Strong and Tunable Near‐Infrared Circular Dichroism in Helical Tetrapyrrole Complexes

Abstract

The selective synthesis of nickel and copper complexes of 19-benzoyl-5,10,15-triphenyl-bilatrien-1-one (H 2 TPBT) is reported, a molecule which crystallizes as a molecular helix of one-and-a-quarter which turns with a 5.7 Å radius and a 3.2 Å pitch, and all 26 participating atoms are sp 2 -hybridized. UV/vis, ECD, ESR and cyclic voltammetry experiments reveal a strong interaction between metal and ligand and partial radical character when copper is coordinated instead of nickel. Strong ECD absorption in the 800 nm range is found which, using TD-DFT calculations as well as literature spectra, is shown to be highly tunable both by metal coordination and variation of the aryl groups in the TPBT periphery. The radical character of the ligand in Cu(TPBT) enables rapid interconversion between (M)-and (P)enantiomers, possibly via intermittent breakage of a CuÀ N bond. The 19-benzoyl group kinetically stabilizes enantiopure (M/P)-Ni(TPBT). The results are interpreted with regard to the application as circularly polarized light (CPL) detectors as well as to the chirality-induced spin-selectivity (CISS) effect which is currently lacking a concise theoretical model.

Calix[4]pyrrolato-germane-(thf)2: Unlocking the Anti-van’t Hoff–Le Bel Reactivity of Germanium(IV) by Ligand Dissociation

“…Interestingly, the reaction cascade is initiated only by deliberating one of the two weakly bound acetonitriles and in line with our hypothesis to achieve reactivities within the anti-Van’t Hof–Le Bel manifold by ligand lability. Compound 8 is also interesting in context of helical chirality in tetrapyrrole systems for optical polarization …”

“…This observation should not be restricted to this specific substance class but urges caution to each experimental verification of Lewis superacidity if concomitant redox chemistry can be expected. By modulation of donor strength, the reactivity can be tuned, and diverse element-ligand cooperative bond activation modes are leveraged by the beneficial interplay with the anti-van’t Hoff–Le Bel induced Lewis acidity enhancement. With acetonitrile as the weakest donor, an unprecedented activation of a CN triple bond is disclosed, which allows attractive transformation of the calix[4]­pyrrolato framework into a germanium­(IV)-constricted helical structure …”