Chiral Thiahelicene-Based Alkyl Phosphine–Borane Complexes: Synthesis, X‑ray Characterization, and Theoretical and Experimental Investigations of Optical Properties

Chiral helical-based
phosphanes are challenging and promising ligands,
with a great potential for the generation of both organic and organometallic
catalysts. We report here the preparation of novel chiral thiahelicene-based
alkyl phosphanes, isolated and characterized as air-stable borane
adducts, and the investigation of their experimental and theoretical
(chir)­optical properties. X-ray characterization of a mono- and a
disubstituted derivative as a racemic mixture has been performed,
which confirms the influence of the number and nature of substituents
on the flexibility of the helix. In addition, the absolute configuration
inferred from CD spectra of the two enantiomers of a diborane complex
has been established from X-ray analysis. State-of-the-art quantum
chemical calculations of vibrationally resolved spectra allow, for
the first time, for an unambiguous assignment of the experimentally
observed peaks in linear absorption and circular dichroism spectra
to excited electronic states of this class of thiahelicene phosphorus
derivatives.