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. 31P NMR investigations [ δ≈160 ppm (major) and δ≈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 CcAAC−P 1.75 Å. This C−P bond length is very close to that of (NHC)2P2 [NHC= N-heterocyclic carbene]. The residual density can be interpreted as a conformational isomer with a shorter CcAAC−P bond similar to a non-conjugated phosphaalkene [R−P=CR2]. Our study shows an unprecedented example of two conformational isomers with different Ccarbene−element bonds. Additionally, Br ( 3c/ 3c'), I ( 4c/ 4c'), and H ( 5c/ 5c') analogues [(Me2-cAAC)P−X; X=Br ( 3), I ( 4), H ( 5)] of 2c/ 2c'[(Me2-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. [ABSTRACT FROM AUTHOR]