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

Abstract

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.

Electrochemistry of hexacoordinate silicon(IV) porphyrin complexes

“…Silicon(iv) porphyrin complexes readily conform to this trend. Single crystal X-ray diffraction studies on (TTP)SiF2 [47] and (TTP)Si(OTf)z [ 171 show that the fluoride derivative is less ruffled than the triflate derivative and our electrochemical studies ( Table 1) have shown that the fluoride derivative is easier to reduce.…”

Main-Group-Element Isophlorin Complexes Revisited: The Question of a Subvalent Central Atom

“…(see the ruffling dihedrals  listed in Table 1). 26,27 A plausible explanation for this difference is that there is less of an energy cost associated with nonplanar deformation of an antiaromatic isophlorin relative to an aromatic porphyrin. 28,29,30 Peripheral bond length alternation and ruffling together result in an overall lower molecular symmetry, D2, for the reduced six-coordinate complexes E(TPP)L2, which may be compared with D2d for Si(TPP)F2…”

“…(RITFOP 26 ) and approximately D4h for Ge(TPP)Cl2 (NIPKEC 27 ). For the four-coordinate complex Ge(TPP), the calculations also led to a delocalized structure, consistent with an aromatic Ge II (Por 2-) formulation and in excellent agreement with the crystal structure (LIJVAC 4 ).…”

Crystallization by Slow Halogen Exchange in Hypercoordinate Silicon Chelates and the first X‐ray Structure of a trans‐Featured Hexacoordinate Difluorosilicon‐bis‐Chelate

“…Formerly published hexacoordinate difluorosilicon complexes with trans-situated fluorine atoms were equipped with rather rigide tetradentate chelating ligands, e.g. salen or porphyrine [5,8]. The conversion of 2c into the cis-configured complex in solution indicates to a small energy gap between both isomers.…”