A room temperature synthesis of perstanna[1.1.1]propellanes and the structure/property relationships revealed by a comparison of two derivatives

Article abstract of DOI:10.1021/ja00044a011

Chemical reduction of hexakis(2,6-diethylphenyl)cyclotristannane (2) with 2.3 equiv of lithium metal in THF provides hexakis(2,6-diethylphenyl)pentastanna[1.1.1]propellane (1) (31% yield) and octakis(2,6-diethylphenyl)tetracyclo-[4.1.0.0^1,5.0 ^2,6]heptastannane (4) (～1% yield). With 1.2 equiv of lithium metal, the same procedure provides 1,2,2,3,3,4,4-heptakis(2,6-diethylphenyl)cyclotetrastannane (3) (85% yield) and tris(2,6-diethylphenyl)stannane (5) (103% yield). A proposed mechanism to account for the formation of 3 proceeds through the intermediacy of the monovalent tin species, [R₂S_n]^- (R = 2,6-diethylphenyl) (9) and 1-lithio-1,2,2,3,3,4,4-heptakis(2,6-diethylphenyl)cyclotetrastannane (14). Evidence for the existence of 9 is provided by an ESR spectrum of a mixture of 2, 0.5% potassium amalgam (1 equiv), and 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (crypt) (1 equiv) in THF which displays a single strong resonance centered at g = 2.024 [a(^119/117Sn) = 152 G]. Compound 14 has been synthesized separately by deprotonation of 3 with lithium diisopropylamide in THF, and it has been isolated as an orange microcrystalline material (43% yield). Reaction of 14 with an excess of lithium metal produces 1 in a 30% yield which supports the observation that this compound appears to be the key intermediate in the transformation of 2 to 1 and 4. Single crystals of 4, obtained from a toluene/acetonitrile solvent mixture at -40°C, are, at 20°C, monoclinic, space group C2/c-C⁶2h with a = 27.968 (7) A?, b = 16.000 (4) A?, c = 38.510(11) A?, β = 103.17 (2)°, V = 16780 (8) A?³, and Z = 8 [d_calcd = 1.501 g cm^-3; μ_a(Mo Kα) = 2.09 mm^-1]. The molecular structure of 4, as obtained from crystallographic analysis (R₁ = 0.047 for 6189 independent reflections), reveals that the [1.1.1]propellane core of this compound is contracted relative to 1 with a mean Sn_bh-S_br bond length value of 2.845 (18) A? and a Sn_bh-Sn_bh, distance of 3.348 (1) A?. On the basis of a correlation between the reduction of this latter value with an hypsochromic shift and increased intensity of an electronic transition, assumed to originate from the HOMO of perstanna[1.1.1]propellanes, in going from 1 to 4, a significant bonding interaction between the two inverted tetrahedral tin atoms in this class of compounds is proposed. Cyclic voltammetry of 4 in THF shows two quasireversible one-electron reduction waves at E_1/2 = -1.35 and -1.90 V (V vs NHE) which correspond to the [4]/[4]^- and the [4]-[4]^2- redox couples, respectively. Finally, chemical reduction of 4 can be achieved with 0.1% potassium amalgam in THF in the presence of crypt to generate, in situ, the complex [4]^-[K,crypt]⁺, and the isotopic ESR spectrum (25°C) of this species displays a single resonance centered at g = 1.95. Simulation of this spectrum can be accomplished by assuming hyperfine interactions with three sets of equivalent tin nuclei with the following parameters: a(^119/117Sn) = 22 G (2 SB atoms); a(^119/117Sn) = 50 G (2 Sn atoms); a(^119/117Sn) = 65 G (3 Sn atoms); line width = 6.5 G.