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 (31)P{(1)H} 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 (31)P{(1)H} variable-temperature NMR spectra. This allowed elucidation of remarkable through-space couplings ((8TS)JPP) 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.