Characterization and reactions of previously elusive 17-electron cations: Electrochemical oxidations of (C6H6)Cr(CO)3 and (C5H5)Co(CO)2 in the presence of [B(C6F5)4]

Article abstract of DOI:10.1021/ja012641f

The electrochemical oxidations of (C₆H₆)Cr(CO)₃, 1, and (C₅H₅)Co(CO)₂, 2, when carried out in CH₂Cl₂/[NBu₄][B(C₆F₅)₄], allow the physical or chemical characterization of the 17-electron cations 1⁺ and 2⁺ at room temperature. The generation of 1⁺ on a synthetic time scale permits an electrochemical switch process involving facile substitution of CO by PPh₃ as a route to (C₆H₆)Cr(CO)₂PPh₃. The radical 2⁺ undergoes a second-order reaction to give a product assigned as the metal-metal bonded dimer dication [Cp₂Co₂(CO)₄]²⁺. The new anodic chemistry of these often-studied 18-electron compounds is made possible by increases in the solubility and thermal stability of the cation radicals in media containing the poorly nucleophilic anion [B(C₆F₅)4]^-, TFAB. Copyright

Full text of DOI:10.1021/ja012641f

Published on Web 05/31/2002
Characterization and Reactions of Previously Elusive 17-Electron Cations:
Electrochemical Oxidations of (C6H6)Cr(CO)3 and (C5H5)Co(CO)2 in the
Presence of [B(C6F5)4]-
Nicole Camire, Ayman Nafady, and William E. Geiger*
Department of Chemistry, UniVersity of Vermont, Burlington, Vermont 05405
Received December 3, 2001
We report the electrochemical generation of the 17-electron
radicals [(C6H6)Cr(CO)3]+, 1+, and [(C5H5)Co(CO)2]+, 2+, under
conditions of enhanced stability which allow the systematic study
of their reactions. The key modification of earlier attempts1 to gen-
erate persistent solutions of these cations is the use of [B(C6F5)4]-,
an ion of limited nucleophilicity, as the supporting electrolyte anion.
Half-sandwich complexes of the type (π-ArL)M(CO)n, where
(π-ArL) is a planar aromatic ligand and n is the number of carbonyl
groups required to achieve an 18-electron configuration,2 are of
both historical and practical importance in organometallic chem-
istry.3 The considerable efforts to generate corresponding 17-elec-
tron cations on synthetically useful time scales have failed except
for derivatives either having substituted π ligands or in which at
least one CO has been replaced by an electron-donating ligand such
as PR3.4 It is generally understood that the electrophilicity of the
17-electron cations leaves them susceptible to cleavage reactions
Figure 1. IR spectrum at room temperature in CO region for 1+ in CH2-
with either donor solvents or other weak nucleophiles, including
Cl2/[NBu4][TFAB] after bulk anodic electrolysis of 2 mM 1.
the supporting electrolyte anions. Perhaps the best understood of
these processes involves 1+, which decomposes with loss of benzene
Scheme 1
after attack by anions such as [ClO4]- [BF4]-, or, more slowly,
,
[PF6]- 1b,5 The recent emergence6 of large fluoroaryl borate anions
to implement the stabilization of reactive cations7 raises the pos-
sibility of their utility in improving anodic reaction chemistry. Very
promising results have already been reported using either [B(C6H3-
(CF3)2)4]-8 or the closely related and even less nucleophilic6b,9 anion
PPh3]+, 3+ (E1/2 ) -0.22 V, matching that of an authentic sample
[B(C6F5)4]-, TFAB.10,11 We now show that the TFAB anion suc-
of 3). Cathodic re-electrolysis of the dark orange solutions of 3+-
ceeds in stabilizing 1+, permitting an electrochemical switch mech-
anism for synthetic-level CO-substitution reactions of (benzene)-
gave neutral 3 in yields of up to 80%. By taking advantage of the
increase in CO substitution rate13 in 1+over 1, the combined forward
Cr(CO)3. The same electrolyte also allows the characterization of
and back electrolyses provide isolable14 3 through an “electrochemi-
the 2/2+ couple by cyclic voltammetry (CV) and permits the study
cal switch” mechanism (Scheme 1) which may be useful in pro-
moting the substitution chemistry of the important class15 of (arene)-
Cr(CO)3 compounds.
In contrast to the extensive previous CV evidence for transient
of dimerization processes and CO-substitution reactions of 2+.
The one-electron oxidation of 1 to 1+ (E1/2 ) 0.44 V vs Fc) is
chemically reversible by CV at room temperature in CH2Cl2/ 0.1
1+, the conceptually related complex 2+ has avoided detection. The
cation 2+ forms a mercurous complex when anodized at a Hg elec-
trode16 and loses CO when generated by Ag(I).17 The CV scans of
some derivatized products such as CpCo(CO)(PPh3)4(b) and (C5Me5)-
M [NBu4][TFAB]. Exhaustive one-electron oxidation (1.00 ( 0.04
F/equiv) at 273 K of 1-2 mM 1 at Eappl ) 0.8 V gave dark yellow
solutions of the monocation 1+. After more than 1 h, cathodic
electrolysis at Eappl ) 0.2 V regenerated 80-85% 1. Solutions of
1+ were removed for EPR analysis (g| ) 2.017, g ) 1.979 at 149
K, silent in fluid solution12) and for IR spectroscopy (Figure 1).
The increase in νCO (1971 and 1891 cm-1 in 1; 2079 and 2009
cm-1 in 1+) is as expected for a one-electron oxidation. Consistent
with previous literature,1anodic electrolysis of 1 in the presence of
[PF6]- gives decomposition of the complex, judging by the absence
of products which show either electroactivity or νCO bands.
Treatment of solutions of 1+ with PPh3, or oxidation of 1 in the
presence of PPh3, gave the substitution product [(C6H6)Cr(CO)2-
16
Co(CO)2 have displayed reversible one-electron oxidations.
The voltammetric behavior of 2 in CH2Cl2/ 0.05 M [NBu4]-
[TFAB] depends on scan rate, temperature and, most dramatically,
on the concentration of 2. Whereas low concentrations (e0.3 mM)
give CV wave shapes resembling those of a reversible one-electron
oxidation, as the concentration of 2 is increased the wave broadens
and resolves into two features (Figure 2). Since IR spectra obtained
after bulk electrolysis (Eappl ) 0.7 V, 298 K, 1.0 F/equiv) of 2 mM
2 show only terminal CO bands (νCO ) 2135 and 2104 cm-1), the
net product of the two waves is likely to be the previously
* To whom correspondence should be addressed. E-mail: wgeiger@zoo.uvm.edu.
unreported metal-metal bonded dimeric dication [Cp2Co2(CO)4]2+
,
9
7260
J. AM. CHEM. SOC. 2002, 124, 7260-7261
10.1021/ja012641f CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
partial donation of Li[B(C6F5)4]. W.E.G. also thanks N.G. Connelly
and C. Amatore for helpful comments.
References
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Figure 2. Experimental (dashed and solid lines) and simulated (circles)
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4
based on the common usage of FAB for structurally related B(C6F5)3.
2 (νCO ) 2024 and 1958 cm-1), with a small amount of Cp2Co+ as
a side product. Although the SOMO of 2+ has some cyclopenta-
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suggest that the oxidation of 4+ (E1/2 ) 0.47 V) is thermodynami-
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)
0.37 V).22 Parallel to the behavior of 1+, CV scans of 2 in the
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[CpCo(CO)(PPh3)]+, identified by its E1/2 value (-0.38 V).4b
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[PF6]- eliminates the features ascribed to the 4+/42+ couple,
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The solubility and stability enhancements acquired with
[B(C6F5)4]--containing electrolytes should be applicable to a wide
range of organic, inorganic, and organometallic anodic processes.
Recent synthetic advances6,24 in the preparation of large anions show
that it is increasingly possible to tailor supporting electrolytes to
specific electrochemical objectives. These aims include not only
the presently addressed goals of kinetic stabilization and dissolution
of highly electrophilic cations but also the desires to effect
electrosynthesis in very low-polarity solvents and to control
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xxxxx.
25
comproportionation equilibria in multielectron processes.
Acknowledgment. We are grateful to the National Science
Foundation for financial support, and to Boulder Scientific Co. for
JA012641F
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