Stable Eight-Coordinate Iron(III/II) Complexes

The chemistry of unusual coordination numbers of transition-metal complexes has been of interest because of their presence in biology and catalytic systems. Herein we describe a systematic and predictable approach toward isolation of stable eight-coordinate (8C) iron(III/II) systems. The 8C (S = 2; high-spin, HS) complex [Fe(L_N4)₂](BF₄)₂ (1) has been synthesized and characterized, displaying a distorted square-antiprism structure. Complex 1 is a unique 8C iron complex that exhibits remarkable stability in solution under various unfavorable conditions. The E_1/2 value of 1 (0.430 V vs Ag/AgCl, MeCN) supports the Fe^II oxidation state; however, the corresponding HS (S = ⁵/₂) 8C Fe^III analogue [Fe(L_N4)₂](NO₃)₃ (3) has also been synthesized via the chemical oxidation of 1. The structural, spectroscopic, and theoretical descriptions of these 8C iron complexes are reported in this work.