FORMATION, STRUCTURE, AND REACTIVITY OF DIBENZO-P-QUINODIMETHANE STABILIZED AS ETA-6-LIGANDS OF (CYCLOPENTADIENYL)RUTHENIUM AND (PENTAMETHYLCYCLOPENTADIENYL)RUTHENIUM CATIONS
Glatzhofer, DT; Liang, YW; Khan, MA; ,
Reaction of [2.2](9,10)anthracenophane (3) with 2 or 4 equiv of [CpRu(CH3CN)3]+PF6- (CP = eta5-cyclopentadienyl) results in thermally forbidden [6 + 6] cleavage of the enthano bridges to form stable complexes (trans-[(CpRu)2(eta6,eta6-dibenzo-p-quinodimethane)]2+(PF6-)2 (7) and [(CpRu)(eta6-dibenzo-p-quinodimethane)]+PF6- (8), respectively) of highly reactive dibenzo-p-quinodimethane (4). X-ray structural analysis of 7 shows it to crystallize in the triclinic space group P1BAR with a = 10.664 (4) angstrom, b = 8.948 (3) angstrom, c = 7.411 (3) angstrom, alpha = 97.14 (2)-degrees, beta = 101.46 (2)-degrees, gamma = 100.60 (2)-degrees, and Z = 1. The dibenzo-p-quinodimethane moiety in 7 is nearly flat, in contrast to the shallow boat conformation determined for its Cp* (Cp* = eta5-pentamethylcyclopentadienyl) analogue 5 and the full boat conformation calculated for 4. Localization of electrons in one benzo ring of 3 as an eta6-ligand of (CpRu+) appears to cause a loss of aromaticity in the remaining anthracene moiety, driving cleavage of the ethano bridges by rearomatization to form the noncomplexed benzo ring. Forming the second aromatic ring is a necessary structural feature in the reaction as shown for anti-[2.2](1,4)naphthalenophane (10), which reacts with [CpRu(CH3CN)3]+PF6- to form the noncleaved trans-[(CpRu)2(exo-eta6,eta6-anti-[2.2] (1,4)-naphthalenophane)]2+(PF6-)2 complex 13. X-ray structural analysis of 13 shows it to crystallize in the monoclinic space group C2/m with a = 23.281 (5) angstrom, b = 9.334 (2) angstrom, c = 7.512 (2) angstrom, beta = 103.45 (3)-degrees, and Z = 2. The ethano bridges of 13 do not show significant elongation relative to the structure of 10. The potential synthetic utility of Ru stabilized dibenzo-p-quinodimethanes was shown by the hydrogenation of [(Cp*Ru)(eta6-dibenzo-p-quinodimethane)]+CF3SO3- (6), followed by photolysis to remove (Cp*Ru+) and give pure cis-9,10-dihydro-9,10-dimethylanthracene. Treatment of 6 with bromine gave 9,10-bis(bromomethyl)anthracene. Photolysis of 6 in acetonitrile removes (Cp*Ru+), generating 4 which polymerizes to form poly((1,9)-anthracenylene).