Synthesis, Characterization, and Reactivity of Iron(III) Complexes Supported by a Trianionic ONO^3– Pincer Ligand

Abstract

Synthetic and characterization results of a new family of Fe(III) compounds stabilized by a trianionic [CF3-ONO](3-) pincer-type ligand are reported. The ligand possesses three negatively charged donors constrained to the meridional positions that provide sufficient electron density to stabilize high-valent metal complexes. Using the redox-insulated [CF3-ONO](3-), pentacoordinated square-pyramidal {[CF3-ONO]FeCl2}{LiTHF2}2 (3), dimeric μ-DME{[CF3-ONO]FeDME}2 (4), trigonal bipyramidal [CF3-ONO]Fe(bpy) (5), and octahedral [CF3-ONO]Fe(bpy)H2O (5·H2O) complexes are synthesized. An interesting feature of the [CF3-ONO](3-) pincer-type ligand is its ability to coordinate the metal center in both the more common meridional positions or occupying a face of a trigonal bipyramidal complex. The molecular structure of 3 contains structural features similar to those of a rare square-planar high-spin Fe(II) complex, and the important role of the counterions in stabilizing a square-plane is emphasized. SQUID magnetometry measurements of 3 reveal its high-spin character, and cyclic voltammetry measurements indicate high oxidation state species are unstable. However, all compounds can be reduced, and in particular 5 displays a reversible reduction event at -2255 mV versus ferrocene (Fc(+)/Fc) that can be assigned to either the Fe(I)/Fe(0) couple or 2,2'-bipyridine reduction.

Solid State Collapse of a High-Spin Square-Planar Fe(II) Complex, Solution Phase Dynamics, and Electronic Structure Characterization of an Fe(II)2 Dimer

“…Stirring the reaction mixture for 2 h, filtering the insoluble residue through a Celite pad, and removing all volatiles under a vacuum generates an analytically pure green powder in 87% yield. A 1 H NMR spectrum in C 6 D 6 confirms the identity of the previously reported complex {[CF 3 -ONO]­FeCl 2 }­{Li­(THF) 2 } 2 ( 4 ) …”

“…The CF 3 groups from the flanking pincer arms exert steric hindrance over these THF molecules, preventing them from binding in a linear fashion (∠O3–Fe1–O5 = 159.17(4)°), thus weakenenig the Fe–THF bonds. The same steric effect is also seen in the related Fe­(III) dimer μ-DME­{[CF 3 -ONO]­FeDME} 2 , where the steric hindrance imposed by the ligand forces the O–Fe–O angles to be even more acute (154.16(6)° and 150.85(7)°).…”

“…After all volatiles were removed under a vacuum, the residue was triturated three times with pentane (6 mL) yielding an analytically pure green powder (0.036 g, 86.7%). 1 H NMR (C 6 D 6 , 300 MHz, 25 °C) δ (ppm): 136.04 (ν 1/2 = 360 Hz), 75.55 (ν 1/2 = 480 Hz), 16.52 (ν 1/2 = 960 Hz), 5.17 (ν 1/2 = 360 Hz), and −148.95 (ν 1/2 = 240 Hz) …”

“…Contributing to this exclusive list, we reported the synthesis and characterization of the high-spin square-planar Fe­(II) trianionic pincer complex {[CF 3 -ONO]­FeCl}­{Li­(Sv) 2 } 2 ( 2 ; Sv = THF, Et 2 O). , Complex 2 breaks the mold of FeO 4 square-planar S = 2 cores. Instead for 2 the core comprises an FeO 2 NCl with a square plane (τ 4 = 0.06) consisting of a trianionic [CF 3 -ONO] 3– pincer-type ligand ,− and a chloride. Most important, complex 2 retains its square-planar geometry and S = 2 spin state in solutions of noncoordinating solvents.…”

Synthesis and characterization of a family of M2+ complexes supported by a trianionic ONO3− pincer-type ligand: towards the stabilization of high-spin square-planar complexes

“…22 Holland illus-trated a set of conditions to promote high-spin square-planar molecular compounds: (1) charge from anionic ligands to reduce the metal acidity and prevent coordination from additional ligands, (2) alkali metal counterions to stabilize the negative charge, (3) strong π-donor ligands to destabilize the sterically preferred tetrahedral geometry, and most importantly, (4) weak σ-donating ligands to minimize the antibonding character of the d x 2 −y 2 orbital. 23 Our group recently reported the trianionic pincer-type ligand 2,2′-(azanediylbis(2,1-phenylene))bis(1,1,1,3,3,3-hexafluoropropan-2-ol) ([CF 3 -ONO]H 3 ; 1) and its coordination to W(VI), [24][25][26][27] Ta(V), 28 Hf(IV), 29 Ti(IV), 29 Fe(III), 30 and Fe(II) 31 metal centers. Ligand 1 contains all the prerequisites described by Holland.…”