Formation of Zn(ii) and Pb(ii) coordination polymers of tetrakis(4-pyridyl)cyclobutane controlled by benzene and toluene

Abstract

Reactions of Zn(NO₃)₂·6H₂O with tetrakis(4-pyridyl)cyclobutane (tpcb) in MeOH followed by addition of benzene or toluene gave rise to two two-dimensional (2D) coordination polymers {[Zn(tpcb)(NO₃)₂]·2S}_n (1: S = benzene; 2: S = toluene). The analogue reactions of Pb(NO₃)₂ with tpcb in similar solvent systems resulted in the formation of {[Pb(tpcb)_0.5(NO₃)₂(MeOH)]·benzene}_n (3) and [Pb(tpcb)(NO₃)₂(MeOH)]_n (4), respectively. Slow evaporation of the MeOH solution containing Pb(NO₃)₂ and tpcb generated [Pb(tpcb)(NO₃)₂(H₂O)]_n (5). Compounds 1 and 2 show similar 2D networks but differ in their packing modes, namely “overlapping” and “crisscross” modes. Compound 3 exhibits a 3D porous framework in which its 1D channels are occupied by benzene molecules. Compound 4 consists of 1D [Pb(tpcb)(NO₃)₂(MeOH)]_n chains which are linked by weak Pb⋯N secondary interactions to form a 2D gridlike network. Compound 5 has a 3D structure constructed by dinuclear [Pb₂(tpcb)₆(NO₃)₄(H₂O)₂] units and tpcb bridges with a Schläfli symbol of (4·6²)·(4²·6¹⁰·8²). The solvent effects may be ascribed to the formation of the five 2D and 3D coordination polymers 1–5. The inclusion of benzene and toluene molecules in the frameworks of 1 and 2 along with the time lengths of their crystal growth may provide an interesting approach to the selective separation of benzene and toluene.