A Monometallic Iron(I) Organoferrate

Article abstract of DOI:10.1021/acs.organomet.6b00841

Tetra-n-butylammonium (TBA) (η⁶-biphenyl)diphenylferrate was formed unexpectedly in the reaction of (TBA)₂[Fe₄S₄Cl₄] with an excess of phenyllithium. This complex belongs to a novel type of organoferrate.

Full text of DOI:10.1021/acs.organomet.6b00841

Communication
pubs.acs.org/Organometallics
A Monometallic Iron(I) Organoferrate
Fedor E. Zhurkin, Matthew D. Wodrich, and Xile Hu*
Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fed
́
er
́
ale de Lausanne (EPFL), 1015 Lausanne, Switzerland
S
* Supporting Information
ABSTRACT: Tetra-n-butylammonium (TBA) (η6-biphenyl)-
diphenylferrate was formed unexpectedly in the reaction of
(TBA)2[Fe4S4Cl4] with an excess of phenyllithium. This
complex belongs to a novel type of organoferrate.
rganoferrates are anionic organometallic iron complexes.1
They are invoked as important intermediates in many
the oxidation states of iron in known organoferrates are limited
to +III, +II, 0, and −II. Here we report a novel Fe(I)
organoferrate which is unsupported by either another metal
cation or a heteroatom-based ligand.
Inoue and co-workers studied the catalytic activity of the
phenyllithium-treated cluster [Fe4S4Cl4]2− in hydrogenation
reactions. They prepared the catalyst by addition of
(TBA)2[Fe4S4Cl4] (10; TBA = tetra-n-butylammonium) to a
solution of PhLi. This catalyst was active for hydrogenation of
some olefins and carbonyl compounds.10,11 In an attempt to
isolate and characterize the active species in this system, we
reacted (TBA)2[Fe4S4Cl4] with 8 equiv of PhLi in ether under
an inert atmosphere at room temperature (Scheme 2). In the
O
iron-catalyzed organic reactions.1,2 Organoferrates devoid of
stabilizing ligands (nonstabilized organoferrates) are scarce.2
Reported examples include tetramethylferrate (1;3 Scheme 1)
Scheme 1. Representative Examples of Nonstabilized
Organoferrates
Scheme 2. Synthesis of Complex 11
first 1−2 min after PhLi addition, the liquid phase turned an
intense orange-black. As the reaction proceeded, a black
precipitate was formed (11) and the solution became nearly
colorless. The precipitate could be dissolved in THF to yield a
blood red solution. Slow diffusion of pentane into this solution
yielded black needle-shaped crystals.
X-ray crystallography revealed the molecular structure of 11
(Figure 1). The compound is a TBA salt of an iron(I) complex
anion, which is coordinated by a π-bound η6-biphenyl ligand
and two σ-bound phenyl ligands. The Fe−C(η1-Ph) distance
(1.971−1.977 Å; see the Supporting Information) in complex
and tetrabenzylferrate(III) (2),4 [Fe(Mes)3]− (3a)5 and
[FeBn3]− (3b),4 tetramethyl- and tetraphenyliron(II) com-
plexes 4 and 5,2 the tetraphenyl dihydride complex 6,6 the
Fe(0) complex 7,7 formally Fe(−II) complexes 8 and 9,8 and
the octanuclear ferrate [MgCl(thf)5][Fe8Me12].9
With the exception of 1, 2, and 3a,b, these complexes often
contain either direct Fe−metal (most often Fe−Li) interactions
or solvated metal cation(s) (most often lithium) in the
proximity of the iron center. The latter usually forms short
contact with the ipso carbons of the organic ligands. Moreover,
Received: November 4, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00841
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
bromobutane (see the Supporting Information) to give the
corresponding C−C coupling products (eq 1), albeit in low
yields. Biphenyl was also formed.
In summary, a novel ferrate complex, (η6-biphenyl)-
diphenylferrate, was isolated. This complex is neither supported
by an alkali-metal ion nor supported by a heteroatom ligand,
making it a unique Fe(I) ferrate.
Figure 1. X-ray crystal structure of the (η6-biphenyl)diphenylferrate
anion in complex 11. Two molecules were found in one asymmetric
unit of 11; only one of them is shown. The TBA cation and hydrogen
atoms are omitted for clarity. The ellipsoids are drawn at the 50%
probability level.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00841.
■
S
11 is slightly shorter than its counterpart in 5 (2.084−2.216 Å)2
and 6 (2.056 Å).6 The distance between the Fe(I) center and
the η6-phenyl ring (1.578 Å) is in line with the values reported
for (η-arene)(Cp)FeII complexes.12,13 It is remarkable that,
despite the presence of lithium ion and diethyl ether in the
reaction mixture, neither is present in complex 11. Formation
of a η6-biphenyl iron(0) complex upon reaction of an iron(II)
bisphosphine complex with phenyl nucleophiles, which
proceeds through reductive elimination, has been reported.14
Complex 11 might be formed in a similar process via an Fe(III)
tetraphenyl intermediate.
Experimental procedures and characterization of complex
11 and computational details (PDF)
Crystallographic data for complex 11 (CIF)
Cartesian coordinates for the calculated structures (XYZ)
AUTHOR INFORMATION
Corresponding Author
*E-mail for X.H.: xile.hu@epfl.ch.
■
Complex 11 is paramagnetic. The solution magnetic moment
in THF is 1.75 μB, determined by the Evans method after
diamagnetic corrections. This value is consistent with a low-
spin Fe(I) center (S = 1/2). DFT computations at the OPBE/
def2-TZVPP levels confirmed the doublet spin multiplicity for
the ground state of 11. The quartet state was found to be 19.8
kcal/mol higher in energy. Moreover, the optimized geometry
of the quartet spin multiplicity deviates significantly from the
crystal structure in Figure 1. An Fe(I) S = 1/2 species was
previously observed by electron paramagnetic resonance
(EPR), but not isolated, in the reaction of FeCl3 with (4-
tolyl)MgBr at −30 °C.4 The N and H contents determined by
elemental analysis agree with the formula of 11; however, the C
content is lower than that predicted, despite repetitive
measurements, including on single crystals. This discrepancy
might be due to decomposition of the complex during
elemental analysis. The Fe content, determined by inductively
coupled plasma atomic emission spectroscopy (ICP-AES), also
agrees with the theoretical formula.
Attempts were made to look for alternative conditions for the
synthesis of complex 11. The reaction solvent could be changed
to benzene. However, the iron precursor was limited to 10.
Replacing 10 with simpler iron salts such as FeCl3, FeCl2, and
TBA[FeCl4] failed to yield complex 11. In fact, reactions of
iron salts with PhLi were previously widely studied,2,6,15 but the
formation of complex 11 in these systems is hitherto unknown.
Complex 11 is insoluble in hydrocarbon solvents and 1,4-
dioxane but shows good solubility in THF and acetonitrile. It
does not react with H2 or catalyze hydrogenation of cis-stilbene,
ruling out its role as the active species in the hydrogenation
system of Inoue.10 Reactions of 11 with CO, CO2, or air led to
its decomposition, with concomitant formation of free
biphenyl. No reaction occurred with chlorobenzene. However,
11 reacted with 2-bromopyridine, p-bromoanisole, and 1-
ORCID
Xile Hu: 0000-0001-8335-1196
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the EPFL and an ERC starting
grant (No. 257096). We thank Drs. Rosario Scopelliti and
Farzaneh Fadaei (EPFL) for the X-ray crystallographic study of
complex 11. The Laboratory for Computational Molecular
Design (EPFL) is acknowledged for providing computational
resources.
REFERENCES
■
(1) Jegelka, M.; Plietker, B. Catalysis by Means of Complex Ferrates.
In Iron Catalysis: Fundamentals and Applications; Plietker, B., Ed.;
Springer: Berlin, Heidelberg, 2011; pp 177−213.
(2) Furstner, A.; Martin, R.; Krause, H.; Seidel, G.; Goddard, R.;
̈
Lehmann, C. W. J. Am. Chem. Soc. 2008, 130, 8773−8787.
(3) Al-Afyouni, M. H.; Fillman, K.; Brennessel, W. W.; Neidig, M. L.
J. Am. Chem. Soc. 2014, 136, 15457−15460.
(4) Bedford, R. B.; Brenner, P. B.; Carter, E.; Cogswell, P. M.;
Haddow, M. F.; Harvey, J. N.; Murphy, D. M.; Nunn, J.; Woodall, C.
H. Angew. Chem., Int. Ed. 2014, 53, 1804−1808.
(5) Irwin, M.; Jenkins, R. K.; Denning, M. S.; Kramer, T.; Grandjean,
F.; Long, G. J.; Herchel, R.; McGrady, J. E.; Goicoechea, J. M. Inorg.
̈
Chem. 2010, 49, 6160−6171.
(6) Bazhenova, T. A.; Kachapina, L. M.; Shilov, A. E.; Antipin, M. Y.;
Struchkov, Y. T. J. Organomet. Chem. 1992, 428, 107−123.
(7) Jonas, K.; Kruger, C. Angew. Chem., Int. Ed. Engl. 1980, 19, 520−
̈
537.
(8) Jonas, K.; Schieferstein, L.; Kruger, C.; Tsay, Y.-H. Angew. Chem.,
̈
Int. Ed. Engl. 1979, 18, 550−551.
(9) Munoz, S. B., III; Daifuku, S. L.; Brennessel, W. W.; Neidig, M. L.
J. Am. Chem. Soc. 2016, 138, 7492−7495.
B
DOI: 10.1021/acs.organomet.6b00841
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
(10) Inoue, H.; Suzuki, M. J. Chem. Soc., Chem. Commun. 1980, 817−
818.
(11) Inoue, H.; Nagao, Y.; Haruki, E. J. Chem. Soc., Chem. Commun.
1985, 501−502.
(12) Houlton, A.; Roberts, R. M. G.; Silver, J.; Wells, A. S.; Frampton,
C. S. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1992, 48, 1018−
1022.
(13) Kundig, E. P.; Jeger, P.; Bernardinelli, G. Inorg. Chim. Acta 2004,
̈
357, 1909−1919.
(14) Daifuku, S. L.; Kneebone, J. L.; Snyder, B. E. R.; Neidig, M. L. J.
Am. Chem. Soc. 2015, 137, 11432−11444.
(15) Jefferis, J. M.; Girolami, G. S. Organometallics 1998, 17, 3630−
3632.
C
DOI: 10.1021/acs.organomet.6b00841
Organometallics XXXX, XXX, XXX−XXX