π-Groups in ion-pair bonding. The molecular structure of bis(tetrahydrofuran)sodium (9,10-dihydro-9,10-anthrylene)dimethylaluminate [Na(C4H8O)2]2[Al(CH3)2C14H10]2, a sodium/trimethylaluminum reduction product of anthracene

The structure of Na(THF)₂]₂Al(CH₃)₂C₁₄H₁₀]₂ has been determined by three-dimensional X-ray crystallographic techniques. The compound crystallizes in the monoclinic space group B2₁/c, and the unit cell dimensions are a=22.610(18), b=11.256(10), c=17.589(15) Å, and β=93.27(5)°. The calculated density for eight Na(THF)₂]Al(CH₃)₂C₁₄H₁₀] monomeric units is 1.196 g/cm³. The structure has been refined by least-squares to a conventional weighted R value of 0.08. The compound exists as a centrosymmetric, contact ion-pair complex with two Na(THF)₂⁺ cations complexed to the dimeric dianion. Two 1,4-dihydro-1,4-anthrylene groups are fused into the dianion via the two dimethylaluminum species. The average Al-C(anthrylene) bond distance of 2.060(8) Å is significantly longer than the average Al-C(methyl) bond distance of 2.002(9) Å. A discussion of the relative importance of steric and electronic effects is given. The importance of steric effects in ion-pairing is also considered.