Hexacoordinate phosphorus. 6. Synthesis and characterization of neutral six-coordinate phosphorus(V) compounds derived from 2-pyridinol and related heteroatomic pyridine ligands

The reaction of the silylated form of the substituted pyridine ligands 2-(methylamino)pyridine, 2-hydroxypyridine, 2-mercaptopyridine, and 2,2′-dipyridylamine with X⁵-halogenophosphoranes yields, via trimethylsilyl halide elimination, a series of neutral hexacoordinate X⁶-phosphorus compounds (1-8), in which a four-membered chelate structure is achieved by donation of the electron pair from the pyridine nitrogen to phosphorus in addition to formation of P-O, P-N, or P-S bonds. The hexacoordinate nature of these compounds is evidenced by their high-field 31P NMR chemical shifts and is further substantiated by the crystal structure of Cl₄P(NC₅H₄)NMe (1). Crystal data for 1: orthorhombic, space group P2₁2₁2₁ (No. 19), a = 7.507 (3) k, b = 11.623 (3) A, c = 12.577 (2) k,V= 1097.4 A3, Z = 4. Final R and R., values for 1 are 0.038 and 0.045, respectively. The molecular structure shows also that the methylamino substituent of pyridine lies flat in the radial plane of the molecule, that the exocyclic nitrogen bound to phosphorus is planar, and that both the endocyclic (1.350 (6) A) and exocyclic (1.344 (6) A) C-N bond lengths have similar values which are furthermore indicative of C-N multiple bonding. P-Cl bond lengths range from 2.077 to 2.153 A, with the longest bonds being those perpendicular to the molecular plane and the shortest trans to the donating pyridine nitrogen. Saturation-transfer NMR experiments indicate that the fluorine exchange in F4P(NC5H4)NMe (2) involves two competitive processes of the opened-ring intermediate with similar energy barriers of 57.8 (1) (pseudorotation) and 56.1 (1) (ligand rotation) kJ/mol. Synthesis, selected reactions, and NMR spectral characterization of these compounds are reported.