Synthesis of [Au3Fe2(CO)8(dppm)]- and [Au5Fe2(CO)8(dppm)2]+: X-ray structures of [NEt4][Au3Fe2(CO)8(dppm)] and [

Article abstract of DOI:10.1021/om960850g

The [Au₃Fe₂(CO)₈(dppm)]^- cluster anion has been isolated from the reaction of either [AuFe₂(CO)₈]^3- or [Fe(CO)₄]^2- with Au₂(dppm)Cl₂ i

Full text of DOI:10.1021/om960850g

© Copyright 1997
Volume 16, Number 4, February 18, 1997
American Chemical Society
Syn th esis of [Au 3F e2(CO)8(d p p m )]- a n d
[Au 5F e2(CO)8(d p p m )2]+: X-r a y Str u ctu r es of
[NEt4][Au 3F e2(CO)8(d p p m )] a n d
[Au 5F e2(CO)8(d p p m )2][BF 4]
Vincenzo G. Albano,† Maria Carmela Iapalucci,‡ Giuliano Longoni,*,‡
Lucia Manzi,‡ and Magda Monari*,†
Dipartimento di Chimica “G. Ciamician”, via F. Selmi 2, 40126 Bologna, Italy, and
Dipartimento di Chimica Fisica ed Inorganica, viale Risorgimento 4, 40136 Bologna, Italy
Received October 7, 1996X
Summary: The [Au3Fe2(CO)8(dppm)]- cluster anion has
been isolated from the reaction of either [AuFe2(CO)8]3-
or [Fe(CO)4]2- with Au2(dppm)Cl2 in THF, whereas the
[Au5Fe2(CO)8(dppm)2]+ cluster cation has been obtained
by oxidation of the former; both new compounds have
been characterized by X-ray studies on their [NEt4][Au3-
Fe2(CO)8(dppm)] and [Au5Fe2(CO)8(dppm)2][BF4] salts,
respectively.
[Au3Fe2(CO)8(dppm)]- has resulted in the synthesis of
the [Au5Fe2(CO)8(dppm)2]+ cluster cation, which has
been characterized by X-ray studies as its tetrafluo-
roborate salt.
The orange-red [Au3Fe2(CO)8(dppm)]- (νCO in THF at
1980 (mw), 1960 (m), and 1890 (s) cm-1) has been
obtained either from reaction of Au2(dppm)Cl2 with
[AuFe2(CO)8]3- as suggested by isolobal analogy con-
cepts (eq 1) or directly from the [Fe(CO)4]2- dianion as
shown in eq 2.
We have recently reported the synthesis and charac-
terization of two bimetallic clusters: [MFe2(CO)8]3- and
[M4Fe4(CO)16]4- (M ) Ag,Au).1,2 The latter has been
envisaged to be formally derived from complexation of
an M22+ moiety with two [MFe2(CO)8]3- cluster ligands,
which are isolobal3 with bis(diphenylphosphino)meth-
ane (dppm). As a consequence of this analogy between
[AuFe2(CO)8]3- + Au2(dppm)Cl2 f
[Au3Fe2(CO)8(dppm)]- + 2Cl- (1)
4[Fe(CO)4]2- + 4Au2(dppm)Cl2 f
[M4Fe4(CO)16]4- and [M2(dppm)2]2+ 4,5
substitution of a
,
2[Au3Fe2(CO)8(dppm)]- + [Au2(dppm)2]2+ + 8Cl-
dppm ligand for an [MFe2(CO)8]3- moiety in [M4Fe4-
(CO)16]4- was expected to afford an [M3Fe2(CO)8(dppm)]-
derivative. The synthesis and structural characteriza-
tion of such a species (for M ) Au) are reported.
Further exploitation of the bonding capability of
(2)
The tetraethylammonium salt of [Au3Fe2(CO)8(dppm)]-
has been isolated by evaporation of the THF reaction
solution and metathesis in methanol with NEt4Cl.6
† Dipartimento di Chimica “G. Ciamician”.
‡ Dipartimento di Chimica Fisica ed Inorganica.
X Abstract published in Advance ACS Abstracts, J anuary 1, 1997.
(1) Albano, V. G.; Azzaroni, F.; Iapalucci, M. C.; Longoni, G.; Monari,
M.; Mulley, S.; Proserpio, D. M.; Sironi, A. Inorg. Chem. 1994, 33, 5320.
(2) Albano, V. G.; Calderoni, F.; Iapalucci, M. C.; Longoni, G.;
Monari, M. J . Chem. Soc., Chem. Commun. 1995, 433.
(3) Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1982, 21, 711.
(4) Ho, D. M.; Bau, R. Inorg. Chem. 1983, 22, 4073.
(5) Liou, L.-S.; Liu, C.-P.; Wuang, J .-C. Acta Crystallogr. 1994, C50,
538.
(6) Solid Na2[Fe(CO)4]‚xTHF (0.85 g; x ∼ 2) and Au2(dppm)Cl2 (1.69
g) were charged in a Schlenk tube under nitrogen. Anhydrous THF
(30 mL) was added. After
1 h of stirring, the resulting orange
suspension was filtered and the solution was evaporated to dryness
in vacuo. The residue was dissolved in methanol. Red crystals of
[NEt4][Au3Fe2(CO)8(dppm)] separated out from the above solution upon
addition of NEt4Cl (2 g) and water (10 mL) and upon standing; yields
62% (based on gold).
S0276-7333(96)00850-3 CCC: $14.00 © 1997 American Chemical Society

498 Organometallics, Vol. 16, No. 4, 1997
Communications
2[Au3Fe2(CO)8(dppm)]- + [Au2(dppm)2]2+
a
2[Au2Fe(CO)4(dppm)]2 (3)
are documented in copper chemistry (e.g. Cu2Fe(CO)4-
(PPh3)4 and Cu2Fe(CO)4(dppe)2; dppe ) bis(diphenylphos-
phino)ethane),12 but not for gold compounds (e.g. Au2-
Fe(CO)4(PcHex3)2,13 Au2Fe(CO)4(PPh3)2,14,15 [AuFe2(CO)8-
(PPh3)]-,16 [AuFe3(CO)11(PPh3)]-,17 and Au2Fe2(CO)8-
(dppm)10).
The orange-red [Au3Fe2(CO)8(dppm)]- anion contains
two cis-divacant octahedral Fe(CO)4 moieties of C2v
symmetry that are capable of further bonding. A slight
stereochemical rearrangement of the above moiety, for
instance that derived from an equatorially trivacant
pentagonal bipyramid, should make available a further
filled σ orbital on each iron, as found in [M5Fe4(CO)16]3-
(M ) Cu, Ag).1,18 Therefore, [Au3Fe2(CO)8(dppm)]- is
potentially a bidentate cluster ligand isolobal with
dppm. This consideration led us to investigate the
further condensation of [Au3Fe2(CO)8(dppm)]- with
Au2(dppm)Cl2 to give a [Au5Fe2(CO)8(dppm)2]+ cluster
cation. Reaction 4 occurs in CHCl3 upon addition of Au2-
Figu r e 1. Molecular geometry of the [Au3Fe2(CO)8(dppm)]-
monoanion. Selected bond distances (Å) and angles (deg):
Au(1)-Au(3), 2.921(2); Au(2)-Au(3), 2.921(2); Au(1)‚‚‚Au-
(2), 3.578(2); Au(2)-Fe(2), 2.545(4); Au(3)-Fe(1), 2.572(4);
Au(3)-Fe(2), 2.576(4); Au(1)-Fe(1), 2.554(4); Au(1)-P(2),
2.274(6); Au(2)-P(1), 2.267(7); Au(1)-Au(3)-Au(2), 75.53-
(4); Fe(1)-Au(3)-Fe(2), 172.7(2); P(2)-Au(1)-Fe(1), 170.8-
(2); P(1)-Au(2)-Fe(2), 165.7(2).
[Au3Fe2(CO)8(dppm)]- + Au2(dppm)Cl2 f
[Au5Fe2(CO)8(dppm)2]+ + 2Cl- (4)
The [Au3Fe2(CO)8(dppm)]- anion,7 as shown in Figure
1, is roughly planar and consists of an isosceles Au3
triangle with two shorter edges, spanned by Fe(CO)4
groups, and a longer one bridged by a dppm ligand
(Au(1)-Au(3) and Au(2)-Au(3) ) 2.921(2) Å, Au(1)‚‚‚
Au(2) ) 3.578(2) Å). The opening up of the metal
triangle cannot be ascribed to the strain imposed by the
rather flexible dppm ligand. In fact shorter Au-Au
bond distances have been reported for the Au2(µ2-dppm)
unit in [Au2Fe2(CO)8(µ-dppm)]10 and [Au2Fe(CO)4-
(dppm)Cl2 in slight excess.19 The red [Au5Fe2(CO)8-
(dppm)2]+ (νCO at 2005 (s) and 1950 (s) cm-1) cation has
been alternatively obtained in good yields by oxidation
in acetonitrile solution of [Au3Fe2(CO)8(dppm)]- with
tropylium tetrafluoroborate.20 The resulting [Au5Fe2-
(CO)8(dppm)2][BF4] salt has been crystallized by layer-
ing of diisopropyl ether, and its molecular structure has
been determined by X-ray crystallography.21
11
(dppm)]2 (2.915(1) and 3.163(1) Å, respectively). The
The structure of the cation [Au5Fe2(CO)8(dppm)2]+ is
shown in Figure 2. The unit cell contains two cations
positioned around nonequivalent inversion centers, and
therefore, each cation conforms to a precise Ci sym-
lengthening of the Au(1)‚‚‚Au(2) contact seems dictated
by the preference for the linear coordination of the Au-
(I) complexes. The idealized molecular symmetry is Cs.
Reaction 2 has previously been reported to afford [Au2-
Fe(CO)4(dppm)]2.11 We have obtained red crystals of the
latter by layering toluene on the nonmetathesized THF
solution of reaction 2. These results stem from the
occurrence in solution of equilibrium 3, which is strongly
influenced by the dielectric constant of the solvent and
is completely shifted to the left in several organic
solvents such as THF, acetone, and acetonitrile. Re-
lated comproportionation-disproportionation equilibria
metry. The idealized symmetry of each cation is C2h
,
(12) Doyle, G.; Eriksen, K. A.; Van Engen, D. J . Am. Chem. Soc.
1986, 108, 445.
(13) Parish, R. V.; Moore, S.; Dens, A. J . D.; Mingos, D. M. P.;
Sherman, D. J . J . Chem. Soc, Dalton Trans. 1988, 781.
(14) Albano, V. G.; Monari, M.; Iapalucci, M. C.; Longoni, G. Inorg.
Chim. Acta 1993, 213, 183.
(15) As deduced from IR monitoring, only very minor amounts of a
related [Au3Fe2(CO)8(PPh3)2]- intermediate were detectable in the
corresponding reaction of Au(PPh3)Cl with the [Fe(CO)4]2- dianion.
(16) Rossel, O.; Seco, M.; J ones, P. G. Inorg. Chem. 1990, 29, 348.
(17) Rossell, O.; Seco, M.; Reina, R.; Font-Bardia, M.; Solans, X.
Organometallics 1994, 13, 2127.
(7) Crystal data for [NEt4][Au3Fe2(CO)8(dppm)]: Mr ) 1441.30,
monoclinic, space group P21/c (No. 14), a ) 17.720(4) Å, b ) 11.785(4)
Å, c ) 23.837(6) Å, â ) 111.60(2)°, V ) 4628(2) Å3, Z ) 4, Dc ) 2.068
Mg m-3, F(000) ) 2712, λ ) 0.710 73 Å, T ) 298 K, µ(Mo KR) ) 10.205
mm-1. Data were collected on an Enraf-Nonius CAD-4 diffractometer
using graphite-monochromated Mo KR radiation (ω scan mode). A total
of 4296 independent reflections were collected to 2θmax ) 40° and
corrected for the effects of decay and absorption. Direct methods
(SHELXS 86)8 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. The metal atoms
were refined anisotropically, and the hydrogen atoms were placed in
calculated positions (C-H ) 0.96 Å). Refinement on F2 (SHELXL 93)9
against 4256 data led to final convergence with R1 ) 0.0599, wR2 )
0.1175, and S ) 1.072 (Fo > 4σ(Fo)) for 212 refined parameters.
(8) Sheldrick, G. M. Program SHELXS 86; University of Gottingen,
Gottingen, Germany, 1986.
(18) Doyle, G.; Eriksen, K. A.; Van Engen, D. J . Am. Chem. Soc.
1985, 107, 7914.
(19) In spite of the stability of [Au5Fe2(CO)8(dppm)2]+ in most
solvents with both high and low dielectric constant, reaction 4 is
strongly affected by the nature of the solvent and affords several other
products, which are currently under investigation.
(20) [NEt4][Au3Fe2(CO)8(dppm)] (1.2 g) was dissolved in acetonitrile
(30 mL), and solid tropylium tetrafluoroborate (0.2 g) was added with
stirring. After 1 h of stirring, the resulting red reaction mixture was
evaporated to dryness. The residue was dissolved in acetonitrile (20
mL), and the suspension was filtered. [Au5Fe2(CO)8(dppm)2][BF4] was
precipitated as red crystals by layering diisopropyl ether (30 mL) on
top of the solution; yield 73% (based on gold).
(21) Crystal data for [Au5Fe2(CO)8(dppm)2][BF4]: Mr ) 2182.20,
triclinic, space group P1h (No. 2), a ) 10.051(4) Å, b ) 10.086(2) Å, c )
33.284(6) Å, R ) 87.79(1)°, â ) 111.60(2)°, γ ) 82.17(2)°, V ) 3318(2)
Å3, Z ) 2, Dc ) 2.185 Mg m-3, F(000) ) 2020, λ ) 0.710 73 Å, T ) 298
K, µ(Mo KR) ) 11.592 mm-1. Data collection, refinement, and solution
were carried out as above.7 A total of 11 672 independent reflections
were collected (2θmax ) 54°). Refinement on F2 against 11 607 data
led to final convergence with R1 ) 0.0794, wR2 ) 0.1770, and S )
1.076 (Fo > 4σ(Fo)) for 316 refined parameters.
(9) Sheldrick, G. M. Program SHELXL 93, University of Gottingen,
Gottingen, Germany, 1993.
(10) Alvarez, S.; Rossell, O.; Seco, M.; Valls, J .; Pellinghelli, M. A.;
Tiripicchio, A. Organometallics 1991, 10, 2309.
(11) Briant, C. E.; Hall, K. P.; Mingos, D. M. P. J . Chem. Soc., Chem.
Commun. 1983, 843.

Communications
Organometallics, Vol. 16, No. 4, 1997 499
rather scattered (Au(center)-Au(periphery) range,
2.630-2.924(1) Å; Au(periphery)-Au(periphery), 3.051
and 3.312(2) Å). Nevertheless, the average values of
all Au-Fe and Au-Au interatomic separations in the
two independent cations are very close. A similar effect
was found for the two deformation isomers of [Au4Fe4-
(CO)16]4- 2
. The Au-P distances are in the range 2.139-
2.398(6) Å. The figures quoted above show great vari-
ability within each set of chemically equivalent bonds;
therefore, the solid-state structures of these soft mol-
ecules are valuable only to assess the general stereo-
geometry, since the bond distances are intrinsically ill-
defined to tenths of an angstrom.
A simple rationalization of the molecular structures
of [Au3Fe2(CO)8(dppm)]- and [Au5Fe2(CO)8(dppm)2]+ is
in terms of relatively strong Au-Fe and Au-P bonds
around linearly coordinated Au(I) units, in addition to
secondary aurophilic22 interactions. The above features
enable a Tinkertoy construction of clusters according to
isolobal analogy concepts. Finally, it is worth noting
that substitution of dppm for PPh3 as ancillary ligand
in the Au-Fe clusters triggers the occurrence in solution
of disproportionation equilibria (e.g. equilibrium 3),
which appear to be tunable as a function of the dielectric
constant of the solvent and/or the phosphine ligand.
F igu r e 2. Structure of one of the two independent cations
(A) of [Au5Fe2(CO)8(dppm)2]+. The cations sit on inversion
centers. Selected bond distances (Å) and angles (deg) for
A and B (in brackets): Au(1)-Au(2), 2.924(1) [2.630(1)];
Au(1)-Au(3), 2.717(1) [2.840(1)]; Au(2)-Au(3), 3.051(2)
[3.312(2)]; Au(1)-Fe(1), 2.533(3) [2.751(3)]; Au(2)-Fe(1),
2.607(3) [2.650(4)]; Au(3)-Fe(1′), 2.750(4) [2.438(3)]; Au-
(2)-P(1), 2.329(6) [2.277(7)]; Au(3)-P(2), 2.398(6) [2.139-
(6)]; Fe(1)-Au(2)-P(1), 166.2(2) [165.9(2)]; Fe(1′)-Au(3)-
P(2), 170.0(2) [168.9(2)].
Ack n ow led gm en t. We thank the MURST and CNR
for financial help. G.L. wishes to thank the HCM
program of the EEC for a grant; V.G.A. and M.M.
acknowledge financial support from the University of
Bologna (project “Molecole e aggregati molecolari intel-
ligenti”).
with the mirror plane containing the iron atoms. The
two independent half-molecules have the same overall
conformations, and therefore, they are chemically equiva-
lent. The metal frameworks of the cations consist of a
“bow tie” of gold atoms whose tips are bridged by dppm
ligands, whereas the Fe(CO)4 units are triply bridging
the central and two apical gold atoms. The central Au
atoms sit on inversion centers, and the gold frames are
therefore planar. The Fe(CO)4 units adopt idealized Cs
symmetry with the two iron atoms pointing upward and
downward with respect to the Au5 plane (distance from
the plane (0.92 Å). One axial carbonyl in each Fe(CO)4
group is leaning on the central gold, while the other is
almost orthogonal to the metal frame. The Au-Fe
(range 2.438(3)-2.750(3) Å) and Au-Au interactions are
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 [NEt4][Au3Fe2(CO)8(dppm)] and [Au5Fe2(CO)8-
(dppm)2][BF4] (29 pages). Ordering information is given on
any current masthead page.
OM960850G
(22) Schmidbaur, H. Pure Appl. Chem. 1993, 65, 691.