Self-adaptation of a conformationally flexible yet restricted “piperazine-pyrazine” building block toward the design of coordination polymers

Abstract

A family of divalent metal–organic coordination polymers (MOCPs) that contain a conformationally flexible N,N′-bis(2-pyrazinyl)piperazine (bpzp) ligand were reported. The MOCPs {[Cu(bpzp)₂(NO₃)₂]·3H₂O}_n (1·3H₂O), {[Co(bpzp)₂(NCS)₂]·CH₂Cl₂·2CH₃OH}_n (2·CH₂Cl₂·2CH₃OH) and {[Ni(bpzp)₂(NCS)₂]_n·4CH₃OH}_n (3·4CH₃OH) possess nearly identical extended rhombic grid-like (4,4)-layered networks, of which the latter two are isostructural and isomorphous. These layers are stacked in an eclipsed AAA arrangement for 1 and in a stagered ABABAB arrangement for 2 and 3. This can be attributed to the different conformations of the bpzp ligand, namely a C-e,e-anti conformer observed in 1 and a C-a,a-anti conformer in 2 and 3. In the case of [Ni(bpzp)₃(NO₃)₂]_n (4) and [Cd(bpzp)₃(ClO₄)₂]_n (5), the bpzp ligand is present as both a bridged bismonodentate C-e,e-anti conformer and a terminal monodentate C-e,e-syn conformer, leading to the formation of nearly identical linear chain structures. On the other hand, the MOCP {[Cd₂(bdc)₂(bpzp)₃(H₂O)₂]·H₂O}_n (6·H₂O, bdc = 1,4-benzenedicarboxylate) exhibits a 2D honeycomb (6,3)-layer structure, comprised of 1D Cd(bdc)-chain subunits and C-a,a-anti configured bpzp linkages. The MOCP [Cd₂(btec)(bpzp)]_n (7, btec = 1,2,4,5-benzenetetracarboxylate) adopts an extended 3D network comprised of Cd(btec)-based layers and C-e,e-anti configured bpzp pillars. These materials are thermally stable at temperatures up to 200 °C. The photoluminescence properties of the free bpzp ligand and the cadmium derivatives were also examined.