Synthesis and X-ray structure of the [{Fe3(CO)9( 3-O)}2H]3- trianion: Dimerization of a metal carbonyl cluster via formation of an exceptionally short hydrogen bond

Article abstract of DOI:10.1039/b703628k

The synthesis, structure and characterization of the [{Fe ₃(CO)₉(₃-O)}₂H]^3- trianion in its [Cs(THF)_0.33]⁺ and [NEt₄] ⁺ salt are reported. The title dimeric cluster has been obtained by protonation in water or in organic solvent of the [Fe₃(CO) ₉(₃-O)]^2- dianion to the hydroxo [Fe ₃(CO)₉(₃-OH)]^- derivative and crystallization. The solid state structure of [Cs(THF)_0.33] ₃[{Fe₃(CO)₉(₃-O)}₂H] is based on ionic packing of [Cs(THF)_0.33]⁺ cations and [{Fe₃(CO)₉(₃-O)}₂H]^3- trianions. The fractional formula is due to the particular packing of Cs ⁺ cations, which are at the vertices of fused cuboctahedral and trigonal antiprismatic polyhedrons. Each cuboctahedron encapsulates a [{Fe ₃(CO)₉(₃-O)}₂H]^3- trianion, whereas each trigonal antiprism encapsulates a THF molecule. The possibility that the structure of the [{Fe₃(CO)₉( ₃-O)}₂H]^3- trianion could be affected by its confinement in the cuboctahedral cage of Cs⁺ ions and the heavy disorder of the THF molecule urged a further structural determination of the trianion with a completely different cation. The corresponding [NEt ₄]₃[{Fe₃(CO)₉(₃-O)} ₂H] salt has been, therefore, prepared and structurally characterized. The [{Fe₃(CO)₉(₃-O)} ₂H]^3- trianion displays an identical structure and almost coincident molecular parameters in both salts. Its most notable feature is represented by the unique hydrogen atom symmetrically bridging the ₃-O atoms of two different [Fe₃(CO)₉( ₃-O)]^2- molecules and displaying one of shortest O...H...O interaction so far reported in organic, inorganic and organometallic literature. The structure of [Cs(THF)]₂[Fe ₄(CO)₁₃], which has been obtained as a by-product of the synthesis of [Cs(THF)_0.33]₃[{Fe₃(CO) ₉(₃-O)}₂H], is also briefly reported. The Royal Society of Chemistry.

Full text of DOI:10.1039/b703628k

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Synthesis and X-ray structure of the [{Fe3(CO)9(l3-O)}2H]3− trianion:
dimerization of a metal carbonyl cluster via formation of an exceptionally
short hydrogen bond†
Cristina Femoni, Maria Carmela Iapalucci, Giuliano Longoni,* Stefano Zacchini and Enrico Zazzaroni
Received 9th March 2007, Accepted 2nd May 2007
First published as an Advance Article on the web 23rd May 2007
DOI: 10.1039/b703628k
The synthesis, structure and characterization of the [{Fe3(CO)9(l3-O)}2H]3− trianion in its
[Cs(THF)0.33]+ and [NEt4]+ salt are reported. The title dimeric cluster has been obtained by protonation
in water or in organic solvent of the [Fe3(CO)9(l3-O)]2− dianion to the hydroxo [Fe3(CO)9(l3-OH)]−
derivative and crystallization. The solid state structure of [Cs(THF)0.33]3[{Fe3(CO)9(l3-O)}2H] is based
on ionic packing of [Cs(THF)0.33]+ cations and [{Fe3(CO)9(l3-O)}2H]3− trianions. The fractional
formula is due to the particular packing of Cs+ cations, which are at the vertices of fused cuboctahedral
and trigonal antiprismatic polyhedrons. Each cuboctahedron encapsulates a [{Fe3(CO)9(l3-O)}2H]3−
trianion, whereas each trigonal antiprism encapsulates a THF molecule. The possibility that the
structure of the [{Fe3(CO)9(l3-O)}2H]3− trianion could be affected by its confinement in the
cuboctahedral cage of Cs+ ions and the heavy disorder of the THF molecule urged a further structural
determination of the trianion with a completely different cation. The corresponding
[NEt4]3[{Fe3(CO)9(l3-O)}2H] salt has been, therefore, prepared and structurally characterized. The
[{Fe3(CO)9(l3-O)}2H]3− trianion displays an identical structure and almost coincident molecular
parameters in both salts. Its most notable feature is represented by the unique hydrogen atom
symmetrically bridging the l3-O atoms of two different [Fe3(CO)9(l3-O)]2− molecules and displaying
one of shortest O · · · H · · · O interaction so far reported in organic, inorganic and organometallic
literature. The structure of [Cs(THF)]2[Fe4(CO)13], which has been obtained as a by-product of the
synthesis of [Cs(THF)0.33]3[{Fe3(CO)9(l3-O)}2H], is also briefly reported.
through
a
[Fe3(CO)9{l3-O–Au(PPh3)}]− intermediate,10 and
Introduction
Fe3(CO)9(l3-O){l3-Au(PPh3)}{l-Au(PPh3)}.12 Unfortunately all
attempts made at that time to isolate and structurally character-
ize the purported [Fe3(CO)9(l3-OH)]−, as well as [Fe3(CO)9(l3-
OR)]− (R = Me, SiMe3, r-allyl) species, have been unsuccessful.
Systematically, only crystals of the [Fe3(CO)9(l3-O)]2− starting
material were isolated, owing to back shift of equilibrium (1)
during crystallization.
Several organometallic oxides and hydroxides of transition metal
are known.1–3 Most of them are derivatives of early transition
metals, though notable examples are also known for late transition
metals such as group 8–10 elements in low oxidation state. Focus-
ing the attention on late transition metals containing carbonyl
groups as ancillary ligands, stable species are mainly found in
group 8. The species so far structurally characterized, we are aware
of, are collected in Table 1.
Several years ago, we reported the synthesis and structure of the
[Fe3(CO)9(l3-O)]2− dianion, which was obtained by deliberate con-
tact with dry air of solutions of [Fe3(CO)11]2−.5 Preliminary studies
indicated that its reactivity was significantly different from that of
the [Fe3(CO)9(l3-S)]2− congener.29–31 As an example, protonation
of [Fe3(CO)9(l3-S)]2− sequentially leads to [Fe3(CO)9(l3-S)(l-H)]−
and Fe3(CO)9(l3-S)(l-H)2 hydride derivatives,26–28 and reaction
with an Au(PPh3)+ moiety (isolobal with H+) accordingly gives rise
to [Fe3(CO)9(l3-S){l-Au(PPh3)}]−.31 In contrast, spectroscopic
evidence suggested that protonation of [Fe3(CO)9(l3-O)]2− was oc-
curring onto the oxygen atom,5 whereas reaction with Au(PPh3)+
was shown to give [Fe3(CO)6(l-CO)3(l3-O){l3-Au(PPh3)}]−,
[Fe3(CO)9(l3-O)]2− + RX ꢀ [Fe3(CO)9(l3-OR)]− + X−
(R = H, X = BF4; R = Me, SiMe3; X = Cl, Br, I)
(1)
However, a [Fe3(CO)9(l3-O–CMe3)]− species has been isolated
and structurally characterized some years later.11 The occurrence
in solution of equilibrium (1) points out the reluctance of oxygen to
share all its valence electrons in low-valent carbonyl compounds,
which is also in keeping with the non-existence of metal carbonyl
clusters containing oxygen as an interstitial atom and the reported
failures in encapsulating the O atom of [Fe3(CO)9(l3-O)]2− even
in bimetallic clusters containing more oxophilic metals. These
latter attempts only led to isolation of [Fe3Rh3(CO)15(l3-O)],12
[Fe3Au2(CO)9(l3-O)(PPh3)2],12 [Fe3M(CO)12(l4-O)]− (M = Mn,
Re)13 and [Fe2Ru3 (CO)14(l4-O)]2−14 clusters displaying exposed
either l3- or l4-O atoms.
Dipartimento di Chimica Fisica e Inorganica, Universita` di Bologna, Viale
Risorgimento 4, 40136, Bologna, Italy. E-mail: longoni@ms.fci.unibo.it;
Fax: +39 051 209 3690; Tel: +39 051 209 3711
† CCDC reference numbers 639851–639853. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b703628k
Our interest in water-soluble metal carbonyl clusters and the
paucity of hydroxo-derivatives of iron led us to re-investigate
the behaviour of [Fe3(CO)9(l3-O)]2− in water. As a first result, we
report the synthesis and structural characterization of the
2644 | Dalton Trans., 2007, 2644–2651
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reported.38 The resulting cloudy solution was filtered and a slow
stream of dry air was directly bubbled through the solution. The
solvent was, then, removed in vacuum yielding an oily residue
of the alkali salts of [Fe3(CO)9(l3-O)]2−. These are soluble in
water, THF, acetone and acetonitrile. Drop-wise addition of a
20% aqueous solution of H2SO4 to their water solutions led to
separation of a mixture of some Fe3(CO)12, [M(H2O)x][HFe3-
(CO)11] and [M(H2O)x][Fe3(CO)9(l3-OH)] and greater amounts
of [M(H2O)x][HFe4(CO)13], as a dark microcrystalline precipitate.
It seems reasonable to speculate that [HFe3(CO)11]− and [HFe4-
(CO)13]− result from further protonation of [Fe3(CO)9(l3-OH)]−,
followed by elimination of water and generation of a {Fe3(CO)9}
species (any eventually coordinated solvent is omitted), according
to eqn (2). The carbonyl-unsaturated {Fe3(CO)9} species readily
disproportionates according to reactions (3) and (4) to tri- and
tetra-nuclear dianions, which are readily protonated to their
corresponding mono-hydride derivatives in the acid conditions of
the reaction. A somehow related disproportionation of Fe2(CO)9
in THF has been reported to give Fe(THF)4[HFe3(CO)11]2.39
Table 1 Structurally characterized oxides and hydroxides of group 8
transition metals
Compound
Ref.
[Fe3(CO)9(l3-O)]2−
Ru3(CO)6(dpAsm)3(l3-O)
Ru3(CO)4(dppm)3(l3-O)
Os4(CO)12(l3-O)4
4
5
6
7
Os6(CO)19(l3-O)
8
9
2
Os6(CO)16(l-H)(l3-O)(C5H5N)(l-g -NC5H4)
[Fe3(CO)9(l3-O)(l3-AuPPh3)]−
[Fe3(CO)9(l3-O–CMe3)]−
10
11
11
12
12
13
13
14
15
16
6
17
18
19
20
21
22
23
24
25
26
27
21
28
Fe3(CO)9(l3-O–CMe3)(l-AuPPh3)
Fe3(CO)9(l3-O)(l3-AuPPh3)(l-AuPPh3)
[Fe3Rh3(CO)12(l3-O)]−
[Fe3Mn(CO)12(l4-O)]−
[Fe3Re(CO)12(l4-O)]−
[Fe2Ru3(CO)14(l4-O)]2−
Fe2(CO)6(l-OH)(l-P(p-Tol)2)
Fe2(CO)6(l-OH)(l-PH(CH(SiMe3)2)
[Ru3(CO)4(dppm)3(l3-OH)]+
Ru4(CO)8(l3-OH)2(COEt)4(CF3SO3)2
Ru4(CO)10(l3-OH)(CCH-iPr)(l-PPh2)
Ru6Cl6(CO)12(COEt)4(l3-OH)2
Ru6(CO)18(l-OH)2(l4-S)
[Fe3(CO)9(l3-OH)]− + H+ ꢀ {HFe3(CO)9(l3-OH)}
→ {Fe3(CO)9} + H2O
4{Fe3(CO)9} → 3Fe2+ + 3[Fe3(CO)11]2− + 3CO
5{Fe3(CO)9} → 3Fe2+ + 3[Fe4(CO)13]2− + 6CO
(2)
(3)
(4)
Os3(CO)8(NMe3)(l-OH)(l-H)(l3-S)
Os3(CO)9(L)(l-OH)(l-H)
Os3(CO)9(l3-C2CPh2)(l-OH)(l-H)
Os3(CO)10(l-OH)(l-NCMe2)
Os4(CO)12(l-OH)H3
[Os4(CO)12(l-OH)H4]+
Os6(CO)18(l4-O)(l-OH)2
Os6(CO)18(l-OH)(l-H)(l4-S)(l3-S)
FeRu2(CO)8(PPh3)2(l-OH)2
Fe3(CO)12 likely derives from concomitant reaction of
{Fe3(CO)9} with the CO set free by reactions (3) and (4).
Alternately, it might result from protonation of [HFe3(CO)11]−
and decomposition of the resulting HFe3(CO)10(CO–H).40
A similar sequence of reactions also occurs in organic solvents
such as MeOH, and THF. IR and 1H NMR monitoring of
the reaction failed to give any spectroscopic evidence of the
purported {HFe3(CO)9(l3-OH)} and {Fe3(CO)9} intermediate
species, probably being too unstable to build up to a detectable
concentration.
[Cs(THF)0.33]3[{Fe3(CO)9(l3-O)}2(l-H)] and [NEt4]3[{Fe3(CO)9-
(l3-O)}2(l-H)] salts, which both contain the notable dimeric
[{Fe3(CO)9(l3-O)}2(l-H)]3− trianion, in which two [Fe3(CO)9(l3-
O)]2− moieties are held together by a linear and symmetric O–H–O
hydrogen bond, ranging among the shortest interactions reported
for organic, inorganic and organometallic compounds.
It was speculated that the above failed attempt to isolate
[Fe3(CO)9(l3-OH)]− from protonation of water solutions of
[Fe3(CO)9(l3-O)]2− might be due to a relatively high solubility
of the Na+ and K+ salts of [Fe3(CO)9(l3-OH)]−, which was not
enabling stabilization by precipitation. Therefore, the reaction
was repeated using a water solution saturated with CsCl. In
these conditions, a mixture of species in their Cs+ salts readily
separates out as a microcrystalline precipitate already from
the beginning of the progressive addition of a water solution
of H2SO4 (20% in volume). The [Fe3(CO)9(l3-OH)]− hydroxo-
derivative is by far the most abundant product. The precipitate
is first extracted with toluene to eliminate trace amounts of
Fe3(CO)12 and then with dichloromethane to eliminate some
[HFe3(CO)11]− and a yet uncharacterized red-violet species. Sub-
sequent extraction in THF yields a red solution which shows
major infrared carbonyl absorptions at 2000(m) and 1960(s) cm−1,
respectively due to [HFe4(CO)13]− and [Fe3(CO)9(l3-OH)]−. Lay-
ering of n-hexane gives rise to separation of few black crystals
of [Cs(THF)]2[Fe4(CO)13] and well shaped orange-red crystals
of [Cs(THF)0.33]3[{Fe3(CO)9(l3-O)}2H]. Isolation of the latter
trianion is prompted by the occurrence in solution of equilibria
(5) and (6), and favoured by the low solubility of its cesium salt in
THF.
Results and discussion
1. Reinvestigation of the chemical behaviour of [Fe3(CO)9-
(l3-O)]2− in water : synthesis and characterization of the
[{Fe3(CO)9(l3-O)}2H]3− trianion
Tetrasubstituted ammonium salts of the [Fe3(CO)9(l3-O)]2− dian-
ion have originally been prepared by reaction of the corresponding
[Fe3(CO)11]2− salts with dry air in THF,5 and are insoluble in water.
The alkali salts of several anionic metal carbonyl clusters, e.g.
[Co6(CO)15]2−32 and [Ni12(CO)21]4−,33 are fairly soluble in water.
In general, the solubility in water of alkali salts of anionic metal
carbonyl clusters increases upon increasing the charge of the anion
and is tuneable as a function of the size and concentration of the
alkali cations. Differential solubility due to the above factors has
been systematically exploited by Chini and Martinengo for their
successful separation of complex mixtures of anionic carbonyl
rhodium clusters, obtained by reduction of Rh4(CO)12 with sodium
hydroxide in alcohol.34,35
In order to obtain the [Fe3(CO)9(l3-O)]2− dianion as alkali
salt, the Na+ and K+ salts of the [Fe3(CO)11]2− dianion have
been synthesized by deprotonation of [NEt3H][HFe3(CO)11]36,37
in methanol with the corresponding hydroxide, as previously
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[Fe3(CO)9(l3-OH)]− ꢀ [Fe3(CO)9(l3-O)]2− + H+
(5)
(6)
[Fe3(CO)9(l3-O)]2− + [Fe3(CO)9(l3-OH)]−
ꢀ [{Fe3(CO)9(l3-O)}2H]3−
The crystals of [Cs(THF)0.33]3[{Fe3(CO)9(l3-O)}2H] have been
mechanically separated from those of [Cs(THF)]2[Fe4(CO)13], on
the basis of their different colour and morphology and both salts
have been characterized by X-ray crystal diffraction. The overall
yield based on starting [NEt3H][HFe3(CO)11] is about 20%. The
heavy disorder of the THF molecule solvating the cesium cation,
though completely unravelled, and the particular packing of the
ions urged a further structure determination on a different ordered
salt to improve reliability of individual molecular parameters and
dismiss the possibility that the presence of [{Fe3(CO)9(l3-O)}2H]3−
ions could be an artefact of the confinement of the cluster in a
cuboctahedral cage of Cs+ ions (see next section).
Tetrasubstituted ammonium and phosphonium salts of
[Fe3(CO)9(l3-OH)]− are readily available by protonation in
methanol with one equivalent of diluted H2SO4 of the corre-
sponding [Fe3(CO)9(l3-O)]2− salts, via the reverse reaction of
equilibrium (5), and precipitation with water. As in the past,
a series of attempted crystallizations in miscellaneous solvents
(acetone–isopropyl alcohol, acetonitrile–di-isopropyl ether, THF–
toluene) of [Fe3(CO)9(l3-OH)]−, in its [NMe4]+, [NMe3CH2Ph]+
and [PPh4]+ salt, only gave rise to separation of crystals of
the starting [Fe3(CO)9(l3-O)]2− dianion. Only in the case of
[NEt4][Fe3(CO)9(l3-OH)], crystallization from a THF–toluene
mixture yielded a mixture of crystals, predominantly constituted
by crystals of [NEt4]2[Fe3(CO)9(l3-O)], accompanied by few crys-
tals of [NEt4]3[{Fe3(CO)9(l3-O)}2H]. Though their amount was
rather meagre (probably ca. 5–10%, as inferred by IR monitoring
of a nujol mull of the mixture), they could be identified and
mechanically separated on the basis of their different colour and
morphology.
Spectroscopic characterization in solution (IR, NMR, ESI-MS)
of the [{Fe3(CO)9(l3-O)}2H]3− trianion has been hampered by
its ready splitting in solution, according to the back reaction of
equilibrium (6). Therefore, only solid state characterization such
as nujol mull IR and X-ray single crystal studies (see next section)
could be carried out. Significantly, the nujol mull IR spectrum
of [{Fe3(CO)9(l3-O)}2H]3− shows IR carbonyl absorptions at
1951(vs), 1920(s) and 1896(m) cm−1, which are fairly close to
the baricentre of those of crude [NEt4][Fe3(CO)9(l3-OH)] (mCO at
1963(vs), 1933(s), 1910(m) cm−1) and [NEt4]2[Fe3(CO)9(l3-O)] (mCO
1931(vs), 1903(s), 1874(m) cm−1), in perfect agreement with its
intermediate charge per carbonyl groups. That points out that the
negative charge is equally delocalized over both Fe3(CO)9(l3-O)
moieties.
Fig. 1 The [{Fe3(CO)9(l3-O)}2H]3− dimer present in the solid state
structure of both [NEt4]3[{Fe3(CO)9(l3-O)}2H] and [Cs(THF)0.33]3-
[{Fe3(CO)9(l3-O)}2H] (the labeling refers to Table 2; iron and hydrogen
atoms are shown as hatched spheres).
of the related [Fe3(CO)9(l3-O–CMe3)]− monoanion is reported in
Table 2. As shown in Fig. 1, the trianion is generated by formal
condensation of a [Fe3(CO)9(l3-O)]2− dianion with a [Fe3(CO)9(l3-
OH)]− monoanion in a staggered conformation (see Fig. 2), via
formation of a symmetric and linear hydrogen bond. By far
the most notable feature is represented by the extremely short
˚
O · · · O separation [2.386(7) and 2.383(9) A] and O–H bond
+
˚
contacts [1.193(5) and 1.191(5) A, respectively, for the [NEt4]
and [Cs(THF)0.33]+ salt]. Homonuclear hydrogen bonds displayed
by organic, organometallic and inorganic compounds in the solid
state have been critically discussed and classified few years ago
by Gilli et al.41 The title trianion falls in the chemical class A,
displaying the shortest O · · · O intermolecular separations, viz., for
˚
Table 2 Comparison between the bond distances (A) in [NEt4]3-
[{Fe3(CO)9(l3-O)}2H] (A), [Cs(THF)0.33]3[{Fe3(CO)9(l3-O)}2H] (B), and
[N(PPh3)2][Fe3(CO)9(l3-O–CMe3)] (C)11
(A)
(B)
(C)a
Fe(1)–C(1)
Fe(1)–C(2)
Fe(1)–C(3)
Fe(1)–O(4)
Fe(1)–Fe(1ꢀ)c
Fe(1)–Fe(1ꢀꢀ)d
C(1)–O(1)
C(2)–O(2)
C(3)–O(3)
O(4)–O(4ꢀ)d
H(4)–O(4)
H(4)–O(4ꢀ)e
1.776(5)
1.743(5)
1.776(5)
1.896(3)
2.4976(11)
2.4976(11)
1.151(5)
1.166(6)
1.156(6)
2.386(7)
1.193(5)
1.193(5)
1.756(4)
1.768 (av)
1.788(3)
b
1.890(3)
1.940 (av)
2.482 (av)
2.4896(10)
2.4896(10)
1.152(5)
1.146 (av)
1.140(4)
b
2. Crystal structure of [Cs(THF)0.33]3[{Fe3(CO)9(l3-O)}2H] and
[NEt4]3[{Fe3(CO)9(l3-O)}2H] salts
Both title salts contain the [{Fe3(CO)9(l3-O)}2(l-H)]3− trianion
displaying almost coincident individual molecular parameters in
both cases.
2.383(9)
1.191(5)
1.191(5)
1.474 (O–CMe3)
a See ref. 11. b Due to the crystal symmetry, only two CO ligands per
Fe atom are independent. c Symmetry transformations used to generate
equivalent atoms: y, z, x for (A), −y + 1, x − y − 1, z for (B). d Symmetry
transformations used to generate equivalent atoms: z, x, y for (A), −x +
y + 2, −x + 1, z for (B). e Symmetry transformations used to generate
equivalent atoms: 0.5 − x, 0.5 − y, 0.5 − z for (A), −x + 2, −y, −z for (B).
The [{Fe3(CO)9(l3-O)}2(l-H)]3− trianion. A first view of the
trianion with the numbering scheme is given in Fig. 1. A
comparison of the individual molecular parameters with those
2646 | Dalton Trans., 2007, 2644–2651
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Fig. 2 View of the [{Fe3(CO)9(l3-O)}2H]3− dimer along the C3 axis.
Fig. 3 The packing of Cs+ ions (blackened Cs+ spheres individuate the
˚
cuboctahedron).
instance, intermolecular alcohol–alcoholate (2.39–2.43 A) adducts
˚
and inorganic acid salts (2.36–2.43 A).
The symmetric nature of the O–H–O bond might appear to
be a consequence of the crystallographic symmetry of the unit
cell of both salts. However the presence of a delocalized 3c-4e
O–H–O bond seems perfectly in keeping with the equivalence of
the two Fe3(CO)9(l3-O) moieties and the observation of perfect
delocalization of the negative charge, inferable from the solid
state infrared spectrum. Indeed, its IR carbonyl absorptions are
intermediate between those of the [Fe3(CO)9(l3-O)]2− dianion
and the [Fe3(CO)9(l3-OH)]− monoanion. Therefore, it seems
reasonable rather to suggest that the symmetric nature of the O–
H–O bond might be at the origin of the high crystallographic
symmetry.
A strength–length relationship is expected to hold for linear or
nearly linear O–H–O bonds41 and would suggest the presence in
the trianion of a relatively strong hydrogen bond. The increasing
strength of O–H–O bonds with O · · · O distances in the 2.432–
˚
2.554 A range has been experimentally substantiated by progres-
sive lowering of their IR O–H stretching frequencies and downfield
shifts of their proton resonances.43 Unfortunately, the fact that the
title trianion is not stable in polar solvents and its salts involve
the presence of tetraethylammonium cations or THF molecules
solvating the Cs+ ions, respectively, makes unsafe any conclusion
based on their IR and 1H NMR.
In agreement with this interpretation, DFT calculations on a
KH2PO4 phase, in which the H position is varied along the O–
The packing of Cs+ cations. It is worth to briefly comment
the packing of Cs+ ions, in order to clarify the formula of the
[Cs(THF)0.33]3[{Fe3(CO)9(l3-O)}2H] salt. As shown in Fig. 3, the
Cs+ ions individuate the vertices of two (non-bonded) polyhedral
arrangements: the non-centred cuboctahedron and the distorted
octahedron. Fusion of cuboctahedrons and octahedrons gives
rise to a 3D space-filling net, as predicted by Andreini.44,45 The
slightly distorted non-bonded cuboctahedron (Cs+ · · · Cs+ 6.55
˚
O axis, reveal that at the short O · · · O distance of 2.405 A there
is a single energy-minimum at the mid-point, whereas as the O–
˚
O distance is elongated to 2.469 and 2.51 A the energy curve
develops a well defined double well with minima separated by 0.34
42
˚
and 0.41 A, respectively.
The fact that the O–O distance is invariant with the cation points
out that it represents an intrinsic feature of the [{Fe3(CO)9(l3-
O)}2H]3− trianion.
and 7.25 A) encapsulates the [{Fe3(CO)9(l3-O)}2(l-H)]3− trianion
˚
(Fig. 4(a)) by interactions of all oxygen atoms of the 18 carbonyl
Fig. 4 The cuboctahedron of Cs+ ions encapsulating the [{Fe3(CO)9(l3-O)}2H]3− trianion (a), the distorted octahedron of Cs+ ions encapsulating the
unique rattling around THF molecules, disordered over 12 positions (b) and the coordination of each Cs+ ion (c) (Cs+ ions are shown as blackened
spheres, iron atoms in (a) and oxygen atoms in (b) and (c) are shown as hatched spheres).
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groups with the surrounding twelve Cs+ ions. The unique bridging
hydrogen atom is located in the centre of the cuboctahedron.
The slightly distorted non-bonded octahedron (Cs+ · · · Cs+ 6.55
originally given by Dahl46 in its seminal paper regarding the
structure of [FePy6][Fe4(CO)13], are shown in Fig. 5. The iron atoms
describe a tetrahedron, one face of which is capped by a l3-CO
group. All other CO groups are essentially linear. The individual
molecular parameters (Table 3) are very similar to those found in
the corresponding [N(PPh3)2] [Fe4(CO)12(l3-CO)] salt.
In contrast to [Cs(THF)0.33]3[{Fe3(CO)9(l3-O)}2H], the Cs+ ions
of [Cs(THF)]2[Fe4(CO)13] do not individuate a simple polyhedral
net. Indeed, the Cs+ ions can be divided in two sets: the first set
consists of a zigzag chain of Cs+ ions. A fragment of this chain is
shown in Fig. 6(a). Each Cs+ displays a coordination number of
9, achieved by interaction with a unique terminally-bonded THF
molecule and eight oxygen atoms belonging to the carbonyl groups
of three distinct [Fe4(CO)13]2− dianions. Notably, two oxygen atoms
of each [Fe4(CO)13]2− unit behave as a bridging ligands for two
consecutive pairs of Cs+ ions.
˚
and 7.25 A) encapsulates a THF molecule, which rattles around in
12 positions, probably in the attempt to saturate the coordination
of all Cs+ ions. As a result, each Cs+ ion is coordinated to
eight oxygen atoms of the carbonyl groups belonging to different
[{Fe3(CO)9(l3-O)}2(l-H)]3− cluster ions. These Cs+–O contacts
+
˚
are divided in two sets averaging 3.144 A (Cs –O contacts) and
+
+
˚
3.396 A (Cs –O–Cs bridging contacts), respectively. Finally, the
coordination sphere of each Cs+ ion displays four coordination
sites partially occupied by a disordered THF molecule [average Cs–
˚
O contact 3.725 A; the occupancy factor for each site is 0.08333]
giving an overall count of 0.33 THF molecules per Cs+ ion.
3. Crystal structure of [Cs(THF)]2[Fe4(CO)12(l3-CO)]
The second set of Cs+ ions are arranged in distinct pairs of
ions. One incomplete pair is shown in Fig. 6(b). The coordination
number of Cs+ is yet 9, owing to the presence of two bridging THF
molecules, two bridging oxygen atoms of the carbonyl groups of
two distinct [Fe4(CO)13]2− ions, and five oxygen atoms belonging
to other two yet different [Fe4(CO)13]2− units. In their turn, these
The structure of [Fe4(CO)12(l3-CO)]2− has been already deter-
mined two times in its [FePy6]2+46 and [N(PPh3)2]+ salt.47 Being
our interest mainly confined to the coordination and packing
of Cs+ ions, only a very brief description of the [Fe4(CO)13]2−
dianion is given. Two views of the latter, corresponding to those
Fig. 5 Two views of the [Fe4(CO)12(l3-CO)]2− dianion.
Fig. 6 A representation of the two different motives generated by the coordination of the two independent Cs+ ions: a fragment of the zigzag chain
(a) and the pair (b) of Cs+ ions described in the text (only the [Fe4(CO)13]2− ions interacting with one Cs+ of the pair are shown for sake of clarity).
2648 | Dalton Trans., 2007, 2644–2651
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[Fe4(CO)13]2− ions interact trough their remaining carbonyl groups
with the Cs+ ions of the first set.
The notable invariance of all bond distances of both
[{Fe3(CO)9(l3-O)}2H]3−and [Fe4(CO)13]2− in their respective Cs+–
[NEt4]+ and Cs+–[N(PPh3)2]+ salts points out that the Cs–O
interactions do not effect significantly the bonding within the
metal carbonyl clusters.
The [NEt3H][HFe3(CO)11] and [Fe3(CO)11]2− and [Fe3(CO)9(l3-
O)]2− salts have been prepared according to the literature.48,38,4
Analysis of Fe was performed by atomic absorption on a Pye-
Unicam instrument. Analyses of C, H and N were obtained with
a ThermoQuest FlashEA 1112NC instrument. IR spectra were
recorded on a Perkin Elmer SpectrumOne interferometer. The
figures were drawn with SCHAKAL99.49
Synthesis of [Cs(THF)0.33]3[{Fe3(CO)9(l3-O)}2H]
Experimental
Solid [NEt3H][HFe3(CO)11] (2.08 g, 2.60 mmol) was dissolved in
an alkaline methanol solution (30 mL) containing KOH (2.20 g,
39.3 mmol) and stirred overnight. Complete deprotonation to give
the dianion [Fe3(CO)11]2− was confirmed by IR spectroscopy. This
was completely converted into [Fe3(CO)9(l3-O)]2− by bubbling
through the solution a slow stream of dry air. An oily residue of
[K(MeOH)x]2[Fe3(CO)9(l3-O)] was, then, obtained after removing
the solvent under reduced pressure. This was dissolved in water
(20 mL). The solution was saturated with CsCl up to incipient
precipitation and diluted H2SO4 (20% in water) was drop-wise
added to the solution resulting in the immediate formation of
a dark microcrystalline precipitate. The solid was first extracted
with toluene to eliminate trace amounts of Fe3(CO)12, and
then with CH2Cl2 to eliminate some [HFe3(CO)11]− and a yet
uncharacterised red-violet species. Subsequent extraction in THF
yielded a red solution containing [HFe4(CO)13]− and [Fe3(CO)9(l3-
OH)]−. Layering of n-hexane resulted in the separation of few
black crystals of [Cs(THF)x]2[Fe4(CO)13] and well-shaped orange-
red crystals of [Cs(THF)0.33]3[{Fe3(CO)9(l3-O)}2H], which were
mechanically separated to give 0.69 g of the latter compound
All reactions and sample manipulations were carried out using
standard Schlenk techniques under nitrogen and in dried solvents.
˚
Table 3 Bond distances (A) of [Cs(THF)]2[Fe4(CO)13]
Fe(1)–Fe(3)
Fe(1)–Fe(2)
Fe(1)–Fe(4)
Fe(3)–Fe(4)
Fe(2)–Fe(4)
Fe(2)–Fe(3)
Fe(1)–C(2)
Fe(1)–C(1)
Fe(1)–C(3)
Fe(2)–C(4)
Fe(2)–C(6)
Fe(2)–C(5)
Fe(2)–C(13)
Fe(3)–C(7)
Fe(3)–C(8)
Fe(3)–C(9)
Fe(3)–C(13)
2.5958(8)
2.6008(8)
2.6079(8)
2.5404(8)
2.5431(8)
2.5596(8)
1.758(4)
1.781(4)
1.788(4)
1.764(4)
1.766(5)
1.782(4)
2.006(4)
1.756(4)
1.764(4)
1.780(4)
2.117(4)
Fe(4)–C(10)
Fe(4)–C(12)
Fe(4)–C(11)
Fe(4)–C(13)
C(1)–O(1)
C(2)–O(2)
C(3)–O(3)
C(4)–O(4)
C(5)–O(5)
C(6)–O(6)
C(7)–O(7)
C(8)–O(8)
C(9)–O(9)
C(10)–O(10)
C(11)–O(11)
C(12)–O(12)
C(13)–O(13)
1.762(4)
1.764(4)
1.774(4)
2.002(4)
1.152(5)
1.166(5)
1.147(5)
1.162(5)
1.160(5)
1.143(5)
1.169(5)
1.150(5)
1.153(5)
1.165(5)
1.150(5)
1.167(5)
1.197(5)
Table 4 Crystal data and experimental details for [NEt4]3[{Fe3(CO)9(l3-O)}2H], [Cs(THF)0.33]3[{Fe3(CO)9(l3-O)}2H] and [Cs(THF)]2[Fe4(CO)13]
Compound
[NEt4]3[{Fe3(CO)9(l3-O)}2H]
[Cs(THF)0.33]3[{Fe3(CO)9(l3-O)}2H]
[Cs(THF)]2[Fe4(CO)13]
Formula
Mr
C42H61Fe6N3O20
1263.04
295(2)
C22H9Cs3Fe6O21
1343.12
295(2)
C21H16Cs2Fe4O15
997.56
100(2)
T/K
˚
k/A
0.71073
0.71073
0.71073
Crystal system
Cubic
Ia3
22.1927(17)
22.1927(17)
22.1927(17)
90
Rhombohedral
R3m
13.0952(3)
13.0952(3)
18.5660(10)
90
90
120
2757.23(17)
3
2.427
Triclinic
P1
¯
¯
¯
Space group
˚
a/A
9.3755(11)
9.4461(11)
17.669(2)
90.984(2)
96.280(2)
101.695(2)
1521.9(3)
2
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
90
90
3
˚
Cell volume/A
Z
10931.3(15)
8
1.535
1.623
5200
0.21 × 0.18 × 0.15
1.84–25.03
−26 ≤ h ≤ 26
−26 ≤ k ≤ 26
−24 ≤ l ≤ 24
44478
1626 [Rint = 0.2068]
1626/0/110
1.012
Dc/g cm−3
l/mm−1
F(000)
2.177
4.292
952
5.312
1890
Crystal size/mm
0.19 × 0.16 × 0.11
2.84–27.98
−16 ≤ h ≤ 17
−16 ≤ k ≤ 16
−24 ≤ l ≤ 24
10703
837 [Rint = 0.0194]
837/31/64
1.108
0.0225
0.0591
0.457/−0.576
0.19 × 0.15 × 0.12
2.20–27.00
−11 ≤ h ≤ 11
−12 ≤ k ≤ 12
−22 ≤ l ≤ 22
16581
6562 [Rint = 0.0351]
6562/108/379
1.020
0.0293
0.0722
1.353/−0.954
h limits/◦
Index ranges
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness on fit on F2
R1 (I > 2r(I))
wR2 (all data)
Largest diff. peak and hole/e A
0.0505
0.1010
0.370/−0.311
−3
˚
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(yield 20% based on [NEt3H][HFe3(CO)11]). Anal. Found (Calcd.):
C 19.91 (19.67), H 0.45 (0.68), Cs 29.41 (29.69), Fe 25.09 (24.95).
3 H. W. Roesky, S. Singh, K. K. M. Yusuff, J. A. Maguire and N. S.
Hosmane, Chem. Rev., 2006, 106, 3813 and ref. therein.
4 A. Ceriotti, L. Resconi, F. Demartin, G. Longoni, M. Manassero and
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5 G. Lavigne, N. Lugan and J.-S. Bonnet, Nouv. J. Chim., 1981, 5, 42.
6 H. A. Mirza, J. J. Vittal and R. J. Puddephatt, Inorg. Chem., 1995, 34,
4239.
Synthesis of [NEt4]3[{Fe3(CO)9(l3-O)}2H]
Solid [NEt4]2[Fe3(CO)9(l3-O)] (1.72 g) was suspended in THF
(30 mL). The suspension was treated drop-wise with diluted
H2SO4 (20% in water) up to complete disappearance of the
IR absorptions of the [Fe3(CO)9(l3-O)]2− dianion. The resulting
orange-red solution (showing IR absorptions at 2025(w), 1963(vs),
1933(s), 1910(m) cm−1) was filtered and precipitated by layering
toluene (50 mL). The resulting red-orange crystalline precipitate
was mainly constituted by crystals of the [NEt4]2[Fe3(CO)9(l3-O)]
starting material. Only a few crystals exhibiting a slightly different
colour and morphology could be noticed, and have been hand
separated. These showed a nujol mull spectrum related to that of
the [{Fe3(CO)9(l3-O)}2H]3− trianion and have been kept apart for
further characterization. Anal. Found (Calcd.): C 39.67 (39.94),
H 4.78 (4.87), N 3.21 (3.33), Fe 26.35 (26.53).
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4537.
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X-Ray crystallographic study
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Crystal data and collection details for [NEt4]3[{Fe3(CO)9(l3-
O)}2H], [Cs(THF)0.33]3[{Fe3(CO)9(l3-O)}2H] and [Cs(THF)]2-
[Fe4(CO)13] are reported in Table 4. The diffraction experiments
were carried out on a Bruker APEX II diffractometer equipped
with a CCD detector using Mo-Karadiation. Data were corrected
for Lorentz polarization and absorption effects (empirical ab-
sorption correction SADABS).50 Structures were solved by direct
methods and refined by full-matrix least-squares based on all
data using F2.51 Hydrogen atoms bonded to C-atoms were fixed
at calculated positions and refined by a riding model. Oxygen
bonded hydrogen atoms were located in the Fourier map and
found to be on an inversion centre for both structures, they
were, then, refined isotropically using the 1.5 fold Uiso value of
the parent O-atom. All non-hydrogen atoms were refined with
anisotropic displacement parameters, unless otherwise stated.
The cation in the structure of [NEt4]3[{Fe3(CO)9(l3-O)}2H] has
imposed two-fold symmetry and the anion has −3 symmetry.
In the case of [Cs(THF)0.33]3[{Fe3(CO)9(l3-O)}2H] the anion has
imposed −3 symmetry and the THF molecule is disordered over 12
symmetry-related positions, therefore, the unique THF molecule
was refined isotropically with a fixed occupancy factor of 0.08333.
Restraints were applied on the C–O and C–C distances of the
THF molecules of both [Cs(THF)0.33]3[{Fe3(CO)9(l3-O)}2H] and
[Cs(THF)]2[Fe4(CO)13], as well as similar U restraints on all oxygen
and carbon atoms, in order to obtain satisfactory models.
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Acknowledgements
The financial contribution of the European Union through the
integrated project NAIMO, NMP4-CT-2004-500355 is gratefully
acknowledged.
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