Geminally Substituted Tris(acenaphthyl) and Bis(acenaphthyl) Arsines, Stibines, and Bismuthine: A Structural and Nuclear Magnetic Resonance Investigation

Tris­(acenaphthyl)-
and bis­(acenaphthyl)-substituted pnictogens (iPr2P-Ace)3E (2–4) (E = As, Sb,
or Bi; Ace = acenaphthene-5,6-diyl) and (iPr2P-Ace)2EPh (5 and 6)
(E = As or Sb) were synthesized and fully characterized by multinuclear
nuclear magnetic resonance (NMR), high-resolution mass spectrometry,
elemental analysis, and single-crystal X-ray diffraction. The molecules
adopt propeller-like geometries with the restricted rotational freedom
of the sterically encumbered iPr2P-Ace
groups resulting in distinct NMR features. In the tris­(acenaphthyl)
species (2–4), the phosphorus atoms are isochronous
in the 31P­{1H} NMR spectra, and the rotation
of the three acenaphthyl moieties around the E–Cipso bond is locked. On the other hand, the bis­(acenaphthyl)
species show a fluxional behavior, resulting in an AX to A2 spin system transition in the 31P­{1H} variable-temperature
NMR spectra. This allowed elucidation of remarkable through-space
couplings (8TSJPP) of 11.5
Hz (for 5) and 25.8 Hz (for 6) at low temperatures.
In addition, detailed line shape analysis of the thermodynamic parameters
of the restricted rotation of the “propeller blades”
in 5 was performed in the intermediate temperature region
and also at coalescence. The lone pairs on the pnictogen atoms in 2–6 are oriented such that they form a bowl-shaped
area that is somehow buried within the molecule.