Formation and Reactivity of a Fleeting NiIII Bisphenoxyl Diradical Species

Abstract Cytochrome P450s and Galactose Oxidases exploit redox active ligands to form reactive high valent intermediates for oxidation reactions. This strategy works well for the late 3d metals where accessing high valent states is rather challenging. Herein, we report the oxidation of NiII(salen) (salen=N,N′‐bis(3,5‐di‐tert‐butyl‐salicylidene)‐1,2‐cyclohexane‐(1R,2R)‐diamine) with mCPBA (meta‐chloroperoxybenzoic acid) to form a fleeting NiIII bisphenoxyl diradical species, in CH3CN and CH2Cl2 at −40 °C. Electrochemical and spectroscopic analyses using UV/Vis, EPR, and resonance Raman spectroscopies revealed oxidation events both on the ligand and the metal centre to yield a NiIII bisphenoxyl diradical species. DFT calculations found the electronic structure of the ligand and the d‐configuration of the metal center to be consistent with a NiIII bisphenoxyl diradical species. This three electron oxidized species can perform hydrogen atom abstraction and oxygen atom transfer reactions.

500 nm, acquired with an iDus-430-BV CCD camera from Andor Technology. The spectral slit width was set to 50 μm. UV-Vis absorption spectra were obtained simultaneous sly using a fiber coupled deuterium/halogen light source and an Avantes-EVO mini spectrometer. Sample cooling was performed using a CoolSpeK USP-203-B Unisoku cryostat.
The reaction mixture was injected (25 μL) in HPLC system (Agilent 1260 infinity machine model) attached with a ZORBAX SB-C-18 reverse phase column (150 × 4.6 mm, 5 μm) and DAD detector. Individual purity of the sample was analyzed prior to reactivity studies using HPLC experiments with the methods described below: A gradient mixture of water and acetonitrile possessing 0.1% trifluoroacetic acid (pre-degassed by applying vacuum and ultrasonication) was used as the mobile phase. Method: 0 min to 20 min, 0.5 mL per min flow rate. For the oxidation reaction of triphenylphosphine, condition A was used and for oxidation of thioanisole and xanthene, condition B was used. C. The addition of Et3N (1.1 mmol) to the above turbid mixture turns the colour from yellow to mustard brown. The reaction was left for stirring for 4 h, following which it was brought to room temperature, and the solvent was evaporated under reduced pressure. Long red needle-like crystals of 1 were obtained in 4-5 days in N, N-dimethylformamide (DMF), and the X-ray crystallographic parameters were found to match those in the literature. [1]

Reactivity conditions in UV-Vis
To 0.05 mM 1, 7 eq. mCPBA was added to generate 440 nm species at -40 C in CH3CN. To the maximum absorbance of 440 nm species, different eq. substrates are added, which caused the decay in its absorbance.

Catalytic reaction conditions for HPLC analysis
To a solution of 1 (concentration mentioned in the captions), 7 eq. mCPBA was added at -40 C in CH3CN to generate 440 nm species. The generation is followed by the addition of 10 eq. substrate to it. The reaction mixture was then left to react completely as per the substrate's reaction profile monitored through UV-Vis absorption spectroscopy ( Figures S27, S30 and S33). The samples were then stored in Liq. N2 to avoid further reaction until the HPLC data was recorded. Under the similiar conditions the blank reaction of 10 eq. substrate with 7 eq. mCPBA without 1, was done at -40 C in CH3CN.

UV-Vis absorption experiments
The majority of the existing reports on 1 have been carried out using CH2Cl2 or DMF as the solvents.
For the present study, we used CH3CN and CH2Cl2 as our choice of solvents. However, due to the lesser solubility of 1 in CH3CN, we performed a series of experiments to evaluate its concentration. 416 nm of 1 in DMF is found to be 7000 M -1 cm -1 . Therefore, 2 mM of 1 was prepared in DMF, from which it was diluted to 1:19 DMF:CH3CN ratio. The 416 nm of the same was recorded to be 6800 M -1 cm -1 which is considerably close to the reported 7000 M -1 cm -1 epsilon at 416 nm in DMF ( Figure S2). Since such dilution experiments yielded consistent results, we estimated 1 to have 416 nm = 7000 M -1 cm -1 in CH3CN. The established 416 nm = 7000 M -1 cm -1 helped us in figuring the exact concentration of 1 in CH3CN for all our experiments. Figure S1. Cyclic voltammograms of 1 in CH3CN at 500 mV/s (black) and 1000 mV/s (red) scan rates at room temperature.  mCPBA. Conditions used: a) 1 eq., b) 3 eq., c) 5 eq., d) 7 eq., and e) 10 eq. mCPBA were added to 0.05 mM 1 in CH3CN at -40 C. Figure S4. The plot of 440 nm against various equivalents of mCPBA. Figure S5. Plot depicting the self-decay of 440 nm species formed from the addition of 7 eq. mCPBA to 0.05 mM 1 at -40 C in CH3CN. The estimated t1/2 is 50 min.

UV-Vis absorption study in CH2Cl2 and DMF
When CH2Cl2 is used as a solvent, the reaction of 1 with 7 eq. mCPBA generated a band at 440 nm with an 440 nm ~ 16,000 M -1 cm -1 ( Figure S8), which is indistinguishable from that of CH3CN ( Figure S3d). The 440 nm species can be assigned as [1 III -L •• ] as in the case of CH3CN. However, in DMF addition of 7 eq. mCPBA did not show any change in the spectra stating the unreactive nature of 1 in DMF ( Figure S10).       *Artifacts due to imperfect solvent subtraction. Condition to generate 440 nm species: 0.2 mM 1 + 7 eq. mCPBA in CH2Cl2 at -80 C.

Computational details
All geometries were optimized in Gaussian 16, [2] using the M06-L density functional [3] with the def2-SVP basis set. [4] Weigend's universal fitting basis set [5] was used via the W06 keyword. The PCM solvation model [6] was used, with acetonitrile specified as the solvent. Frequency analysis confirmed all located stationary points were minima. An UltraFine grid was specified, along with increased two-electron integral accuracy (acc2e=16). See below for an example input. All converged solutions were tested for stability using the Stable=Opt keyword, and the geometries were reoptimized in the case an instability was found. In every 5-coordinate case, the LS state was found to be lowest in energy (energetics discussed further below).
At the optimized (M06-L/def2-SVP/PCM) geometries, single point (SP) calculations were performed in ORCA 5.0.1. [7] These calculations employed the local density functional M06-L, [3a] the hybrid generalized gradient approximation (GGA) PBE0 [8] , and the hybrid meta-GGA PW6B95. [9] The self-consistent D4 model [10] was used for the calculations with PBE0 and PW6B95. All the SP calculations employed the def2-TZVPP basis set, [4] the cPCM solvation model [11] with acetonitrile specified as the solvent, and the resolution of the identity (RI) approximation to speed up the Coulomb integrals, [12] in combination with Weigend's universal fitting basis set (def2/J). [5] The calculations with the hybrid functionals (PBE0 and PW6B95) also employed the COSX approximation for the exchange integrals, again with Weigend's universal fitting basis set (def2/J). [5] All these calculations used an SCF convergence criterion of 10 -8 a.u. (TightSCF). See below for an example input line. Every method used also found the LS state to be the lowest in energy of the five-coordinate species (Table S1). All orbital isosurfaces were plotted in IboView v2021 [13] to enclose 80% of the electron density. The following notation is used to label the IBOs: s(Γ-IBO) O , where s=(α,β) is the spin, Γ=(σ,π,δ) is the symmetry and O=(1,2) is the occupation. All spin density isosurfaces were also rendered with IboView, at +/-0.004 in green/purple, respectively. Qualitative molecular orbital diagrams for the LSA and LSB states of [Ni-MeCN] 3+ (Figures S22-23) were made by using the corresponding orbital transformation. [14] Example input line: Table S1. Key bond distances (in Å) of 1 were obtained via X-ray diffraction (XRD) and DFT (M06-L/def2-SVP/PCM).

Structural agreement with experiments
Bond distance Ni-Oa Ni-Ob Ni-Na Ni-Nb XRD [ Satisfied with the structural performance of the chosen M06-L/def2-SVP/PCM(Acetonitrile) methodology (Table S1), which we have previously applied to high-valent Nickel species, [15] we proceeded to examine the electronic structure of 1 and its three electron oxidized counterpart, [1-L] 3+ , via an Intrinsic Bonding Orbital (IBO) analysis.

Electronic structure
The formal assignment of 1 as a Ni(II) compound is fully consistent with our calculated electronic structure, obtained via single-point calculations at the optimized geometry with a larger basis set (def2-TZVPP). As expected, 1 exhibit an intrinsic d 8 Ni(II) configuration and dative σ-bonding in the first coordination sphere of the metal center ( Figure S14).  Figure S18) or a d 7 Ni(III) center with a doubly oxidized ligand [1 III -L •• ] (with PW6B95-D4, Figure S19). As expected for the d 6 LS state, where three alpha electrons are aligned antiparallel to three beta electrons, reduced spin density is found at the metal center ( Figure S20, top) as compared to the d 7 LS state ( Figure S20, bottom), which has an unpaired electron at the metal center.