Synthesis of [2Fe-2S] and [4Fe-4S] clusters having terminal amide ligands from an iron(II) amide complex

Article abstract of DOI:10.1246/cl.2005.172

The reaction of iron(II) bis-amide Fe{N(SiMe₃)₂} ₂ with elemental sulfur afforded a direct entry to new [Fe ₂S₂] and [Fe₂S₄] clusters with terminal amide groups, Fe₄S₄{N(SiMe₃) ₂}₄ (1) and Fe₂S₂{N(SiMe ₃)₂}₂(tmtu)₂ (2), whose structures have been determined by X-ray crystallography. Tetrameric cubane cluster 1 exhibits one reversible and one quasireversible processes in the cyclic voltammetry, whereas dimeric rhombus complex 2 reveals one irreversible reduction process. Copyright

Full text of DOI:10.1246/cl.2005.172

172
Chemistry Letters Vol.34, No.2 (2005)
Synthesis of [2Fe–2S] and [4Fe–4S] Clusters Having Terminal Amide Ligands from an Iron(II)
Amide Complex
Yasuhiro Ohki, Yusuke Sunada, and Kazuyuki Tatsumiꢀ
Department of Chemistry, Graduate School of Science and Research Center for Materials Science,
Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602
(Received November 10, 2004; CL-041348)
The reaction of iron(II) bis-amide Fe{N(SiMe3)2}2 with el-
Si
N
Si
emental sulfur afforded a direct entry to new [Fe2S2] and [Fe4S4]
clusters with terminal amide groups, Fe4S4{N(SiMe3)2}4 (1) and
Fe2S2{N(SiMe3)2}2(tmtu)2 (2), whose structures have been de-
termined by X-ray crystallography. Tetrameric cubane cluster
1 exhibits one reversible and one quasireversible processes in
the cyclic voltammetry, whereas dimeric rhombus complex 2 re-
veals one irreversible reduction process.
S
N
Si
Si
Fe
S
Fe
S
S
S
Fe
Fe
N
Si
N
Si
Si
Si
Si
Si
Si
Si
N
Fe N
1
N
Si
N
N
S
The extensive development of the chemistry of iron–sulfido
cluster complexes has resulted in the isolation and detailed char-
acterization of several structural types of complexes such as
Fe2S2, Fe3S4, and Fe4S4, which are analogous to the redox sites
in iron–sulfur proteins.1 Despite the development of the chemis-
try of Fe2S2 and Fe4S4 clusters since these types were first syn-
thesized in 1970’s,2 there has been only one Fe4S4 cluster having
terminal amide groups on iron (unpublished),3 and no Fe2S2–
amide cluster. Recently we have demonstrated that the iron(II)
bis-amide complex Fe{N(SiMe3)2}24 serves as a suitable precur-
sor for a Fe8S7 cluster in which core structure represents the
P-cluster core of nitrogenases. It was synthesized from a self-
assembly reaction using Fe{N(SiMe3)2}2, thiols, elemental sul-
fur, and tetramethylthiourea (tmtu).5 The successful isolation of
metastable Fe8S7 cluster presumably relies on the property of
amide group. The amide group promotes the solubility in non-
polar solvents and is readily replaced with thiolate by a pro-
ton-transfer reaction with thiols. In our attempts to develop syn-
thetic routes to new iron–sulfido clusters, we have examined the
reactions of Fe{N(SiMe3)2}2 with elemental sulfur, and this re-
action system has afforded direct entry to the ferric Fe2S2 and
Fe4S4 clusters having terminal amide moiety. Herein we report
the synthesis, structures, and redox properties of Fe2S2 and
Fe4S4 amide clusters.
S
S
Si
S
N
N
Fe
Fe
S
Si N
Si
S
N
N
2
Scheme 1.
Cluster 1 consists of a Fe4S4 core and four terminal amide li-
gands bound to iron. The Fe–Fe distances varying in the range
of 2.8667(7)–3.0014(5) A are significantly longer than those in
the representative [Fe4S4(SR)4]2ꢂ clusters (2.71–2.82 A) prob-
ably owing to the steric repulsion among the amide ligands. The
geometry around amide nitrogen is planar, with variation of
356.2–360.0ꢁ for the angle around the nitrogen atom. This re-
sults in the strong ꢀ-donation to iron which electronically stabil-
izes the ferric centers in the Fe4S4 core. Indeed, the Fe–N dis-
ꢀ
1
ꢀ
N1
S2
N2
Fe1
Fe3
Fe2
S1
Fe4
S3
S4
Treatment of iron(II) bis-amide Fe{N(SiMe3)2}2 with one
equiv. of elemental sulfur in toluene afforded a dark brown solu-
tion, from which an all-ferric Fe4S4 cluster Fe4S4{N(SiMe3)2}4
(1) was isolated in 31% yield as black crystals (Scheme 1). Clus-
ter 1 is alternatively synthesized from the reaction of ferric
N4
N3
6
amide complex (THF)FeCl{N(SiMe3)2}2 with NaSH (in 86%
Figure 1. Molecular structure of Fe4S4{N(SiMe3)2}4 (1) with
thermal ellipsoids at 50% probability level. Selected bond dis-
ꢀ
tances (A): Fe1–Fe2 2.8724(7), Fe1–Fe3 2.9425(5), Fe1–Fe4
2.8831(5), Fe2–Fe3 2.8823(5), Fe2–Fe4 3.0014(5), Fe3–Fe4
2.8667(5), Fe1–S1 2.3124(8), Fe1–S2 2.2799(8), Fe1–S3
2.2967(7), Fe2–S1 2.2800(8), Fe2–S2 2.3096(8), Fe2–S4
2.3045(7), Fe3–S1 2.3032(7), Fe3–S3 2.2985(8), Fe3–S4
2.2728(7), Fe4–S2 2.3092(7), Fe4–S3 2.2543(8), Fe4–S4
2.3247(8), Fe1–N1 1.866(2), Fe2–N2 1.865(2), Fe3–N3
1.863(2), Fe4–N4 1.866(2).
yield as crystals in our case).3 In contrast to the typical synthetic
Fe4S4 clusters, cluster 1 is well soluble in nonpolar solvents such
as hexane and toluene, and is very sensitive toward moisture.
The stoichiometry of the reaction between Fe{N(SiMe3)2}2
and elemental sulfur can be explained by concomitant formation
of iron(III) tris-amide Fe{N(SiMe3)2}3 which was evaporated as
dark green liquid from the residue under vacuum at 130 ꢁC.
The structure of 1 was determined by an X-ray diffraction
study using single crystals obtained from hexane (Figure 1).7
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.2 (2005)
173
exhibit irreversible reduction peak in the CV spectrum due to
the lability of halide ion.11
The ready availability of iron–sulfide–amide clusters from
the reaction of iron–amide precursor with elemental sulfur
may provide useful reaction system for the synthesis of yet larger
iron–sulfur clusters.
S2
S1*
Fe*
N*
N
Fe
S1
This research was financially supported by Grant-in-Aids
for Scientific Research (No. 14078211 and 15750047) from
the Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan.
S2*
Figure 2. Molecular structure of Fe2S2{N(SiMe3)2}2(tmtu)2
(2) with thermal ellipsoids at 50% probability level. Selected
bond distances (A) and angles ( ): Fe–Fe 2.783(2), Fe–S1
2.212(2), Fe–S2 2.421(2), Fe–N 1.930(5), Fe–S1–Feꢀ 78.11(6),
S1–Fe–S1ꢀ 101.89(6).
References and Notes
a) P. V. Rao and R. H. Holm, Chem. Rev., 104, 527 (2004). b) H.
1
ꢁ
ꢀ
ꢀ
Beinert, R. H. Holm, and E. Munck, Science, 277, 653 (1997). c)
¨
R. H. Holm, Adv. Inorg. Chem., 38, 1 (1993). d) R. H. Holm, Chem.
Soc. Rev., 10, 455 (1981). e) J. M. Berg and R. H. Holm, in ‘‘Iron-
Sulfur Proteins,’’ ed. by T. G. Spiro, Wiley-Interscience, New York
(1982), Vol. 4, Chap. 1. f) R. H. Holm and J. A. Ibers, in ‘‘Iron-
Sulfur Proteins,’’ ed. by W. Lovenberg, Academic Press, New York
(1977), Vol. 3, Chap. 7. g) R. H. Holm, Acc. Chem. Res., 10, 427
(1977).
ꢀ
tances (1.863(2)–1.866(2) A) are shorter than the other known
II
8
ꢀ
terminal Fe –N(SiMe3)2 distances (1.88–1.98 A).
Whereas treatment of Fe{N(SiMe3)2}2 with elemental sul-
fur led to the formation of a tetrameric cubane cluster 1, similar
reaction under the presence of tetramethylthiourea gave a ferric
dimer Fe2S2{N(SiMe3)2}2(tmtu)2 (2), which can be crystallized
from toluene and hexane in 33% yield. Thus in the reaction of
Fe{N(SiMe3)2}2 with elemental sulfur, redistribution of amide
ligand occurs to give tris-amide Fe{N(SiMe3)2}3 and transient
‘‘FeIIIS{N(SiMe3)2}’’ species in solution, the latter of which
makes available the iron–sulfide–amide clusters of tetrameric
[Fe4S4] cubane 1 and [Fe2S2] rhombus 2 as a result of associa-
tion. Tetramethylthiourea coordinates to the dimeric Fe2S2{N-
(SiMe3)2}2 intermediate, giving rise to 2.
The dimeric nature of 2 was proven by a crystal structure de-
termination (Figure 2).7 The entire structure has centrosymmetry
with a planar Fe2S2 core. The terminal amide and tmtu ligands
on each iron atom are located anti with respect to the Fe2S2 plane
to minimize the steric repulsion. Within the Fe2S2 core, the Fe–
S–Fe angle is 78.11(6)ꢁ, which results in a shorter Fe–Fe dis-
2
3
a) J. J. Mayerle, R. B. Frankel, R. H. Holm, J. A. Ibers, W. D.
Phillips, and J. F. Weiher, Proc. Natl. Acad. Sci. U.S.A., 70, 2429
(1973). b) T. Herskovitz, B. A. Averill, R. H. Holm, J. A. Ibers,
W. D. Phillips, and J. F. Weiher, Proc. Natl. Acad. Sci. U.S.A.,
69, 2437 (1972).
a) C. R. Sharp, J. S. Duncan, and S. C. Lee, unpublished result. b)
The compound is mentioned in the recent review without the prod-
uct yield. S. C. Lee and R. H. Holm, Chem. Rev., 104, 1135 (2004).
R. A. Andersen, K. Faegri, Jr., J. C. Green, A. Haaland, M. Lappert,
W.-P. Leung, and K. Rypdal, Inorg. Chem., 27, 1782 (1988).
Y. Ohki, Y. Sunada, M. Honda, M. Katada, and K. Tatsumi, J. Am.
Chem. Soc., 125, 4052 (2003).
4
5
6
7
J. S. Duncan, T. N. Nazif, A. K. Verma, and S. C. Lee, Inorg.
Chem., 42, 1211 (2003).
Crystal data for 1: monoclinic, P21=c, a ¼ 14:934ð2Þ, b ¼
ꢁ
ꢀ
ꢀ 3
12:481ð2Þ, c ¼ 27:162ð4Þ A, ꢁ ¼ 97:9441ð8Þ , V ¼ 5013:9ð1Þ A ,
Z ¼ 4, Dcalcd ¼ 1:348 gcmꢂ1
;
11362 reflections (5:5ꢁ ꢃ 2ꢂ ꢃ
55ꢁ), 8670 observed with F > 2ꢃðFÞ, 397 parameters; R ¼
0:033, Rw ¼ 0:049, GOF ¼ 0:98. Crystal data for 2: monoclinic,
ꢀ
P21=c, a ¼ 12:439ð6Þ, b ¼ 15:712ð8Þ, c ¼ 10:405ð5Þ A, ꢁ ¼
ꢀ
tance of 2.783(2) A than those in 1. The Fe–tmtu distance of
108:601ð8Þꢁ, V ¼ 1927ð1Þ A , Z ¼ 2, Dcalcd ¼ 1:311 g cmꢂ1
;
ꢀ 3
ꢀ
2.421(2) A is longer than that in the Fe8S7 cluster (2.3488(7)
4403 reflections (5:5ꢁ ꢃ 2ꢂ ꢃ 55ꢁ), 2508 observed with
F > 2ꢃðFÞ, 172 parameters; R ¼ 0:060, Rw ¼ 0:066, GOF ¼ 1:10.
a) M. M. Olmstead, P. P. Power, and S. C. Shoner, Inorg. Chem.,
30, 2547 (1991). b) U. Siemeling, U. Vorfeld, B. Neumann, and
H.-G. Stammler, Organometallics, 17, 483 (1998). c) R. A. Bartlett,
J. J. Ellison, P. P. Power, and S. C. Shoner, Inorg. Chem., 30, 2888
(1991). d) R. Hauptmann, R. Kliss, and G. Henkel, Angew. Chem.,
Int. Ed., 38, 377 (1999). e) H. Chen, M. M. Olmstead, D. C.
Pestana, and P. P. Power, Inorg. Chem., 30, 1783 (1991). f) T.
Komuro, H. Kawaguchi, and K. Tatsumi, Inorg. Chem., 41, 5083
(2002). g) F. M. MacDonnell, K. Ruhlandt-Senge, J. J. Ellison,
R. H. Holm, and P. P. Power, Inorg. Chem., 34, 1815 (1995). h)
D. J. Evans, M. S. Hill, and P. B. Hitchcock, Dalton Trans.,
2003, 570. i) M. A. Putzer, B. Neumuller, K. Dehnicke, and J.
Magull, Chem. Ber., 129, 715 (1996). j) U. Siemeling, U. Vorfeld,
B. Neumann, and H.-G. Stammler, Inorg. Chem., 39, 5159 (2000).
k) A. K. Verma and S. C. Lee, J. Am. Chem. Soc., 121, 10838
(1999). l) A. Oanda, M. Stender, R. J. Wright, M. M. Olmstead,
P. Klavins, and P. P. Power, Inorg. Chem., 41, 3909 (2002).
T. O’Sullivan and M. M. Millar, J. Am. Chem. Soc., 107, 4096
(1985).
5
ꢀ
A), indicating that the coordination of tmtu is labile.
The redox properties of 1 and 2 have been examined by
cyclic voltammetry (CV) in THF at room temperature. Cluster
1 exhibits one reversible and one quasireversible reduction
processes at E1=2 ¼ ꢂ0:088 and ꢂ1:22 V vs Cp2Feþ/Cp2Fe, re-
spectively, which are ascribed to one-electron reduction process-
es to the corresponding [Fe4S4{N(SiMe3)2}4]ꢂ and [Fe4S4{N-
(SiMe3)2}4]2ꢂ species. The CV spectrum of 1 suggests that the
unusually high oxidation state in the FeIII4S4 cluster is stabilized
by the strong ꢀ donation from amide ligands, whereas the typical
redox process for [Fe4S4(SR)4]nꢂ (n ¼ 2, 3) clusters are between
FeII2FeIII2 and FeII3FeIII, and they can not be oxidized to the all-
ferric form. Even in the FeIII3FeII state, there has been only one
isolated example to date.9 Dinuclear complex 2 reveals one irre-
versible reduction process at Ep ¼ ꢂ1:13 V vs Cp2Feþ/
Cp2Fe. Since the reduction potential is comparable to those for
known [Fe2S2(SR)4]2ꢂ complexes which generally afford a re-
versible reduction process with E1=2 between ꢂ0:8 to ꢂ1:1 V
vs SCE,10 liberation of tmtu ligand following the reduction is
a possible reason for irreversibility. It is notable that the dinu-
clear complexes with halides [Fe2S2(X)4]2ꢂ (X = Cl, Br) also
8
9
10 J. J. Mayerle, S. E. Denmark, B. V. DePamphilis, J. A. Ibers, and
R. H. Holm, J. Am. Chem. Soc., 97, 1032 (1975).
11 G. B. Wong, M. A. Bobrik, and R. H. Holm, Inorg. Chem., 17, 578
(1978).
Published on the web (Advance View) January 8, 2005; DOI 10.1246/cl.2005.172