Synthesis of Unique Phosphazane Macrocycles via Steric Activation of C–N Bonds

Herein we describe
that oxidation reactions of the dimeric cyclophosphazanes,
[{P­(μ-NR)}2(μ-NR)]2, R = tBu (1), to produce a series of diagonally
dioxidized products P4(μ-NtBu)6E2 [E = O (2), S (3), and Se (4)] and tetraoxidized frameworks.
The latter display an unexpected C–N bond activation and cleavage
to produce a series of novel phosphazane macrocyclic arrangements
containing newly formed N–H bonds. Macromolecules P4(μ-NtBu)4(μ-NH)2O4 (5) and P4(μ-NtBu)3(μ-NH)3E4, E = S (6) and Se (7), dicleaved
and tricleaved products, respectively, are rare examples of dimeric
macrocycles containing NH bridging groups. Our theoretical and experimental
studies illustrate that the extent to which these C–N bonds
are cleaved can be controlled by modification of steric parameters
in their synthesis, by adjusting either the steric bulk of the substituents
in the parent framework or the size of the chalcogen element introduced
during the oxidation process. Our findings represent new synthetic
pathways for the synthesis of otherwise-elusive macrocycle arrangements
within the phosphazane family.