II. Results and Discussion
II.5: N 2 -splitting in a flow cell
Parts of this Chapter are published in:,Implementierung einer Flow-Zelle in die elektrochemische Stickstoffspaltung’, C.M.G.K. von Petersdorff-Campen, 2020, Bachelorthesis, Georg-August-Universität Göttingen.
The electrochemical N2-splitting, C-N bond functionalisation and subsequent MeCN release starting from the (un)saturated platform [ReCl2/3(P(=)N(=)P)] can only move closer towards a catalytic future if the separate reactions are carried out at isolated reaction sites, preventing all incompatible reagents to directly react with each other. Initially, separating the reductive N2 -splitting and the oxidative MeCN-release could be envisioned in a regular CPE set up, where both reactions occur and the transport of the corresponding Re-species occurs via a membrane or porous filter. Considering the saturated platform, the applied potentials of oxidative MeCN release (Eappl. =−0.3 V from 6) and reductive N2-splitting (Eappl. = −1.9 V from 8, respectively) would allow that the required Re-species are not reacting at the opposite electrode: reduction of 6 is at very negative potentials (Ep = −3.1 V) and 8, which would be the starting platform of choice, is first oxidised at E1/2 = 0.05 V. Yet, the C-N bond functionalisation should also be incorporated (which is a rather slow reaction), and complicates the one CPE cell set up.
Furthermore, intermediates (such as 1Cl during N2-splitting), might not be compatible with these potentials. It should be stated that EtOTf is first reduced starting from > −2.5 V in 1,2-difluorobenzene (see Appendix A2), and it is imagined to be at least partially compatible with electrochemical N2-splitting. A solution can be found in a flow cell set up, where both reactions are separated and coupled to a separate reaction vessel where the electrophilic ethylation is performed, as very simply sketched in Scheme 60. As counter reaction, Fc oxidation and re-reduction is envisioned, that can occur at high concentrations, to ensure that there are no limitations from the counter electrode side. A cyclic flow could be generated from a peristaltic pump.
It is noted that the simple sketch of this envisioned flow cell set up disregards many challenges, for instance many compatibility questions, i.e. formation of MeCN & N2-splitting, EtOTf & THF, excess Cl− ions & N2-splitting, just to name some. Addressing those is beyond the scope of this work. As final part of this thesis, a flow cell set up was explored for the N2-splitting reduction. In general, the diamagnetic nitride result of such a reaction is easier to quantify compared to the MeCN release reaction. As found in Chapter II.4, the unsaturated platform is performing better for the MeCN release, giving at least some of =8. Yet, we perform test reactions on 1Cl in THF, as high nitride yields were obtained.
Scheme 60. Schematic set up of the Re-mediated N2-splitting and oxidative MeCN release, coupled via a separate reaction vessel where the chemical C-N bond formation reagents are injected.
Figure 62. Flow cell set up with the syringe pump as used for initial attempts of electrochemical N2-splitting from 1Cl. From left to right comes first the syringe pump with a solution of 1Cl (purple, back) and a solution of Fc (yellow, front) in THF. The syringes are connected via PTFE tubes to the stainless steel entrances of the flow cell. The flow cell is connected via crocodile clamps to the potentiostat. After flowing passed the electrodes, the solutions are gathered in the attached vials on the right.
First, we recognised that the commercially obtained flow cell was made from mainly two materials that are not compatible with most organic solvents (EPDM, CPVC), and had to be modified to a PTFE set up, which is extensively described in Section IV.4.2. Meanwhile, we wanted to correlate the flow rates with conversion via UV-vis spectroscopy for a simple test system, either using the [Fc]+/0 couple in organic solvents or [Fe(CN6)]2−/3− in water. Yet, we encountered several obstacles that hampered a decent study at this point. Amongst the problems are mainly the facts that we a) explored graphite electrodes for most test runs that required high overpotentials to observe some current density, and b) the presence of stainless steel entrances of the flow cell as described in Section IV.4.2, which strangely proved incompatible with [Fc]+.
Despite lacking information about the flow rate/conversion correlation, we explored a first attempt for dinitrogen activation in the flow cell set up in the MBraun glovebox under a N2
atmosphere, as depicted in Figure 62 and Scheme 61. Instead of a peristaltic pump, a syringe pump was used for these initial tests, since its handling proved easier.
Scheme 61. Reactions examined in the flow cell: electrochemical N2-splitting from 1Cl into 3Cl at the cathode and Fc oxidation at the anode, with an overall cell potential of Ecell = –1.9 V.
We confirmed that both 1Cl and 3Cl are stable while flowing through the stainless steel entrances of the flow cell, as judged from identical UV-vis traces before and after a run through the flow cell. Afterwards, we charged the syringes with separate solutions of 1Cl and Fc, and applied a flowrate of 0.04 mL min-1. This corresponds to a dwell time for the Re-species at the electrode surface of circa 7/8 times lower compared to a classic steady state CPE experiment. An anion-exchange membrane was applied to secure charge compensation and to prevent Fc+ migrating in the cathode frame to oxidise 1Cl. Based on literature examples, we got the impression that large overpotentials were required,123 and CPE was initially examined at a large overpotential of Eappl. = −3.0 V vs. the Ag-wire (which very roughly corresponds to −3.4 V vs. Fc+/0). Although the colour of the Re-containing solution changed promisingly to orange, no 3Cl was found by NMR spectroscopy. Instead, it showed a mixture of at least 7 unknown species. Supported by full conversion of 1Cl in this attempt, a modest overpotential was explored next at Eappl. = −1.6 V vs.
the Ag-wire (which very roughly corresponds to ≈ −2.0 V vs. Fc+/0). The colour on the Re-side changed to yellow, accompanied by a current density of ≈ 1.6 mA cm-2. In the 31P{1H} NMR spectrum we observe a feature at 84.2 ppm, which matches to formation of 3Cl, as shown in Figure 63. The nitride is formed alongside many other species as judged by the appearance of many signals in the 31P{1H} NMR spectrum, and was not quantified. The main species is found at 80 ppm, which based on its chemical shift corresponds to a Re(I) or Re(V)-species.
Unfortunately, the mixture of compounds hinder clear allocation of 3Cl via 1H NMR spectroscopy: with a very optimistic eye, the backbone features at 3.10 and 3.69 ppm might be
recognised, yet also clearly obscured by other peaks. Surprisingly, the 1H NMR spectrum of the cathodic side shows the presence of Fc, which was not expected based on the anion-exchange membrane. Besides, paramagnetic species must be present, judged from the appearance of broad features in the range of −5 to −11 ppm. Via LIFDI mass spectroscopy, no clear feature for the nitride was observed, although the region is obscured by other features at 598 m/z and 594 m/z (calculated for 3Cl is 596 m/z). Although the outcome now only relies on one method, we believe it is a promising start for future research.
Figure 63. NMR spectroscopy of electrochemical N2-splitting in a flow cell at roughly Eappl. = −2.0 V vs. Fc+/0 with a
flow rate of 0.04 mL min-1. Left: 31P{1H} NMR, right: 1H NMR.