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

Abstract

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 -C₆H₄^tBu-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.