We recently reported a new synthesis of azophosphines via the reaction of arenediazonium salts ([ArN2][BF4]; Ar = Mes or (p-NMe2C6H4)) with deprotonated secondary phosphine-boranes (HPR2•BH3), although the P-substituent was limited to R = t Bu and Ph. The same synthetic procedure was used here, and was shown to tolerate smaller alkyl Psubstituents. The secondary phosphine-boranes HPR2•BH3 (R = i Pr, Cy (cyclohexyl)) were synthesised from the corresponsing dialkylchlorophosphines by a modified literature procedure (Scheme 1A). Deprotonation of these substrates with sec-BuLi at -78 °C, followed by reaction with mesitylenediazonium tetrafluoroborate ([MesN2][BF4]) allowed for the formation of the target azophosphine-boranes (MesN2PR2•BH3) 1•BH3 (R = i Pr) and 2•BH3 (R = Cy) as a purple oil or purple solid, respectively, in moderate yields following purification (1•BH3 = 41%, 1•BH3 = 44%; Scheme 1B). The borane protecting group could then be fully removed via reaction with pyrrolidine to yield the free azophosphines (MesN2PR2) 1 (R = i Pr) and 2 (R = Cy) in good yields (1 = 78%, 2 = 71%; Scheme 1), both as red oils. With azophosphines 1 and 2 in hand, as well as the previously reported azophosphine 3 (MesN2P t Bu2), we next investigated the cycloaddition reactions of these systems. The reaction of azophosphines 1-3 with one equivalent of the electron-poor alkyne diethyl acetylenedicarboxylate (C(CO2Et))2 in toluene gave an immediate colour change from red (1, 2) or purple (3) to pale yellow, and formation of the five-membered heterocycles (R = i Pr (4), Cy (5), t Bu (6); Scheme 2). Reaction monitoring by 31 P{ 1 H} NMR spectroscopy showed quantitative conversion from azophosphine to product; removal of the solvent in vacuo yielded analytically pure compounds 4-6 in good isolated yields (80-85%). ) Å] observed in the previously reported solid state structure for its azophosphine-borane precursor 3•BH3. The P-N bond length of 6 exists somewhere between a single and double bond [1.6461( ) Å] and N-P [1.6442( ) Å] bond lengths to 6. The planarity and bond lengths of both 4 and 6 are in good agreement with the N-heterocyclic iminophosphoranes reported by Cummins, which also displayed planar heterocyclic cores and comparable N-N and P-N bond distances. To corroborate these findings, density functional theory (DFT) and natural bond orbital (NBO) analyses were carried out (see SI for details). The optimised structures are in good agreement with the observed solid-state structures, with similarly planar heterocyclic cores and N-N and P-N bond metrics. Natural population analysis (NPA) showed a highly electron-rich N2 nitrogen centre (-0.859 for 4, -0.874 for 6) and electron-deficient phosphorus centre (+1.809 for 4, +1.841 for 6), with calculated Wiberg bond indices for the P-N bonds of 1.07 (4) and 1.05 (6). These data are suggestive of more single bond than double bond character in the P-N bond. ), N2-P1 1.6461( ), C10-C11 1.401(2), N1-N2-P1-C11 -5.77 (11), N1-C10-C11-P1 -2.50 (18). 4 crystallised with two product molecules in the asymmetric unit; only one is shown here for clarity and bond metrics are very similar for both independent molecules so only one set is given above. Thermal ellipsoids were drawn at the 50% probability level. Given the partial single-bond character of the P-N bond in five-membered rings 4-6, we hypothesised that this bond could be cleaved by certain substrates to yield larger phosphorus-containing heterocycles, by analogy with the aforementioned ring expansion chemistry. Reactions of an P-N bond within fivemembered rings has previously been reported with ketones, ketenes, and isocyanates, although in all cases either acyclic or bicyclic structures were observed instead of ring expansion. 28-30 Cummins also reported one example of P-N bond cleavage in an N-heterocyclic iminophosphorane with Ph3SiH, although this resulted in loss of the heterocyclic structure and formation of a Ph3Si-N(R)-H moiety. Five-membered heterocycle 4 (R = i Pr) was thus reacted with an excess (4 equiv.; Scheme 3A) of (C(CO2Et))2 in toluene. Monitoring by P{ 1 H} NMR spectroscopy showed a new species forming, product 7, with quantitative conversion within 48 hours at room temperature; the solution also became a visibly brighter yellow colour. 7 could also be formed directly from azophosphine 1, by reacting 1 with the same excess of (C(CO2Et))2. Monitoring this latter reaction by P{ 1 H} NMR spectroscopy showed 4 initially forming at 74.9 ppm, before giving way to 7 at 60.4 ppm. Single crystals of 7 were produced by slow diffusion of n-hexane into a concentrated THF solution of 7 at -35 °C, for which single-crystal X-ray diffraction revealed an unusual seven-membered ring structure featuring cleavage of the P-N bond by a second equivalent of alkyne (Figure ). Analogous reactivity was observed with azophosphine 2 (MesN2PCy2), which yielded seven-membered heterocycle 8 via 5, the structure of which was also confirmed by SXRD (Figure ). Removal of the solvent in vacuo yielded analytically pure compounds 7 and 8 in excellent isolated yields (7 = 88%, 8 = 92%), with no further work-up necessary. However, when the sterically more hindered azophosphine 3 (MesN2P t Bu2) was used, no seven-membered ring could be detected by P{ 1 H} NMR spectroscopy. Even with a large excess of alkyne (10 equiv.) and heating to 60 °C, only formation of five-membered ring 6 was observed (Scheme 3A); heating above 60 °C gave an intractable mixture of products. ), C12-C13 1.443(2); C13-P1 1.7407 (16). Thermal ellipsoids were drawn at the 50% probability level. In contrast to the planar geometries of five-membered rings 4 and 6, the seven-membered rings of 7 and 8 feature non-planar, puckered cores (Figure and). Three major resonance structures can be considered for these seven-membered rings (α, β, γ; Scheme 3B; atom labelling the same as in Figure ). Analysis of the solid-state bond metrics of 7 revealed N2-C12 to be almost entirely double-bond in character