The perils and opportunities of reactive building blocks: Attempted synthesis of new Hg(CN)2-based coordination polymers and the structures of the resulting products

The addition of Hg(CN)2 to an aqueous solution of CoBr2 and tris(2-aminoethyl)amine (tren) yields an oxidized peroxocobalt(III)-containing ionic complex {[Co(tren)(CN)]2(μ-O2)}[HgBr4], which has been characterized structurally. The bromide ligands have migrated to the Lewis acidic Hg(CN)2 moiety with concomitant transfer of the cyanide groups to the cobalt(III) centre. The potentially non-innocent nature of the Hg(CN)2 building block is also manifested in its reaction with CuCl2 and N,N-dimethylethylenediamine (dmeda); recrystallization of the initial precipitate from ammonia yielded the oxalate-containing coordination polymer {(dmeda)2Cu2(μ-C2O4)[Hg(CN)2Cl]2Hg(CN)2}·(H2O)1.5. The structure consists of 1D chains of [(dmeda)2Cu2(μ-C2O4)][Hg(CN)2Cl]2 units, which are formed by mercury-bridging chloride ligands as well as N-cyano coordination to the opposing sites of the copper dimer. The chains are connected into a 3D CdSO4-type array via the second Hg(CN)2 unit binding to the chlorides; adjacent chains are inclined at 59.0°. The structure can also be considered as a pair of inclined (4,4)-grids fused at [Hg(CN)2Cl2]2− centroids. The oxalate might be generated in situ as a result of the NH3-triggered release of CN− from Hg(CN)2, its reaction with available copper(II) to yield cyanogen, and the subsequent hydrolysis of (CN)2. These two products illustrate examples of potentially undesirable side-reactivities of building blocks that must be considered when rationally designing the synthesis of new coordination polymers.