Geminally substituted tris(acenaphthyl) and bis(acenaphthyl) arsines, stibines and bismuthine : a structural and nuclear magnetic resonance investigation

Abstract

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–Cipso 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 8TSJPP 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.