Double Deprotonation of CH3CN by an Iron‐Aluminium Complex

Abstract Herein we present the first double deprotonation of acetonitrile (CH3CN) using two equivalents of a bimetallic iron‐aluminium complex. The products of this reaction contain an exceeding simple yet rare [CHCN]2− dianion moiety that bridges two metal fragments. DFT calculations suggest that the bonding to the metal centres occurs through heavily polarised covalent interactions. Mechanistic studies reveal the intermediacy of a monomeric [CH2CN]− complex, which has been characterised in situ. Our findings provide an important example in which a bimetallic metal complex achieves a new type of reactivity not previously encountered with monometallic counterparts.[1, 2] The isolation of a [CHCN]2− dianion through simple deprotonation of CH3CN also offers the possibility of establishing a broader chemistry of this motif.

α-Nitrile anions date back to the late 1800s.The deprotonation of CH 3 CN, by reaction with Na, was first reported by Holtzwart in 1889. [3]7] Tae and co-workers recently reviewed the use of these anions in organic synthesis. [8][19][20] The [CHCN] 2À dianion represents one of the simplest chemical building blocks available, this fragment contains only four atoms and 16 valence electrons.There are unanswered fundamental questions about the electronic structure of [CHCN] 2À and its binding modes to metal centres; theoretically, several structural motifs can be considered (Figure 1).These include 1,1-dianion (I), 1,3-dianion (II) and metallocarbene (III) species.To the best of our knowledge, only two structurally characterised compounds have been reported that contain the [CHCN] 2À motif; a diiron complex [19] with connectivity reminiscent of I and tungsten cluster [20] that appears to contain four units of III.A third, closely related species, Li 2 [(Me 3 Si)CCN] was reported in the late 1980s, [17] as well as [Mes 2 Ge(C(Ph)CN)] 2 in the 1990s. [18]In the solid-state, Li In this paper, we report the double deprotonation of CH 3 CN to form a [CHCN] 2À dianion.This was achieved by reaction of CH 3 CN with a highly basic FeÀ Al bimetallic complex recently developed by our group. [21,22] he resulting products contain two bimetallic metal fragments bridged by a 1,3-dianion (structure type II).We analyse the electronic structure of the [CHCN] 2À dianion, the nature of its binding to the metals, and its mechanism of formation.Our findings provide new insight into the fundamental chemistry of an uncommon and understudied, yet extremely simple species, [CHCN] 2À .
The reaction of 1 a with acetonitrile (1 equiv.) in C 6 D 6 at 25 °C proceeded with an immediate colour change from deep red to orange-brown, corresponding to the formation of an intermediate (see below).Over the course of 3 days, 2 a crystallised out of the reaction mixture in 33 % yield (Scheme 1).Single crystal X-ray crystallographic analysis revealed that 2 a is an unusual metal complex containing two FeÀ Al units bridged by a μ 2 -k C -k N -[CHCN] 2À dianion. [23]Al nucleus.The 13 CHCN resonance appeared as a doublet due to coupling to the S = 1/2 13 C ( 1 J H-C = 127.4Hz).IR spectroscopy of 2 b revealed a strong band at 2038 cm À 1 assigned as the C=N vibration.This is at a markedly lower wavenumber than the C�N vibration in CH 3 CN (2251 cm À 1 ).The experimental spectroscopic data is well reproduced by DFT calculations (Table S8).
The solid-state structures of 2 a-b are depicted in Figure 2.  [24] A Natural Bond Orbitals (NBO) analysis was carried out on 2 b to gain insight into electronic structure of the [CHCN] 2À dianion (Figure 3). [25]In 2 b, the [CHCN] 2À dianion adopts a heterocumulene [HC=C=N] 2À motif.The [HC=C=N] 2À fragment is characterised by Wiberg Bond Indicies (WBIs) consistent with two conjugated double bonds (2 b: C=N 2.10, C=C 1.71). [26]Using the same level of theory  S6 for details).These interactions were confirmed and quantified by ETS-NOCV (Extended Transition State -Natural Orbitals for Chemical Valence) calculations. [27]ithin the ETS-NOCV approach the coordination modes were separated out, and binding of the N-terminus and C-terminus of the dianion considered independently.The total interaction energies were calculated to be ΔE orb -(Al 2 À N) = À 181.9 kcal mol À 1 and ΔE orb (Al 1 À C) = À 197.9 kcal mol À 1 respectively.In both cases the most prominent contributions to the orbital interaction were identified as donation of electron density from sp 2 hybrid orbitals of the terminal C/N atom of the dianion fragment into the Al p z orbitals.Specifically, for the NÀ Al 2 interaction: Δρ 1 (N sp 2 z !Al 2 p z ) = À 26.1 kcal mol À 1 , Δρ 2 (N sp 2 x !Al 2 p y ) = À 10.6 kcal mol À 1 and for the CÀ Al 1 interaction: Δρ 1 (C sp 2 z !Al 1 p z ).= À 50.0 kcal mol À 1 (see Table S7 for details on ETS-NOCV/fragmented NBO analysis of the bonding).
In combination the calculations suggest that 2 b features a [CHCN] 2À dianion fragment possessing a C=C=N cumu-  DFT calculations were also undertaken on the mechanism of the reaction of CH 3 CN with 1 b. [21]The computed pathway supports a stepwise double deprotonation of CH 3 CN by two equivalents of 1 b (Figure 4).Initial, endergonic (ΔG = 6.5 kcal mol which is consistent with a reaction that is complete in < 5 min at 25 °C.This activation barrier is comparable to that calculated for the CÀ H activation of pyridine by 1 a, which goes through a very similar anchored transition state. [21]Once formed,  Both the first and second deprotonation steps involve early transition states and are highly exergonic (ΔG � À 30 kcal mol À 1 ).An alternative pathway involving a direct deprotonation of CH 3 CN without pre-coordination to Al has also been considered computationally but has significantly higher barriers and is unlikely to be operating (Figure S2).
NBO calculations provide further detail on the changes to electronic structure as CH 3 CN is doubly deprotonated (see Table S2  Following the reaction between 1 b and 9 equiv. of CH 3 CN by NMR spectroscopy in C 6 D 6 revealed 94 % conversion to the postulated intermediate 3 b in < 5 min at 25 °C (Scheme 2). 3 b shows a broadened singlet hydride resonance at δ H = À 16.00 ppm, and a singlet 31 P signal at δ P = À 28.8 ppm.This latter resonance is characteristic for Al-μ-H 3 -Fe species derived from proton transfer reactions. [21,22] e CH 2 CN 1 H NMR signal integrating to 2H can be found at δ H = 1.81 ppm which is downfield of the corresponding resonance for CH 3 CN (δ H = 0.58 ppm in C 6 D 6 ). [28]his resonance appears as a doublet with In conclusion, we present the first double deprotonation of CH 3 CN, achieved by reaction with two equivalents of a bimetallic FeÀ Al complex.The products of deprotonation contain a coordinated [CHCN] 2À dianion.DFT calculations support charge localisation on the termini of the [CHCN] 2À dianion and its formulation as a [HC=C=N] 2À cumulene structure bound to the aluminium centres through polarised covalent interactions.Mechanistic studies suggest that the product is formed via a [CH 2 CN] À monoanion intermediate.This species could be spectroscopically characterised in situ.DFT calculations suggest that the unique reactivity can be conceptualised in terms of two successive deprotonation steps, both of which are driven by the high basicity and unusual properties of the FeÀ Al bimetallic complex.Our findings open up the possibility of a wider chemistry of simple [CHCN] 2À dianions including potential applications as chemical building blocks for synthesis.

2 a
is poorly soluble in common laboratory solvents precluding detailed solution-state characterisation.Prepara-tion of analogue addressed both these issues.Hence, reaction of 1 b with CH 3 CN (0.5 equiv.)led to isolation of 2 b, a direct analogue of 2 a bearing 2,6-xylyl groups in place of mesityl on the ligand periphery.2 b could be unambiguously characterised in both solution and solid state.NMR analysis of 2 b revealed a highly shielded resonance of δ H = 0.56 ppm for the CHCN proton. 2 b also featured two chemically inequivalent environments for the Al-μ-H 3 -Fe hydrides, visible through two broadened resonances at δ H = À 15.54 and À 15.87 ppm, each integrating to 3H.Despite 2 b showing good solubility in benzene, neither the 13 C or 14 N NMR resonances of the dianion fragment, nor any couplings in the respective 2D NMR spectra were observed.Variable temperature NMR experiments down to À 50 °C in toluene-[D 8 ] yielded no further information.These observations are similar to those reported for Li 2 -[(Me 3 Si)CCN], for which only the SiMe 3 carbon resonance was reported. [17] 13C isotopic labelling of the CH 3 CN fragment in 2 b-[ 13 C] was achieved through reaction of 1 b with 13 CH 3 CN. 2 b-[ 13 C] was characterised by a broad singlet at δ C = 22.1 ppm (FWHM = 51 Hz) assigned to the CHCN resonance broadened due to the adjacent quadrupolar S = 5/ 2 2 a crystallises in the P1 ̄, and 2 b in the P2 1 /n space group, both with one solvent molecule in the asymmetric unit.Their structures are very similar, the main exception is the difference in the FeÀ AlÀ AlÀ Fe torsion (2 a: 180.00 °; 2 b: 68.57(9) °) which is likely caused by crystal packing effects.Both structures feature short C=C (2 a: 1.34(2) Å; 2 b: 1.300(6) Å; CH 3 CN: 1.436(12) Å) and C=N (2 a: 1.16(2) Å; 2 b: 1.206(6) Å; CH 3 CN: 1.149(12) Å) bond lengths clearly indicating a delocalised cumulene-type structure of the [CHCN] 2À dianion.
CH 3 CN shows discrete WBIs close to those expected for single and triple bonds (CH 3 CN: C�N 2.91, CÀ C 1.08).Inspection of the NPA (Natural Population Analysis) charges shows near even charge localisation on the two terminal atoms of the [CHCN] 2À dianion (2 b: N À 0.93, C À 1.18, CH 3 CN: N À 0.33, C À 0.81).NBO calculations also provide information on the nature of binding of the [CHCN] 2À dianion within 2 b.The AlÀ C and AlÀ N WBIs are both lower than unity (2 b: Al 1 À C 0.44, Al 2 À N 0.35) and there is large positive charge localised on the Al nuclei (2 b: Al 1 1.69, Al 2 1.74).Second Order Perturbation theory reveals a series of donor-acceptor interactions between the termini of the [CHCN] 2À dianion and the two Al centres (see Table

1 .
À 1 ) coordination of CH 3 CN to one molecule of 1 b yields an unstable intermediate INT-1.INT-1 can undergo an intramolecular deprotonation through TS-In this transition state the substrate adopts a bent conformation.TS-1 leads directly to an Al ketene imide complex 3 b containing the [CH 2 CN] À anion.3 b is predicted to be experimentally accessible (see below).The first deprotonation step has a barrier of ΔG � = 14.0 kcal mol À 1

3 bFigure 3 .
Figure 3. Bonding analysis of 2 b. a) NPA charges and b) Wiberg bond indices from the NBO analysis; c) deformation density plots from ETS-NOCV calculations (charge flow from blue to red).

Figure 4 .
Figure 4. a) Calculated free energy profile for the double CÀ H activation of acetonitrile, Gibbs free energies in kcal mol À 1 ; b) computed structure of TS-1; c) computed structure of TS-2.