Phospholes, first synthesised in 1959, have emerged as highly valuable compounds in numerous fields ranging from photophysical applications to coordination chemistry, and often display aromaticity. Utilising phosphorus alongside its lighter group 15 congener, nitrogen, can bestow further reactivity in the heterocycle, particularly when featuring a direct P V =N bond. This arises due to the highly polarised nature of the P V =N bond, which yields significant single-bond character and a more basic nitrogen centre. Such phosphorus and nitrogen containing heterocycles have subsequently been used in areas including luminescent materials and as building blocks for inorganic polymers. The majority of currently reported phosphorus/nitrogen containing heterocycles are based on either five-or six-membered rings. Sevenmembered rings containing both phosphorus and nitrogen and comparatively scarce. The most common approach for preparing seven-membered P/N rings relies on the condensation of amines with halophosphine derivatives, with concomitant release of HCl, which necessarily affords products with direct P-N bonds. There are fewer examples of P/N heterocycles that contain solely P-C bonds, but heteroocycles fo this type recently have been applied as ligands for Ni in electrocatalysts or as chelating ligands for lanthanide ions, and in medicinal applications (Figure ). Azaphosphepines are unsaturated seven-membered P/N heterocycles, and convenient and high-yielding synthetic routes to such heterocycles remain limited in scope. The most pertinent examples of azaphosphepine synthesis for this current study are ring-expansions via P-N cleavage of an appropriate five-membered ring precursor. In 1990, Ried et al. reported the ringexpansion of a five-to seven-membered ring via the cleavage of a P-N bond with electronically activated alkynes. The products were characterised crystallographically and spectroscopically, although the mechanism of reaction was not discussed. In 2019, Lozovskiy et al. also reported intramolecular ring-expansion by P-N bond cleavage in a fivemembered ring to yield a seven-membered ring. The only prior example of 1,2,5-diazaphosphepine chalcogenides was reported by Barkallah et al., where 2,2'-(phenylphosphoryl)bis(cyclopentan-1-one) (or the sulfur and selenium analogues) was reacted with hydrazine and acetic acid (Figure ), although the products were not structurally characterised. Azophosphines are phosphorus-containing analogues of triazenes with the chemical formula ArN=N-PR2, with a trivalent phosphorus centre that can be exploited for further reactivity. A limited range of azophoshines were synthesised in the 1970s and 1980s, but no further reactivity was explored. This class of compounds was ignored until 2021, when Cummins and co-workers synthesised MesN2PA (Mes = mesityl, A = anthracene). This molecule was shown to undergo a range of cycloaddition reactions with unsaturated substrates, including [Na(dioxane)2.5][PCO] and AdCP (Ad = adamantyl), with loss of the labile anthracene moiety. The following year, the same group isolated azophosphines of the form ArN2PR2 (R = i Pr, Cy, Ph, NMe2), and demonstrated the cycloaddition reactions of these species with cyclooctyne, which yielded fivemembered N-heterocyclic iminophosphoranes. One example of a 1,3-dipolar cycloaddition of the azophosphine (p-Me)C6H4N2PPh2 with the electronically activated alkyne dimethyl acetylenedicarboxylate (C(CO2Me))2 was also reported. Very recently, we reported a general synthetic route to azophosphines which allowed for tolerance of bulky P-substituents, and demonstrated the use of these systems as ligands in Ru complexes via the phosphorus and nitrogen centres. Motivated by these results, we reasoned that azophosphines may be able to serve as precursors to a wider range of phosphorus and nitrogen containing heterocycles beyond those currently reported. In this manuscript, we report the synthesis and characterisation of five-and seven-membered phosphorus-and nitrogen-containing rings from azophosphines. The seven-membered rings are the first crystallographically characterised 1,2,5diazaphosphepines, and the mechanism of this transformation has been probed both computationally and experimentally. The interactions of these species with the Lewis acidic borane B(C6F5)3 has also been explored, which was shown to catalyse the formation of the seven-membered heterocycle for the i Prderivtive and result in the trapping of a key intermediate for the t Bu-derivative.