Seletive C-X (X=F, Cl, H) Bond Activation And Functionalization Of Aromatics By Cobalt And Nickel Complexes Supported With Phosphine Ligands

Cobalt and nickel belong to VIII family of transition metals. Because of the low toxicity and lower price, they have been widely used in organic synthesis, industrial catalysis. Trimethylphosphine is a kind of strong support ligands, which has good coordination ability, simple preparation, and simple spectroscopic characteristics. It can form stable complexes with transition metal, and is widely used in organic synthesis. Cobalt and nickel complexes supported by trimethylphosphine have good reactivities and can selectively activate the C-Cl、C-F and C-H bonds. In this paper, nickel and cobalt compounds supported with phosphine ligands are used as raw materials and the main research work concludes the following five aspects:1.The reactions of ortho-chlorinated compounds bearing imine as anchoring groups with Ni(PMe3)4, Co(PMe3)4were investigated. By activation of the C-Cl bonds novel ortho-chelated nickel (Ⅱ)8,9were obtained and characterized. The double-metal ortho-chelates Ni (Ⅱ)10,11and Co(Ⅱ)14complexes were also isolated by oxidative addition of two aromatic C-Cl bonds. While the reductive elimination product cobalt (Ⅰ)12was formed through the reaction of6with Co(PMe3)4. The proposed formation mechanism of the complexes12was discussed. From another point of view, complex12is a CNN type pincer cobalt(Ⅰ) complex, and it can be used as catalyst for Kumada cross coupling reaction. The crystal and molecular structures of complexes9,10,12and14were determined by X-ray diffraction.2. The C-Cl bonds of benzamides15,16,17,18and29were successfully activated by tetrakis(trimethylphosphine)nickel(0) and tetrakis(trimethylphosphine)cobalt(0) respectively. The trans-four-coordinate nickel(Ⅱ) chloride complexes30as C-Cl bond activation products were obtained without coordination of the amide groups. In the case of benzamide15, ionic cobalt(Ⅱ) chloride,25as a C-Cl bond activation product was isolated. Instead of the n-butyl group on the amide-N atom (15), if the substituent on the amide-N atom is phenyl or its derivatives (16-18), the hexa-coordinate bis-chelate cobalt(Ⅲ) complexes26-28could be obtained via the reaction of the amides16-18with Co(PMe3)4and CoMe(PMe3)4. The phosphonium salt31was obtained by reaction of complexes30as a starting material under1bar of CO at room temperature.3. The aryne cobalt complex [Co(η2-C6Cl4)(PMe3)3(35) was generated from the reaction of hexachlorobenzene (32) with2eqiv. Co(PMe3)4through selective activation of double C-Cl bonds of hexachlorobenzene, meanwhile the byproduct CoCl2(PMe3)2was also confirmed through IR spectra. Under the similar reaction condition, the nickel(Ⅱ) complex33with a single C-Cl bond activation was isolated by the reaction of hexachlorobenzene (32) with the stoichiometric amount or two equivalents of Ni(PMe3)4, respectively, in good yields. In order to verify the reaction mechanism, the intermediate complex [CoCl(C6Cl5)(PMe3)3](34) of the proposed mechanism to aryne complex was isolated and structurally characterized by the reaction of hexachlorobenzene (32) with the stoichiometric amount of Co(PMe3)4. The structures of complexes33,34and35were determined by X-ray single crystal diffraction. Successful selective hydrodechlorination of hexachlorobenzene were carried out in the presence of cobalt catalyst supported by trimethylphosphine and with sodium formate as reducing agent in toluene or DMSO. The hydrodechlorination products C6Cl5H and C6Cl4H2were isolated in high yield, respectively, by changing the reaction conditions. The cross coupling reaction products38-41were obtained through the reaction of format reagent and halogenated hydrocarbon with Co(PMe3)4as catalysis.4. The C-H bond activation product48was obtained from the reaction of pentafluorobenzene C6F5H with Co(PMe3)4. As the byproducts hydrodefluororination product1,2,4,5-C6F4H2and F2PMe3were also observed. The reaction mechanism of C6F5H with Co(PMe3)4is proposed and partly-experimentally verified. Under similar reaction conditions, The C-H bond activation product52was obtained from the reaction of1,2,4,5-C6F4H2, with Co(PMe3)4. The reactions of C6F5H or1,2,4,5-C6F4H2with Co(PMe3)4under1bar CO at room temperature afforded hydrido dicarbonyl cobalt(II) complex (C6F5)Co(H)(CO)2(PMe3)2(50) and (C6F4H)Co(CO)2(PMe3)2(54). Treatment of the mixtures of C6F5H/Co(PMe3)4or with hexachlorobenzene, C6C16, resulted in (C6F5)CoCl(PMe3)3(51) and (C6F4H)CoCl(PMe3)3(55) via C-H bond cleavage with the hydrodechlorination product pentachlorobenzene C6Cl5H, and1,2,4,5-tetrachlorobenzene, C6Cl4H2. The crystal and molecular structures of complexes50,51,54and55were determined by X-ray diffraction.5. According to the stoichiometric reaction results and mechanism study, we designed a catalytic hydrodefluorination of perfluoroarenes with sodium formate as reducing agent under mild conditions. Several derivatives of fluoroarenes were obtained through catalytic hydrodefluorination. It is not easy to prepare some of the products directly through classic organic synthesis route. In this cobalt-catalyzed C-F bond activation system, the hydrodefluorination of octafluorotoluene, pentafluoropyridine, hexafluorobenzene, pentafluororbenzene and perfluorobiphenyl as substrates were investigated with tetrakis(trimethylphosphine)cobalt(0) as catalyst and sodium formate as reducing agent. The hydrodefluorination mechanism is proposed and discussed with the support from in situ IR experiments and structural data of the intermediates.