Improved Access to 1,8-Diformyl-carbazoles Leads to Metal-Free Carbazole-Based [2 + 2] Schiff Base Macrocycles with Strong Turn-On Fluorescence Sensing of Zinc(II) Ions.

Development of a new and high yielding synthetic route to 1,8-diformyl-carbazoles 3 (3a 3,6-di- tert-butyl substituted; 3b 3,6-unsubstituted) is reported. Use of a Heck coupling reaction, followed by ozonolysis, has greatly facilitated the preparation of these interesting head units in useful quantities. An initial foray into the new generations of Schiff base macrocycles that ready access to these head units (3) opens up, has led to the direct (i.e., metal-free) synthesis of two [2 + 2] macrocycles from 3a or 3b with 1,2-diaminoethane, H2LtBu (4a) and H2LH (4b), respectively, obtained as yellow powders in high yields (87-88%). The dizinc complex [Zn2LH(OAc)2] (5b) was isolated as a bright yellow solid in 83% yield, by 1:2:2 reaction of H2LH with zinc(II) acetate and triethylamine. Aldehydes 3a and 3b, macrocycle H2LH, and complex [Zn2LH(OAc)2] (5b) have been structurally characterized. The carbazole NH makes bifurcated hydrogen bonds with the pair of flanking 1,8-diformyl-moieties in 3, or 1,8-diimine-moieties in H2LH, leading to a flat, all- cis conformation. The stepped conformation of the metal-free macrocycle H2LH is retained in [Zn2LH(OAc)2], despite deprotonation and binding of two zinc(II) centers within the two tridentate pockets. The N3O2 coordination of the zinc ions is completed by one μ1,1- and one μ1,3- bridging acetate anion. Excitation of nanomolar [Zn2LH(OAc)2] in DMF at 335 nm results in clearly visible blue fluorescence (λmax = 460 nm). Further studies on the H2LH macrocycle revealed turn-on fluorescence, with selectivity (over Ca2+, Mg2+ and a range of 3d dications) and nanomolar sensitivity for zinc(II) ions, highlighting one of the many potential applications for these new carbazole-based Schiff base macrocycles.