Cobalt(III) Carbene Complex with an Electronic Excited-State Structure Similar to Cyclometalated Iridium(III) Compounds

Many organometallic iridium(III) complexes have photoactive excited states with mixed metal-to-ligand and intraligand charge transfer (MLCT/ILCT) character, which form the basis for numerous applications in photophysics and photochemistry. Cobalt(III) complexes with analogous MLCT excited-state properties seem to be unknown yet, despite the fact that iridium(III) and cobalt(III) can adopt identical low-spin d6 valence electron configurations due to their close chemical relationship. Using a rigid tridentate chelate ligand (LCNC), in which a central amido π-donor is flanked by two σ-donating N-heterocyclic carbene subunits, we obtained a robust homoleptic complex [Co(LCNC)2](PF6), featuring a photoactive excited state with substantial MLCT character. Compared to the vast majority of isoelectronic iron(II) complexes, the MLCT state of [Co(LCNC)2](PF6) is long-lived because it does not deactivate as efficiently into lower-lying metal-centered excited states; furthermore, it engages directly in photoinduced electron transfer reactions. The comparison with [Fe(LCNC)2](PF6), as well as structural, electrochemical, and UV–vis transient absorption studies, provides insight into new ligand design principles for first-row transition-metal complexes with photophysical and photochemical properties reminiscent of those known from the platinum group metals.

Scheme S1. Synthesis of the ligand precursor.
Synthesis of compound 2. Compound 2 was synthesized following a modified literature procedure. 1 3,6-Di-tertbutylcarbazole, 1 (2.000 g, 7.158 mmol, 1.0 equiv.) was dissolved in 100 mL glacial acetic acid in a 250 mL round bottom flask. To this mixture, Br 2 (2.402 g, 15.032 mmol, 2.1 equiv.) in 10 mL acetic acid was added dropwise, and the resulting mixture was stirred at 25 C for 3 h. Thereafter, the reaction mixture was added to 500 mL cold water in a beaker. The resulting off-white solid was filtered off and washed with water (3 × 100 mL). Then the off-white solid was dissolved in 100 mL dichloromethane and washed with saturated aqueous NaHCO 3 solution (3 × 50 mL). The organic phase was dried over anhydrous Na 2 SO 4 , and the solvent was evaporated to dryness to obtain compound 2 as an off-white solid (3.000 g, 6.862 mmol, 96%). 1  Synthesis of compound 3. Compound 3 was synthesized following a modified literature procedure. 3 A 100 mL Schlenk flask containing a stir bar was charged with compound 2 (1.000 g, 2.287 mmol, 1.0 equiv.), imidazole (0.934 g, 13.722 mmol, 6.0 equiv.), K 2 CO 3 (1.90 g, 13.722 mmol, 6.0 equiv.), CuSO 4 ·5H 2 O (114 mg, 0.457mmol, 0.2 equiv.). The flask was evacuated and backfilled with N 2 (3 times) and then 5 mL anhydrous dimethylsulfoxide was added. The resulting mixture was refluxed at 140 °C for 48 h. After cooling the reaction mixture to ambient temperature, 10 mL saturated aqueous NH 4 Cl solution was added and stirred for 10 min, then the mixture was diluted with 100 mL water. The mixture was then extracted with dichloromethane (3 × 50 mL). The combined organic phases were washed with water (3 × 50 mL), dried over anhydrous Na 2 SO 4 and filtered. The solvent was removed in vacuo to yield compound 3 (908 mg, 2.206 mmol, 96%) as an off-white solid. 1

S5
Synthesis of [Fe(L CNC ) 2 ](PF 6 ). To a 100 mL dry Schlenk flask was added (H 3 L CNC )(PF 6 ) 2 (50 mg, 0.068 mmol, 1 equiv.) and KO t Bu (25 mg, 0.225 mmol, 3.3 equiv.), and the flask was evacuated and backfilled with N 2 . To this mixture, dry THF (5 ml) was added under N 2 at 0 C, and the resulting suspension was stirred for 30 min at ambient temperature. FeBr 2 (7.3 mg, 0.034 mmol, 0.5 equiv.) was added to another dry flask in the glove box and 5 mL dry THF was added to it. Then the FeBr 2 solution was added to the first flask under N 2 and the resulting mixture was heated to 55 C for 18 h. After cooling the reaction mixture to ambient temperature, it was stirred in air for 1 h. Then the mixture was dried and 20 mL water was added. Thereafter, the resulting suspension was extracted with 20 mL dichloromethane, dried and washed with diethyl ether.
The resulting product was re-dissolved in dichloromethane and filtered through a pad of                   6 . The UV-vis absorption spectrum recorded prior to applying the potential served as a baseline. The four difference spectra (red, blue, green, ocher) were recorded after different durations S17 of applied potential, corresponding to increasing amounts of Co IV oxidation product. Signals marked by asterisks are artefacts caused by the instrument and the presence of air.
As discussed in the main paper, the first oxidation process of [Co(L CNC ) 2 ] + is attributable to a metal-centered redox event, in which cobalt(III) is oxidized to cobalt(IV), analogously to a recently reported hexacarbene cobalt(III) complex. 7 Due to the combination of two strongly -donating amido donors and four strongly -donating (weakly -accepting) NHC donors in our cobalt(III) complex, that metal-centered oxidation process occurs at even less positive potential than in the previously studied hexacarbene cobalt(III) complex. For the cobalt(III) analog of the iron(II) complex in Figure 1c, 8 the situation is very different. In this case, there are two amido donors and four only weakly -donating (as well as strongly -accepting) phenanthrine-quinoline-amido donors, leading to a situation in which the first oxidation process is not metal-centered but rather ligand-based. The resulting aminyl radical formed upon one-electron oxidation of that cobalt(III) phenanthridinequinoline-amido complex at 0.31 V vs Fc +/0 manifested by an UV-vis absorption band extending from 1000 to 2300 nm. 8 For our [Co(L CNC ) 2 ] + complex, no such NIR absorption is observable upon electro-chemical oxidation at 0.46 V vs Fc +/0 ( Figure S18b), in line with the view that the first oxidation wave is metal-centered in our complex.
Another important difference between our [Co(L CNC ) 2 ] + complex and the previously studied cobalt(III) phenanthrinequinoline-amido compound is that in our new complex the first reduction is ligand-based whereas in the previously studied compound this is a metal-centered reduction of cobalt(III) to cobalt(II). The fact that in [Co(L CNC ) 2 ] + the first oxidation is metal-centered and the first reduction is ligand-based leads to an energetically lowest excited state with substantial MLCT character.

Computational calculations:
Quantum chemical calculations were performed to investigate the electronic transitions of the investigated cobalt(III) complex by using density functional theory (DFT). The singlet ground state geometries were optimized using the Gaussian 09 software 9 with Kohn-Sham orbital DFT in combination with the B3LXP functional 10 and the def2SVP basis set [11][12] for all atoms in vacuum to obtain full geometrical freedom. Vibrational frequency analysis confirmed the local minima of the optimized geometries by obtaining only positive eigenvalues. The frontier orbitals of the electronic ground state were obtained by single point calculations with the same level of theory, using the optimized vacuum ground-state geometry.
Time-dependent DFT calculations were used to compute the first sixty vertical transitions (1-60 in Table S3) by single point calculations on the vacuum ground-state geometry in acetonitrile, employing the polarizable continuum model (PCM) and the same level of theory. All 60 vertical transitions are shown in Figure S30 with an additional (arbitrary) shift of 2140 cm -1 to the red region of the spectrum to better match with the experimental UV-Vis spectrum. Such arbitrary shifts of computed relative to experimental spectra seem to be more or less common practice. 13 Furthermore, it seems that discrepancies between computed and experimental spectra are not uncommon for charge transfer transitions. [14][15][16] In addition, for all 60 vertical transitions differential electron densities maps are visualized for the largest contribution of the corresponding transitions. By visual inspection, all transitions were assigned as summarized in Table S3.     Based on the data in Table S4, the HOMO has a metal contribution of 7%, whereas the two N amido atoms contribute with 22% to the HOMO. In the LUMO, the four weakly -accepting imidazolidene (NHC) units contribute with 18%. Based S31 on this data (illustrated by Figure 4d in the main paper) it seems that the MLCT character of the lowest energy transition is in fact limited. However, the experimental data clearly signals a metal-centered primary oxidation event and a ligandbased primary reduction process (Figure 3a, Figures S18a/b and associated discussion above). Aside from the cyclic voltammetry and spectro-electrochemistry data, femtosecond transient absorption spectroscopy supports the assignment of the lowest excited state as an electronic transition with substantial MLCT character ( Figure 6).