Chemistry of a new trispyrazolylborate ligand with some group 1 group 2 ions

by Yaman, Gül?ah

Abstract (Summary)
Single-site metal alkoxides are used to catalyze the controlled ring-opening polymerization (ROP) of lactides and other cyclic esters. In this study, a new trispyrazolylborate ligand, Tp*, containing a 3-substituted pyrazole with ?CMe2CH2OMe group has been synthesized. By synthesizing this new ligand we aimed that this ligand can form a pocket around Ca^2+ in order to protect the metal center against ligand scrambling, transesterification, or deactivation by impurities, such as water. First, the synthesis of 3-(2-methoxy-1,1-dimethylethyl)pyrazole, pz*H is described together with its reactions with the borohydrides MBH4, where M = Li, Na, and K, under melt conditions. At 180°C, this procedure leads to a mixture of products for M = Li, and at higher temperatures, a derivative LiTp'pz*H is isolated, wherein a B-H bond and a methyl group have been eliminated and a B-O bond has been formed. For M = Na, the reaction proceeds to give NaTp* but at higher temperatures NaB(pz*)4 is obtained. The reactions involving KBH4 and pz*H yield the dinuclear complex K2(Tp*)2pz*H. The reaction between NaTp* and TlOAc in dichloromethane at room temperature gives TlTp* along with NaOAc. TlTp* reacts with methyllitium in diethylether to give LiTp* and thallium metal, and, similarly, TlTp*and KH react in THF to give KTp* and Tl(0). In addition, the heavier alkaline earth metal iodides (Tp*MI where M = Ca, Sr and Ba) were synthesized by using TlTp* and the corresponding metal diiodide, MI2 in THF. However, Tp*MgI and Tp*ZnI were obtained from the reaction of MI2 and NaTp* in THF and dichloromethane, respectively. The reaction between Ca[N(SiMe33)](THF)2 and TlTp*yieldsTp*Ca[N(SiMe3)]2-via [CaTp*]+{Ca[N(SiMe3)2)3]}-.-Heating-Tp*CaN(SiMe3)2-gives-CaTp*2-by disproportionation. Whereas MgTp*2 is prepared from MgBu2 and TlTp* in THF. Both CaTp*3 and MgTp*2 exist as salts in the solid state: [Tp*M]^+[Tp*], but in solution the CaTp*3 undergoes dynamic Tp* exchange on the NMR time-scale. Slower ligand exchange in MgTp*2 is studied by variable temperature ^13C{^1H}-NMR spin saturation transfer experiments. The reaction between Tp*CaN(SiMe3)2 and p-cresol in benzene gives Tp*CaOC6H*4-p-Me which can initiate the ROP of L-LA, rac-LA, trimethylene carbonate (TMC) and ?-caprolactone (?-CL) in benzene-d6.
Bibliographical Information:


School:The Ohio State University

School Location:USA - Ohio

Source Type:Master's Thesis

Keywords:trispyrazolylborate hemilabile ether appendages group 1 and 2 ions thallium i zinc ii ring opening polymerization


Date of Publication:01/01/2008

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