Synthesis and Ethylene Reactivity of Dinuclear Iron and Cobalt Complexes Supported by Macrocyclic Bis(pyridine-diimine) Ligands Containing o‑Terphenyl Linkers

The synthesis and ethylene reactivity of a new family of dinuclear Co₂Br₄ and Fe₂Br₄ complexes supported by binucleating macrocyclic bis­(pyridine-diimine) (PDI) ligands that contain 4,4″-R₂-3,3″-o-terphenyl linkers (1, R = H; 2, R = Me; 3, R = ⁱPr) are described. In the solid state, (1–3)­M₂Br₄ (M = Zn, Fe, Co) adopt C_s-skew-syn structures in which the (PDI)­M planes are skewed 49–82° relative to each other and both middle rings of the o-terphenyl bridges are on the same side of the molecule. The metal–metal distances range between 5.7600(8) and 6.232(1) Å. In solution, (1)­M₂Br₄ (M = Zn, Co, Fe) undergo a fluxional process that permutes the two inequivalent (PDI)­M units, while (2)­M₂Br₄ and (3)­M₂Br₄ are static and adopt C_s-symmetric structures similar to those observed in the solid state. Activation of (2)­Fe₂Br₄ and (3)­Fe₂Br₄ with MMAO-12 or triisobutylaluminum (TIBA) in the presence of ethylene generates catalysts that produce solid polyethylene (M_w = 4500–280000 Da), which contrasts with the reported production of α-olefins by analogous mononuclear (PDI)­FeCl₂ catalysts. (3)­Fe₂Br₄/TIBA and (3)­Fe₂Br₄/MMAO-12 produce polyethylenes with broad molecular weight distributions (MWDs) due to chain transfer to Al. (3)­Co₂Br₄/1000 TIBA and (3)­Co₂Br₄/1000 MMAO-12 also produce polyethylenes with broad MWDs. However, in these cases chain transfer to Al is not operative and the broad MWDs result from multisite behavior.