Synergic Catalysis: the Importance of Intermetallic Separation in Co(III)K(I) Catalysts for Ring Opening Copolymerizations

Dinuclear polymerization catalysts can show high activity and control. Understanding how to design for synergy between the metals is important to improving catalytic performances. Three heterodinuclear Co(III)K(I) catalysts, featuring very similar coordination chemistries, are prepared with different intermetallic separations. The catalysts are compared for the ring-opening copolymerization (ROCOP) of propene oxide (PO) with CO₂ or with phthalic anhydride (PA). The catalyst with a fixed, wide intermetallic separation, L_wideCoK(OAc)₂ (Co-K = 8.06 Å), shows very high activity for PO/PA ROCOP, but is inactive for PO/CO₂ ROCOP. On the other hand, the catalyst with a fixed, narrow intermetallic separation, L_shortCoK(OAc)₂ (Co-K, 3.59 Å), shows high activity for PO/CO₂ ROCOP, but is much less active for PO/PA ROCOP. A bicomponent catalyst system, comprising a monometallic complex L_monoCoOAc used with an equivalent of KOAc[18-crown-6], shows high activity for both PO/CO₂ and PO/PA ROCOP, provided the catalyst concentration is sufficiently high, but underperforms at low catalyst loadings. It is proposed that the two lead catalysts, L_wideCoK(OAc)₂ and L_shortCoK(OAc)₂, operate by different mechanisms for PO/PA and PO/CO₂ ROCOP. The new wide separation catalyst, L_wideCoK(OAc)₂, shows some of the best performances yet reported for PO/PA ROCOP, and suggests other catalysts featuring larger intermetallic separations should be targeted for epoxide/anhydride copolymerizations.