Synthesis and Electrochemistry of Heterobimetallic Ruthenium/Platinum and Molybdenum/ Platinum Complexes

Article abstract of DOI:10.1021/ic9510977

As starting materials for heterobimetallic complexes, [RuCp(PPh₃)CO(PPh₂H)]PF₆ and [RuCp(PPh₃)CO(η¹-dppm)]-PF₆ were prepared from RuCp(PPh₃)(CO)Cl. In the course of preparing [RuCp(η²-dppm)(η¹-dppm)]Cl from RuCp(Pha₃P)(η¹-dpprn)Cl, the new monomer RuCpCl(η¹-dppm)₂ was isolated. The uncommon coordination mode of the two monodentate bis(phosphines) was confirmed by X-ray crystallography [a = 11.490(1) A?, b = 14.869-(2) A?, c = 15.447(2) A?, α = 84.63(1)°, β = 70.55(1)°, γ = 72.92(1)°, V = 2378.7(5) A?³, d_calc = 1.355 g cm^-3 (298 K), triclinic, P1?, Z = 2]. The dppm-bridged bimetallic complexes RuCp(PPh3)Cl(M-dppm)Pta2, RuCpCK/tdppm)₂PtCl₂, and [RuCp(PPh₃)CO(μ-dppm)PtCl₂]PF₆ each exhibit electrochemistry consistent with varying degrees of metal-metal interaction. The cationic heterobimetallic complexes [Mo(CO)₃(μ-dppm)₂Pt(H)]PF₆ and [MoCp-(CO)₂(μ-PPh₂)(μ-H)Pt(PPh ₃)(MeCN)]PF₆ were prepared by chloride abstraction from the corresponding neutral bimetallic species and show electrochemical behavior similar to the analogous Ru/Pt complexes.

Full text of DOI:10.1021/ic9510977

916
Inorg. Chem. 1996, 35, 916-922
Synthesis and Electrochemistry of Heterobimetallic Ruthenium/Platinum and Molybdenum/
Platinum Complexes
Stephen D. Orth, Michael R. Terry, Khalil A. Abboud, Brian Dodson, and
Lisa McElwee-White*
Department of Chemistry, University of Florida, Gainesville, Florida 32611
ReceiVed August 23, 1995X
As starting materials for heterobimetallic complexes, [RuCp(PPh3)CO(PPh2H)]PF6 and [RuCp(PPh3)CO(η1-dppm)]-
PF6 were prepared from RuCp(PPh3)(CO)Cl. In the course of preparing [RuCp(η2-dppm)(η1-dppm)]Cl from
RuCp(Ph3P)(η1-dppm)Cl, the new monomer RuCpCl(η1-dppm)2 was isolated. The uncommon coordination mode
of the two monodentate bis(phosphines) was confirmed by X-ray crystallography [a ) 11.490(1) Å, b ) 14.869-
(2) Å, c ) 15.447(2) Å, R ) 84.63(1)°, â ) 70.55(1)°, γ ) 72.92(1)°, V ) 2378.7(5) Å3, dcalc ) 1.355 g cm-3
(298 K), triclinic, P1h, Z ) 2]. The dppm-bridged bimetallic complexes RuCp(PPh3)Cl(µ-dppm)PtCl2, RuCpCl(µ-
dppm)2PtCl2, and [RuCp(PPh3)CO(µ-dppm)PtCl2]PF6 each exhibit electrochemistry consistent with varying degrees
of metal-metal interaction. The cationic heterobimetallic complexes [Mo(CO)3(µ-dppm)2Pt(H)]PF6 and [MoCp-
(CO)2(µ-PPh2)(µ-H)Pt(PPh3)(MeCN)]PF6 were prepared by chloride abstraction from the corresponding neutral
bimetallic species and show electrochemical behavior similar to the analogous Ru/Pt complexes.
Introduction
Interest in bimetallic compounds as models for surface and
catalytic reactions has led to extensive research in this area.1,2
Heterobinuclear complexes are of particular interest since the
differing reactivities of the metals may be exploited in chemical
transformations.3,4 However, it is often difficult to determine
whether observed reactivity is due to the bimetallic complex
itself or to monometallic complexes formed upon fragmentation
of the starting complexes under the reaction conditions.3 In
order to address this problem, various synthetic strategies have
been employed to ensure the integrity of the bimetallic structure.
One of the most successful of these utilizes bridging ligands,
commonly bidentate phosphines such as dppm5-9 or µ-phos-
phido3,10 moieties. A representative example of this approach
is shown in eq 1.8a The monomeric iron complex is constructed
with the pendant dppm attached. The second metal is then
added as an unsaturated fragment generated in situ by loss of
the labile COD ligand. Another common approach to heter-
obinuclear compounds uses “bridge-assisted synthesis”2,6 in
preparing µ-phosphido ligands (eq 2).10a In this strategy, a metal
with a coordinated secondary phosphine is deprotonated, and
the resulting terminal phosphido ligand displaces a labile ligand
from the second metal affording the µ-phosphido-bridged dimer.
Our interest in methanol oxidation at platinum electrodes
bearing either molybdenum or ruthenium atoms11 has led us to
investigate the properties of Mo/Pt and Ru/Pt bimetallic
complexes. Although heterobinuclear complexes are common
in the literature, the number of Ru/Pt complexes is rather small
X Abstract published in AdVance ACS Abstracts, January 15, 1996.
(1) (a) Bruce, M. I. J. Organomet. Chem. 1985, 283, 339-414. (b) Bruce,
M. I. J. Organomet. Chem. 1983, 242, 147-204. (c) Roberts, D. A.;
Geoffroy, G. L. In ComprehensiVe Organometallic Chemistry; Wilkin-
son, G., Stone, F. G. A., Eds.; Pergamon: Oxford, 1982; Chapter 40.
(2) (a) Soriaga, M. P. Chem. ReV. 1990, 90, 771-793. (b) Schwank, J.
Gold Bull. 1985, 18, 2-10. (c) Gonzalez, R. D. Appl. Surf. Sci. 1984,
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1992, 2793-2801. (b) Powell, J.; Fuchs, E.; Gregg, M. R.; Phillips,
J.; Stainer, M. V. R. Organometallics 1990, 9, 387-393. (c) Powell,
J.; Fuchs, E.; Sawyer, J. F. Organometallics 1990, 9, 1722-1729. (d)
Powell, J.; Couture, C.; Gregg, M. R.; Sawyer, J. F. Inorg. Chem.
1989, 28, 3437-3444. (e) Loeb, S. J.; Taylor, H. A.; Gelmini, L.;
Stephan, D. W. Inorg. Chem. 1986, 25, 1977-1982.
(11) (a) Enyo, M.; Machida, K.-i.; Fukuoka, A.; Ichikawa, M. In Electro-
chemistry in Transition; Murphy, O. J., Ed.; Plenum Press: New York,
1992; pp 359-369. (b) Ross, P. N. Electrochim. Acta 1991, 36, 2053-
2062. (c) Leger, J.-M.; Lamy, C. Ber. Bunsen-Ges. Phys. Chem. 1990,
94, 1021-1025. (d) Goodenough, J. B.; Hamnett, A.; Kennedy, B. J.;
Manoharan, R.; Weeks, S. A. J. Electroanal. Chem. 1988, 240, 133-
145. (e) Beden, B.; Kadirgan, F.; Lamy, C.; Leger, J. M. J. Electroanal.
Chem. 1981, 127, 75-85. (f) Swathirajan, S.; Mikhail, Y. M. J.
Electrochem. Soc. 1991, 138, 1321-1326. (g) Wang, J.; Nakajima,
H.; Kita, H. Electrochim. Acta 1990, 35, 323-328. (h) Wang, J.;
Nakajima, H.; Kita, H. J. Electroanal. Chem. 1988, 250, 213-217.
(i) Kita, H.; Nakajima, H.; Shimazu, K. J. Electroanal. Chem. 1988,
248, 181-191. (j) McNicol, B. D. Proc. Electrochem. Soc. 1978, 79
(2), 93-122. (k) Adzˇic´, R. R. In Energy Storage, Trans. Int. Assem.;
Silverman, J., Ed.; Pergamon: Oxford, 1980; pp 212-220.
(3) He, Z.; Jugan, N.; Neibecker, E.; Mathieu, R.; Bonnet, J.-J. J.
Organomet. Chem. 1992, 426, 247-259.
(4) Delavaux, B.; Chaudret, B.; Devillers, J.; Dahan, F.; Commenges, G.;
Poilblanc, R. J. Am. Chem. Soc. 1986, 108, 3703-3711.
(5) Abbreviations: dppm ) bis(diphenylphosphino)methane, dppe ) bis-
(diphenylphosphino)ethane, Cp ) cyclopentadienyl, TBAH ) tet-
rabutylammonium hexafluorophosphate, COD ) 1,5-cyclooctadiene,
DME ) dimethoxyethane.
(6) Puddephatt, R. J. Chem. Soc. ReV. 1983, 99-127.
(7) Chaudret, B.; Delavaux, B.; Poilblanc. R. Coord. Chem. ReV. 1988,
86, 191-243.
(8) (a) Jacobsen, G. B.; Shaw, B. L.; Thornton-Pett, M. J. Chem. Soc.,
Chem. Commun. 1986, 13-15. (b) Jacobsen, G. B.; Shaw, B. L.;
Thornton-Pett, M. Inorg. Chim. Acta 1986, 121, L1-L2. (c) Fontaine,
X. L. R.; Jacobsen, G. B.; Shaw, B. L.; Thornton-Pett, M. J. Chem.
Soc., Dalton Trans. 1988, 741-750.
(9) (a) Braunstein, P.; Oswald, B.; DeCian, A.; Fischer, J. J. Chem. Soc.,
Dalton Trans. 1991, 2685-2692. (b) Shaw, B. L.; Blagg, A. J. Chem.
Soc., Dalton Trans. 1987, 221-226. (c) Blagg, A.; Hutton, A. T.;
Pringle, P. G.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1984, 1815-
1822.
0020-1669/96/1335-0916$12.00/0 © 1996 American Chemical Society

Heterobimetallic Ru/Pt and Mo/Pt Complexes
Inorganic Chemistry, Vol. 35, No. 4, 1996 917
(CH2Cl2): Epa ) 1.95 V. Anal. Calcd for C36H31F6OP3Ru‚0.5CH2-
Cl2: C, 52.80; H, 3.86. Found: C, 53.24; H, 3.59.
(Mo/Pt complexes are more common). The electrochemical
properties of these compounds, for the most part, remain
uninvestigated. We report here the synthesis, structure, and
electrochemical characterization of some new Mo/Pt and Ru/
Pt heterobinuclear complexes with bis(phosphine) and phosphido
bridges. The focus of the work is characterization of the dimeric
species using cyclic voltammetry and correlation of the observed
redox potentials with those of the analogous monomers to gain
insight into the oxidation behavior of the binuclear complexes.
Preparation of [RuCp(PPh3)CO(η1-dppm)]PF6 (2). RuCp-
(PPh3)(CO)Cl (0.5 g, 1 mmol), dppm (1.17 g, 3.05 mmol), and NH4-
PF6 (0.50 g, 3.1 mmol) were dissolved in MeOH (100 mL). The
mixture was heated to 60 °C for 8 h and then allowed to cool to room
temperature. The solvent was removed to give a bright yellow residue,
CH2Cl2 (25 mL) was added, and the supernatant was filtered through
Celite. The volatile components were removed to give a yellow solid
that was washed with hexane (3 × 10 mL) and recrystallized twice
from 2:5 CH2Cl2/ether to give 0.14 g of 2 as an off-white solid (14%
yield). Note: The product contained 0.25 equiv of CH2Cl2 as indicated
by 1H NMR. 1H NMR (CDCl3): δ 7.8-6.9 (m, 35H, PPh3 and Ph2P-
CH2-PPh2), 4.96 (s, 5H, Cp), 2.70 (br d, 1H, JHH ) 16 Hz, Ph2P-
CH2-PPh2), 1.79 (dd, 1H, JHH ) 16 Hz, JPH ) 10 Hz, Ph2P-CH2-
PPh2). 31P NMR (CDCl3): δ 44.9 (d, JPP ) 27 Hz, PPh3), 34.5 (dd,
JPP ) 47, 27 Hz, Ru-PPh2-CH2-PPh2), -28.6 (d, JPP ) 47 Hz, Ru-
PPh2-CH2-PPh2). IR (CH2Cl2): νCO ) 1976 (s) cm-1. CV (CH2-
Cl2): Epa ) 1.91 V. Anal. Calcd for C49H42F6OP4Ru‚0.25CH2Cl2: C,
58.73; H, 4.22. Found: C, 58.85; H, 4.28.
Experimental Section
General Methods. Standard Schlenk/vacuum techniques were used
throughout. Hexane, petroleum ether, chloroform, and methylene
chloride were distilled from CaH2. Diethyl ether, THF, toluene, and
dimethoxyethane were distilled from Na/Ph2CO. All NMR solvents
were degassed by three freeze-pump-thaw cycles. Benzene-d6 was
vacuum transferred from Na/Ph2CO. CDCl3 was stored over 3 Å
molecular sieves. All other starting materials were purchased in reagent
grade and used without further purification. 1H, 31P, and 13C NMR
spectra were recorded on Varian VXR-300 or Gemini-300 NMR
spectrometers. IR spectra were recorded on a Perkin-Elmer 1600
spectrometer. Elemental analyses were performed at the University
of Florida.
Electrochemical experiments were performed under nitrogen using
a PAR Model 273 potentiostat/galvanostat or an IBM EC225 voltam-
metric analyzer. Cyclic voltammograms were recorded at room
temperature in a standard three-electrode cell with a glassy carbon
working electrode. All potentials are reported vs NHE and were
determined in CH2Cl2 or dimethoxyethane (DME) using 0.5 or 0.1 M
TBAH, respectively. Ferrocene (E1/2 ) 0.55 V), decamethylferrocene
(E1/2 ) 0.04 V), or cobaltocenium hexafluorophosphate (E1/2 ) -0.78
V) was used in situ as a calibration standard. Bulk electrolyses were
performed using 2.0 × 3.5 cm stainless steel plates for the working
and auxiliary electrodes in a standard three-electrode cell. The stirred
solutions were electrolyzed at the appropriate potential until a color
change was observed. RuCp(Ph3P)(CO)Cl,12 RuCp(Ph3P)(η1-dppm)-
Cl,13 Pt(COD)Cl2,14 Pt(PhCN)2Cl2,15 Mo(CO)3(µ-dppm)2Pt(H)Cl,16 and
MoCp(CO)2(µ-PPh2)(µ-H)Pt(PPh3)Cl17 were prepared by literature
methods. Ru3(CO)12, K2PtCl4, PtCl2, and RuCl3‚xH2O were obtained
from Johnson Matthey and used as received.
Preparation of RuCp(η1-dppm)2Cl (3). In the course of preparing
[RuCp(η2-dppm)(η1-dppm)]Cl from RuCp(Ph3P)(η1-dppm)Cl (2.41 g,
2.84 mmol) and dppm (1.64 g, 4.26 mmol),13 the orange mother liquor
obtained after precipitation of this complex was evaporated to dryness
to give an orange solid. The solid was redissolved in a minimal amount
of CH2Cl2 (5 mL) and filtered through Celite. Hexane (15 mL) was
added and the solution allowed to stand undisturbed over several hours,
resulting in the formation of a red/orange crystalline solid. A second
recrystallization of the resulting solid from CH2Cl2/hexane gave 3 as
0.92 g of red/orange crystals (33% yield). The remainder of the
ruthenium from the reaction was identified as 3, RuCp(η2-dppm)Cl,18
and starting material (Vide infra). Attempts to separate these complexes
via further chromatography failed. 3. 1H NMR (CDCl3): δ 7.3-6.8
(m, 40H, Ph2P-CH2-PPh2), 4.39 (s, 5H, Cp), 3.70 (br d, 2H, JHH
)
15 Hz, Ph2P-CH2-PPh2), 1.91 (br d, 2H, Ph2P-CH2-PPh2). 13C
NMR (CDCl3): δ 133.2 (d, JCP ) 56 Ηz), 132.9 (d, JCP ) 55 Ηz),
129.0 (d, JCP ) 9 Ηz), 128.3 (d, JCP ) 22 Ηz), 128.1 (d, JCP ) 30
Ηz), 128.0 (d, JCP ) 2 Ηz), 127.6 (m), 126.9 (m), 84.3 (s, Cp), 25.8
(m). 31P NMR (CDCl3): δ 39.3 (overlapping d of virtual t, JPP ) 41,
34 Hz, Ru-PPh2-CH2-PPh2), -25.2 (overlapping d of virtual t, JPP
) 41, 32 Hz, Ru-PPh2-CH2-PPh2). CV (CH2Cl2): E1/2 ) 0.50 V.
Anal. Calcd for C55H49ClP4Ru: C, 68.08; H, 5.09. Found: C, 68.19;
H, 5.08.
Preparation of [RuCp(PPh3)CO(PPh2H)]PF6 (1). RuCp(PPh3)-
(CO)Cl (0.5 g, 1 mmol) was added to a flask containing MeOH (20
mL) and PPh2H (0.95 g, 5.1 mmol, 0.88 mL). A solution of NH4PF6
(0.50 g, 3.1 mmol) in MeOH (15 mL) was added via cannula and the
mixture heated to 60 °C for 12 h. After cooling to room temperature,
the volatile components were removed in Vacuo and CH2Cl2 (20 mL)
was added to the residue giving a bright yellow solution and a white
precipitate. The supernatant was filtered through Celite and concen-
trated to ca. 10 mL, and ether (15 mL) was added, resulting in the
formation of a white microcrystalline solid over 1 h. After the mother
liquor was removed via cannula, the solid was washed with ether,
redissolved in a minimal amount of CH2Cl2, and reprecipitated with
ether to give 1 as 0.49 g of a white solid (61% yield). Note: The
Preparation of RuCp(PPh3)Cl(µ-dppm)PtCl2 (4). A Schlenk flask
was charged with RuCp(PPh3)(η1-dppm)Cl (1.5 g, 1.8 mmol) and CH2-
Cl2 (50 mL). A solution of Pt(COD)Cl2 (0.66 g, 1.8 mmol) in CH2Cl2
(25 mL) was then added via cannula to give a red/orange solution.
The solution was stirred overnight at room temperature and filtered
through Celite. Removal of solvent afforded a red/orange solid which
was washed with 1:1 hexane/ether to give an orange/brown residue.
The residue was dissolved in CH2Cl2 with gentle heating and repre-
cipitated with 1:1 hexane/ether, giving 4 as 1.51 g of an orange powder
(77% yield). 1H NMR (CDCl3): δ 8.0-6.0 (m, 35H, Ph2P-CH2-
PPh2 and PPh3), 4.59 (s, 5H, Cp), 2.71 (overlapping m, 2H, Ph2P-
CH2-PPh2). 13C NMR (CD2Cl2): δ 137.9-127.2 (aromatic), 82.0 (s,
Cp), 59.4 (m, PPh2-CH2-PPh2). 31P NMR (CDCl3): δ 49.1 (dd, JPP
) 21, 36 Hz, Ru-PPh2-CH2-PPh2), 37.8 (d, JPP ) 36 Hz, Ru-PPh3),
-2.9 (d, JPP ) 20 Hz, JPPt ) 3826 Hz, Ru-PPh2-CH2-PPh2). CV
(CH2Cl2): E1/2 ) 1.13 V, Epa ) 1.78 V. Anal. Calcd for C48H42Cl3P3-
PtRu: C, 51.74; H, 3.80. Found: C, 51.32; H, 3.80.
1
product contained 0.5 equiv of CH2Cl2 as indicated by H NMR. 1H
NMR (CDCl3): δ 7.5-7.0 (m, 25H, PPh3 and PPh2H), 6.8 and 5.5
(1H, dd, JPH ) 384, 8 Hz, PPh2H), 5.11 (s, 5H, Cp). 13C NMR
(CDCl3): δ 200.2 (t, JCP ) 17 Hz, CO), 133.0 (d, JCP ) 11 Hz), 132.6
(d, JCP ) 11 Hz), 131.9 (s), 131.6 (d, JCP ) 2 Hz), 130.6 (d, JCP ) 19
Hz), 129.5 (d, JCP ) 19 Hz), 129.4 (d, JCP ) 20 Hz), 129.1 (d, JCP
)
Preparation of RuCpCl(µ-dppm)2PtCl2 (5). RuCp(η1-dppm)2Cl
(3, 0.15 g, 0.16 mmol) was dissolved in 10 mL of CH2Cl2, and a solution
of Pt(PhCN)2Cl2 (0.07 g, 0.16 mmol) in CH2Cl2 (5 mL) was added.
After the mixture was stirred overnight, the solvent was removed,
11 Hz), 89.5 (s, Cp). 31P NMR (CDCl3): δ 48.1 (d, JPP ) 32 Hz, PPh3),
30.0 (d, JPP ) 32 Hz, PPh2H). IR (CH2Cl2): νCO ) 1989 (s) cm-1. CV
(12) Davies, S. G.; Simpson, S. J. J. Chem. Soc., Dalton Trans. 1984, 993-
994.
(13) Bruce, M. I.; Humphrey, M. G.; Patrick, J. M.; White, A. H. Aust. J.
Chem. 1983, 36, 2065-2072.
(14) Clark, H. C.; Manzer, L. E. J. Organomet. Chem. 1973, 59, 411-
428.
(18) See ref 13 for original report of the complex. Our spectral data for
RuCp(η2-dppm)Cl differ from those reported. The identity of the
compound was confirmed with an X-ray crystal structure (Terry, M.
1
R.; Abboud, K.; McElwee-White, L. Unpublished results.) H NMR
(15) Hartley, F. R. Organomet. Chem. ReV., Sect. A 1970, 6, 119-137.
(16) Blagg, A.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1987, 221-226.
(17) Powell, J.; Sawyer, J. F.; Smith, S. J. J. Chem. Soc., Dalton Trans.
1992, 2793-2801.
(CDCl3): δ 7.7-7.2 (m, 20H, Ph2P-CH2-PPh2), 5.06 (dt, 1H, JHH )
14.7 Hz, JPH ) 10.2 Hz, Ph2P-CH2-PPh2), 4.69 (s, 5H, Cp), 4.34
(dt, 1H, JHH ) 14.7 Hz, JPH ) 11.1 Hz, Ph2P-CH2-PPh2). 31P NMR
(CDCl3): δ 13.6 (s, PPh2-CH2-PPh2).

918 Inorganic Chemistry, Vol. 35, No. 4, 1996
Orth et al.
affording an orange solid. The solid was reprecipitated from CH2Cl2/
hexane to give 5 as 0.18 g of an orange powder (91% yield). Note:
The product contained 1 equiv of CH2Cl2 as indicated by 1H NMR. 1H
NMR (CDCl3): δ 8.12 (m, 8H), 7.46-6.77 (aromatic, 32H), 5.03 (s,
5H, Cp), 3.21 (m, 4H, Ph2P-CH2-PPh2). 13C NMR (CDCl3): δ
applied on the basis of measured crystal faces using SHELXTL plus:19
absorption coefficient, µ ) 0.56 mm-1 (minimum and maximum
transmission factors are 0.883 and 0.946, respectively).
The structure was solved by the heavy-atom method in SHELXTL
plus from which the location of the Ru atom was obtained. The rest
of the non-hydrogen atoms were obtained from a subsequent difference
Fourier map. The structure was refined in SHELXTL plus using the
full-matrix least-squares method. The non-H atoms were treated
anisotropically, whereas the positions of the hydrogen atoms were
calculated in ideal positions and their isotropic thermal parameters were
fixed. A total of 550 parameters were refined, and ∑w(|Fo| - |Fc|)2
133.9-127.5 (aromatic), 91.3 (s, Cp), 61.8 (m, Ph2P-CH2-PPh2). 31
P
NMR (CDCl3): δ 43.2 (d, JPP ) 24 Hz, Ru-PPh-CH2-PPh2), -2.6
(d, JPP ) 19 Hz, JPPt ) 2372 Hz, PPh2-CH2-PhP-Pt). CV (CH2-
Cl2): Epa1 ) 1.13 V, Epa2 ) 1.45 V. Anal. Calcd for C55H49Cl3P4Pt-
Ru‚CH2Cl2: C, 50.90; H, 3.86. Found: C, 50.70; H, 3.78.
Preparation of [RuCp(PPh3)CO(µ-dppm)Pt(Cl)2]PF6 (6). RuCp-
(PPh3)Cl(µ-dppm)PtCl2 (4, 0.143 g, 0.129 mmol) was partially dissolved
in DME, and then CO was bubbled through the mixture for 15 min. A
CO-saturated solution of TlPF6 (0.055 g, 0.16 mmol) in DME was added
and the mixture stirred under ambient CO pressure for 10 h. The
mixture was filtered and the filtrate evaporated to dryness. Reprecipi-
tation from DME and Et2O gave 6 as 0.11 g of a yellow solid (73%
yield). 1H NMR (CDCl3): δ 7.98 (m, 4H), 7.66-7.22 (aromatic, 31H),
5.59 (s, 5H, Cp), 2.17 (m, 2H, Ph2P-CH2-PPh2). 13C NMR
(CDCl3): δ 218.4 (s, CO), 133.0-132.2, 129.8-128.1 (aromatic), 90.7
(s, Cp), 71.8 (m, Ph2P-CH2-PPh2). IR (CH2Cl2): νCO ) 1979 (s)
cm-1. CV (CH2Cl2): Epa1 ) 1.43 V, Epa2 ) 1.68 V. Anal. Calcd for
C49H42Cl2F6P4PtRu: C, 50.13; H, 3.58. Found: C, 49.81; H, 3.40.
Preparation of [Mo(CO)3(µ-dppm)2Pt(H)]PF6 (7). Mo(CO)3(µ-
dppm)2Pt(H)Cl (0.75 g, 0.64 mmol) was dissolved in CH2Cl2 (20 mL)
and MeCN (10 mL). A slurry of TlPF6 (0.22 g, 0.64 mmol) in CH2-
Cl2 (10 mL) was added, resulting in an orange/brown solution over an
off-white precipitate. The mixture was stirred for 30 min and then
filtered through Celite. Ether (15 mL) was added to the filtrate, giving
a microcrystalline solid over 30 min. The product was recrystallized
from CH2Cl2/ether and dried under vacuum to give 7 as 0.74 g of an
orange microcrystalline solid (90% yield). Note: The product contained
was minimized: w ) 1/(σ|Fo|)2, σ(Fo) ) 0.5kI -1/2{[σ(I)]2
+
(0.02I)2 }1/2, I (intensity) ) (I peak - Ibackground)(scan rate), σ(I) ) (Ipeak
+ Ibackground)
1/2(scan rate), k is the correction due to decay and Lp effects,
and 0.02 is a factor used to down-weight intense reflections and to
account for instrument instability. The linear absorption coefficient
was calculated from values from the International Tables for X-ray
Crystallography.20 Scattering factors for non-hydrogen atoms were
taken from Cromer and Mann21 with anomalous-dispersion corrections
from Cromer and Liberman,22 while those of hydrogen atoms were
from Stewart, Davidson, and Simpson.23
Results and Discussion
Synthesis of Mononuclear Complexes 1-3. Reaction of
RuCp(PPh3)(CO)Cl with NH4PF6 in MeOH in the presence of
a phosphine results in substitution of phosphine for chloride to
yield RuCp(PPh3)(CO)L+ [L ) PPh2H (1) or dppm (2)] (eq 3),
1
1 equiv of CH2Cl2 as indicated by H NMR. 1H NMR (CD2Cl2): δ
7.5-7.1 (m, 40H, Ph2P-CH2-PPh2), 3.12 (br m, 4H, Ph2P-CH2-
PPh2), -2.86 (tt, JHPt ) 1453 Hz, JHP ) 11 Hz, Pt-H). 13C NMR
(CDCl3): δ 232.1, 191.6, 190.8 (CO), 149.3, 135.1-128.3 (Ph2P-
CH2-PPh2), 66.2 (Ph2P-CH2-PPh2). 31P NMR (CD2Cl2): δ 42.9 (t,
JPP ) 46 Hz, Mo-Ph2P-CH2-PPh2), 22.1 (d, JPP ) 46 Hz, JPPt
)
2452 Hz, Pt-Ph2P-CH2-PPh2). IR (CH2Cl2): νCO ) 1998 (s), 1844
(s), 1806 (s) cm-1. CV (CH2Cl2): E1/2 ) 0.79 V, Epa ) 1.83 V. Anal.
Calcd for C53H45F6MoO3P5Pt‚CH2Cl2: C, 47.17; H, 3.42. Found: C,
47.80; H, 3.32.
similar to reactions of this starting material reported by Davies
and Simpson.12 The resulting cations, 1 and 2, are produced in
good yield when L ) PPh2H (61%), but much lower yield with
L ) dppm (14%). Alternatively, 1 could be prepared by
reacting RuCp(PPh3)(CO)Cl and PPh2H with TlPF6 in CH2Cl2,
although the yield is slightly lower (58%). Attempts to prepare
2 under the TlPF6/CH2Cl2 conditions proved unsuccessful.
Analysis of these reaction mixtures, after stirring overnight at
room temperature or refluxing 4 h, indicated only unreacted
starting material.
Preparation of [MoCp(CO)2(µ-PPh2)(µ-H)Pt(PPh3)(MeCN)]PF6
(8). A Schlenk flask was charged with MoCp(CO)2(µ-PPh2)(µ-H)Pt-
(PPh3)Cl (0.50 g, 0.56 mmol, 5:1 mixture of isomers, see eq 8) and
MeCN (15 mL). A solution of TlPF6 (0.19 g, 0.56 mmol) in MeCN
(5 mL) was added, giving a yellow/orange solution and an off-white
precipitate. The mixture was stirred for 30 min, concentrated to ca.
10 mL, and filtered through Celite. Evaporating the filtrate to dryness
under reduced pressure afforded an orange solid. The solid was
recrystallized from CH2Cl2/hexane to give 0.55 g of 8 as a mixture of
isomers (94% yield). Isomer A. 1H NMR (CD2Cl2): δ 7.6-7.1 (m,
25H, PPh3 and µ-PPh2), 5.13 (s, 5H, Cp), 2.09 (s, 3H, NCCH3), -9.5
(dd, JPP ) 27, 77 Hz, JPPt ) 477 Hz, µ-H). 13C NMR (CD2Cl2): δ
243.6, 193.4 (CO), 134-129 (aromatic carbons), 91.9 (Cp), 23.0
(virtual t, JCPt ) 658 Hz, NCCH3). 31P NMR (CD2Cl2): δ 97.6 (s, JPPt
) 3051 Hz, µ-PPh2), -35.5 (d, JPP ) 70 Hz, JPPt ) 4099 Hz, Pt-
PPh3). Isomer B. 1H NMR (CD2Cl2): δ 4.76 (s, Cp), 1.89 (s, NCCH3),
-16.6 (dd, JPP ) 9, 18 Hz, JPPt ) 680 Hz, µ-H). 13C and 31P NMR
peaks for this isomer were not sufficiently resolved for a confident
assignment. Mixture 8A,B. CV (CH2Cl2): Epa1 ) 0.94 V, Epa2 ) 1.13
V, Epa3 ) 1.65 V, Epa4 ) 1.82 V. IR (CH2Cl2): νCO ) 1971 (s), 1905
(s) cm-1. Anal. Calcd for C39H34NF6MoO2P3Pt: C, 44.82; H, 3.26;
N, 1.34. Found: C, 44.71; H, 3.34; N, 1.14.
The 1H NMR spectra of both 1 and 2 show numerous
aromatic resonances overlapping between 7.8 and 6.9 ppm. Each
spectrum also displays a characteristic singlet for the Cp
resonance at 5.11 (1) or 4.96 ppm (2). The phosphine proton
of 1 is seen as a doublet of doublets with PH coupling constants
of 384 Hz for the attached phosphorus and 8 Hz for the ligated
PPh3. The methylene protons of 2 are seen at 2.70 and 1.79
ppm, also as doublets of doublets, with PH coupling constants
of 16 and 10 Hz. In the IR spectrum of 2, the CO stretch is
observed at 1976 cm-1, while that of 1 is at 1989 cm-1
,
consistent with their formulation as Ru(II) complexes (e.g., for
Crystal Structure Determination of RuCp(η1-dppm)2Cl (3). Data
were collected at room temperature on a Siemens P3m/V diffractometer
equipped with a graphite monochromator utilizing Mo KR radiation
(λ ) 0.710 73 Å). Thirty-two reflections with 20.0° e 2θ e 22.0°
were used to refine the cell parameters, and 8848 reflections were
collected using the ω-scan method. Four reflections were measured
every 96 reflections to monitor instrument and crystal stability
(maximum correction on I was <1%). Absorption corrections were
(19) Sheldrick, G. M. SHELXTL plus; Nicolet XRD Corporation: Madison,
WI, 1990.
(20) International Tables for X-ray Crystallography; Kynoch Press: Bir-
mingham, 1974; Vol. IV, p 55.
(21) Cromer, D. T.; Mann, J. B. Acta Crystallogr. 1968, A24, 321-324.
(22) Cromer, D. T.; Liberman, D. J. Chem. Phys. 1970, 53, 1891-1898.
(23) Stewart, R. F.; Davidson, E. R.; Simpson, W. T. J. Chem. Phys. 1965,
42, 3175-3187.

Heterobimetallic Ru/Pt and Mo/Pt Complexes
Inorganic Chemistry, Vol. 35, No. 4, 1996 919
Table 1. Crystallographic Data for 3
formula
a, Å
b, Å
C55H49P4ClRu
11.490(1)
14.869(2)
15.447(2)
84.63(1)
70.55(1)
72.92(1)
2378.7(5)
2
fw, g mol-1
space group
T, °C
970.34
P1h (No. 2)
25
0.71073
1.355
c, Å
λ, Å
R, deg
â, deg
γ, deg
V, Å3
Z
F
calc, g cm-3
µ cm-1
R
Rw
0.56
0.0368a
0.0415a
a R ) ∑(||Fo| - |Fc||)/∑|Fo|, Rw ) [∑w(|Fo| - |Fc|)2/∑|Fo| ]1/2
.
2
Table 2. Selected Bond Lengths (Å) of Compound 3
Cl-Ru
P1-Ru
P3-Ru
C1-Ru
2.452(1)
2.319(1)
2.314(1)
2.191(4)
C2-Ru
C3-Ru
C4-Ru
C5-Ru
2.229(5)
2.226(5)
2.221(4)
2.194(3)
Figure 1. Molecular structure of 3, with 50% probability ellipsoids,
showing the atom numbering scheme. The phenyl groups are symbol-
ized by “Ph” with a number that refers to the C atom bonded to P.
Table 3. Selected Bond Angles (deg) of Compound 3
Cl-Ru-P1
Cl-Ru-P3
89.91(3)
87.97(3)
P1-Ru-P3
97.20(4)
RuCp(PPh3)(CO)Cl νCO ) 1959 cm-1).12 The lower energy
stretch of the η1-dppm complex reflects the slightly greater
donating ability (or lesser π-acidity) of the dppm ligand relative
to that of PPh2H. This electronic difference is also observed
in the oxidation potentials of the compounds (Vide infra).
Isolation of RuCp(η1-dppm)2Cl (3) from the literature prepa-
ration of [RuCp(η2-dppm)(η1-dppm)]Cl13 arose from an attempt
to determine the fate of the remaining starting material after
the reaction. After precipitation of [RuCp(η2-dppm)(η1-dppm)]-
Cl in low yield (ca. 20%), 3 was obtained from the filtrate in
moderate yield (33%) by crystallization. Attempts to improve
the yield of 3 by reaction of either RuCp(PPh3)2Cl or RuCp-
(PPh3)(η1-dppm)Cl with dppm under a variety of conditions
gave mixtures of the products noted above in varying ratios;
however, the yield of 3 was not improved beyond 33%.
In addition to the aromatic signals for the dppm phenyl
groups, the 1H NMR spectrum of 3 exhibits methylene
resonances for the dppm ligand at 3.70 and 1.91 ppm. Both
are doublets broadened by 31P coupling. The 31P NMR spectrum
shows two widely separated resonances reflecting the differing
environments of the phosphorus atoms. The peak at 39.3 ppm
is assigned to the Ru-bound phosphorus atoms, while the
resonance at -25.2 ppm is assigned to the pendant phosphorus.
These assignments correspond with those reported for the closely
related compound, RuCp(PPh3)(η1-dppm)Cl.13
Structure of RuCp(η1-dppm)2Cl (3). Shown in Figure 1
is the thermal ellipsoid drawing of complex 3. The phenyl
groups of the dppm ligands have been omitted for clarity,
although the phenyl ipso carbons (denoted with Ph#) are shown
to indicate their position. The unusual η1 binding mode of both
dppm ligands can be seen in Figure 1. There are many examples
of bidentate binding of dppm to Ru, but there are only four
reported complexes of ruthenium where dppm is in a mono-
dentate binding mode.24 Although monodentate binding of two
dppm ligands has been observed previously in complexes of
other metals (i.e., Mo25 or Re26), complex 3 is the first structure
reported to have two monodentate dppm ligands coordinated
to Ru. The coordination geometry around the metal center is
similar to that of [RuCp(η2-dppm)(η1-dppm)]+ 24a and is the
piano-stool configuration common to four-coordinate Ru-Cp
complexes.27 The Ru atoms in the three other ruthenium
structures with a monodentate dppm24b-d all display octahedral
geometry. Tables 1-3 detail crystallographic data, bond
lengths, and bond angles for 3, respectively.
Synthesis of Heterobimetallic Complexes 4-8. RuCp-
(PPh3)Cl(η1-dppm) reacts readily with Pt(COD)Cl2 to give the
heterobinuclear complex RuCp(PPh3)Cl(µ-dppm)PtCl2 (4) (eq
4) in which the diene has been displaced from platinum. This
yellow compound is air stable as a solid, although solutions
decompose slowly over the course of a day when exposed to
air. The 31P NMR spectrum of complex 4 has resonances at
49.1 and 37.8 ppm assigned to the Ru-bound dppm phosphorus
and the PPh3, respectively. A peak at -2.9 ppm shows satellites
(JPPt ) 3826 Hz) characteristic of platinum coupling and is
therefore assigned to the dppm phosphorus bound to Pt. The
1H and 13C NMR spectra are less diagnostic, yet do display
signals at 2.71 and 82 ppm, respectively, due to the dppm
methylene bridge.28
In a reaction similar to the preparation of 4, the bis η1-dppm
ruthenium compound, 3, reacts with Pt(PhCN)2Cl2 in CH2Cl2
to give the neutral bimetallic complex, RuCpCl(µ-dppm)2PtCl2
(5) (eq 5). However, when 3 is combined with Pt(COD)Cl2,
no bimetallic complex is formed. Rather, transfer of dppm from
RuCpCl(η1-dppm)2 (3) to platinum occurs, leading to RuCpCl-
(η2-dppm) and Pt(η2-dppm)Cl2. Confirmation of the identity
of these products was made by comparison of the spectral data
with that reported in the literature.13,29
(25) (a) Cano, M.; Campo, J. A.; Perez-Garcia, V.; Gutierrez-Puebla, E.;
Alvarez-Ibarra, C. J. Organomet. Chem. 1990, 382 (3), 397-406. (b)
Riera, V.; Ruiz, M. A.; Villafan˜a, F.; Bois, C.; Jeannin, Y. J.
Organomet. Chem. 1990, 382 (3), 407-417. (c) Hor, T. S. A.; Chee,
S.-M. J. Organomet. Chem. 1987, 332 (1), 23-28. (d) Klendworth,
D. D.; Welters, W. W.; Walton, R. A. Organometallics 1982, 1, 336-
343.
(26) Hartl, F.; Vlcˇek, A., Jr. Inorg. Chem. 1992, 31, 2869-2876.
(27) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D.
G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1-S83.
(28) A reviewer has suggested an alternative structure for 4 in which the
ruthenium-chloride is bridging. This structure cannot be excluded
on the basis of the spectroscopic data. However, subsequent manipula-
tion of the complex suggests the Ru-bound chloride is the most reactive
of the three chlorides (Vide infra). In the absence of crystallographic
data, we conclude that the structure pictured in eq 4 is more likely.
(24) (a) Bruce, M. I.; Cifuen, M. P.; Grundy, K. R.; Liddell, M. H.; Snow,
M. R.; Tiekink, E. R. T. Aust. J. Chem. 1988, 41, 597-603. (b) Lugan,
N.; Bonnet, J.-J.; Ibers, J. A. Organometallics 1988, 7, 1538-1545.
(c) Ball, R. G.; Domazetis, G.; Dolphin, D.; James, B. R.; Trotter, J.
Inorg. Chem. 1981, 20, 1556-1562. (d) Singleton, E.; van Rooyen,
P. H.; de V. Steyn, M. M. S. Afr. J. Chem. 1989, 42, 57-63.

920 Inorganic Chemistry, Vol. 35, No. 4, 1996
Orth et al.
yield (90%). No incorporation of CH3CN was indicated by 1H
NMR analysis of the complex when this solvent was used.
On the basis of spectroscopic data, assignment of structure 7
is as shown in eq 7. The 31P NMR spectrum shows the multiplet
characteristic of PF6- at 350.2 ppm which confirms the cationic
nature of the compound. The IR spectrum shows two CO bands
The aromatic region of the 1H NMR spectrum of 5 is
predictably crowded. However, the Cp resonance is well
resolved at 5.03 ppm, as is the multiplet for the methylene
protons of the dppm bridges at 3.21 ppm. The 13C NMR
spectrum exhibits a similar pattern in that the aromatic carbon
resonances are overlapping while the Cp and methylene
resonances are more distinct at 91.3 and 61.8 ppm, respectively.
The platinum satellites bracketing the doublet at -2.6 ppm (JPPt
) 2372 Hz) in the 31P spectrum identify the resonance as that
for the Pt-bound phosphorus. The resonance at 43.2 ppm is
similar to those of other Ru-bound phosphines.
As one goal of this work is to ligate putative intermediates
from the oxidation of methanol, methoxide (or other alkoxides)
and CO were chosen as target ligands. Attempts to substitute
an alkoxide for a chloride on complex 4 with TlOEt, TlOMe,
TlOtBu, or NH4PF6 in CH2Cl2, MeOH, or combinations of these
solvents produced no identifiable substitution products. In
addition, mixtures of 4 in MeOH with either a catalytic or
equimolar amount of a hindered amine (proton sponge, NEt3)
failed to produce any substitution product. The extremely low
solubility of 4 in MeOH, however, may preclude reactions in
this solvent.
typical for terminal carbonyl ligands30 (1998 and 1844 cm-1
)
and one band of lower energy at 1806 cm-1. In addition, the
13C NMR spectrum shows a peak at 190.8 ppm, which is at
lower field than the typical carbonyl carbon resonance. The
presence of an open coordination site on Pt, as well as these
data, suggests that one carbonyl may be bridging or semibridg-
ing. However, spectral data for terminal carbonyls overlap31
with those observed here, making a conclusive assignment of
the bonding of one CO impossible in the absence of structural
data.
In a related reaction, MoCp(CO)2(µ-PPh2)(µ-H)Pt(PPh3)Cl
(mixture of isomers) was combined with TlPF6 in CH3CN. In
this case, chloride abstraction occurred with incorporation of
acetonitrile to give [MoCp(CO)2(µ-PPh2)(µ-H)Pt(PPh3)(CH3-
CN)]PF6 (8) (eq 8). The product is a mixture of isomers (A,
Reaction of 4 with TlPF6 in the presence of CO results in
substitution of a single chloride on 4 at the ruthenium center to
form [RuCp(CO)(PPh3)(µ-dppm)PtCl2]PF6 (6) (eq 6). Assign-
ment of the site of CO binding is based on spectroscopic and
electrochemical data for the complex. The carbonyl peak in
the 13C NMR spectrum shows no platinum satellites; however,
we cannot rule out the possibility that such satellites may be
lost in the baseline due to the low signal to noise ratio of the
peak. The IR spectrum of 6 shows a CO band (νCO ) 1979
cm-1) remarkably similar to that of the similarly ligated
compound [RuCp(PPh3)CO(η1-dppm)]PF6 (2) (νCO ) 1976
cm-1). In addition, there is a significant shift in the oxidation
potential of the ruthenium center (see Electrochemistry section).
These data suggest the structure as drawn in eq 6 rather than
one with the carbonyl bound to platinum.
With 1 equiv of TlPF6, 6 is the only CO-substituted product
isolated. When 2 equiv of TlPF6 are used in the reaction, a
mixture of products containing 6 and a number of other
compounds results. Three equivalents of TlPF6 give only
unidentified decomposition products. If the reaction is carried
out in CH2Cl2 instead of DME, TlPF6 reacts with 4 to give a
product in which chloride has been abstracted, as evidenced by
the TlCl filtered from the reaction. The product could not be
fully characterized, but an IR spectrum of the product indicates
no incorporation of CO.
PPh3 and µ-PPh2 are cis; B, MeCN and µ-PPh2 are cis) in a ca.
5:1 ratio (A:B) and reflects the ratio observed in the starting
material.
The 31P NMR spectrum of the mixture of 8A and 8B shows
the bridging phosphido resonance of 8A at 97.6 ppm, markedly
shifted from the analogous resonance in the starting material
(167 ppm). The bridging hydride resonances observed in the
1H NMR spectrum of the mixture were assigned by comparison
of the chemical shifts and 195Pt-1H coupling constants (A, -9.5
ppm, JPtH ) 477 Hz; B, -16.6 ppm, JPtH ) 680 Hz) with those
of the analogous isomers of the starting material (A, -8.40 ppm,
JPtH ) 445 Hz; B, -15.6 ppm, JPtH ) 684 Hz).10a
Electrochemistry. Cyclic voltammetric results for the
ruthenium monomers 1-3 are summarized in Table 4. The
compounds with a coordinated CO (1 and 2) exhibit irreversible
oxidation potentials at 1.95 and 1.91 V, respectively. The
potentials reflect the electronic trend of the PPh2H ligand and
η1-coordinated dppm ligand observed in the νco values.
(30) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, 4th ed.; Wiley & Sons: New York, 1986; p
292.
The known heterobinuclear complex, Mo(CO)3(µ-dppm)2Pt-
(H)Cl, reacts with TlPF6 in CH3CN or CH2Cl2 to yield TlCl
and the cation [Mo(CO)3(µ-dppm)2Pt(H)]PF6 (7) (eq 7) in high
(31) Mann, B. E.; Taylor, B. F. Carbon-13 NMR Data for Organometallic
Compounds; Academic Press: New York, 1981.
(32) Herring, F. G.; Legzdins, P.; Richter-Addo, G. B. Organometallics
1989, 8, 1485-1493.
(33) Anker, M. W.; Colton, R.; Tomkins, I. B. Aust. J. Chem. 1968, 21,
1143-1147.
(29) Brown, M. P.; Puddephatt, R. J.; Rashidi, M.; Seddon, K. R. J. Chem.
Soc., Dalton Trans. 1977, 951-955.
(34) Isaacs, E. E.; Graham, W. A. G. Inorg. Chem. 1975, 14, 2560-2561.

Heterobimetallic Ru/Pt and Mo/Pt Complexes
Inorganic Chemistry, Vol. 35, No. 4, 1996 921
Table 4. Summary of Redox Potentials for Monomeric
Compoundsa
redox
change
E1/2
(V vs NHE) ref
compound
[RuCp(Ph3P)(CO)(PPh2H)]PF6 (1)
Ru(II/III)
1.95b
1.91b
0.50
c
c
c
[RuCp(Ph3P)(CO)(η1-dppm)]PF6 (2) Ru(II/III)
RuCpCl(η1-dppm)2 (3)
RuCpCl(PPh3)(η1-dppm)
MoCp(NO)I2(PMePh2)
MoCp(NO)Cl2
Ru(II/III)
Ru(II/III)
Mo(IV/III)
Mo(IV/III)
Mo(II/III)
Mo(II/I)
Pt(II/IV)
Pt(II/IV)
Pt(II/IV)
Pt(II/IV)
Pt(II/IV)
Pt(II/IV)
0.56
13
32
32
33
34
35
35
35
35
36
29
-1.22
-0.34
1.20b
0.42
Mo(CO)2Cl2(η1-dppm)(η2-dppm)
Mo(CO)3(η1-dppm)(η2-dppm)
cis-PtCl2(PMePh2)2
2.19b,d
1.59b,d
2.24b,d
1.79b,d
1.68b
2.01b
trans-PtCl2(PMePh2)2
cis-PtCl2(PPh3)2
trans-PtCl2(PPh3)2
trans-Pt(PPh3)2 (H)Cl
Pt(η2-dppm)Cl2
Figure 2. Cyclic voltammograms of 4 in CH2Cl2/TBAH (V vs NHE,
Glassy carbon electrode, 100 mV/s, ambient T).
a All values obtained in CH2Cl2/TBAH unless otherwise noted.
b Irreversible wave, Epa reported. c This work. d CH3CN/TBAP, Pt
working electrode.
in the solvent window of CH2Cl2 (>2.0 V). Interestingly,
compounds which should contain a more electron rich platinum
coordination environment than 4 due to the presence of two
phosphines [e.g., PtCl2(PMePh2)2, Pt(PPh3)2(H)Cl] display
oxidation potentials similar to that observed in 4 (see Table 4).
This also suggests that electron density is donated from Ru to
Pt.
Replacement of a chloride on the bimetallic Ru/Pt compound
4 with CO leads to 6 and produces a significant shift in the
oxidation potentials of the compound. The Ru(II/III) potential
of 6 shifts 300 mV positive of that of 4, while the Pt(II/IV)
oxidation shifts 100 mV negative of that of the starting material.
Both shifts are consistent with the notion of the CO being ligated
to ruthenium and support the structure postulated earlier. The
π-back-bonding nature of CO, as well as the cationic Ru center,
contributes to the more positive oxidation potential of the metal.
The negative shift in the irreversible Pt wave may be rationalized
by solvent coordination in the absence of a suitable metal-
metal interaction. Conversely, if the carbonyl were bound to
platinum, the potentials would both be expected to shift positive
of that of the starting material, with the Pt shift being greater
than that of Ru.
The cyclic voltammogram of [Mo(CO)3(µ-dppm)2Pt(H)]PF6
(7) exhibits a reversible couple at 0.79 V and an electrochemi-
cally irreversible wave at 1.83 V. The reversible couple is
assigned to the molybdenum center while the irreversible wave
is assigned to the platinum center. The oxidation potential of
the cationic Pt center is nearly 400 mV positive of that of the
neutral bimetallic starting material, Mo(CO)3(µ-dppm)2Pt(H)-
Cl, though similar to the neutral monomer, trans-PtCl2(PPh3)2,
at 1.79 V.35 The Mo(0/I) potential of 7 is in marked contrast
to both the irreversible oxidation potential of the similar
monomer, Mo(CO)3(η1-dppm)(η2-dppm) (see Table 4), at 0.42
V and the reversible Mo(0/I) couple of the starting material,
Mo(CO)3(µ-dppm)2Pt(H)Cl, at 0.38 V. Loss of a chloride from
7 creates both a cationic complex and an open coordination site
on Pt. The shift in the Mo(0/I) potential could be explained by
a new metal-metal interaction or a bridging or semibridging
carbonyl. A bridging carbonyl alone would satisfy the Pt
coordination sphere, but would not account for the positive shift
in Mo potential, since a bridging CO is poorer at π-back-bonding
than a terminal CO. These data, taken along with the
spectroscopic evidence, suggest that a structure with a bridging
carbonyl, as well as a Mo/Pt interaction, is most consistent with
the data.
Table 5. Summary of Redox Potentials for Bimetallic Compounds
(CH2Cl2/TBAH)
E1/2
redox
(V vs
compound
change
NHE) ref
RuCp(PPh3)Cl(µ-dppm)PtCl2 (4)
Ru(II/III)
Pt(II/IV)
Ru(II/III)
Pt(II/IV)
Ru(II/III)
Pt(II/IV)
Mo(0/I)
1.13
b
1.78a
1.13a
1.45a
1.43a
1.68a
0.79
RuCpCl(µ-dppm)2PtCl2 (5)
b
[RuCp(PPh3)CO(µ-dppm)PtCl2]PF6 (6)
[Mo(CO)3(µ-dppm)2Pt(H)]PF6 (7)
Mo(CO)3(µ-dppm)2Pt(H)Cl
b
b
Pt(II/IV)
Mo(0/I)
1.83a
0.38
16
b
Pt(II/IV)
1.44a
[MoCp(CO)2(µ-PPh2)(µ-H)Pt(PPh3)-
(CH3CN)]PF6 (8)
Mo(II/III) 0.94a
Mo(II/III) 1.13a
Pt(II/IV)
Pt(II/IV)
1.65a
1.82a
MoCp(CO)2(µ-PPh2)(µ-H)Pt(PPh3)Cl
Mo(II/III) 0.98a 17
Mo(II/III) 1.15a
Pt(II/IV)
Pt(II/IV)
1.36a
1.53a
[MoCp(CO)2(µ-PPh2)(µ-H)Pt(PPh3)2]PF6 Mo(II/III) 1.16a 17
Pt(II/IV)
2.03a
a Irreversible wave, Epa reported. b This work.
The oxidation potential of RuCpCl(η1-dppm)2 (3) is negative
of that of 1 and 2 at 0.50 V. Such a shift is expected for a
compound with greater electron density at the metal center.
Cyclic voltammetric results for the heterobimetallic com-
pounds (4-8) are summarized in Table 5. The cyclic voltam-
metric scan of RuCp(PPh3)Cl(µ-dppm)PtCl2 (4) (Figure 2)
shows a couple at 1.13 V vs NHE and an electrochemically
irreversible oxidation wave at 1.78 V in CH2Cl2. The 1.13 V
couple is fully reversible if the switching potential of the scan
is <1.6 V, and is assigned to the ruthenium(II/III) couple, while
the irreversible wave is assigned to the Pt(II/IV) oxidation. The
II/III wave of the monomeric ruthenium compound, RuCp-
(PPh3)Cl(η1-dppm), is observed at 0.56 V.13 Considering the
minor change in the ligand geometry about ruthenium, the nearly
600 mV shift positive in its oxidation potential indicates a
significant loss in electron density at the metal. Donation of
this density to a Ru-Pt interaction accommodates the coordi-
natively unsaturated platinum center and suggests the structure
of the compound as drawn in eq 4. Further evidence of such
an interaction comes from the oxidation wave from the platinum
center. The Pt oxidation at 1.78 V contrasts with that of the
starting material, Pt(COD)Cl2, which shows no oxidation wave
(35) Mazzocchin, G.; Bontempelli, G.; Nicolini, M.; Crociani, B. Inorg.
Chim. Acta 1976, 18, 159-163.

922 Inorganic Chemistry, Vol. 35, No. 4, 1996
Orth et al.
Unlike the Mo(0)/Pt(II) complex (7), the CV of [MoCp(CO)2-
(µ-PPh2)(µ-H)Pt(PPh3)(CH3CN)]PF6 (8) shows two irreversible
waves at 0.94 and 1.13 V for the Mo(II/III) oxidations of the
trans and cis isomers, respectively. The Pt(II/IV) oxidations
are observed at 1.65 and 1.82 V for the trans and cis isomers,
respectively. These assignments correspond with reported trends
for trans and cis forms of four-coordinate Pt oxidation poten-
tials.35,37 Substitution of CH3CN for Cl- at the platinum center
has little effect on the oxidation potentials of the molybdenum
center, as can be seen when the Mo(II/III) potentials of MoCp-
(CO)2(µ-PPh2)(µ-H)Pt(PPh3)Cl and 8 are compared (Table 5).
However, a positive shift in the Pt potentials is observed,
generally consistent with the notion of a cationic compound
being more difficult to oxidize. Also noteworthy is the
difference in the Mo oxidation potentials of 8A,B and [MoCp-
(CO)2(µ-PPh2)(µ-H)Pt(PPh3)2]PF6. The complexes are each
cationic and the molybdenum coordination spheres identical,
yet there is a difference of 220 mV in the oxidation potential
of one isomer (8B) and the platinum bis(phosphine) compound,
while the potential of the second isomer (8A) is nearly identical
to that of the reference compound. These differences in
oxidation potential emphasize the propensity of the metal centers
to “communicate” through the bridging ligands, since the only
change in the compounds occurs at platinum.
metal interaction is precluded. For example, compare the Pt
oxidation potential of 5 (1.45 V) with that of Mo(CO)3(µ-
dppm)2Pt(H)Cl (1.44 V). Although the metal bridged to
platinum in the two complexes is different, as is the oxidation
state of each, the redox potentials of the similarly ligated Pt
centers are nearly identical. When an opportunity for a metal-
metal interaction exists, the Pt oxidation potential displays a
large dependence on the second metal’s redox state, illustrating
the influence each center has on the other. The shift in the
oxidation potentials of 4 and 6 exemplify this influence. One
should be able, in such complexes, to exploit this dependence
by doing chemistry at one metal center with the intent of tuning
the redox potential of the second metal. Such an approach may
provide further insight into the redox properties of heterobi-
metallic systems in which one metal is a reactive site but
chemically inaccessible through common methods.
Acknowledgment. Funding for this work was provided by
the Division of Sponsored Research, University of Florida. K2-
PtCl4, PtCl2, and RuCl3‚xH2O were generously supplied by the
Johnson Matthey Metals Loan Program.
Supporting Information Available: Complete thermal ellipsoids
drawing, tables of bond lengths and angles, and crystallographic data
for 3 (15 pages). Ordering information is given on any current masthead
page.
Conclusions
IC9510977
In the compounds investigated here, the platinum oxidation
potentials appear to be relatively insensitive to the redox
potential of the second metal when the possibility for a metal-
(36) Bailar, J. C.; Itatani, H. Inorg. Chem. 1965, 4, 1618-1620.
(37) Davies, J. A.; Uma, V. Inorg. Chim. Acta 1983, 76, L305-L307.