Catalytic Nitrogen Fixation via Direct Cleavage of Nitrogen Nitrogen Triple Bond of Molecular Dinitrogen under Ambient Reaction Conditions

Transcription

1 Catalytic Nitrogen Fixation via Direct Cleavage of Nitrogen Nitrogen Triple Bond of Molecular Dinitrogen under Ambient Reaction Conditions Kazuya Arashiba, Aya Eizawa, Hiromasa Tanaka, Kazunari Nakajima, Kazunari Yoshizawa,* and Yoshiaki Nishibayashi* 2017 The Chemical Society of Japan

2 General Methods. All manipulations were carried out under an atmosphere of nitrogen or argon by using standard Schlenk techniques or glovebox techniques unless otherwise stated. Toluene was distilled from dark-blue Na/benzophenone ketyl solution, degassed, and stored over molecular sieves 4Å in a nitrogen-filled glove box. Other solvents were dried by general methods, and degassed before use. C 5 NH 3 (CH 2 P t Bu 2 ) 2 (PNP), 1 [MoX 3 (thf) 3 ] (X = Cl 2, Br 3, and I 3 ), [MoCl 3 (PNP)] (1b) 4, [Mo(N)Cl(PNP)] (2b) 5, [Mo(N 2 ) 2 (PNP)] 2 ( -N 2 ) (3) 4, [MoCl 3 ( tbu PPP)] (4b; tbu PPP = PhP(CH 2 CH 2 P t Bu 2 ) 2 ) 6, [ColH]OTf (Col = 2,4,6-trimethylpyridine) 6, CrCp* 2 (Cp* = 5 -C 5 Me 5 ), 7 and CoCp* 2 8 were prepared according to the literature methods. Other reagents were purchased commercially and used as received. 1 H NMR (270 or 400 MHz), 31 P{ 1 H} NMR (109 or 162 MHz), and 15 N{ 1 H} NMR (27 or 41 MHz) spectra were recorded on a JEOL Excalibur 270 or ECS400 spectrometer in suitable solvents, and spectra were referenced to residual solvent ( 1 H) or external standard ( 31 P{ 1 H}: 85% H 3 PO 4, 15 N{ 1 H}: CH 3 NO 2 ). Magnetic susceptibility was measured in CD 2 Cl 2 using the Evans method 9. IR spectra were recorded on a JASCO FT/IR 4100 Fourier Transform infrared spectrometer. Absorption spectra were recorded on a Shimadzu MultiSpec Mass spectra were recorded on a JEOL Accu TOF JMS-T100LP. Evolved dihydrogen was quantified by a gas chromatography using a Shimadzu GC-8A with a TCD detector and a SHINCARBON ST (6 m x 3 mm). Elemental analyses were performed at Microanalytical Center of The University of Tokyo. Preparation of [MoI 3 (PNP)] (1a). A mixture of [MoI 3 (thf) 3 ] (69.5 mg, 0.10 mmol) and PNP (42.3 mg, 0.11 mmol) in THF (5 ml) was stirred at 50 C for 18 h. The resultant reddish brown solution was filtered through Celite, and the filter cake was washed with THF (2 ml x 5). The combined filtrate was concentrated in vacuo, and the residue was washed with Et 2 O (1 ml x 3) to afford 1a as a brown solid (55.7 mg, mmol, 64% yield). Magnetic susceptibility (Evans S2

3 method): μ eff = 3.5 μ B in CD 2 Cl 2 at 294 K. Anal Calcd for C 23 H 43 I 3 MoNP 2 : C, 31.67; H, 4.97; N, Found: C, 31.78; H, 4.96; N, Preparation of [MoBr 3 (PNP)] (1c). A mixture of [MoBr 3 (thf) 3 ] (55.8 mg, 0.10 mmol) and PNP (43.7 mg, 0.11 mmol) in THF (5 ml) was stirred at 50 C for 18 h. The resultant orangebrown solution was filtered through Celite, and the filter cake was washed with THF (1 ml x 5). The combined filtrate was concentrated in vacuo, and the residue was washed with Et 2 O (1 ml x 3) to afford 1c as an orange solid (54.7 mg, mmol, 75% yield). Magnetic susceptibility (Evans method): μ eff = 3.6 μ B in CD 2 Cl 2 at 294 K. Anal Calcd for C 23 H 43 Br 3 MoNP 2 : C, 37.78; H, 5.93; N, Found: C, 37.48; H, 5.63; N, Preparation of [Mo(N)I(PNP)] (2a). A mixture of [MoCl 3 (thf) 3 ] (126 mg, 0.30 mmol) and Me 3 SiN 3 (45 μl, 0.34 mmol) in THF (9 ml) was stirred at 50 C for 1 h. The resultant reddish brown solution was concentrated under reduced pressure. To the residue were added PNP (119 mg, 0.30 mmol) and THF (15 ml), and then the mixture was stirred at 50 C for 4 h. After cooling to room temperature, to the brown cloudy solution were added KC 8 (42.2 mg, 0.31 mmol) and NaI (136 mg, 0.91 mmol) and stirred at room temperature for 20 h. Volatiles were removed in vacuo, and benzene (8 ml) was added to the residue. The benzene solution was filtered through Celite, and the filter cake was washed with benzene (2 ml x 5). After the combined filtrate was concentrated to about 8 ml, slow addition of hexane (15 ml) afforded 2a as brown crystals, which were collected by filtration, washed with hexane (1 ml x 3), and dried in vacuo (34.1 mg, mmol, 18% yield). 31 P{ 1 H} NMR (THF-d 8 ): 98.9 (s). 1 H NMR (THF-d 8 ): (m, ArH, 1H), 7.60 (d, J = 7.3 Hz, ArH, 2H), 3.82 (br s, CH 2 P, 4H), 1.56 (pseudo t, J = 6.5 Hz, P t Bu 2, 18H), 1.23 (pseudo t, J = 6.5 Hz, P t Bu 2, 18H). IR (KBr): 1033 cm 1 ( MoN ). ESI-TOF-MS (THF): 634 (m/z). Anal Calcd for C 23 H 43 IMoN 2 P 2 : C, 43.68; H, 6.85; N, Found: C, 43.50; H, 6.79; N, S3

4 Preparation of [Mo( 15 N)I(PNP)] (2a (50% 15 N labeled)). A mixture of NaN 3 (1-15 N) (39.2 mg, 0.59 mmol) and Me 3 SiCl (150 L, 1.19 mmol) in THF (3 ml) was stirred at room temperature for 18 h. The resultant white suspension was filtered through Celite and the filter cake was washed with THF (2 ml x 3). To the filtrate was added [MoCl 3 (thf) 3 ] (168 mg, 0.40 mmol), and then the mixture was stirred at 50 C for 1 h. The resultant dark reddish brown solution was concentrated under reduced pressure. To the residue were added PNP (158 mg, 0.40 mmol) and THF (12 ml), and then the mixture was stirred at 50 C for 4 h. After cooling to room temperature, to the reaction mixture were added KC 8 (55.7 mg, 0.41 mmol) and NaI (301 mg, 2.0 mmol), and the mixture was stirred at room temperature for 20 h. The solution was concentrated under reduced pressure. To the residue was added benzene (5 ml), and the solution was filtered through Celite and the filter cake was washed with benzene (2 ml x 10). The combined filtrate was concentrated to ca. 15 ml, and slow addition of hexane (5 ml) afforded 3-15 N as brown crystals (42.1 mg, mmol, 17% yield). 15N{1H} NMR (THF-d 8 ): (s, Mo 15 N). IR (KBr, cm 1): 1033 ( Mo14N ), 1005 ( Mo15N ). Anal Calcd for C 23 H 43 IMoN N 0.5 P 2 : C, 43.65; H, 6.85; N, Found: C, 43.55; H, 7.12; N, Reaction of 1a with 3.3 equiv of CoCp* 2 under N 2. A mixture of 1a (8.9 mg, mmol) and CoCp* 2 (11.2 mg, mmol) in toluene (1 ml) was stirred at room temperature for 2 h under N 2 (1 atm). After volatiles were removed in vacuo, the residue was washed with pentane (1 ml x 3). The yields of [Mo(N)I(PNP)] (2a) in residual solid and [Mo(N 2 ) 2 (PNP)] 2 ( -N 2 ) (3) in washings were determined by 1 H NMR in THF-d 8 using pentamethylbenzene as an internal standard (75% for 2a and 4% NMR yield for 3, respectively). Reaction of 1a with 3.3 equiv of CoCp* 2 under 15 N 2. A mixture of [MoI 3 (PNP)] (17.0 mg, mmol) and CoCp* 2 (21.4 mg, mmol) in toluene (2 ml) in 20 ml Schlenk flask fitted with septum was evacuated and then exposed to 15 N 2. To the mixture under 1 atm of 15 N 2 was added toluene (under Ar) by syringe from the septum, and then the mixture was stirred at room S4

5 temperature for 13 h. After volatiles were removed in vacuo, the residue was washed with pentane (1 ml x 3). IR (KBr, cm 1): 1005 ( Mo15N ). ESI-TOF-MS (THF): 635 (m/z). Preparation of [Mo(N)Br(PNP)] (2c). A mixture of [Mo(N)Cl(PNP)] (55.5 mg, 0.10 mmol) and NaBr (105 mg, 1.02 mmol) in THF (5 ml) was stirred at room temperature for 40 h. The resultant brown slurry was concentrated under reduced pressure, and then benzene (8 ml) was added to the residue. The benzene solution was filtered through Celite, and the filter cake was washed with benzene (2 ml x 4). Slow addition of hexane (15 ml) to the filtrate afforded 2c as brown crystals, which were collected by filtration, washed with hexane (1 ml x 3), and dried in vacuo (32.0 mg, mmol, 54% yield). 31 P{ 1 H} NMR (THF-d 8 ): 99.2 (s). 1 H NMR (THF-d 8 ): (m, ArH, 3H), (m, CH 2 P, 4H), 1.53 (pseudo t, J = 6.5 Hz, P t Bu 2, 18H), 1.21 (pseudo t, J = 6.5 Hz, P t Bu 2, 18H). IR (KBr): 1031 cm 1 ( MoN ). Anal Calcd for C 23 H 43 BrMoN 2 P 2 : C, 47.19; H, 7.40; N, Found: C, 46.91; H, 7.63; N, Preparation of [MoI 3 ( tbu PPP)] (4a). A mixture of [MoI 3 (thf) 3 ] (69.0 mg, 0.10 mmol) and tbu PPP (0.1 M THF solution, 1.1 ml, 0.11 mmol) in THF (4 ml) was stirred at 50 C for 18 h. The resultant brown solution was filtered through Celite, and the filter cake was washed with THF (1 ml x 3). The combined filtrate was concentrated in vacuo, and the residue was washed with Et 2 O (1 ml x 3) to afford 4a as a brown solid (70.5 mg, mmol, 76% yield). Anal Calcd for C 26 H 49 I 3 MoP 3 : C, 33.53; H, Found: C, 33.90; H, Catalytic reduction of dinitrogen into ammonia by molybdenum complexes. Typical experimental procedure. In a nitrogen-filled glove box, to a mixture of 1a (8.9 mg, mmol) and [ColH]OTf (130 mg, 0.48 mmol) in a 50 ml Schlenk flask was added toluene (1 ml). Then a solution of CoCp* 2 (118 mg, 0.36 mmol) in toluene (4 ml) was slowly added to the stirred suspension in the Schlenk flask with a syringe pump over a period of 1 h. After the addition of CoCp* 2, the mixture was further stirred at room temperature for 19 h. The amount of dihydrogen of S5

6 the catalytic reaction was determined by gas chromatography (GC). Potassium hydroxide aqueous solution (30 wt%; 5 ml) was added to the reaction mixture. The mixture was evaporated under reduced pressure, and the distillate was trapped in dilute H 2 SO 4 solution (0.5 M, 10 ml). amount of ammonia present in the H 2 SO 4 solution was determined by the indophenol method. 10 The No hydrazine was detected by the p-(dimethylamino)benzaldehyde method. 11 Table S1. Solvents and reaction conditions. Catalytic reactions were performed in various solvents and conditions using typical experimental procedure. S6

7 Table S2. Reductants. Catalytic reactions were performed in the presence of various reductants using typical experimental procedure. Table S3. Proton sources. Catalytic reactions were performed in the presence of various proton sources using typical experimental procedure. S7

8 Table S4. Catalysts. Catalytic reactions were performed in the presence of various catalysts using typical experimental procedure. S8

9 Catalytic reduction of dinitrogen into ammonia under 15 N 2. In a 50 ml Schlenk were placed 1a (8.8 mg, mmol) and [ColH]OTf (131 mg, 0.48 mmol). After the mixture was cooled at 196 C, toluene (1 ml) was added to the mixture by trap-to-trap distillation. The Schlenk flask was warmed to room temperature, and the mixture was exposed to 15 N 2. Then a solution of CoCp* 2 (119 mg, 0.36 mmol) in toluene (4 ml) was slowly added to the stirred suspension in the Schlenk flask with a syringe pump over a period of 1 h. After the addition of CoCp* 2, the mixture was further stirred at room temperature for 19 h. KO t Bu (4 mmol) in MeOH (5 ml) was added to the resultant yellow-brown suspension, and the mixture was stirred at room temperature for 20 min. The volatile components in the mixture were collected by trap-to-trap distillation to the Schlenk flask to which was added a solution of 2 M HCl in Et 2 O (5 ml, 10 mmol). The obtained colorless solution was dried up in vacuo to afford a colorless solid which contains 15 NH 4 Cl and [ColH]Cl. The yield of 15 NH 4 Cl was determined by 1 H NMR in DMSO-d 6 using 1,1,2,2-tetrachloroethane as an internal standard (65% NMR yield based on CoCp* 2, 7.7 equiv/mo). 1 H NMR spectrum of this solid is shown in Fig. S1. In this mixture, no molybdenum species presented. 1 H NMR (DMSOd 6 ): 7.48 (d, J NH = 71.0 Hz, 15 NH 4 Cl). 15 N{ 1 H} NMR (DMSO-d 6 ) : (s, 15 NH 4 Cl). S9

10 Fig. S1. 1 H NMR (DMSO-d 6 ) spectra of (a) catalytic reaction by 1a under 15 N 2, (b) authentic sample of the mixture of 15 NH 4 Cl and [ColH]Cl, and (c) authentic sample of the mixture of 14 NH 4 Cl and [ColH]Cl. S10

11 Catalytic reaction using larger amounts of CoCp* 2 and [ColH]OTf. A typical procedure is as follows. In a 50 ml Schlenk flask were placed 1a (0.002 mmol) and [ColH]OTf (480 equiv/mo). Toluene (1 ml) was added to the mixture under N 2 (1 atm), and then a solution of CoCp* 2 (360 equiv/mo) in toluene (4 ml) was slowly added to the stirred mixture in the Schlenk flask with a syringe pump over a period of 1 h. After addition of CoCp* 2 (1st), [ColH]OTf (480 equiv/mo) was added to the stirred mixture, and then a solution of CoCp* 2 (360 equiv/mo) in toluene (4.0 ml) was slowly added with a syringe pump over a period of 1 h. After the addition of CoCp* 2 (2nd), the mixture was further stirred at room temperature for 18 h. These results for the various catalyst loading (0.001 or mmol) of 1a or 2a and drop time of a toluene solution of CoCp* 2 (2 to 12 h; total reaction time (20 h)) are shown in Table S5. Table S5. The amounts of ammonia and molecular dihydrogen catalyzed by 1a using larger amounts of CoCp* 2 and [ColH]OTf. S11

12 Time profiles of the formation of ammonia and molecular dihydrogen. A typical procedure is as follows. In a 50 ml Schlenk flask were placed 1a ( mmol), [ColH]OTf (144 equiv/mo), and toluene (1 ml) under N 2 (1 atm). A solution of CoCp* 2 (108 equiv/mo) in toluene (4 ml) was slowly added to the stirred mixture in the Schlenk flask with a syringe pump over a period of 1 h, and then the mixture was further stirred at room temperature for 19 h. After the indicated time (0.33 h, 0.67 h, 1 h, 2 h, and 20 h), the amount of molecular dihydrogen in the catalytic reaction was determined by GC analysis, and then 30wt% potassium hydroxide aqueous solution (5 ml) was added to the reaction mixture. The mixture was evaporated under reduced pressure, and the distillate was trapped in dilute H 2 SO 4 solution (0.5 M, 10 ml). The amount of ammonia present in the H 2 SO 4 solution was determined by the indophenol method. 10 The results are summarized in Tables S6 S9. Table S6. The amounts of ammonia and molecular dihydrogen catalyzed by 1a. S12

13 Table S7. The amounts of ammonia and molecular dihydrogen catalyzed by 1b. Table S8. The amounts of ammonia and molecular dihydrogen catalyzed by 1c. S13

14 Table S9. The amounts of ammonia and molecular dihydrogen catalyzed by 3. Catalytic reaction using by 3 in the presence of NaX. Catalytic reactions by 3 were performed in the presence of NaX (1 equiv/mo in 3) using typical experimental procedure. The results are summarized in Table S10. Table S10. The amounts of ammonia and molecular dihydrogen catalyzed by 3 in the presence of NaX. S14

15 Kinetic study for the formation of ammonia. A typical procedure is as follows. To a mixture of catalyst (0.005 mmol, mmol, and mmol based on Mo atom), CoCp* 2 (0.36 mmol), and [ColH]OTf (0.48 mmol) in a 50 ml Schlenk flask was added toluene (5.0 ml) and the mixture was stirred at room temperature under N 2 (1 atm). After 10 min, 30wt% potassium hydroxide aqueous solution (5 ml) was added to the reaction mixture. The mixture was evaporated under reduced pressure, and the distillate was trapped in dilute H 2 SO 4 solution (0.5 M, 10 ml). The amount of ammonia present in the H 2 SO 4 solution was determined by the indophenol method. 10 The results are summarized in Table S11. Plots of (NH 3 ) against [Mo] are shown in Fig. S2. Table S11. Amount of ammonia for various concentration of molybdenum catalyst 1a (a) and 3 (b). S15

16 Fig. S2. Rate measurement for the formation of ammonia from molecular dinitrogen with 1a (red) and 3 (green) as catalyst at different initial Mo concentration. ESI-TOF-MS analysis. The reaction of 1a with excess amounts of CoCp* 2 and [ColH]OTf under N 2 is as follows. To a mixture of 1a (8.8 mg, mmol) and [ColH]OTf (130 mg, 0.48 mmol) in a 50 ml Schlenk flask was added toluene (1 ml). Then a solution of CoCp* 2 (119 mg, 0.36 mmol) in toluene (4 ml) was slowly added to the stirred suspension in the Schlenk flask with a syringe pump over a period of 1 h. After the addition of CoCp* 2, the mixture was further stirred at room temperature for 1 h. The resultant yellow-brown suspension was filtered, and the filtrate was concentrated in vacuo to afford a yellow-brown oily solid. The residue was washed with pentane (1 ml x 3) to give a yellow-brown solid. ESI-TOF-MS of the solid in THF showed ion peaks at m/z = 634, which were assigned as those of 2a (Fig. S3). S16

17 Fig. S3. ESI-TOF-MS of (a) after the catalytic reaction and (b) simulated spectrum of 2a. S17

18 Reaction of 2a with CoCp* 2 and [ColH]OTf under Ar. To a mixture of 2a (25.3 mg, mmol), CoCp* 2 (39.7 mg, 0.12 mmol, 3 equiv/mo), and [ColH]OTf (43.5 mg, 0.16 mmol, 4 equiv/mo) was added toluene (5 ml) under Ar atmosphere, and the mixture was stirred at room temperature for 2 h. The amount of ammonia was mmol (0.6 equiv/mo, 60% yield) determined by the indophenol method utilized procedure described previously 10. Reaction of 3 with I 2 under N 2. To a dark green solution of 3 (11.1 mg, mmol) in THF (1 ml) cooled to 78 C was added a solution I 2 (3.0 mg, mmol) in THF (1 ml) cooled to -78 C. The mixture was stirred at 78 C for 10 min, and then warmed gradually to room temperature with stirring. After 1 h of stirring at room temperature, the dark brown solution was dried up in vacuo. The yields of [Mo(N)I(PNP)] (2a) in residual solid was determined by 1 H NMR in THF-d 8 using pentamethylbenzene as an internal standard (0.008 mmol, 40% NMR yield based on 3). Electrochemistry. Cyclic voltammograms were recorded on an ALS/Chi model 610C electrochemical analyzer with platinum working electrode in THF containing 1 mm of sample and 0.1 M of [ n Bu 4 N]BF 4 as supporting electrolyte at a scan rate of 0.1 V/s at room temperature. All potentials were measured against an Ag 0/+ electrode and converted to the values vs ferrocene/ferrocenium (Fc 0/+ ). The cyclic voltammograms of 2a 2c are shown in Fig. S4. S18

19 Fig. S4. Cyclic Voltammograms of 2a (a), 2c (b), and 2b (c). S19

20 X-ray crystallography. Crystallographic data of 1a (CCDC ), 2a (CCDC ), and 2c (CCDC ) are summarized in Table S12. ORTEP drawings and selected bond lengths and angles of 1a, 2a, and 2c are shown in Fig. S5-Fig. S7. Diffraction data were collected at 100 C (for 1a and 2c) or 75 C (for 2a) on a Rigaku RAXIS RAPID imaging plate area detector with graphite-monochromated Mo K radiation ( = Å). Reflections were collected for the 2 range of 5 to 55. Intensity data were collected for Lorenz-polarization effects and for empirical absorption (REQAB). The structure solution and refinements were carried out by using the CrystalStructure crystallographic software package. 12 The positions of the non-hydrogen atoms were determined by direct methods (SIR for 1a and 2c, SHELX for 2a), and subsequent Fourier syntheses (DIRDIF ) and were refined F 2 o using all unique reflections by full-matrix least-squares with anisotropic thermal parameters. All hydrogen atoms were placed at the calculated positions with fixed isotropic parameter. S20

21 Table S12. X-ray crystallographic data for [MoI 3 (PNP)] (1a), [Mo(N)I(PNP)] (2a), and [Mo(N)Br(PNP)] (2c). 1a CH 2 Cl 2 2a 2c 1.5C 6 H 6 chemical formula C 24 H 45 Cl 2 I 3 MoNP 2 C 23 H 43 IMoN 2 P 2 C 23 H 43 BrMoN 2 P 2 formula weight crystal system monoclinic monoclinic monoclinic space group P2 1 /c P2 1 /n C2/c a, Å (3) (5) (14) b, Å (9) (7) (6) c, Å (4) (8) (8), deg , deg (8) (13) (11), deg V, Å (2) (2) (5) Z calcd, g cm F(000) , cm trans. factors range no. reflections measured no. unique reflections 7582 (R int = ) 6290 (R int = ) 7517 (R int = ) no. parameters refined R1 (I > 2 (I)) a wr2 (all data) b GOF (all data) c max diff peak / hole, e Å / / / 1.11 CCDC number a R1 = F o F c / F o. b wr2 = [ w(f 2 o F 2 c ) 2 / w(f 2 o ) 2 ] 1/2, w = 4F 2 o /[p (F 2 o )] [p = 2.8 (1a CH 2 Cl 2 ); p = 5.5 (2a); p = 4.3 (2c 1.5C 6 H 6 )]. c GOF = [ w(f 2 o F 2 c ) 2 /(N o N params )] 1/2. S21

22 Fig. S5. Molecular structure of [MoI 3 (PNP)] (1a). Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Mo1 N (3), Mo1 P (12), Mo1 P (11), Mo1 I (4), Mo1 I (4), Mo1 I (4), P1 Mo1 P (3). S22

23 Fig. S6. Molecular structure of [Mo(N)I(PNP)] (2a). Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Mo1 N (3), Mo1 N (3), Mo1 P (8), Mo1 P (9), Mo1 I (5), P1 Mo1 P (3), N1 Mo1 I (7). S23

24 Fig. S7. Molecular structure of [Mo(N)Br(PNP)] (2c). Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Mo1 N (4), Mo1 N (4), Mo1 P (14), Mo1 P (12), Mo1 Br (5), P1 Mo1 P (5), N1 Mo1 I (9). S24

25 Computational Details. DFT calculations were performed with the Gaussian 09 program (Rev. E01). 16 Geometry optimizations were carried out with the B3LYP functional with the Grimme s dispersion correction (B3LYP-D3) The SDD (Stuttgart/Dresden pseudopotentials) basis set 22,23 and 6-31G(d) basis set were employed for the Mo and I atoms and the other atoms, respectively. Optimized structures were confirmed to have the appropriate number of imaginary frequencies by vibrational analysis. For the N N bond cleavage of intermediate B yielding two molecules of 2a, an appropriate connection between a reactant complex (RC) and a product complex (PC) was confirmed by the intrinsic reaction coordinate (IRC) method The open-shell singlet state of PC was not optimized due to the high stability of the closed-shell singlet state. The optimized structures of RC, TS, and PC and Mulliken spin densities assigned to the Mo atoms and the bridging N 2 ligand are shown in Fig. S8. To obtain free energy profiles at K ( G 298 ), single-point energy calculations were performed at the optimized geometries by using the G(d,p) basis set instead of 6-31G(d). Solvation effects of toluene were taken into account by using the polarizable continuum model (PCM). 34 For the N N bond cleavage of [MoI 2 (PNP)] 2 ( -N 2 ) (D) yielding two molecules of trans-[mo( N)I 2 (PNP)] (D ), Fig. S9 presents optimized structures of RC (D, quintet), TS (TS D/D, triplet), and PC (2 D, triplet) with their relative free energies. For the N N bond cleavage of the bridging N 2 ligand in [Mo(N 2 ) 2 (PNP)] 2 ( -N 2 ) 3, Fig. S10 shows optimized structures of 3, the corresponding Mo nitride complex [Mo( N)(N 2 ) 2 (PNP)] (3 ). For the protonation of the N 2 ligand in [MoI(N 2 )(PNP)] (A) by ColH +, Fig. S11 shows optimized structures and relative free energies of RC, TS and PC, where TS is lower in energy than PC after thermal corrections at K. Cartesian coordinates of the optimized structures in the present study are listed in Tables S S25

26 closed-shell singlet open-shell singlet triplet P Mo I N Mo N N Mo RC (intermediate B) Mo N N Mo TS Mo N N Mo PC (2 2a) Fig. S8. Optimized structures of intermediates and transition state (TS) for the N N bond cleavage of intermediate B (RC) yielding two molecules of 2a (PC). Hydrogen atoms are omitted for clarity. Selected bond distances are given in Å. Mulliken spin densities assigned to the Mo atoms and the bridging N 2 ligand are presented in italics. S26

27 RC (D) quintet 0.0 kcal/mol Mo N N = TS (TS D/D ) triplet kcal/mol PC (2 D ) triplet kcal/mol Fig. S9. Optimized structures and relative energies ( G 298 in kcal/mol) of intermediates (RC and PC) and transition state (TS) for the N N bond cleavage of [MoI 2 (PNP)] 2 ( -N 2 ) D. Hydrogen atoms are omitted for clarity. Selected bond distances are given in Å. S27

28 closed-shell singlet doublet Fig. S10. Optimized structures of [Mo(N 2 ) 2 (PNP)] 2 ( -N 2 ) 3 and trans-[mo( N)(N 2 ) 2 (PNP)] 3. Hydrogen atoms are omitted for clarity. Selected bond distances are given in Å. S28

29 RC TS PC doublet doublet doublet 0.0 (0.0) kcal/mol +6.0 (+7.0) kcal/mol +6.8 (+5.7) kcal/mol Fig. S11. Optimized structures and relative energies ( G 298 in kcal/mol) of intermediates RC and PC for protonation of the N 2 ligand in [Mo(N 2 )I(PNP)] A by ColH +. Energies in parenthesis present relative SCF energies in kcal/mol (without zero-point energy and thermal corrections). Hydrogen atoms on carbon are omitted for clarity. Selected bond distances are given in Å. S29

30 Table S13. Cartesian coordinate of intermediate B (RC) in the closed-shell singlet state. Units are presented in Å. SCF energy = hartree Thermal correction ( K) = hartree SCF energy (in toluene) = hartree Atom Coordinates (Angstroms) X Y Z Mo Mo I I P P P P N N N N C C C C C C C C S30

31 C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C S31

32 C C C C C C C H H H H H H H H H H H H H H H H H H H H H H H H S32

33 H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H S33

34 H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H S34

35 S35

36 Table S14. Cartesian coordinate of intermediate B (RC) in the open-shell singlet state. Units are presented in Å. SCF energy = hartree Thermal correction ( K) = hartree SCF energy (in toluene) = hartree Atom Coordinates (Angstroms) X Y Z Mo Mo I I P P P P N N N N C C C C C C C S36

37 C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C S37

38 C C C C C C C C H H H H H H H H H H H H H H H H H H H H H H H S38

39 H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H S39

40 H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H S40

41 H S41

42 Table S15. Cartesian coordinate of intermediate B (RC) in the triplet state. Units are presented in Å. SCF energy = hartree Thermal correction ( K) = hartree SCF energy (in toluene) = hartree Atom Coordinates (Angstroms) X Y Z Mo Mo I I P P P P N N N N C C C C C C C S42

43 C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C S43

44 C C C C C C C C H H H H H H H H H H H H H H H H H H H H H H H S44

45 H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H S45

46 H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H S46

47 H S47

48 Table S16. Cartesian coordinate of the transition state (TS) of the N N bond cleavage of B in the closed-shell singlet state. Units are presented in Å. SCF energy = hartree Thermal correction ( K) = hartree SCF energy (in toluene) = hartree Imaginary frequency: 676i cm Atom Coordinates (Angstroms) X Y Z Mo Mo I I P P P P N N N N C C C C C C S48

49 C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C S49

50 C C C C C C C C C H H H H H H H H H H H H H H H H H H H H H H S50

51 H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H S51

52 H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H S52

53 H H S53

54 Table S17. Cartesian coordinate of the transition state (TS) of the N N bond cleavage of B in the triplet state. Units are presented in Å. SCF energy = hartree Thermal correction ( K) = hartree SCF energy (in toluene) = hartree Imaginary frequency: 1546i cm Atom Coordinates (Angstroms) X Y Z Mo Mo I I P P P P N N N N C C C C C C C S54

55 C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C S55

56 C C C C C C C C H H H H H H H H H H H H H H H H H H H H H H H S56

57 H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H S57

58 H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H S58

59 H S59

60 Table S18. Cartesian coordinate of PC in the closed-shell singlet state. Units are presented in Å. SCF energy = hartree Thermal correction ( K) = hartree SCF energy (in toluene) = hartree Atom Coordinates (Angstroms) X Y Z Mo Mo I I P P P P N N N N C C C C C C C S60

61 C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C S61