SYNTHESIS OF CYCLIC BIFUNCTIONAL ORGANOMAGNESIUM COMPOUNDS - X-RAY CRYSTAL-STRUCTURES OF TETRAMERIC ORGANOMAGNESIUM CLUSTERS

Three cyclic bifunctional organomagnesium compounds were prepared by three different routes. Reaction of 1,8-dibromo- or 1,8-diiodonaphthalene with metallic magnesium gave the corresponding di-Grignard reagents 3, from which the halide-free 1,8-naphthalenediylmagnesium (4) was isolated. o-Phenylenemagnesium (10) was prepared by transmetalation from o-phenylenemercury with magnesium. The reaction of freshly sublimed magnesium with diphenylacetylene gave a mixture of cis- and trans-diphenylvinylenemagnesium; cis-diphenylvinylenemagnesium (15) was isolated by crystallization from THF. The X-ray crystal structures of 4, 10, and 15 showed these compounds to be tetrameric in the solid state. Crystal data are as follow: 4, C56H56Mg4O4.5C4H8O, tetragonal, space group I4(1)/a, with cell dimensions a = 14.429(2) and c = 34.927(4) angstrom, final R = 0.068, R(w) = 0.083; 10, C40H48Mg4O4, Monoclinic, space group I2/a, with cell dimensions a = 17.071(3), b = 11.877(1), and c = 18.809(1) angstrom, beta = 99.97(1)degrees, final R = 0.066, R(w) = 0.092; 15, C72H72Mg4O4.2C4H8O, tetragonal, space group P42(1)cBAR, with cell parameters a = 17.464(1) and c = 11.911(2) angstrom, final R = 0.067, R(w) = 0.089. Their cores consist of distorted tetrahedrons of magnesium atoms. Each organic fragment is positioned above the face of such a tetrahedron and is bonded to three magnesium atoms via two carbon atoms. One carbon is single-bonded to a magnesium, and the other bridges two magnesiums by a 3-center, 2-electron bond. Each magnesium atom in the three structures has a pseudotetrahedral coordination; it is coordinated to one single-bonded and two bridging carbon atoms and a THF molecule. The arrangement of four organic groups over the four faces of a magnesium tetrahedron can be of either C2 or of S4 symmetry. Compounds 4 and 15 were found to have S4 symmetry, whereas 10 has C2 symmetry.