Umpolung in a Pair of Cobalt(III) Terminal Imido/Imidyl Complexes

Abstract Reaction of the CoI complex [(TIMMNmes)CoI](PF6) (1) (TIMMNmes= tris‐[2‐(3‐mesityl‐imidazolin‐2‐ylidene)‐methyl]amine) with mesityl azide yields the CoIII imide [(TIMMNmes)CoIII(NMes)](PF6) (2). Oxidation of 2 with [FeCp2](PF6) provides access to a rare CoIII imidyl [(TIMMNmes)Co(NMes)](PF6)2 (3). Single‐crystal X‐ray diffractometry and EPR spectroscopy confirm the molecular structure of 3 and its S=1/2 ground state. ENDOR, X‐ray absorption spectroscopy and computational analyses indicate a ligand‐based oxidation; thus, an imidyl‐radical electronic structure for 3. Migratory insertion of one ancillary NHC to the imido ligand in 2 gives the CoI N‐heterocyclic imine (4) within 12 h. Conversely, it takes merely 0.5 h for 3 to transform to the CoII congener (5). The migratory insertion in 2 occurs via a nucleophilic attack of the imido ligand at the NHC to give 4, whereas in 3, a nucleophilic attack of the NHC at the electrophilic imidyl ligand yields 5. The reactivity shunt upon oxidation of 2 to 3 confirms an umpolung of the imido ligand.

[(TIMMN mes )Co I ](PF6) (1) was further characterized by single-crystal X-ray diffraction analysis. In the crystal structure of 1, the asymmetric unit contained two independent molecules of the complex salt. For one of the complex cations, the molecular structure is shown in Figure S2. According to the crystallographic analysis, the cobalt center is in a distorted trigonal pyramidal, C3-symmetric coordination environment, with the cobalt atom located 0.349/0.322 Å above the plane of the three carbenes (doop). The TIMMN mes ligand coordinates in a tetradentate fashion with an average Co-Ccarbene bond distance of 1.9606(13)/1.9741 (13) Å and a Co-N bond distance of 2.2504(11)/2.2364(11) Å.

[(TIMMN mes )*Co I (NMes)'](PF6) (4):
The THF solution (10 mL) of [(TIMMN mes )Co III (NMes)](PF6) (2) (150 mg, 0.16 mmol) was stirred at room temperature overnight, and the color of the solution changed from green to dark yellow. Removal of the solvent under vacuum afforded a yellow residue, which was extracted with benzene (20 mL). Filtration of the mixture and lyophilization of the benzene phase afforded the product as a yellow solid (130 mg, 87%). Single-crystals suitable for X-ray diffraction analysis were obtained by diffusion of n-pentane into a solution of [(TIMMN mes )*Co I (NMes)'](PF6) (4) in a solvent mixture of THF/Benzene at room temperature overnight.    Complex 4 was further characterized by SQUID magnetization measurement. According to the SQUID measurement, cobalt(I) complex 4 possesses a magnetic moment, µeff, of 3.10 µB at 300 K; thus, supporting the high-spin S = 1, Co I , d 8 electronic structure assignment ( Figure S15). This value is higher than the calculated spinonly value of 2.83 µB for a high-spin d 8 (S = 1) system. This deviation is generally considered to be caused by a substantial orbital angular momentum contribution from the cobalt ion. VT-VF SQUID measurements were fitted with an average g-value of 2.24 and a zero-field splitting parameter, D, of 11.5 cm -1 . [(TIMMN mes )*Co I (NMes)'](PF6) (4) was further characterized by single-crystal X-ray diffraction analysis. Its molecular structure is represented in Figure S16. In the molecular structure of 4, the modified TIMMN* ligand coordinates to the Co ion through two of its remaining carbenoid carbons, the anchoring nitrogen and the newly formed imine N atom. In this ligand environment, the cobalt center is located 0.153 Å above the trigonal plane. The distance between the N-anchor and the metal ion is 2.2155 (14) Å and, consequently, indicative for a bonding interaction.

[(TIMMN mes )*Co II (NMes)'](PF6)2 (5):
[(TIMMN mes )Co(NMes)](PF6)2 (3) (132 mg, 0.12 mmol) was dissolved in THF (8 mL) at -35 °C, and the mixture was further stirred at room temperature for additional 0.5 h. The color of the solution gradually changed from brown to yellow-green. Removal of the solvent under vacuum afforded a yellowgreen residue, which was washed with n-pentane (3 × 3 mL) and dried in vacuo to yield 125 mg (95%) of product as yellow-green solid. Single-crystals suitable for X-ray diffraction analysis were obtained by diffusion of diethyl ether into a solution of [(TIMMN mes )*Co II (NMes)'](PF6)2 (5) in a solvent mixture of DCM/Toluene at room temperature overnight.    [(TIMMN mes )*Co II (NMes)'](PF6)2 (5) was further characterized by single-crystal X-ray diffraction analysis ( Figure  S20). Complex 5 features a similar coordination geometry as that of complex 4 but with two well-separated PF6counter anions ( Figure S20). In this ligand environment, the Co center is located 0.173 Å above the trigonal plane. Compared to 4, and as expected, dicationic 5 exhibits shorter Co-N(anchor) (2.1947(18) Å in 5, 2.2155(14) Å in 4) and Co-N(imine) (1.9218(19) Å in 5, 2.0128(14) Å in 4) bonds. Interestingly, despite the smaller radius of the Co(II) ion, the average Co-C bond distance of 2.016(2) Å is significantly longer than that of Co(I) cation 4 (1.9400(16) Å); likely due to enhanced p-backbonding in the case of lower-valent 4.   Complex 5 was further characterized by SQUID magnetization measurement. According to the SQUID measurement, cobalt(II) complex 5 possesses a magnetic moment, µeff, of 4.35 µB at 300 K ( Figure S22). This value is higher than the calculated spin-only value of 3.87 µB for a high-spin d 7 (S = 3/2) system. Such deviation is generally considered to be caused by a substantial orbital angular momentum contribution from the cobalt ion. VT-VF SQUID measurements were fitted with an average g-value of 2.24 and a zero-field splitting parameter, D, of 19.3 cm -1 .

Reactivity Studies
Reactivity studies of 2 with styrene, cyclohexene, 1,4-cyclohexadiene, triphenylphosphine as well as CO2 and CO were carried out. However, these efforts turned out rather unproductive. For example, 2 reacted under 1 atm. of CO at room temperature overnight to give yellow, diamagnetic [(TIMMN mes )*Co I (CO)2(NMes)'](PF6) (6) (Scheme S1) rather than the anticipated intermolecular imido transfer products mesityl isocyanate (MesNCO) and the corresponding cobalt(I) complex. Complex 6 was characterized by NMR as well as UV/Vis electronic absorption and IR vibrational spectroscopy, SC-XRD analysis ( Figures S23-S27), and CHN elemental analysis. The formation of the cobalt dicarbonyl 6 can be rationalized by the sequential processes of intramolecular imido migratory insertion, followed by CO coordination to cobalt (Scheme S1). Indeed, reacting [(TIMMN mes )*Co I (NMes)'](PF6) (4) with 1 atm. of CO cleanly produced 6 within 2 mins. These results suggest that the intramolecular imido insertion reaction in 2 is favored over the intermolecular imido transfer to CO. The bent imido linkage (157.97(13)°) observed for 2 closely resembles that of the previously reported uranium(V) mesityl imido complex, [(( tBu ArO)3tacn)U(NMes)], [12] which features a similarly bent imido ligand with a U-N-C(Mes) angle of 154.7(8)°. In this case, the bent imido fragment is reactive and undergoes multiple bond metathesis with CO2 to produce the corresponding U(V) oxo species and free mesityl isocyanate (MesNCO). Indeed, 2 behaves similarly, and, under 1 atm. of CO2, yields complicated mixtures of cobalt species and free MesNCO (15% NMR yield, Scheme S1), as revealed by 1 H NMR and IR spectroscopy by comparison to an authentic sample of MesNCO ( Figures S28, S29). The low yield may result from secondary reactivity of MesNCO with 2. The mechanism of formation is likely similar to that described for the above-mentioned U(V) imido complex; namely a retro-cycloaddition reaction of a fourmembered metallacycle (via a cobalt(III) carbimate species) formed from the [2p + 2p] cycloaddition of 2 with CO2, along with the cobalt(III) oxo species [(TIMMN mes )Co III (O)] + as another product. Attempts to isolate the proposed cobalt(III) carbimate intermediate and oxo complex proved futile. In any case, this imido/oxo exchange reaction further corroborates the nucleophilic character of the imido nitrogen of 2.
The reactivity of 3 was also probed in reactions with an excess of styrene, cyclohexene, 1,4-cyclohexadiene, triphenylphosphine, CO, CO2, CS2 and phenyl isocyanate. However, either no reaction or no intermolecular imido transfer reactions were observed at -35 °C and room temperature during an extended period of time. These results prove the intramolecular insertion reaction in 3 to be much faster than the intermolecular imido transfer.
In a typical procedure, compound 2 (or 3, 4, 5) (15 mg) was dissolved in 0.4 mL of THF-d8 at -35 °C, and to this solution an excess of reagent A (see below) dissolved in 0.2 mL of cold THF-d8 was added. The reaction mixture was standing in the freezer (-35 °C) overnight. Afterwards, the reaction mixture was transferred to a J-Young NMR tube and was slowly warmed up to room temperature before being inserted into the probe of JEOL 400 MHz spectrometer. The reaction progress was followed by 1 H and, if applicable, with 31 P NMR spectroscopy.
Alternatively, compound 2 (or 3, 4, 5) (15 mg) was dissolved in 0.5 mL of THF-d8 in a J-Young NMR tube at -35 °C. The solution was degassed and exposed to 1 atm. of CO or CO2 at -35 °C. Afterwards, the reaction mixture was slowly warmed up to room temperature before being inserted into the probe of JEOL 400 MHz spectrometer. The reaction progress was followed by 1 H NMR spectroscopy.
The conversion of 2 to 4 (3 to 5) was monitored by UV-Vis spectroscopy. The initial concentration of complex 2 or 3 was held constant (0.70 mM for 2, 0.58 mM for 3) for each run, the reaction temperature was varied. In a typical experiment, a J. Young absorption cell (pathlength of 1 cm) was charged with 0.70 mM of 2 (0.58 mM of 3) in THF in an N2-filled glovebox. The cell was placed in a UV-Vis spectrometer, and the electronic absorption spectra were recorded in 1-minute-intervals at a certain temperature. The rate of conversion of 2 to 4 (3 to 5) at a certain temperature was determined by monitoring the disappearance of the band at 710 nm of 2 (515 nm of 3) over time. The linear plots of ln(Abs) vs. time show the reactions are first-order in [2] (or [3]). The slope represents the reaction rate constant at the measured temperature.          In all cases, the first three to four excited states (depending on the number of b holes in the 3d shell) correspond to excitations from Co(1s) to molecular orbitals with large Co(3d) character. The main exception are states 1 and mainly state 2 of the calculates spectrum of the dication in 3, where the large N mes character can be observed from the plots on Figure S41, right panel. The larger N mes character in the acceptor NTO of state 2 is consistent with the excitation of a β electron to N radical bearing a spin-up electron.
While the experimental pre-edge peak of [(TIMMN mes )Co II (Cl)](PF6) has clearly two features, the TDDFT calculation shows only one. This is due to the known multiplet structure of d 7 systems, which the particle-hole approximation of TDDFT cannot capture. [13] In all cases, the first feature at the rising edge is attributed to transitions from Co(1s) to low-lying doubly degenerate p* orbitals located at the tricarbene ligand according to TDDFT. The acceptor NTOs in this region are composed of linear combinations of p* molecular orbitals located and N-C-N portion with antiphase combination of the 2p atomic orbitals perpendicular to the plane of the five-membered ring.
The energy shift of +93.2 eV was applied to all calculated spectra in order to match the position of the pre-edge peaks. To match the position of the rising edge features, a shift of +91.9 eV would have been appropriate, however the former was preferred because the TDDFT protocol can more reliably calculate pre-edge transitions. The 1.3 eV difference indicate a systematic error in the calculations of intra-metal transitions (characteristic of the pre-edge peak) and metal to ligand charge transfer (characteristic of the rising edge features in all tricarbene systems investigated). In fact, the relative position of the 1s to 3d transitions and MLCT transitions has been previously shown to be systematically dependant on the Hartree-Fock exchange content of the DFT functional, [14] which is 20% for B3LYP. Tuning this fraction could potentially result in better reproduction of the experimental differences between pre-edge and rising edge features, but is outside the scope of the present work and provides no further insights into the electronic structure.  [15] as well as copper nitrene [( EMind L)Cu(N(C6H4 t Bu))] [16] in the solid state.  Suitable single crystals were embedded in protective perfluoropolyalkylether oil and transferred to the cold nitrogen gas stream of the diffractometer. Intensity data of all investigated crystals of the (TIMMN mes )-ligand system were collected at 100 K using MoKα radiation (λ = 0.71073 Å) either on a Bruker Kappa PHOTON 2 IμS Duo diffractometer equipped with QUAZAR focusing Montel optics (for 1, 3, 4, and 5) or on a Bruker Smart APEX 2 diffractometer using a Triumph curved graphite monochromator (for 2 and 6). Data were corrected for Lorentz and polarization effects, and, in addition, semi-empirical absorption corrections were performed on the basis of multiple scans using SADABS. [17] The structures were solved by direct methods (SHELX XT 2014/5) [18] and refined by fullmatrix least-squares procedures on F 2 using SHELXL 2018/3. [19] All non-hydrogen atoms were refined with anisotropic displacement parameters. All other hydrogen atoms were placed in positions of optimized geometry. The isotropic displacement parameters of all H atoms were tied to those of the corresponding carrier atoms by a factor of either 1.2 or 1.5.
In the crystal structure of 1, the asymmetric unit contained two independent molecules of the complex salt. One of the PF6anions was disordered. Two alternative orientations were refined and resulted in site occupancies of 58 (2) and 42(2) % for the atoms P1 -F16 and P1A -F16A, respectively. The compound crystallized with a total of two molecules of toluene in its asymmetric unit. While one of the toluene molecules was located on a general position the other was situated on two different crystallographic inversion centers and was accordingly disordered. Similarity restraints and pseudo-isotropic restraints were applied in the refinement of the anisotropic displacement parameters of the atoms of toluene C301 -C307. In the crystal structure of 2, the PF6anion was disordered. Two alternative orientations were refined and resulted in site occupancies of 56.6(5) and 43.4(5) % for the atoms F11 -F16 and F11A -F16A, respectively. Similarity restraints were applied to the anisotropic displacement parameters of the disordered fluorine atoms. The compound crystallized with two molecules of CH2Cl2 per formula unit. One of these solvent molecules was disordered. Two alternative orientations were refined and resulted in site occupancies of 43(2) and 57(2) % for the atoms C100 -Cl12 and C110 -Cl14, respectively. The crystals of 3 suffered from an extremely rapid loss of included solvent and had to be handled at low temperatures all the time. One of the PF6anions was disordered. Two alternative orientations were refined and resulted in site occupancies of 57 (2) and 43(2) % for the atoms P2 -F26 and P2A -F26A, respectively. The compound crystallized with five molecules of THF and half a molecule of n-pentane per formula unit. The n-pentane molecule was situated on a crystallographic inversion center and, correspondingly, was disordered. Three out of the five THF molecules were disordered. Two alternative orientations each were refined and resulted in the following site occupancies for the affected atoms: 54.4 (19) and 45.6(19) % for O100 -C104 and O110 -C114; 44.7 (19) and 55.3 (19) % for O400, C401, C402, C404 and O410, C411, C412, C414; 61.1(9) and 38.9(9) % for O500 -C504 and O510 -C514, respectively. Similarity restraints were applied to the anisotropic displacement parameters of the disordered atoms. For the n-pentane additional pseudo-isotropic restraints were applied. Compound 4 crystallized with one molecule of benzene per formula unit. The PF6anion was disordered. Two alternative orientations were refined and resulted in site occupancies of 74.8(5) and 25.2(5) % for the atoms F11 -F16 and F11A -F16A, respectively. Similarity restraints were applied to the anisotropic displacement parameters of the disordered atoms. Compound 5 crystallized with one molecule of CH2Cl2 per formula unit. One of the PF6anions was disordered. Two alternative orientations were refined and resulted in site occupancies of 53.0(5) and 47.0(5) % for the atoms F23 -F26 and F23A -F26A, respectively. All examined crystals of compound 6 proved to be multiply twinned. For the present crystal a twin rotation about the reciprocal 0 0 1 axis of 180° was detected as the major twin element. The refinement resulted in twin individual mass fractions of 0.954(1) and 0.046 (1).
Crystallographic data, data collection, and structure refinement details for the crystal structure determinations are given in Tables S5 and S6. Representations of the molecular structures are shown in Figures S42-S47.

Computational Details
All calculations were performed with ORCA v. 5.0.1, 5.0.2 and 5.03. [20] Only the cations are used for calculations.

1.) CASSCF calculations:
The positions of all hydrogen atoms were optimized ("optimizehydrogens true") with constrained coordinates of all other atoms using the geometric parameters as obtained by the solid-state structures. CASSCF calculations were performed at the triple-z (ZORA-def2-TZVPP on all atoms) [21] level using CAS(10,7) for 2 and CAS(11,8) for 3 according to Figures 6, 7 (Figures S56, S57 and S58, respectively). The difference between the two active spaces relates to inclusion of the ligand-centered, s-donor orbital which combines with the d(x 2 -y 2 ) d-orbital. Despite of repeated attempts, adding the imido ligand's s-(p(z), respectively) orbital in 2 top the active space led to poor convergence (undesired rotation, respectively), likely due to strong s-character, low energy and mixing with the aryl substituent. Scalar relativistic effects were modeled within the "Zeroth Order Regular Approximation" (ZORA). [22] The RIJCOSX approximation with the related auxiliary basis set SARC/J (autoaux for /C) was used to speed up the calculations. [23] Tighter-than-default scf ("tightscf") criteria were chosen. In the manuscript, the results for calculations without state-averaging are given. For the calculation of the absorption spectra, state-averaged calculations (2, 50 singlet, 45 triplet and 5 quintet roots; 3, 75 doublet, 24 quartet, 1 sextet root) were performed and energies were corrected using the 2 nd order perturbation theory NEVPT2 with frozen core electrons ( Figures S59, S60). [24] Spin-orbit coupling was considered by quasi-degenerate perturbation theory (QDPT; soc true), yet does not have a strong effect on the absorption spectra and is not included in Figures S59 and S60.

2.) DFT calculations:
The geometric parameters of all isolated compounds were optimized at the ZORA-DFT-D3BJ/def2-SVP level of theory using the PBE, TPSS, PBE0 and TPSSh functionals. [25] The transition states were modeled using only the PBE (results given below) and TPSSh functionals. For cobalt, the ZORA-def2-TZVP basis set was used. The RI approximation (RIJCOSX, respectively) with the related auxiliary basis set SARC/J was used to speed up the calculations. [23] Tighter-than-default scf ("tightscf") and optimization criteria ("tightopt") were chosen. All calculated structures were verified as true minima by the absence (N imag = 0) of negative eigenvalues in the harmonic vibrational frequency analysis. The PBE, TPSS, and TPSSh functional gave reasonable agreement with the structural parameters in the solid state, whereas PBE0 proved less accurate (Table S7). Spin contamination with PBE and TPSSh was moderate (Deviation from ideal S(S+1) was between 0.01 a.u. and 0.03 a.u.). The energies of all compounds were corrected by single-point calculations at the triple-z level of theory using the TPSSh functional and SMD solvation model for THF. [26] Intrinsic bond orbitals were calculated according to Knizia. [27] For the reaction of 2 on the singlet potential energy surface, unrestricted (and broken symmetry) calculations were performed with the PBE functional, which however repeatedly converged to the closed-shell wavefunction. Generally, switching from PBE to TPSSh (and even more so PBE0) systematically favors the high-spin-versus the low-spin states in line with increasing admixture of exact exchange. The values obtained at the TPSSh//PBE level of theory gave the best fit with the experimentally determined barriers and are thus shown in the manuscript. See below Figures S61, S62 for the results obtained with the PBE//PBE level of theory, which are consistent (yet with "later" spin-crossover for the transformation from 2 to 4).
Further, single-point calculations at the ZORA-DLPNO-CCSD(T)//def2-TZVPP level of theory were conducted [default pno settings (normalpno), including correction for solvation in THF (SMD)]. However, high T1 values up to 0.03 were obtained (Table S8), and we thus judge these values rather unreliable.

3.) EPR calculations:
The EPR spectrum of 3 was modeled with the TPSSh functional at the triple-z level of theory using the structural parameters from the solid-state (vide supra). For the calculation of the HFC tensor, 2 nd order contributions from spin-orbit-coupling ("aorb") were included for the cobalt ion.

4.) K-edge X-ray absorption calculations:
The K-edge XAS spectra were calculated with time-dependent density functional theory (TDDFT) within Tamm-Dancoff approximation, as previously described, [28] using B3LYP [29] exchange-correlation functional on structures derived from crystallography where only the H atoms were optimized. Scalar-relativistic effects were included by Douglas-Kroll-Hess formalism. [30] The DKH all-electron adapted version of Aldrich's triple-zeta basis function DKH-def2-TZVP(-f) [21] were used together with the SARC/J general-purpose auxiliary basis set for Coulomb fitting. [31] The SCF convergence criteria were set with "TightSCF" keyword. The oscillator strength was taken as the sum of electric dipole, electric quadrupole and magnetic dipole contributions. The transitions were assigned based on the natural transition orbitals. [32] The natural transition orbital plots were produced with VMD. [33]