Electronic Communication in Binuclear Osmium- and Iridium-Polyhydrides

Reactions of polyhydrides OsH6(PiPr3)2 (1) and IrH5(PiPr3)2 (2) with rollover cyclometalated hydride complexes have been investigated in order to explore the influence of a metal center on the MHn unit of the other in mixed valence binuclear polyhydrides. Hexahydride 1 activates an ortho-CH bond of the heterocyclic moiety of the trihydride metal–ligand compounds OsH3{κ2-C,N-[C5RH2N-py]}(PiPr3)2 (R = H (3), Me (4), Ph (5)). Reactions of 3 and 4 lead to the hexahydrides (PiPr3)2H3Os{μ-[κ2-C,N-[C5RH2N-C5H3N]-N,C-κ2]}OsH3(PiPr3)2 (R = H (6), Me (7)), whereas 5 gives the pentahydride (PiPr3)2H3Os{μ-[κ2-C,N-[C5H3N-C5(C6H4)H2N]-C,N,C-κ3]}OsH2(PiPr3)2 (8). Pentahydride 2 promotes C—H bond activation of 3 and the iridium-dihydride IrH2{κ2-C,N-[C5H3N-py]}(PiPr3)2 (9) to afford the heterobinuclear pentahydride (PiPr3)2H3Os{μ-[κ2-C,N-[C5H3N-C5H3N]-N,C-κ2]}IrH2(PiPr3)2 (10) and the homobinuclear tetrahydride (PiPr3)2H2Ir{μ-[κ2-C,N-[C5H3N-C5H3N]-N,C-κ2]}IrH2(PiPr3)2 (11), respectively. Complexes 6–8 and 11 display HOMO delocalization throughout the metal–heterocycle-metal skeleton. Their sequential oxidation generates mono- and diradicals, which exhibit intervalence charge transfer transitions. This notable ability allows the tuning of the strength of the hydrogen–hydrogen and metal–hydrogen interactions within the MHn units.


■ INTRODUCTION
The hydrogen atoms of the MH n units of L m MH n transition metal polyhydride complexes interact with one another and with the metal center, forming Kubas type dihydrogens (d H−H = 0.8−1.0 Å), elongated dihydrogens (d H−H = 1.0−1.3 Å), compressed hydrides (d H−H = 1.3−1.6 Å), or classical hydrides (d H−H ≥ 1.6 Å). These interactions are governed by the electron density of the metal center, which is regulated by the coligands L m . 1 Thus, the ability of such compounds to reversibly release the H 2 molecule requires L m ligands, which are able to modify the electron density of the metal center, in order to allow reversible changes in the inner interactions of the MH n units. The design of such ligands is certainly a challenge of the first magnitude.
An attractive approach to the solution of this challenge is the use transition metal complexes, which should display frontier orbitals involving substantial mixing with a π-ligand backbone, whereas such a ligand should also bear atoms with free electrons. The coordination of this metal−ligand to an MH n unit would generate species with frontier orbitals delocalized between the two metal centers connected by the π-linker. Thus, the metals can be viewed as being electronically coupled and therefore the changes in electron density at one site should perturb the electron density at the other. 2 The search for efficient π-linkers (bridging ligands), which promote the cooperative effect between the redox active centers through electronic coupling pathways, is central for success. Unsaturated carbon chains, 3 aromatic polycycles, 3c,4 aromatic Nheterocycles, 5 bisdioxolenes, 6 bisdithiolenes, 7 dithiolate, 8 cyanide, 9 and cyanamides 10 have been mainly employed so far, as bridging ligands, to provide electronical coupling between metals.
The interactions between the metal centers have been grouped into three categories, according to the Robin−Day classification: weak, moderate, and strong. 11 Compounds displaying weak interaction form class I, and their redox centers mostly behave as separated sites. On the other hand, strong interaction affords a complete electron density delocalization, and complexes with this ability are grouped as class III. Species exhibiting moderate interaction between their redox centers constitute class II. 3d,12 The degree of interaction is efficiently assessed by means of the analysis of the intervalence charge transfer (IVCT) band in the UV−vis− NIR spectra on the basis of the Marcus−Hush theory. 13 At the electrochemistry level, the redox potential separation between successive redox processes is also a frequently used measure, although it often presents misinterpretation issues. 14 We have recently shown that the platinum group polyhydrides OsH 6 (P i Pr 3 ) 2 (1) and IrH 5 (P i Pr 3 ) 2 (2) promote the activation of CH bonds of the rings of 2,2′-bipyridines and related heterocycles to afford rollover cyclometalated trihydride-and dihydride-derivatives (Scheme 1), 15 in agreement with the ability of polyhydrides of the platinum group metals to activate σ-bonds 1f and in particular the d 2hexahydride OsH 6 (P i Pr 3 ) 2 . 16 In the context of the rollover cyclometalation, we noted that in a few cases the resulting ligands underwent an additional cylometalation promoted by a second metal complex, to form binuclear species bearing a bridging rollover bis-cyclometalated heterocycle. 17 Although evidence of the ability of these bridges to provide electronic coupling pathways has not been reported, these findings inspired us to prepare osmium-and iridium-polyhydrides with this class of bridging ligands and to use them as models to check our proposed approach toward the control of reversible changes in the existing interactions within the MH n units. This paper proves that rollover cyclometalated 2,2′bipyridine heterocycles provide electronic coupling pathways between the metals of (P i Pr 3 ) 2 H n Os(μ-L)OsH n (P i Pr 3 ) 2 (n = 2 or 3) and (P i Pr 3 ) 2 H 2 Ir(μ-L)IrH 2 (P i Pr 3 ) 2 complexes and that changes in the electron density of a metal center influence the inner interactions of the MH n unit of the other.

■ RESULTS AND DISCUSSION
Metal−Ligand C−H Bond Activation. Osmium-hexahydride complex 1 is able to activate CH bonds of the rollover cyclometalated trihydride derivatives OsH 3 {κ 2 -C,N-[C 5 RH 2 Npy]}(P i Pr 3 ) 2 (R = H (3), Me (4), Ph (5)) in toluene under reflux (Scheme 2) and in agreement with its ability to promote σ-bond activation reactions. Complexes 3 and 4 afford the binuclear-hexahydride compounds (P i Pr 3 ) 2 H 3 Os{μ-[κ 2 -C,N-[C 5 RH 2 N−C 5 H 3 N]-N,C-κ 2 ]}OsH 3 (P i Pr 3 ) 2 (R = H (6) Me (7)), as a result of the coordination of the free nitrogen atom of the rollover cyclometalated heterocycle and the ortho-CH bond activation of the other ring, whereas the reaction with the phenyl-derivative 5 leads to the pentahydride (P i Pr 3 ) 2 H 3 Os{μ-[κ 2 -C,N-[C 5 H 3 N-C 5 (C 6 H 4 )H 2 N]-C,N,C-κ 3 ]}OsH 2 (P i Pr 3 ) 2 (8). In contrast to 6 and 7, complex 8 bears two different osmium(IV) OsH n (P i Pr 3 ) 2 moieties, OsH 3 (P i Pr 3 ) 2 and OsH 2 (P i Pr 3 ) 2 . In this case, the bridging ligand acts in a dual manner: monoanionic C,N-chelate with OsH 3 (P i Pr 3 ) 2 and dianionic C,N,C-pincer with OsH 2 (P i Pr 3 ) 2 . The difference is a consequence of the hexahydride being also able to activate the phenyl substituent of the rollover cyclometalated heterocycle of 5. The three binuclear products can be also prepared by treatment of 1 with 0.5 equiv of the 2,2′-bipyridine. Both methods, via intermediates 3−5 and the one-pot synthesis procedures, afford the quantitative formation of the binuclear species, which were isolated as orange solids in about 80% yield. Complexes 6 and 8 were characterized by X-ray diffraction analysis. Figure 1 shows the structure of 6, which can be described as two equivalent OsH 3 (P i Pr 3 ) 2 units linked by a rollover biscyclometalated 2,2′-bipyridine. The coordination polyhedron around each osmium atom is the typical pentagonal bipyramid for osmium(IV) OsH 3 (Y-X)(P i Pr 3 ) 2 species 18 with axial phosphines (P(1)−Os-P(2) = P(2A)Os(A)P(1A) = 160.82(2)°), whereas the hydride ligands lie at the joint base of the bipyramid coplanar to the heterocycle. The OsN and OsC bond lengths of 2.1665 (18) and 2.144(2) Å are similar to those of the precursor 3. 15 In agreement with the high symmetry of the molecule, the 31 P{ 1 H} NMR spectrum of this compound in toluene-d 8 displays a singlet at 23.1 ppm for the four equivalent phosphines. In the 1 H NMR spectrum, the most noticeable feature is the hydride resonances, which appear between −5 and −13 ppm displaying the typical behavior observed for the inequivalent hydrides of OsH 3 (Y-X)(P i Pr 3 ) 2 complexes, involved in a thermally activated site exchange process. 18 The 31 P{ 1 H}, 1 H, and 13 C{ 1 H} NMR spectra of 7 in toluene-d 8 reflect the asymmetry imposed by the methyl substituent of the heterocycle. Thus, in contrast to 6 the 31 P{ 1 H} NMR spectrum shows two singlets at 22.7 and 21.4 ppm, whereas resonances corresponding to inequivalent OsH 3 (P i Pr 3 ) 2 units are observed between −5 and −14 ppm in the 1 H NMR spectrum. The 13 C{ 1 H} NMR spectrum displays two triplets ( 2 J C−P ≈ 6 Hz) at 173.9 and 168.9 ppm for the inequivalent metalated carbon atoms. The structure of 8 ( Figure 2) proves the dual coordination of the heterocycle in this complex, C,N-chelate to a metal center (Os(1)) and C,C,N-pincer to the other (Os (2)). The coordination polyhedron around both metal centers can be also idealized as a pentagonal bipyramid. However, there are significant differences between the bipyramids, which are associated with the acting fashion of the bridging ligand. The polyhedron around Os(1) resembles that of 6 with a P(1) Os(1)P(2) angle of 158.86(3)°. Phosphine ligands attached to Os(2) also occupy the axial positions of the bipyramid, forming a P(3)Os(2)P(4) angle of 164.05(3)°, whereas the pincer lies at the perpendicular joint base, coplanar to the hydride ligands, acting with a C(1)Os(2)C(12) angle of 150.41(12)°, which slightly deviates from the ideal value of 144°. The Os(1)C (7) and OsN(1) distances of 2.143(3) and 2.185(2) Å are similar to those found in 6, whereas the Os(2)C(1), Os(2)C(12), and Os (2)N (2) bond lengths compare well with the observed ones for osmium compounds bearing C,N,C-pincer ligands. 16c,15,19 31 P{ 1 H}, 1 H, and 13 C{ 1 H} NMR spectra of 8 in dichloromethane-d 2 are consistent with the structure shown in Figure 2. Thus, the 31 P{ 1 H} NMR spectrum contains two singlets at 24.1 and 1.8 ppm, assigned to the OsH 3 (P i Pr 3 ) 2 and OsH 2 (P i Pr 3 ) 2 units, respectively. In the 1 H NMR spectrum, the resonances of the OsH 3 (P i Pr 3 ) 2 unit display the typical pattern for the cyclometalated OsH 3 (Y-X)(P i Pr 3 ) 2 species, between −6 and −14 ppm, along with two temperature invariant doublets ( 2 J H−H = 11.3 Hz) of triplets ( 2 J H−P = 15.1 and 17.2 Hz) at −8.48 and −9.19 ppm corresponding to the hydride ligands of the OsH 2 (P i Pr 3 ) 2 unit. The 13 C{ 1 H} NMR spectrum shows three triplets ( 2 J C−P = 6.1−8.5 Hz) at 169.9, 168.0, and 165.5 ppm due to the metalated carbon atoms.
The 31 P{ 1 H}, 1 H, and 13 C{ 1 H} NMR spectra of 10 in toluene-d 8 strongly support the structure proposed for this compound, in Scheme 3. The 31 P{ 1 H} NMR spectrum contains two singlets at 30.3 and 22.4 ppm, one for each     Figure 3 shows a view of the structure.
The molecule is formed by two chemically equivalent IrH 2 (P i Pr 3 ) 2 moieties connected to each other through a rollover bis-cyclometalated 2,2′-bipyridine linker. It is a d 6 −d 6 counterpart of the d 4 −d 4 complex 6 and the d 4 −d 6 derivative 10. The coordination polyhedron around each iridium center is the expected octahedron with trans phosphines (PIrP = 156.94(3)°). In agreement with its structure, the 31 P{ 1 H} NMR spectrum of this highly symmetrical molecule shows a singlet at 29.9 ppm for the four equivalent phosphines, the 1 H NMR spectrum contains two doublets ( 2 J H−H = 4.1 Hz) of triplets ( 2 J H−P = 21.2 and 19.3 Hz) at −12.93 and −22.00 ppm for the inequivalent hydrides of the equivalent IrH 2 (P i Pr 3 ) 2 units, whereas the 13 C{ 1 H} NMR spectrum displays a triplet ( 2 J C−P = 6.5 Hz) at 163.6 for the equivalent metalated carbon atoms.
Frontier Orbitals and Photophysical Properties. The UV−vis spectra for 1 × 10 −4 M solutions of the mononuclear precursors 3−5 and 9 and binuclear derivatives 6−8, 10, and 11 in 2-methyltetrahydrofuran (MeTHF) were recorded. Figure 4 shows the spectra of 10 and their mononuclear building blocks 3 and 9, whereas the rest are shown in Figures S19−S27. In addition, time-dependent DFT calculations (B3LYP-GD3//SDD(f)/6-31G**) were performed to their rationalization, considering tetrahydrofuran as solvent. Selected absorptions are collected in Table 1 The spectra of the osmium mononuclear precursors 3-5 show bands in three different regions of the spectrum: <300, 300−500, and >500 nm. The absorptions at the highest energy region correspond mainly to 1 π−π* intraheterocycle transitions, whereas the bands between 300 and 500 nm are due to transitions from the metal to the heterocycle mixed with from the heterocycle to the heterocycle. These bands mainly result from HOMO−1-to-LUMO, and HOMO-to-LUMO transitions. Both HOMO−1 and HOMO are essentially located at the metal center and the metalated heterocycle. For HOMO, the percentage of the former is between 52% and 59% and that of the second one lies in the range 28−38%. The LUMO is almost exclusively centered on the metalated heterocycle (95%). The very weak absorption tails after 500 nm are assigned to formally spin forbidden 3 MLCT transitions caused by the large spin−orbit coupling introduced by osmium. The spectrum of the mononuclear iridium complex 9 is similar to those of 3-5. The absorptions of <300 nm should be assigned to 1 π−π* ligand-to-ligand transitions, whereas those between 300 and 450 nm are due to spin-allowed iridium-to-heterocycle charge transfer ( 1 MLCT) mixed with heterocycle-to-heterocycle transitions. The absorption tails after 450 nm correspond to formally spin forbidden 3 MLCT transitions, which are produced by the large spin−orbit coupling introduced, in this case, by the iridium center.
Complexes 3−5 and 9 display a HOMO involving substantial mixing with a π-ligand backbone (Figures S28− S30 and S34). Thus, they fulfill the main requirement in order to serve as metal−ligand species, which allow building binuclear compounds bearing metals electronically coupled, where the new HOMO is delocalized between the metal centers connected by the π-linker. As a proof-of-concept validation, the HOMO of the homobinuclear derivatives 6−8 and 11 is clearly delocalized throughout the metal−heterocycle-metal system ( Figure 5) with similar participation percentage of the three moieties. As in the mononuclear precursors, the LUMO is almost exclusively centered on the heterocycle. The UV−vis spectra of the binuclear osmium compounds 6−8 show bands between 274 and 496 nm corresponding to osmium-to-heterocycle charge transfer ( 1 MLCT) mixed with heterocycle-to-heterocycle transitions and weak absorption tails after 500 nm due to formally spin forbidden 3 MLCT transitions, whereas the spectrum of the binuclear iridium derivative 11 contains bands between 249 and 431 nm assigned to iridium-to-heterocycle charge transfer ( 1 MLCT) mixed with heterocycle-to-heterocycle transitions  Inorganic Chemistry pubs.acs.org/IC Article and weak absorption tails after 440 nm due to formally spin forbidden 3 MLCT transitions. The HOMO delocalization throughout metal−heterocyclemetal of the binuclear complexes requires not only the HOMO delocalization along metal-heterocycle of the metal−ligand mononuclear precursor but also electronic compatibility between the metal fragments linked by the heterocycle. This is given in complexes 6−8, where the heterocycle links two d 4metal fragments, and in complex 11 formed by two d 6 -metal moieties. In contrast to complexes 6−8 and 11, the heterocycle of the heterobinuclear derivative 10 associates fragments of two different ions, d 4 and d 6 , which appear to be inconsistent to produce electronic coupling. Thus, the HOMO of this compound ( Figure S35) is essentially centered on the osmium atom (45%) and the heterocycle (33%), whereas the iridium center has only a residual contribution (5%). Despite this difference, the UV−vis spectrum of 10 can be rationalized in a similar manner to those of 7 and 8.
The mononuclear complexes 3−5 and binuclear derivatives 6−8 are osmium(IV) phosphorescent emitters in the orangered spectral region (546−728 nm) upon photoexcitation. Emission spectra in doped poly(methyl methacrylate)  Table 2 shows the experimental and calculated wavelengths, observed lifetimes, quantum yields, and radiative and nonradiative rate constants. The spectra of the six compounds are very similar, which is consistent with the scarce differences found for the DFT-calculated HOMO− LUMO gaps (3.26−3.54 eV, see Table 2). Because the emissions can be attributed to T 1 excited states, there is good agreement between the experimental wavelengths and those calculated through the estimation of the difference in energy between the optimized triplet states T 1 and the singlet states S 0 in tetrahydrofuran. The observed lifetimes are in the range of 1.5−5.2 μs. Quantum yields are modest and higher for the binuclear compounds. This poor efficiency could be related to the low value of the radiative rate constants. Phosphorescent emitters based on osmium 20 are comparatively much less frequent than those of iridium 21 and platinum 22 in particular the osmium(IV) ones. 19,23 Complexes 6−8 are the first reported binuclear osmium(IV) emitters. In contrast to 3−8, the iridium derivatives 9−11 are not emissive. Electrochemical Properties. The redox properties of the osmium precursors 3−5, the mononuclear iridium complex 9, and the binuclear derivatives 6−8, 10, and 11 were evaluated by cyclic voltammetry performed under argon atmosphere in a 0.1 M [NBu 4 ]PF 6 dichloromethane solution, and the potentials were referenced versus Fc/Fc + . Table 3    Calculated according to the equations k r = Φ/τ obs and k nr = (1 − Φ)/τ obs , where k r is the radiative rate constant, k nr is the nonradiative rate constant, Φ is the quantum yield, and τ obs is the excited-state lifetime.
Inorganic Chemistry pubs.acs.org/IC Article value of 2.90 × 10 6 , which lies within the range found for 6−8.
On the other hand, the separation between the second oxidation peak and the third one is shorter. It only allows calculating a K c value of 9.86 × 10 3 . The heterobinuclear complex 10 ( Figure S62) has three quasi-reversible oxidation peaks at −0.32, 0.07, and 0.31 V, corresponding to independent events on each metal. According to the contribution of the metal centers to the HOMO of the species generated in the process and their respective spin density maps ( Figure S121), the first oxidation appears to take place on the osmium center, whereas the second and third ones should occur on the iridium center.  6 were carried out in order to corroborate the formation of mixed valence species, suggested by the electrochemical study, as a result of the performed oxidations. In contrast to 7, 8, 10, and 11, the solubility of the symmetrical complex 6 in the usual organic solvents is not enough to carry out the same spectroelectrochemical study with this compound.
The comparison of the spectra of the monocations [M 2 ] + with those of the neutral complexes reveals interesting findings. The spectrum of the monocation [7] + ( Figure S85) shows growing of the absorption bands in the visible region between 450 and 550 nm, with regard to that of 7 ( Figure S84), together with the appearance of a broader absorption centered at 1746 nm in the NIR region. This behavior is ascribed to a HOMO(B)-to-LUMO(B) intervalence charge transfer transition (IVCT) signature by a mixed-valence species. An IVCT band is also observed in the spectrum of [8] + (Figure 7, green line). It appears at 1705 nm, slightly red-shifted with regard to [7] + by about 40 nm, being much more intense. In contrast, the spectrum of the diiridium cation [11] + ( Figure S93) has a much less intense IVCT band at 912 nm, blue-shifted. The spectrum of the heterobimetallic Os−Ir cation [10] + ( Figure  S98) does not contain any perceptible IVCT band, in spite of that DFT calculations predict a weak transition at 1066 nm.
The oxidation of the monocations to the [M 2 ] 2+ species gives rise to the disappearance of the IVCT band in some cases. Spectra of the dications [7] 2+ and [11] 2+ do not contain any IVCT band ( Figures S86 and S94). However, an intense IVCT transition at 2000 nm is observed in the spectrum of [8] 2+ (Figure 7, red line); which is about 300 nm red-shifted with regard to that of [8] where Δν 1/2 and Δν max are the bandwidth at the half height and the maximum absorption, respectively, for a Gaussianshaped ICTV band (cm −1 ). Table 4 collects the values of the delocalization parameter, calculated according to eq 2, for the ITCV bands previously mentioned. Values of Γ < 0.5 indicate mixed-valence complexes of class II, while values of Γ > 0.5 are characteristic of compounds of class III. Complexes in the borderline class II/class III display values of Γ ≈ 0.5. According to this criteria, cation [7] 3+ belongs to class II, whereas ITCV bands of the cations resulting from the three sequential oxidations of the asymmetrical homobinuclear osmium complex 8 and the diiridium cation [11] + give Γ values, which fit to class III. Cation [7] + appears to be a species of the borderline class II/ class III with a Γ-value of 0.51.
Nature of the MH n Units upon Oxidation. The dissociation energy of a hydrogen molecule from a polyhydride complex depends upon the electron density of the metal. This energy increases as the hydrogen−hydrogen interaction decreases and therefore it is higher for hydride forms than for dihydrogen ones. This is a direct consequence of the metaldihydrogen bonding situation. Similar for all σ-complexes, the interaction between the coordinated hydrogen molecule and the transition metal in the dihydrogen compounds involves σdonation from the σ-orbital of the coordinated bond to empty orbitals of the metal and back bonding from the metal to the σ*-orbital of the bond. The balance between donation and back-donation determines the oxidative addition degree, which has been fit to the separation between the coordinated hydrogen atoms. 1 To gain insight into the influence of the sequential oxidation of the binuclear complexes 6−8, 10, and 11 on the respective MH n units, we comparatively analyzed the hydrogen−hydrogen separations in the optimized structures of the generated cations ( Figures S64−S83). Chart 1 gives a view of these structures, whereas Table 5 gathers the separation between the hydrogen atoms of the MH n units.
The neutral complexes are in the four cases classical hydrides with separations between their hydride ligands longer than 1.6 Å. The monocations [M 2 ] + are also pure hydrides, although it should be mentioned that subtle but significant differences are observed between them. Two of the hydride ligands of a half of [6] + approach about 0.1 Å to form a compressed dihydride (H(1) and H (2)). The same behavior is observed in the OsH 3 (P i Pr 3 ) 2 moiety of [7] + linked to the nitrogen atom of the unsubstituted pyridyl ring and in the OsH 3 (P i Pr 3 ) 2 moiety of [10] + . In contrast, the hydrides of [8] + and [11] + are not affected. This difference in behavior appears to be connected with the distribution of the frontier orbitals of the cations (Figure 8). The SOMO of [6]  The previous observations suggest that the MH n units of d 4osmium fragments are more sensitive to the oxidation than those of d 6 -iridium fragments and that the metal center of the MH n unit that undergoes the transformation is that with the highest contribution to the LUMO of the binuclear species.
■ EXPERIMENTAL SECTION General Information. All reactions were carried out with exclusion of air using Schlenk-tube techniques or in a drybox. Instrumental methods and X-ray details are given in the Supporting Information. In the NMR spectra (Figures S1−S18) the chemical shifts (in ppm) are referenced to residual solvent peaks ( 1 H, 13 C{ 1 H}) or external 85% H 3 PO 4 ( 31 P{ 1 H}). Coupling constants J and N (N = J P−H + J P'-H for 1 H and N = J P−C + J P'-C for 13 C{ 1 H}) are given in hertz.
UV−vis−NIR Spectroelectrochemical Investigations. Spectroelectrochemical experiments combine UV−vis−NIR spectroscopic measurements and redox processes at the same time. Thus, they allow obtaining the spectra of specific controlled oxidation states. The electrochemical measurements were performed with a micro-Autolab FRA2 Type III (Methrom, Utrecht, Netherlands) potentiostat controlled by NOVA (v.2.1.4) software. For the optical measurements, a JASCO V670 spectrophotometer using quartz (1 mm optical path length) was used. The spectroelectrochemical cell (1 mL volume) was a DRP-PTGRID-TRANSCELL (DropSens). It contains an optically transparent Pt grid working electrode (0.6 × 0.4 cm) which allows the bulk electrolysis of the solution contained in the cell, a Ag/AgCl reference electrode, and a platinum counter electrode. The experiments were performed under argon and protected from the light in dichloromethane solution (10 −