Platform modifications to the unsaturated ligand

To explore (electro)chemical release of MeCN from unsaturated [Re(NCHCH2)Cl(P=N=P)] (⁼6), its synthesis needs to be developed. First, the PCET route was examined by addition of excess TBP to either 5 or 6 at reaction temperatures RT and 50 °C. However, no conversion was observed at any condition. This implies that the (P=N=P)-ligand backbone protons have a higher BDFE_(CPNP-H) compared to the O-H of the TBP (BDFETBP (THF) = 74.4 kcal mol^-1).¹⁹⁵ Subsequently, the electrophilic functionalisation was started from nitride [ReNCl(P=N=P)]

(⁼3^Cl) with EtOTf. Notably, such reactivity was already established for the similar reaction with MeOTf. Several conditions were explored to obtain full conversion of which 10 eq. of EtOTf at 80°C represents the fastest route; no starting material remains after 90 minutes. Towards a (pseudo-)catalytic application, functionalisation at RT is elegant, and this could be achieved, yet with more severe reaction conditions (20 eq. of EtOTf and 48 h reaction time). The ethylation conditions for ⁼3^Cl are more harsh compared to ethylation of 3^Cl, for which at RT only 1.2 eq. of EtOTf are required for full conversion after 24 h. This is attributed to the reduced nucleophilic character of ⁼3^Cl because of backbone desaturation. In the crude ³¹P {¹H} NMR spectrum after reaction with EtOTf, two species are observed of which the major feature comes at 87.0 ppm, which resembles the saturated imido analogue (δ31P {1H} 5 = 90.1 ppm). This species can be isolated via crystallisation from THF/Et2O in 55 % yield, and LIFDI mass spectroscopy confirms the formation of [Re(NCH2CH3)Cl(PNP)]OTf (⁼5). By ¹H NMR spectroscopy, the incorporated

ethyl fragment is clearly assigned to the nitride-N, since no 2D NMR coupling between the protons of the (P=N=P)-backbone and the ethyl moiety is observed. XRD of single crystals provides the molecular structure of ⁼5, where the Re coordinates in a distorted square pyramidal geometry (τ5 =0.14), see Figure 51.^m,171 Compared to the parent nitride, the Re-Nimido bond is basically untouched (Re-N2 =1.647(18) Å (⁼3^Cl)⁷² and 1.680(5) Å (⁼5)).

Figure 51. Top left: Synthesis of ⁼5 from ⁼3^Cl with different eq. of EtOTf in chlorobenzene with 11 as side-product.

Top right: crude ³¹P{¹H} NMR spectrum after reaction of ⁼3^Cl with EtOTf. Bottom left: ¹H NMR spectrum of ⁼5 in CD2Cl2. Bottom right: ORTEP plot of ⁼5 with anisotropic displacement parameters drawn at the 50% probability level.

Hydrogen atoms and the anion are omitted for clarity. Selected bond lengths (Å) and angles (deg.): Re-Cl1:

2.3608(14), Re-N2: 1.680(5), Re-N1: 2.005(4), Re-P1: 2.4832(15), Re-P2: 2.4695(15), N2-C21: 1.449(7), P2-Re-P1:

151.54(5), N1-Re-N2: 110.4(2), N1-Re-Cl1: 143.11(13).

Besides ⁼5, a second species (δ31P{1H} = 75.8 ppm) is always present in the crude reaction mixtures in circa 35 %, independent of the applied synthesis route (Figure 51). Additionally, its formation is also solvent independent, as an identical ratio between ⁼5 and this species was observed in C6D6, chlorobenzene and THF (quickly measured before THF polymerisation occurred). It is

m XRD performed by Dr. J. Abbenseth

Et2O-soluble, which is distinctively different than imido ⁼5, and a relatively purified batch shows Cs-symmetry on the NMR time scale, see Figure 52. Initially, the presence of a signal in the ¹⁹F NMR spectrum triggered us to suspect triflation of the nitride-N. Especially because in case of the saturated nitride, such reactivity was observed upon reaction of 3^Cl with Tf2O to form [Re(NSO2CF3)Cl(PNP)]OTf. To probe this hypothesis, ⁼3^Cl was reacted with 1 eq. Tf2O in chlorobenzene (Scheme 57).

Scheme 57. Attempted synthesis of [ReCl(NSO2CF3)(P=N=P)]OTf to test the hypothesis of its formation during synthesis of ⁼5.

However, no initial conversion was observed and only after substantial heating (75 °C) and the addition of a large excess of Tf2O (circa 100 eq.), full conversion was achieved into a species with a ³¹P{¹H} NMR resonance at 75.8 ppm, precisely similar to the unknown side-product. This is in sharp contrast to the straightforward reactivity of 3^Cl with Tf2O. Although this could indicate that the side-product is [ReCl(NSO2CF3)(P=N=P)]OTf, two signals are to be expected in the ¹⁹F NMR and a cationic species is not in agreement with Et2O-solubility. Furthermore, in the reaction of ⁼3^Cl with EtOTf, the side product is readily formed by only using 1 eq., in contrast to the many equivalents of Tf2O needed here. Therefore, a different explanation was sought. Taking a closer look at the ¹H NMR spectrum reveals the close resemblance of this side-product to the parent nitride, see Figure 52. A crystallisation attempt yielded the structure of [ReN(P=N=P)(MeCN)]OTf. This finding is unexpected, since it does not match with the ¹H NMR spectroscopy of our side-product. Furthermore, MeCN is not regularly used in the synthesis glovebox used within this work. Its formation however indicates that the chloride of nitride complex ⁼3^Cl is labile and can be substituted, i.e. with a triflate. A LIFDI mass of the side product showed indeed a major peak at 706.1 m/z, which corresponds to [ReN(OTf)(P=N=P)] (11) (calculated mass 706.1 m/z). Assignment of this neutral species as side product is in line with the NMR spectra, the resemblance to ⁼3^Cl, and the observed solubility. Furthermore, the addition of excess (nHe4N)Cl to a sample of 11 in C6H6 results in qualitative regeneration of ⁼3^Cl, confirming the anion-exchange from chloride to triflate to form 11. Notably, Dr. I. Scheibel reported the exchange from chloride to cyanide.⁷⁹

Figure 52. Left: NMR spectra of 11 in C6D6: ³¹P{¹H} NMR (top), ¹H NMR (bottom). Right: NMR spectra of ⁼3^Cl in C6D6: ³¹P{¹H} NMR (top), ¹H NMR (bottom).

The saturated imido 5 was synthesised in up to 80 % yield without the mentioning of a repeatedly observed side-product. It cannot be fully excluded if some anion exchange occurs, but it is for sure less prominent in comparison to the unsaturated platform. This difference is likely the result of different reaction conditions between ethylation of 3^Cl or ⁼3^Cl, since for the latter either large excess of EtOTf or higher reaction temperatures are needed, which will both favour anion exchange. Furthermore, it can be stated that nitride 11 is clearly not capable of ethylation, since no [Re(NCH2CH3)(OTF)(P=N=P)]OTf is formed. This is in line with weaker donor properties of triflate compared to chloride, as reflected in their respective Lever parameters (−0.22 V (Cl⁻) vs. +0.13 V (OTf⁻))⁶⁴ or ligand donor properties (15.05 ± 0.29 kcal mol^-1 (Cl⁻) 15.75 ± 0.29 kcal mol^-1 (OTf⁻), see Section II.2.8 for details),¹⁹¹ making the nitride less nucleophilic.

Since the reverse reaction of 11 to ⁼3^Cl by addition of excess chloride proved successful, ethylation was examined in presence of (nHe4N)Cl to increase the imido yield. However, several C1-symmetric products were formed besides ⁼5 as judged by the ³¹P{¹H} NMR spectrum. Upon deprotonation these do not form ⁼6. As alternative strategy, ethylation was performed in a repetitive approach, where after each run, 11 was separated from ⁼5 and reacted with (nHe4N)Cl.

Subsequent extraction with pentane affords clean ⁼3^Cl, which can be recycled for the reaction sequence (see Scheme 58). It was noted that during ethylation of ⁼3^Cl, some protonated nitride [9^Cl]⁺ is formed, which upon addition of (nHe4N)Cl also reacts to ⁼3^Cl. In this reaction, the chloride can be assumed to act as a base. Via this repetitive synthesis route, which was done three times, ⁼5 was isolated in 77 % yield. The theoretical maximum after three runs is only

slightly higher (82 %). Although the material and time consumption of this approach is substantial, the yield of ⁼5 is higher.

Scheme 58. Repetitive ethylation, separation, and chlorination cycles to increase the overall yield of the ethylation from ⁼3^Cl to ⁼5 by recycling the side-product 11.

To circumvent formation of 11, experiments were initiated with Meerwein salts (OEt3)BF4 or (OEt3)BAr^F24, of which the first one contained significant traces of water, leading to quantitative protonation of the nitride. Upon reaction of ⁼3^Cl with the BAr^F24 salt, a ³¹P{¹H}NMR peak at 86.9 ppm was observed, which is basically identical to ⁼5, that could not be isolated (likely due to its Et2O solubility). Deprotonation of the mixture resulted in partial formation of ⁼6 as proven by ³¹P{¹H} NMR spectroscopy (vide infra for characterisation of ⁼6), proving that [Re(NCH2CH3)Cl(P=N=P)]BAr^F24 was formed, accompanied by side-products. Due to challenging purification of the crude imido mixture, the synthesis of ⁼5 using (OEt3)BAr^F24 was not further pursued.

Upon establishing the unsaturated ethyl imido species, the electrochemistry of nitrido ⁽⁼⁾3 and imido ⁽⁼⁾5 can be compared. The nitrides are irreversible reduced at very cathodic potential (Ep

≈ −3.4 V (3^Cl, Table A2), Ep ≈ −3.1 V (⁼3^Cl))²⁰⁶, both at ν = 0.1 Vs^-1), whereas the imido reveals a much milder potential for the first reduction (Ep ≈ −1.8 V at ν = 0.1 Vs^-1 (5, Table A23), E1/2 ≈ −1.3 V (⁼5, Table A22)). This represents a striking shift (by circa 40 kcal mol^-1), taking into account that both species are formal Re(V), and the ethyl-group is assumed to be redox-inactive (thereby rendering the reductions at least in the saturated case metal-based).

=5 was deprotonated with an excess NEt3 in chlorobenzene to form [Re(NCHCH3)Cl(P=N=P)]

(⁼6). ³¹P{¹H} NMR spectroscopy revealed full conversion after 24 h and the appearance of two peaks at δ31P{1H} = 34.1 and 33.2 ppm, which couple via ³¹P-¹H HMBC spectroscopy to two ethyl-fragment containing Cs-symmetric species. Between these species, only the ethyl-peaks seem to be in a different chemical environment, as their chemical shift deviates (Δδ ≈ 0.3 ppm), in

contrast to the overlapping backbone protons (Δδ ≈ 0-0.1 ppm). Since in LIFDI mass spectroscopy only one main peak is found at 620.3 m/z, the reaction mixture is explained by formation of two isomers of ⁼6 as present in solution, which was shown before for the saturated analogue 6. Deprotonation of the imido ligand of ⁼5 leads to an azavinylidene ligand with a formal N-C double bond that hinders rotation: one isomer has the methyl group pointing towards the ligand and vice versa.

Figure 53. Top left: Deprotonation of ⁼5 with NEt3 yields ⁼6 in quantitative amounts. Top right: ¹H NMR spectrum of ⁼6 in C6D6. Bottom left: ³¹P-¹H HMBC of ⁼6 in d8-THF. Bottom right: ORTEP plot of ⁼6 with anisotropic displacement parameters drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg.): Re-Cl1: 2.3718(9), Re-N2: 1.794(3), Re-N1: 2.011(3), Re-P1(#): 2.4114(15), N1-C11:

1.268(5), P1-Re-P1#: 160.60(3), N1-Re-N2: 109.01(13), N1-Re-Cl1: 144.77(9), N2-C11-C12: 122.6(4), τ5 = 0.26.

Single crystals for XRD could be grown from a saturated pentane solution,ⁿ and show a distorted square pyramidal coordination geometry around the Re-centre (τ5 = 0.26).¹⁷¹ Compared to imido

=5, the Re-Nimido bond is elongated (1.680(5) Å for ⁼5 and 1.794(3) Å for ⁼6) and the Nimido-C bond is shortened (1.449(7) Å for ⁼5 and 1.268(5) Å for ⁼6), in agreement with a lower bond

n XRD performed by Dr. C. Würtele.

order between Nimido-C upon deprotonation. Comparing this structure to the saturated PNP-ligand analogue 6, especially the coordination geometry strikes the eye. The τ5 value of 6 is 0.45, which is mainly the result of the increased Namide-Re-Cl angle (144.77(9)° (⁼6) compared to 135.07(15)° (6)). The ligand-bite angle is barely touched: P-Re-P decreases from 162.01(5)° (6) upon desaturation to 160.60(3)° (⁼6). Since the Namide,P=N=P is a weaker trans-donor compared to Namide,PNP, this increases the angles of the chloride with respect to the ligand, likely favourable due to decreased strain of the proximity to the tert-butyl moieties.

At RT in solution, both isomers of ⁼6 are present (ratio circa 1:3), indicating that the isomerisation rate is slower compared to the timescale of NMR. It was noted that in most solvents, a set of tert-butyl moieties appeared broad in the ¹H and ¹³C{¹H} NMR spectra (e.g. at δ1H = 1.3 ppm in Figure 54, right). This could indicate that at RT, the NMR time scale is close to the isomerisation rate. Therefore, we measured VT-NMR of ⁼6 in d8-THF to 60 °C. However, instead of additional broadness, as expected in case the increasing temperature would allow the isomerisation rate to come closer to the experimental conditions, the tert-butyl moiety sharpens.

The same behaviour was observed when ⁼6 was heated in d8-toluene to 95 °C. This indicates that the isomerisation rate is clearly lower as can be examined with straightforward NMR spectroscopy. The observed broadness at RT is attributed to additional dynamic processes.

Figure 54. VT-NMR of ⁼6 in d8-THF. Left: ³¹P{¹H} NMR spectra, right: ¹H NMR spectra.