Ruthenium(V) terminal arylimido corroles: isolation, spectroscopic characterization and reactivity

Abstract

Terminal Ru(V)–imido species are thought to be as reactive to group transfer reactions as their Ru(V)–oxo homologues, but are less studied. With the electron-rich corrole ligand, relatively stable and isolable Ru(V)–arylimido complexes [Ru(^tBu–Cor)(NAr)] (H₃(^tBu–Cor) = 5,15-diphenyl-10-(p-tert-butylphenyl)corrole, Ar = 2,4,6-Me₃C₆H₂ (Mes), 2,6-(ⁱPr)₂C₆H₃ (Dipp), 2,4,6-(ⁱPr)₃C₆H₂ (Tipp), and 3,5-(CF₃)₂C₆H₃ (BTF)) can be prepared from [Ru(^tBu–Cor)]₂ under strongly reducing conditions. This type of Ru(V)–monoarylimido corrole complex with S = ½ was characterized by high-resolution ESI mass spectrometry, X-band EPR, resonance Raman spectroscopy, magnetic susceptibility, and elemental analysis, together with computational studies. Under heating/light irradiation (xenon lamp) conditions, the complexes [Ru(^tBu–Cor)(NAr)] (Ar = Mes, BTF) could undergo aziridination of styrenes and amination of benzylic C(sp³)–H bonds with up to 90% product yields.

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Introduction

Fe(V)- and Ru(V)–imido species are widely proposed but underexplored reactive intermediates in many Fe(III) and Ru(III)/Ru(IV)-catalysed C(sp³)–H amination reactions.¹ Such reactive Fe(V)/Ru(V)–imido species can undergo atom-efficient functionalization of C–H bonds as their oxo-congeners (Fe(V)– and Ru(V)–oxo species),² but are unstable and short-lived in solution.^1b,3 Spectroscopic characterization and isolation of the Fe(V)/Ru(V)–imido complexes reactive towards hydrocarbons are essential for understanding their bonding, electronic structure, and reactivity. In the literature, although there are ample examples of well-characterized Ru(V)–oxo species,^2a,4 terminal Ru(V)–imido species are sparse.⁵ Wilkinson and co-workers reported [ⁿPr₄N][Ru(NCy){OCEt₂C(O)O}₂],^5d but the reactivity of this isolable Ru–imido complex towards hydrocarbons has not been documented. Literature examples of spectroscopically characterized M(V)–imido complexes that can aziridinate alkenes and/or aminate C(sp³)–H bonds, including Fe(V)–tosylimido species with the tetraamide ligand^1b and Ru(V)–acylimido porphyrin^1e complexes, are generated in solutions without isolation (Fig. 1a).

Fig. 1 Examples of (a) in the literature and (b) in this work for spectroscopically characterized M(V)–imido species that can undergo alkene aziridination and/or C–H amination reactions.

In order to obtain novel spectroscopically characterized M(V)–imido species reactive with hydrocarbons, and to improve the stability of such species, we attempted to focus on Ru(V) (considering the high reactivity of Ru(V)–acylimido porphyrin species^1e) and the use of the trianionic corrole (Cor) ligand; the latter was previously shown to stabilize the [M^V(Cor)(NAr)] (M = Cr, Mn, Re) species.⁶ One hurdle to overcome is that most of the reported Ru corroles are dimers with strong Ru≡Ru triple bonds,⁷ in contrast to the case of Ru porphyrins, where Ru₂(II,II) porphyrin dimers are highly sensitive to oxygen and are useful precursors for Ru–carbene, –oxo or –imido species.^8a,f Here we report reductive activation of Ru₂(III,III) corrole dimers followed by reaction with aryl azides to give Ru(V)–arylimido (Ru^V(NAr)) complexes; the latter have been isolated and characterized by X-band EPR, resonance Raman (rR), HR-ESI-MS, UV-vis spectroscopy, elemental analysis, and/or DFT calculations. This type of Ru^V(NAr) complex can undergo alkene aziridination and C(sp³)–H amination reactions, both of which have not been reported for [M^V(Cor)(NAr)] (M = Cr, Mn, Re).⁶

Results and discussion

Synthesis of Ru(V)–arylimido complexes

The Ru₂(III,III) corrole dimer [Ru(^tBu–Cor)]₂ (1, Scheme 1) was prepared in 65% yield and characterized by NMR, UV-vis, HR-ESI-MS, cyclic voltammetry, elemental analysis and X-ray crystallography (Fig. S1 in the ESI†). We attempted to generate Ru(V)–imido species by reaction of 1 with aryl azide, but the dimer was quantitatively recovered. We envisage weakening the Ru≡Ru bond of 1 by reducing this complex. Reduction of 1 with excess potassium graphite (KC₈) in rigorously deoxygenated and dried THF, followed by overnight stirring at RT (Pyrex® Spinbar), afforded a species 2, tentatively the reduced dimer {[Ru(^tBu–Cor)]₂}²⁻. The effective magnetic moment of 2 (in situ generated) was determined to be 2.81μ_B using the Evans method, which indicates that 2 is paramagnetic with two unpaired electrons (S = 1). This is in contrast to the dimer 1 (S = 0) and mononuclear Ru(II) corrole species, which are expected to be diamagnetic, similar to the Ru(II) porphyrin complexes [Ru(Por)(L)₂] (L = vacant or L = N₂, CH₃CN, THF, Et₂O).^8g Based on its magnetic moment (2.81μ_B), complex 2 is likely to be a Ru₂(II,II) corrole dimer, whose electronic configuration (S = 1) is similar to that (σ²π⁴(δ_nb)⁴(π*)²) of the Ru₂(II,II) porphyrin dimers reported by Collman and co-workers.^8d Redox titration of 2 with [Cp₂Fe]PF₆ showed that ca. 2 equiv. of Cp₂Fe and 1 equiv. of 1 were generated. In a control experiment, 1 was not able to reduce [Cp₂Fe]PF₆ under the same reaction conditions. The UV-vis spectrum of 2 in THF shows pairwise Soret bands at 411 and 445 nm, which are red-shifted from that of 1 at 329 and 399 nm (Fig. S4 in the ESI†). A Ru₂(II,II) species, {[Ru(Cor)]₂}²⁻, was previously proposed as the two-electron reduced product of [Ru(Cor)]₂ by Guilard and co-workers.^7b

Scheme 1 Synthesis of Ru(V)–arylimido corrole complexes 3a–3d.

As shown in Scheme 1, treatment of 2 with aryl azides (ArN₃, Ar = Mes, Dipp, Tipp, BTF) afforded a set of Ru^V(NAr) complexes [Ru(^tBu–Cor)(NAr)] (3); [Ru(^tBu–Cor)(NBTF)] (3d) bearing the CF₃-substituted arylimido ligand is one of the rare examples of terminal imido complexes of late transition metals featuring strongly electron-withdrawing CF₃ substituents on their imido ligands.⁹ The reactions using MesN₃ (for 3a) and BTFN₃ (for 3d) took only a few minutes, whereas the reactions with DippN₃ (for 3b) and TippN₃ (for 3c) bearing sterically more hindered aryl groups required stirring for 0.5 h at RT. Complexes 3 are likely formed via initial generation of [Ru^IV(^tBu–Cor)(NAr)]⁻ followed by disproportionation or one-electron oxidation. In the literature, Ru(IV)–oxo porphyrin was reported to undergo disproportionation;^8f,10 Fe(IV)–oxo porphyrins and Mn(IV)–oxo corroles have been reported to convert into their one-electron oxidized counterparts in solution.¹¹

MALDI-TOF MS was used to monitor the reaction. In initial trials using “1 + MesN₃” in refluxing benzene solution, a cluster peak attributable to 3a was not observed. By using “2 + MesN₃” (Fig. S6 in the ESI†), the ESI-MS spectrum of the reaction mixture showed a strong cluster peak at m/z 814.2479, which was attributed to the parent ion of [Ru(^tBu–Cor)(NMes)] (3a, calcd for C₅₀H₄₂N₅Ru: 814.2492; Fig. S12 in the ESI†), giving the simulated isotope pattern that matches that observed experimentally. As expected, the peak is isotopically sensitive, with the isotope pattern changing upon using a 50% ¹⁵N-enriched sample (^14,15N–3a, prepared from ¹⁵N–MesN₃; Fig. 2). Other Ru–arylimido complexes (3b–3d) have also been isolated and characterized by ESI-MS/MALDI-TOF MS. Complexes 3a–3d were found to gradually decompose within 24 h even under an inert atmosphere and at low temperature (argon filled glove box, refrigerator pre-equilibrated at −35 °C), as evidenced by UV-vis spectroscopy and ESI-MS analysis. Attempts to obtain diffraction-quality crystals of these complexes for X-ray crystal structure determination were not successful.

Fig. 2 (a) HR-ESI-MS (positive-ion mode) spectrum of [Ru(^tBu–Cor)(NMes)] (3a). (b) Experimental isotope pattern of “[Ru(^tBu–Cor)(¹⁴NMes)]⁺ + [Ru(^tBu–Cor)(¹⁵NMes)]⁺” in a 1 : 1 ratio (50% ¹⁵N enrichment). (c) Simulated pattern of (b).

EPR and electronic spectroscopy

After measuring their magnetic susceptibility (ca. 1.73μ_B using the Evans method), X-band EPR experiments were performed to elucidate the electronic structure of these Ru–arylimido corrole complexes 3. An anisotropic EPR signal for 3a (Fig. S16 in the ESI†) was found at frozen glass temperature (100 K, benzene). Upon treatment with 5 equiv. of PPh₃, this EPR signal diminished, and [Ru(^tBu–Cor)(PPh₃)]⁺ (Fig. S8 in the ESI,†m/z 943.4) and MesN=PPh₃ were found (Fig. S9 in the ESI;†m/z calcd: 396.1858, found: 396.1881; ³¹P NMR in CDCl₃: −13.85 ppm).

The EPR spectrum of 3a was simulated using the parameters of g_x = 2.010, g_y = 2.005, g_z = 1.880; A_x, A_y, A_z = 53, 14, 14 MHz (¹⁴N). Simulation for the EPR spectrum of 3b (Fig. S17 in the ESI†) was also carried out, by employing the g and A values of g_x = 2.000, g_y = 1.980, g_z = 1.870; A_x, A_y, A_z = 50, 14, 14 MHz (¹⁴N). The EPR spectrum of 3c (Fig. 3a) is similar to that of its structurally similar complex 3b (g and A values for the simulated spectrum: same as those of 3b), and the signal pattern is assigned to the superhyperfine coupling to the ¹⁴N nucleus of the imido ligand, similar to that of Fe(V)–imido species.¹² For the 50% ¹⁵N-enriched sample, ^14,15N–3c (prepared from ¹⁵N–TippN₃), the corresponding EPR spectrum (Fig. 3b) shows a signal which can be reasonably fitted by summing the simulated spectra of 3c and ¹⁵N–3c, with a 1 : 1 ratio (Fig. 3b; ¹⁴N: I = 1, ¹⁵N: I = ½). The EPR signals of 3a–3c are characterized by g_iso (g_x + g_y + g_z)/3) values of 1.950–1.965, which can be assigned to Ru(V) complexes with S = ½.^1e,2a These g_iso values are similar to those of Ru(V)–acylimido and Ru(V)–oxo porphyrin complexes (g_iso = 1.94,^1e 1.970 (ref. 2a)), but smaller than those commonly observed for Ru(III) complexes (g_iso = 2.1–2.3 (ref. 2c and 13)).

Fig. 3 X-band EPR spectra of isolated samples of (a) 3c and (b) ^14,15N–3c recorded at 100 K.

Complexes 3a–3d each exhibit one Soret band (at 406–411 nm) in the UV-vis spectrum; like mononuclear [Ru^VI(Cor)(N)] complexes each complex also has one Soret band (at 417–419 nm),^7d but unlike the Ru₂(III,III) dimer 1, which exhibits two Soret bands (at 329 and 399 nm). In accord with the [Ru^V(Cor)(NAr)] formulation, the UV-vis spectra of 3a–3d (Fig. S5 in the ESI†) do not show typical absorption bands of corrole π–cation radical (Cor⁺˙) complexes (above 650 nm¹⁴). For example, a band at 676 nm was observed for [Mn^V(Cor⁺˙)(N)],¹⁵ the EPR spectrum of which (at 4 K) showed an isotropic radical signal, in contrast to the anisotropic EPR signals of 3a–3c.

Vibrational spectroscopy (IR and resonance Raman)

The vibrational spectra of 3a and its 50% ¹⁵N-enriched form ^14,15N–3a were recorded. No bands over 3200 cm⁻¹ were found in the FTIR spectra (isolated solid samples, KBr pellets), in agreement with the absence of the amino/amido group in the complex. The band could possibly be obscured by an intense band of the coordinated corrole ligand at 1016 cm⁻¹; based on the FTIR spectroscopy, we tentatively assign the band of ^14,15N–3a at 972 cm⁻¹. To observe the and bands, we measured the resonance Raman (rR) spectra of 3a and ^14,15N–3a (isolated samples dissolved in CH₂Cl₂). The rR spectrum of ^14,15N–3a (Fig. 4b, λ_excitation: 416 nm) shows pairwise bands (ca. 1 : 1 ratio due to 50% ¹⁵N-enrichment) at ∼949 and 921 cm⁻¹ with Δν of ∼28 cm⁻¹ agreeing with the value of 28 cm⁻¹ estimated using Hooke's law. These two bands are assigned to and respectively, and the band at 921 cm⁻¹ is absent in the rR spectrum of 3a (Fig. 4a). By considering literature examples of rR studies on metal–arylimido (M(NAr), e.g., Fe(NPh)¹⁶) complexes, other ¹⁵N-isotopically sensitive bands within 820–1000 cm⁻¹ may be associated with the coordinated arylimido ligand, where ¹⁵N-labelling resulted in a downshift of the bands involving M–N_imido and aryl groups.

Fig. 4 rR spectra of the isolated samples (dissolved in dry CH₂Cl₂) of (a) 3a and (b) ^14,15N–3a. (c) DFT-calculated Raman spectra of 3a, ¹⁵N–3a and ^14,15N–3a.

N-group transfer reactivities

Complex 3a, which has a moderately hindered mesitylimido ligand with electron-donating Me groups, was reacted with styrene at 85 °C for 24 h (entry 1 in Table 1) to give 1-mesityl-2-phenylaziridine in 69% yield. Under the same conditions, the reaction of 3d bearing the 3,5-bis(trifluoromethyl)phenylimido ligand with styrene gave the aziridine product in much higher yield (90%; entry 2 in Table 1). This is attributed to the lower steric hindrance and higher electrophilicity of the 3,5-bis(trifluoromethyl)phenylimido ligand compared with the mesitylimido ligand. Reactive M(NAr) complexes with Ar = 3,5-bis(trifluoromethyl)phenyl (BTF) or other groups (e.g. C₆F₅, CF₃–C₆F₄ and NO₂–C₆H₄) with electron-withdrawing substituents are reactive intermediates involved in metal-catalysed N-group transfer reactions using the corresponding aryl azides.¹⁷ Reaction of 3d with cis-stilbene at 85 °C for 24 h gave trans-aziridine in 62% yield; cis-aziridine was not detected (entry 3 in Table 1). The reaction of 1,1-diphenylethylene with 3d gave diphenylaziridine in lower yield (49%, entry 4 in Table 1). This is attributed to the steric effect, as α-methylstyrene (which is sterically less hindered than 1,1-diphenylethylene) was found to react with 3d to give methylphenylaziridine in higher yield (76%, entry 5 in Table 1). 3d was reacted with excess cyclohexene at 85 °C for 24 h, and neither aziridination of the C=C bond nor amination of the allylic C(sp³)–H bond was observed.

Table 1 Stoichiometric reaction of Ru(V)–arylimido corrole complexes with alkenes/C–H bonds^a

a Ru(V)–arylimido complex (5.50 μmol) and the substrate (1.65 mmol) in benzene (1 mL); the product yields under UV-vis irradiation are presented in pink colour whereas the black ones refer to the yields obtained at 85 °C.

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We then examined the amination of benzylic C(sp³)–H bonds with 3d. Treatment of 3d with excess isochroman gave the amination product in <1% yield, whereas the reaction of indane and phthalane with 3d gave the amination product in 12% and 18% yields, respectively (entries 7–9 in Table 1; 85 °C for 24 h). The reaction of 3d with tetralin gave the amination product in 31% yield (entry 10 in Table 1; 29% yield for a 1 h reaction). Complex 3d exhibited a relatively high reactivity towards indene (61% yield of the amine product; entry 11 in Table 1). We found that light irradiation at RT (xenon lamp) promoted the 3d-mediated amination of indane, phthalane and tetralin as revealed by the higher product yields obtained (50–66% yield; entries 8–10 in Table 1). Indane and phthalane gave higher yield of the amination product than isochroman, which may be related to the statistical factor because isochroman has a smaller number of equivalent C–H bonds for the amination. In the amination of tetralin and indane by 3d, the product yield was found to decrease with increasing dissociation energy of the reactive C–H bond (cf. 82.9 and 85.3 kcal mol⁻¹ for tetralin and indane, respectively¹⁸).

Catalytic N-group transfer reactions were examined using Ru corrole complexes 1 and 2 as catalysts. For the 1-catalysed reaction of styrene with aryl azide BTFN₃ in benzene at 85 °C, the aziridination product was obtained in 5% yield. Upon changing the catalyst to 2, a much higher product yield of 46% was achieved (entries 1 and 2 in Table 2). Complex 2 also catalysed the aziridination of para-substituted styrenes to aziridines in 38–54% yields (entries 3–6 in Table 2). Using cis-stilbene as the substrate, the reaction afforded trans-aziridine as the major product (trans/cis ratio: 2.9 : 1.0, 48% yield, entry 8 in Table 2).

Table 2 Catalytic aziridination of alkenes using a Ru corrole complex as the catalyst^a

a Reaction conditions: 2/ArN₃/styrene = 1 : 50 : 250; yields were determined by ¹H NMR analysis. b Using 1 as the catalyst. c No metal catalyst was used.

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Regarding the mechanism of the 3d-mediated aziridination of alkenes, we conceive that the aziridination proceeds in a stepwise manner due to the formation of trans-aziridine from cis-stilbene (entry 3 in Table 1). For the aziridination of para-substituted styrenes by 3d, the plot of log(k_rel) against σ_mb and using a dual parameter equation developed by Jiang and co-workers,¹⁹ shows a good linearity (R² = 0.9979) featuring ρ_mb = −0.54 and (Fig. 5), with the linearity being better than that for the plot of log(k_rel) against σ⁺ (Fig. S15 in the ESI;†R² = 0.9356). This is consistent with a stepwise mechanism in which a carboradical is involved. A positive value of indicates the spin density of the radical centre delocalized by the para-substituent, while a negative value of ρ_mb supports electrophilic attack of styrene by 3d.

Fig. 5 Hammett correlation for the aziridination of para-substituted styrenes mediated by 3d using a dual-parameter method (σ_mb and ).

Furthermore, we examined the deuterium kinetic isotope effect (KIE) for 3d-mediated C(sp³)–H amination by using equimolar and excess of tetralin and d₄-tetralin. The k_H/k_D value determined for such amination by 3d at 383 K is 4.0(2), comparable to that reported for the C(sp³)–H amination of ethylbenzene by [Ru^VI(TMP)(NNs)₂] (k_H/k_D = 4.8) at 298 K.²⁰ We propose that the C–H amination mediated by 3d involves a stepwise mechanism similar to that for the [Ru^VI(TMP)(NNs)₂]-mediated amination,²⁰ characterized by H atom abstraction followed by radical rebound.

Computational studies

We first calculated the Raman spectrum of 3a and ^14,15N–3a to check the accuracy and reliability of the computational studies. The calculated energy bands of and are located at 944 and 923 cm⁻¹, respectively (Fig. 4c), which are consistent with the respective experimental values (∼949 and 921 cm⁻¹). By converting the Raman shifts to bond distances, 3a has a calculated Ru–N_imido distance of 1.756 Å. The calculated Ru–N_imido distances for 3b and 3d are 1.779 and 1.763 Å, respectively. In terms of the Wiberg bond index (WBI), 3a, 3b and 3d have predicted Ru–N_imido bond orders of 2.05, 1.96 and 2.04, respectively, indicating a multiple bond character. This is in contrast to the cobalt complex with the imidyl ligand NDipp⁺˙, which is characterized by a calculated single Co–N_imidyl bond character (WBI ≈ 1.0).²¹

Similar to the previously reported Ru(V)–oxo porphyrin,^2a both 3a and 3b adopt an S = 1/2 ground state with (d_xy)²(d_xz,d_yz)¹(d_z²)⁰(d_x²−y²)⁰ configuration. In both species, the singly occupied molecular orbital (SOMO) corresponds to d_π, while the π orbitals of the corrole ligand are ca. 0.4–0.5 eV lower than the SOMO, indicating that the corrole ligand is not redox-active (Fig. S18 in the ESI†). We calculated g values for 3a by DFT calculations; the calculated results (g = 2.010, 2.000, 1.890) are in good agreement with the values based on the experimental EPR spectrum (g = 2.010, 2.005, 1.880) (Table S3 in the ESI†). By DFT calculations, the % spin density on Ru/N_imido atom (% SD_Ru/N) of 3a, 3b, and 3d is 44/27, 35/36, and 53/19, respectively, and that on the coordinated arylimido ligand (% SD_NAr) of 3a/3b/3d is 40/54/26 (Fig. 6).

Fig. 6 DFT-optimized spin density distributions (contour value: 0.005), bond angles, bond distances and out-of-plane deviations.

The decrease of spin density on the arylimido ligand (% SD_NAr) along the series 3b > 3a > 3d is in line with the decrease of the Ru–N_imido–C_Ar angle along the same series (160°, 146° and 138° for the DFT-optimized structures of 3b, 3a, and 3d, respectively; Fig. 6). The arylimido ligand of 3b bears bulky 2,6-substituents (ⁱPr) and adopts a basically linear coordination mode, which minimizes steric conflicts between these substituents and the corrole ligand. Complex 3a has less bulky 2,6-substituents (Me); its arylimido ligand can accommodate a relatively bent coordination mode.²² A more bent coordination mode of the arylimido ligand corresponds to 3d which possesses no 2,6-substituents and bears electron-withdrawing 3,5-CF₃ groups, like the case of [Ru^VI(TPP)(NBTF)₂] (Ru–N_imido–C_Ar angles: 139.8(3)° and 143.7(4)°).²³ The bending of the Ru–N_imido–C_Ar moiety would reduce spin delocalization along the z-axis (and into the aryl ring), thus decreasing the spin density on the arylimido group.

As the spin density on N_imido is the highest in 3b among 3a, 3b, and 3d, we performed multi-reference calculations for 3b using the complete active space self-consistent field (CASSCF) method to check if Ru(IV)-imidyl character is involved.^17a,24 The calculation results revealed that 3b can be described as “85.9% of [Ru^V(^tBu–Cor)(NDipp)] + 3.6% of [Ru^IV(^tBu–Cor)(NDipp⁺˙)]” (Fig. 7). For the Ru–N_imido bond of 3b, a calculated bond order of 2.04 was obtained by considering the matrix of atomic orbital coefficients for all relevant molecular orbitals. Based on the occupation numbers of orbitals within the active space, the calculated Ru–N_imido bond order is 2.27, close to the formal bond order of 2.5 with σ²π⁴(π*)¹ configuration. The spin population obtained from the CASSCF calculation is 0.55 on Ru and 0.35 on N_imido (Fig. S19 in the ESI†), indicating that 3b has a doublet ground state with the unpaired electron mainly residing on the Ru centre. This provides support for the formulation of 3b, and also 3a and 3d, as a formal Ru(V)–arylimido corrole species bearing considerable spin density on N_imido.

Fig. 7 Multi-configurational calculations (CASSCF method) using an active space of 11-electron, 9-orbital, i.e., CASSCF(11,9) to depict leading configurations of the doublet ground state of 3b. The remaining configurations each occupy <1% of the ground state. The natural orbitals associated with the assignment, occupation numbers (in parentheses) and atomic orbital contributions are shown in the upper panel (not according to relative energies).

We compared the DFT calculation results for the Ru(NAr) complexes having an S = 1/2 ground state, including % spin density on Ru/NAr (% SD_Ru/NAr) and the Ru–N_NAr distance (Table S4 in the ESI†). The % SD_Ru/NAr of [(SiP^iPr₃)Ru(N–p-CF₃C₆H₄)]^25a and [Ru(PNP)(NPh)] (PNP = [(^tBu₂PCH₂SiMe₂)₂N]⁻)^25b with different types of monoanionic supporting ligands is 40/54 and 30/68, respectively. In the case of complexes 3 with the same trianionic ^tBu–Cor supporting ligand, a direct comparison of % SD_Ru/NAr among various Ar groups can be made (3a: 44/40, 3b: 35/54, and 3d: 53/26); the % SD_NAr decreases along the 3b > 3a > 3d series as discussed above. The Ru–N_NAr distances in [(SiP^iPr₃)Ru(N–p-CF₃C₆H₄)] and [Ru(PNP)(NPh)] (ca. 1.81–1.87 Å) are longer than in complexes 3 (ca. 1.76–1.78 Å).

The electronic structure of the Ru(V)–arylimido complex 3b calculated by CASSCF is different from that of the Fe(V)–tosylimido species [Fe^V(TAML)(NTs)]⁻ calculated by DFT;^1b,26 the properties of the M–N_imido π- and π*-bonding of these two complexes are different. For 3b with an essentially linear M–N_imido–C_Ar moiety (bond angle: 160°), two sets of π and π* orbitals between M-d_xz/yz and N-p_x/y are involved, reminiscent of the corresponding CASSCF calculation results for other M(NAr) complexes with linear M–N_imido–C_Ar moieties.²⁷ However, for [Fe^V(TAML)(NTs)]⁻ having a bent Fe–N_imido–S_Ts moiety (bond angle: 126° (ref. 1b)), only the π and π* orbitals between M-d_yz and N-p_y are involved, and the M-d_xz is non-bonding.²⁶

DFT calculations were also carried out for the aziridination of cis-stilbene by 3d, which was experimentally found to favour the formation of the trans-aziridine product. The calculated reaction profile (Fig. S20 in the ESI†) involving the formation of a carboradical intermediate (Int-trans and Int-cis for trans- and cis-configuration, respectively) is consistent with the proposed stepwise mechanism. The ring-closure barrier for Int-trans and Int-cis is 14.1 and 13.3 kcal mol⁻¹, respectively, both of which are larger than the barrier of 10.7 kcal mol⁻¹ required for isomerization of Int-cis to the thermodynamically more favourable Int-trans by rotation about the C_Ph–C_Ph bond, allowing such rotation to take place prior to the ring closure, giving trans-aziridine as the major product.²⁸

Conclusions

The electron-rich trianionic corrole ligand ^tBu–Cor is capable of stabilizing reactive M(V)–imido species, enabling the isolation and spectroscopic characterization of reactive Ru(V)–arylimido complexes capable of undergoing N-group transfer with hydrocarbons. The relatively stable Ru(V)–arylimido corroles [Ru(^tBu–Cor)(NAr)] (3) isolated in this work have been characterized by X-band EPR, rR, ESI or MALDI-TOF MS, and elemental analysis. Their electronic structures were examined by single- (DFT) and multi-reference (CASSCF) computational studies. Complexes 3 could aziridinate alkenes and aminate benzylic C(sp³)–H bonds via a stepwise mechanism under heating/irradiation conditions, with product yields of up to 90% and 66%, respectively. The results of this work provide useful insights into the development of alkene and C(sp³)–H bond functionalization via reactive Ru(V)–arylimido intermediates.

Author contributions

CMC conceived the idea and directed the project. KPS and LQ performed the experiments. LLW carried out the computational studies. KPS, LQ, JSH and CMC analysed the data. KPS, JSH and CMC wrote the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Hong Kong Research Grants Council (HKU 17304019) and Basic Research Program-Shenzhen Fund (JCYJ20180508162429786). We thank Dr Xiao-Yong Chang for assistance in X-ray crystal structure determination of complex 1.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Abbreviations, experimental procedures, characterization of compounds, computational details, Tables S1–S4, Fig. S1–S21, NMR spectra of compounds, and Cartesian coordinates from DFT calculations. CCDC 2214628. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3sc02266h

‡ These authors contributed equally.

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