Conformational and Isomeric Preferences of Six-Membered Inorganic Heterocycles [EtNP(E)(OR)]₃ (E = Lone Pair, O, S, or Se): A Synthetic, Spectroscopic, Structural, and Computational Study

A silicophosphonate bearing four hydroxyl groups, [RSi(OH){OP(O)(H)(OH)}]2O (2; R = (2,6-iPr2C6H3)N(SiMe3)), has been isolated from the reaction between RSi(OH)3 and [EtNPCl]3. In contrast, the reaction between the sodium salt of substituted phenols and [EtNPCl]3 yields [EtNP(OAr)]3 [Ar = −C6H3iPr2-2,6 (3), −C6H2Me3-2,4,6 (4), −C6H3Me2-2,6 (5), −C10H7-1 (6), and −C6H4tBu-4 (7)]. The cis isomers 3a−7a preferentially crystallize out from the cis/trans isomeric mixture. Single-crystal X-ray diffraction studies carried out for 3a, 4a, and 5a reveal that the P3N3 ring adopts a flattened-chair conformation with the aryloxy substituent on all three phosphorus atoms occupying the axial position. Oxidation of the cis isomers 3 and 5 by H2O2, sulfur, and selenium yields the trichalcogenides [EtNP(E)(OC6H3iPr2-2,6)]3 (E = O (8), S (10), Se (12)) and [EtNP(E)(OC6H3Me2-2,6)]3 (E = O (9), S (11), Se (13)). The reaction leading to the formation of the triselenide 12 also produces small quantities of the diselenide [Et3N3P3(OR)3Se2] (R = −C6H3iPr2-2,6) (14). The P3N3 ring in these λ5-phosphazanes is highly distorted and resembles more of a twist-boat conformation. The DFT calculations on model systems [HNP(S)(OMe)]3 (15) and [HNP(Se)(OMe)]3 (16) indicate five low-lying unique conformers A−E. The chair conformer with a triaxial-OMe group is identified as the lowest-energy conformer in both cases.