Abstract
We have labelled as “bridge-flipped isomers” those pairs of molecules that differ only in the orientation of a bridge of atoms linking two larger parts of the molecules. Among the benzylideneanilines, this isomerism is R-CH=N-R′ vs. R-N=CH-R′. Pairs of these isomers that are isomorphous might be capable of forming solid solutions with tailorable properties. Although differences in molecular space-filling properties between bridge-flipped benzylideneanilines are minimal, isomorphous pairs are rare. Differences in crystal structure can result from differences in molecular conformation due to steric interaction between the bridge hydrogen atom and an ortho hydrogen atom on the aniline ring, forcing the aniline ring farther than the benzylidene ring out of coplanarity with the bridge. We have determined the crystal structures of some 2,2′-disubstituted benzylideneanilines to determine whether this substitution pattern can offset the conformational difference and thereby encourage isomorphism. We have found 2-bromobenzylidene-2′-chloroaniline (Br)CH=N(Cl) and 2-chlorobenzylidene-2′-bromoaniline (Br)N=CH(Cl) to assume crystal structures in space group Pbcn isomorphous with each other and with our previously published 2-methylbenzylidene-2′-bromoaniline (CH3)CH=N(Br) (RIHMOL) and 2-bromobenzylidene-2′-methylaniline (CH3)N=CH(Br) (RIFYEL) isomorphous pair, as well as with 2-methylbenzylidene-2′-chloroaniline (CH3)CH=N(Cl) and its bridge-flipped isomer 2-chlorobenzylidene-2′-methylaniline (CH3)N=CH(Cl) (ZAHYAL, published by previous workers). In this same Pbcn isomorphous group are 2-bromobenzylidene-2′-bromoaniline (Br)CH=N(Br), 2-chlorobenzylidene-2′-chloroaniline (Cl)CH=N(Cl), 2-chlorobenzylidene-2′-cyanoaniline (Cl)CH=N(CN), and 2-fluorobenzylidene-2′-bromoaniline (F)CH=N(Br) (FOBKIS, published by previous workers). (CH3)CH=N(Cl) also exists as a P212121 structure isomorphous with 2-methylbenzylidene-2′-methylaniline (CH3)CH=N(CH3). (Cl)CH=N(CN) is not isomorphous with its bridge-flipped isomer (Cl)N=CH(CN), which assumes a P21/n structure isomorphous with (Br)N=CH(CN) and (I)N=CH(CN). (I)N=CH(CN) is not isomorphous with its bridge-flipped isomer (I)CH=N(CN), which crystallizes in space group P(-1), although both packing arrangements involve close I…NC intermolecular contacts. Relatively strong or directional intermolecular contacts between ring substituents may prove to impede isomorphism rather than facilitate it.