Reactions of Dilithium Dibenzopentalenides with Cr(CO)₃(CH₃CN)₃: Unexpected Formation of a Cubic Tetramer of an Anionic Hydrodibenzopentalenyl Complex

Abstract

DedicatedtoProfessor Norihiro Tokitoh on the occasiono fh is 60th birthdayTo explore the coordination chemistry of dibenzopentalene dianion, reactions of two dilithium dibenzopentalenides having different silyl substituents with Cr(CO) 3 (CH 3 CN) 3 were investigated. The products were unexpected anionic complexes,T he proton at the 9-position is derived from Cr(CO) 3 (CH 3 CN) 3 ,a se videnced by the use of Cr(CO) 3 (CD 3 CN) 3 .T he X-ray diffraction analysis revealed that the chromiumi sc oordinated by an anionic fivemembered ring of the pentalene skeleton, and the lithium atom is coordinated by oxygen atoms of the carbonyl groups. The complexes form ad imer or ac age-liket etramer via carbonyl-lithium interactions,d epending on the bulk of the silyl groups.T he cubic tetramer appears to retain its cage structure in nonpolarsolvents such as benzene.Benzene and cyclopentadienyl anion (Cp À )s erve as 6p-electron-donating ligandsf or transition metals to form h 6 -arene and h 5

Cations and Anions of Dibenzo[a,e]pentalene and Reduction of a Dibenzo[a,e]pentalenophane

“…This may be due to the steric hindrance through the mesityl groups, which do not allow for an η 5 ‐coordination of the potassium ions to the reduced DBP core. In all reported solid‐state structure of DBP dianions in the literature with alkaline counter cations, both metal ions η 5 ‐coordinated to the DBP core [16, 18, 19] …”

“…Coordination of the first potassium ion is hindered by the two mesityl groups (see structure of [K(THF) 6 ] + 1 .− above), and the second potassium ion would need to bind from the bottom side of the DBP, which is sterically inaccessible. In all reported solid‐state structures of DBP dianions in the literature with alkaline counter cations, both metals coordinate from opposite sides of the DBP [16, 18, 19] . Furthermore, due to the strained structure of DBP‐phane 2 , a planarization of the DBP unit upon twofold reduction is not possible, which would be required to fully profit from the obtained aromatic character.…”

“…In all reported solid-state structure of DBP dianions in the literature with alkalinec ounter cations, both metal ions h 5 -coordinated to the DBP core. [16,18,19] For the oxidation of 1 as trongy et innocent oxidant, facilitating an outer-sphere electron transfer,w as required. Based on its high oxidation potential (see above), we used the perfluorinated dihydrophenazine cation radicalo fs alt 3,w hich was recently introduced as as trong oxidant.…”

“…The other five-membered ring contains the tetrahedral carbon atom, as can be seen from the close-up view in Figure 6C.T his proton abstraction is reminiscent of the reported reactiono fadianionic DBP speciesw ith Cr(CO) 3 (CH 3 CN) 3 ,w here protona bstraction from a CH 3 CN ligand led to ac ompound similar to 4 À À with Cr(CO) 3 h 5coordinated to the reduced five-membered ring and at etrahedral C-atom in the other five-membered ring. [19] The reason for the instability of ad ianion 2 2À À may lie in the fact that h 5 -coordination of two potassium ions is not possible due to the incorporation of the DBP into ac yclophanes tructure. Coordination of the first potassium ion is hindered by the two mesityl groups (see structure of [K(THF) 6 ] + 1C À À above), and the second potassium ion would need to bind from the bottom side of the DBP,w hich is sterically inaccessible.I na ll reported solidstate structures of DBP dianions in the literature with alkaline counter cations,b oth metals coordinate from opposite sides of the DBP.…”

“…[11] Rabinovitz [12][13][14] as well as Edlund [15] and co-workers followed with more detailed NMR-and EPR-spectroscopic studies, confirming an increased aromatic character of the oxidized and reduced DBP.F inally,p roof for the planar structure of the DBP dianion in the solid state, af urther indication for its aromaticity,w as obtained by Saito et al in 2007. [16] More examples of dianionic DBPs in the solids tate followed [17][18][19] as well as the structure of aD BP anion radical. [20] Similarly,t he structures of the reduction products of p-extended indenofluorenes werer eported.…”