A new class of binuclear and trinuclear iron-sulfur clusters derived from bis[bis(trimethylsilyl)amido]iron(II)

Article abstract of DOI:10.1021/ic7023742

The compound Fe[N(SiMe₃)₂]₂ is shown to be a useful precursor to dinuclear and trinuclear iron-sulfur-silylamido complexes by reaction with thiols or thiols and sulfur in tetrahydrofuran (THF) or toluene. Reaction with 1 equiv of p-tolylthiol affords [Fe₂(μ ₂-S-p-tol)₂(N(SiMe₃)₂) ₂(THF)₂] (1); with 0.5 equiv of adamantane-1 -thiol, [Fe₂(μ₂-S-1-Ad)(μ₂-N(SiMe ₃)₂)(N(SiMe₃)₂)₂] (2) is formed. The clusters [Fe₃(μ₃-Q)(μ₂-SR) ₃(N(SiMe₃)₂)₃] are available by three methods: (i) self-assembly in the systems Fe[N(SiMe₃) ₂]₂/RSH/S or Se [Q = S, R = p-tol (3) and 1-Ad (5)]; (ii) reaction of 1 with Q = S or Se to yield 3 or [Fe₃Se(S-p-tol) ₃(N(SiMe₃)₂)₃] (4); (iii) reaction of 2 with 1-AdSH and S to give 5. Structures of 1-5 are presented. Complexes 1 and 2 contain planar Fe₂S₂ and Fe₂SN rhombs. Clusters 3-5 contain a mixed-valence Fe₃Q(SR)₃ core with trigonal (cuboidal) geometry. Of known iron-sulfur clusters, these most closely resemble previously reported [Fe₃S(S-R-S)₃]^2- stabilized by bidentate thiolate ligands. Complexes 1-5, together with a small set of recently described clusters of nuclearities 2, 4, and 8, constitute a new class of iron-sulfur-silylamido clusters. Complexes 3-5 constitute a new structure type of mixed-valence iron-sulfur clusters.

Full text of DOI:10.1021/ic7023742

Inorg. Chem. 2008, 47, 3366-3370
A New Class of Binuclear and Trinuclear Iron-Sulfur Clusters Derived
from Bis[bis(trimethylsilyl)amido]iron(II)
Ruslan Pryadun and R. H. Holm*
Department of Chemistry and Chemical Biology, HarVard UniVersity, Cambridge,
Massachusetts 02138
Received December 6, 2007
The compound Fe[N(SiMe3)2]2 is shown to be a useful precursor to dinuclear and trinuclear iron-sulfur-silylamido
complexes by reaction with thiols or thiols and sulfur in tetrahydrofuran (THF) or toluene. Reaction with 1 equiv of
p-tolylthiol affords [Fe2(µ2-S-p-tol)2(N(SiMe3)2)2(THF)2] (1); with 0.5 equiv of adamantane-1-thiol, [Fe2(µ2-S-1-Ad)(µ2-
N(SiMe3)2)(N(SiMe3)2)2] (2) is formed. The clusters [Fe3(µ3-Q)(µ2-SR)3(N(SiMe3)2)3] are available by three methods:
(i) self-assembly in the systems Fe[N(SiMe3)2]2/RSH/S or Se [Q ) S, R ) p-tol (3) and 1-Ad (5)]; (ii) reaction of
1 with Q ) S or Se to yield 3 or [Fe3Se(S-p-tol)3(N(SiMe3)2)3] (4); (iii) reaction of 2 with 1-AdSH and S to give 5.
Structures of 1-5 are presented. Complexes 1 and 2 contain planar Fe2S2 and Fe2SN rhombs. Clusters 3-5
contain a mixed-valence Fe3Q(SR)3 core with trigonal (cuboidal) geometry. Of known iron-sulfur clusters, these
most closely resemble previously reported [Fe3S(S-R-S)3]2- stabilized by bidentate thiolate ligands. Complexes
1-5, together with a small set of recently described clusters of nuclearities 2, 4, and 8, constitute a new class of
iron-sulfur-silylamido clusters. Complexes 3-5 constitute a new structure type of mixed-valence iron-sulfur
clusters.
Introduction
dimer, [Fe2(SR)2(SR′)2] derived from exceptionally capacious
thiols, upon reaction with sulfur affords the high-nuclearity
species [Fe8S7(SR)4(SR′)2],8,10 significant because it is a
structural analogue of the PN cluster of nitrogenase. Other
applications utilizing bulky thiols have resulted in the linear
cluster [Fe3(µ2-SR)4(N(SiMe3)2)2],10 with PhNHNHPh as the
coreactant the cubane [Fe4(NPh)4(SR)4],11,12 and with sulfur
the cubane [Fe4S4(N(SiMe3)2)4].13 The compound exists in
the monomer–dimer equilibrium 2Fe[N(SiMe3)2]2 a Fe2[N-
(SiMe3)2]4 in toluene, with the monomer the predominant
species at ambient temperature.2 This finding, together with
the isolation of Fe[N(SiMe3)2]2(THF) from synthesis in
tetrahydrofuran (THF),2 makes probable that the monomer
is the prevailing reactive form in the examples cited and the
reactions described in this work.
The compound bis[bis(trimethylsilyl)amido]iron(II)1,2 has
proven to be a versatile precursor in the synthesis of
polynuclear iron-sulfur species, often of unusual structural
types, by reactions with thiols and/or sulfur in aprotic
solvents. As examples, reactions with sterically encumbered
thiols afford the two-coordinate monomers [Fe(SR)2]3,4 and
three-coordinate dimers [Fe2(µ2-SR)2(SR)2].4–9 One such
* To whom correspondence should be addressed. E-mail: holm@
chemistry.harvard.edu.
(1) Andersen, R. A.; Faegri, K., Jr.; Green, J. C.; Haaland, A.; Lappert,
M. F.; Leung, W.-P.; Rypda, K. Inorg. Chem. 1988, 27, 1782–1786.
(2) Olmstead, M. M.; Power, P. P.; Shoner, S. C. Inorg. Chem. 1991, 30,
2547–2551.
(3) Ellison, J. J.; Ruhlandt-Senge, K.; Power, P. P. Angew. Chem., Int.
Ed. Engl. 1994, 33, 1178–1180.
(4) MacDonnell, F. M.; Ruhlandt-Senge, K.; Ellison, J. J.; Holm, R. H.;
Power, P. P. Inorg. Chem. 1995, 34, 1815–1822.
(5) Ruhlandt-Senge, K.; Power, P. P. Bull. Soc. Chim. Fr. 1993, 129, 594–
598.
In this laboratory, we are investigating the utility of
Fe[N(SiMe3)2]2 and other reduced and potentially highly
reactive FeII complexes in the formation of high-nuclearity
(6) Sydora, O. L.; Wolczanski, P. T.; Lobkovsky, E. B. Angew. Chem.,
Int. Ed. 2003, 42, 2685–2687.
(7) Sydora, O. L.; Henry, T. P.; Wolczanski, P. T.; Lobkovsky, E. B.;
Rumberger, E.; Hendrickson, D. N. Inorg. Chem. 2006, 45, 609–626.
(8) Ohki, Y.; Ikagawa, Y.; Tatsumi, K. J. Am. Chem. Soc. 2007, 129,
10457–10465.
(10) Ohki, Y.; Sunada, Y.; Honda, M.; Katsada, M.; Tatsumi, K. J. Am.
Chem. Soc. 2003, 125, 4052–4053.
(11) Verma, A. K.; Lee, S. C. J. Am. Chem. Soc. 1999, 121, 10838–10839.
(12) Duncan, J. S.; Nazif, T. M.; Verma, A. K.; Lee, S. C. Inorg. Chem.
2003, 42, 1211–1224.
(9) Hauptmann, R.; Kliss, R.; Schneider, J.; Henkel, G. Z. Anorg. Allg.
Chem. 1998, 624, 1927–1936.
(13) Ohki, Y.; Sunada, Y.; Tatsumi, K. Chem. Lett. 2005, 34, 172–173.
3366 Inorganic Chemistry, Vol. 47, No. 8, 2008
10.1021/ic7023742 CCC: $40.75  2008 American Chemical Society
Published on Web 03/07/2008

Binuclear and Trinuclear Iron-Sulfur Clusters
Chart 1. Complex Designations and Abbreviationsa
(0.40 mmol) of sulfur suspended in 2 mL of toluene. The reaction
mixture was stirred overnight. Slow removal of the solvent led to
a sticky black solid, which was washed with hexanes and dried to
1
yield the product as 0.25 g (60%) of a black solid. The H NMR
spectrum of this material was identical with that from method A.
[Fe3(µ3-S)(µ2-S-1-Ad)3(N(SiMe3)2)3]. Method A. This com-
pound was synthesized according to method A of the preceding
preparation with use of adamantane-1-thiol and was isolated in 30%
yield as a black crystalline solid. Absorption spectrum (toluene):
λmax (ꢀM) 306 (12 900), 425 (8000) nm. Anal. Calcd for
C48H99Fe3N3S4Si6: C, 48.75; H, 8.44; Fe, 14.17; N, 3.55; S, 10.85.
Found: C, 48.53; H, 8.30; Fe, 14.06; N, 3.36; S, 10.42.
a Abbreviations: Ad ) adamantyl, LS3 ) 1,3,5-tris[(4,6-dimethyl-3-mer-
captophenyl)thio]-2,4,6-tris(p-tolylthio)benzene(3-), mes ) mesityl, S2-o-
xyl ) o-xylyl-R,R′-dithiolate(2-), tol ) tolyl.
Method B. A solution of 0.33 g (0.20 mmol) of adamantane-
1-thiol in 2 mL of toluene was added dropwise to a stirred solution
of 0.15 g (0.20 mmol) of [Fe2(S-1-Ad)(N(SiMe3)2)3] in 3 mL of
toluene. The reaction mixture was stirred for 1 h; a suspension of
4.0 mg (0.13 mmol) of sulfur was added, forming a black solution.
The solution was stirred for 6 h, the solvent was removed in vacuo,
the sticky black residue was dissolved in hexanes, and the solution
was filtered. Removal of the solvent afforded the product as 0.38 g
iron-sulfur clusters of potential biological relevance. Here
we report the first stage of this investigation involving two
thiols of differing steric properties.
Experimental Section
Preparation of Compounds. All reactions and manipulations
were performed under a pure nitrogen atmosphere using either
Schlenk techniques or an inert-atmosphere glovebox. Solvents
were passed through an Innovative Technology or an MBraun
solvent purification system prior to use. All drying and volume
reduction steps were performed in vacuo. Abbreviations are given
in Chart 1.
[Fe2(µ2-S-p-tol)2(N(SiMe3)2)2(THF)2]. A solution of 0.30 g (0.80
mmol) of [Fe(N(SiMe3)2)2]1,2 was added to a solution of 0.098 g
(0.80 mmol) of p-tolylthiol in 3 mL of THF. The green-brown
reaction mixture was stirred for 2 h and maintained at -28 °C
overnight. The solid was collected, washed with copious amounts
of hexanes, and dried to afford the product as light-green-yellow
crystals. The filtrate was concentrated and a second crop of crystals
was obtained, affording a total of 0.59 g (90%). Absorption
spectrum toluene: λmax (ꢀM) 305 (8800) nm. 1H NMR (THF-d8, -5
°C): δ 16.75 (s, 3), 14.99 (s, 2), 13.89 (s, 18), 3.63 (THF), 1.80
(THF), -17.91 (s, 2).
1
(25%) of a black solid. The H NMR spectrum of this material
was identical with that from method A.
[Fe3(µ3-Se)(µ2-S-p-tol)3(N(SiMe3)2)3]. A suspension of 0.032 g
(0.45 mmol) of gray selenium was added to a solution of 0.25 g
(0.30 mmol) of [Fe2(S-p-tol)2(N(SiMe3)2)2(THF)2] in 4 mL of
toluene, forming a red-black solution. The reaction mixture was
stirred for 3 days and filtered, and the filtrate was reduced to
dryness. The sticky residue was dissolved in a minimal volume of
hexanes maintained at -28 °C overnight. The product was isolated
1
as 0.13 g (28%) of a black crystalline solid. H NMR (C6D6): δ
29.71 (s, 3), 25.57 (s, 2), 3.66 (s, 18), -13.55 (s, 2).
In the sections that follow, complexes are designated as in
Chart 1.
X-ray Structure Determinations. The structures of the five
compounds in Table 1 were determined. Diffraction-quality crystals
were obtained by maintaining saturated solutions in the indicated
solvents at -30 °C for 24 h: 1, THF; 2 and 3, toluene; 4 and 5,
hexanes. Data were collected at 193 K on a Siemens (Bruker)
SMART CCD-based diffractometer. Cell parameters were retrieved
using SMART software and refined using SAINT on all observed
reflections. Data were collected at 0.3° intervals in Φ and ω for 30
s/frame such that a hemisphere of data was collected. No crystal
decay was observed upon re-collection of the first 50 frames at the
end of the data collection. Data sets were reduced with SAINT
and corrected for Lorentz and polarization effects; absorption
corrections were applied with SADABS. Structures were solved
by direct methods using SHELX-97. Metal and bound ligand atoms
were located from E maps. Other non-hydrogen atoms were found
in alternating difference Fourier syntheses and least-squares refine-
ment cycles and were refined anisotropically. Hydrogen atoms were
placed in calculated positions and refined as riding atoms with a
uniform value of Uiso. Crystallographic parameters are collected in
Table 1.14
[Fe2(µ2-S-1-Ad)(µ2-N(SiMe3)2)(N(SiMe3)2)2]. To a solution of
0.60 g (1.6 mmol) of [Fe(N(SiMe3)2)2] in 3 mL of toluene was
added 0.12 g (0.80 mmol) of adamantane-1-thiol. The reaction
mixture was stirred vigorously overnight and developed a deep-
red color. Slow removal of the solvent led to the separation of a
yellow crystalline solid, which was recrystallized from toluene to
give the product as 0.37 g (30%) of large yellow prisms. Absorption
1
spectrum (toluene): λmax (ꢀM) 307 (11 730), 424 (2000) nm. H
NMR (C6D6): δ 1.00 (36), -0.31 (18), -3.62 (s, 3), -6.60 (s, 3),
-8.94 (s, 3), -32.48 (s, 6). Anal. Calcd for C28H69Fe2N3SSi6: C,
44.24; H, 9.15; Fe, 14.69; N, 5.53; S, 4.22. Found: C, 44.14; H,
9.21; Fe, 14.50; N, 5.40; S, 4.30.
[Fe3(µ3-S)(S-p-tol)3(N(SiMe3)2)3]. Method A. To a solution of
0.60 g (1.6 mmol) of [Fe(N(SiMe3)2)2] in 3 mL of toluene was
added 0.20 g (1.6 mmol) of p-tolylthiol in 2 mL of toluene followed
by 0.017 g (0.53 mmol) of sulfur. The reaction mixture was stirred
vigorously overnight, during which it assumed a deep-red-black
color. Slow removal of solvent caused the separation of the product
as a black crystalline solid, which was recrystallized from hexanes/
toluene to give the product as 0.67 g (40%) of large well-formed
black prisms. Absorption spectrum (toluene): λmax (ꢀM) 323 (26 500),
430 (25 200), 770 (5200) nm. 1H NMR (C6D6): δ 29.72 (s, 3), 25.94
(s, 2), 3.49 (s, 18), -13.23 (s, 2). Anal. Calcd for C39H75Fe3N3S4Si6:
C, 44.60; H, 7.20, N, 4.00; S, 12.21. Found: C, 44.31; H, 7.08; N,
3.86; S, 12.20.
The structure of compound 1 shows a minor 2-fold disorder
of the two carbon atoms (C9 and C10) not bonded to oxygen in
the THF ligand. These were modeled over two positions with an
occupancy ratio of 3:1. In compound 2, the Fe1-(S1-Ad)-Fe2
portion is disordered over two nearly identical positions with a 3:1
occupancy. Given the quality and limited resolution of the data for
compound 5, the ether solvate molecule was refined isotropically.
One adamantylthiolate ligand is rotationally disordered about the
Method B. To a solution of 0.50 g (0.60 mmol) of [Fe2(S-p-
tol)2(N(SiMe3)2)2(THF)2] in 3 mL of toluene was added 0.013 g
(14) See the Supporting Information.
Inorganic Chemistry, Vol. 47, No. 8, 2008 3367

Pryadun and Holm
Table 1. Crystallographic Dataa for Clusters 1-5
1
2
3
4
5
formula
fw
cryst syst
space group
Z
C34H66Fe2N2O2S2Si4
823.07
C28H69Fe2N3SSi6
760.16
C39H75Fe3N3S4Si6
1050.36
monoclinic
P21/n
C39H75Fe3N3S3SeSi6
1097.25
monoclinic
P21/n
C52H109Fe3N3OS4Si6
1256.75
monoclinic
P21/c
triclinic
triclinic
j
j
P1
P1
1
2
4
4
4
a, Å
b, Å
c, Å
9.111(1)
11.697(2)
12.785(2)
63.335(2)
82.641(3)
68.006(2)
1127.6(3)
0.783
9.068(2)
14.670(3)
16.193(3)
87.869(3)
82.904(3)
88.375(3)
2135.5(7)
1.050
13.768(3)
16.455(4)
24.499(5)
90
90.48(3)
90
5550(2)
0.998
0.0495, 0.0956
13.786(1)
16.429(2)
24.572(3)
90
90.918(2)
90
5565(1)
1.095
0.057, 0.1066
12.708(1)
23.124(2)
23.020(2)
90
93.909(2)
90
6749.3(9)
1.034
0.0498, 0.1125
R, deg
ꢀ, deg
γ, deg
V, Å3
GOF (F2)
R1,b wR2c
0.0456, 0.0851
0.0541, 0.1182
a Obtained at 193 K with Mo KR2 radiation (λ ) 0.710 73 Å). b R1 ) ∑(|Fo| - |Fc|)/∑|Fo|. c wR2 ) {∑[w(Fo - Fc )2]/∑[w(Fo )2]}1/2; w ) 1/[σ2(Fo )
2
2
2
2
2
2
+ (aP)2 + bP], P ) [max(Fo or 0) + 2(Fc )]/3.
1, Fe-S-Fe ) 81.74(3)° and S-Fe-S ) 98.26(3)°. The
opposite angular core distortion in the former complex versus
1 arises from steric interactions of the larger mesityl group
with neighboring ligands, resulting in an opening of the angle
at the µ2-sulfur atom.
2Fe[N(SiMe3)2]2 + 2p-tolSH + 2THF f
[Fe2(µ2-S-p-tol)2(N(SiMe3)2)2(THF)2] + 2(Me3Si)2NH (1)
An equimolar reaction of Fe[N(SiMe3)2]2 and 1-adaman-
tylthiol in THF did not yield a tractable product. The use of
0.5 equiv of thiol led to isolation of 2 in moderate yield
(30%). This molecule (Figure 2) contains a planar Fe2NS
core with thiolate and silylamide bridges, an Fe · · ·Fe
separation of 2.823 Å, and planar three-coordination at the
iron sites. The latter feature arises from the reaction
stoichiometry and ligand size properties. The µ2-N(SiMe3)2
bridging functionality is less frequently encountered than
terminal binding but is precedented in dinuclear complexes
with at least six metals.2,8,10,16–19 Of the prior examples
involving iron, Fe2[N(SiMe3)2]4, containing planar FeII and
a rhombic Fe2N2 core with an Fe · · · Fe distance of 2.663 Å,2
is pertinent here. Dimensions of the Fe-N-Fe core portion
are similar to that of 2. In the two complexes, Fe-N bridge
bond lengths differ by only 0.02 Å, but the smaller bridge
angle of 79.4° vs 85.2° in 2, occasioned by the presence of
a µ2-sulfur atom, is largely responsible for the longer
metal-metal distance in the latter species.
Trinuclear Clusters. Although not obtained in high yields,
these clusters are accessible by three methods (Figure 1),
which proceed with the apparent stoichiometries of reactions
2–4. Self-assembly reaction 2 (R ) p-tol and 1-Ad) leads to
the formation of clusters 3 (40%) and 5 (30%). Reactions 3
(Q ) S and Se) and 4 effectively increase cluster nuclearity
and lead to 3 (60%) and 4 (28%) and to 5 (25%),
respectively. The formation of a trinuclear cluster from
dinuclear species is an unusual reaction. With no other source
of iron, cleavage of the dinuclear cluster is required,
Figure 1. Scheme depicting the synthesis of dinuclear complexes 1 and 2
from Fe[N(SiMe3)2]2 and trinuclear complexes 3–5 from 1 and 2.
S-C bond in two orientations that were refined with equal
populations. In the treatment of all disorders, appropriate constraints
were applied to bond lengths and anisotropic displacement para-
meters.
Results and Discussion
The utility of Fe[N(SiMe3)2]2 as a precursor is made
evident by the reactions in Figure 1 that afford dinuclear
complexes 1 and 2 and trinuclear clusters 3-5. Structures
of the reaction products are provided in Figures 2 and 3.
Dinuclear Complexes. The reaction of Fe[N(SiMe3)2]2
with an equimolar amount of p-tolylthiol in THF results in
thiolate-bridged dinuclear complex 1 (90%) by the stoichi-
ometry of reaction 1. The molecule (Figure 2) is centrosym-
metric with distorted tetrahedral coordination at the FeII sites
and a rhombic Fe2S2 core with an Fe · · · Fe and mean Fe-S
distances of 3.145 and 2.403 Å, respectively. Although
thiolate-bridged FeII dimers abound, this complex is closely
precedented only by [Fe2(µ2-Smes)2(N(SiMe3)2)2(THF)2]15
whose core is distinctly different from that of 1. In this
complex, the Fe-S bond lengths differ by only 0.03 Å from
1, but the rhomb is elongated along the Fe-Fe vector such
that the Fe · · · Fe separation is 3.534 Å and the Fe-S-Fe
and S-Fe-S angles are 94.45° and 85.55°, respectively. In
(15) Verma, A. K.; Lee, S. C. J. Am. Chem. Soc. 1999, 121, 10838–10839.
(16) Murray, B. D.; Power, P. P. Inorg. Chem. 1984, 23, 4584–4588.
(17) Cummins, C. C.; Schrock, R. R.; Davis, W. M. Organometallics 1991,
10, 3781–3785.
(18) Gru¨tzmacher, H.; Steiner, M.; Pritzkow, H.; Zsolnai, L.; Huttner, G.;
Sebald, A. Chem. Ber. 1992, 125, 2199–2207.
(19) Angermaier, K.; Schmidbaur, H. Chem. Ber. 1995, 128, 817–822.
3368 Inorganic Chemistry, Vol. 47, No. 8, 2008

Binuclear and Trinuclear Iron-Sulfur Clusters
Figure 2. Structures of dinuclear complexes showing 50% probability
ellipsoids and atom-labeling schemes; selected interatomic distances (Å)
and angles (deg) are indicated. (Top) [Fe2(µ2-S-p-tol)2(N(SiMe3)2)2(THF)2]:
Fe1-S1 2.399(1), Fe1-S1′ 2.407(1), Fe1-N1 1.940(3), Fe1-O1 2.092(2),
Fe1-Fe1′3.145,Fe1-S1-Fe1′81.74(3),S1-Fe1-S1′98.26(3),N1-Fe1-O1
104.5(1), N1-Fe1-S1 125.48(9), N1-Fe1-S1′ 123.34(8), O1-Fe1-S1
99.55(7), O1-Fe1-S1′ 101.02(7). (Bottom) [Fe2(µ2-S-1-Ad)(µ2-N(SiMe3)2)-
(N(SiMe3)2)2]: Fe1-S1 2.337(2), Fe2-S1 2.340(3), Fe1-N1 2.066(3),
Fe2-N1 2.094(4), Fe1-N2 1.906(4), Fe2-N3 1.885(4), Fe1-Fe2 2.816,
Fe1-S1-Fe2 74.04(9), Fe1-N1-Fe2 85.2(1), S1-Fe1-N1 98.7(1).
Disordered ligands are shown in their majority positions.
Figure 3. Structures of trinuclear complexes showing 50% probability
ellipsoids and atom-labeling schemes. (Top) [Fe3(µ3-S)(µ3-S-p-tol)3(N-
(SiMe3)2)3]. (Bottom) [Fe3(µ3-S)(µ3-S-1-Ad)3(N(SiMe3)2)3]. The disordered
adamantylthiolate ligand is shown in one of two orientations.
Table 2. Structural Parameters of [Fe3S(S-p-tol)3(N(SiMe3)2)3]
presumably leading to other products (not isolated) and
contributing to the usually modest yields.
Fe1-S1
Fe2-S1
Fe3-S1
Fe1-S2
Fe1-S3
Fe2-S3
Fe2-S4
Fe3-S2
Fe3-S4
Fe1-N1
Fe2-N2
Fe3-N3
Fe1-Fe2
Fe1-Fe3
Fe2-Fe3
N-Fe-S1
2.262(1)
2.303(1)
2.317(1)
2.362(1)
2.364(1)
2.363(1)
2.360(1)
2.381(1)
2.388(1)
1.876(2)
1.892(2)
1.900(2)
2.947(1)
2.960(1)
3.033(1)
118.2–120.8a
Fe1-S1-Fe2
Fe1-S1-Fe3
Fe2-S1-Fe3
Fe1-S2-Fe3
Fe1-S3-Fe2
Fe2-S4-Fe3
S1-Fe1-S2
S2-Fe1-S3
S1-Fe1-S3
S1-Fe2-S3
S1-Fe2-S4
S3-Fe2-S4
S1-Fe3-S2
S1-Fe3-S4
S2-Fe3-S4
N-Fe-S2–4
80.41(3)
80.51(3)
82.03(3)
77.23(3)
77.15(3)
79.36(3)
101.87(3)
96.21(3)
101.15(3)
99.98(3)
99.31(3)
97.11(3)
99.68(3)
98.09(3)
94.83(3)
116.3–120.8b
3Fe[N(SiMe3)2]2 + 3RSH + S f
[Fe3S(SR)3(N(SiMe3)2)3] + 3(Me3Si)2NH (2)
3[Fe2(S-p-tol)2(N(SiMe3)2)2(THF)2] + 2Q f
2[Fe3Q(S-p-tol)3(N(SiMe3)2)3] + 6 THF (3)
3[Fe2(S-1-Ad)(N(SiMe3)2)3] + 3Ad-1-SH + 2S f
2[Fe3S(S-1-Ad)3(N(SiMe3)2)3] + 3(Me3Si)2NH (4)
Clusters 3-5 contain the mixed-valence [Fe3(µ3-Q)(µ2-
S)3] core (2FeIII + FeII) with a triply bridging sulfide or
selenide atom, doubly bridging thiolate ligands, and idealized
trigonal symmetry (Figure 3). The structures of 3 and 5 are
very similar despite the steric bulk of the ligands and differ
in the conformational disorder of one adamantylthiolate
ligand over two sites. The metric features of 3 are sum-
marized in Table 2.14 The structure of 3 consists of the µ3-
S1 atom bonded to three iron atoms with a mean Fe-S-Fe
angle of 81.0°, each of which is bridged by two µ2-S-p-tol
ligands and terminally coordinated by a bis(trimethylsilyl-
amido) group. The core is thus built of three edge-fused
rhombs that include iron atoms with distorted tetrahedral
a Range of 3. b Range of 6.
coordination. Because of bridging, the S-Fe-S angles
within the rhombs (98.1–101.9°) are substantially smaller
than the N-Fe-S angles (116.3–120.8°), which appear also
to reflect steric interactions between the terminal silylamido
ligands and thiolate substituents. Corresponding dimensions
of 5 are nearly identical, and those of 4 differ only because
of the larger radius of the µ3-selenium ligand.14 The mean
Fe-Se bond length is 2.42(3) Å. The pattern of isotropically
shifted resonances of the p-tolyl group in 3 (o-H, +15.5 ppm;
Inorganic Chemistry, Vol. 47, No. 8, 2008 3369

Pryadun and Holm
m-H, -23.7 ppm; p-Me, -27.4 ppm) is indicative of
metal-sulfur antiparallel spin transfer and dominant contact
interactions, characteristic features of all iron-sulfur thiolate
clusters.
clearities of 2,13,15 4,10,13 and 8.8,10 Complexes 3-5, while
topologically precedented, constitute a new class of mixed-
valence iron-sulfur clusters. The electronic properties of 3
are currently under investigation. Lastly, the occurrence of
the [Fe3(µ3-S)(µ2-SR)3] core in five different molecules
suggests that a corresponding [Fe3(µ3-S)(µ2-SCys)3] might be
a viable biological site. While there remain structurally
unidentified protein-bound iron-sulfur sites, no information
has been forthcoming as to the occurrence of this type of
trinuclear entity.
The cores of 3-5 present a cuboidal shape resembling
that of the [Fe3S4]0 core in [Fe3S4(LS3)]3-, in which the
metals are in the same average oxidation state (Fe2.67+).20
As is the case in nearly all iron-sulfur clusters,21 no
structurally distinct iron sites with different oxidation states
are observed. A closer structural relationship is found with
22
the all-ferrous cluster [Fe3S(S2-o-xyl)3]2- and a nearly
identical complex derived from durene-1,2-dithiol.23 In these
cases, the chelating dithiolate supplies both bridging and
terminal ligands, probably contributing to marked differences
in Fe-Fe distances and Fe-S-Fe angles compared to 3
and 5.
Acknowledgment. The research was supported by NIH
Grant GM 28846. We thank Dr. C. L. Nygren, Dr. S.
Groysman, and Dr. D. Ho for crystallographic assistance.
Supporting Information Available: X-ray crystallographic
information for the five compounds in Table 1. This material is
available free of charge via the Internet at http://pubs.acs.org.
Summary
Complexes 1-5 are the most recent additions to a small
class of iron-sulfido/thiolato-silylamido clusters with nu-
IC7023742
(20) Zhou, J.; Hu, Z.; Mu¨nck, E.; Holm, R. H. J. Am. Chem. Soc. 1996,
118, 1966–1980.
(22) Hagen, K. S.; Christou, G.; Holm, R. H. Inorg. Chem. 1983, 22, 309–
314.
(21) Holm, R. H. Iron-Sulfur Clusters. In Biocoordination Chemistry; Que,
L., Jr., Tolman, W. A., Eds.; Elsevier: Oxford, U.K., 2004; pp 61–90.
(23) Henkel, G.; Tremel, W.; Krebs, B. Angew. Chem., Int. Ed. Engl. 1981,
20, 1033–1034.
3370 Inorganic Chemistry, Vol. 47, No. 8, 2008