2,2′-Azobispyridine in Phosphorus Coordination Chemistry: A New Approach to 1,2,4,3-Triazaphosphole Derivatives

Oxidative addition of 2,2'-azobispyridine (abpy) to PCl3 in CH2Cl2 or THF gave the 1:1 addition product, containing phosphorus in high oxidation state (+5) and a reduced form of the ligand. 2,2'-Hydrazobispyridine (hbpy) was prepared by reduction of abpy with hydrazine-hydrate in 63% yield. Interaction of hbpy with PCl3 in the presence of triethylamine gave (abpy)2-PCl (2) in 25% preparative yield. A similar reaction of hbby with (Et2N)(2)PCl afforded (abpy)2-PNEt2 (4) in 89% yield. Compound 4 after work-up with PCl3 or PBr3 gave 2 and (abpy)2-PBr (6) respectively in high yields. Diethylamino-derivative 4 formed (kappa(2)-N,N) adduct with SiCl(4)7 (coordination by Py and azo-functions), while the chloro-derivative 2 did not. Reaction of 2 with PCl5 is accompanied with liberation of PCl3 and formation of spirocyclic ate complex [(abpy)P-2-(2)]+PCl6-. All structurally characterized compounds demonstrated short distances between pyridyl nitrogen and the phosphorus atom. However, the QTAIM analysis did not reveal the presence of appropriate bond critical points (3, -1) for the intramolecular noncovalent interactions NP in 2, 4, and 6. We theoretically estimated values of the rotation barriers for the pyridyl and Et2N moieties in 4 using the relaxed potential energy surface scan at the B3LYP/6-31G(d) level of theory. The values of rotation barriers are very close to each other, viz. 13.2 (pyridyl) and 13.0 (Et2N) kcal/mol.