Organometallics
Article
likely be weaker in intensity relative to the bridging
counterparts.30
Scheme 3. Possible Dimerization Routes after Reduction
CONCLUSIONS
■
The electrochemical behavior of the title complex [CpFe-
(dppf)(CO)]+ showed a reversible oxidation that occurred at
the ferrocenediyl moiety of the dppf ligand and not at the
CpFe moiety. The location of this oxidation event was
confirmed by IR and UV−vis SEC and was further supported
by DFT calculations. The reduction event observed in the
cyclic voltammogram is proposed to give an intermediate with
significant radical character at the CpFe moiety, and limited
spin density is located at the CO ligand. This FeI species can
undergo dimerization through displacement of one dppf ligand
followed by bridging the CO ligands. We are currently
investigating the exchange of Cp for Cp* in the iron carbonyl
moiety as well as modifications to substituents on the P
donors.
atoms in this complex and the SEC IR spectrum (Figure 5) did
not fit with that of the reported (μ-M(OC)2)M. Thus, this
structure was abandoned to pursue the more promising option
of dppf displacement and subsequent dimerization.
A second, more feasible dimer based on the bands νCO
=
1710(vs) and 1667(vw) cm−1 can be attributed to the
formation of a Fe−Fe bond through the displacement of one
of the dppf ligands. The energy for the dechelation of dppf
from κ2 to κ1 was calculated to be ∼2.4 kcal mol−1 for [1]0
according to a potential energy surface scan (Figure S15). This
would allow for the formation of a Fe−Fe bond), where
similarly structured complexes [Cp2Fe2(μ-CO)2(μ-P−P)] (P−
P = 1,n-bis(diphenylphosphino)-CnH2n, n = 3 (dppp), 2
(dppe), 1 (dppm) have also been described showing
νCO(dppp) = 1725 (vw), 1687 (vs), νCO(dppe) = 1730
(vw), 1693 (vs),27,28 and νCO(dppm) = 1734 (vw), 1690 (s)
(Chart 1).27,28 The complex [Cp2Fe2(μ-CO)2(μ-(P-
EXPERIMENTAL SECTION
Information concerning instrumentation and spectrometers can be
■
Density functional theory (DFT) calculations were performed
using the solid-state structure of [1]+. A computational analysis was
performed by means of restricted Kohn−Sham density functional
theory (DFT) using the B3LYP31 functional in combination with the
D3 dispersion correction32 with the def2-TZVP and Weigend J
auxiliary basis set.33,34 Geometry optimizations were realized with the
ORCA program package with TightSCF convergence (1.0e-7 au).35
Solvation in CH2Cl2 was modeled using the CPCM solvation
model.36 Open-shell calculations on the structures [1]2+ and [1]0
were determined by removing or adding 1e−, respectively, and
calculations were performed by means of unrestricted Kohn−Sham
DFT using the same basis set, functionals, and solvent models as for
[1]+. For the numerical frequency calculations, only the fast
contribution of the solvent response to the molecular vibrations
(defined through electronic polarizations) was taken into account.
1,1′-Bis(diphenylphosphino)ferrocene (dppf) (Carbosynth) was
crystallized from CH2Cl2/EtOH, and the resulting orange crystals
were dried at 50 °C under vacuum. n-BuOH (Sigma-Aldrich) was
sparged with N2 prior to use. NaB(ArF)4 was synthesized according to
a literature procedure.37
Chart 1. Reported CpFe(CO) Dimers
Tetrabutylammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]-
borate (Bu4NB(ArF)4) was made prior to use and passed through a
neutral alumina column using CH2Cl2 as eluent to afford a white
crystalline solid. The solid is best stored over a drying agent such as
CaSO4. The syntheses of [CpFe(CO)2]2 and [CpFe(CO)2I] were
done according to a literature procedure,38 and details can be found in
Synthesis of [CpFe(CO)(dppf)]I. [CpFe(CO)2I] (1.00 g, 3.29
mmol) was dissolved in EtOH (80 mL), and dppf (1.82 g, 3.28
mmol) was added to the solution under a flow of nitrogen. The
reaction mixture turned dark red after heating to reflux for 120 h or by
heating to 160 °C for 2 h in a microwave reactor. This solution was
cooled and filtered through Celite. The solvent was reduced to 10 mL
under vacuum. Et2O was added to the concentrated solution, and a
brown-orange solid precipitated. This solid was collected by filtration
and recrystallized using CH2Cl2 overlayered with n-hexane, and the
final product was dried under vacuum. Yield: 2.11 g (78%). IR
(CH2Cl2): νCO = 1966 cm−1. Anal. Calcd for C40H33Fe2P2OI·
0.25C6H14·CH2Cl2: C, 54.49; H, 4.14. Found: C, 54.48; H, 4.16.
Synthesis of [CpFe(CO)(dppf)]B(ArF)4 ([1]+). To a solution of
[CpFe(CO)(dppf)]I (508 mg, 0.61 mmol) in MeOH (100 mL) was
added NaB(ArF)4 (575 mg, 0.65 mmol). The mixture was stirred for
30 min with gentle heating at 40 °C. Demineralized water was added
slowly to the cooled solution until no more solid precipitate formed.
The pale yellow-brown solid was collected by filtration, washed with
(OEt)2)2O)] showed two bands at 1712 (vs) and 1754 (vw)
cm−1 (Chart 1).29 The molecular structure of [Cp2Fe2(μ-
CO)2(μ-dppf)] was calculated and is the likely explanation of
the bands at νCO = 1710 (vs) and 1667 (vw) cm−1 shown in
the IR SEC spectrum at −30 °C (Figure 5); the proposed
electrochemical mechanism is shown in Scheme 3.
The final bands at 1876 cm−1 are thought to be due to the
higher energy configuration that contains two terminal CO
ligands, [Cp2Fe2(CO)2(μ-dppf)]. The later appearance of the
higher energy band is likely due to its weaker intensity, where
the similarly structured complex [Cp2Fe2(CO)2(H2CCCH2)]
showed νCO = 1999 (m) (Chart 1).30 A DFT analysis found
that [Cp2Fe2(CO)2(μ-dppf)] is about 20 kcal mol−1 higher in
energy than [Cp2Fe2(μ-CO)2(μ-dppf)], which would be
consistent with less of this species at νCO = 1876 cm−1 being
formed during reduction. The terminal CO ligands would also
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Organometallics 2021, 40, 760−765