Synthesis and characterization of new heterometallic iron sulfur nitrosyl clusters

Article abstract of DOI:10.1016/j.poly.2007.06.015

The reactivity of the (PPN)₂[Fe₈S₆(NO)₈] and (PPN)₂[Fe₆S₆(NO)₆] clusters is explored and new derivative clusters have been synthesized and structurally characterized. The unique (PPN)₂Fe₄S₄(NO)₆ "open-cubane" cluster with a chair like Fe₄S₄ core is obtained along with the mixed metal pentandite-like clusters (PPN)₂[Mo₂Fe₆S₆(NO)₆(CO)₆], (PPr₃)₂Cu₂Fe₆S₆(NO)₆, (PPr₃)₄Cu₄Fe₄S₆(NO)₄, (PPr₃)₂Ni₂Fe₆S₆(NO)₆, (PPr₃)₃Ni₃Fe₄S₆(NO)₄. The rich electrochemistry of the mixed metal clusters is presented as well.

Full text of DOI:10.1016/j.poly.2007.06.015

Polyhedron 26 (2007) 4765–4778
www.elsevier.com/locate/poly
Synthesis and characterization of new heterometallic iron sulfur
nitrosyl clusters
*
Harris Kalyvas, Dimitri Coucouvanis
Department of Chemistry, University of Michigan, Ann Arbor, MI 48109, USA
Received 11 April 2007; accepted 13 June 2007
Available online 28 June 2007
Dedicated to Professor Dieter Fenske on the occasion of his 65th Birthday
Abstract
The reactivity of the (PPN)2[Fe8S6(NO)8] and (PPN)2[Fe6S6(NO)6] clusters is explored and new derivative clusters have been synthe-
sized and structurally characterized. The unique (PPN)2Fe4S4(NO)6 ‘‘open-cubane’’ cluster with a chair like Fe4S4 core is obtained along
with the mixed metal pentandite-like clusters (PPN)2[Mo2Fe6S6(NO)6(CO)6], (PPr3)2Cu2Fe6S6(NO)6, (PPr3)4Cu4Fe4S6(NO)4, (PPr3)2Ni2-
Fe6S6(NO)6, (PPr3)3Ni3Fe4S6(NO)4. The rich electrochemistry of the mixed metal clusters is presented as well.
Published by Elsevier Ltd.
Keywords: Iron sulfur clusters; Nitrosyl coordination
1. Introduction
nitric oxide plays in living systems as a vasodilator and a
signaling molecule [10–12]. Various iron sulfur nitrosyl
clusters are known and (NH4)[Fe4S3(NO)7] [13] also known
as the Roussin black salt is the oldest and the most well
studied member of this class. This molecule is also recog-
nized as an important physiological NO donor due to its
ability to penetrate membranes of cells and slowly release
NO [14–16].
In a recent communication [17] we have reported the
synthesis of (PPN)2[Fe8S6(NO)8], (PPN)2[Fe6S6(NO)6]
and (PPN)2[Fe4S4(NO)4] and in this article we report fur-
ther on the reactivity and derivatives of the PPN+ salts
of the [Fe8S6(NO)8]2À and [Fe6S6(NO)6]2À clusters and
the isolation of the unique (PPN)2[Fe4S4(NO)6] ‘‘open-
cubane’’ cluster with a chair like Fe4S4 core, and also we
report the synthesis of new M/Fe/S/NO clusters (M@Ni,
Cu, Mo) with a cubeoctahedral M8S6 core.
The diversity of Fe/S cluster chemistry is expressed in a
multitude of stoichiometries, ligand (L) types and struc-
tures and in a general broad sense the Fe/S/L clusters
can be classified as Fe/S/X (X = halogens, thiols), Fe/S/
CO, Fe/S/PR3 and Fe/S/NO species. Examples of these
clusters (Scheme 1) include the [Fe4S4(L)4]nÀ cubanes
[1,2], the sulfur voided [Fe4S3] cube of (Ph4As)[Fe4S3(NO)7]
[3], the iron voided [Fe3S4] cube in (Et4N)3[Fe3S4(LS3)],
(LS3 = 1,3,5-Tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,
4,6-tris(p-tolylthio)benzene) [4], the Fe6S6 prism in the
[Fe6S6(L)6]nÀ prismanes (L = halogens, MeOC6H4O or
nitrosyl) [5–7], the ‘‘bow-tie’’ [Fe6S6] core in (PPN)2-
[Fe6S6(CO)12] [8] (PPN = Bis(triphenylphospine)iminium),
the edge-fused double cubane [Fe8S8] core in Fe8S8(PCy3)6
[9] (PCy3 = Tricyclohexylphosphine).
Interest in the coordination chemistry of metal-nitrosyl
clusters stems from the recognition of the important role
The cubeoctahedral M8S6 ‘‘pentandite-like’’ core with a
cubic array of metal atoms inscribed in an S6 octahedron of
l4-sulfido ligands has been observed in minerals [18] and
also in molecular clusters with halides [19–23] as terminal
ligands.
*
Corresponding author. Tel.: +1 734 7647339.
E-mail address: dcouc@umich.edu (D. Coucouvanis).
0277-5387/$ - see front matter Published by Elsevier Ltd.
doi:10.1016/j.poly.2007.06.015

4766
H. Kalyvas, D. Coucouvanis / Polyhedron 26 (2007) 4765–4778
Cl
Cl
Yield 0.41 mmol 94% of (PPN)2[Fe8S6(NO)8] yield based
on [Fe4S3(NO)7]À as the only source of Fe. Elemental Anal.
Calc. for (PPN)2[Fe8S6(NO)8] Æ DMF C75H67N11O9P4S6-
Fe8: C, 44.39; H, 3.33; N, 7.59. Found: C, 44.75, H, 3.48,
N, 7.36%. IR: 3054(w), 2690(m), 2928(m), 2868(w),
1684(vs)(mN–O), 1427(m), 1263(m), 1113(m), 797(w),
Cl
Fe
S
S
S
Cl
S
Cl
Fe
Fe
S
Fe
S
S
Fe
Fe
Fe
Fe
Fe
S
Cl
Fe
Cl
S
S
Cl
Cl
Cl
(Et4N)2[Fe4S4Cl4]
(Et4N)3[Fe6S6Cl6]
NO
Fe
744(w), 723(m), 690(m), 550(m), 532(m), 526(w), 495(w).
À
MS: (ESI À) 878.6 Fe8S6(NO)8À, 788.2 Fe8S6(NO)5
,
S
S
S
707.2 Fe6S6(NO)6À, 647.3 Fe6S6(NO)4À, 617.3 Fe6S6-
(NO)3À, 587.3 Fe6S6(NO)2À, 471.5 Fe4S4(NO)4À, 441.5
Fe4S4(NO)3À, 411.52 Fe4S4(NO)2À. MS: (ESI +) 537.8
(PPN+). CV: (DMF solution, 0.017 M) E1(rev) = À482 mV,
S
S
S
Fe
Fe
Fe
Fe
SL
Fe
Fe
LS
LS
S
ON
NO
ON ON
NO
NO
(Ph4As)[Fe4S3(NO)7]
(Et4N)3[Fe3S4(LS3)]
E2(rev) = À616 mV, E3(rev) = À1202 mV,
E4(rev) = À1295
Cy
P
3
PCy
S
3
CO
CO
Fe
mV.
Fe
S
CO
CO
S
Cy P
3
OC
OC
OC
OC
S
Fe
S
S
Fe
Fe
S
Fe
S
S
S
Fe
Fe
Fe
Fe
Fe
S
S
S
CO
CO
PCy
Fe
2.2. Bis(Bis(triphenylphospine)iminium) hexasulfido-
hexairon-hexanitrosyl, (PPN)2[Fe6S6(NO)6)]
3
Fe
Fe
S
Cy
P
3
PCy
CO
3
CO
Fe8S8(PCy3)6
(PPN)2[Fe6S6(CO)12
]
2.2.1. Method A (Scheme 2, Reaction 2)
Scheme 1. Examples of different arrangements of Fe/S clusters.
An amount of (PPN)[Fe4S3(NO)7] (0.50 g, 0.47 mmol)
and (PPN)2[Fe4(CO)13] (0.78 g, 0.47 mmol) were dissolved
in 20 ml MeCN, and Bz2S3 (0.39 g, 1.40 mmol) was added.
The resulting mixture was heated under reflux for 24 h. The
precipitate formed was washed with 5 ml MeCN, dissolved
in 15 ml DMF and layered with ca. 150 ml ether, affording
0.88 g of a black crystalline product. Yield 0.49 mmol, 79%
of (PPN)2[Fe6S6(NO)6], based on total Fe content.
2. Experimental
All Reactions have been performed under a dinitrogen
atmosphere in a glove box and standard shlenck line tech-
niques. All solvents have been distilled and degassed,
except of DMF that was not distilled.
(NH4)[Fe4S3(NO)7] [24], (PPN)2[Fe4(CO)13] [25], Benzyl
trisulfide (Bz2S3) [26], Mo(CO)3(MeCN)3 [27], [Ni-
(MeCN)6](BF4)2 [28] were synthesized according to pub-
lished procedures. [Cu(MeCN)4]PF6, Fe(CO)5, SCl2 and
Bu4NSH were purchased from commercial sources.
IR spectra were obtained on a Perkin–Elmer BX FT-IR
spectrometer (mid-IR) and a Nicolet 740 FT-IR spectro-
meter (Far-IR) in KBr pellets. Mass Spectra were collected
on a Micromass LCT Time-of-Flight mass spectrometer.
Elemental analyses were performed by the Microanalytical
Laboratory at the University of Michigan. Microbeam
Electron Analysis was performed at The University of
Michigan Electron Microbeam Analysis Laboratory
(EMAL). and Cyclic Voltammetry experiments were car-
ried out on a Glass Carbon working electrode and Ag/
AgCl reference electrode with 0.1 M Bu4NPF6 supporting
electrolyte on a EG&G Princeton Potentiostat/Galvano-
stat model 263 A. The reduction potentials are reported
against a standard calomel electrode SCE.
2.2.2. Method B (Scheme 2, Reaction 4)
An amount of (PPN)2[Fe8S6(NO)8] (0.23 g, 0.12 mmol)
was dissolved in 10 ml DMF and Bz2S3 (0.1 g, 0.36 mmol)
was added and the solution stirred for 24 h. The solution
was filtered and layered with ca. 100 ml ether, affording
0.18 g of crystalline product. Yield 0.10 mmol, 84% of
(PPN)2[Fe6S6(NO)6]. Elemental Anal. Calc. for (PPN)2-
[Fe6S6(NO)6], C72H60N8O6P4S6Fe6: C, 48.46; H, 3.39; N,
6.28. Found: C, 47.90; H, 3.65; N, 7.33%. IR: 2690(m),
2928(m), 2868(w), 1679(vs) (mN–O), 1669(vs) (mN–O),
1435(m), 1262(m), 1116(m), 803(w), 743(w), 723(m),
692(m), 687(m), 546(m), 531(m), 523(w), 498(w). MS:
(ESI À) m/z 1245.31 (PPN)Fe6S6(NO)6À, 707.26 Fe6S6-
(NO)6À, 647.27 Fe6S6(NO)4À, 617.28 Fe6S6(NO)3À, 587.28
Fe6S6(NO)2À, 557.31 Fe6S6(NO)À, 471.49ÀFe4S4(NO)4
,
À
441.49 Fe4S4(NO)3À, 411.52 Fe4S4(NO)2 and several
smaller molecular weight fragments were observed below
400. MS: (ESI +): 537.78 (PPN+). CV: (DMF solution,
0.019 M) E1(rev) = À690 mV, E2(rev) = À1521 mV.
2.1. Bis(Bis(triphenylphospine)iminium) hexasulfido-
octairon-octanitrosyl, (PPN)2[Fe8S6(NO)8)]
2.3. Bis(Bis(triphenylphospine)iminium) tetrasulfido-
tetrairon-tetranitrosyl, (PPN)2[Fe4S4(NO)4]
An amount of (NH4)[Fe4S3(NO)7] (0.50 g, 0.87 mmol)
and (PPN)2[Fe4(CO)13] (2.17 g 1.30 mmol) were dissolved
in 20 ml MeCN and allowed to stir for 24 h. The precipitate
obtained by filtration was washed with 5 ml MeCN, dis-
solved in 15 ml DMF and the resulting solution layered
with ca. 150 ml ether, affording 0.80 g of a black crystalline
product.
2.3.1. Method A (Scheme 2, Reaction 3)
An amount of (PPN)[Fe4S3(NO)7] (0.50 g, 0.47 mmol)
and (PPN)2[Fe4(CO)13] (1.17 g, 0.70 mmol) were dissolved
in 20 ml MeCN, followed by the addition of Bu4NSH
(0.13 g, 0.47 mmol) in 5 ml MeCN and the mixture was
heated under reflux for 24 h. After cooling at room temper-

H. Kalyvas, D. Coucouvanis / Polyhedron 26 (2007) 4765–4778
4767
NO
-
⎤
NO
S
Fe
Fe
S
S
Fe
NO
Fe
NO
ON
NO
NO
1
Fe4(CO)2-
+Bz2S3
2-
13
13
Fe (CO)
4
+Bu4NSH
Reaction3
Reaction2
2-
13
NOBF4
Fe (CO)
4
Reaction1
Reaction6
NOBF
4
Reaction6
NO
Fe
2-
⎤
NO
ON
2-
2-
NO
⎤
⎤
S
S
Fe
Fe
S
S
S
NO
Reaction5
Bu4NSH
Fe(H2 O)BF4 /NOBF4
Reaction7
ON
S
NO
Fe
S
S
Fe
S
NO
Fe
Fe
Fe
S
S
Fe
ON
ON
S
SF
e
Fe
S
Fe
S
Fe
Fe
NO
Fe
Fe
Fe
NO
S
ON
ON
NO
Bz2S3
Reaction4
Fe
ON
ON
NO
2
8
7
Scheme 2. Schematic representations of the transformations of Fe/S/NO clusters discussed in this article.
ature the mixture was filtered, and the precipitate was
washed with MeCN and extracted with ca. 15 ml DMF
and layered with ca. 100 ml ether, affording 0.35 g of micro-
crystalline product. Yield 0.23 mmol 48.9% of (PPN)2-
Fe4S4(NO)4 based on Fe4S3(NO)7À. Elemental Anal. Calc.
for (PPN)2[Fe4S4(NO)4] C72H60N6O4P4S4Fe4: C, 55.39; H,
3.90; N, 5.43. Found: C, 51.49; H, 3.49; N, 5.96%.
filtered, and the precipitate was washed with MeCN and
extracted with ca. 5 ml DMF and layered with ca. 50 ml
ether, left for 2 days in the freezer, filtered again to remove
any (PPN)2[Fe4S4(NO)4] that formed and the filtrate layered
with additional 100 ml ether, affording 0.08 g of green brown
microcrystalline product. Yield 0.07 mmol, 14% of
(PPN)2[Fe4S4(NO)6] based on [Fe4S3(NO)7]À. Elemental
Anal. Calc. for (PPN)2[Fe4S4(NO)6] Æ 2DMF C78H74N10-
O8P4S4Fe4: C, 53.38; H, 4.25; N, 7.98. Found: C, 53.92; H,
4.20; N, 7.33%. IR:, 2962(w), 2930(w), 2861(w), 1700(vs)
(mN–O), 1668(vs) (mN–O), 1435(m), 1262(m), 1115(m),
2.3.2. Method B (Scheme 2, Reaction 5)
An amount of (PPN)2[Fe8S6(NO)8] (0.20 g, 0.10 mmol)
and Bu4NSH (0.17 g, 0.62 mmol) were dissolved in 10 ml
DMF and heated at 80 ꢀC for 24 h. The resulting mixture
was cooled at room temperature, filtered, and the solution
was layered with ca. 100 ml ether, affording 0.22 g of
microcrystalline product. Yield 0.14 mmol, 68.75% of
(PPN)2Fe4S4(NO)4. Elemental Anal. Calc. for (PPN)2-
[Fe4S4(NO)4] C72H60N6O4P4S4Fe4: C, 55.39; H, 3.90; N,
5.43. Found: C, 52.04; H, 4.10; N, 5.88%. IR:, 2960(m),
2928(m), 2868(w), 1652(vs) (mN–O), 1435(m), 1262(m),
1116(m), 803(w), 743(w), 723(m), 692(m), 687(m), 546(m),
531(m), 523(w), 498(w). MS: (ESI À) m/z 471.5 Fe4S4-
(NO)4À, 441.5 Fe4S4(NO)3À, 411.5 Fe4S4(NO)2À and several
smaller molecular weight fragments were observed below
400. MS: (ESI +): 537.7 (PPN+). CV: (DMF solution,
0.022 M) E1(rev) = À202 mV, E2(rev) = À1279 mV.
840(w), 746(w), 723(m), 692(m), 687(m), 546(m), 531(m),
À
523(w), 496(w). MS: (ESI À): m/z 525.4 Fe4S4(NO)6À
495.6 Fe4S4(NO)5À, 467.8 Fe4S4(NO)4À 438.1 Fe4S4(NO)3
MS: (ESI +): m/z 535.3 (PPN+).
,
.
2.5. Bis(Bis(triphenylphospine)iminium) hexasulfido-
hexairon-hexanitrosyl bis(tricarbonyl-molybdenum)
(PPN)2[Mo2Fe6(NO)6(CO)6]
An amount of (PPN)2[Fe6S6(NO)6] (0.5 g, 0.28 mmol)
and Mo(CO)3(MeCN)3 (0.17 g, 0.56 mmol) were dissolved
in 15 ml dichloroethane and the solution stirred overnight.
The mixture was filtered, and the filtrate layered with
ca. 60 ml ether, affording 0.45 g of crystalline material.
Yield 0.20 mmol, 71.3% of (PPN)2[(CO)6Mo2Fe6S6(NO)6].
Elemental Anal. Calc. for (PPN)2[(CO)6Mo2Fe6S6(NO)6]
C78H60N8O12P4S6Fe6Mo2: C, 43.68; H, 2.82; N, 5.22.
Found: C, 43.48; H, 2.70; N, 5.16%. IR:, 3055(w), 2957(w),
2926(w), 2861(w), 1926(vs) (mC–O), 1877(mCÀO), 1723(sh)
(mN–O), 1695(vs) (mN–O), 1670(sh) (mN–O), 1437(m), 1384(m),
1258(m), 1113(m), 744(w), 723(m), 691(m), 690(m), 594(m)
(mMo–C), 548(m), 532(m), 498(w). CV: (DMF solution,
0.031 M) E1(rev) = – 641 mV, E2(rev) = À1247 mV.
2.4. Bis(Bis(triphenylphospine)iminium) tetrasulfido-
tetrairon-hexanitrosyl, (PPN)2[Fe4S4(NO)6]
An amount of (NH4)[Fe4S3(NO)7] (0.20 g, 0.35 mmol)
and (PPN)2[Fe4(CO)13] (0.87 g 0.52 mmol) were dissolved
in 15 ml MeCN, the mixture was cooled at À20 ꢀC and a
solution of Bu4NSH (0.05 g, 0.19 mmol) in 2 ml MeCN
was added. The mixture was stirred for 8 h at À20 ꢀC,

4768
H. Kalyvas, D. Coucouvanis / Polyhedron 26 (2007) 4765–4778
2.6. Hexasulfido-hexairon-hexanitrosyl
bis(tripropylphosphino-copper) (PPr3)2Cu2Fe6S6(NO)6
and Hexasulfido-tetrairon-tetranitrosyl
tetrakis(tripropylphosphino-copper)
(PPr3)4Cu4Fe4S6(NO)4
Fe6S6(NO)6. The filtrate was taken to dryness and the res-
idue stirred in ether affording a colored solution and a
white powder identified as PPNBF4. The resulting ether
solution was dried and washed with hexanes and ether
and extracted with THF using silica gel as a filtration
agent, affording approx. 0.04 g of oily (PPr3)3Ni3-
Fe4S6(NO)4. Crystals suitable for X-ray diffraction were
obtained by recrystalization from THF/Hexanes.
In 5 ml MeCN, [Cu(MeCN)4]PF6 (0.21 g, 0.56 mmol)
was dissolved and tripropyl phosphine (PPr3) (0.12 ml,
0.60 mmol) was added dropwise and the solution stirred
for 30 min. This solution was added dropwise to a dichlo-
roethane solution (15 ml) containing (PPN)2[Fe6S6(NO)6]
(0.5 g, 0.28 mmol) and the mixture was stirred overnight.
The mixture was filtered and the precipitate was washed
with small amounts of ether, extracted with a THF/Ether
1:1 mixture and layered with hexanes affording 0.2 g of
crystalline (PPr3)2Cu2Fe6S6(NO)6. The filtrate was taken
to dryness and the residue stirred in ether affording a
colored solution and a white powder identified as
PPNPF6. Following filtration, the ether solution was
dried affording approx. 0.05 g of oily (PPr3)4Cu4-
Fe4S6(NO)4. Crystals suitable for X-ray diffraction were
obtained by redissolving in ether and allowing for slow
evaporation to occur.
(PPr3)2Cu2Fe6S6(NO)6 Elemental Anal. Calc. for
(PPr3)2Cu2Fe6S6(NO) C18H42N6O6P2S6Fe6Cu2: C, 18.72;
H, 3.67; N, 7.28. Found: C, 18.51; H, 3.63; N, 7.05%. Yield
0.17 mmol, 60% of (PPr3)2Cu2Fe6S6(NO)6 based on Fe.
IR:, 2957(m), 2927(m), 2867(w), 1766(sh) (mN–O), 1750(sh)
(mN–O), 1729(vs) (mN–O), 1660(sh) (mN–O), 1450(m),
1410(m), 1080(m), 1000(m), 851(w), 736(w). Microprobe
analysis confirms the 2:6 ratio of Cu to Fe. CV: (THF solu-
tion, 0.029 M) E1(rev) = À169 mV, E2(rev) = À1035 mV,
E3(rev) = À1667 mV.
(PPr3)2Ni2Fe6S6(NO)6 Elemental Anal. Calc. for
(PPr3)2Ni2Fe6S6(NO)6 C18H42N6O6P2S6Fe6Ni2 C, 18.88;
H, 3.70; N, 7.34. Found: C, 19.12; H, 3.77; N, 7.26%. Yield
0.13 mmol, 46% of (PPr3)2Ni2Fe6S6(NO)6 based on Fe. IR:
2958(m), 2927(m), 2868(w), 1763(s) (mN–O), 1745(sh) (mN–O),
1725(vs) (mN–O), 1663(sh) (mN–O), 1449(m), 1405(m),
12181(m), 1076(m), 1041(m), 851(m), 762(w), 731(m),
586(w), 435(w). Microprobe analysis confirms the 2:6 ratio
of Ni to Fe. CV: (THF solution, 0.029 M) E1(rev) = À1 mV,
E2(rev) = À717 mV, E3(rev) = À1431 mV.
(PPr3)3Ni3Fe4S6(NO)4 Elemental Anal. Calc. for
(PPr3)3Ni3Fe4S6(NO)4C27H63N4O4P3S6Fe4Ni3 C, 27.19;
H, 5.32; N, 4.70. Found: C, 31.57; H, 5.91; N, 5.31%. Yield
0.033 mmol, 17.83% (PPr3)3Ni3Fe4S6(NO)4 based on Ni.
IR: 2957(m), 2927(m), 2868(w), 1720(s) (mN–O), 1698(vs)
(mN–O), 1679(s) (mN–O), 1452(m), 1404(m), 1221(m),
1073(m), 1038(m), 849(m), 724(m), 584(w), 440(w). Micro-
probe analysis confirms the 3:4 ratio of Ni to Fe.
2.8. Reactions of (PPN)2[Fe6S6(NO)6] with Fe2+ or
Fe0
2.8.1. With Fe(H2O)6(BF4)2/ PPr3 (Scheme 2, Reaction 7)
An amount of (PPN)2[Fe6S6(NO)6] (0.2 g, 0.11 mmol)
and Fe(H2O)6(BF4)2 (0.08 g, 0.24 mmol) were suspended
in 20 ml of THF/MeCN 1:1 ratio, and PPr3 (0.05 ml,
0.25 mmol) was added dropwise. The mixture was stirred
overnight, filtered, and the solution layered with ether afford-
ing 0.12 g of crystalline material. The material was identified
as (PPN)2[Fe8S6(NO)8] by IR, MS and cyclic voltammetry,
without any evidence suggesting the presence of a different
species. Yield 0.06 mmol, 55% of (PPN)2[Fe8S6(NO)8].
(PPr3)4Cu4Fe4S6(NO)4 Elemental Anal. Calc. for
(PPr3)4Cu4Fe4S6(NO)4 C36H84N4O4P4S6Fe4Cu4: C, 30.22;
H, 5.92; N, 3.92%. Found: C, 32.90; H, 6.35; N, 3.08%.
Yield 0.04 mmol 28.5% (PPr3)4Cu4Fe4S6(NO)4 based on
Cu. IR: 2957(m), 2928(m), 2869(w), 1724(vs) (mN–O),
1712(vs) (mN–O), 1457(m), 1080(m), 840(w). Microprobe
analysis confirms the 1:1 ratio of Cu to Fe. CV: (THF solu-
tion, 0.011 M) E1(rev) = À153 mV, E2(rev) = À1021 mV,
E3(rev) = À1537 mV, E4(irr) = À2077 mV.
2.8.2. With Fe(CO)5
An amount of (PPN)2[Fe6S6(NO)6] (0.2 g, 0.11 mmol)
was dissolved in a solution of 20 ml THF/MeCN 1:1 ratio
and Fe(CO)5 (0.03 ml, 0.23 mmol) was added dropwise.
The mixture was stirred overnight, filtered, and the filtrate
layered with ether affording 0.10 g of a microcrystalline
material. The material was identified as (PPN)2-
[Fe8S6(NO)8] by IR and MS. Yield 0.05 mmol, 46% of
(PPN)2[Fe8S6(NO)8].
2.7. Hexasulfido-hexairon-hexanitrosyl
bis(tripropylphosphino-nickel)(PPr3)2Ni2Fe6S6(NO)6 and
Hexasulfido-tetrairon-tetranitrosyl tris(tripropylphosphino-
nickel) (PPr3)3Ni3Fe4S6(NO)4
In 5 ml MeCN, [Ni(MeCN)6](BF4)2 (0.27 g, 0.56 mmol)
was dissolved and PPr3 (0.12 ml, 0.60 mmol) was added
dropwise and the solution stirred for 30 min. This solution
was added dropwise to a dichloroethane solution (15 ml)
containing (PPN)2[Fe6S6(NO)6] (0.5 g, 0.28 mmol) and
the mixture was stirred overnight. The mixture was filtered
and the precipitate was washed with small amounts of ether
and extracted with a THF/Ether 1:1 mixture and layered
with hexanes affording 0.15 g of crystalline (PPr3)2Ni2-
2.9. Reactions of (PPN)2[Fe8S6(NO)8] with oxidizing
agents
2.9.1. With O2
An amount of (PPN)2[Fe8S6(NO)8] (0.2 g, 0.11 mmol)
was dissolved in 10 ml DMF and O2 was bubbled through

Table 1
Crystallographic and refinement table of the compounds presented herein
[Fe4S4(NO)6]2À
[(CO)6Mo2Fe6S6(NO)6]2À
(PPr3)2Cu2Fe6S6(NO)6
(PPr3)4Cu4Fe4S6(NO)4
(PPr3)2Ni2Fe6S6(NO)6
(PPr3)3Ni3Fe4S6(NO)4
Color
brown
black
black
black
black
black
Habit
Size (mm)
Formula
needle
0.60 · 0.04 · 0.04
needle
polyhedron
0.44 · 0.40 · 036
C18H42Cu2Fe6N6O6P2S6
1155.06
plate
polyhedron
0.38 · 0.34 · 030
C18H42Fe6Ni2N6O6P2S6
block
044 · 0.44 · 0.34
C90H86Fe6Mo2N12O16P4S6
2434.93
0.30 · 0.28 · 0.05
C36H84Cu4Fe4N4O4P4S6
1430.87
monoclinic
P21/c
0.40 · 0.24 · 0.16
C24H63Fe4N4Ni3O4P3S6
1192.61
C
78H74Fe4N10O8P4S4
Weight
1754.99
triclinic
Crystal system
Space group
Unit cell dimensions
triclinic
cubic
cubic
cubic
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
P1
P1
Pa3
Pa3
Pa3
˚
a (A
10.951(6)
12.345(7)
28.632(15)
90.021(9)
94.034(9)
90.127(9)
3861(4)
12.373(2)
14.386(3)
15.495(3)
97.037(3)
91.936(3)
110.014(2)
2563.4(8)
1
15.9081(9)
15.9081(9)
15.9081(9)
90.00
90.00
90.00
11.299(3)
15.292(5)
17.404(5)
90.00
98.193(4)
90.00
15.7773(10)
15.7773(10)
15.7773(10)
90.00
90.00
90.00
21.3741(18)
21.3741(18)
21.3741(18)
90.00
90.00
90.00
˚
b (A)
˚
c (A)
a (ꢀ)
b (ꢀ)
c (ꢀ)
Volume
4025.8(4)
4
2976.4(15)
2
3927.3(4)
4
9764.8(14)
8
Z
2
Temperature (K)
Absorption coefficient
F(000)
123(2)
0.991
1808
123(2)
1.311
1234
108(2)
3.563
2320
123(2)
2.700
1472
108(2)
3.528
2312
108(2)
2.675
4912
h Range (ꢀ)
Reflections
Limiting indices
1.80–19.27
32467
À10 < h < 10,
À11 < k < 11,
À26 < l < 26
0.1486
1.88–28.39
81490
À16 < h < 16,
À19 < k < 19,
À20 < l < 20
0.0292
2.22–28.29
82778
À21 < h < 21,
À21 < k < 21,
À21 < l < 21
0.0332
1.78–26.43
51829
À14 < h < 14,
À19 < k < 19,
À21 < l < 21
0.0806
2.24–28.30
85115
À21 < h < 21,
À20 < k < 20,
À21 < l < 21
0.0383
1.65–28.32
194707
À28 < h < 28,
À28 < k < 28,
À28 < l < 28
0.0571
Rint
Data/restraints/ parameters
R1, wR2 [I > 2r(I)]
R1, wR2 (all data)
Goodness-of-fit (F2)
6428/0/978
0.0573, 0.1372
0.1235, 0.1781
1.029
12725/0/617
0.0396, 0.1050
0.0456, 0.1098
1.024
1671/0/72
0.0134, 0.0343
0.0142, 0.0348
1.090
6079/0/286
0.0738, 0.1753
0.1290, 0.2281
0.990
1636/0/71
0.0147, 0.0395
0.0158, 0.0399
1.139
4069/0/157
0.0199, 0.0413
0.0345, 0.0497
1.239

4770
H. Kalyvas, D. Coucouvanis / Polyhedron 26 (2007) 4765–4778
the solution for 5 min. the mixture was stirred for 1 h and
filtered to remove a light brown powder and layered with
ether affording 0.10 g of crystalline material. The material
was identified as [Fe(DMF)6][Fe6S6(NO)6] by IR, MS and
Unit Cell determination. Yield 0.08 mmol, 75.64% of
[Fe(DMF)6][Fe6S6(NO)6].
3. Results and discussion
The synthesis of the octanuclear iron sulfur nitrosyl
cluster (PPN)2[Fe8S6(NO)8] proceeds through the reaction
of (NH4)[Fe4S3(NO)7] with (PPN)2[Fe4(CO)13] in a 2:3
molar ratio affording (PPN)2[Fe8S6(NO)8] in high yields.
The byproducts of this reaction, that were isolated and ver-
ified by IR and MS are mainly (PPN)[Fe(CO)3(NO)] and a
small amount of (PPN)[Fe4N(CO)12]. The reaction appears
to proceed through the abstraction of nitrosyl ligands from
[Fe4S3(NO)7]À by the [Fe4(CO)13]2À cluster and the possi-
ble formation of an [Fe4S3(NO)4]À intermediate which sub-
sequently self couples to form the [Fe8S6(NO)8]2À cluster.
(PPN)[Fe(CO)3(NO)] seems to be the main product formed
by the reaction of the [Fe4(CO)13]2Àcluster with NO+. The
generation of the [Fe4N(CO)12]À cluster also is possible
considering the reported synthesis of this cluster by the
reaction of Fe3(CO)12 with [Fe(CO)3(NO)]À [30,31].
In the synthesis of (PPN)2[Fe6S6(NO)6] (Reaction 2,
Scheme 2), the yield of the Fe6S6 ‘‘prismane’’ is unreason-
ably high, assuming that [Fe4S3(NO)7]À is the only source
of Fe. It appears therefore that the [Fe4(CO)13]2À cluster
also provides Fe in the formation of (PPN)2[Fe6S6(NO)6].
In addition, it is known that [Fe(CO)3(NO)]À upon reac-
tion with S0 produces [Fe4S4(NO)4]À [32] so the pathway
to the formation of (PPN)2[Fe6S6(NO)6] through reaction
2 may be more complicated than the initial formation of
(PPN)2[Fe8S6(NO)8] and further reaction with Bz2S3 as
shown by reactions 1 and 4 (Scheme 2). The [Fe6S6-
(NO)6]2À cluster has been synthesized previously with dif-
ferent counter ions but in lower yields [7].
2.9.2. With [Fe(Cp)2]PF6
An amount of (PPN)2[Fe8S6(NO)8] (0.2 g, 0.11 mmol)
was dissolved in 10 ml DMF and [Fe(Cp)2]PF6 (0.14 g,
0.42 mmol) was added. The mixture was stirred overnight,
filtered, and the filtrate layered with ether affording 0.05 g
of crystalline [Fe(DMF)6][Fe6S6(NO)6] as identified by IR
and MS. Yield 0.04 mmol, 37.8% of [Fe(DMF)6]-
[Fe6S6(NO)6].
2.9.3. With SCl2
An amount of (PPN)2[Fe8S6(NO)8] (0.2 g, 0.11 mmol)
was dissolved in 20 ml dichloroethane and a 10% SCl2
solution in dichloroethane (0.07 ml, 0.11 mmol) was
added dropwise and the solution was stirred for 1 h.
The mixture was filtered and the precipitate was
dissolved in DMF and layered with ether affording a
powder that was identified by IR as [Fe(DMF)6]-
[Fe6S6(NO)6].
The filtrate of this solution was layered with ca. 20 ml
ether, filtered to remove a white powder of PPNCl, and
then layered with additional 40ml ether to obtain 0.08 g
light green needle shaped crystals.
The unit cell was identical to the known (PPN)-
[Fe(CO)3(NO)] complex [29], and IR showed characteristic
The [Fe4S4(NO)4]2À cluster has not been isolated in
this oxidation level previously. The IR spectrum shows
a NO stretching vibration at 1652 cmÀ1 which is lower
when compared to the 1760 cmÀ1 and 1700 cmÀ1
reported for Fe4S4(NO)4 and [Fe4S4(NO)4]À respectively
[32]. The lower frequency of the NO stretching vibra-
tion in (PPN)2[Fe4S4(NO)4] is consistent with the
expected increase in the Fe–NO backbonding in the
dianionic (PPN)2[Fe4S4(NO)4]. Elemental analysis also
is consistent with the (PPN)2[Fe4S4(NO)4] formula, how-
ever we were not able to obtain a satisfactory crystal
structure due to high distortion of the PPN+ cations
in the crystal lattice. Nevertheless, the presence of a
[Fe4S4(NO)4]2À cluster and two PPN+ cations has been
confirmed.
peaks at 1794 cmÀ1 (mN–O
)
and 350 cmÀ1 (mFe–Cl).
Elemental analysis confirmed that the crystals are (PPN)-
[FeCl3(NO)], apparently isomorphous to the (PPN)[Fe-
(CO)3(NO)] complex. Yield 0.17 mmol, 80% of
(PPN)[FeCl3(NO)].
Elemental Anal. Calc. for (PPN)[FeCl3(NO)]C36H30-
N2OP2Cl3Fe C, 59.17; H, 4.14; N, 3.83. Found: C, 58.86;
H, 3.99; N, 3.66%. IR:, 3052(w), 2917(w), 2848(w),
1794(vs) (mN–O), 1436(s), 1244(s), 1114(s), 998(w), 795(w),
749(m), 723(s), 692(s), 550(s), 533(m), 500(m), 350(s)
(mFe–Cl).
2.10. Crystallographic data
The synthesis of the [Fe4S4(NO)6]2À (Fig. 1) and [Fe4S4-
(NO)4]2À clusters from the Roussin anion is not unex-
pected. The reactions of [Fe4S3(NO)7]À with SHÀ, in polar
solvents (L), may well lead in cluster fragmentation and
generation of dimeric species such as {Fe2S2(NO)3(L)}À
and {Fe2S2(NO)2(L)2}À. Coupling of these units can lead
to the formation of the [Fe4S4(NO)6]2À and [Fe4S4(NO)4]2À
clusters respectively. As in the case of all [Fe4S3(NO)7]À/
[Fe4(CO)13]2À reactions, the main byproduct in the
synthesis of either [Fe4S4(NO)4]2À or [Fe4S4(NO)6]2À is
[Fe(CO)3(NO)]À. The apparent role of [Fe4(CO)13]2À as a
Diffraction data were collected on a Siemens SMART
CCD-based X-ray diffractometer, equipped with a Mo
Ka probe on a wavelength of 0.71073 A. Space groups
˚
were determined on systematic absences and intensity sta-
tistics. Full Matrix least-squares refinement based on F2
was performed. All non-hydrogen atoms were refined
anisotropically, and hydrogen atoms were placed in ideal
positions and refined as rigid atoms with individual
isotropic thermal displacement parameters. The specific
crystallographic data for each compound are presented in
Table 1.

H. Kalyvas, D. Coucouvanis / Polyhedron 26 (2007) 4765–4778
4771
3.1. Transformations of (PPN)2[Fe8S6(NO)8]
The (PPN)2[Fe8S6(NO)8] cluster, stable at high tempera-
tures, upon oxidation transforms to the [Fe6S6(NO)6]2À
‘‘prismane’’. This occurs using O2, SCl2 and [Fe(Cp)2]PF6.
In all three cases it seems that there is loss of two {Fe(NO)}
fragments rather than rearrangement of the clusters. In
coordinating solvents, the main product isolated is
[FeII(L)6][Fe6S6(NO)6] (L = DMF, THF, MeCN). The
[Fe6S6(NO)6]2À anion is structurally related to the [Fe8S6-
(NO)8]2À pentlandite. The former is obtained from the lat-
ter when two {Fe(NO)} moieties are removed from the
body diagonal of the pentlandite cube. In addition to the
[Fe6S6(NO)6]2À prismane, a yellow brown powder also
forms. In the case of the SCl2 reaction this byproduct
was identified and characterised as (PPN)[FeCl3(NO)] both
analytically and spectroscopically (by the NO stretch at
1794 cmÀ1, the Fe–Cl stretch at 350 cmÀ1 and characteris-
tic PPN stretches in the 800–500 cmÀ1 region). The unit cell
dimensions of this byproduct were nearly identical to those
of the known (PPN)[Fe(CO)3(NO)] complex [29]. Although
this particular complex has not been synthesized previ-
ously, its crystallographic structure determination and fur-
ther characterizations were not of particular interest in this
study and were not further pursued. The [Fe(L)6]-
[Fe6S6(NO)6] complex isolated has exactly the same infra-
red spectral pattern for the NO stretching vibrations as
the pattern for the previously reported [Fe(DMF)6]-
[Fe6S6(NO)6] [7]. A counter-ion exchange of [Fe(DMF)6]n+
with PPN+ gives (PPN)2[Fe6S6(NO)6]. The latter is verified
by elemental analysis and confirms that the iron in the
[Fe(L)6]n+ counter-ion is divalent (n = 2).
Fig. 1. Ortep diagram of [Fe4S4(NO)6]2À with 50% probability ellipsoids.
NO abstracting reagent is affected by both time and tem-
perature and this explains the formation and isolation of
[Fe4S4(NO)6]2À at low temperatures.
The formation of Fe4S4(NO)4 from the reaction of
[Fe4S3(NO)7]À and elemental sulfur has been reported
[32] and this cluster was studied several years ago,
although the mechanism of this reaction is not clear.
Based on experimental observations and theoretical calcu-
lations by Glidewell and Hoffmann [33,34] the formation
of Fe4S4(NO)4 from [Fe4S3(NO)7]À could proceed through
a mechanism of fragmentation and reassembly of the lat-
ter cluster.
The isolation of (PPN)2[Fe4S4(NO)6] has been achieved
by repeating the reaction that yields (PPN)2[Fe4S4(NO)4]
but at low temperatures. According to the formal oxidation
levels, there is a two electron oxidation from [Fe4S3(NO)7]À
to [Fe4S4(NO)6]2À and an additional two electron oxida-
tion to [Fe4S4(NO)4]2À. The isolation of this edge fused
double dimer can be envisioned as the coupling product
of two {Fe2S2(NO)3À} fragments while the cubane as the
product of two {Fe2S2(NO)2À} fragments.
For the reaction of (PPN)2[Fe8S6(NO)8] with eight
equivalents of NOBF4 the products isolated are
Fe(DMF)6[Fe6S6(NO)6], (PPN)[Fe4S3(NO)7] and PPNBF4
suggesting that in order to form the more reduced [Fe4S3-
(NO)7]À cluster some of the [Fe8S6(NO)8]2À cluster has to
be oxidized to produce the necessary electrons since NO+
cannot act as a reducing agent.
In contrast, when gaseous NO is used both [Fe8S6-
(NO)8]2À and [Fe6S6(NO)6]2À produce [Fe4S3(NO)7]À in
nearly quantitative yields.
The procedure reported here for the synthesis of
(PPN)2[Fe4S4(NO)6] is not an easily reproducible process
and seems to be sensitive to time and temperature of crys-
tallization. Both the hypothetical {Fe2S2(NO)3(L)}À and
{Fe2S2(NO)2(L)}À fragments possibly involved in the for-
mation of [Fe4S4(NO)6]2À and [Fe4S4(NO)4]2À respectively,
may be preset at the same time in various amounts. There-
fore isolation of [Fe4S4(NO)6]2À or the more stable [Fe4S4-
(NO)4]2À will depend on the exact conditions of the exper-
iment, and such conditions are not easily reproduced/
obtained. Although [Fe4S4(NO)6]2À has been detected
several times in mixtures obtained from this reaction, ana-
lytically pure crystalline material has only been obtained
once thus far.
3.2. Reactivity of (PPN)2[Fe6S6(NO)6]
The cyclohexane-like Fe3S3 units of the [Fe6S6] core can
serve as ligands for two additional metal ions and form
[M2Fe6S6] clusters, [19,35,36]. The latter were obtained in
reactions carried out with coordinatively-unsaturated com-
plexes containing Fe, Mo, Cu and Ni.
In the reaction of (PPN)2[Fe6S6(NO)6] with [Fe-
(H2O)6]2+/PPr3, or Fe(CO)5, the only product isolated
was [Fe8S6(NO)8]2À. The formation of [Fe8S6(NO)8]2À even
at NO deficient conditions can be explained if fragmenta-
tion of the cluster to {Fe(NO)} units occurs. The later
under reducing conditions form the [Fe8S6(NO)8]2À
preferentially.

4772
H. Kalyvas, D. Coucouvanis / Polyhedron 26 (2007) 4765–4778
In order to increase the yield of [Fe8S6(NO)8]2À the reac-
tion of (PPN)2[Fe6S6(NO)6] with [Fe(H2O)6]2+ was carried
out in the presence of NOBF6. The only products isolated
were (PPN)[Fe4S3(NO)7] and [Fe(DMF)6][Fe6S6(NO)6]
suggesting that the formation of [Fe4S3(NO)7]À is more
favorable in the presence of NO+.
As in the case of (Et4N)2[Fe6S6Cl6], [35] the reaction of
(PPN)2[Fe6S6(NO)6] with Mo(CO)3(MeCN)3 yields
(PPN)2[(CO)6Mo2Fe6S6(NO)6] (Fig. 2) even in molar ratios
less than 1:1 suggesting that the [MoFe6S6] core, if it forms
at all, disproportionates to the [Mo2Fe6S6] and [Fe6S6]
clusters.
The reaction of (PPN)2[Fe6S6(NO)6] with [Cu(Me-
CN)4]+/PPr3 affords (PPr3)2Cu2Fe6S6(NO)6 (Fig. 3) as the
main product, and in a lower yield the (PPr3)4Cu4-
Fe4S6(NO)4 (Fig. 4) cluster. Formation of the (PPr3)4Cu4-
Fe4S6(NO)4 cluster could be explained in terms of
fragmentation of the [Fe6S6(NO)6]2À cluster to smaller
Fe/S/NO units (i.e. {Fe2S2(NO)2} units), The latter in the
presence of Cu(PR3) form (PR3)CuS2Fe(NO) units which
couple to form the final product. In both cases the Fe6Cu2
and Fe4Cu4 clusters form due to the superior thermody-
namic stability of the Cu–S bond relative to the Fe–S bond.
Previously, we have reported on the substitution of Fe
atoms by Cu atoms in Fe/S clusters [19,37].
Fig. 3. Ortep diagram of (PPr3)2Cu2Fe6S6(NO)6 showing ellipsoids with
50% probability. The carbons of the phosphines have been omitted for
clarity.
These two products form regardless of the ratio of the
Cu used, and their separation was achieved due to their
slightly different solubilities. Similar reactions of [Fe6S6-
(NO)6]2À with [Ni(MeCN)6]2+/PPr3, give the neutral
(PPr3)2Ni2Fe6S6(NO)6 (Fig. 5) cluster as the main product,
but unlike the case of copper where the byproduct formed
Fig. 4. Ortep diagram of (PPr3)4Cu4Fe4S6(NO)4 showing ellipsoids with
50% probability. The carbons of the phosphines have been omitted for
clarity.
is (PPr3)4Cu4Fe4S6(NO)4, with nickel the other isolable
byproduct is the unique Ni/Fe/S cluster (PPr3)3Ni3-
Fe4S6(NO)4 (Fig. 6). This cluster contains the ‘‘voided–
cubic’’ M7S6 core which previously has been found in
(PPr3)4Fe7S6Cl3 [38], Co7S6I3(PEt3)4 [39], and Co7S6-
Fig. 2. Ortep diagram of [(CO)6Mo2Fe6S6(NO)6]2À showing ellipsoids
with 50% probability.

H. Kalyvas, D. Coucouvanis / Polyhedron 26 (2007) 4765–4778
4773
Fig. 5. Ortep diagram of (PPr3)2Ni2Fe6S6(NO)6 showing ellipsoids with
50% probability. The carbons of the phosphines have been omitted for
clarity.
X3(PPh3)4 (X@ClÀ [23], BrÀ [40]) but not in mixed metal
systems. The (PPr3)3Ni3Fe4S6(NO)4 is the only byproduct
isolated from a mixture that appears to contain three
compounds. The separation of these species is relatively
difficult as their solubilities are relatively similar, with
(PPr3)3Ni3Fe4S6(NO)4 being less soluble in ether and hex-
anes than the other two species. Unfortunately we ware
not able to obtain any crystals from the other two byprod-
ucts as they seem relatively unstable. Nevertheless these
two species also contain phosphine and nitrosyls as shown
by infrared spectroscopy, with the main N–O stretching
peak at 1696 cmÀ1 for (PPr3)3Ni3Fe4S6(NO)4 (THF frac-
tion), 1739 cmÀ1 (Ether fraction) and 1711 cmÀ1 (Hexanes
Fraction). The IR spectra of the Ether fraction is very
similar to the pattern of (PPr3)4Cu4Fe4(NO)4 and since
the [Ni4Fe4S4] core is known for (PPr3)4Ni4Fe4S4Cl4 [19]
Fig. 6. Ortep diagram of (PPr3)3Ni3Fe4S6(NO)4 (side and top view)
showing ellipsoids with 50% probability. The carbons of the phosphines
have been omitted for clarity.
and (PPh Me)4Ni4Fe4S4I4 [20], we expect the nitrosyl clus-
2
ter to have a similar structure.
3.3. Structures
the Mo atoms (Fig. 2). The cluster topology is identical
to the MoFe/S/Cl analogues (Et4N)3[(CO)6Mo2Fe6S6Cl6]
and (Et4N)4[(CO)6Mo2Fe6S6Cl6] [35].
Both of the Cu/Fe/S/NO structures exhibit the typical
cubic arrangement of metals capped by sulfur atoms and
have structures similar to those of their Cu/Fe/S/Cl
analogues (Bu4N)2[(PPr3)2Cu2Fe6S6Cl6] and (PPr3)4Cu4-
Fe4S6Cl6 [19]. In the case of (PPr3)2Cu2Fe6S6(NO)6
(Fig. 3) the average Cu–Fe and Fe–Fe distances are
Crystallographic determination of (PPN)2[Fe4S4(NO)6]
reveals a Fe4S4 core that can be described as the fusion
of two [(NO)2Fe(l–S)2Fe(NO)]À dimers to give a nido
[Fe4S4(NO)6]2À cluster (Fig. 1) with an inter-dimer Fe–Fe
˚
distance of 2.77 A and an average intra-dimer Fe–Fe dis-
˚
tance of 2.68 A. The Fe–NO distances are almost equiva-
˚
˚
lent at 1.66 A (1.65–1.67 A) with average Fe–N–O bond
angles of 167ꢀ within a range of 163ꢀ–171ꢀ.
˚
˚
2.70 A and 2.65 A and the Cu–S and Fe–S bonds are found
In the (PPN)2[(CO)6Mo2Fe6S6(NO)6] cluster, the M8S6
cuboctahedral core exhibits average Mo–Fe and Fe–Fe dis-
˚
˚
at 2.38 A and 2.24 A. In (PPr3)4Cu4Fe4S6(NO)4 (Fig. 4) the
metals in the Cu4Fe4 core are arranged in D2h geometry
(mutually perpendicular Fe4 and Cu4 squares) with average
metal–metal distances of 2.74 A and average Cu–S and Fe–
S bond distances of 2.36 A and 2.26 A respectively.
˚
˚
tances are at 2.89 A and 2.69 A, respectively, and Mo–S
˚
˚
and Fe–S distances at 2.58 A and 2.25 A, respectively. As
expected, by comparison to [Fe8S6(NO)8]2À, the Mo2Fe6
cluster is elongated along the three-fold axis that contains
˚
˚
˚

4774
H. Kalyvas, D. Coucouvanis / Polyhedron 26 (2007) 4765–4778
˚
The neutral (PPr3)2Ni2Fe6S6(NO)6 (Fig. 5) possesses the
same core structure as the copper analogue, but is slightly
more compact with average Ni–S and Fe–S distances of
2.74 A from the capping iron. The Ni–Fe distances and
˚
Ni–Ni distances are at found at 2.61 and 2.79 A, respec-
˚
tively, and the Fe–S and Ni–S distances are 2.23 A and
˚
˚
˚
2.25 A and 2.26A, respectively, and Ni–Fe and Fe–Fe dis-
2.25 A, respectively.
˚
˚
tances at 2.59A and 2.65A, respectively. A similar Ni2Fe6
core is also found in the Ni/Fe/S/halide analogues,
(Et4N)2[(PPr3)2Ni2Fe6S6Cl6] [19] and [PhCH2NEt3]2[(Ph2-
MeP)2Ni2Fe6S6I6] [20].
As shown in Table 2, the M/Fe/S (M@Mo,Cu,Ni) clusters
reported in this paper have shorter metal–metal and iron–
sulfur distances in a more compact overall structure than
their corresponding analogues with halogens. An exception
is the (PPr3)3Ni3Fe4S6(NO)4 cluster which is of similar size
to the structurally similar [Fe7S6] and [Co7S6] clusters. In
the M7S6 clusters the ‘‘uncapped’’ M3S3 unit shows a trian-
The (PPr3)3Ni3Fe4S6(NO)4 (Fig. 6) cluster can be viewed
as a Fe4S3(NO)4 unit coupled with a nearly trigonal planar
Ni3S3(PPr3)3 unit. In this structure the Iron atoms show a
tetrahedral coordination geometry while the Nickel atoms
show a trigonal pyramidal geometry. The Fe–Fe distances
in the equatorial triangle (defined by Fe1, Fe2 and Fe3)
˚
gular M3 array with distances nearly 1 A shorter than the dis-
tances found in the hexagonal faces of the M6S6 prismanes.
At present we are exploring the possible function of the
‘‘uncapped’’ unit as a ligand to other metal atoms.
˚
have a mean value of 3.82 A. These irons are located
Table 2
Comparison of selected bond distances between different M/Fe/S clusters with the range of distances given in parenthesis along with esd’s
˚
Selected bond distances (in A)
[(CO)6Mo2Fe6S6(NO)6]2À
[(CO)6Mo2Fe6S6Cl6]3À
[(CO)6Mo2Fe6S6Cl6]4À
Mo–Fe
Fe–Fe
Octahedron Volume
Mo–S
Fe–S
2.893(2.884(6)–2.906(6))
2.672(2.669(6)–2.674(6))
21.535
2.580(2.576(8)–2.674(8))
2.247(2.230(8)–2.279(8))
2.930(2)
2.742(3)
22.810
2.582(3)
3.005(2.980(1)–3.031(2))
2.761(2.740(2)–2.781(2))
23.983
2.619(2.611(2)–2.625(2))
2.353 (2.279(2)–2.336(3))
2.292(2.280(4)–2.312(4))
(PPr3)2Cu2Fe6S6(NO)6
[(PEt3)2Cu2Fe6S6Cl6]2À
Cu–Fe
Fe–Fe
M8 volume
Cu–S
2.699(3)
2.647(3)
19.098
2.381(3)
2.784(2.756(10)–2.811(10))
2.772(2.750(11)–2.791(10))
21.435
2.385(2.384(15)–2.392(15))
2.310(2.297(16)–2.322(16))
Fe–S
2.240(2.230(3)–2.247(3))
(PPr3)4Cu4Fe4S6(NO)4
(PPr3)4Cu4Fe4S6Cl6
Cu–Cu
Cu–Fe
Fe–Fe
2.747(16)
2.747(2.765(17)–2.739(19))
2.710(19)
2.733(6)
2.787(2.765(6)–2.823(6))
2.784(6)
M8 volume
Cu–S
Fe–S
20.593
21.431
2.366(2.351(8)–2.380(8))
2.269(2.265(8)–2.275(8))
2.236(2.341(29)–2.376(23))
2.264(2.251(29)–2.281(27))
(PPr3)2Ni2Fe6S6(NO)6
[(PPr3)2Ni2Fe6S6Cl6]2À
[(Ph2MeP)2Ni2Fe6S6I6]2À
Ni–Fe
Fe–Fe
M8 volume
Ni–S
2.586(3)
2.649(3)
17.933
2.250(3)
2.615(2.613(15)–2.617(15))
2.752(2.747(18)–2.759(19))
19.328
2.255(2.245(25)–2.267(25))
2.296(2.297(26)–2.309(26))
2.645(2.624(2)–2.658(2))
2.719(2.707(2)–2.726(2))
19.281
2.261(2.257(1)–2.265(1))
2.289 (2.277(2)–2.300(2))
Fe–S
2.257(2.236(3)–2.271(3))
(PPr3)3Ni3Fe4S6(NO)4
(PEt3)4Fe7S6Cl3
Co7S6Br3(PPh3)4
Ni–Nia
2.790(4)
2.746(3)
2.883(4)
Ni–Fe
2.618(2.605(4)–2.630(4))
3.823(4)
2.744(4)
2.144(2.140(5)–2.148(5))
2.470(5)
2.624(2.622(2)–2.626(2))
4.148(2)
2.584(2)
2.179(3)
2.231(3)
2590 (2.583(3)–2.596(4))
3.955(4)
2.607(3)
2.139 (2.129(6)–2.149(5))
2.197(6)
Fe–Feb(planar)
Fe–Fe(capping)
Ni–S(equatorial)
Ni–S(axial)
Fe–S(capping)
Fe–S(equatorial)
Fe–S(axial)
2.223(5)
2.242(5)
2.204(5)
2.193(2)
2.363(2.361(3)–2.366(3))
2.276(3)
2.168(4)
2.254 (2.249(5)–2.258(5))
2.184(5)
The volume of the metal formed cuboidal unit has been calculated approximating it as a cube, using average metal–metal distances.
a
The distances of interest are denoted as shown in Fig. 6 using the equivalent metal positions for Fe7S6 and Co7S6 since in the case of Ni3Fe4S6 they are
more easily defined and recognized.
b
The Fe–Fe plane is the trigonal plane defined by Fe1–Fe2–Fe3, and the Fe–Fe(capping) is the distance of Fe4 to the trigonal Fe plane. Fe–S (capping)
is the Fe4–S3 distance, Fe–S (equatorial) is the Fe3–S3 distance and Fe–S (axial) is the Fe3–S4 distance.

H. Kalyvas, D. Coucouvanis / Polyhedron 26 (2007) 4765–4778
4775
3.4. The NO ligands
In metal nitrosyl complexes it is difficult to determine
precise oxidation states for the metal atoms since NO could
be described as NO+, NO or NOÀ. Thus, in [Fe4S3(NO)7]À
two descriptions have been proposed for the cluster based
on the assignment of the NO ligand state. These are:
4Fe3+/NOÀ [41] and 3FeÀ1,Fe+/NO+ [42].
Spectroscopic and structural data for the Fe/S/NO and
M/Fe/S/NO clusters (Table 3) show the N–O stretching
vibrations in the range from 1652 to 1766 cmÀ1, the M–
N–O angle in the range from 171.8ꢀ to 180.0ꢀ, the Fe–N
˚
˚
bond in the range from 1.67 A to 1.71 A and the N–O bond
˚
˚
between 1.14 A and 1.18 A. Within 3r (at times as high as
.01–.02 A) the N–O and Fe–NO distances are nearly indis-
˚
tinguishable. The Fe–NO bond is short enough consistent
with a considerable amount of multiple bond character.
The high standard deviations often associated with the N–
O bonds make the bond order difficult to evaluate. The
M–N–O angles are all near 180ꢀ which is expected for
NO+ coordination as opposed to 120ꢀ for NOÀ taken in
extreme cases, but on the basis of the IR data the NO
ligands may be even considered as NOÀ given that the typ-
ical range for lineal NO is 1450–1950 cmÀ1 and 1400–
1720 cmÀ1 for bent NO [10]. Unfortunately there is great
overlap in ranges, and given that in
a series of
[M(NO)(CN)5]nÀ complexes (M@V, Cr, Mn, Fe and
n = 2, 3) the position of mNO depends upon the cation pres-
ent and variations up to 100 cmÀ1 have been observed for a
given anion [43,44]. This makes the mNÀO vibration unreli-
able for determining NO oxidation. As described by Ene-
mark and Feltham [45] the transition metal-nitrosyl
systems are better described as highly covalent entities using
a {M–NO}n notation with n the sum of metal d and NO p*
electrons. Any reference to change of oxidation states of the
clusters in this article is in accordance with this notation.
An examination of the structural parameters in the
{[(R3P)M]xFe8Àx(L)8ÀxS6}nÀ clusters for L = NO vs ClÀ
(Table 2) shows that the NO clusters are characterized by sig-
nificantly shorter Fe–M, Fe–Fe and Fe–S bond lengths. The
data are consistent with the expected p back bonding from
the Fe atoms into the NO p* orbital which in turn results
in electron deficient Fe atoms, significant M–Fe and Fe–Fe
bonding and Fe–S multiple bonding. At present 57Fe Moess-
bauer spectroscopy appears to be needed for an assignment
of oxidation states to the Fe metals and the NO ligands, by
a comparison of the Moessbauer spectra of the NO clusters
to those of the corresponding chloro clusters. The Moess-
bauer spectra of the Cu2Fe6, Cu4Fe4 and Ni2Fe6 clusters
(L = ClÀ), with isomer shifts of 0.345 mm/s, 0.341 mm/s
and 0.355 mm/s, respectively, are consistent with an Fe oxi-
dation state around +3 and Cu+1, Niꢀ or Ni+1
.
3.5. Electrochemistry
Electrochemical studies of the clusters reported here,
show mainly reversible, multielectron reductions (Table 4).

4776
H. Kalyvas, D. Coucouvanis / Polyhedron 26 (2007) 4765–4778
Table 4
Reduction potentials of the four M2 Fe6S6 clusters and comparison with their Fe/S/Cl equivalents
Compound
0/À1 couple
E1/2 (mV)
À169
À1/À2 couple
À2/À3 couple
À3/À4 couple
DE
ipc/ipa
E1/2 (mV)
DE
ipc/ipa
E1/2 (mV)
DE
ipc/ipa
E1/2 (mV)
DE
ipc/ipa
(PPr3)2Cu2Fe6S6(NO)6
(Bu4N)2[(PPr3)2Cu2Fe6S6Cl6]
(PPr3)4Cu4Fe4S6(NO)4
(PPr3)4Cu4Fe4S6Cl6
(PPr3)2Ni2Fe6S6(NO)6
(Et4N)2[(PPr3)2Ni2Fe6S6Cl6]
(PPN)2[(CO)6Mo2Fe6S6(NO)6]
(Et4N)3[(CO)6Mo2Fe6S6Cl6]
152
0.84
À1035
148
0.99
À1667
À228
152
26
116
1.02
À555
120
À153
À17
À1
172
124
168
0.39
1.10
À1021
À635
À717
156
130
156
0.57
0.61
À1537
0.73
0.71
0.70
À2065
irr
À1431
À585
À641
334
156
192
À1247
176
92
0.68
1.00
+50
All reductions are reversible unless denoted otherwise.
The (PPr3)2Cu2Fe6S6(NO)6 cluster shows three revers-
ible reduction waves (Fig. 7) at À169 mV, À1035 mV,
À1667 mV. Following a cyclic scan from 1.8 V to À2 V,
the voltammetry shows the appearance of an additional
reduction wave at about À633 mV, which suggest that
the small peak that appears in that region in the 0 to
À2.0 V scan is due to partial oxidation of the cluster
during the experiment rather than to the presence of an
impurity.
In the case of (PPr3)4Cu4Fe4S6(NO)4, although the CV
appears to be very similar to (PPr3)2Cu2Fe6S6(NO)6 as
the first two reductions coincide, (Fig. 8) at high potentials
there are different peaks. In addition when two consecutive
full scans from À2 V to 1.8 V are performed, there is no
appearance of the peak at À0.5 V indicating that it is not
a mixture of two species.
For (PPr3)2Ni2Fe6S6(NO)6 the cyclic voltammogram
(Fig. 9) shows only two reductions at À717 mV and
À1431 mV but there is also a reversible wave at À1 mV
(not shown), and overall the voltammogram of (PPr3)2Ni2-
Fe6S6(NO)6 is similar to that of (PPr3)2Cu2Fe6S6(NO)6
shifted by approx 0.2 V towards more positive potentials.
Again after a full range scan there is an emergence of a
new reversible wave at À235 V which is also evident in
the initial scan but with smaller intensity suggesting that
part of the sample had been oxidized during the experimen-
tal process.
Fig. 7. Cyclic voltammogram of (PPr3)2Cu2Fe6(NO)6.
Contamination of the (PPr3)3Ni3Fe4S6(NO)4 cluster
with the other unidentified Ni/Fe/S species, did not allow
for conclusive electrochemical measurements of this
cluster.
For (PPN)2[(CO)6Mo2Fe6S6(NO)6], there are two reduc-
tion waves observed at À641 mV and À1247 mV (Fig. 10).
In addition there is no significant effect of oxidation as
there are no new peaks appearing but the two reductions
change in intensity after a full range scan.
As shown in Table 4, by comparison to the correspond-
ing M/Fe/S/Cl species and equivalent couples, the clusters
presented herein are more difficult to reduce which is rea-
sonable given that these clusters appear to be more electron
rich than their corresponding halogen analogues due to the
more covalent nature of the Fe–NO bond that eventually
Fig. 8. Cyclic voltammogram of (PPr3)4Cu4Fe4S6(NO)6 in comparison
with (PPr3)2Cu2Fe6S6(NO)6.

H. Kalyvas, D. Coucouvanis / Polyhedron 26 (2007) 4765–4778
4777
ucts obtained through various reactions seem to be the
four basic and thermodynamically stable core topologies
of Fe4S3, Fe4S4 Fe6S6 and Fe8S6, however the isolation of
the unique (PPN)2[Fe4S4(NO)6], ‘‘open-cubane’’ and the
(PPr3)3Ni3Fe4S6(NO)4 clusters provides additional evi-
dence that it is possible to obtain new core topologies by
varying reaction conditions and stabilizing inherently
metastable structures.
Acknowledgements
We thank Dr. Jeff Kampf for X-ray data collection and
Carl Henderson for the Electron Microbeam Analysis. The
authors acknowledge the support of this work by a grant
from the National Institutes of Health (GM 33080).
Appendix A. Supplementary material
Fig. 9. Cyclyc voltammogram of (PPr3)2Ni2Fe6(NO)6.
CCDC 643468, 643469, 643470, 643471, 643472 and
643473 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Supplementary
data associated with this article can be found, in the online
version, at doi:10.1016/j.poly.2007.06.015.
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