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 and mass spectrometry. 31P NMR investigations [~160 ppm (major) and ~130 ppm (minor)] structural determination reveal that there are two different P environments of 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 A. 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 non-conjugated phosphaalkene [R-P=CR2]. Our study represents 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 are further studied by theoretical calculations.