Electrochemical and structural characterization of cis-and trans-[Cp(CO)Ru(μ-As(C6H5)2)]2, isomers that undergo two-electron-transfer oxidations

Article abstract of DOI:10.1021/om9709998

The structures and oxidations of the cis and trans isomers of the doubly bridged dinuclear species [Cp(CO)Ru(μ-AsPh₂)]₂ (Ph = C₆H₅) have been studied by X-ray crystallography, electrochemistry, and IR and NMR spectroscopies. Each complex oxidizes in a single two-electron voltammetric process, the E_1/2 values being -0.36 V for the cis isomer (1) and -0.30 V for the trans isomer (2) in CH₂Cl₂/0.1 M [NBu₄][PF₆] (referenced to ferrocene). These are apparently the first comparative redox potentials published for cis and trans isomers of bridged dinuclear organometallic complexes. Oxidation of 1 to 1²⁺ was cleanly accomplished either by electrolysis or by oxidation of 1 by 2 equiv of ferrocenium, allowing isolation of the dication. The split carbonyl absorptions in the IR spectra of 1²⁺ (ν_CO = 2032, 2050 cm^-1) are consistent with formation of a Ru-Ru bond in the oxidation reaction. The electrode reaction 1/1⁺ + e^- is much slower than the reaction 1⁺/1²⁺ + e^-, implying that the metal-metal bond is formed in the former process. This conclusion is supported by the observation that the inner-sphere activation barrier, ΔG?, is about 8.5 kcal/mol, close to that (～10 kcal/ mol) estimated for a one-electron oxidation involving formation and cleavage of a Ru-Ru bond. The electron-transfer (ET) activation barrier is higher in these Ru complexes than in analogous Fe complexes, which are known to undergo large ET-induced changes in metal-metal bond lengths, most likely because M-M bond strengths are larger when M = Ru than when M = Fe.

Full text of DOI:10.1021/om9709998

Organometallics 1998, 17, 1169-1176
1169
Electr och em ica l a n d Str u ctu r a l Ch a r a cter iza tion of cis-
a n d tr a n s-[Cp (CO)Ru (µ-As(C6H5)2)]2, Isom er s Th a t
Un d er go Tw o-Electr on -Tr a n sfer Oxid a tion s†
Anthony-J oseph DiMaio,‡ Arnold L. Rheingold,*,‡ Teen T. Chin,§
David T. Pierce,§, and William E. Geiger*,§
Departments of Chemistry, University of Delaware, Newark, Delaware 19716, and
University of Vermont, Burlington, Vermont 05405
Received November 13, 1997
The structures and oxidations of the cis and trans isomers of the doubly bridged dinuclear
species [Cp(CO)Ru(µ-AsPh2)]2 (Ph ) C6H5) have been studied by X-ray crystallography,
electrochemistry, and IR and NMR spectroscopies. Each complex oxidizes in a single two-
electron voltammetric process, the E1/2 values being -0.36 V for the cis isomer (1) and -0.30
V for the trans isomer (2) in CH2Cl2/0.1 M [NBu4][PF6] (referenced to ferrocene). These are
apparently the first comparative redox potentials published for cis and trans isomers of
bridged dinuclear organometallic complexes. Oxidation of 1 to 12+ was cleanly accomplished
either by electrolysis or by oxidation of 1 by 2 equiv of ferrocenium, allowing isolation of the
dication. The split carbonyl absorptions in the IR spectra of 12+ (νCO ) 2032, 2050 cm-1
)
are consistent with formation of a Ru-Ru bond in the oxidation reaction. The electrode
reaction 1/1+ + e- is much slower than the reaction 1+/12+ + e-, implying that the metal-
metal bond is formed in the former process. This conclusion is supported by the observation
that the inner-sphere activation barrier, ∆Gq, is about 8.5 kcal/mol, close to that (∼10 kcal/
mol) estimated for a one-electron oxidation involving formation and cleavage of a Ru-Ru
bond. The electron-transfer (ET) activation barrier is higher in these Ru complexes than in
analogous Fe complexes, which are known to undergo large ET-induced changes in metal-
metal bond lengths, most likely because M-M bond strengths are larger when M ) Ru
than when M ) Fe.
In tr od u ction
ena. Examples are known in which the overall two-
electron process has the voltammetric characteristics of
The electron-transfer (ET) properties of several di-
nuclear complexes bridged by group 15 or group 16
moieties have been reported.1,2 A number of such
complexes can be prepared, or at least generated, having
electron counts differing by two, e.g., Fe2(CO)6(µ-S2) and
the corresponding dianion.3 The factors affecting both
the ET stoichiometries (i.e., one or two ET steps between
two-electron products) and energetics are not, however,
well understood. The most basic question is whether
or not a thermodynamically stable one-electron inter-
mediate (charge ) m + 1 in the drawing) is formed as
the molecules transit between the outlying oxidation
states, m and (m + 2):
2
a single Nernstian two-electron wave, with E1/2
,
≈
.
E1/21,4 a pair of unresolved one-electron waves (E1/2
2
E1/21),5 two well-separated one-electron waves (E1/2
2
E1/21),6,7 or an ECE process.8 Although one expects that
structural changes in the ET series, specifically changes
in M-M bond length and M-bridging atom-M bond
1
angles, will influence the values of ∆E1/2(dE1/22-E1/2
)
and the charge-transfer rates of the couples,6,7,9,10 the
relationships among these factors are still unclear.5,7
(1) Geiger, W. E.; Connelly, N. G. Advances in Organometallic
Chemistry; Stone, F. G. A., West, R., Eds.; Academic Press, Inc.:
Orlando, FL, 1985; Vol. 24, pp 98-110.
(2) Astruc, D. Electron-Transfer and Radical Processes in Transition
Metal Chemistry; VCH Publishers: New York, 1995; pp 247 ff.
(3) (a) Seyferth, D.; Henderson, R. S.; Song, L. J . Organomet. Chem.
1980, 192, C1; (b) Organometallics 1982, 1, 125.
(4) [(CO)3Fe(µ-PPh2]20/2-: Collman, J . P.; Rothrock, R. K.; Finke, R.
G.; Moore, E. J .; Rose-Munch, F. Inorg. Chem. 1982, 21, 146.
(5) [(CO)4W(µ-SBz)]22-/0 (Bz ) benzyl): (a) Fernandes, J . B.; Zhang,
L. Q.; Schultz, F. A. J . Electroanal. Chem. 1991, 297, 145. (b) Hill, M.
G.; Rosenhein, L. D.; Mann, X. R.; Mu, X. H.; Schultz, F. A. Inorg.
Chem. 1992, 31, 4108.
Electrochemistry (cyclic voltammetry, CV, in particu-
lar) is a powerful method for probing these ET phenom-
(6) (CO)6Mn2(µ-SR)3-/0/+ (a) McDonald, J . W. Inorg. Chem. 1985, 24,
1734. (b) Lyons, L. J .; Tegan, M. H.; Haller, K. J .; Evans, D. H.;
Treichel, P. M. Organometallics 1988, 7, 357.
† Structural Consequences of Electron-Transfer Reactions. 34. For
part 33, see: Geiger, W. E.; Shaw, M. J .; Wu¨nsch, M.; Barnes, C. E.;
Foersterling, F. H. J . Am. Chem. Soc. 1997, 119, 2804.
‡ University of Delaware.
(7) [Cp(CO)Fe(µ-PPh2)]2, [Cp(CO)Fe(µ-SMe)]2, and [CpMo(µ-SMe)2]2:
Gennett, T.; Geiger, W. E.; Willett, B.; Anson, F. C. J . Electroanal.
Chem. 1987, 222, 151.
§ University of Vermont.
(8) Cp*2Fe2S40/2+: Inomata, S.; Tobita, H.; Ogino, H. Inorg. Chem.
1991, 30, 3039.
Present address: Department of Chemistry, University of North
Dakota, Grand Forks, ND 58202.
(9) Marcus, R. A. Annu. Rev. Phys. Chem. 1964, 15, 155.
S0276-7333(97)00999-0 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 02/26/1998

1170 Organometallics, Vol. 17, No. 6, 1998
DiMaio et al.
νCO ) 1958(s), 1925(m); UV/vis (CHCl3, 4.4 × 10-5 M) bands
observed at 205 (ꢀ ) 9773), 245 (26 704), and 330 nm (6477)
In an attempt to learn more about this problem, we
have investigated the oxidation of cis-[Cp(CO)Ru(µ-
AsPh2)]2 (1) and (less extensively) its trans isomer, 2.
1
(the 330-nm band tails off at 470 nm); H NMR (CDCl3) δ )
7.62 (dd, l0H), 7.35 (m, 10H), 4.59 (s, 10H); 13C NMR (CDCl3)
δ ) 207 (s), 145 (s), 138 (s), 134 (s), 131 (s), 127 (q), 82 (s); MS
parent ion at m/ z ) 847 (isotope pattern for two Ru atoms).
Elemental analysis: C, 50.67; H, 3.44; As, 16.9 (calcd C, 51.07,
H, 3.57, As, 17.70) (Galbraith). For 2: IR (CHCl3) νCO
)
1957(s); UV/vis (CHCl3, 2.48 × 10-5 M) bands at 240 (ꢀ )
23 790) and 337 nm (6250) (the 337-nm band tails off at 450
1
nm); H NMR (CDCl3) δ ) 7.63 (dd, 10H), 7.32 (s, 16H), 4.52
(s, 10H); 13C NMR (CDCl3) δ ) 141 (s), 139 (s), 134 (s), 133
(s), 129 (d), 127 (d), 83 (s). Elemental analysis: C, 50.85; H,
3.72; As, 16.8 (Galbraith).
In contrast to their first-row analogue [Cp(CO)Fe(µ-
PPh2)]2, which undergoes two well-separated (∆E1/2
≈
350 mV) one-electron oxidations,7 the two diruthenium
complexes show single two-electron anodic waves that
deviate from Nernstian behavior only at higher CV
sweep rates. Although the dication 12+ proved isolable,
X-ray quality crystals of it were not obtained. The
crystal structures of the two neutral complexes were
obtained, however, providing a rare comparison of the
molecular structure of cis- and trans-isomers of a
bridged dinuclear system. The dication was character-
ized by spectroscopic methods, and the couple 1/12+ was
shown to constitute a chemically reversible two-electron
redox system, with the neutral-to-monocation ET being
the rate-determining step.
[cis-{Cp (CO)Ru (µ-AsP h 2)}2][P F 6]2. A CH2Cl2 solution
containing 30 mg (35 µmol) of 1 and 23 mg (70 µmol) of
ferrocenium hexafluorophosphate13 was stirred at room tem-
perature for 1 h. During this time, the color of the solution
changed from the green of 1 to yellow-green, and a precipitate
was deposited. After filtration, the solid was washed copiously
with diethyl ether to remove ferrocene. Recrystallization from
CH3NO2/ethyl ether at 243 K gave small yellow crystals of the
desired dication (19 mg, 41%) (elemental analysis by Robertson
Laboratories: C, calcd 38.02, found 36.80; H, calcd 2.64, found
2.52): 1H NMR (CD3NO2) δ ) 6.03 (s, Cp), 7.30-7.55 (m, Ph);
IR (CH2Cl2) νCO ) 2050, 2032 cm-1; IR Nujol) νCO ) 2043, 2023
cm-1; PF6 at 850 cm-1
.
-
Electr och em istr y. Measurements were conducted under
an atmosphere of dinitrogen within a drybox, using solvents
and procedures previously described.14 The working electrodes
were commercially available disks, except in the case of
rotating platinum electrode scans, for which a homemade Pt
bead mounted through soft glass was employed. A synchro-
nous rotator (Sargent, 1800 rpm) was used in rotating elec-
trode experiments. The disk diameters were nominally 1 mm,
except for the larger (Beckman) electrode used for chrono-
amperometry experiments. The electrochemical area of the
larger electrode was calibrated as 0.393 cm2 using the oxida-
tion of K4[Fe(CN)6] in aqueous 2 M KCl.15 Disk electrodes were
polished with a series of diamond pastes, finishing with a
particle size of 0.25 µm.
Potentials in the paper are quoted vs the ferrocene-
ferrocenium couple, as recommended by IUPAC.16 Ferrocene
was added to the analyte solutions as an internal standard at
an appropriate point in the experiment. Conversion to the
aqueous SCE scale may be achieved by addition of 0.46 V for
CH2Cl2 solutions and 0.40 V for DMF solutions. In all cases,
the supporting electrolyte was 0.1 M [NBu4][PF6]. Digital
simulations of CV scans were performed originally with a
program for EE mechanisms written at the University of
Vermont, based on the explicit finite difference method of
Feldberg,17 but later confined using DIGISIM (Bioanalytical
Systems).
Exp er im en ta l Section
11
12
Ma ter ia ls. [CpRu(CO)2]2 and cyclo-(C6H5As)6 were pre-
pared according to literature procedures. Hydrocarbon sol-
vents were distilled under nitrogen from sodium/benzophe-
none, while halogenated solvents were dried over molecular
sieves.
Th er m olysis of [Cp Ru (CO)2]2 w ith cyclo-(C6H5As)6. A
solution in a 40-mL heavy-wall Carius tube containing 0.308
g of [CpRu(CO)2]2 (0.695 mmol), 0.640 g of cyclo-(C6H5As)6
(0.701 mmol), and 15 mL of toluene was degassed using three
freeze-pump-thaw cycles and then flame sealed. The tube
was placed inside an end-capped steel pipe with several small
vents. (Ca u tion : Pressures within the tube may reach 10-20
atm at the maximum temperature. Ruptures occur in about
10% of the tubes prepared in this manner. Tubes can be most
safely opened after cooling the contents in liquid nitrogen.)
The tube was heated in an oven at 180 °C for 65 h and then
slowly cooled to room temperature. The tube was opened, and
its contents were filtered and washed with toluene. The
solvent was removed from the filtrate to give a reddish tar.
The tar was redissolved in a minimum volume of CH2Cl2 and
then chromatographed on an alumina column. After an initial
flush of the column with hexanes to remove excess cyclo-
(C6H5As)6, a gradient mixture of CH2Cl2 in hexanes was used
to elute the column. The first major product fraction appeared
as a yellow band with 20% CH2Cl2. 1H NMR spectra showed
the presence of two cyclopentadienyl compounds in this and
subsequent bands that varied quanititatively, indicating the
presence of two products. All fractions containing these two
proton resonances were combined, and a second alumina
column, eluted with 15% CH2Cl2 in hexanes, separated the
mixture into two products, both yellow-orange. Both samples
were recrystallized from acetone/hexanes. X-ray diffraction
identified these compounds as the cis (1, 30% yield) and trans
(2, 10%) isomers of [Cp(CO)Ru(µ-AsPh2)]2. For 1: IR (CHCl3)
X-r a y Str u ctu r a l Ch a r a cter iza tion . Crystallographic
data for 1 and 2 are collected in Table 1. Both were found to
possess 2/m Laue symmetry. For 1, systematic absences in
the diffraction data and the presence of two-fold rotational
symmetry along an axis aligned with the crystallographic b
axis indicated that the correct space group was C2/c. For 2,
systematic absences uniquely indentified the space group as
P21/c. Empirical corrections for absorption was made using
ψ-scan data. Both structures were solved by direct methods
and refined with anisotropic parameters for all non-hydrogen
(13) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877.
(14) Chin, T. T.; Geiger, W. E.; Rheingold, A. L. J . Am. Chem. Soc.
1996, 118, 5002.
(15) Adams, R. Electrochemistry at Solid Electrodes; Marcel Dek-
ker: New York, 1969; p 124.
(16) Gritzner, G.; Kuta, J . Pure Appl. Chem. 1984, 56, 461.
(17) Feldberg, S. W. In Electroanalytical Chemistry; Bard, A. J ., Ed.;
Marcel Dekker: New York, 1969; Vol. 3, p 199.
(10) Geiger, W. E. In Progress in Inorganic Chemistry; Lippard, S.
J ., Ed.; J ohn Wiley: New York, 1985; Vol. 33, p 275.
(11) (a) Humphries, A. P.; Knox, S. A. R. J . Chem. Soc., Dalton
Trans. 1975, 1710. (b) Blackmore, T.; Bruce, M. L.; Stone, F. G. A. J .
Chem. Soc. A 1968, 2158.
(12) Palmer, C. S.; Scott, A. B. J . Am. Chem. Soc. 1928, 50, 536.

cis- and trans-[Cp(CO)Ru(µ-As(C6H5)2)]2
Organometallics, Vol. 17, No. 6, 1998 1171
Ta ble 1. Cr ysta llogr a p h ic Da ta for cis- a n d
tr a n s-[Cp Ru (CO)As(C6H5)2]2
cis
trans
(a) Crystal Parameters
formula
C
36H30O2As2Ru2
C36H30O2As2Ru2
monoclinic
P21/c
21.860(1)
15.544(3)
18.92(1)
crystal system
space group
a, Å
b, Å
c, Å
monoclinic
C2/c
9.443(3)
36.037(12)
9.663(2)
98.34(2)
3253(2), 4
b, deg
96.98(4)
6380(5), 8
V, Å3, Z
cryst dimens, mm
crystal color
D(calc), g cm3
m(Mo KR), cm-1
temp, K
0.15 × 0.30 × 0.35 0.24 × 0.25 × 0.42
orange
1.73
orange
1.76
49.74
296
50.72
296
(b) Data Collection
Nicolet R3
diffractometer
Nicolet R3
2θ scan range, deg 4 g 2θ g 50
no. rflns colltd,
indpt, obsvd
4 g 2θ g 50
12 075, 11 242, 6771
3108, 2878, 2057
(c) Refinement
3.40, 4.53
0.481
1.04
R(F), R(wF) %a
∆(F), e Å-3
GOF
4.40, 5.16
1.015
1.05
F igu r e 1. Molecular structure of cis-[CpRu(CO)(µ-AsPh2)]2
(1), with atoms represented as 40% thermal ellipsoids.
a
Quantity minimized ) ∑∆2; R ) ∑∆/∑(Fo); R(w) ) ∑∆w1/2
∑(Fow1/2), ∆ ) |(Fo - Fc)|.
/
atoms. Hydrogen atoms were treated as idealized contribu-
tions. All computations used SHELXTL (ver. 4.2, G. Sheldrick,
Siemens XRD, Madison, WI).
Resu lts a n d Discu ssion
Syn th esis a n d Str u ctu r es. The reaction of [CpRu-
(CO)2]2 with the homocycle cyclo-(AsC6H5)6 in a sealed
tube at 145 °C in toluene leads to a disproportionation
of the phenylarsenido group to form diphenylarsenido-
bridged and naked arsenic clusters. No naked arsenic
clusters were isolable, but, from solution spectroscopic
data and analogy to earlier work with cyclopentadienyl
Fe18 and Mo19 carbonyl reactions under similar condi-
tions, it is likely that these are tetrahedrane analogues
of general formula [CpRu(CO)]mAsn. The bridged diphen-
ylarsenido complexes formed have been crystallographi-
cally characterized as the cis (1) and trans (2) isomers
of [CpRu(CO)(µ-AsPh2)]2 (Figures 1 and 2). 1 and 2 are
separable by column chromatography and are produced
in a 3:1 ratio:
F igu r e 2. Molecular structure of trans-[CpRu(CO)(µ-
AsPh2)]2 (2), with atoms represented as 40% thermal
ellipsoids. One of two independent molecules is shown.
[CpRu(CO)2]2 + cyclo-(AsC6H5)6 f
+ [CpRu(CO)]mAsn
[CpRu(CO)µ-As(C6H5)2]2
cis (1) and trans (2)
coordination sites. The most acute angle found is at
As-Ru-As(a), 75.2(1)°. A slightly distorted tetrahedral
environment is seen at arsenic; the smallest angle found
about arsenic is that between the two phenyl rings,
C(16)-As-C(26) ) 99.9(1)°. The phenyl rings about
arsenic are situated such that those on the same side
of the Ru2As2 plane as the Cp rings are rotated to fit
the space available. The rings are contained in an
approximate plane formed by the As and ipso C atoms.
The other two phenyl rings are twisted so that they are
essentially perpendicular to this plane.
The cis isomer 1 crystallizes in the monoclinic space
group C2/c; selected bond parameters are given in Table
2. There is a crystallographically imposed two-fold
rotation axis located at the centroid of the Ru2As2
rhombus, which it generates and to which it is perpen-
dicular. The average ruthenium-arsenic bond is 2.455
Å. The Ru2As2 framework is virtually planar; the angle
formed by the two RuAs2 planes in the rhombus is
179.3(1)°. Ruthenium exhibits a distorted octahedral
environment if the Cp ring is viewed as occupying three
For the trans isomer 2, two independent molecules
crystallize in the monoclinic space group P21/c; selected
bond parameters are given in Table 2. There are only
minor deviations in the bond lengths and angles for the
(18) Sinclair, J . D. Ph.D. Dissertation, University of Wisconsin,
Madison, WI, 1972.
(19) Sullivan, P. J .; Rheingold, A. L. Organometallics 1982, 1, 1547.

1172 Organometallics, Vol. 17, No. 6, 1998
DiMaio et al.
Ta ble 2. Selected Bon d Dista n ces a n d An gles for cis- (1) a n d tr a n s-[Cp Ru (CO)µ-As(C6H5)2]2 (2)
2
1
molecule A
molecule B
(a) Bond Distances (Å)
Ru(1)-As(1)
Ru-As
2.457(1)
2.453(1)
2.485(1)
2.478(1)
2.461(1)
2.459(1)
1.886(6)
1.900(7)
3.872(1)
2.466(1)
2.469(1)
2.457(1)
2.463(1)
1.894(7)
1.891(7)
3.843(1)
Ru-As(a)
Ru(1)-As(2)
Ru(2)-As(1)
Ru(2)-As(2)
Ru(1)-CNT1
Ru(2)-CNT2
Ru(1)‚‚‚Ru(2)
Ru-CNT
1.897(8)
3.884(1)
Ru‚‚‚Ru(a)
(b) Bond Angles (deg)
As(1)-Ru(1)-As(2)
As(1)-Ru(1)-C(6)
As(2)-Ru(1)-C(6)
As(1)-Ru(1)-CNT1
As(2)-Ru(1)-CNT1
CNT1-Ru(1)-C(6)
As(1)-Ru(2)-As(2)
As(1)-Ru(2)-C(12)
As(2)-Ru(2)-C(12)
As(1)-Ru(2)-CNT2
As(2)-Ru(2)-CNT2
CNT2-Ru(2)-C(12)
Ru(1)-As(1)-Ru(2)
C(26)-As(1)-C(36)
Ru(1)-As(2)-Ru(2)
C(46)-As(2)-C(56)
As-Ru-As(a)
As-Ru-C(1)
AS(a)-Ru-C(1)
As-Ru-CNT
As(a)-Ru-CNT
CNT-Ru-C(1)
75.2(1)
88.5(2)
89.6(2)
130.7(2)
127.3(2)
128.6(2)
75.8(1)
96.9(3)
93.7(2)
125.2(3)
128.2(2)
124.1(2)
76.5(1)
88.8(2)
86.3(2)
130.3(2)
128.8(3)
128.8(3)
103.1(1)
96.4(2)
103.3(1)
97.3(2)
74.3(1)
91.9(3)
100.6(3)
129.6(2)
122.2(3)
124.9(2)
74.6(1)
85.1(3)
89.7(3)
132.7(2)
129.8(3)
127.4(3)
102.6(1)
101.1(2)
102.3(2)
97.8(3)
Ru-As-Ru(a)
C(16)-As-C(26)
104.5(1)
99.9(1)
two molecules. The average Ru-As bond is 2.467 Å,
which is slightly longer than that found in 1. Again,
the Ru2As2 rhomboid is flat; the angle formed by the
two RuAs2 planes is 179.3(1)°. The local Ru atom
geometry is again distorted octahedral, with the largest
angles found about the Cp centroid-ruthenium vector,
and the smallest angles found within the As-Ru-As
portion of the rhombus. Arsenic again exhibits distorted
tetrahedral geometry, with the phenyl-As-phenyl
angle consistently being acute. The phenyl rings about
arsenic in 2 are oriented in a slightly different manner
from those of 1. Each pair of phenyl rings lies tilted
slightly in deference to the Cp ring located on that side
of the Ru2As2 plane. The Ru‚‚‚Ru distances in both 1
(3.884(1) Å) and 2 (av 3.858 Å) preclude direct metal-
metal interactions.
bond breaking/Mo inversion scenario was proposed for
the cis f trans photochemical process in [CpMo(CO)2-
(µ-AsMe2)]2.22 These results imply that 1 is the kinetic
product, while 2 is the thermodynamically favored
product.
Electr och em ica l Oxid a tion of 1: Tw o-Electr on
Stoich iom etr y. Complex 1 displays a single anodic
wave (Figure 3) in CH2Cl2 or DMF with E1/2 values
(computed from average of Epa and Epc) of -0.36 and
-0.30 V, respectively (Table 3). The oxidation was
established as a chemically reversible two-electron
process (eq 1) by cyclic voltammetry (CV), double-
1 h 12+ + 2e-
(1)
potential step chronoamperometry (CA), and bulk cou-
lometry experiments.
The structures of 1 and 2 are the first published for
iron triad metals in bridged dinuclear complexes of the
type [CpM(CO)(µ-ER2)]2 and only the second for a
related complex of any dinuclear cyclopentadienyl metal
complex with bridging diorganoarsenido groups, the first
being [CpMo(CO)2(µ-AsMe2)]2.20 Additionally, isomers
1 and 2 constitute only the second example of an isolated
and crystallographically characterized set of cis-trans
isomers for any bis-diorganopnictido-bridged structure
of the very general formula [CpMn(ER2)]2, the first being
[CpCr(NO)N(CH3)2]2.21
Conversion of 1 to 2, or vice versa, by thermal means
was not possible. Even after 72 h at 160 °C, there was
no evidence for conversion of a toluene solution as
monitored by 1H NMR. However, it is possible to
convert the cis isomer 1 to the trans isomer 2 irrevers-
ibly and in 90% yield after a 4-h exposure at room
temperature with a low-pressure photochemical ap-
paratus emitting at about 360-440 nm. The photo-
chemical conversion of cis to trans may proceed via Ru-
As bond breaking and inversion at Ru. The Mo-As
CV scans in CH2Cl2 at ambient temperatures gave a
ratio of ic/ia of slightly above unity with scan rates as
low as 0.05 V/s. CA gave a reverse-to-forward current
ratio, irev/ifwd, at the end of a 5-s step time, of 0.29, in
agreement with expectations for formation of a stable
electrode product.23 The CV current function (ipa/ν1/2 24
)
was exactly twice that of equimolar decamethylfer-
rocene.25 CV peak separations, ∆Ep ()Epa - Epc), were
about 40 mV, and the anodic peak breadth, δEp ()Ep -
Ep/2), was 34 mV, with ν ) 0.05 V/s. The two-electron
nature of the process was finally confirmed by bulk
coulometry, in which 2.1 F/equiv was released when 1
was electrolyzed to 99% completion at a Pt basket, with
Eappl ) 0 V at T ) 273 K.
(22) Malisch, W.; Kuhn, M.; Albert, W.; Ro¨ssner, H. Chem. Ber. 1980,
113, 3318.
(23) Theory for a completely reversible couple, 0.293 (Schwarz, W.
M.; Shain, I. J . Phys. Chem. 1965, 69, 30); experiment for ferrocene
under our conditions, 0.284.
(24) Geiger, W. E. In Laboratory Techniques in Electroanalytical
Chemistry, 2nd ed.; Kissinger, P. T., Heineman, W. R., Eds.; Marcel
Dekker: New York, 1996; p 683.
(25) This demonstrates that the oxidation of 1 is a two-electron
process if the diffusion coefficients of 1 and decamethylferrocene are
similar.
(20) Gross, E.; Burschka, C.; Malisch, W. Chem. Ber. 1986, 119, 378.
(21) Bush, M. A.; Sim, G. A. J . Chem. Soc. A 1970, 611.

cis- and trans-[Cp(CO)Ru(µ-As(C6H5)2)]2
Organometallics, Vol. 17, No. 6, 1998 1173
reversible two-electron process of eq 1 [E1/2 ) -0.30 V;
ic/ia ) 1.05 at ν ) 0.05 V/s; irev/ifwd ) 0.29 for 5-s CA
step time; Do ) 2.5 × 10-6 cm2/s (from CA)]. The isola-
ted dication (see below) also had E1/2 ) -0.30 V in DMF.
In every published example of which we are aware, a
two-electron wave for a bridged dinuclear complex has
been ascribed to the making and breaking of a metal-
metal bond accompanying the overall charge trans-
fer(s).30 The voltammetry is, therefore, interpreted in
light of such a model; as the next section shows, the
structural changes are likely apportioned between the
two one-electron steps, with the major change occuring
in the first electron transfer:
Devia tion s fr om Rever sible Volta m m etr y: Use
of H igh er Sca n R a t es To E st im a t e E lect r on -
Tr a n sfer Ra te Con sta n ts for 1. The CV responses
of 1 at higher scan rates are of interest because, under
quasi-reversible charge-transfer conditions, some infor-
mation may be extracted about the individual one-
electron processes that make up the overall two-electron
transfer.14,27,30 ∆Ep values increased with increases in
ν for either CH2Cl2 or DMF solutions of 1. Anodic peak
potentials (Epa) also increased, and anodic current
functions (øa ) ipa/ν1/2) decreased at higher scan rates.
No new peaks or other features were detected up to ν )
100 V/s, nor were the voltammetric wave shapes affected
by changes in concentration of 1 from 0.28 to 1.10 mM.
The two functions Epa, and øa, though less widely
employed as diagnostics than ∆Ep values, shed light on
the ET reversibility of the electrode reaction. Figure 4
shows their values as the oxidation process goes from
nearly Nernstian behavior (low ν) to one which displays
irreversible charge-transfer kinetics (high ν). The cur-
rent function is expected to decrease according to eq 2,
F igu r e 3. Cyclic voltammetry scans of 0.5 mM 1 in DMF/
0.1 M [NBu4][PF6] at 295 K: top, ν ) 0.05 V/s; bottom, ν )
1 V/s.
Ta ble 3. Electr och em ica l P oten tia ls of 1 a n d 2
(Volts vs F c)
complex
product
solvent
E1/2
D0 (cm2/s)
1
1
12+
12+
1
CH2Cl2
DMF
DMF
DMF
CH2Cl2
-0.36
-0.30
-0.30
-2.3b
-0.30
a
2.5 × 10-6
12+
1
a
a
12- b
22+
2
8.7 × 10-6
øa(irrev) ) 1.11(RnR)1/2øa(rev)
(2)
a
b
Not measured. Irreversible reduction wave of two-electron
height; Epc at ν ) 0.1 V/s given.
Solutions of 12+ generated coulometrically were stable
in which R is the charge-transfer coefficient, nR is the
number of electrons transfered in the rate-determining
step, and ø(irrev) and ø(rev) are the current functions
for electrode reactions in the irreversible and Nernstian
limits, respectively. The limiting current function for
1 at high sweep rates is consistent with R ) 0.4 and nR
) 1.
The shifts in anodic peak potentials at high ν also are
consistent with R ) 0.4, nR ) 1, as shown by the straight
line in Figure 4 computed from eq 3:
in CH2Cl2, and the back-reduction of 12+ to 1 at Eappl
)
-0.6 V was efficient (2.1 F/equiv also found in the
cathodic electrolysis of 12+). Only about 10% lower
current was found in RPE voltammograms of 1 after
the bulk oxidation/reduction cycle. Every indication,
therefore, points to the adequacy of eq 1 as a description
of this redox process.
It is worth noting that the diffusion coefficient (D0)
of 12+ is, apparently, substantially lower than that of
1. This conclusion is indicated by the fact that a current
plateau of 3.7 µA was measured for 1 prior to bulk
electrolysis, but only 2.7 µA was observed for 12+ after
the anodic reaction. Considerably reduced diffusion
coefficients have been found previously for dications in
CH2Cl2 compared to their neutral precursors.14,26-29
Complex 1 was also studied in DMF, a solvent
superior to CH2Cl2 for quantitative fast voltammetry.
Its oxidation was again described by the chemically
d(Ep)/d(log ν) ) 30 mV/RnR
(3)
(28) In well-defined hydrodynamic circumstances (e.g., a rotating
disk electrode), the relative values of the diffusion coefficients for the
reduced and oxidized forms of the redox couple may be obtained from
the plateau currents prior to and subsequent to, respectively, an anodic
electrolysis. Quantitative comparisons are less certain when the
(empirically shaped) Pt bead electrode is used.29 A reasonable estimate
is that the diffusion coefficient of 12+ is about one-half of that of 1 in
CH2Cl2.
(29) Sawyer, D. T.; Roberts, J . L., J r. Experimental Electrochemistry
for Chemists; J ohn Wiley and Sons: New York, 1974; p 91.
(30) For leading citations, see refs 5, 6, 10, and 14.
(26) Edwin, J .; Geiger, W. E. J . Am. Chem. Soc. 1990, 112, 7104.
(27) Pierce, D. T.; Geiger, W. E. J . Am. Chem. Soc. 1992, 114, 6063.

1174 Organometallics, Vol. 17, No. 6, 1998
DiMaio et al.
F igu r e 4. Anodic current function (9, left ordinate) and
anodic peak potential (b, right ordinate) as a function of
CV scan rate for 1 in DMF at 295 K. The current function
is ratioed to the value at low sweep rates (the Nernstian
limit). Line a gives the expected slope in the irreversible
limit for a process with R ) 0.4, and line b gives the
expected current function in the irreversible limit for R )
0.4.
The data in Figure 4 tell us that the quasi-reversible
charge-transfer regime lies at scan rates below 10 V/s
for the oxidation of 1, so we now turn to this scan rate
region to estimate the quantitative electron transfer
parameters of 1.
Figure 5 shows a CV recorded at ν ) 1 V/s. The fact
that the anodic peak height is less than the cathodic
peak height means that the first charge-transfer step
is rate limiting,31 consistent with the dominant struc-
tural change occurring in the process 1/1+. In digital
F igu r e 5. Fits between theory (solid lines) and experiment
(dots) for the oxidation of 1.0 mM 1 in DMF at 298 K and
(top) 10 V/s and (bottom) 1 V/s. Simulation parameters:
E1/21 ) -0.27 V, R1 ) 0.42, kS(1) ) 0.007 cm/s; E1/22 ) -0.45
V, R2 ) 0.50, kS(2) ) 0.1 cm/s.
2
simulations, we set the value of (E1/2 - E1/21) at -180
mV, based on the nearly Nernstian behavior of the
couple32 at slow sweep rates and the value of ks(2) at
0.1 cm/s, consistent with findings for related systems.5a,7
In fact, the simulations have little sensitivity to the
precise value of ks(2), since the value of ks(l) is rate
limiting. Simulations such as those shown in Figure 5
were consistent with the values ks(l) ) 0.007 ( 0.002
cm/s and R(1) ) 0.42 ( 0.05. As a test of internal
consistency, the R value agrees with that implied from
the changes in current function and anodic peak poten-
tial with scan rate (Figure 4, vide ante).
Electr och em ica l P r op er ties of 2. The trans iso-
mer 2 also undergoes a two-electron oxidation in CH2-
Cl2 to a persistent dication.33 Its potential (E1/2 ) -0.30
V) was 60 mV positive of that of the cis isomer 1 in the
same medium. The dication 22+ did not appear to be
as stable as 12+: about one-third of 2 was lost in a bulk
coulometry experiment in which 22+ was generated at
273 K and then re-reduced back to neutral 2.
Sign ifica n ce of Electr on -Tr a n sfer P r op er ties.
The difference in E1/2 values for the two isomers,
although small (60 mV), shows that cis and trans
isomers of dinuclear systems may have resolvable
formal potentials. There is no evidence for intercon-
version of the two isomers in either of the higher
oxidation states. Although worth noting, this observa-
tion may not be general for dinuclear bridged complexes,
since the formation of a metal-metal bond in the
dication may impart isomeric rigidity to the complexes
that might be otherwise lacking.
P r ep a r a tion of 12+ by Ch em ica l Oxid a tion . Oxi-
dation of 1 using 2 equiv of ferrocenium hexafluoro-
phosphate in CH2Cl2 gave a yellow-green solid which,
when recrystallized from CH3NO2/diethyl ether, pro-
duced fine needles which analyzed as [1][PF6]2. When
this reaction was carried out in an NMR tube in CD2Cl2,
1
the Cp H resonances of 1 (δ ) 4.63) and 12+ (δ ) 6.03)
were observed, without evidence of intermediates or side
products. IR spectra of 12+ in CH2Cl2 gave a pair of
CO stretches at 2050 and 2032 cm-1, shifted an average
of +87 cm-1, from the single νCO of 1 at 1954 cm-1. The
split band in the dication is significant, since the implied
vibronic coupling between the CO groups is consistent
Of, perhaps, greater interest is the contrast between
the present diruthenium complexes and their diiron
analogues having sulfido or phosphido bridges. Both
[Cp(CO)Fe(µ-PPh2]2 (3) and [Cp(CO)Fe(µ-SMe)]2 (4) may
with the presence of a metal-metal bond in 12+
.
The dication [1][PF6]2 displayed the expected vol-
tammetry in DMF, i.e., a single reduction wave at E1/2
) -0.30 V (cf. Figure 6).
(33) In CV at ν ) 0.05 V/s, ic/ia ) 1.08, ∆Ep ) 34 mV, Ep - Ep/2 ) 28
mV; double-potential step chronoamperometry, irev/ifwd ) 0.27 (5-s
pulses); bulk coulometry, 1.9 F/equiv for 99% electrolysis (all data at
Pt electrodes).
(31) Ryan, M. D. J . Electrochem. Soc. 1978, 125, 547. See also
citations contained in ref 27.
(32) Polcyn, D. S.; Shain, I. Anal. Chem. 1966, 38, 370.

cis- and trans-[Cp(CO)Ru(µ-As(C6H5)2)]2
Organometallics, Vol. 17, No. 6, 1998 1175
rier is one-fourth of the inner-sphere reorganizational
energy, λin (eq 5).
∆Gq ) λin/4
(5)
IS
In the limit that there is localized bond making and
breaking in the ET transition state, the inner-sphere
reorganizational energy is essentially equal to the bond
enthalpy.34 The latter may be estimated for a Ru-Ru
bond of ≈2.8 Å from the work of Connor35 as ≈20 kcal/
mol. This suggests a value of ∆Gq ≈ 5 kcal/mol if the
IS
Ru-Ru bond is fully formed in the first one-electron
transfer 1/1+. Using an estimate of 5 kcal/mol for
36
∆Gq
,
an overall value of ≈10 kcal/mol is predicted
OS
for a one-electron process involving full formation of a
Ru-Ru single bond in the transition state. Given the
uncertainties involved in these estimates, the observed
value of 8.5 kcal/mol is respectably close to the predic-
tion of this model.37
Con clu sion s
The cis and trans isomers of [Cp(CO)Ru(µ-AsPh2)]2
have been prepared and characterized by X-ray crystal-
lographic methods.
Both isomers display single anodic voltammetric
waves of two-electron stoichiometry, the cis isomer being
60 mV easier to oxidize than the trans isomer. There
is little previous literature on the question of whether
cis and trans isomers of bridged dinuclear complexes
have measureably different redox potentials.38
The dication of the cis isomer, 12+, has been isolated
and characterized by NMR and IR spectroscopies. The
presence of two CO stretches in the IR spectrum of 12+
is evidence that the dication possesses the expected Ru-
Ru bond.
F igu r e 6. Cyclic voltammetric scans for 1 (top) and 12+
(bottom) in identical media (CH2Cl2/0.1 M [NBu4][PF6])
prior to (top) and after (bottom) exhaustive electrolysis
(Eappl ) 0 V) of a 1 mM solution of 1. Scan rate, 0.2 V/s; T
) 273 K.
The CV waves are essentially Nernstian at slow
sweep rates but become quasi-reversible at higher scan
rates. The wave shapes of the cis isomer are fit
be oxidized to dications, but their oxidations differ from
those of 1 and 2 in (i) being well-separated one-electron
processes and (ii) exhibiting fast electrode kinetics.7
2
quantitatively by E1/21, ) -0.27 V vs Fc, E1/2 ) -0.45
V, R1 ) 0.42, R2 ) 0.5, kS(1) ) 0.007 cm/s, and kS(2) >
0.1 cm/s.
The fact that the first oxidation is the rate-determin-
ing anodic process suggests that a major structural
change occurs in electron transfer between the neutral
and monocationic complexes. An activation barrier,
∆Gq, of 8.5 kcal/mol is consistent with the metal-metal
bond being formed largely in this step.
The differences in electrode behavior between the
present diruthenium complexes and their diiron ana-
logues are explicable in terms of the presence of a
stronger M-M bond in the one-electron intermediate
formed upon oxidation of the diruthenium complexes.
Factor i argues that the energy of the one-electron
(monocation) intermediate is lowered more in the Ru2
case than in the Fe2 case, compared to the two outlying
oxidation states. Factor ii, when contrasted with the
slow:fast character of the two one-electron transfers
1/1+:1+/12+, argues that the major part of the structural
change in the diruthenium system comes in the first of
these ET steps, in contrast to a more distributed set of
structural changes in the diiron systems. Stated in
another way, the M-M bond formation is more gradual
in the sequence Fe2/Fe2+/Fe22+ than in the sequence Ru2/
Ru2+/Ru22+, where M2 stands for the dinuclear com-
plexes under discussion.
(34) (a) Saveant, J . M. In Advances in Electron Transfer Chemistry;
Mariano, P. S., Ed.; J AI Press: New York, 1994; Vol. 4, p 53. (b)
Andrieux, C. P.; Robert, M.; Saveant, J . M. J . Am. Chem. Soc. 1995,
117, 9340 and references therein.
(35) Connor, J . A. In Transition Metal Clusters; J ohnson, B. F. G.,
Ed.; J ohn Wiley and Sons: Chichester, 1980; p 356.
(36) Weaver, M. J .; Gennett, T. Chem. Phys. Lett. 1985, 113, 213.
(37) From a value of 0.007 cm/s for ks(1) and an assumed preexpo-
nential term of 104, ∆Gq ) 0.38 eV (8.5 kcal/mol) at room temperature.
For an introductory treatment of these concepts, see: Hale, J . M. In
Reactions of Molecules at Electrodes; Hush, N. S. Ed.; Wiley-Inter-
science: London, 1971; p 229.
The activation barrier to ET, ∆Gq, consists of inner-
sphere and outer-sphere contributions, ∆GqIS and ∆Gq
,
OS
respectively (eq 4). Furthermore, the inner-sphere bar-
(38) The inorganic dinuclear complex [(triphos)(H)Rh(µ-Cl)]2 has
been reported to display a single irreversible reduction wave for a 1:1
solution of trans:cis isomers: Bianchini, C.; Meli, A.; Laschi, F.;
Ramirez, J . A.; Zanello, P.; Vacca, A. Inorg. Chem. 1988, 27, 4429.
∆Gq ) ∆Gq + ∆Gq
(4)
IS
OS

1176 Organometallics, Vol. 17, No. 6, 1998
DiMaio et al.
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
coordinates, complete bond distances and angles, and aniso-
tropic thermal parameters for 1 and 2 (15 pages). Ordering
information is given on any current masthead page.
Ack n ow led gm en t. This work was supported in part
by the donors of the Petroleum Research Fund, admin-
istered by the American Chemical Society, and in part
by the National Science Foundation (CHE 91-16332 and
CHE 94-16611).
OM9709998