Synthesis, structure and chemistry of a twisted olefinic bis-didentate proligand: 5,5 '-bi-5H-cyclopenta[2,1-b : 3,4-b ']dipyridinylidene

The elusive chiral twisted alkene, 5,5'-bi-5H-cyclopenta[2.1-b : 3,4-b']dipyridinylidene, known also as 9,9'-bi-4,5-diazafluorenylidene (BDAF), has been prepared in racemic form from 9-bromo-4,5-diazafluorene and from 9,9'-bi-4,5-diazafluorenyl and fully characterised. X-Ray measurements show that there is a twist of 37.8 degrees about the double bond between the 4,5-diazafluorenylidene units. A 1:1 charge-transfer compound with 7,7,8,8-tetracyanoquinodimethane (TCNQ) contains an unusual packing arrangement which is centred around the formation of spiral stacks. Each BDAF molecule contributes one 4,5-diazofluorenylidene unit to the backbone of the stack, while the second half is involved in hydrogen-bonding interactions and additional stacking with TCNQ. Examples of complexes containing the axially symmetric tetradentate ligand binding to one and to two metal ions. [M(bdaf)Cl-2] and [(MCl2)(2)(bdaf)] (M = Co, Ni, Zn), are reported.