Structural and Variable-Temperature NMR Studies of 9-Diisopropylphosphanylanthracenes and 9,10-Bis(diisopropylphosphanyl)anthracenes and Their Oxidation Products

The diisopropylphosphanyl-substituted anthracenes i-Pr₂P(C₁₄H₉) (1a), i-Pr₂P(C₁₄H₈)Br (2a), and (i-Pr₂P)₂(C₁₄H₈) (3a) and some of their oxidation products were prepared from 9-bromoanthracene and 9,10-dibromoanthracene, respectively. Low-temperature ¹H NMR spectra of the 9-monophosphanyl-substituted anthracenes 1a and 2a are in accordance with a staggered conformer, while above room temperature dynamic processes occur. The low-temperature NMR spectrum of the 9,10-diphosphanylanthracene 3a indicates the presence of two different rotational isomers. The rotational barrier for 1a was determined from variable-temperature ¹H NMR spectra to be 56 kJ mol⁻¹ (ΔG_298K). The crystal structure determinations show the solid-state conformers to be consistent with the prevailing conformer at low temperature.