Catalytic Formylation of Amines with Co2 and H2 Using Complexes of Abundant Metals
Catalytic formylation of amines with CO2 and H2 is an efficient and selective way to produce formamides, which have many applications in the industry. However, all of the highly active catalysts reported in the literature have used precious metals, which are expensive, scarce, at risk of depletion, and toxic. Recently, research has focused on formylation of the amine with CO2 and hydrogenation using inexpensive abundant-metals. In this regard, this thesis focuses on the development of homogeneous catalysts for the hydrogenation of CO2 to formamides using abundant-metals. Firstly, in situ formation of catalysts from abundant-metal salts and phosphine ligands was tested for the formylation of dimethylamine, 2-ethylhexylamine, morpholine, aniline, and N-methylaniline with H2 and CO2. From the screening tests, efficient in-situ catalysts such as Ni(acac)2 and Ni(OAc)2·4H2O/dmpe (acac- = acetylacetonato; dmpe = 1,2-bis(dimethylphosphino)ethane) were identified for the formylation of morpholine and 2-ethylhexylamine, respectively. The hydrogenation of CO2 to formamide using a primary/secondary amine was also tested in the presence of preformed catalysts ([NiX2(dmpe)] or [NiX(dmpe)2]X, where X = Cl-, CH3CO2-, acac-, BPh4-) in DMSO. Morpholine and 2-ethylhexylamine are formylated at 100 °C and 135 °C, respectively at a total pressure of 100 bar with TONs (TON = turnover number) up to 18,000, and 1,610, the highest TON using any abundant-metal phosphine complex to date. With the appropriate selection of catalyst and reaction conditions, >90-98% conversion of amine to formamide was achieved, and the effect of changing the metal to amine ratio was also investigated. Nickel complexes have not been much explored for the hydrogenation of CO2 to formic acid derivatives. We describe the syntheses of Ni(II) complexes, [Ni(dmpe)2Cl]Cl; [Ni(dmpe)Cl2]; [Ni(Me2(O)P(CH2)2P(O)Me2)(acac)2]n; and [Ni(acac)(MeO)(MeOH)]4. The preformed Ni(II) complexes were also tested as homogeneous catalysts for the hydrogenation of CO2 to formamide using morpholine and dimethylamine as substrates. It was found that DMF formed with a TON value of 6,300, the highest TON for that product using abundant-metal phosphine complexes to date. Co(II)/Ni(II)-PPN (PNN = (2-(di-tert-butylphosphinomethyl)-6- diethylaminomethyl)pyridine) complexes have also been prepared and characterized, and tested for CO2 hydrogenation to formamide. Unfortunately, these complexes were not catalytically active.
URI for this recordhttp://hdl.handle.net/1974/24248
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