Two Structurally Characterized Conformational Isomers with Different C-P Bonds

The cyclic alkyl(amino) carbene (cAAC) bonded chlorophosphinidene (cAAC) P-Cl (2/2') was isolated from the direct reaction between cAAC and phosphorus trichloride (PCl3). Compound 2/2' has been characterized by NMR spectroscopy and mass spectrometry. P-31 NMR investigations [delta approximate to 160 ppm (major) and delta approximate to 130 ppm (minor)] reveal that there are two different P environments of the P-Cl unit. X-ray single-crystal determination suggests a co-crystallization of two conformational isomers of (cAAC) P-Cl (2/2'); the major compound possessing a cAAC-PCl unit with C-cAAC-P 1.75 angstrom. This C-P bond length is very close to that of (NHC)(2)P-2 [NHC= N-heterocyclic carbene]. The residual density can be interpreted as a conformational isomer with a shorter C-cAAC-P bond similar to a non-conjugated phosphaalkene [R-P= CR2]. Our study shows an unprecedented example of two conformational isomers with different C-carbene-element bonds. Additionally, Br (3c/3c'), I (4c/4c'), and H (5c/5c') analogues [(Me-2-cAAC) P-X; X= Br (3), I (4), H (5)] of 2c/2c'[(Me-2-cAAC) P-Cl] were also synthesized and characterized by NMR spectroscopy suggesting similar equilibrium in solution. The unique property of cAAC and the required electronegativity of the X (X= Cl, Br, I, and H) atom play a crucial role for the existence of the two isomers which were further studied by theoretical calculations.