Noncovalent polymer-like structures of 1,1 '-bicyclohexylidene-4,4 '-dionedioxime and 1,1 '-bicyclohexyl-4,4 '-dione dioxime. Chiral recognition in the solid state

As a consequence of the high activation barrier for oxime nitrogen inversio n, the dioximes 1,1'-bicyclohexylidene-4,4'-dione dioxime (1) and 1,1'-bicy clohexyl-4,4'-dione dioxime (2) as well as their related monooximes 1,1'-bi cyclohexylidene-4-one oxime (3) and 1,1'-bicyclohexyl-4-one oxime (4) consi st of a mixture of stereoisomers. For 1, the configurational stereoisomers 1-E and 1-Z were identified in solution in a ratio 1:1 (C-13 NMR). Moreover , in the solid state, 1-Z as well as 3 also consist of conformational enant iomers, viz. 1-Z(RR)/1-Z(SS) and 3-R/3-S, respectively, which rapidly inter convert in solution due to ring inversions of the cyclohexyl moieties. Wher eas dioxime 2 consists of an enantiomer pair (2-RR/2-SS) and a meso form (2 -RS/2-SR) in a ratio 1:1:2, for monooxime 4 an enantiomer pair (4-R/4-S) in a ratio 1:1 is found (chiral HPLC). Upon recrystallization of crude 1-4 no segregation of stereoisomers occurs [H-1 and C-13 NMR, FT-IR, (chiral) HPL C, and WAXD]. Both dioximes 1 and 2 crystallize in the achiral P2(1)/c spac e group and possess nearly identical structures build up from stacked layer s consisting of parallel oriented noncovalent polymer-like chains formed vi a intermolecular oxime dimer formation by (self) complementary hydrogen bon ding [R-2(2)(6) motif in graph-set notation]. In the unit cell both molecul es possess C-i symmetry. In addition, C-i symmetry is found in the six-memb ered rings resulting from hydrogen bonding of two oxime groups. Similar cen trosymmetric oxime dimerization is also observed for 3 and 4 in the solid s tate. The C-i symmetry and the presence of multiple stereoisomers can be re conciled if chiral recognition is operational in the solid state, i.e, inte rmolecular dimeric hydrogen bonding occurs only between oxime groups of uni ts possessing opposite configurations. This poses restrictions on the stere oisomer distribution within the noncovalent polymerlike chains, viz. it dic tates their linear rodlike topology. The oxime disorder observed in all cry stal structures is attributed to random stacking of differently oriented no ncovalent polymer-like chains (1 and 2) or dimers (3 and 4).