Two Structurally Characterized Conformational Isomers with Different C−P Bonds

Abstract

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. ³¹ P 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 C_cAAC -P 1.75 Å. This C-P bond length is very close to that of (NHC)₂ P₂ [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 [(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.