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The synchrony of black particle formation and deposition suggested that the black particles and the electrode-adsorbed species were the same material. X-ray photoelectron spectroscopy (XPS) was used to further investigate the composition of the film and insoluble material (Appendix A.5). To prepare film samples, glassy carbon plates were soaked in a solution of [Co(bdt)2]- and 1 or 2 equivalents p-

cyanoanilinium for 1 week and then rinsed with acetonitrile. Samples of the black particulates were prepared by filtering a solution of [Co(bdt)]- _{and 2 equivalents p-cyano-anilinium that had been left to stir for one}

week, washing the resulting material with acetonitrile, and dropcasting the material onto a glassy carbon plate. These samples were compared to the XPS of dropcast [NBu4][Co(bdt)2] (Appendix A.5).

XPS analysis of the film and particles showed that all samples contained cobalt, sulfur, and nitrogen. The peaks in the Co 2p, S 2p, and N 1s region overlap for all samples, providing evidence that the film and black particles are the same material (Figure 2.6, Appendix A.5). The Co 2p region for the XPS spectra of the film, particles, and molecular [Co(bdt)2]- contain only one doublet, indicating that only one type of cobalt species is present (Figure 2.6A, Appendix A.5). The S 2p region of dropcast [NBu4][Co(bdt)2]

(Figure 2.6B) contains a single doublet indicating that only one type of sulfur is present, as expected given the symmetry of the molecule. By contrast, the S 2p region of the film and particle samples contains a broad doublet with a higher binding energy than that seen for [Co(bdt)2]- (Figure 2.6B). Further analysis of the S

2p region of the film and particle samples shows that the peak is comprised of multiple overlapping doublets indicating that more than one sulfur environment exists. It is notable that rinse tests where a working electrode was submerged in a solution of suspended black particles showed minimal binding of the precipitated material onto the glassy carbon electrode, suggesting that direct nucleation of the material on the electrode surface is more favorable than adsorption of the particles after formation (Appendix A.6).

Figure 2.6 High resolution XPS spectra of the (A) Co 2p region and (B) S 2p region for dropcast [Co(bdt)]-

(blue trace), film formed with 1 equivalent p-cyano-anilinium (green trace), film formed with 2 equivalents p-cyano-anilinium (black trace), and dropcast black particles (red trace).

Prompted by the similarities in the XPS of the heterogeneous deposits and molecular [Co(bdt)2]-,

more rigorous characterization of the black particulates and film was attempted. The insolubility of the particulates and films in a diverse array of solvents as well as the non-crystalline nature of the particulates precluded characterization of this material by more traditional means. Rinse tests showed that the film is stable under acidic and basic conditions (Appendix A.6), but readily desorbs as a diffusional species upon application of reducing potential in an electrolyte-only solution (Figure 2.7A). The nature of this desorption product was analyzed with the goal of gaining greater insight into the heterogeneous material. A 10 cm x 20 cm x 2 cm glassy carbon plate was submerged in a solution of [Co(bdt)2]- and p-cyanoanilinium for one

week, rinsed and then subjected to reducing conditions in a pure electrolyte solution (Appendix A.6). The UV-vis absorbance spectrum of the electrolyte after application of a reducing potentials contained peaks located at nearly identical positions as the spectrum for [Co(bdt)2]-, suggesting that [Co(bdt)2]- was

regenerated in its molecular form (Figure 2.7B). Chemical reduction of the isolated black particles, formed with either anilinium or non-anilinium acids, by benzophenone radical anion (Eo_{′ = –2.2 V vs Fc}+/0₎120

corroborated these results; UV-vis absorbance spectrum of the resulting solution also contained features with nearly identical peak positions, supporting the regeneration of molecular [Co(bdt)2]- (Figure 2.7C).

While the relative absorbance of the peaks at 615 and 656 nm were consistent with [Co(bdt)2]- for spectra

collected after both electrochemical reduction of the film or chemical reduction of the black particulates, the feature at 360 nm was slightly elevated in both cases. This discrepancy may be the results of residual

conjugate base (e.g., p-cyanoanilinium absorbs below 330 nm) or, in the case of the chemically reduced samples, benzophenone which absorbs weakly between 380-310 nm and strongly below 310 nm.

Figure 2.7(A) Electrochemistry of a film-modified electrode. A 10 cm x 20 cm x 2 cm glassy carbon plate was soaked in a solution of [Co(bdt)2]- and p-cyanoanilinium for 1 week and rinsed with acetonitrile. Linear

sweep voltammogram collected in 0.25 M [NBu4][PF6] acetonitrile shows a large, amorphous stripping

current (blue). If the same plate is used to collect a second linear sweep voltammogram in the same solution, the resulting wave resembles that of a diffusion-controlled species (red). Background voltammogram of a freshly polished and pretreated plate shown for comparison (grey). All voltammograms collected at 0.2 V s-1 _{in 0.25 M [NBu4][PF6] acetonitrile and are not referenced to Fc}+/0_{. (B) Normalized UV-}

vis absorbance spectrum of electrolyte solution after application of reducing potentials to a 10 cm x 20 cm x 2 cm glassy carbon plate that had been soaked in a solution of [Co(bdt)2]- and p-cyanoanilinium for 1

week (blue) overlaid with spectrum of 0.25 mM solution of [Co(bdt)2]- in CH3CN (red). (C) UV-vis absorbance

spectrum of a solution of benzophenone radical anion and black particles in CH3CN shows regeneration of

[Co(bdt)2]- upon chemical reduction of black particles, formed with either 5 mM [Co(bdt)2]- and 2 equivalents

p-cyanoanilinium (blue) or 0.5 mM [Co(bdt)2]- and 100 equivalents TFA.

The regeneration of molecular [Co(bdt)2]- upon reduction of the film or particles suggests that

[Co(bdt)2]- does not undergo irreversible degradation or demetallation upon addition of acid and that,

instead, protonation of [Co(bdt)2]- instigates a solubility change which leaves the integrity of the molecular

intact. *The break in the symmetry in the S 2p region observed in the XPS spectra of the film and particulates suggests that one or more of the sulfur sites has been protonated (Figure 2.6B, Appendix A.5). After our original report, a similar pathway – in which protonation results in precipitation of the molecular species which is regenerated upon reduction – was proposed for the selenium-only analogue of [Co(bdt)2]-

.34 _{This reaction pathway would also be in line with prior reports on [Fe(bdt)}

2]2- which dimerizes and

precipitates as a blue-grey solid upon protonation. However, in this case, H2 is released in the absence of

an external reductant to generate a dimer composed of two [FeIII_(bdt)

2]- units.121 This behavior is in stark

contrast to the [Ni(bdt)2]- analogue which irreversible degrades under reducing and protic conditions to

generate a Ni-S film.89

coordinated to the metal center. This information could also not be definitely gleaned from XPS (Appendix A.5). However, rinse tests were able to provide indirect insight into the role of acid during film formation. In these trials, electrodes were soaked in a solution containing 2.5 mM [Co(bdt)2]- and either TFA, HCl, or 4-

chloroanilinium. The electrodes were then rinsed and used to collect voltammograms in a solution containing 2.5 mM 4-chloroanilinium. For all three modification procedures, the resulting rinse test voltammograms had comparable shapes, but the peak potential for feature E was different. This experimental procedure was repeated using four different anilinium acids in the scanning solution, but keeping the acids used during film formation the same (Appendix A.3). The same general trend was observed across these trials: the peak potential of E became more positive as the acid pKa of the anilinium

in the scanning solution decreased (Figure 2.8). However, in all cases, the actual peak potentials of E were different for films formed with different acids (Figure 2.8B). This suggests that the redox properties of the film is acid dependent, however this does not allow us to distinguish whether the acid is incorporating in the film structure. Even without precise information about the structure of the reaction intermediate, the formation of the intermediate upon protonation and the regeneration of the complex upon reduction allows us to postulate possible mechanisms.

Figure 2.8 Comparison of rinse test data for of films formed with para-substituted anilinium and non- anilinium acids. Working electrodes were pretreated in a solution of 2.5 mM [Co(bdt)2]- and 1 equivalent of

either 4-chloroanilinium (blue), (trifluoro)acetic acid (green), or HCl (purple). Electrodes were then rinsed with acetonitrile and used to collect a cyclic voltammogram in a solution containing 2.5 mM of either (1) anilinium, (2) 4-chloroanilinium, (3) 4-bromoanilinium, (4) 4-trifluoromethoxyanilinium, (5) 4- methylbenzoateanilinium. The peak potential for feature E is plotted as a function of the pKa of the anilinium

acid used in the scanning solution. Rinse tests with TFA and HCl were not performed with (2) or (4). All voltammograms were collected at 0.2 V s-1 _{in 0.25 M [NBu}

4][PF6] acetonitrile.