Ligand exchange and abstraction reactions involving titanium isopropoxide with tris(pentafluorophenyl)borane and -alane: ramifications for ring-opening polymerization of propylene oxide

In situ mixing of Ti(OiPr)4 and B(C6F5)3 generates a very efficient catalyst system for ring-opening polymerization (ROP) of propylene oxide (PO) with a turnover frequency (TOF) >1350/h, whereas the mixture of Ti(OiPr)4 and Al(C6F5)3 is inactive for the same polymerization. The inactivity of the Ti(OiPr)4/Al(C6F5)3 mixture is due to the formation of the stable isopropoxy-bridged bimetallic species Ti(OiPr)3(μ-OiPr)Al(C6F5)3 (1), the structure of which has been confirmed by X-ray diffraction. The products of the Ti(OiPr)4 + B(C6F5)3 reaction, however, depend on the Ti(OiPr)4:B(C6F5)3 ratio. The 1:1 ratio reaction in toluene at ambient temperature is rapid and produces the ligand exchange products: Ti(OiPr)3C6F5 and iPrOB(C6F5)2 (2), along with a small amount of (iPrO)2BC6F5. The two resulting boranes are inseparable by recrystallization or vacuum distillation, and the formation of the undesired (iPrO)2BC6F5 is either significantly enhanced upon heating the reaction in toluene to 80 °C or nearly exclusive by carrying out the reaction in THF. By employing 1.2 equiv. of B(C6F5)3 in the reaction with Ti(OiPr)4, however, the formation of (iPrO)2BC6F5 is suppressed, enabling the isolation of the new borane 2 in its pure state. The excess of B(C6F5)3 added to the reaction apparently slows down the exchange reaction by stabilizing the intermediate Ti(OiPr)3(μ-OiPr)B(C6F5)3 (4), as shown by the 1:2 Ti(OiPr)4:B(C6F5)3 reaction which initially forms the ligand abstraction product 4 followed by subsequent slow ligand exchange to give the final products Ti(OiPr)3C6F5 and 2. The studies of these individual reactions, in combination with control polymerization runs, reveal that the active species responsible for the catalytic activity of the Ti(OiPr)4/B(C6F5)3 mixture is the isopropoxy borane 2. Thus, the isolated 2, in the absence or presence of a hydroxylic initiator, serves as a very effective catalyst for the ROP of PO, producing PPOs with Mn=2000–3000, Mw/Mn=1.30–1.43, and TOF >1400/h. The MALDI-TOF MS analyses of the PPOs formed show the linear PPO structures having the initiator and water molecules as end groups, demonstrating the control over the PPO structure.