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, Bi; Ace = acenaphthene-5,6-diyl) and (iPr₂P-Ace)₂EPh (5 and 6), (E = As, Sb) were synthesised and fully characterised by multinuclear NMR, HRMS, 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 E–C_ipso bond is locked. On the other hand, the bis(acenaphthyl) species show a fluxional behaviour, resulting in an AX to A₂ spin system transition in the ³¹P{¹H} VT NMR spectra. This allowed elucidation of remarkable through-space couplings of ^8TSJ_PP 11.5 Hz (for 5) and 25.8 Hz (for 6) at low temperature. In addition, detailed lineshape 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 which is somehow buried within the molecule.