Bridge-Flipped Isomers: Searching for Isomorphous Benzylideneanilines and Phenylhydrazones Reveals Unexpected Structures

Abstract

Examination of a series of "bridge-flipped" halogen-and nitrile-substituted benzylideneanilines for isomorphism by single-crystal X-ray diffraction has resulted in the identification of three non-isomorphous pairs:Their crystal structures are rendered unique by differences in solid-state molecular conformation and intermolecular interactions. The crystal structures of N-(4-cyanobenzylidene)-2-bromoaniline (2Bran) and N-(4-cyanobenzylidene)-2-iodoaniline (2Ian) were determined, but upon attempts to prepare their bridge-flipped isomers the unexpected dianilines 1-(2-bromophenyl)-N,N′-bis(4-cyanophenyl)methanediamine (2Brdian) and 1-(2-iodophenyl)-N,N′-bis(4-cyanophenyl)methanediamine (2Idian) were obtained. The crystal structures of 2Brdian and 2Idian were found to be isomorphous, as were the crystal structures of the three pairs 2Clan/2Bran, 2Ian/2CH 3 an (N-(4-cyanobenzylidene)-2methylaniline), and 2CNan/XOXLOM (N-(4-methylbenzylidene)-2-cyanoaniline). Isomorphism involving iodine/methyl and nitrile/methyl exchanges is rare in the Cambridge Structural Database (CSD). The unexpected product meso-(E,E)-1,1′-[1,2bis(4-methylphenyl)ethane-1,2-diyl]-bis [(2-trifluoromethyl)phenyldiazene] (2CF 3 azo) was obtained in an attempt to prepare the phenylhydrazone, and its crystal structure is reported here. Only one other example of this type of compound currently appears in the CSD. Examples of isomorphous bridge-flipped isomers among benzylideneanilines and phenylhydrazones remain rare in spite of their conformational flexibility and steric similarity.

Probing the Electronic Properties and Interaction Landscapes in a Series of N-(Chlorophenyl)pyridinecarboxamides

“…Of note are the isostructural Clmp ( Z ′ = 4) and Brmp ( Z ′ = 2) with two sets of similar unit cell axes in Clmp ( a , b ) and Brmp ( b , c ) and with the third axis ( a ) halved in Brmp (Supporting Information Table S4b). This represents the extent of overlap within these classes of functionalized benzamides. − There is an extensive ’ pp ’ series with several closely related crystal structures. , There are, however, no pairs of isomorphous NxxCl / Clxx structures involving amide-bridge swapped isomers, , e.g ., NomCl / Clmo , as noted in the NmmM and Mmm crystal structures. , Ojala and co-workers have commented on bridge-flipped isomers in an extensive series of benzylideneanilines and phenylhydrazones. , In our series, the amide group N–H dominates as a pivot in the crystal structures and together with the N pyridine and halogen X reduces the possibility of bridge-flipping or amide-bridge swapping. ,, …”

“… 51 , 53 Ojala and co-workers have commented on bridge-flipped isomers in an extensive series of benzylideneanilines and phenylhydrazones. 88 , 89 In our series, the amide group N–H dominates as a pivot in the crystal structures and together with the N pyridine and halogen X reduces the possibility of bridge-flipping or amide-bridge swapping. 50 , 51 , 54 …”

“… 52 − 54 There is an extensive ’ pp ’ series with several closely related crystal structures. 52 , 56 There are, however, no pairs of isomorphous NxxCl / Clxx structures involving amide-bridge swapped isomers, 88 , 89 e.g ., NomCl / Clmo , as noted in the NmmM and Mmm crystal structures. 51 , 53 Ojala and co-workers have commented on bridge-flipped isomers in an extensive series of benzylideneanilines and phenylhydrazones.…”

“…51,53 Ojala and coworkers have commented on bridge-flipped isomers in an extensive series of benzylideneanilines and phenylhydrazones. 88,89 In our series, the amide group N−H dominates as a pivot in the crystal structures and together with the N pyridine and halogen X reduces the possibility of bridge-flipping or amide-bridge swapping. 50,51,54 The general trend for Cl/Br pairs of isomorphous structures is interesting, 18,49 and in some cases, a methyl analogue (NxxM) 51 is isomorphous with the Cl/Br pairs (NmoCl/ NmoBr; 8 4 NmoM/ NmoBr 8 6 with NmoBr polymorphs 84,86,87 ).…”