II. Results and Discussion
1.1. Dinuclear N 2 -bridged intermediate and its splitting into nitrides
Chemical N2-splitting starting from a molecular Re-complex was established in our group using the platform [ReCl2(PNP)] (1Cl) forming the terminal nitride [ReNCl(PNP)] (3Cl) in yields up to 85 % ((PNP) = N(CH2CH2PtBu2)2−), as elaborated in Section I.1.4. This directly raised the question for the existence of a possible N2-bridged intermediate, as initiated experimentally by Dr. F. Wätjen (né Schendzielorz); chemical reduction of 1Cl at low temperatures (−40 °C) forms C2-symmetric [{ReCl(PNP)}2(µ-N2)] (2Cl), showing coupling doublets by 31P{1H} NMR spectroscopy (Scheme 39). Upon warming, this species converts to 3Cl, proving its role as intermediate in N2-splitting. 1H NMR spectroscopy showed paramagnetically shifted, yet sharp signals in the range of +12 to −17 ppm, among which by 1H COSY NMR spectroscopy eight signals for the PNP-pincer backbone proton are identified. The allocation of the four tert-butyl groups remained unclear, due to their broadness at low temperatures. 2Cl could not be isolated, yet crystals suitable for XRD spectroscopy were obtained, that confirm its structure.71 From this already elaborate starting point, the properties of 2Cl and its role in N2-splitting are examined further within this work.
Scheme 39. Reduction of 1Cl at −30 °C with Na/Hg affords 2Cl in 75 % spectroscopic yield, which splits into 3Cl upon warming up in a first order manner.
Figure 10. Left: NMR spectroscopy of a mixture of 2Cl and 3Cl at −30 °C, 31P{1H} NMR spectrum (top) and 15N{1H}
NMR spectrum (bottom). Right: 1H NMR spectrum of 2Cl in d8-THF at −15 °C with addition of hexamethylbenzene as internal standard to quantify 2Cl.
First, the intense red colour of a solution of 2Cl was examined by UV-vis spectroscopy showing two absorption maxima at 533 and 375 nm. Resonance Raman with excitation wavelengths at 467 and 633 nm did unfortunately not reveal a N2-stretching frequency that could complement the N-N bond length for insight into the degree of N2-activation. Furthermore, the NMR characterisation of 2Cl was completed. Upon measuring NMR spectroscopy at higher temperatures (−15/−10 °C), the tBu-groups sharpen, and we were able to identify four moieties via coupling by 1H COSY NMR spectroscopy. As already stated, 2Cl is not isolatable, and its yield was therefore determined spectroscopically to be circa 75 %. Since 3Cl is formed in 85 % yield via reduction at RT, our spectroscopic yield of 2Cl indicates formation of some additional side products upon performing the reduction at low temperatures (which are observed around 50 ppm, see Figure 10). Rarely, another set of coupling doublets was visible by NMR spectroscopy in low amounts (δ31P{1H} [ppm] = 64.8 (d, 2JPP = 225 Hz), −10.8 (d, 2JPP = 225 Hz)).
It will not be discussed here, yet it is briefly addressed in Section II.2.4, where a similar and more persistent compound is found when reducing the bromide analogue. Reduction of 1Cl under a
15N2-atmosphere revealed a 15N{1H} singlet at 211 ppm, alongside the signal of 3Cl at 369 ppm, which is consistent with a symmetrically bound N2 (in contrast to a terminal N2 ligand) (Figure 10). Although this value lies in the typical range for coordinated N2,156 it notably differs for the isostructural [{MCl(PNP)}2(µ-N2)] (δ15N = 69 ppm (M = Mo), and δ15N = 30 ppm (M = W)).25,66
The unusual strongly shifted, yet narrow NMR lines as found for 2Cl are tentatively attributed to an expression of temperature independent paramagnetism (TIP). This arises from mixing of excited states via spin-orbit coupling into a thermally well separated (>> kBT) ground state. This
phenomenon is well-known for d4 Re(III) phosphine complexes, i.e. [ReCl3(PMe2Ph)3], [ReCl3(PEt2Ph)3]157 and [ReCl3(HN(CH2CH2PiPr2)2)] or [{ReCl2(HN(CH2CH2PiPr2)2}2(μ-N2)]80 of which the latter is isoelectronic to 2Cl. Variable-Temperature (VT-) NMR spectra of 2Cl between −55 and −5 °C show that most signals are basically temperature independent, supporting the TIP assignment, see Figure A1. Only the 31P NMR resonance at −120 ppm and three 1H NMR signals of the PNP-ligand backbone shift temperature-dependently (Δδ > 1.0 ppm), yet show a linear behaviour when plotted vs. T-1 (Curie Plots, Figure A1). The tert-butyl signals exhibit significant broadening at lower temperatures, suggesting that additional dynamic processes might be responsible for the temperature dependence of these signals, such as freezing out of bond rotations. It must be stated that the temperature window for this VT-NMR analysis was chosen rather small (ΔT = 50 °C). Unfortunately, its electronic structure cannot be further analysed by SQUID magnetometry due to its limited stability at RT and only 75 % spectroscopic purity.
Due to these spin-orbit coupling effects, DFT is unable to describe 2Cl properly. Yet, the DFT calculations as performed by Dr. M. Finger could fully reproduce the molecular geometry as found by Dr. F. Wӓtjen. Within a localised description of the core, the rhenium-centred spin density agrees with a ReII-(N2)-ReII, or alternative a ReIII-(N2)2−-ReIII formulation. However, oxidation states can be meaningless in case of high covalent metal-ligand multiple bonding, as was addressed in literature for nitrosyl and nitride complexes.158 A more covalent binding picture of such a {MNNM}-fragment was presented in Section I.1.3. The 14 valence electrons that such a {ReIII-(N2)-ReIII}2− fragment offer are distributed over the Re-N-N-Re manifold and non-bonding δ-orbitals, see Scheme 40. The thereof resulting δ4π10 configuration was substantiated by computations.
Such a π10-configuration is associated with the formation of stable, closed-shell nitrides, as discussed in Section I.1.4. From 2Cl, it was calculated that the transition state for splitting into nitrides exhibits a Re-N-N-Re in-plane zig-zag structure. Upon approaching this transition state, the high lying σReN-σ*NN-σReN gains considerable σReN and σ*NN character and is gradually stabilised with respect to the π-manifold. Population of this molecular orbital from the π*ReN-πNN-π*ReN level leads to N-N bond weakening and simultaneous Re-N strengthening, and ultimately N-N cleavage.
Scheme 40. Qualitative MO-scheme representing the δ4π10 configuration of L4ReII(N2)ReIIL4 (representing 2Cl), and subsequent N2-splitting into terminal nitrides. The MO’s of the nitride are depicted on the right.
The activation parameters for this process can be obtained via NMR spectroscopic analysis of splitting from 2Cl into 3Cl, justified by the observation of 2Cl in high spectroscopic yields at low temperatures and its selective splitting into 3Cl. Therefore, we monitored decay of 2Cl by 1H NMR spectroscopy at temperatures ranging from −15 to +7.5 °C. Plotting this conversion agreed with first-order kinetics over two half-lives; exemplarily conversion traces for each temperature are shown in Figure 11. An Eyring analysis over this temperature range provides activation parameters for N2-splitting: ΔH‡ = 24 ± 1 kcal mol-1 and ΔS‡ = 14 ± 3 cal mol-1 K-1, in agreement with a short lived intermediate at RT. In the context of this in-depth study, Dr. M. Finger performed calculations that reflect the full N2-splitting mechanism of 1Cl (vide infra).70 The computed free energy of activation is higher than the experimental value (ΔG‡298K, Cacld. = +26.9 kcal mol-1, ΔG‡298K, Exp. = +19.8 kcal mol-1), but the calculated enthalpy of activation is in excellent agreement with the experiment (ΔH‡298K, Cacld. = +26.2 kcal mol-1) and only the activation entropy is deviating from the calculated value (ΔS‡ Cacld. = −3 cal mol-1 K-1).
The experimental value suggests that formation of the proposed zig-zag transition state is accompanied by an increase in entropy. In contrast, the calculated value describes an entropically neutral process upon splitting via this transition state. These computations are supported by comparison to the activation entropy for N2-splitting of related compounds [{Mo(HPNP)Cl}2(μ-N2)], [{W(CO)(PNP)}2(μ-N2)] by Schneider, or the pioneering example of [{Mo(N(C(CD3)CH3)(3,5-C6H3(CH3)2))3}2(μ-N2)] by Cummins (ΔS‡ exp [cal mol-1 K-1] = −5.7, +2.3, or +2.9, respectively).43,66,67 The activation entropy is very sensitive towards the fit of the data,
since it is calculated from the y-intercept. Due to the small temperature range that was examined (ΔT = 22.5 °C) the experimental value as derived herein should be handled with care.
Figure 11. Left: Exemplary conversion plots vs. time. Right: Eyring plot for the conversion of 2Cl to 3Cl in the temperature range from −15 to +7.5 °C.
With 2Cl identified as the intermediate that splits N2 towards nitride 3Cl, reactions potentially relevant to its assembly were examined, i.e. dinitrogen coordination to 1Cl. Collaboration partnersa examined possible N2-coordination by IR spectroscopy. A solution of 1Cl in THF under 1 atm N2 gave no indication of end-on binding of dinitrogen. In addition, UV-vis spectra under Ar or N2 are identical, even upon cooling from RT to −78 °C.70 In the context of this work, 1Cl was examined by NMR spectroscopy as recorded under Ar or N2 (4 atm), showing no difference even upon cooling to −95 °C (see Figures A2). From −40 °C and lower, broadening, disappearance and splitting of the tBu-moieties is observed, indicating freezing out of the P-C bond rotation and the appearance of the individual methyl resonances. These experiments speak against a pre-equilibrium to form [ReCl2(N2)(PNP)] and by NMR spectroscopy an upper limit for this N2 pre-equilibrium to 1Cl can be estimated (Keq < 1 M-1, see experimental Section IV.6.1).
Additionally, potential reactivity of 1Cl with chloride anions is assessed, as it is released during the N2-splitting reaction. NMR spectroscopic monitoring of a solution of 1Cl under N2 in presence of 5 eq. of (nBu4N)Cl indicate no chloride association over more than 48 h (Figure A3). These coordination studies demonstrate that the initial step most likely consists of reduction of rhenium(III). Therefore, the formation pathway of 2Cl was examined via electrochemical methods.
a IR and UV-vis spectroscopy performed by Dr. B.M. Lindley, under supervision of Prof. Dr. A.J.M. Miller, University of North Carolina at Chapel Hill.