Abstract
Diphenylamine-2,2’-dicarboxaldehyde (1a) was prepared in three steps from ethyl-2-iodobenzoate and ethyl-2-aminobenzoate (38% overall yield) following the literature synthesis. The intermediate diphenylamine-2,2’-ethylbenzoate (3a) is central to accessing three new derivatives (1b-1d). Bromo- and iodo-groups were introduced into 3a, by reactions with NBS (di-bromo 3b and mono-bromo 3c) and NIS (di-iodo 3e), respectively. Acetylene groups were introduced by a Sonogashira-Hagihara coupling of 3e with phenylacetylene to give 3d. The diesters 3a-3d were reduced with LiAlH4 to the dialcohols which were then oxidised to the new dialdehyde head-units 1b-1d. Carbazole-1,8-dicarboxaldehyde (2a) was prepared from 3a in three steps (19% overall yield).
The HLEt, HLd-Br, HLm-Br and HLPA Schiff-base macrocycles were readily generated, as yellow to orange crystalline solids, by the direct condensation of diethylene triamine and 1a, 1b, 1c and 1d, respectively. The carbazole macrocycle HLCz was generated from 2a and diethylene triamine in an analogous manner. The synthesis of HLPr from dipropylene triamine and 1a proved to be non-trivial but it was prepared microanalytically pure in the presence of acid (acetic, formic and tosylic), as HLPr•2HOAc, HLPr•2HCO2H and HLPr•C7H8SO3. Reduction of HLEt gave the amine macrocycle HLaa.
These macrocycles provide an N4-donor set regardless of whether or not the diphenylamine NH is deprotonated. Both the template and the metallation of the preformed macrocycle methods were employed to access a total of thirty one mononuclear and two dinuclear complexes of mid to late first row transition metal ions of these macrocycles (seven complexes of HLEt, nine complexes of HLaa, seven complexes of HLPr, six complexes of HLd-Br and HLm-Br, two complexes of HLCz and two dinuclear manganese complexes of HLPr).
Twelve complexes were structurally characterised. These macrocycles usually provide the metal ion with an equatorial plane of N4-donors, imposing square planar coordination on copper(II) and nickel(II) when deprotonated. In contrast, the neutral HLaa macrocycle binds facially to the octahedral nickel(II) ion in [NiIIHLaa(NCS)2]•0.3DCM•0.15MeOH•0.15H2O, adopting a highly folded conformation in order to do so. This unusual capping binding mode was also seen for the deprotonated macrocycles in the structure of [MnIV2LEt2(O)2](ClO4)2•3DMF.
The cobalt(II) and iron(III) complexes of (LEt)- are in magnetically interesting, low- and intermediate-spin states, respectively, consistent with this macrocycle providing a strong field ligand. The HLaa macrocycle undergoes base catalysed oxidative dehydrogenation in air in the presence of nickel(II) to form complexes of a mixed-Schiff-base-amine macrocycle and in doing so tunes the spin state, as increasing the number of Schiff-base bonds from zero to two converts nickel(II) from the high-spin to the low-spin state.
Electrochemical measurements revealed a number of quasi-reversible and irreversible redox processes for the (LEt)- complexes. Interestingly, after controlled potential coulometry a new reversible or quasi-reversible process was observed for [CuIILEt](BF4)•H2O, [NiIILEt](BF4)•H2O and [ZnIILEt(py)](BF4). In contrast, the analogous complexes of (LPr)-, in particular [NiIILPr](BF4) and [CuIILPr](BF4), exhibit some reversible processes.
Finally, the copper(II) and nickel(II) complexes of the new macrocycle (LCz)-, prepared by the metallation of HLCz and the template method, respectively, provide a promising start to accessing families of the analogous carbazole-based macrocyclic complexes.