Monovalent iron in a sulfur-rich environment

Article abstract of DOI:10.1021/ic7023378

A series of low-coordinate, paramagnetic iron complexes in a tris(thioether) ligand environment have been prepared. Reduction of ferrous {[PhTt^tBu]FeCl}₂ [1; PhTt^tBu = phenyltris((tert-butylthio)-methyl)borate] with KC₈ in the presence of PR₃ (R = Me or Et) yields the high-spin, monovalent iron phosphine complexes [PhTt^tBu]Fe(PR₃) (2). These complexes provide entry into other low-valent derivatives via ligand substitution. Carbonylation led to smooth formation of the low-spin dicarbonyl [PhTt^tBu]Fe(CO) ₂ (3). Alternatively, replacement of PR₃ with diphenylacetylene produced the high-spin alkyne complex [PhTt ^tBu]Fe(PhCCPh) (4). Lastly, 2 equiv of adamantyl azide undergoes a 3 + 2 cycloaddition at 2, yielding high-spin dialkyltetraazadiene complex 5.

Full text of DOI:10.1021/ic7023378

Inorg. Chem. 2008, 47, 1889-1891
Monovalent Iron in a Sulfur-Rich Environment
Michael T. Mock,† Codrina V. Popescu,*,‡ Glenn P. A. Yap,† William G. Dougherty,†
and Charles G. Riordan*,†
Department of Chemistry and Biochemistry, UniVersity of Delaware, Newark, Delaware 19716,
and Department of Chemistry, Ursinus College, CollegeVille, PennsylVania 19426
A series of low-coordinate, paramagnetic iron complexes in a
tris(thioether) ligand environment have been prepared. Reduction
of ferrous {[PhTttBu]FeCl}2 [1; PhTttBu ) phenyltris((tert-butylthio)-
methyl)borate] with KC8 in the presence of PR3 (R ) Me or Et)
yields the high-spin, monovalent iron phosphine complexes
[PhTttBu]Fe(PR3) (2). These complexes provide entry into other low-
valent derivatives via ligand substitution. Carbonylation led to
smooth formation of the low-spin dicarbonyl [PhTttBu]Fe(CO)2 (3).
Alternatively, replacement of PR3 with diphenylacetylene produced
the high-spin alkyne complex [PhTttBu]Fe(PhCCPh) (4). Lastly, 2
equiv of adamantyl azide undergoes a 3 + 2 cycloaddition at 2,
yielding high-spin dialkyltetraazadiene complex 5.
the preparation and examination of high-spin monovalent iron
complexes5,6 in a sulfur-rich ligand environment. While the
spectroscopic characteristics of heme and nonheme iron
complexes have been extensively examined, similar data for
low-coordinate, i.e., CN < 5, low-valent, high-spin com-
plexes are limited. Thus, a comparison with the data derived
from the examination of metalloproteins is tenuous. Herein,
we present the synthesis, structure, electron paramagnetic
resonance (EPR), Mössbauer and magnetic properties of a
series of monovalent iron complexes supported by the
tris(thioether)borate ligand [PhTttBu].7 Ligand substitution
allows for the introduction of a range of donors including
those that are potentially redox-active and, thus, confounds
simple electronic structure descriptions.8
In contrast to the synthesis of [PhTttBu]MX derivatives of
Ni, Co, Zn, and Cd, entry into [PhTttBu]-ligated iron
chemistry is quite sensitive to the nature of the metal salt.7,9
After canvassing a number of potential precursors, we
established that FeCl2(THF)1.5 leads to the target complex,
albeit in a stepwise fashion. The addition of [PhTttBu]Tl to
FeCl2(THF)1.5 in THF generates the colorless “ate” complex
{[κ2-PhTttBu]FeCl2}Tl ·THF (see the Supporting Information
for details). Removal of the volatiles followed by dissolu-
tion in toluene precipitates TlCl, generating light-yellow
{[PhTttBu]FeCl}2 (1). Unlike the Co, Ni, and Zn analogues,
1 is dimeric in the solid state as revealed by X-ray diffraction
(Figure S6 in the Supporting Information, SI). The magnetic
susceptibility of 1 (SQUID; Figure S1 in the SI) follows the
Curie law, with µeff ) 5.1 µB per Fe (10–290 K), indicating
no detectable exchange coupling between the metals. The
4.5 K Mössbauer parameters of 1, at δ ) 0.96(3) mm/s and
∆EQ ) 3.45(2) mm/s, are in the range expected for a high-
spin ferrous site and similar to those of ferrous rubredoxin.10
The monovalent oxidation state of iron is receiving
increasing attention because of its implication in biocatalytic
hydrogen and ammonia production and its potential in
promoting group-transfer reactions. The iron-only hydroge-
nase enzymes reduce protons to H2 at an active site of
composition Fe2(CO)3(CN)2(µ-S2(CH2)2X).1 The unusual
organometallic diiron subcluster is proposed to redox cycle
via a number of states including reduced states that are
formally iron(I).2 The more structurally complex iron
molybdenum cofactor of nitrogenase is a metallocluster
featuring low-coordinate iron sites in sulfur-rich environ-
ments that facilitate N2 reduction.3 These developments have
stimulated a high level of activity within the coordination
chemistry community4,5 wherein emphasis can be placed on
elucidating fundamental aspects of the geometric and elec-
tronic structure and establishing how such structural features
dictate chemical reactivity. Our interests in this regard are
* To whom correspondence should be addressed. E-mail: cpopescu@
ursinus.edu (C.V.P.), riordan@udel.edu (C.G.R.).
(5) (a) Hendrich, M. P.; Gunderson, W.; Behan, R. K.; Green, M. T.;
Mehn, M. P.; Betley, T. A.; Lu, C. C.; Peters, J. C. Proc. Natl. Acad.
Sci. U.S.A. 2006, 103, 17107. (b) Stoian, S. A.; Yu, Y.; Smith, J. M.;
Holland, P. L.; Bominaar, E. L.; Münck, E. Inorg. Chem. 2005, 44,
4915. (c) Stoian, S. A.; Vela, J.; Smith, J. M.; Sadique, A. R.; Holland,
P. L.; Münck, E.; Bominaar, E. L. J. Am. Chem. Soc. 2006, 128, 10181.
(6) (a) Brown, S. D.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003,
125, 322. (b) Yu, Y.; Smith, J. M.; Flaschenriem, C. J.; Holland, P. L.
Inorg. Chem. 2006, 45, 5742.
†
University of Delaware.
Ursinus College.
‡
(1) Peters, J. W. Opin. Struct. Biol. 1999, 9, 670.
(2) Darensbourg, M. Y.; Lyon, E. J.; Zhao, X.; Georgakaki, I. P. Proc.
Natl. Acad. Sci. U.S.A. 2003, 100, 3683.
(3) Einsle, O.; Tezcan, F. A.; Andrade, S. L. A.; Schmid, B.; Yoshida,
M.; Howard, J. B.; Rees, D. C. Science 2002, 297, 1696.
(4) (a) For example, see: Georgakaki, I. P.; Thomson, L. M.; Lyon, E. J.;
Hall, M. B.; Darensbourg, M. Y. Coord. Chem. ReV. 2003, 238, 255.
(b) Rauchfuss, T. B. Inorg. Chem. 2004, 43, 14. (c) Sadique, A. R.;
Gregory, E. A.; Brennessel, W. W.; Holland, P. L. J. Am. Chem. Soc.
2007, 129, 8112..
(7) Schebler, P. J.; Riordan, C. G.; Guzei, I.; Rheingold, A. L. Inorg.
Chem. 1998, 37, 4754.
(8) Muresan, N.; Chlopek, K.; Weyhermuller, T.; Neese, F.; Wieghardt,
K. Inorg. Chem. 2007, 46, 5327.
10.1021/ic7023378 CCC: $40.75
Published on Web 02/15/2008
2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 6, 2008 1889

COMMUNICATION
Figure 1. Molecular structures of 2-5 as determined by X-ray diffraction. Thermal ellipsoids are at the 30% level, and H atoms are omitted.
nearly identical values, νCO ) 1979 and 1914 cm-1,6a whereas
Scheme 1
Chirik’s formally iron(0) complexes have expectedly lower
energy bands.12 The molecular structure of 3 (Figure 1) is that
of a square pyramid, with two thioether sulfurs and two
carbonyls defining the equatorial plane. The apical thioether
bond length is longer, at Fe-S1 ) 2.361(2) Å, than the
equatorial Fe-S bonds, 2.307(2) and 2.327(2) Å. This observa-
tion is surprising given the strong trans influence of CO ligands.
In [PhBP3]Fe(CO)2, the apical Fe-P bond length is shorter than
the equatorial ones. The room temperature magnetic moment
of 3, determined in solution by the method of Evans,13 is µeff
1
) 1.7(1) µB, consistent with an S ) /2 spin state. Moreover,
complex 3 displays an isomer shift of 0.21 mm/s (162 K),
consistent with a low-spin complex and a rhombic EPR signal,
g ) 2.13, 2.07, and 2.00 (Figure S3 in the SI), similar to the
signal reported for the oxidized state of the H cluster of
hydrogenase II, g ) 2.078. 2.027, and 1.99.14 A sample of 3
prepared with 13CO exhibits superhyperfine coupling to the two
13C nuclei, with A1 ) 30 MHz, A2 ) 30.5 MHz, and A3 ) 36
MHz.
The addition of 3 equiv of PR3 (R ) Me or Et) to 1 generates
a light-yellow solution. Subsequent stirring over KC8 produces
a deep-blue solution from which monovalent [PhTttBu]Fe(PR3)
(2R) is isolated in moderate yields. The molecular structure of
2Et is depicted in Figure 1. The molecule is pseudo-C3-
symmetric, with the phosphine residing on the 3-fold axis. The
phosphine complexes display µeff ) 3.9(1) µB for 2Me and 4.1(1)
µB for 2Et, consistent with three unpaired electrons, i.e., S )
3/2. Both derivatives exhibit axial EPR signals at 5 K with
effective g values of 4.26 and 2.05 and E/D ) 0 (Figure S2 in
the SI). There is no discernible superhyperfine coupling to the
The addition of diphenylacetylene to 2 generates an olive-
green alkyne complex, 4. The geometry at the iron is square
pyramidal, τ ) 0.06, 15 when considering the alkyne carbons
in two of the equatorial positions. The Fe-C bond lengths,
1.971(3) and 1.961(3) Å, reflect symmetric alkyne coordination.
The C-C bond length, 1.280(4) Å, of the diphenylacetylene
ligand is lengthened compared to the free alkyne, 1.210(3) Å.16
The C-C bond length is indistinguishable from that in
(iPrPDI)Fe(PhCCPh)17 while somewhat longer than that found
in several lower coordinate ꢀ-diketiminate derivatives.6b The
νCC mode at 1804 cm-1 is at much lower energy than for the
free ligand, 2217 cm-1.18 Thus, there is significant electronic
back-donation to the diphenylacetylene ligand, with the ferrous
state relevant. The paramagnetically shifted 1H NMR spectral
features of 4 are more similar to those of other ferrous
31P NMR nucleus, suggesting that little unpaired spin density
1
resides on that nucleus. The EPR spectrum of S )
/
2
[PhTttBu]Ni(PMe3) also does not display such coupling.11
Mössbauer spectra of 2Me exhibit δ ) 0.76(3) mm/s and ∆EQ
) 1.88(3) mm/s at 4.5 K. The isomer shift is higher than those
reported for the few iron(I) complexes published.5
2 has proven to be a useful synthon via ligand substitution
for the preparation of derivatives containing the [PhTttBu]Fe
fragment (Scheme 1). Exposure of a pentane solution of 2 to a
CO atmosphere results in conversion to the dicarbonyl, 3. The
IR spectrum contains two intense νCO bands at 1984 and 1911
cm-1, which appear at 1938 and 1867 cm-1 in samples prepared
from 13CO. For comparison, Peters’ [PhBP3]Fe(CO)2 exhibits
(12) Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J. Inorg. Chem. 2006, 45,
7252.
(13) Evans, D. F. J. Chem. Soc. 1959, 2003.
(14) Popescu, C. V.; Münck, E. J. Am. Chem. Soc. 1999, 121, 7877.
(15) τ ) (ꢀ-R)/60, where R and ꢀ are the two largest L-M-L angles
with ꢀgR; τ ) 0 for square pyramidal, and τ ) 1 for trigonal
bipyramidal. Addison, A. W.; Rao, T. N.; Reedijk, J.; Vanrijn, J.;
Verchoor, G. C. J. Chem. Soc., Dalton Trans. 1984, 1349.
(16) Zanin, I. E.; Antipin, M. Y.; Struchkov, Y. T. Kristallografiya 1991,
36, 411.
(9) Chiou, S.-J.; Innocent, J.; Lam, K.-C.; Riordan, C. G.; Liable-Sands,
L.; Rheingold, A. L. Inorg. Chem. 2000, 39, 4347.
(10) Yoo, S. J.; Meyer, J.; Achim, C.; Peterson, J.; Hendrich, M. P.; Münck,
E. J. Biol. Inorg. Chem. 2000, 5, 475.
(17) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004,
126, 13794.
(11) Mandimutsira, B. S.; Riordan, C. G. , unpublished results.
(18) Hiura, H.; Takahashi, H. J. Phys. Chem. 1992, 96, 8909.
1890 Inorganic Chemistry, Vol. 47, No. 6, 2008

COMMUNICATION
R2N4)]–, in which the unpaired electron was determined to reside
in a delocalized π* orbital of the metallacycle based on XR
calculations.22 While we consider resonance form B as a
description consistent with the experimental data, it can be
problematic to overemphasize such assignments. As noted by
Trogler, “. . . tetrazabutadiene complexes defy a simple descrip-
tion.”21 Recent reports of the complex redox behavior and
electronic structures of iron R-diimine complexes highlight this
cautionary note.8 Experiments aimed at the characterization of
the electronic structures of 4 and 5, augmented by density
functional theory calculations, are in progress.
A plausible reaction pathway leading to tetraazadiene com-
plexes was proposed first by Stone et al.23 The formation of 5
would initiate with group transfer to iron(I) generating a putative
imidoiron(III) intermediate, [PhTttBu]Fe(NAd). Clear precedents
of similar adducts are available in the systematic studies of
Peters and co-workers that demonstrate that four-coordinate
imidoiron complexes may be accessed in the ferrous, ferric,
and iron(IV) oxidation states.24 In the present case, the imi-
doiron(III) intermediate reacts further with a second 1 equiv of
AdN3 via a 3 + 2 cyclization, leading to 5. This last step may
be favored because of the ability of [PhTttBu] to deligate one
thioether substituent, thus providing sufficient space for the 3
+ 2 cycloaddition to proceed. Additional studies are warranted
to further investigate the mechanism of this reaction.
Figure 2. Resonance structures of 5: high-spin iron(I) with (N4Ad2)0 (A),
high-spin iron(II) with an antiferromagnetically coupled N4Ad2 radical anion
(B), and intermediate-spin iron(III) with (N4Ad2)2- (C).
complexes, i.e., 1 and [PhTttBu]Fe(Me),19 than to those of the
monovalent species 2 and 3. 4 is high-spin, S ) 3/2, as indicated
by its magnetic moment, µeff ) 4.1(1) µB, and rhombic EPR
signals, g ) 6.42, 1.64, and 1.29 and E/D ) 0.228 (Figure S4
in the SI). Preliminary Mössbauer spectra of 4 reveal δ )
0.62(3) mm/s and ∆EQ ) 1.62(2) mm/s, which do not contradict
its assignment as an iron(I) S ) 3/2 species. The pseudo-three-
coordinate LFe(PhCCH) prepared by Holland et al. displays
parameters δ ) 0.44(2) mm/s and ∆EQ ) 2.02(2) mm/s.5b
Lastly, we are interested in preparing higher valent
complexes of [PhTttBu]Fe, specifically imidoiron(III), using
the group-transfer approach.6a,20 It was surprising that the
addition of 1-adamantyl azide to 2 did not yield imido-
iron(III) [PhTttBu]Fe(NAd). Instead, the dark-orange diada-
mantyltetraazadiene, 5, was isolated. 5 has an S ) 3/2 ground
state as indicated by its magnetic moment, µeff ) 3.8(1) µB
and its rhombic EPR spectrum with g ) 5.47, 2.28, and 1.57
and E/D ) 0.305 (Figure S4 in the SI). The similarities
between the EPR and spectral data of 4 and 5 provide
empirical evidence for similar degrees of charge transfer onto
the ligands PhCCPh and Ad2N4, respectively.
The molecular structure of 5 (Figure 1) features a four-
coordinate iron site of roughly tetrahedral stereochemistry with
the [PhTttBu] ligand coordinated in a κ2 fashion. The Ad2N4 unit
binds symmetrically through its 1 and 4 nitrogens, Fe-N1 )
1.943(3) Å and Fe-N4 ) 1.947(3) Å. The resulting five-
membered metallacycle is planar with N-N bond lengths of
N1-N2 ) 1.324(4) Å, N3-N4 ) 1.317(4) Å, and N2-N3 )
1.337(5) Å. The similarity of the N-N bond distances suggests
a resonance structure with electron delocalization across the ring
[Figure 2 (resonance form B)].21 These structural parameters
are inconsistent with limiting resonance contributors in which
the Ad2N4 ligand is either neutral (A) or dianionic (C) because
each predicts localized N-N and NdN bonds. Interestingly,
the electronic spectrum of 5 contains a low-energy absorption
band in the near-IR, λmax ) 950 nm (ꢀ ) 112 M-1 cm-1). A
similar feature was reported for the 19-electron anions, [CpCo(1,4-
In summary, we have established entry into monovalent iron
coordination complexes in a sulfur-rich ligand environment.
High-spin iron(I) complex 2 is a synthon for a number of new
complexes via ligand substitution. While 2 and 3 appear to be
bona fide monovalent iron species, high-spin 4 and 5 exhibit
distinct spectral and structural characteristics indicative of back-
donation to the alkyne and tetraazadiene ligands, respectively.
The interesting and more complex electronic structures of 4
and 5 are the subject of ongoing studies.
Acknowledgment. We thank the NSF (CHE-0518508 to
C.G.R. and CHE-0421116 to C.V.P.) for financial support.
S. Bobev, S. Xia, and P. Tobash are thanked for assistance
in acquiring the SQUID data for 1, M. Hendrich is thanked
for the use of the program SpinCount and for the simulations
of the EPR spectra of 4 and 5, and J. Dykins and H. B.
Linden are thanked for collection of the LIFDI-MS data.
Supporting Information Available: Synthetic procedures and
characterization data including EPR and Mo¨ssbauer spectra (PDF)
and X-ray diffraction refinement data for 1-5 (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
(19) Mock, M. T. Synthesis and Reactivity of Thioether-Supported Orga-
noiron and Low-Valent Iron Complexes and Cyanide Bridged Bi-
nuclear Complexes. Thesis, University of Delaware, Newark, DE,
2008.
IC7023378
(20) (a) For selected recent examples in late metal chemistry, see: Eckert,
N. A.; Vaddadi, S.; Stoian, S.; Lachicotte, R. J.; Cundari, T. R.;
Holland, P. L. Angew. Chem. 2006, 45, 6868. (b) Kogut, E.; Wiencko,
H. L.; Zhang, L. B.; Cordeau, D. E.; Warren, T. H. J. Am. Chem.
Soc. 2005, 127, 11248. (c) Shay, D. T.; Yap, G. P. A.; Zakharov,
L. N.; Rheingold, A. L.; Theopold, K. H. Angew. Chem. 2005, 44,
1508.
(22) Maroney, M. J.; Trogler, W. C. J. Am. Chem. Soc. 1984, 106, 4144.
(23) Ashley-Smith, J.; Green, M.; Stone, F. G. A. J. Chem. Soc., Dalton
Trans. 1972, 1805.
(24) (a) Mehn, M. P.; Peters, J. C. J. Inorg. Biochem. 2006, 100, 634. (b)
Thomas, C. M.; Mankad, N. P.; Peters, J. C. J. Am. Chem. Soc. 2006,
128, 4956.
(21) Trogler, W. C. Acc. Chem. Res. 1990, 23, 426.
Inorganic Chemistry, Vol. 47, No. 6, 2008 1891