Remarkable catalytic activity of dinitrogen-bridged dimolybdenum complexes bearing NHC-based PCP-pincer ligands toward nitrogen fixation

Intensive efforts for the transformation of dinitrogen using transition metal–dinitrogen complexes as catalysts under mild reaction conditions have been made. However, limited systems have succeeded in the catalytic formation of ammonia. Here we show that newly designed and prepared dinitrogen-bridged dimolybdenum complexes bearing N-heterocyclic carbene- and phosphine-based PCP-pincer ligands [{Mo(N2)2(PCP)}2(μ-N2)] (1) work as so far the most effective catalysts towards the formation of ammonia from dinitrogen under ambient reaction conditions, where up to 230 equiv. of ammonia are produced based on the catalyst. DFT calculations on 1 reveal that the PCP-pincer ligand serves as not only a strong σ-donor but also a π-acceptor. These electronic properties are responsible for a solid connection between the molybdenum centre and the pincer ligand, leading to the enhanced catalytic activity for nitrogen fixation.

Scheme for preparation of Bim-PCP [1] and Me-Bim-PCP [1]. (b) Scheme for preparation of Im-PCP [2].    (4) Mo (1)  All manipulations were carried out under an atmosphere of nitrogen by using standard Schlenk techniques or glovebox techniques unless otherwise stated. Toluene was distilled from a dark-blue Na/benzophenone ketyl solution and degassed, and stored over molecular sieves 4A in a nitrogen-filled glove box. Other solvents were dried by general methods, and degassed before use. CoCp 2 (Aldrich) was sublimed before use. 3

Preparation of 4c
Compound 4c was prepared according to a similar procedure to 4a. 1

Preparation of 5a and 5c
A typical procedure for the preparation of 5a is described below. A solution of o-C 6  Hz, 36H, P t Bu 2 ). 13

Preparation of 6a and 6c
A typical procedure for the preparation of 6a is described below. To a mixture of 5a (2.56 g, 5.25 mmol) and NH 4 PF 6 (864 mg, 5.30 mmol) was added CH(OEt) 3 (13 mL, 78 mmol) and stirred at 120 °C for 3 h. The volatiles were removed under vacuum. The resultant solid was washed with CH 2 Cl 2 /Et 2 O = 2/5 (v/v, 35 mL) and Et 2 O (5 mL x 2) and dried under vacuum to give an analytically pure 6a as a white solid (3.23 g, 5.01 mmol, 95%).

Preparation of 6b
To a suspension of 9b (280 mg, 1.26 mmol) in toluene (6 mL

Reaction of 1a with 15 N 2
To a 20 mL Schlenk flask was placed 1a. The Schlenk flask was evacuated and 15 N 2 gas was backfilled. Then C 6 D 6 was added to the flask and the reaction mixture was stirred at room temperature for 1 h. The 15 N NMR spectrum of the reaction mixture was recorded on a JEOL JNM-ECS 400 spectrometer. 15 N NMR (C 6

Catalytic reduction of dinitrogen to ammonia under N 2 atmosphere
A typical experimental procedure for the catalytic reduction of dinitrogen into ammonia using the dinitrogen complex 1a is described below. The reaction was carried out in a nitrogen-filled glove box. In a 50 mL Schlenk flask were placed 1a·1.3C 4 H 8 O·0.4C 6 H 14 (12.9 mg, 0.0097 mmol) and 2,6-lutidinium trifluoromethanesulfonate  Table 1 in the Article.
For the experiments shown in Table 2, 2.0 µmol of catalyst 1a was used, and the same procedure was applied.
For the experiments shown in Figure 5,

Time profiles for the formation of ammonia
A typical procedure is as follows. In a 50 mL Schlenk flask were placed 1a or 1c (0.0033 mmol) and [LutH]OTf (0.96 mmol). Toluene (1.0 mL) was added under N 2 (1 atm), and then a solution of CrCp* 2 (0.72 mmol) in toluene (4.0 mL) was slowly added to the stirred mixture in the Schlenk flask with a syringe pump at a rate of 4.0 mL per hour. After the indicated time (0.33 h, 0.67 h, 1 h, 2 h, and 20 h), the amount of molecular dihydrogen produced in the catalytic reaction was determined by GC analysis. The amount of ammonia was determined by the indophenol method utilized procedure previously described. 8 The results are summarized in Supplementary Table 10. Supplementary Figure 12a shows the IR spectra of 2,6-lutidine. Supplementary Figure 12b shows the IR spectra of [LutH]OTf.

X-ray crystallography.
Crystallographic data of 1a, 1b, 1c, 3b·1/3C 6 H 14 , 3c·0.5CH 2   has ruled out the possibility of both non-merohedral and merohedral twins. As the beta angle of 3c is almost 90 degrees, the R int value is not good.

Computational Methods.
DFT calculations were performed with the Gaussian 09 program (Rev. C01). 17 The functional and basis sets adopted in the present study are basically similar to those in our previous works on the catalytic activity of dinitrogen-bridged dimolybdenum complexes 18 for comparison. Geometry optimizations were carried out with the B3LYP* functional. The B3LYP* functional is a reparametrized version of the B3LYP hybrid functional [19][20][21][22] developed by Reiher and co-workers for proper estimation of the energy difference between different spin states. 23,24 The B3LYP and B3LYP* energy expressions are given as Supplementary Equation (1): where a 0 = 0.20 (B3LYP) or 0.15 (B3LYP*), a x = 0.72, a c = 0.81 and in which E X HF is the Hartree-Fock exchange energy; E X LSDA is the local exchange energy from the local spin density approximation; E X B88 is Becke's gradient correction 19 to the exchange functional; E C LYP is the correlation functional developed by Lee et al. 21 ; and E C VMN is the correlation energy calculated using the local correlation functional of Vosko, Wilk and Nusair (VWN) 22 . The SDD (Stuttgart/Dresden pseudopotentials) basis set 25,26 and 6-31G(d) basis sets [27][28][29][30] were employed for the Mo atoms and the other atoms, respectively. Optimized structures were confirmed to have the appropriate number of imaginary frequencies by vibrational analysis. Calculated vibrational frequencies were corrected with a scaling factor of 0.960. 31 An appropriate connection between a reactant and a product for each reaction step was confirmed by IRC 32-34 and quasi-IRC calculations. In the quasi-IRC calculation, the geometry of a transition state was at first shifted by perturbing the geometries very slightly along the reaction coordinate and released for equilibrium optimization. Cartesian coordinates of optimized intermediates and transition states are shown in Supplementary Tables 11-32. To discuss the energetics of the first protonation processes, single-point energy calculations at the optimized geometries were performed by using the 6-311+G(d,p) basis set 35-37 instead of 6-31G(d). Solvation effects of toluene were taken into account by using the polarizable continuum model (PCM). 38 Zero-point energy corrections were applied for energy changes and activation energies calculated for each reaction step in the protonation processes.