Electronic structure and reactivity of nickel(i) pincer complexes: their aerobic transformation to peroxo species and site selective C–H oxygenation

The dinuclear nickel peroxo complexes undergo autoxidation giving a cyclic alkoxo or peroxo complex in the presence or absence of O2, respectively.


Preparation of the μ-1,2-peroxo complex [Lig tBu (iso)NiO] 2 (3c)
The solution of the nickel(I) complex 1c in Et 2 O was cooled to −78 °C, subjected to oxygen and held at low temperature for 15 min. After removal of the solvent at low temperature, the product was obtained as red solid. The obtained complexes are thermally instable and rapid decomposition was found to occur at temperatures above 0 °C.

Preparation of the hydroperoxo complex Lig iPr (iso)NiOOH (5b)
A solution of the ethyl complex 12 (28 mg, 0.069 mmol) in 0.6 ml toluene-d8 was subjected to 4 bar O 2 at −78 °C and analyzed by NMR spectroscopy at 233 K. Next to the hydroperoxo complex 5b the formation of ethylene was observed. Compound 5b was found to slowly convert to the 1,2-μ-peroxo complex 3b at 233 K and attempts of isolation were unsuccessful. Oxygen was bubbled through a solution of the nickel(I) complex 1a (53 mg, 0.11 mmol) in 2 ml THF at −78 °C for 5 min. Subsequently, the reaction mixture was allowed to warm up to room temperature and stirred for 18 hours. The solution was flushed through a silica plug (washed with THF) and the solvents were removed under reduced pressure. After fractional precipitation from a toluene/pentane mixture, both products 6 and 6' were obtained as a mixture in 42% yield (22 mg

Preparation of cyclic alkoxo complex 8
Oxygen was bubbled through a solution of the nickel(I) complex 1b (50 mg, 0.13 mmol) in 3 ml diethyl ether at −78 °C for 10 min. Subsequently, the solvent was removed during which the temperature was held below −20 °C. After the addition of THF the reaction was allowed to warm up to room temperature under an atmosphere of argon and was stirred for 4 h. The solution was flushed through a silica plug (washed with THF) and the solvents were removed. The crude product was recrystallized from pentane at −40 °C to give an orange crystalline solid in 33% yield (17 mg

Preparation of the cyclic peroxo complex 9
Oxygen was bubbled through a solution of the nickel(I) complex 1b (98.0 mg, 0.26 mmol) in 2 ml toluene at −78 °C for 5 min. Subsequently, the reaction mixture was stirred at −20 °C for 10 min, allowed to warm up to room temperature and stirred for another hour. After the solvents were removed under reduced pressure, THF was added and the solution was flushed through a silica plug. After removal of the solvents, the product was recrystallized from pentane at −80 °C to give an orange crystalline solid in 48% yield (48.5 mg).

Preparation of the oxazolinylcarboxylato complex 10
Oxygen was bubbled through a solution of the nickel(I) complex 1c (30 mg, 0.074 mmol) in 2 ml toluene at −78 °C for 5 min. Subsequently, the reaction mixture was stirred at −20 °C for 10 min, allowed to warm up to room temperature and stirred for another 8 hours. After the solvents were removed under reduced pressure, the product was purified by fractional crystallization from a toluene/pentane mixture to give a brown crystalline solid in 28% yield (12 mg

Preparation of the ethyl complex Lig iPr (iso)NiEt (12)
To a solution of the chlorido complex Lig iPr (iso)NiCl (50 mg, 0.12 mmol) in 3 ml THF, EtMgCl (0.16 mmol, 1.3 eq) was added at −78 °C. After 5 min the cooling bath was removed and the reaction was stirred for another 20 min. After removal of the solvents, the residue was treated with a diethyl ether/pentane mixture (1/50) and the solution was filtrated. After removal of the solvents, the crude was recrystallized from hexane at −40 °C to give a yellow crystalline solid in 78% yield (38 mg).

Preparation of the hexenyl complex 13
After the activation of Mg turnings (50 mg) using I 2 , 6-bromohexene (70 mg, 0.43 mmol) dissolved in 5 ml THF was added and stirred for 6 h at room temperature. The reaction mixture was filtered and canulated to a solution of the nickel chlorido complex Lig Ph (iso)NiCl (90 mg, 0.19 mmol) at 0 °C. After 20 min the solvents were removed and the residue was treated with pentane. The solution was stirred for 15 min and filtrated. After removal of the solvent, the product was obtained as yellow powder in 83% yield (83 mg

Preparation of the formato complex Lig Ph (iso)NiOOCH (16)
To a solution of the hydroxo complex 7a (20 mg, 0.15 mmol) in 3 ml toluene a drop of HOOCH was added and stirred for 10 min. After removal of the volatiles, the crude was recystallized from a toluene/pentane mixture to give a red crystalline solid in 75% yield (16 mg).

S1.3. Additional Experiments Comparison of the decomposition rates of 3b under argon and 8 bar oxygen pressure
In both experiments 0.4 ml of a stock solution of 20.2 mg of the nickel(I) complex 1b in 1.6 ml THF were used. The samples were subjected to 8 bar O 2 at −78 °C and held at low temperature for 15 min. In first experiment the sample was allowed to warm to room temperature (295 K) under 8 bar oxygen pressure and the reaction was monitored by 1 H NMR spectroscopy. The integral of the 1 H NMR signal at 5.71 ppm (H 6 , THF, 295 K) 5 was used to determine the concentration of 3b throughout the course of the reaction. In the second experiment the sample was degassed at low temperature prior to allowing it to warm to room temperature. No significant difference in the decomposition rate in both experiments was observed (see Figure 4).

Determination of the aerobic decomposition rates of 3a and 3b
The corresponding solutions of the nickel(I) complexes (11.5 mg (1a) and 9.7 mg (1b) and 1,4dimethoxybenzene in 0.5 ml toluene-d8) were subjected to 8 bar O 2 at −78 °C und held for 15 min at low temperature. Subsequently, the samples were allowed to warm to 283 K and the decomposition reaction was monitored by 1 H NMR spectroscopy (integrals of the 1 H NMR signals at 5.98 ppm (H 6 ) 5 for 3b and 6.37 ppm (H 6 ) 5 for 3a in toluene at 285 K were used, respectively).

Reaction of the hydroperoxo complex 5a with the hydrido complex 11a
The solution of the hydroperoxo complex 5a (6,4 mg) in 0.5 ml THF-d8 and the solution 0.2 ml THF-d8 of the in situ generated hydrido complex 11a (6.2 mg of 1a, 10 bar H 2 ) were combined at room temperature and kept under 10 bar H 2 pressure. NMR analysis after less than 10 min showed a near complete conversion to the hydroxo complex 7a.

Reaction of the nickel(I) complex 1b with N 2 O
A solution of the nickel(I) complex 1b (8 mg) in 0.4 ml toluene-d8 was cooled to −78 °C and subjected to N 2 O. After about 10 min, the solution was analyzed by NMR spectroscopy at −30 °C. The formation of the hydrido complex 11b (roughly 0.3 eq.) as well as 7b, 9 and other species was observed.

Reaction of alkyl complexes 12 and 13 with oxygen
The solution of the ethyl complex 12 (28 mg) in 0.4 ml toluene-d8 was subjected to 5 bar O 2 at −78 °C in an NMR tube. After approximately 15 min the sample was analyzed by NMR spectroscopy at 233 K which showed that a clean formation to 5b and ethylene had occurred. The latter slowly converted into the 1,2-μ-peroxo complex 3b.
The solution of the hexenyl complex 13 (20 mg) in 0.4 ml toluene-d8 was subjected to 5 bar O 2 at 0 °C in an NMR tube. After approximately 5 min the sample was analyzed by NMR spectroscopy at 0 C which showed that a clean formation to 5b and 1,5-hexadiene had occurred.

Thermal decomposition of the methylperoxo complex 15
The solution of the methylperoxo complex 15 (5 mg) in 0.4 ml toluene-d8 was held at room temperature for a week and was subsequently analyzed by NMR spectroscopy. The formation of the formato complex 16, the hydroxo complex 7a and methanol was observed.

Thermal decomposition of the formato complex 16
The solution of the formato complex 16 (5 mg) in 0.4 ml toluene-d8 was held at 110 C for 2 days. The subsequently NMR spectroscopic analysis at room temperature revealed that the nickel(I) complex 1a and traces of the hydrido complex 11a and H 2 .

S2.1. X-ray crystal structure determinations
Crystal data and details of the structure determinations are compiled in Table S1. Full shells of intensity data were collected at low temperature with a Bruker AXS Smart 1000 CCD diffractometer (Mo-K a radiation, sealed X-ray tube, graphite monochromator; compounds 8 and 10) or an Agilent Technologies Supernova-E CCD diffractometer (Mo-or Cu-K a radiation, microfocus X-ray tube, multilayer mirror optics; all other compounds). Data were corrected for air and detector absorption, Lorentz and polarization effects; 7,8 absorption by the crystal was treated numerically (Gaussian grid) 8,9 or with a semiempirical multiscan method. [10][11][12][13] The structures were solved by the heavy atom method combined with structure expansion by direct methods applied to difference structure factors 14,15 or by the charge flip procedure 16,17 and refined by full-matrix least squares methods based on F 2 against all unique reflections. [18][19][20] All non-hydrogen atoms were given anisotropic displacement parameters. Hydrogen atoms were generally input at calculated positions and refined with a riding model. When justified by the quality of the data the positions of some hydrogen atoms were taken from difference Fourier syntheses and refined. The disordered formato ligand in complex 16 was subjected to suitable geometry and adp restraints.
CCDC 1439102-1439106 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.