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

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 <i>N</i>,<i>N</i>′-dialkyl-substituted 4,5-dimethylimidazole-2-ylidenes (^RNHC^Me) and benzimidazole-2-ylidenes (^RNHC^B) (R = Me, Et, iPr) have been fully characterized. We found that increasing <i>N</i>-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