Investigating the formation of metal nitride complexes employing a tetradentate bis-carbene bis-phenolate ligand†

Romain Kunert ^ab, Diego Martelino ^a, Samyadeb Mahato ^a, Nicholas M. Hein ^a, Jason Pulfer ^a, Christian Philouze ^b, Olivier Jarjayes *^b, Fabrice Thomas *^b and Tim Storr *^a
aDepartment of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada. E-mail: tim_storr@sfu.ca
^bUniv. Grenoble Alpes, CNRS, DCM, F-38000, Grenoble, France. E-mail: Olivier.Jarjayes@univ-grenoble-alpes.fr; Fabrice.Thomas@univ-grenoble-alpes.fr

First published on 8th November 2024

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1. Introduction

High valent metal nitrido (N³⁻) species are of significant interest as proposed intermediates in biological and industrial processes,^1–3 as well as N-atom transfer chemistry.^4–13 A number of bioinspired Fe nitride complexes have been studied in the context of nitrogen fixation,^14–20 and recent advances understanding synthetic FeS clusters have provided important information on the identity of activated Fe–N species.^21–25 The stability and reactivity of discrete transition metal nitrides depends on factors such as metal identity and oxidation state, ligand identity, and overall geometry.^26–28 Early transition metal nitrides are generally stabilized,^29,30 and exhibit nucleophilic reactivity from either filled MN π orbitals or the nitride lone pair.^27,31–33 In contrast, late transition metal nitrides can be either isolated or described as transient intermediates, exhibiting electrophilic reactivity via MN π* orbitals.^28,34–42 Interestingly, a number of metal nitride complexes display ambiphilic reactivity,^43–45 and subtle changes to the coordination environment can result in tunable reactivity at the nitride.^{33,35,46–49}

We have reported that oxidation of nitrido metal salen complexes (see Scheme 1 for example salen H₂Sal^R) results in an electronic structure that can be modulated via alteration of the electron-donating ability of para-R phenolate substituents, without changing the geometry at the metal center.^33,50–52 Salen ligands are redox-active and have been documented to undergo oxidation or reduction at the ligand in place of the metal.^53–56 In certain exceptional cases both metal and ligand oxidation can occur.^57–59 We, and others, have also reported that the formation of a Mn^VI salen nitride results in rapid homocoupling of the nitride to form N₂,^52,60,61 however formation of a ligand radical (employing strongly electron-donating para-R substituents such as R = NMe₂) precludes this homocoupling.⁵² Recent work with Ru⁶² and Mo^63,64 nitrides has explored the homocoupling pathway and documented the formation of μ-nitrido products. In addition to homocoupling, Mn salen nitrides have been investigated in the context of the hydrogen atom bond dissociation free energy of the associated imido complexes,⁶⁵ generation of ammonia by proton-coupled electron transfer,^66–68 and as a catalyst for ammonia oxidation.⁶⁹

In the current work we investigate the formation and stability of Mn and Cr nitride complexes using a tetradentate bis-phenol bis-N-heterocyclic carbene ligand H₂L^C2O2,⁷⁰ and compare to our recently published work using salen ligands (Scheme 1).^33,50–52 The strong σ-donating capability (and potential for π-backbonding) of NHC ligands results in facile binding to electron-rich metals for catalytic applications.^71,72 A number of high oxidation state metal complexes employing NHC ligands have also been characterized, including those of V^V,⁷³ Co^IV,^38,74 Cu^III,⁷⁵ Ni^III,^70,76,77 Mn^IV/V,⁷⁸ Fe^IV–VII,^14,16,19,20 Mo^VI,⁷⁹ and Nb^V.⁸⁰ Of particular relevance to the work herein is the use of tripodal carbene ligand frameworks by Smith and Meyer to stabilize high oxidation state Mn, Fe, and Co nitrides.^{14,16,19,20,78} In certain cases the nitride ligand has been shown to insert into the M–C_NHC bond to form an imine.^16,20,38

Redox-active NHC ligands have been described,^81–88 and the presence of arylphenoxide units results in a significant lowering of the oxidation potential of the ligand. Indeed, a series of Ni^II ligand radical complexes of the HL^CO, H₂L^OCO, and H₂L^C2O2 (Scheme 1) ligands have been characterized in the solid state.^70,76,77 The different bonding properties of the imine and NHC donors in the salen and H₂L^C2O2 ligands (Scheme 1) juxtaposed with the similar metal binding geometry and capacity for ligand-based oxidation presents an interesting opportunity to compare the formation and stability of the respective Cr and Mn nitride complexes.

2. Results and discussion

2.1. Synthesis of Mn precursors

Reaction of the bromide salt of the tetradentate bis-carbene bis-phenolate ligand H₂L^C2O2·2HBr with Mn(OAc)₂ and NEt₃ in CH₃CN solvent afforded the Mn^III complex MnL^C2O2(Br) (Scheme 2). The synthesis of MnL^C2O2(Br) using CH₃OH as the solvent was recently reported,⁸⁹ and a Mn^III complex of the H₂L^OCO ligand (Scheme 1) was previously studied by Bellemin-Laponnaz and co-workers.⁷³ The complex was characterized by ESI-MS and EA. Dark brown crystals suitable for X-ray crystallography were obtained by slow evaporation of a concentrated CH₂Cl₂ solution. The solid-state structure of MnL^C2O2(Br) was recently reported,⁸⁹ however, certain metrical parameters differ due to the crystallization solvent. Thus, we will briefly describe the metrical parameters (Fig. S1 and Table S1†) in order to compare with the other solid-state structures herein. The Mn ion lies in a distorted square pyramidal geometry with the Mn center coordinated by two phenolate oxygen atoms (O1/O2) and two carbons of the NHC units (C7/C17) in equatorial positions. The anionic bromide ligand occupies the apical position, with the Mn atom 0.417 Å above the C₂O₂ ligand plane. The angle between the NHC and phenolate rings are 25° and 28° in the structure, and in addition the ligand backbone adopts an umbrella shape with an angle between the two NHC units of 32°. This is in contrast to the neutral Ni^II complex of the same ligand reported by us, in which the angle between the two NHC units is only 3°, the angle between the NHC and phenolate rings are ∼9° and the complex is essentially flat.⁷⁰ In addition, the Ni–C_NHC bonds are significantly shorter (1.843 Å) in comparison to the Mn–C_NHC bonds in MnL^C2O2(Br) (2.03 Å). The ligand structural changes demonstrate significant binding flexibility, thereby accommodating different metal cations and different geometries.

The azido complex MnL^C2O2(N₃) was synthesized from a concentrated CH₃CN solution of MnL^C2O2(Br) in the presence of 1.5 equiv. of NaN₃ (Scheme 2). Solvent removal, addition of CH₂Cl₂ and filtration removed the excess NaN₃ affording MnL^C2O2(N₃) as a brown solid. IR (ν(N₃) = 2040 cm⁻¹), ESI-MS, and EA analysis confirmed product formation. Red/brown crystals suitable for X-ray diffraction were obtained via slow evaporation of a concentrated CH₃CN solution of MnL^C2O2(N₃). The solid-state structure of MnL^C2O2(N₃) is presented in Fig. 1 (see ESI, Table S1† for selected crystallographic data). The distorted square pyramidal structure is similar to MnL^C2O2(Br), with the azide ligand in the apical position, and an angle between the two NHC units of 33°. When dissolved in CH₂Cl₂ both MnL^C2O2(Br) and MnL^C2O2(N₃) display a brownish color and broad absorptions in their UV-vis-NIR spectra (Table 1 and Fig. S2†).

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2.2. Attempted synthesis of Mn nitride complex MnL^C2O2(N)

With the two Mn^III complexes in hand, we next investigated two different synthetic methods to prepare the same Mn nitride product MnL^C2O2(N) (Scheme 1). Due to the susceptibility of nucleophilic attack at the methylene position between the two NHC rings of the ligand, we did not attempt to use NH₄OH/bleach as described by Carreira and co-workers.⁹⁰ Initially, we employed azide photolysis of MnL^C2O2(N₃) as this is a common method to produce nitrides (Scheme 3). Using the same protocol as one we had successfully used for Mn salen variants (λ = 312 nm, benzene solvent)⁵² resulted in solution color changes, however, the Mn nitride complex MnL^C2O2(N) could not be isolated. Evans method analysis of the crude reaction mixture afforded a magnetic susceptibility of μ_eff ∼ 4 (or ∼3 unpaired electrons) consistent with the presence of a paramagnetic Mn species, and an absence of the expected diamagnetic d² Mn^V nitride complex. While ESI-MS analysis of the crude reaction mixture showed the expected MnL^C2O2(N) peak at 624.3 m/z, attempted purification via chromatography and/or recrystallization was unsuccessful (vide infra). Irradiation at lower energy (λ > 350 nm), or low temperature (λ = 312 nm, CH₃CN, 235 K), resulted in no reaction.

We thus turned to a nitride exchange reaction, using a Mn^V salen nitride (Mnsalen(N)) complex as the nitride exchange reagent, which has been used to synthesize a variety of metal nitrides,^91–93 including nitride exchange to other Mn^III complexes (Scheme 3).⁹⁴ Mixing of 1 equiv. of the nitride exchange reagent with MnL^C2O2(Br) in CH₃CN resulted in the formation of a brown precipitate identified as Mn^IIIsalen(Br) (ESI-MS, 321.04 m/z) which was removed via filtration. ESI-MS analysis of the filtrate exhibited the expected MnL^C2O2(N) peak at 624.3 m/z (Fig. S3†), however, we were again unable to isolate the desired nitride complex via chromatography and/or recrystallization. Attempting the nitride insertion reaction at low temperature (198 K, CH₂Cl₂) afforded the same result. Intriguingly, in these reactions a peak in the ESI-MS at 570.38 m/z indicated N-insertion into the ligand framework (Fig. S3†), pointing to a possible decomposition pathway for the initially formed MnL^C2O2(N) complex. Nitrido ligands have been reported to undergo intramolecular N-insertion into the ligand backbone of Fe,^16,20 Co,^38,95 U,⁹⁶ and Ni⁴¹ complexes. In the majority of cases, population of antibonding π* MN orbitals destabilizes the nitride, and thus intramolecular nitride insertion was unexpected for the putative d² Mn^V complex in this work. Meyer in a series of elegant studies has demonstrated intramolecular nitride insertion at a carbenic center in a series of Co³⁸ and Fe^16,20 complexes employing tripodal carbene-based ligands. Interestingly, the analogous Mn^V nitride complexes did not undergo the same insertion chemistry.⁷⁸

2.3 Isolation of nitride insertion product

To further investigate the potential nitride insertion product, we solubilized the crude reaction mixture from a representative nitride exchange reaction in diethyl ether and washed with aqueous 10% HCl and then EDTA. Analysis by ESI-MS showed the loss of the Mn complex peak at 624.3 m/z (Fig. S4†). Further washing of a CH₃CN solution with hexane, and recrystallization by slow evaporation of CH₃CN/CH₂Cl₂ solution afforded pure ligand insertion product in 47% isolated yield. The same product can be isolated from the azide photolysis reaction, however in lower (∼15%) yield. ¹H NMR of the product (Fig. S5†) showed the compound to be symmetric in solution (Fig. 2A), with the expected mass of 570.38 m/z (Fig. S6†). Further NMR analysis (¹³C, ¹³C DEPT and ¹H–¹³C gHSQC NMR) provides confirmation of the structure in solution (Fig. S7–S9†). Interestingly, ¹H NMR in CDCl₃ results in broadened resonances indicative of hindered rotation of the phenol due to dynamics in the fast-exchange regime (Fig. S10†). Analysis of the product by X-ray crystallography showed that the nitride insertion product is a cation [H₂L^C2O2(N)]⁺ with chloride as the counterion (Fig. 2B). In addition, nitride insertion into both carbenic carbons has occurred, resulting in three fused rings including two imidazoles and a central triazone core. The nitride nitrogen (N1) exhibits similar short bond lengths to both carbenic carbons (N1–C7 and N1–C17: 1.335 and 1.334 Å respectively) and the three fused rings are essentially flat, with an 11° angle between the planes of the two imidazole rings. These structural features indicate the presence of a conjugated π system located across the N2–C7–N1–C17–N5 atoms (Fig. 2C). The hydrogen bond network involving the two phenols, a lattice water, and the chloride counterion ensures the secondary structure of the crystal (Fig. S11†). Interestingly, Bullock and co-workers have reported that H-atom abstraction from a bound NH₃ in [Mn(dmpe)₂(CO)(NH₃)]⁺ (dmpe = 1,2-bis(dimethylphosphino)ethane) results in double insertion of the N-atom into the diphosphine ligand to form a cyclophosphazenium cation,⁹⁷ with theoretical calculations supporting insertion of an NH_x species rather than the nitride herein.

¹⁵N isotopic labeling was employed to further investigate the N-atom insertion into the carbene ligand backbone. We used the ¹⁵N-labeled nitride exchange reagent Mnsalen(¹⁵N) in a reaction with MnL^C2O2(Br) and subsequent ESI-MS analysis showed a one Dalton m/z shift for the two ions at 571.4 (100%) and 625.3 (20%) (Fig. S12†) in comparison to the unlabeled reaction (Fig. S3†). The ¹⁵N-inserted product [H₂L^C2O2(¹⁵N)]Cl was isolated in a similar manner to the unlabeled analogue in a comparative yield (56%). To further confirm that the N-insertion is a result of intramolecular N-insertion from a transient MnL^C2O2(N) nitride complex, and not for example from an intermolecular process, we synthesized the ¹⁵N-labeled azide complex MnL^C2O2(^14/15N₃) using 50% terminally ¹⁵N-labeled NaN₃. Photolysis of MnL^C2O2(^14/15N₃) (λ = 312 nm) afforded a 1:1 mixture of N-inserted ligand products [H₂L^C2O2(¹⁵N)]Cl and [H₂L^C2O2(¹⁴N)]Cl by ESI-MS as well as an additional peak at 573.4 m/z likely corresponding to an O-inserted ligand product (Fig. S13†). The O-inserted product was not investigated further in this work. Overall, the two different isotopic labeling experiments provide further evidence that intramolecular N-atom insertion from an unstable MnL^C2O2(N) nitride complex results in formation of the [H₂L^C2O2(N)]Cl product.

2.4. Synthesis of the Cr nitride complex

Based on our previous work exploring the electronic structure and reactivity of the analogous Mn and Cr nitride complexes with salen ancillary ligands,^33,51,52 and the increased stability of the Cr derivatives more generally,^33,52 we investigated the preparation of the Cr nitride complex CrL^C2O2(N). Following our previous work with salen ligands,³³ and the general preparation outlined by Bendix,⁹² we added a solution of the H₂L^C2O2·2HBr ligand to an easily complexed [CrN]²⁺ synthon with labile auxillary ligands (CH₃CN and Cl⁻) (Scheme 4).

A bright orange powder was isolated and confirmed to be the CrL^C2O2(N) complex based on IR (ν(NCr) = 1025 cm⁻¹), ESI-MS (621.32 m/z, [CrL + H]⁺, 100%; Fig. S14†), and X-ray crystallography. The UV-vis absorption spectrum of CrL^C2O2(N) displayed typical features of a d¹ Cr^V species in a square pyramidal geometry, including an intense LMCT transition at 30770 cm⁻¹, and a weak transition (ε < 300 M⁻¹ cm⁻¹) at 20800 cm⁻¹ corresponding to a transition from the non-bonding d¹_xy into empty d_xz or d_yz π* orbitals (Fig. S15†). In addition, frozen solution X-band EPR measurements displayed an axial pattern, with simulation parameters (g_⊥ = 1.990, A^53Cr = 49; g_|| = 1.949, A^53Cr = 125) indicative of a d¹ Cr^V complex (Fig. 3).⁹⁸

Single crystals suitable for X-ray diffraction were obtained by slow evaporation of a concentrated CH₂Cl₂/CH₃CN solution of CrL^C2O2(N) (Fig. 4 and Table S2†). The Cr ion is in a distorted square pyramidal environment with the Cr center coordinated to two phenolate oxygens (O1/O2), two NHC carbons (C7/C17), and the nitride ligand (N1). The Cr–nitride bond length of 1.558 Å is similar to that reported for other Cr^V nitrides.^33,99 The Cr center is shifted out of the ligand C₂O₂ plane by ca. 0.56 Å, which is slightly more than that reported for the salen analogues.^33,99 Similarly to the X-ray structures for both MnL^C2O2(Br) and MnL^C2O2(N₃) the ligand backbone for CrL^C2O2(N) adopts an umbrella shape with an angle between the two NHC units of 25°. Based on the marked difference in stability of the MnL^C2O2(N) and CrL^C2O2(N) complexes investigated herein we turned to theoretical calculations to further investigate their predicted electronic structure and reactivity differences.

2.5. Theoretical calculations on CrL^C2O2(N) and MnL^C2O2(N)

The predicted metrical parameters for CrL^C2O2(N) were determined to be within ±0.03 Å of the experimental X-ray data (Table 2). Similarly to the X-ray structure, a square pyramidal shape is predicted with the Cr atom out of plane towards the nitride (Fig. S16†). The CrN bond length is predicted to be ca. 0.02 Å shorter in comparison to the X-ray data. Overall, the predicted structure for MnL^C2O2(N) (Fig. S17†) is similar to the Cr analogue, however, certain key differences are noted. For example, the computed MnN bond length is ca. 0.03 Å shorter in comparison to CrN, however the Mayer bond order for the Mn derivative is slightly lower (2.72 vs. 2.79). In addition, the Mn center is closer to the C₂O₂ plane, by ca. 0.05 Å (Table 2). By moving closer to the C₂O₂ plane the Mn center interacts more strongly with the carbene ligands, and the predicted M–C(7/17) carbene bond lengths are ca. 0.08 Å shorter for the Mn complex in comparison to Cr. The shorter metal–carbene bond lengths in MnL^C2O2(N) likely facilitates the observed nitride insertion reactivity.

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We compared the relative stability of MnL^C2O2(N) and CrL^C2O2(N) by investigating the initial insertion of the nitride into one of the M–C_NHC bonds. We considered this as the first step in the reaction pathway to eventually form the doubly inserted product [H₂L^C2O2(N)]⁺ (Scheme 3). The reaction pathway is shown in Fig. 5 with the triplet transition state (³TS) calculated to be 24.17 kcal mol⁻¹ higher in energy than the singlet MnL^C2O2(N) starting material. Note that by analyzing the potential energy surface (PES) for the reaction (Fig. S18†), it is apparent that the triplet energy surface (unpaired electrons in d¹_xy and d¹_xz/yz rather than d²_xy) is slightly lower in energy at the TS, with the ¹TS slightly higher in energy at 26.12 kcal mol⁻¹. Further analysis of the relative energies of the ¹TS versus the ³TS using different DFT functionals/basis sets also predicts the ³TS to be of lower energy (Table S3†), however such comparisons should be viewed with caution.¹⁰⁰ The nitride N is bridging the Mn–C_NHC bond at the TS (Fig. 5) supporting a N-migratory insertion pathway. The quintet nitride inserted Mn^III imido reaction product MnL^CNO2 is stabilized by 9.72 kcal mol⁻¹ relative to the MnL^C2O2(N) starting material (Fig. 5). In contrast, for the doublet CrL^C2O2(N) complex the ²TS is calculated to be significantly higher at 35.22 kcal mol⁻¹, with the resulting quartet nitride inserted Cr^III imido reaction product CrL^CNO2 de-stabilized by 3.03 kcal mol⁻¹ relative to the CrL^C2O2(N) starting material (Fig. 5). Spin crossover to the quartet manifold is calculated to occur after the ²TS (Fig. S18†). These calculations highlight the greater susceptibility of MnL^C2O2(N) to nitride insertion relative to CrL^C2O2(N), as observed experimentally. We note that Meyer et al. isolated a protonated imine reaction product in their studies,^16,38,101 however, we did not further evaluate this possibility as we have no experimental verification for either the imido or imine species herein. Further, we did not investigate the second N-carbene insertion to form the [H₂L^C2O2(N)]⁺ product due to the uncertainty in the identity of the final Mn product(s).

We next investigated the orbitals involved in the nitride insertion reaction to gain further insight. We analyzed the stabilization energy (E⁽²⁾) in the equilibrium structures of ML^C2O2(N) (M = Mn, Cr) between different donating (filled) and accepting (empty) orbitals associated with the MN and M–C_NHC bonds using second-order perturbation theory (SOPT) from natural bond order (NBO) calculations.^102,103 This analysis predicts that while a donor–acceptor interaction occurs from filled NHC orbitals to the π* MN orbitals, the most significant interaction is in the opposite direction, and in particular π MN → σ* M–C_NHC (Table 3 and Fig. 5). It is interesting to note that the predicted donor–acceptor interactions are significantly increased for the Mn derivative in comparison to Cr (Table 3), which is in line with the shorter predicted M–C_NHC bond distance for the Mn derivative, and the observed reactivity. Further, we investigated the change in predicted orbital occupancy via NBO analysis for MnL^C2O2(N) as the N–Mn–C_NHC bond angle is decreased from the equilibrium structure (ca. 90°) to the TS (ca. 60°) along the reaction coordinate, and in line with the SOPT analysis, the most significant decrease occurs for the π MN orbitals, with a concomitant increase in occupancy for both σ* Mn–C_NHC and π* NHC (Fig. S19†). Overall, these calculations support the observed nitride insertion reactivity for the MnL^C2O2(N) complex, and lack thereof for CrL^C2O2(N), and that the reactivity is primarily driven by nitride donation into σ*/π* orbitals associated with the NHC ligand.

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2.6. Oxidation of Cr nitride complex

Upon isolation and characterization of the neutral nitride complex CrL^C2O2(N) we next investigated its oxidation chemistry to better understand the stability and electronic structure of the one-electron oxidized form. Cyclic voltammetry experiments exhibited a quasi-reversible one-electron redox process at 0.14 V vs. Fc⁺/Fc (Fig. 6), with no other redox processes observed in the electrochemical window (Fig. S20†). Based on previous work, additional redox processes would be expected in the case of ligand-based oxidation.^33,70 Interestingly, this redox potential is between that reported for CrNSal^OiPr (E_1/2 = 0.38 V vs. Fc⁺/Fc) and CrNSal^NMe2 (E_1/2 = −0.04 V vs. Fc⁺/Fc) with the former characterized as a Cr^VI species and the latter as a Cr^V ligand radical, upon oxidation.³³

Based on the quasi-reversible oxidation process, and the relatively low redox potential, we further investigated the bulk oxidation of CrL^C2O2(N) to [CrL^C2O2(N)]⁺ with tris(2,4-dibromophenyl)aminium hexafluoroantiminoate ([N(C₆H₃Br₂)₃]˙⁺[SbF₆]⁻; E_1/2 = 1.1 V vs. Fc⁺/Fc).¹⁰⁴ Upon sequential addition of a solution of the oxidant to CrL^C2O2(N) at 253 K in CH₂Cl₂ a new band is observed at 26500 cm⁻¹ along with a lower energy transition at 11200 cm⁻¹ (2800 M⁻¹ cm⁻¹) (Fig. 7). The oxidized complex is stable for at least one hr at 253 K (Fig. S21†). The UV-vis-NIR spectrum of [CrL^C2O2(N)]⁺ is similar to that reported for the Cr^VI nitride complexes with salen ancillary ligands, in which a comparable low energy feature was assigned as a ligand-to-metal charge transfer (LMCT) band.³³ It is unlikely that the 11200 cm⁻¹ transition is a ligand radical intervalence charge transfer (IVCT) band as the single one-electron redox feature discounts a Class II regime, and analysis of the band properties (Δν_1/2 = 6500 cm⁻¹, ε = 2800 M⁻¹ cm⁻¹) are not in line with a Class III ligand radical (Δν_1/2 ≤ 2000 cm⁻¹, ε > 5000 M⁻¹ cm⁻¹).¹⁰⁵

EPR analysis of [CrL^C2O2(N)]⁺ shows ca. 5% signal intensity in comparison to a concentration-matched sample of CrL^C2O2(N), with the remaining signal mostly due to unoxidized CrL^C2O2(N) (Fig. 3). Loss of the EPR signal can be attributed to (1) formation of a d⁰ Cr^VI complex, (2) an antiferromagnetically coupled Cr^V ligand radical species (S = 0, open-shell singlet), or (3) a ferromagnetically coupled Cr^V ligand radical species (S = 1, triplet) exhibiting large zero-field splitting.^106,107 We further investigated the predicted change in metrical parameters upon oxidation and relative energy of the different plausible electronic structures using theoretical calculations. The metal oxidized Cr^VI species was predicted to be lowest in energy, with the antiferromagnetically coupled Cr^V ligand radical species (broken-symmetry, BS) higher in energy by +6.03 kcal mol⁻¹, and the ferromagnetically coupled Cr^V ligand radical species (triplet, T) highest in energy by +13.95 kcal mol⁻¹ (Table 4). The ligand radical is predicted to be localized to form a phenoxyl/phenolate species for the BS and T solutions (Fig. S22†), with the expected difference in Cr–O1/Cr–O2 bond lengths (Table 5). We further investigated the predicted low energy transitions for the three possible electronic structures based on the experimentally observed UV-vis-NIR feature at 11200 cm⁻¹ (Fig. 7). Both ligand radical electronic structures (BS, T) predict low energy transitions in the NIR (Table 4), and no transitions of appreciable intensity are observed experimentally in this energy region (Fig. 7). However, the Cr^VI singlet solution correctly predicts a ligand to metal charge transfer (LMCT) transition at 12098 cm⁻¹ (Fig. 7 and Table 4), providing further support that this is the correct electronic structure of [CrL^C2O2(N)]⁺.

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3. Summary

In this work we studied the synthesis and properties of the Mn and Cr nitrides of a tetradentate bis-phenol bis-N-heterocyclic carbene ligand H₂L^C2O2. The tetradentate platform of H₂L^C2O2 exhibits similarities to the common salen analogue H₂Sal^R (Scheme 1), however the different donating/accepting properties of the NHC units in comparison to imine functionalities of the salen framework provide for an interesting comparison. Indeed, while the Mn^V nitride salen complexes have been reported extensively,^108,109 the Mn^V nitride MnL^C2O2(N) could not be isolated herein. Our results suggest that MnL^C2O2(N) forms, yet is unstable, and the nitride rapidly inserts into a Mn–C_NHC bond. A second insertion reaction results in the isolation of the doubly inserted ligand product [H₂L^C2O2(N)]⁺. Interestingly, the Cr analogue CrL^C2O2(N) can be readily prepared and does not exhibit nitride N-insertion reactivity. Theoretical investigations predict that the transition state for the first N-insertion is ca. 10 kcal mol⁻¹ lower in energy for the Mn derivative, and the reactivity is primarily driven by nucleophilic attack of the terminal nitride on the carbene (π MN → σ* M–C_NHC). Theoretical calculations predict the Mn–C_NHC bond lengths to be ∼0.08 Å shorter in comparison to the Cr–C_NHC bonds for ML^C2O2(N) (M = Mn, Cr) (Table 2), providing rationale for the facile N-insertion reactivity observed for MnL^C2O2(N). In addition, the observed N-insertion reactivity is in accord with the general lower stability and more extensive reactivity of Mn nitrides in comparison to Cr nitrides.^{33,52,91–93} Finally, we investigated the one-electron oxidation of the CrL^C2O2(N) complex, and determined that metal-based oxidation occurs to form the d⁰ Cr^VI product [Cr^VIL^C2O2(N)]⁺.

4. Experimental section

4.1. Materials and methods

All chemicals were of the highest quality grade and purified whenever necessary. The ligand H₄L^C2O2Br₂ was synthesized following a literature procedure.⁷⁰ The atom-transfer reagents Mnsalen(N) and CrCl₃(THF)₃ were also prepared according to published protocols.^90,92 Dichloromethane and acetonitrile were dried by refluxing over calcium hydride and distilled prior to use. The tris(2,4-dibromophenyl)aminium hexafluoroantimonate radical oxidant [N(C₅H₃Br₂)₃][SbF₆] was synthesized according to published protocols.¹¹⁰ Electronic spectra were obtained using a Cary 5000 spectrophotometer. Mass spectrometry (ESI positive mode) was performed on an Agilent 6210 TOF ESI-MS system. ¹H nuclear magnetic resonance (NMR) spectroscopy as well as magnetic susceptibility via Evans Method were carried out on a Bruker AVANCE III 500 MHz instrument. Elemental analysis (C, H, N) were performed at Simon Fraser University on a Carlo Erba EA1110 CHN elemental analyser. All electron paramagnetic resonance (EPR) were recorded on a Bruker EMXplus spectrometer operating with a premium X-band microwave bridge and an HS resonator. EPR spectra were simulated using the EasySpin package in Matlab.¹¹¹ Solid state infrared spectra (IR) were measured on a Thermo Nicolet Nexus 670 FT-IR spectrometer equipped with a Pike MIRacle attenuated total reflection (ATR) sampling accessory. Cyclic voltammetry (CV) was performed on a PAR-263A potentiometer equipped with a silver wire reference electrode, a platinum disk counter electrode and glassy carbon working electrode. Tetrabutylammonium perchlorate (0.1 M) was used as the supporting electrolyte, and decamethylferrocene was used as an internal standard in CH₂Cl₂.¹¹² One-electron processes were confirmed via comparison of the electrochemical response of one equiv. of the Cr complex versus decamethylferrocene.

4.2. Synthesis

4.3. X-ray crystallography

Diffraction data for brown needle single crystals of MnL^C2O2Br, red blade crystals of MnL^C2O2(N₃), and orange prism crystals of CrL^C2O2(N) were collected at the University of British Columbia (Canada) by Dr Brian Patrick on a Bruker X8 Apex II diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) at 296 K. Data were integrated using the Bruker SAINT software package (Bruker, V8.40B, 2016) and absorption corrections were performed using the multiscan technique SADABS-2016/2 (Bruker, 2016/2). The structures were solved by Dr Nicholas Hein and Jason Pulfer using the SHELX software¹¹³ implemented by Olex2.¹¹⁴ All non-hydrogen atoms were anisotropically refined and hydrogen atoms were placed at calculated positions and refined as riding atoms with isotropic displacement parameters. It is noted that for MnL^C2O2(N₃) a residual peak was modeled at 2.55 Å as a Br from the starting material. The refined occupancies are azide (0.967(3)) and Br (0.033(3)). Further data is shown in Tables S1 and S2. Diffraction data for the yellow prism single crystal of solvated [H₂L^C2O2(N)]Cl were collected at Université de Grenoble Alpes (France) by Dr Christian Philouze on a Bruker-AXS-Enraf-Nonius Kappa Apex II diffractometer with multilayer mirrors monochromated Mo-Kα radiation (λ = 0.71073 Å) from an Incoatec high brilliance micro-source at 210 K. Data were integrated using the Bruker EvalCCD software package and absorption corrections were performed using the multiscan technique SADABS-2004/1. The structure was solved by using the SHELX software¹¹³ implemented by Olex2.¹¹⁴ All non-hydrogen atoms were anisotropically refined and hydrogen atoms were placed at calculated positions and refined as riding atoms with isotropic displacement parameters. The structure displays a solvation site which may be modeled with two dichloromethane and one acetonitrile molecules spread over 3 different positions and with respective partial occupancy: 0.3968, 0.35167 and 0.25 Further data is shown in Table S1 and Fig. S11.† The crystallographic data for the new structures have been deposited with Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 2359452, 2359453, 2359455, and 2359456.†

4.4. Theoretical calculations

Geometry optimizations were all performed using the Gaussian 16 program (Revision A.03)¹¹⁵ employing the B3LYP functional, the D3 dispersion correction¹¹⁶ in combination with the 6-31g* basis set. Frequency calculations performed on the same functional/basis set confirmed optimized structures were at a global minimum. Single-point calculations as well as time-dependant DFT calculations were performed using the BP86-GD3 functional and the TZVP basis set of Ahlrichs.^117,118 Natural bond order (NBO) calculations were completed using the single-point calculations in the Gaussian 16 program.¹¹⁹ For reaction profile calculations single-point energies were converted to Gibbs free energies using corrections from the frequency calculations on the optimized coordinates. Calculated transition states were confirmed to have one negative frequency associated with N–C bond formation. All calculations employed a polarizable continuum model (PCM) for CH₂Cl₂ (ε = 8.93) for all atoms.¹²⁰

Data availability

Compound characterization, ¹H NMR and ESI-MS data, as well as X-ray crystallography details are available in the ESI.† Calculation data is compiled in a zip folder.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by Natural Sciences and Engineering Research Council (NSERC) Discovery Grants (RGPIN-2019-06749 and RGPAS-2019-00054 to T. S.). Digital Alliance Canada is thanked for access to computational resources. D. M. acknowledges Mitacs Canada for a Globalink Fellowship (FR43327). Dr Brian Patrick is thanked for assistance with solving the X-ray structures. This work is also supported by the French National Research Agency in the framework of the “Investissements d'avenir” program (ANR-15-IDEX-02) and Labex ARCANE and CBH-EUR-GS (ANR-17-EURE-0003) for financial support. The NanoBio-ICMG platforms (FR 2607) are acknowledged for their support.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2359452, 2359453, 2359455 and 2359456. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt01765j

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