Demott, Jessica Carman (2015-12). Unsaturated Pt Pincer Complexes and Attempts toward the Isolation of a Methane ?-Complex. Doctoral Dissertation. Thesis uri icon

abstract

  • Alkane ?-complexes have been identified as key intermediates in C-H bond activation and subsequent functionalization of alkanes. Methane and larger saturated alkanes, however, are sterically and electronically disinclined to bind to transition metals. In particular, a thermodynamically stable methane ?-complex intermediate has not been attained or structurally characterized. This research aimed to make progress toward the development and isolation of stable organometallic methane ?-complexes. 16-electron d^8 Pt(II) complexes are typically stable square planar, 4-coordinate species that require ligand dissociation in order to provide an open coordination site for methane. Accessing stable three coordinate Pt(II) species, however, is a challenge. Novel Pt silylene pincer ligand scaffolds, however, were found to undergo a net 1,2-migration of R from Pt to Si, yielding highly unsaturated Pt cations with weakly coordinating carborane anions. Apparent protodesilylation of mesitylene resulted in two Pt cationic units linked together by a siloxane bridge where unsaturation at the Pt centers was retained. Attempts to isolate unsaturated three-coordinate Pt species with bis(phosphine)diarylamido-based (PNP) pincer ligands were frustrated by rapid C-H activation of various arene reaction solvents. When dichloromethane was used as a solvent, formation of the paramagnetic species [(PNP)PtCl]^+ was observed. This is consistent with the radical abstraction of a chlorine atom and oxidation of the ligand, demonstrating the system's potential for metalloradical behavior. Overall, the suspected (PNP)Pt^+ intermediate is a highly reactive species that exhibits both strongly electrophilic and metalloradical behavior. Exploring alternative pincer ligand scaffolds for generating stable methane ?-complexes lead us to the use of carbazole pincer ligands that have lower oxidation potentials, a less basic central nitrogen, and a new steric profile. In contrast to previously synthesized Pt triflate complexes, facile displacement of triflate by water was observed for a Pt complex of a bis(imino)carbazolyl (NNN) pincer ligand containing mesityl sidearms. The steric bulk projected by the mesityl groups of the NNN ligand may favor the coordination of smaller ligands such as water. Unexpectedly facile cyclometallation of the mesityl sidearm was also observed.
  • Alkane ?-complexes have been identified as key intermediates in C-H bond activation and subsequent functionalization of alkanes. Methane and larger saturated alkanes, however, are sterically and electronically disinclined to bind to transition metals. In particular, a thermodynamically stable methane ?-complex intermediate has not been attained or structurally characterized. This research aimed to make progress toward the development and isolation of stable organometallic methane ?-complexes.

    16-electron d^8 Pt(II) complexes are typically stable square planar, 4-coordinate species that require ligand dissociation in order to provide an open coordination site for methane. Accessing stable three coordinate Pt(II) species, however, is a challenge. Novel Pt silylene pincer ligand scaffolds, however, were found to undergo a net 1,2-migration of R from Pt to Si, yielding highly unsaturated Pt cations with weakly coordinating carborane anions. Apparent protodesilylation of mesitylene resulted in two Pt cationic units linked together by a siloxane bridge where unsaturation at the Pt centers was retained.

    Attempts to isolate unsaturated three-coordinate Pt species with bis(phosphine)diarylamido-based (PNP) pincer ligands were frustrated by rapid C-H activation of various arene reaction solvents. When dichloromethane was used as a solvent, formation of the paramagnetic species [(PNP)PtCl]^+ was observed. This is consistent with the radical abstraction of a chlorine atom and oxidation of the ligand, demonstrating the system's potential for metalloradical behavior. Overall, the suspected (PNP)Pt^+ intermediate is a highly reactive species that exhibits both strongly electrophilic and metalloradical behavior.

    Exploring alternative pincer ligand scaffolds for generating stable methane ?-complexes lead us to the use of carbazole pincer ligands that have lower oxidation potentials, a less basic central nitrogen, and a new steric profile. In contrast to previously synthesized Pt triflate complexes, facile displacement of triflate by water was observed for a Pt complex of a bis(imino)carbazolyl (NNN) pincer ligand containing mesityl sidearms. The steric bulk projected by the mesityl groups of the NNN ligand may favor the coordination of smaller ligands such as water. Unexpectedly facile cyclometallation of the mesityl sidearm was also observed.

publication date

  • December 2015