Synthesis, chemical characterization, and molecular structures of Ag8Fe4(CO)16(dppm)2 and Ag4Au4Fe4(CO)16(dppe)2

Article abstract of DOI:10.1021/om980151z

The new Ag₈{Fe(CO)₄}₄(dppm)₂ and Ag₄Au₄{Fe(CO)₄}₄(P∧P) ₂ (P∧P = dppm, dppe) neutral cluster compounds have been isolated in good yields by condensation in CH₃CN or THF of M₂(P∧P)²⁺ (M = Ag, Au) cations with [Ag₄{Fe(CO)₄}₄]^4- anions and characterized by IR, NMR, and microanalyses. The structure of both Ag₈{Fe(CO)₄}₄(dppm)₂ and Ag₄Au₄{Fe(CO)₄}₄-(dppe)₂ has been determined by X-ray diffraction studies. The former represents a very rare example of a neutral Ag-Fe cluster containing phosphine as ancillary ligand for silver.

Full text of DOI:10.1021/om980151z

4438
Organometallics 1998, 17, 4438-4443
Syn th esis, Ch em ica l Ch a r a cter iza tion , a n d Molecu la r
Str u ctu r es of Ag8F e4(CO)16(d p p m )2 a n d
Ag4Au 4F e4(CO)16(d p p e)2
Vincenzo G. Albano,† M. Carmela Iapalucci,‡ Giuliano Longoni,*,‡
Magda Monari,† Alessandro Paselli,‡ and Stefano Zacchini‡
Dipartimento di Chimica “G.Ciamician”, University of Bologna, via F. Selmi 2,
40126 Bologna, Italy, and Dipartimento di Chimica Fisica ed Inorganica,
University of Bologna, viale Risorgimento 4, 40136 Bologna, Italy
Received March 2, 1998
The new Ag8{Fe(CO)4}4(dppm)2 and Ag4Au4{Fe(CO)4}4(P∧P)2 (P∧P ) dppm, dppe) neutral
cluster compounds have been isolated in good yields by condensation in CH3CN or THF of
M2(P∧P)2+ (M ) Ag, Au) cations with [Ag4{Fe(CO)4}4]4- anions and characterized by IR, NMR,
and microanalyses. The structure of both Ag8{Fe(CO)4}4(dppm)2 and Ag4Au4{Fe(CO)4}4-
(dppe)2 has been determined by X-ray diffraction studies. The former represents a very
rare example of a neutral Ag-Fe cluster containing phosphine as ancillary ligand for silver.
In tr od u ction
(CO)4}4(dppe)2 compounds (dppm ) bis(diphenylphos-
phine)methane, dppe ) bis(diphenylphosphine)ethane).
The molecular structures of two of them, namely, Ag8-
{Fe(CO)4}4(dppm)2 and Ag4Au4{Fe(CO)4}4(dppe)2, have
been determined by X-ray diffraction studies. To our
knowledge, Ag8{Fe(CO)4}4(dppm)2 represents the second
example of a neutral Ag-Fe cluster containing phos-
phine as ancillary ligand for silver. Moreover, its
structure suggests that the nature of the elusive
[Ag6Fe4(CO)16]2-7 could be different from that of the
[Cu6Fe4(CO)16]2- congener16,17 and points out a pos-
sible reaction pathway from [Ag4Fe4(CO)16]4- to [Ag13Fe8-
(CO)32]3- that preserves the favored ν2-square Ag4{Fe-
(CO)4}4 motif throughout the reaction.
The chemistry of Ag-Fe clusters containing phos-
phines as ancillary ligands for silver is very poor.1-3 No
neutral compound is known, besides Ag6Fe3(CO)12-
{(PPh2)3CH}, which contains the tridentate triphos
ligand.4 A few addition products of AgL+ moieties to
anionic iron carbonyl compounds have also been re-
ported, but none of them have been structurally
characterized.1-3 The paucity of Ag-Fe-phosphine
compounds is in contrast with the existence of several
anionic silver clusters stabilized uniquely by Fe(CO)4
groups5-8 and the wide variety of Au-Fe clusters in
which gold is coordinated by phosphine ligands.9-15
We now report the synthesis of the new Ag8{Fe(CO)4}4-
(dppm)2, Ag4Au4{Fe(CO)4}4(dppm)2, and Ag4Au4{Fe-
Resu lts a n d Discu ssion
† Dipartimento di Chimica “G.Ciamician”, University of Bologna.
‡ Dipartimento di Chimica Fisica ed Inorganica, University of
Bologna.
(1) Salter, I. D. Adv. Organomet. Chem. 1989, 29, 249.
(2) Hall, K. P.; Mingos, D. M. P. Progr. Inorg. Chem. 1984, 32, 237.
(3) Holloway, C. E.; Nevin, W. A.; Melnik, M. J . Coord. Chem. 1994,
31, 191.
(4) Briant, C. E.; Hall, K. P.; Mingos, D. M. P. J . Chem. Soc., Chem.
Commun. 1983, 843.
(5) Albano, V. G.; Grossi, L.; Longoni, G.; Monari, M.; Mulley, S.;
Sironi, A. J . Am. Chem. Soc. 1992, 114, 5708
(6) Albano, V. G.; Azzaroni, F.; Iapalucci, M. C.; Longoni, G.; Monari,
M.; Mulley, S.; Proserpio, D. M.; Sironi, A. Inorg. Chem. 1994, 33, 5320.
(7) Albano, V. G.; Calderoni, F.; Iapalucci, M. C.; Longoni, G.;
Monari, M.; Zanello, P. J . Cluster Sci. 1995, 6, 107.
(8) Calderoni, F.; Iapalucci, M. C.; Longoni, G.; Testoni, U. In The
Synergy between Dynamics and Reactivity at Clusters and Surfaces;
Farrugia, L. J ., Ed.; NATO ASI Series 465; 1995; p 335.
(9) Coffey, C. E.; Lewis, J .; Nyholm, R. S. J . Chem. Soc. 1964, 1741.
(10) Albano, V. G.; Monari, M.; Iapalucci, M. C.; Longoni, G. Inorg.
Chim. Acta 1993, 213, 183.
(11) Albano, V. G.; Iapalucci, M. C.; Longoni, G.; Manzi, L.; Monari,
M. Organometallics 1997, 16, 497.
(12) Rossell, O.; Seco, M.; J ones, P. G. Inorg. Chem. 1990, 29, 348.
(13) Alvarez, S.; Rossell, O.; Seco, M.; Valls, J .; Pellinghelli, M. A.;
Tiripicchio, A. Organometallics 1991, 10, 2309.
The exploitation of the additional bonding capability
of the µ2-Fe(CO)4 groups of [Ag4Fe4(CO)16]4- leading to
the synthesis of [Ag5Fe4(CO)16]3- has demonstrated that
the former can act as an 8-crown-2 ligand for Ag+ ions.6
The consideration that pinning of M+ ions with an
ancillary ligand could trigger outward rather than
inward bonding capability of the µ2-Fe(CO)4 groups of
[Ag4Fe4(CO)16]4- (see Scheme 1) led us to investigate
the potential of the latter as a tetradentate ligand
toward M2(P∧P)2+ (M ) Ag, Au) moieties.
The reaction of [Ag4Fe4(CO)16]4- salts with M2(P∧P)2+
species greatly depends on the nature of the coinage
metal, the ancillary phosphine ligand, and the reaction
solvent. For instance, mixing of ca. 2 mol of Ag2(dppm)-
(NO3)2 per mole of [NEt4]4[Ag4Fe4(CO)16] in acetonitrile
affords a mixture containing the neutral Ag8{µ3-Fe-
(CO)4}4(dppm)2 species and comparable amounts of the
[Ag13Fe8(CO)32]3-/4- and [Ag6Fe4(CO)16]2- salts. The red
(14) Rossell, O.; Seco, M.; Reina, R.; Font-Bardia, M.; Solans, X.
Organometallics 1994, 13, 2127.
(15) Briant, C. E.; Smith, R. G.; Mingos, D. M. P. J . Chem. Soc.,
Chem. Commun. 1984, 586.
(16) Doyle, G.; Eriksen, K. A.; Van Engen, D. J . Am. Chem. Soc.
1986, 108, 445.
(17) Doyle, G.; Eriksen, K. A.; Van Engen, D. J . Am. Chem. Soc.
1985, 107, 7914.
S0276-7333(98)00151-4 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 09/04/1998

Ag8Fe4(CO)16(dppm)2 and Ag4Au4Fe4(CO)16(dppe)2
Organometallics, Vol. 17, No. 20, 1998 4439
Sch em e 1
The corresponding reaction of [NEt4]4[Ag4Fe4(CO)16]
with 2 mol of Ag2(dppe)(NO3)2 gives rise to a red-brown
solution, from which only the [Ag(dppe)2]+ salt of the
[Ag13Fe8(CO)32]3- trianion could be isolated upon pre-
cipitation. IR monitoring of the above reaction suggests
that labile Ag-Fe clusters containing phosphines as
ancillary ligands may be intermediately obtained. Un-
fortunately, these decompose to the most stable [Ag-
(dppe)2]3[Ag13Fe8(CO)32] salt with time or upon precipi-
tation. It is worth mentioning that also all attempts to
obtain an Ag2Fe(CO)4(PPh3)2 species, corresponding to
the well-known Au2Fe(CO)4(PPh3)2 compound,9,10 or Ag8-
Fe4(CO)16(PPh3)4, by reaction of Ag(PPh3)NO3 with
either Na2[Fe(CO)4]‚xTHF or [NEt4]4[Ag4Fe4(CO)16] in
the required stoichiometric ratios, afforded only [Ag-
(PPh3)4]3[Ag13Fe8(CO)32]. The ionic nature of this prod-
uct showing an Ag2Fe(CO)4(PPh3)1.5 empirical formula
has been confirmed by an X-ray diffraction study.20
Ag8{µ3-Fe(CO)4}4(dppm)2 species has been separated by
differential solubility in organic solvents and crystal-
lized from acetone-isopropyl alcohol (νCO in THF at
1982 s, 1920 sh, and 1902 s cm-1). Although its
absorption wavenumbers are very close to those of the
[Ag13Fe8(CO)32]4- tetraanion,5,7 the two are readily
distinguishable on the basis of their different IR pattern
and solubility in THF. The 31P{1H} NMR spectrum of
Ag8{µ3-Fe(CO)4}4(dppm)2 displays a AA′XX′ signal cen-
tered at δ +8.4 ppm from H3PO4 with a pattern very
similar to that of Ag2(dppm)2X2 (X) R-COO, BF4)18 and
has been correspondingly interpreted, obtaining the
parameters given in the Experimental Section.
The Ag8{Fe(CO)4}4(dppm)2 species has also been
obtained by reacting Na2[Fe(CO)4]‚xTHF with Ag2-
(dppm)(NO3)2 in a ca. 1:1 molar ratio in THF as solvent.
Even if this reaction gives rise to [Ag4Fe4(CO)16]4- as
initial product, the composition of the final reaction
mixture is even more complicated than above due to the
additional presence of some [Ag3Fe(CO)4(dppm)3][NO3],19
which further hampers separation and purification of
Ag8{Fe(CO)4}4(dppm)2. Formation of the above cation
is due to the too high dppm/Ag ratio of this alternative
reagent mixture and can be avoided by in situ prepara-
tion of [Ag4Fe4(CO)16]4- from a 1:1 molar mixture of Na2-
[Fe(CO)4]‚xTHF and AgNO3, followed by addition of a
further equivalent of silver as Ag2(dppm)(NO3)2 salt.
However, under these conditions the [Ag13Fe8(CO)32]3-/4-
and [Ag6Fe4(CO)16]2- side products were present as
sodium salts, and their solubility was too similar to that
of Ag8{Fe(CO)4}4(dppm)2 to allow a convenient separa-
tion. This could only be achieved by metathesis of the
mixture with tetraethylammonium chloride.
These results, as well as the synthesis of Ag6-
Fe3(CO)12{(PPh2)3CH}4 and [Ag3Fe(CO)4(dppm)3]+,19 out-
line the importance of a polydentate nonchelating
phosphine, as well as the concomitant presence of the
highest possible fractional positive charge on silver, in
stabilizing Ag-Fe carbonyl clusters containing phos-
phine as ancillary ligand for the latter.
The nature of the diphosphine ligand in the M2(P∧P)2+
fragment is less relevant upon substitution of silver with
gold, even if the resulting Ag4Au4{µ3-Fe(CO)4}4(P∧P)2
is less stable. Indeed, the condensation reaction of
Au2(P∧P)2+ moieties onto [Ag4Fe4(CO)16]4- is much more
simple and can be carried out by using [Ag4Fe4(CO)16]4-
as sodium salt (obtained in situ by reaction of Na2[Fe-
(CO)4]‚xTHF and AgNO3) both in THF and acetonitrile.
In THF solution the condensation reaction of Na4[Ag4-
Fe4(CO)16] with 2 equiv of Au2(P∧P)Cl2 (P∧P ) dppm
and dppe) gives Ag4Au4{Fe(CO)4}4(P∧P)2 in almost
quantitative yields, and the compound has been isolated
in 65-70% yields (based on gold) by filtration of the
reaction solution and crystallization by precipitation
with toluene. The corresponding condensation in ac-
etonitrile directly leads to a red precipitate of the Ag4-
Au4{Fe(CO)4}4(P∧P)2 neutral derivative and a solution
that shows the additional presence of a species display-
ing infrared carbonyl absorptions in CH3CN at 1973 and
1895 cm-1. The latter might be either the [Ag4Au2{Fe-
(CO)4}4(P∧P)]2- 1:1 condensation product or, more likely,
[Ag4Au2Fe4(CO)16]4-. Indeed, a species showing coin-
cident infrared carbonyl absorptions has also been
obtained by addition of 2 equiv of Au(SEt2)Cl to
[Ag4Fe4(CO)16]2-. Owing to the presence of the above
side product, under these experimental conditions Ag4-
Au4{Fe(CO)4}4(P∧P)2 could be recovered by filtration
only in 40-50% yields.
Once isolated in a pure state, Ag8{Fe(CO)4}4(dppm)2
is stable in several organic solvents, such as THF and
acetone, under an inert atmosphere, even in the pres-
ence of free dppm added in 5-fold molar excess. This
suggests that the [Ag13Fe8(CO)32]3-/4- and [Ag6Fe4-
(CO)16]2- side products arise either from disproportion-
ation of an intermediate species such as the expected
[Ag6{Fe(CO)4}4(dppm)]2- or from side reactions of the
starting [Ag4Fe4(CO)16]4- salt with Ag+ ions set free by
Ag2(dppm)(NO3)2. On the contrary, Ag8{Fe(CO)4}4-
(dppm)2 readily disproportionates to [Ag13Fe8(CO)32]3-
and [Ag2(dppm)2]2+ in dichloromethane solution.
Both Ag4Au4{Fe(CO)4}4(dppm)2 (νCO in THF at 1992
s, 1922 s cm-1) and Ag4Au4{Fe(CO)4}4(dppe)2 (νCO in
THF at 1991 s, 1921 s cm-1) have been conveniently
crystallized as dark red crystals by extraction in THF
and precipitation with toluene. Crystallization in other
solvent mixtures results in Ag4Au4{Fe(CO)4}4(P∧P)2
crystalline precipitates contaminated by other species
and partial decomposition. For instance, crystallization
of Ag4Au4{Fe(CO)4}4(dppe)2 in acetone-isopropyl alco-
hol leads to mixtures of red crystals of the starting
material, smaller amounts of yellow crystals of Au4{Fe-
(18) Van Der Ploeg, A. F. M. J .; Van Koten, G. Inorg. Chim. Acta
1981, 51, 225.
(19) Albano, V. G.; Castellari, C.; Iapalucci, M. C.; Longoni, G.;
Monari, M.; Paselli, A.; Zacchini, S. J . Organomet. Chem., in press.

4440 Organometallics, Vol. 17, No. 20, 1998
Albano et al.
F igu r e 1. Molecular geometry of Ag8{Fe(CO)4}4(dppm)2 and labeling of the independent atoms. The molecule sits on a
2-fold rotation axis lying along the short diagonal of the central Ag4 rhombus. Contacts between coinage atoms are shown
by dashed lines.
F igu r e 2. Molecular geometry and atom labeling of one of the two independent molecules (A) of Ag4Au4{Fe(CO)4}4(dppe)2.
Ta ble 1. Selected Bon d Len gth s (Å) a n d An gles
(CO)4}2(dppe)2,15 and a silver mirror. Moreover, Ag4-
(d eg) for Ag8{F e(CO)4}4(d p p m )2
Au4{Fe(CO)4}4(P∧P)2 is rapidly decomposed by contact
Ag(1)-Ag(2)
Ag(2)-Ag(4)
Ag(4)-Ag(5)
Ag(3)‚‚‚Ag(5)
Ag(1)-P(2)
Ag(4)-Fe(1)
Ag(2)-Fe(2)
Ag(3)-Fe(2)
3.138(1)
2.829(1)
2.802(1)
3.701(1)
2.404(2)
2.660(2)
2.608(1)
2.666(1)
Ag(1)-Ag(4)
Ag(3)-Ag(4)
Ag(4)‚‚‚Ag(4′)
Ag(2)-P(1)
Ag(1)-Fe(1)
Ag(5)-Fe(1)
Ag(4)-Fe(2)
Ag‚‚‚C(CO)
2.833(1)
2.796(1)
4.201(2)
2.400(2)
2.617(1)
2.644(1)
2.664(1)
2.58-2.79
with air or upon deliberate addition of chloride ions. IR
monitoring of the latter reaction shows the formation
of [Au3{Fe(CO)4}2(P∧P)]- 11 and [HFe(CO)4]-; concomi-
tantly, a precipitate of silver chloride and a silver mirror
separates out.
The 31P{1H} NMR spectrum of Ag4Au4{µ3-Fe(CO)4}4-
(P∧P)2 in THF or acetone solution is more complicated
than expected. It consists of two poorly resolved dou-
blets and one singlet (P∧P ) dppm, THF, δ +42.8 (d,
1.0), +42.7 (d, 4.1), and +37.6 (s, 1.1); P∧P ) dppe,
acetone, δ +48.2 (d, 1.0), +47.7 (d, 3.5), and +38.3 (s,
1.1) ppm; figures in parentheses represent the relative
intensities). The apparent coupling constants of the
doublets are comprised in the 2-3.8 Hz range of values
and can only be due to coupling with the 107/109Ag atoms
at formal two (P-Au-Ag) or three (P-Au-Fe-Ag) bond
distance from phosphorus. Conformational isomers are
very unlikely owing to the preference for linear Fe-M-
Fe and Fe-M-P coordination of both gold and silver.
The observed multiplicity of the signals and their
relative intensities indicate the coexistence in solution
of Ag4Au4{Fe(CO)4}4(P∧P)2 (most intense doublet) and
solvent-separated or -unseparated [Ag4Au2{Fe(CO)4}4-
(P∧P)]2- (least intense doublet) and [Au2(P∧P)]2+ (sin-
glet) ion pairs. Indeed, a stretching of the Au-Fe bonds
of one [Au2(P∧P)]2+ unit could be sufficient to lose the
above very weak coupling. Moreover, the known 31P
chemical shifts of Au2(dppm)Cl2 (δ +23.5 in CDCl3)21
Fe(2)-Ag(3)-Fe(2′) 158.69(6) Fe(1)-Ag(5)-Fe(1′) 154.99(6)
P(1)-Ag(2)-Fe(2) 162.06(7) P(2)-Ag(1)-Fe(1) 163.97(7)
and Au2(dppe)Cl2 (δ +31.5 in CDCl3)22 are of little help
in making the assignment. Solvent-unseparated ion
pairs seem, however, more likely since the IR spectra
do not display any new feature. Variable-temperature
NMR and solvent effect studies have been precluded by
their limited solubility and stability. Furthermore,
addition of chloride ions in the NMR tube, aimed at
trapping the [Au2(P∧P)]2+ unit, led to decomposition.
The molecular structures of both Ag8{Fe(CO)4}4(dppm)2
and Ag4Au4{Fe(CO)4}4(dppe)2 have been investigated by
X-ray diffraction studies and are reported in Figures 1
and 2, respectively. The most relevant interatomic
distances are reported in Tables 1 and 2. The molecular
connectivity and conformation in the two species are
strictly comparable, and the metal atom framework can
be described as a two-dimensional triangulated ribbon
twisted around the elongation direction. This confor-
(21) Lin, I. J . B.; Hwang, J . M.; Feng, D.-F.; Cheng, M. C.; Wang,
Y. Inorg. Chem. 1994, 33, 3467.
(20) Albano, V. G.; Monari, M.; Iapalucci, M. C.; Longoni, G.
Unpublished results.
(22) Berners-Price, S. J .; Mazid, M. A.; Sadler, P. J . J . Chem. Soc.,
Dalton Trans. 1984, 969.

Ag8Fe4(CO)16(dppm)2 and Ag4Au4Fe4(CO)16(dppe)2
Organometallics, Vol. 17, No. 20, 1998 4441
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles (d eg) for A a n d B In d ep en d en t Molecu les of
Ag4Au 4{F e(CO)4}4(d p p e)2
A
B
A
B
Ag(1)-Ag(2)
Ag(3)-Ag(4)
Ag(1)-Au(1)
Au(1)-Au(2)
Ag(4)-Au(3)
Ag(2)‚‚‚Ag(3)
Au(4)-P(4)
2.782(4)
2.767(4)
2.793(2)
3.381(2)
2.793(3)
3.473(4)
2.265(9)
2.304(8)
2.679(7)
2.551(5)
2.648(5)
2.691(7)
2.565(5)
2.665(5)
2.53-2.85
2.751(4)
2.765(4)
2.767(2)
3.518(2)
2.786(2)
3.294(4)
2.272(9)
2.308(8)
2.708(7)
2.546(5)
2.650(5)
2.711(7)
2.561(5)
2.643(5)
2.53-2.85
Ag(1)-Ag(3)
Ag(2)-Ag(4)
Ag(1)-Au(2)
Ag(4)-Au(4)
Au(3)-Au(4)
Ag(1)‚‚‚Ag(4)
Au(3)-P(3)
Au(2)-P(2)
Ag(1)-Fe(2)
Ag(2)-Fe(1)
Au(1)-Fe(1)
Ag(4)-Fe(4)
Ag(2)-Fe(3)
Au(3)-Fe(3)
2.768(4)
2.791(4)
2.815(3)
2.792(3)
3.367(2)
4.334(4)
2.300(8)
2.256(9)
2.681(5)
2.669(5)
2.621(5)
2.708(5)
2.674(5)
2.621(5)
2.762(4)
2.773(4)
2.824(3)
2.794(3)
3.488(2)
4.434(4)
2.295(8)
2.27(1)
2.657(5)
2.691(5)
2.605(5)
2.691(5)
2.695(5)
2.608(5)
Au(1)-P(1)
Ag(3)-Fe(2)
Au(2)-Fe(2)
Ag(1)-Fe(1)
Ag(3)-Fe(4)
Au(4)-Fe(4)
Ag(4)-Fe(3)
Ag/Au‚‚‚C(CO)
P(2)-Au(2)-Fe(2)
P(4)-Au(4)-Fe(4)
Fe(2)-Ag(3)-Fe(4)
169.9(3)
169.4(3)
160.0(2)
172.9(3)
171.1(3)
160.3(2)
P(1)-Au(1)-Fe(1)
P(3)-Au(3)-Fe(3)
Fe(1)-Ag(2)-Fe(3)
162.5(3)
163.4(3)
155.3(2)
162.7(3)
165.7(3)
154.9(2)
mational feature makes the molecules chiral, and the
crystals contain racemic mixtures of enantiomers deriv-
ing from opposite spiralization. The carbonyl ligands
belong primarily to the Fe(CO)4 groups in C2v idealized
geometry with only secondary interactions with the
coinage metal atoms. The diphosphine ligands act as
bidentate and saturate the terminal coinage metals,
thus preventing further condensation of the constituent
units (vide infra). Both molecules conform to a C2
symmetry that is crystallographically imposed for the
Ag8 molecule and only idealized for the Ag4Au4 species.
If carbon and oxygen atoms are ignored, the idealized
symmetry of the heavy atom frames is D2. The metal
atom array is organized around a central Ag4 diamond
whose edges are spanned by Fe(CO)4 groups lying
alternately above and below the Ag4 plane. The op-
posite vertexes defining the long diagonal of the dia-
mond are bonded to Ag2(dppm) and Au2(dppe) frag-
ments, respectively. The latter units are significantly
rotated with respect to the diamond plane [ 55.20(3)°
for Ag8{Fe(CO)4}4(dppm)2 and 57.6av° for Ag4Au4{Fe-
(CO)4}4(dppe)2] according to the 2-fold symmetry axis
located along the short diagonal of the diamond. This
rationalization of the metal atom assembly points out
[Cu6Fe4(CO)16]2- 16
.
Indeed, the condensation of [Ag4Fe4-
(CO)16]4- ions with nonbonded Ag2 moieties (as well
as the rearrangement of [Ag5Fe4(CO)16]3- upon addition
of an Ag+ ion) could give rise to [{Ag}2n{Ag4Fe4-
(CO)16}n]2n- (n g 1) oligomers, which preserve the
favored ν2-square Ag4Fe4 motif, rather than involve the
less favorable ν2-triangular M3Fe3 moiety of the related
[Cu3Fe3(CO)12]3- and [Cu6Fe4(CO)16]2- pair of com-
pounds.16,17 For instance, wrapping of two [{Ag}2{Ag4Fe4-
(CO)16}]2- moieties around an Ag+ ion could directly give
rise to the structure of [Ag13Fe8(CO)32]3-. This alterna-
tive reaction pathway would take into account the
insolubility of all purported [Ag6Fe4(CO)16]2- salts.7 In
conclusion, it seems reasonable to suggest that the
twisted ribbonlike metal frame of Ag8{Fe(CO)4}4(dppm)2
represents a fragment of the above oligomers inter-
cepted and stabilized by ancillary phosphine ligands.
Furthermore, the present synthesis of Ag4Au4{Fe-
(CO)4}4(P∧P)2 from the [Ag4Fe4(CO)16]4- ion and the
previously reported synthesis of its [Au4Fe4(CO)16]4-
congener23 may anticipate the possible existence of a
related Au8{Fe(CO)4}4(P∧P)2 neutral species.
2+
Exp er im en ta l Section
that the molecules are formed by simple condensation
All reactions including sample manipulations were carried
out using standard Schlenk techniques under nitrogen and in
carefully dried solvents. The [NEt4]4[Ag4Fe4(CO)16] and Na2-
[Fe(CO)4]‚xTHF salts have been prepared according to the
literature.7,24 Analyses of Fe, Ag, and Au were performed by
atomic absorption. Infrared spectra were recorded using CaF2
cells.
Syn th esis of Ag8{F e(CO)4}4(d p p m )2. Solid [NEt4]4[Ag4-
Fe4(CO)16] (1.51 g, 0.93 mmol) and Ag2(dppm)(NO3)2 (1.45 g,
2.0 mmol) were mixed in a 100 mL flask under a nitrogen
atmosphere. Anhydrous CH3CN (30 mL) was added, and the
suspension was stirred for 6 h. After this period of time a
precipitate of [NEt4]2[Ag6Fe4(CO)16] is formed, and IR monitor-
ing of the surnatant solution showed the presence of Ag8{Fe-
(CO)4}4(dppm)2, [Ag13Fe8(CO)32]3-, and trace amounts of Fe-
(CO)4(dppm). The suspension was filtered, and the resulting
orange solution was evaporated to dryness under vacuum. The
residue was washed with water (20 mL) and toluene (10 mL).
2+
of two [Ag2(dppm)]2+ or [Au2(dppe)]
dications on
opposite atoms of the square tetraanion [Ag4Fe4(CO)16]4-.6
No bond breaks down in the process, and a major change
is a squeezing of the Ag4 rectangle into a rhombus
(contacts along the short diagonal 3.701(1) Å in the Ag8
species and 3.473 and 3.294(4) Å in the Ag4Au4 species).
The out-of-plane positions of the Fe(CO)4 groups are
caused by the tilted conformation of the M2(P∧P) units.
The twisted overall geometry of the molecules seems
dictated by the need to achieve the preferred linear
coordination of the Ag and Au atoms [P-Ag-Fe(av)
163.0° or P-Au-Fe(av) 167.2°, Fe-Ag-Fe(av) 164.3°].
Con clu sion s
The structure of the Ag8{µ3-Fe(CO)4}4(dppm)2 species
envisions a possible reaction course from [Ag4Fe4(CO)16]4-
to [Ag13Fe8(CO)32]3- upon addition of Ag+ ions, which
does not implicate the intermediate formation of a
purported [Ag6Fe4(CO)16]2- species,7 isostructural with
(23) Albano, V. G.; Calderoni, F.; Iapalucci, M. C.; Longoni, G.;
Monari, M. J . Chem. Soc., Chem. Commun. 1995, 433.
(24) Collman, J . P.; Finke, R. G.; Cawse, J . N.; Brauman, J . I. J .
Am. Chem. Soc. 1977, 99, 2515.

4442 Organometallics, Vol. 17, No. 20, 1998
Albano et al.
Ta ble 3. Cr ysta l Da ta a n d Exp er im en ta l Deta ils for Ag8{F e(CO)4}4(d p p m )2 a n d Ag4Au 4{F e(CO)4}4(d p p e)2
formula
M
C66H44Ag8Fe4O16 P4
2303.25
C68H48Ag4 Au4Fe4O16 P4
2687.69
temp, K
293(2)
293(2)
wavelength, Å
crystal symmetry
space group
a, Å
b, Å
0.71069
0.71069
triclinic
tetragonal
I41/a (No. 88)
19.981(5)
19.981(5)
37.024(6)
90
P1h (No. 2)
19.621(7)
20.672(7)
21.181(6)
97.49(3)
89.61(3)
114.72(2)
7726
c, Å
R, deg
â, deg
90
90
γ, deg
cell volume, Å3
Z
14 782(6)
8
2.070
2.976
8864
4
Dc , Mg m-3
2.311
9.421
5008
µ(Mo KR), mm-1
F(000)
crystal size, mm
θ limits, deg
scan mode
no. of rflns collected
no. of unique obsd rflns [Fo > 4σ(Fo)]
goodness of fit on F2
R1(F),a wR2(F2)b
weighting scheme a, b
largest diff peak and hole, e Å-3
0.20 × 0.21 × 0.22
0.12 × 0.20 × 0.27
2.5-25
2.5-20
ω
ω
25 291((h, (k, (l)
14 743((h, (k, +l)
3708
1.053
6467
0.993
0.0449, 0.0790
0.0244, 93.3550
0.637 and -0.815
0.0580, 0.1370
0.0954, 109.2737b
1.638 and -1.615
R1 ) ∑||Fo| - |Fc|/∑|Fo|. wR2 ) [∑w(Fo - Fc2)2/∑w(Fo2)2]1/2, where w ) 1/[σ2(Fo2) + (aP)2 + bP], where P ) (Fo + 2Fc2)/3.
a
b
2
2
Extraction of the residue in THF (30 mL) followed by filtration
toluene (40 mL) to give 0.47 g of Ag4Au4{Fe(CO)4}4(dppe)2 as
red crystals. Another 0.18 g of product was collected by
crystallization of the residue obtained upon evaporation of the
mother liquor. Anal. Found (Calcd): Ag 16.18 (16.05), Au
29.16 (29.31), Fe 8.21 (8.31), C 30.43 (30.47), H 1.69 (1.79).
The compound is soluble in THF and acetone, sparingly soluble
in acetonitrile, and insoluble in toluene, n-heptane, alcohol,
and water.
afforded a orange-red solution containing crude Ag8{Fe(CO)4}4-
(dppm)2. This was purified by evaporating the THF in vacuo,
dissolving the residue in acetone (25 mL), and precipitation
by diffusion of isopropyl alcohol (40 mL). Yields: 0.42 g of
Ag8{Fe(CO)4}4(dppm)2 as shiny red crystals. Anal. Found
(Calcd): Ag 37.25 (37.47), Fe 9.54 (9.70), C 34.56 (34.39), H
1.98 (1.91). 1H NMR: δ 3.7 (m, 2H, CH2), 7.07-7.65 (m, 10H,
Ph). 31P {1H} NMR: AA′XX′ pattern centered at δ +8.43
Cr ysta llogr a p h y. Crystal data and experimental details
for Ag8{Fe(CO)4}4(dppm)2 and Ag4Au4{Fe(CO)4}4(dppe)2 are
given in Table 3. The diffraction experiments were carried
out at room temperature on a fully automated Enraf-Nonius
CAD-4 diffractometer using graphite-monochromated Mo KR
radiation. The unit cell parameters were determined by a
least-squares fitting procedure using 25 reflections. Data were
corrected for Lorentz and polarization effects. A significant
decay (40%) was observed in Ag4Au4{Fe(CO)4}4(dppe)2, for
which the data collection was limited at θ ) 20° and corrected.
An empirical absorption correction was applied in both
compounds by using the azimuthal scan method.25 Direct
methods (SHELXS 86)26 identified the positions of the metal
atoms and iterative cycles of least-squares refinement (on F2),
and difference Fourier synthesis located the remaining non-
hydrogen atoms. All non-hydrogen atoms were refined aniso-
tropically, and the hydrogen atoms were placed in calculated
positions (C-H ) 0.97 Å) for Ag8{Fe(CO)4}4(dppm)2. The
molecule sits around a 2-fold symmetry axis in the crystal.
3
(J P- Ag 477, J P- Ag 414, J P-P 167, 3J P- Ag -4.3, and J P-
109
107
109
107
Ag
-1.8 Hz). The compound is soluble in THF and acetone,
sparingly soluble in acetonitrile, and insoluble in toluene,
n-heptane, alcohol, and water.
Syn th esis of Ag4Au 4{F e(CO)4}4(d p p m )2. Solid Na2[Fe-
(CO)4]‚xTHF (x ∼ 2, 0.68 g, ca. 1.9 mmol) and AgNO3 (0.31 g,
1.82 mmol) were mixed in a 100 mL flask under a nitrogen
atmosphere. Anhydrous THF (30 mL) was added, and the
suspension was rapidly stirred for 6 h. After this period of
time IR monitoring showed the presence of the carbonyl
absorptions of [Ag4Fe4(CO)16]4-. Solid Au2(dppm)Cl2 (0.77 g,
0.91 mmol) was added, and the supension was stirred for ca.
3 h until quantitative formation of Ag4Au4{Fe(CO)4}4(dppm)2
was monitored by IR. The suspension was filtered under
nitrogen, and the solution was precipitated by diffusion of
toluene (40 mL) to give 0.58 g of Ag4Au4{Fe(CO)4}4(dppm)2 as
red crystals. Another 0.27 g of product was collected after
crystallization of the residue obtained upon evaporation of the
mother liquor of the first crystallization. Anal. Found (Cal-
cd): Ag 16.09 (16.23), Au 29.37 (29.63), Fe 8.25 (8.40), C 29.85
(29.79), H 1.74 (1.65). The compound is soluble in THF and
acetone, sparingly soluble in acetonitrile, and insoluble in
toluene, n-heptane, alcohol, and water.
Two independent molecules of Ag4Au4{Fe(CO)4}4(dppe)2
have been found in the crystal; they have the same stereoge-
ometry including the conformations of the diphosphine rings.
Some differences are observed only in the softer metal-metal
interactions and conformational details of the phenyl rings.
The final refinement (on F2) of all positional and thermal
parameters for Ag4Au4{Fe(CO)4}4(dppe)2 proceeded by full-
matrix least-squares calculations (SHELXL 93)27 using aniso-
tropic thermal parameters for the heavy atoms (Au, Ag, Fe,
Syn th esis of Ag4Au 4{F e(CO)4}4(d p p e)2. Solid Na2[Fe-
(CO)4]‚xTHF (x ∼ 2, 0.52 g, ca. 1.45 mmol) and AgNO3 (0.24
g, 1.41 mmol) were mixed in a 100 mL flask under a nitrogen
atmosphere. Anhydrous THF (30 mL) was added, and the
suspension was rapidly stirred for 6 h. After this period of
time IR monitoring showed the presence of the carbonyl
absorptions of [Ag4Fe4(CO)16]4-
. Solid Au2(dppe)Cl2 (0.62 g,
(25) North, A. C.; Philips, D. C.; Mathews, F. S. Acta Crystallogr.
1968, A24, 351.
(26) Sheldrick, G. M. SHELXS 86, Program for crystal structure
solution; University of Go¨ttingen, 1986.
(27) Sheldrick, G. M. SHELXL 93, Program for crystal structure
refinement; University of Go¨ttingen, 1993.
0.72 mmol) was added, and the supension was stirred for ca.
3 h until quantitative formation of Ag4Au4{Fe(CO)4}4(dppe)2
was monitored by IR. The suspension was filtered under
nitrogen, and the solution was precipitated by diffusion of

Ag8Fe4(CO)16(dppm)2 and Ag4Au4Fe4(CO)16(dppe)2
Organometallics, Vol. 17, No. 20, 1998 4443
P) and isotropic for all the remaining atoms due to unfavorable
observations-to-parameters ratio.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data and data collection details, fractional coordinates and U
values, bond distances and angles, and anisotropic thermal
parameters for Ag8{Fe(CO)4}4(dppm)2 and for Ag4Au4{Fe-
(CO)4}4(dppe)2 (40 pages). Ordering information is given on
any current masthead page.
Ack n ow led gm en t. We acknowledge financial sup-
port from the EEC, MURST, and the University of
Bologna (projects “Sintesi, modelli e caratterizzazione
per materiali funzionali” and “Molecole e aggregati
molecolari intelligenti”) and thank Dr. Carla Boga for
recording the NMR spectra.
OM980151Z