Single Electron Transfer to Diazomethane–Borane Adducts Prompts C−H Bond Activations

Abstract

While (Ph₂CN₂)B(C₆F₅)₃ is unstable, single electron transfer from Cp*₂Co affords the isolation of stable products [Cp*₂Co][Ph₂CNNHB(C₆F₅)₃] 1 and [Cp*Co(C₅Me₄CH₂B(C₆F₅)₃)] 2. The analogous combination of Ph₂CN₂ and BPh₃ showed no evidence of adduct formation and yet single electron transfer from Cp*₂Cr affords the species [Cp*₂Cr][PhC(C₆H₄)NNBPh₃] 3 and [Cp*₂Cr][Ph₂CNNHBPh₃] 4. Computations showed both reactions proceed via transient radical anions of the diphenyldiazomethane–borane adducts to effect C−H bond activations.

Single electron transfer to diphenyldiazomethane–borane adducts generates transient radical anions that effect C−H bond activations. Such reductions provide a strategy to stabilize weak and even undetectable donor–acceptor adducts.

The activation of small molecules has been a major driver of organometallic chemistry over the last 60 years. Such efforts have spawned great interest and important developments. In recent years, such inquiries have begun to permeate main group chemistry. One avenue of main group chemistry exploited for the activation of small molecules has been frustrated Lewis pair (FLP) chemistry.1 While this initially emerged from the finding of the heterolytic activation of H₂ by combinations of Lewis acids and bases,2 subsequent efforts demonstrated reactivity of FLPs with a wide range of small molecules.3 Noticeably absence from these investigations have been studies involving dinitrogen.

Organometallic chemists have studied metal–N₂ systems since the seminal report of A. D. Allen4 who described the first transition metal–dinitrogen complex. Over the past 50 years numerous advances have emerged from the luminaries of organometallic chemistry including Schrock,5 Cummins,6 Peters,7 Fryzuk,8 Evans,9 Gambarotta,10 Nishibiashi,11 Holland,12 Chatt,13 and Liddle14 among others.15 Avenues to metal‐mediated N₂ chemistry have typically involved stoichiometric reductants.8d More recently, in 2017 the Szymczak16 and Simonneau17 groups demonstrated the utility of a Lewis acidic borane in promoting reactivity of metal‐bound N₂ fragments, effecting protonation, borylation and silylation of N₂ bound between the metal and boron.

Main group interactions with N₂ have drawn much less attention. A number of computational studies have addressed the interactions of N₂ with Lewis acids, while the species (N₂)BF₃ was spectroscopically characterized upon generation by supersonic expansion at 600 torr and 170 K.18 The compound Ph₃PNNPPh₃ 19 although not derived from N₂, was controversially described as N₂ stabilized by two phosphine donors.20 However, in a truly seminal finding, Braunschweig et al.21 described the first metal‐free capture of N₂ using a cAAC‐stabilized borylene (cAAC: cyclic (alkyl)(amino) carbene).

In our own efforts towards main group‐N₂ chemistry, we initiated studies of diazomethanes which liberate N₂. Such systems may provide insight for the design of main group‐N₂ systems.22 In 2012, we reported the insertion of diazomethanes into B−C bonds of electrophilic boranes with the liberation of N₂ (Scheme 1).23 Such insertions were recently exploited in organic synthesis by Melen et al.24 In recent work,25 we showed that the sterically‐encumbered diazomethane, Ph₂CN₂, does not insert but rather forms a highly reactive, yet isolable borane‐adduct, (Ph₂CN₂)B(C₆F₅)₃. Moreover, we also showed weak Lewis acid–base adducts were stabilized by stoichiometric reduction.26 This notion was also exploited by the Erker group in the isolation of Lewis acid stabilized radicals.27 Herein, we probe the impact of reduction on the reactivity of the unstable (Ph₂CN₂)B(C₆F₅)₃, demonstrating that single electron transfer to diazomethane‐borane adducts stabilizes weak B⋅⋅⋅N interactions providing reactive transient radicals which effect C−H bond activation.

Scheme 1.

Interactions of main group systems with N₂‐fragments.

A 1:1 combination of diphenyldiazomethane (Ph₂CN₂) with B(C₆F₅)₃ in chlorobenzene was stirred at −35 °C. Addition of an equal molar amount of Cp*₂Co immediately gave a yellow solution. The crude ¹⁹F NMR spectrum showed two sets of resonances at −134.0, −163.9, and −167.3 ppm and −130.2, −161.9, −162.7 ppm, attributable to inequivalent C₆F₅ rings. The ¹¹B NMR spectrum showed two resonances at −7.6 and −13.0 ppm attributable to two tetra‐coordinated boron species, 1 and 2, respectively. ¹H NMR data showed resonances at 6.61 and 2.43 ppm attributable to NH and CH₂ fragments. Fractional recrystallization permitted formulation of the two products 1 and 2 by X‐ray crystallographic analysis. Compound 1 was found to be the salt [Cp*₂Co][Ph₂CNNHB(C₆F₅)₃] (Figure 1 a). While the cation was unexceptional, the diazoborate anion was derived from the interaction of the hydrazide bound to borane. The B−N(H) bond length in 1 is 1.539(7) Å, while the N−N and N−C bonds lengths is 1.342(5) and 1.303(7) Å, respectively. The B−N−N angle was determined to be 118.2(3)° while the C−N−N angle is 120.9(4)°. The second isolated product was confirmed to be [Cp*Co(C₅Me₄CH₂B(C₆F₅)₃)] 2 (Figure 1 b). In this species, one of the hydrogen atoms in one of the Cp* methyl groups has been replaced by borane, affording the zwitterionic Co^III‐borate 2, with a methylene–boron B−C bond length of 1.66(1) Å.

Figure 1.

POV‐Ray depiction of the anion of a) 1 and b) 2. The cation and hydrogen atoms (except NH) are omitted for clarity. C: black, N: blue, F: pink, B: yellow‐green, Co: green, and H: grey.

Collectively, the identification of 1 and 2 is consistent with two possible reaction mechanisms involving single electron transfer from a Co^II center to either B(C₆F₅)₃ 28 or the diazomethane adduct of the borane, (Ph₂CN₂)B(C₆F₅)₃.25 It is noteworthy that although C−H activation by the radical [B(C₆F₅)₃)]^.−,29 is expected to give the anion [HB(C₆F₅)₃]⁻, independent combination of diazomethane with [HB(C₆F₅)₃]⁻ showed no reaction. This supports the view that compound 1 is formed through hydrogen atom abstraction from Cp*₂Co by the transient diazomethane‐borane adduct radical anion [Ph₂CN₂B(C₆F₅)₃]^.− (Scheme 2), consistent with the overall reaction ratio of diazomethane:Cp*₂Co:B(C₆F₅)₃ of 1:2:2.

Scheme 2.

Reactions of Ph₂CN₂ with B(C₆F₅)₃ and Cp*₂Co, and with BPh₃ and Cp*₂Cr.

Ph₂CN₂ was combined with BPh₃ in chlorobenzene at −35 °C. Monitoring the solution by multinuclear NMR spectroscopy revealed no evidence of adduct formation. This is consistent with the poor Lewis basicity of the diazomethane and the weaker Lewis acidity of the BPh₃ in comparison to B(C₆F₅)₃, in line with the computed free energies (see Supporting Information). Addition of Cp*₂Cr to a mixture of BPh₃ and diazomethane at −35 °C generated an orange solution. The ¹¹B NMR spectrum showed resonances at 23.0 and −3.5 ppm consistent with the formation of two products, 3 and 4 which were isolated by fractional crystallization. An X‐ray diffraction study revealed species 3 to be [Cp*₂Cr][PhC(C₆H₄)NNBPh₃] (Scheme 2, Figure 2 a). While the cation was typical, the anion of 3 was shown to be a borate with a substituent derived from the cyclization of the N₂ fragment onto the ortho position of one of the aryl rings on the diazomethane carbon. The resulting five membered ring which is fused to the aryl ring is 1,3‐disubstituted with and phenyl ring on carbon and BPh₃ bound to nitrogen. The resulting N−B bond is 1.566(6) Å, while the N−N and new N−C bond distances are determined to be 1.312(5) and 1.351(5) Å. The second product 4 was also characterized crystallographically revealing its formulation as [Cp*₂Cr][Ph₂CNNHBPh₃] (Scheme 2, Figure 2 b). The B−N and N−N distances in the anion of 4 were determined to be 1.562(4) Å and 1.322(3) Å, respectively.

Figure 2.

POV‐Ray depiction of the anion of a) 3 and b) 4. The cation and hydrogen atoms (except NH) are omitted for clarity. C: black, N: blue, and B: yellow‐green.

In contrast, the reaction of 9‐diazofluorene ((C₁₂H₈)CN₂) with B(C₆F₅)₃ did not form an adduct but led to loss of N₂ (Scheme 3) and the formation of the carboboration product (C₁₂H₈)C(C₆F₅)(B(C₆F₅)₂) as confirmed spectroscopically and crystallographically (see Supporting Information). This is consistent with observations seen for less sterically encumbered diazomethanes.23, 30 However, monitoring the reaction of (C₁₂H₈)CN₂ with BPh₃ and Cp*₂Cr by ¹¹B NMR spectroscopy revealed the generation of three products as evidenced by the resonances at 26.2, 2.4, and −1.7 ppm. The peaks at −1.7 and 2.4 ppm were unambiguously assigned to [Cp*₂Cr][C₁₂H₈CNNHBPh₃] 5 and [Cp*Cr(C₅Me₄CH₂BPh₃)] 7 by NMR and crystallographic methods (Figure 3, see Supporting Information). The remaining resonance at 26.2 ppm, was attributed to the species [Cp*₂Cr][C₁₃H₇N₂BPh₃] 6 by analogy to 3. These results suggest that the electron transfer to the weak adducts gives a transient radical [(C₁₂H₈)CN₂BPh₃]^.−, which reacts further through competitive pathways involving either intramolecular cyclization or intermolecular H‐atom abstraction.

Scheme 3.

Reactions of (C₁₂H₈)CN₂ with BPh₃ and Cp*₂Cr.

Figure 3.

POV‐Ray depiction of the anion of 5. The cation and hydrogen atoms (except NH) are omitted for clarity. C: black, N: blue, and B: yellow‐green.

The mechanism of these reactions were probed by density functional theory (DFT) calculations at the PW6B95‐D3/def2‐QZVP + COSMO‐RS// TPSS‐D3/def2‐TZVP + COSMO level of theory in chlorobenzene solution.31 The reaction of Ph₂CN₂ with BPh₃ and Cp*₂Cr is initiated by single electron transfer from Cp*₂Cr to the unstable Ph₂CN₂⋅BPh₃ adduct (Figure 4) affording the radical anion [Ph₂CNNHBPh₃]^.− INT1 (spin on N next to B: 0.53e) is 4.3 kcal mol⁻¹ endergonic. Alternative pathways involving electron transfers to separated BPh₃ and Ph₂CN₂ species (14.8 and 8.9 kcal mol⁻¹) are significantly less favorable. Similarly, further electron transfer to INT1 is unlikely (12.2 kcal mol⁻¹ endergonic). The N‐centered radical INT1 may then add intramolecularly to the ortho position of a phenyl ring (via transition structure TS1) to give INT2 affording delocalization of the spin onto the ring. From here, a highly exergonic H‐transfer to another Ph₂CN₂ molecule (via TS2) gives the anion of 3 and the neutral N‐radical (Ph₂CNNH)^. (spin on N next to H: 0.54e) with a moderate overall barrier of 23.5 kcal mol⁻¹. The latter radical is readily reduced by Cp*₂Cr through electron transfer and trapped by BPh₃ giving the anion of 4 (−66.4 kcal mol⁻¹).

Figure 4.

DFT‐computed free energy profile (kcal mol⁻¹, at 298 K and 1 mol L⁻¹ reference concentration) for the formation of anion of 3 and 4. Selected bond lengths are given in angstroms while selected C: grey, H: white, N: blue, and B: pink.

For the reaction of Ph₂CN₂ with the stronger Lewis acid B(C₆F₅)₃ and the more reductive Cp*₂Co,32 electron transfer from Cp*₂Co to the reversible adduct (Ph₂CNN)B(C₆F₅)₃ is −15.3 kcal mol⁻¹ exergonic affording the radical anion [Ph₂CN₂B(C₆F₅)₃]^⋅−. This intermediate is computed to effect H‐atom from one methyl group of Cp*₂Co over a barrier of 16.6 kcal mol⁻¹ to form the stable anion of 1 (see Supporting Information). The alternative intramolecular pathway, analogous to that above encounters a higher overall barrier of 23.3 kcal mol⁻¹ (see Supporting Information).

To garner further support for the computed mechanism, efforts to observe the transient radical adducts in the reactions were undertaken, but were unsuccessful. However, monitoring the reaction of (C₁₂H₈)CN₂, Cp*₂Fe and Al(C₆F₅)₃ in C₆H₅Cl at room temperature by EPR spectroscopy revealed a pentet resonance at g(iso)=2.0039, with (¹⁴N) hyperfine couplings of 3.70 G and 3.58 G. This signal was similar to the related N‐based radicals33 and was attributed to the radical species [Ph₂CN₂Al(C₆F₅)₃]^.−. This signal slowly degrades at room temperature over 5 h, leaving a broad resonance attributed to an organic radical (see Supporting Information). Subsequent addition of Ph₃SnH generated a mixture of products, from which [Cp*₂Fe][(C₁₂H₈)CNNHAl(C₆F₅)₃] 8 was identified by NMR spectroscopy while single crystals of [Cp*₂Fe][(C₁₂H₈)CHAl(C₆F₅)₃] 9 were obtained from the reaction mixture (Figure 5). Compound 8 was independently prepared and crystallographically characterized from the reaction of (C₁₂H₈)CNNH₂, Cp*₂Fe, and Al(C₆F₅)₃. The formulations of 8 and 9 are consistent with the generation of the spectroscopically observed radical anions [(C₁₂H₈)CN₂Al(C₆F₅)₃]^.− and [(C₁₂H₈)CAl(C₆F₅)₃]^.− (Scheme 4).

Figure 5.

POV‐Ray depiction of the anion of a) 8 and b) 9. The cation and hydrogen atoms (except the NH and the Al‐bound CH) are omitted for clarity. C: black, N: blue, Al: Cyan, and F: pink.

Scheme 4.

Reactions of (C₁₂H₈)CN₂, Al(C₆F₅)₃ and Cp*₂Fe with Ph₃SnH.

In conclusion, we have demonstrated that single electron transfer to unstable diazomethane‐borane adducts, accesses reactive radical anions that effect H‐atom abstraction from C−H bonds. The resulting anionic species are significantly more stable than the corresponding neutral adducts, suggesting that in situ reductions may be a useful strategy to infer the presence of weakly bound adducts. We suggest that this strategy could be exploited in developing main group‐N₂ chemistry. At the same time, the potential utility of main group radical anions in C−H bond homolysis offers an interesting prospect for C−H functionalization.

Conflict of interest

The authors declare no conflict of interest.

Supporting information

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Supplementary

L. L. Cao, J. Zhou, Z.-W. Qu, D. W. Stephan, Angew. Chem. Int. Ed. 2019, 58, 18487.