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Development of New Cobalt Pincer Complexes for Catalytic Reduction Reactions

Li, Yingze
http://orcid.org/0000-0002-0159-2365
http://rave.ohiolink.edu/etdc/view?acc_num=ucin1554215914263187

Abstract Details

2019, PhD, University of Cincinnati, Arts and Sciences: Chemistry.
Several new cobalt complexes bearing bis(phosphinite)-based (POCOP) pincer ligands have been synthesized through direct C–H bond activation. Various cobalt complexes or salts including Co2(CO)8, Co(PMe3)4H, Co(PMe3)4Me and CoCl2 have been tested as the precursors. A series of cobalt(I) POCOP pincer complexes were isolated from the reaction of Co2(CO)8 with various POCOP pincer ligands. The resulting pincer complexes, (RPOCOP)Co(CO)2 (R for substituents on the phosphorus donors) for short, can be oxidized by I2 to yield Co(III) complexes. Another series of cobalt(I) POCOP pincer complexes featuring two PMe3 ligands have been synthesized through the reaction of the POCOP ligands with Co(PMe3)4H or Co(PMe3)4Me. The dissociation of one PMe3 ligand enables the cobalt center to react with phenylsilane to form a cobalt(III) complex. CoCl2 has also been demonstrated to be an effective precursor to activate the central C–H bond of a POCOP pincer ligand to afford a cobalt(II) pincer complex. In addition to the POCOP pincer system, a bis(phosphinomethyl)pyrrolide-based (PNP) cobalt(I) pincer complex has also been synthesized using PMe3 as an ancillary ligand to stabilize the metal complex. Among these newly synthesized cobalt pincer complexes, complexes (RPOCOP)Co(CO)2 prove to be efficient catalysts for the catalytic hydrosilylation of aldehydes with (EtO)3SiH at elevated temperatures (=100°C) in a closed system or at 50 °C in an open system. Mechanistic investigation has suggested that the reaction is initiated by CO dissociation, which is kinetically feasible even at room temperature but thermodynamically unfavorable. Detailed mechanistic studies focusing on the cobalt dicarbonyl complexes catalyzed hydrosilylation of aldehydes have been conducted. The cobalt dicarbonyl complexes are shown to be pre-activated by an aldehyde at elevated temperatures in an open system. The “same tube” and “separate tubes” competition experiments support either a decomposition mechanism or an “aldehyde pre-activation” mechanism. The analogous (iPrPOCOP)Co(PMe3)2 catalyzed hydrosilylation of aldehydes is also shown to proceed via a ligand dissociation mechanism; however, in this case, a cobalt(III) intermediate has been independently synthesized. In our previous research, iron and nickel complexes supported by the same POCOP pincer ligands, (iPrPOCOP)Fe(H)(PMe3)2, (iPrPOCOP)Fe(H)(CO)2, and (iPrPOCOP)NiH, were also studied for catalytic hydrosilylation of aldehydes. Among these three different metal-based catalytic systems, the cobalt and iron complexes catalyze the reactions through the dissociation of the ancillary ligand (CO or PMe3), even though in the latter system a hydride ligand is present. In contrast, for the nickel system, the catalytic reaction starts with C=O insertion into the Ni–H bond. To compare the reactivity and mechanisms of these three different metal-based catalytic systems, a more detailed mechanistic investigation of the nickel and iron systems has been conducted. Furthermore, catalytic hydrogenation of aldehydes with a well-defined, square planar cobalt complex, (CyPNP)CoCl, has been studied. A cobalt hydride intermediate has been successfully isolated. Even though an aldehyde insertion mechanism has been established with the Co(II) hydride, a detailed mechanistic study also supports the involvement of a Co(I) intermediate.
Hairong Guan, Ph.D. (Committee Chair)
Michael Baldwin, Ph.D. (Committee Member)
David Smithrud, Ph.D. (Committee Member)
139 p.
Chemistry
cobalt; pincer complexes; hydrogenation; hydrosilylation; C-H bond activation

Recommended Citations

Citations

  • Li, Y. (2019). Development of New Cobalt Pincer Complexes for Catalytic Reduction Reactions [Doctoral dissertation, University of Cincinnati]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1554215914263187

    APA Style (7th edition)

  • Li, Yingze. Development of New Cobalt Pincer Complexes for Catalytic Reduction Reactions. 2019. University of Cincinnati, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ucin1554215914263187.

    MLA Style (8th edition)

  • Li, Yingze. "Development of New Cobalt Pincer Complexes for Catalytic Reduction Reactions." Doctoral dissertation, University of Cincinnati, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1554215914263187

    Chicago Manual of Style (17th edition)

ucin1554215914263187
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This open access ETD is published by University of Cincinnati and OhioLINK.