[Re(η5-C5H5)(CO)3] + family of 17-electron compounds: Monomer/dimer equilibria and other reactions

Article abstract of DOI:10.1021/ja710513n

The anodic electrochemical oxidations of ReCp(CO)₃ (1, Cp = η⁵-C₅H₅), Re(η⁵-C ₅H₄NH₂)(CO)₃ (2), and ReCp*(CO)₃ (3, Cp* = η⁵-C ₅Me₅), have been studied in CH₂Cl₂ containing [NBu₄][TFAB] (TFAB = [B(C₆F₅) ₄]^-) as supporting electrolyte. One-electron oxidations were observed with E_1/2 = 1.16, 0.79, and 0.91 V vs ferrocene for 1-3, respectively. In each case, rapid dimerization of the radical cation gave the dimer dication, [Re₂Cp^γ₂(CO) ₆]²⁺ (where Cp^γ represents a generic cyclopentadienyl ligand), which may be itself reduced cathodically back to the original 18-electron neutral complex ReCp^γ(CO)₃. DFT calculations show that the SOMO of 1⁺ is highly Re-based and hybridized to point away from the metal, thereby facilitating the dimerization process and other reactions of the Re(II) center. The dimers, isolated in all three cases, have long metal-metal bonds that are unsupported by bridging ligands, the bond lengths being calculated as 3.229 A for [Re ₂Cp₂(CO)₆]²⁺ (1₂ ²⁺) and measured as 3.1097 A for [Re₂(C ₅H₄NH₂)₂(CO)₆] ²⁺ (2₂²⁺) by X-ray crystallography on [Re ₂(C₅H₄NH₂)₂(CO) ₆][TFAB]₂. The monomer/dimer equilibrium constants are between K_dim = 10⁵ M^-1 and 10⁷ M^-1 for these systems, so that partial dissociation of the dimers gives a modest amount of the corresponding monomer that is free to undergo radical cation reactions. The radical 1⁺ slowly abstracts a chlorine atom from dichloromethane to give the 18-electron complex [ReCp(CO) ₃Cl]⁺ as a side product. The radical cation 1⁺ acts as a powerful one-electron oxidant capable of effectively driving outer-sphere electron-transfer reactions with reagents having potentials of up to 0.9 V vs ferrocene.

Full text of DOI:10.1021/ja710513n

Published on Web 02/05/2008
[Re(η5-C5H5)(CO)3]+ Family of 17-Electron Compounds:
Monomer/Dimer Equilibria and Other Reactions
Daesung Chong,† Derek R. Laws,† Ayman Nafady,†,| Paulo Jorge Costa,‡
Arnold L. Rheingold,§ Maria Jose´ Calhorda,‡ and William E. Geiger*,†
Department of Chemistry, UniVersity of Vermont, Burlington, Vermont 05405, Departamento de
Qu´ımica e Bioqu´ımica, Faculdade de Cieˆncias, UniVersidade de Lisboa, 1749-016, Lisboa,
Portugal, and Department of Chemistry and Biochemistry, UniVersity of California at San
Diego, La Jolla, California 92093
Received November 29, 2007; E-mail: william.geiger@uvm.edu
Abstract: The anodic electrochemical oxidations of ReCp(CO)3 (1, Cp ) η5-C5H5), Re(η5-C5H4NH2)(CO)3
(2), and ReCp*(CO)3 (3, Cp* ) η5-C5Me5), have been studied in CH2Cl2 containing [NBu4][TFAB] (TFAB )
[B(C6F5)4]-) as supporting electrolyte. One-electron oxidations were observed with E1/2 ) 1.16, 0.79, and
0.91 V vs ferrocene for 1-3, respectively. In each case, rapid dimerization of the radical cation gave the
dimer dication, [Re2Cpγ (CO)6]2+ (where Cpγ represents a generic cyclopentadienyl ligand), which may be
2
itself reduced cathodically back to the original 18-electron neutral complex ReCpγ(CO)3. DFT calculations
show that the SOMO of 1+ is highly Re-based and hybridized to point away from the metal, thereby facilitating
the dimerization process and other reactions of the Re(II) center. The dimers, isolated in all three cases,
have long metal-metal bonds that are unsupported by bridging ligands, the bond lengths being calculated
as 3.229 Å for [Re2Cp2(CO)6]2+ (122+) and measured as 3.1097 Å for [Re2(C5H4NH2)2(CO)6]2+ (222+) by
X-ray crystallography on [Re2(C5H4NH2)2(CO)6][TFAB]2. The monomer/dimer equilibrium constants are
between Kdim ) 105 M-1 and 107 M-1 for these systems, so that partial dissociation of the dimers gives a
modest amount of the corresponding monomer that is free to undergo radical cation reactions. The radical
1+ slowly abstracts a chlorine atom from dichloromethane to give the 18-electron complex [ReCp(CO)3Cl]+
as a side product. The radical cation 1+ acts as a powerful one-electron oxidant capable of effectively
driving outer-sphere electron-transfer reactions with reagents having potentials of up to 0.9 V vs ferrocene.
1. Introduction
are essentially unexplored. Hershberger et al. reported briefly
that the oxidation of ReCp(CO)2L, L ) pyridine or PPh3, is
irreversible by voltammetry,5 but there appears to have been
no report on the oxidation of the parent compound 1. We became
interested in exploring the putative 17-electron radical cation
1+. Given the well-known ability of rhenium to adopt seven
coordination,2 it was thought that 1+ might have a potent
Re(II) reaction center, unencumbered by steric effects owing
to the presence of the three carbonyl ligands.
The isoelectronic neutral second- and third-row group 6
forerunners of 1+, namely MCp(CO)3, M ) Mo, W, have a
rich and well-explored chemistry that is heavily influenced by
metal-based radical reactions, including metal-metal dimer-
izations.6 Group 7 cationic analogues of these radicals were
expected to encounter two issues that might compromise their
generation and long-term stability. The fact that significantly
more positive potentials5 would be necessary to oxidize the Re-
(I) compound suggested that the Re(II) product would be quite
electrophilic, raising the likelihood of its encountering nucleo-
philic attack7 by a traditional supporting electrolyte anion such
as [PF6]- or [BF4]-. An additional difficulty, more method-
ological in nature, is that, unless there are ligand substituents
The prototypical third-row piano stool complex ReCp(CO)3,
1 (Cp ) η5-C5H5), continues to draw new interest in diverse
areas, including pharmacological applications.1 Although its
thermal2 and photolytic3,4 reactions have been well studied, the
anodic electrochemical properties of 1 and its close analogues
† University of Vermont.
‡ Universidade de Lisboa.
§ University of California at San Diego.
| Permanent Address: Chemistry Department, Faculty of Science, Sohag
University, Sohag, Egypt 82524.
(1) (a) Cesati,R. R., III; Tamagnan, G.; Baldwin, R. M.; Zoghbi, S. S.; Innis,
R. B.; Kula, N. S.; Baldessarini, R. J.; Katzenellenbogen, J. A. Bioconjugate
Chem. 2002, 13, 29.(b) Mull, E. S.; Sattigeri, V. J.; Rodriguez, A. L.;
Katzenellenbogen, J. A. Bioorg. Med. Chem. 2002, 10, 1381.
(2) (a) Caulton, K. G. Coord. Chem. ReV. 1981, 38, 1. (b) O’Connor, J. M. In
ComprehensiVe Organometallic Chemistry II; Abel, E. W., Stone, F. G.
A., Wilkinson, G., Eds.; Pergamon Press: Oxford, 1994, Volume 6; pp
210 ff. (c) Boag, N. M.; Kaesz, H. D. In ComprehensiVe Organometallic
Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon
Press: Oxford, 1982; Vol. 4, pp 205 ff.
(3) (a) Bergman, R. G.; Cundari, T. R.; Gillespie, A. M.; Gunnoe, T. B.;
Harman, W. D.; Klinckman, T. R.; Temple, M. D.; White, D. P.
Organometallics 2003, 22, 2331. (b) Yang, H.; Asplund, M. C.; Kotz, K.
T.; Wilkens, M. J.; Frei, H.; Harris, C. B. J. Am. Chem. Soc. 1998, 120,
10154.
(4) (a) Lawes, D. J.; Darwish, T. A.; Clark, T.; Harper, J. B.; Ball, G. E. Angew.
Chem., Int. Ed. 2006, 45, 4486. (b) Lawes, D. J.; Geftakis, S.; Ball, G. E.
J. Am. Chem. Soc. 2005, 127, 4134. (c) Childs, G. I.; Grills, D. C.;
Nikiforov, S. M.; Poliakoff, M.; George, M. W. Pure Appl. Chem. 2001,
73, 443. (d) Geftakis, S.; Ball, G. E. J. Am. Chem. Soc. 1999, 121, 6336.
(5) Hershberger, J. W.; Amatore, C.; Kochi, J. K. J. Organomet. Chem. 1983,
250, 345.
9
2692
J. AM. CHEM. SOC. 2008, 130, 2692-2703
10.1021/ja710513n CCC: $40.75 © 2008 American Chemical Society

[Re(η
5-C5H5)(CO)3]+ Family of 17-Electron Compounds
A R T I C L E S
that enhance their solubilities in low polarity solvents,6y dileptic8
metal carbonyl cations tend to be poorly soluble as salts of the
traditional anions in otherwise attractive low-donor solvents such
as dichloromethane, adding difficulty to the voltammetric
analysis of the oxidation processes.9,10c Both of these potential
problems are likely to be circumvented by the use of weakly
coordinating electrolyte anions (WCEAs).7c, 9-11 Taking advan-
tage of this medium modification, we now describe the one-
electron oxidation of 1 and two of its derivatives, namely the
aminocyclopentadienyl complex Re(C5H4NH2)(CO)3, 2, and the
pentamethylcyclopentadienyl complex ReCp*(CO)3, 3, Cp* )
(η5- C5Me5), in CH2Cl2/WCEA electrolytes. The radical cation
which delineate electronic and structural properties of this Re
system. A preliminary account of the work has appeared.11
2. Experimental Section
Experimental procedures were performed under nitrogen using either
standard Schlenk conditions or a Vacuum Atmospheres drybox.
Reagent-grade dichloromethane and 1,2-dichloroethane were twice
distilled from CaH2, the second distillation being carried out, bulb-to-
bulb, under static vacuum. Other solvents were purified and dried by
distillation from potassium (diethyl ether, hexanes, and pentane) or
potassium benzophenone (THF).
The moisture sensitivity of the dimer dications required that
particularly close attention be paid to the dryness of the solvents and
glass vessels employed. NMR solvents were used as received from
CIL (CD2Cl2) or Acros (CD3NO2). Glassware used for electrochemical
experiments was cleaned in aqua regia, followed by copious rinsings
by nanopure water and subsequent drying for at least 12 h in a 120 °C
oven. The warm glassware was loaded into the drybox antechamber
and allowed to cool under vacuum. Ferrocene, ReCp(CO)3, 1, and
ReCp*(CO)3, 3 (Cp* ) η5-C5Me5), were purchased from Strem
Chemical Co. and used as received. The supporting electrolytes, [NBu4]-
[PF6]9a and [NBu4][TFAB]10b (TFAB ) [B(C6F5)4]), were prepared as
previously described, the latter being metathesized from alkali metal
salts purchased from Boulder Scientific Co.
2.1. Re(C5H4NH2)(CO)3, 2. Although this compound had been
reported earlier by Nesmeyanov et al.,12a the improved approach to
aminocyclopentadienyl complexes used by Barybin et al.12b for the
preparation of Mn(C5H4NH2)(CO)3 was adapted here for synthesis of
the rhenium analogue. After cooling a solution of 0.20 g (0.60 mmol)
of ReCp(CO)3 in 3.2 mL of THF to 195 K, 0.4 mL (0.64 mmol) of
n-BuLi (Acros, 1.6 M in hexanes) was added with stirring. After 50
min, 0.25 g (1.27 mmol) of tosyl azide12c was added, and the resulting
solution was allowed to warm to room temperature over the course of
1 h, followed by stirring for an additional 12 h. At this point an NMR
spectrum was obtained which indicated the presence of the cyclopen-
tadienyl azide complex (1H NMR (CD2Cl2): δ (ppm vs TMS) 5.33 (t,
2H), 5.22 (t, 2H)). The solvent was removed in Vacuo, and the
remaining brown solid was dissolved in 4.0 mL of 95% EtOH. To this
was slowly added 0.095 g (2.51 mmol) of NaBH4 in 3.5 mL of 95%
EtOH, during which an exothermic reaction was evident. After 45 min,
the solvent was removed and the light brown solid was extracted with
4 × 20 mL of diethyl ether. Evaporation gave a white solid that
contained a mixture of starting material and 2. Recrystallization from
CH2Cl2/hexanes gave pure 2 in 83-94 mg (40-45%) yield. Anal. Calcd
for 2: C, 27.51; H, 1.73; N, 4.01. Found: C, 27.65; H, 1.66; N, 3.89.
IR (CH2Cl2): νCO 2017 vs, 1919 vs cm-1. 1H NMR (CD2Cl2): δ (ppm
vs TMS) 4.99 (t, 2H), 4.87 (t, 2H), 3.31 (broad s, 2H).
2.2. [ReCp(CO)3Cl][B(C6F5)4], 4[TFAB]. This salt was prepared
by metathesis of the corresponding hexachloroantimonate complex,
which had been synthesized from 1 according to the literature method.12d
To 90 mg (0.13 mmol) of [ReCp(CO)3Cl][SbCl6] in 5 mL of
spectrograde nitromethane 90 mg (0.12 mmol) of K[B(C6F5)4] in 5 mL
of nitromethane were added dropwise, with stirring, over 10 min, giving
a homogeneous yellow solution. The solid remaining after vacuum
evaporation was extracted with 10 mL of CH2Cl2, followed by filtration
through Celite. Evaporation of the resulting filtrate provided a yellow-
orange powder which was recrystallized from CH2Cl2/hexanes, giving
1+ does, indeed, form positively charged adducts with one-
electron donors, allowing the metal to return to its 18-electron,
Re(I), electronic state. In the absence of other agents, the radical
1+ exists in equilibrium with the weakly Re-Re bonded dimeric
dication [Re2Cp2(CO)6]2+, 122+. The former is shown to function
as a strong one-electron oxidant and to be capable of initiating
homolytic cleavage reactions of C-H and C-halogen bonds.
DFT calculations and X-ray crystallographic results are reported
(6) (a) Adams, R. D.; Collins, D. E.; Cotton, F. A. J. Am. Chem. Soc. 1974,
96, 749. (b) Landrum, J. T.; Hoff, C. D. J. Organomet. Chem. 1985, 282,
215. (c) Hackett, P.; O’Neill, P. S.; Manning, A. R. J. Chem. Soc., Dalton
Trans. 1974, 1625. (d) Madach, T.; Vahrenkamp, H. Z. Naturforsch. 1979,
34b, 573. (e) Goh, L.-Y.; D’ Aniello, M. J.; Slater, S.; Muetterties, E. L.;
Tavanaiepour, I.; Chang, M. I.; Fredrich, M. F.; Day, V. W. Inorg. Chem.
1979, 18, 192. (f) Cooley, N. A.; Watson, K. A.; Fortier, S.; Baird, M. C.
Organometallics 1986, 5, 2563. (g) Keller, H. J. Z. Naturforsch. 1968, 23b,
133. (h) Madach, T.; Vahrenkamp, H. Z. Naturforsch. 1978, 33b, 1301. (i)
McLain, S. J. J. Am. Chem. Soc. 1988, 110, 643. (j) Cooley, N. A.;
MacConnachie, P. T. F.; Baird, M. C. Polyhedron 1988, 7, 1965. (k) Jaeger,
T. J.; Baird, M. C. Organometallics 1988, 7, 1965. (l) Wrighton, M. S.;
Ginley, D. S. J. Am. Chem. Soc. 1975, 97, 4246. (m) Hughey, J. L.; Bock,
C. R.; Meyer, T. J. J. Am. Chem. Soc. 1975, 97, 4440. (n) Laine, R. M.;
Ford, P. C. Inorg. Chem. 1977, 16, 388. (o) Watkins, W. C.; Jaeger, T.;
Kidd, C. E.; Fortier, S.; Baird, M. C.; Kiss, G.; Roper, G. C.; Hoff, C. D.
J. Am. Chem. Soc. 1992, 114, 907. (p) Goh, L. Y.; Habley, T. W.;
Darensbourg, D. J.; Reibenspies, J. J. Organomet. Chem. 1990, 381, 349.
(q) Lin, G.; Wong, W.-T. J. Organomet. Chem. 1996, 522, 271. (r) Clegg,
W.; Compton, N. A.; Errington, R. J.; Norman, N. C. Acta Crystallogr.
1988, C44, 568. (s) Rheingold, A. L.; Harper, J. R. Acta Crystallogr. 1991,
C47, 184. (t) Richards, T. C.; Geiger, W. E.; Baird, M. C. Organometallics
1994, 13, 4494. (u) Woska, D. C.; Ni, Y.; Wayland, B. B. Inorg. Chem.
1999, 38, 4135. (v) O’ Callaghan, K. A. E.; Brown, S. J.; Page, J. A.;
Baird, M. C.; Richards, T. C.; Geiger, W. E. Organometallics 1991, 10,
3119. (w) Yao, Q.; Bakac, A.; Espenson, J. H. Organometallics 1993, 12,
2010. (x) Kadish, K. M.; Lacombe, D. A.; Anderson, J. E. Inorg. Chem.
1986, 25, 2246. (y) Brownie, J. H.; Baird, M. C.; Laws, D. R.; Geiger, W.
E. Organometallics 2007, 26, 5890.
(7) (a) Alvarez, M. A.; Garc´ıa, G.; Garc´ıa, E.; Riera, V.; Ruiz, M. A.;
Lanfranchi, M.; Tiripicchio, A. Organometallics 1999, 18, 4509. (b) Beck,
W.; Su¨nkel, K. Chem. ReV. 1988, 88, 1405. (c) Camire, N.; Nafady, A.;
Geiger, W. E. J. Am. Chem. Soc. 2002, 124, 7260.
(8) Dileptic describes the metal complex as containing only two types of ligands
(Cp and CO in this case) and emphasizes its “parental”, i.e., unsubstituted,
character. For a recent review on homoleptic metal carbonyl cations, see:
Xu, Q. Coord. Chem. ReV. 2002, 231, 83.
(9) Nafady, A.; Costa, P. J.; Calhorda, M. J.; Geiger, W. E. J. Am. Chem. Soc.
2006, 128, 16587.
120 mg (90%) of [4][B(C6F5)4]. IR (CH2Cl2,) νCO : 2139, 2084 cm-1
;
ν(B(C6F5)4): 1643, 1515, 1461, 1275, 1080, 979 cm-1. 1H NMR (CD2-
Cl2): δ (ppm vs TMS) 6.56 (s). Anal. Calcd for [ReCp(CO)3Cl][TFAB]:
C, 36.61; H, 0.48. Found: C, 36.70; H, 0.59.
(10) (a) Barrie´re, F.; Geiger, W. E. J. Am. Chem. Soc. 2006, 128, 3980. (b)
LeSuer, R. J.; Buttolph, C.; Geiger, W. E. Anal. Chem. 2004, 76, 6395. (c)
Camire, N.; Mueller-Westerhoff, U. T.; Geiger, W. E. J. Organomet. Chem.
2001, 637-639, 823. (d) LeSuer, R. J.; Geiger, W. E. Angew. Chem., Int.
Ed. 2000, 39, 248. (e) Hill, M. G.; Lamanna, W. M.; Mann, K. R. Inorg.
Chem. 1991, 30, 4687.
(12) (a) Nesmeyanov, A. N.; Anisimov, K. N.; Kolobova, N. E.; Markarov,
Yu. V. IzVetiya Akademii Nauk SSSR, Seriya Khimicheskaya 1968, 6, 1421.
(b) Holovics, T. C.; Deplazes, S. F.; Toriyama, M.; Powell, D. R.;
Lushington, G. H.; Barybin, M. V. Organometallics 2004, 23, 2927. (c)
Regitz, M.; Hocker, J.; Liedhegener, A. Org. Synth. 1973, 5, 179. (d) King,
R. B. J. Inorg. Nucl. Chem. 1967, 29, 2119.
(11) Chong, D.; Nafady, A.; Costa, P. J.; Calhorda, M. J.; Geiger, W. E. J. Am.
Chem. Soc. 2005, 127, 15676.
9
J. AM. CHEM. SOC. VOL. 130, NO. 8, 2008 2693

A R T I C L E S
Chong et al.
2.3. [Re2Cp2(CO)6][B(C6F5)4]2, [12][TFAB]2. 37 mg (0.11 mmol)
of ReCp(CO)3 in 15 mL of CH2Cl2/0.05 M [NBu4][B(C6F5)4] were
added to the working electrode compartment (see electrochemical
section below) and electrolyzed, with stirring, using Eappl ) 1.3 V at
243 K. The reduced temperature takes advantage of the poor solubility
of the product at this temperature, allowing the yellow dimer dication
to precipitate as the oxidation proceeds. The progress of the electrolysis
was followed by coulometry, which required precisely 1.0 F for
completion. At that point, the only product waves observed in CV scans
chemistry was carried out under Schlenk-type conditions, and other
electrochemical experiments were conducted inside a Vacuum Atmo-
spheres drybox, which was outfitted with a cooling bath capable of
controlling solution temperatures to about 1 °C. Oxygen levels in the
drybox were typically 1-5 ppm during the course of an experiment.
Bulk electrolyses were carried out in a three-compartment “H-type”
cell having counter and working compartments separated by a fine glass
frit. The working electrode was a basket-shaped Pt gauze which had
been treated with nitric acid, washed copiously with distilled water,
and dried by first leaving it overnight at 120 K, followed by evacuation.
Voltammetry scans were recorded using a glassy carbon working
electrode disk of either 1 or 2 mm diameter (Bioanalytical Systems),
the effective areas of which were checked by chronoamperometric
measurements of ferrocene in acetonitrile/0.1 M [NBu4][PF6], for which
a diffusion coefficient of 2.4 × 10-5 cm2 s-1 has been reported.14 The
disks were pretreated using a standard sequence of polishing with
diamond paste (Buehler) of decreasing sizes (3 to 0.25 µm) interspersed
by washings with nanopure water, and finally vacuum drying. All
potentials given in this paper are referred to the ferrocene (FcH)/
ferrocenium reference couple.15 Mechanistic aspects of redox reactions
were probed by applying the appropriate diagnostic criteria to cyclic
voltammetry (CV) data, using procedures described elsewhere.16
2+
were those of the slightly soluble 12 (Epc ) 0.55 V, scan rate 0.2 V
s-1) and the soluble side product [ReCp(CO)3Cl]+, 4 (Epc ) -0.16 V).
The contents of the working electrode compartment were then filtered
through a fine glass frit and washed three times with 4 mL portions of
cold CH2Cl2, providing 78 to 89 mg (70-80%) of analytically pure
[12][TFAB]2, which was stable indefinitely if kept in the drybox at
room temperature. Electrolyses were also conducted on a slightly larger
scale, starting with about 60 mg of 1, with somewhat lower yields (ca.
55-65%) of dimer. In this case, precipitation onto the platinum gauze
caused periodic electrode passivation, typically two or three times during
the electrolysis, at which time the working electrode was removed and
cleaned by rinsing it with CH2Cl2 or C2H4Cl2 at ambient temperature.
Anal. Calcd for [12][TFAB]2: C, 37.88; H, 0.50. Found: C, 37.55; H,
0.50. IR (CH2Cl2): νCO 2105, 2031 cm-1; IR (nujol): νCO 2113, 2098,
2.8. Digital Simulations. Digital simulations of the background-
subtracted and iR-compensated CV data for compounds 1-3 were
performed using Digisim 3.0 (Bioanalytical Systems). Fitting the initial
anodic process involving the one-electron oxidation (e.g., 1/1+) and
coupled dimerization (e.g., 2 1+ f 122+), an ECdim process, gave reliable
values for E1/2 and the lower limit of Kdim (Vide infra; see discussion
of simulations for 1). The reverse cathodic feature, arising from the
1
2046, 2037 cm-1. H NMR (CD2Cl2): δ (ppm vs TMS) 6.40 (s).
2.4. [Re2(C5H4NH2)2(CO)6][B(C6F5)4]2, [22][TFAB]2. An electrolytic
procedure at 243 K using Eappl ) 1.0 V, but otherwise similar to that
detailed above for [12] [TFAB]2, gave 50-60% yield of the pure yellow
dimer [22][TFAB]2. The electrolysis solution commonly became
supersaturated with the dimer dication, and a way around this slow
precipitation of the desired product was found by happenstance (see
discussion in text). Anal. Calcd: C, 37.33; H, 0.59; N, 1.36. Found:
C, 36.80; H, 0.84; N, 1.40. IR (C2H4Cl2): νCO 2071, 2007 (with
asymmetry on high-frequency side) cm-1. IR (KBr): νCO 2083 vs, 2022
2+
reduction of the dimer dication 12 back to the starting material 1,
was only simulated for the purpose of having a reasonable visual fit.
As seen in the figures to follow, this cathodic feature was broad and
highly electrochemically irreversible. It almost certainly involves a net
transfer of two electrons, most likely through an EirrevCirrevE mechanism,
in which the chemical step, Cirrev, is cleavage of the Re-Re bond in
an intermediate dimer monocation 12+. It is not likely that a unique fit
of the electrochemical and chemical parameters could be found for such
a mechanism. Therefore we modeled these cathodic back-reactions with
the simpler EirrevE model, using values for Eirrev that befit a very slow
charge-transfer rate (ks ca. 10-5 cm s-1), with charge-transfer coefficients
(R) less than 0.5 (typically, 0.3 to 0.35) and diffusion coefficients for
1
sh, 2012 vs cm-1. H NMR (CD3NO2): δ (ppm vs TMS) 6.54 (broad
s, 4H), 6.22 (t, 4H), 5.47 (t, 4H). X-ray quality crystals of [22][TFAB]2
were grown from a solution of the dimer dication in nitromethane, to
which a small amount of dichloromethane had been added until the
first observed level of cloudiness. Kept overnight at 253 K, the desired
crystals were obtained.
2.5. [Re2Cp*2(CO)6][B(C6F5)4]2, [32][TFAB]2. As 50 mg of ReCp*-
(CO)3 dissolved in 10 mL of CH2Cl2/0.05 M [NBu4][TFAB] were
electrolyzed at 243 K, Eappl ) 1.1 V, the solution turned deep yellow.
Near the end of the electrolysis, when the current was only a few % of
its initial value, precipitation of the yellow-orange solid occurred. After
being kept cold for another 20 min, the solution was filtered and washed
with cold CH2Cl2 and hexanes to remove electrolyte and any unreacted
starting material. Evaporation gave 105 mg (79%) of analytically pure
[Re2Cp*2(CO)6][TFAB]2. νCO(CH2Cl2) ) 2062, 2018, 2010 cm-1, Anal.
Calcd: C, 40.97; H, 1.38. Found: C, 40.76; H 1.28.
2+
12 and the other dimers that simply fit with the observed cathodic
peak currents and shapes. Only the values derived for the initial ECdim
process should be taken to have quantitative significance.
2.9. Spectroscopy. IR spectra were recorded with an ATI-Mattson
Infinity Series FTIR interfaced to a computer employing Winfirst
software at a resolution of 4 cm-1, and NMR spectra were recorded
using a Bruker ARX 500 MHz spectrometer. In situ IR spectro-
electrochemistry17 was performed using a mid-IR fiber-optic “dip” probe
(Remspec, Inc) with a standard H-type electrolysis cell under argon.
In these experiments, the bulk electrolysis was intermittently halted at
selected Coulombic points in order to record the spectrum of the
electrolysis solution. The spectra typically took 2 to 3 min to record,
after which the electrolysis was continued to another stopping point
until the electrolysis current was 1% or less of the original value. UV-
vis spectra were recorded with an Olis Cary-14 spectrometer.
2.6. [Fe(C5H4COCH3)2][B(C6F5)4]. To a solution of 19 mg (7.0
µmol) of bis(acetylcyclopentadienyl)iron (Aldrich) in 1 mL of CH2Cl2
a solution of 7 mg (3.45 µmol) of [Re2Cp2(CO)6][B(C6F5)4]2 in 2 mL
of CH2Cl2 was added slowly, with stirring, at 263 K. An immediate
color change from orange to green occurred. The solvent was removed
in Vacuo to provide a green solid which was washed 10 times with
n-hexane to remove ReCp(CO)3 and any of the unreacted ferrocene
derivative. Vacuum evaporation provided 59 mg (89%) of analytically
pure green microcrystals of [Fe(C5H4COCH3)2][B(C6F5)4]. Anal. Calcd.
for C38H14O2F20BFe: C, 48.08; H, 1.49. Found: C, 47.95; H, 1.60.
λmax (CH2Cl2) ) 656 nm (ꢀ ) 270 M-1 cm-1). IR (nujol): νCO 1686
cm-1 (lit.: 1687 cm-1).13
2.10. X-ray Crystallography. Crystallographic data are collected
in Table 1. Data were collected on a Bruker D8 platform diffractometer
equipped with an APEX CCD detector. The structure was solved by a
(14) Hershberger, J. W.; Klingler, R. J.; Kochi, J. K. J. Am. Chem. Soc. 1983,
105, 61.
2.7. Electrochemistry. A standard three-electrode configuration was
employed in conjunction with a PARC 273A potentiostat interfaced to
a personal computer through homemade software. IR spectroelectro-
(15) (a) Gritzner, G.; Kuta, J. Pure Appl. Chem. 1984, 56, 461. (b) Connelly,
N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877.
(16) Geiger, W. E. In Laboratory Techniques in Electrochemistry, 2nd ed.;
Kissinger, P. T., Heineman, W. R., Eds.; Marcel Dekker: New York, 1996;
Chapter 23.
(13) Guillon, C.; Vierling, P. J. Organomet. Chem. 1994, 464, C42.
(17) Shaw, M. J.; Geiger, W. E. Organometallics 1996, 15, 13.
9
2694 J. AM. CHEM. SOC. VOL. 130, NO. 8, 2008

[Re(η
5-C5H5)(CO)3]+ Family of 17-Electron Compounds
A R T I C L E S
Table 1. Crystal Data and Structure Refinement for
[22][TFAB]2‚2H2O
Table 2. Listing of ν(CO) Infrared and Redox Potentials for Re
Compounds
1
(CO)/cm-
E1
Epc
empirical formula
formula weight
temperature
crystal system
space group
C64H12B2F40N2O6Re2‚2H2O
ν
/
2
2094.82
100(2) K
triclinic
P1h
/+
compounds
in CH2Cl2
in nujol
V vs Cp2Fe0
1
12
2024, 1926
2105, 2031
1.16
0.79
0.91
2+
2113, 2098,
2046, 2037
0.55
unit cell dimensions
a ) 11.9709(5) Å
b ) 12.1176(5) Å
c ) 12.3692(6) Å
1570.85(12) Å3
1
R ) 94.268(1)°
â) 92.209(1)°
γ ) 118.233(1)°
2
22
2017,1919
2+
2071, 2007a
2083, 2012
(in KBr)
-0.01
volume
Z
density (calculated)
absorption coefficient
(Mo KR)
F(000)
crystal size
crystal color, habit
θ range for data
collection
3
32
2005, 1906
2062, 2015
2140, 2085
2.214 g/cm3
2+
2062, 2015
0.15
-0.16
4.031 mm-1
4
998
a These data were obtained in dichloroethane, in which the dimer dication
is more soluble. Values of 2076 and 2010 cm-1 were obtained in situ when
222+ was electrolytically generated in CH2Cl2, and these values were used
to compute the raw carbonyl shift discussed in the text.
0.31 × 0.21 × 0.11 mm3
yellow, plate
1.66 to 28.31°
reflections collected
independent reflections
data/restraints/
parameters
13063
6866 [R(int) ) 0.0215]
6866/3/544
O, 1.520 Å; H, 1.200 Å.25 The oxidation potentials were calculated as
described elsewhere.9 Free energy changes (∆G), as well as inclusion
of the zero-point energy (ZPE), should be considered for the M f M+
+ e- reaction and for the calculation of dimerization energies. This
requires, however, the demanding computation of the vibrational
frequencies. As Ziegler has shown26 that the electrochemical behavior
of cyclooctatetraene is mainly determined by electronic enthalpies, we
calculated redox potentials based on electronic enthalpies, rather than
free energies. The good agreement between calculated and experimental
data validated this approximation. The dimerization energies were also
computed based on electronic enthalpies, although the entropic
contributions are expected to be relevant for an associative process.
goodness-of-fit on F2
1.065
final R indices [I > 2σ(I)] R1 ) 0.0241, wR2 ) 0.0607
R indices (all data) R1 ) 0.0257, wR2 ) 0.0616
largest diff. peak and hole 1.552 and -0.712 e Å-3
Patterson projection. All non-hydrogen atoms were refined anisotro-
pically, and the hydrogen atoms, except for those on the water molecule,
were placed in idealized locations. The asymmetric unit contained two
molecules of water from unknown sources for each Re dimer. The
hydrogen atoms on the unique water molecule were located, and the
geometry of the water molecule was restrained during refinement. All
software was contained in the SMART, SAINT, and SHELXTL
libraries distributed by Bruker AXS, Madison, WI.
3. Results and Discussion
3.1. General Anodic Behavior. The half-sandwich com-
plexes 1-3 share the same primary voltammetric characteristic,
namely a partly chemically reversible one-electron oxidation
at fairly positive potentials which gives, in addition to the radical
cation, a product having a broad and irreversible cathodic wave
about 0.6 to 0.8 V negative of the original anodic wave. The
characteristics of the voltammetry vary with changes in tem-
perature, scan rate, and concentrations of the analyte, arising,
as will be shown below, from an ECdim process (eqs 1 and 2)
for the anodic wave and an overall two-electron, electrochemi-
cally irreversible, process (eq 3) for the cathodic wave.
2.11. DFT Calculations. All Density Functional Theory calcula-
tions18 were performed using the Amsterdam Density Functional
program package (ADF, versions 2004 and 2005).19 Gradient corrected
geometry optimizations20 (gas-phase and solvent) were performed
without symmetry constraints using the Local Density Approximation
of the correlation energy (Vosko-Wilk-Nusair),21a augmented by the
exchange-correlation functional of Perdew-Wang (PW91).21b Triple-ú
Slater-type orbitals (STOs) were used to describe the valence shells of
C, O, H, and Re, with a set of two polarization functions (p,f for Re;
d,f for C, O; and p,d for H). The core orbitals were frozen for Re ([1-
4]s, [2-4]p, [3-4]d), C, and O (1s). The relativistic effects were treated
with the ZORA approximation.22 The solvent effects were included
using the COnductor like Screening MOdel (COSMO)23 implemented
in ADF. The rigid sphere radius of the CH2Cl2 solvent molecules was
taken to be 2.449 Å,24 and the dielectric constant was 8.93. The van
der Waals radii of the atoms were used for the atomic radius of the
solute (or atomic radius in the case of Re): Re, 1.370 Å; C, 1.700 Å;
ReCpγ(CO)3 - e- h [ReCpγ(CO)3]+
E1/2 (1)
where Cpγ ) Cp (1), C5H4(NH2) (2), or Cp* (3)
2 [ReCpγ(CO)3]+ h [Re2Cpγ (CO)6]2+
Kdim (2)
Epc (3)
2
[Re2Cpγ (CO)6]2+ + 2 e- f 2 ReCpγ(CO)3
2
(18) Parr, R. G.; Young, W. Density Functional Theory of Atoms and Molecules;
Oxford University Press: New York, 1989.
(19) (a) te Velde, G.; Bickelhaupt, F. M.; van Gisbergen, S. J. A.; Guerra, C.
F.; Baerends, E. J.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22,
931. (b) Guerra, C. F.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Theor.
Chem. Acc. 1998, 99, 391. (c) ADF2005.01, SCM, Theoretical Chemistry,
Vrije Universiteit, Amsterdam, The Netherlands, http://www.scm.com.
(20) (a) Versluis, L.; Ziegler, T. J. Chem. Phys. 1988, 88, 322. (b) Fan, L.;
Ziegler, T. J. Chem. Phys. 1991, 95, 7401.
(21) (a) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (b)
Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M.
R.; Singh, D. J.; Fiolhais, C. Phys. ReV. 1992, B46, 6671.
(22) van Lenthe, E.; Ehlers, A.; Baerends, E. J. J. Chem. Phys. 1999, 110, 8943.
(23) (a) Klamt, A.; Schu¨u¨rmann, G. J. Chem. Soc., Perkin Trans. 1993, 2, 799.
(b) Klamt, A. J. Phys. Chem. 1995, 99, 2224. (c) Klamt, A.; Jones, V. J.
Chem. Phys. 1996, 105, 9972. (c) Pye, C. C.; Ziegler, T. Theor. Chem.
Acc. 1999, 101, 396.
Although the monomer/dimer equilibrium (eq 2) lies in favor
of the latter, partial dissociation of the dimer gives, at least for
the Cp and Cp* complexes, modest amounts (several percent,
depending on temperature and concentration) of the very reactive
17 e- monomeric cation radical. The dimer dications, which
have Re-Re bonded 18 e- metal centers unsupported by
bridging ligands, are fully characterized, isolable, species.
Relevant electrochemical potentials and infrared frequencies are
collected in Table 2.
(24) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and
Liquids, 4th ed.; McGraw-Hill International: 1987; Appendix B pp 733-
734.
(25) Bondi, A. J. Phys. Chem. 1964, 68, 441.
(26) Baik, M-H.; Schauer, C. K.; Ziegler, T. J. Am. Chem. Soc. 2002, 124, 11167.
9
J. AM. CHEM. SOC. VOL. 130, NO. 8, 2008 2695

A R T I C L E S
Chong et al.
Figure 1. Cyclic voltammograms (CVs) of 1 in CH2Cl2/0.05 M [NBu4][TFAB] at 1 mm glassy carbon electrode (gce), under various conditions of concentration
of 1, scan rate, and temperature: (a) 0.64 mM, 1 V s-1, 273 K; (b) 1.5 mM, 0.2 V s-1, 298 K; (c) 1.0 mM, 0.2 V s-1, 243 K; and (d) 5 mM, 0.2 V s-1, 295
K. There is significant ohmic distortion at the highest concentration (d).
Table 3. Digital Simulation Parameters for Anodic Processes of 1,
2, and 3 at Room Temp
3.1.1. ReCp(CO)3, 1. Figure 1, which gives a sampling of
CV scans taken at different scan rates for varying analyte
E
°
ks
(cm s-
D0
(cm2 s-
Kdim
kdim
concentrations, is representative of the voltammetric behavior
of this family of compounds, wherein the observed chemical
reversibility of the [ReCp(CO)3]0/+ couple (E1/2 ) 1.16 V)
decreases at slower scan rates (Figure 1b) and higher analyte
concentrations (Figure 1d, distorted by ohmic loss). The effect
of temperature changes on the chemical reversibility of 1/1+ is
more complex, owing to competition between the kinetic and
thermodynamic aspects of the dimerization. Generally, however,
the irreversible cathodic wave for the dimer is enhanced at lower
temperatures (see Figure 1c, a slow-scan CV of 1 mM 1 at 243
K). The position (Epc) and peak width (δEp ) Ep - Ep/2) of
1
1
1
1
(V)
)
R
)
(M-
)
(M-1 s-
)
10/+ 1.165
20/+ 0.790
30/+ 0.905
0.07
0.07
0.05
0.4 2.10 × 10-5 1.0 × 106 1.0 × 104
0.5 2.10 × 10-5 1.0 × 107 1.2 × 105
0.4 1.73 × 10-5 1.0 × 105 8.5 × 103
Experimental Section). We therefore approached the digital
simulations with two goals in mind: to obtain the value of Kdim
giving the optimum fit with experiment and to also find the
highest Value of Kdim that failed to fit the experimental data.
The former values are given as the “determined” dimerization
equilibrium constants in Table 3, and the latter are quoted in
the text as the lower limits of Kdim. For the Cp compound, a
value of Kdim ) 106 M-1 (Figure 2) gave optimal results, Kdim
) 105 M-1 (lower limit) gave fits that were still judged to be
within experimental error, and Kdim ) 104 M-1 was clearly too
low to account for the experimental data over a range of scan
rates.
2+
this cathodic wave for the reduction of the dimer dication 12
back to 1 are quite temperature and scan rate dependent,
consistent with the behavior of an electrochemically irreversible
process,27 with values of Epc ) 0.55 V and δEp ) 97 mV being
observed for V ) 0.2 V s-1 at ambient temperatures.
There are significant limitations in determining equilibrium
constants for reversible chemical follow-up reactions which
strongly favor the follow-up product, as is the case in the present
monomer/dimer equilibrium. Focusing on the oxidation of 1,
the key voltammetric measureable is the degree of chemical
reversibility of the 1/1+ anodic/cathodic pair, which is small
under our experimental conditions. This analytical problem is
not ameliorated by measurement of the height of the cathodic
wave of the follow-up product (122+), which does not easily
inform us about the concentration of the dimer dication (see
Bulk anodic electrolysis of 1 at Eappl ) 1.3 V in CH2Cl2/
0.05 M [NBu4][TFAB] at room temperature passed precisely
1.0 F and resulted in an orange solution having as the dominant
voltammetric feature the cathodic wave of the dimer dication
at Epc ) 0.55 V. When solutions were re-electrolyzed at Eappl
) 0.3 V, 70-80% of the starting material 1 was regenerated.
If the anodic electrolysis was carried out at 243 K, or the solution
was cooled to that temperature after an electrolysis at 273 K, a
yellow solid was obtained that was analyzed for pure dimer
dication, [12][TFAB]2, typically in 70-80% isolated yield. An
(27) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.; John Wiley
& Sons: New York, 2000; pp 234 ff.
9
2696 J. AM. CHEM. SOC. VOL. 130, NO. 8, 2008

[Re(η
5-C5H5)(CO)3]+ Family of 17-Electron Compounds
A R T I C L E S
Figure 2. Comparison of experimental (solid) and simulated (circles) CV curves for 1.07 mM 1 in CH2Cl2/0.05 M [NBu4][TFAB] at 296 K, 1 mm (diam)
gce, 0.5 V s-1. Important simulation parameters in Table 3.
Figure 3. CVs of 1 before electrolysis (arrow pointing left) and after bulk anodic electrolysis at 1.3 V (arrow pointing right), showing cathodic peak for
[ReCp(CO)3Cl]+ side product at Epc ) -0.16 V. Conditions: 1 mM in 1, 1 mm gce, scan rate 0.2 V s-1, room temperature.
electroactive secondary product (Epc ) -0.16 V), later shown
to be the chloride complex [ReCp(CO)3Cl]+, 4 (Vide infra),
accounted for no more than 15% of the products at or below
243 K but as much as 30% when the electrolysis was conducted
at room temperature and/or over longer time frames (Figure 3).
The reaction of 1+ with dichloromethane is described in more
detail below. Bulk electrolytic reduction of this secondary
product using Eappl ) -0.4 V also regenerated 1, through the
reaction 4 + e- f 1 + Cl•.28
The carbonyl-range infrared spectra were investigated under
a number of conditions. A nujol mull spectrum (Figure 4) of
isolated [12][TFAB]2 showed four absorption bands at 2113,
2098, 2046, and 2037 cm-1 which are attributed to a cis/trans
mixture of the dimer dication. Only two bands were observed
(2105, 2031 cm-1) in CH2Cl2 (Supporting Information, Figure
SM1), most likely arising from the thermodynamically more
stable trans isomer.29 A “raw” average shift30 of + 97 cm-1 is
therefore observed for the carbonyl bands in going from 1 to
122+. A fiber-optic spectroelectrochemical experiment shed
further light on the IR properties of the dimer. Electrolysis of
a 2 mM solution of 1 at 243 K gave a single product having
νCO ) 2097 and 2038 cm-1 as the neutral compound was
converted to 122+, but after completion of about three-fourths
of the electrolysis, this pair of bands disappeared (Figure 5),
being replaced by a pair at 2105 and 2031 cm-1, identical to
the those of the isolated product in solution. The original pair
at 2097 and 2038 cm-1 almost certainly does not arise from
the monocation 1+, which is not present in large amounts at
this temperature. Since the in situ IR-electrochemistry method
(29) Room-temperature IR spectra of [12][TFAB]2 in CH2Cl2 showed that the
dimer dication slowly reverted to the neutral complex 1 over a period of a
few hours. No other decomposition or side products were observed in this
process.
(30) The expression “raw” refers to the fact that we are calculating the simple
difference in energies of the two observed bands without taking into account
their degeneracies. The symmetric and asymmetric carbonyl stretches for
the neutral compounds 1-3 are given in Table 2.
(28) The fate of the chlorine atom is unknown. A referee has pointed out that
if Cl• is reduced to Cl- as it is formed, the cathodic reduction of 4 is a net
two-electron process.
9
J. AM. CHEM. SOC. VOL. 130, NO. 8, 2008 2697

A R T I C L E S
Chong et al.
Figure 4. IR spectrum of [Re2Cp2(CO)6][TFAB]2 in nujol.
Figure 5. Fiber-optic IR spectra of 2 mM solution of 1 in CH2Cl2/0.05 M [NBu4][TFAB] as the anodic electrolysis proceeded at 243 K, Eappl ) 1.3 V. The
last spectrum, showing the more intense band at 2105 cm-1, is a superposition of the final two spectra recorded in this sequence.
allows for the voltammetric characterization of the electrolysis
solution at given points in electrolysis time, we were able to
confirm by CV and linear scan voltammetry that the solution
contained mostly the dimer dication at the time of recording
1
0/+) and chemical reversibility (higher scan rates were necessary
to observe cathodic reverse currents for 2+). The irreversible
2+
cathodic peak for reduction of the dimer dication 22 was
observed at Epc ) -0.01 V (ambient temperature, scan rate 0.1
V s-1). Digital simulations (Figure 6) at scan rates from 0.5 to
2 V s-1 at 298 K were best fit with values of Kdim ) 107 M-1
and kdim ) 1.2 × 105 M-1 s-1, consistent with the monomer/
dimer equilibrium of eq 2 lying slightly more in favor of the
dimer for the aminocyclopentadienyl complex compared to its
counterparts having either Cp or Cp* (Vide infra) ligands (for
comparative values, see Table 3). A lower limit of Kdim ) 106
the next-to-last IR scan of Figure 5 (90% electrolysis). We think
2+
it likely that the kinetically favored isomer cis-12
was
responsible for the initial pair of CO bands at 2097 and 2038
cm-1
.
1
The H NMR spectrum of isolated [12][TFAB]2 in CD2Cl2
gave a sharp resonance at δ ) 6.40 ppm. A small singlet at δ
) 5.33 ppm was also present, owing to the slow reduction of
M-1 was obtained for the 2/22 equilibrium.
2+
2+
12 to 1 at room temperature. Addition of ferrocene to this
solution quantitatively reduced the dimer dication to 1.
Bulk anodic electrolysis of 1.1 mM 2 at Eappl ) 0.9 V in
CH2Cl2/0.06 M [NBu4][TFAB] at 298 K passed approximately
1.0 F and resulted in a dark yellow solution having as the
dominant voltammetric feature the cathodic wave attributed to
the dimer dication. The presence of at least two additional minor
3.1.2. Re(C5H4NH2)(CO)3, 2. The aminocyclopentadienyl
complex 2 also undergoes a one-electron oxidation in CH2Cl2/
[NBu4][TFAB], with the 20/+ couple differing from 10/+ in terms
of oxidation potential (E1/2 of 0.79 V, compared to 1.16 V for
9
2698 J. AM. CHEM. SOC. VOL. 130, NO. 8, 2008

[Re(η
5-C5H5)(CO)3]+ Family of 17-Electron Compounds
A R T I C L E S
Figure 6. Experimental and simulated CV curves of 0.5 mM 2 in CH2Cl2/0.1 M [NBu4][TFAB] at room temperature. The most important parameters are
given in Table 3; scan rate ) 0.5 V s-1
.
products was indicated by irreversible cathodic peaks at Epc
)
are within an order of magnitude of those calculated for the
dimerization of 1+ (Table 3), showing that the physical
parameters of the dimerization process are not greatly affected
by permethylation of the Cp ring. In terms of generating stable
solutions of the dimer dication 322+, bulk anodic electrolyses
were somewhat less successful than those used for the Cp and
(C5H4NH2) analogues. A room-temperature electrolysis of 0.82
mM 2 (1.35 F) gave about a 50% yield of secondary products
having cathodic waves at Epc ) -0.50 and -0.85 V. Back-
electrolysis at an Eappl value negative of both side products
regenerated 90% of the starting material 3. The overall chemical
reversibility of these processes suggests that atom-abstraction
reactions producing [ReCp*(CO)3X]+, X ) halide or hydride,
may also be responsible for the side reactions in this case.
Improved yields of the dimer dication were obtained at lower
temperatures: 1.1 F was released for electrolysis of a 1.5 mM
solution at 243 K, with only 10% secondary products being
observed.
-0.43 and -1.18 V. Infrared spectroscopy of samples removed
from this solution showed three significant absorptions in the
metal carbonyl range: two high intensity absorptions attributed
to 222+ at 2076, 2010 cm-1 and one weaker band at 2093 cm-1
attributed to a side product. Bulk cathodic electrolysis at Eappl
) -1.7 V of the oxidized solution yielded the starting material
2 in about 80% yield, as determined by coulometry and linear
scan voltammetry.
,
Anodic electrolysis using a lower temperature (243 K) and
higher (9.6 mM) concentration of 2 allowed isolation of the
dimer dication 222+. As precisely 1.0 F was passed, a yellow-
orange solution was generated and the bright yellow precipitate
that formed was collected,31 washed with CH2Cl2 at 273 K, and
determined to be [22][TFAB]2 by 1H NMR, IR, elemental
analysis, and X-ray diffraction studies. The isolated material
exhibited an IR spectrum (KBr) with absorptions at 2083, 2012
cm-1 and shoulders at ca. 2092, 2022 cm-1 (Supporting
Information, Figure SM2), similar to spectra in C2H4Cl2 (2071,
2007 cm-1, shoulders at ca. 2091, 2034 cm-1) and CH2Cl2
(2076, 2010 cm-1, Table 2). Based on the two most intense
peaks, a raw average shift30 for νCO of +75 cm-1 was calculated
At higher concentrations (5-12 mM) and a temperature of
240 K, anodic electrolyses proceeded smoothly until late in the
electrolysis, when pure [32][TFAB]2 precipitated from the
solution. An isolated yield of 79% was obtained in the
electrolysis of 12 mM 3 at 243 K (see Experimental Section).
IR spectra of this solid in nujol had the highest energy νCO band
in going from 2 IR (2017, 1919 cm-1) to 22 (in CH2Cl2).
2+
3.1.3. ReCp*(CO)3, 3. The original anodic peak (E1/2 ) 0.91
V for 30/+) and the cathodic product peaks (Epc ) 0.15 V for
at 2062 cm-1 and a barely resolved pair at 2018 and 2010 cm-1
.
2+
Unlike the case of 122+, only one isomer was observed,
presumably the trans, in concert with the increased steric
requirements of the Cp* ligand. In dichloromethane, the IR
bands appeared at 2062 and 2015 cm-1, giving a raw average
carbonyl shift30 of +83 cm-1 for 3 f 322+, comparable to the
values of +97 cm-1 and 75 cm-1 observed for the Cp and
amino-Cp complexes 1 and 2, respectively. These carbonyl shifts
are somewhat smaller than those commonly observed for metal
carbonyl compounds differing by one electron. A close analogue
of the present compounds is that of the cobalt system recently
described,9 in which the average νCO shift was +126 cm-1 in
going from CoCp(CO)2 to the metal-metal bonded dimer
dication [Co2Cp2(CO)4]2+. It is useful to keep in mind, however,
32 and Epc ) -0.50 for a secondary product) are shifted
negative compared to 1 in response to the electronic effect of
permethylation of the Cp ring, the shift of -0.25 V in going
from 10/+ to 30/+ being exactly as predicted.32 CV scan rates as
high as 5 V s-1 did not completely outrun the influence of the
dimerization process when concentrations of 1 mM or greater
were studied. Digital simulations (Figure 7) gave good fits of
the CV curves from 0.2 to 5 V s-1, being successfully modeled
by a dimerization equilibrium constant of Kdim ) 105 M-1 and
a dimerization rate constant of 8.5 × 103 M-1 s-1. These values
(31) The electrolysis solution appeared to reach supersaturation, in that
precipitation of [22][TFAB]2 did not occur until minutes after completion
of the electrolysis.
(32) Lu, S.; Stelets, V. V.; Ryan, M. F.; Pietro, W. J.; Lever, A. B. P. Inorg.
Chem. 1996, 35, 1013.
2+
that whereas the IR spectra of 12 and its analogues contain,
9
J. AM. CHEM. SOC. VOL. 130, NO. 8, 2008 2699

A R T I C L E S
Chong et al.
Figure 8. Cationic structure for [22][TFAB]2‚2H2O drawn with 30%
thermal ellipsoids.
and 2.312(3) Å [to C(4)]. Rationalization of the Cp bending is
made more difficult by the fact that the structure of the parent
18-electron complex 2 has not been reported. One cannot rule
out the influence of the water molecule (source unknown) which
is strongly hydrogen bonded to the amino group: O(4)‚‚‚HN-
(1), 2.87 Å.
3.2. Homolytic Cleavage Reactions of [ReCp(CO)3]+. As
mentioned above, an electroactive secondary product (Epc
)
-0.16 V) of roughly 20-30% yield was formed in room-
temperature bulk anodic electrolyses of 1 in dichloromethane.
It was characterized as the 18-electron chlorocomplex [ReCp-
1
(CO)3Cl]+, 4, based on a comparison of its IR and H NMR
properties with an authentic sample of [ReCp(CO)3Cl][TFAB]
(the [SbCl6]- salt12d is insoluble in dichloromethane). To this
end, electrolysis of 1 at room temperature gave, in addition to
the IR bands noted above for 122+, a pair at 2140 and 2085
cm-1 (a raw carbonyl shift30 of +137 cm-1) that was identical
to those measured separately for 4. The NMR spectrum of
authentic 4[TFAB] in CD2Cl2 has its Cp resonance at δ ) 6.56
ppm, the precise position of a singlet obtained after bulk
electrolysis of 1 in CD2Cl2/0.02 M [NBu4][TFAB] at 243 K.
The chlorocomplex 4 is apparently formed through the ho-
molytic cleavage reaction shown in Scheme 1.
Figure 7. Representative comparisons of experimental (line) and simulated
(circles) cyclic voltammograms for 0.86 mM 3 in CH2Cl2/0.1 M [NBu4]-
[TFAB] at room temperature. Parameters as given in Table 3; also: electrode
area 0.0785 cm2; Ru ) 1500 Ω; Cdl ) 5 × 10-8 F; Do of 322+, 8.6 × 10-6
cm2 s-1; scan rate 0.5 V s-1 (top) and 5 V s-1 (bottom).
Scheme 1
in principle, six fundamental CO bands, only two are resolved,
thereby limiting the comparative analysis of the vibrational
behavior of the Re complexes.
3.1.4. X-ray Structure of [22][TFAB]2. The crystalline
structure of the amino-dimer dication, shown in Figure 8,
confirms the transoid orientation of the aminocyclopentadienyl
rings and establishes 3.1097(2) Å as the Re-Re bond length.
Tables of the measured bond lengths and angles are available
as Supporting Information (Tables SM1-SM4), and some
structural aspects are discussed below in comparison to the
computational results on the parent dimer, 122+. One interesting
aspect of the amino-Cp structure is the significant out-of-plane
bend observed for the cyclopentadienyl ring in 222+, for which
C(5) is elevated 0.22 Å above the nearly perfect plane of the
other four ring carbons. The Re-to-C(5) distance lengthens to
2.554(3) Å as a consequence, over 0.3 Å more than the Re-
C(2) and Re-C(3) distances of 2.242(3) and 2.225(3) Å,
respectively. The other Re-C distances are 2.324(3) Å [to C(1)]
In order to probe the generality of such homolytic reactions
for 1+, CV experiments were performed in which the radical
cation was generated in the presence of organic compounds
having C-X or C-H bonds that are weaker than that of the
C-Cl bond in CH2Cl2 (bond dissociation energy, BDE, of 81
kcal/mol).33 Addition of 1 equiv or more of CH3I, CPh3Cl, or
9,10-dihydroanthracene (DHA) resulted, in each case, in
complete disappearance of the cathodic wave of the dimer
2+
dication 12 and appearance of a new irreversible cathodic
feature in the range Epc ) -0.08 to -0.20 V, which we ascribe
to the 18-electron products [ReCp(CO)3I]+, [ReCp(CO)3Cl]+,
9
2700 J. AM. CHEM. SOC. VOL. 130, NO. 8, 2008

[Re(η
5-C5H5)(CO)3]+ Family of 17-Electron Compounds
A R T I C L E S
and [ReCp(CO)3H]+, respectively. The CV observed upon DHA
addition is shown in Supporting Information Figure SM3,
wherein the reversible anodic oxidation (E1/2 ) 0.83 V) of the
anthracene produced through eq 4 is observed. Bulk anodic
electrolysis of 1 in the presence of DHA gave an apparently
complete conversion to [ReCp(CO)3H]+ (Epc ) -0.20 V
assigned to the hydride) and the anthracene radical cation, owing
to the fact that anthracene itself undergoes anodic oxidation (eq
5) at the Eappl value sufficient to oxidize 1.
form of “magic blue”, E1/2 ) 0.70 V) and bis(acetylcyclopen-
tadienyl)ferrocene (E1/2 ) 0.49 V) as target substrates for
electron-transfer reactions. The radical cations of these systems
are among the more popular one-electron oxidizing agents.15b,35
When a 1 mM CH2Cl2 solution of either thianthrene or N(C6H4-
Br-4)3 was treated with a nominally 0.5 mM solution of [12]-
[TFAB]2, the solution changed color immediately to the purple
of the thianthrene cation radical or the blue of [N(C6H4Br-4)3]+,
respectively. Although the oxidized organic products were not
isolated, measurements of the optical spectra of these solutions
showed that the reactions were quantitative in the production
of the organic radical cation.36 The bis(acetylcyclopentadienyl)
ferrocenium complex [Fe(C5H4COMe)2]+ was, however, iso-
lated as the pure TFAB salt in high yield by this method. See
the Experimental Section for details. This general procedure is
promising as a way of readily obtaining TFAB salts of oxidized
systems having potentials (vs FcH) that are about 0.9 V or less.
The oxidizing power of the Re system could be raised by
preparing a Re(II) system substituted on the Cp with electron-
withdrawing groups or by shutting off the dimerization of 1+,
perhaps through surface immobilization of the Re system. These
possibilities are under investigation.
2 [ReCp(CO)3]+ + DHA f
2 [ReCp(CO)3H]+ + anthracene
(4)
(5)
anthracene - e- f [anthracene]+
Although the production of anthracene might possibly proceed
Via an electron-transfer reaction (DHA to 1+) followed by loss
of proton from the [DHA]+ radical cation, it is more likely to
be initiated by loss of an H-atom from DHA, which is known
to be a facile H-atom donor.34 Homolytic cleavage reactions
appear to be a common reaction pathway of organic hydrides
and halides with 1+.
3.4. DFT Calculations. Calculations18,19 were performed on
the cyclopentadienyl system without symmetry constraints to
get the optimized geometries of ReCp(CO)3, [ReCp(CO)3]+, and
the dimer [Re2Cp2(CO)6]2+. Two isomers, cis and trans, were
calculated for the dimeric dication, the trans form being more
stable by 3.7 kcal mol-1. We also analyzed the monocation
radical dimer [Re2Cp2(CO)6]+ (12+). Although it was not
detected electrochemically, it is formally analogous to the radical
dimer [Co2Cp2(CO)4]+ recently reported as the radical-substrate
dimerization product resulting from the oxidation of CoCp-
(CO)2.9 Owing to the fact that solvent interactions are likely to
be important for half-sandwich metal carbonyl cations,9 the
calculations were carried out using the COSMO model24 with
CH2Cl2 as the solvent. The differences in geometry (distances
and angles) from the gas phase to CH2Cl2 are negligible, but
the energies are significantly corrected. The results described
here refer to solvent-based calculations, unless otherwise stated.
The optimized geometries (ADF) of 1 and 1+ are shown in
Figure 9, with some relevant distances and angles.
3.3. Use of [ReCp(CO)3]+ as a One-Electron Oxidant. In
view of the notably positive potential of 10/+ (E1/2 ) 1.16 V),
the feasibility of employing 1+ as a one-electron oxidant for
synthetic purposes was probed. The fact that the dimer dication
[12][TFAB]2 partially dissociates in solution (eq 2) provides the
monomer 1+ for a redox reaction with added substrates, denoted
as Red in eq 6. Based on a Kdim value between 106 and 105
M-1, in a nominally 0.5 mM solution of the pure dimer dication
at 298 K, between 4.5% and 14.6% of the ions at equilibrium
are the monomer radical 1+.
1+ + Red f 1 + [Red]+
(6)
The fact that the monomer/dimer equilibrium is coupled to
one member of the redox pair 1/1+ reduces the effective
oxidizing power of the radical, since the overall ECdim process
of eq 7 has a formal potential, E°′, which is less than the
measured E1/2 of the 10/+ couple, E1/2(Re/Re+) (eq 8).
2 1 h 122+ + 2 e-
E°′
(7)
The calculated structure of 1 is in good agreement with that
determined by X-ray,37 with three C(O)-Re-C(O) angles very
close to 90° and experimental Re-C(O) distances of 1.888,
1.899, and 1.894 Å, compared to the calculated one of 1.917 Å
(although no symmetry constraints were considered, the opti-
mized geometry reflects some symmetry, except for 1+, and
only part of the numbers is thus given).
E°′ ) E1/2(Re/Re+) - 0.0295 log Kdim
(T ) 298 K) (8)
Given a range of 105 to 106 M-1 for Kdim, the oxidizing
potential of the Re system is thus reduced from its intrinsic
value of 1.165 V for the 10/+ couple to 1.01 to 0.99 V when the
oxidant 1+ equilibrates with 122+, which occurs rapidly under
normal reaction conditions. For synthetic applications of the
Re system as an outer-sphere one-electron oxidant, we therefore
chose thianthrene (E1/2 ) 0.81 V), N(C6H4Br-4)3 (the reduced
2+
The structure of the dimer dication 12 can be compared
with the one described above for the amino analogue 222+. The
2+
calculated Re-Re distance in 12 (3.229 Å) is only slightly
2+
longer than the experimental value of 3.0197 Å for 22 (for
(33) Some bond dissociation energies were obtained from the summary in
jhu.edu/chem./lectka and were based on values found in the following
papers: (a) Goldsmith, C. R.; Jonas, R. T.; Stack, D. P. J. Am. Chem. Soc.
2002, 124, 83. (b) Blanksby, S. J.; Ellison, G. B. Acc. Chem. Res. 2003,
36, 255. (c) Senosiain, J. P.; Han, J. H.; Musgrave, C. B.; Golden, D. M.
J. Chem. Soc., Faraday Disc. 2001, 119, 173. (d) McGivern, W. S.;
Derecskei-Kovacs, A.; North, S. W. J. Phys. Chem. A 2000, 104, 436. BDE
values: C-I in CH3I, 57.6 kcal/mol; C-H in 9,10-dihydroanthracene, 78
kcal/mol. For C-Cl in CPh3Cl (50 kcal/mol), see: Ko¨cher, J.; Lehnig,
M.; Neumann, W. P. Organometallics 1988, 7, 1201.
(35) The value for the oxidation of thianthrene was obtained by measurement
in our laboratory in CH2Cl2/0.05 M [NBu4][TFAB]. A somewhat higher
value, 0.86 V, was reported earlier for an acetonitrile/[NBu4][BF4] medium.
See ref 15b and Hammerich, O.; Parker, V. D. Electrochim. Acta 1973,
18, 537. An apparently quantitative oxidation (based on optical spectros-
copy) was also seen when the analyte was Mn(C5H4Me)(CO)3, which has
a potential of 0.87 V in this medium.
(36) (a) Thianthrene cation: Murata, Y.; Shine, H. J. Org. Chem. 1969, 34,
3368. (b) Magic blue: O’Connor, A. R.; Nataro, C.; Golen, J. A.; Rheingold,
A. L. J. Organometal. Chem. 2004, 689, 2411.
(37) Fitzpatrick, P. J.; Le Page, Y.; Butler, I. S. Acta Crystallogr., Sect. B 1981,
37, 1052.
(34) Gupta, R.; MacBeth, C. E.; Young, V. C., Jr.; Borovic, A. S. J. Am. Chem.
Soc. 2002, 124, 1136.
9
J. AM. CHEM. SOC. VOL. 130, NO. 8, 2008 2701

A R T I C L E S
Chong et al.
Figure 9. Optimized structures of, left to right, ReCp(CO)3 (1), [ReCp(CO)3]+ (1+), [Re2Cp2(CO)6]2+(122+), and [Re2Cp2(CO)6]+• (12+).
Table 4. Calculated (ADF) Absolute (En) and Relative (E°n vs
2+
FcH) Oxidation Potentials for the Pairs 1/1+ and 12+/12
Experimental E°n (Bold), and Dimerization Enthalpies ∆Hfn (kcal
mol-1
,
)
E1
E
°
∆
Hf1
∆
Hf2
E2
12+/ 12
E°
2
1
+
2
+
2+
1/1+
(1
+
1+
f
12
)
(1+
+
1+
f
12
)
6.21
1.23
-6.4
(-19.7)
-16.6
5.76 0.78
(10.20) (5.22)
ReCp(CO)3 (8.14)a (3.16)
(27.8)
1.16
-6.8b
Figure 10. HOMO (left) of CpRe(CO)3 (1) and SOMO (right) of [CpRe-
(CO)3]+ (1+).
b
a Gas-phase values. Free energy estimated from Keq ) 105 M-1 9
.
comparison, the gas-phase calculated Re-Re bond length is
3.271 Å for 122+). This may, in fact, reflect the presence of a
somewhat weaker Re-Re bond in 122+ compared to 222+, which
would be qualitatively consistent with the fact that the dimer
of the amino compound dissociates less in solution. The Re-
C(O) bond lengths and angles (Figure 8) show only negligible
deviations from the experimental values. Even the asymmetry
in the C(O)-Re-C(O) angles is well reproduced, with one
wider angle (108.2°) and two narrow ones (∼80°). The trans
form is more stable by 3.7 kcal mol-1 and is indeed the isomer
found in all experimentally available structures of the family.
each other, but the solvent introduces a good correction (-16.6
kcal mol-1). From Keq ) 105 M-1 we can estimate the
experimental value for ∆Gf 2 as -6.8 kcal mol-1. The agreement
between the calculated and experimental values is reasonable
since we are not considering the entropy effects (∆S < 0 for an
associative process) in the calculation. The inclusion of this term
would make ∆Hf 2 less negative. Also, a better solvent model
would probably improve the value.
It can be seen that the formation of the radical dimer 12
+
,
from 1 and 1+, is attractive in the absence of solvent, but the
enthalpy change for the formation of 122+ is more negative than
that associated with formation of 12+. Even if 12+ were formed,
it would be promptly oxidized, as its potential is lower that that
of 1. These data are qualitatively different from those observed
in the related CpCo(CO)2, which we discussed in detail
previously.9
2+
The dicationic complex 12 was initially identified by IR
spectroscopy (Figure 4), and the calculation of frequencies
supported this assignment, as the calculated frequencies agree
2+
with experimental values for both 1 and 12
.
The loss of one electron to form the cation [CpRe(CO)3]+
(1+) introduces some asymmetry into the molecule, as well as
some lengthening of the Re-C(O) bonds and shortening of the
C-O bonds (Figure 9), as a reflection of reduced back-donation
from the oxidized metal center. Indeed, the one-electron loss is
strongly concentrated on the metal, as can be observed in the
representation of the HOMO of 1 (Figure 10), which has also
some metal-carbonyl and metal-Cp bonding character. The
asymmetry concerns the opening of one C(O)-Re-C(O) angle
from ∼90° to ∼100°, while the other two become narrower
(∼85°). At the same time, one of the Cpcent-Re-C(O) angles,
which were ∼125° (octahedral angle), drops to 116.9°, and the
others widen by ca. 1°-2°. This enables the SOMO of 1+ to
change its directionality and become more hybridized away from
the metal. The SOMO of the radical cation 1+ thus explains
the easy dimerization to form 122+and other reactions involving
seven-coordinate products or intermediates.
4. Conclusions
As an important member of the piano-stool family of
organometallic compounds, the robust,2c IR-active, easily
derivatized,1-3,38 and potentially radioactively labelled1 ReCp-
(CO)3 has become widely scrutinized in recent years. However,
its electrochemical behavior is only now being seriously probed.
The basic molecular orbital model for ReCp(CO)3 describes
it as having a quasi-octahedral electronic structure that is
reminiscent of ferrocene, at least in terms of its highest occupied
orbitals.39 On that basis, ReCp(CO)3 might be expected to
undergo a one-electron oxidation, albeit at a comparatively
positive potential owing to its three highly electron-withdrawing
carbonyl ligands. The potential of 1.16
V found for
[ReCp(CO)3]0/+ is in concert with this expectation. Less
predictable, however, were the chemical properties of the 17-
electron Re(II) species. Compared to the ferrocenium ion, the
metal center in the rhenium system is much more likely to take
The oxidation potentials were also calculated for the mono-
meric and the dimeric systems, and the results are shown in
Table 4. The experimental oxidation potential (1.16 V) for the
pair 1/1+ is very well reproduced by calculations (1.23 V), as
long as solvent is considered. In the gas-phase calculation, there
is a larger deviation. The other experimental parameter that can
also be calculated is the dimerization enthalpy of 122+ from two
radical cations 1+. The gas-phase value is repulsive (27.8 kcal
mol-1), since two positively charged species are approaching
(38) (a) Djukic, J-K.; Michon, C.; Heiser, D.; Kyritsakas-Gruber, N.; de Cian,
A.; Do¨tz, K. H.; Pfeffer, M. Eur. J. Inorg. Chem. 2004, 2107 (b) Cesati,
R. R.; Katzenellenbogen, J. A. J. Am. Chem. Soc. 2001, 123, 4093. (c)
Minutolo, J. A.; Katzenellenbogen, J. A. Angew. Chem., Int. Ed. 1999, 38,
1617.
(39) (a) Veiros, L. F. J. Organometal. Chem. 1999, 587, 221. (b) Albright, T.
A.; Burdett, J. K.; Whangbo, M-H. Orbital Interactions in Chemistry; John
Wiley & Sons: New York, 1985; pp 384-386, 392-394.
9
2702 J. AM. CHEM. SOC. VOL. 130, NO. 8, 2008

[Re(η
5-C5H5)(CO)3]+ Family of 17-Electron Compounds
A R T I C L E S
2+
on an additional coordinative bond, even in the absence of Cp
slippage, owing to its greater (third-row) size.40 With 1+,
coordinative expansion occurs through formation of a Re-Re
12 cleaves the Re-Re bond (calculated Re-Re distance of
4.37 Å in 12+, Figure 9), ripping the dimer apart and accounting
for the slow heterogeneous charge transfer associated with the
electrochemically irreversible cathodic process.
2+
bond in the dimer dication 12 or, more slowly, by atom-
addition reactions, giving 18-electron seven-coordinate species.
The DFT results on 1 and 1+ give a clear picture of the orbital
makeup that leads to this behavior. Removal of one electron
from the mostly metal-based HOMO of 1 significantly lowers
the symmetry of the compound, with the resulting rehybridized
SOMO of 1+ being directed away from the metal, prompting
the high reactivity of the Re(II) center.
The 17-electron monocations of all three Re systems studied,
1-3, were detected by cyclic voltammetry and shown to
undergo rapid dimerization giving dications in which the Re-
Re bond was unsupported by bridging ligands. The metal-metal
bonds of these systems are sufficiently weak to allow partial
dissociation in solution, resulting in an equilibrium between
monomers and dimers, about 4-14% monomer being calculated
for half-millimolar solutions of 122+. The dimer dissociation was
less prominent for the amino-sustituted compound, perhaps due
to the higher metal electron density conveyed by the electron-
releasing NH2 group. The dimer dications underwent highly
irreversible two-electron reductions, regenerating the corre-
sponding original neutral 18-electron complexes. DFT calcula-
tions showed that addition of one electron to the dimer dication
The very positive potentials of the monomer cations make
them very strong outer-sphere oxidants which can be readily
employed in electron-transfer reactions by taking advantage of
the dissociation of the more easily isolated dimer dications.
Given the potent and geometrically unencumbered nature of the
Re(II) center, the reactions of 1+ with added substrates promise
to be of interest, and we are currently pursuing inquiries into
possible inner-sphere ET reactions with organic systems.
Acknowledgment. We are grateful to the National Science
Foundation (CHE 04-11703) for support of this work at the
University of Vermont and to FCT and FEDER (POCI/QUIM/
58925/2004) for the work in Lisbon. P.J.C. acknowledges FCT
for the grant SFRH/BD/10535/2002, and W.E.G. thanks Boulder
Scientific Co. for a partial donation of electrolyte anions.
Supporting Information Available: Three figures, two show-
ing the IR spectra recorded for isolated [12][TFAB]2 in
dichloromethane and of [22][TFAB]2 in KBr, the third showing
the CV curve when dihydroanthracene is added to a solution
of 1; four tables giving bond lengths and angles from X-ray
analysis of [Re2(C5H4NH2)2(CO)6][TFAB]2. This material is
available free of charge via the Internet at http://pubs.acs.org.
(40) Veiros, L. F. Organometallics 2000, 19, 5549.
JA710513N
9
J. AM. CHEM. SOC. VOL. 130, NO. 8, 2008 2703