (0.6 eV) than in 2a (0.3 eV). The spin density distribution on the optimized T₁ is mainly located on one of the cyclometalated groups with a lower metallic contribution in relation to the pbt-derivative 2a (0.0809 3a vs 0.2125 2a), supporting primarily ³ILCT nature for the phosphorescent emission (Table S9). As was expected, the ratio F/P clearly increases in oxygenated solution due to a partial quenching of the low energy phosphorescent band (Figure 9a, blue line). The determined phosphorescence quantum yield in degassed solution is 17% and is reduced to 2% in air equilibrated solution. The excitation spectra of both bands correlate with the absorption spectrum in the low energy region, indicating that both emissions came from the same complex. However, the excitation spectra are not exactly identical in the region around 365–400 nm, where there is a notable MLCT contribution. This suggests that, while the fluorescent HE band mainly proceeds of excitation of the ¹ILCT, the phosphorescent LE band is also notably populated from high-energy S_n excited states. Interestingly, we also observe that the emission depends on the excitation wavelength. Thus, upon excitation at 365 nm, a dual emission is also observed formed by a short-lived blue-shifted fluorescence band (1.6 ns) located at 405 nm and the structured low energy phosphorescent at 570 nm, which is extremely air sensitive being nearly absent in air-equilibrated THF solution (decreases from 23% in degassed solution to less of 1% in air equilibrated). The new fluorescence band is related to an excitation manifold located at 350 nm, whereas the excitation spectrum when detected the LE band is identical to that observed for the LE band upon exciting in the low energy region (400–450 nm) (Figure 9b). Our calculations suggest that the excitation S₇, at 351.3 nm, has mixed ¹MLCT/¹LC nature with a remarkable platinum contribution and minor contribution of the NMe₂ (Figure 10b). In agreement with this, the high quantum efficiency in 3a upon excitation to 365 nm (ϕ = 23%) is mainly attributable to its higher k_r (2.6 × 10⁴) and lesser k_nr (8.9 × 10⁴) in relation to those obtained upon excitation to 400–450 nm (Table 2), probably related to the higher metal contribution in the LE emission using the high-energy excitation wavelengths. This wavelength dependence suggests the occurrence of hyper-intersystem crossing (HISC) from S₇ (¹MLCT/¹LC) to T₁, i.e., relaxation from S₇ to T₁ clearly competes with internal conversion (IC) to S₁. It seems that the energy transfer from relaxed (¹MLCT/¹LC)* to ¹ILCT* is nonefficient. This relatively rare behavior has been previously observed in some platinum complexes, being related to the presence of relaxed S₁ states having strong ππ* characters with an essentially null metal contribution.^17d,32 At 77 K, the fluorescence is lost and only the phosphorescence emission band at 565 nm is developed regardless of the wavelength used in the excitation.

Figure 9.

Excitation (····) and emission (—) spectra of complex 3a in oxygenated and deoxygenated THF 5 × 10^–4 M solution at 298 K upon excitation at (a) 400–450 and (b) 365 nm.

Interestingly, we observed that in aerated THF solution, upon prolonged photoexcitation at 365 nm, a continuous enhancement of the phosphorescent band is observed, rising its maximum intensity in ca 27 min (Figure 10b). The process is also visible with a hand UV–vis lamp in which the initial violet emission, attributed to ¹MLCT/¹LC fluorescence, was gradually changing to a final orange enhanced emission (Figure 10a). The intense orange emission was switched off by simply shaking the solution allowing its oxygenation. A similar process, which is reversible and can be repeated several times, was also observed in DMSO but does not take place in other solvents, such as acetonitrile, toluene, or MeOH. This relatively rare behavior has been previously observed for us^17d and other groups,³³ being explained by the occurrence of a local sensitization caused by energy transfer from the low energy triplet to ³O₂ producing singlet ¹O₂ able to selectively react with the solvent (THF³⁴ or DMSO^17d), thus creating a free oxygen microenvironment that switch-on the phosphorescence (³ILCT). The strong sensibility of the phosphorescent emission in this complex (see Table 2) encourages us to determine its efficiency as an ¹O₂ sensitizer. The singlet oxygen generation of complex 3a was examined in acetonitrile solution (5 × 10^–4 M) on the infrared region detecting the characteristic emission profile of ¹O₂ at λ_em ∼ 1274 nm (Figure S21) upon excitation at both 365 and 400 nm, respectively. Phenalenone (PN), a universal reference compound which can be used in various solvents,³⁵ has been employed. The measured quantum yield (ϕ_Δ) of ¹O₂ was notably higher upon exciting at 365 nm than upon exciting at 400 nm (37% vs 11%), in agreement with the higher ³ILCT phosphorescence efficiency (23% vs 17%, Table 2). These values indicate that this complex can be used as a photosensitizer.

In the solid state, this complex (3a) displays a broader emission red-shifted at low temperatures (Figure S19), indicating some additional ³MMLCT contribution.

Because the initial structural report of a diplatinum d⁷–d⁷ complex, K₂[Pt₂(SO₄)₄(H₂O)₂],³⁶ different types of high-valent diplatinum d⁷–d⁷, either with bridging and unbridged ligands, have been reported.³⁷ These complexes have attracted a great interest in many efficient catalyst processes, such as the oxidation of unsaturated organic molecules, facilitated by the strong electron-withdrawing ability of the unusually high oxidation state of the Pt^III atom.^37a Initial suggestions³⁸ and recent contributions^13,39 support that these complexes can be sometimes key intermediate in the oxidation pathway from Pt^II to Pt^IV. Indeed, their rich reactivity indicates that the electron distribution along the Pt–Pt bond can be viewed as a resonance structure between Pt^III–Pt^III and Pt^II–Pt^IV.^37e,40 Despite their rich chemistry, and also the well-known emissive properties of either Pt^II and Pt^IV complexes, reports on emissive d⁷–d⁷ derivatives are quite rare, mainly due to the fact that in these complexes the target orbital and therefore, the lowest-lying excited state possess metal–metal antibonding (dσ*_M-M) character,²⁸ being therefore short life and nonemissive. To the best of our knowledge, only three types of luminescent binuclear Pt^III complexes have been reported: (i) of the type [Pt₂ (μ-pop)₄X₂]^4– (pop = P,P-pyrophosphite, P₂O₅H₂^2–; X = Cl, Br, SCN) or [Pt₂ (μ-pop)₄X₂]^2– (X = py);⁴¹ (ii) type [Pt₂ (μ-C₆H₃-5R-2-AsPh₂)₄X₂] (R = methyl o isopropyl, X = Cl, Br, I)^14a and (iii) [Pt(C^N)(μ-pxdt)]₂ (pxdt = oxadiazole-thiol). In these latter, donor–acceptor bridging ligands have been employed giving rise to emission from an excited state having ligand-to-metal–metal charge transfer-(LMMCT) character.^14b

The Pt^III–Pt^III complexes (4a and 5a) are weakly emissive, showing in PS rigid media and in glassy THF solution (only 4a), a structured low-efficient (2% 4a and <1% 5a in PS) emission profile associated with a ³IL character centered on the R-pbt group (550 4a and 566 nm 5a) (Figure 11). The lowest TD-DFT (T_1,2 for 4a and T_1–3 for 5a) vertical excitations at the S₀ geometry (Table S4) are of LMMCT/LXCT in nature and are expected to be not emissive, in accordance with the lack of emission in fluid solution. The weak emission observed in rigid media is tentatively associated with close higher excited states (T₃ for 4a and T₄ for 5a) having a mainly ³ILCT character with a minor ³MLCT/³XLCT additional contribution for 4 (see Table S4). The calculated values (487.5 nm 4a and 603 nm 5a) agree with the experimental red shift observed for 5a relative to 4a attributed to the incorporation of the donor NMe₂ group, which increases the energy of the HOMO decreasing the gap of the transition.

Figure 11.

Emission spectra (λ_ex 365–420 nm) of (a) 4a in THF solution (5 × 10^–5 M) at 77 K and (b) of 4a and 5a in the PS film (10% wt).

Electrochemical Properties

The electrochemical properties of complexes 2a–5a were investigated using cyclic voltammetry in anhydrous CH₂Cl₂ with (NBu₄)PF₆ as a supporting electrolyte in the dark.

The potentials and HOMO/LUMO energy estimations are listed in Table 3 and voltammograms covering the anodic (2a, 3a) and cathodic (2a–5a) regions are depicted in Figure S22. Cyclometalated diplatinum complexes with butterfly or half-lantern shape exhibit mainly a two-electron oxidation process, assignable to the oxidation of the divalent species Pt₂(II) to trivalent species Pt₂(III);^8b,8d,12,42 although in some cases, two one-electron redox waves corresponding to the Pt₂(III)/Pt₂(II) couple were observed.⁴³ The Pt^II–Pt^II complexes 2a and 3a show in the anionic window two bad resolved quasi-reversible or irreversible process with E^ox 0.63, 0.96 V 2a, 0.73, and 1.07 V 3a (vs Ag/AgCl), probably due to two steps of one-electron oxidation from Pt₂(II,II) to Pt₂(III,II) and to Pt₂(III,III). The irreversibility of the oxidation processes is caused by the nucleophilic reactions of the coordinating solvent to the electrogenerated Pt^III species.¹² The Pt^III–Pt^III complexes 4a and 5a do not exhibit significant redox peaks by scanning the potential in the anodic direction.

Table 3. Electrochemical Data^a and HOMO/LUMO Energy Estimations for Complexes 2a–5a.

	cyclic voltammetry				DFT calculations
	Eoxb (V)	Eredc (V)	EHOMOd (eV)	ELUMOd (eV)	EHOMOe (eV)	ELUMOe (eV)
2a	0.63	–1.01	–4.98	–3.35	–5.46	–2.00
	0.96
3a	0.73	–1.10	–5.07	–3.24	–5.06	–1.67
	1.07
4a		–0.69		–3.13	–6.28	–2.97
		–1.21
5a		–1.07		–3.27	–5.12	–2.84

^aAll measurements were carried out in dark conditions at 298 K in 0.1 M solution of (NBu₄)PF₆ in dry CH₂Cl₂ at 100 mV s^–1 vs Ag/AgCl reference electrode.

^bQuasi-reversible or irreversible anodic peaks.

^cIrreversible cathodic peaks.

^dEstimated HOMO/LUMO energy by electrochemistry data [E_HOMO/LUMO = −(E^ox/red +4.8 – E^Fc/Fc+)].

^eEstimated HOMO/LUMO energy by DFT calculations.

Complexes constructed with the NMe₂-pbt cyclometalating ligand show an irreversible reduction wave with shape and potential values that are similar (E^red −1.10 V Pt₂^II3a, −1.07 V Pt₂^III5a). However, the pbt complexes displays different reduction behavior. Thus, whereas the Pt₂^II complex 2a shows an irreversible wave at −1.01 V, two reduction waves at −0.69 and −1.21 V were resolved for 4a (Pt₂^III). HOMO and LUMO energy levels were estimated from these CV data by using the relationship E_HOMO/LUMO = −(E^ox/red + 4.8 – E^Fc/Fc+), where E^Fc/Fc+ (0.45 V) is the potential of ferrocene vs Ag/AgCl and 4.8 eV is the energy level of ferrocene to the vacuum energy level. The calculated LUMO energies are between −3.13 and −3.35 V, being similar for the NMe₂-pbt complexes (−3.24 3a, −3.27 V 5a), what is an accordance with a LUMO mainly located in the cyclometalated ligand. Notwithstanding, the estimated HOMO and LUMO energies do not correlate well with those obtained by DFT calculations.

Photocatalytic Studies

In recent years, visible light photocatalysis has received a great attention allowing to furnish new molecules and structural motifs with lower energy consuming when compared with reactions under thermal or ultraviolet (UV).⁴⁴ In this area, luminescent cyclometalated transition metals (Ru^II, Ir^III and Pt^II) are among the most frequently employed sensitizers for energy transfer and photoredox catalysis.⁴⁵ In particular, some of these complexes have been previously described as efficient photosensitizers for the generation of reactive oxygen species (¹O₂, O₂^•–), being successfully employed as photocatalysts for the photooxidation of different organic molecules.⁴⁶ Among the various oxidation reactions, the photo generation of sulfoxides from sulfides using oxygen as oxidant is of great interest,^{21,23a,23c,47} mainly due to its relevance in the synthesis of biologically active compounds used in the pharmaceutical industry and also in organic synthesis.¹⁸ We recently reported the ability of dimethylphenylbenzothiazole platinum complexes to generate, under photoexcitation, sensitized singlet oxygen (¹O₂) able to induce oxidation of DMSO to DMSO₂.^17d In this line, the good ability of complex 3a to photo sensitize singlet oxygen encouraged us to estimate its photocatalytic activity. In particular, we investigated the induced photooxidation ability of p-bromothioanisole (Scheme 4) under visible light (blue light; λ = 460 nm) in the presence of 3a and oxygen as a model for heterogeneous catalysis.⁴⁸ The evolution of the catalysis was monitored by NMR spectroscopy. Photosensitizer 3a showed a good photostability under irradiation of blue light in suspension and solid state (measured by UV–vis spectroscopy from 0 to 50 h, Figure S23). This reaction was evaluated in different conditions and molar ratio of catalyst and substrate (Table 4, entries 1 and 2), with 1 and 5 mol % of the metal complex. In both cases, the reaction occurred with similar results allowing >95% conversion after 50 h of reaction. However, with 5% the reaction shows slight greater efficiencies at shorter times (3–4 h of reaction, Table S11). As illustration, after 15 h of reaction with 1% showed a 45% conversion, while with 5% a 55%. These results allow us to conclude that the increase of the amount of catalyst does not produce remarkable changes in the efficiencies.

Scheme 4. Photooxidation of p-Bromothioanisole to the Corresponding Sulfoxide.

Reagents and conditions: (a) CD₃OD, Blue LED (460 nm), photosensitizer (3a).

Table 4. Heterogeneous Visible-Light Oxidative Reactions with Different Conditions.

entry	% photosensitizer (%)	light	atmosphere	time (h)	conversion (%)
1	1	+	air	50	>95
2	5	+	air	50	>95
3a	5	+	air	36	>95
4	1	+	N2	50	–
5	1	–	air	50	–
6	–	+	air	50	–
7b	1	+	air	50	10
8c	1	+	air	50	23

^aDouble amount of reagent (p-bromothioanisole) and photosensitizer than in the entry 2.

^bIn the presence of DABCO.

^cIn the presence of BQ.

When the reaction was carried out with double amount of catalyst and substrate (Table 4, entry 3), the 95% of conversion was reached at 36 h, revealing that the increment in the concentration of the photocatalytic reactions improves the efficiency. Under hypoxic conditions, the conversion was considerably reduced because of the impossibility to produce ROS in the absence of oxygen (Table 4, entry 4). In the absence of light (Table 4, entry 5), no conversion was observed and, similarly, without a catalyst (3a) in the presence of air the reaction does not take place (Table 4, entry 6).

Generally, two mechanisms involving two main ROS intermediates such as ¹O₂ and O₂^•– have been proposed for the photocatalytic oxidation of sulfides:⁴⁹ (a) The photooxygenation promoted by ¹O₂ generated by a photosensitizer and (b) a photosensitized electron-transfer (ET) oxidation using ³O₂ through O₂^*– as an intermediate. In both mechanisms, a similar zwitterionic persulfide intermediate (R₁R₂S⁺O–O^–) is proposed, which reacts with a second molecule of R₁R₂S leading to the formation of two molecular sulfoxides. Discrimination between both mechanisms is not an easy task.²¹ To get insights into the catalytic mechanism, several control experiments were carried out. Thus, the photocatalytic reaction was evaluated out in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO), a well-known singlet oxygen quencher, to evaluate the potential production of these species. As can be observed in the Table 2-entry 7, in the presence of DABCO (3 equiv), reagents (sulfide, 3a), and light, the reaction is notably reduced (10%) and only traces of oxidized product were obtained, pointing to a key role of complex 3a as an oxygen sensitizer. On the other hand, the addition of a superoxide radical (O₂^•–) quencher like 1,4-benzoquinone (BQ, 3 equiv) to the photooxidation process (Table 4, entry 8) also results in a remarkable decrease of the reaction yield to 23%, revealing a key role of these reactive oxygen species too. These observations suggest that both ¹O₂ and superoxide radicals play an important role in this photocatalytic reaction.

Conclusions

In summary, we have prepared novel series of mononuclear (1a–c), bimetallic (Pt^II–Pt^II) (2a–c, 3a), and (Pt^III–Pt^III) (4a, 5a) complexes incorporating phenylbenzothiazole (pbt) and 2-(4-dimethylaminophenyl)benzothiazole (Me₂N-pbt) as cyclometalating chromophore groups and pyrazole (1) or pyrazolate bridging ligands (2–5). Experimental data and computational studies reveal the negligible influence of the pyrazole or pyrazolate bridging ligand on the optical properties of complexes 1a–c and 2a,b, which exhibit typical low-lying IL/MLCT electronic transitions. Only complex 2c, incorporating the bulky 3,5-ⁱPr₂pz bridging groups exhibits in the solid state an emission with some contribution of the ³MMLCT excited state, which is clearly predominant at low temperatures (560 nm at 298 K; 600 nm 77 K). 3a incorporating the donor–acceptor Me₂N-pbt ligand displays unusual dual, Fluorescence (¹ILCT or ¹MLCT/¹LC) and phosphorescence (³ILCT) emissions depending on the excitation wavelength. The efficiency of the population of the triplet manifold increases upon photoexcitation of excited states having a higher metal d contribution. The phosphorescence can be reversibly photoinduced in oxygenated THF and DMSO solutions upon continuous excitation (365 nm, ∼ 15 min) and quenched by shaking. The complex also photosensitizes ¹O₂, with a higher quantum yield at λ_ex of 365 nm than at 400 nm (37 vs 11%). Computed results for the low-lying T₁–T₃ excited states in complexes 2a and 3a indicate that T_1,2 have a mixed ³IL/³MLCT nature in 2a and mainly ³ILCT character in 3a. In both complexes, the T₃ has a ³MMLCT character and lies more separated from T₁ (0.6 eV) in 3a than in 2a (0.3 eV). The diplatinum complexes 4a and 5a increase the small number of luminescent d⁷–d⁷ compounds reported, with a weak emission, in rigid media, ascribed to ³ILCT. Finally, complex 3a, which demonstrates the ability to photosensitize singlet oxygen, has been further applied for the photooxidation of p-bromothioanisol under visible light (460 nm). Control of this reaction suggests that both, ¹O₂ and superoxide radicals, play an important role in this photocatalytic reaction.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.3c03532.

• Characterization of complexes (NMR spectra, crystal data), photophysical properties and computational details, and photocatalytic studies (PDF)

The authors declare no competing financial interest.