Origin and Use of Hydroxyl Group Tolerance in Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Catalysts

Abstract The origin of hydroxyl group tolerance in neutral and especially cationic molybdenum imido alkylidene N‐heterocyclic carbene (NHC) complexes has been investigated. A wide range of catalysts was prepared and tested. Most cationic complexes can be handled in air without difficulty and display an unprecedented stability towards water and alcohols. NHC complexes were successfully used with substrates containing the hydroxyl functionality in acyclic diene metathesis polymerization, homo‐, cross and ring‐opening cross metathesis reactions. The catalysts remain active even in 2‐PrOH and are applicable in ring‐opening metathesis polymerization and alkene homometathesis using alcohols as solvent. The use of weakly basic bidentate, hemilabile anionic ligands such as triflate or pentafluorobenzoate and weakly basic aromatic imido ligands in combination with a sterically demanding 1,3‐dimesitylimidazol‐2‐ylidene NHC ligand was found essential for reactive and yet robust catalysts.


Synthesis of Organometallic Complexes
Scheme S1. Synthesis of 3d and 4d from previously not reported precursors.  [10] (71.1 mg, 0.0472 mmol, 1 equiv) was dissolved in 3 mL THF and cooled to -35 °C. To the cold solution solid DFTOH (20.1 mg, 0.0472 mmol, 1 equiv) was added as a solid. The red solution turned yellow and was stirred at room temperature for 30 min. All volatiles were removed in vacuo. The dark residue was washed with 5 mL of pentane. The crude product was recrystallized from 2 mL of a 1:1 mixture of diethyl ether and pentane at -35 °C. The product precipitated as a crystalline yellow solid (61 mg, 73%). 1 [10] (65 mg, 0.043 mmol, 1 equiv) was dissolved in 3 mL CH2Cl2. At room temperature 1,3,5-trimethylpyrazole (24 mg, 0.22 mmol, 5 equiv) was added as a solid and the mixture was stirred for 15 mins. The color changed from dark red to yellow. All volatiles were removed in vacuo and the dark yellow residue was washed with 10 mL pentane to obtain a yellow solid (61 mg, 96%). The product can be recrystallized from diethyl ether/pentane. 1  Mo(N-2,6-Cl2-C6H3)(CHCMe3)(IMes)(OTf)2 [15] (120.4 mg, 0.130 mmol, 1 equiv) was dissolved in 5 mL CH2Cl2. At room temperature solid NaB(Ar F )4 (114.9 mg, 0.130 mmol, 1 equiv) was added. The mixture was stirred for 30 min. Subsequently all solids were filtered off and the filtrate was reduced to dryness. The oily product was dissolved in 1 mL CH2Cl2 and filtered once more. The CH2Cl2 was removed in vacuo and the oily product was triturated with pentane until a yellow solid formed. Yield: 184 mg, 87%.
(100 mg, 0.061 mmol, 1 equiv.) in in 2 mL CH2Cl2 was quickly added to a solution of LiOC(CF3)3 (14.7 mg, 0.061 mmol, 1 equiv.) in 2 mL of CH2Cl2 and a few drops of acetonitrile. The resulting solution was stirred overnight, and the solvent was removed in vacuo. The residue was triturated with pentane to give a yellow suspension. The pentane was decanted, and the residue dried in vacuo. CH2Cl2 was added and the resulting suspension was filtered through a pad of celite. The solvent was removed, and the residue was triturated with pentane, yielding the product as a yellow solid.
At room temperature solid NaB(Ar F )4 (73.8 mg, 0.083 mmol) was added to a solution of Mo(N-2,6-Cl2-C6H3)(CHCMe3)(IMesH2)(OTf)2 [15] (77.5 mg, 0.083 mmol, 1 equiv.) in 6 mL of CH2Cl2. The vial was rinsed with an additional 4 mL of CH2Cl2 and the resulting suspension was stirred for five hours at room temperature. It was then filtered through a pad of celite and the solvent was evaporated in vacuo. The oily residue was triturated with pentane, which led to the formation of a yellow solid. The solid was filtered off and washed with pentane to yield the product as a yellow solid. Yield: 124 mg (91%). Mo(N-2,6-Cl2-C6H3)(CHCMe3)(IMesCl2)(OTf)2, 3d-p1 A cold (-35 °C) solution of IMesCl2 (261 mg, 0.70 mmol) in 8 mL of toluene was added to a cold (-35 °C) solution of Mo(N-2,6-Cl2-C6H3)(CHCMe3)(DME)(OTf)2 [18] (500 mg, 0.70 mmol, 1 equiv.) in 25 mL of toluene. The mixture was stirred at room temperature for four hours during which time a yellow solid precipitated. The suspension was filtered and the solid was washed with 2 mL of toluene and 2 mL of pentane to yield the product as a yellow solid. Yield: 455 mg (65%). Complex 4a was reported previously. [11] Crystallization of the dihydrate succeeded via a slightly altered procedure by employing non predried NaB(Ar F )4 which contains approx. 2.5 equiv. of water. [6] A cold solution (-35 °C) of NaB(Ar F )4 (28.8 mg, 0.031 mmol, 1 equiv.) in diethyl ether (1 mL) was slowly added to a cold solution of Mo(N-2-tBu-C6H4)(IMes)(CHCMe2Ph)(OTf)2 [13] (30 mg, 0.031 mmol, 1 equiv.) in CH2Cl2 (1 mL). The solution was stirred at room temperature for three hours and the solvent was evaporated. The residue was dissolved in CH2Cl2 and filtered through a pad of celite. The product crystallized from a 1:1 mixture of CH2Cl2 and n-pentane (0.5 mL) at -35 °C over the course of a few days as yellow crystals. [11] (100 mg, 0.10 mmol) in 2 mL of CH2Cl2 was added to a suspension of NaB(Ar F )4 (90 mg, 0.10 mmol, 1 equiv.) in 2 mL CH2Cl2 containing three drops of acetonitrile. The mixture was stirred for two hours at room temperature and the solvent was removed in vacuo. The residue was triturated with n-pentane which led to the formation of a yellow solid. The pentane was decanted and the solid was dried in vacuo. The residue was extracted with CH2Cl2 and filtered through pad of celite. n-Pentane was added to the solution and upon storage of the solution at -35 °C overnight, a yellow solid precipitated. The suspension was filtered and the solid washed with pentane to yield the product as a yellow solid. Yield: 90 mg (52 %). Mo(N-2,6-F2-C6H3)2(Cl)2(DME), 4d-p1 Ammonium dimolybdate (1.00 g, 2.94 mmol, 1 equiv.) was suspended in 30 mL acetonitrile. Triethylamine (2.38 g, 23.52 mmol, 8 equiv.), chlorotrimethysilane (5.45 g, 50.17 mmol, 17 equiv.) and 2,6-difluoroaniline (1.52 g, 11.77 mmol, 4 equiv.) were added sequentially under stirring. The reaction mixture was stirred at room temperature for 36 h to afford a dark red solution with some precipitate. The solvent was removed in vacuo and the remaining solid material was stirred with a minimum amount of 1,2dimethoxyethane (DME) for 12 hours. Subsequently the solvent was removed in vacuo and co-evaporated with n-pentane several times. The resulting black-red solid was extracted with a Soxhlet apparatus using DME as a solvent. The DME was removed in vacuo and a minimum amount of dichloromethane was added to the obtained black-red solid and stirred for one hour. From this mixture a crystalline red solid was filtered off. The product was washed with cold dichloromethane and recrystallized from a mixture of DCM/diethyl ether to get a crystalline bright orange solid (0.72 g, 47% yield). 1 H NMR (400 MHz, CD2Cl2) d = 7.11 (m, 2H, p-Ar-Imido), 6.92 (m, 4H, m-Ar-Imido), 4.01 (s, 4H, CH2-DME), 3.99 (s, 6H, CH3-DME). 13 Mo(N-2,6-F2-C6H3)2Cl2(DME) 4d-p1 (0.131 g, 0.25 mmol, 1 equiv.) was dissolved in 15 mL THF and cooled to -30 °C. Neophylmagnesium chloride (1.21 M in diethyl ether, 0.4 mL, 0.50 mmol, 2 equiv.) was added dropwise to the stirring solution. The reaction mixture was stirred overnight at room temperature and filtered through celite. The solvent was removed in vacuo and the red-orange solid was recrystallized from pentane (0.14 g, 92% yield). 1  Mo(N-2,6-F2-C6H3)(CHCMe2Ph)(OTf)2DME, 4d-p3 Mo(N-2,6-F2C6H3)2(CH2CMe2Ph)2 4d-p2 (0.140 g, 0.23 mmol, 1 equiv.) was dissolved in 20 mL 1,2-dimethoxyethane (DME) and cooled to -30 °C. The chilled triflic acid (0.10 g, 0.69 mmol) and 1 mL of 1,2-dimethoxyethane was mixed and cooled to -30 °C. After one hour, the cooled triflic acid solution was added dropwise to the solution of educt. The reaction mixture was stirred overnight and DME was evaporated in vacuo. Benzene was added to the residue and the suspension was stirred for 15 minutes. Subsequently the precipitate was removed by filtration through celite. The benzene was removed in vacuo and diethyl ether was added. The yellow precipitate was filtered and washed with cold diethyl ether. The product was recrystallized from a mixture of CH2Cl2/Et2O/pentane to obtain a yellow crystalline solid (0.11 g, 64% yield). The product consists of two isomers in a 1:0.5 ratio, as can be seen in NMR spectra. We assign the two isomers to complexes with cis and trans coordinated triflates. 1 H NMR ( 11 mmol, 1 equiv.) was dissolved in 5 mL benzene and a solution of 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (0.033 g, 0.11 mmol, 1 equiv.) in 1 mL benzene was added dropwise. The reaction mixture was stirred for 3 h at room temperature. The solvent was removed in vacuo. The yellow solid was recrystallized from a mixture of dichloromethane and pentane to obtain a yellow crystalline solid (0.070 g, 67% yield).
Mo(N-2,6-F2-C6H3)(CHCMe2Ph)(OTf)2(IMes) 4d-p4 (0.050 g, 0.052 mmol. 1 equiv.) was dissolved in 4 mL dichloromethane and cooled to -30 °C for about 30 min. Then Ag(CH3CN)2B(Ar F )4 (0.057 mg, 0.052 mmol, 1 equiv.) was added in small portions as a solid and the reaction mixture was stirred for 2 h at room temperature. The reaction mixture was filtered through a pad of celite and CH2Cl2 was removed in vacuo. The residue was co-evaporated with pentane to yield a yellow solid. The product was recrystallized from a mixture of CH2Cl2/Et2O/pentane to obtain a crystalline yellow solid (0.080 g, 89% yield). Mo(N-2,6-Me2-C6H3)(CHCMe2Ph)(IMes)(OTf)2 [15] (70.7 mg, 0.0744 mmol, 1 equiv) was dissolved in 5 mL CH2Cl2. At room temperature solid NaB(Ar F )4 (66 mg, 0.0744 mmol, 1 equiv) was added. The mixture was stirred for 30 min. Subsequently all solids were filtered off over celite and the filtrate was reduced to dryness. The oily product was dissolved in 1 mL CH2Cl2 and filtered once more. The CH2Cl2 was removed in vacuo and the oily product was triturated with pentane until a yellow solid formed. Yield: 103 mg, 83%. 1 H NMR ( Li(OC(CF3)3) (54.9 mg, 0.2 mmol, 1 equiv.) was dissolved in a mixture of CH2Cl2 (1 mL) and acetonitrile (1 mL) and both mixtures were cooled to -35 °C. Then, the lithium alkoxide solution was slowly added to the metal complex solution. The mixture was stirred at room temperature for 3 hours, upon which the solution turned cloudy. All volatiles were removed and the residue was triturated with CH2Cl2 (~12 mL), resulting in a yellow suspension. LiOTf was removed by filtration over Celite. The filtrate was evaporated to dryness. For crystallization, the residue was dissolved in CH2Cl2 (2 mL) and diethyl ether (4 mL). Then, pentane (~10 mL) was added until the solution started getting cloudy upon addition of more pentane. Crystals of 5b were obtained from this solution at -35 °C after 12 hours. Mo(N-tBu)(CHCMe2Ph)(IMes)(OTf)(C6F5CO2), 10 Mo(N-tBu)(CHCMe2Ph)(IMes)(OTf)2 [14] (111 mg, 0.123 mmol, 1 equiv) was dissolved in 5 mL CH2Cl2 and cooled to -35 °C. To the cold solution solid silver pentafluorobenzoate (40 mg, 0.123 mmol, 1 equiv) was added. The mixture was stirred for one hour in the dark. All solids were filtered off and the filtrate was reduced to dryness. The oily residue was redissolved in 1 mL CH2Cl2 and filtered once more. All volatiles were removed in vacuo. The residue was triturated with 5 mL pentane until a pale yellow solid precipitated. Mo(N-tBu)(CHCMe2Ph)(IMes)(OTf)(C6F5CO2) 10 (53 mg, 0.055 mmol, 1 equiv) was dissolved in 3 mL CH2Cl2. At room temperature solid NaB(Ar F )4 (48.7 mg, 0.055 mmol, 1 equiv) was added. The mixture was stirred for 30 min. Subsequently all solids were filtered off and the filtrate was reduced to dryness. The oily product was dissolved in 1 mL CH2Cl2 and filtered once more. The CH2Cl2 was removed in vacuo and the dark oily product (pure by NMR) was triturated with pentane until a pale yellow solid formed. Yield: 86 mg, 93% Mo(N-2,6-Me2-C6H3)(CHCMe2Ph)(IMes)(OTf)2 [15] (53 mg, 0.0558 mmol, 1 equiv) was dissolved in 3 mL CH2Cl2 and cooled to -35 °C. To this solution solid LiAl(OC(CF3)3)4 (54.3 mg, 0.0558 mmol, 1 equiv) was added and the solution was stirred for 15 min at room temperature. All solids were filtered off and solid Lithium pentafluorobenzoate (15.8 mg, 0.0725 mmol, 1.3 equiv) was added to the filtrate. The mixture was stirred for one hour. Subsequently the reaction mixture was filtered and reduced to dryness. The residue was triturated with pentane until a yellow solid precipitated. The product can be recrystallized from chloroform. Yield: 82 mg, 80%. Mo(N-3,5-Me2-C6H3)(CHCMe2Ph)(IMes)(OTf)2 [16] (120 mg, 0.126 mmol, 1 equiv) was dissolved in 6 mL CH2Cl2 and cooled to -35 °C. To this solution solid LiAl(OC(CF3)3)4 (123 mg, 0.126 mmol, 1 equiv) was added and the solution was stirred for 15 min at room temperature. All solids were filtered off and solid lithium pentafluorobenzoate (35.8 mg, 0.164 mmol, 1.3 equiv) was added to the filtrate. The mixture was stirred for one hour. Subsequently the reaction mixture was filtered and reduced to dryness. The residue was triturated with pentane until a yellow solid precipitated. The product can be recrystallized from chloroform. Yield: 192 mg, 83%.                                                                                                                       (6) and 1959262 (15b) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre.

Comment on checkcif:
The B level alert in the analysis of compound 4a*H2O stems from the water molecules; one hydrogen of each water molecule lacks a classic acceptor for hydrogen bonding (here a carbon). This is routinely indicated by the software.