Electrochemical MeCN release from [Re(NCHCH 3 )Cl(PNP)]

The N2-derived nitride [ReNCl(PNP)] (3^Cl) was functionalised via C-N bond formation by EtOTf, as elaborated in Section I.2.2.3. Subsequent deprotonation with the strong base KHMDS yields azavinylidene [Re(NCHCH3)Cl(PNP)] (6), from which MeCN was released by oxidation and deprotonation in presence of chloride ions to regenerate [ReCl3(PNP)] (8). In the established method, all three required reagents (oxidant, base, and chloride source) are provided by the multifunctional reagent NCS, see Scheme 55.¹⁵¹ The mechanism of this oxidation is not well understood: two equivalents of H-succinimide are formed although only one proton originates from 6 to release MeCN. As alternative to a chemical reagent, the required oxidation equivalents can originate from electrochemistry, where the potential can be chosen selectively. To probe for promising conditions to transfer this MeCN release step towards electrochemical conditions, the electrochemistry of 6 was examined in presence of a base and a chloride source.

Scheme 55. The previously established N-C bond functionalisation and MeCN release from 3^Cl.¹⁵¹

The CV of 6 in THF shows 4 oxidative events, see Figure 47. The first event at E1/2 = −0.49 V is fully reversible, diffusion controlled and assigned to a metal-centred Re(IV/III) oxidation (Figure A36). Its one-electron character was confirmed in a separate coulometric experiment.

The subsequent three oxidations are at Ep = 0.50 V, 0.61 V, and 0.95 V at ν = 0.1 Vs^-1. After scanning through the latter two events, the Re(IV/III) oxidation loses its reversibility, indicating that these are coupled to irreversible chemical reactions. They are therefore assigned to ligand backbone oxidation, as corroborated by their absence in the CV comparison between 6 and unsaturated [Re(NCHCH2)Cl(P=N=P)] (⁼6) (Figure 47, vide infra for its synthesis). In addition, 6 shows an irreversible, far negative reduction at Ep = −3.14 V at ν = 0.1 Vs^-1 (Figure A36).

Figure 47. CV of the oxidative region of 6 (top) and ⁼6 (bottom) in THF with 0.2M (nBu4N)PF6, ν = 0.1 Vs^-1.

To enable electrochemical MeCN generation and release, the CV of 6 was examined in presence of base and chloride source (nHe4N)Cl. The electrochemistry of the explored additives was tested to secure redox-inertness in our region of interest, as presented in Appendix A2. A successful base for this reaction would be capable of deprotonating [6]⁺, of which the product can be oxidised again to formally form the [ReCl(NCCH3)(PNP)]⁺-adduct via an ECE-mechanism. This is indicated in the CV by the loss of reversibility of the Re(IV/III) oxidation of 6 and the appearance of an additional oxidation wave. The addition of 4 eq. of NEt3 has only little effect on the first oxidation of 6, see Figure 48. A slight current increase is observed at ν = 0.1 Vs^-1, and the reversibility slightly decreases (ip,a/ip,c changes from 1.0 to 1.25 at ν = 0.1 Vs^-1 upon addition of 4 eq. of NEt3). At ν = 1.0 Vs^-1 no influence is observed, indicating that NEt3 is most likely close to the pKa of [6]⁺ (pKa NEt3 (THF) = 14.0)²⁰⁴, and the reaction is in equilibrium. More eq. of NEt3 should have been added to examine this hypothesis, which was not tested in the context of this work. Subsequent addition of Cl⁻ ions results in an anodic peak potential shift of the Re(IV/III) oxidation, and the appearance of an additional, irreversible oxidative wave. Notably, the same behaviour was observed when no base was present. These observations strongly indicate that chloride association can be coupled to the Re(IV/III) oxidation of 6, yielding [ReCl2(NCHCH3)(PNP)], which is subsequently irreversibly oxidised at Ep = −0.32 V (ν = 1.0 Vs^-1). This Re(V/IV) oxidation appears at only a slightly more positive potential compared to the Re(IV/III) oxidation of 6, attributed to the newly coordinated, electron-donating chloride ligand. This species and the origin of its irreversible oxidation were not investigated further.

Figure 48. CV of the oxidation of 6 in presence of 4 eq. NEt3 and subsequently 4 eq. (nHe4N)Cl ν = 0.1 Vs^-1 (left) ν = 1.0 Vs^-1 (right) in THF with 0.2 M (nBu4N)PF6.

Since NEt3 proved too weak to induce significant changes in the CV, stronger bases were explored. Addition of 10 eq. of DBU (pKa = 18.1 (THF))²⁰⁴ to 6 and measuring the Re(IV/III) oxidation shows an anodic shift of circa 0.03 V, a complete loss of reversibility and a strong (> twofold) current increase (Figure 49). All observations indicate the coupling of the Re(IV/III) oxidation to deprotonation and the subsequent oxidation of the product. Notably, 4 eq. of DBU already showed this behaviour. Subsequent addition of chloride causes the first redox event to shift even more anodically and a second oxidation is shown in proximity. Although this initially looks similar to the CV of 6 and (nHe4N)Cl alone (or additional presence of NEt3 as shown in Figure 48, left), the current on the second oxidation is substantially higher and increases further thereafter when DBU is present, reflecting the influence of the base. Similar results were obtained by using the slightly weaker tetramethylguanidine (TMG) as base (pKa = 17.0 (THF))²⁰⁴. However, the CV appearance changed over time and this reagent was therefore not examined further.

With DBU as base of choice, scanning through this oxidative wave hopefully generates MeCN and leaves a Re complex-fragment that is trapped by the Cl⁻ anions to form [ReCl3(PNP)] (8).

This might be visible in the cathodic reverse trace in the CV. The irreversible Re(IV/III) reduction of 8 is found at Ep = −1.10 V at ν = 0.1 Vs^-1, which shifts cathodically in presence of Cl -(vide infra, Section II.4.5). In this region, only a very small wave at Ep = −1.12 V is found in the cathodic reverse trace of the CV of 6 in presence of DBU and Cl^- (Figure 49). We therefore state that at least no significant amount of 8 is formed on the CV timescale.

Figure 49. Top left: CV of the Re(IV/III)-oxidation of 6 at ν = 0.1 Vs^-1 in presence of 10 eq. DBU and subsequently 10 eq. (nHe4N)Cl in THF with 0.2 M (nBu4N)PF6. Top right: CPE of the previous mixture at Eappl. = −0.30 V with a zoom of the region between −0.9 V and −1.2 V. The dashed line represents the potential of the reduction of 8. Bottom left: ³¹P{¹H} NMR spectrum of the crude inorganic fraction of the CPE in normal THF. Bottom right: CV of the oxidative region of 5 at ν = 0.1 Vs^-1 in presence of 5 eq. DBU and 5 eq. (nHe4N)Cl (left) in THF with 0.2 M (nBu4N)PF6.

Performing a CPE of this mixture at the peak potential of the second oxidation wave at Eappl. = −0.30 V results in a colour change from brown to orange and a transfer of circa 7 electrons per rhenium metal (as averaged over three experiments) (Scheme 56). During electrolysis, no significant wave appears close to the Re(IV/III) reduction of 8. The before mentioned small feature at Ep = −1.12 V remains around circa 0.5 μA peak current until 90 min.

of CPE time after which it decreases. A small wave at Ep = −0.96 V appears, which is too electropositive to correspond to the reduction of 8. After electrolysis, the volatiles are vacuum transferred to a separate container and checked for the presence of MeCN by GC chromatography. Although obtaining a calibration curve for quantifying MeCN in THF proved challenging (see Section IV.3 for details), a reproducible release of circa 25% MeCN is established (based on two runs of 20 % and 32 % yield). This yield is only low and it is considered

if MeCN acts as a ligand to coordinatively unsaturated species. Therefore, we added in a separate experiment 5 eq. of a strong ligand (tBuCN) to the crude inorganic fraction, which could release MeCN as hypothetical ligand. Yet, no additional acetonitrile was obtained. On the inorganic side, the crude fraction was measured by NMR spectroscopy, and unfortunately, no 8 was found in the ¹H NMR spectrum. Even though a large excess of electrolyte is present, 8 should be easily identified due to its paramagnetic nature. This is in agreement with the absence of any peak in the right potential range for 8 in the CVs of the CPE. Furthermore, no broad paramagnetic peaks are observed. Instead, the ³¹P{¹H} NMR spectrum shows various compounds (Figure 49). No possible doublet pairs with the same coupling constant could be identified, indicating it to be at least >10 individual species. Extraction attempts with Et2O proved that most of these species were rather polar and not separable from the electrolyte, which inhibits mass spectroscopic analysis. To check if too little chloride source was present during standard CPE conditions (10 eq. of (nHe4N)Cl) to form 8, the reaction was repeated with 60 eq., yet with the same results.

Scheme 56. Electrochemical oxidation of 6 (either isolated or in situ formed from deprotonation of 5) in presence of DBU and (nHe4N)Cl to release acetonitrile and a mixture of unknown Re-complexes.

In the course of this work, the deprotonation of the imido species [Re(NCH2CH3)Cl(PNP)]OTf (5) towards 6 was reconsidered, since the published route¹⁵¹ relies on the strong base KHMDS (pKa HMDS (THF) ≈ 26).²⁰⁵ Ideally and more economically, the same base would be used for electrochemical MeCN release and deprotonation of 5. To our positive surprise does DBU readily deprotonate 5 to quantitatively form 6 as characterised by NMR spectroscopy. The same is displayed by electrochemistry: 5 is irreversibly oxidised at a far anodic potential (Ep = 0.81 V, ν = 0.1 Vs^-1). Upon addition of 5 eq. DBU, the colour changes from green to brown and the first oxidation is found at Ep = −0.47 V, indicating in situ formation of 6. Subsequent addition of 5 eq. of (nHe4N)Cl leads to the same oxidative feature as starting from isolated 6 (Figure 49), and oxidative CPE on this mixture gives the same results. Although the electrolysis results do not invite to think towards catalytic N2-splitting and functionalisation on this platform, modifying this deprotonation towards a milder reagent is noteworthy.

After establishing electrochemical MeCN release from 6, initial considerations about the mechanism can be made. The transformation of the azavinylidene 6 to MeCN is accompanied by a deprotonation (C) and two oxidation steps (2xE). Isolated 6 is not deprotonated by DBU and based on the current increase in the CV upon addition of base, these fundamental steps can be arranged in an ECE mechanism: oxidation to [6]⁺, deprotonation and subsequent oxidation.

The overall outcome of these steps is generation of [Re(NCH2CH2)Cl(PNP)]OTf (7), see Figure 50. The synthesis of 7 is already established via chemical oxidation and subsequent PCET reactivity (Figure 50), and by its NMR characteristics found to be best described as the Re(V) vinylimido tautomer. In the proposed ECE-mechanism, the potential of the second oxidation should correspond to the Re(V/IV) reduction potential of 7. Since no CV of this compound was measured, its synthesis using AgOTf and TBP was repeated. Unfortunately, 7 was not synthesised as clean as in the published procedure but could only be isolated in circa 90% purity based on ³¹P{¹H}NMR spectroscopy (Figure A37). Still, CV was measured of this batch, to get an estimation of the Re(V/VI) reduction potential and its relevance in the CV of 6 upon addition of base. 7 shows one irreversible reduction at Ep = −1.38 V at ν = 0.1 Vs^-1 (Figure 50). Although this value seems quite negative for a cationic Re(V) compound, it is not unprecedented: cationic Re(V) imido compounds 5 and ⁼5 are reduced in the same potential region (Ep = −1.84 V at ν = 0.1 Vs^-1 (5), E1/2 = −1.33 V (⁼5)) (Figure A35). Besides, 7 displays an irreversible oxidation with multi-electron character at Ep = 0.83 V at ν = 0.1 Vs^-1, attributed to PNP-ligand backbone oxidation. Considering the basic ECE-oxidative mechanism, this indicates a remarkable potential inversion: the first E (oxidation of 6) is at E1/2 = −0.49 V, compared to the second E (analogue to the reduction of 7) at Ep = −1.38 V. To probe if this second E can be the oxidation envisioned to form 7, we briefly turned to digital simulation. As presented in Appendix Figure A37, the Re(IV/III) oxidation of 6 is simulated in agreement with the experimentally observed E1/2 = −0.49 V. In a next step, this oxidation of 6 is coupled to deprotonation and subsequent oxidation at E1/2 = −1.38 V to mimic the [7]^0/- oxidation in this process, see Figure 50. This reveals a current increase and anodic shift in such a manner that it is considered realistically that such a strong potential inversion appears. From the simulations, it becomes clear that a current increase of a two-electron process is too small to account for the experimental observed increase after addition of base. This strongly suggest that additional electron transfers occur within this oxidation process.

Figure 50. Top left: Synthesis of 7 as published by Schneider.¹⁵¹ Top right: CV of 7 in THF with 0.2 M (nBu4N)PF6 at ν = 0.1 Vs^-1. Inset: scan rate dependence of the first reduction. Bottom left: Simulation attempt of the Re(IV/III) oxidation of 6 before and after addition of 10 eq. of DBU. The electron transfers where simulated with E1/2 = −0.49 V, and −1.38 V respectively, where the latter represents the reduction of 7 as found by CV. The parameters for the chemical reactions used for this simulation are: Kdeprotonation = 10, kdeprotonation = 1∙10⁴ M^-1 s^-1, KMeCN release= 1 M, kMeCN release: 1∙10⁴ s^-1. For more simulation parameters, see Figure A37. Bottom right: the simulated pathway: proposed ECE-mechanism from 6 to form 7 and subsequent MeCN release to a redox-inactive species X. The PNP-ligand is omitted for clarity.

To recapitulate the findings so far: oxidative CPE of 6 (either added as isolated material or formed in situ by deprotonation of 5) in presence of DBU and (nHe4N)Cl affords 25 % of MeCN and a mixture of Re-compounds, yet no 8. Importantly, circa seven electrons per rhenium are needed to fully convert the starting material by CV, whereas only three are necessary to successfully release MeCN and form 8. These findings are in sharp contrast to chemical MeCN release, where 8 is found is circa 90% yield. One possibility for not generating 8 during CPE is an over-oxidation to the Re(V) stage, since an unlimited (and maybe drifting) oxidative potential can be present under CPE conditions. This would explain the need of more than three electrons.

Yet, the quasi-reversible Re(V/IV) oxidation of 8 comes at E1/2 = +0.05 V, thus the >0.3 V less oxidising CPE potential most likely prevents further oxidation. The origin of the over-oxidation

is therefore attributed to reaction of the PNP-ligand backbone. Especially in presence of excess base, the oxidation can be coupled to follow-up chemistry (deprotonation) via PCET, shifting it to milder potentials (as shown for 1^Cl in Chapter II.3.1). Such further oxidations would also support the higher current deviating from a two-electron process observed after addition of base to 6 in Figure 50. Backbone based reactivity was also suspected by Dr. I. Scheibel (née Klopsch), who examined the oxidation of 7 with CuCl2 upon which a C1-symmetric species was generated by ³¹P{¹H} NMR spectroscopy.⁷⁹ Although we have no clear indication for the presence of C1 -symmetric species under CPE conditions, multiple (redox-active) follow-up reactions must occur to account for the mixture of various Re-compounds and the high amount of transferred electrons. Since the regeneration of a Re-species capable of reductive N2-fixation is a pre-requisite towards a (pseudo-)catalytic approach, the same (electro)chemical MeCN release was explored with the unsaturated (P=N=P)-ligand, as described in the next Sections.