II. Results and Discussion
1.5. Discussion of the N 2 -splitting mechanism
The minimal scenario that could adequately simulate the CV data is the ECN2CClECdim.-pathway, as presented in Scheme 43. Reduction of 1Cl is followed by N2-binding to form [ReCl2(N2)(PNP)]−. This Re(II) species undergoes chloride loss to neutral [ReCl(N2)(PNP)], which is reduced once more and ultimately comproportionates with parent 1Cl. Subsequent or simultaneous chloride loss would form the N2-bridged 2Cl, but for simplicity, this process was not included in the simulations. All steps will be discussed extensively in this Section. The only trend that is not accounted for within this model is the rising current of the first reduction of 1Cl upon increasing the N2-pressure. The conversion of [ReICl(N2)(PNP)]− to bis-dinitrogen complex [ReICl(N2)2(PNP)]− through an additional N2-binding equilibrium can account for this observation, as indicated with K6 and k6 in Scheme 43. Formation of such a Re(I) might inhibit dimerization to 2Cl, thereby preventing the depletion of 1Cl in the diffusion layer, which results in higher currents at higher N2-pressure. This additional step can be included in the model leaving the simulations for ambient pressures virtually unchanged.
Scheme 43. Minimal mechanistic model for the simulation of the CV data of 1Cl under Argon (black box), or N2 (red box). The PNP-ligand is omitted for clarity.
The simulation parameters were chosen to maintain the parameters from the Ar data (K1, k2, E1, E2), and to minimise the number of variables in the simulations. The numerical ranges of the fit parameters under N2 (E3, k3, K3, k4, K4, k5, k6, K6) are presented in Table 4. The simulated and experimental CV data are in good agreement over a wide scan rate and chloride ion concentration range. In addition, this pathway (and its alternatives) was calculated by DFT, showing a reasonable agreement. In general, the values are less well defined compared to the ECClE-mechanism under Ar, and are therefore mostly given as range or lower limits in Table 4.
The N2-coordination equilibrium (K3) and subsequent chloride loss (k4) interdepend to a certain extend; a lower k4 is partially compensated by a larger K3. The potential value E3 is less well defined by the simulation, since changes of ± 0.02 V do not change the simulation quality significantly.
Table 4. Thermodynamic and kinetic parameter ranges for the reduction of 1Cl presented in Scheme 43 as obtained via simulation of CV data under N2. Values in parentheses are used for the simulation as depicted in Figure 16.f
f Digital Simulation of the CV data was performed by Prof. Dr. I. Siewert.
1Cl /
Figure 16. Overlay of experimental (black solid lines) and simulated (green dashed lines) CV data for 1Cl under N2 in THF with 0.2 M (nBu4N)PF6. Simulation according to Scheme 43 (without K6), with the thermodynamic and kinetic parameters given in Table 4. Left: 1.0 mM 1Cl. Right: 1Cl in presence of 20 eq. of (nBu4N)Cl.
The initial reaction is reduction of 1Cl at E1, as also present under Ar. By DFT computations, only small structural changes are found within the [ReCl2(PNP)]0/− couple, which is in line with an electrochemical reversible electron transfer. Upon reduction, this anionic platform is now capable of N2-coordination, which is both thermodynamically favourable (K3 ≈ 104 M-1) as kinetically very rapid (k3 > 5∙107 M-1s-1). This is in sharp contrast to the unobserved binding of dinitrogen to the Re(III) starting platform (Keq < 1.0 M-1). This increased affinity for the π-accepting ligand N2 is anticipated for the more electron rich, anionic Re(II) complex. Similar trends are well known and were quantified by Peters and Mock (Figure 17). Peters found an increase in N2-binding affinity by six orders of magnitude upon reduction of a low-valent bimetallic iron complex.159 Their rate of N2-binding (kN2 = 3∙106 M-1s-1) is in the similar range as found for [ReCl2(PNP)]−. In addition, the group of Mock established a N2-coordination series including three oxidation states, where the equilibrium constant is improved by the order of 27 upon reduction of FeII to Fe0.160
Figure 17. Increasing N2-affinity upon reduction as shown within this work on 1Cl (left), and examples on Fe from the group of Peters (middle) and Mock (right).159,160 All equilibrium constants are in M-1.
This model reveals two key findings about N2-coordination: it occurs on the formal Re(II) oxidation state, and before chloride dissociation. Rapid N2-binding to Re(II) is a prerequisite to avoid the unimolecular decay after chloride loss as described by k2. Our bulky pincer ligand, which enforces a five-coordinate geometry of 1Cl, might be instrumental here for productive N2 -activation: no ligand has to detach before N2-coordination can occur at a generally instable Re(II) oxidation state.
Subsequently, the six-coordinate anionic species [ReCl2(N2)(PNP)]− undergoes chloride loss, which is thermodynamically not favoured (K4 ≈ 10-2 M), but rapid (k4 > 5∙102 s-1). The rate is comparable or larger to chloride dissociation from [ReCl2(PNP)]− under Ar (k1 = 1∙103 s-1), and considerably faster than chloride loss from [ReCl3(PMe2Ph)3]−, as reported with k = 0.9 s-1, or from [ReCl3(dppe)(PPh3)]−, as reported with k ≈ 1 s-1.161,162 Chloride dissociation for our complexes is likely facilitated by the bulky tert-butyl moieties and the strong amide π-donating ability of the PNP-ligand. Especially the influence of the latter should not be underestimated.
For instance, the ReIII/II reductions of [ReCl3(HPNPiPr)] and [ReCl3(PONpyOP)]
(PONpyOP = 2,6-(OPiPr)2NC5H3) with dramatically decreased Npincer to Re electron-donation, appear reversible.80,84 Chemical reduction of the latter even afforded the stable rhenate [ReCl3(PONpyOP)]− complex, showcasing the decreased preference for chloride loss.84 The result of these two reaction steps is an associative substitution to form the five-coordinate [ReCl(N2)(PNP)].
Due to this overall associative ligand-exchange from a π-donating chloride- to a π-accepting dinitrogen ligand, the resulting complex can be reduced once more at a slightly milder potential compared to 1Cl (E3 = −1.86 V). This potential inversion leads to the characteristic narrow, multi-electron reduction wave for 1Cl under N2 and could also be backed up by the respective calculated potentials (E1,calc. = −2.08 V, E3,calc. = −2.02 V).70 This subsequent reduction to the more stable Re(I) state, likely prevents decomposition at the Re(II) state (as also found within this work as expressed with k2). An even larger potential inversion was found for [ReCl2(dppe)2]+, which is reduced in DMF at E1/2 = −0.70 V.163 The dinitrogen bound species [ReCl(N2)(dppe)2] is reversible oxidised at E1/2 = −0.29 V in THF, corresponding to the ReII/I reduction.164 Despite the formal lower oxidation state of the latter, it is reduced at substantially more positive potential. The onset of reversibility that is found for the first reduction feature of 1Cl under N2 upon increasing ν is assigned to this [ReCl(N2)(PNP)]0/− couple, since the initial reduction within this wave ([ReCl2(PNP)]0/−) coupled to chloride loss remains irreversible as was shown when measuring the ν-dependency under Ar.
Formation of 2Cl requires a bimolecular reaction step. Since the overall transformation from 1Cl into 2Cl required one electron per rhenium, and Re(II) is easily reduced to Re(I), the proposal involves comproportionation of Re(III) and Re(I) by reaction of 1Cl with [ReCl(N2)(PNP)]−. These two species can react as the anionic Re(I) complex diffuses away from the electrode and 1Cl (which concentration is at that point depleted at the electrode surface) diffuses towards the electrode, and is simulated as an irreversible reaction with k5 ≈ 7*103 M-1 s-1. At this point, it should be stated that during CPE the solution is stirred in contrast to unstirred CV measurements. In such a situation, the mechanic force dominates the mass transport, analogous to i.e. applying a rotating disc electrode. Indeed, the stirred CV using the stirrers as used for CPE shows a plateau current, indicating a concentration profile that reaches a steady state. Still, the existence of a diffusion layer justifies the described reactivity. The result of this coupling reaction
is [{ReCl(PNP)}{ReCl2(PNP)}(μ-N2)]−, which is the chloride adduct of 2Cl. To prevent over-parameterisation, the required chloride loss was not accompanied in the simulations, since
no information about this reaction is obtained via CV. DFT calculations suggest that the combined dimerization and chloride dissociation is strongly exergonic (ΔG˚calc. = −12.0 kcal mol-1).
Accordingly, at least five steps occur during the sweep of a CV to form N2-bridged intermediate 2Cl from molecular N2. After establishing this pathway, we can discuss the by DFT calculated
degree of N2-activation along the pathway, see Table 1 for reference values.g N2-coordination to form [ReCl2(N2)(PNP)]− is accompanied by moderate N-N bond activation as reflected in a calculated stretching frequency of 1975 cm-1. The hypothetical Re(III) congener of this species is calculated at 2105 cm-1. Subsequent chloride loss and reduction to [ReCl(N2)(PNP)]− is accompanied by a slight bathochromic shift to 1935 cm-1 and some electron density accumulation on the terminal nitrogen atom. However, due this moderate degree of activation, this species is still best described as ReI-(N2) as opposed to a ReIII-(N2)2- diazenido complex.
Upon dimerization into 2Cl the N-N bond distance (1.20 Å (DFT), and 1.202(10) Å (XRD)) and the calculated stretching frequency (1771 cm-1) range between typical values for a moderately activated N2-ligand and a diazenido (N2)2--bridge.16 Considerable N2-activation is first obtained upon dimerization to 2Cl, thereby avoiding unfavourable one-electron reduction of N2.165
g DFT calculations performed by Dr. M. Finger