Inorganic Chemistry
Article
of 1.29 The QS parameter decreases from 1 to [4]+, revealing a
more symmetrical charge distribution in the oxidized complex
[4]+. Similarly low values are found for FeIII phenolate [6]+.
The QS value is also affected by the electronic properties of the
para substituent in nitroso complexes [3]+. Exchange of a p-
NO2 group in [3m]+ by p-NMe2 in [3o]+ increases the QS
value from 0.845(16) to 1.113(10), caused by the reduced π
acidity of the ligand in [3o]+, resulting in a more asymmetrical
charge distribution around the metal.
carbonyl complex [Fe{N(O)-Ph}(CO)2Cp]+ (1.226(3) Å),18
indicating greater Fe−N(O) back-bonding. The Fe−ONR
binding mode was not observed in any of the compounds
studied here.8 The donor−acceptor electronic effects observed
in the NMR and optical properties are also reflected in the C−
N bond lengths toward the para substituents, which decrease
from 1.465(3) (p-NO2, [3m]+) to 1.363(4) Å (p-NMe2,
[3o]+).
The strength of the Fe−N interaction in the nitroso
Although both values are similar to those of FeIII complexes
[4]+ and [6]+, the sharp signals in NMR experiments reveal a
diamagnetic FeII configuration for the nitroso complexes.
Clearly the QS values are sensitive to both the nature of the
ligand environment and the oxidation state of the metal ion.
The half-height width (line broadening, Γ) increases in the
order FeII (0.13) < [3o]+ (0.16) ≈ [3m]+ (0.17) < FeIII−Cl
(0.27) < FeIII−O (0.44). Thus, although a comparison of
complexes with ligands of different nature can be misleading,31
Γ reveals that nitroso complexes [3m,o]+ exhibit properties
similar to those of other FeII compounds, evidencing a FeII/
N(O)R0 metal/ligand couple.
complexes was investigated by displacement reactions with
acetonitrile. Complexes [3m][BArF ] (p-NO2) and [3o]-
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[BArF ] (p-NMe2), representing both extremes of the
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electronic scale, were chosen by way of example. Electron-
poor [3m]+ displayed a high barrier toward displacement of
the nitroso ligand and converted only slowly to the acetonitrile
complex [Fe(NCMe)(dppe)Cp]+ in refluxing NCMe, with
10% conversion after 4 h and 90% of [3m]+ being recovered. A
demonstrating the high stability of [3m]+. In addition, it
should be noted that 1,4-dinitrobenzene was also formed in
this reaction, by reoxidation of the released p-nitroso
nitrobenzene ligand. In contrast, the blue color of electron-
rich [3o]+ vanishes completely within 10−20 min in
acetonitrile at ambient temperature. These results also help
explain the lower yields of the electron-rich nitroso complexes
[3h,o,p]+ (Scheme 2), which are more prone toward a release
of the nitroso ligand during the workup and purification
processes.
Compounds [3d,m,o,s]+, [4]+, [5]+, [6]+, and [9]2+ ([9]2+
=
The electronic differences of the nitroso ligand substituents
are also reflected in the Fe−N distances and degree of η1/η2
disorder in the solid-state structures. The more labile p-NMe2
derivative, [3o]+, offers a relatively long Fe−N distance
(1.841(3) Å) and absence of the η2 coordination mode of
the N(O)R moiety in the structural model, consistent with
limited Fe−N back-bonding. The derivatives bearing more
electron withdrawing p-NO2 ([3m]+) and p-CCPh ([3s]+)
moieties offer shorter Fe−η1-N(O) distances (1.820(2) and
1.707(9) Å, respectively) and feature more significant
contributions from the disordered η2-N(O) component in
the structural models ([3m]+, 10%; [3s]+, 52%). The C6−N
O torsion angle of the η1-bonded nitroso functionality toward
the phenyl plane follows the trend of a decrease of electron
density in the order [3o]+ > [3s]+ > [3m]+ via 47.9(2)° <
61.0(7)° < 66.58(14)°. The η2-coordination mode induces a
rather coplanar orientation of the NO and phenyl fragments
(Table 2). The Fe−O distances decrease in the order ∼2.7 Å
(η1 entities) ≳ 2.1 Å (η2 entities) ≳ 1.87 Å for phenolate
complexes [5]+ and [6]+.
Figure 2. ORTEP drawings of the molecular structures of [3m]+ (top
left; probability level 30%), [6]+ (top right; probability level 50%),
[3s]+ (bottom left; probability level 50%), and [5]+ (bottom right;
probability level 30%) showing their atom-numbering scheme. H
−
atoms, the [BArF ]− (3m,s, 5) or [BArCl
]
(3s) counterion,
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disordered parts (see ESI) and the phenyl rings of the dppe
fragments have been atoms are omitted for clarity. Selected bond
properties are summarized in Table 2. Selected bond properties (Å):
[5]+, Fe−P 2.154(9)−2.2757(7), Fe−O 1.863(2), Fe−CtCp
1.7355(4); [6]+, Fe1−P1 2.2460(8), Fe1−P2 2.2589(8), Fe−O
1.8828(18), C−N 1.457(3), N−O 1.228(3) and 1.233(3), Fe−CtCp
1.7495(4). For a comparison of bond parameters of nitroso
complexes [3]+, see Table 2.
Recent investigations by Ishii and co-workers highlighted the
use of Na[BArF ] for the activation of 1 by halide abstraction
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in nonpolar/noncoordinating solvents. The electronically and
coordinatively unsaturated [Fe(dppe)Cp]+ complex is able to
activate internal alkynes via an alkyne−vinylidene rearrange-
ment.21 Other halide abstracting agents such as AgPF6 and
NaBPh4 resulted in a complex mixture of products. However,
could be further characterized by single-crystal X-ray
diffraction. The Fe−P distances (Table 2) in [3d,m,o,s]+ are
comparable to those of other Fe nitroso complexes19 and
cationic, N-ligated complexes, such as [Fe(NCMe)(dppe)Cp]+
(Figure S8), which further supports the description of
compounds [3]+ in terms of an FeII metal atom and a neutral
nitroso ligand.
The Fe−N and N−O bond lengths are in good agreement
with those of Fe porphyrin nitroso complexes featuring a low-
spin FeII center.10 The N−O distances of [3d,m,o,s]+
(1.251(3)−1.272(12) Å), featuring more electron donating
dppe ancillary ligands, are slightly elongated relative to the
the use of [BArF ]− is not without difficulty, due to its rather
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elaborate purification and high propensity for disorder of the
CF3 groups in the solid state,32 which can complicate
crystallization processes.
To address some of these potential concerns, the non-
fluorinated, but still weakly coordinating, [B(3,5-Cl2-C6H3)4]−
anion was explored in these studies.32 A comparison between
reactions promoted by the more soluble Na[BArF ] salt and
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Inorg. Chem. 2021, 60, 4986−4995