Electronic interactions in iron- and ruthenium-containing heterobimetallic complexes: Structural and spectroscopic investigations

Article abstract of DOI:10.1021/om0610750

The heterobimetallic Ru complexes Cp(CO)Ru(μ-I)(μ-dppm)PtI ₂ (8), Cp(CO)Ru(μ-I)(μ-dppm)PdI₂ (10), and Cp(CO)RuI(μ-dppm)AuI (12) and their isoelectronic Fe analogues Cp(CO)Fe(μ-I)(μ-dppm)PtI₂ (9), Cp(CO)Fe(μ-I)(μ-dppm) PdI<

Full text of DOI:10.1021/om0610750

Organometallics 2007, 26, 3085-3093
3085
Articles
Electronic Interactions in Iron- and Ruthenium-Containing
Heterobimetallic Complexes: Structural and Spectroscopic
Investigations
Daniel Serra, Khalil A. Abboud, Casie R. Hilliard, and Lisa McElwee-White*
Department of Chemistry and Center for Catalysis, UniVersity of Florida, GainesVille, Florida 32611-7200
ReceiVed NoVember 24, 2006
The heterobimetallic Ru complexes Cp(CO)Ru(µ-I)(µ-dppm)PtI2 (8), Cp(CO)Ru(µ-I)(µ-dppm)PdI2 (10),
and Cp(CO)RuI(µ-dppm)AuI (12) and their isoelectronic Fe analogues Cp(CO)Fe(µ-I)(µ-dppm)PtI2 (9),
Cp(CO)Fe(µ-I)(µ-dppm)PdI2 (11), and Cp(CO)FeI(µ-dppm)AuI (13) were prepared by the reactions of
Cp(CO)M(κ1-dppm)I (6, M ) Ru; 7, M ) Fe) with Pt(COD)I2, Pd(COD)I2, and AuI, respectively. All
six complexes were characterized by cyclic voltammetry, IR, UV, and NMR (1H and 31P) spectroscopy,
and elemental analysis. The structures of the I-bridged compounds 8-11 were determined by X-ray
crystallography. Electronic interaction between the two metals is significant for the iodide-bridged
compounds 8-11, as evidenced by the variation in their carbonyl stretching frequencies and UV-vis
spectra, as well as in the shifts of their redox potentials in comparison to the shifts for mononuclear
model compounds. In contrast, compounds 12 and 13, which have only dppm bridges, exhibit limited
interactions between the two metals.
activities or can result in unique properties not observed in
monomeric models.12,15-19
Introduction
A similar effect has been noted for the electrooxidation of
methanol upon comparison of various electrode materials:20-22
initially Pt anodes23-25 and then more complex alloys.25-32
The chemistry of heterobimetallic complexes has attracted
attention, due to the possibility of combining the different
reactivities of the two metals in chemical transformations.1-7
It has been long recognized that interesting characteristics such
as cooperative behavior and/or different mechanistic roles of
the metal centers can be observed when two metals are in close
proximity.2,8-14 It has also been shown that the cooperative
effect in heterobimetallic complexes can enhance catalytic
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10.1021/om0610750 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 05/18/2007

3086 Organometallics, Vol. 26, No. 13, 2007
Serra et al.
Surface studies on Pt anodes showed that Pt is poisoned by
adsorbed CO, which is formed as an intermediate during the
electrooxidation process.33-35 Addition of a second metal
improved the anode behavior, with RuPt26,36-39 systems proving
to be particularly effective. In the “bifunctional mechanism”
initially proposed by Watanabe and Motoo,40 the Pt sites are
responsible for the binding and dehydrogenation of methanol,
while the Ru sites activate water through formation of Ru-
oxo intermediates, which are involved in the conversion of
surface-bound CO to CO2.
Experimental Section
General Procedures. All reactions and manipulations were
performed under an argon atmosphere using standard Schlenk
techniques. Pentane and ethyl ether were dried by distillation from
Na/Ph2CO. Acetonitrile and 1,2-dichloroethane were dried by
distillation from CaH2. Benzene and dichloromethane were dried
with an MBraun solvent purification system. All solvents were
saturated with argon prior to use. All deuterated solvents for NMR
measurements (Cambridge Isotope Laboratories) were degassed via
freeze-pump-thaw cycles and stored over molecular sieves (4 Å).
1H and 31P{1H} NMR spectra were recorded at room temperature
on a Varian Mercury 300 spectrometer operating at 300 and 121
MHz, respectively, with chemical shifts (δ, ppm) reported relative
to tetramethylsilane (1H NMR) or 85% H3PO4 (31P NMR). IR
spectra were obtained as neat films on NaCl using a Perkin-Elmer
Spectrum One FT-IR spectrophotometer. UV-vis spectra were
recorded on a Shimadzu UV-1650PC spectrophotometer using silica
quartz cells (1 cm path length). Elemental analyses (C, H) were
performed by Robertson Microlit Laboratories, Madison, NJ. CpRu-
(PPh3)(µ-I)(µ-dppm)PtI2 (12),53 CpRu(PPh3)I(κ1-dppm) (17),53 CpRu-
(PPh3)I(µ-dppm)AuI (16),53 CpFe(CO)(κ1-dppm)I (7),54 CpRu-
(PPh3)(CO)I (4),55 CpFe(PPh3)(CO)I (5),56 PPh3AuI (18),57 and
CpRu(CO)2I58 were prepared as previously described. All other
starting materials were purchased in reagent-grade purity and used
without further purification.
Electrochemical experiments were performed at ambient tem-
perature in a glovebox using an EG&G PAR Model 263A
potentiostat/galvanostat. Cyclic voltammograms (CV) were recorded
in 3.5 mL of DCE/0.1 M tetrabutylammonium hexafluorophosphate
(TBAH) or tetrabutylammonium triflate (TBAT) at ambient tem-
perature under nitrogen. All potentials are reported versus NHE
and referenced to Ag/Ag+. The reference electrode consisted of a
silver wire immersed in an acetonitrile solution containing freshly
prepared 0.01 M AgNO3 and 0.1 M TBAH or TBAT. The Ag+
solution and silver wire were contained in a 75 mm glass tube fitted
at the bottom with a Vycor tip. Cyclic voltammetry was performed
in a three-compartment H-cell separated by a medium-porosity
sintered-glass frit in 2.5-3.5 mL of DCE/0.1 M TBAH or TBAT
at room temperature under nitrogen. A glassy-carbon electrode
(diameter 3 mm) was the working electrode, and a platinum flag
was used as the counter electrode. All electrochemical measure-
ments were performed inside the glovebox. The E° values for the
ferrocenium/ferrocene couple in DCE/0.1 M TBAH and DCE/0.1
M TBAT were +0.67 and +0.68 V.
Cooperative interactions in heterobinuclear systems have been
the primary target in the development of our catalysts for the
electrooxidation of alcohols and led us to prepare the hetero-
bimetallic complexes Cp(PPh3)Ru(µ-Cl)(µ-dppm)PtCl2 (1),41
Cp(PPh3)Ru(µ-Cl)(µ-dppm)PdCl2 (2),42 and Cp(PPh3)RuCl(µ-
dppm)AuCl (3).42 These complexes were found to be effective
catalysts for electrooxidation of methanol, resulting in much
higher current efficiencies in comparison with those obtained
with the model compound CpRu(PPh3)2Cl.43 Clearly, the
enhancement of the catalytic activity is due to the presence of
the second metal. As a continuation of our work, the replacement
of ruthenium by its congener iron was investigated. Catalysis
by iron complexes has been of recent interest, due to their
reactivity and the possibility of replacing expensive precious
metals with the more readily available first-row metals.44-52 We
now report the synthesis and characterization of new carbonyl-
containing Ru/Pt, Ru/Pd, and Ru/Au derivatives of 1-3, as well
as their isoelectronic Fe/Pt, Fe/Pd, and Fe/Au analogues. X-ray
crystallography, cyclic voltammetry, and IR, UV-vis, and NMR
(1H and 31P) spectroscopy were used to examine the electronic
interaction between the two metal centers.
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CpRu(CO)(κ1-dppm)I (6). In a Schlenk flask a solution of
CpRu(CO)2I (1.08 g, 3.10 mmol) and dppm (1.25 g, 3.25 mmol)
in 150 mL of benzene was refluxed for 12 h. The solvent was
removed under vacuum. Purification of the product was achieved
by column chromatography on silica gel using CH2Cl2 as eluent to
afford 1.53 g (70.1%) of 6 as an orange powder. Anal. Calcd for
C31H27IOP2Ru: C, 52.78; H, 3.86. Found: C, 53.04; H, 3.78.
CpRu(CO)(µ-I)(µ-dppm)PtI2 (8). A solution of 6 (0.400 g,
0.567 mmol) in CH2Cl2 (15 mL) was added to a suspension of
Pt(COD)I2 (0.347 g, 0.625 mmol) in CH2Cl2 (5 mL). The resulting
solution was stirred for 1 h at ambient temperature. The solvent
was removed under vacuum, and purification was achieved by
column chromatography on silica gel using CH2Cl2 as eluent.
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W. Organometallics 2004, 23, 5037-5047.
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944-948.

Fe- and Ru-Containing Heterobimetallic Complexes
Organometallics, Vol. 26, No. 13, 2007 3087
Scheme 1. Synthetic Routes to the Heterobimetallic Complexes
Chart 1
CpRu(CO)I(µ-dppm)AuI (12). A solution of 6 (0.25 g, 0.36
mmol) in CH2Cl2 (15 mL) was added at -20 °C to a solution of
AuI (0.11 g, 0.36 mmol) in CH2Cl2 (15 mL). The reaction mixture
was stirred for 1 h at ambient temperature, and after evaporation
of the solvent, purification was achieved by extraction with
approximately 30 mL of ethyl ether, leading to 12 as an orange
solid. Yield: 0.29 g, 78.5%. Anal. Calcd for C31H27AuI2OP2Ru:
C, 36.17; H, 2.64. Found: C, 36.20; H, 2.45.
CpFe(CO)I(µ-dppm)AuI (13). In the same manner described
for 12, a solution of 7 (0.70 g, 10.6 mmol) and AuI (0.40 g, 10.6
mmol) in CH2Cl2 (50 mL) was stirred for 1 h. After evaporation
of the solvent, purification was achieved by column chromatography
on silica gel using CH2Cl2 as eluent to afford 13 as a dark green
solid. Yield: 0.67 g, 64.3%. Anal. Calcd for C31H27AuFeI2OP2:
C, 37.83; H, 2.77. Found: C, 37.53; H, 2.57.
CpRu(PPh3)(µ-I)(µ-dppm)PdI2 (15). A solution of CpRu(PPh3)-
(κ1-dppm)I (17; 0.40 g, 0.43 mmol), Pd(COD)Cl2 (0.13 g, 0.45
mmol), and NaI (0.32 g, 2.16 mmol) in 50 mL of CH2Cl2 was
reacted in a manner similar to that described above for 10. After
evaporation of the solvent, the purification was achieved by
recrystallization from dichloromethane/pentane to yield 15 as a dark
brown powder. Yield: 0.35 g, 63.5%. 1H NMR (CDCl3): δ 8.09-
5.81 (m, 35H, C6H5), 4.62 (s, 5H, C5H5), 3.49 (m, 1H, PCH2P),
2.94 (m, 1H, (PCH2P)). 31P{1H} NMR (CDCl3): δ 48.23 (dd, Ru-
PPh2, JPP ) 35.4, 20.7 Hz), 40.40 (dd, Ru-PPh3, JPP ) 35.4, 6.7
Hz), 12.7 (dd, Pd-PPh2, JPP ) 20.7, 6.7 Hz). HRMS (FAB): m/z
calcd for C48H42I2P3PdRu, 1172.8667 (M-I)•+; found, 1172.8758.
Yield: 0.37 g, 57.6%. Anal. Calcd for C31H27I3OP2PtRu: C, 32.25;
H, 2.36. Found: C, 32.28; H, 2.26.
CpFe(CO)(µ-I)(µ-dppm)PtI2 (9). Using the same procedure as
for 8, a solution of CpFe(CO)(κ1-dppm)I (0.500 g, 0.756 mmol)
and Pt(COD)I2 (0.463 g, 0.831 mmol) in 20 mL of CH2Cl2 was
stirred for 1 h at ambient temperature. After purification, 9 was
obtained as a pale green solid. Yield: 0.493 g, 62.3%. Anal. Calcd
for C31H27FeI3OP2Pt: C, 33.57; H, 2.45. Found: C, 33.50; H, 2.44.
CpRu(CO)(µ-I)(µ-dppm)PdI2 (10). To a solution of 6 (0.400
g, 0.567 mmol) in CH2Cl2 (20 mL) was added a solution of Pd-
(COD)Cl2 (0.178 g, 0.623 mmol) in CH2Cl2 (20 mL). After the
mixture was stirred for 1 h at ambient temperature, 10 equiv of
NaI (0.85 g, 5.67 mmol) was added and the resulting suspension
was stirred for 24 h. After evaporation of the solvent under vacuum,
purification was achieved by column chromatography on silica gel
(CH2Cl2) to give 10 as a dark brown powder. Yield: 0.503 g,
83.2%. Anal. Calcd for C31H27I3OP2PdRu: C, 34.94; H, 2.55.
Found: C, 34.76; H, 2.34.
CpFe(CO)(µ-I)(µ-dppm)PdI2 (11). A solution of CpFe(CO)-
(κ1-dppm)I (0.200 g, 0.303 mmol), Pd(COD)Cl2 (0.103 g, 0.303
mmol), and NaI (0.450 g, 3.03 mmol) in 50 mL of CH2Cl2 was
reacted in a manner similar to that described above for 10. Similar
purification afforded 11 as a dark brown solid. Yield: 0.203 g,
65.6%. Anal. Calcd for C31H27FeI3OP2Pd: C, 36.49; H, 2.67.
Found: C, 36.56; H, 2.31.
Results and Discussion
Synthesis of Complexes 6 and 7. Details of the preparation
and spectroscopic data for CpFe(CO)(κ1-dppm)I (7) have been
reported.54 The ruthenium analogue was synthesized by fol-
lowing a similar procedure. Reaction of CpRu(CO)2I with dppm
in refluxing benzene afforded CpRu(CO)(κ1-dppm)I (6) in
greater than 70% yield.
Synthesis of Heterobimetallic Complexes 8-13. Reactions
of CpRu(CO)(κ1-dppm)I (6) or CpFe(CO)(κ1-dppm)I (7) with
Pt(COD)I2 in CH2Cl2 afford the I-bridged Ru/Pt complex 8 (57%
yield) and Fe/Pt complex 9 (62% yield), respectively (Scheme
1; see also Chart 1). During the reactions, small amounts of
Pt(κ2-dppm)I2 were formed by dppm transfer to Pt(COD)I2. This
byproduct can be detected by 31P NMR as a singlet with Pt
satellites at ca. -70 ppm. Complexes 8 and 9 were obtained as
a bright orange solid and a pale green solid, respectively. These

3088 Organometallics, Vol. 26, No. 13, 2007
Serra et al.
Table 1. Selected NMR (1H, 31P{1H}) and IR Data for
Complexes 4-13
Table 2. Crystal Data and Structure Refinement Details for
Complexes 8 and 9
1H NMR (δ)a
31P NMR (δ)a
8
9
IR
(cm-1
νCO′
empirical formula
C33H31Cl2I3O-
P2PtRu
1253.28
173(2)
0.710 73
monoclinic
P212121
11.8206(8)
12.9648(9)
24.4701(17)
90
90
90
3750.1(4)
4
2.220
C33H31Cl2I3O-
P2PtFe
1278.96
173(2)
0.710 73
monoclinic
P212121
13.7053(9)
14.9673(9)
18.7364(12)
90
99.6020(10)
90
3789.6(4)
4
2.242
6.908
b
Ru-
Fe-
)
Cp
-CH2-
PPh2-
PPh2- M-PPh2
Mr
T/K
λ/Å
cryst syst
space group
a/Å
b/Å
c/Å
4
5
6
7
8
9
4.92
4.42
49.87 (s)
1950
1941
68.44 (s)
4.85 3.81-3.56 (m)
4.38 3.59 (m), 3.16 (m)
44.3 (d)
-23.8 (d) 1952
63.3 (d) -24.0 (d) 1946
5.02 4.10 (m), 3.73 (m) 43.5 (d)
4.51 3.72 (m)
10 5.07 4.01 (m), 3.64 (m) 46.7 (d)
11 4.56 3.72 (m), 3.44 (m)
12 4.82 4.70 (m), 3.63 (m) 40.6 (d)
13 4.36 4.46 (m), 3.19 (m)
-3.3 (d)
65.3 (d) -4.3 (d)
12.6 (d)
69.2 (d) 12.2 (d)
26.9 (d)
1969
1963
1968
1960
1952
1945
R/deg
â/deg
γ/deg
62.0 (d) 27.6 (d)
V/Å3
a Spectra measured in CDCl3, at room temperature. b Neat film.
Z
Dc/Mg m-3
µ/mm-1
F000
6.852
2328
complexes are moderately stable in the solid state but decompose
slowly in solution if stored outside a glovebox. Since Pd(COD)-
I2 could not be isolated, the I-bridged Ru/Pd complex 10 (83%
yield) and Fe/Pd complex 11 (65% yield) were prepared by
reacting 6 and 7 with Pd(COD)Cl2 in CH2Cl2, followed by
treatment with 10 equiv of NaI. The Ru/Au complex 12 (78%
yield) and Fe/Au complex 13 (64% yield), in which the two
metal centers are linked only via the dppm bridge, were prepared
by direct reaction of 6 and 7 with AuI. Although pure samples
of the Ru/Au complex 12 could be isolated, it was found to be
the most sensitive of the heterobimetallic complexes. Degrada-
tion of 12 could be observed in the solid state even when it
was stored under nitrogen.
2392
cryst size/mm
θ range/deg
index ranges
0.28 × 0.21 × 0.10 0.18 × 0.09 × 0.03
1.66-27.50
- 13 e h e 15
- 14 e k e 16
- 28 e l e 31
22 870
1.71-27.50
- 17 e h e 15
- 19 e k e 12
- 20 e l e 24
24 440
no. of rflns collected
no. of indep rflns (Rint
completeness to θ )
27.49°/%
)
8433 (0.0462)
99.4
8580 (0.0909)
98.4
abs cor
integration
0.5224/0.2130
8433/0/399
integration
0.8179/0.3668
8580/0/352
max/min transmissn
no. of data/restraints/
params
GOF on F2 a
R1b
1.054
0.0264
0.0587
0.899
0.0732
0.1035
2.911, -1.983
Synthesis of Complex 15. The heterobimetallic Ru/Pd
complex 15 was prepared in a manner similar to that for 10
and obtained as a dark brown solid (63%). The stability of
complex 15 was also found to be similar to those of other
I-bridged complexes synthesized in this work.
wR2c
largest diff peak, hole/e Å-3 1.134, -1.043
2
2
2
a GOF ) S ) [w(Fo - Fc )2]/(n - p)]1/2; w ) 1/[σ2(Fo ) + 0.0337p)2
+ 1.24p]; p ) [max(Fo , 0) + 2Fc2]/3. b R1 ) ∑(||Fo| - |Fc||)/∑|Fo|. c wR2
2
) [∑[w(Fo - Fc )2]/∑[w(Fo )2]1/2
.
2
2
2
NMR Data. The 31P NMR spectra of the iron complexes 9,
11, and 13 showed features for the phosphine ligands similar
to those previously reported for CpFe(CO)(µ-I)(µ-dppm)M(CO)4
(M ) Cr, Mo, W).59 A pair of doublets is always observed,
with the downfield resonance corresponding to the Fe-bound
phosphorus, while the upfield doublet is assigned to phosphorus
coordinated to the second metal (Pt, Pd, or Au). The spectra of
the ruthenium complexes 8, 10, and 12 are similar and can be
assigned in an analogous fashion. Chemical shifts (δ) as well
as coupling constants are reported in Table 1.
It has been noted that for metal phosphine complexes of the
same structure and oxidation state, one generally observes a
shift of the 31P resonance to higher field as one descends in a
given group.60 Accordingly, we clearly see a decrease in δP as
one descends from Fe to Ru. Interaction between Fe or Ru
and the second metal can be detected by careful examination
of the M-bound phosphorus (M ) Fe, Ru). We also see a
decrease in δP for the Fe-bound phosphorus from 69.2 to 65.3
ppm as one descends from Pd to Pt in complexes 11 and 9,
respectively. Variation in δP also follows this trend for the Ru
series. When the M′-bound (M′ ) Pt, Pd) phosphorus shifts
are compared, we can clearly see a decrease in δP for the Ru-
bound phosphorus from 46.7 to 43.5 ppm when the metal goes
from Pd to Pt for compounds 10 and 8, respectively. Although
a similar trend in the phosphorus shifts from Fe (62.0 ppm) to
Ru (40.6 ppm) can be observed for the Ru/Au and Fe/Au
complexes 12 and 13, no further comparison with the I-bridged
compounds can be made, due to the differences in structures
and oxidation states compared to those for the I-bridged
complexes.
1
Selected H NMR data for 4-13 are reported in Table 1.
The spectra for the iron complexes 9, 11, and 13 exhibit
chemical shift values similar to those previously published.59
A sharp singlet in the range 4.36-4.51 ppm is observed for the
Cp protons, and the diastereotopic methylene protons of the
bridging dppm appear as two sets of doublet of doublet of
doublets centered in the range 3.19-4.46 ppm as a result of
the coupling to each other and to the two adjacent phosphorus
atoms. The spectra for the ruthenium analogues 8, 10, and 12
are similar, with a sharp singlet for the Cp protons observed in
the range 4.82-5.07 ppm and the two sets of multiplets for the
diastereotopic methylene protons centered in the range 3.63-
4.70 ppm.
IR Spectroscopy. The IR data for compounds 4-13 are
reported in Table 1. The infrared spectra of all the complexes
displayed a single carbonyl stretching frequency in the range
1945-1969 cm-1, characteristic of terminal CO ligands.
Examination of νCO provides a powerful tool to explore the
electronic interactions in the bimetallic complexes. As electron
donation from metal d orbitals to π*CO increases, there is a
concomitant decrease in νCO. On this basis, the infrared data
are consistent with an increase of the electron density at the
carbonyl along the following compound series: Ru/Pt (8) ≈
Ru/Pd (10) < Fe/Pt (9) < Fe/Pd (11) < Ru monomer (4)
≈ Ru monomer (6) ≈ Ru/Au (12) < Fe/Au (11) ≈ Fe
monomer (5) ≈ Fe monomer (7). A possible explanation for
(59) Hsu, M.-A.; Yeh, W.-Y.; Chiang, M. Y. J. Organomet. Chem. 1998,
552, 135-143.
(60) Garrou, P. E. Chem. ReV. 1981, 81, 229-266.

Fe- and Ru-Containing Heterobimetallic Complexes
Organometallics, Vol. 26, No. 13, 2007 3089
Figure 2. ORTEP drawing of the molecular structure of Cp(CO)-
Fe(µ-I)(µ-dppm)PtI2 (9). Thermal ellipsoids are plotted at the 50%
probability level. Hydrogen atoms are omitted for clarity.
Figure 1. ORTEP drawing of the molecular structure of Cp(CO)-
Ru(µ-I)(µ-dppm)PtI2 (8). Thermal ellipsoids are plotted at the 50%
probability level. Hydrogen atoms are omitted for clarity.
Table 3. Selected Bond Distances (Å) and Angles (deg) for
the tendencies in the bridged complexes is that the two metals
communicate inductively through the σ bonds of the iodide
bridge. This would be consistent with the bridging iodide being
an important conduit for the shifts in electronic density from
the Ru or Fe center to Pd or Pt that are observed in complexes
8-11. On the other hand, the values of νCO for the Ru/Au and
Fe/Au complexes 12 and 13, which are bridged only by dppm,
are nearly identical with those of the corresponding mononuclear
Ru and Fe complexes 4-7. This is consistent with limited
interactions between the Ru or Fe and Au through the dppm
bridge.
CpRu(CO)(µ-I)(µ-dppm)PtI2 (8)
Ru1-I1
Pt1-I1
C1-O1
Ru1-C1
Ru1-C2
2.6620(6)
2.5944(4)
1.147(7)
1.864(6)
2.248(8)
Ru1-C3
Ru1-C4
Ru1-C5
Ru1-C6
2.227(7)
2.168(7)
2.156(10)
2.190(10)
Pt1-I1-Ru1
I1-Pt1-I3
I1-Pt1-I2
108.233(15)
173.163(12)
88.38(9)
O1-C1-Ru1
C5-Ru1-I1
174.6(5)
148.6(4)
Table 4. Selected Bond Distances (Å) and Angles (deg) for
CpFe(CO)(µ-I)(µ-dppm)PtI2 (9)
X-ray Crystallography. X-ray structures were obtained for
the I-bridged complexes 8-11. Crystallographic details for the
Ru/Pt complex 8 and the Fe/Pt complex 9 are provided in Table
2. Crystallographic details for the Ru/Pd complex 10 and the
Fe/Pd complex 11 are available in the Supporting Information.
All of these complexes show structures similar to those of
I-bridged complexes previously reported,42,53,59 where the
coordination about the iron and ruthenium atoms retained a
piano-stool configuration and the coordination about the Pt and
Pd atoms is distorted square planar.
Shown in Figures 1 and 2 are the ORTEP drawings of the
Ru/Pt and Fe/Pt compounds 8 and 9, respectively. Selected bond
distances and bond angles for compounds 8 and 9 appear in
Tables 3 and 4, respectively. Structural data for the Ru/Pd
complex 10 and the Fe/Pd complex 11 are available in the
Supporting Information. In each complex, the two metals are
linked by the dppm ligand and iodine atom to form a distorted
six-membered ring. Consistent with the IR data, the carbonyl
ligands for 8-11 are terminal. As previously noted for hetero-
bimetallic iron complexes,59 the cyclopentadienyl ring is bound
asymmetrically to Ru or Fe, with the Ru-C(Cp) distances
ranging from 2.168(7) to 2.248(8) Å, while the Fe-C(Cp)
distances range from 2.071(8) to 2.102(8) Å. The shortest
metal-carbon bond lengths (Ru1-C5 and Fe-C5) are due to
the trans influence from the iodide ligand (C5-Ru1-I1 ) 148°
and C5-Fe-I1 ) 150°). The M-I-M′ angles and bond
distances in 8-11 fall within the range of expected values,61
Fe-I1
Pt-I1
C7-O1
Fe-C7
Fe-C1
2.6001(12)
2.6071(5)
1.138(8)
1.783(8)
2.102(8)
Fe-C2
Fe-C3
Fe-C4
Fe-C5
2.100(7)
2.088(6)
2.073(7)
2.071(8)
Fe-I1-Pt
I1-Pt-I3
I1-Pt-I2
108.46(3)
176.86(2)
87.399(17)
O1-C7-Fe
C5-Fe-I1
173.3(7)
150.1(2)
with asymmetric M-I and M′-I distances varying between
2.5944(4) and 2.6962(5) Å. The Ru-I distance for 8, 2.6620-
(6) Å, is comparable to the value of 2.6749(5) Å reported
previously for the related complex CpRu(PPh3)(µ-I)(µ-dppm)-
PdCl2.41
Cyclic Voltammetry. Cyclic voltammetry was performed on
heterobimetallic complexes 8-13 as well as the monometallic
complexes 4 and 5. The monometallic complexes CpRu(CO)-
(κ1-dppm)I (6) and CpFe(CO)(κ1-dppm)I (7) were not used for
comparison, since after multiple CV cycles for 6 or 7, a new
oxidation wave at the redox potential of the substitution product
CpM(κ2-dppm)I grows in. In addition to these complexes,
electrochemical experiments were also carried out with ruthe-
nium triphenylphosphine complexes 14 and 16,53 as well as with
the Ru/Pd complex 15, which was prepared for comparison
purposes. The CV data for complexes 4, 5, 8-16, and 18 are
given in Table 5, while representative cyclic voltammograms
of complexes 5, 8, and 13 are provided in Figures 3-5,
respectively. The cyclic voltammograms for the heterobimetallic
complexes 8-13 each display three waves in the 1.00-2.27 V
range (e.g., Figures 4 and 5). These redox processes have been
(61) Blake, A. J.; Lippolis, V.; Parsons, S.; Schro¨der, M. J. Chem. Soc.,
Chem. Commun. 1996, 2207-2209.

3090 Organometallics, Vol. 26, No. 13, 2007
Table 5. Formal Potentials for Complexes 4-17a
Serra et al.
complex
couple
E
p,a/V
E1/2b/V
couple
Ep,a/V
E1/2b/V
couple
Ep,a/V
ref
Fc/Fc+
4
5
Fe(II/III)
Ru(II/III)
Fe(II/III)
Ru(II/III)
Fe(II/III)
Ru(II/III)
Ru(II/III)
Fe(II/III)
Ru(II/III)
Ru(II/III)
Fe(II/III)
Ru(II/III)
0.76
1.15
0.95
0.96
0.97
1.10
0.90
0.95
1.29
0.99
0.99
0.97
0.67
i
i
i
i
i
53
i
i
i
i
i
53
Ru(III/IV)
Fe(III/IV)
Ru(III/IV)
Fe(III/IV)
Ru(III/IV)
Ru(III/IV)
Fe(III/IV)
Ru(III/IV)
Ru(III/IV)
Fe(III/IV)
Ru(III/IV)
1.93
2.18
2.00
2.20
1.98
1.99
2.27
1.98
2.02
1.99
1.80
0.88
8
9
Pt(II/IV)
Pt(II/IV)
Pt(II/IV)
Pd(II/IV)
Pd(II/IV)
Pd(II/IV)
Au(I/III)
Au(I/III)
Au(I/III)
Au(I/III)
1.85
1.54
1.49
1.87
1.57
1.55
1.78
1.61
1.54
1.62
1.76
1.48
1.43
1.79
1.51
1.50
1.72
1.49
14c,d,h
10
11
15c,e,h
12
13c
16c,f,h
18c,g,h
0.92
0.89
i
a All potentials obtained in 0.1 M DCE/TBAH (tetrabutylammonium hexafluorophosphate) unless otherwise specified and reported vs NHE. ν ) 10
b
mV/s. E1/2 reported for reversible waves. c Potentials obtained in 0.1 M DCE/TBAT (tetrabutylammonium triflate). d CpRu(PPh3)(µ-I)(µ-dppm)PtI2.
e CpRu(PPh3)(µ-I)(µ-dppm)PdI2. f CpRu(PPh3)I(µ-dppm)AuI. g PPh3AuI. h ν ) 50 mV/s. i This work.
Figure 3. Cyclic voltammograms of complex 5 (5 mM; blue line)
and ferrocene (5 mM; red line) in DCE/0.1 M TBAH at a scan
rate of 10 mV/s.
Figure 5. Cyclic voltammogram of complex 13 (5 mM) in DCE/
0.1 M TBAT at a scan rate of 10 mV/s.
metal. The irreversibility of some of the oxidation waves is
consistent with a chemical reaction following oxidation. Whether
this is ligand loss/exchange or dissociation into monometallic
species is not yet clear.
Electronic interactions between the two metals in the I-bridged
heterobimetallic complexes 8-11 are evidenced by the differ-
ences in redox potential of the second metal (Pt, Pd, or Au)
when a Ru complex is compared to its Fe analogue. These
differences are significantly greater when the two metals are
bridged by both iodide and dppm (cf. 8 and 9, or 10 and 12)
than they are with only the dppm bridge (12 and 13). The cyclic
voltammograms of the Ru/Pt complex 8 and the Fe/Pt complex
9 exhibit irreversible oxidation waves at 0.96 V vs NHE for 8,
assigned to Ru(II/III), and at 0.97 V for the Fe(II/III) couple of
9. The Ru/Pt complex 8 exhibits a second quasi-reversible wave
at 1.76 V assigned to the Pt(II/IV) couple, overlapping with an
irreversible oxidation wave at 2.00 V assigned to the Ru(III/
IV) couple. For the Fe/Pt analogue 9, the quasi-reversible Pt-
(II/IV) wave is shifted negatively about 280 mV (E1/2 ) 1.48
V) in comparison to the analogous wave from complex 8. This
shift is evidence of a more significant electron donation from
Fe to Pt through the I bridge in Fe/Pt complex 9 in comparison
to its Ru/Pt analogue 8. The carbonyl stretching frequencies
for complexes 8 and 9 differ by 6 cm-1, which is roughly the
difference between νCO(Ru) and νCO(Fe) for all the congeneric
pairs of complexes, including monomers 4-7. A similar
difference (about 10 cm-1) has also been reported for the
terminal carbonyls in the heterobimetallic complexes MCl(CO)-
Figure 4. Cyclic voltammogram of complex 8 (5 mM) in DCE/
0.1 M TBAH at a scan rate of 10 mV/s.
previously described for several Ru heterobimetallic complexes
having similar structures.41,42,53 For the ruthenium complexes
8, 10, and 12, the first and third waves are assigned to the Ru-
(II/III) and Ru(III/IV) couples, respectively, while the second
wave is assigned to the redox couple of the second metal. For
iron complexes 9, 11, and 13, the trend is very similar to that
for the ruthenium analogue, with the first and third waves
assigned to the Fe(II/III) and Fe(III/IV) couples, respectively,
and the middle one assigned to the redox couple of the second

Fe- and Ru-Containing Heterobimetallic Complexes
Organometallics, Vol. 26, No. 13, 2007 3091
(µ-CO)(µ-dppm)2PtCl (M ) Fe,62 Ru63). Hence, the difference
in the electronic interactions between Ru/Pt and Fe/Pt that results
in the potential shift for Pt(II/IV) cannot be detected in the IR
data.
data, which suggest limited interactions between the two metals
of 12 and 13 through the dppm bridge.
UV-Vis Spectroscopy. In order to gain further insight into
the electronic structures of these bimetallic complexes, the UV-
vis spectra of the heterobimetallic complexes 8-13 were
recorded in acetonitrile, THF, dichloromethane, and benzene
solutions (Table 6) and compared with those of the correspond-
ing monometallic derivatives 6 and 7. All of the compounds
exhibit intense absorptions in the UV range below 270 nm.
Similar high-energy bands in half-sandwich complexes bearing
phosphine ligands have been assigned to ligand-centered π f
π* electronic transitions.64
The spectrum for the mononuclear Ru complex 6 possesses
two major features: a low-energy band in the visible region at
436 nm (band I, ꢀ ) 428 M-1 cm-1) and a higher energy band
at 324 nm (band II, ꢀ ) 3140 M-1 cm-1). A third band (band
III) is also observed around 275 nm, appearing as a shoulder
on the higher energy feature. The iron complex 7 shows features
similar to those of Ru complex 6. Complex 7 exhibits a low-
energy absorption (band I, 612 nm, ꢀ ) 191 M-1 cm-1) and
two absorptions at higher energy (band II, 438 nm, ꢀ ) 802
M-1 cm-1; band III, 385 nm, ꢀ ) 800 M-1 cm-1). These
transitions are red-shifted by approximately 176, 114, and 110
nm, respectively, from those of Ru complex 6, and their
extinction coefficients are 2-5 times lower. An analogous effect
has been reported for Cp*M(PMe3)2X (M ) Fe, Ru).65 An
additional absorption (band IV, ꢀ ≈ 2700 M-1 cm-1) is also
observed for 7, on the high-energy side, appearing as a shoulder
around 334 nm. The relatively weak intensities of these bands,
low solvent dependence, and band shifts to higher energy in
the series Fe < Ru65,66 allows assignment of bands I-IV as
d-d transitions.
Additional studies on CpRu(PPh3)(µ-I)(µ-dppm)PtI2 (14)
show that the more electron-donating ligand triphenylphosphine
induces a positive shift of the Ru(II/III) redox couple (about
140 mV) and a negative shift of the Pt(II/IV) redox wave (about
340 mV) for the Ru/Pt complex 14, in comparison to the
carbonyl compound 8. This behavior suggests that the PPh3
ligand on Ru leads to stronger electron donation from
Ru to the Pt through the I bridge. On the other hand, the iron
complex 9 shows very similar behavior compared to the
heterobimetallic complex 14, suggesting that the combination
Fe-carbonyl/Pt in complex 9 exhibits equivalent electronic
interactions compared to the combination Ru-PPh3/Pt in
complex 14.
The cyclic voltammograms for the Ru/Pd complex 10 and
the Fe/Pd complex 11 exhibit irreversible oxidations at 0.90
and 0.95 V vs NHE, assigned to the Ru(II/III) and Fe(II/III)
couples, respectively. The Ru/Pd complex 10 exhibits a quasi-
reversible Pd(II/IV) redox wave around 1.79 V vs NHE, which
appears as a shoulder on the irreversible Ru(III/IV) wave (Ep,a
) 1.98 V). As seen for the Fe/Pt complex 9, significant electron
donation through the I bridge is also observed for the Fe/Pd
complex 11, with a quasi-reversible Pd(II/IV) redox wave of
11 that is more easily oxidized by 280 mV (E1/2 ) 1.51 V) in
comparison to the Ru/Pd complex 10. Comparison of complexes
10 and 11 with the triphenylphosphine Ru/Pd compound 15
shows a trend similar to that for the Pt complexes 8, 9, and 14.
The more electron donating triphenylphosphine ligand induces
a significant positive shift of the potential of the Ru(II/III) center
for 15 in comparison to the shift observed for the Ru/Pt complex
14. The Ru(II/III) wave of 15 is shifted to higher potential by
390 mV compared to the value for 14, probably as a conse-
quence of greater electronic donation through the iodide
bridge to the Pd center. This observation is consistent with
the shift of the Pd(II/IV) wave of 15 to lower potential with
respect to the value for the Ru-CO compound 10. As observed
for the analogous Pt complexes 9 and 14, similar behavior is
observed between the Ru-PPh3/Pd 15 and the Fe-carbonyl/
Pd compound 11.
As expected, the absorption spectra of the bimetallic Ru/Au
complex 12 are very similar to those of Ru complex 6 and a
similar relationship is observed for the Fe complex 7 and the
Fe/Au complex 13. These results are consistent with those
obtained in IR and cyclic voltammetry, once again confirming
that the electronic interactions between the two metals through
the dppm bridge are negligible.
Previous studies with the PPh3-substituted compounds Cp-
(PPh3)Ru(µ-X)(µ-dppm)MX2 (M ) Pt, Pd) have shown that the
heterobimetallic complexes tend to exhibit band structures
comparable to those of the corresponding monometallic Ru
complexes.53 These results suggest that the low-energy transi-
tions are localized at the Ru center, with the differences in
absorption attributed to the electronic interactions between the
two metals. Spectra of the Ru/Pt and Fe/Pt heterobimetallic
complexes 8 and 9 are similar to those of 6 and 7, respectively.
Three transitions are observed for the Ru/Pt complex 8. The
first band (band I, 432 nm, ꢀ ) 3010 M-1 cm-1) appears as a
shoulder and has an extinction coefficient 7 times that of the
monometallic Ru complex 6. Transitions II and III are similar
in intensity to those from the Ru monometallic 6 but are slightly
red-shifted. As for the monometallic Fe complex 7, the Fe/Pt
complex 9 exhibits four transitions which are not well resolved.
Band I appears as a tail around 571 nm with a relatively low
extinction coefficient (ꢀ ≈ 270 M-1 cm-1), while band II (417
nm, ꢀ ) 4690 M-1 cm-1), band III (350 nm, ꢀ ) 6450 M-1
The Ru/Au complex 12 exhibits irreversible waves at 0.99
and 2.02 V vs NHE for the Ru(II/III) and Ru(III/IV) couples,
respectively, and a quasi-reversible wave at 1.72 V for the Au-
(I/III) couple. This compound was found to be the least stable
heterobimetallic complex under the conditions of cyclic volta-
mmetry, with degradation evident after several successive cycles.
The cyclic voltammogram of the Fe/Au analogue 13 (Figure 5)
also displays three redox waves: two quasi-reversible at 0.92
and 1.49 V vs NHE assigned to the Fe(II/III) and Au(I/III) redox
couples, respectively, and an irreversible wave at 1.99 V for
the Fe(III/IV) couple. The small wave at 1.13 V corresponds to
the redox potential of CpFe(κ1-dppm)(CO)I (7), a degradation
product of 13. As previously observed for CpRu(PPh3)I(µ-
dppm)AuI (16),53 the waves for the non-iodide-bridged Ru/Au
and Fe/Au complexes 12 and 13 are very similar to those of
the mononuclear compounds CpRu(CO)(PPh3)I (4), CpFe(CO)-
(PPh3)I (5), and PPh3AuI (18). This is consistent with the IR
(64) Cordiner, R. L.; Albesa-Jove´, D.; Roberts, R. L.; Farmer, J. D.;
Puschmann, H.; Corcoran, D.; Goeta, A. E.; Howard, J. A. K.; Low, P. J.
J. Organomet. Chem. 2005, 690, 4908-4919.
(62) Braunstein, P.; Rose, J. In Metal Clusters in Chemistry; Braunstein,
P., Oro, L. A., Raithby, P. R., Eds.; Wiley-VCH: Weinheim, Germany,
1999; Vol. 2, pp 616-677.
(63) Sterenberg, B. T.; Jennings, M. C.; Puddephatt, R. J. Organome-
tallics 1999, 18, 3737-3743.
(65) Bray, R. G.; Bercaw, J. E.; Gray, H. B.; Hopkins, M. D.; Paciello,
R. A. Organometallics 1987, 6, 922-925.
(66) Gerloch, M.; Constable, E. C. Transition Metal Chemistry: The
Valence Shell in d-Block Chemistry; VCH: Weinheim, Germany, 1994.

3092 Organometallics, Vol. 26, No. 13, 2007
Serra et al.
Table 6. UV-Vis Spectral Data of Complexes 6-13
abs, λmax/nm (ꢀ/M-1 cm-1
)
complex
medium
IV
III
II
I
6
benzene
THF
CH2Cl2
acetonitrile
279 (7830) sh
276 (8490) sh
275 (9780) sh
276 (9720) sh
324 (3480)
325 (3110)
324 (3140)
324 (3250)
436 (472)
438 (432)
436 (428)
439 (438)
7
8
benzene
THF
CH2Cl2
acetonitrile
328 (2850) sh
332 (2940)
324 (2700) sh
323 (2430) sh
388 (920) sh
396 (1130) sh
385 (800) sh
381 (720) sh
439 (860)
442 (950)
438 (802)
438 (770)
612 (193)
620 (206)
612 (191)
612 (161)
benzene
THF
CH2Cl2
acetonitrile
334 (3860) sh
328 (8620) sh
335 (7510) sh
331 (5580) sh
388 (3140) sh
387 (4350)
388 (4300)
384 (3170)
431 (2720)
428 (3620) sh
432 (3010) sh
423 (2300) sh
9
benzene
THF
CH2Cl2
acetonitrile
288 (12 120) sh
324 sh
321 (9025) sh
354 (6190)
391 (5970)
350 (6450) sh
326 (7430) sh
438 (3611) sh
423 (5670) sh
417 (4690)
tail
tail
tail
571 (270)
404 (3911)
10
11
12
13
benzene
THF
CH2Cl2
acetonitrile
324 (8830)
524 (2050)
520 (3480)
495 (4720)
482 (4300)
624 (660) sh
641 (885) sh
622 (985) sh
618 (816) sh
327 (14 100)
330 (18 330)
329 (17 380)
397 sh
396 sh
389 (6320) sh
benzene
THF
CH2Cl2
acetonitrile
329 (14 250)
430 (3950) sh
424 (4780) sh
424 (4700) sh
421 (4930) sh
520 (3220)
513 (4370)
499 (4540)
495 (5160)
647 (946) sh
643 (1280) sh
618 sh
337 (14 570) sh
351 (11 790) sh
352 (12 570) sh
613 (1840) sh
benzene
THF
CH2Cl2
acetonitrile
280 (6600)
308 (3820) sh
304 (3650) sh
304 (3760) sh
306 (3350) sh
414 (215)
418 (248)
418 (245)
420 (197) sh
274 (7680) sh
274 (8950) sh
274 (7920) sh
benzene
THF
CH2Cl2
acetonitrile
326 (2950) sh
325 (2950)
325 (3340) sh
327 (1430) sh
386 (996) sh
375 (1660) sh
375 (1160) sh
377 (869) sh
442 (741)
437 (724)
441(814)
440 (697)
607 (182)
610 (149)
611 (199)
612 (158)
cm-1), and band IV (310 nm, ꢀ ) 9025 M-1 cm-1) are similar
to those from complex 7 but exhibit higher intensities. A blue
shift of the d-d transitions is generally expected for isostructural
compounds when the metal is changed from Fe to Ru, due to
increased electronic density at Ru.65 Comparison of the transi-
tions between the two platinum complexes 8 and 9 reveals the
expected blue shift of all the bands. However (except for band
I), the intensities are comparable between the Ru/Pt complex 8
and the Fe/Pt complex 9, which suggests some charge-transfer
character associated with the d-d transitions.
The spectrum for the Fe/Pd complex 11 shows features similar
to those of the Fe complex 7, but the transitions are slightly
red-shifted and are 5-11 times more intense. Three transitions
are observed in the visible region of the spectrum for complex
11. Band I appears as a shoulder around 613 nm, with an
extinction coefficient that is difficult to resolve (ꢀ ≈ 946-1840
M-1 cm-1), followed by two bands with higher intensities (band
II, 499 nm, ꢀ ) 4540 M-1 cm-1; band III, 424 nm, ꢀ ) 4700
M-1 cm-1), while a more intense absorption is also observed
in the near-UV region at 351 nm (band IV, ꢀ ) 11 790 M-1
cm-1). A significant difference is observed for Ru/Pd complex
10 as compared to the monometallic Ru complex 6 and the
heterobimetallic complexes 8 and 12. The spectrum of 10 is
dominated by two transitions: band II in the visible region (495
nm, ꢀ ) 4720 M-1 cm-1) and band IV, appearing at higher
energy (330 nm, ꢀ ) 18 330 M-1 cm-1). Shoulders are also
observed on the lower energy sides of band II (band I, 622 nm,
ꢀ ) 985 M-1 cm-1) and band IV (band III, 396 nm, ꢀ ≈ 6000
M-1 cm-1). First of all, the expected blue shift observed
for the heterobimetallic Ru/Pt and Fe/Pt pair 8 and 9 as
well as that for the heterobimetallic Ru/Au and Fe/Au pair 12
and 13 is not observed for complexes 10 and 11. In fact, the
transitions for the Ru/Pd complex 10 are red-shifted with
respect to those of the model compound 6 and the Ru/Pt
complex 8, which makes them similar to those from the Fe/Pd
complex 11. These spectral characteristics for complex 10
are consistent with charge-transfer transitions. Also consistent
with MLCT is the presence of a solvatochromic effect67,68 for
band II of complexes 10 and 11, in which a blue shift of the
maximum absorption is observed when the polarity of the
solvent increases. These results are consistent with a major role
for the iodide bridge in the electronic structure of the com-
pounds, since the non-iodide-bridged complexes 12 and 13
exhibit spectra very similar to those of the monometallic
complexes 6 and 7.
Conclusion
In summary, this work describes the synthesis and charac-
terization of a new series of heterobimetallic Ru/Pt, Ru/Pd,
and Ru/Au complexes as well as their isoelectronic Fe/Pt, Fe/
Pd, and Fe/Au analogues. The structures of compounds 8-11
were determined by X-ray crystallography and were found to
be similar to those of previously reported halide-bridged
complexes.
The heterobimetallic complexes 8-11 also illustrate the effect
of an I-bridged ligand in mediating the electronic interactions
between the two metals, as can be seen by comparison of the
(67) Szklarzewicz, J.; Makula, A.; Matoga, D.; Fawcett, J. Inorg. Chem.
Acta 2005, 358, 1749-1761.
(68) Fischer, H.; Szesni, N. Coord. Chem. ReV. 2004, 248, 1659-1677.

Fe- and Ru-Containing Heterobimetallic Complexes
Organometallics, Vol. 26, No. 13, 2007 3093
redox potentials, carbonyl stretching frequencies, and UV-vis
transitions of heterobimetallic complexes 8-11 with those of
the monometallic compounds 4-7 and also with those of the
dppm-bridged complexes 12 and 13. Investigations on methanol
oxidation studies using these complexes will be reported in due
course.
X-ray equipment. C.R.H. thanks the Arnold and Mabel Beckman
Foundation for support. We thank Christophe R. G. Grenier for
helpful discussions.
Supporting Information Available: Text, tables, figures, and
CIF files giving experimental procedures for X-ray crystallography,
crystallographic data for 6-9 (including atomic coordinates, bond
lengths and angles, and anisotropic displacement parameters), and
1H and 31P{1H} NMR spectra for 13. This material is available
free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. This work was supported by NASA
Glenn Research Center under Grant No. NAG 3-2930. K.A.A.
wishes to acknowledge the National Science Foundation and
the University of Florida for funding of the purchase of the
OM0610750