Influence of Phosphorus Substituents and DoubleBond Configuration on the Nitrogen Chemical Shifts of Bis(imino)phosphoranes: An Experimental and Quantum Chemical Study

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The 15N NMR spectra of a series of bis(imino)phosphoranes X—P(=NR′)=NR″ [X = tBu, NHtBu, OMes*, P(tBu2), StBu (2a–e)], Br—P(=NMes*) (2f) and Mes*(Me3Si)N—P(=NtBu)=NSiMe3 (2g) were measured and assigned using a combination of 1H- and 31P-based INEPT techniques. The data reveal a close correlation between the changes in δ15N and 1J(PN), with an increase in δ15N being generally associated with a decrease in ∣1J(PN)∣. For 2d, the signs of 2J(PN) were obtained from a 2D-31P, 15N shift correlation and allowed the deduction of the rotational orientation of the P(tBu)2 substituent. A more detailed understanding of the connection between nitrogen shielding and structural features of bis(imino)phosphoranes was obtained from a study of ab initio-calculated nitrogen chemical shifts of the model compounds X′—P(=NH)2 [X′ = CH3, NH2, OH, PH2, SH, Br (3a–f)]. The analysis of the observed trends allowed the identification of two major sources of the variation in δN, viz. (i) the Z- or E-configuration of the double bond and (ii) the nature of the phosphorus substituent. Comparison of δ15N for 2a–f with δNcalc of the corresponding model compounds suggested that 2a–e exhibit a Z-configuration of both double bonds, and that the observed variation in δ15N of all compounds is determined exclusively by the substituent effects which may be transmitted by both inductive and mesomeric interactions. The observation of a close correlation between both δ15N (for 2a-f) and δNcalc (for 3a–f) with the ‘bond polarity’ of the phosphorus–nitrogen bond as defined by the difference in the calculated charge densities at the nitrogen and the adjacent phosphorus atoms in 3a–f revealed a relationship between an increase in nitrogen shielding and a higher ‘ylidic’ character of the phosphorus–nitrogen double bond.