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

摘要

Single-site metal alkoxide initiators with a general formula of LM-OR, where L is a bulky inert ligand, are currently used to catalyze the ring-opening polymerization (ROP) of lactides and other cyclic esters with good control. In this study, a new trispyrazolylborate ligand, Tp*, containing a 3-substituted pyrazole with --CMe2CH2OMe group has been synthesized. The Tp* ligand is a potentially hexadentate kappa6-N 3,O3-donor ligand. By synthesizing this new ligand we aimed that this ligand can form a pocket around Ca2+ 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 the tris-pyrazolylborate derivative NaTp* but at higher temperatures the tetra-pyrazolylborate complex NaB(pz*)4 is obtained. The reactions involving KBH 4 and pz*H yield the dinuclear complex K2(Tp*)2pz*H. The reaction between NaTp* and TlOAc in dichloromethane at room temperature leads to the formation of TlTp* along with NaOAc. Thallium(I) cation is an efficient ligand transfer agent for Tp* ligand. TlTp* reacts with methyllithium 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 easily synthesized by using TlTp* and the corresponding metal diiodide, MI2, in THF. However, Tp*MgI and Tp*ZnI are obtained from the reaction of MI2 and NaTp* in THF and dichloromethane, respectively. In the solid state structures of the magnesium and calcium complexes there are no close contacts to the iodide and thus they exist as salts: [Tp*M]+I-. While strontium and barium complexes of the form Tp*M(I) have iodide bound to the metal with one other donor ligand such as THF. Natural bond orbital (NBO) analyses revealed that the metal ligand-bonding in pseudo-octahedral MTp* complexes is largely ionic.; The reaction between Ca[N(SiMe3)2]2(THF) 2 and TlTp* yields Tp*CaN(SiMe3)2 via [CaTp*] +{lcub}Ca[N(SiMe3)2]3{rcub}-. Heating Tp*CaN(SiMe3)2 gives CaTp*2 by disproportionation. Whereas MgTp*2 is prepared from MgBu2 and TlTp* in THF. Both CaTp*2 and MgTp*2 exist as salts in the solid state: [Tp*M]+[Tp*], but in solution the calcium compound undergoes dynamic Tp* exchange on the NMR time-scale. Slower ligand exchange in MgTp* 2 is studied by variable temperature 13C{lcub}1H{rcub}-NMR spin saturation transfer experiments.; The reaction between Tp*CaN(SiMe3)2 and p-cresol in benzene gives Tp*CaOC6H4-p-Me, a seven-coordinate calcium alkoxide. Tp*CaOC6H4-p-Me is a useful initiator for ROP of L-LA in benzene-d6 at room temperature, and it can sustain the polymerization with further stepwise loadings of L-LA. In addition, it is active for ROP of rac-LA, trimethylene carbonate (TMC) and epsilon-caprolactone (epsilon-CL) in benzene-d6.; All of the compounds have been characterized by elemental analysis, NMR spectroscopy, and by single-crystal X-ray studies, the latter of which reveal the versatile modes of binding for this new ligand bearing hemilabile ether appendages.