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)]center dot 3H(2)O}(n) (1 center dot 3H(2)O), {[Co(bpzp)(2)(NCS)(2)]center dot CH2Cl2 center dot 2CH(3)OH}(n) (2 center dot CH(2)Cl(2 center dot)2CH(3)OH) and {[Ni(bpzp)(2)(NCS)(2)](n)center dot 4CH(3)OH}(n) (3 center dot 4CH(3)OH) 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 {[Cd-2(bdc)(2)(bpzp)(3)(H2O)(2)]center dot H2O}(n) (6 center dot 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 [Cd-2(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 degrees C. The photoluminescence properties of the free bpzp ligand and the cadmium derivatives were also examined.[[note]]SC