Group 12 Metal Complexes of an 18-Membered N2O2S2 Macrocycle Incorporating Two Pyridines: First Examples of an Infinite Mercury(I) Complex and a Dumbbell-Shaped Heteronuclear Complex with a Macrocyclic Ligand.

Abstract

Homo- and heteronuclear group 12 metal (Zn2+, Cd2+, and Hg2+) complexes 1-6 containing a newly designed 18-membered N2O2S2 macrocycle incorporating two pyridine subunits (L) were prepared and structurally characterized. The individual complexes isolated exhibit unusual stoichiometries, geometries, oxidation states, and structural topologies and include an infinite mercurous complex and a heteronuclear dumbbell-shaped complex. Both the Zn(II) complex [Zn(L)][ZnBr4] (1) and the Cd(II) complex [Cd(L)Br2] (2) are mononuclear, with the metal ion located inside the macrocyclic cavity. The six-coordinated Zn(II) center in 1 adopts an octahedral geometry and is shielded from the anion and solvent by the strongly bound macrocycle. The Cd(II) center in 2 is seven-coordinate, being bound equatorially to two N donors, two O donors, and a S donor from the macrocycle and axially to two bromide ions on opposite sides of the macrocyclic plane, adopting a pentagonal-bipyramidal geometry. In the Hg(II) complexations, the configuration adopted by the macrocycle L shows a dependence of the nuclearity on the anion used. When mercury(II) bromide was used, the dinuclear complex [HgII2(L)Br4] (3) was obtained, while the reaction with mercury(II) nitrate afforded the unexpected Hg(I) complex {[HgI2(L)](NO3)2}n (4) with a one-dimensional polymeric structure. In heterometallic complexation experiments, one-pot reaction of L with a mixture of ZnBr2 and CdBr2·4H2O resulted in the stepwise isolation of two pure solubility-dependent Cd(II) complexes (2 and 5), including the half-dumbbell-type complex [Cd(L)(μ-Br)(CdBr3)] (5), while a mixture of CdBr2·4H2O and HgBr2 yielded the heterometallic bis(macrocycle) product [(CdL)2(μ-Hg2Br6)](Hg2Br6) (6). This is the first example of a heteronuclear dumbbell-shaped complex in which two terminal macrocyclic Cd(II) complexes are linked by a hexabromodimercury(II) cluster via Cd-Br-Hg bonds. The heterometallic dumbbell 6 can be considered as a good example of competition and collaboration between Cd(II) and Hg(II) ions because its formation is associated with the higher coordination affinity of Cd(II) toward the macrocycle and the formation of the (Hg2Br6)2- cluster, which links the two endocyclic Cd(II) complexes directly. Both NMR titration and comparative NMR data indicate a relatively higher coordination affinity of Cd(II) toward the macrocycle than occurs for Hg(II), in parallel to the situation observed in the solid state.