Conformational and Isomeric Preferences of Six-Membered Inorganic Heterocycles [EtNP(E)(OR)]_3 (E = Lone Pair, 0, S, or Se): A Synthetic, Spectroscopic, Structural, and Computational Study

摘要

A silicophosphonate bearing four hydroxyl groups, [RSi(OH){OP(0)(H)(OH)}]_20 (2; R = (2,6-~1Pr_2C-6H_3)N(SiMe_3)), has been isolated from the reaction between RSi(OH)_3 and [EtNPC1]_3. In contrast, the reaction between the sodium salt of substituted phenols and [EtNPCI]_3 yields [EtNP(OAr)]_3 [Ar =-C_6H_3 Pr_2-2,6 (3), -C_6H_2Me_3-2,4,6 (4), -C_6H_3M_e2-2,6 (5), -C_10H_7,-1 (6), and -C_6H_4B_u-4 (7)]. The cis isomers 3_a-7_a preferentially crystallize out from the cis/trans isomeric mixture. Single-crystal X-ray diffraction studies carried out for 3_a, 4_a, and 5_a reveal that the P_3N_3 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 H_20_2, sulfur, and selenium yields the trichalcogenides [EtNP(E)(OC_6H_3'Pr_2-2,6)]_3 (E = 0(8), S(10),Se(12)) and [EtNP(E)(OC_6H_3Me_2-2,6)]3 (E = 0 (9), S (11), Se (13)). The reaction leading to the formation of the triselenide 12 also produces small quantities of the diselenide [Et_3N_3P_3(OR)_3Se_2] (R =-C_6H_3 Pr_2-2,6) (14). The P_3N_3 ring in these λ~5-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.