Results and Discussion

Chapter II: Electrochemistry in Ionic Liquids: Case Study of a Manganese Corrole

2.3 Results and Discussion

We investigated four ionic liquids as solvents (EMIm-EtOSO3, EMIm-OTf, EMIm-TCB, BMIm- TFSI). We thought it likely that IL anions would play a role in tuning (tpfc)Mn^IV/III formal potentials, as there was an earlier report of Mn^IV/III cathodic shifts upon addition of OAc⁻, Cl⁻, OH⁻, and CN⁻.¹⁷ Additionally, axial coordination of Br⁻ to a Mn(III) corrole has been established by X-ray analysis.¹⁹ Of interest is that the electronic ground states of five-coordinate Mn(III) corroles are high-spin (S = 2): in the case of a prototypal complex, (tfpc)Mn(OPPh3), the magnetic moment is 4.88 µB.^19,20

Figure 2.2. (Left) Mn^III/II and (Right) Mn^IV/III redox processes in ionic liquids with different anions. Inset:

Structure of (tpfc)Mn. All scan rates were 100 mV s⁻¹. All potentials were referenced to Fc^+/0

Cyclic voltammograms of the Mn^IV/III and Mn^III/II redox couples of (tpfc)Mn are shown in Figure 2.2 (formal potential values for the Mn^IV/III oxidation and Mn^III/II reduction of (tpfc)Mn in the four ILs are given in Table 2.1). Complete scan rate dependence data for all three redox events for (tpfc)Mn in BMIm-TFSI are given in Figure 2.3. We found that Mn^IV/III formal potentials decrease in the order OTf⁻ < EtOSO3− < TFSI⁻< TCB⁻. It was surprising that (tpfc)Mn oxidation in EMIm-OTf occurred at lower potential than that in EMIm-EtOSO as, according to Schmeisser et al., ethylsulfate is a stronger donor than triflate (in ILs).²¹

Figure 2.3. Scan rate dependence of the three redox couples of (tpfc)Mn in BMIm-TFSI. All potentials referenced to Fc^+/0.

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Table 2.2 presents the peak-to-peak separations for each redox process, calculated by subtracting the peak anodic potential from the peak cathodic potential, and potential separations between the peak oxidation and reduction potentials, calculated by subtracting the formal reduction potential (or peak potential obtained from differential pulse voltammetry) from the formal oxidation potential, of (tpfc)Mn for each IL.

aPeak cathodic potential.

Table 2.1. Formal potentials for (tpfc)Mn in different ionic liquids. All potentials referenced to Fc^+/0.

aCalculated using the peak potential obtained from differential pulse voltammetry.

bReference 21.

The separations can be attributed to IL uncompensated resistance and/or viscosity, rather than electrochemical irreversibility of the electron transfer process, with Mn^IV/III in EMIm-EtOSO3

exhibiting the largest peak-to-peak difference. For reference, the peak-to-peak separation for Fc^+/0 in EMIm-EtOSO3 is 85 mV at a scan rate of 0.1 V s⁻¹. The separation between the first oxidation and reduction peaks of (tpfc)Mn was found to depend on the extent of axial anion coordination to the complex, with smaller values an indication of stronger binding.^17,21

Redox Process

EMIm- TCB

EMIm- EtOSO3

BMIm- TFSI

EMIm- OTf

MeCN Reduction -1.375 V -1.423 V -1.422 V -1.458 V -1.48 V Oxidation 0.353 V 0.269 V 0.336 V 0.249 V 0.24 V

Ionic Liquid ∆EReduction ∆EOxidation ∆EOx−Red Donor Strength (kcal mol⁻¹)

EMIm-TCB 66 mV 69 mV 1.728 V 20.8

BMIm-TFSI 80 mV 68 mV 1.758 V 10.2

EMIm-EtOSO3 72 mV 106 mV 1.692 V 22.3

EMIm-OTf --- 76 mV 1.630 V^a 20.4

Table 2.2. Peak-to-peak separations at 100 mV s⁻¹ for (tpfc)Mn and donor strength of each ionic liquid.

The Mn^IV/III redox couple has been shown to be electrochemically reversible.¹⁷ To probe the effects of IL viscosity and anion binding strength on the electrochemical behavior of this couple, voltammetry at a range of scan rates was conducted; the Randles-Ševćik plots for (tpfc)Mn in each IL are displayed in Figure 2.4.²² Diffusion coefficients were calculated using the Randles-Ševćik equation (Equation 2.1),

𝑖_! = 0.4463𝑛^"/$𝐹^"/$𝐴𝐶0^%&_'( (2.1) where 𝑛 is the number of electrons (𝑛 = 1 e⁻), 𝐴 is the electrode surface area (𝐴 = 0.0707 cm²), 𝐷 is the diffusion coefficient of the electroactive species (cm² s⁻¹), 𝑣 is the scan rate (V s⁻¹), and 𝐶 is the bulk concentration of analyte (mol cm⁻³).²² Table 2.3 lists the anodic and cathodic diffusion coefficients for (tpfc)Mn in each IL.

aReference 23 ^bReference 24 ^cReference 25 ^dReference 26

The diffusion coefficients obtained for the first oxidation of (tpfc)Mn vary substantially, owing to charge effects and viscosities of the ionic liquids.^23-26 Diffusion-controlled, electrochemically reversible electron transfer behavior was displayed by (tpfc)Mn in EMIm-TCB and BMIm-TFSI. In these ILs, the diffusion coefficient ratio was found to depend on the net charge and size of the complex as well as on the IL dynamic viscosity. The diffusion coefficient ratios for (tpfc)Mn(IV) for ILs EMIm-TCB and BMIm-

Ionic Liquid DO,a

(10⁻¹² m² s⁻¹) DO,c

(10⁻¹² m² s⁻¹) DO,c/DO,a Ionic Liquid Viscosity (cP)

EMIm-TCB 7.9 ± 0.2 5.7 ± 0.2 0.7 20^a

BMIm-TFSI 3.9 ± 0.2 3.2 ± 0.2 0.8 52^b

EMIm-EtOSO3 0.88 ± 0.1 1.1 ± 0.1 1.3 131^c

EMIm-OTf 1.9 ± 0.7 3.8 ± 0.1 2.0 43^d

Table 2.3 Anodic and cathodic diffusion coefficients of the

Mn^IV/III redox couple in each ionic liquid and their ratios.

Values obtained from the first oxidation of the complex.

31 TFSI are very different from those for EMIm-EtOSO3 and EMIm-OTf, suggesting that the latter complexes are axially ligated prior to oxidation.

Figure 2.4. Peak current versus square root of scan rate plots for anodic and cathodic components of the Mn^IV/III redox couple in various ionic liquids.

One question of interest is whether the metal center is singly- or doubly-coordinated to an IL anion.

To address this question, UV-vis absorption spectra for (tpfc)Mn in the four different ionic liquids were obtained and are displayed in Figure 2.5. It is well established that the LMCT band in the 470–

500 nm region is a reporter of Mn(III) coordination environment.²⁷ The proposal is that the ratio of Soret and LMCT band intensities distinguish single versus double axial anion coordination (the extinction coefficient of the LMCT band being greater than that of the Soret when Mn(III) is six- coordinate).¹⁷ For EtOSO3−, splitting of the Soret and lower LMCT intensity suggests that Mn(III) is five-coordinate.¹⁷ In contrast, the higher LMCT extinction coefficient (relative to the Soret) of (tpfc)Mn in EMIm-OTf indicates that two triflates could be bound to Mn(III) in that IL. We suspect that the nature of axial coordination to Mn(III) in these ILs is not as clear cut as this “single vs double”

proposal, although these tentative assignments are consistent with our finding that the EMIm-OTf diffusion coefficient ratio is higher than the one in EMIm-EtOSO3.

Figure 2.5. UV-visible absorption spectra for (tpfc)Mn in four different ionic liquids.

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