Photochemical nitrogenation of alkanes and arenes by a strongly luminescent osmium(VI) nitrido complex

The search for a highly active nitrido complex that can transfer its nitrogen atom to inert organic molecules remains a challenge to chemists. In this regard, the use of solar energy to generate a reactive nitrido species is an appealing strategy to solve this problem. Here we report the design of a strongly luminescent osmium(VI) nitrido compound, [OsVI(N)(NO2-L)(CN)3]− (NO2-OsN) with emission quantum yield (Φ) and life time (τ) of 3.0% and 0.48 μs, respectively in dichloromethane solution. Upon irradiation with visible light, this complex readily activates the aliphatic C-H bonds of various hydrocarbons, including alkanes. The excited state of NO2-OsN can undergo ring-nitrogenation of arenes, including benzene. Photophysical and computational studies suggest that the excited state of NO2-OsN arises from O^N ligand to Os ≡ N charge transfer transitions, and as a result it possesses [Os = N•] nitridyl character and is highly electrophilic. Activation of C-H bonds through visible light irradiation remains a challenge in attaining energy efficient organic transformations. Here the authors show an osmium(VI) nitrido complex which can perform nitrogenation of cyclic alkanes and arenes by acting as a strong electrophile in the excited state.

Apart from the above systems, the use of solar energy to generate highly reactive nitrido complex in the excited state for the nitrogenation of organic substrates is an appealing approach. A number of d 2 nitrido complexes, such as those of Re V and Os VI , have long-lived emissive excited states [15][16][17][18][19][20][21] ; the emissions of these complexes were shown to originate mainly from metalcentered ligand field 3 [(d xy ) 1 (d π* ) 1 ] excited states. Although some of these complexes are strong one-electron oxidants in their excited states, nitrogen atom transfer reactions of these complexes, especially towards inert organic substrates, have not been demonstrated.
It readily undergoes aliphatic C-H bond activation of hydrocarbons and ring-nitrogenation of arenes.

Results
Synthesis and characterization of NO 2 -OsN. The synthesis of NO 2 -OsN is summarized in Fig. 1.
NO 2 -OsN was isolated as light yellow PPh 4 + salt in 35% yield. Consistent with its (d xy ) 2 ground state electronic configuration, this compound is diamagnetic (μ eff~0 μ B ), as evidenced by the sharp resonances in the normal range in the 1 H nuclear magnetic resonance (NMR) spectrum ( Supplementary Fig. 1). The electrospray ionization/mass spectrometry (ESI/MS) of NO 2 -OsN in MeOH (−ve mode) shows the parent anion [M] − at m/z 539, which is shifted to m/z 540 in the 15 N-labeled complex NO 2 -Os 15 N ( Supplementary Fig. 2). In the infrared (IR) spectrum, the v(Os≡N) stretch is found at 1074 cm −1 , which is shifted to 1046 cm −1 upon 15 N labeling.
The molecular structure of NO 2 -OsN has been determined by X-ray crystallography (Fig. 2a Photophysical properties of OsN and NO 2 -OsN. The two Os(VI) nitrido complexes show intense ligand-centered π→π* transitions of the bidentate O^N ligand (L/NO 2 -L) at 230-310 nm with molar extinction coefficients (ε) of the order of 10 4 dm 3 mol −1 cm −1 (Fig. 3a). For OsN, there is a moderately intense broad absorption band at 357 nm with molar absorptivity (ɛ)~1.2 × 10 4 dm 3 mol −1 cm −1 and a shoulder at 378 nm tailing down to about 500 nm. In NO 2 -OsN, the corresponding absorption band is more intense (~3.2 × 10 4 dm 3 mol −1 cm −1 ) and is slightly blue-shifted with λ max at 365 nm. The lowest-energy absorption band in the two complexes is tentatively assigned to O^N ligand to Os≡N charge transfer (LML ′CT) transitions, probably mixed with metal-centered d-d transitions [(d xy ) 2 →(d xy ) l (d π *) 1 ].
The LML′CT assignment is further supported by resonance Raman (RR) spectroscopic study of NO 2 -OsN (Fig. 3c), which shows enhancements of Raman signals corresponding to the Os≡N stretching mode (assigned based on the normal Raman spectra of the unlabeled and 15 N-labeled complexes) and C=C, C=N, NO 2 stretches of the bidentate N^O ligand upon 355 nm excitation. This is indicative of the involvement of both the bidentate N^O ligand and Os≡N in the electronic transition of the lowest-energy absorption band and is consistent with LML′ CT [π(N^O)→dπ*(Os≡N)] character. This is further supported by the results of the density functional theory (DFT)/timedependent DFT (TDDFT) calculations.     Table 4; the corresponding simulated ultraviolet (UV) spectra ( Supplementary Fig. 5) computed by TDDFT calculations are in good agreement with the experimental absorption spectra of these complexes. The S 1 /S 2 states for the two complexes are predominantly derived from the HOMO→-LUMO/LUMO+1 transitions and therefore can be considered as LML′CT states. For S 3 /S 4 states, they are derived from HOMO−1 [d xy (Os)]→LUMO/LUMO+1, which can be considered as metalcentered ligand field dd states. By introducing the strong electronwithdrawing −NO 2 group, the energy level of the S 1 -S 4 states are increased. Such trend is consistent with the absorption data. Since both the geometry optimizations and TDDFT calculations were performed in the gas phase, we have also simulated the absorption of OsN using the linear response polarizable continuum model with dichloromethane matrix to examine the effect of the solvent. As shown in Supplementary Fig. 6, the simulated spectra did not show obvious improvement.
Despite close resemblance of these two complexes, they exhibit very different emission properties (Fig. 3b). In the solid state, OsN exhibits a weak orange emission at 611 nm (Φ < 0.1%, τ = 0.14 μs), while NO 2 -OsN exhibits a much stronger and longerlived emission at 591 nm (Φ = 11.7%, τ = 1.90 μs). With submicrosecond lifetimes, these emissions are attributed to phosphorescence derived from LML′CT excited state. The blue-shifted emission observed in NO 2 -OsN is in agreement with its LML′CT character, due to the stabilization of the π orbitals of NO 2 -L by the strong electron-withdrawing and π-conjugating NO 2 group. In CH 2 Cl 2 solution, OsN shows a very weak emission at 620 nm (Φ < 0.001%), while NO 2 -OsN also displays a much strong emission at 594 nm (Φ = 3.0%, τ = 0.48 μs). The enhanced phosphorescence properties of NO 2 -OsN over that of OsN is possibly due to the strong electron-withdrawing effect of the -NO 2 group that could effectively stabilize the d π (Os) orbitals and raise the energy of the ligand field excited state. In 77 K EtOH-MeOH (4:1, v/v) glassy medium, the emissions of OsN and NO 2 -OsN remain structureless with maxima at 594 nm (τ = 1.38 μs) and 577 nm (τ = 5.90 μs), respectively (Supplementary Fig. 7 and Table 5), which are blue-shifted relative to those in CH 2 Cl 2 solution. This is due to the rigidochromic effect in the lowtemperature glassy medium, typically observed in phosphorescence of a charge transfer state.
Nanosecond transient absorption spectroscopy was carried out to provide insights into their emissive excited states. As shown in Supplementary Fig. 8, OsN exhibits no absorption features after 355 nm nanosecond laser excitation, while NO 2 -OsN shows two absorption features at ca. 300-340 nm and 400-550 nm, with strong ground state bleaching at 340-400 nm in the transient absorption difference spectra. The observation of transient absorption feature in the visible region for NO 2 -OsN is suggestive of the radical character involving the conjugated ligands in its emissive excited state, which is supportive of the LML′CT excited state origin.
Cyclic voltammetry of OsN and NO 2 -OsN. The redox properties of these two complexes were studied by cyclic voltammetry (CV). The CV of OsN shows an irreversible oxidation wave at E pa = 1.44 V and an irreversible reduction wave at E pa = −1.17 V vs. Saturated Calomel Electrode (SCE), which are tentatively assigned as the metal-centered Os VII/VI and Os VI/V process, respectively (Supplementary Fig. 9). Similar irreversible oxidation and reduction waves are also found for NO 2 -OsN at E pa = 1.88 V and −0.99 V, respectively. From the estimated E 0−0 emission (OsN 2.05 eV, NO 2 -OsN 2.15 eV) and electrochemical data (E pc : OsN −0.93 V, NO 2 -OsN −0.75 V vs. Normal Hydrogen Electrode (NHE)), the excited state redox potentials are estimated to be 1.13 and 1.40 V for OsN and NO 2 -OsN, respectively (Supplementary Note 1) 15 . In order to support these assignments, further calculations have been conducted regarding the redox properties of the OsN and NO 2 -OsN. As shown in Supplementary  26,27 , which would make it highly electrophilic. Silica chromatography of the product solutions afforded two osmium products in each case, an osmium(IV) iminato and an osmium(III) ammine complex, as illustrated in Fig. 5.
Molecular structures. The molecular structure of the osmium (III) ammine complex [PPh 4 ]1 was determined by X-ray crystallography ( Fig. 2b and Supplementary Table 1). The distorted octahedral arrangement of the parent nitrido complex is retained in the ammine product; the three cyano ligands are in merconfiguration. The Os-N (NH3) (Os1-N5) bond length is 2.110(3) Å, typical of a Os-N single bond.
The molecular structures of the osmium(IV) iminato complexes 2, 3, 4 and 5 have also been determined by X-ray crystallography. As shown in Fig. 2c-f, the distorted octahedral arrangement of the ligands around the Os center in the parent ESI/MS of photochemical reactions. The progress of the photochemical reactions was followed by ESI/MS. The ESI/MS of NO 2 -OsN in CH 2 Cl 2 (−ve mode) shows a predominant parent peak at m/z 539. The ESI/MS for the photochemical reaction mixture of NO 2 -OsN with DHA (Fig. 6) shows the appearance of a peak at m/z 719, which is assigned to the amido species [NO 2 -Os-(NH-DHA (−H) )] − (see Fig. 9a). This peak is gradually shifted to m/z 717, which is consistent with the iminato species 2, [NO 2 -Os-(N = DHA (−2H) )] − . These results suggest that the initially formed amido species undergoes oxidative dehydrogenation to give an iminato complex. After 12 h, the peak at m/z 717 becomes predominant, while the peak at m/z 539 is further decreased and the isotopic distribution shows that it is a mixture of parent (m/z 539) and the ammine complex 1 (m/z 542). Similar observations were also found for xanthene, ethylbenzene and cyclooctane .
UV/Vis absorption and emission spectroscopy of photochemical reactions. The photochemical reactions of NO 2 -OsN with excess XAN, DHA and EB were also monitored by UV/Vis spectroscopy (Fig. 7). In the case of DHA, the absorbance at 374 nm due to NO 2 -OsN decreases with time, while the absorbance from around 410-600 nm increases with time, consistent with the formation of NO 2 -Os=(DHA (−2H) ) and NO 2 -OsNH 3 with λ max at 444 nm and 459, 551 nm, respectively. Similar UV/Vis spectral changes were observed for the photoreaction of NO 2 -OsN with XAN or EB ( Supplementary Fig. 21).
The emission spectrum of NO 2 -OsN is greatly interfered by the emission of the aromatic substrates. On the other hand, the photochemical reaction of NO 2 -OsN with cyclooctane in CH 2 Cl 2 could be followed by both UV/Vis absorption and emission spectroscopy. As shown in Fig. 7b, c, there is a much larger change in emission intensity than in absorbance of the reaction mixture.
Kinetic isotope effects. The kinetic isotope effects (KIE) for the photochemical reaction of NO 2 -OsN with hydrocarbons were determined by ESI/MS. KIE was found to be~4.1 from competition experiments using an equimolar mixture of cyclohexane (c-C 6 H 12 ) and d 12 -cyclohexane (c-C 6 D 12 ) as substrate (Fig. 8). The KIE was estimated from the ratios of the most intense peaks for the two osmium(IV) iminato products, assuming that the spraying and ionization efficiencies of the two ions are similar. Similarly, the KIE for ethylbenzene was found to be~7.7 using an equimolar mixture of ethylbenzene (C 6 H 5 CH 2 CH 3 ) and d 10ethylbenzene (C 6 D 5 CD 2 CD 3 ) as substrate. These large KIE values indicate that C-H bond cleavage occurs in the rate-determining step.
Photochemical reaction mechanism. Based on all the experimental results, a mechanism for C-H bond activation by NO 2 -OsN* is proposed, using DHA as an example (Fig. 9a). NO 2 -OsN* first abstracts a H-atom from DHA; this is followed by a N-rebound process to give an Os IV amido species. These processes are reminiscent of C-H bond activation by metal-oxo species [33][34][35] . The Os IV amido species undergoes further H-atom abstractions by NO 2 -OsN* to generate the Os IV iminato product 3; this is accompanied by the formation of NO 2 -OsNH 3 (1). For substrates with weak C-H bonds, such as XAN and DHA, the amido species should be formed more rapidly than its subsequent dehydrogenation, and hence both the amido and the iminato species can be observed by ESI/MS. On the other hand, for ethylbenzene, cyclooctane and cyclohexane, which have stronger C-H bonds, the amido species should be formed much more slowly, hence only the iminato species can be observed.  [28][29]. The X-ray crystal structure of [PPh 4 ]8 shows Os-N5 bond distance of 1.758(11) Å (Fig. 2h). The C=N and C=O bond lengths in the newly formed ligand are 1.306(18) and 1.231(19) Å, respectively.

Discussion
We have designed a highly luminescent osmium(VI) nitrido complex (NO 2 -OsN) with long-lived excited state. Experimental results and DFT calculations indicate that the emissive excited state of this complex exhibits LML′CT [π(N^O)→dπ*(Os≡N)] character, which is different from reported luminescent osmium nitrido complexes with predominant d-d transition character. In accordance with the LML′CT character of its emission, NO 2 -OsN* possess [Os=N • ] nitridyl character, which to our knowledge is the most oxidizing/electrophilic nitrido species reported to date. NO 2 -OsN* readily undergoes C-H bond activation with alkanes and nitrogenation of arenes, including benzene. In the reaction with alkanes and alkylaromatics, the proposed mechanism involves initial H-atom abstraction from the organic substrate by NO 2 -OsN*, followed by a N-rebound process; which is reminiscent of C-H bond activation by metaloxo species. On the other hand, reaction with arenes occur by