1,5-DILITHIATED ARENES - DOUBLE METAL BRIDGING VERIFIED BY 3 X-RAY STRUCTURES AND MNDO CALCULATIONS

The crystal structures of three 1,5-doubly lithium bridged heteroatom- linked diaryl systems with sulfur (4), oxygen (5), and nitrogen (6) as flexible centers have been determined. All are monomers in the crysta ls. Each lithium bridges both deprotonated ortho carbons symmetrically . The coordination sphere of the metal is completed by the bidentate l igand tetramethylethylenediamine (TMEDA). The pi-system is coplanar in o,o'-bis(lithio.TMEDA)diphenyl thioether (4b), but the rings are tilt ed markedly in o,o'-bis(lithio.TMEDA)diphenyl ether (5b) and in 1,2'-b is(lithio.TMEDA)-N-phenylcarbazole (6b). The distance between the meta lated carbons is responsible for the degree of twisting of the phenyl rings. The structural features of 4b and 6b are reproduced well by MND O calculations (with ethylenediamine (EDA) (4a-6a) as a model for TMED A), but less so for the oxygen derivative 5b. MNDO emphasizes the ther modynamic preference for double lithium bridging in o,o'-dilithiated d iphenyl systems. The MNDO relative energies of 4a-6a agree well with a b initio results with regard to the stabilization effects of each of t he three heteroatoms ortho to a negative charge. Gilman's claim that N -phenylcarbazole gave the o,o'-dilithiated-N-phenyl product could not be confirmed; instead, one of the adjacent sites on the carbazole ring is metalated. The principle of symmetrical doubly lithium bridging ha s been evaluated computationally (MNDO) for two aromatic systems dilit hiated in the 1,4-positions. The symmetrically doubly bridged rotamer of o,o'-bis(lithio.EDA)biphenyl (9a')(with perfectly coplanar pi-syste ms) is 8.8 kcal/mol more stable than the isomer with perpendicular phe nyl rings (9a''). Steric proximity of H8/H8' in doubly bridged 2,2'-bi s(lithio-EDA)-1,1'-binaphthyl (10a') prevents coplanarity of the napht hyl moieties. A slight preference (0.7 kcal/mol) for the perpendicular conformation (10a'') results. While the pi-systems are not coplanar i n 10a', this is compensated by the tilting of the naphthyl moieties. T hus, the distance between both anionic centers in 10a' is still approp riate for lithium double bridging. X-ray structures of 5b and 6b and t he MNDO geometry of 10a emphasize the preference for symmetrical doubl e lithium bridging, even if the pi-systems are twisted.