The bonding situation in 1,3,2‐diazaphospholene derivatives as studied by solid‐state NMR spectroscopy

The 31 P chemical shift (CS) tensors of the 1,3,2‐diazaphospholenium cation 1 and the P ‐chloro‐1,3,2‐diazaphospholenes 2 and 3 and the 31 P and 19 F CS tensors of the P ‐fluoro‐1,3,2‐diazaphospholene 4 were characterized by solid‐state 31 P and 19 F NMR studies and quantum chemical model calculations. The computed orientation of the principal axes system of the 31 P and 19 F CS tensors in the P ‐fluoro compound was found to be in good agreement with experimentally derived values obtained from evaluation of P–F dipolar interactions. A comparison of the trends in the chemical shifts of 1 – 4 with further available literature data confirms that the unique high shielding of δ 11 in the cation 1 can be related to the effective π‐conjugation in the five‐membered heterocycle, and that a further systematic decrease in δ 11 for the P ‐halogen derivatives 2 – 4 is attributable to the increased perturbation of the π‐electron distribution by interaction with the halide donor. Copyright © 2002 John Wiley & Sons, Ltd.