The synthesis of BCPIs 3 was attempted via the Staudinger reaction between Nphosphanyl-substituted imidazolylidene 1, bearing a 2,6-diisopropylphenyl group (Dipp), and a boryl azide (Figure ), which was designed based on our previous synthesis of N-phosphineimide-substituted imidazolylidenes (PimIms). Treatment of 1 with 9-azido-9-borafluorene (N3-B f , where B f is a 9-borafluorenyl group) in toluene resulted in the formation of carbene-borane adduct 2a, which was isolated in 71% yield. Then, we synthesized 3a by heating a toluene solution of 2a at 100 °C and 3a was isolated in 90% yield. BCPI 3a was also obtained from the reaction between 1 and N3-B f without the isolation of 2a in 76% yield. Similarly, we also prepared 3b in 95% yield using bis(pentafluorophenyl)boryl azide (N3-B(C6F5)2), where the formation of both the C-B and N=P bonds proceeded at room temperature. We were not able to confirm the generation of a carbene-borane adduct even though we monitored the formation of 3b using NMR spectroscopy. Compounds 2a, 3a, and 3b were unambiguously characterized using multinuclear NMR spectroscopy as well as single-crystal X-ray diffraction (SC-XRD) analysis. In the 31 P NMR spectrum of 3a, a resonance showing the formation of the phosphinimidoyl moiety is observed at P 69.9, which is significantly upfield shifted compared to 2a (P 95.2). A nearly identical chemical shift was observed in the case of 3b (P 70.2), which is clearly different from that of (C6F5)2B-NP t Bu3 (P 43.6), 11a thus demonstrating the difference in the electronic structures around the P atoms of BCPIs and N-boryl phosphine imides. For comparison, we prepared 5, a silane analogue of BCPIs, using B(C6F5)3 for the methyl abstraction from PimIm 4 bearing a Ntrimethylsilyl group (Figure ). Compound 5 demonstrates a resonance at P 83.5, downfield shifted in comparison with 3a and 3b. These results manifest the critical roles that both the substituents on the N atom (i.e. borane, silyl, or boryl group) and the cyclic structure play in determining the electronic and steric properties of the phosphinimidoyl moieties. In the 11 B NMR spectra, the resonance that supports the generation of a four-coordinate boron species is observed for 3a at B -1.6 and for 3b at B -5.9. The molecular structures of 2a and 3a determined by SC-XRD analysis are shown in Figure and 2D, respectively, demonstrating that the framework of BCPI is constructed via the formation of the carbene-boron bond before the formation of the N=P bond. The molecular structure of the cationic species in 5 is also given in Figure . The geometrical parameters of 3a and 3b (Figure ) are nearly identical (with the exception of the parameters that are derived from each B-diaryl substituent), and the following discussions thus use those for 3a unless otherwise noted. In 3a and 5, the imidazolium and the C1-N2-P-N3-X (X = B for 3a; X = Si for 5) rings are almost coplanar, i.e. the angles between the two five-membered rings are 3.5º in 3a and 0.8º in 5.