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Convenient Preparation and Detailed Analysis of a Series of NHC-Stabilized Phosphorus(I) Dyes and Their Derivatives

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posted on 28.06.2016, 19:23 by Charles L. B. Macdonald, Justin F. Binder, Ala’aeddeen Swidan, Jennifer H. Nguyen, Stephanie C. Kosnik, Bobby D. Ellis
A safe, convenient, and P-atom-efficient synthesis of N-heterocyclic-carbene- (NHC-) stabilized phosphorus­(I) bromide salts is reported that involves P+ transfer from an easily prepared triphosphenium precursor. The resulting family of phosphamethine cyanine dyes featuring N,N′-dialkyl-substituted 4,5-dimethylimidazole-2-ylidenes (RNHCMe) and benzimidazole-2-ylidenes (RNHCB) (R = Me, Et, iPr) have been fully characterized. We found that increasing N-alkyl group size causes increased twisting of the carbene fragments from the C–P–C plane, which decreases the magnitude of hyperconjugation between the π-type lone pair on phosphorus and the carbene fragments. This decrease is manifested as changes in the 31P NMR chemical shifts, magnitudes of the P–C coupling constants, and electronic spectra of the cations. The reactivities of these ions toward various oxidants are reported: Oxidation by sulfur generates dithiophosphinium salts, protonation or methylation gives dicationic phosphines, and coordination to 1 or 2 equiv of gold­(I) chloride results in mono or bimetallic complexes, respectively.