3 years ago

Two Structurally Characterized Conformational Isomers with Different C−P Bonds

Two Structurally Characterized Conformational Isomers with Different C−P Bonds
Bin Li, Herbert W. Roesky, Dietmar Stalke, Debasis Koley, Sayan Dutta, Regine Herbst-Irmer, Kartik Chandra Mondal, Sudipta Roy, Christian J. Schürmann, Subrata Kundu
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. An unprecedented dual bonding situation is demonstrated by the cyclic alkyl(amino) carbene (cAAC) bound chlorophosphinidene (cAAC)P-Cl (2/2'). The conformational form with a C−P bond length of 1.75 Å is the major conformer (2) possessing a donor–acceptor coordinate (CcAACP−Cl) bond. The minor conformer (2') with a C−P bond length of 1.61 Å contains a double bond (CcAAC=P−Cl). The presence of two resonance forms has been confirmed by NMR studies. The bonding situation and stability of 2/2' have been studied by theoretical calculations.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201702870

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