trans-Difluorosilicon(IV) Complexes of Tetra-p-tolylporphyrin and Tetrakis(p-(trifluoromethyl)phenyl)porphyrin: Crystal Structures and Unprecedented Reactivity in Hexacoordinate Difluorosilanes1

Single-crystal X-ray structures have been determined for the difluoro(porphyrinato)silicon(IV) complexes trans-(Por)SiF(2) (Por = the dianions of tetra-p-tolylporphyrin (TTP) and tetrakis(p-(trifluoromethyl)phenyl)porphyrin). Crystallographic data for (TTP)SiF(2).Et(2)O: monoclinic, space group C2/c, a = 30.228(4) Å, b = 9.913(4) Å, c = 15.474(5) Å, alpha = gamma = 90 degrees, beta = 114.58(3) degrees, V = 4217(2) Å(3), Z = 4, R1 = 0.0588. Crystallographic data for (TTFP)SiF(2): monoclinic, space group C2/c, a = 31.557(2) Å, b = 9.546(1) Å, c = 15.941(1) Å, alpha = gamma = 90 degrees, beta = 115.83(1) degrees, V = 4322.4(8) Å(3), Z = 4, R1 = 0.0489. In both structures, the silicon lies in a slightly distorted octahedral geometry (average distances: Si-F 1.642 Å and Si-N 1.919 Å) with the fluorides in a trans configuration, and the porphyrin is in a ruf nonplanar form. The trans-(Por)SiF(2) structures were compared to the structures of related hexacoordinate cis-difluorosilanes and other group 14 metalloporphyrins. (TTP)SiF(2) readily reacts with excess MeMgBr or LiPh to give (TTP)SiMe(2) or (TTP)SiPh(2), respectively, in contrast to related hexacoordinate cis-difluorosilanes which do not react with strong nucleophiles. The enhanced reactivity of (TTP)SiF(2) may be a combination of a trans-effect, even though the structural parameters for cis and trans Si-F bonds and Si-N bonds are essentially the same, and a single-electron transfer process involving the porphyrin ligand.