ABSTRACT
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.
ABSTRACT
The very different multi-step routes to the closely related pair of diformyl-carbazole head units, 1,8-diformyl-3,6-di-tert-butyl-9H-carbazole (1tBu) and 1,8-diformyl-9H-carbazole (1H), are detailed and compared. The first examples of Schiff base macrocycles derived from diformyl-carbazole head units are reported. Specifically, the direct cyclisation of 1tBu or 1H with diethylenetriamine gives the two metal-free [1 + 1] Schiff base macrocycles HLH and HLtBu in high yields. Four carbazole-based macrocyclic complexes, [CuIIL(OH2)]OAc and [NiIIL]OAc, where L = LH or LtBu, were accessed either by metallation of these macrocycles, or by metal templated reaction of the macrocycle components. [CuIILtBu(OH2)]OAc·0.5(Ether) and [NiLH]OAc·EtOH, were structurally characterised, confirming the nickel(ii) complexes are square planar (both show diamagnetic NMR spectra) and that the copper(ii) complexes are square pyramidal with a water molecule bound in the axial site. Like porphyrins, both of these N4-donor macrocycles, which differ only in the R group present at the 3 and 6 positions (H or tBu), impose a strong ligand field.
