All manipulations were performed under an inert atmosphere of argon by using Schlenk techniques or in a MBraun inert-gas glovebox. The solvents were purified according to standard procedures [29]. The deuterated solvents were purchased from Aldrich and dried over 3 Å molecular sieves. The ligands 1,1’-(2-bromo-1,3-phenylene)-bis-(N,N-diethylamine) (N(C-Br)NCH2-Et) (1) and 1,1’-(2-bromo-1,3-phenylene)-bis-(N,N-diisopropylamine) (N(C-Br)NCH2-iPr) (2) and anhydrous CrBr2 were synthesized according to literature [30–32]. All other materials are known compounds and were used as obtained from commercial suppliers. 1H, 13C{1H} and 19F{1H} NMR spectra were recorded on Bruker AVANCE-250, AVANCE-400, and AVANCE-600 spectrometers. 1H and 13C{1H} NMR spectra were referenced internally to residual protio-solvent and solvent resonances, respectively, and are reported relative to tetramethylsilane (δ = 0 ppm). 19F{1H} NMR spectra were referenced externally to CFCl3.
High resolution-accurate mass spectra were recorded on a hybrid Maxis Qq-aoTOF mass spectrometer (Bruker Daltonics, Bremen, Germany) fitted with an ESI-source. Measured accurate mass data of the [M]+ ions for confirming calculated elemental compositions were typically within 5 ppm accuracy. The mass calibration was done with a commercial mixture of perfluorinated trialkyl-triazines (ES Tuning Mix, Agilent Technologies, Santa Clara, CA, USA).
Electron Paramagnetic Resonance (EPR) spectra were recorded on an X-band Bruker Elexsys-II E500 CW-EPR spectrometer (Bruker Biospin GmbH, Rheinstetten, Germany) equipped with a high sensitivity cavity (SHQE1119) at 100 ± 1 K. The instrument parameters were set as follows: microwave frequency, 9.43 GHz; modulation frequency, 100 kHz, and microwave power, 15.9 mW. The spectra were analyzed using Xepr software and the Anisotropic SpinFit simulation program (both Bruker Biospin GmbH).
Synthesis of [2,3-bis[(diethylamino)methyl]phenyl- C,N,N′](dichloro) (tetrahydrofuran)chromium(III), [Cr(κ3NCN-NCNCH2-Et)(Cl)2(THF)] (3)
A solution of (N(C-Br)NCH2-Et) (1) (400 mg, 1.23 mmol) in THF (13 cm3) was cooled to -90°C. After stirring for 10 minutes, n-BuLi (1.52 cm3, 1.6 M in hexanes, 2.46 mmol) was added dropwise via a syringe and the light-yellow solution was stirred at -90°C for 2.5 h. After warming up to -30°C, [CrCl3(THF)3] (460 mg, 1.23 mmol) was added portion-wise under stirring resulting in the formation of a dark green solution. The solution was allowed to warm to room temperature and was stirred for 12 h. After removal of the solvent under reduced pressure, the residue dissolved in toluene (7 cm3) and filtered through celite. After removal of the toluene, the residue was washed three times with n-pentane (3x10 cm3). Drying under reduced pressure yielded a light-purple solid. Yield: 340 mg, 63%). Dark green crystals suitable for X-ray crystallography were obtained by slow diffusion of n-pentane into a saturated THF solution of 3. µeff = 3.7(2)µB (Evans Method, THF), HR-MS (ESI+, THF) m/z calcd for C16H27Cl2CrN2 [M-THF + H]+ 370.1029, found 370.1031.
Synthesis of Bis-[2,3-bis[(di iso propylamino)methyl]phenyl- C,N,N′]chromium(II), [Cr(κ2NC-NCNCH2-iPr)2] (4)
A solution of (N(C-Br)NCH2-iPr) (2) (100 mg, 0.26 mmol) in THF (7 cm3) was cooled to -90° C. After stirring at that temperature for 10 min, n-BuLi (0.18 cm3, 1.6 M, 0.30 mmol) was added dropwise via a syringe. A suspension of anhydrous CrCl2 (38 mg, 0.30 mmol) in THF (4 cm3) was added in a dropwise fashion at -20°C. After the addition was completed, all volatiles were removed under reduced pressure and the residue was extracted with n-pentane (6 cm3) and filtered through a syringe filter (PTFE, 0.2 µm). The solvent was removed under vacuum yielding a dark blue solid. Yield: 45 mg (37%). Blue crystals suitable for X-ray crystallography were obtained by cooling a saturated n-pentane solution of 4 to -30° C. µeff = 4.7(3) µB (Evans Method, benzene). HR-MS spectra could not be obtained due to the highly air-sensitive nature of the compound.
Synthesis of [2,3-bis[(di iso propylamino)methyl]phenyl- C,N,N′](chloro) chromium(II), [Cr(κ3NCN-NCNCH2-iPr)Cl] (5a)
Anhydrous CrCl2 (38 mg, 0.30 mmol) was suspended in THF (15 cm3). The suspension was exposed to ultrasonic irradiation for 1 h to form a light-blue clear solution. The ligand (N(C-Br)NCH2-iPr) (2) (100 mg, 0.26 mmol) was dissolved in THF (5 cm3) and subsequently cooled to -90° C. After stirring at that temperature for 10 minutes, n-BuLi (0.18 cm3, 1.6M, 0.30 mmol) was added dropwise via a syringe. The orange solution was stirred for 30 min before it was allowed to warm to 0° C while stirring another hour. The lithiated intermediate was added in a dropwise fashion to the THF-solution of CrCl2via a syringe. The solution was allowed to stir for 15 min at room temperature before all volatiles were evaporated under vacuum. The residue was extracted into n-pentane (7 cm3) and filtered through a syringe filter (PTFE, 0.2 µm). The volume of the filtrate was reduced to about 1.5 cm3 and stored at -30° C to afford 5 as dark purple crystalline plates suitable for X-ray diffraction analysis. Yield: 75 mg (73%). µeff = 4.8(2) µB (Evans Method, benzene). HR-MS spectra could not be achieved due to the highly air-sensitive nature of the compound.
Synthesis of [2,3-bis[(di iso propylamino)methyl]phenyl- C,N,N′](bromo) chromium(II), [Cr(κ3NCN-NCNCH2-iPr)Br (5b)
Complex 5b was prepared analogously to complex 5a utilizing anhydrous CrBr2 (58 mg, 0.30 mmol) and N(C-Br)NCH2-iPr (2) (100 mg, 0.26 mmol) as starting materials. Yield: 85.2 mg (75%). µeff = 4.7(2) µB (Evans Method, benzene). HR-MS spectra could not be obtained due to the highly air-sensitive nature of the compound.
Synthesis of [2,3-bis[(di iso propylamino)methyl]phenyl- C,N,N′](phenyl)(methyl)) chromium(II), [Cr(κ3NCN-NCNCH2-iPr)(CH2Ph)](6)
Method A: A suspension of anhydrous CrCl2 (38 mg, 0.30 mmol) in THF (15 cm3) was placed in an ultrasonic bath for 1 h whereupon the suspension turned into light blue solution. (N(C-Br)NCH2-iPr) (2) (100 mg, 0.26 mmol) was dissolved in THF (5 cm3) and subsequently cooled to -90° C. After stirring for 10 min, n-BuLi (0.18 cm3, 1.6M, 0.30 mmol) was added dropwise and the solution was stirred for additional 30 min. The solution was allowed to warm to 0° C and was stirred for 1 h. The lithiated intermediate was then added in a dropwise fashion to THF solution of CrCl2 and a dark colored solution was formed immediately. The solution was stirred for 15 min at room temperature and all volatiles were then evaporated under vacuum. The residue was dissolved in benzene and the solution was filtered through a syringe filter (PTFE, 0.2 µm). A solution of PhCH2MgCl (0.1 cm3, 1M in THF) was then added and the suspension was stirred for 1.5 h. All volatiles were removed under reduced pressure. The remaining residue was extracted with n-pentane (3 cm3) and filtered through syringe filter (PTFE, 0.2 µm). Evaporation of the solvent afforded 6 as a dark brown solid. Cooling of a saturated n-pentane solution of 6 to -30° C yielded crystals suitable for X-Ray diffraction. Yield: 81 mg (72%). µeff = 4.9 (1) µB (Evans Method, benzene).
Method B. Complex 6 was also obtained by reacting isolated 5a (50 mg, 0.12 mmol) in benzene (2 cm3) with PhCH2MgCl (0.14 cm3, 1 M in THF) and stirring for 1.5 h at room temperature. After evaporating the solvent under reduced pressure, redissolving the residue in n-pentane (2 cm3) and subsequent filtration (syringe filter PTFE, 0.2 µm) and removal of the solvent, 6 could be isolated as a dark brown solid. Yield: 41 mg (72%). HR-MS spectra could not be achieved due to the highly air-sensitive nature of the compound.
Synthesis of [2,3-bis[(di iso propylamino)methyl]phenyl- C,N,N′](trimethylsilane) (methyl)chromium(II), [Cr(κ3NCN-NCNCH2-iPr)(CH2SiMe3)] (7)
Method A. Complex 7 was prepared in analogous fashion to 6 utilizing LiCH2TMS (0.28 cm3, 0.28 mmol, 1 M in n-pentane). Complex 7 was obtained as dark brown solid. Yield: 76 mg (68%). µeff = 4.8 (1)µB (Evans Method, benzene).
Method B: A solution of Complex 5a (50 mg, 0.12 mmol) in benzene (2 cm3) was treated with LiCH2TMS (0.13 cm3, 0.28 mmol, 1 M in n-pentane) and stirring for 1.5 h at room temperature. After evaporation of the solvent, redissolving the residue in n-pentane (2 cm3) and subsequent filtration (syringe filter PTFE, 0.2 µm) and removal of the solvent, 7 could be isolated as a dark brown solid. Yield: 39 mg (69%). HR-MS spectra could not be obtained due to the highly air-sensitive nature of the compound.
Synthesis of Bis-[2,3-bis[(di iso propylamino)methyl]phenyl- C,N,N′])(µ2-hydrido) chromium(II)], [Cr(κ2NC-NCNCH2-iPr)(µ2-H)]2 (8)
A suspension of anhydrous CrCl2 (38 mg, 0.30 mmol) in THF (15 cm3) was placed in an ultrasonic bath for 1 h, whereupon a light blue solution was obtained. The ligand precursor N(C-Br)NCH2-iPr (2) (100 mg, 0.26 mmol) was dissolved in THF (5 cm3) and cooled to -90° C. After stirring for 10 min, n-BuLi (0.18 cm3, 1.6 M, 0.30 mmol) was added dropwise via a syringe and the orange solution was further stirred for 30 min and then allowed to warm to 0° C and stirred for an additional 1 h. The solution containing lithiated N(C-Br)NCH2-iPr (2) was added dropwise to the THF-solution of anhydrous CrCl2 whereupon the solution became dark purple. The solution was allowed to stir for 15 min at room temperature before all volatiles were evaporated under vacuum. The residue was dissolved in benzene and filtered through a syringe filter (PTFE, 0.2 µm) and volatiles were evaporated under reduced pressure. The residue was dissolved in precooled THF (-30°C, 3 cm3) and treated with NaHBEt3 (0.91 cm3, 0.91 mmol, 1M in THF) and stirred for 30 min. After removal of the solvent, the remaining residue was redissolved in n-pentane (10 cm3), filtered through a syringe filter (PTFE, 0.2 µm) and the volume of the solution was reduced to approximately 1 cm3. Storage of the light brown solution at -30° C for 12 h led to the formation of bright orange crystals. Yield: 64 mg (35%). µeff = 6.9 (3) µB (Evans Method, benzene-d6). HR-MS (ESI+, THF) m/z calcd for C40H72Cr2N4 [M-C20H36CrN2]+ 356.2279, found 356.2278.
General Procedure for the Hydrosilylation of Ketones
To a solution of the substrate (0.67 mmol, 1 equiv.) and HSi(OMe)3 (174 mm3, 1.35 mmol, 2 equiv) in toluene (3 cm3) complex 6 was added (3 mg, 6.7 µmol, 1 mol%) causing an immediate color change. The mixture was stirred at room temperature for 1 h and then a saturated methanolic K2CO3 solution (3 cm3) was added to the mixture. After stirring for 5 h volatiles were evaporated under reduced pressure. The residue was dissolved in CH2Cl2 (6 cm3) and filtered through a short plug of silica. After evaporation of the solvent the reaction products were analyzed by the means of 1H, 13C {1H} and 19F{1H} NMR spectroscopy.
X-ray Structure Determination
X-ray diffraction data of 3, 4, 5a (CCDC 2285377–2285379) and 8 (CCDC 2285381) were collected at T = 100 K in a dry stream of nitrogen on a Bruker Kappa APEX II diffractometer system using graphite-monochromatized Mo-Kα radiation (λ = 0.71073 Å) and fine sliced φ- and ω-scans. Data were reduced to intensity values with SAINT and an absorption correction was applied with the multi-scan approach implemented in SADABS [33]. Data of 6 (CCDC 2285380) were collected at T = 100 K on a Rigaku XtaLAB Synergy, Dualflex diffractometer system equipped with a HyPix hybrid photon counting detector using Cu-Kα radiation (λ = 1.54184 Å). Data were reduced and an absorption correction applied using the multi-scan approach with the CrysAlisPro software [34]. The structures were solved by the dual-space approach implemented in SHELXT [35] and refined against F2 with SHELXL [36]. Non-hydrogen atoms were refined with anisotropic displacement parameters. H atoms attached to C were placed in calculated positions and thereafter refined as riding on the parent atoms. The positions of the hydride Hs in 8 were freely refined. The halogenide ligands in 3 and 5a were modelled as occupationally disordered Cl/Br sites with the total occupation of each sit e constrained to 1. Molecular graphics were generated with the program MERCURY [37].
Computational Details
The computational results presented have been achieved in part using the Vienna Scientific Cluster (VSC). Calculations were performed using the GAUSSIAN 09 software package [38] and the OPBE [39–44] functional without symmetry constraints, the Stuttgart/Dresden ECP (SDD) basis set to describe the electrons of the chromium atom and a standard 6-31G** basis for all other atoms as already previously described [45]. Population Analysis (NPA) [46] and the resulting Wiberg indices [28] were used to study the electronic structure and bonding of the optimized species. The NPA analysis was performed with the NBO 5.0 program [47]