CHEMISTRY OF C-TRIMETHYLSILYL-SUBSTITUTED MAIN-GROUP HETEROCARBORANES .7. REACTIVITY OF CLOSO-GERMACARBORANES AND CLOSO-PLUMBACARBORANES TOWARD A BIS(BIDENTATE) LEWIS BASE, 2,2'-BIPYRIMIDINE - SYNTHESES AND STRUCTURES OF 1 OR 1,1'-(2,2'-C8H6N4)[CLOSO-1-M-2-(ME3SI)-3-(R)-2,3-C2B4H4]N (N = 1, M = GE, N = 2, M = PB, R = ME3SI, ME, H)

The unbridged germacarborane complexes 1-Ge-(2,2’-C8H6N4)-2,3-(SiMe3)2-2,3-C2B4H4 (I), 1-Ge-(2,2’-C8H6N4)-2-(SiMe3)-3-(Me)-2,3-C2B4H4 (II), and 1-Ge-(2,2’-C8H6N4)-2-(SiMe3)-3-(H)-2,3-C2B4H4 (III) were synthesized, in yields of 78-84%, from reactions involving the corresponding closo-germacarborane and 2,2’-bipyrimidine in a respective ratio of 2:1 at room temperature in benzene solution over a period of 7-8 days. The complexes I, II, and III were characterized on the basis of 1H, 11B, and 13C pulse Fourier transform NMR, IR, and mass spectroscopy. The crystal structure of I with one molecule of cocrystallized 2,2’-bipyrimidine shows that the complex consists of only one distorted germacarborane unit, and a bipyrimidine molecule, which acts unusually as a monodentate ligand, is bonded to the apical Ge atom. This Ge-bound 2,2’-bipyrimidine base is not situated exactly opposite the C–C(cage) bond; rather, it is tilted toward the two basal borons above the C2B3 face. The nonbonded contacts between the atoms of I and the cocrystallized 2,2’-bipyrimidine exceed the sum of their van der Waals radii, and consequently, the geometry of the complex was unaffected by the presence of an extra molecule of the ligand in the crystal lattice. Complex I crystallized in the monoclinic space group P21/n with a = 7.765 (9) Å, b = 37.69 (6) Å, c = 9.651 (16) Å, β = 101.41 (1)°, V = 2768 (2) Å3, and Z = 4. Full-matrix least -squares refinements of I converged at R = 0.065 and Rw = 0.077. Although the closo-plumbacarboranes form the expected bridged complexes 1,1’-(2,2’-C8H6N4)[closo-1-Pb-2,3-(SiMe3)2-2,3-C2B4H4]2 (IV), 1,1’-(2,2’-C8H6N4)[closo-1-Pb-2-(SiMe3)-3-(Me)-2,3-C2B4H4]2 (V), and l,l’-(2,2’-C8H6N4)[closo-1-Pb-2-(SiMe3)-3-(H)-2,3-C2B4H4]2 (VI) with 2,2’-bipyrimidine in 48-58% yields, their slow formation is consistent with that of the germacarborane system in which complexation between the heterocarborane and the Lewis base was kinetically controlled. The complexes IV, V, and VI were characterized by1H,11B, and13C NMR, IR, and mass spectroscopy, and the single-crystal X-ray analyses were performed on two crystal modifications, IVA and IVB, of the complex IV. The calculated density of IVA is greater than that of IVB and is consistent with the crystal packing. The crystal structures show that the molecular geometries of both modifications are identical in that the apical lead is slipped significantly from the centroidal position above the C2B3 face in a distorted-pentagonal-bipyramidal geometry. The direct relationship between the crystal packing and the ligand–ligand dihedral angle was also evident in these structures. Complexes IVA and IVB crystallized in the monoclinic space groups P21/n and P21/a with a = 8.488 (5) and 13.882 (5) Å, b = 24.46 (2) and 9.854 (3) Å, c = 9.540 (7) and 16.104 (8) Å, β = 96.20 (5) and 105.91 (4)°, V = 1969 (2) and 2118.4 (15) Å3, and Z =2 and 2, respectively. Full-matrix least-squares refinements of IVA and IVB converged at R = 0.037 and 0.064 and Rw = 0.064 and 0.081, respectively. © 1990, American Chemical Society. All rights reserved.