Are Monophospha(III)amidines and ‐guanidines with Ionizable Hydrogens Tautomeric? Towards a Deeper Understanding of Two Related Hetero‐element Functional Groups

Abstract

This paper presents definitive structural evidence for N,P(III)‐monophosphaamidines in P=C and N=C isomeric forms from a combination of new syntheses, single‐crystal X‐ray diffraction (SC‐XRD), solid‐state NMR and FTIR. Evidence is also provided for C‐amino‐(σ2,λ3)‐phosphaalkene and C‐(σ3,λ3)‐phosphinoimine tautomerism in solution using multi‐nuclear NMR methods. Synthesis and SC‐XRD structure determination of a trisubstituted N,N’,P(III)‐monophosphaguanidine is presented, the first structure of a phospha(III)guanidine with two ionizable H atoms. The structural evidence is convincing for an N=C geometry, resulting in both N−H and P−H in the molecule. A detailed computational investigation using DFT methods is presented, with the goal of understanding the tautomeric structure preferences both at the fundamental level (parent molecules with all substituents on the heteroatoms being hydrogen) and using the full structures containing the very bulky 2,6‐diisopropylphenyl (Dipp) substituents employed in this study. Arguments are espoused for treating phospha(III)amidines and ‐guanidines as new types of functional groups, similar to but distinct from the familiar organic analogues. Limited reactivity studies and a voltammetry study of one phospha(III)amidine are included with the supporting information.

N,N’‐Diaryl‐Sulfurdiimides are Strongly Redox Tuned

“…Previous work, particularly of Stalke and co‐workers, has provided irrefutable experimental and computational evidence against this idea, [11] brilliantly confirming that the Lewis resonance formulation (Figure 1) is fully valid, whilst also elucidating the real differences in structures and reactivity that result from an allylic resonance with a third‐period central atom. This situation is notably different from other heteroallylic systems, such as phospha(III)amidines and diphospha(III)amidines, wherein the central atom remains C [51] . Hence, there is not only sulfur diimide, but also an extensive triimide chemistry [6a,b,52] .…”

“…Previous work, particularly of Stalke and co-workers, has provided irrefutable experimental and computational evidence against this idea, [11] brilliantly confirming that the Lewis resonance formulation (Figure 1) is fully valid, whilst also elucidating the real differences in structures and reactivity that result from an allylic resonance with a third-period central atom. This situation is notably different from other heteroallylic systems, such as phospha(III)amidines and diphospha(III)amidines, wherein the central atom remains C. [51] Hence, there is not only sulfur diimide, but also an extensive triimide chemistry. [6a,b,52] Although Pauling is strongly associated with the now-deprecated notion of valence expansion by hybridization (i. e. dsp 3 models for S IV ), [53] it seems to be forgotten that his even more fundamental concept of element electronegativity was directly based on the concept of enhanced strengths of bonds with partial ionic character.…”