Coordination polymers based on substituted terpyridine ligands: synthesis, structural diversity, and highly efficient and selective catalytic oxidation of benzylic c-h bonds

Reaction of two related rigid terpyridine ligands, 4′-(4-cyanophenyl)-4,2′:6′,4-terpyridine (L¹) and its derivative, 4′-(4-carboxyphenyl)-4,2′:6′,4-terpyridine (L²), with transition metal ions (Co²⁺ or Cu²⁺/Cu⁺), afforded four novel coordination compounds: Co(L¹)Cl₂ (1), Co₃(L¹)₃Cl₆ (2), Cu^I₉(L¹)_4.5(CN)₉ (3), and [Cu^II₃(L²)₆(H₂O)₆]·4H₂O (4). Crystal structure analysis reveals that 1 is comprised of single-stranded 2₁ helical chains, and weak interactions involving C-H···Cl weak hydrogen bonding and stacking interactions exist in the structure. The structure of 2 is comprised of 3₂ helices, which is compared with the structure of compound 1. 3 shows a three-dimensional (3D) 4-fold interpenetration structure. 4 exhibits a one-dimensional gridlike belt structure, which further builds 3D supramolecular architecture via stacking interactions. These novel coordination compounds show exceptional catalytic activity for the oxidation of benzylic C-H bonds. Notably, compound 4 shows the best catalytic properties for the oxidation of benzylic hydrocarbons up to 99% conversion and 99% selectivity.