Heptacoordinated Antimony(V) Cations

Heptacoordinated pnictogen cations are quite rare, and structurally characterized Sb(V) cations have remained elusive. Here, we report on the synthesis, isolation, and structural characterization of salts containing two heptacoordinated Sb(V) cations, i.e., [(ppy)3Sb–X]+ (X = F; [1–F]+, X = CH3; [1–CH3]+, ppy = 2-(2-pyridyl)phenyl). Fluorination of neutral [(ppy)3Sb] (1) followed by treatment with BF3·OEt2 afforded [1–F][BF4], while methylation of 1 followed by counteranion exchange furnished [1–CH3][PF6]. In the solid state, [1–F]+ adopts a pentagonal-bipyramidal (PBP) geometry, whereas [1–CH3]+ adopts a tricapped-tetrahedral (TCT) one. Density functional theory (DFT) calculations for the [(ppy)3Sb–X]+ ([1–X]+) series revealed that the PBP structure is more stable for X = F, Cl, and Br, whereas the TCT structure is more stable for X = CF3, H, and CH3. Calculated fluoride ion affinity (FIA) and pyridine affinity (PyA) of simplified [Ph3Sb–X]+ models revealed substituent-dependent Lewis acidity and donor-binding-site preferences that broadly correlate with the PBP/TCT preferences of [1–X]+. These results suggest that substituent X influences the relative stability of the PBP and TCT structures in the heptacoordinated [1–X]+ series.