SYNTHESIS AND STRUCTURAL SYSTEMATICS OF NITROGEN BASE ADDUCTS OF GROUP-2 SALTS .2. SOME ADDUCTS OF GROUP-2 SALTS WITH PYRIDINE

Recrystallization of Group 2 halides from pyridine (py) solution has y ielded, in a number of cases, crystalline adducts MX(2).npy which have been the subject of room-temperature single-crystal X-ray structural characterization. MgBr2.6py (1) is orthorhombic, Ccca, a 11.575(2), b 14.999(4), c 17.292(2) Angstrom, Z = 4; conventional R on \F\ for N-o = 752 'observed' (I > 3 sigma(I)) reflections was 0.043. The compound is formulated as [(py)(4)MgBr2].2py, the metal atom on a site of 222 s ymmetry having a trans-X(2)MN(4) coordination environment. With CaCl2, a 1:2 adduct (2) is formed, orthorhombic, Fdd2, a 33.83(1), b 18.143( 2), c 3.981(7) Angstrom, Z = 8 f.u., R 0.040 for N-o 451; the complex is an infinite polymer along c, [(py)(2)CaCl2]((infinity/infinity)), t he ... CaCl2CaCl2Ca ... spine having six-coordinate, octahedral calciu m environments in which the pyridine nitrogen atoms are bound trans to either side, with the calcium site symmetry being 2. With SrBr2, a 1: 5 adduct (3) is found, mononuclear, [(py)(5)SrBr2], with the metal ato m in a pentagonal bipyramidal coordination environment (orthorhombic, Pcab, a 18.611(5), b 17.768(7), c 17.411(7) Angstrom, Z = 8; R 0.060 f or N-o 949). With BaI2, a 1:6 adduct (4) is obtained, [(py)(6)BaI2], t he eight-coordinate barium atom lying on a crystallographic 2 axis in a monoclinic C2/e cell, Z = 4, a 18.311(2), b 9.960(3), c 18.856(8) An gstrom, beta 99.28(3)degrees (R 0.031 for N-o 2279), with the iodide a toms occupying a pair of B sites in one of the trapezoidal planes of a quasi-dodecahedral array. [(2-mpy)(2)MgBr2] (5), 2-mpy = 2-methylpyri dine, is of interest, offering a four-coordinate N2MgBr2 array (tricli nic, P1, a 8.046(3), b 8.873(3), c 11.475(1) Angstrom, alpha 86.25(2), beta 78.82(5), gamma 67.88(5)degrees, Z = 2; R 0.069 for N-o 2091), w hile [(py)(6)/(4-mpy)(6)Ba(O2ClO2)(2)] (6) and (7), perhaps surprising ly, are isomorphous with (4), with O,O'-bidentate perchlorates replaci ng iodide in the coordination sphere (R 0.031 and 0.052 for N-o 2401 a nd 2204 respectively). With 3,5-dmpy (3,5-dimethylpyridine), strontium iodide yields a trans-six- rather than seven-coordinate adduct, [(3,5 -dmpy)(4)SrI2] (8), the molecule conforming to crystallographic 4/m sy mmetry in the tetragonal space group I 4/m, a 9.808(4), c 17.196(8) An gstrom, Z = 2; R 0.066 for N-o 608.