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Unable to suppress this parallel pathway(s), we decided to pinpoint the reactivity underpinning this variability. The increasingly erratic electrochemistry at higher acid concentrations suggested a protonation- initiated side reaction was driving irreproducibility in the electrochemical data. One possibility is that, under highly acidic conditions, [Co(bdt)2]- can be protonated prior to reduction. UV-vis absorbance spectroscopy

was used to determine whether protonation of [Co(bdt)2]- was possible. In the absence of acid, [Co(bdt)2]-

absorbs strongly in the visible and near UV with prominent features at 360, 615, and 656 nm. Addition of p-cyanoanilinium (pKa = 7118) to a 0.25 mM solution of [Co(bdt)2]- results in the formation of black particles

and the loss of solution absorbance. The magnitude of the solution bleaching is proportional to acid concentration; complete loss of UV-vis signal is observed after 30 minutes upon addition of >1 equivalents of p-cyanoanilinium under atmospheric conditions (Figure 2.3). The degree of solution bleaching varied slightly when the same experiment was performed under an inert atmosphere; incomplete consumption of [Co(bdt)2]- was observed when the same solution was monitored via UV-vis absorbance spectroscopy in a

nitrogen-filled glovebox. The rate of solution bleaching is slower with weaker acids, as expected for a protonation-initiated reaction (Appendix A.2). Particulate precipitation was also observed with non-anilinium acids. Under atmospheric conditions, addition of 100 equivalents TFA (pKa = 12.65117) results in nearly

complete solution bleaching after 30 minutes and the instantaneous precipitation of black particulates with a slower loss of signal also observed upon addition of 1 equivalent TFA (Appendix A.2).

Figure 2.3 UV-vis absorbance spectra of 0.25 mM [Co(bdt)2]- in acetonitrile 30 minutes after the addition

of p-cyanoanilinium under atmospheric conditions. Solutions were filtered prior to obtaining spectra to reduce scattering arising from the formation of black particles.

Visually monitoring electrodes soaked in a solution of [Co(bdt)2]- with 4-chloroanilinium in the

absence of an applied potential suggested that a protonation-initiated reaction could also lead to film formation; the working electrode surface becomes increasingly discolored with a black film with longer soak times (Appendix A.2). Rinse test (see Section 1.3.3) s were conducted in the absence of an applied potential to evaluate whether an electrode-adsorbed electroactive species is generated under acidic conditions. A freshly polished and pretreated electrode was soaked in a 2.5 mM [Co(bdt)2]- solution containing 1

equivalent 4-chloroanilinium for 30 minutes, rinsed with CH3CN, and then scanned in an electrolyte only

solution. The cyclic voltammogram in the electrolyte-only solution contains three features, evidence that supports formation of an electroactive, adsorbed material (Figure 2.4A).

A second set of rinse tests were conducted to determine whether the film displayed any additional reactivity in the presence of acid. Freshly polished electrodes were soaked in a solution of 2.5 mM [Co(bdt)2]- with 1 equivalent 4-chloroanilinium, rinsed with CH3CN, and scanned in an electrolyte solution

containing only 4-chloroanilinium (Figure 2.4B, Appendix A.3). Multiple redox features (E, F, G, H) are observed in the cyclic voltammogram in the acid-only solution all of which are more positive than the potential for direct reduction of 4-chloroanilinium by glassy carbon.48 _{Crucially, no voltammetric response}

was observed in the rinse test if the electrode was soaked in a solution containing only [Co(bdt)2]- or a

solution containing only acid (Appendix A.3). These series of control studies indicate that an electroactive material is only deposited upon treatment of the electrode in a solution containing both [Co(bdt)2]- and 4-

chloranilinium. Additional rinse tests were conducted with 4-bromoanilnium and a series of non-anilinium acids to determine whether film formation is specific to 4-chloroanilinium. Working electrodes were soaked in a solution containing [Co(bdt)2]- and 1 equivalent of either 4-bromoanilnium, TFA, toluenesulfonic acid

(pKa = 8.6119), or (trichloro)acetic acid (pKa = 10.56117). The electrodes were then rinsed with CH3CN and

used to collect a voltammogram in an electrolyte solution containing the same acid that was used to form the film. Voltammetric responses could be observed in all rinse tests (Appendix A.3), confirming that film formation is not exclusive to 4-chloroanilinium specifically or anilinium acids more generally.

Figure 2.4 (A) Cyclic voltammogram recorded in an electrolyte only solution using a freshly polished electrode (blue trace) and an electrode treated in a solution of 2.5 mM [Co(bdt)2]- and 2.5 mM 4-

chloroanilinium for 30 min (black trace). (B) Cyclic voltammograms recorded in a 2.5 mM 4-chloroanilinium electrolyte solution using electrodes treated with 2.5 mM [Co(bdt)2]- and 2.5 mM 4-chloroanilinium.

Pretreatment times ranged from 15 seconds to 10 minutes.

Next, rinse tests were conducted to determine how the electrochemical response of the film varied as a function of the amount of time the electrode was soaked in an acidic solution of [Co(bdt)2]-. The

electrode was soaked in a solution of 2.5 mM [Co(bdt)2]- with 1 equivalent 4-chloroanilinium for times

ranging from 15 seconds to 20 minutes, rinsed with CH3CN, and used to collect voltammograms in a

solution of 2.5 mM 4-chloroanilinium (Figure 2.4B, Appendix A.3). The formation of an electroactive, electrode-adsorbed material was observed after soaking the working electrode for only 15 seconds and the electrochemical response of the film evolved as a function of soak time. This data shows that the film is formed on an experimentally relevant timescale and suggests that this reactivity may contribute to the variability observed in voltammograms collected under catalytic conditions (Figure 2.2B). Of particular interest was the redox feature E because it overlapped with the putative catalytic wave A observed in catalytic voltammograms collected under comparable conditions (i.e., 2.5 mM [Co(bdt)2]- with 1 equivalent

4-chloroanilinium) (Figure 2.5A). As the soak time used during rinse tests was increased, the current of redox feature E increased in a non-linear manner (Figure 2.5B). The peak potential of E also varied somewhat as a function of soak time, spanning a potential range of ca. 25 mV (-0.994 V to -1.020 V). This suggests that the variance in the location and broadness of redox feature A can be at least partially attributed to the rapid formation of the redox active film.

Figure 2.5 (A) Cyclic voltammogram of 2.5 mM [Co(bdt)2]- and 2.5 mM 4-chloroanilinium pre-treatment

solution (black trace) overlayed with cyclic voltammogram of 2.5 mM 4-chloroanilinium solution obtained with an electrode that had been pre-treated 10 min (blue trace). (B) Rinse test current (blue) and potential (red) of peak E recorded as a function of pre-treatment time. Dashed lines indicate the current and potential of prewave A for the 2.5 mM [Co(bdt)2]- and 2.5 mM 4-chloroanilinium solution used to pretreat the

electrodes. Cyclic voltammogram of pre-treatment solution obtained with an electrode that was submerged in the solution for less than 90 seconds.

Optical monitoring and rinse tests provide strong support for a reaction scheme in which protonation of [Co(bdt)2]- upon addition of acid leads to formation of both the black particulates and electrode film. The

increased rate of precipitation as acid strength is increased (Figure 2.3, Appendix A.2) further supports this conclusion. We also considered the possibility that precipitation is promoted by exchange of the [NBu4]+

cation of [NBu4][Co(bdt)2] with cationic anilinium acids in solution. This is not supported by studies showing

that precipitation and film formation is also observed with neutral acids such as TFA, (trichloro)acetic acid, and tosic acid (Appendix A.2 and A.3). To investigate whether cation exchange contributes to the formation of the heterogeneous material when using cationic acids, N,N,N-trimethylbenzenaminium tetrafluoroborate ([BF4][An-(CH3)3]), the N-methylated analogue of anilinium, was synthesized. One would anticipate that

[BF4][An-(CH3)3] would form the black particulates and film if precipitation was the result of cation exchange.

However, no precipitation or bleaching of the [Co(bdt)2]- signal was observed via UV-vis absorbance

spectroscopy upon addition of up to 25 equivalent [BF4][An-(CH3)3] (Appendix A.4) and rinse tests using

electrodes pretreated in [Co(bdt)2]- and 1 equivalent [BF4][An-(CH3)3] showed only a small increase in

current (Appendix A.4), which control experiments demonstrate can be attributed to the deposition of [BF4][An-(CH3)3] (Appendix A.4). These rinse tests indicate that cation exchange is not the operative