Spirocyclic Sulfur and Selenium Ligands as Molecular Rigid Rods in Coordination of Transition Metal Centers

A set of analogous chalcogen-containing spirocycles, 2,6-dithiaspiro[3.3]heptane, 2,6-diselenaspiro[3.3]heptane, and 2-thia-6-selenaspiro[3.3]heptane [E₂C₅H₈, E = S (1), Se (2), and S/Se (3)], has been prepared and fully characterized by spectroscopic methods and by X-ray diffraction. The structural characterization of 2 was presented by us earlier, while the crystal structures of 1 and 3 are reported here for the first time. Molecules 1−3 are built around the central tetrahedral carbon atom and therefore are nonplanar. The E···E separation ranges from 4.690(1) Å in 1 to 4.906(1) Å in 2. Molecule 3 has statistically mixed positions of sulfur and selenium atoms in the solid state with all geometric characteristics being intermediate between those of 1 and 2. Compounds 2 and 3 have been tested as molecular rigid rod ligands in coordination reactions with transition metal complexes such as Cu(hfac)₂ (4), cis-Co(hfac)₂·2H₂O (5), and cis-Ni(hfac)₂·2H₂O (6) (hfac = hexafluoroacetylacetonate). Several coordination products built of two building blocks, M(hfac)₂ (M = Cu, Co, and Ni) and Se₂C₅H₈ (2), have been prepared in crystalline form and structurally characterized. The copper-based product (7) is comprised of the oligomeric units {[Cu(hfac)₂]₃·2μ²-Se₂C₅H₈-Se,Se‘} built on the axial Cu···Se interactions averaged at 2.909 Å. These units are further assembled into 1D polymeric chains via intermolecular Cu···F contacts at 2.829 Å. The SSeC₅H₈ (3) ligand was also used in the reaction with Cu(hfac)₂ to afford an analogue of 7, namely {[Cu(hfac)₂]₃·2μ²-SSeC₅H₈-S,Se} (8). Complex 8 exhibits statistically mixed positions of the donor sulfur and selenium atoms to give an average axial Cu···S/Se contact at 2.892 Å. In contrast to the copper complexes of composition 3:2, the stoichiometries of the isolated cobalt and nickel products are 1:1, [M(hfac)₂·Se₂C₅H₈] (M = Co (9) and Ni (10)). Complexes 9 and 10 exhibit 1D polymer structures having alternating metal units cis-M(hfac)₂ and ligands 2 with intermolecuar M···Se separations of 2.6046(8) and 2.5523(16) Å, respectively. In all products 7−10 the initial cis or trans geometry of M(hfac)₂ complexes is preserved and the spiro[3.3]heptane ligands act as bidentate linkers bridging transition metal centers via both donor ends. The magnetic properties of this series of new Cu^II, Co^II, and Ni^II complexes have been tested by variable-temperature magnetic susceptibility measurements.