Syntheses and properties of phosphine-substituted ruthenium( II ) polypyridine complexes with nitrogen oxides †

Four novel phosphine-substituted ruthenium( II ) polypyridine complexes with nitrogen oxides — trans -(P,NO 2 )-[Ru(trpy)(Pqn)(NO 2 )]PF 6 ( trans -NO 2 ), cis (P,NO 2 )-[Ru(trpy)(Pqn)(NO 2 )]PF 6 ( cis -NO 2 ), [Ru(trpy)(dppbz)-(NO 2 )]PF 6 ( PP-NO 2 ), and cis (P,NO)-[Ru(trpy)(Pqn)(NO)](PF 6 ) 3 ( cis -NO ) — were synthesised (trpy = 2,2 ’ :6 ’ ,2 ’’ terpyridine, Pqn = 8-(diphenylphosphanyl)quinoline, and dppbz = 1,2-bis(diphenylphosphanyl)benzene). The in ﬂ uence of the number and position of the phosphine group(s) on the electronic structure of these complexes was investigated using single-crystal X-ray structural analysis, UV-vis absorption spectroscopy, and electrochemical measurements. The substitution lability of the nitrogen oxide ligand of each complex is discussed in comparison with that of the corresponding acetonitrile complexes. the results encouraged us to investigate in more detail the influence of the P atom on the physical properties of Ru-based metal complexes. In this study, we investigated the reaction of phosphine-containing ruthenium complexes with the nitrogen oxides, NO and NO 2 − , because of their biological roles as signalling molecules 22 and reservoirs, 23 in addition to the fundamental interest in their coordination chemistry. Here, we show the syntheses, structural characterisation, and electrochemical and spectroscopic properties of a series of ruthenium( II )

In our previous report, 21 we synthesised and structurally characterised for the first time a series of phosphine-containing ruthenium(II) polypyridine complexes of the type [Ru(TL)-(BL)(L)] n+ , with L = acetonitrile, TL = 2,2′:6′,2″-terpyridine (trpy), and BL = 8-(diphenylphosphanyl)quinoline (Pqn) or 1,2-bis(diphenylphosphanyl)benzene (dppbz) (Scheme 1).The influence of the number and position of phosphine donors on the structures and electronic properties was characterised, and unique isomerisation behaviours of these complexes were observed.The coordinating phosphorus ligands played a crucial role in these isomerisation reactions, and the results encouraged us to investigate in more detail the influence of the P atom on the physical properties of Ru-based metal complexes.
In this study, we investigated the reaction of phosphinecontaining ruthenium complexes with the nitrogen oxides, NO and NO 2 − , because of their biological roles as signalling molecules 22 and reservoirs, 23 in addition to the fundamental interest in their coordination chemistry.Here, we show the syntheses, structural characterisation, and electrochemical and spectroscopic properties of a series of ruthenium(II) polypyridine complexes containing Pqn or dppbz with nitric oxides.Three novel nitrito-κN complexes-trans(P,NO 2 )-and cis(P,NO 2 )-[Ru(trpy)(Pqn)(NO 2 )]PF 6 (trans-NO 2 and cis-NO 2 ), and [Ru(trpy)(dppbz)(NO 2 )]PF 6 (PP-NO 2 )-were successfully synthesised.Described here are the single-crystal X-ray structural determinations, UV-vis absorption spectra, and electrochemical measurements for these complexes, and the preparation of the nitrosyl complex cis(P,NO)-[Ru(trpy)(Pqn)-(NO)](PF 6 ) 3 (cis-NO) from cis-NO 2 and its properties are examined.The present study allows us to probe systematically the chemical and structural properties of phosphine-containing ruthenium complexes with nitrogen oxides by the use of the geometric isomers of the Pqn complex.Additionally, other reactivity properties and comparisons with those of the corresponding acetonitrile complexes are presented.

Syntheses and characterisation
The synthetic procedures to obtain trans-NO 2 , cis-NO 2 , and PP-NO 2 are shown in Scheme 2. The precursors, trans-MeCN, cis-MeCN, and PP-MeCN, were synthesised according to the method that we previously reported. 2126b The resulting products were characterised by 1 H NMR and 31 P{ 1 H} NMR spectroscopy and elemental analysis.Conversions of the nitrito-κN complexes to the corresponding ruthenium nitrosyls were attempted by adding an excess of HPF 6 to acetone solutions of the nitrito-κN species at 0 °C.The preparations of trans-NO and PP-NO from trans-NO 2 and PP-NO 2 , respectively, were not successful because of the instability of the nitrosyl complex or of a reaction intermediate under acidic conditions (for details, see Fig. S1 and S2 in the ESI †).However, cis-NO was isolated in 85% yield and was characterised by 1 H and 31 P{ 1 H} NMR spectroscopy and elemental analysis.The 31 P{ 1 H} NMR spectrum of cis-NO in acetone-d 6 gave a singlet at δ 54.23.cis-NO immediately converted to a solvent-coordinated complex in acetonitrile (Fig. S3 in the ESI †) but was meta-stable in weaker-coordinating solvents, such as acetone, γ-butyrolactone and ethylene glycol (Fig. S4 in the ESI †).The difference in the stability of these nitrosyl complexes will be discussed in the "Substitution lability of nitrogen oxide" section.

Crystal structures
Single crystals of trans-NO 2 and PP-NO 2 suitable for structural determination were obtained by recrystallisation from diethyl ether/methanol/acetonitrile.Single crystals of the cis-nitrito-κN complex were obtained as the BPh 4 -salt, cis(P,NO 2 )-[Ru(trpy) (Pqn)(NO 2 )]BPh 4 (cis-NO 2 ′), by adding an excess of NaBPh 4 , instead of NH 4 PF 6 , after the reaction.The molecular structures of trans-NO 2 , cis-NO 2 ′, and PP-NO 2 determined by singlecrystal X-ray crystallography and the summary of crystallographic data are shown in Fig. 1  two crystallographically independent ruthenium complexes, two BPh 4 − anions, and one acetonitrile molecule as the crystal solvent in the asymmetric unit of the triclinic P1 space group.The asymmetric unit of the monoclinic P2 1 /c crystal of PP-NO 2 contained one cationic ruthenium complex, one PF 6 anion, and two methanol molecules.For each complex, the ratio of ruthenium to counter anion indicated that the oxidation state of the ruthenium centre was +2.The coordination geometry of each Ru atom was that of a distorted octahedron composed of a meridionally coordinated terpyridine ligand, a bidentate ligand, and a nitrito ligand.The bond distances between the ruthenium and nitrogen atoms of the nitrito ligand of trans-NO 2 and PP-NO 2 were 2.146(4) (Ru1-N5) and 2.124(2) Å (Ru1-N4), respectively (Fig. 2), and were longer than those found in [Ru(trpy)(bpm) (NO 2 )]PF 6 (2.034(5) Å, bpm = 2,2′-bipyrimidine). 25By contrast, the Ru-N(NO 2 ) distances in cis-NO 2 ′ (2.0362(18) and 2.0290 (18) Å for Ru1-N5 and Ru2-N10, respectively, Fig. 2) were similar to those of [Ru(trpy)(bpm)(NO 2 )]PF 6 (2.034(5) Å).These results indicated a stronger trans influence of the phosphorus atom of Pqn or dppbz compared with that of the nitrogen atom of bpm or bpy.This tendency was also observed in trans-MeCN, cis-MeCN, and PP-MeCN in our previous study. 21-vis absorption spectra Fig. 3 shows the UV-Vis absorption spectra of the nitrito-κN complexes, trans-NO 2 , cis-NO 2 , and PP-NO 2 , in acetonitrile solution.The spectral data for these complexes and related compounds are listed in Table 2.All complexes displayed intense absorption bands in the UV region that were assigned to ligand-based π-π* transitions.Additionally, a moderately intense band was observed in the visible region for each complex.TD-DFT calculations that were performed at the B3LYP/LANL2DZ and B3LYP/SDD level of theory indicated that the visible region band could be assigned to metal-to-ligand charge transfer (MLCT) transitions from the dπ orbitals of ruthenium to the π* orbitals of trpy and Pqn or dppbz (for details, see Table S1 and Fig. S5-9 in the ESI †).The molar absorption coefficient of PP-NO 2 was nearly half of those of trans-NO 2 and cis-NO 2 .The absorption maximum (λ max ) of the MLCT transition of trans-NO 2 , cis-NO 2 , and PP-NO 2 was 443, 431, and 402 nm, respectively, and was blue-shifted compared with that of [Ru(trpy)(bpm)(NO 2 )]PF 6 , 25 suggesting the stabilisation of the dπ orbitals of the ruthenium centre upon introduction of the phosphine donors.Note that the MLCT band of cis-NO 2 was more blue-shifted than that of trans-NO 2 despite their isomeric relationship.A similar pattern was noted for the acetonitrile complexes, trans-MeCN, cis-MeCN, and PP-MeCN.21 The UV-vis absorption spectra of the cis-isomers with different ligands L, cis(P,L)-[Ru(trpy  3).For the different Ls, the lowest energy MLCT bands of the cis-isomers are in the following order: L = Cl − (468 nm) < NO 2 − (423 nm) < MeCN (413 nm) < NO + (383 nm).This result can be attributed to the competing σ-donor/π-acceptor properties of the respective ligands.

Electrochemical properties
The cyclic voltammograms (CVs) of trans-NO 2 , cis-NO 2 , and PP-NO 2 are shown in Fig. 5, and electrochemical data for these complexes and related compounds are listed in Table 4.The CVs were measured in 0.1 M tetraethylammonium perchlorate (TEAP)/acetonitrile. cis-NO 2 displayed one reversible oxidation wave in the positive region at E 1/2 = 0.79 V vs. ferrocene/ferrocenium (Fc/Fc + ), which was assigned to a Ru(III)/Ru(II) redox couple.By contrast, trans-NO 2 and PP-NO 2 exhibited one irreversible (E pa = 0.79 for trans-NO 2 and 0.95 V for PP-NO 2 ) and one reversible (E 1/2 = 0.97 for trans-NO 2 and 1.27 V for PP-NO 2 ) redox waves in the positive region.The former irreversible oxidation peak can be attributed to oxidation of the (Ru-NO 2 ) + centre, and the latter reversible wave was observed exactly at the same potential as the Ru(III)/Ru(II) redox couple of the corresponding acetonitrile complex (E 1/2 = 0.97 for trans-MeCN and 1.27 V for PP-MeCN, Table 4 and Fig. S10b and S10d in the ESI †). 21These observations suggest that the one-electron oxidation of trans-NO 2 and PP-NO 2 results in the release of NO 2 , owing to the labilising effect of the trans-phosphine in each case.This leads to the formation of the respective ruthenium(II) acetonitrile complexes, which are oxidised reversibly on further sweep to more positive potential (Scheme 3).In the absence of a trans-labilising phosphine, the oxidation of the (Ru-NO 2 ) + centre of cis-NO 2 occurs at a similar potential to the trans-isomer, but reversibly (Fig. 5 and Table 4).
In the negative potential region, cis-NO 2 displayed two reversible reduction waves at −1.70 and −1.99 V, which were assigned to the trpy/trpy − and Pqn/Pqn − redox couple, respectively.trans-NO 2 exhibited two redox waves at E 1/2 = −1.77and −2.13 V, and these redox potentials were the same as that of trans-MeCN (Table 4 and Fig. S10a in the ESI †).PP-NO 2 displayed two irreversible waves (E pc = −1.70V and E pa = −1.47V) and one reversible (E 1/2 = −2.49V) redox wave in the negative region.The reversible wave at E 1/2 = −2.49V and the  irreversible anodic wave at E pa = −1.47V were similar to those observed for PP-MeCN (Table 4 and Fig. S10c in the ESI †).These redox behaviours of trans-NO 2 and PP-NO 2 revealed that the reduction of these complexes induced the dissociation of NO 2 − and the formation of MeCN-coordinated species; this behaviour was similar to that observed in the positive potential region.The electrochemical behaviours of the nitrito-κN complexes are summarised in Scheme 3. cis-NO displayed one reversible redox wave at E 1/2 = 0.05 V and one irreversible reduction peak at E pc = −0.61V (Fig. 6).Comparison with similar nitrosyl compounds 24,25 revealed that the former redox wave was attributed to the NO + /NO • redox couple and the latter peak could be assigned to the reduction of NO • to NO − .Note that a Ru(III)/Ru(II) redox couple was not observed in the potential region between −1.6 and 1.5 V due to the low HOMO energy level originating from the poor donating ability of the NO + ligand.
Cyclic voltammograms of cis-isomers with various ligands L are shown in Fig. 7.The redox potentials of a Ru(III)/Ru(II) redox couple for each complex were observed at 0.49, 0.79, and 1.05 V for cis(P,Cl)-[Ru(trpy)(Pqn)(Cl)] + (cis-Cl), cis-NO 2 , and cis-MeCN, respectively.This result indicated the increase in the HOMO     Dalton Transactions Paper energy level by electron donation from monodentate ligands and was consistent with the UV-vis absorption spectroscopy.

Photostability of a nitrosyl complex
The photostability of cis-NO was investigated by UV-vis absorption spectroscopy and by using a Sievers nitric oxide analyser (NOA) to evaluate NO release.When a solution of cis-NO in ethylene glycol (0.05 mM) was irradiated at λ irr = 355 nm, the slow spectral changes seen in Fig. 8 were observed.The quantum yields of NO release, Φ NO , were quite low, 0.0048 in air saturated solution and 0.003 under helium (ESI Fig. S11 and S12 †).These Φ NO values are about two orders of magnitude smaller than that measured by Silva et al. for the photolysis of the analogous ruthenium nitrosyl complex, [Ru(tpy)(bpy)-(NO)] 3+ (bpy = 2,2′-bipyridine). 27Notably, exhaustive photolysis led to a nearly quantitative conversion to a spectrum ana-logous to that of trans-L (L = solvent, see Fig. S13 in the ESI †), which suggests that the photodissociation of NO and the isomerization of the complex from cis to trans form proceeds in a step-wise manner.The result is consistent with the multi-step spectral change shown in Fig. 8.It should further be noted that a Ru(II) complex, rather than the Ru(III) species is obtained upon NO photodissociation from cis-NO (ESI Fig. S13 †).There are several possible explanations, one being that the Ru(III) intermediate is readily reduced by the solvent.Another is that the principal photo-reaction is release of NO + rather than NO 27 owing to phosphine stabilization of the low-valent Ru(II) state.However, this question was not explored in greater detail.

Substitution lability of nitrogen oxide
The substitution lability of the monodentate ligand L in the [Ru(TL)(BL)(L)] n+ -type complexes is an important factor in    determining the reactivity of the complexes in various catalytic and photo-induced reactions.Several experimental results described above enabled us to discuss in detail the lability of the monodentate ligand in the complexes.First, the reaction of cis-NO 2 with HPF 6 afforded the desired cis-NO.However, similar reactions of trans-NO 2 and PP-NO 2 resulted in the formation of trans-MeCN and PP-MeCN via the dissociation of a monodentate labile ligand, N(O)OH or NO + ( probably the former).Second, UV-vis absorption spectroscopy revealed that the nitrito ligands of trans-NO 2 , cis-NO 2 , and PP-NO 2 did not dissociate, even in strongly coordinating solvents such as acetonitrile, whereas ligand exchange reactions of trans-MeCN and PP-MeCN easily occurred under analogous conditions (Fig. S14 in the ESI †).cis-NO was not stable in coordinating solvent and was easily converted to a solvent-coordinated form (Fig. S4 in the ESI †).
The difference in lability of the oxidised and reduced state can also be clarified by the results of the electrochemical measurements.In the one-electron oxidised states, trans-NO 2 and PP-NO 2 were labile and were converted to the solventcoordinated forms, trans-MeCN and PP-MeCN, respectively.By contrast, cis-NO 2 was inert during the oxidation process, and a reversible redox wave was observed in the electrochemical measurement.Similarly, in the reduced states, the nitrito ligands of trans-NO 2 , and PP-NO 2 easily dissociated, although cis-NO 2 was stable during the whole electrochemical process.However, this stability of cis-NO 2 was quite different from that of cis-MeCN: the acetonitrile ligand of cis-MeCN became labile upon reduction, and the dissociation of the ligand resulted in the isomerisation of cis-MeCN to trans-MeCN. 21The stability of the monodentate labile site for each complex is shown in Scheme 4.
The difference in the lability of these complexes can be explained by considering the following factors.First, the σ-donor character of the phosphine group significantly elongates the bond length between the ruthenium centre and the ligand trans to the phosphine group.This trans influence of the phosphine group was clearly observed in the X-ray structure for the series of nitrito-κN complexes; the bond distances between the ruthenium and nitrogen atom of the nitrite ligand are 2.141(3), 2.124(2), and ca.2.03 Å for trans-NO 2 , PP-NO 2 , and cis-NO 2 , respectively.Therefore, the trans-isomers and PP complexes exhibited greater lability compared with the corresponding cis-isomers.Second, the electron donation from labile ligands can stabilise Ru-L bonds.The donating ability of labile ligands was confirmed by the comparison of the HOMO energy levels obtained from the electrochemical measurements of the cis-isomers and follows the order NO 2 − > MeCN > NO + and was in accordance with the stability of the complexes.Third, oxidation decreases the electron density of the Ru centre, and the π back-donating ability of Ru centres should be weakened.DFT calculations revealed that π back-donation from Ru to the nitrito ligand occurs and stabilises the Ru-L bond (Fig. S6 in the ESI †).
Finally, the reduction of the complexes stabilises the five-coordinated species, as rationalised for the Ru(II)-MeCN complexes in our previous report. 21The formation of fivecoordinated species in MeCN results in ligand exchange in trans-NO 2 and PP-NO 2 or isomerisation of cis-MeCN to trans-MeCN, whereas no observable chemical process exists in the case of trans-MeCN and PP-MeCN.These results suggest that the (1) number and position of P atom(s), (2) coordinating ability of the monodentate ligand, and (3) oxidation state of the complexes are all factors in defining the lability of complexes.

Conclusions
This study describes the syntheses, crystal structures and spectroscopic and electrochemical properties of a series of phos- phine-substituted ruthenium(II) polypyridine complexes with nitrogen oxides.Three nitrito-κN complexes, trans-NO 2 , cis-NO 2 , and PP-NO 2 , were synthesised by the reaction of the corresponding acetonitrile complex with NaNO 2 in an ethanol/water mixed solution, and a nitrosyl complex, cis-NO, was obtained by the reaction of cis-NO 2 with HPF 6 in acetone.Crystallographic, spectroscopic, and electrochemical analyses for these complexes revealed that the σ-donating and π-accepting characters of the phosphine ligands clearly affected the dσ and dπ orbitals of the ruthenium centre, respectively.The investigation of the substitution lability of the monodentate ligand of each complex suggested that the (1) number and position of the phosphine groups, (2) coordinating ability of the monodentate ligand, and (3) oxidation state of the metal centre are all factors in determining the lability of the complex.As a further extension of our studies, investigations on various catalytic and photo-induced reactions of the phosphine-substituted ruthenium(II) polypyridine complexes are in progress in our laboratories.Measurements 1 H and 31 P{ 1 H} NMR spectra were recorded at room temperature on a JEOL JNM-LA500 spectrometer using tetramethylsilane as an internal reference for the 1 H NMR spectra and phosphoric acid as an external reference for the 31 P{ 1 H} NMR spectra.UV-vis absorption spectra were obtained on a Shimadzu UV-2450SIM spectrophotometer at room temperature.

Experimental
Elemental analyses were carried out on a Yanagimoto MT-5 elemental analyser.Infrared data were obtained using a Perkin-Elmer Spectrum 100 FT-IR spectrometer.ESI-TOF mass spectra were recorded on a JEOL JMS-T100LC mass spectrometer.All the ESI-TOF mass spectrometric measurements were recorded in the positive ion mode at a cone voltage of 20 V. Typically, each sample solution was introduced in the spectrometer at a flow rate of 10 mL min −1 using a syringe pump.Cyclic voltammograms were measured at room temperature on a BAS ALS Model 650DKMP electrochemical analyser in acetonitrile ([complex] = 0.5 mM; 0.1 M tetraethylammonium perchlorate (TEAP)).A glassy carbon disk, platinum wire, and Ag/Ag + electrode (Ag/0.01M AgNO 3 ) were used as the working, auxiliary, and reference electrodes, respectively.The redox potentials of the samples were calibrated against the redox signal for the ferrocene/ ferrocenium (Fc/Fc + ) couple.The photochemical experiments shown in Fig. 8 were made using a photolysis apparatus consisting of a LS-2134UTF Nd/YAG laser (Tokyo Instruments, INC.) with excitation provided by the third harmonic at λ = 355 nm.The pulse width was 5 ns, the beam diameter incident on the sample was 6 mm, and the repetition rate was 5 Hz.

X-ray crystallography
The X-ray data collection and processing was performed on a Kappa APEX II CCDC diffractometer by using graphite-monochromated Mo-Kα radiation (0.71075 Å) for trans-NO 2 and PP-NO 2 .Single-crystal X-ray diffraction measurement of cis-NO 2 ′ was performed with a RAXIS-RAPID Imaging Plate diffractometer equipped with confocal monochromated Mo-Kα (0.71075 Å) radiation, and the data were processed using RAPID-AUTO (Rigaku).The structure was solved by the direct methods using SIR-92 28 and refined on F 2 with the full-matrix least squares technique using SHELXL-2014. 29ll non-hydrogen atoms were refined anisotropically.Molecular graphics were generated using ORTEP-3 for Windows 30 and POV-RAY. 31A summary of the crystallographic data and structure refinement parameters is given in Table 1.
The crystallographic data have been deposited at the Cambridge Crystallographic Data Centre: deposition numbers CCDC 1040452, 1040453, and 1040454 for trans-NO 2 , cis-NO 2 ′, and PP-NO 2 , respectively.

DFT calculations
Calculations were performed using the DFT method implemented in the Gaussian 09 package. 32The structures were fully optimised using the hybrid B3LYP method, which uses Becke's three-parameter exchange functional 33 with the correlation energy functional of Lee, Yang, and Parr. 34All calculations were performed using the standard double-ζ type LANL2DZ basis set 35a-c or SDD basis set 35d implemented in Gaussian 09, without adding any extra polarisation or diffuse functions.The LANL2DZ basis set also uses a relativistic effective core potential (RECP) for the Ru atom to account for the scalar relativistic effects of the inner 28 core electrons ([Ar] 3d 10 ).All calculations were performed using the polarisable continuum model (PCM) 36 to compute the structures in acetonitrile media.All stationary points were characterised as minima of the potential energy surface by their harmonic vibrational frequencies.The free energies at 298 K and 1 atm were obtained through thermochemical analysis of the frequency calculation using the thermal correction to Gibbs free energy as implemented in Gaussian 09.The excited states were calculated using the TDDFT 37 method within the Tamm-Dancoff approximation as implemented in Gaussian 09.These calculations employ the hybrid B3LYP functional along with the basis sets described above.A minimum of 100 excited states was computed in each calculation.To obtain the simulated spectrum of each species, transition energies and oscillator strengths were interpolated by a Gaussian convolution with a common σ value of 0.2 eV.

Quantum yield measurements
Nitric oxide was detected and analysed using a GE Sievers model 280i nitric oxide analyser (NOA). 38Known volumes of the gases from the solution headspace were injected into the NOA purge vessel, and these gases were entrained to the detector using helium or medical grade air.The NO present in the sample was quantified using a calibration curve generated from the reaction of NaNO 2 with acidic KI.Chemical actinometry was performed with ferric oxalate solutions. 39The photolysis source was the output from a 200 W high-pressure mercury lamp passed through an IR filter and collimated with lenses.An appropriate interference filter was used to select the desired λ irr .A shutter shielded the sample from the arc lamp.A sample of a known volume in a 1 cm square cuvette with a magnetic stirring bar was irradiated for determined time periods. 40The NO quantum yields (Φ NO ) were calculated based on the nitric oxide release measured using the NOA.
and NO + ), in ethylene glycol solution are shown in Fig. 4.These complexes each exhibited ligand-based π-π* transitions in the UV region and MLCT transitions in the visible region.The MLCT transition of cis-NO was observed at λ max = 383 nm and is comparable to similar Ru-NO complexes (Table

a
Ref. 26. b Ref. 25. c Absorption shoulder.d Data not collected.

Fig. 8
Fig.8UV-Vis absorption spectra of cis-NO in ethylene glycol at room temperature during photolysis using a Nd/YAG laser operating at 355 nm."Times" indicates the number of the laser pulses used to irradiate the sample.Scheme 4 Changes in lability upon oxidation or reduction.

Table 3
UV-Vis absorption data (λ max /nm (10 −3 ε/M −1 cm −1 )) in acetonitrile, infrared data (v/cm −1 ), and redox potentials (E 1/2 /V vs. Fc/Fc + ) in acetonitrile for cis-NO and related compounds at room temperature f a UV-Vis absorption data and redox potentials in ethylene glycol and γ-butyrolactone, respectively.b Ref. 25. c Ref. 24. d Absorption shoulder.e E pc value for the irreversible process.f Data not collected.