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This article highlights the utilization of phosphine-containing redox-active ligands for efficient hydrosilylation catalysis. Manganese, iron, cobalt, and nickel precatalysts featuring these chelates have been described and leading activities for carbonyl, carboxylate, and ester C–O bond hydrosilylation have been achieved. Mechanistic studies have provided insight into the importance of phosphine hemilability.Free, publicly-accessible full text available November 16, 2022
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The electronic structure of a dimeric manganese hydride catalyst supported by β-diketiminate ligands, [( 2,6-iPr2Ph BDI)Mn(μ-H)] 2 , was investigated with density functional theory. A triple bond between the manganese centres was anticipated from simple electron-counting rules; however, calculations revealed Mn–Mn Mayer bond orders of 0.21 and 0.27 for the ferromagnetically-coupled and antiferromagnetically-coupled extremes, respectively. In accordance with experimentally determined Heisenberg exchange coupling constants of −15 ± 0.1 cm −1 (SQUID) and −10.2 ± 0.7 cm −1 (EPR), the calculated J 0 value of −10.9 cm −1 confirmed that the ground state involves antiferromagnetic coupling between high spin Mn( iimore »
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The manganese hydride dimer, [( 2,6-iPr2Ph BDI)Mn(μ-H)] 2 , was found to mediate nitrile dihydroboration, rendering it the first manganese catalyst for this transformation. Stoichiometric experiments revealed that benzonitrile insertion affords [( 2,6-iPr2Ph BDI)Mn(μ-NCHC 6 H 5 )] 2 en route to N , N -diborylamine formation. Density functional theory calculations reveal the precise mechanism and demonstrate that catalysis is promoted by monomeric species.
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N,N-Diborylamines have emerged as promising reagents in organic synthesis; however, their efficient preparation and full synthetic utility have yet to be realized. To address both shortcomings, an effective catalyst for nitrile dihydroboration was sought. Heating CoCl2 in the presence of PyEtPDI afforded the six-coordinate Co(II) salt, [(PyEtPDI)CoCl][Cl]. Upon adding 2 equiv of NaEt3BH, hydride transfer to one chelate imine functionality was observed, resulting in the formation of (κ4-N,N,N,N-PyEtIPCHMeNEtPy)Co. Single-crystal X-ray diffraction and density functional theory calculations revealed that this compound possesses a low-spin Co(II) ground state featuring antiferromagnetic coupling to a singly reduced imino(pyridine) moiety. Importantly, (κ4-N,N,N,N-PyEtIPCHMeNEtPy)Co was found tomore »
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The phosphine-substituted α-diimine Ni precursor, ( Ph2PPr DI)Ni , has been found to catalyze alkene hydrosilylation in the presence of Ph 2 SiH 2 with turnover frequencies of up to 124 h −1 at 25 °C (990 h −1 at 60 °C). Moreover, the selective hydrosilylation of allylic and vinylic ethers has been demonstrated, even though ( Ph2PPr DI)Ni is known to catalyze allyl ester C–O bond hydrosilylation. At 70 °C, this catalyst has been found to mediate the hydrosilylation of ten different gem -olefins, with turnover numbers of up to 740 under neat conditions. Prior and current mechanistic observationsmore »
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A (BDI)Mn catalyst has been found to hydrosilylate olefins and the observed selectivity can be attributed to alkene insertion.
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The synthesis of alkylphosphine-substituted α-diimine (DI) ligands and their subsequent addition to Ni(COD) 2 allowed for the preparation of ( iPr2PPr DI)Ni and ( tBu2PPr DI)Ni . The solid state structures of both compounds were found to feature a distorted tetrahedral geometry that is largely consistent with the reported structure of the diphenylphosphine-substituted variant, ( Ph2PPr DI)Ni . To explore and optimize the synthetic utility of this catalyst class, all three compounds were screened for benzaldehyde hydrosilylation activity at 1.0 mol% loading over 3 h at 25 °C. Notably, ( Ph2PPr DI)Ni was found to be the most efficient catalystmore »
