Synthesis and structure of the tetradeca-iron(III) oxide-alkoxide cluster [Bu4N]2[Fe14O8(OCH2CH3)20Cl8]

Article abstract of DOI:10.1016/j.inoche.2006.07.027

The iron(III) oxide-alkoxide cluster, di-tetra-n-butylammonium octachloro-tetra(μ₃-oxo)-tetra(μ₄-oxo)- icosa(μ₂-ethoxo)-tetradeca-iron(III), has been isolated and its X-ray crystal structure determined.

Full text of DOI:10.1016/j.inoche.2006.07.027

Inorganic Chemistry Communications 9 (2006) 1204–1206
www.elsevier.com/locate/inoche
Synthesis and structure of the tetradeca-iron(III) oxide–alkoxide
cluster [Bu4N]2[Fe14O8(OCH2CH3)20Cl8]
*
Craig A. Grapperhaus , Martin G. O’Toole, Mark S. Mashuta
Department of Chemistry, University of Louisville, 2320 So. Brook St., Louisville, KY 40292, United States
Received 26 June 2006; accepted 25 July 2006
Available online 2 August 2006
Abstract
The iron(III) oxide–alkoxide cluster, di-tetra-n-butylammonium octachloro-tetra(l3-oxo)-tetra(l4-oxo)- icosa(l2-ethoxo)-tetradeca-
iron(III), has been isolated and its X-ray crystal structure determined.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Iron; Oxo compounds; Alkoxides; Cluster compounds; Sulfur; Oxidation; Oxygen
The oxidation of iron compounds to iron oxides is a well
known, yet complex process. During our studies of the oxy-
gen sensitivity of iron–thiolate complexes, we reproducibly
generate an insoluble orange powder upon exposure of
LFeCl (1) (L = 4,7-bis(20-methyl-20-mercaptopropyl)-1-
thia-4,7-diazacyclononane) to dioxygen in solution [1,2].
The oxidation product is insoluble in all common solvents
preventing full spectroscopic characterization, but it dis-
plays a diagnostic IR spectrum. Similar products were
reported by others studying iron–thiolate oxygen sensitiv-
ity, but the product was never fully characterized [3–6].
Herein we report the isolation and structure of the iron(III)
oxide–alkoxide cluster, [Bu4N]2[Fe14O8(OCH2CH3)20Cl8]
(2).
Complex 2 is the largest iron(III) oxide–alkoxide cluster
known to date. Previously, clusters with 5–10 iron centers
have been isolated and structurally characterized [7–15].
Typically, these clusters are derived from iron(III) chloride
and the appropriate alkoxide. Chloride from the iron
source may be present or absent in the product. These clus-
ters have been studied for a variety of reasons including
their magnetic properties [8,14,15] and application to
homogenous catalysis [7,9–11]. Very recently, XAS data
on whole blood cells from Perophora annectens reveal the
presence of an iron alkoxide cluster, [Fe4-l-(OR)5(OR)9–10
[16].
Previously, we reported the synthesis of 1 from H2L,
NEt3, and [NBu4][FeCl4] in ethanol [1]. Complex 1 precip-
itates as a blue solid, which is isolated by filtration leaving a
dark brown filtrate. The filtrate was allowed to stand in air
over a period of several days yielding 2 as orange block
shaped crystals with an infrared spectrum identical to the
oxidation product of 1. The structure of 2 was determined
by X-ray crystallography [17–22].
]
Compound 2 crystallizes in the tetraganol space group
1
˚
˚
ꢀ
P421c with a = 17.0996(10) A and c = 21.027(3) A. Since
crystals of 2 rapidly degrade upon removal from the
mother liqueur, all manipulations were preformed in a cold
room. Numerous crystals were selected and mounted in an
attempt to obtain a more precise crystal structure, however
a higher quality data set could not be obtained. The data
1
Crystal data for 1: [Bu4N]2[Fe14O8(OCH2CH3)20Cl8], FW =
2579.62 g molꢀ1 tetragonal, space group P421c, a = 17.0996(10) A,
ꢀ
˚
c = 21.027(3) A, V = 6148.1(9) A , Z = 2, l = 1.827 mmꢀ1
, q =
3
˚
˚
1.393 g cmꢀ3. Data were collected on a Bruker Smart Apex CCD using
Mo Ka radiation. For all 5447 unique reflections (R(int) = 0.2003), the
final anisotropic full-matrix least-squares refinement on F2 for 240
variables data converged at R1 = 0.1018 and wR2 = 0.2276 with a GOF
of 1.075.
*
Corresponding author. Tel.: +1 502 852 5932; fax: +1 502 852 8149.
E-mail address: grapperhaus@louisville.edu (C.A. Grapperhaus).
1387-7003/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2006.07.027

C.A. Grapperhaus et al. / Inorganic Chemistry Communications 9 (2006) 1204–1206
1205
Table 1
set was collected at low temperature, 100 K. A representa-
˚
tion of the [Fe14O8(OCH2CH3)20Cl8]2ꢀ dianion is shown in
Fig. 1.
Selected bond distances (A) for 2
Fe1a–O1a
Fe1a–O4a
Fe1a–O5a
Fe1a–O6b
Fe1a–O7d
Fe1a–Cl1a
Fe2a–O1a
Fe2a–O2a
Fe2a–O3a
Fe2a–O7d
Fe2a–Cl2a
2.093(9)
1.971(10)
1.952(10)
1.954(9)
2.097(10)
2.341(4)
1.956(11)
1.946(11)
1.953(11)
1.992(9)
2.241(5)
Fe3a–O3a
Fe3a–O4a
Fe3a–O7d
2.010(10)
2.051(10)
1.973(9)
Compound 2 contains four unique iron centers, labeled
Fe1a, Fe2a, Fe3a, and Fe4a (Fig. 1). There are 10 additional
symmetry generated iron centers. The seven unique oxygen
atoms include five l2-ethoxides (O1a–O5a), a l3-oxo (O6a),
and a l4-oxo (O7a). For each O-donor, symmetry generates
three additional sites. The Fe–OC2H5 bond distances
Fe4d–O2a
Fe4d–O5d
Fe4d–O6a
Fe4d–O6d
Fe4d–O7d
2.047(10)
1.976(9)
1.853(10)
1.924(10)
1.962(9)
˚
(Table 1) fall in the range of 1.946(11)–2.051(10) A with
˚
an average of 2.00(1) A that is identical within error to a
˚
previously reported Fe–OC2H5 (l2) distance of 2.009(1) A
[14]. The Fe–k3O distances fall within error of related struc-
be formed with Fe1a, Fe2a, Fe3a, and Fe4d. Fe3a sits on
the special position (0.5,0.5,z) and serves as the joining
point for the second tetrahedron, which also contains
Fe1c, Fe2c, and Fe4b. The tetrahedra are also connected
by O6 linkages between Fe1c/Fe4d and Fe1a/Fe4b. The
O6 linkage also extends out to Fe4 positions in the second
Fe7-ditetrahedra, which sits across the cluster and perpen-
dicular to the first. The two Fe7 cores are also linked
together by four l2-ethoxides, consisting of Fe1a, O5a,
and Fe4a and their symmetry generated equivalents.
˚
˚
tures, 1.911(1) A, and range from 1.853(10) to 1.954(9) A
4
˚
with an average of 1.91(1) A [14]. The Fe–k O distances
vary from 1.962(9) to 2.097(10) A, with an average of
2.00(1) A near the reported value of 2.011(7) A [14]. The
cluster also contains two unique terminal chlorides, Cl1a
and Cl2a, and six additional symmetry generated chlorides.
The Fe1–Cl1 and Fe2–Cl2 distances of 2.341(4) and
˚
˚
˚
˚
2.241(5) A are within the expected range. Overall, the dian-
ion of 2 contains a cluster of 14 iron centers, 20 l2-ethox-
ides, four l3-oxo bridges, four l4-oxo bridges, and eight
terminal chlorides. Based on charge balance, each iron
exists in the ferric state.
The 14 iron centers of 2 are arranged in a pair of point-
sharing Fe7-ditetrahedra (Fig. 2). An Fe4-tetrahedron can
An isolated Fe4-tetrahedron is shown in Fig. 3. As
shown, Fe2a sits atop of the tetrahedron in a triganol bipy-
ramidal environment with l2-ethoxides in all three equato-
rial positions. The ethoxides O1a, O2a, and O3a bridge the
Fig. 2. A PLUTO representation of the dianion of 2. Carbon atoms have
been omitted. The Fe7-ditetrahedra are outlined in gray. The atoms from
the asymmetric unit are labeled as a. Other positions are symmetry
generated; b (y,1 ꢀ x,ꢀz), c (1 ꢀ x,1 ꢀ y,z), d (1 ꢀ y,x,ꢀz).
Fig. 1. A PLUTO representation of the dianion of 2. The atoms from the
asymmetric unit are labeled as a. Other positions are symmetry generated;
b (y,1 ꢀ x,ꢀz), c (1 ꢀ x,1 ꢀ y,z), d (1 ꢀ y,x,ꢀz).

1206
C.A. Grapperhaus et al. / Inorganic Chemistry Communications 9 (2006) 1204–1206
identified as an iron(III) oxo-alkoxide cluster. The tetra-
decairon(III) cluster has an IR spectrum consistent with
the thiolate oxidation product upon oxidation of the fil-
trate from the synthesis of 1. This cluster, 2, is the largest
member of the growing family of iron(III) oxo-alkoxides.
Acknowledgement
Acknowledgment is made to the National Science Foun-
dation (CHE-0238137) for funding provided to C.A.G.
CCD X-ray equipment was purchased through funds pro-
vided by the Kentucky Research Challenge Trust Fund.
Appendix A. Supplementary data
Fig. 3. A PLUTO representation of a Fe4-tetrahedron from 2. The atoms
from the asymmetric unit are labeled as a. Other positions are symmetry
generated; b (y,1 ꢀ x,ꢀz), c (1 ꢀ x,1 ꢀ y,z), d (1 ꢀ y,x,ꢀz).
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.inoche.
2006.07.027.
References
[1] C.A. Grapperhaus, M. Li, A.K. Patra, S. Poturovic, P.M. Kozlowski,
M.Z. Zgierski, M.S. Mashuta, Inorg. Chem. 42 (2003) 4382–4388.
[2] C.A. Grapperhaus, M.G. O’Toole, unpublished results.
[3] T.C. Harrop, P.K. Mascharak, Acc. Chem. Res. 37 (2004) 253–260.
[4] C.M. Lee, C.H. Hsieh, A. Dutta, G.H. Lee, W.F. Liaw, J. Am. Chem.
Soc. 125 (2003) 11492–11493.
[5] C.M. Lee, C.H. Chen, H.W. Chen, J.L. Hsu, G.H. Lee, W.F. Liaw,
Inorg. Chem. 44 (2005) 6670–6679.
[6] R.M. Theisen, J. Shearer, W. Kaminsky, J.A. Kovacs, Inorg. Chem.
43 (2004) 7682–7690.
[7] S. Asirvatham, M.A. Khan, K.M. Nicholas, Inorg. Chem. 39 (2000)
2006–2007.
[8] H.K. Chae, C. Hwang, Y. Dong, H. Yun, H.G. Jang, Chem. Lett.
(2000) 992–993.
Fig. 4. Infrared spectrum of 2 prepared as a KBr pellet at 4 cmꢀ1
resolution.
[9] K. Hegetschweiler, H. Schmalle, H.M. Streit, W. Schneider, Inorg.
Chem. 29 (1990) 3625–3627.
[10] K. Hegetschweiler, H.W. Schmalle, H.M. Streit, V. Gramlich, H.U.
Hund, I. Erni, Inorg. Chem. 31 (1992) 1299–1302.
[11] B.J. O’Keefe, S.M. Monnier, M.A. Hillmyer, W.B. Tolman, J. Am.
Chem. Soc. 123 (2001) 339–340.
[12] G.A. Seisenbaeva, S. Gohil, E.V. Suslova, T.V. Rogova, N.Y.
Turova, V.G. Kessler, Inorg. Chim. Acta 358 (2005) 3506–3512.
[13] J. Spandl, M. Kusserow, I. Brudgam, Z. Anorg. Allg. Chem. 629
(2003) 968–974.
[14] M. Veith, F. Gratz, V. Huch, Eur. J. Inorg. Chem. (2001) 367–368.
[15] M. Veith, F. Gratz, V. Huch, P. Gutlich, A. Ensling, Z. Anorg. Allg.
Chem. 630 (2004) 2329–2336.
[16] P. Frank, A. DeTomaso, B. Hedman, K.O. Hodgson, Inorg. Chem.
45 (2006) 3290–3931.
[17] SMART (v.5628), Bruker Advanced X-ray Solutions, Inc., Madison,
WI, 2002.
[18] SAINT (v6.36), Bruker Advanced X-ray Solutions, Inc., Madison,
WI, 2002.
[19] G.M. Sheldrick, SADABS (v2.02), University Gottingen, Gottingen,
Germany, 2001.
Fe2a–Fe1a, Fe2a–Fe3a, and Fe2a–Fe4d edges of the tetrahe-
dron, respectively. A terminal chloride, Cl2a, and a l4-oxo,
O7d, sit in the axial positions of Fe2a. All four iron centers
are coordinated to O7d, which is located on the interior of
the Fe4-tetrahedron. A fourth l2-ethoxide, O4a, sits along
the Fe1a–Fe3a edge. The pseudo-octahedral environment
about Fe3a is completed by the symmetry generated equiv-
alents O3c, O4c, and O7b in the adjoining point-sharing
Fe4-tetrahedron. The environment around Fe1a is com-
pleted by O6b, a l3-oxo bridge, which joins a single Fe1
with two Fe4 sites. A l2-ethoxy bridge, O5, between Fe1
and Fe4 completes a diamond core and the coordination
environment of Fe1 and Fe4.
The infrared spectrum of 2 is shown in Fig. 4. The IR
shows weak bands at 457 and 511 cmꢀ1, which are similar
to those in a-Fe2O3 [23]. The more intense bands at 806
and 893 cmꢀ1 are consistent with the presence of alkoxide
in the sample. This is further confirmed by the array of
bands at 1053, 1095, 1379, and 1471 cmꢀ1 [12]. Finally,
C–H bending modes are observed at 2875, 2929, and
2960 cmꢀ1 (not shown).
[20] G.M. Sheldrick, SHELXS-90, Acta Crystallogr. A46 (1990) 467.
[21] G.M. Sheldrick, SHELXL-97, University Gottingen, Gottingen,
Germany, 1997.
[22] SHELXTL (v6.12). Bruker Advanced X-ray Solutions, Inc. Madison,
WI (2001).
In conclusion, the orange, insoluble product from the
oxidation of iron–thiolate complexes in alcohol has been
[23] R. Balasubramaniam, A.V.R. Kumar, Corros. Sci. 45 (2003) 2451–
2465.