A structural study of seven salt forms of sulfonated azo dyes containing nitrile functional groups

The crystal structures of seven salt forms of nitrile functionalised monosulfonated azo dyes, as isolated from aqueous solution, are described. The presence of nitrile gives unique structures for the Na salts examined. The normally observed Na–O3S bonding is absent and has been replaced by Na–NC interactions, giving supramolecular structures not previously seen for similar azo colourants. A highly unusual side-on nitrile coordination mode is observed and this bonding is observed both crystallographically and by IR spectroscopy. However, the alkaline earth element salts are all of structural types seen before for other non-nitrile bearing sulfonated azo dyes. Thus the Mg salts are solvent-separated ion-pair species based on the [Mg(OH2)6] ion and the Ca and Sr species are simple complexes of the type [M(L)2(OH2)6], where L is the azo dye anion bonded to the metal centre through a single O atom of the sulfonate group.

Graphical abstractIntroducing nitrile functionality to monosulfonated azo dye species replaces solid-state Na-sulfonate bonding with Na–NC bonding and so gives very different supramolecular structures from non-nitrile bearing species. However, alkaline earth element salts do not form M–NC bonds and thus have supramolecular structures similar to those of their non-nitrile analogues.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Crystal structures of seven nitrile functionalised monosulfonated azo dye salts. ► Na to nitrile bonds replace the expected Na–O3S bonds. ► Dimeric [(L)(H2O)3Na(μ-H2O)2Na(H2O)3(L)] has unusual side-on nitrile bonding.